Current understanding of the rewarding and addictive effects of opioids involves mu-opioid receptor (MOR) binding within the nucleus accumbens (NAcc), a region of the ventral striatum. GABAergic neurotransmission in the NAcc potentiates the rewarding response to opioids, and in fact, drugs that stimulate GABAergic activity are also addictive, a phenomenon mediated in part by endogenous opioids. However, the neuroanatomical relationship between opioid and GABA systems is still unclear, and further study of the interaction between these neurotransmitter systems in the reward pathway is warranted. We report evidence supporting the direct interaction between GABAergic and opioidergic neurotransmitter systems within the mouse NAcc. Male and female FVB/NJ mice (12-16 months of age) were euthanized via carbon dioxide inhalation and brains processed for histology and immunohistochemistry (IHC). Coronal cryosections (10-12 um in thickness) were taken through the NAcc at the level of the anterior commissure. A mouse monoclonal antibody against GAD67, an enzyme catalyzing GABA production, was used in conjunction with an anti-mouse rhodamine red-X-labeled secondary antibody to identify GABAergic neurons in the NAcc. Alternating sections were stained for MOR using a fluorescein isothiocyanate (FITC)-conjugated rabbit polyclonal anti-MOR1 antibody. DAPI (4′,6-diamidino-2-phenylindole) was used to counterstain the nuclei. As expected, fluorescence microscopy results show that GAD67 staining is localized predominately in the neuronal cytoplasm, with approximately 40% of the population staining. The MOR1-FITC stain showed localization within the cytoplasm and plasma membrane as expected, however, there was also significant staining in the nuclear membrane and neuronal nucleus. MOR1-FITC was present in approximately 35% of the neuronal population. In separate experiments, we used double-immunostaining to study the co-expression of MOR1 and GAD67 within the same NAcc neurons. A similar localization pattern was detected with a small subset, approximately 2%, of neurons expressing both labels. There are few published reports of GAD67 and MOR1 co-expression within neurons of the NAcc. Previous studies of MOR expression show the receptor to be localized to the plasma membrane and, to a smaller degree, intracellularly. Here we found the MOR1 staining to be predominantly in the nucleus and nuclear membrane, with minimal expression on the plasma membrane. Further studies are in progress to validate the nuclear expression of MOR in GABAergic NAcc neurons. Results of the study suggest that a subset of mouse NAcc neurons express both MOR1 and GAD67, providing a direct intracellular link between opioid and GABAergic systems in the reward pathway. Preliminary intranuclear localization of MOR suggests a novel signaling pathway that may be important in fully elucidating neurobiological mechanisms underlying behaviors related to reward and addiction.