σ1 Receptor Modulation of G-Protein-Coupled Receptor Signaling: Potentiation of Opioid Transduction Independent from Receptor Binding

sigma Ligands modulate opioid actions in vivo, with agonists diminishing morphine analgesia and antagonists enhancing the response. Using human BE(2)-C neuroblastoma cells that natively express opioid receptors and human embryonic kidney (HEK) cells transfected with a cloned mu opioid receptor, we now demonstrate a similar modulation of opioid function, as assessed by guanosine 5'-O-(3-[S-35]thio) triphosphate ([S-35]GTP gamma S) binding, by sigma(1) receptors. sigma Ligands do not compete opioid receptor binding. Administered alone, neither sigma agonists nor antagonists significantly stimulated [S-35]GTP gamma S binding. Yet sigma receptor selective antagonists, but not agonists, shifted the EC50 of opioid-induced stimulation of [S-35]GTP gamma S binding by 3- to 10-fold to the left. This enhanced potency was seen without a change in the efficacy of the opioid, as assessed by the maximal stimulation of [S-35]GTP gamma S binding. sigma(1) Receptors physically associate with mu opioid receptors, as shown by coimmunoprecipitation studies in transfected HEK cells, implying a direct interaction between the proteins. Thus, sigma receptors modulate opioid transduction without influencing opioid receptor binding. RNA interference knockdown of sigma(1) in BE(2)-C cells also potentiated mu opioid-induced stimulation of [S-35] GTP gamma S binding. These modulatory actions are not limited to mu and delta opioid receptors. In mouse brain membrane preparations, sigma(1)-selective antagonists also potentiated both opioid receptor and muscarinic acetylcholine receptor-mediated stimulation of [S-35]GTP gamma S binding, suggesting a broader role for sigma receptors in modulating G-protein-coupled receptor signaling.