Involvement of PKCα and G‐protein‐coupled receptor kinase 2 in agonist‐selective desensitization of µ‐opioid receptors in mature brain neurons

Background and purpose:  The ability of an agonist to induce desensitization of the µ‐opioid receptor (MOR) depends upon the agonist used. Furthermore, previous data suggest that the intracellular mechanisms underlying desensitization may be agonist‐specific. We investigated the mechanisms underlying MOR desensitization, in adult mammalian neurons, caused by morphine (a partial agonist in this system) and DAMGO (a high‐efficacy agonist). Experimental approach:  MOR function was measured in locus coeruleus neurons, by using whole‐cell patch‐clamp electrophysiology, in rat and mouse brain slices (both wild‐type and protein kinase C (PKC)α knockout mice). Specific isoforms of PKC were inhibited by using inhibitors of the receptors for activated C‐kinase (RACK), and in vivo viral‐mediated gene‐transfer was used to transfect neurons with dominant negative mutants (DNMs) of specific G‐protein‐coupled receptor kinases (GRKs). Key results:  Morphine‐induced desensitization was attenuated by using RACK inhibitors that inhibit PKCα, but not by other isoform‐specific inhibitors. Further, the PKC component of morphine‐induced desensitization was absent in locus coeruleus neurons from PKCα knockout mice. The PKC‐enhanced morphine‐induced desensitization was not affected by over‐expression of a GRK2 dominant negative mutant (GRK2 DNM). In contrast, DAMGO‐induced MOR desensitization was independent of PKC activity but was reduced by over‐expression of the GRK2 DNM but not by that of a GRK6 DNM. Conclusions and implications:  In mature mammalian neurons, different MOR agonists can induce MOR desensitization by different mechanisms, morphine by a PKCα‐mediated, heterologous mechanism and DAMGO by a GRK‐mediated, homologous mechanism. These data represent functional selectivity at the level of receptor desensitization.