Mu Opioid Receptor mRNA and Protein Localization Across the Rat and Mouse Habenula

The habenula (Hb) has high-intensity mu opioid binding and receptor (MOR) expression. It contains medial and lateral subdivisions (MHb and LHb, respectively), yet the details of MOR localization across these regions remain debated. MHb and LHb participate in largely non-overlapping neural circuits; therefore, accurately resolving MOR expression across them is critical for understanding how MOR ligands impact behaviors. Here we utilized in situ hybridization (ISH) and immunocytochemistry (ICC) to systematically map Oprm1 mRNA and MOR protein throughout the habenular complex. We studied rat and mouse tissue to evaluate expression across two common research species. We also performed parallel mapping in Oprm1-^Venus/Venus mice. Importantly, we found mRNA and protein in both MHb and LHb in both species. In rat, 39 ± 3% and 21 ± 4% of cells expressed Oprm1 in MHb and LHb, respectively. These proportions were greater in mouse: 57 ± 1% (MHb) and 32 ± 4% (LHb). Oprm1 puncta per positive cell were greater in MHb compared to LHb for both rat and mouse (p < 0.0001). The highest intensity labeling was localized along the lateral edge of the MHb for all methods. ICC showed MOR localized to fibers and somata in both regions. In LHb, MOR labeling was most dense in intermediate sections along the anterior-posterior (AP) axis. In rats we also observed greater labeling in dorsal LHb at intermediate AP levels and medial LHb more posteriorly. These results indicate that both MHb and LHb can contribute to MOR-mediated actions through their respective circuits.