THE EFFECTS OF CHOLINOCEPTOR AGONISTS AND ANTAGONISTS ON C-FIBER EVOKED-RESPONSES IN THE SUBSTANTIA-GELATINOSA OF NEONATAL RAT SPINAL-CORD SLICES

1 The effects of cholinoceptor agonists and antagonists were studied on neurones in the substantia gelatinosa (SG) of an in vitro spinal cord slice and nerve preparation from neonatal rats. 2 Bath application of carbachol (1-50 muM) reduced, in a dose-related manner, the amplitude and duration of the excitatory postsynaptic potentials (e.p.s.ps) evoked in response to nerve stimulation. 3 The latencies and stimulation thresholds required to evoke these e.p.s.ps suggested that the majority were due to C-fibre activation. 4 The reduction in e.p.s.p. amplitude and duration produced by carbachol was reversed by the muscarinic antagonists, atropine (in 8 out of 11 cells), pirenzepine (in 7 out of 9 cells) and methoctramine (in 8 out of 9 cells) and by the nicotinic antagonist mecamylamine (in 3 out of 7 cells). 5 Injection of small hyperpolarizing or depolarizing pulses was associated with no change in conductance in 19 out of 26 (73%) of cells tested, suggesting that an action at a site presynaptic to the neurone studied could account for part of the effect of carbachol. 6 It is proposed that some of the cholinoceptors associated with the e.p.s.p. depression are located on C-fibres.