Multiple circadian oscillators in the photosensitive pike pineal gland: A study using organ and cell culture

Bolliet V, Bégay V, Ravault J‐P, Ali MA, Collin J‐P, Falcón J. Multiple circadian oscillators in the photosensitive pike pineal gland: A study using organ and cell culture. J. Pineal Res. 1994: 16: 77–84. The fish pineal organ contains typical and, in some species, modified photoreceptor cells involved in the photoperiodic control of melatonin production. In the majority of species studied, the rhythm in melatonin production is driven by an intra‐pineal circadian oscillator synchronized by the light: dark cycle. In the present study, it is shown that the endogenous rhythm in melatonin release of super fused pike pineals maintained under constant darkness is expressed at temperatures of 19°C, 20°C, 25°C, and 30°C (period > 24 hr), but not at temperatures of 10°C and 15°C. Under constant darkness, pineal fractions containing either typical photoreceptors, modified photoreceptors, or both behaved like total organs. Dissociated pike pineal cells, cultured statically at 20°C, expressed a high amplitude rhythm in melatonin secretion under a light: dark cycle. Under constant darkness, circadian oscillations, which appeared better sustained than in organ culture, were also observed. This study provides the first evidence that the rhythmic production of melatonin, by a fish pineal, is driven by a population of circadian oscillators or clocks. It is hypothesized that each typical and modified photoreceptor might be the locus of a circadian clock. Damping of the overall rhythm under constant darkness might reflect the resynchronization (uncoupling) between these clocks and/or damping of individual oscillators.