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Monoamine oxidase inhibitors in Amazonian hallucinogenic plants : ethnobotanical, phytochemical, and pharmacological investigations McKenna, Dennis Jon

Abstract

Ethnobotanical, phytochemical, and pharmacological investigations of two Amazonian hallucinogens are presented. The extant literature on the botany, chemistry, and ethnopharmacology of the Malpighiaceous and Myristicaceous hallucinogens is reviewed. The hallucinogenic beverage ayahuasca is prepared from the woody liana Banisteriopsis caapi (Malpighiaceae) and various admixture plants which strengthen or modify the effect. The genus Virola (Myristicaceae) is the source of the other hallucinogens investigated; the cambial resin of certain Virola spp. is made into hallucinogenic snuffs by some Amazonian tribes, while others prepare an orally-ingested drug from the resin. Although derived from entirely different botanical sources, both ayahuasca and the Vi rola drugs owe their hallucinogenic activity to indole alkaloids, viz., tryptamines and β-carbolines. The major tryptamines found in these preparations are N,N-dimethyltryptamine and/or 5-methoxy-N,N-dimethyltryptamine; both are potent hallucinogens but are inactive when ingested orally, presumably due to oxidative deamination by visceral monoamine oxidase (MAO). The β-carboline alkaloids, although having limited activity as hallucinogens, are potent reversible inhibitors of MAO; thus they may protect the tryptamines from visceral MAO and render them orally active. This mechanism may underly the oral activity of ayahuasca and the orally-ingested Myristicaceous drugs. Ayahuasca is an integral part of mestizo folk medicine among the lower socioeconomic classes living in semi-urban Amazonian centers, but the utilization of Virola spp. as the source of hallucinogenic preparations is confined to a few indigenous Amazonian tribes, and even among some of these groups is a rapidly disappearing practice. The pharmacology and biochemistry of tryptamine and 0-carboline derivatives is reviewed (Chapter II). Their biosynthesis, distribution, structure/activity relationships, interactions with MAO inhibitors, endogenous synthesis and degradative metabolism in mammals, hallucinogenic properties, and other biological activities are reviewed. Results of ethnographic and ethnobotanical fieldwork in the Amazon Basin are presented (Chapter III). Herbarium voucher collections are documented (Appendix II), and the methods used in the collection of drug samples and plant materials for chemical analysis are described. Ethnographic observations on the use of ayahuasca and the orally-ingested Virola pastes are presented, and the methods used in their preparation are documented.. The folk-medical use of ayahuasca by an ayahuasquero living near Pucallpa, Peru, is described. The procedures followed by Bora and Witoto informants in the preparation of the orally-ingested Virola pastes are also described. Observations on the biological activity of ayahuasca and the Virola pastes in self-experiments are included. The contemporary use of ayahuasca in mestizo folk medicine is compared to the use of the oral Myristicaceous drugs, which has remained ethnologically restricted to the Bora and Witoto tribes. The possibility is raised that the somewhat unreliable pharmacological activity of the Myristicaceous drugs, a reflection of their chemical variability, may have contributed to a decline in the use of the pastes. The alkaloids of a number of ayahuasca brews, cultivars of B. caapi, and admixture plants were qualitatively and qualitatively analyzed using thin-layer chromatography (TLC), high-pressure liquid chromatography (HPLC) and gas chromatography/mass spectrometroy (GC/MS) The ayahuasca samples contained insufficient levels of β-carbolines to account for their hallucinogenic properties at the doses typically used; however in most samples the concentration of DMT was well above the threshold level, assuming that it is orally activated by the blockade of visceral MAO. Different batches of ayahuasca had similar alkaloid compositions, however the concentrations of total alkaloids and the proportions of individual constituents varied considerably in batches of ayahuasca prepared by different ayahuasqueros in various parts of Peru. Different batches prepared by the same practitioner were remarkably consistent both in total alkaloid concentration and in the proportions of constituents. Considerable variation in alkaloid concentration in several recognized cultivars of B. caapi were found but may be due to environmental factors rather than genetic differences. Substantial concentrations of DMT were found in all Psychotr ia viridi s samples analyzed, and in one collection of Diplopterys cabrerana but DMT was not detected in Psychotria carthagenensis. DMT was the single major base detected in these admixtures; only traces of other alkaloids were detected. Several uncommon admixture plants were screened for alkaloids, but only one, Abuta grandifolia, (Menispermaceae) gave an unambiguously positive reaction. Alkaloids in twenty-eight Myristicaceous bark and leaf samples were qualitatively and quantitatively determined using TLC, GC, precipitation tests, and GC/MS. Sixteen of the twenty-eight samples contained detectable alkaloids. DMT and 5-MeO-DMT were the major bases, with much smaller amounts of NMT and/or tryptamine also present in most samples. Detectable levels of β-carbolines were not found in the bark and leaf samples. Fourteen of the eighteen Virola samples contained alkaloids; none of the six Iryanthera species contained detectable alkaloids. An indolic base, identified as N-methyl-tryptophan methyl ester, was found in Osteophloem platyspermum. Seven samples of orally-ingested drugs made from Virola spp. were analyzed. All but one contained substantial amounts of tryptamines, but the types and proportions varied greatly between samples. Samples of Virola snuff including various admixtures were analyzed and all but one contained tryptamines. Drug samples with the highest concentrations of alkaloids contained 15-20 mg/g d wt; the bark and leaf samples had concentrations ranging from 0.04 to 0.25 mg/g d wt. Only two Virola paste samples contained detectable levels of β-carbolines, which were identified as MTHβC and DMTHβC. The β-carbolines were trace constituents. Methods were devised for the in vitro assay of rat-liver monoamine oxidase (MAO) using ¹⁴-C-serotonin as substrate. Structure/activity relationships of various tryptamine and 0-carboline derivatives as MAOI were determined. The MAOI activity of β-carboline derivatives was several orders of magnitude greater than the activity measured with tryptamine derivatives. DMT was the most active MAOI of the tryptamines tested while harmine was the most active of the β-carbolines. Multi-component mixtures ofβ-carbolines were not significantly more effective than the single most active component, indicating an additive rather than a synergistic mechanism of action. Samples of ayahuasca were highly active as MAOI even when diluted by several orders of magnitude. The activity was comparable to mixtures of β-carbolines having similar concentrations and proportions. Samples of orally-ingested Virola pastes were less effective than ayahuasca as MAOI. The inhibition which they elicited was closely matched by mixtures of tryptamine standards having comparable proportions and concentrations. An alkaloid-free paste sample and a crude lignan fraction from V. elongata elicited only a slight degree of non-specific inhibition at the highest concentrations. These observations indicate that the limited MAOI activity of the pastes is due primarily to the tryptamines; the traces of β-carbolines or non-nitrogenous inhibitors present probably do not contribute significantly to the total inhibition. These results suggest that the inhibition of peripheral MAO by β-carbolines may explain the oral activity of ayahuasca, but it is unlikely that this mechanism can account for the oral activity of the Myristicaceous pastes. Some alternative mechanism must therefore be considered. One possible alternative is that Virola spp. may contain non-alkaloid constituents which are active as inhibitors of hepatic microsomal mixed-function oxidases (MFOs). Experimental evidence is reviewed which indicates that these enzymes, rather than hepatic MAO, may actually be more important in the peripheral metabolism and inactivation of orally-administered DMT and related compounds.

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