No abstract

Summary. Data on the cell biology of pineal transducers (chief cells: typical and modified photoreceptors, pinealocytes) which belong to the paraneuron family, are reviewed in the vertebrate series. In spite of major changes throughout phylogeny, it is proposed that pineal chief cells share a common feature: they somehow transform the information derived from the light/ dark cycle into daily rhythms of neural (an excitatory neurotransmitter) and/or hormonal (melatoninergic) output and appear invariably involved in the temporal organization of physiological and behavioral processes.

The light/dark cycle influences the rhythmic production of melatonin by the trout pineal organ through a modulation of the serotonin N-acetyltransferase (NAT) activity. In static organ culture, cyclic AMP (cAMP) levels (in darkness) and NAT activity (in darkness or light) were stimulated in the presence of forskolin, isobutylmethylxanthine, or theophylline. Analogues of cAMP, but not of cyclic GMP, induced an increase in NAT activity. Light, applied after dark adaptation, inhibited NAT activity. This inhibitory effect was partially prevented in the presence of drugs stimulating cAMP accumulation. In addition, cAMP accumulation and NAT activity increase, induced by forskolin, were temperature dependent. Finally, melatonin release, determined in superfused organs under normal conditions of illumination, was stimulated during the light period of a light/dark cycle by adding an analogue of cAMP or a phosphodiesterase inhibitor. However, no further increase in melatonin release was observed during the dark phase of this cycle in the presence of the drugs. This report shows for the first time that cAMP is a candidate as intracellular second messenger participating in the control of NAT activity and melatonin production by light and temperature.