MODULATION OF RHYTHMIC CONTRACTIONS BY MELATONIN VIA CYCLIC-GMP IN THE COELENTERATE RENILLA-KOELLIKERI

1. The action of melatonin was investigated in a colonial anthozoan, the sea pansy Renilla koellikeri, in which rhythmic contractions underlying peristalsis were previously shown to be potentiated by serotonin (5-HT) and cAMP analogues (Anctil 1989). 2. Melatonin (2-1000-mu-mol.l-1) consistently and reversibly depressed the amplitude of rhythmic contractions, the latter all but vanishing at the most effective concentration (10-mu-mol.l-1). In addition, the frequency of rhythmic contractions incurred a 25% reduction in some preparations exposed to melatonin (10-20-mu-mol.l-1). Vaseline gap experiments showed that this response was restricted to tissues directly exposed to melatonin. 3. The potentiating effect of 5-HT on rhythmic contractions was eliminated in a reversible manner in the presence of melatonin. The latter also caused a 50% reduction of the amplitude of contractions elicited by field electrical stimulation. 4. Dibutyryl or 8-bromo cGMP, as well as the phosphodiesterase inhibitor IBMX, mimicked the melatonin-induced response and the effects of melatonin on the 5-HT potentiation or contractile response to electrical stimulation. cGMP levels, measured by radioimmunoassay (RIA) in rachidial tissues, increased 3-fold above basal levels in the presence of melatonin (0.01 mmol.l-1) and 2-fold with IBMX (1 mmol.l-1). In contrast, RIA measurements of cAMP levels showed the latter to fall to 50 and 40% of control values in the presence of melatonin (0.01 mmol.l-1) and IBMX (0.1 mmol.l-1), respectively. These results and other findings suggest that a cGMP-mediated melatonergic mechanism participates in an antagonistic fashion with 5-HT in the modulation of rhythmic contractions and peristalsis of the sea pansy.