Chronic treatment with antidepressants: protentiation of clonidine-induced aggression in mice via noradrenergic mechanism.

The chronic (10 mg/kg i.p. twice daily, 10 days)-and not the acute-administration of amitriptyline, maprotiline or zimelidine enhances aggressiveness induced by clonidine in mice. An analogous potentiation of clonidine-induced aggressiveness was obtained with chronic administration (the schedule as above) of levomepromazine (2 mg/kg) or thioridazine (5 mg/kg) but not of spiperone (0.2 mg/kg). Fluoxetine (10 mg/kg), atropine (5 mg/kg), propranolol (10 mg/kg) or metergoline (0.5 mg/kg) given chronically (the schedule as above) also had no effect. The enhancement of clonidine aggressiveness induced by prolonged treatment with imipramine (10 mg/kg) was prevented by cycloheximide, an inhibitor of protein synthesis. The results supply further evidence for the previously proposed hypothesis that chronic administration of antidepressants enhances the responsiveness of central postsynaptic noradrenaline receptors.