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UBC Theses and Dissertations
Analysis of excitatory amino acid receptors in the rat spinal cord in vivo and in vitro Magnuson, David Stuart Keith
Several endogenous amino acids including L-glutamate and L-aspartate have potent excitatory effects in the central nervous system. They are thought to act as synaptic transmitters in many neural pathways including those in the spinal cord. Three distinct receptors have been described through which these excitatory amino acids exert their effects. These are referred to as quisqualate, kainate and N-methyl-D-aspartate (NMDA) receptors, after the exogenous excitants most specific for each. In addition, sub-types of the NMDA receptor have been proposed to account for differences observed in the actions of the endogenous excitant quinolinate (2,3-pyridine dicarboxylate) in various regions of the nervous system. The characterization of excitant amino acid receptors has been accomplished primarily using two or more potent antagonists which include D-(-)-2-amino-5-phosphonovalerate (APV), a specific NMDA antagonist, and kynurenate, a compound related to quinolinate which potently attenuates the actions of NMDA- and kainate-like excitants. Structure-activity studies of amino acid receptors were undertaken using standard extracellular recording and iontophoretic techniques in the dorsal horn of the spinal cord in vivo, and compared with the neocortex of the rat. In addition, a spinal cord slice preparation was developed wherein dorso-ventral longitudinal slices were prepared from the lumbar enlargement of weanling rats (50 - 125 g). The slices were maintained in an "interface" tissue bath of novel design. Extracellular recording of several hours duration and up to 8 hours after slice preparation were routinely possible. Conformationally restricted analogues of glutamate, aspartate and quinolinate were examined for agonist and antagonist actions in the rat spinal cord in vivo and in vitro. Compounds found to be excitants were compared directly with quisqualate, kainate, and NMDA for sensitivity to blockade by APV and kynurenate applied both iontophoretically and in the bathing medium; antagonist dose-response curves were constructed for the actions of APV and kynurenate against quisqualate, kainate, quinolinate and NMDA. The conformationally restricted compounds found to be antagonists were examined to determine their potency and specificity against excitations elicited by quisqualate, kainate, quinolinate and NMDA. Although quinolinate is known to be NMDA-like in the hippocampus and cortex, when compared to quisqualate, kainate and NMDA in the spinal cord in vitro, it proved to be unique. A fourth receptor (the "QUIN" receptor) is proposed to account for its actions in the spinal cord. Three of the isomers of 1-amino-1,3-cyclopentane dicarboxylate (ACPD), conformationally restricted analogues of glutamate, were potently blocked by APV and KYNA and were therefore classified as NMDA-like. The fourth, D-trans-ACPD. was indistinguishable from quinolinate in terms of both potency and sensitivity to antagonists. The (-) isomer of trans-1-amino-1,2-cyclopentane dicarboxylate proved to be an antagonist with greater potency against excitations elicited by quisqualate and kainate than those of NMDA. These findings are, in many ways, different from what has been observed in the hippocampal slice. Several pyridine derivatives were examined; 2,5- and 2,6-pyridine dicarboxylate were weak excitants behaving like quisqualate in the presence of APV and kynurenate. No other pyridines were excitatory; however 2,4-pyridine dicarboxylate was observed to be a weak, non-specific antagonist similar in action to acridinate (an antagonist closely related to kynurenate). None of the pyridine derivatives, save quinolinate, are excitatory in the hippocampus. Structural analysis of the active compounds tested, in consideration of previous studies, shows that three points of attachment (two carboxyl and one amino group) are necessary for activation of NMDA, quisqualate and quinolinate receptors in the spinal cord. The location of the distal or y-carboxyl group relative to the a ionic groups appears to be the primary factor determining the activity of a conforrnationally restricted compound. The absolute distance between the Y-carboxyl and α-carbon appears to play a secondary role in determining the action of a compound.
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