Interactions between delta opioid receptors and alpha-adrenoceptors.

1. Several studies have reported functional interactions between different subtypes of opioid and alpha2A-adrenoceptors in the induction of spinal cord analgesia. The mechanisms underlying this phenomenon are not well characterized. We propose that direct receptor-receptor associations could account for some of the observed functional interactions. In the present study, we examined the presence of delta opioid receptors and alpha2A-adrenoceptors in interacting complexes and the functional implications of such interactions on receptor activity. 2. Using the proximity based bioluminescence resonance energy transfer (BRET) assay, we found that the delta opioid receptors and alpha2A-adrenoceptors are in close enough proximity (< 100 A) in live cells that can foster physical interactions. 3. Using coimmunoprecipitation of differentially epitope-tagged receptors, we found that delta opiate receptors exist in interacting complexes with alpha2A-adrenoceptors in heterologous cells. 4. Finally, using receptor activity mediated neurite outgrowth in Neuro 2A cells as a physiological readout, we found that interactions between delta opiate receptors and alpha2A-adrenoceptors have functional consequences. The expression of alpha2A-adrenoceptors is sufficient to promote delta opiate receptor-mediated neurite outgrowth, suggesting that the presence of inactive alpha2A-adrenoceptors can enhance delta opiate receptor-mediated signalling. 5. Taken together, these findings suggest that modulation of receptor function as a result of physical associations between delta opiate receptors and alpha2A-adrenoceptors may account for the observed synergy between opiate and adrenergic agonists in spinal analgesia.