Functional independence of endogenous mu- and delta-opioid receptors co-expressed in cholinergic interneurons

Abstract

Class A G protein-coupled receptors (GPCRs) normally function as monomers, although evidence from heterologous expression systems suggests they may form homodimers and/or heterodimers. Detection of GPCR dimers in native tissues has however been challenging due to the lack of suitable tools. - and {delta}-Opioid receptors (MORs and DORs) co-expressed in transfected cells has been reported to form heterodimers. The co-localization of MORs and DORs in neurons has been studied in knock-in mice expressing genetically engineered receptors fused to fluorescent proteins. Here we report that single cholinergic neurons in the mouse striatum endogenously express both MORs and DORs. The receptors were fluorescently labeled in live brain slices with a ligand-directed labeling reagent, NAI-A594. The selective activation of MORs and DORs, with DAMGO (-agonist) and deltorphin ({delta}-agonist) inhibited spontaneous firing in all cells examined. In the continued presence of agonist, the firing rate returned to baseline with the application of deltorphin but was persistently inhibited with the application of DAMGO. In addition, agonist-induced internalization of DORs but not MORs was detected. When MORs and DORs were activated simultaneously with [Met5]enkephalin, desensitization of MORs was facilitated but internalization was not increased. Together, these results indicate that while MORs and DORs are expressed in single striatal cholinergic interneurons, the two receptors function independently.