OCULAR MELATONIN RHYTHMS IN THE GOLDFISH, CARASSIUS-AURATUS

Ocular melatonin rhythms in the goldfish were studied and compared to those in the pineal organ and plasma. Under Light:dark (LD) of 12 h li ght:12 h dark, melatonin contents in the eye as well as the pineal org an and plasma exhibited clear day-night changes with higher levels at mid-dark than at mid-light. However, melatonin contents in the eye at mid-light and mid-dark were approximately 100 and 9 times greater than those in the pineal organ, respectively. Day-night changes of ocular melatonin persisted after pinealectomy, which abolished those in plasm a melatonin under LD 12:12. Ocular melatonin contents in the pinealect omized fish at mid-light were significantly higher than those in the s ham-operated control. Under constant darkness (DD), circadian melatoni n rhythms were observed in the eye but damped on the 3rd day, whereas plasma melatonin rhythms generated by the pineal organ persisted for a t least 3 days. Under constant light, ocular melatonin contents exhibi ted a significant fluctuation with a smaller amplitude than that under DD, whereas plasma melatonin remained at low levels. These results in dicate the involvement of LD cycles, a circadian clock, and the pineal organ in the regulation of ocular melatonin rhythms in the goldfish.