AN EXAMINATION OF THE 5-HT3 RECEPTOR MEDIATING CONTRACTION AND EVOKED [H-3] ACETYLCHOLINE-RELEASE IN THE GUINEA-PIG ILEUM

1. The relative contributions of two classes of 5-hydroxytryptamine (5-HT) receptor (5-HT2 and 5-HT3) to the contractile action of 5-HT, 2-methyl-5-hydroxytryptamine (2-methyl-5-HT) and α-methyl-5-hydroxytryptamine (α-methyl-5-HT) were studied in the guinea-pig ileum longitudinal muscle-myenteric plexus strip (LMMP) preparation. Contractility studies were combined with an analysis of the effects of the three agonists on [3H]-acetylcholine ([3H]-ACh) release from preparations preincubated with [3H]-choline. 2. In contracting the LMMP, 5-HT was approximately one order of magnitude more active than 2-methyl-5-HT and α-methyl-5-HT, with relative activities for 5-HT: 2-methyl-5-HT: α-methyl-5-HT of 1.00:0.13:010. 3. Ketanserin (1 μM) was without effect on the concentration-response curves for contraction to 5-HT, 2-methyl-5-HT or α-methyl-5-HT, whilst ondansetron (GR38032F; 1 μM) produced a parallel rightward displacement of the upper part of the concentration-response curves to 5-HT and α-methyl-5-HT and of the entire curve to 2-methyl-5-HT. 4. Increasing the spontaneous release of [3H]-ACh from the LMMP, 5-HT was again approximately one order to magnitude more active than 2-methyl-5-HT and α-methyl-5-HT with relative activities for 5-HT:2-methyl-5-HT:α-methyl-5-HT of 1.00:0.19:0.11. 5. Ondansetron (1 μM) greatly attenuated the increase in spontaneous [3H]-ACh release evoked by all three agonists. pK(B) estimates of 7.62 ± 0.12 and 7.64 ± 0.09 were obtained for ondansetron antagonism of 5-HT and 2-methyl-5-HT-evoked increases respectively. 6. These data suggest that the contractile action of 5-HT, 2-methyl-5-HT and α-methyl-5-HT in the guinea-pig ileum can, under these conditions, be accounted for largely in terms of 5-HT3 receptor activation. Estimates for pK(B) obtained with ondansetron are in accordance with those previously obtained from contractility studies in this preparation and these findings are discussed in terms of the postulated existence of subtypes of 5-HT3 receptors.