Effects of naloxone and diprenorphine on amphetamine-stimulated behavior in guinea pigs and rats.

Amphetamine (0.1-10 mg/kg), naloxone (0.1-10 mg/kg) and diprenorphine (0.03-10 mg/kg) were studied for their ability to modulate locomotor behavior in the guinea pig. Naloxone, administered alone, caused a non-significant decrease in locomotor activity and had a similar non-significant effect on amphetamine-stimulated activity. Diprenorphine induced a significant suppression of locomotor activity, the magnitude of which was inversely related to dose: smaller doses of diprenorphine caused a greater suppression of locomotor activity than larger doses. Two doses of diprenorphine (0.1 and 1.0 mg/kg) were tested in combination with amphetamine in the guinea pig. They significantly reduced amphetamine-stimulated behavior and were equipotent in this regard. In contrast, diprenorphine had no effect on amphetamine-stimulated activity in rats. However, in keeping with other reports, naloxone (10 mg/kg) significantly reduced amphetamine-stimulated behavior. The differences in the actions of diprenorphine and naloxone on the behavior of guinea pigs and rats may reflect a different underlying distribution of subtypes of opioid receptor in the two species.