ON THE NATURE OF THE 5-HT RECEPTOR SUBTYPE INHIBITING ACETYLCHOLINE-RELEASE IN THE GUINEA-PIG ILEUM

1 The nature of the 5-hydroxytryptamine (5-HT) receptor subtype controlling acetylcholine release and contraction induced by stimulation of the neurokinin NK3 receptor has been studied in the longitudinal muscle-myenteric plexus preparation from guinea-pig ileum. 2 In preparations preloaded with [H-3]-choline, the selective NK3 agonist, senktide, produced a concentration-dependent increase in tritium overflow, an index of [H-3]-acetylcholine release. Low concentrations of neurokinin B, also markedly increased tritium efflux. 3 The senktide-induced acetylcholine release was markedly increased by the same concentration of methysergide and mesulergine. The 5-HT2A/(2C) agonist DOI (1 mu M) inhibited the tritium overflow while 8-OH-DPAT, sumatriptan and ketanserin (1 mu M each) were without effect on the senktide-induced tritium efflux. 4 The contractile response to senktide in the guinea-pig ileum was attenuated by atropine, 0.1 mu M. Methysergide, a 5-HT1/2, receptor antagonist, and mesulergine, a 5-HT2A/2B/(2C) receptor antagonist, (1 mu M each), enhanced the contractile effect of the NK3 receptor agonist. 5 It is concluded that the acetylcholine release induced by a NK3 receptor agonist is inhibited by stimulation of a 5-HT receptor, possibly of the 5-HT2C or 5-HT2B subtype.