Mechanism of 5-hydroxytryptamine-induced biphasic responses of fowl rectum.

The present study investigated the mechanism of the biphasic action of 5-hydroxytryptamine (5-HT) on isolated fowl rectum. 5-HT (2.6 X 10(-6) M) very often produced a biphasic response i.e., an initial relaxation followed by contraction. Tetrodotoxin completely abolished the relaxatory component and partially inhibited the contraction. On the Remak's nerve denervated preparation (in which the non-adrenergic inhibitory and non-cholinergic excitatory neural elements are eliminated), 5-HT produced only contraction. Lysergic acid diethylamide or methysergide completely blocked the contractile component and significantly inhibited the relaxation. Caffeine, given alone, reduced the relaxation without affecting the contraction caused by 5-HT. Atropine, mepyramine, propranolol, pentolinium or indomethacin did not alter the biphasic responses to 5-HT. Thus, it is concluded that the 5-HT-induced relaxation is mediated by non-adrenergic inhibitory neurons, while the contractions of the fowl rectum are partly due to a direct myogenic action and partly mediated by an indirect stimulation of non-cholinergic excitatory neurons.