Plasma membrane localization of the μ-opioid receptor controls spatiotemporal signaling

Differences in plasma membrane mobility or clustering of an opioid receptor may underlie the different effects of its agonists. Spatiotemporal opioid receptor signaling The μ-opioid receptor (MOR) is a GPCR that mediates the effects of endogenous opioids and opioid analgesics, such as morphine. Different MOR agonists produce different biological effects, in part by differentially regulating receptor phosphorylation and internalization. In cells transfected with MOR, Halls et al. examined downstream signaling in the absence of receptor internalization. Whereas the synthetic opioid DAMGO stimulated receptor movement within the plasma membrane and transiently increased ERK activity in both the cytosol and nucleus, morphine stimulated a protein kinase C–dependent pathway that restricted MOR movement and produced prolonged cytosolic ERK activity. Similar effects were observed in mouse dorsal root ganglion neurons, suggesting that the differences in plasma membrane mobility or clustering of MOR may underlie the differential effects of its agonists in vivo. Differential regulation of the μ-opioid receptor (MOR), a G protein (heterotrimeric guanine nucleotide–binding protein)–coupled receptor, contributes to the clinically limiting effects of opioid analgesics, such as morphine. We used biophysical approaches to quantify spatiotemporal MOR signaling in response to different ligands. In human embryonic kidney (HEK) 293 cells overexpressing MOR, morphine caused a Gβγ-dependent increase in plasma membrane–localized protein kinase C (PKC) activity, which resulted in a restricted distribution of MOR within the plasma membrane and induced sustained cytosolic extracellular signal–regulated kinase (ERK) signaling. In contrast, the synthetic opioid peptide DAMGO ([d-Ala2,N-Me-Phe4,Gly5-ol]-enkephalin) enabled receptor redistribution within the plasma membrane, resulting in transient increases in cytosolic and nuclear ERK activity, and, subsequently, receptor internalization. When Gβγ subunits or PKCα activity was inhibited or when the carboxyl-terminal phosphorylation sites of MOR were mutated, morphine-activated MOR was released from its restricted plasma membrane localization and stimulated a transient increase in cytosolic and nuclear ERK activity in the absence of receptor internalization. Thus, these data suggest that the ligand-induced redistribution of MOR within the plasma membrane, and not its internalization, controls its spatiotemporal signaling.