METHYLPHENIDATE - DIURNAL EFFECTS ON LOCOMOTOR AND STEREOTYPIC BEHAVIOR IN THE RAT

The dose-response relationship and time course of effect on motor acti vity after a single dose of methylphenidate given at different times o f the light/dark cycle was investigated using a computerized infrared activity analysis system. After 5 to 7 days of acclimation and 2 days of baseline activity recording, rats received a single subcutaneous in jection of vehicle (saline) or of 0.6, 2.5, 10 or 40 mg/kg methylpheni date at 08:00, 14:00, 20:00, or 02:00. Recording was then resumed for an additional 36 to 48 hours. The locomotor indices analyzed were hori zontal activity, total distance, vertical activity, stereotypic activi ty, and number of stereotypic movements. Saline and 0.6 mg/kg did not alter motor activity, but 2.5, 10 and 40 mg/kg significantly increased (P < 0.01) motor activity. The time to the maximum effect and the dur ation of effect increased with dose. Ten mg/kg had the most robust eff ect on locomotor activity, while the largest dose, 40 mg/kg, elicited a more focused stereotyped activity that limited the amount of forward ambulation. A single injection of methylphenidate had only transient effects. The locomotor stimulating effects of the lower doses were sim ilar whether given during the Light or dark phase, despite the large d iurnal variations in baseline activity between the activity phases. Th e stereotypic effects of the highest dose of methylphenidate, however, varied between the light and dark phase, with a smaller stereotypic e ffect during the dark phase when compared to administration during the light phase. (C) 1997 Elsevier Science B.V.