Naloxone

Naloxone is a drug that acts as an opiate antagonist. It has a high affinity for the endorphin receptors that serve the body's endogenous opiates, or exogenous drugs like morphine or heroin, a particularly concentrated opiate.

Interest in the addictive properties of opiates has stimulated research into naloxone, which has been associated with immediate withdrawal symptoms in users of heroin. Naloxone may counter the behavioral suppression of opiates (e.g. the euphoria and lethargy following morphine injection) by its competition with opiates at endorphin receptors. It is also possible that naloxone itself exerts some behavioral influence, either in the form of general activity enhancement or as a suppressor of behavior. Experiments with the hamster suggest that naloxone injected before morphine can drive activity levels even above those of saline controls. But other research on naloxone has shown that it, like morphine, may suppress some behaviors, e.g. sexual behavior in male rats. In order for naloxone to depress activity it appears to be necessary that it be injected at relatively high doses (e.g. 10 mg/kg and above in rats).