Voltage modulates the effect of μ‐receptor activation in a ligand‐dependent manner

BACKGROUND AND PURPOSE: Various GPCRs have been described as being modulated in a voltage‐dependent manner. Opioid analgesics act via activation of μ receptors in various neurons. As neurons are exposed to large changes in membrane potential, we were interested in studying the effects of depolarization on μ receptor signalling. EXPERIMENTAL APPROACH: We investigated potential voltage sensitivity of μ receptors in heterologous expression systems (HEK293T cells) using electrophysiology in combination with Förster resonance energy transfer‐based assays. Depolarization‐induced changes in signalling were also tested in physiological rat tissue containing locus coeruleus neurons. We applied depolarization steps across the physiological range of membrane potentials. KEY RESULTS: Studying μ receptor function and signalling in cells, we discovered that morphine‐induced signalling was strongly dependent on the membrane potential (V(M)). This became apparent at the level of G‐protein activation, G‐protein coupled inwardly rectifying potassium channel (K(ir)3.X) currents and binding of GPCR kinases and arrestin3 to μ receptors by a robust increase in signalling upon membrane depolarization. The pronounced voltage sensitivity of morphine‐induced μ receptor activation was also observed at the level of K(ir)3.X currents in rat locus coeruleus neurons. The efficacy of peptide ligands to activate μ receptors was not (Met‐enkephalin) or only moderately ([D‐Ala(2), N‐Me‐Phe(4), Gly(5)‐ol]‐enkephalin) enhanced upon depolarization. In contrast, depolarization reduced the ability of the analgesic fentanyl to activate μ receptors. CONCLUSION AND IMPLICATIONS: Our results indicate a strong ligand‐dependent modulation of μ receptor activity by the membrane potential, suggesting preferential activity of morphine in neurons with high neuronal activity.