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UBC Theses and Dissertations

Development and validation of a new preclinical analgesic assay Asiri, Yahya I. A. 2018

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 Development and Validation of a New Preclinical Analgesic Assay   by Yahya I. A. Asiri B.Pharm, King Khalid University  A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF  DOCTOR OF PHILOSOPHY in THE FACULTY OF GRADUATE AND POSTDOCTORAL STUDIES (Pharmacology)  THE UNIVERSITY OF BRITISH COLUMBIA (Vancouver) August NOPQ  © Yahya I. A. Asiri, NOPQ  ii   The following individuals certify that they have read, and recommend to the Faculty of Graduate and Postdoctoral Studies for acceptance, the dissertation entitled:  Development and Validation of a New Preclinical Analgesic Assay  submitted by Yahya I. A. Asiri  in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Pharmacology  Examining Committee: Dr. Bernard A. MacLeod Co-supervisor Dr. Alasdair M. Barr Co-supervisor  Dr. Ernest Puil Supervisory Committee Member Dr. Michael J. A. Walker University Examiner Dr. John H. McNeill University Examiner   Additional Supervisory Committee Members: Dr. Stephan K. W. Schwarz  Supervisory Committee Member Dr. Brian E. Cairns Supervisory Committee Member   iii  Abstract Current preclinical analgesic assays suffer from major weaknesses that hamper the discovery of new drugs. This thesis describes the development of a new mouse assay, the Hypertonic Saline Analgesia Assay (HSAA). Chapter P provides an overview for current preclinical analgesic assays and discusses their strengths and weaknesses. It also discusses the impact of sex, strain, and circadian rhythm on nociception and antinociception.  Chapter N describes the development of the assay. We evaluated hypertonic saline (HS) as a nociceptive agent, its optimal concentration, the responses, and the optimal time for evaluation. We also assessed sex related differences and tissue damage.  Chapter ] describes the validation of the assay. Analgesics known to be effective clinically were assayed to establish the validity of the HSAA. HSAA demonstrated that responses were attenuated by a range of clinical analgesics, produces minimal tissue damage and has lower variability compared to other assays. Chapter ^ examines the refinement of the assay. We determined the effects of repeated testing, strain, sex, and diurnal rhythm on the reproducibility of HSAA. The absence of tissue damage raised the possibility of repeated testing in a mouse to reduce the number of animals. The antinociceptive action of morphine was unchanged when HSAA was used at different times in the same hind paw of a mouse. No histological damage was observed following multiple injections of HS. Repeated testing in the same mouse reduces both the number of animals required and the variance of the results.   Initial development and validation were performed with CD-P mice. To evaluate the widespread utility of HSAA, we evaluated its ability to detect analgesics in another commonly iv  used murine strain, C_`BL/b. HSAA detected the dose-dependent attenuation of responses by morphine in C_`BL/b which was equivalent to responses found with CD-P mice. Additionally, female mice were demonstrated to have greater response than males. The diurnal cycle had little effect.    In conclusion, the HSAA rapidly detects standard analgesics and is reproducible within an animal and between strains. This assay minimizes tissue damage and the number of animals required. HSAA is a useful addition to the armamentarium of preclinical analgesic assays.   v  Lay Summary The opioid crisis highlights the need to develop new and safer non-opioid analgesics. Preclinical development of new analgesics requires the use of experimental animal assays to assess effectiveness and toxicity. Existing assays used to screen for analgesics are weak detectors of effects in humans, costly, and distressful to animals.  This thesis describes a new analgesia assay using a non-toxic irritant—hypertonic saline—in mice. An injection of a small volume of hypertonic saline into the mouse footpad causes brief pain-like responses such as licking of the paw. These responses were effectively reduced by analgesics such as aspirin and morphine in a dose-related manner. The responses to repeated injections were unchanged, allowing the same animal to be reused, thereby reducing the total number needed. The feasibility of this reuse is supported by the absence of tissue damage. In conclusion, the HSAA is an inexpensive tool that efficiently (fewer animals, less time required) detects analgesics without inflicting undue suffering.    vi  Preface  The following work was undertaken in the Hugill Anesthesia Research Centre at the Department of Anesthesiology, Pharmacology & Therapeutics, University of British Columbia under the principal supervision of Dr. Bernard A. MacLeod. Results from chapter N and ] have been published in the following publications: o Asiri YI, Fung T, Schwarz SKW, Asseri KA, Welch ID, Schuppli CA, Barr AM, Wall RA, Puil E, Macleod BA. An Intraplantar Hypertonic Saline Assay in Mice for Rapid Screening of Analgesics. Anesth Analg. NOPQ Aug;PN`(N):_^Q-___.  Dr. Bernard A. MacLeod conceived the overall project idea and helped with the experimental design. I finalized the experimental design and conducted all in vivo experiments and data analysis. I performed tissues collection for histological examinations and Drs. Ian D. Welch and Catherine A. Schuppli conducted histological analysis. Writing of the first draft of the manuscript was performed by myself and modified/edited sequentially by all authors.  o Asiri YI, Fung T, Puil E, and Macleod BA. Antinociceptive effects of Dg- tetrahydrocannabinol and Cannabidiol Alone and in Combination in the Mouse Hypertonic Saline Foot Assay. Proceedings of the British Pharmacological Society at http://www.paNonline.org/abstract/abstract.  Drs. Bernard A. Macleod, and Michael J. A. Walker conceived the project. Dr. Timothy Fung and I were equally responsible for the design and the execution of the experiments, as well as data collection and analysis. I wrote the first draft of the abstract and was sequentially edited by all authors.  The first section of chapter ^ has been published:  o Asiri YI, Fung T, Schwarz SKW, Barr AM, Puil E, Macleod BA. Repeated Testing with the Hypertonic Saline Assay in Mice for Screening of Analgesic Activity.  Anesth Analg. Epub a head of print. July, NOPQ. In this chapter, I conceived the project with the help of Dr. Bernard A. MacLeod and the Hugill Anesthesia Research Centre team. I solely designed and executed all experiments. I performed data analysis and wrote the first draft of the manuscript. The manuscript underwent sequential editing and revisions by all authors.  The second section of Chapter ^ is a manuscript under consideration for publication: o  Asiri YI, Fung T, Schwarz SKW, Barr AM, Puil E, Macleod BA. Effects of Animal Strain on Sex, Circadian Rhythm, and Antinociception on Nociceptive Responses of Hypertonic Saline Analgesia Assay in Mice. I conceived the project with the help of Dr. Bernard A. MacLeod and the Hugill Anesthesia Research Centre team. I designed and executed all experiments. I performed the data analysis and wrote the first draft of the manuscript.   Animal usage and experimental procedures conformed to the guidelines of the Canadian Council on Animal Care (CCAC) and were approved by The University of British Columbia Animal Care Committee (Certificate numbers: AP_-OP__, AP_-OP`b, APb-ON``, AP`-OPOb).  vii  Table of contents   Abstract .................................................................................................................................... iii Lay Summary ............................................................................................................................. v Preface ...................................................................................................................................... vi Table of contents..................................................................................................................... vii List of tables ............................................................................................................................. xi List of Figures .......................................................................................................................... xii List of Abbreviations .............................................................................................................. xiii List of symbols ........................................................................................................................ xvi Acknowledgements .............................................................................................................. xvii Dedication ............................................................................................................................ xviii Chapter O: Introduction .......................................................................................................... O P.P Pain prevalence .......................................................................................................... 1 P.N Pain definition and classification ................................................................................ 1 P.] Preclinical non-human in vivo assays of nociception and antinociception ................ 2 P.^ Acute nociceptive assays............................................................................................ 6 P.^.P Thermal nociceptive assays ................................................................................ 7 P.^.P.P The tail flick assay ........................................................................................... 7 P.^.P.N The hot plate assay ....................................................................................... 10 P.^.P.] The cold plate assay ...................................................................................... 13 P.^.P.^ The paw withdrawal assay (Hargreaves’ test).............................................. 14 P.^.N Mechanical nociceptive assays ......................................................................... 16 P.^.N.P The tail clip assay (Tail pinch test) ................................................................ 17 P.^.N.N The von Frey assay ........................................................................................ 18 P.^.N.] The Randall-Selitto paw pressure assay........................................................ 19 viii  P.^.] Chemical nociceptive assays ............................................................................. 20 P.^.].P The abdominal constriction assay (the writhing assay)................................ 21 P.^.].N Formalin foot assay ....................................................................................... 22 P.^.].] Capsaicin foot assay ...................................................................................... 26 P.^.].^ Hypertonic saline as an experimental nociceptive irritant in pain research . 27 P._ Factors influencing nociception and antinociception in preclinical nociceptive assays ............................................................................................................................... 34 P._.P Effect of strain differences on nociception and antinociception ..................... 34 P._.N Effect of sex on nociception and antinociception ............................................ 35 P._.] Effect of circadian rhythm on nociception and antinociception ...................... 37 P.b Summary and thesis rationale .................................................................................. 39 Chapter P: Development of the Hypertonic Saline Analgesic Assay in Mice ..................... QO N.P Introduction ............................................................................................................. 41 N.N Methods ................................................................................................................... 42 N.N.P Animals ............................................................................................................. 42 N.N.N Drugs and chemicals ......................................................................................... 43 N.N.] Characterization of the response to hypertonic saline .................................... 43 N.N.^ Hind paw histopathology ................................................................................. 44 N.N._ Statistical analysis ............................................................................................. 45 N.] Results ...................................................................................................................... 45 N.].P Concentration dependence and time course of nociceptive responses to saline 45 N.].N Sex difference in nociceptive responses to hypertonic saline ......................... 48 N.].] Histopathology ................................................................................................. 50 N.^ Discussion ................................................................................................................. 52 Chapter R: Validation of the Hypertonic Saline Assay in Mice for Rapid Detection of Analgesics ............................................................................................................................ TU ].P Introduction ............................................................................................................. 56 ].N Methods ................................................................................................................... 57 ix  ].N.P Animals ............................................................................................................. 57 ].N.N Drugs and chemicals ......................................................................................... 57 ].N.] Hypertonic saline analgesia assay..................................................................... 58 ].N.^ Statistical Analysis ............................................................................................ 59 ].] Results ...................................................................................................................... 60 ].].P Hypertonic saline analgesia assay sensitivity to established analgesics .......... 60 ].^ Discussion ............................................................................................................. 67 Chapter Q: Refinement of the Hypertonic Saline Analgesic Assay ..................................... VW ^.P Repeated Testing with the Hypertonic Saline Analgesic Assay in Mice for Screening of Analgesic Activity ......................................................................................................... 70 ^.P.P Introduction ...................................................................................................... 70 ^.P.N Methods............................................................................................................ 72 ^.P.N.P Animals ......................................................................................................... 72 ^.P.] Drugs and chemicals ......................................................................................... 72 ^.P.^ Hypertonic saline analgesia assay..................................................................... 72 ^.P.^.P Evaluation of repeated intraplantar injection of PO% HS at different time intervals .................................................................................................................... 72 ^.P.^.N Evaluation of the nociceptive response in other paws following hind paw injection of PO% HS .................................................................................................... 73 ^.P.^.] Utility of repeated injection procedure for detecting morphine antinociception......................................................................................................... 73 ^.P.^.^ Hind paw histopathology .......................................................................... 73 ^.P._ Statistical analysis ............................................................................................. 74 ^.P.b Results .............................................................................................................. 76 ^.P.b.P Reproducibility of nociceptive responses induced by PO% HS in the same hind paw at different time intervals ......................................................................... 76 ^.P.b.N Prior right hind paw injection did not affect nociceptive responses of the right forepaw or left hind paw ................................................................................. 78 x  ^.P.b.] Effects of repeated injection of PO% HS on the antinociceptive effect of morphine .................................................................................................................. 80 ^.P.b.^ Histopathology ......................................................................................... 81 ^.P.` Discussion ......................................................................................................... 84 ^.N Effects of Animal Strain on Sex, Circadian Rhythm, and Antinociception on Nociceptive Responses of Hypertonic Saline Analgesia Assay in Mice............................ 88 ^.N.P Introduction ...................................................................................................... 88 ^.N.N Methods............................................................................................................ 90 ^.N.N.P Animals ......................................................................................................... 90 ^.N.N.N Drugs and chemicals ................................................................................. 90 ^.N.N.] Hypertonic saline analgesic assay ............................................................. 90 ^.N.N.^ Statistical analysis ..................................................................................... 92 ^.N.] Results .............................................................................................................. 93 ^.N.].P Differences in nociceptive responses between sexes in C_`BL/b mice ........ 93 ^.N.].N Assay sensitivity to morphine in C_`BL/b mice ......................................... 97 ^.N.].] Circadian rhythm modulation of responses induced by PO% HS in C_`BL/b mice 99 ^.N.^ Discussion ....................................................................................................... 101 Chapter T: General Discussion ............................................................................................OWT _.P Summary of results ................................................................................................ 105 _.N HSAA for analgesic evaluation ............................................................................... 106 _.] Repeated testing procedure of HSAA for assessing analgesic activity .................. 109 _.^ Hypertonic saline analgesic assay - pros and cons ............................................. 110 _._ Recommendations for future studies .................................................................... 115 _.b Conclusion .......................................................................................................... 116 References ............................................................................................................................. 117  xi  List of tables  Table P. Overview of the acute nociceptive assays .................................................................. ]P Table N. Summary for the advantages and disadvantages of using HSAA ............................. PP]   xii  List of Figures Figure P. Studies utilizing acute nociceptive assays found in the PubMed database. ............. 30 Figure N. Custom restrainer for intraplantar injections ........................................................... 44 Figure ]. Dose-dependent relationship and time course of nociceptive responses to intraplantar saline injection ..................................................................................................... 47 Figure ^. Comparison of nociceptive sensitivity of female and male mice to hind paw injection of PO% saline ............................................................................................................... 49 Figure _. Histology of the hind paw following hind paw injection of PO% hypertonic saline or _% formalin ............................................................................................................................... 51 Figure b. Effects of centrally and peripherally acting µ-opioid receptor agonists in the HSAA ................................................................................................................................................. 61 Figure `. Effects of NSAIDs in the HSAA .................................................................................. 62 Figure Q. Effects of acetaminophen in the HSAA .................................................................... 63 Figure g. Effects of lidocaine in the HSAA ............................................................................... 64 Figure PO. Effects of pure cannabinoids in the HSAA ............................................................... 65 Figure PP. The interaction between CBD and THC in the HSAA ................................................ 66 Figure PN. Repeatability of nociceptive responses induced by PO% HS into the same hind paw at different time intervals ........................................................................................................ 77 Figure P]. Prior right hind paw injection of PO% HS did not alter nociceptive responses of other paws ............................................................................................................................... 79 Figure P^. The utility of a repeated testing procedure of HSAA for assessing the antinociceptive effect of morphine ......................................................................................... 81 Figure P_. Histology of the hind paw following repeated hind paw injections of PO% HS or _% formalin .................................................................................................................................... 83 Figure Pb. Strain and sex differences in the HSAA ................................................................... 96 Figure P`.  HSAA sensitivity to morphine in C_`BL/b mice ....................................................... 98 Figure PQ. Circadian rhythm effects in the HSAA ................................................................... 100 Figure Pg. Licking and biting response distribution in HSAA ................................................. 114  xiii  List of Abbreviations AM                            Ante meridiem ANOVA                    Analysis of variance BMLAO                  Brain and muscle ARNT-like-P  C                                Celsius Captisol           Sulfobutyl ether β-cyclodextrin CBO                             Cannabinoid receptor P CBD                           Cannabidiol CCAC                      Canadian council on animal care  Cf Confer  CFA                           Complete Freund adjuvant CI                               Confidence interval CLOCK                  Circadian locomotor output cycles kaput cm                              Centimeter CNS                           Central nervous system DMSO                       Dimethyl sulfoxide e.g.                             Exemplī grātiā g                                 Gram h                                Hour HS Hypertonic saline  HSAA                            hypertonic saline analgesic assay  IASP                        International Association for the Study of Pain xiv   i.e.                             Id est i.p.                          Intraperitoneal i.v.                          Intravenous kg                           Kilogram mg                         Milligram min                        Minute ml                          Millilitre mM                       Millimolar Na+                          Sodium ion NaCl                     Sodium chloride NaOH                  Sodium hydroxide NKO                      Neurokinin P NSAIDs               Non-steroidal antiinflammatory drugs PEG QWW             Polyethylene glycol ^OO PGEP                           Prostaglandin EN  PM                       Post meridiem  RM ANOVA     Repeated measures analysis of variance SD                     Standard deviation SNC Suprachiasmatic nucleus  THC                  Tetrahydrocannabinol TREK-O             TWIK-related K+ channel P  xv  TRPAO                Transient receptor potential cation channel, subfamily A, member P TRPVO            Transient receptor potential cation channel, subfamily V, member P TRPVQ Transient receptor potential cation channel, subfamily V, member ^  xvi  List of symbols k           kappa D           delta µ           mu µl          microliter xvii  Acknowledgements I feel very fortunate to have studied and gained valuable experiences in a prestigious institution such as The University of British Colombia.  