RHYTHMIC SECRETION OF MELATONIN BY THE SUPERFUSED PIKE PINEAL ORGAN -THERMOPERIOD AND PHOTOPERIOD INTERACTION

In the pineal organ of the pike (Esox lucius, teleost), the rhythmic p roduction of melatonin by the photoreceptor cells is governed by a pop ulation of endogenous circadian oscillators, synchronized and entraine d by the light/dark (L/D) cycle. Production of melatonin is inhibited by light and highly increased in the dark. In the present study, isola ted and superfused pike pineals were exposed to a 24-hour temperature cycle of high (10 degrees C) or low (4 degrees C) amplitude, either un der L/D, or under constant darkness. Under L/D, photoperiod is the dom inant entraining stimulus for the melatonin secretion rhythm. It was h igh during the dark phase and low during the light phase, either under cold light/warm dark or under warm light/cold dark conditions. Under a warm light/cold dark cycle, the amplitude of the melatonin rhythm wa s reduced. In pineals cultured at 10 degrees C, a 20 degrees C tempera ture pulse potentiated or reduced the night-induced rise in melatonin production when applied, during the dark or during the preceding light phase, respectively. Under constant darkness, high- or low-amplitude temperature cycles could synchronize the rhythmic production of melato nin, which peaked with the high temperature. However, the shape of the oscillation could appear modified under warm subjective-L/cold subjec tive-D, depending on the experimental conditions. Finally, the rhythmi c release of melatonin synchronized by a temperature cycle under const ant darkness was no longer observed after removal of the external zeit geber. The present study provides direct evidence that the pineal acts as a photothermotransducer. Photo- and thermoperiod interact on the p ineal to determine the amplitude, phase, and duration of the melatonin oscillations. Thermoperiod is not as powerful as photoperiod in entra ining the circadian clocks which drive the melatonin-producing rhythm. We provide strong support for the hypothesis that melatonin might be involved in mediating the effects of the fish pineal organ on thermode pendent physiological and behavioral processes.