Specific mu 2 opioid isoreceptor regulation of nigrostriatal neurons: in vivo evidence with naloxonazine.

The irreversible mu 1 opiate antagonist, naloxonazine, totally blocked the analgesic actions (tail-flick assay) of morphine (8 mg/kg) in the rat. However, this treatment was without effect on the morphine-dependent (4 mg/kg) increases in striatal dopamine metabolism. These data support our previous studies which have suggested mu 2 and delta opioid receptor regulation of the rat nigrostriatal pathway.