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Inhibition of forebrain μ-opioid receptor signaling by low concentrations of rimonabant does not require cannabinoid receptors and directly involves μ-opioid receptors

Institute of Biochemistry, Biological Research Centre, Hungarian Academy of Sciences, Temesvari krt. 62, H-6726 Szeged, Hungary.
Neurochemistry International (Impact Factor: 2.65). 05/2012; 61(3):378-88. DOI: 10.1016/j.neuint.2012.05.015
Source: PubMed

ABSTRACT Increasing number of publications shows that cannabinoid receptor 1 (CB(1)) specific compounds might act in a CB(1) independent manner, including rimonabant, a potent CB(1) receptor antagonist. Opioids, cannabinoids and their receptors are well known for their overlapping pharmacological properties. We have previously reported a prominent decrease in μ-opioid receptor (MOR) activity when animals were acutely treated with the putative endocannabinoid noladin ether (NE). In this study, we clarified whether the decreased MOR activation caused by NE could be reversed by rimonabant in CB(1) receptor deficient mice. In functional [(35)S]GTPγS binding assays, we have elucidated that 0.1mg/kg of intraperitoneal (i.p.) rimonabant treatment prior to that of NE treatment caused further attenuation on the maximal stimulation of Tyr-d-Ala-Gly-(NMe)Phe-Gly-ol (DAMGO), which is a highly specific MOR agonist. Similar inhibitory effects were observed when rimonabant was injected i.p. alone and when it was directly applied to forebrain membranes. These findings are cannabinoid receptor independent as rimonabant caused inhibition in both CB(1) single knockout and CB(1)/CB(2) double knockout mice. In radioligand competition binding assays we highlighted that rimonabant fails to displace effectively [(3)H]DAMGO from MOR in low concentrations and is highly unspecific on the receptor at high concentrations in CB(1) knockout forebrain and in their wild-type controls. Surprisingly, docking computational studies showed a favorable binding position of rimonabant to the inactive conformational state of MOR, indicating that rimonabant might behave as an antagonist at MOR. These findings were confirmed by radioligand competition binding assays in Chinese hamster ovary cells stably transfected with MOR, where a higher affinity binding site was measured in the displacement of the tritiated opioid receptor antagonist naloxone. However, based on our in vivo data we suggest that other, yet unidentified mechanisms are additionally involved in the observed effects.

