Simultaneous activation of the δ opioid receptor (δOR)/sensory neuron-specific receptor-4 (SNSR-4) hetero-oligomer by the mixed bivalent agonist bovine adrenal medulla peptide 22 activates SNSR-4 but inhibits δOR signaling

Hetero-oligomerization among G protein-coupled receptors has been proposed to contribute to signal integration. Because sensory neuron-specific receptors (SNSRs) and the opioid receptors (OR) share a common ligand, the bovine adrenal medulla peptide (BAM) 22, and have opposite effects on pain modulation, we investigated the possible consequences of delta OR/ SNSR-4 hetero-oligomerization on the signaling properties of both receptor subtypes. Bioluminescence resonance energy transfer revealed that the human delta OR has similar propensity to homo-oligomerize and to form hetero-oligomers with human SNSR-4 when coexpressed in human embryonic kidney 293 cells. The hetero-oligomerization leads to a receptor form displaying unique functional properties. Individual activation of either delta OR or SNSR-4 in cells coexpressing the two receptors led to the modulation of their respective signaling pathways; inhibition of adenylyl cyclase and activation of phospholipase C, respectively. In contrast, the delta OR/SNSR-4 bivalent agonist BAM22, which could activate each receptor expressed individually, fully activated the SNSR-4-dependent phospholipase C but did not promote delta OR-mediated inhibition of adenylyl cyclase in delta OR/SNSR-4-coexpressing cells. Likewise, concomitant activation of the delta OR/SNSR-4-hetero-oligomer by selective delta OR and SNSR-4 agonists promoted SNSR-4 but not delta OR signaling, revealing an agonist-dependent dominant-negative effect of SNSR-4 on delta OR signaling. Furthermore, the delta OR selective antagonist naltrexone trans-inhibited the SNSR-4-promoted phospholipase C activation mediated by BAM22 but not by the SNSR-4-selective agonists, suggesting a bivalent binding mode of BAM22 to the delta OR/SNSR-4 hetero-oligomer. The observation that BAM22 inhibited the Leu-enkephalin-promoted cAMP inhibition in rat dorsal root ganglia neurons supports the potential physiological implication of such regulatory mechanism.