The role of beta-arrestin2 in the severity of antinociceptive tolerance and physical dependence induced by different opioid pain therapeutics
Introduction
Opioid analgesics are commonly used to treat moderate to severe pain. Long-term administration, however, is associated with the development of undesirable side effects including analgesic tolerance and physical dependence. A number of studies have demonstrated that the physiological actions of morphine and other clinically used opiates are mediated primarily through activation of the mu opioid receptor (MOR), a G protein-coupled receptor (Matthes et al., 1996, Sora et al., 1997, Roy et al., 1998, Kieffer, 1999). While diverse signaling components and complex neuronal adaptations contribute to the development of analgesic tolerance and physical dependence, considerable evidence suggests that regulation of MOR signaling can affect these adaptive responses.
Previously, we have shown that βarrestin2, an important G protein-coupled receptor regulatory protein, differentially regulates opiate effects in a manner that is determined by the agonist. In βarrestin2 knockout (βarr2-KO) mice, acute morphine and heroin-induced antinociception is enhanced and prolonged, while acute antinociceptive responses to etorphine, methadone, and fentanyl are similar to those observed in their wild type (WT) counterparts (Bohn et al., 1999, Bohn et al., 2004). Further, βarr2-KO mice also do not develop antinociceptive tolerance in a hot plate test following treatment with a single high dose of morphine or after chronic treatment using either repeated injections of morphine for 9 days or implantation with a 75 mg morphine pellet for 3 days (Bohn et al., 2000, Bohn et al., 2002). However, the extent of physical dependence produced following a 72 h treatment with the 75 mg morphine pellet is equivalent between both WT and βarr2-KO mice (Bohn et al., 2000). These earlier studies demonstrate a complex role for βarrestin2 in the regulation of morphine-induced antinociceptive tolerance and physical dependence; however, its role in MOR regulation with other agonists following chronic administration has not been previously investigated.
In this study, we have evaluated the development of tolerance and dependence in WT and βarr2-KO mice following chronic opiate infusion using implanted osmotic pumps. Since patients suffering from moderate to severe pain are commonly treated with sustained released formulations of morphine (MS Contin®), fentanyl (Duragesic®), and oxycodone (Oxycontin®), a continuous drug infusion paradigm used in these studies may closely mimic the exposure to opiate drugs in a clinical setting. Furthermore, while prior studies suggest that βarrestin2 does not play a significant role in morphine-induced physical dependence or in the display of somatic withdrawal signs, morphine was administered using a 75 mg pellet implantation in those studies (Bohn et al., 2000). Since this high dosing regimen may have occluded βarrestin2 contributions to this adaptive response, the effects of lower doses of morphine, as well as several doses of methadone, fentanyl and oxycodone, are assessed in this current study.
Section snippets
Animals
Male WT and βarr2-KO mice were generated from heterozygous breeding as previously described (Bohn et al., 1999). Mice were age matched (3–6 months old), weighed between 20 and 35 g, were group housed in a temperature-controlled room, and were maintained on a 12 h reversed light/dark cycle. All behavioral studies were conducted during the light phase of the animal’s circadian cycle. Mice had free access to food and water prior to experiments. Both genotypes were tested in parallel and each mouse
Results
Antinociceptive tolerance in WT and βarr2-KO mice was evaluated using two different treatment paradigms. In the first paradigm, mice were assessed for their response latencies to a thermal hot plate stimulus every other day during 5 days of chronic agonist treatment. While the half-life of morphine is approximately 2 h in mice (Aceto et al., 1997), the half-lives of methadone, fentanyl, and oxycodone are much shorter at 60, 45 and 30 min respectively (Duttaroy and Yoburn, 1995, Kalvass et al.,
Discussion
In this study several different opioid analgesics were compared for their ability to induce antinociceptive tolerance and physical dependence in WT and βarr2-KO mice following chronic infusion via osmotic pumps. Under these dosing conditions, morphine produces robust tolerance in WT mice, while this effect is greatly attenuated in the βarr2-KO mice. At high doses of morphine both WT and βarr2-KO mice display equivalent somatic signs of withdrawal, while at lower doses, the βarr2-KO mice display
Acknowledgements
This work was supported by the National Institute on Drug Abuse grants DA021952 (K.M.R), DA14600 and DA18860 (L.M.B.)
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