Excitatory amino acid receptor mediation of sensory inputs to functionally identified dorsal horn neurons in cat spinal cord

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 α-chloralose-anesthetized cats. The tests included iontophoretic application of the N-methyl- d-aspartate (NMDA) receptor antagonist 2-amino-5-phosphonovaleric acid (APV), thenon- 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)-α-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-aspartate24/27(89%), quisqualate12/13(92%) and domoate6/7(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-aspartate9/32(28%), quisqualate50/54(93%), (RS)-α-amino-3-hydroxy-5-methyl-4-isoxazolepropionate19/23(83%) and domoate17/38(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. Application of subthreshold currents of quisqualate and domoate potentiated responses to hair stimulation and noxious mechanical pinch, while application of subthreshold currents of N-methyl- d-aspartate was without any effect on synaptically elicited responses. In each of seven decerebrated animals, chloralose was administered once responses of a neuron to the agonists had been fully classified. In all cases, the excitatory responses to glutamate, N-methyl- d-aspartate and quisqualate, were depressed following chloralose administration (40 and 60 mg/kg, i.v.). However, inhibition of the response to N-methyl- d-aspartate following chloralose was greater than the inhibition of the response to quisqualate. The results implicate the involvement of both N-methyl- d-aspartate andnon- N-methyl- d-aspartate receptors in the mediation of non-noxious mechanical hair afferent inputs and of N-methyl- d-aspartate receptors in the mediation of noxious thermal inputs to dorsal horn neurons and indicate that the responses mediated via excitatory amino acid receptors, preferentially N-methyl- d-aspartate receptors, are depressed by chloralose.