SECRETION OF METHOXYINDOLES FROM TROUT PINEAL ORGANS IN-VITRO - INDICATION FOR A PARACRINE MELATONIN FEEDBACK

Synthesis and release of the pineal hormone melatonin is in all verteb rates primarily regulated by the light/dark cycle. In pineal organs of teleost fish, like in other non-mammalian vertebrates, melatonin form ation is regulated by a direct photoreception of the pineal organ. We performed measurements in explanted, perifused pineal organs of the ra inbow trout, Oncorhynchus mykiss, to examine whether melatonin can inf luence its own production. For this purpose we have continuously perif used isolated pineal organs under light- and dark-adapted conditions a nd measured the release of melatonin and other methoxy-indoles by HPLC with electrochemical detection. Addition of 2-iodomelatonin to the pe rifusate in a concentration of 2 ng/ml significantly inhibited melaton in release in light-, as well as in dark-adapted organs. The release o f 5-methoxytryptamine and 5-methoxytryptophol was also significantly r educed in light-adapted organs. These results indicate that extracellu lar melatonin may act as a paracrine or autocrine feedback signal and may be important for the illumination-dependent melatonin production.