First and foremost, I would like to express my enduring gratitude for the guidance and mentorship of my supervisor, Dr. Bernard A. MacLeod, throughout my Ph.D. program. His style of mentorship has allowed me to think freely and enabled me to hone my skills and become an independent scientist. Without his invaluable support during the many difficult times, I would not have been able to reach this milestone.  I would also like to thank my co-supervisor, Dr. Alasdair M. Barr, for his mentorship and candid advice.  I am grateful to my committee members, Dr. Ernest Puil, Dr. Brian Cairns, and Dr. Stephan Schwarz, for their commitment and constructive suggestions to improve the quality of my work.    Many thanks are owed to the Hugill Anesthesia Research Centre (HARC) and its members for facilitating my laboratory needs. Special thanks to the Dr. Jean Templeton Hugill Chair in Anesthesia, Dr. Stephan Schwarz, for his push for scientific excellence.  I owe particular thanks to Dr. Ernest Puil, who taught me how to think and write in a scholarly fashion. Our long discussions have been instrumental in shaping the way I perceive science. Thank you for your patience.  I am thankful to Dr. Michael J. A. Walker for broadening my vision of pharmacology and biostatistics. His takes on many scientific issues have truly transformed me and made me a better scientist.   Many thanks to my former laboratory member, Dr. Khalid Asseri, for his help in teaching me some in vivo techniques and for his helpful comments throughout my degree.  I am also grateful to my former laboratory member, Dr. Timothy Fung, for his help with blinding experiments, analyzing data, and proof-reading manuscripts. Thank you for all your input, the discussions on my work, and the good times we had in the lab. I am very fortunate to have known you both as a colleague and as friend.  Thank you to my current laboratory member, Mr. Desmond Fung, for the valuable input and feedback pertaining to this thesis.   Thank you to both current and former staff members in the department of Anesthesiology, Pharmacology and Therapeutics for their unwavering support. In particular, thank you to Mrs. Wynne Leung who was extremely dedicated to smoothing out all administrative issues, and Mr. Andy Jeffries who I have always enjoyed chatting with and who was always there to help when it was needed.   I extend my thanks to King Khalid University represented by the Saudi Cultural Bureau, Canada, for the financial support over the years. Without this support I would not have been able to complete my degree. Thank you to the past and present supervisors in the Saudi Cultural Bureau for their invaluable help.  I must thank my beloved mother, Aisha, for her unconditional love and support. Thank you to my siblings and relatives for their endless support.  Last but, never least, special thanks are owed to my lovely wife, Monera, who has always been the biggest source of support and encouragement throughout this long arduous journey. Your presence in my life has truly been a great blessing. Thank you!   xviii  Dedication       To the fond memories of my beloved father who instilled in me the passion of science and encouraged me to pursue my dreams no matter how difficult they may appear                         xix             “Fullness of Knowledge always means some understanding of the depths of our ignorance; and that is always conducive to humility and reverence”—Robert E. Millikan “What I believe: Living Philosophy – II” The Forum, PgNg, Vol. QN, No. ^, p Pgg.   1  Chapter O: Introduction  O.O Pain prevalence While pain serves as a primary defensive role in protecting against tissue injury, it unequivocally has, when poorly unmanaged, a negative psychoeconomic impact on patients’ wellbeing. Even excluding people who were suffering from cardiovascular and cancer diseases, the Institute of Medicine of the National Academics noted that an estimated POO million Americans reported having experienced some level of pain lasting for more than N^ hours (Simon, NOPN). Pain is detrimental to one’s ability to lead a normal active life, it affects mood, and it disrupts sleep. As a result, hundreds of millions of dollars are spent in direct health care coverage and millions more lost due to job losses and loss of work productivity (Simon, NOPN). Advancements in discovering new analgesics have been limited despite pain prevalence, in part due to a poor predictability of preclinical pain assays (Berge, NOPP).   O.P Pain definition and classification  Human pain is regarded as sensory and emotional experience wherein a multitude of factors (pathophysiological and psychological) intermingle and influence each other. The International Association for the Study of Pain (IASP) describes pain as “an unpleasant sensory and emotional experience associated with actual or potential tissue damage or described in terms of such damage” (Loeser and Treede, NOOQ). Pain can be classified according to many aspects including etiology, location, and duration. With respects to duration, pain is classified into two major types; acute and chronic. Acute pain usually occurs  2  as a result of transient injury and subsides upon healing whereas chronic pain is long-lasting (> b months) and may occur with or without tissue injury (Loeser and Treede, NOOQ). The appropriateness of this definition of pain applied to preclinical research using animals, is challenging because of the difficulty in assessing the emotional subjective component in animals. Based on the IASP definition of pain, Zimmermann provided a definition that is more suitable to animals which describes pain in animals as “an aversive sensory experience caused by actual or potential injury that elicits progressive motor and vegetative reactions, results in learned avoidance behaviour, and may modify species specific behaviour, including social behaviour” (Sneddon et al., NOP^). As pain cannot be measured in animals directly due to the inability to verbally convey their pain, indirect assessment can be deduced from behaviours induced by noxious stimuli (e.g., tail withdrawal or paw licking). Experimentally, pain in animals can be classified on the basis of etiology (nociceptive, neuropathic, inflammatory), type of stimulus (chemical, thermal, mechanical), anatomy (subcutaneous, visceral, muscular, neuraxial), and duration (Acute, chronic) (Gregory et al., NOP]).  O.R Preclinical non-human in vivo assays of nociception and antinociception  Over the past POO years, a large number of preclinical assays of nociception have been devised to better understand pain and increase the likelihood of identifying potential analgesics. In developing an animal nociceptive assay, Le Bars et al. laid out a number of criteria that need to be considered to ensure assay success (Le Bars et al., NOOP) including,   3  P- Specificity of the assay: which means that the stimulus should be nociceptive and the resulting nociceptive response can easily be distinguished from non-nociceptive behaviour (i.e., unique to the stimulus).  N- Validity of the assay: the assay must demonstrate its ability to assess what it is supposed to assess. The assay should also possess some attributes of human pain.   ]- Sensitivity of the assay: the measured response must correlate with the intensity of the stimulus. The nociceptive response must also be sensitive to a wide range of antinociceptive agents/analgesics. ^- Reproducibility of the assay: the nociceptive response should be consistent either in the same animal when retested or across animals from the same species or strain. Importantly, the assay must be easily replicated by other investigators.  These criteria have not been met by a single assay, yet new nociceptive assays continue to evolve whilst existing ones are improved in the quest for an optimal assay. The improvement of present assays largely focuses on enhancing accuracy, reducing variability, and employing non-reflexive measures of nociception (Barrot, NOPN; Cobos and Portillo-Salido, NOP]). The following sections will summarize the current state of preclinical pain assays. These assays can be categorized in five major categories listed according to their etiology and historic precedence.  The first category of nociceptive assays is commonly referred to as acute assays. Acute nociceptive assays were the first to be developed and are still widely used today. These assays have been critical not only for the progression of knowledge about pain mechanisms, but also  4  the discovery of improved analgesics (Barrot, NOPN). They involve direct application of noxious stimuli (thermal, electrical, chemical, mechanical) to a certain bodily area of an animal (e.g., tail or hind paw), prompting the animal to exhibit brief nociceptive reactions (seconds to P hour). Examples of these assays include the tail flick, Randall-Selitto paw pressure, and hot plate tests. Despite their shortcomings (which will be discussed thoroughly in sections below), they remain the most widely used assays for assessing the analgesic profile of novel agents (Barrot, NOPN; Gregory et al., NOP]).   The second category consists of preclinical assays assumed to emulate some aspects of inflammation seen in humans. Animal assays of inflammation utilize a number of chemical agents applied to diverse anatomical structures, including subcutaneous, muscle, and joint tissues. These agents trigger long-lasting alterations in the sensory system which manifest as heat and mechanical hypersensitivity, lasting for hours or weeks. The widely used assays are complete Freund adjuvant (CFA), carrageenan, zymosan and mustard oil. The choice of assay may depend on the onset, peak, and the duration of inflammation (Zhang and Ren, NOPP).  The third category is thought to recapitulate neuropathic pain in humans. Rodent neuropathic pain assays were established either surgically via blunt injury or compression to a nerve(s) situated in the central nervous system (CNS) or in the periphery, or chemically by systemic administration of chemotherapeutic agents such as vincristine. Subsequent somatosensory changes that last for months are demonstrated by hypersensitivities to various stimuli (e.g., heat) (Jaggi et al., NOPP). Notable examples of surgery-induced neuropathy assays include, but are not limited to, chronic constriction injury, spinal nerve ligation, and spared nerve injury. These assays are widely employed as they enable in-depth  5  examination of specific injuries to specific sites (Jaggi et al., NOPP; Challa, NOP_). Additionally, drug-induced neuropathy assays involve the use of anti-cancer and anti-human deficiency virus (anti-HIV) agents. These assays are less favored compared to surgery-induced assays. This is partly because of toxicities as well as difficulty establishing measurable neuropathy since variable dosing regimens are employed by different investigators (Jaggi et al., NOPP).  While the surgically-induced neuropathic assays yield valuable insight into mechanisms underpinning pain and have helped identify new analgesic targets, they resemble rare conditions that are less relevant to prevalent pain conditions (Mogil, NOOg). Thus, the fourth category of pain assays was established with an attempt to model common painful diseases. This was accomplished by inducing disease or injury in animals with the aim to feature pain- like symptoms that are analogues to specific human pain conditions. Some of the common disease-induced assays include painful diabetic neuropathy, post herpetic neuralgia, cancer pain, spinal cord injury pain, muscle pain, and postoperative pain. (Mogil, NOOg; Cobos and Portillo-Salido, NOP]).  The fifth category consists of assays that are thought to recapitulate aspects of the emotional element of the human pain experience. These assays involve incorporating alternative non-reflexive behavioural responses to the conventional simple evoked behaviour (Cobos and Portillo-Salido, NOP]). The non-reflexive responses allow  assessing the impact of nociception and antinociception on animals’ physical disability (e.g., weight bearing), innate behaviour (e.g., burrowing), facial expression (e.g., grimace scale), motivation (e.g., conditioned place preference by analgesia), and psychological distress (e.g., sleep disturbances) (Barrot, NOPN; Cobos and Portillo-Salido, NOP]). Although the non-reflexive  6  indicators of nociception are sensitive to clinical analgesics, they are influenced by non-analgesic drugs and therefore may not be pain-specific. Combining typical reflexive with non-reflexive measures for assessing nociception in animals may provide better estimation for the analgesic profile of candidate analgesics (Cobos and Portillo-Salido, NOP]).  O.Q Acute nociceptive assays  These assays are the first-line choice for researchers when screening for possible analgesic action of a new chemical entity as they are quick and easy to perform. In general, nociception in these assays is evoked by a variety of high-intensity noxious stimuli inducing brief responses (seconds to P h). These noxious stimuli can be thermal (usually applied to the animal’s tail or hind paw), mechanical (applied to the tail or hind paw), or chemical (injected subcutaneously into the animals’ tail or the hind paw, intraperitoneally or intramuscularly) (Gregory et al., NOP]). The nature of the resulting nociceptive responses is dependent on the type and intensity of stimulus, as well as the anatomical location. In addition to their use in naïve animals, some of these assays can also be used to assess hyperalgesia and allodynia in animals suffering from inflammatory or neuropathic conditions.   This section will discuss a number of preclinical acute assays of nociception in rodents. Some assays will be described in more detail than others due to their prevalent use. The description of these assays will be on the basis of the physical characteristics of the stimulus being used—thermal, mechanical, and chemical.    7  O.Q.O Thermal nociceptive assays  Assays for measuring thermal nociception involves subjecting a small skin region of the animal (mainly the tail or paw) to a thermal stimulus. These can be applied through focused heat lamps, laser beams, or hot water. Consequently, the stimulus will prompt the animal to react through withdrawal or licking of the tail or paw, respectively. The time until the animal reacts to the stimulus is used as a measure of nociception. These tests permit the animal to avoid the stimulus which would normally result in tissue damage. O.Q.O.O The tail flick assay  This assay was among the earliest to be characterized in the preclinical pain research field and is widely used today. This assay can be used for different experimental scenarios which include the determination of basal nociception, antinociception of putative analgesics, tolerance, and development of hyperalgesia (Barrot, NOPN). It comprises evaluating an evoked response after cutaneous application of a thermal stimulus on the tail of a rodent. Two popular versions of the tail flick assay are used currently. The first version is the classical method originally developed by D’Amour and Smith which utilizes a radiant heat source that is directly focused on the tip of the animal’s tail (D’Amour and Smith, Pg^P). The animal vigorously flicks the tail upon feeling discomfort and the time elapsed until the tail flicks (i.e., latency time) is noted as an index of nociception. The perceived intensity of radiant heat is confirmed indirectly via the change in latency time to withdraw of the tail. The latency time is sensitive to changes in the intensity of radiant heat such that higher intensities yield shorter latency times and lower intensities yield longer latency times (Ness and Gebhart, PgQb; Carstens and Wilson, Pgg]). The latency time is also sensitive to location of the applied  8  stimulus with the tip of the tail being most sensitive and the base of the tail being least sensitive (Yoburn et al., PgQ^; Ness et al., PgQ`).   From a practical viewpoint, this assay is rapid, easy to conduct and requires minimal time for training. The test is terminated once the animal flicks its tail away from the radiant heat stimulus inflicting no tissue injury. Furthermore, the nociceptive response in the tail flick assay is consistent when repeated testing is utilized (Mogil et al., NOOP; Barrot, NOPN). The stability of the nociceptive response upon repeated testing is useful in studying the mechanisms underlying known clinical conditions such as opioid induced tolerance (Dighe et al., NOOg). Pharmacologically, the tail flick assay is an efficient detector of opioid analgesics and regarded as a standard test for its ability to differentiate centrally acting opioids from non-opioid or peripherally acting agents (Whiteside et al., NOP]). The radiant heat-based version of the tail flick assay has some shortcomings. This assay has a narrow range of sensitivity only being able to detect opioids (Negus et al., NOOb; Whiteside et al., NOP]). Several reports demonstrated that the area of the tail being stimulated produces different response readouts which subsequently affects the degree of antinociception (Yoburn et al., PgQ^; Ness et al., PgQ`). The tail flick latencies are highly variable among laboratories due to diverse radiant heat devices used (custom-made or commercial), creating dissimilar time and temperature ranges. In addition, it has been confirmed that tail pigmentation significantly affects latency time. This confounding factor causes spurious results in some strains (e.g., C_`BL/b) known to have a considerable fraction of their population with non-pigmented patches along the tail (Wen et al., NOOg). The assay is also profoundly influenced by changes in ambient temperature wherein the latency time and  9  temperature are negatively correlated leading to an increase in the variability of the basal response and misinterpretations of the degree of antinociceptive activity of tested drugs (Tjølsen and Hole, Pgg]; Pincedé et al., NOPN).  Another important limitation of the assay is that it requires the animal to be forcibly restrained, a procedure that has been demonstrated to cause stress-induced antinociception (Miller, PgQQ; d’Amore et al., PggN).   The second popular version of the tail flick assay is the hot water tail immersion assay. Originally developed by Ben-Bassat (Ben-Bassat et al., Pg_g), this assay consists of dipping the third distal portion of the rodent’s tail into a temperature-controlled warm water bath (^b–_N°C). Similar to the radiant heat version, submersion of the tail in hot water triggers vigorous tail withdrawal and the length of time lapsed until the tail is withdrawn is monitored.   The use of the hot water tail immersion assay offers some additional merits over the radiant heat version. From an economic perspective, it is cheaper than the radiant heat assay, necessitating fewer materials and expertise to set up. From a scientific perspective, it has been suggested to be more suitable in studies that aim to unravel subtle genetic differences and detect weak analgesics for the following reasons. (P) unlike the radiant heat where the location of the heat source generates differing readouts, the area of thermal stimulation is uniform delivering somewhat comparable thermal transfer which conceivably produces less variability of the response. (N) the intensity of the thermal noxious stimuli is known from the temperature of the hot water whereas radiant heat is inferred from the resulting latency time. (]) the tail pigmentation does not affect the overall response making it suitable to test basal nociceptive and pharmacological differences in a variety of rodent strains (Mogil et al., NOOP; Wen et al., NOOg).   10  As with the radiant heat version, the hot water tail immersion version of the tail flick assay suffers from some major limitations. Sensitivity to known analgesics is limited to opioids. Some researchers have suggested lowering the intensity of the stimulus by lowering the water temperature to allow the detection of mild analgesics. However, weaker intensities introduce much more variation in the responses, making it more difficult to achieve statistical significance in the detection of antinociceptive activity (Tjølsen and Hole, Pgg`). This test is also sensitive to changes in ambient temperature. While this can be fixed in the radiant heat variant by warming up the tail to the normal tail degree, the tail immersion test can only be controlled by thoroughly wiping the affected area with a towel which may not be sufficient to maintain normal tail temperature (Tjølsen and Hole, Pgg]). Furthermore, although far less than in the radiant heat assay, the tail water immersion assay requires restraining the animal. Thus, stress-induced antinociception is inherent in this assay, a confound that may affect the interpretation of the presence of antinociception. O.Q.O.P The hot plate assay  The hot plate assay was first established by Woolfe and MacDonald (Woolfe and MacDonald, Pg^^) and was later improved into today’s version by Eddy and Leimbach (Eddy and Leimbach, Pg_]). This assay involves the use of a plate made of metal or porcelain that is heated and maintained at constant temperature (_O–__°C). The animal, confined in a transparent enclosure, is placed on the heated plate surface and the length of time elapsed until observing the nociceptive behaviour is monitored and analyzed. The determination of what is an appropriate nociceptive behaviour induced by the hot plate is contested. The contention arises from what constitutes the nociceptive behaviour and what behaviour upon  11  which the test is terminated. Major nociceptive responses triggered by a hot plate includes licking the forepaw, licking the hind paw, lifting the hind paw, fluttering the hind paw, and jumping (Espejo and Mir, Pgg]). Some additional behavioural responses are posited, including rearing, freezing, grooming, and climbing the wall of the testing chamber (Hammond, PgQg) . While a significant number of studies have opted to use one or more of hind paw licking, hind paw lifting, hind paw fluttering, and jumping as an indicator of nociception, it remains debatable as to which particular behaviour best demonstrates antinociception (Deuis et al., NOP`).  The hot plate assay possesses some advantages. It is quick and easy to set up and perform. Unlike the tail flick assay, the hot plate is performed without animal restraint, thereby reducing stress that can influence the response. As the test terminates upon observing the nociceptive response, the occurrence of tissue damage and further animal suffering is minimized (Deuis et al., NOP`). With respect to the predictability of possible analgesics, the hot plate assay consistently detects opioids while it has a limited ability to detect other analgesics such as acetylsalicylic acid (Joshi and Honore, NOOb; Whiteside et al., NOP]) Despite the previously addressed advantages, there are some major demerits pertaining to the use of the hot plate assay. Similar to the tail flick assay, it is only sensitive to one analgesic class, the opioids. It has been suggested that by lowering the temperature of the plate, non-opioid analgesics can be detected; however, lowering the temperature significantly increases the variability of responses, rendering it statistically underpowered when using standard numbers of animals in each group (Tjølsen and Hole, Pgg`). As  12  mentioned above, defining an appropriate nociceptive endpoint is a challenge. It seems though that there is a general agreement to use either one or more of the hind paw behavioural responses (hind paw licking, hind paw lifting, hind paw fluttering) and jumping as endpoints, but it is more difficult to decide which behaviour should end the test. Some investigators suggested the use of the hind paw lick and jumping, whichever appeared first, while others chose the hind paw lick or flutter, whichever occurred first (Deuis et al., NOP`). The use of jumping as a measure of nociception raises some scientific and ethical concerns. The jumping response takes a longer time to occur which adds unnecessary animal suffering (Barrot, NOPN). In addition, repeated testing procedures using the hot plate assay have indicated a significant reduction of the latency time rendering it unsuitable for time course or long-term studies (Espejo and Mir, Pgg^; Plone et al., Pggb)  A modified version of the hot plate, known as the ramped hot plate, was described by Hunskaar and colleagues (Hunskaar et al., PgQb). The ramped hot plate consists of an aluminum plate equipped with a Peltier device to control for heating and cooling allowing easier manipulation of the rate of temperature increase. The plate temperature is programmed below the level of the nociceptive threshold, and the heating rate is set to increase gradually in a linear fashion (e.g., ] °C per min) until observing the nociceptive behaviour. This modification is purported to reduce the variability of the responses seen in the standard hot plate and widen the range of sensitivity to analgesics other than opioids (Hunskaar et al., PgQb).  