Mu-opioid receptor activation underpins clinical analgesia and is the central event in the abuse of narcotics. Continued opioid use produces tolerance to the acute effects of the drug and adaptations that lead to physical and psychological dependence. Continued mu-receptor signaling provides the engine for these adaptations, with most evidence suggesting that chronic agonist treatment produces only limited alterations in primary mu-opioid receptor signaling. Here we examine agonist regulation of mu-opioid receptor function, and whether this is altered by chronic treatment. Receptor phosphorylation is thought to be the key initial event in agonist regulation of the mu-opioid receptor, providing a signal for acute receptor desensitization and also subsequent receptor resensitization. Morphine appears to produce qualitatively and quantitatively different mu-receptor phosphorylation than other agonists, but the consequences of this remain obscure, at least in neurons. There is no evidence that agonist-induced mu-opioid receptor phosphorylation changes in chronically morphine-treated animals, although receptor regulation appears to be altered. Thus, as receptor phosphorylation and resensitization appear to maintain continued signaling through the mu-opioid receptor, these two events are crucial in facilitating adaptations to chronic opioid treatment, and the possibility that agonist-specific phosphorylation can contribute to the development of different adaptations remains open.
"Using a [ 32 P]orthophosphate metabolic pre-labelling approach, in conjunction with site-directed mutagenesis, previous studies have identified amino acids in MOPr that are phosphorylated under basal conditions or in response to opioid agonist (Deng et al. 2000; El Kouhen et al. 2001; Wang et al. 2002, 2007; Schulz et al. 2004; Ozsoy et al. 2005; Chu et al. 2008). However, there remains disagreement as to the identity of phosphorylated residues, as well as the role of such residues in the regulation of MOPr (Pak et al. 1997; Burd et al.1998; Deng et al. 2000; El Kouhen et al. 2001; Wang et al. 2002, 2007; Schulz et al. 2004; Johnson et al. 2005; Ozsoy et al. 2005; Chu et al. 2008). This may in part be because of the previous reliance on the Received August 10, 2012; revised manuscript received October 10, 2012; accepted October 18, 2012. "
"Interestingly, morphine exposure generally results in less phosphorylation of the MOR than other, more efficacious agonists, such as DAMGO. This may be due to specific agonist-induced phosphorylation at different residues (Johnson et al., 2005). Therefore, weak phosphorylation-inducing agonists, such as morphine, might desensitize the receptor via pathways other than those involving GRK and β-arrestins (Chu et al., 2010; Zheng et al., 2010). "
[Show abstract][Hide abstract] ABSTRACT: The abuse of intravenous drugs, such as heroin, has become a major public health concern due to the increased risk of HIV-1 infection. Opioids such as heroin were originally identified and subsequently abused for their analgesic effects. However, many investigations have found additional effects of opioids, including regulation of the immune system. As such, chronic opioid abuse has been shown to promote HIV-1 pathogenesis and facilitate HIV-1-associated neurocognitive dysfunction. Clinical opioids, such as morphine and methadone, as well as illicit opioids, such as heroin, exert their effects primarily through interactions with the µ-opioid receptor (MOR). However, the mechanisms by which opioids enhance neurocognitive dysfunction through MOR-mediated signaling pathways are not completely understood. New findings in the regulation of MOR expression, particularly epigenetic and transcriptional regulation as well as alternative splicing, sheds new insights into possible mechanisms of HIV-1 and opiate synergy. In this review, we identify mechanisms regulating MOR expression and propose novel mechanisms by which opioids and HIV-1 may modulate this regulation. Additionally, we suggest that differential regulation of newly identified MOR isoforms by opioids and HIV-1 has functional consequence in enhancing HIV-1 neurocognitive dysfunction.
"The mechanism behind this AC adaptation is not known, but kinase enzymes such as Raf-1 and PKC that could phosphorylate AC have been implicated. Numerous studies have shown the m-opioid receptor itself is a target for phosphorylation leading to desensitization (Johnson et al., 2005). Additionally, phosphorylation of the receptor has been implicated in AC sensitization (Wang et al., 1994; 1999; Sadee and Wang, 1995). "
[Show abstract][Hide abstract] ABSTRACT: Adenylyl cyclase sensitization occurs on chronic agonist activation of mu-opioid receptors and is manifested by an increase in cAMP levels (overshoot) on challenge with antagonist. It has been proposed that a long lasting constitutively active receptor is formed on chronic mu-opioid exposure and that antagonists with inverse agonist activity rapidly return the receptor to a basal state causing a cAMP overshoot and a more severe withdrawal response in vivo. This hypothesis depends on an accurate characterization of neutral and inverse agonist properties of opioid antagonists.
C6 glioma and HEK293 cells expressing mu-opioid receptors were used. Opioid antagonists were examined for their ability to induce a cAMP overshoot following chronic treatment with the agonist DAMGO ([D-Ala(2),N-Me-Phe(4),Glyol(5)]-enkephalin). The compounds were also characterized as agonists, inverse agonists or neutral antagonists by using assays for competitive binding, [(35)S]GTPgammaS (guanosine-5'-O-(3-[(35)S]thio)triphosphate) binding and changes in cell surface receptor expression.
Naltrexone, 6beta-naltrexol and naloxone were indistinguishable to the mu-opioid receptor in the opioid-naïve or dependent state and acted as neutral antagonists. The delta-opioid receptor inverse agonist RTI-5989-25 [(+)-N-[trans-4'-(2-methylphenyl)-2'-butenyl]-(3R,4R)-dimethyl-4-(3-hydroxyphenyl)piperidine], a 3,4-dimethyl-4-(3-hydroxyphenyl)-piperidine, was an inverse agonist at the mu-opioid receptor, and the peptide antagonist CTAP (H-D-Phe-Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH(2)) showed variable, assay-dependent properties. All the antagonists precipitated the same degree of cAMP overshoot in opioid-dependent cells.
Antagonists at the mu-opioid receptor may be neutral or show inverse agonist activity. Formation of a constitutively active mu-opioid receptor is not a requirement for the development or expression of adenylyl cyclase sensitization.
British Journal of Pharmacology 03/2009; 156(7):1044-53. DOI:10.1111/j.1476-5381.2008.00035.x · 4.84 Impact Factor
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