5-HT IS PRESENT IN NERVES OF GUINEA-PIG SPHINCTER OF ODDI AND DEPOLARIZES SPHINCTER OF ODDI NEURONS

This study involved immunohistochemistry and intracellular electrophys iology to investigate serotonergic neurotransmission in the sphincter of Oddi (SO). 5-Hydroxytryptamine (HT)-positive neurons (14 cells/prep aration) and nerve fibers were observed in the ganglionated plexus. Se rotonergic nerve fibers, which persisted under 2- to B-day organ cultu re, were densely distributed, with varicose endings encircling some SO neurons. When 5-HT was applied to SO neurons, it elicited three diffe rent responses: 1) a fast depolarization to 5-HT in 31 of 62 cells was mimicked by 2-methyl-5-HT and blocked by LY-278584 (1 mu M); 2) a pro longed depolarization to 5-HT in 21 of 62 cells evoked an increase in input resistance and was attenuated by the 5-HT1P antagonist renzaprid e (1 mu M) but not by the 5-HT4 antagonist SDZ-205557 (0.1-10 mu M); a nd 3) an indirect depolarization blocked by TTX or atropine was observ ed in 32 of 62 cells. 5-HT superfusion elicited a dose-dependent monop hasic depolarization (EC50 = 2 mu M, n = 14). In conclusion, 5-HT is p resent in nerves of the SO and elicits both 5-HT3 and 5-HT1P receptor- mediated depolarizations, supporting the concept that 5-HT plays a rol e in SO regulation.