Multiple 5-hydroxytryptamine receptors on feline ileum and ileocecal sphincter

The effects of 5-hydroxytryptamine (5-HT) on the feline ileum and ileocecal sphincter (ICS) were studied in vivo using simultaneous myoelectric and pressure recordings. Both inhibitory and excitatory responses were seen in the ileum and ICS, with the inhibitory response being seen with lower doses of 5-HT (EDmax for inhibition was 5.0 micrograms/kg and EDmax for contractile response was 80.0 micrograms/kg). The inhibitory and excitatory responses were antagonized separately by different 5-HT antagonists. Methysergide inhibited the ICS relaxation response only, whereas cinanserin inhibited the ICS contractile response. Neither effect was inhibited by tetrodotoxin, suggesting that 5-HT acts directly on the smooth muscle at at least two separate receptors. In addition, methysergide alone resulted in a usually tonic increase (8.6 +/- 0.6 mmHg) in basal ICS pressure (P less than 0.05) without an ileal effect, and tetrodotoxin alone induced phasic ileal and ICS contractions (delta ICS = 7.3 +/- 1.5 mmHg). These data suggest the action of at least two inhibitory neurons tonically at the ICS.