Effects of naloxone on heroin-, amphetamine- and caffeine-stimulated locomotor activity in the rat.

We investigated the effects of naloxone on the locomotor activating properties of heroin (0.25 mg/kg SC), d-amphetamine (0.25 mg/kg SC) and caffeine (7.5 mg/kg SC). Naloxone eliminated heroin-stimulated locomotion at doses approximately six times lower than those that blocked amphetamine-stimulated locomotion. Caffeine-induced locomotor activation was insensitive to naloxone at all doses tested. These results suggest that central opiate systems are differentially involved in the behavioral activation produced by heroin, amphetamine and caffeine.