Resultsand Discussion

Acute exposure of dark-adapted chicks to UV-A light produced a marked suppression of nighttime NAT activity of the pineal gland. The magnitude of the observed decline in the enzyme activity was dependent on the duration of the light pulse and age of chicks, with two weeks old birds being more sensitive than seven weeks old animals (Figure 1).

In another set of experiments, after a 5 min pulse of UV-A light chicks were transferred to darkness and a time course of NAT reactivation was determined for the pineal gland. When the exposure took place at the beginning of the fourth hour of the dark phase, i.e., when levels of both NAT-mRNA and NAT activity are increasing (5), the small UV-A light-evoked decline in the enzyme activity gradually deepened during the first 40min treatments of the birds with darkness, from 9% (at 0min) to 43% (at 40min). After 40min of darkness, NAT activity of the chick pineal gland began to rise, reaching complete restoration within 2 hours (Figure 2A). When the UV-A pulse of the same duration was applied in the middle of the ninth hour of the dark phase, i.e., during a declining portion of circadian oscillation in NAT-mRNA and NAT activity of the chick pineal gland (5), it produced a significant (by 23%) but short-lasting (up to 15min) decline in the pineal NAT activity (Figure 2B).

Earlier studies have demonstrated that noradrenaline acting on postsynaptic a2-adrenergic receptors plays an important role in the regulation of MLT biosynthesis in the chick pineal gland (6). Stimulation of these receptors have been shown to mediate, at least partially, the suppressive action of white light on pineal NAT activity and MLT level. Therefore, in the present studies we examined a possible involvement of an a2-noradrenergic signal in the action of UV-A light on NAT activity of the chick pineal

Exposure to UV-A light (min)

Figure 1. Effects of near-ultraviolet (UV-A) light on the nocturnal serotonin N-acetyltransferase activity of the chick pineal gland. Data shown are means ± SEM. N = 5-10/group. The enzyme activity in the dark control group (in nmol/hr/pineal gland): two weeks old chicks, 17.52 ± 0.93; seven weeks old chicks, 33.21 ± 2.83. *P < 0.05 vs dark control, **P < 0.01 vs dark control. ***P < 0.001 vs dark control.

Exposure to UV-A light (min)

Figure 1. Effects of near-ultraviolet (UV-A) light on the nocturnal serotonin N-acetyltransferase activity of the chick pineal gland. Data shown are means ± SEM. N = 5-10/group. The enzyme activity in the dark control group (in nmol/hr/pineal gland): two weeks old chicks, 17.52 ± 0.93; seven weeks old chicks, 33.21 ± 2.83. *P < 0.05 vs dark control, **P < 0.01 vs dark control. ***P < 0.001 vs dark control.

0 15 30 40 60 120

0 15 30 40 60 120

Time after a 5 min exposure to UV-A light (min)

Figure 2. Time course of reactivation of serotonin N-acetyltransferase activity of the chick pineal gland after a short exposure to UV-A light. At the beginning of the fourth hour (A) or in the middle of the ninth hour (B) of the dark phase of the LD cycle the animals were exposed to a 5min pulse of UV-A light, and then returned to darkness. Dark control refers to the enzyme activity in pineal glands isolated from chicks kept under dark conditions throughout the experiment. Data shown are means ± SEM. N = 5-15/group. **P < 0.01 vs dark control, ***P < 0.001 vs dark control.

gland. Pretreatment of chicks with a-methyl-p-tyrosine, an effective in vivo inhibitor of tyrosine hydroxylase, significantly reduced the UV-A light-evoked decline in the chick pineal NAT activity (7). Furthermore, yohimbine (2mg/kg, ip), a blocker of a2-adrenergic receptors, antagonized the suppressive action of UV-A radiation on the enzyme activity [NAT activity (in nmol/hr/pineal): dark, 16.15 ± 1.13; dark + yohimbine, 18.22 ± 1.71; dark + 20min UV-A light pulse, 10.11 ± 0.95a; dark + yohimbine + UV-A light, 17.28 ± 1.23b. N = 6-l0/group. aP < 0.001 vs dark, bP < 0.001 vs dark + UV-A light].

In summary, results of our study indicate that UV-A radiation, similar to visible light, potently suppresses the nocturnal NAT activity of the chick pineal gland with the a2-noradrenergic signal playing a role of an intermediate in this action.

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