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

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• 作者：Ferenc Z¨¢dor ; Ferenc ?tv?s ; S¨¢ndor Benyhe ; Andreas Zimmer ; Eszter P¨¢ldy
• 关键词：Rimonabant ; CB1 cannabinoid receptor ; ¦Ì-Opioid receptor ; [35S]GTP¦ÃS binding ; Radioligand binding ; Docking ; CB1 receptor knockout mouse ; CB1/CB2 receptor double knockout mouse ; Forebrain
• 刊名：Neurochemistry International
• 出版年：2012
• 出版时间：August, 2012
• 年：2012
• 卷：61
• 期：3
• 页码：378-388
• 全文大小：899 K

文摘

Increasing number of publications shows that cannabinoid receptor 1 (CB₁) specific compounds might act in a CB₁ independent manner, including rimonabant, a potent CB₁ receptor antagonist. Opioids, cannabinoids and their receptors are well known for their overlapping pharmacological properties. We have previously reported a prominent decrease in ¦Ì-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 CB₁ receptor deficient mice. In functional [³⁵S]GTP¦ÃS binding assays, we have elucidated that 0.1 mg/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 CB₁ single knockout and CB₁/CB₂ double knockout mice. In radioligand competition binding assays we highlighted that rimonabant fails to displace effectively [³H]DAMGO from MOR in low concentrations and is highly unspecific on the receptor at high concentrations in CB₁ 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.