Hippocampal mu-receptors mediate opioid reinforcement in the CA3 region

Dependence on reinforcing chemicals is manifested when drug-seeking and drug-taking behaviors come to dominate the response repertoire. Clinical observations suggest that the craving and compulsive drug-seeking that characterize drug dependence are aroused by memories of the reinforcing drug experience. If so, a brain structure intimately associated with memory--the hippocampus--would be a plausible substrate for drug reinforcement effects. We report here that drug-naive rats rapidly learn to self-administer the opioid peptide dynorphin A in the CA3 region of hippocampus, and that this behavior is blocked by co-administration of the non-selective opiate antagonist naloxone. Subsequent studies demonstrated that coadministration of mu-, but not kappa- or delta-opioid antagonists also blocked self-administration behavior. We conclude that mu-receptors in the CA3 region of hippocampus may be important target sites for opioid dependence.