Recovery from μ-Opioid Receptor Desensitization after Chronic Treatment with Morphine and Methadone

Abstract

Chronic treatment with morphine results in a decrease in mu-opioid receptor sensitivity, an increase in acute desensitization and a reduction in the recovery from acute desensitization in locus coeruleus neurons. With acute administration, morphine is unlike many other opioid agonists in that it does not mediate robust acute desensitization or induce receptor trafficking. This study compares mu-opioid receptor desensitization and trafficking in brain slices taken from rats treated for 6–7 days with a range of doses of morphine (60, 30, 15 mg/kg/day) and methadone (60, 30, 5 mg/kg/day) applied by subcutaneous implantation of osmotic mini pumps. Mice were treated with 45 mg/kg/day. In morphine treated animals, recovery from acute [Met]5enkephalin-induced desensitization and receptor recycling was diminished. In contrast, recovery and recycling were unchanged in slices from methadone treated animals. Remarkably the reduced recovery from desensitization and receptor recycling found in slices from morphine treated animals were not observed in animals lacking β-arrestin2. Further, pharmacological inhibition of GRK2, while not affecting the ability of [Met]5enkephalin to induce desensitization, acutely reversed the delay in recovery from desensitization produced by chronic morphine treatment. These results characterize a previously unidentified function of the GRK/arrestin system in mediating opioid regulation in response to chronic morphine administration. They also suggest that the GRK/arrestin system, rather then serving as a primary mediator of acute desensitization, controls recovery from desensitization by regulating receptor reinsertion to the plasma membrane after chronic treatment with morphine. The sustained GRK/arrestin dependent desensitization is another way in which morphine and methadone are distinguished.

Evidence that Behavioral Phenotypes of Morphine in β-arr2−/− Mice Are Due to the Unmasking of JNK Signaling

“…Such constitutive activity also explains the reduced VDCC inhibition by mu agonists in b-arr2À/À neurons (Table 1; Figure 5; Lam et al, 2011;Walwyn et al, 2007). The well-known attenuated morphine desensitization and tolerance of the mu receptor in b-arr2À/À mice may result from b-arrestin 2 inhibition of mu receptor resensitization, independent of receptor internalization (Dang et al, 2011;Quillinan et al, 2011). This could explain the lack of desensitization of DAMGO-VDCC inhibition following chronic morphine treatments in b-arr2À/À neurons seen in Figure 5.…”

“…As morphine does not induce significant internalization of the mu opioid receptor, the effects of morphine in b-arr2À/À mice cannot be explained by this prototypical role of barrestin 2. However, some progress has recently been made in revealing several arrestin-dependent, yet internalizationindependent, signaling pathways of the mu opioid receptor that explain some, but not all, effects of morphine in mice lacking b-arrestin 2 (Arttamangkul et al, 2008;Dang et al, 2011;Quillinan et al, 2011;Walwyn et al, 2007).…”

Desensitization and Trafficking of μ-Opioid Receptors in Locus Ceruleus Neurons: Modulation by Kinases

“…Adult, male, SpragueDawley rats (150-250 g; Charles River Laboratories, Wilmington, MA), FlagMOPR-transgenic mice (Arttamangkul et al, 2008), and transgenic mice with a GRK isoform (GRK2as5) that is selectively inhibited by the nucleotide analog NaPP1 (Quillinan et al, 2011) were used for electrophysiological experiments. Brain slices were prepared as described previously (Williams et al, 1984).…”

“…Experiments examining the role of arrestin 3 used knockout (ArrKO) animals that were initially crossed with FlagMOPR-transgenic animals. In a series of subsequent crosses, animals that were hemizygous FlagMOPR-transgenic and homozygous ArrKO(Ϫ/Ϫ) were obtained (Arttamangkul et al, 2008;Quillinan et al, 2011). Brain slices (200 m) were prepared as described for electrophysiological experiments.…”

“…The activation of protein kinase C was shown to facilitate the desensitization induced by morphine but not DAMGO, whereas inhibition of GRK blocked the desensitization induced by DAMGO but not morphine (Bailey et al, 2004; reviewed by Kelly et al, 2008). Other studies found that inhibition of GRK2 activity in locus ceruleus (LC) neurons had no effect on the acute desensitization induced by [Met 5 ]enkephalin (ME) Quillinan et al, 2011). Dang et al (2009) found that blockers of both GRK2 and extracellular signal-regulated kinases 1 and 2 were required to reduce desensitization induced by ME.…”