Excitatory amino acid receptor mediation of sensory inputs to functionally identified dorsal horn neurons in cat spinal cord
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abstract
As excitatory amino acid receptors have been implicated in nociceptive sensory transmission, the principal objective of the present study was to investigate the effects of various excitatory amino acid antagonists on naturally evoked responses in spinal dorsal horn neurons. Extracellular single unit activity was recorded from functionally identified, spinal dorsal horn neurons in unanesthetized, decerebrated cats and in alpha-chloralose-anesthetized cats. The tests included iontophoretic application of the N-methyl-D-aspartate (NMDA) receptor antagonist 2-amino-5-phosphonovaleric acid (APV), the non-N-methyl-D-aspartate receptor antagonists 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and kynurenate, and also the intravenous administration of the N-methyl-D-aspartate receptor antagonist, ketamine. In addition, attempts were made to determine the effects on these neurons of iontophoretic application of the excitatory amino acid agonists, L-glutamate, N-methyl-D-aspartate, quisqualate, (RS)-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) and domoate. Marked differences were noted in the actions of agonists and antagonists between the responses observed in the unanesthetized, decerebrated and the anesthetized animals. In decerebrated cats, responses to hair afferent stimulation were blocked by kynurenate, 6-cyano-7-nitroquinoxaline-2,3-dione and 2-amino-5-phosphonovaleric acid. Responses to noxious thermal stimulation were attenuated by 2-amino-5-phosphonovaleric acid and in one unit also by ketamine. Neither 6-cyano-7-nitroquinoxaline-2,3-dione nor kynurenate affected the responses to noxious thermal stimulation. The proportion of cells responding to the agonists were: N-methyl-D-aspartate 24/27 (89%), quisqualate 12/13 (92%) and domoate 6/7 (86%). In chloralose-anesthetized cats, responses to hair afferent stimulation were blocked by 6-cyano-7-nitroquinoxaline-2,3-dione and kynurenate but not by 2-amino-5-phosphonovaleric acid. Responses to noxious thermal stimulation were not affected by any of these antagonists, while the response to non-noxious thermal stimulation was blocked by 2-amino-5-phosphonovaleric acid, ketamine and kynurenate in the one neuron studied. The proportion of cells excited by the agonists differed from those observed in decerebrated cats: N-methyl-D-aspartate 9/32 (28%), quisqualate 50/54 (93%), (RS)-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate 19/23 (83%) and domoate 17/38 (45%). Application of the putative endogenous excitatory amino acid precursor N-acetyl-aspartyl-glutamate (NAAG) did not elicit a response in any of the neurons studied.(ABSTRACT TRUNCATED AT 400 WORDS)
