Naltrexone blocks nicotine-induced prolactin release.

Two receptor populations involved in the release of prolactin were examined in conscious, freely moving, male rats bearing indwelling jugular cannulae. The intravenous administration of either nicotine or morphine increased plasma prolactin levels. Pretreatment with the nicotinic antagonist mecamylamine blocked the prolactin response to nicotine only. In contrast, the opiate antagonist naltrexone blocked the prolactin response to both nicotine and morphine. These findings indicate that the nicotine stimulated release of prolactin is dependent not only on functional nicotinic cholinergic receptors but on opiate receptors as well. This suggests that nicotine and morphine release prolactin via a common pathway containing nicotinic cholinergic and opiate synapses in series.