The Effect of Serotonin, N‐Acetylserotonin, and Melatonin on Spontaneous Contractions of Isolated Rat Intestine

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Abstract
A dose-dependent increase in tone and reduction in amplitude of contractions was observed after serotonin (5-HT) was administered to isolated segments of rat ileum, incubated in Locke''s solution at 38.degree. C. Melatonin (M) reduced the tone but not the amplitude or frequency of contractions. Addition of M (administered in doses 20 to 100 .times. higher than 5-HT) relieved the spasm induced by 5-HT. Furthermore, pretreatment with M significantly reduced the 5-HT effect. N-acetylserotonin (NAS) exhibited delayed but similar effects to M. Neither M nor NAS could prevent or relieve acetylcholine-induced contractions or influence relief of intestinal contractions by adrenaline. This indicates that 5HT and M act via a different mechanism than that of adrenaline and acetycholine system. Serotonin muscle receptor blocker methysergide reduced 5-HT effect but was not able to abolish it completely. As methysergide could not reduce the muscle tone and did not relieve spasm caused by 5-HT, it is speculated that M is not acting as antagonist of 5HT-stimulatory receptors but rather as agonist of 5-HT-inhibiting neuronal receptors.