The ramped hot plate assay is rapid, easy to perform, and does not cause histological damage. It also features less variability of the response than the conventional hot plate assay  13  (Hunskaar et al., PgQb; Tjølsen et al., PggP). The test is conducted using freely moving animals which minimizes the impact of stress on the nociception as well as antinociception. As far as the detection of the antinociceptive agents is concerned, the assay is sensitive not only to morphine but also to other known analgesics such as acetaminophen (Hunskaar et al., PgQb). The ramped hot plate assay is not without limitations. For one, it is more expensive than the standard hot plate assay. In addition, as with the conventional hot plate assay, the nociceptive responses provoked by the ramped hot plate assay are not easily defined. While the ability to detect non-opioid analgesics such as nonsteroidal anti-inflammatory drugs (NSAIDs) has been shown, other investigators could not replicate this finding (Mogil et al., NOOP). O.Q.O.R The cold plate assay Nociceptive testing using a cold stimulus is rarely used in normal rodents. However, it is frequently utilized to assess hypersensitivity (i.e., cold allodynia) in rodent models of neuropathic pain since cold sensitivity is a common feature of human neuropathic pain. The cold plate assay was originally described in 1988 by Bennett and Xie and has undergone numerous modifications (Bennett and Xie, 1988; Jasmin et al., 1998; Allchorne et al., 2005). The assay involves placing an animal onto a temperature controlled cold aluminum plate (- 5–P_ °C) and the latency to exhibit nociceptive behaviour is recorded. Nociceptive responses elicited by the cold plate include shaking, flinching, guarding the hind paw, and jumping (Bennett and Xie, 1988; Jasmin et al., 1998; Allchorne et al., 2005). Other behavioural responses such as forepaw grooming and walking backward are posited; however, it is yet to be comprehensively validated.  14  The cold plate assay is preferred over other cold nociceptive tests owing to its speed and ability to maintain an accurate temperature. The assay is also performed without animal restraint. The assay has shown strong pharmacological validity as a number of clinically known analgesics (morphine, gabapentin, amitriptyline and lidocaine) are able to attenuate responses (Bennett and Xie, PgQQ; Walczak and Beaulieu, NOOb; Mika et al., NOP_; Gong and Jasmin, NOP`). There are limitations with the cold plate assay. Similar to the hot plate, defining the nociceptive response is inconsistent in the literature. There is no agreement on what behaviour should be considered as an endpoint and how that behaviour should be scored, thus hampering the reproducibility of results (Deuis et al., NOP`). The cold plate assay is less favoured when compared to the hot plate due to the higher variability of observed responses (Barrot, NOPN).  O.Q.O.Q The paw withdrawal assay (Hargreaves’ test)  This assay first conceived by Hargreaves et al., has some of the characteristics of the hot plate and tail flick assays (Hargreaves et al., PgQQ). In this assay the ventral aspect of the hind paws of rodents are stimulated by a radiant heat source positioned beneath a transparent glass surface. The radiant heat source is equipped with a rheostat to control the intensity of the radiant heat. The resultant nociceptive response is defined as rapid withdrawal of the hind paw followed by occasional hind paw licks. The length of time until the animal withdraws its hind paw is used as a measure of nociception.  In conjunction with the mechanical paw pressure assay, the Hargreaves test has been extensively used to assess thermal hypersensitivity in inflammatory and neuropathic pain assays.  15  The Hargreaves test offers several advantages over other thermally induced assays. The thermal stimulation is more precise and confined to a defined area (Deuis et al., NOP`). Since the assay is terminated upon noticing the nociceptive response, it minimizes histological damage and animal suffering. As well, the assay is performed without the need to restrain the animal, thereby avoiding stress related issues that has been proven to interfere with animal behavioural assessments. Unlike the hot plate assay, the nociceptive behaviour is easily defined. Also, the Hargreaves test allows testing of both hind paws. This feature allows the animal to act as its own control if a single hind paw is injured (Barrot, NOPN; Deuis et al., NOP`). The repeated testing procedure does not change the latency time providing a stable assay for prolonged assessment of hyperalgesia induced by inflammation or nerve injury. In non-injured animals, the assay only detects opioids (Inoue et al., NOO^). However, the range of sensitivity is enhanced to other known classes of analgesics when used to assess thermal hyperalgesia induced by neuropathy (surgical or chemical) or inflammation (Hargreaves et al., PgQQ; Inoue et al., NOO^). Although the Hargreaves paw withdrawal assay is somewhat superior to the hot plate and tail flick assays, it has several weaknesses. The test is laborious, requiring the continuous presence of the experimenter. The animal needs to be still in order to precisely apply the stimulus. Consequently, a prolonged habituation period is needed, rendering the assay time- consuming (Barrot, NOPN). In addition, it has been demonstrated that the glass surface temperature influences the paw temperature and can act as a heat sink, a confound that can significantly influence the latency time (Dirig et al., Pgg`). As reported by Mogil and colleagues (NOOP), applying the thermal stimulus at different anatomical locations within the hind paw  16  (the heel vs near the toes) was shown to yield differing latency time. In addition, animals guard the injured paw in neuropathic and inflammatory pain states. As a result, contact with the stimulated surface is often reduced resulting an underestimation of both the hyperalgesia and the effectiveness of tested analgesics. A new variant of the Hargreaves assay known as the thermal probe test has been recently described by (Deuis and Vetter, NOPb). This test purports to provide comparable assessment while reducing cost and habituation time. In this test, a rounded probe (N mm diameter) is pre-set at ]`°C and applied directly by the investigator to the animal’s hind paw. Once the probe is applied to the hind paw surface, the intensity of the temperature increases at a rate of N._°C per second until the animal withdraws its hind paw (Deuis and Vetter, NOPb). The test has been shown to be sensitive to opioids in the carrageenan assay of inflammation. It is unknown however whether this assay can detect the antinociceptive activity of opioids or other known analgesics in non-hyperalgesic states.   O.Q.P Mechanical nociceptive assays Before the introduction of thermal assays, mechanical nociceptive assays were the mainstay assays for studying nociception and evaluating possible analgesics (Franklin and Abbott, PgQg). In all mechanical nociceptive assays, a mechanical stimulus (typically a nylon monofilament or pressure by a clip or an applicator) is applied to the rodent’s tail or hind paws. Depending on the type and intensity of the mechanical stimulus, animals swiftly react by exhibiting distinctive nociceptive behaviour which may include one or more of vocalization, biting the stimulus or attempting to escape, and paw withdrawal. The length of time between  17  stimulus application and observing the response is regarded as an indicator of nociception. These assays are often used to assess an animal’s mechanical sensitivity after injection of an inflammatory agent or nerve injury. O.Q.P.O The tail clip assay (Tail pinch test)  This assay was first developed by Haffner and hence also is referred to as Haffner’s test (Haffner, PgNg). Haffner noticed that mice exhibiting an appearance of a straub tail after morphine treatment were insensitive to mechanical stimuli applied to their tails. Ever since, variants of the assay have been described (Franklin and Abbott, PgQg) for assessing antinociception in awake or anesthetized animals as well as mechanical hyperalgesia associated with inflammation in rodents (Hirosi et al., Pgbb; Eger et al., NOO]; Choi et al., NOO`; Whitehead et al., NOP_). In this assay a mechanical stimulus, commonly an artery or alligator clip, is applied to the base of the rodent’s tail. Upon feeling discomfort, the animal will frantically attempt to dislodge the clip via biting the clip itself or the area of the tail around the clip. The duration of time between clip application and attempting to dislodge the clip is recorded and analyzed.   Besides its simplicity, low cost, short duration, and lack of animal restraint, the tail clip test has demonstrated strong sensitivity to a number of opioids. The application of the tail clip causes no tissue injury rendering animals suitable for reuse (Franklin and Abbott, PgQg).  While the nociceptive response induced by the tail clip test may seem straightforward to score, several reports have questioned the reliability of the response for evaluating antinociception. The response is strikingly bimodal with some animals trying to dislodge the clip seconds after its application while others seem completely unaware of the clip presence  18  (Wilson and Mogil, NOOP). As such, the presence of responders and non-responders can overestimate or underestimate the degree of antinociceptive activity of tested drugs. It has also been demonstrated that repeated testing using the tail clip assay produced an extremely variable latency time making it undesirable to use in long term studies (Wilson and Mogil, NOOP). With respect to sensitivity to analgesics, the tail clip assay has a narrow range of sensitivity showing only reliable detection of opioids while failing to demonstrate any appreciable detection of other known analgesics (Franklin and Abbott, PgQg). O.Q.P.P The von Frey assay  This is perhaps the oldest assay of nociception, which was first described by von Frey in PQgb (Von Frey, PQgb). Instead of using actual hairs from a squirrel tail as described by von Frey, Weinstein used a series of nylon fibres for assessing the mechanical sensitivity in humans (Weinstein, PgbN). The use of von Frey fibres was then adapted for use with experimental animals whereby a set of calibrated von Frey filaments are applied directly to the animal’s hind paw. In this test, the animal is confined in a small enclosure and placed on a wire mesh floor to allow easy access to the hind paw for von Frey hair application. A series of von Frey hairs with varying stiffness are applied through the mesh into the middle of the ventral hind paw. The nociceptive response caused by von Frey hair includes withdrawing and shaking the hind paw followed by a brief period of licking. Several protocols are used to determine the nociceptive threshold in the von Frey test. The most reliable protocol commonly used is the one developed by Chaplan,  which is based on Dixon’s up-down method (Dixon, PgQO; Chaplan et al., Pgg^).  Another variant of this assay consists of using an electronic von Frey probe in which one filament is applied with a gradual increase in force, eventually yielding a paw  19  withdrawal response. This method is comparable to the classical method. The von Frey test is extensively utilized to determine mechanical hypersensitivity in inflammatory and neuropathic pain assays and rarely utilized to assess antinociception in naïve animals (Deuis et al., NOP`).  The von Frey assay is simple and does not inflict tissue damage or animal suffering. The determination of the mechanical threshold for both hind paws is feasible when using the von Frey test, a feature that allows the animal to control for itself if one side is injured or a local treatment was given. The von Frey test has been shown to detect a wide range of known analgesics when utilized in animals that are hypersensitive due to neuropathy (Miyazaki and Yamamoto, NOPN)  .  In terms of limitations, the von Frey assay is time consuming and laborious requiring a lengthy habituation period. It is also inherently subjective as the definition of the response may differ from one experimenter to another. Moreover, the threshold is sensitive to the anatomical location at which a given fiber is applied (Deuis et al., NOP`).  O.Q.P.R The Randall-Selitto paw pressure assay  This test was initially described by Randall-Selitto in Pg_` to assess the mechanosensitivity of a rat’s hind paw after plantar injection of an inflammatory agent (Randall, Pg_`). To a lesser extent the assay has recently been employed in mice (Bang et al., NOPN; Elhabazi et al., NOP^; Zulazmi et al., NOP`; Rasheed et al., NOPQ). The assay use can be expanded to assess the development of hyperalgesia and tolerance after long term opioid treatment (Elhabazi et al., NOP^). Typically, the animal is restrained and a blunt probe is applied with ascending pressure to a body region (commonly the medial portion of the dorsal surface  20  of the hind paw but it can also be applied to the tail) until the animal struggles to withdraw or squeals. The force (in grams) at which the animal tries to withdraw or squeal serves as an index of nociception (Franklin and Abbott, PgQg; Deuis et al., NOP`).  The Randall-Selitto assay is reproducible within and between animals. Similar to the von Frey assay, the ability of the Randall-Selitto assay to detect a wide range of known analgesics is contingent on the presence of injury, or inflammatory or neuropathic states (Franklin and Abbott, PgQg; Deuis et al., NOP`).   The Randall-Selitto assay has some drawbacks. The mechanical paw pressure assay is done while the animal is severely restrained in an unnatural position, resulting in a profound stress, a disadvantage that can significantly affect the interpretation of nociception and antinociception. Although the response is reproducible within animals, this reproducibility requires extensive animal habituation and training. While this assay is extensively utilized in rats, it requires a skilled experimenter with mice due to difficulty of handling and habituation to the assay (Barrot, NOPN).  O.Q.R Chemical nociceptive assays  While thermal and mechanical assays have enhanced basic knowledge regarding the nociception mechanisms, they are brief and involve simple withdrawal reflexes that remain inadequate to feature important characteristics of human pain — prolonged, spontaneous and not localized (Gregory et al., NOP]). Therefore, chemical assays of nociception were developed featuring inescapable and spontaneous nociceptive behaviour. These involve injection of an algogen into the rodent’s peritoneal cavity, muscle, the hind paw or the tail.  21  Depending on the concentration and the anatomical location of the injected algogen, the nociceptive responses produced are inescapable and long in duration (minutes to P hour). Nociceptive responses include hind paw licking, hind paw flinching, hind paw biting, stretching or writhing (Gregory et al., NOP]). O.Q.R.O The abdominal constriction assay (the writhing assay)  Abdominal stretches, twisting of the trunk and hind limb extension following intraperitoneal (i.p.) injection of phenlbenzoquinone in mice was first described by Siegmund (Siegmund et al., Pg_`). Early reports named these responses as “writhing” or “cramping” but later described as “abdominal constriction response” since the earlier names implicated an emotional component that cannot be measured in animals (Collier et al., PgbQ). Siegmund found that abdominal constrictions induced by phenlbenzoquinone were lessened by a number of opioids and suggested this assay is a reliable assay for testing candidate analgesics (Siegmund et al., Pg_`). The assay was modified a number of times using a variety of algogens (e.g., acetylcholine, acetic acid, magnesium sulfate) to further simplify the assay and improve its ability to reveal potential analgesics. In this technique, every algogen differs in its onset, time course, and degree of sensitivity to clinically known analgesics (Collier et al., PgbQ). The most widely used algogen that produces stable abdominal constrictions that are sensitive to a wide range of analgesics is acetic acid. The injection of acetic acid (usually O.b%, although other concentrations are used) into the peritoneal cavity produces typical abdominal constrictions within the first _ min and declines over ]O min. The number of constriction episodes is usually recorded for ]O min and thought to indicate nociception (Gregory et al., NOP]).  22   Besides its simplicity, the main advantage of the acetic acid-induced abdominal constriction assay is its sensitivity to NSAIDs which might be the reason for its widespread use (Tjølsen and Hole, Pgg`; Cobos and Portillo-Salido, NOP]). Indeed, the ID_Os of NSAIDs in the acetic acid induced abdominal constrictions assay are lower than other assays. A likely explanation for this sensitivity is the occurrence of peritoneal inflammation, since magnesium sulfate induces constrictions without causing subsequent inflammation and was unaffected by NSAIDs (Collier et al., PgbQ). The acetic acid-induced abdominal constriction assay has some serious limitations. The assay has shown to be sensitive to an array of clinically used analgesics including opioids, NSAIDs, antidepressants and gabapentinoids (Collier et al., PgbQ; Spiegel et al., PgQ]; Feng et al., NOO]; Stepanovic-Petrovic et al., NOOQ; Meymandi and Keyhanfar, NOP]). However, the response was also attenuated by non-analgesic drugs such as anticholinergics and antipsychotics raising serious questions regarding specificity (Hammond, PgQg).  Collier and others reported that Pb% of mice exhibited virtually none of the nociceptive responses typically observed after i.p. injection of acetic acid. Consequently, the lack of response may then be mistaken as antinociceptive action increasing the likelihood of obtaining false positives (Collier et al., PgbQ; Mogil et al., NOOP). O.Q.R.P  Formalin foot assay This by far is the most extensively employed acute nociceptive assay in the field of pain research (Figure P). The formalin foot assay was initially established by Dubuisson and Dennis in rats and cats (Dubuisson and Dennis, Pg``), and later characterized in mice by Hunskaar and colleagues (Hunskaar et al., PgQ_) to study the mechanism of nociceptive processes and assay  23  possible analgesics. The formalin test has also been established in other mammals including guinea pigs (Takahashi et al., PgQ^), monkeys (Alreja et al., PgQ^), rabbits (Carli et al., PgQP), and sheep (Dolan et al., NOPP). In rodents, this assay involves subcutaneous injection of a small volume of dilute formalin (]`% w/w solution of formaldehyde) into the dorsal or ventral surface of the hind paw, triggering a spontaneous nociceptive behaviour that persists for an hour. These distinctive nociceptive responses occur concurrently or intermittently throughout the test and include licking/biting, guarding, shaking, or flinching of the affected hind paw. The frequency of these responses varies with the concentration of formalin and murine species/strain. The frequency or duration of the response is used a measure of nociception (Sawynok and Liu, NOO]). Besides its use in healthy naïve animals, it is utilized to assess nociception in neuropathic animals (Courteix et al., Pgg]; Masocha, NOP^; Michot et al., NOP^). The unique features of the formalin test that makes it extraordinarily popular are the unusual biphasic nature of the response and its dissimilar sensitivity to known analgesics. These features have been observed in nearly all experimental animals including mice and rats. Plantar injection of formalin causes an early acute phase of nociception (O–_ min after injection) followed by quiet interphase (_–P_ min) where little or no nociceptive response is observed. Subsequently, the late phase resumes (P_–bO min) featuring an increase in the frequency of the response (Sawynok and Liu, NOO]). The induction of the biphasic response is concentration-dependent. Low concentrations of formalin (O.ON–O.N%) trigger only the early phase while higher concentration (>P%) trigger both phases (Rosland et al., PggO). Mechanistically, these two phases are thought to be dissimilar and reflect different underlying  24  pathologies (Sawynok and Liu, NOO]). The early phase is attributed to direct activation of transient receptor potential cation channels, subfamily A, member P (TRPAP) on nociceptors (McNamara et al., NOO`) whereas the late tonic phase is thought to be due to central sensitization ensued by initial sensory fiber discharge during the early phase and the release of inflammatory mediators (Sawynok and Liu, NOO]). The quiet phase is said to involve spinal or supraspinal inhibition and/or hyperpolarization and temporary silencing of sensory neurons (Sawynok and Liu, NOO]; Fischer et al., NOP^).  While the formalin-induced responses are easily distinguished, the selection of an appropriate index of nociception and how behaviours are scored (i.e., score or number of episodes) are not well characterized. In mice, multiple regression analysis of a number of nociceptive responses showed that the licking and biting response is the dominant behaviour and the cumulative time of this response is the best method to score while the flinching is thought to be rare and difficult to quantify (Sufka et al., PggQ). However, the dominant behaviour might vary between strains. For example, many investigators who used the C_`BL/b mice strain reported flinching while others reported flinching and licking/biting (Kayser et al., NOO`; McNamara et al., NOO`; Bai et al., NOPN; Sawynok and Reid, NOPN; Liu et al., NOP]). In rats, numerous scoring methods have been described. Early studies preferred the use of a weighted score method which consists of four categories (O= the injected hind paw is not favored; P= little or no weight bearing on the injected paw; N = the injected hind paw is elevated; ] = the animal exhibited licking/biting/shaking). In recent years, many investigators have opted to use either the number of flinching episodes or the length of time exhibiting a  25  licking/biting response as they are easier to quantify and continuous variables in nature (Sawynok and Liu, NOO]).  The ability to establish an assay that features acute and tonic pain after a single administration of the stimulus is a major advantage of the formalin foot assay. The dissociation between formalin phases has proven valuable for analgesic assessment. Several published reports have demonstrated that specific drugs are mostly effective against the late phase and ineffective in the acute phase; these drugs include NSAIDs, amitriptyline, pregabalin and gabapentin (Hunskaar et al., PgQ_; Sawynok et al., Pggg; Heughan and Sawynok, NOON; Bardin et al., NOPO).  The formalin test is simple, performed in non-restrained animals and highly sensitive to range of known analgesics, but it is laborious and time-consuming (Sawynok and Liu, NOO]). Formalin injection also causes substantial tissue damage and long term hyperalgesia lasting for weeks which renders the animal unusable for repeated testing procedures (Rosland et al., PggO; Fu et al., NOOP; Vergnolle et al., NOPO; Salinas-Abarca et al., NOP`). The nociceptive response is very variable particularly in the late phase which may potentially obscure the detection of small but genuine antinociceptive effects. While there is general agreement on the presence of two phases in the formalin foot assay, there is less agreement about the beginning and the end of each phase. There exists disagreement when it comes to defining the late phase, making it difficult to reproduce by different laboratories (Sawynok and Liu, NOO]). A number of researchers have divided the late phase into two sub phases (A and B) without evidence that they are pathologically different (Pini et al., Pgg`; Tegeder et al., NOOP; Heughan and Sawynok, NOON; McNamara et al., NOO`; Lee et al., NOP_). Another confounding  26  factor, often ignored, is the effect of ambient temperature. Rosland showed that the late phase response magnitude is particularly sensitive to changes in room temperature in that the magnitude of the response in animals tested at NO°C was three times lower than those tested at N_°C (Rosland, PggP). O.Q.R.R Capsaicin foot assay  Capsaicin, a major component of the  hot chilli pepper that gives it its pungent taste, was first described by Simone and colleagues as a useful assay for studying pain and analgesia in humans (Simone et al., PgQg). Later, Sakurada adapted a similar use in mice in which capsaicin (commonly P.b µg) is injected intraplantarly into the ventral region of the animal’s hind paw, triggering a brief nociceptive behaviour that peaks during the first _ min and gradually diminishes within P_ min (Sakurada et al., PggN). The typical capsaicin-induced responses that are considered indicative of nociception include licking/biting and flinching. This assay has been also used  in rats (Gilchrist et al., Pggb). In addition to its use in healthy animals to assess the neurophysiological mechanisms of pain and inflammation, this assay has  been utilized in neuropathic animals (Rashid, NOO]).  