Enhanced apomorphine-induced fighting in rats after prolonged oral treatment with drugs altering noradrenergic transmission.

Male Wistar rats were given orally for seven days water, clonidine (0.125 or 0.25 mg/kg bid) or S3341, a new clonidine-like drug (2.5 or 5.0 mg/kg bid) and afterwards pairs of rats were injected ip with 2.5 mg/kg of apomorphine (APO). Water-treated animals did not behave aggressively. Clonidine and S3341 both induced clear aggression when combined with a subthreshold dose of APO. Some degree of hyperirritability developed in these two groups during drug treatment when observed in the home-cage. No significant changes in aggressive behavior were noted upon acute administration of both drugs, though some tendency to augment aggressivity could be observed after clonidine (0.25 mg/kg po). It is suggested that noradrenergic neurons play an inhibitory role in APO-induced fighting.