In contrast to the CPNE4-positive ICjs, the RXFP1-positive islands had larger nuclei that were less densely packed together and did not appear different from nearby D1-MSNs (Figures S6E and S6F). Likewise, RXFP1-positive islands were not restricted to the border regions of the VS, rather, they were found throughout the NAc, putamen, and near the adjacent septal nuclei (Figure 6E, orange arrows and dashed black box, Figures S5G-S5I, S6H, S6I, and S7A). RXFP1-positive cells located in these VS islands exhibited high levels of DRD1 expression, but no detectable DRD2 expression (Figures S7A and S7B). Moreover, cells in the RXFP1-positive islands expressed high levels of the gene for the μ-opioid receptor (OPRM1) compared to regular D1- MSNs located outside of the D1-exclusive island (Figures 7A and 7B). Interestingly, expression of the κ-opioid receptor gene (OPRK1) was almost completely absent from these islands, compared to surrounding tissues (Figure S7D). Using immunohistochemistry, we confirmed that MOR was expressed in the RXFP1-positive islands (Figure 7C). Based on their distribution and the upregulation of μ-opioid receptor – upregulation that was not present in the ICjs (Figures