The assay is quick, easy to perform and has low variance. Although not as commonly used as previously described assays, the capsaicin foot assay has been shown to robustly detect clinically used analgesics including morphine, indomethacin and aspirin. The level of antinociception produced in this test is comparable to that of the formalin foot test (Ferreira et al., NOOO; Iida et al., NOO]; Guimarães et al., NOPO; Zakaria et al., NOPb). Although the spontaneous nociceptive response subsides quickly, capsaicin produces neurogenic  27  inflammation and, mechanical and thermal hypersensitivities that last variably from ^ to N^ h post injection (Gilchrist et al., Pggb; Kim et al., NOOP; Saade et al., NOON).  O.Q.R.Q Hypertonic saline as an experimental nociceptive irritant in pain research  Since its first introduction by Kellgren and Lewis in Pg]Q,  hypertonic saline (HS), a noninflammatory algogen, has been utilized predominantly for studying human muscular pain and for evaluating potential pharmacological interventions (Kellgren, Pg]Q). A single intramuscular injection of HS into muscles located in different bodily regions has been shown to produce transitory local and referred pain (Ashton-Miller et al., PggO; Arendt-Nielsen et al., Pggb; Graven-Nielsen et al., Pgg`a; b; Cairns et al., NOOP). This assay is useful for studying acute and referred muscle pain but it is inadequate to assay the clinically prevalent chronic muscle pain. Several continuous HS infusions protocols have been employed to assay chronic deep tissue pain. Sustained infusions of HS  were demonstrated to induce a longer pain experience, which allowed assessing common features of chronic muscle pain such as widespread hyperalgesia (Zhang et al., Pgg]; Stohler et al., NOOP; Samour et al., NOP`). The sustained infusion of the HS technique have been shown to reliably detect a number of drugs with proven analgesic efficacy including opioids, NSAIDs, and local anesthetics, making it a suitable tool for evaluating potential agents aimed for treating muscle pain (Schulte et al., NOOb; Arendt-Nielsen et al., NOO`; Olesen et al., NOO`; Bendixen et al., NOPO; Lei and You, NOPN). HS has also been used to induce muscle nociception in preclinical studies. Ro et al. described an assay for craniofacial muscle nociception in rats. Under light anesthesia, intramuscular injection (POO µl) of HS into the masseter muscle evoked intense hind paw  28  shaking that lasted for gO s, with the maximal response occurring during the first ]O s after injection. The paw shaking response is thought to resemble what would be the rubbing response in an awake animal (Ro et al., NOO]). This assay is reliable and has been shown to detect standard analgesics such morphine (Sánchez et al., NOPO; Bagues et al., NOP`).  It is also repeatable in the same animal since the hind paw shaking response is unchanged following multiple injections, allowing reduction in the number of animals used and minimizing variability of the data (Ro et al., NOO]; Sánchez et al., NOPO). While this assay is useful, the nociceptive response is observed in animals under anesthesia. Thus, effects due to the anesthetic are difficult to exclude (Capra and Ro, NOOQ).   The use of HS as an assay for subcutaneous nociception in preclinical studies is limited. Hwang and Wilcox described the first acute cutaneous nociceptive HS assay in mice  (Hwang and Wilcox, PgQb) for screening candidate analgesics. Intradermal injection of HS (`_ µl) into the lower abdominal region (O._ cm above the genitals) triggered nociceptive reactions in a concentration-dependent manner for a brief duration (] min). These nociceptive reactions comprised of licking, biting, and scratching the site of injection. The number of times the animal exhibited these reactions was scored as indicative of nociception. Nociceptive reactions induced by intradermal HS  were dose-dependently attenuated by morphine and pentazocine but were not blocked by aspirin or indomethacin (Hwang and Wilcox, PgQb).   Intradermal injection of HS into the lower abdominal region has a narrow range of sensitivity to analgesics and a large variability of the response. Also, the intradermal injection technique is much more difficult to perform than subcutaneous injection due the short anatomical distance that separate the dermis from the subcutaneous layer (Labrie, NOO_). The  29  animal must be heavily restrained for the intradermal injection, causing the large variation of the response.  Alessandri-Haber et al. described an acute nociceptive HS assay in mice. Intraplantar injection of HS through the ventral aspects of the mouse hind paw provoked licking and biting behaviour lasting up to _ min (Alessandri-Haber et al., NOO_). This technique was used only to study the role of transient receptor potential cation channel, subfamily V, member ^ (TRPV^) in the transduction of nociceptive signals induced by HS. Up to now, no study has assessed intraplantar HS as an assay for analgesics.          30     Figure O. Studies utilizing acute nociceptive assays found in the PubMed database. A) Number of studies appearing between NOO` to NOPb for commonly used acute nociceptive assays in rodents. Keywords entered in PubMed database for each assay are “formalin”, “hot plate”, “tail flick”, “tail immersion”, and “writhing”. B) Overall percentage of publications in A for each assay (Asiri, NOPQ, unpublished)  Y e a r# of published papers2 0 0 7 2 0 0 8 2 0 0 9 2 0 1 0 2 0 1 1 2 0 1 2 2 0 1 3 2 0 1 4 2 0 1 5 2 0 1 605 01 0 01 5 02 0 02 5 03 0 0F o rm a lin H o t p la te W rith in g R a d ia n t h e a t ta il f lic kH o t w a te r ta il im m e rs io nF o rm a linH o t p la teT a il f l ic k  ( ra d ia n t h e a t)T a il f l ic k  (h o t w a te r  ta il im m e rs io n )W r ith in gT o ta l= 5 3 6 93 4 %2 3 %1 9 %2 0 %4 %AB 31   Table '. Overview of the acute nociceptive assays Ö: sensitive; ´: not sensitive; Ö/´: questionable    Assay  Sensitivity to analgesics  Pros  Cons Opioids NSAIDs Acetaminophen Gabapentinoids Antidepressants   Tail flick  (Radian heat)  Ö  ´  ´  ´  ´ Simple Stable response in repeated testing No tissue damage   Narrow range of sensitivity to analgesics Ambient temperature affects the latency time Tail pigmentation affects latency time Requires severe restraint leading to stress induced antinociception (SIA) Radiant heat is not uniform and requires precise heat placement on the tail   Tail flick  (Hot water tail immersion)  Ö  ´  ´  ´  ´ Simple Stable response in repeated testing No tissue damage The heat stimulation is uniform Tail pigmentation does not affect latency time Narrow range of sensitivity to analgesics Ambient temperature affects the latency time Requires severe restraint leading to SIA   Hot plate  (constant temperature)  Ö  ´  ´  ´  ´ Does not require restraint Quick  No histological damage  Narrow range of sensitivity to analgesics Difficulty in determining the appropriate nociceptive endpoint Not reliable for repeated testing Large variability in the response   Hot plate  (ramped temperature)  Ö  Ö/´  Ö/´  ´  ´ Does not require restraint Quick  No histological damage Purported to detect weak analgesics Response is less variable  Narrow range of sensitivity to analgesics Difficulty in determining the appropriate nociceptive endpoint Not reliable for repeated testing Need special equipment to control for temperature increase    Tail clip    Ö        ´                   ´                        ´                       ´  Simple Brief  Does not require restraint No histological damage Can be used in awake and anesthetized animals   Narrow range of sensitivity to analgesics Response is unreliable (bimodal) Not reliable for repeated assessment Very subjective   32   Assay  Sensitivity to analgesics  Pros  Cons Opioids NSAIDs Acetaminophen Gabapentinoids Antidepressants  Thermal paw withdrawal (Hargreaves test)    Ö   ´                  ´                     ´                    ´   Detection increases in the presence of inflammation  or nerve injury Does not require restraint Quick  No histological damage Deliver precise and localized stimulation Response is less variable Allows testing both hind paws Response is easily defined  Reliable for repeated testing  Narrow range of sensitivity to analgesics in naïve animals Requires animal to be motionless® longer habituation time  Labor intensive Require precise beam placement® latency time differs from one area to another within the paw Ambient temperature affects the latency time® glass surface causes heat sink Animal’s excrement accumulation may affect the latency response     Von Frey hairs  ´             ´                 ´                     ´                   ´  Detection increases in the presence of inflammation or nerve injury Simple Does not require restraint No histological damage Allows testing both hind paws Reliable for repeated testing   Narrow range of sensitivity to analgesics in naïve animals Requires animal to be motionless® longer habituation time  Labor intensive Instability of response in mice upon repeated testing Threshold varies based on anatomical location within the paw Very subjective The response is highly variable    Paw pressure (Randall-Selitto test)  ´             ´                 ´                     ´                   ´  Detection increases in the presence of inflammation or nerve injury No histological damage Allows testing both hind paws Reliable for repeated testing  Requires severe restraint Narrow range of sensitivity to analgesics in naïve animals Requires habituation to obtain stable response  Requires more expertise to perform in mice The response (vocalization) may not be nociceptive   Abdominal constriction  Ö    Ö         Ö        Ö     Ö  Simple Does not require restraint Sensitive to a wide range of analgesics  Not specific for analgesics (response blocked by non-analgesics) Highly variable response  Large number of population are non-responders Not suitable for repeated testing Histological damage  Ö: sensitive; ´: not sensitive    33   Ö: sensitive; ´: not sensitive; ND: not determined Assay  Sensitivity to analgesics  Pros  Cons Opioids NSAIDs Acetaminophen Gabapentinoids Antidepressants  Capsaicin foot  Ö  Ö  Ö  ND  ND Simple Quick Sensitive to a wide range of analgesics Does not require restraint Less variable response   Not suitable for repeated testing Causes thermal and mechanical hyperalgesia    Intradermal abdominal  hypertonic saline  Ö  ´  ´  ´  ´  Brief   Narrow range of sensitivity to analgesics Highly variable response Intradermal injection needs expertise  Intradermal injection may require sever restraint   Formalin foot  Ö  Ö  Ö  Ö  Ö Simple Sensitive to a wide range of analgesics Unique biphasic pattern of the response Does not require restraint  Not suitable for repeated testing Response is highly variable, particularly phase X Causes histological damage Laborious Time consuming Inconsistent reporting of phase X Ambient temperature greatly affects the response      34 !.# Factors influencing nociception and antinociception in preclinical nociceptive assays  Aside from major elements that directly affect the measured nociceptive response (anatomical location, duration of the response, type and intensity of the stimulus), there are other factors that are often overlooked and have shown to profoundly impact results of nociceptive experiments. These factors are believed to be the source of conflicting results and large variability, which includes strain, sex, and the time of the day of testing.  !.#.! Effect of strain differences on nociception and antinociception   Over the last twenty years, ample evidence points towards the importance of considering an appropriate mouse or rat strain when assessing the analgesic activity because of the large variability observed in response to noxious stimuli, as well as in the degree of antinociception. Using a variety of acute nociceptive assays, Mogil and collaborators found variability between EE mouse strains in which FG–IJ % of that variability was attributed to genetic differences (i.e. heritability). In the thermal assays, the heritability ranged from MN–IO% with CJIBL/N mice being one of the most sensitive while AKR mice were the least sensitive strain (Mogil et al., EVVVa). The estimated heritability in basal von Frey response was NV% and CJIBL/N mice were the most resistant while BALB/c mice were the most sensitive strains. Assessed by the formalin foot assay, the heritability ranged from FV–MN% wherein the CJIBL/N strain demonstrated strong sensitivity while “A” strain exhibited low sensitivity in the same test (Mogil et al., EVVVa). Strain differences in response to opioid treatment has also been reported where highly sensitive strains demonstrated high degrees of resistance to antinociception indicating that the genetic difference in basal nociceptive response may be     35 related to the differences in the degree of functionality of endogenous opioids (Mogil et al., OGGG; Wilson, OGGF).   Strain related differences have also been observed in rats which have been much less studied than in mice. For instance, when comparing four commonly utilized rat strains in the hot plate assay, Schaap et al. observed that the basal responses obtained from male Wistar Kyoto and Lewis rats were lower than those obtained from male Fawn Hooded and Brown Norway rats, which led to these authors proposing the use of sensitive strains when assessing antinociception (Schaap et al., OGEF). However, the high sensitivity to nociceptive stimulation may not always translate into a high degree of antinociception. Woolfolk and Holtzman surveyed five rat strains (Sprague Dawley, Long Evans, Lewis, Wistar, and Fischer FMM) in the tail flick assay and found that Sprague Dawley rats required less morphine than other strains despite having the greatest latency to the application of heat in the tail flick assay  (Woolfolk and Holtzman, EVVJ). The strain of the animal has major effects on both nociception and antinociception.  !.#.8 Effect of sex on nociception and antinociception  There is a burgeoning body of evidence demonstrating qualitative and quantitative differences between male and female rodents in response to nociception and antinociception. Female rodents are far more sensitive than male rodents with respect to their basal response to different nociceptive stimuli including, thermal (Chesler et al., OGGO; Mitrovic et al., OGGF; Terner et al., OGGF; Leo et al., OGGd), mechanical (Barrett et al., OGGO),  and chemical stimuli (Aloisi et al., EVVM; Gaumond et al., OGGO; Perissin et al., OGGF). The cause of these     36 differences is not well understood. A paucity of reports has demonstrated the critical role of gonadal steroid hormones in modulating nociceptive signals. For instance, the higher sensitivity observed in female rodents was abolished with ovariectomized animals suggesting that estrogen plays a pro-nociceptive function (Craft et al., OGGM). There is evidence that male and female animals may utilize neurotransmitters differently when modulating nociceptive signals. Using pharmacological manipulations in the formalin test, Mifflin and colleagues reported that male mice utilized the serotonergic and noradrenergic systems to attenuate the nociceptive behaviour while females relied substantially on the GABAergic system (Mifflin et al., OGEN). Recent data has shown that male and female mice use different mechanisms in the immune system to mediate inflammatory or neuropathic pain signals. Microglia are the dominant immune cells regulating nociceptive signals in males whereas T cells are the dominant cells in females despite upregulation of both cell types in both sexes (Sorge et al., OGEJ).  The processing of the nociceptive signals is different between sexes.  The responses to specific antinociceptive agents differ between the sexes. Opioids acting preferentially at µ- or k- receptors were more potent in males than in females (Craft, OGGFb). NSAIDs efficacy may be sex-dependent as inflammatory signs following complete Freund’s adjuvant (CFA) were greater in female mice lacking cyclooxygenase E or O enzymes than males lacking cyclooxygenase E or O enzymes (Chillingworth, OGGN). Cannabinoids acting on cannabinoid E receptors (CBE) were far more potent in females than males (Cooper and Craft, OGEd). Sex differences in response to antinociceptive agents are likely due to differences in drug pharmacokinetics (e.g., metabolism) and drug pharmacodynamics (e.g., drug affinity for receptors and receptor expression) (Craft, OGGFa).       37 Despite evidence showing the impact of sex on nociception and antinociception in preclinical pain research, the vast majority of published studies used male animals (Mogil and Chanda, OGGJ; Sorge and Totsch, OGEI). This preference arises from the assumption that estrous cycle increases variability in females. This has been refuted (for more details, see Mogil and Chanda, OGGJ). Since sex associated differences in preclinical observations have been replicated in humans (Craft, OGGFb),  the special interest group on Sex, Gender, and Pain of the IASP highly recommends the use of either both sexes, or females if limited to one sex, to improve translation from the preclinical to clinical (Greenspan et al., OGGI).  !.#.: Effect of circadian rhythm on nociception and antinociception The majority of pain studies involving rodents is conducted during animals’ resting period, i.e., during the day. This can potentially affect the outcome of nociceptive and analgesic assessments. The basal nociceptive responses have been shown to significantly fluctuate over the span of a OM-hour period in the hot plate and tail flick assays, with some investigators describing higher basal sensitivity during the night time whereas others reported the opposite pattern (Oliverio et al., EVdO; Kavaliers and Hirst, EVdF; Castellano et al., EVdJ). Similar inconsistency has been observed for the magnitude of morphine antinociception. Morphine has been shown to  have greater antinociceptive activity in the hot plate and tail flick assays during the dark phase in some studies and during the light phase in others (Oliverio et al., EVdO; Kavaliers and Hirst, EVdF; Castellano et al., EVdJ). Some investigators have found the nocifensive responses induced by formalin to be greater in mice tested at night than during the day (Perissin et al., OGGG, OGGF, OGGM). However, Zhang and     38 colleagues found that the formalin induced response was lower at night than the day (Zhang et al., OGEO). The reason for these discrepancies is not clear. Since these studies used different mouse strains, genetic differences in the function of the hypothalamic suprachiasmatic nucleus (SNC), which regulates circadian rhythm, and its subsequent impact on the release of a number of crucial pain modulating substances such as melatonin and endogenous opioids, may explain this discrepancy (Castellano et al., EVdJ; Segal et al., OGEI).                     39 !.= Summary and thesis rationale  There exist several preclinical nociceptive assays that are commonly used for screening potential analgesics. They can be classified according to the type of stimulus being applied; thermal, mechanical, and chemical. The most commonly used assays are the tail flick, hot plate and formalin tests (Figure E).  An optimal nociceptive assay would (E) have a wide range of detection of clinical analgesics, (O) be reproducible within and between animal species, (F) be triggered exclusively by nociceptive stimuli, M) and cause minimal harm to animals.   Current assays fail in one or more of these criteria. Some have a narrow range of sensitivity to known analgesics (e.g., tail flick and hot plate assays only detect opioids). Most lack reproducibility within animals (only the tail flick assay can reliably be used). Many have a large variability of the response (e.g., formalin response particularly in phase O may obscure weak antinociceptive effects). Some produce long term tissue damage (e.g., formalin test).  There hence exists a compelling need for a better analgesic assay that is efficient, less variable, reproducible within/between animals, minimizes animal suffering, and is sensitive to clinically used analgesics. Hypertonic saline (HS) has been utilized in humans and experimental animals as a non-toxic noxious stimulus that induces different forms of pain. One of the assays utilizing HS is the intraplantar HS as described in OGGJ by Alessandri-Haber. However, there has been no prior investigation of this technique as a screen for new compounds with potential analgesic activity.  My thesis investigates the use of murine intraplantar HS as a screening assay for analgesic agents. Chapters O and F describe the development and validation of the assay. The     40 development of the assay focuses on assessing the concentration–response relationship, time course, sex difference of animals’ responses, and tissue damage. The assay is subsequently validated by evaluating its screening ability to detect a range of established analgesics belonging to different classes.   Within and between animal reproducibility is not always attained in a single assay. Chapter M is dedicated to refining the HS assay. The first section of chapter M explores the reproducibility of the HS response within an animal upon repeated injections of HS. The hypothesis was that repeated intraplantar HS in the same hind paw or other paws will produce stable amplitude responses and that the use of this modified technique will allow the detection of the antinociceptive effect of morphine. The second section focuses on assessing the reproducibility of the HS assay in another commonly used mouse strain, CJIBL/N mice. I characterized the response and validated the assay using morphine. I also examined the impact of sex and circadian rhythm on the HS response. The hypotheses were that in another commonly used murine strain, CJIBL/N, the HS assay a) detects analgesic efficacy; and, b) is influenced by gender and circadian rhythm.     41 Chapter 8: Development of the Hypertonic Saline Analgesic Assay in Mice  8.! Introduction While existing preclinical in vivo nociceptive assays have improved our understanding of how pain is processed and have revealed potential analgesic targets, they suffer from numerous practical and translational limitations (Mao, OGGV; Cobos and Portillo-Salido, OGEF). For instance, thermal assays such as the hot plate and tail flick are only reliable for detecting opioids (Negus et al., OGGN; Barrot, OGEO). False positives are common when using thermal assays since they are usually performed on animals under severe restraint or without acclimatization causing stress-induced antinociception (SIA) (Deuis et al., OGEI). While the formalin foot assay, which is the most utilized assay in pain research (cf. Figure E), detects a wide range of clinically known analgesics, inconsistency in the distinction between the test phases, variability of drug effects on phase O, labor costs, and permanent tissue damage resulting in prolonged animal suffering are limitations of this test (Kim et al., EVVV; McNamara et al., OGGI). A need exists for developing better preclinical analgesic assays that overcome the shortcomings of current assays.  Injection of hypertonic saline (HS) has been known for some time to produce pain in humans and animals. Injected into the abdominal wall of mice, HS has been shown to induce licking and scratching for a brief duration and reveal the analgesic activity of opioids, but not of NSAIDs. It produces damage to the abdominal wall, and results in a very large variance of the response (Hwang and Wilcox, EVdN). In one study, HS injected subcutaneously through the ventral surface of the mouse hind paw produced licking and biting lasting up to J min     42 (Alessandri-Haber et al., OGGJ). However, the authors used this approach to examine osmosensitive receptor systems. No study has previously investigated the utility of this as an assay for analgesics.  The goal of this study was to develop an analgesic assay that is effective, inexpensive, and produces minimal tissue damage or animal suffering. During assay development, we established the nature and magnitude of animals’ responses to the HS concentration, the responses time course and determined the optimum time for evaluation. We also assessed local tissue damage and the impact of animal’s sex on HS induced response.    8.8 Methods  !.!.# Animals Animal usage and experimental procedures conformed to the guidelines of the Canadian Council on Animal Care (CCAC) and The University of British Columbia Animal Care Committee (Vancouver, British Columbia, Canada). Experiments were performed using adult CD-E mice (OJ–FG g; both sexes). Mice were housed (five per cage) in The University of British Columbia Animal Resources unit where temperature and humidity were maintained at OM ºC and MJ–JJ%, respectively, under a EO-hour light/dark cycle with the lights switched on at I:GG AM and experiments performed between V:GG AM and J:GG PM. Laboratory food and water were accessible ad libitum. Each mouse was used once and euthanized immediately upon the completion of the experiment. Mice were allowed at least one week of acclimatization in the animal facilities.      