Differential Effects of Sodium on Agonist-Induced Conformational Transitions and Signaling at μ and κ Opioid Receptors

Sodium ions are classically conceptualized as negative allosteric modulators for G protein-coupled receptors, although there have been reports of either positive allosteric modulation or no effect of sodium on GPCR function. Here, we identified opposing actions of sodium on μ and κ opioid receptors. We utilized a variety of methods including radioligand binding, real-time conformational monitoring of transitions using bioluminescence resonance energy transfer, and signaling assays using the TRUPATH resource. At the μ receptors, sodium behaved as a negative allosteric modulator of binding, conformational transitions, and signaling. Intriguingly, bitopic μ agonists displayed transducer-specific effects on conformational transitions and signaling sodium concentrations. By contrast, at the κ opioid receptor, sodium negatively modulated agonist binding and positively modulated conformational transitions and signaling. Taken together, these findings support the notion that the differential sensitivities to sodium concentrations will result in opposing effects on the cell surface and intracellular signaling.