Two distinct forms of desensitization of G-protein coupled inwardly rectifying potassium currents evoked by alkaloid and peptide mu-opioid receptor agonists

Abstract

Mu-opioid receptors (MORs) activate G-protein coupled inwardly rectifying potassium (GIRK) channels. The peptide agonist [D-Ala(2), NMe-Phe(4), Gly(5)-ol]enkephalin (DAMGO), but not the alkaloid morphine (MS), leads to acute desensitization of this response. Furthermore, DAMGO, as opposed to MS, triggers rapid internalization of MORs. Given this dichotomy, we probed the relationship between receptor internalization and GIRK current desensitization in neurons of the locus coeruleus (LC) using acute rat brain slices. Interfering with MOR recycling by selective impairment of dynamin-dependent endocytosis left GIRK current desensitization unchanged. Conversely, coapplication of MS with a low concentration of DAMGO, a cocktail reported to enhance MOR internalization, revealed competition between the two agonists and normal desensitization. We also examined the case of methadone (MD), an alkaloid that has been reported to strongly trigger endocytosis. Interestingly, MD and other alkaloids agonists did induce GIRK current desensitization, but only at suprasaturating concentrations. Furthermore, responses to alkaloids were blunted in LC neurons and the same agonists inhibited GIRK currents in cells that do not express MORs. Our results indicate that two distinct forms of MOR-evoked GIRK current desensitization exist. Peptide agonists induce receptor-mediated desensitization while alkaloid agonists lead to apparent desensitization by receptor-independent inhibition of GIRK channels.