43 !.!.! Drugs and chemicals  Sodium chloride (NaCl) was obtained from BDH Chemicals (Toronto, ON, Canada) and dissolved in distilled water. Buffered formalin (EG%) was purchased from Sigma-Aldrich Inc. (St. Louis, MO, USA). Formic acid (TBD-O™) was purchased from Thermo Fisher Scientific (Calgary, AB, Canada).  !.!.4 Characterization of the response to hypertonic saline  In a quiet room with the temperature maintained at OF ºC, mice were habituated individually in darkened cylindrical Plexiglas chambers (height, EV cm; diameter, V cm; Associated Plastics and Supply, Vancouver, BC, Canada) on a glass plane for two consecutive hours. For each mouse, the testing chambers were kept unwashed between habituation and testing to maintain the animal’s scent and hence reduce stress induced by a novel environment. Mice were gently restrained with a ventilated cylindrical restrainer (length, V cm; diameter, F cm; custom made from a JG ml syringe, BD, Mississauga, ON, Canada [Figure O]) with the tail and hind paws protruding. Subcutaneous injections (EG µl) of HS were injected into the middle of ventral aspect of the right hind paw, using a OVG, G.F ml disposable insulin syringe (BD, Mississauga, ON, Canada). The intraplantar injection procedure was performed within OG s to minimize stress induced by the injection and novel environment. Each mouse was immediately placed back in its chamber. Activity was recorded for FG min using a video camera (Lorex by FLIRÒ, ON, Canada) placed beneath the chamber. Nociceptive responses were defined as licking or biting at the injection site or the toes. From the video records, the     44 length of time the animal exhibited licking and biting behaviour in J-minute bins after HS injection was determined. For determination of the optimal HS concentration, time course and optimum evaluation time, female mice were randomized to receive different concentrations of HS (G.V, J, EG, or FG% saline) using readily accessible website (www.random.org). The experimenter and the analyzer were blinded.    After establishing the concentration of HS that produced the maximal and least variable response, the responses in male and female mice were assessed.   Figure 8. Custom restrainer for intraplantar injections    !.!.< Hind paw histopathology To evaluate local tissue damage after the injections, mice were euthanized and hind paws were amputated at the ankle. Tissues were fixed in EG% formalin and then decalcified in formic acid (TBD-O™) for Md h. Four F mm sections including the injection site and the surrounding tissue were embedded in paraffin for Md h and then a single transverse section (J µm thick) of the section was cut. The slides were stained with hematoxylin and eosin and 9 cm 3 cm     45 assessed with light microscopy for histological features of tissue damage by a pathologist and clinical veterinarian. The evaluators were blinded to the treatments.  Hind paws from mice were examined EJ min after injection of G.V% (n = F), and EG% (n = F) saline. Controls included two mice without needle placement. For comparison with a known inflammatory agent, mice (n = O) were euthanized NG min after J% formalin injection.  !.!.? Statistical analysis Data were analyzed using Prism N.GE software (GraphPad Software Inc., La Jolla, CA, USA). The mean and VJ% CI for the sum of the time spent licking and biting for saline concentrations were calculated in J min bins over FG min. Parametric one-way ANOVA followed by post hoc Tukey’s test was used to determine the difference between different saline concentrations. To determine sex difference, ordinary two-way ANOVA was utilized (saline concentration x sex) followed by Tukey’s post hoc analysis.   Group size calculations were determined based on a previous study (Alessandri-Haber et al., OGGJ). Data are presented as a scatter plot of individual responses with error bars representing mean and VJ% CI. P values were reported and P<G.GJ was regarded as statistically significant.  8.: Results  !.4.# Concentration dependence and time course of nociceptive responses to saline  Subcutaneous injection of HS into the ventral surface of hind paw induced vigorous licking and biting that peaked during the first J min and diminished over time. As Figure OA shows, intraplantar injection of HS induced vigorous licking and biting that was dose-    46 dependent, with a maximum mean response after EG% HS in the G-J min interval (G.V% NS control, O.J s [VJ% CI, -G.I–J] versus J% saline, NG s [VJ% CI, FI–dJ; P = G.GGGE]; EG% HS, EEF s [VJ% CI, VJ–EFG] versus control, P<G.GGGE; FG% HS, EOJ s [VJ% CI, EGE–EMV] versus control, P<G.GGGE). Licking and biting behaviour was not increased further by FG% HS (P = G.IF; Figure OA). The licking and biting responses rapidly diminished over the subsequent OJ min to less than OG% of their initial values (Figure OB). In view of these results we used EG% HS in subsequent experiments.      47  Figure :. Dose-dependent relationship and time course of nociceptive responses to intraplantar saline injection A) Responses represent total time spent licking and biting during the first J min binned intervals. Each point represents individual animal responses during the post-injection G–J min interval. Summary bars denote means VJ% CI (one-way ANOVA followed by Tukey’s test; * P<G.GJ versus control; n = EG per group). B) Time course of licking and biting induced by hind paw injection of different saline concentrations. Each point represents the mean of a J min bin of responses (vertical error bars denote VJ% CI; n = EG per group).        48 8.:.8 Sex difference in nociceptive responses to hypertonic saline Nociceptive responses were greater in amplitude and of longer durations in female than male mice (Figure F). The mean responses in both groups was greater at J min after HS injection and gradually decreased over FG min (Figure F B). The two-way ANOVA of the data in Figure F A revealed a significant effect of sex (F E,FN = OV, P<G.GGGE), saline concentration (FE,FN = dF.F, P <G.GGGE), and interaction (F E,FN = OF.II, P<G.GGG). Tukey’s post hoc analysis revealed that both sexes showed significant increases in mean licking and biting responses due to intraplantar EG% HS compared to G.V% NS during the first J min (female control, mean EG s [VJ% CI, -E–OE] versus EG% HS, VN s [VJ% CI, IN–EEN; P<G.GGGE]; male control, I s [VJ% CI, F–EE] versus EG% HS, FO s [VJ% CI, Ed–Md; P = G.GOM]). As shown Figure F A, the mean response magnitude of female mice to EG% HS was twice that of male mice during the first J min (female, EG% HS, mean VN s [VJ% CI, IN–EEN] versus male, EG% HS, FO s [VJ% CI, Ed–Md; P<G.GGGE]).       49  Figure M. Comparison of nociceptive sensitivity of female and male mice to hind paw injection of !O% saline A) Licking and biting responses during the first J min following hind paw injections of G.V and EG% saline. The mean response to EG% HS was significantly greater in female than male mice injected with EG% saline. Each point represents an individual response during the post-injection G–J min interval. Horizontal bars are means and vertical error bars denote VJ% CI (two-way ANOVA followed by Tukey’s test; * P<G.GJ; n = EG per group). B) Time course of licking and biting induced by hind paw injections of G.V and EG% saline in male and female mice. Each point represents the mean of J min binned responses (vertical error bars denote VJ% CI; n = EG per group).     50 !.4.4 Histopathology HS injection caused no tissue damage. Hind paws from mice with no needle placement (Figure M A) and needle placement with injection of G.V% saline (Figure M B) showed normal histology. Close examination of plantar tissue revealed no significant pathology except moderate subcutaneous edema in the hind paw of the EG% HS group, compared to the G.V% saline group (black arrow, Figure M C).  The above results were compared to hind paw injection of J% formalin which produced profound histological changes [cf. (Rosland et al., EVVG)]. A single injection of formalin resulted in severe edema in subcutaneous tissue examined after NG min (Figure M D).          51   Figure #. Histology of the hind paw following hind paw injection of !O% hypertonic saline or #% formalin  Representative images (Mx magnification) of tissue specimens of mouse hind paw show histological changes at EJ min post-injection of EG% saline and at NG min post-injection of J% formalin.  A) Control without needle placement shows normal histology.  B) Single injection of G.V% saline appears similar to control. C) Moderate edema (black arrow) is evident after single injection of EG% saline.  D) Severe edema (black arrows) is evident after single injection of J% formalin.  Calibration bar indicates OGG µm.        52 8.M Discussion The aim of the work summarized in this chapter was to develop and optimize the use of a new preclinical assay for analgesic agents based on hind paw injection of HS. HS injection into the hind paw produced typical licking and biting response that reached its maximum at EG% HS. The licking and biting response lasted for FG min wherein the early J min after injection demonstrated the peak followed by gradual diminution within the subsequent OJ min. The response to EG% HS was greater in female than male mice. The response to HS was easily determined and produced minimal animal suffering and histological damage. The development study showed that intraplantar injection of HS into the ventral surface of the mouse hind paw induced nociceptive response for short duration. The HS-induced nociceptive response is mainly licking and biting of the injected hind paw. Other behavioural responses were also observed such as favouring, flinching and lifting the injected hind paw. While these responses have been previously scored alone or in combination and utilized as behavioural indicators of nociception, Sufka and others showed that cumulative licking and biting time is the best measure of nociceptive and antinociceptive responses in mice (Sufka et al., EVVd). This behaviour is easily scored and provides an objective assessment for assessing analgesics.  The present results showed that the nociceptive effects of EG% HS in female mice had twice the magnitude of the corresponding effects in male animals. This observation is consistent with studies showing that female mice are more sensitive to different modalities of pain (Leo et al., OGGd). Similarly in humans, female subjects respond to a subcutaneous injection of J% HS with higher sensory scores than males (Henderson et al., OGGd). Reasons for     53 the greater sensitivity of female to HS injection are unclear. A post hoc analysis of a large data set encompassing different strains of mice shows that the mean licking time of phase E in response to intraplantar formalin is significantly greater in female than male mice (Mogil and Chanda, OGGJ). Nociceptive processing in females and males with neuropathy exhibit distinct differences in neurotransmitter release after formalin injection or Toll-like receptor activation (Sorge et al., OGEE; Mifflin et al., OGEN). Gonadal hormones also may influence processing of nociceptive signals (Craft et al., OGGM). The current majority of preclinical pain studies use male animals (Mogil and Chanda, OGGJ). The larger responses and reduced variability of responses observed with HS injection suggests preferred use of female mice. The special interest group on Sex, Gender, and Pain of the International Association for the Study of Pain highly recommends the use of females if limited to one gender (Greenspan et al., OGGI).  This chapter’s experiments show that single injection of HS produced few gross changes in tissue structure. Moderate subcutaneous edema was present in the plantar hind paw at EJ min after a single EG% HS injection. This is consistent with previous observations, showing that inflammation is not seen at NG min after hind paw injection of EG or OG% HS injection into rat hind paw (Antonijevic et al., EVVJ; Rittner et al., OGGV). However, this may have been too early to see more advanced changes such as leukocytic infiltration in the cellular phase of acute inflammation. Vergnolle et al. reported that intraplantar injection of EG% HS into the mouse hind paw caused structural disruption and severe edema measured at intervals between FG min and Mh (Vergnolle et al., OGEG). While the reason for the difference is not clear, occasional edema in the present experiments may be attributable to unabsorbed saline rather than inflammatory microvascular exudation of fluids resulting from the solution     54 hypertonicity. Overall, the acute injection of EG% HS produces no long-lasting histopathology other than moderate edema, while formalin injection induces severe edema and significant destruction of tissue structure (Rosland et al., EVVG; Fu et al., OGGE). The hypertonic saline analgesic assay (HSAA) has advantages over other assays of chemically induced nociception. The time required for the assay is short. The assay requires only J min compared to NG min for the formalin foot test. Similarly, the prostaglandin EO (PGEO) foot test requires significantly more time (MJ versus J min) (Kassuya et al., OGGI). The HSAA response variation is low therefore few animals are required. While the variability of responses to HSAA is similar to the early phase of the formalin test, the late phase formalin has a greater variance. The capsaicin foot test has similar variance compared to HS. In assessing PGEO-induced nociception, some studies have measured allodynia using von Frey filaments, a subjective procedure, which results in large variation in responses (Yang and Gereau, OGGF; Kassuya et al., OGGI; Asseri et al., OGEJ). The HSAA results in minimal tissue damage and resulting stress compared to formalin, capsaicin, and PGEO.  In brief, the outstanding features of the HS assay are the rapid assessment, low response variability, lack of tissue damage and minimal suffering.   The mechanism by which HS produces nociception is unclear. Current evidence suggests that HS causes shrinkage of cell structures in the area of the bolus injection. As a consequence, HS may directly activate stretch-inactivated Na+ channels (Schumacher et al., OGGG). Transduction of this physical stimulus may result in depolarization of fine nerve     55 endings, initiating action potentials along axons of small diameter primary afferent neurons which leads to a nociceptive response (Schumacher et al., OGGG). Other osmosensitive systems such as TREK-E channels of sensory neurons also may participate in polymodal nociception (Alloui et al., OGGN). While both O% and EG% HS produce nociceptive responses in wild type mice, only the responses to O% HS are reduced in vanilloid TRPVM  knockout mice (Alessandri-Haber et al., OGGJ). The prolonged effect observed suggests two possible mechanisms sustaining the nociceptive response. (E) hypertonicity may stimulate pronociceptive substances release such as substance P and glutamate from C-fiber terminations, exciting the primary sensory afferents (Garland et al., EVVJ; Louca et al., OGEM). (O) unabsorbed NaCl provided high level of hypertonicity that maintained direct activation of nociceptors. Rittner and colleagues showed that NaCl concentrations were greater than normal for EJ min following hind paw injection of HS (Rittner et al., OGGV). This observation may explain the prolonged response observed in our study. In summary, induction and maintenance of HS induced response might be mediated by different mechanisms.      In conclusion, this thesis’ new analgesic assay, HSAA, is an easily performed rapid assay with low variability of response which does not produce tissue damage and minimizes suffering.         56 Chapter :: Validation of the Hypertonic Saline Assay in Mice for Rapid Detection of Analgesics   :.! Introduction Despite enormous advances in in vitro and in vivo preclinical techniques, the translation rate of new analgesics from benchside to bedside remains low (Kissin, OGEG; Mao, OGEO). The low success rate for developing new analgesics is believed to be a combination of factors such as limited understanding of mechanisms underlying pain and the shortcomings of existing preclinical animal analgesic assays (Whiteside et al., OGGd; Kissin, OGEG). The development of improved animal analgesic assays may lead to new safer analgesics to prevent dangerous overuse of prescription opioids (Fischer et al., OGEJ). During the development of a new analgesic assay, certain criteria must be taken into consideration to ensure its success. Optimally, nociceptive stimulation should elicit responses that are: (a) consistent in magnitude with minimal variation between animals; (b) reproducible upon repeated measurements in the same or different animals; (c) sensitive to depression by established clinical analgesics; and, (d) readily duplicated by researchers in different laboratories (Le Bars et al., OGGE).  Since present assays do not meet all of these criteria, a need exists for better preclinical analgesic assays to detect novel compounds which are potential clinical analgesics, decrease the requirement for opioids, and improve patient safety. The non-toxic irritant HS has been utilized to induce muscular and superficial pain in humans and experimental animals (Hwang and Wilcox, EVdN; Graven-Nielsen et al., EVVIa; Ro et al., OGGF; Henderson et al., OGGd).  In chapter E, we developed and optimized a new     57 nociceptive assay based on the use of HS, the HSAA. We showed HS injection into the mouse’s hind paw produced licking and biting behaviour that lasted for FG min, with the first J min period representing the maximal response with low variability. Histological examination of hind paws that received HS showed normal histology.     The objective of the present investigations was to validate the utility of HSAA as an assay for analgesics using a range of established analgesics.   :.8 Methods  4.!.# Animals As in section O.O.E  4.!.! Drugs and chemicals Morphine hydrochloride, purchased from the MacFarlan Smith Ltd (Edinburgh, UK), was dissolved in G.V% saline (NS). Acetylsalicylic acid and naproxen, purchased from Sigma-Aldrich Inc. (St. Louis, MO, USA) were dissolved in NS. Sodium hydroxide (E M NaOH) was added to enhance the solubility of acetylsalicylic acid and naproxen, with a final solution pH of EG. Acetaminophen was purchased from Sigma-Aldrich Inc. (St. Louis, MO, USA) and dissolved in a vehicle composed of J% polyoxyl FJ castor oil, EG% polyethylene glycol MGG (PEG MGG), and dJ% NS. Loperamide hydrochloride was purchased from Sigma-Aldrich Inc. (St. Louis, MO, USA) and dissolved in EGG% ethanol to prepare a stock solution of OGG mM. For i.p. injections, the stock solution was diluted with NS to the appropriate concentration with a maximum ethanol content of E.MJ%. For intraplantar injection of loperamide, working solutions were diluted from the stock using EG% Captisol® (sulfobutyl ether β-cyclodextrin;     58 Ligand Pharmaceuticals Inc., La Jolla, CA, USA) and NS. The vehicle control for intraplantar loperamide was composed of the following: EG% ethanol, MJ% Captisol®, and MJ% NS. Lidocaine hydrochloride, purchased from Astra Pharmaceutical Products Inc. (Westborough, MA, USA), was combined with EG% NaCl for intraplantar injection. Delta-V- tetrahydrocannabinol (DV-THC) and cannabidiol (CBD) were purchased from CanniMed Therapeutics Inc. (Saskatoon, SK, Canada). THC and THC+CBD were dissolved in a vehicle comprised of the following: ethanol, ethoxylated castor oil, and saline (in a ratio of E:E:Ed, respectively). CBD was dissolved in EGG% dimethyl sulfoxide (DMSO). Each vehicle was tested during the optimization phase to ensure a lack of toxicity. Intraperitoneal injections were given in a volume of N.N ml/kg. Intravenous tail vein (i.v.; OVG, G.F ml disposable insulin syringe, BD, Mississauga, ON, Canada)) injections were administered in a volume of E ml/kg except for CBD where it was given in a volume of G.F ml/kg. Hind paw injections were given in a volume of EG µl.   Polyoxyl FJ castor oil was obtained from Toronto Research Chemicals (Toronto, ON, Canada). Polyethylene glycol MGG (PEG MGG) was obtained from Fischer Scientific (Fair Lawn, NJ, USA). Buffered formalin (EG%) was purchased from Sigma-Aldrich Inc. (St. Louis, MO, USA). Sodium chloride (NaCl) and sodium hydroxide (NaOH) obtained from BDH Chemicals (Toronto, ON, Canada) were dissolved in distilled water.   4.!.4 Hypertonic saline analgesia assay The HSAA was performed as described in section O.O.F.        59 To validate the utility of HSAA, a wide range of drugs with or without proven analgesic efficacy were assessed using female mice. Antinociceptive responses were determined at FG min after i.p. injection of morphine, naproxen, or loperamide; at EJ min after acetylsalicylic acid; and at OG min after acetaminophen. The antinociceptive actions of THC, CBD, or THC+CBD combination was assessed J min after treatment. Lidocaine (G.GGF–E%) was co-administered with intraplantar EG% HS. Loperamide was intraplantarly applied EJ min before testing with EG% HS. The length of time the animal exhibited licking and biting behaviour during the J min period following HS injection was recorded and used to measure antinociception.   4.!.< Statistical Analysis Data were analyzed using Prism N.GE software (GraphPad Software Inc., La Jolla, CA, USA). The mean and VJ% confidence intervals (CI) for the sum of the time spent licking and biting in the first J min period was calculated for each study. Parametric one-way ANOVA followed by post hoc Tukey’s test for multiple comparisons or dose-response curves generation were used, as appropriate, for the assessment of the antinociceptive effects of drugs. Dose response curves were generated using non-linear least squares fits of a log [inhibitor] vs response [variable slope] by applying the following equation:  Y= Bottom + (Top-Bottom)/ (E+EG [(LogIDJG-X) * Hill Slope])   in which Top and Bottom are plateau values on the Y axis and the IDJG is the concentration halfway between the Top and Bottom values. To determine the interaction between THC and CBD, the THC and THC+CBD combination dose response relationship curves were fit together     60 using the above equation and IDJGs and slopes were compared using extra sum of squares F test.   Data are presented as a scatter plot of individual responses with error bars representing mean and VJ% CI. P values were reported and P<G.GJ was regarded as statistically significant.  :.: Results 4.4.# Hypertonic saline analgesia assay sensitivity to established analgesics HSAA detected the antinociceptive activity of a number of clinical analgesics and differentiated them from non-analgesics. Morphine administration (i.p.) produced a dose-dependent attenuation of the mean nociceptive response to EG% HS with an estimated IDJG of M mg/kg (VJ% CI, O.O–N; Figure J A). Loperamide, an opioid with peripherally restricted activity, reduced nociceptive responses when injected intraplantarly (e.g., control, NM s [VJ% CI, FI–V] versus EGG µg/paw, d s [VJ% CI, G.N–EM; P<G.GGGE]; Figure J B). However, systemic loperamide did not attenuate nociceptive behaviour (Figure J C).          61     Figure =. Effects of centrally and peripherally acting µ-opioid receptor agonists in the HSAA In A, B, and C, each point represents an individual response (G–J min) after HS injection, with summary bars presented as means and VJ% CI. A) Morphine attenuated licking and biting time induced by HS in a dose-dependent manner (n = EG per group). B) Loperamide (intraplantar) at EJ min prior to injection of HS reduced licking and biting time (one-way ANOVA followed by Tukey’s test; * P<G.GJ versus control, n = d per group). C) Loperamide (i.p.) did not reduce licking and biting induced by HS (one-way ANOVA followed by Tukey’s test; no significant difference was found; n = EG per group).   NSAIDs decreased nociceptive responses induced by HS. Acetylsalicylic acid at FGG mg/kg decreased the mean nociceptive responses induced by intraplantar HS (control, VM s [VJ% CI, dO–EGJ] versus FGG mg/kg, MF s [VJ% CI, OM–NE; P<G.GGGE]; Figure NA). Naproxen was     62 antinociceptive at EJG mg/kg, reducing the mean response at the EJG mg/kg (control, VM s [VJ% CI, II–EEG] versus EJG mg/kg, MO s [VJ% CI, OI–JI]; P<G.GGE; Figure NB).     Figure R. Effects of NSAIDs in the HSAA A) Acetylsalicylic acid at FGG mg/kg (i.p.) reduced licking and biting time induced by HS (one-way ANOVA followed by Tukey’s test; * P<G.GJ, n = EG per group). B) Naproxen at EJG mg/kg (i.p.) reduced licking and biting time induced by HS (one-way ANOVA followed by Tukey’s test; * P<G.GJ, n = d per group).  In A and B, each point represents an individual response (G–J min) after HS injection, with summary bars presented as means and VJ% CI.      63  Acetaminophen, an aniline class analgesic, attenuated nociceptive responses induced by HS. Acetaminophen at FGG mg/kg reduced the mean nociceptive response (control, VN s [VJ% CI, IV–EEM] versus FGG mg/kg, JI s [VJ% CI, FV–IN]; P = G.GGJ; Figure I).  