Mu opioid receptors are extensively co-localized with parvalbumin, but not somatostatin, in the dentate gyrus.

In the rat dentate gyrus, mu opioid receptor (MOR) agonists disinhibit principal cells, promoting excitation, but whether MOR protein is differentially distributed to interneuron subtypes is unknown. Here, the distribution of MOR immunoreactivity was semi-quantitatively examined in neurochemically identified interneurons using fluorescence microscopy. We find that MOR- and parvalbumin-immunoreactivities are frequently co-localized, while MOR- and somatostatin-immunoreactivities are less commonly co-localized. This suggests that MORs are most frequently on interneurons specialized to inhibit granule cell output, and are on a limited number of interneurons that inhibit granule cell distal dendrites.