Suppression of play fighting by amphetamine: effects of catecholamine antagonists, agonists and synthesis inhibitors.

Moderate doses of amphetamine and methylphenidate profoundly depress play fighting in juvenile rats. To test the idea that this behavioral effect was dependent on the release of catecholamines (CAs) we administered haloperidol (0.05-0.8 mg/kg), chlorpromazine (0.5-5 mg/kg), phenoxybenzamine (0.5-20 mg/kg) or propranolol (0.5-20 mg/kg) alone or in combination with 0.5 or 1 mg/kg d-amphetamine sulfate. None of these CA antagonists reversed the suppression of play fighting (indexed by pinning) caused by amphetamine, but at higher doses haloperidol, chlorpromazine and phenoxybenzamine depressed both pinning and rearing. The presynaptic NE agonist clonidine (0.05-0.2 mg/kg) also failed to block the effects of amphetamine on play; instead it too depressed both pinning and rearing. Finally the CA synthesis inhibitor, alpha-methyltyrosine (total dose: 100 mg/kg) did not attenuate the suppression of play by amphetamine. Ephedrine (10-80 mg/kg) mimicked the effects of amphetamine on pinning, but apomorphine did not. At doses from 0.125-0.5 mg/kg apomorphine stimulated pinning while 1 mg/kg had no effect. The present findings confirm earlier reports that amphetamine suppresses play fighting but the mechanism of action remains obscure.