Regulation of melatonin secretion in a photoreceptive pineal organ: an in vitro study in the pike.

The pineal organ (or pineal) of a teleost fish, the pike, contains typical (cone-like) and modified photoreceptor cells. Both are involved in indole metabolism, including melatonin production. How photoperiod controls melatonin biosynthesis in organs containing mainly photoreceptor cells, remains unclarified. To tackle this question we have used cultured pike pineal organs to investigate the variations in (1) the activity of the arylalkylamine-N-acetyltransferase (NAT), which is involved in the biosynthesis of melatonin, in static culture and (2) melatonin release in a perifusion system. Serum melatonin was also quantified in pike kept under a 24 hr light/dark (LD) cycle. Under LD conditions, NAT activity, melatonin release, and serum melatonin levels were high during the scotophase and low during the photophase. High-amplitude rhythms in NAT activity and melatonin release were maintained during three 24 hr cycles in constant darkness and a low-amplitude rhythm of NAT persisted in constant illumination. Midnight illumination induced a dramatic decrease of NAT activity and melatonin release. Darkness at midday did not induce a rise of the melatonin release, which occurred only at the onset of the subjective scotophase. From the present data it is strongly suggested that the pineal of the pike contains a circadian oscillator--synchronized by the photoperiod--which generates the rhythms of NAT activity and of melatonin release. Melatonin release, which reflects the rhythmic activity of NAT, might largely contribute to the melatonin circulating levels. The circadian oscillations observed under constant conditions suggest that the oscillator might behave differently in the pike, compared with intrapineal oscillators of 2 other species (lizard and chicken) under investigation.(ABSTRACT TRUNCATED AT 250 WORDS)