Neurons with Mu Opioid Receptors Interact Indirectly with Enkephalin-Containing Neurons in the Rat Dentate Gyrus

In the dentate gyrus, mu opioid receptors (MORs) and their enkephalin agonists have overlapping distributions and influence excitability and plasticity. Released endogenous enkephalins can activate at least some of these MORs; however, whether these interactions involve synaptically associated profiles or more distant associations and whether some subcellular compartments (e.g., terminals or dendrites) are more likely to be targeted than others are not known. To elucidate the relationships between potential sites of enkephalin release and MORs, MOR1 and leucine–enkephalin (LE) immunoreactivities were localized in the hilus by electron microscopy, using immunoperoxidase and immunogold markers. Of the 573 MOR-immunoreactive (ir) profiles analyzed, most were axons and terminals (51 and 30%, respectively), and fewer were dendrites (12%), glia (3%), or unclassifiable (4%). Most MOR-ir profiles resembled interneuron processes, while most LE-ir terminals resembled mossy fibers. One third of MOR-ir profiles were within 3 μm and approximately half were within 4 μm of the nearest LE-ir profile. In contrast, few (3%) MOR-ir profiles contacted LE-ir profiles; only 16% of these contacts included observable synapses, and very few profiles (0.5%) colocalized MOR and LE immunoreactivity. MOR-ir axons, terminals, and dendrites were not distributed differently relative to LE-ir profiles. These results suggest that activation of hilar MORs by LE usually involves short-range volume transmission and that dendritic MORs are as likely as axonal and terminal MORs to be activated by released LE. However, the greater abundance of MOR-ir axons and terminals compared to dendrites indicates that presynaptic profiles are a more prominent target for enkephalins and exogenous MOR agonists such as morphine.