Immunohistochemical distribution of delta opioid receptors in the rat central nervous system: Evidence for somatodendritic labeling and antigen-specific cellular compartmentalization

Many studies have reported on the distribution of delta opioid receptors (δOR) in the mammalian central nervous system (CNS) by using a variety of techniques. However, no general consensus has emerged with regards to the localization of this receptor due to inconsistencies in the immunohistochemical literature. In the present study, we analyzed the cellular and subcellular distribution of immunoreactive δOR in the rat CNS using two different antibodies (directed against a sequence in the C‐terminus or N‐terminus of the rat δOR). By using Western blotting, these two antibodies recognized similar forms of the δOR in COS‐7 cells transfected with this receptor, but distinct forms in membranes from the rat spinal cord. By using light microscopic immunohistochemistry, both antibodies recognized identical populations of nerve cell bodies throughout the CNS; the distribution of these cell bodies conformed to that of δOR mRNA‐expressing cells detected by in situ hybridization. However, whereas the C‐terminus‐directed antibody recognized predominantly perikarya and proximal dendrites, the N‐terminus–directed antibody also labeled extensively dendritic and terminal arbors. Furthermore, by using electron microscopy, the two antibodies were found not only to label differentially somatodendritic versus axonal compartments, but also plasma membrane versus cytoplasmic ones, suggesting that distinct immunological forms of the receptor are being targeted preferentially to different cellular and subcellular domains. J. Comp. Neurol. 440:65–84, 2001. © 2001 Wiley‐Liss, Inc.