Effect of midbrain stimulation on amphetamine-induced stereotypy in rats

Behavioral changes were induced in rats by administration of high doses of amphetamine (5 mg/kg): among these was the development of rapid sniffing. The effect of electrical stimulation of the midbrain periaqueductal gray was then examined. This stimulation is known to release an enkephalin-like substance into the ventricular spinal fluid and to induce analgesia. Stimulation blocked amphetamine-induced sniffing. This effect was blocked in turn by pretreatment with naloxone, a specific opiate antagonist. We discuss this finding in the context of opiate-catecholamine antagonism in the CNS.