Effect of serotonin on prolactin- and MSH-secreting cells in the eel. Comparison with the effect of 5-hydroxytryptophan.

As parachlorophenylalanine appears to reduce release of prolactin (PRL) and melanocyte-stimulating hormone (MSH) in the eel, a stimulating serotoninergic control of these adenohypophysial secretions was suspected. Eels were therefore injected with serotonin (5-HT) (7.5--20 mg/kg) and compared with eels injected with the precursor of serotonin, 5-hydroxytroptophan (5-HTP) (20 mg/kg). In both cases, a darkening of the skin was observed, more rapid and intense with 5-HTP than with 5-HT. Similarly, MSH cell degranulation is more completely in 5-HTP-treated eels, which show simultaneously an unexplained dilatation of the swim-bladder not elicited by 5-HT treatment. In both cases, PRL cells are stimulated, and the nuclear area of PRL and MSH cells increases significantly. These results suggest that a serotoninergic pathway stimulates PRL and MSH release in the eel. An antagonism between the serotoninergic system and the dopaminergic system previously demonstrated in the same species seems apparent, but the interaction of other organs or factors, such as the pineal, are considered.