Circadian rhythms of melatonin secretion from superfused goldfish (Carassius auratus) pineal glands in vitro.

A flow-through, whole-organ culture (superfusion) system was developed, and goldfish pineal glands were maintained at 25 degrees under light-dark (LD) 12:12 cycles, reversed LD 12:12 cycles, continuous dark (DD), or continuous light (LL) conditions for 48 hr. Under LD 12:12 and reversed LD 12:12 cycles, superfused pineal glands showed a rhythmic melatonin secretion: Scotophase was associated with high titers and photophase with low titers. The melatonin secretion rhythms persisted for two cycles under DD conditions, whereas nocturnal rises were suppressed under LL conditions. After the transition from LL to DD conditions on the third day, melatonin showed a nocturnal increase. These results indicate that melatonin secretion from the superfused goldfish pineal gland is directly photosensitive and that the goldfish pineal gland harbors a circadian oscillator which generates melatonin secretion rhythms.