The effects of dopaminergic agonists and antagonists were investigated on withdrawal signs in lorazepam-dependent rats. Physical dependence was developed by giving lorazepam admixed with food in the following dose schedules: 10 x 4, 20 x 4, 40 x 4, 80 x 4 and 120 x 7 mg/kg, daily for x days. 2. The parameters observed during the periods of administration of lorazepam and after its withdrawal were spontaneous locomotor activity (SLA), reaction time to pain, foot shock aggression (FSA) and audiogenic seizures. 3. During the withdrawal period, rats were divided into groups of 10 rats each. One group did not receive any drug and served as the control withdrawal groups. Three other groups received, separately, one of the following dopamimetic drugs: (i) 200 mg/kg per day, i.m., L-dihydroxyphenylalanine (DOPA; +50 mg/kg per day, i.m., carbidopa); (ii) 2 mg/kg per day, i.m., amphetamine; or (iii) 1 mg/kg per day, i.m., apomorphine. The remaining groups received one of the following dopamine antagonists: (i) 0.1 mg/kg per day, i.m., SCH 23390; (ii) 0.5 mg/kg per day, i.m., haloperidol; (iii) 0.5 mg/kg per day, i.m., centbutindol; and (iv) either 1 or 20 mg/kg per day, i.m., clozapine. 4. The withdrawal signs observed in the control group were hyperkinesia, hyperaggression and audiogenic seizures. 5. L-Dihydroxyphenylalanine (+ carbidopa), amphetamine and apomorphine potentiated hyperaggression and audiogenic seizures. The dopamine D2 receptor antagonists haloperidol, centbutindol and clozapine (at 20 mg/kg, i.m.) blocked all withdrawal signs. The D1 receptor antagonist SCH 23390 inhibited hyperkinesia and hyperaggression. The D4 receptor antagonist clozapine (at 1 mg/kg, i.m.) had no effect on any of the withdrawal signs. 6. It may be concluded that dopamine D2 receptors exert a dominant facilitatory influence, with partial contribution of D1 receptors, on the benzodiazepine withdrawal syndrome.
