Effects of restricted food access on circadian fluctuation of serotonin N-acetyltransferase activities in hereditary microphthalmic rats.

The characteristics in diurnal fluctuation of serotonin N-acetyltransferase activity were examined in normal and microphthalmic mutant rats of the Donryu strain under ad lib or restricted feeding conditions. Under a 12:12-h light:dark (12-h LD) cycle with free access to food, normal-sighted rats exhibited typical nocturnal increases in the activity of pineal serotonin N-acetyltransferase, being more than 50-fold higher in the dark period than that in the light period, but hereditary blind rats showed nonperiodic change in the pineal enzyme activity in the average, suggesting that the rhythms in individuals have become free-running, asynchronous. When the subjective night or subjective day of the mutants was discerned by active or inactive in the locomotor activity, the pineal enzyme activities in the mutants increased at the subjective night but depressed at the subjective daytime. When food access was restricted only for 6 h in the light period of the LD cycle, normal rats still showed the nocturnal increases in the pineal enzyme activity, but hereditary blind rats manifested a blunt peak in the activity of the pineal enzyme at eating time in the light period. The results suggest that microphthalmic mutant rats maintain the ability to shift and to synchronize their circadian phases induced by restricted access to food, even if they completely lack their optic nerve and visual input to the circadian clock.