Production of melatonin by the gilthead sea bream pineal: An in vivo and in vitro study

This study deals with the variations of plasma melatonin content and of in vitro pineal melatonin secretion, in one year-old male sea bream. Plasma melatonin levels varied from almost undetectable levels during the day to 150-200 pg ml(-1) during the night, in animals maintained under a light/dark (12L/12D) cycle. The duration of the nocturnal surge corresponded to the duration of the night. A similar pattern of melatonin secretion was obtained in vitro, with pineals cultured under a 24h L/D cycle. When the photoperiod cycle was advanced by 8h, the first nocturnal peak in melatonin secretion was more acute compared to the profile described above, but the rhythmic variations were immediately synchronized to the new L/D conditions. Under constant darkness (D/D), circadian variations in melatonin release were observed for no more than two 24h cycles. No melatonin was detected in the culture media from retinas cultured under LID or under DID. These results strongly suggest that the pineal is the major source of plasma melatonin in the gilthead sea bream, and that the 24h photoperiod cycle is a powerful synchronizer of the rhythmic production of melatonin by this organ.