How important is protein kinase C in μ-opioid receptor desensitization and morphine tolerance?

Morphine is one of the most potent analgesic drugs. However, the utility of morphine in the management of chronic pain is limited by its rapid development of tolerance. Morphine exerts all of its pharmacological effects via the -opioid receptor. In many systems, tolerance is associated with phosphorylation and desensitization of G-protein-coupled receptors (GPCRs). In case of the -opioid receptor, phosphorylation occurs in an agonist-selective manner. High-efficacy agonists such as [D-Ala 2 -MePhe 4 -Gly-ol]enkephalin (DAMGO), fentanyl, or etonitazene stimulate the phosphorylation of both C-terminal threonine 370 (T370) and serine 375 (S375). In contrast, morphine promotes the phosphorylation of S375 but fails to stimulate T370 phosphorylation. Here, we have assessed the contribution of S375 phosphorylation to the development of antinociceptive tolerance to high-and low-efficacy agonists in vivo. We show that S375 phosphorylation of the -opioid receptor occurs in intact mouse brain in a dose-dependent manner after administration of morphine, fentanyl, or etonitazene. In knock-in mice expressing the phosphorylation-deficient S375A mutant of the -opioid receptor, morphine and fentanyl exhibited greater dose-dependent antinociceptive responses than in wild-type mice. However, acute and chronic tolerance to morphine was retained in S375A mutant mice. In contrast, antinociceptive tolerance after repeated subcutaneous application of etonitazene or repeated intracerebroventricular application of DAMGO was diminished. Thus, tolerance to agonists with different efficacies develops through distinct pathways. Whereas tolerance induced by DAMGO or etonitazene requires agonist-driven phosphorylation of S375, the development and maintenance of antinociceptive tolerance to morphine occurs independent of S375 phosphorylation.

Section: Discussionmentioning

confidence: 99%

“…However, the molecular mechanisms of morphine tolerance are far from being understood (Nestler, 1996;Koch et al, 2005;Bailey et al, 2006;Martini and Whistler, 2007). For many clinically used drugs, tolerance has been associated with phosphorylation, desensitization and downregulation of their G-protein-coupled receptors (GPCRs).…”

Section: Introductionmentioning

confidence: 99%

Analgesic Tolerance to High-Efficacy Agonists But Not to Morphine Is Diminished in Phosphorylation-Deficient S375A μ-Opioid Receptor Knock-In Mice

Grecksch¹,

Just²,

Pierstorff³

et al. 2011

“…Depending on the experimental conditions, MOR desensitization can be modulated by protein kinases such as PKC and PI3K (Tan et al, 2003;Bailey et al, 2006). However, these studies showed that blocking PKC or PI3K individually failed to affect acute desensitization mediated by endogenous opioid peptides.…”

Section: Erk1/2 Mediates Acute Mor Desensitization In Lc Neurons Of ␤mentioning

confidence: 99%

“…mice. LC neurons homogenously express MOR but no other opioid receptors, making them useful for studying MOR regulation in native neurons (Connor et al, 2004;Bailey et al, 2006). In LC neurons, MOR activates GIRK channels via a rapid, membrane delimited interaction with G␤␥ subunits, providing a sensitive and direct assay of receptor function and desensitization when studied with appropriate agonist concentrations (Connor et al, 2004).…”

Section: Introductionmentioning

confidence: 99%

Two Distinct Mechanisms Mediate Acute μ-Opioid Receptor Desensitization in Native Neurons

Dang

Napier²,

Christie³

2009

Sustained stimulation of G-protein coupled receptors (GPCRs) leads to rapid loss of receptor function (acute desensitization). For manyGPCRs including the -opioid receptor (MOR), an accepted mechanism for acute desensitization is through G-protein coupled receptor kinase (GRKs) mediated phosphorylation of the receptor, which facilitates the binding of ␤-arrestins (␤arrs) to the receptor and then promotes endocytosis. However, the mechanism(s) that mediate acute desensitization have not yet been well defined in native neurons. This study used whole-cell patch clamp recording of G-protein coupled inward-rectifying potassium (GIRK) currents to assay MOR function and identify mechanisms of acute MOR desensitization in locus ceruleus (LC) neurons. The rate and extent of MOR desensitization were unaffected by ␤arr-2 knock-out. Disruption of GRK2 function via inhibitory peptide introduced directly into neurons also failed to affect desensitization in wild type or ␤arr-2 knock-outs. Inhibition of ERK1/2 activation alone had little effect on acute desensitization. However, when both GRK2-␤arr-2 and ERK1/2 functions were disrupted simultaneously, desensitization of MOR was nearly abolished. Together, these results suggest that acute desensitization of MOR in native LC neurons is determined by at least two molecular pathways, one involving GRK2 and ␤arr-2, and a parallel pathway mediated by activated ERK1/2.

“…Since previous studies showed the involvement of PKC isoforms in tolerance to morphine (Bilsky et al, 1996;Bailey et al, 2006), we next examined the activation of PKC following morphine administration. Among PKC isoforms, PKC␣ and in the dorsal root ganglion, as well as PKC␣ and ␥ in the spinal cord, were known to take part in morphine-induced tolerance (Smith et al, 2007).…”

Section: Morphine-induced Translocation Of Pkc Was Attenuated In Nox1mentioning

confidence: 99%

Involvement of NOX1/NADPH Oxidase in Morphine-Induced Analgesia and Tolerance

Ibi¹,

Matsuno²,

Matsumoto³

et al. 2011