Figure T. Effects of acetaminophen in the HSAA Each point represents an individual response (G–J min) after HS injection, with summary bars presented as means and VJ% CI. Acetaminophen at FGG mg/kg (i.p.) reduced licking and biting time induced by injection of EG% HS. (one-way ANOVA followed by Tukey’s test; * P<G.GJ versus control, n = EG per group).   The widely used local anesthetic, lidocaine, also reduced nocifensive responses induced by plantar injection of HS.  Intraplantar co-injections of lidocaine (G.GGF, G.GF, G.F, and E%) with HS reduced the nociceptive responses (control, EEO s [VJ% CI, VN–EOV] versus G.GGF%, Nd s [VJ% CI, MF–VM; P = G.GEO]; G.GF%, MN s [VJ% CI, OG–IF] versus control, P<G.GGGE; G.F%, FI s [VJ% CI, EN–Jd] versus control, P<G.GGGE; E.G%, FO s [VJ% CI, OO–MF] versus control, P<G.GGGE; Figure d).     64  Figure U. Effects of lidocaine in the HSAA Intraplantar co-administration of multiple doses of lidocaine with EG% HS reduced licking and biting time. Each point represents an individual response (G–J min) after HS injection, with summary bars presented as means and VJ% CI (one-way ANOVA followed by Tukey’s test; * P < G.GJ versus control, n = EG per group).  Clinical and preclinical studies have suggested that administration of THC, alone or in combination with CBD, may have analgesic efficacy (Casey and Vaughan, OGEd). Hence, the ability of the HSAA to detect antinociceptive actions of THC and CBD, alone or in combination, was assessed. THC (G.GF–EG mg/kg, i.v.) produced a dose-dependent reduction of the nociceptive responses induced by HS, with an estimated IDJG of G.J mg/kg (VJ% CI, G.F–E) and a Hill slope of -E (VJ% CI, -O– -E.N) (Figure V A). In contrast, CBD (N–MJ mg/kg, i.v.) did not attenuate HS-induced responses (Figure V B).      65  Figure !O. Effects of pure cannabinoids in the HSAA  A) Dose-response relationship for THC. THC dose-dependently decreased the time spent licking and biting of the hind paw following injection of EG% HS.  B) Dose response-relationship for CBD.  CBD produced no detectable antinociceptive effects at any dose tested. One-way ANOVA revealed no differences in the time spent licking and biting of the hind paw following hind paw injection of HS.  In A and B, each point represents an individual response (G–J min) after HS injection, with summary bars presented as means and VJ% CI (n = d per group).   The combination of THC and CBD (in a E:EG ratio, G.GE/G.E–E/EG mg/kg, i.v.) produced a dose-dependent reduction of nociceptive behaviour, with an IDJG of G.M mg/kg (VJ% CI, G.EF–G.V) and a Hill slope of -O (VJ% CI, -N– -G.F) (Figure EG A). Post hoc comparison between the dose-response curves of THC alone versus THC+ CBD in combination using the F extra sum square test revealed no difference between the two curves and indicated no interaction (F O,VJ = G.OJ, P= G.d; Figure EG AE).     66  Figure !!. The interaction between CBD and THC in the HSAA A) Dose-response relationship for a combination of THC+CBD (E:EG). The THC+CBD combination dose-dependently decreased the time spent licking and biting of the hind paw following HS injection. Each point represents an individual response (G–J min) after HS injection, with summary bars presented as means and VJ% CI. A!) Determination of interaction between CBD and THC. Dose-response curves from Figure V A & EG A were fitted together post hoc as above, indicating no interaction. The IDJGs and Hill slopes were compared using the extra sum of squares F test. Data were normalized to the control mean and doses expressed as those of THC. Circles (filled and open) and error bars denote means and their VJ% CI. n = d per group.                67 :.M Discussion The results summarized in this chapter demonstrate that the HSAA detects the antinociceptive activity of centrally- and peripherally-acting opioids, NSAIDs, a local anesthetic, and cannabinoids.  Intraperitoneal morphine produced a dose-dependent decrease in nociceptive responses induced by HS. The IDJG of morphine was M mg/kg (i.p.) in this assay, comparable to IDJG values in standard nociceptive assays (Woode et al., OGEE). Morphine blocks nociception mainly through µ-opioid receptor activation in the CNS, with a small contribution from peripheral receptors (Khalefa et al., OGEO). Systemic administration (E-EG mg/kg, i.p.) of the peripherally-restricted µ-opioid agonist, loperamide (Khalefa et al., OGEO), did not block the nociceptive responses.  Intraplantar loperamide attenuated the nociception only at high doses (equivalent to F mg/kg). Previous studies suggest that loperamide is only antinociceptive when inflammation is present (DeHaven-Hudkins et al., EVVV; Khalefa et al., OGEO), but HS injections produce only minimal inflammation (Rittner et al., OGGV; Vergnolle et al., OGEG). However, it is possible that EG% HS opens perineurial barriers formed by tight junctions to facilitate the antinociceptive action of opioids on peripheral nerves (Rittner et al., OGEO). This observation may partly explain the ability of high intraplantar doses of loperamide to reduce nociceptive responses induced by HS.  The present findings indicate that the HSAA has an ability to detect the antinociceptive activity of NSAIDs such as acetylsalicylic acid and naproxen, as well as acetaminophen. All three agents reduced nociception at higher doses, with no apparent adverse effects on     68 behaviour. The antinociceptive activity at these dose is detected with the use of the formalin assay, but not consistently so with the tail flick and hot plate assays (Hunskaar et al., EVdN; Hunskaar and Hole, EVdI; Vaz et al., EVVN). Although NSAIDs are generally thought to act against inflammatory pain, they have been shown to be effective in non-inflammatory nociceptive assays (Bustamante et al., EVVI). A lack of anti-inflammatory activity distinguishes acetaminophen from NSAIDs. Acetaminophen’s mechanism of analgesic action involves modulation of receptor systems different from NSAIDs (Bertolini et al., OGGN). The present data suggest that both acetylsalicylic acid, naproxen, and acetaminophen do not require inflamed tissue to produce antinociceptive effects. This study demonstrates the ability of HSAA to detect the antinociceptive activity of cannabinoids such as THC and to discriminate between two major cannabinoid constituents, THC and CBD. THC, but not CBD, dose-dependently reduced nociception induced by HS. The efficacy of THC in the HS assay is comparable to that previously reported in the mouse formalin test (Takahashi et al., OGGF). The mechanism by which THC produces its antinociceptive effect is thought to involve CBE receptor activation at a number of spinal and supraspinal structures critically involved in modulating nociceptive signals (Takahashi et al., OGGF; Cichewicz, OGGM). While THC is potent against different modalities of noxious stimuli, the therapeutic window between antinociception and behavioural and respiratory side effects is very narrow in rodents (Schmid et al., OGGF; Cichewicz, OGGM). Several studies have demonstrated that co-administration of CBD synergistically enhances THC antinociception while reducing its side effects (Casey et al., OGEI; King et al., OGEI). The present experiments showed neither potentiation of THC antinociception nor apparent attenuation of behavioural     69 side effects resulting from a combination of THC and CBD. The inability of HSAA to detect CBD antinociception or potentiation of THC may be due to differences in the nature of acute and chronic pain assays (Casey et al., OGEI). In summary, these results suggest that THC is the main driving force for producing antinociceptive activity in HSAA. The validation of HSAA was demonstrated by an ability of the HSAA to detect a number of established analgesics. It also correctly discerned the antinociceptive activity of two different opioids with different clinical analgesic profiles (morphine vs loperamide). While this is a powerful method for validation, the number of drugs assessed here was finite. Thus, future assessment of other agents and classes such as those with significant supraspinal action (e.g., amitriptyline) would provide further information regarding the scope of detection. In conclusion, this chapter has shown that HSAA is a valid assay of analgesic activity, allowing rapid detection of a wide range of analgesics. These findings suggest that this assay will be a useful addition to the armamentarium of preclinical analgesic assays, which may ultimately lead to the discovery and development of safer analgesic agents.          70 Chapter M: Refinement of the Hypertonic Saline Analgesic Assay    M.! Repeated Testing with the Hypertonic Saline Analgesic Assay in Mice for Screening of Analgesic Activity  M.!.! Introduction Experimental animals in preclinical pain research play a pivotal role in unraveling pain mechanisms and revealing possible analgesics (Mogil, OGGV; Gregory et al., OGEF) While non-animal alternatives hold promise (Wainger et al., OGEM), they remain unable to fully imitate the multifaceted aspects of pain in humans, which emphasizes the necessity for studying animal pain assays (Mogil et al., OGEG). Nonetheless, both Refinement and Reduction of animal use are feasible—and routinely mandated by Animal Care Committees—in preclinical pain research (Aske and Waugh, OGEI). In an ideal pain assay, a nociceptive response triggered by a chemical, thermal or mechanical stimulus should be (E) reproducible in magnitude on repeated testing and cause no tissue damage in the same animal; (O) readily replicated by different laboratories; and, (F) diminished by clinical analgesics (Le Bars et al., OGGE). For the most part, many preclinical animal screens have largely failed the reproducibility criterion while others were not sensitive to current analgesics. Improvements of the features (E-F) in a new assay would reduce the number of animals necessary for the study of pain as well as analgesic research.   Reproducibility of the nociceptive response is possible using non-chemically induced nociceptive assays such as the hotplate and tail flick tests. Although thermal assays allow for     71 the reuse of animals, several reports have demonstrated that these assays are susceptible to acquired learning as a result of repetitive testing, and poorly detect clinical analgesics (Barrot, OGEO). During performance of these assays, inappropriate environmental settings may play a major role in generating stress-induced analgesia/antinociception (Mogil et al., OGGE; Deuis et al., OGEI).  The number of studies demonstrating a reproducible response following a second injection of formalin is limited. Rosland et al showed that repeated injection of a low formalin concentration (G.O%) produces consistent phase E responses without noticeable histological changes (Rosland et al., EVVG). Repeated injection of a higher concentration of formalin triggers inflammatory sequelae such as long-lasting hyperalgesia and alteration of the biphasic response, coupled with gross histological damage (Coderre et al., EVVG; Rosland et al., EVVG; Fu et al., OGGE; Huh and Cho, OGEJ). An alternative to formalin is hypertonic saline (HS), which has been used to produce nociception in humans and animals (Hwang and Wilcox, EVdN; Graven-Nielsen et al., EVVIa; Ro et al., OGGF; Henderson et al., OGGd; Hodges et al., OGGV). In Chapters E and O, we described a new assay based on the intraplantar injection of EG% HS in mice that is capable of detecting a range of known analgesics. As evidenced histologically, single hind paw injections of EG% HS resulted in no tissue damage (cf. chapter O). The absence of tissue damage seen after acute administration of EG% HS led to our hypothesis that repeated testing, in an animal, would induce nociceptive responses that will be reproducibly attenuated by morphine. Therefore, the number of animals needed to establish antinociceptive activity would be reduced.     72 In the present investigations, we assessed the reproducibility of nociceptive responses to repeated EG% HS injections at various intervals into the same paw. We also determined the effects of intraplantar EG% HS in the contralateral hind paw or ipsilateral forepaw after its injection in the ipsilateral hind paw. We used the antinociceptive activity of morphine as a point of reference for validating the sensitivity of this modified HS assay. Histopathologic studies were performed to assess potential tissue damage.  M.!.8 Methods  M.!.8.! Animals  As in section O.O.E  <.#.4 Drugs and chemicals  Morphine hydrochloride, obtained from the MacFarlan Smith Ltd (Edinburgh, UK), was dissolved in normal saline. Sodium chloride (NaCl) obtained from BDH Chemicals (Toronto, ON, Canada), and was dissolved in distilled water.   <.#.< Hypertonic saline analgesia assay  As described in section O.O.F, the HSAA in this study was performed using female mice.  M.!.M.! Evaluation of repeated intraplantar injection of #E% HS at different time intervals  After O h acclimatization post-injection, mice received second hind paw injections of EG% HS (I days, OM h, and M h, respectively) or three injections (J-day interval between each injection) after the initial injection. The sum of the time the animal spent licking and biting the injected hind paw was recorded for J min.     73  M.!.M.8 Evaluation of the nociceptive response in other paws following hind paw injection of #E% HS   Thirty minutes after right hind paw injection of EG% HS, a group of mice receive EG µl injections of saline (EG% or G.V%) into the ventral surface of the left hind paw and responses were video recorded for J min.   In a separate experiment, two groups of naïve mice initially received i.pl injections of either EG% or G.V% saline into the right hind paw. The licking and biting responses were recorded for J min. Thirty minutes later, the same mice received EG µl injections of EG% HS into the ventral surface of the right forepaw. The licking and biting responses of the forepaw were video recorded for J min. M.!.M.: Utility of repeated injection procedure for detecting morphine antinociception Ten mice were habituated for O h. Morphine (G, M, and EG mg/kg) was then given intraperitoneally once every J days. The treatment order was assigned randomly following a Latin square design in which each mouse received each dose in a different order. After thirty min, EG µl of EG% HS was injected into the right hind paw and the responses over the subsequent J min were recorded and used to measure the antinociceptive effect. Morphine doses were based on a previous investigation in which M and EG mg/kg represented the IDJG and maximal effective dose, respectively (cf. Figure J). M.!.M.M Hind paw histopathology  Histological examination was conducted as described in section O.O.M. To examine for a potential inflammatory response to repeated injections of HS, three mice had EG% HS     74 injected every J days for a total of M injections. For comparison with a well-known inflammatory substance, hind paws were collected from mice (n = O) that received hind paw injections of J% formalin I days earlier.   <.#.? Statistical analysis Data analysis was performed using Prism N.GE software (GraphPad Software Inc., La Jolla, CA, USA). The mean and VJ% confidence intervals (CI) for the sum of the time spent licking and biting was analyzed for the first J min period after EG% HS injection. To analyze the effect of repeated testing of EG% HS on the same hind paw, one-way repeated measures analysis of variance (RM ANOVA) followed by Tukey’s multiple comparisons test and paired t-test were utilized as appropriate. For assessing the effect of EG% HS into the left hind paw preceded by right hind paw injection of EG% HS, a paired t-test was utilized. To evaluate the EG% HS induced ipsilateral forepaw responses preceded by an ipsilateral hind paw injection of EG% or G.V% saline, an unpaired t-test was used. One-way RM ANOVA followed by post hoc Tukey’s test were used to evaluate the antinociceptive effect of morphine. To determine the order effect of morphine treatment, the effect of each dose was analyzed separately using one-way ANOVA followed by Tukey’s post hoc test for the difference in the mean responses between first, second, and third treatment.  G*Power software (Christian-Albrechts-Universität Kiel, Kiel, Germany) was used to calculate sample sizes. A pilot study using d mice was initially conducted in which mice received a second intraplantar injection of EG% HS at I days after the initial injection (similar to the procedure described above). Based on sample standard deviations (SD) and means (mean     75 ± SD; EOG s ± FE for initial injection and dJ s ± OO for the second injection, with SD of difference of ± JE and within animal correlation of -G.d), the calculated effect size was E.GO. The estimated sample size to establish differences between matched pairs (assuming paired t-test) with a power of G.d and a= G.GJ was EG animals. Data are presented as a scatterplot of individual responses with error bars representing mean and VJ% CI. P values are reported and P<G.GJ is regarded as statistically significant.         76 M.!.= Results  M.!.=.! Reproducibility of nociceptive responses induced by #E% HS in the same hind paw at different time intervals   We initially asked whether a second injection of HS performed in F groups at I days, OM h, or M h after the first intraplantar injection produced responses that were of comparable magnitude to those of the first injection. The second administration was injected into the same hind paw. As illustrated in Figure EE A–C, there was no significant change in magnitude after a subsequent EG% HS injection. There was no statistically significant difference in mean licking and biting time between a second injection given I days after the initial injection and initial injection itself (prior mean, EEO s versus after, EEF s, mean difference [VJ% CI] = -G.F [-EO–EE]; paired t-test, n = EG, P = G.VJ; Figure EE A). The mean responses to the first and second (at OMh) injections of EG% HS were not significantly different (prior mean, EOO s versus after, EEF s, -N [-OM–EO]; paired t-test, n = EG, P = G.J; Figure EE B). In addition, there was no significant difference in the magnitude of responses after a second injection (at M h after the first injection) compared with the initial injection (prior mean, dM s versus after, IJ s, -V [-MG–OF]; paired t-test, n = EG, P = G.N; Figure EE C). Multiple injections of HS into the same hind paw of each animal (n = EG) produced responses of comparable magnitude at J-day intervals. As shown in Figure dD, the nociceptive responses to repeated intraplantar injections of EG% HS did not show evidence of change in mean amplitude (F (F,OM) = G.GFM, P >G.VV). Analysis with Tukey’s post hoc test indicated no significant differences in mean licking and biting times between repeated injections and the initial EG% HS injection (initial EG% HS mean, EOO s versus second, EOE s, mean difference [VJ% CI]     77 = -G.F [-Od–OI] P >G.VV; initial, EOO s versus third, EEd s, O.J [-FN–ME] P = G.VV; initial, EOO s versus fourth, EEV s, O [-FN–Fd] P = G.VV, n = EG; Figure EE D). The above results suggest the utility of repeated testing procedures using the HSAA.   Figure !8. Repeatability of nociceptive responses induced by !O% HS into the same hind paw at different time intervals   A–C) The nociceptive response induced by the second intraplantar injection of EG% HS administered I d, OM h, or M h after the initial injection into naïve mice. The mean licking and biting time after the second injection was not statistically different from that of the initial injection (paired sample t-test for the EG% HS group; n = EG per group).  D) Repeatability of the nociceptive response following multiple hind paw injections of EG% HS. Each injection was administered to the same hind paw in J-day intervals. No significant difference was found between groups (one-way RM ANOVA followed by Tukey’s test for EG% HS group; n = EG per group). In A–D, each point represents an individual response (G–J min) after HS injection, with summary bars presented as means and VJ% CI.        78 M.!.=.8 Prior right hind paw injection did not affect nociceptive responses of the right forepaw or left hind paw  We performed two tests for establishing the specificity of the HS response. Firstly, we determined if a single injection of HS into the hind paw would affect the response to a later injection into the contralateral hind paw. Right hind paw injection of EG% HS produced a licking and biting response that did not change the left hind paw response induced FG min later by EG% HS injection (left hind paw mean, VN s versus right hind paw, VF s, mean difference [VJ% CI] = -F [-OG–EF]; paired t-test, n = EG, P = G.I; Figure EO A).  Secondly, we determined whether a single injection of HS into the hind paw would affect the response induced by EG% HS injected FG min later into the ipsilateral forepaw. As expected, there was a significant difference between the mean licking and biting responses of the hind paw between control (G.V%) and EG% HS (control mean, F s versus EG% HS mean, VG s, mean difference [VJ% CI] = dI [JV–EEJ]; unpaired t-test, n = EG per group, P <G.GGGE; Figure EO B E). As illustrated in Figure EO B O, right hind paw injections of control or EG% HS did not significantly change the responses to EG% HS injected FG min later into the right forepaw (forepaw mean after EG% HS into hind paw, EMN s versus forepaw mean after G.V% saline into hind paw, EMV s, -F [-Od–OO]; unpaired t-test, n = EG per group, P = G.d). In summary, an initial hind paw injection of HS does not affect responses induced by subsequent HS injection into either the contralateral hind paw or ipsilateral forepaw.     79  Figure !:. Prior right hind paw injection of !O% HS did not alter nociceptive responses of other paws A) Effect of a right hind paw injection on the response of the left hind paw. No statistically significant difference was found between the mean licking and biting time of the left hind paw and the right hind paw (paired sample t-test for the EG% HS group; n = EG). HP denotes hind paw.   B) Effects of hind paw pretreatment on the forepaw response induced by EG% HS.  Open circles represent a group, closed circle represent a separate group. B!) Licking and biting responses of the right hind paw during the first J min after intraplantar injection of G.V% and EG% saline. Significant responses were produced by EG% saline (unpaired sample t-test;* P < G.GJ, n = EG per group). B8) The mean licking and biting time after an injection of EG% HS into the forepaw was not altered by an injection of saline (G.V% or EG% HS) into the right hind paw administered FG min prior (unpaired sample t-test;* P < G.GJ, n = EG per group). In A and B (BE, BO), each point represents an individual response during the post-injection G-J min interval and summary bars are means and VJ% CI.         80 M.!.=.: Effects of repeated injection of !O% HS on the antinociceptive effect of morphine   To validate the repeated procedure of HSAA, we determined whether the responses to the EDJG and maximal dose of morphine (cf. chapter F) were changed by F repeated injections of HS. Morphine dose-dependently attenuated the mean amplitude of licking and biting behaviour induced by EG% HS injections (one-way RM ANOVA (FO,EJ = Ed.GM, P = G.GGGO); Tukey’s post hoc  test, control mean, VM s versus M mg/kg, NN s, mean difference [VJ% CI] = OV [-I–NM] P = G.EO; versus EG mg/kg, OI s, NI [MM–VG] P <G.GGGE; M mg/kg versus EG mg/kg, NI [MM–VG] P = G.GF, n = EG; Figure EF).   The responses to the morphine injections suggested that the order of injection did not make a difference in the responses to HS, as revealed by one way ANOVA (control: F O,I = G.F, P = G.I; Tukey’s post hoc test, first treatment mean, EGG s versus second, VJ s, mean difference [VJ% CI] = J [-FV–Md] P >G.VV, versus third, dI s, EF [-FO–JV] P = G.I, n = F–M) (M mg/kg: FO,I = G.d, P = G.J; first treatment mean, JI s versus second, Jd s, -E [-NI–NJ] P >G.VV, versus third, dM s,   -OI [-VF–Fd] P = G.J, n = F–M) (EG mg/kg: FO,I = G.O, P = G.d; first treatment mean, OE s versus second, OI s, -J [-Md–Fd] P = G.V, versus third, FE s, -V [-JG–FE] P = G.d, n = F–M; Figure EG). As shown in figure F, the magnitude of EG% HS response in the control group is comparable to the magnitude of normal naïve responses obtained from MdJ mice (control, VM s [VJ% CI, dO–EGI] versus MdG naïve mice response, dN s [VJ% CI, dF–dV]; P = G.J; unpaired t-test). In brief, the above results suggest that the repeated HSAA procedure permits detection of the antinociceptive action of morphine and facilitates experimental design that allows for a reduction of animals used for antinociceptive screening (cf. Discussion).       