Involvement of PKC alpha and G-protein-coupled receptor kinase 2 in agonist-selective desensitization of mu-opioid receptors in mature brain neurons.
OBJECTIVE
The ability of an agonist to induce desensitization of the mu-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).


METHODS
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)alpha 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).


RESULTS
Morphine-induced desensitization was attenuated by using RACK inhibitors that inhibit PKCalpha, but not by other isoform-specific inhibitors. Further, the PKC component of morphine-induced desensitization was absent in locus coeruleus neurons from PKCalpha 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
In mature mammalian neurons, different MOR agonists can induce MOR desensitization by different mechanisms, morphine by a PKCalpha-mediated, heterologous mechanism and DAMGO by a GRK-mediated, homologous mechanism. These data represent functional selectivity at the level of receptor desensitization.
Bailey
C P
CP
Department of Pharmacy & Pharmacology, University of Bath, Bath, UK. c.p.bailey@bath.ac.uk
Oldfield
S
S
Llorente
J
J
Caunt
C J
CJ
Teschemacher
A G
AG
Roberts
L
L
McArdle
C A
CA
Smith
F L
FL
Dewey
W L
WL
Kelly
E
E
Henderson
G
G
D000367:Age Factors; D000818:Animals; D001921:Brain / Q000187:drug effects / Q000201:enzymology*; D020875:Enkephalin, Ala(2)-MePhe(4)-Gly(5)- / Q000494:pharmacology; D005260:Female; D054769:G-Protein-Coupled Receptor Kinase 2 / Q000037:antagonists & inhibitors / Q000502:physiology*; D008297:Male; D051379:Mice; D018345:Mice, Knockout; D009474:Neurons / Q000187:drug effects / Q000201:enzymology*; D051571:Protein Kinase C-alpha / Q000037:antagonists & inhibitors / Q000502:physiology*; D047428:Protein Kinase Inhibitors / Q000494:pharmacology; D051381:Rats; D017208:Rats, Wistar; D017450:Receptors, Opioid, mu / Q000819:agonists* / Q000502:physiology*
D047428:Protein Kinase Inhibitors; D017450:Receptors, Opioid, mu; D020875:Enkephalin, Ala(2)-MePhe(4)-Gly(5)-; D051571:Protein Kinase C-alpha; D054769:G-Protein-Coupled Receptor Kinase 2