Influence of pinealectomy on plasma and extrapineal melatonin rhythms in young chickens (Gallus domesticus).

A specific radioimmunoassay was validated for the quantitative measurement of melatonin (MT) levels in plasma and homogenates of the pineal gland, Harderian gland, or retinae of young chickens. Single-comb White Leghorn (SCWL) cockerels were raised under a 12L:12D light/dark cycle for two experiments. In Experiment 1, 12 birds were bled and immediately killed for their pineal glands at 4-hr intervals during a single light/dark cycle at 25 days of age (25 da) for simultaneous determination of changes in MT levels in the plasma and pineal gland. Plasma MT levels were low during photophase (100 pg/ml) and reached a distinct peak (390 pg/ml) at mid-scotophase. A parallel MT rhythm was found in pineal homogenates where the average MT content during scotophase (7.4 ng/gland) was 10 times higher than the average MT content of pineal glands obtained during photophase. In Experiment 2, SCWL cockerels were either pinealectomized or sham-operated (PN) at 8 to 10 da. At 25 da, six birds from each surgical treatment group, including unoperated controls (C), were bled at 4-hr intervals, corresponding to those in Experiment 1, during a single light/dark cycle. Immediately after being bled, each bird was killed and the eyes and Harderian glands were removed for measurement of their MT contents. Pinealectomy completely abolished the plasma MT rhythm which in intact chicks (PN and C) reached a sharp peak (298 pg/ml) at mid-scotophase. Although not affected by surgical treatment, retinal MT levels showed a higher amplitude rhythm with a prominent peak (4 ng/retina) at mid-scotophase that was 15 times higher than the average retinal MT content during photophase. A modest nocturnal MT rhythm was found in the Harderian gland where the average MT level for all surgical treatment groups during scotophase (89 pg/100 mg wet wt) was only 51% higher than that observed for photophase. These data indicate that the plasma MT rhythm in chickens is derived solely from MT secreted into blood by the pineal gland and that the extrapineal MT produced rhythmically in both the retina and Harderian gland does not contribute to the plasma MT rhythm.