81    Figure !M. The utility of a repeated testing procedure of HSAA for assessing the antinociceptive effect of morphine Each animal received, in a random order, morphine (G, M, EG mg/kg, i.p.). Morphine dose dependently reduced the mean licking and biting time induced by the intraplantar injection of EG% HS (one-way RM ANOVA followed by Tukey’s test; * P < G.GJ, n = EG). The mean licking and biting time in G mg/kg dose was not statistically different from the global mean. The global control data are G- J min responses to EG% HS in naïve animals (n = MdJ), collected from previous experiments. Each point represents an individual response during the post-injection G-J min interval. Colored circles in each column indicate the order of the treatment each animal received (blue, first; red, second; black, third). Horizontal bars represent means and vertical error bars denote VJ% CI.  M.!.=.M Histopathology  Hind paws from mice that received four repeated injections of EG% HS showed no significant tissue damage. Similar to acute hind paw injection of EG% HS (Figure M C), close examination of hind paw tissue revealed no significant damage except moderate subcutaneous edema in the hind paw (black arrow, Figure EM A). Moderate edema with normal     82 muscle cells (striped arrow) was present in the hind paw after M repeated injections of EG% HS (Figure EM C).  We compared the above results to J% formalin which produced profound histological damage. After I days, a deep ulcer with tissue necrosis, including adjacent skeletal muscle, marked infiltration of inflammatory cells (e.g., macrophages, lymphocytes), with signs of healing such as a large fibrin crust and early stage re-epithelialization was observed (Figure EM B, D).                83  Figure !#. Histology of the hind paw following repeated hind paw injections of EG% HS or #% formalin   A) Moderate edema (black arrow) is evident after repeated injections of EG% saline separated by J days and collected EJ min after the Mth injection.  B) Deep ulcer and associated, necrotizing cellulitis and myositis (asterisk) with a large fibrin crust (black arrow) are evident after single injection of J% formalin at I d.  C) EGx magnification of A shows normal muscle cells (striped arrow).  D) EGx magnification of B shows ulcer and fibrin crust (located between stars). Calibration bar indicates OGG µm (A, B) and EGG µm (C, D).         84 M.!.R Discussion  In this study, we have demonstrated that the nociceptive responses to repeated intraplantar injections of EG% HS were reproducible in magnitude. This method permits an experimental design that includes a reduction in the number of animals needed to establish an antinociceptive effect, providing an efficient method for assaying analgesics. Reproducibility of the responses after multiple injections of HS allowed for the detection of morphine’s antinociceptive effect in EG mice rather than FG, the number which would be required if using single injections (cf. chapter F). Modulation of nociceptive responses was not evident after subsequent injections of EG% HS into the opposite hind limb and forepaw, suggesting that EG% HS saline does not cause central sensitization. The features of the repeated testing of EG% HS suggest that the HSAA procedure has an important utility for analgesic screens.  The reproducibility of measurements on repeated EG% HS injection and their variation demonstrate that this assay can reduce the total number of animals required for analgesic assays. Three factors contributed to the reduction: (E) the magnitude of responses to EG% HS was not different when tested at various intervals up to EJ days; (O) variances of these responses to EG% HS were low; (F) there was little or no tissue damage or detectable contribution of central modulation or learning in the nociceptive responses. A reduction in animals for pain assays has not been a priority in the literature although the tail flick and hot plate tests in an animal may allow repeated testing that produces stable responses (Barrot, OGEO). The reproducibility of responses within each animal enables researchers to study the mechanisms underlying well-known clinical problems such as opioid tolerance and     85 hyperalgesia (Langerman et al., EVVJ; Menéndez et al., OGGO; Dighe et al., OGGV; Vardanyan et al., OGGV).  A concern that animals learn to avoid the stimulus over repeated exposures tends to deter a reuse of animals (King et al., EVVI; Gunn et al., OGEE). The current findings demonstrate that the HSAA can be performed appropriately with a reduced number of animals without compromising scientific rigor. Repeated intraplantar testing with EG% HS at different time intervals either in the same or different paws did not show change in the mean nociceptive response, suggesting little or no involvement of learning. The nociceptive responses evoked in the same hind paw did not vary significantly when administered for a second or third time over intervals up to EJ days. The nociceptive responses in the ipsilateral forepaw or contralateral hind paw showed no evidence of being affected by prior administration of the ipsilateral hind paw. The lack of differences in the amplitude of the responses suggests that learning is not a factor. This consistent with  previous observation of the amplitude response to repeated testing using (G.O%) of formalin (Rosland et al., EVVG).  This study suggests a practicality of using repeated testing of HSAA for determining antinociceptive activity by using a smaller number of CD-E mice than in our previous study (cf. Chapter F). Repeated administration of morphine in low and high doses at J-day intervals reduced nociceptive responses to EG% HS. Morphine at EG mg/kg produced a similar level of antinociception in naïve animals challenged with single injections of EG% HS (IO% vs II% in naïve mice). Our analysis of morphine treatment showed no impact of the administration order on the ability of each dose to produce an expected effect. We found that the magnitude of the response in the control group fell within normal responses obtained from MdJ naïve mice     86 demonstrating a stable result. While we cannot generalize to other mice strains (Leo et al., OGGd), the ability to establish a morphine effect with reduced number of animals using repeated nociceptive challenge constitutes a valid basis for determining analgesic activity. The HSAA is cost-effective and appropriate for retaining transgenic mice for reuse. Transgenic mice used to determine analgesic targets are costly and unusable after assays that are complicated or injurious. Using the HSAA, repeated testing of the same paw or other paws facilitates assessment of the analgesic activity of systemically or intrathecally administered drugs, including their duration of action. The implementation of repeated testing for screening potential analgesics allows an experimental design that animals serve as their own controls. This minimizes data variability and enhances the sensitivity of the statistical analysis used to detect a difference (Chow and Liu, EVVd).  The current study is not without weaknesses. The data presented were obtained solely from female mice. Sex related differences in preclinical nociceptive assays have been previously reported (Leo et al., OGGd; Asiri et al., OGEI). Thus, we suggest caution when extrapolating the current findings to male mice. We demonstrated that repeated injections of HS, similar to that in Figure ED, caused minimal histological damage. However, it is unknown whether this holds true for all time points examined in the present study. Although there was no evidence of any statistical difference in responses after repeated injections of HS in the same, or other paws, the results should be interpreted with caution. Small changes may exist between groups that were undetected due to low statistical power. In addition, the mechanisms by which HS induces nociception is unclear. Therefore, it is difficult to draw a connection to a specific form of clinical pain.     87 We conclude that the current modification of the HSAA is an efficient, valid procedure for assessing analgesic activity. The present findings support the use of the HSAA for rapid and repeatable procedure for detecting analgesic activity. This modified HSAA fulfills the concept of “reduction and refinement” for animal use while decreasing variance and minimizing animal suffering.                   88 M.8 Effects of Animal Strain on Sex, Circadian Rhythm, and Antinociception on Nociceptive Responses of Hypertonic Saline Analgesia Assay in Mice   M.8.! Introduction  Screening for potential analgesics and decoding intricate pathophysiological mechanisms of pain necessitates the use of experimental non-human mammals. Although non-mammals such as fish (Sneddon, OGGF) and non-animal in vitro pain assays are available (Wainger et al., OGEM), fully mimicking the multidimensional nature of pain or even reaching a similar level to that attained with laboratory mammals remains a challenge, which underlines the importance of improving mammalian assays in pain research (Key, OGEJ). The success of a preclinical nociceptive assay is contingent on its ability to meet certain criteria. The nociceptive response induced by a stimulus should be consistent in magnitude, sensitive to diminution by clinically used analgesics, and reproducible across laboratories and different strains within a species (Le Bars et al., OGGE). Because many current preclinical assays show divergent basal nociceptive and antinociceptive responses across different strains of mice, a comprehensive assessment of newly developed assays in background strains frequently used in knockout studies would improve its validity and generalizability of results.   Within species differences have been well documented in rodent assays of pain. These variations arise from many factors including strain, sex, and nature of stimulus. Large variations in baseline readings were found in EE commercially available mice strains tested in thermal assays including standard thermal hot plate and tail flick tests, with CJIBL/N being one the most sensitive to pain and most resistant strains to opiate treatment (Mogil et al., EVVVa; Wilson, OGGF). Although this study was conducted exclusively on male mice, an     89 observation by Leo et al. showed similar strain differences with some assays exhibiting gender differences as well (Leo et al., OGGd). In the case of chemical nociceptive assays, many reported strain differences in the standard formalin foot test where both sexes of CJIBL/N mice exhibited higher nocifensive responses in phase I and II compared to CD-E and NMRI mice (Leo et al., OGGd). In contrast, the CJIBL/N strain was mostly resistant to thermal and mechanical hyperalgesia in inflammatory and neuropathic pain models suggesting fundamental genetic variations may account for the observed differences in response to different stimuli (Mogil et al., EVVVb; a).   Hypertonic saline (HS) has been used experimentally in human and laboratory animals to induce pain/nociception and evaluate the efficacy of innovative therapeutic agents (Hwang and Wilcox, EVdN; Graven-Nielsen et al., EVVIa; Ro et al., OGGI; Hodges et al., OGGV; Samour et al., OGEI). While the vast majority of studies utilized HS to study muscle pain, a limited number of clinical and preclinical studies have exploited it for the induction of transient subcutaneous nociception. Recently, we developed a new nociceptive assay in mice based on a plantar injection of EG% HS, HSAA (cf. chapter O and F). This assay detected the antinociceptive activities of a range of established analgesics and caused minimal histological damage. Furthermore, this assay is highly reproducible in the same animal and produced consistent responses regardless of retesting which allows for the reduction of animals needed for detecting analgesics.   Since the literature shows that the strain of the mouse has a great influence on the basal and antinociceptive responses, we assessed the HSAA in another frequently used strain,     90 CJIBL/N, in addition to CD-E. We first evaluated the responses to intraplantar injection of EG% HS in both male and female CJIBL/N mice. We then assessed the effects of morphine in this assay. Lastly, we assessed the effect of circadian rhythm on the responses induced by HS. For comparison, parallel experiments were conducted in CD-E mouse strain.  M.8.8 Methods   M.8.8.! Animals Adult CD-E and CJIBL/N mice of both sexes (OJ–FG, OG–OJ g, respectively; Charles River, Québec, Canada) were utilized. (See Section O.O.E for details)   M.8.8.8 Drugs and chemicals Morphine sulfate obtained from Toronto Research Chemicals (Toronto, Ontario, Canada) dissolved in normal saline. Sodium chloride (NaCl) was obtained from BDH Chemicals (Toronto, Ontario, Canada) was dissolved in distilled water.   M.8.8.: Hypertonic saline analgesic assay As described in Section O.O.F <.!.!.4.# Characterization of the nociceptive response and evaluation of sex effect in HSAA in C?JBL/N mice   Male and female CJIBL/N mice were habituated individually for two consecutive hours. Thereafter, mice received right hind paw injections of EG% HS (EG µl) and were immediately returned to their respective tubes and video recorded for FG min. The videos were analyzed     91 post hoc by an analyst who recorded, in J min bins, the length of time each animal licked and bit the affected paw and the number of times the affected hind paw flinched. The same procedure was performed in CD-E mice for comparison.   <.!.!.4.! Validation of HSAA in C?JBL/N mice   To assess the antinociceptive activity of morphine in CJIBL/N mice, females were utilized.  Mice were habituated for O h then morphine (G, G.N, O, N, and OG mg/kg) was given intravenously (i.v.) in a volume of FG µl. After J min, EG µl of EG% HS was injected into the ventral aspects of the right hind paw and the nociceptive responses over the following J min were recorded. The number of flinches or the length of time animals licked and bit were recorded and used to measure the antinociceptive effect. To compare the potency of morphine antinociception, a similar experiment was conducted in female CD-E mice.   <.!.!.4.4 The effect of circadian rhythm on HSAA   The influence of circadian rhythm was investigated utilizing female mice. Mice were divided into two groups and tested separately at EO:GG PM and EO:GG AM. These time points were shown to represent the peak of rest and activity and produced differences in response to a number of nociceptive stimuli (Castellano et al., EVdJ; Turek et al., OGEF). After O h habituation, mice received EG µl injections of either EG% or G.V% saline into the ventral surface of the right hind paw and were video recorded for J min. The camera was capable of recording in the dark. For animals tested at EO:GG AM, the entire experiment was performed in a dark room and a red light was used briefly when administering solutions. The number of flinches     92 and the length of time animals exhibited licking and biting responses were recorded afterwards by a blinded observer. For comparison, a similar procedure was performed in female CD-E mice.  M.8.8.M Statistical analysis All statistical analyses were carried out using Prism N.GE software (GraphPad Software Inc., La Jolla, CA, USA). The number of flinches and the time that animals spent licking and biting the affected paw in the first J min period after intraplantar injection were used for analysis. To determine the effect of sex in each mouse strain, Two-way ANOVA (factors; saline concentration x sex) followed by Tukey’s post hoc test was utilized. To compare the responses of male and females between strains to EG% HS, O-way ANOVA (factors; strain x sex) followed by Tukey’s post hoc test was utilized. For assessing the antinociceptive effect of morphine, data were first normalized to the mean of the control. Dose–response curves for estimating IDJGs were then constructed by fitting the data using nonlinear least squares log[inhibitor] vs. response [variable slope] equation: (Y= Bottom + (Top-Bottom)/ (E+EG [(LogIDJG-X)*Hill Slope]). Morphine dose–response curves (for licking and biting) from CJIBL/N and CD-E mice were fitted together. Slopes and IDJGs were compared using extra sum of squares F test. Two-Way ANOVA (factors; saline treatment x time of day) was used to determine the effect of circadian rhythm on nociceptive responses induced by plantar injection of EG% HS. Tukey’s post hoc test was applied when a significant interaction detected. Estimations of sample sizes were projected based on our previous study. Individual responses are plotted with mean and VJ%     93 confidence intervals (VJ% CI). P values are reported and P<G.GJ is set as the criterion for statistical significance.  M.8.: Results M.8.:.! Differences in nociceptive responses between sexes in C?JBL/N mice  Intraplantar injection of EG% HS induced intense nociceptive responses. Unlike the nociceptive response observed in CD-E mice, CJIBL/N mice produced two distinct nociceptive responses, namely licking/biting and flinching (Figure EJ A-C). As shown in figure EJ C, the licking and biting response peaked during the first J min period after injection and gradually subsided over FG min. The flinching response was only observed in the early J min following the injection. Both responses were greater in females than in males. With regards to the licking and biting response, two-way ANOVA revealed a significant interaction (F E,FN = EE.F, P = G.GGO), sex effect (F E,FN = V, P <G.GGJM), and significant effect of saline concentration (F E,FN = dG, P <G.GGGE). The Tukey post hoc test indicated that both male and female mice exhibited significant increases in mean licking and biting behaviour in response to EG% HS compared to G.V% saline during the early J min (female, control mean, J s [VJ% CI, E–V] versus EG% HS, dI s [VJ% CI, NM–EEG], P <G.GGGE]; male, control mean, d s [VJ% CI, -O–Ed] versus EG% HS, MJ s [VJ% CI, OV–NE], P = G.GGO; n = EG per group; Figure EJ A). The magnitude of licking and biting responses in female mice injected with EG% HS was significantly higher than the corresponding response in male mice (female, EG% HS, dI s [VJ% CI, NM–EEG] versus male, EG% HS, MJ s [VJ% CI, OV–NE], P = G.GGGM; n = EG per group; Figure EJ A). Similarly, flinching was observed in both sexes with a greater magnitude in females than in males. Two-way ANOVA of the flinching response     94 indicated a significant interaction (F E,FN = EE, P = G.GGO), sex (F E,FN = N, P <G.GO), and saline concentration (F E,FN = EJE, P <G.GGGE). Tukey’s multiple comparison test revealed that a higher saline concentration triggered more flinches in both sexes during the J min following injection (female control mean, E [VJ% CI, G.J–O] versus EG% HS, FE [VJ% CI, OJ–FN], P <G.GGGE]; male control, F [VJ% CI, -O–I] versus EG% HS, OG [VJ% CI, EM–OJ], P <G.GGGE; n = EG per group; Figure EJ B). The number of flinches in female mice that received EG% HS was significantly higher than in males (female, EG% HS, FE [VJ% CI, OJ–FN] versus male, EG% HS, OG [VJ% CI, EM–OJ], P = G.GGE; n = EG per group; Figure EJ B)  Consistent with our previous observation in CD-E mice, the dominant nociceptive response, licking and biting, in both sexes peaked during the early J min after EG% HS injection and faded after OJ min (Figure EJ E). The mean response was larger in magnitude in female than in male mice (Figure EE D). As revealed by two-way ANOVA, a significant interaction (F E,FN = OI, P <G.GGGE), sex (F E,FN = OE, P <G.GGGE), and saline concentration effect (F E,FN = EdM, P <G.GGGE). Further analysis of the data using Tukey’s post hoc test demonstrated that in both sexes the increase in saline concentration triggered vigorous nociceptive response compared to control (female control mean, E s [VJ% CI, -E–O] versus EG% HS, EEGs [VJ% CI, VJ–EON], P <G.GGGE]; male control, M s [VJ% CI, -E–EG] versus EG% HS, JF s [VJ% CI, FO–IM], P <G.GGGE; n = EG, per group; Figure EJ D). As shown in Figure EJ D, the mean magnitude of licking and biting in female mice after EG% HS was twice that of the corresponding response in male mice (female EG% HS, EEG s [VJ% CI, VJ–EON] versus male EG% HS, JF s [VJ% CI, FO–IM], P <G.GGGE; n = EG per group).       95  When comparing the magnitude of licking and biting in males and females between strains, there was a significant effect of sex (F E,FN = FM, P <G.GGGE). No interaction between sex (F E,FN = G.I, P = G.F) and none between strains (F E,FN = F.J, P = G.GI). No difference was found within sex between both strains (female CJIBL/N, EG% HS, dI s [VJ% CI, NM–EEG] versus female CD-E, EEG s [VJ% CI, VJ–EON], P = G.O; n = EG per group; male CJIBL/N, EG% HS, MJ s [VJ% CI, OV–NE] versus male CD-E, JF s [VJ% CI, FO–IM], P = G.V; n = EG per group)      96  Figure !=. Strain and sex differences in the HSAA A) Comparison of the magnitude of EG% HS induced licking and biting response between male and female CJIBL/N mice. The mean licking and biting magnitude of female mice to EG% HS was greater than that in male mice.     97 B) Comparison of the magnitude of EG% HS induced flinching response between male and female CJIBL/N mice. Flinches in female mice were greater than in male mice.  C) Time course of observed licking and biting response after intraplantar injection of saline in male and female CJIBL/N mice (same animals in A). D) Comparison of the magnitude of EG% HS induced licking and biting response between male and female CD-E mice. The mean licking and biting magnitude of female mice to EG% HS was greater than in male mice. E) Time course of observed licking and biting response after intraplantar injection of saline in male and female CD-E mice (same animals in D). In C and E, summary bars are mean and VJ% CI.  In A, B, and D, each point represents an individual animal response during the early J min after HS injection. Summary bars denote means with VJ% CI (O-way ANOVA followed by Tukey’s post hoc test; *P<G.GJ; n = EG per group).  M.8.:.8 Assay sensitivity to morphine in C?JBL/N mice  Morphine administered intravenously (i.v.) produced a dose-dependent attenuation of nociceptive responses (licking/biting and flinching) triggered by intraplantar injection of EG% HS (Figure EN A&B). Figure EN A shows that morphine reduced mean duration of time animals spent licking their affected hind paw. The effect was dose-dependent an IDJG of G.N mg/kg (VJ% CI, G.F–E.J). As well, morphine dose-dependently attenuated the number of flinches induced by hind paw injections of EG% HS (IDJG = O.F mg/kg; VJ% CI, E–M.I; Figure EN B).  By comparison, morphine (i.v.) produced a similar dose-dependent reduction of licking and biting time following EG% HS injection in female CD-E mice, with an IDJG of = O.J mg/kg (VJ% CI, E.J–M) (Figure EN C). Comparison of the morphine dose response relationships using the extra sum of squares F test revealed no statistical differences (F O,IO = O.O, P = G.E) between the parameters analyzed (CJIBL/N, [IDJG, VJ% CI = G.I, G.O–O.O; Hill slope, VJ% CI = -G.d, -O–G.M] versus CD-E, [IDJG, VJ% CI = O.d, G.V–d.d; Hill slope, VJ% CI = -E, -O.O–G.GGE]).       98  Figure !R.  HSAA sensitivity to morphine in C#RBL/= mice   A) Morphine dose dependent attenuation of licking and biting response in CJIBL/N mice. Each point represents a normalized individual response during the first J min period after EG% HS injection. Horizontal bars and vertical error bars represent mean and VJ% CI, respectively (n = d per group).  B) Morphine dose dependent attenuation of the concurrent flinching response in CJIBL/N mice. Each point represents a normalized individual response during the first J min period after EG% HS injection. Horizontal bars and vertical error bars represent mean and VJ% CI, respectively (n = d per group). C) Morphine dose response curve for the attenuation of licking and biting response produced by EG% HS in CD-E mice. Each point represents a normalized individual response during the first J min period after EG% HS injection. Horizontal bars and vertical error bars represent mean and VJ% CI, respectively (n = d per group).        99 M.8.:.: Circadian rhythm modulation of responses induced by #E% HS in C?JBL/N mice  The licking and biting response produced by EG% HS injection was sensitive to the time of day when the animal was tested, while flinching response was unaffected (Figure EI A & B). With the licking and biting response, significant effect of time (F E,FN = J.O, P = G.GF), saline concentration effect (F E,FN = NN, P <G.GGGE), and interaction (F E,FN = J, P = G.GM) were detected. The mean licking and biting behaviour in animals that were tested at EO:GG AM was significantly higher than those tested at EO:GG PM (EG% HS at EO:GG AM, NM s [VJ% CI, MG–dd] versus EG% HS at EO:GG PM, FI s [VJ% CI, OM–JE], P = G.GO, n = EG per group; Figure EI A). As illustrated in Figure EI B, flinching events were not influenced by the time of day in which EG% HS was given. No time of day effect and no interaction were found  (F E,FN = O, P = G.O, and (F E,FN = E, P = G.F, respectively). Contrary to CJIBL/N mice, licking and biting response in CD-E mice was not altered by the test time of day. As shown in Figure EI C, the statistical analysis with O-way ANOVA indicated a significant saline treatment effect (F E,FN = JV, P <G.GGGE) with no interaction between treatment x time of day (F E,FN = G.GGM, P = E), and no time of day effect (F E,FN = G.GI, P = G.d).     