REGULATION OF MELATONIN PRODUCTION BY PINEAL PHOTORECEPTOR CELLS - ROLE OF CYCLIC-NUCLEOTIDES IN THE TROUT (ONCORHYNCHUS-MYKISS)

The light/dark cycle influences the rhythmic production of melatonin b y the trout pineal organ through a modulation of the serotonin N-acety ltransferase (NAT) activity. In static organ culture, cyclic AMP (cAMP ) levels (in darkness) and NAT activity (in darkness or light) were st imulated in the presence of forskolin, isobutylmethylxanthine, or theo phylline. Analogues of cAMP, but not of cyclic GMP, induced an increas e in NAT activity. Light, applied after dark adaptation, inhibited NAT activity, This inhibitory eff ect was partially prevented in the pres ence of drugs stimulating cAMP accumulation. In addition, cAMP accumul ation and NAT activity increase, induced by forskolin, were temperatur e dependent. Finally, melatonin release, determined in superfused orga ns 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 prese nce of the drugs. This report shows for the first time that cAMP is a candidate as intracellular second messenger participating in the contr ol of NAT activity and melatonin production by light and temperature.