Analyses of signal transduction cascades reveal an essential role of calcium ions for regulation of melatonin biosynthesis in the light-sensitive pineal organ of the rainbow trout (Oncorhynchus mykiss).

Signal transduction processes regulating melatonin production in the light-sensitive trout pineal organ were investigated by immunocytochemical and immunochemical demonstration of phosphorylated cyclic AMP-responsive element-binding protein (pCREB) and measurements of cyclic AMP, melatonin, and calcium levels. Melatonin levels were tightly controlled by light and darkness. Elevation of cyclic AMP levels by 8-bromo-cyclic AMP, forskolin, and 3-isobutyl-1-methylxanthine increased the levels of pCREB and melatonin in light- or dark-adapted pineal organs in vitro. Without pharmacological treatment, the levels of pCREB and cyclic AMP remained constant for several hours before and after light onset. Inhibition of cyclic AMP-dependent proteasomal proteolysis by lactacystin, MG 132, and calpain inhibitor I did not prevent the rapid, light-induced suppression of melatonin biosynthesis. However, changes in the intracellular calcium concentration by drugs affecting voltage-gated calcium channels of the L type and intracellular calcium oscillations (cobalt chloride, nifedipine, Bay K 8644) had dramatic effects on the rapid, light-dependent changes in melatonin levels. These effects were not accompanied by changes in cyclic AMP levels. Thus, the rapid, light-dependent changes in melatonin levels in the trout pineal organ are regulated apparently by a novel calcium signaling pathway and do not involve changes in cyclic AMP levels, cyclic AMP-dependent proteasomal proteolysis, or phosphorylation of cyclic AMP-responsive element-binding protein.