Functional interactions between mu opioid and alpha 2A-adrenergic receptors.

Adrenergic and opioid receptors belong to the rhodopsin family of G-protein coupled receptors, couple to analogous signal transduction pathways, and affect the nociceptive system. Although a number of previous studies have reported functional interactions between these two receptors, the basis for this has not been well explored. We propose that direct receptor-receptor interactions could account, in part, for opioid-adrenergic cross-talk. In this report, we have addressed this using biophysical, biochemical, and pharmacological studies. We show that mu opioid and alpha2A adrenergic receptors reside in close proximity in live cells using the bioluminescence resonance energy transfer assay. These receptors colocalize to proximal dendrites in primary hippocampal neurons. mu-alpha2A Receptor complexes can be isolated from heterologous cells or primary neurons coexpressing these receptors. In these cells, the activation of either mu or alpha2A receptor leads to a significant increase in the level of immunoprecipitable mu-alpha2A complexes, whereas activation of both receptors leads to a significant decrease. The implications of these effects on signaling were examined using the agonist-mediated increase in G-protein activity and mitogen-activated protein kinase activity. We find that activation of either mu or alpha2A receptors leads to an increase in the extent of signaling, whereas activation of both receptors leads to a decrease. The increase in signaling by individual ligands and decrease by a combination of ligands is also seen in primary spinal cord neurons endogenously expressing these receptors. Taken together, these results suggest that physical associations between mu and alpha2A receptors could play a role in the functional interactions between these receptors.