Vasoactive intestinal polypeptide (VIP) has been shown to stimulate melatonin synthesis in mammalian pineal; however, a regulatory role for VIP in the avian pineal has not been explored. Immunocytochemical and physiological response experiments were performed to investigate whether 1) immunoreactive VIP fibers innervated the avian pineal gland ; 2) VIP had a specific effect on melatonin release that was mediated by cAMP stimulation; and 3) α2-adrenergic signal transduction was associated with a reduction in cAMP levels. Immunocytochemical experiments demonstrated the presence of both tyrosine hydroxylase- and VIP-immunoreactive fibers in the avian pineal gland. Treatment of dispersed chick pineal cell cultures with VIP stimulated melatonin release (maximum 6-fold increase; EC50 = 1.8 nM) when administered during the 12-h light period of a 12-h light, 12-h dark cycle. Of the other four peptides tested [porcine VIP- (10–28), porcine peptide histidine isoleucine, porcine secretin, and human glucagon), only peptide histidine isoleucine stimulated melatonin release (EC50 = 30 nM). The effect of VIP was mediated by a time- and dosedependent increase in cAMP accumulation (maximum 4-foldincrease). The specific α2-agonist UK-14,304 reduced cAMP accumulation (maximum, 43% reduction) and inhibited melatonin release (EC50 = 19 nM) in the presence of 3 × 10−8 M VIP. Norepinephrine-induced inhibition of nocturnal melatonin release was blocked by the elevation of cAMP achieved through the administration of forskolin (EC50 = 0.2 μM), isobutylmethylxanthine (EC50 = 112 μM), or 8-bromo-cAMP (EC50 = 166 μM). Collectively, these results demonstrate the presence and functional significance of VIP in the avian pineal gland, and the interaction of VIP and norepinephrine at the level of cAMP in the regulation of melatonin biosynthesis.
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