PHOTORECEPTOR CELLS OF THE PIKE PINEAL ORGAN AS CELLULAR CIRCADIAN OSCILLATORS

In the pike pineal, the rhythm of melatonin (MEL) secretion is driven by a population of cellular circadian oscillators, synchronized by the 24 h light/dark (LD) cycle. Because the pineal photoreceptor contains both the input and output pathways of the clock, this cell is likely to be a cellular circadian system by itself. To support this idea, we have dissociated and cultured pike pineal cells as well as purified ph otoreceptors. In culture, the pineal cells reassociated in follicles, surrounded by collagen fibres. At the electron microscopic level, they appeared well preserved. Total cells consisted mainly of photorecepto rs and glia. Purified cells corresponded exclusively to photoreceptors . Under LD, MEL production was rhythmic. Under constant darkness (DD), the rhythm was well sustained for at least six 24 h cycles (tau = 24/ 27 h) with 1 x 10(6) total cells/well or below; with 2 x 10(6) total c ells/well, a strong damping occurred towards high levels as soon as af ter the second cycle. At the density of 0.5 x 10(6) cells/well, purifi ed photoreceptors produced less MEL than an equivalent amount of total cells. However, the pattern of the oscillations was similar to that o bserved with 2 x 10(6) total cells, i.e. a damping occurred rapidly. D ecreasing the density to 0.125 x 10(6) photoreceptors/well resulted in a loss of homogeneity among replicates. The production of melatonin b y single photoreceptors was monitored by means of the reverse haemolyt ic plaque assay. Both under LD and under DD, the number of photorecept ors releasing melatonin,was higher during the (subjective) dark than d uring the (subjective) light. The results provide strong support to th e idea that the pike pineal photoreceptor is a cellular circadian syst em. Expression of the oscillations seemed to depend on several factors , including cell to cell contacts between photoreceptors. There is ind ication that also MEL and glia might be involved.