Effects of imipramine on open-field behavior in relation to initial level of ambulatory activity

Rats were divided into 2 groups according to their ambulation scores in an open-field test. The procedure required a water-deprived rat to walk about and explore freely during a 3-min period in an open-field. The ambulation score was obtained in each rat. Imipramine was then administered 3 times at 24-hr intervals (40, 80, and 160 mg/kg, p.o., respectively), and each animal was tested at 4, 8 and 12 hr after each administration. Rats with high initial ambulation scores, i.e., innately active rats, showed a greater post imipramine decrease than those with low initial scores. An analysis of variance of these data showed significant effects of the type of subject (in terms of the level of initial ambulation scores), dose, and their interaction. Interval conditions also differentiated the inhibitory effects of imipramine when the dose level was increased. It was demonstrated that the greater the initial ambulation scores the greater the decrease in the level of ambulation by imipramine.