Circadian rhythm of apomorphine-induced stereotypy in rats.

The existence of circadian variations of apomorphine-induced stereotypy was determined. Male Wistar rats, standardized to a light-dark cycle (lights on from 7:00--19:00) for three weeks, were injected with apomorphine hydrochloride (1 mg/kg or 3 mg/kg) at one of six times (9:00, 13:00, 17:00, 21:00, 1:00 and 5:00). A significant time-of-day effect was found for apomorphine-induced stereotypy, with highest stereotypic score following injection at 13:00 or 17:00. The circadian rhythm of apomorphine-induced stereotypy was significantly fitted to a single cosine curve with a 24-hr cycle using the least squares method.