MELATONIN SIGNAL-TRANSDUCTION IN THE GOLDFISH, CARASSIUS-AURATUS

Generation and reception of melatonin signals in the goldfish, Carassi us auratus, are reviewed. The photoreceptive pineal gland of the goldf ish generates circulating melatonin rhythms according to a given photo period under light-dark cycles and in a circadian manner under continu ous dark conditions. Melatonin is also produced in the retina in a sim ilar fashion. Melatonin produced in the pineal gland and retina is con sidered to act as internal zeitgeber in the brain and retina, respecti vely, controlling various physiological events via specific melatonin binding sites that are coupled with G protein. The goldfish exhibit cl ear diurnal locomotor activity rhythms under light-dark cycles and fre e-running rhythms under constant conditions. However, the relationship between melatonin and locomotor activity rhythms in the goldfish rema ins unclear. Further studies should be required to demonstrate the rol es of melatonin in the circadian system in this species.