REGULATION BY LIGHT AND DARKNESS OF MELATONIN SECRETION FROM THE SUPERFUSED MASU SALMON (ONCORHYNCHUS-MASOU) PINEAL ORGAN

The pineal organ of masu salmon Oncorhynchus masou was maintained in a . now-through, whole-organ culture (superfusion) system and melatonin secretory profiles were determined at 15 degrees C under light-dark cy cles of 12:12 h (LD 12:12) or the same in combination with constant da rkness (DD) for 72 h. Under LD 12:12, superfused pineal organs showed a rhythmic melatonin secretion with high and low rates during the dark phase and the light phase. respectively. When the pineal organs maint ained under LD 12:12 for 24 h were transferred to DD. melatonin secret ion was consistently activated and no endogenous component was evident . When the pineal organs maintained under DD for 48 h were transferred to LD 12:12, melatonin secretion was reduced only during the light ph ase. These results indicate that melatonin secretion from the superfus ed pineal organ of masu salmon is regulated not by an intra-pineal cir cadian oscillator but by the environmental LD cycles, via local photor eceptors.