SUBTYPES OF TACHYKININ RECEPTORS ON TONIC AND PHASIC NEURONS IN CELIAC GANGLION OF THE GUINEA-PIG

1 Intracellular recording techniques were used to investigate the char acteristics of tachykinin receptors and their subtypes in tonic and ph asic neurones, which constituted two major neuronal populations in the coeliac ganglion of the guinea-pig. 2 In 95% of phasic neurones a lon g-lasting after-hyperpolarization (LAH), 5-8 s in duration and 10-20 m V in amplitude, was observed following action potentials evoked by pas sing a train of depolarizing current pulses into the neurones. In cont rast, LAH was observed in only 4% of tonic neurones. 3 In most tonic n eurones, substance P (SP), neurokinin A (NKA) and senktide induced dep olarizations, whereas in phasic neurones they usually inhibited LAH bu t rarely induced depolarization. 4 Tonic and phasic neurones were furt her classified into three groups based on their responses (depolarizat ion for tonic neurones and LAH inhibition for phasic neurones) to thes e tachykinin receptor agonists: (1) neurones responsive to SP, NKA and senktide (71-78%); (2) those responsive to senktide but not to SP and NKA (12-23%) and (3) those not responsive to any of the three agonist s (7-11%). 5 GR71251 (5 mu M), an NK1-selective tachykinin receptor an tagonist, depressed the depolarization in tonic neurones and the LAH i nhibition in phasic neurones induced by SP and NKA, but not those indu ced by senktide. 6 Selective NK2 receptor agonists, [Nle(10)]NKA(4-10) ,[beta-Ala(8)]NKA(4-10) and GR64349, were without effect in both tonic and phasic neurones. Furthermore, an NK2 receptor antagonist, L659,87 7, did not inhibit the depolarization induced by NKA, SP or senktide i n tonic neurones. 7 It is suggested that NK1 and NK3 receptors are pre sent on a large proportion of coeliac ganglion neurones. In tonic neur ones both subtypes of tachykinin receptors are coupled to membrane dep olarization, whereas in phasic neurones activation of these receptors leads to inhibition of LAH. The present study also suggests that NKA e vokes the depolarization in tonic neurones and the LAH inhibition in p hasic neurones via NK1, but not NK2 receptors.