Contractile effects of 5-hydroxytryptamine and 5-carboxamidotryptamine in the equine jejunum

1 The use of human prokinetic drugs in colic horses leads to inconsistent results. This might be related to differences in gastrointestinal receptor populations. The motor effects of 5-hydroxytryptamine (5-HT; serotonin) on the equine mid-jejunum were therefore studied. Longitudinal muscle preparations were set up for isotonic measurement. 2 5-HT induced tonic contractions with superimposed phasic activity; these responses were not influenced by tetrodotoxin and atropine, suggesting a non-neurogenic, non-cholinergic pathway. 3 The 5-HT receptor antagonists GR 127935 (5-HT1B,D), ketanserin (5-HT2A), SB 204741 (5-HT2B), RS 102221 (5-HT2C), granisetron (5-HT3), GR 113808 (5-HT4) and SB 269970 (5-HT7) had no influence on the 5-HT-induced response; the 5-HT1A receptor antagonists NAN 190 (pK(b) = 8.13 +/- 0.06) and WAY 100635 (pK(b) = 8.69 +/- 0.07), and the 5-HT1,2,5,6,7 receptor antagonist methysergide concentration-dependently inhibited the 5-HT-induced contractile response. 4 The 5-HT1,7 receptor agonist 5-carboxamidotryptamine (5-CT) induced a contractile response similar to that of 5-HT; its effect was not influenced by tetrodotoxin and atropine, and SB 269970, but antagonised by WAY 100635. 8-OHDPAT, buspiron and flesinoxan, which are active at rat and human 5-HT1A receptors, had no contractile influence. 5 These results suggest that the contractile effect of 5-HT in equine jejunal longitudinal muscle is due to interaction with muscular 5-HT receptors, which cannot be characterised between the actually known classes of 5-HT receptors.