100  Figure !T. Circadian rhythm effects in the HSAA A) Effects of time of day on licking and biting response induced by G.V% and EG% in CJIBL/N mice. The mean licking and biting response in mice tested at night was significantly higher than that of the corresponding response at day (O-way ANOVA followed by Tukey’s post hoc test; *P<G.GJ; n = EG per group). B) Effects of time of day on the concurrent flinching response induced by G.V% and EG% in CJIBL/N mice. The mean flinching response in mice tested at night was not different from those tested at day (O-way ANOVA followed by Tukey’s post hoc test; *P<G.GJ; n = EG per group).  C) Effects of time of day on licking and biting response induced by G.V% and EG% in CD-E mice. The mean licking and biting response in mice tested at night was not different from those tested at day (O-way ANOVA; no significant difference was found; n = EG per group). In A-C, each point represents an individual animal response during the early J min after injection. Summary bars denote means with VJ% CI.           101 M.8.M Discussion The aim of our study was to validate HSAA in CJIBL/N mice, a strain extensively used for generating knockout mice, and compare the responses of CJIBL/N mice to CD-E mice with respects to sex, analgesia, and circadian rhythm. Our results showed that the EG% HS assay is reproducible in CBJIBL/N mice and detected the antinociceptive action of the archetypical opioid morphine. Unlike in CD-E mice, intraplantar EG% HS produced two noticeable nociceptive behaviours in CJIBL/N mice: licking/biting and flinching. Similar to CD-E mice, the licking and biting response lasted for FG min, whereas flinching responses preceded licking/biting and dominated the J min period immediately after injection of EG% HS. These nociceptive responses were greater in magnitude in female mice than male mice, corroborating with observations in CD-E mice. The impact of circadian rhythm on the nociceptive responses induced by EG% HS in CJIBL/N was confined to the licking and biting response with mice showing a slightly greater response at night than during the day. Circadian rhythm had no apparent impact on flinching behaviour. These results, together with previous data, further support the utility of the HS foot assay for analgesic screens.  The nociceptive responses to EG% HS in CJIBL/N mice are composed of two distinctive and easily quantified behaviours, licking/biting and flinching. Licking/biting responses induced by algogens (e.g., HS and formalin) are prominent in CD-E mice and are recommended as an index of nociception whereas flinching is rare and difficult to quantify (Sufka et al., EVVd). In CJIBL/N mice, flinching alone and in combination with licking/biting response have been used as a measure of nociception (McNamara et al., OGGI; Bai et al., OGEO; Liu et al., OGEF). The discrepancy of reporting of the nociceptive response in CJIBL/N mice is unclear. It is possible     102 that the weak and highly variable licking/biting response increased the propensity to rely on flinching as a marker for nociception instead. For example, Bai et al. observed weak licking/biting responses in phase O of the formalin test compared to a more robust biphasic flinching response (Bai et al., OGEO). Indeed, we observed a large variation in licking/biting response across many samples tested compared to a stable flinching response. Although both behaviours were sensitive to morphine and their IDJGs were not different from each other or from those  in CD-E mice, we recommend reporting both behaviours as they may be modulated differently which could have different sensitivities to pharmacological manipulations (Aloisi and Carli, EVVN).   This study demonstrates that female mice are more sensitive, regardless of strain, to EG% HS stimulus than their corresponding male mice. These observations are in agreement with our previous study as well as other studies in a wide variety of pain modalities showing female rodents have a propensity to exhibit exacerbated nociceptive responses more than males (Leo et al., OGGd; Mogil and Bailey, OGEG). A similar pattern is observed in humans where female subjects reported higher sensory scores than males after subcutaneous or intramuscular injections of J% HS (Henderson et al., OGGd). While a considerable body of evidence clearly indicates sex differences in preclinical assays of pain, the mechanism(s) underlying these differences are not well-understood. Gonadal steroid hormones have been shown to contribute to the modulation of nociceptive signals and response to treatment (Craft et al., OGGM). Neurochemical recruitment involved in modulating nociceptive signals such as neurotransmitters are observed to be sex-dependent (Lei et al., OGEE; Mifflin et al., OGEN). Recent studies found that both male and female animals use dissimilar mechanisms in     103 the microglial system for regulating inflammatory and neuropathic nociceptive signals (Sorge and Totsch, OGEI). The majority of preclinical pain studies exclusively involves the use of male rodents despite evidence of substantial difference between sexes in responses to stimuli and analgesic drugs (Mogil and Chanda, OGGJ). Consequently, for the betterment of translating preclinical findings, the special interest group on Sex, Gender, and Pain of the International Association for the Study of Pain strongly endorses the use of both sexes or at least females if limited to one sex (Greenspan et al., OGGI).  The impact of circadian rhythm on nociceptive responses induced by EG% HS in our study is strain specific where CJIBL/N mice exhibited a higher licking and biting response during the dark than the light phase compared to no change in CD-E mice. The slight increase of licking/ biting response to EG% HS during the dark phase was similar to phase Ι responses of the formalin test reported by Zhang et al. (OGEO) where the diurnal cycle effect was weaker in phase Ι than phase ΙΙ (Zhang et al., OGEO). Circadian rhythm effects on other assays of pain are largely dependent on the species, strain, and type of stimulus. In the thermal assays, CJIBL/N  mice had higher sensitivity during the dark phase than the light phase, contrary to other observations in albino mice (Swiss Webster and CF-E) (Oliverio et al., EVdO; Castellano et al., EVdJ). Mechanical sensitivity to von Frey hairs was greater for the period of the dark active phase in naïve, but not in neuropathic CJIBL/N mice (Minett et al., OGEM; Xia et al., OGEN). In addition, mechanical threshold in neuropathic but not sham CD-E mice fluctuated over OM h with higher sensitivity observed during the late hours of the day (Kusunose et al., OGEG). While the circadian oscillation effect on formalin induced nociceptive behaviour is moderate in CJIBL/N (Zhang et al., OGEO), a profound exacerbation of formalin-induced nociceptive     104 responses during the dark phase was detected in CBA mice (Perissin et al., OGGG, OGGF, OGGM). The divergent effects of circadian rhythm on regulating nociceptive processing in different mice strains is elusive. The hypothalamic suprachiasmatic nucleus (SNC) has been known as the master clock that synchronizes circadian rhythms through secreting mediators to modulate various hormones (e.g., melatonin), cytokines, and clock genes expression (brain and muscle ARNT-like-E (BMLAE) and circadian locomotor output cycles kaput (CLOCK)) (Jahanban-Esfahlan et al., OGEI; Segal et al., OGEI). Thus, it is likely that phenotypic differences in the ability of SNC to regulate circadian rhythm may account for the discrepancies seen in diverse mouse strains in response to different nociceptive stimuli.  Although HS assay was able to detect the antinociceptive effects of morphine in female CJIBL/N and CD-E mice, the sensitivity of male mice to standard analgesic drugs in this assay is unknown. Several studies demonstrated that male mice are more sensitive to pharmacological manipulations than female mice (Mogil, OGGV). Hence, further studies are needed to evaluate the HS sensitivity to analgesic drugs in male animals. We also did not test whether the treatment of morphine is influenced by the circadian rhythm although other reports have suggested that antinociceptive efficacy of drugs varied depending on the time of the day even if animals exhibited no differences in their basal nociceptive behaviour (Rasmussen and Farr, OGGF; Kusunose et al., OGEG; Akamine et al., OGEJ).    In summary, the current study demonstrates that HSAA is an efficient, reproducible assay for screening potential analgesics in CJIBL/N background mice strain. The present findings together with previous data further support the utility of the HSAA in different mouse strains for analgesic screens.      105 Chapter #: General Discussion  #.! Summary of results   This thesis developed the HSAA as a new analgesic assay in mice. The murine nociceptive assays currently employed to characterize putative analgesics fail principally due to their inability to detect known classes of analgesics and poor reproducibility within and between animals. The thesis assessed the feasibility of the new assay in mice, focusing on two important criteria, its sensitivity to a wide range of analgesics and its reproducibility. In CD-E mice, we determined that intraplantar injection of various concentrations of saline produced nocifensive responses (licking and biting) with EG% HS producing the maximal and least variable responses. The magnitude of the response was concentration dependent and notably influenced by sex. However, both sexes exhibited a similar response time pattern, peaking at J minutes after the injection and gradually diminishing over the subsequent OJ minutes. HSAA detected a number of agents representing different classes of analgesics. There was no tissue damage shown by histological assessment following EG% HS injection. In view of the absence of histopathology, we examined the intra-individual reproducibility of HS responses. We showed that the magnitude of the HS response did not significantly change after repeated injections at different time intervals in the same hind paw. The responses were stable on repeated testing, allowing for the detection of morphine’s antinociceptive effect using fewer animals than required by conventional methods. Prior hind paw injection did not appear to affect subsequent responses in either the contralateral hind     106 paw or ipsilateral forepaw suggesting that nociception was localized to the site of injection and was not subject to learning by the nervous system. The strain of the mice showed a high degree of variability in nociceptive sensitivity and potency of antinociceptive treatment (Mogil et al., EVVVa; Wilson, OGGF). This thesis aimed to assess the reproducibility of the HSAA in another commonly used mouse strain, CJIBL/N. This involved determining the HS response and the ability of morphine to attenuate this response. Furthermore, we examined the impact of sex and circadian rhythm on HS induced responses. We found that EG% HS produced two distinct nociceptive responses — licking/biting and flinching. Both responses were significantly different in female and male animals, but exhibited similar time response patterns; the response peaked during the first J minutes and slowly diminished within OJ minutes. HSAA detected analgesics in CJIBL/N mice and morphine dose-dependently attenuated the responses with potency comparable to values obtained from CD-E mice. The circadian rhythm effect on the HS response seemed to only affect the licking/biting responses which were higher during the night compared to the day. HSAA equally detected analgesic activities of drugs in both CJIBL/N and CD-E strains.    #.8 HSAA for analgesic evaluation This thesis establishes the utility of HSAA to rapidly screen for potential analgesics. The nociceptive responses were attenuated by a number of widely used analgesics. These agents are known to decrease nociceptive responses in mammals via actions on different receptor systems suggesting a wider range of sensitivity than existing assays. The assay was sensitive to morphine, the archetypical µ-opioid receptor agonist, which predominantly exerts its     107 antinociceptive effects via the activation of µ-opioid receptors in the CNS (Khalefa et al., OGEO). However, the assay failed to detect systemic loperamide, an opioid with peripherally restricted activity. While some preclinical reports showed that the antinociceptive action of loperamide was more pronounced in the presence of inflammation (Shannon and Lutz, OGGO; Khalefa et al., OGEO), the insensitivity of the HSAA to systemic treatment of loperamide is consistent with its poor analgesic activity in clinical trials. Furthermore, the HSAA is capable of detecting the antinociceptive effects of NSAIDs (aspirin and naproxen),  which showed potencies comparable to observations in other chemical assays (Hunskaar et al., EVdN; Vaz et al., EVVN). NSAIDs are mainly efficacious in attenuating inflammatory pain by blocking cyclooxygenase enzymes. Since HS injection does not cause inflammation (Rittner et al., OGGV; Asiri et al., OGEI), the antinociceptive effects of NSAIDs could be due to direct action on nociceptors (Bustamante et al., EVVN, EVVI). The aniline analgesic acetaminophen also demonstrated an ability to attenuate the HS induced response at a dose that did not cause adverse effects. The mechanism by which acetaminophen exerts its antinociceptive action is not well understood. Several studies have indicated the involvement of different receptor systems, such as the serotonergic descending inhibitory system in the brainstem and spinal cord, interacting with TRPVE, endocannabinoid and opioidergic systems (Smith, OGGV). HSAA detected a number of agents with different mechanisms of action. The question arises regarding the predictive validity of HSAA for the development of innovative analgesics. While the HSAA demonstrated the ability to detect a number of clinically utilized analgesics, we may speculate about its predictive potential for detecting clinically useful analgesics. Currently utilized preclinical analgesic assays, all with proven     108 sensitivity to a wide range of analgesics, have been increasingly blamed for the failure of successful translation to humans. An example of the poor predictive ability of analgesia assays is the neurokinin E (NKE) antagonist (substance P antagonist) which shows strong efficacy in a variety of analgesic assays but has no appreciable analgesic efficacy in humans (Whiteside et al., OGGd). A failure to predict analgesic efficacy in humans may not necessarily result from preclinical analgesic assays per se. Several factors are postulated to contribute to this failure including incorrect association of the identified target to pain, lack of drug selectivity that ultimately causes untoward side effects, species related differences with respect to the relative functionality of the receptor target, and inappropriate results from the use of  a single nociceptive assay (Whiteside et al., OGGd). Targeting the wrong patient populations and poor experimental design in initial clinical trials are also contributing factors for the lack of successful clinical trials (Whiteside et al., OGGd). The ability of the HSAA to quickly detect different analgesics that act at different receptor sites suggests a wide range of sensitivity, but its predictive validity can only be confirmed by demonstrating that it can identify new clinically effective analgesic agents.   Using a single assay to assess potential analgesic activity is an oversimplification of pain conditions which are extremely complex and multidimensional in nature. Hence, it is recommended to utilize a battery of assays that represent different modalities of nociception in order to enhance the predictability for clinical application of novel analgesics and minimize false positives (Mogil, OGGV; Cobos and Portillo-Salido, OGEF). HSAA serves as a quick analgesic screening tool in early stages but does not preclude or substitute for the utilization of other assays.       109 #.: Repeated testing procedure of HSAA for assessing analgesic activity  The first refinement of HSAA investigated the reproducibility of the HS response (i.e., intra-animal reproducibility) with repeated testing. We found that nociceptive responses following repeated administration of HS into the same hind paw remained unchanged. When this repeated testing procedure was employed to assess the antinociceptive activity of morphine, the assay detected the antinociception of morphine with sensitivity similar to that observed in singly treated animals. Interestingly, injecting HS into either the contralateral hind paw or ipsilateral forepaw produced responses that were unaffected by a preceding HS injection into the ipsilateral hind paw. These results suggest the feasibility of utilizing this modified procedure to evaluate potential analgesics. Minimizing the number of animals used to obtain comparable information is recommended in preclinical pain research whenever possible. This can be achieved by implementing proper experimental design, statistical analysis, and the utilization of non-tissue damaging techniques (Council, OGEG). The reduction of animal numbers in the HSAA was possible due to: (E) the lack of abnormal histology after one or multiple injections of HS; (O) the magnitude of responses to EG% HS was unchanged following repeated injections into the same site; (F) the lack of sensitization or learning evidenced by unaltered nociceptive responses after injections of EG% HS. Repeated testing with HS therefore facilitated the detection of morphine’s antinociceptive effects and allowed for an efficient experimental design in which each animal served as its own control.   It is important to note that while we found no change in the magnitude of the licking and biting response after repeated testing with HS, it does not necessarily confirm the lack     110 of local sensitivity to mechanical or thermal stimulus (i.e., mechanical allodynia or thermal hyperalgesia). It is unlikely that repeated testing with HS would cause prolonged thermal or mechanical hypersensitivity similar to that observed after the injection of formalin (Fu et al., OGGE; Ambriz-Tututi et al., OGEE) owing to the absence of inflammation. Rittner et al. reported that a single intraplantar injection of EG% HS did not alter the paw pressure withdrawal threshold in rats which supports the lack of inflammation (Rittner et al., OGGV, OGEO). However, HS is known to facilitate the depolarization of small diameter primary afferent neurons resulting in the release of pro-inflammatory mediators known to be pivotal for the maintenance of mechanical and thermal hyperalgesia (e.g., substance P and glutamate) (Garland et al., EVVJ; Schumacher et al., OGGG; Louca et al., OGEM). Repeated application of HS may possibly exacerbate the release of these pro-inflammatory mediators which could ultimately cause mechanical and thermal hypersensitivity. Future studies addressing this concern are needed.   #.M Hypertonic saline analgesic assay - pros and cons The utilization of HSAA offers advantages over other nociceptive assays. The advantages are translational as well as practical (cf. Table O). From a translational point of view, the HSAA is capable of detecting a wide range of clinically utilized analgesics. The HSAA closely resembles the subcutaneous HS assay in humans described by Henderson and colleagues in terms of onset, intensity, and duration (Henderson et al., OGGd). This suggests the underlying mechanisms for processing HS induced nociceptive signals are analogous to those in humans. This is not the case for the majority of preclinical nociceptive assays. Even     111 considering the differences in pain experience and analgesia between humans and mice, results from the HSAA in mice may provide preliminary translational evidence for analgesic action of drugs in humans. From a practical point of view, the HSAA is simple, quick (requires EG min to test and analyze) and may be performed in non-restrained animals. The HS injection does not cause histological damage, a feature that minimizes animal suffering and allows for animal reuse. In addition, the nociceptive response in the HSAA is stable with repeated testing. The stability of the nociceptive response with repeated testing allows for the assessment of potential analgesics with a reduced number of animals. Another advantage of this assay is the possibility of using other anatomical sites without the influence of prior stimulation, a feature that can be useful for assessing the dosing intervals necessary to maintain a drug effect. Statistically, the continuous nature of HS responses avoids statistical limitations associated with the use of the arbitrary cut-offs frequently employed in thermal and mechanical assays. In addition, the HS response is normally distributed (cf. Figure EI) which makes the use of more sensitive statistical tests possible.  The outstanding advantages of the HSAA are the rapid detection of analgesic activity, reproducible intra- and inter-animal responses, and lower cost to perform.   A number of limitations exist with regards to the HS assay (cf. Table O). One limitation is its relevance to all of the types of human pain. In preclinical pain assays, this assay would be classified as an acute assay due to the short duration of the response. However, the term “acute pain” in the clinic carries different meaning. Human acute pain (e.g., injury pain) is associated with the recruitment of a host of inflammatory and sensitization processes (Berge, OGEE). These inflammatory and sensitization processes are not evident in HSAA nor in its     112 analogous human version. It is most likely that the HS response resulted from a physiologic nociceptive response to a hypertonic saline solution. Thus, this assay may be best described as a nociceptive assay. Another limitation is related to the nature of the HS response. The HS response is evoked while human pain is spontaneous and multidimensional. Thus, it is difficult to equate the observed response to a specific pain condition. Also, the HSAA is only capable of assessing the sensory component of pain experience whilst human pain is complex, involving sensory and affective components. We demonstrated the effectiveness of the HSAA to detect clinical analgesics and validated its reproducibility within and between animals in female mice. Male and female differences in response to nociceptive and analgesics have been well-documented in preclinical pain assays. Thus, this suggests further studies before extrapolating similar conclusions in male mice.      113    Table '. Summary for the advantages and disadvantages of using HSAA Ö: sensitive; ´: not sensitive; ND: not determined      Assay   Sensitivity to analgesics   Pros  Cons Opioids NSAIDs Acetaminophen Gabapentinoids Antidepressants Cannabinoids THC CBD    Hypertonic saline analgesic assay   Ö       Ö                 Ö           ND           ND    Ö   X  Simple and inexpensive Quick Sensitive to a wide range of analgesics Does not require restraint Minimal experimenter subjectivity Reproducible within animals and between strains  Does not cause tissue damage thereby allowing animal reuse     Induces brief nociception Does not represent specific form of inflammatory or neuropathic pain Response is evoked       114      Figure (). Licking and biting response distribution in HSAA Histogram of frequency distribution data for the first 7 min licking and biting response following <=% HS injection in AB= naïve CD-< mice. The data were fitted using non-linear least square fit (Gaussian model equation) and passed the D'Agostino & Pearson normality test criteria.              115 9.9 Recommendations for future studies This thesis characterized the utility of HSAA for analgesic screening in mice. However, rats are increasingly utilized in the preclinical pain research field (Mogil, U==V). Thus, studies aimed to evaluate the utility and reproducibility of the assay in rats is necessary to improve the external validity of the data and to strengthen the assay. Assessing agents such as gabapentin and amitriptyline that act supraspinally and are used for managing neuropathic pain would add important information regarding the assay’s sensitivity. Furthermore, since HS injection does not inflict tissue damage and the response is unaltered upon repeated injections, it would be of interest to examine the HS response in a neuropathic pain model. Using licking and biting response as an indicator of nociception may provide more objective assessment than the endpoints commonly used with mechanical or thermal stimuli. The next step would be to examine the translatability of rodent findings in the HSAA to the human subcutaneous HS assay by assessing the sensitivity of the assay to a number of clinically used analgesics. This would provide invaluable information regarding the predictableness of the preclinical assay.             116 9.= Conclusion HSAA is rapid and capable of detecting a range of clinical analgesics. It is repeatable within animals allowing a reduction in number of animals and minimizing the variability. It is reproducible within strains and it causes minimal animal suffering and tissue damage. 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