Coexpression of α_2A-adrenergic and δ-opioid receptors in substance P-containing terminals in rat dorsal horn

Agonists acting at α₂-adrenergic and opioid receptors (α₂ARs and ORs, respectively) inhibit pain transmission in the spinal cord. When coadministered, agonists activating these receptors interact in a synergistic manner. Although the existence of α₂AR/OR synergy has been well characterized, its mechanism remains poorly understood. The formation of heterooligomers has been proposed as a molecular basis for interactions between neuronal G-protein-coupled receptors. The relevance of heterooligomer formation to spinal analgesic synergy requires demonstration of the expression of both receptors within the same neuron as well as the localization of both receptors in the same neuronal compartment. We used immunohistochemistry to investigate the spatial relationship between α₂ARs and ORs in the rat spinal cord to determine whether coexpression could be demonstrated between these receptors. We observed extensive colocalization between α ₂A-adrenergic and δ-opioid receptors (DOP) on substance P (SP)-immunoreactive (-ir) varicosities in the superficial dorsal horn of the spinal cord and in peripheral nerve terminals in the skin. α_2AAR- and DOP-ir elements were colocalized in subcellular structures of 0.5 μm or less in diameter in isolated nerve terminals. Furthermore, coincubation of isolated synaptosomes with α₂AR and DOP agonists resulted in a greater-than-additive increase in the inhibition of K⁺-stimulated neuropeptide release. These findings suggest that coexpression of the synergistic receptor pair α _2AAR-DOP on primary afferent nociceptive fibers may represent an anatomical substrate for analgesic synergy, perhaps as a result of protein-protein interactions such as heterooligomerization.