CO-EXPRESSION OF ALPHA-2A-ADRENERGIC AND DELTA-OPIOID RECEPTORS IN SUBSTANCE P 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 co-administered, 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 hetero-oligomers has been proposed as a molecular basis for interactions between neuronal G-protein-coupled receptors. The relevance of hetero-oligomer 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 if co-expression could be demonstrated between these receptors. We observed extensive co-localization between α2A-adrenergic and delta-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. α2AAR- and DOP-ir elements were co-localized in subcellular structures of 0.5 μm or less in diameter in isolated nerve terminals. Furthermore, co-incubation 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 co-expression of the synergistic receptor pair α2AAR-DOP on primary afferent nociceptive fibers may represent an anatomical substrate for analgesic synergy, perhaps due to protein-protein interactions such as hetero-oligomerization.