MELATONIN AND 5-METHOXYTRYPTAMINE INDUCED MUSCULAR-CONTRACTION IN SEA-ANEMONES

Many cnidarians exhibit a diurnal rhythm of expansion and contraction. Previous studies correlated this behavior to various environmental fa ctors, including prey-catching, the photosynthetic requirement of the zooxanthellae, and tidal movement in the intertidal zone. Whether intr insic factors affect the diurnal rhythm of expansion/contraction were not addressed. The present study investigated the effect of the photop eriod hormone melatonin (N-acetyl-5-methoxytryptamine) and its homolog ue on the expansion and contraction of a sea anemone. Addition of exog enous melatonin at 10(-4) M induced the expansion of the oral disk. At an exogenous concentration of 10(-3) M, protrusion of the actinophary nx was induced. Melatonin thus exerted its effect in the sea anemone i n a dose-dependent manner. Addition of another indoleamine, 5-methoxyt ryptamine at a concentration of 10(-4) M resulted in the thythmic cont raction of the body column, resembling the respiratory rhythm of coele nteric flushing. Serotonin (5-hydroxytryptamine), however, did not ind uce any apparent effect at an exogenous concentration of 10(-3) M. Sim ultaneous addition of melatonin and 5-methoxytryptamine induced both t ypes of responses, suggesting that their effects are not antagonistic with each other. This and their different responses suggested that the two indoleamines acted on two different sites in the sea anemone. At a concentration of 10(-4) M, melatonin was also able to modulate the e ffect of Antho-RF-amide in the contraction of the oral disk. Eserine, an inhibitor of melatonin deacetylation, also induced protrusion of th e actinopharynx. This strongly supported the concept that melatonin wa s produced endogenously and probably at or near the site of action. (C ) 1997 Wiley-Liss, Inc.