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Available from: Ferenc Zádor, Feb 21, 2014
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    • "For the in vivo assays we choose a dose of 0.1 mg/kg, to avoid its non-CB 1 mediated effects which occur upon chronic and/or high micromolar dose application. The duration time was chosen according to our previous findings where rimonabant altered MOR function 24 h after a single 0.1 mg/kg i.p. treatment (Z ador et al., 2012). We used CB 1 knockout mice to understand the impact of the CB 1 receptors in the observed effects, and we first followed the changes in KOR signaling and protein expression. "
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    ABSTRACT: There is an increasing number of studies demonstrating the direct effect of the cannabinoid receptor 1 (CB1) antagonist/inverse agonist rimonabant on the opioid system. The kappa opioid receptors (KORs) are well known to mediate depression- and anxiety-like behavior. Clinical studies on chronic rimonabant administration have revealed that rimonabant leads to a very similar pathophysiology, suggesting a potential impact of rimonabant on KORs.Objectives Our objectives were to examine the putative effects of rimonabant on KOR ligand binding, G-protein activity, protein expression and how all these contribute to the development of depression- and anxiety-like behavior.ResultsIn Chinese hamster ovary (CHO) cell membranes transfected with rat KOR (CHO-rKOR) rimonabant inhibited KOR agonist [3H]U69593 binding in the micromolar range in competition binding experiments and specifically reduced KOR basal activity at lower micromolar concentrations in [35S]GTPγS binding assays. Rimonabant significantly inhibited dynorphin (1–11)-induced [35S]GTPγS binding in micromolar range in CHO-rKOR cells, CB1 knockout (CB1 K.O.) and CB1/CB2 double knockout mouse forebrain membranes. A single dose of i.p. 0.1 mg/kg rimonabant significantly reduced dynorphin (1–11)-induced KOR G-protein activity and KOR protein expression levels 24 h following the administration in both wild type and CB1 K.O. mice forebrain. Furthermore, in elevated plus maze mice showed an anxiolytic-like effect upon rimonabant injection that could be reversed by 1 mg/kg KOR antagonist norbinaltorphimine. The anxiolytic-like effects were further confirmed with the light–dark box test.Conclusion Rimonabant reduced KOR ligand binding, receptor mediated G-protein activity and protein expression level, which overall leads to altered anxiety-like behavior.
    Neuropharmacology 10/2014; 89. DOI:10.1016/j.neuropharm.2014.10.008 · 4.82 Impact Factor
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    • "RIM, however, is thought to have multiple sites of action in the central nervous system (CNS). In particular, a recent report demonstrated that RIM also acts on m-opioid receptors at a concentration comparable to that effective at the CB 1 receptor (Zádor et al, 2012). To determine whether the m-opioid receptor is involved in the fearattenuating effect of the 5% krill diet, we administered the m-opioid antagonist NLX (3 mg/kg, subcutaneously). "
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    ABSTRACT: Though the underlying mechanism remains unknown, several studies have suggested benefits of n-3 long-chain polyunsaturated fatty acid (PUFA) for patients with anxiety disorders. Elevated fear is thought to contribute to the pathogenesis of particular anxiety disorders. The aim of the present study was to evaluate whether the dietary n-3 to n-6 PUFA (3/6) ratio influences fear memory. For this purpose, the effects of various dietary 3/6 ratios on fear memory were examined in mice using contextual fear conditioning, and the effects of these diets on central synaptic transmission were examined to elucidate the mechanism of action of PUFA. We found that fear memory correlated negatively with dietary, serum and brain 3/6 ratios in mice. The low fear memory in mice fed a high 3/6 ratio diet was increased by the cannabinoid CB1 receptor antagonist rimonabant, reaching a level seen in mice fed a low 3/6 ratio diet. The agonist-sensitivity of CB1 receptor was enhanced in the basolateral nucleus of the amygdala (BLA) of mice fed a high 3/6 ratio diet, compared with that of mice fed a low 3/6 ratio diet. Similar enhancement was induced by pharmacological expulsion of cholesterol in the neuronal membrane of brain slices from mice fed a low 3/6 ratio diet. CB1 receptor-mediated short-term synaptic plasticity was facilitated in pyramidal neurons of the BLA in mice fed a high 3/6 ratio diet. These results suggest that the ratio of n-3 to n-6 PUFA is a factor regulating fear memory via cannabinoid CB1 receptors.Neuropsychopharmacology accepted article preview online, 12 February 2014; doi:10.1038/npp.2014.32.
    Neuropsychopharmacology: official publication of the American College of Neuropsychopharmacology 02/2014; DOI:10.1038/npp.2014.32 · 7.83 Impact Factor
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    • "Pharmacology endothelial cells (Sugiura et al., 1998; Liu et al., 2000; Rajesh et al., 2007; 2008a). Cannabinoids, endocannabinoids (endogenous cannabinoids) and related endocannabinoidlike compounds also interact with other receptors including an uncloned GPCR located on the endothelium (CBe, Jarai et al., 1999), the orphan receptor GPR55 (Ryberg et al., 2007; Lauckner et al., 2008), the orphan receptor GPR119 (Overton et al., 2006), transient receptor potential (TRP) channels (Zygmunt et al., 1999; Jordt et al., 2004; De Petrocellis et al., 2007; Qin et al., 2008; Alexander et al., 2013b), PPARα,β,γ (reviewed in O'Sullivan, 2007; Alexander et al., 2013c), opioid receptors (Seely et al., 2012; Zador et al., 2012), adrenoceptors (Cascio et al., 2010) and 5-HT receptors (Russo et al., 2005). However, the pharmacological profiling of these compounds is complicated, as outlined by Alexander and Kendall (2007), and the effects of these ligands can vary according to cell/ tissue type, whether the receptor is native or overexpressed, whether allosteric modulators are present, and can display agonist bias at target sites. "
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    ABSTRACT: Application of cannabinoids and endocannabinoids to perfused vascular beds or individual isolated arteries results in changes in vascular resistance. In most cases, the result is vasorelaxation, although vasoconstrictor responses are also observed. Cannabinoids also modulate the actions of vasoactive compounds including acetylcholine, methoxamine, angiotensin II and U46619 (thromboxane mimetic). Numerous mechanisms of action have been proposed including receptor activation, potassium channel activation, calcium channel inhibition and the production of vasoactive mediators such as calcitonin gene-related peptide, prostanoids, nitric oxide, endothelial-derived hyperpolarising factor and hydrogen peroxide. The purpose of this review is to examine the evidence for the range of receptors now known to be activated by cannabinoids. Direct activation by cannabinoids of CB1 , CBe , TRPV1 (and potentially other TRP channels) and PPARs in the vasculature has been observed. A potential role for CB2, GPR55 and 5HT1A has also been identified in some studies. Indirectly, activation of prostanoid receptors (TP, IP, EP1 and EP4 ) and the CGRP receptor are involved in the vascular responses to cannabinoids. The majority of this evidence has been obtained through animal research, but recent work has confirmed some of these targets in human arteries. Vascular responses to cannabinoids are enhanced in hypertension and cirrhosis, but are reduced in obesity and diabetes, both due to changes in the target sites of action. Much further work is required to establish the extent of vascular actions of cannabinoids and the application of this research in physiological and pathophysiological situations.
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