The avian pineal gland releases melatonin in a cyclic manner, with the highest level at night and the lowest level during the daytime. Mechanisms regulating melatonin secretion in birds are very complex, probably due to the phylogenetic position of the avian pineal gland as an intermediate form between the pineals of lower vertebrates and mammals. Avian pinealocytes possess an endogenous oscillator, formed by a self-regulated system of cock genes, that controls the transcription of several enzymes, among them arylalkylamine N-acetyltransferase (AA-NAT), the enzyme limiting melatonin synthesis. These cells are also directly photosensitive due to the presence of photopigments, pinopsin and melanopsin, as well as corresponding signal transduction systems. Light acting via pinopsin induces a cascade of events that leads to the decrease in cGMP and cAMP levels, AA-NAT activity and melatonin secretion. Melanopsin is probably involved in an entrainment of the circadian oscillator to the environmental light conditions. The function of the avian pineal gland is also regulated by light acting indirectly via the retina as well as by the extrapineal oscillator located in the suprachiasmatic nucleus. Both structures influence the pinealocyte activity via a common multisynaptic pathway, which ends in the gland as the sympathetic nerve fibers. Norepinephrine released from these fibers stimulates α₂-adrenoreceptors in pinealocyte plasmalemma and inhibits adenylate cyclase activity and melatonin secretion. The significance of direct and indirect routes of light perception as well as intra- and extra-pineal oscillators in the regulation of melatonin secretion may differ between species, but this problem is poorly recognized.
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