Turkey retina and pineal gland differentially respond to constant environment

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.