Selective dopamine D1 and D2 agonists independently affect different components of the free-running circadian rhythm of locomotor activity in rats.

The effects of selective dopamine D1 (SKF 38393) and D2 (4-propyl-9-hydroxynaphthoxazine or PHNO) agonists on free-running rhythms of locomotor activity in rats were investigated under either constant light (LL) or dark (DD) condition. Rats under the DD condition exhibited clear circadian patterns of activity, while circadian patterns of activity in animals kept under the LL condition were more ambiguous. Under the DD condition, SKF 38393 significantly increased the length of the period of free-running rhythms of locomotor activity but had no effect on the amplitude and the mesor, while PHNO significantly increased the amplitude and the mesor of free-running rhythms but had no effect on the length of the period.