Demonstration and effect of melatonin in fish

Abstract

Melatonin ( N-acetyl-5-methoxytryptamine) was identified in the pineal organ of the Pacific salmon Oncorhynchus tshawytscha by thin-layer chromatography and detectable stores were found to be restricted to this organ. The amount of melatonin stored in the pineal was measured fluorometrically following the differential extraction of this indole from the pineals of mature and immature salmon. The store of pineal melatonin in immature salmon was approximately six times as great as that present in mature salmon and it is suggested that this store is related to gonad function. Intraperitoneal injections of melatonin (20 μg/fish for 50 days) into goldfish inhibited the increase in gonad size which accompanied increased daily light exposure in those animals which received placebo injections. The discovery that the melatonin-treated fish had larger pituitary gonadotrophs suggested a possible mode of action of the melatonin. Together, the evidence attests to the possibility that the pineal organ of at least some fishes is involved in the mediation of the gonadal changes effected by changing photoperiods. Melatonin, produced or stored specifically within the pineal organ, apears to exert an inhibitory effect upon gonad functions, possibly by inhibiting the release of a gonadotropic factor.