TACHYKININ NK(1) BUT NOT NK(2) RECEPTORS MEDIATE NONCHOLINERGIC EXCITATORY JUNCTION POTENTIALS IN THE CIRCULAR MUSCLE OF GUINEA-PIG COLON

1 The effect of tachykinin NK1 and NK2 receptor antagonists on nonchol inergic excitatory junction potentials (e.j.ps) evoked by electric fie ld stimulation (EFS) in the circular muscle of the guinea-pig proximal colon was investigated by means of a sucrose-gap technique. 2 In the presence of 1 muM atropine, submaximal EFS (10 Hz, 20-30 V, 0.5 ms pul se width, 1 s train duration) evoked an inhibitory junction potential (i.j.p.) followed by e.j.p. with superimposed action potentials (APs) and contraction. Addition of either N(G)-nitro-L-arginine (L-NOARG, 0. 1 mM) or apamin (0.1 muM) inhibited the evoked i.j.p. and the combined administration of the two agents almost abolished it. In the presence of both L-NOARG and apamin, an atropine-resistant e.j.p. was the only electrical response evoked by EFS in 50% of cases and a small i.j.p. (10% of original amplitude) followed by e.j.p. was evident in the rema inder. 3 In the presence of L-NOARG and apamin, the tachykinin NK1, re ceptor antagonists, (+/-)-CP 96,345 and GR 82,334 (10 nM-3 muM) concen tration-dependently inhibited the atropine-resistant e.j.p. and accomp anying contraction evoked by EFS. EC50 values were: 0.77 muM (e.j.p. i nhibition) and 0.22 muM (inhibition of contraction) for (+/-)-CP 96,34 5; 0.61 muM (e.j.p. inhibition) and 0.20 muM (inhibition of contractio n) for GR 82,334. The tachykinin NK2 receptor antagonists, MEN 10,376 (up to 3 muM) and SR 48,968 (up to 1 muM) had no effect on the atropin e-resistant e.j.p. MEN 10,376 (3 muM) but not SR 48,968 produced a sli ght inhibition of the evoked contraction. 4 (+/-)-CP 96,345 (3 muM) an d GR 82,334 (3 muM) markedly reduced (81 and 89% inhibition, respectiv ely) the atropine-resistant e.j.p. in the absence Of L-NOARG and apami n, without affecting the i.j.p. MEN 10,376 (3 muM) and SR 48,968 (1 mu M) had no significant effect on noncholinergic i.j.p. and e.j.p. evoke d in the absence of apamin and L-NOARG. 5 The electrical and mechanica l responses to the NK1 receptor agonist [Sar9]substance P (SP) sulfone were blocked by (+/-)-CP 96,345 (3 muM) or GR 82,334 (3 muM) which, a t the same concentration, failed to affect the responses to the NK2 re ceptor agonist [betaAla8] neurokinin A (NKA) (4-10). In contrast, MEN 10,376 (3 muM) or SR 48,968 (1 muM) blocked the response to [betaAla]N KA(4-10) without affecting the response to [Sar9]SP sulfone. 6 In the presence of L-NOARG and apamin, and in the absence of atropine, EFS of low pulse width (0.02-0.03 ms, other parameters as above) produced ch olinergic e.j.ps and contraction which were unaffected by GR 82,334 (3 muM). (+/-)-CP 96,345 (3 muM) produced 24% reduction in the area of t he atropine-sensitive e.j.p. without affecting the peak amplitude of e .j.p. or contraction. 7 These findings demonstrate that the noncholine rgic e.j.ps and accompanying contraction of the circular muscle of the guinea-pig colon are produced through activation of intramural tachyk ininergic nerves and that the resultant smooth muscle response is almo st entirely mediated through NK1 receptors.