BACKGROUND AND PURPOSE: Various GPCRs (G-protein coupled receptors) have been described as being modulated in a voltage-dependent manner. Opioid analgesics act via activation of μ-receptors (MOP) in various neurons. As neurons are exposed to large changes in membrane potential, we were interested in studying the effects of depolarization on MOP signalling.
EXPERIMENTAL APPROACH: We investigated potential voltage-sensitivity of MOP 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 MOP function and signalling in cells we discovered that morphine-induced signalling was strongly dependent on the membrane potential (VM ). This became apparent at the level of G-protein activation, G-protein coupled inwardly rectifying potassium channel (Kir 3.X) currents and binding of G-protein coupled receptor kinases and Arrestin3 to MOP by a robust increase in signalling upon membrane depolarization. The pronounced voltage-sensitivity of morphine-induced MOP activation was also observed at the level of Kir 3.X currents in rat locus coeruleus neurons. The efficacy of peptidergic ligands to activate MOP was not (Met-enkephalin) or only moderately (DAMGO) enhanced upon depolarization. In contrast, depolarization reduced the ability of the analgesic fentanyl to activate MOP.
CONCLUSION AND IMPLICATIONS: Our results indicate a strong ligand-dependent modulation of MOP activity by the membrane potential, suggesting preferential activity of morphine in neurons with high neuronal activity.