Effect of brefeldin A on melatonin secretion of chick pineal cells

Melatonin is secreted from the pineal gland in a circadian manner. It is we ll established that the synthesis of melatonin shows a diurnal rhythm refle cting a daily change in serotonin N-acetyltransferase (NAT) activity, and t he overall secretion of melatonin requires a cellular release process, whic h is poorly understood. To investigate the possible involvement of Golgi-de rived vesicles in the release, we examined the effect of brefeldin A (BFA), a reversible inhibitor of Golgi-mediated secretion, on melatonin secretion of cultured chick pineal cells. We show here that treatment with BFA compl etely disassembles the Golgi apparatus and reduces melatonin secretion. In more detailed time course experiments, however, the inhibition of melatonin secretion is only observed after the removal of BFA in parallel with the r eassembly of the Golgi apparatus. This inhibition of melatonin secretion is not accompanied by accumulation of melatonin in the cells. These observati ons indicate that chick pineal melatonin is released independently of the G olgi-derived vesicles, and suggest inhibition of melatonin synthesis after the removal of BFA. By measuring the activities and mRNA levels of melatoni n-synthesizing enzymes, we found that the removal of BFA specifically inhib its NAT activity at the protein level. On the other hand, BFA causes no det ectable phase-shift of the chick pineal oscillator regulating the circadian rhythm of melatonin secretion. The results presented here suggest that the Golgi-mediated vesicular transport is involved in neither the melatonin re lease nor the time-keeping mechanism of the circadian oscillator, but rathe r contributes to the regulation of NAT activity.