Near-ultraviolet radiation suppresses melatonin synthesis in the chicken retina - A role of dopamine

Effects of near-ultraviolet radiation (UV-A; 325-390 nm, peak at 365 nm) on melatonin content and activity of serotonin N-acetyltransferase (AA-NAT; a key regulatory enzyme in melatonin biosynthesis) were examined in the reti na of chickens. Acute exposure of dark-adapted animals to UV-A light produc ed a marked decline in melatonin content and AA-NAT activity of the retina. The magnitude of the observed changes was dependent upon duration of the l ight pulse and age of chickens, with 1-2-week old birds being more sensitiv e to UV-A action than 6-7-week old ones. The decrease in the nocturnal AA-N AT activity evoked by a 5-min UV-A pulse gradually deepened during the firs t 30 min after the return of chickens to constant darkness, then the enzyme activity began to rise, reaching nearly complete restoration within 2.5 hr . Systemic administration to chickens of alpha-methyl-p-tyrosine tan inhibi tor of catecholamine synthesis; 0.3 g/kg) blocked the suppressive effect of W-A light on retinal AA-NAT activity. Haloperidol, sulpiride (blockers of D2-family of dopamine (DA) receptors) and 2-chloro-11-(4-methylpiperazino)d ibenz[bf] (an antagonist of D-4-DA receptors), given intraocularly (1-100 n mol/eye) prevented the UV-A light-evoked decrease in AA-NAT activity in the chicken retina in a dose-dependent manner, while raclopride (300 nmol/eye) , an antagonist of D-2/D-3-DA receptors, was ineffective. In dark-adapted c hickens exposure to UV-A light increased the DA content of the retina. It i s concluded that UV-A radiation, similar to visible light, potently suppres ses melatonin biosynthesis in the retina of chicken, with a D-4-dopaminergi c signal playing the role of an intermediate in this action. (C) 2000 Elsev ier Science Inc. All rights reserved.