Ethanol and Naltrexone Have Distinct Effects on the Lateral Nano-organization of Mu and Kappa Opioid Receptors in the Plasma Membrane

The complex spatiotemporal organization of proteins and lipids in the plasma membrane is an important determinant of receptor function. Certain substances, such as ethanol, can penetrate into the hydrophobic regions of the plasma membrane. By altering protein–lipid and protein–protein interactions, these substances can modify the dynamic lateral organization and the function of plasma membrane receptors. To assess changes in plasma membrane receptor organization, we used photoactivated localization microscopy (PALM). This single molecule localization microscopy technique was employed to quantitatively characterize the effects of pharmacologically relevant concentrations of ethanol and naltrexone (an opioid receptor antagonist and medication used to treat alcohol use disorders) on the lateral nano-organization of mu and kappa opioid receptors (MOR and KOR, respectively). Ethanol affected the lateral organization of MOR and KOR similarly: It reduced the size and occupancy of opioid receptor nanodomains and increased the fraction of opioid receptors residing outside of nanodomains. In contrast, naltrexone affected MOR and KOR lateral organization differently. It significantly increased KOR surface density, nanodomain size, and the occupancy of KOR nanodomains. However, naltrexone marginally affected these parameters for MOR. Pretreatment with naltrexone largely protected against ethanol-induced changes in MOR and KOR lateral organization. Based on these data, we propose a putative mechanism of naltrexone action that operates in addition to its canonical antagonistic effect on MOR- and KOR-mediated signaling.