Substantial evidence suggests that dopamine and melatonin are mutually inhibitory factors that act in the retina as chemical analogs of day and night. Here, we show an impact of environmental light, biological clock, and melatonin on retinal levels of dopamine and its major metabolite 3,4-dihydroxyphenylacetic acid (DOPAC) in the turkey. In turkeys held under different light (L) to dark (D) cycles (16L:8D, 12L:12D, 8L:16D), retinal levels of dopamine and DOPAC fluctuated with daily rhythms. High levels of dopamine and DOPAC were observed during light hours and low during dark hours. Under the three photoperiodic regimes, rhythms of dopamine and DOPAC were out of phase with daily oscillation in retinal melatonin content. In constant darkness, dopamine and DOPAC levels oscillated in circadian rhythms. Light deprivation resulted, however, in a significant decline in amplitudes of both rhythms. Injections of melatonin (0.1-1 mumol/eye) during daytime significantly reduced retinal levels of DOPAC. This suppressive effect of melatonin was more pronounced in the dark-adapted than light-exposed turkeys. Quinpirole (a D(2)/D(4)-dopamine receptor agonist; 0.1-10 nmol/eye) injected to dark-adapted turkeys significantly decreased retinal melatonin. Our results indicate that in the turkey retina: (1) environmental light is the major factor regulating dopamine synthesis and metabolism; (2) dopaminergic neurones are controlled, in part, by intrinsic circadian clock; and (3) dopamine and melatonin are components of the mutually inhibitory loop.

Dynamics of rhythmic oscillations in the activity of arylalkylamine N-acetyltransferase (AA-NAT, the penultimate and key regulatory enzyme in melatonin biosynthesis) were examined in the retina and pineal gland of turkeys maintained for 7 days in the environment without daily light-dark (LD) changes, namely constant darkness (DD) or continuous light (LL). The two tissues differentially responded to constant environment. In the retina, a circadian AA-NAT activity rhythm disappeared after 5 days of DD, while in the pineal gland it persisted for the whole experiment. No circadian rhythm was observed in the retinas of turkeys exposed to LL, although rhythmic oscillations in both AA-NAT and melatonin content were found in the pineal glands. Both tissues required one or two cycles of the re-installed LD for the full recovery of the high-amplitude AA-NAT rhythm suppressed under constant conditions. It is suggested that the retina of turkey is less able to maintain rhythmicity in constant environment and is more sensitive to changes in the environmental lighting conditions than the pineal gland. Our results indicate that, in contrast to mammals, pineal glands of light-exposed galliformes maintain the limited capacity to rhythmically produce melatonin.