N-bromoacetyltryptamine strongly and reversibly inhibits in vitro melatonin secretion from mammalian pinealocytes

Objectives: Cell-permeable and specific inhibitors of melatonin secretion are sill lacking among tools of the pineal research. Recently, a large effort has been made in the development of arylalkylamine N-acetyltransferase inhibitors, but in most cases the new drugs were tested exclusively using cell-free assays or non-pineal cells. The aim of the present study was to characterize the effect of N-bromoacetyltryptamine (BAT), the first synthesized cell-permeable inhibitor of arylalkylamine N-acetyltransferase, on melatonin secretion from rat and pig pineal glands. Methods: The studies were performed in the superfusion cultures of rat and pig pineal explants. Melatonin secretion was determined by radioimmunoassay (RIA). Results: BAT strongly inhibited the non-stimulated and norepinephrine-stimulated melatonin secretion from the pig and rat pineal explants, with ED₅₀ ≈ 0.3-0.7 μM. The adrenergic stimulation did not modify significantly the inhibitory potency of BAT on the melatonin release. The decline in melatonin secretion induced by the BAT-treatment was biphasic in both rat and pig pinealocytes, with an initial rapid phase followed by a slow one. The half-time of BAT-induced decline in the non-stimulated and norepinephrine-stimulated melatonin secretion was ca. 25-35 minutes. The inhibitory effect of BAT was reversible in pinealocytes of both investigated mammals. Conclusions: The results show that BAT is a potent and reversible inhibitor of the melatonin secretion in the mammalian pineal gland and open the way for the use of this inhibitor in investigations on the pinealocyte physiology performed in vitro.