The circadian rhythm of rat motor activity and the effect of brain monoamine inhibitors (PCPA and nialamide).

The study of the circadian motor rhythm in free-running conditions of 500 g rats during 6 successive days shows that the endogenous period of the rhythm in these conditions is 23.40 hrs. The two poisons studied (PCPA in 40 mg/100 g dose and nialamide 5 and 10 mg/100 g doses) have both a deep effect on the functioning of the endogenous rhythm. PCPA reduces the amplitude of the increase of night over day activity to a fourth of the control value. The reducation may be attributed either to increased day activity or to reduced night activity. Nialamide reduces the amplitude of this increase to 80 or 70/100 of control value; this was observed with 5 or 10 mg nialamide/100 g body weight. The most important effect of nialamide on the endogenous rhythm lies in the disappearance of the regular shortening of nearly 20 min of the endogenous rhythm in DD conditions. The abrupt transition for LD 12:12 conditions to DD disturbs the endogenous period for 24 hrs.