A comparison of the actions of agonists and antagonists at non‐NMDA receptors of C fibres and motoneurones of the immature rat spinal cord in vitro

1 The shift in d.c. potential in dorsal roots (EC50 8.0 μm ± 0.9 s.e.mean, n = 5) or depression of the C elevation of the compound action potential (EC50 3.0 μm ± 0.3, n = 7) have been used to measure the depolarizing action of kainate on dorsal root C fibres of immature (3 to 5 day old) rats. Depolarization of motoneurones was measured from the shift in d.c. potential in ventral roots. 2 6‐Cyano‐7‐nitroquinoxaline,2–3,dione (CNQX) (pA2 5.78 ± 0.06, n = 8) and 6‐nitro‐7‐suplhamobenzo(f)quinoxaline‐2,3‐dione (NBQX) (pA2 5.75 ± 0.04, n = 7) had similar potencies as antagonists of kainate at dorsal root fibres. The potency of NBQX as a kainate antagonist was similar also at motoneurones (pA2 5.72 ± 0.07, n = 3). At motoneurones, NBQX was less potent as an antagonist of domoate (pA2 5.29 ± 0.05) and more potent as an antagonist of S‐α‐3‐hydroxy‐5‐methyl‐4‐isoxazolepropionic acid (AMPA) (pA2 6.80 ± 0.09) than as an antagonist of kainate. 3 Application of l‐glutamate, quisqualate and RS‐AMPA to dorsal roots produced only short lasting depolarizations but kainate concentration‐effect plots were shifted to the right in the presence of these three agonists (pA2 5.08 ± 0.08, (n = 3), 5.59 ± 0.04, (n = 4) and 4.46 ± 0.04 (n = 4) respectively). Slopes of dose‐ratio against concentration were significantly less than one for the latter antagonism. 4 The amplitude of depolarizations induced by l‐glutamate, AMPA and quisqualate were increased up to ten fold and those induced by kainate up to two fold following treatment of dorsal roots with concanavalin A. The duration of the responses was increased also by the latter treatment. Folowing 85 s applications of glutamate, quisqualate, AMPA and kainate the mean respective times (s ± s.e.mean (n)) taken for responses to decay to half the peak amplitude were increased from 63 ± 7 (10), 86 ± 17 (4), 95 ± 19 (4) and 135 ± 3 (12) to 202 ± 49 (10), 147 ± 7 (4), 160 ± 13 (6) and 163 ± 10 (10). Under similar conditions the mean decay time of γ‐aminobutyric acid‐induced responses was 145 ± 7 (10). This was not significantly altered by concanavalin A treatment. 5 Application to dorsal roots of l‐aspartate at concentrations up to 5 mm (with or without concanavalin A treatment), the selective metabotropic agonist 1S,3R‐trans‐1‐aminocyclopentane‐1,3‐dicarboxylate (1 mm,) and d‐serine (20 μm) in the presence or absence of N‐methyl‐d‐aspartate (NMDA, 500 μm) neither depolarized the preparations nor shifted the kainate concentration‐effect plot. 6 It is concluded that primary afferent C fibres possess only one type of non‐NMDA receptor which is activated strongly by domoate or kainate but only weakly by AMPA. This receptor is readily desensitized by glutamate, quisqualate or AMPA and it is less readily desensitized by kainate.