p38 MAPK and β-Arrestin 2 Mediate Functional Interactions between Endogenous μ-Opioid and α2A-Adrenergic Receptors in Neurons

Formation of receptor complexes between mu-opioid and alpha(2A)-adrenergic receptors has been demonstrated in transfected cells. The functional significance and underlying mechanisms of such receptor interactions remain to be determined in neuronal systems. We examined functional interactions between endogenous mu and alpha(2A) receptors in mouse dorsal root ganglion neurons. Acute application of the mu agonist [D-Ala(2), N-MePhe(4), Gly-ol(5)] enkephalin (DAMGO) or the alpha(2) agonist clonidine inhibited voltage-gated Ca2+ currents in these neurons. Prolonged treatment with either DAMGO or clonidine induced a mutual cross-desensitization between mu and alpha(2A) receptor-mediated current inhibition. The cross-desensitization was closely associated with simultaneous internalization of mu and alpha(2A) receptors. Morphine, a mu agonist triggering little mu receptor endocytosis, induced neither cross-desensitization nor internalization of alpha(2A) receptors. Furthermore, inhibition of p38 MAPK prevented the cross-desensitization as well as cointernalization of mu and alpha(2A) receptors. Changes in receptor trafficking profiles suggested that p38 MAPK activity was required for initiating mu receptor internalization and maintaining possible mu-alpha(2A) association during their cointernalization. Finally, the mu-alpha(2A) cross-desensitization was absent in dorsal root ganglion neurons lacking beta-arrestin 2. These findings demonstrated p38 MAPK- and beta-arrestin 2-dependent cross-regulation between neuronal mu and alpha(2A) receptors. By promoting receptor cross-desensitization and cointernalization, such functional interactions may serve as negative feedback mechanisms triggered by prolonged agonist exposure to modulate the signaling of functionally related G protein-coupled receptors.