Effect of naloxone and morphine on dopamine agonist-induced stereotypy in rats and guinea pigs.

Naloxone (0.80 mg/kg) and morphine (7.5 mg/kg) were given to rats or guinea pigs with increasing doses of amphetamine (0.5-6.0 mg/kg) to determine their respective effects on amphetamine-induced stereotypy. In contrast to the inhibiting and potentiating effect of these agents on apomorphine-induced stereotyped behavior, naloxone enhanced and morphine markedly attenuated amphetamine-induced stereotypy. Since other investigators have reported that drugs inhibiting release of dopamine block amphetamine-induced stereotypy and enhance apomorphine-induced stereotypy, whilst compounds stimulating the release of dopamine potentiate amphetamine-induced stereotypy and inhibit apomorphine-elicited stereotyped behavior, it is likely that naloxone stimulates and morphine inhibits the release of DA following their acute administration.