ULTRASTRUCTURAL LOCALIZATION OF μ-OPIOID RECEPTOR IMMUNOREACTIVITY AND RELATIONSHIP TO ENKEPHALIN IN CERVICAL DORSAL HORN OF THE RAT SPINAL CORD

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Abstract
We used a polyclonal rabbit antiserum against an 18 amino acid sequence from the carboxyl terminus of the cloned μ-opioid receptor (MOR) to determine the potential cellular sites for the analgesic actions of opiates in the rat cervical spinal cord. By light microscopy, MOR-like immunoreactivity (MOR-LI) was most intensely localized, using either peroxidase or gold-silver labeling methods, to processes in lamina I and II of the dorsal horn. Electron microscopic examination of this region revealed prominent localization of immunogold-silver labeling for MOR specifically along plasma membranes of axon terminals, small unmyelinated axons, and dendrites. We have also shown by dual labeling that Leu5-enkephalin (LE) was sometimes present in axon terminals which synaptically contacted dendrites showing MOR-LI. These results provide the first direct evidence that MOR is preferentially localized to extrasynaptic sites along the membrane of both dendrites and axons within the superficial layers in the dorsal horn of the rat cervical spinal cord. Activation of MOR by LE or exogenous opiates in this area may thus produce analgesia through modulation of the postsynaptic receptivity or presynaptic release of other transmitters.

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