The effects of near-ultraviolet light on serotonin N-acetyltransferase activity in the chick pineal gland

Effects of near-ultraviolet light (UV-A; 325-390 nm, peak at 365 nm) on the activity of the pineal serotonin N-acetyltransferase (NAT, a key regulatory enzyme in melatonin biosynthesis) were examined in chicks. Acute exposure of dark-adapted animals to UV-A radiation produced a marked decline in NAT activity of the pineal gland. The magnitude of this suppression was dependent upon duration of the light pulse and the age of the animals. The decrease in the nighttime NAT activity evoked by a 5 min pulse of UV-A light applied during the fourth hour of the dark phase of the 12 hr light:12 hr dal li cycle (LD) gradually deepened during the first 40 min after the return of animals to constant darkness, then the enzyme activity began to riser reaching control values by 2 hr. Exposure of chicks to a 5 min pulse of UV-A light during the ninth hour of the dark phase produced a marked decline in pineal NAT activity, which was reversible after 15 min of darkness. Pretreatment of animals with an inhibitor of catecholamine synthesis, x-methyl-p-tyrosine (300 mg kg. i.p.). or with a blocker of x(2)-adrenergic receptors, yohimbine (2 mg kg, i.p.), antagonized the suppressive effect of UV-A light on nighttime NAT activity of the chick pineal gland. It is concluded that UV-A irradiation, similar to visible light, potently suppresses melatonin biosynthesis in the chick pineal gland, with an x(2)-noradrenergic signal playing the role of an intermediate in this action.