Effects of apomorphine administration on rearing activity of control and experimental rats withdrawn from long-term...

Rats were administered increasing doses of haloperidol or control solution for 25 days. 72 hr after drug withdrawal they were observed for rearing activity in an open field with or without apomorphine treatment. Rearing frequency was higher in haloperidol withdrawn rats. In control rats rearing frequency decreased 15 min after apomorphine administration in a dose-dependent way. In haloperidol withdrawn animals the apomorphine effects were dual, first an increase in rearing frequency induced by the smaller apomorphine doses (0.025-0.1 mg/kg) was observed and secondly a progressive and dose-dependent decrease was scored. Apomorphine also induced different motor effects not necessarily similar in control and experimental rats. Supersensitivity of dopaminergic receptors induced by the long-term neuroleptic administration was considered to be involved with the difference observed.