TACHYKININERGIC SYNAPTIC TRANSMISSION IN THE CELIAC GANGLION OF THE GUINEA-PIG

1 The responses of coeliac ganglion neurones of the guinea-pig to elec trical stimulation of the mesenteric nerves and applications of tachyk inin receptor agonists were investigated by use of intracellular recor ding techniques. 2 Ganglion neurones were classified into three groups based on firing patterns in response to a depolarizing current pulse: phasic (38% of the population), tonic (39%) and atypical (23%). In th e majority of phasic neurones (91%) a long after-hyperpolarization (LA H) lasting 5-8 s followed action potentials induced by a train of depo larizing current pulses. In contrast, LAK was rarely observed in tonic neurones (5%). 3 In most of tonic neurones (90%) slow excitatory post -synaptic potentials (e.p.s.ps) lasting 3-10 min were evoked by repeti tive electrical stimulation of the mesenteric nerves. Prolonged depola rizations were also evoked in most tonic neurones by applications of s ubstance P (SP), neurokinin A (NKA) or senktide, a tachykinin NK3 rece ptor agonist. 4 In most of phasic neurones (73%), mesenteric nerve sti mulation did not induce an obvious depolarization but induced a prolon ged inhibition of LAH lasting 3-10 min. Bath-applied tachykinin recept or agonists similarly induced an inhibition of LAH without causing dep olarization in most of the phasic neurones. 5 GR71251 (5 mu M), a tach ykinin NK1 receptor antagonist, partially depressed the nerve-evoked s low e.p.s.ps in tonic neurones and the nerve-evoked LAH inhibition in phasic neurones. 6 Capsaicin (0.1-5 mu M) induced a prolonged depolari zation in tonic neurones and an inhibition of LAH in phasic neurones. 7 A mixture of peptidase inhibitors potentiated the depolarization and the LAH inhibition evoked by nerve stimulation, SP and NKA, but not t hose evoked by senktide. 8 It is concluded that tonic neurones respond to repetitive mesenteric nerve stimulation preferentially with slow e .p.s.ps and that phasic neurones respond preferentially with LAH inhib ition. The present study further suggests that SP and NKA, released fr om axon collaterals of primary afferent neurones, produce slow e.p.s.p s in tonic neurones and the LAH inhibition in phasic neurones via NK1 receptors.