Preferential cytoplasmic localization of delta-opioid receptors in rat striatal patches: Comparison with plasmalemmal mu-opioid receptors

The activation of delta -opioid receptors (DORs) in the caudate-putamen nuc leus (CPN) produces regionally distinct changes in motor functions, many of which are also influenced by opioids active at mu -opioid receptors (MORs) . These actions most likely occur in MOR-enriched patch compartments in the CPN. To determine the functional sites for DOR activation and potential in teractions involving MOR in these regions, immunoperoxidase and immunogold- silver labeling methods were applied reversibly for the ultrastructural loc alization of DOR and MOR in single rat brain sections containing patches of the CPN. DOR immunoreactivity was commonly seen within the cytoplasm of sp iny and aspiny neurons, many of which also expressed MOR. In dendrites and spines, DOR labeling was preferentially localized to membranes of the smoot h endoplasmic reticulum and spine apparatus, whereas MOR showed a prominent plasmalemmal distribution. DOR- and/or MOR-labeled spines received asymmet ric, excitatory synapses, some of which showed no-table perforations, sugge sting the involvement of these receptors in activity-dependent synaptic pla sticity. DORs were more frequently detected than were MORs within axon term inals that formed either asymmetric synapses with spine heads or symmetric synapses with spine necks. Our results suggest that in striatal patches, DO Rs, often in cooperation with MORs, play a direct modulatory role in contro lling the postsynaptic excitability of spines, whereas presynaptic neurotra nsmitter release onto spines is mainly influenced by DOR activation. In com parison with MOR, the prevalent association of DOR with cytoplasmic organel les that are involved in intracellular trafficking of cell surface proteins suggests major differences in availability of these receptors to extracell ular opioids.