Effects and interactions of naloxone and amphetamine on self-stimulation of the prefrontal cortex and dorsal tegmentum.

Naloxone (0.1 to 10 mg/kg) caused a dose-dependent depression of self-stimulation of the medial prefrontal cortex (PFC) lateral hypothalamus (LH) and a region in the dorsal tegmentum lateral to the central gray (DT). The DT contains many enkephalin fibers and is a site for stimulation-produced analgesia, while the PFC contains little enkephalin and does not support stimulation-produced analgesia. However, self-stimulation rates of the PFC, DT and LH were all equally depressed by naloxone. In order to study possible opiate-dopamine interactions, we examined the effects of naloxone on the facilitatory effects of D- and L-amphetamine (1.0 mg/kg) on self-stimulation of the DT and PFC. If amphetamine mildly facilitated self-stimulation (L-amphetamine on DT self-stimulation and D-amphetamine on PFC self-stimulation) then the addition of naloxone was without effect. If amphetamine greatly increased self-stimulation (D-amphetamine on DT self-stimulation), naloxone caused a depression of the amphetamine effect. It is argued that naloxone's effects in this and other reports reviewed is related to the level of self-stimulation performance, and not to the level of enkephalin at the self-stimulation site, nor to amphetamine's effects on dopamine activity.