Specific opioid-amphetamine interactions in the caudate putamen

Bilateral microinjection of morphine (0.003-3 μg/side) into the caudate putamen enhances the behavior induced by the IP injection of 1 mg/kg d-amphetamine phosphate in a dose-related manner. The duration of activity was prolonged and ambulation was changed to d-amphetamine stereotypy, a behavior normally associated with higher doses of d-amphetamine. The opioid activity was stereospecific in that levorphanol was active, whereas dextrorphan was not. The enhancement of d-amphetamine-induced behavior by the opioids was blocked by naloxone. d-ala²-met-Enkephalin also enhanced the amphetamine-induced behavior. This enhancement appears to be specific to the caudate putamen because the oral stereotypy observed appears to be a unique action of amphetamine in this region of the brain.