REPEATED HEROIN ADMINISTRATION INCREASES EXTRACELLULAR OPIOID PEPTIDE-LIKE IMMUNOREACTIVITY IN THE GLOBUS-PALLIDUS VENTRAL PALLIDUM OF FREELY MOVING RATS

Microdialysis was used to investigate the effects of heroin administra tion on extracellular opioid peptide levels in the globus pallidus/ven tral pallidum of freely moving rats. Two injections of heroin (0.6 mg/ kg IP) were given 3 h apart. The first injection had no significant ef fect on opioid peptide levels, bur: the second injection produced a tr ansient yet significant increase (+268%) in opioid peptide-like immuno reactivity in pallidal dialysates, peaking 1 h after injection. This e ffect was blocked by administration of naloxone (3 mg/kg IP) prior to the second injection. The implications of these data are discussed wit h regards to the role of the endogenous opioid peptide system in opiat e reward.