AM-251 and rimonabant act as direct antagonists at mu-opioid receptors: Implications for opioid/cannabinoid interaction studies

Abstract

Mu-opioid and CB1-cannabinoid agonists produce analgesia; however, adverse effects limit use of drugs in both classes. Additive or synergistic effects resulting from concurrent administration of low doses of mu- and CB1-agonists may produce analgesia with fewer side effects. Synergism potentially results from interaction between mu-opioid receptors (MORs) and CB1 receptors (CB1Rs). AM-251 and rimonabant are CB1R antagonist/inverse agonists employed to validate opioid-cannabinoid interactions, presumed to act selectively at CB1Rs. Therefore, the potential for direct action of these antagonists at MORs is rarely considered. This study determined if AM-251 and/or rimonabant directly bind and modulate the function of MORs. Surprisingly, AM-251 and rimonabant, but not a third CB1R inverse agonist AM-281, bind with mid-nanomolar affinity to human MORs with a rank order of affinity (Ki) of AM-251 (251 nM) > rimonabant (652 nM) > AM281 (2135 nM). AM-251 and rimonabant, but not AM-281, also competitively antagonize morphine induced G-protein activation in CHO-hMOR cell homogenates (Kb = 719 or 1310 nM, respectively). AM-251 and rimonabant block morphine inhibition of cAMP production, while only AM-251 elicits cAMP rebound in CHO-hMOR cells chronically exposed to morphine. AM-251 and rimonabant (10 mg/kg) attenuate morphine analgesia, whereas the same dose of AM-281 produces little effect. Therefore, in addition to high CB1R affinity, AM-251 and rimonabant bind to MORs with mid-nanomolar affinity and at higher doses may affect morphine analgesia via direct antagonism at MORs. Such CB1-independent actions of these antagonists may contribute to reported inconsistencies when CB1/MOR interactions are examined via pharmacological methods in CB1-knockout versus wild-type mice.

Synergistic antinociceptive interaction between palmitoylethanolamide and tramadol in the mouse formalin test

“…Many studies have shown this synergistic antinociceptive interaction between cannabinoid and opioid systems (Manzanares et al, 1998(Manzanares et al, , 1999Cichewicz 2004;Cox et al, 2007;Seely et al, 2012). However, the use of cannabis is frequently underreported in our society.…”

Effects of centrally administered endocannabinoids and opioids on orofacial pain perception in rats

“…The cannabinoid system comprises the two major cannabinoid receptors, CB 1 and CB 2 receptors, their endogenous ligands, anandamide (AEA) and 2-arachidonoylglycerol (2-AG) and enzymes, fatty acid amide hydrolase (FAAH) and monoacylglycerol lipase (MAGL), which regulate their biosynthesis and degradation (Di Marzo et al, 2004). Simultaneous activation of opioid and cannabinoid receptors can produce synergistic analgesic effects (Smith et al, 1998;Seely et al, 2012). Opioids and cannabinoids produce antinociception through separate (although possibly interrelated) mechanisms (Cichewicz, 2004;Tham et al, 2005;Roberts et al, 2006).…”

Endocannabinoids, through opioids and prostaglandins, contribute to fever induced by key pyrogenic mediators

“…In addition, in vitro studies have shown that opioid receptor expression is induced by cannabinoids in Jurkat E6.1 cells via a CB2-dependent mechanism (Borner et al, 2006). Seely et al, (2012) showed that, differently from AM281, a selective CB1 receptor antagonist, AM251 could also acts on µ-opioid receptors. However, the present study clearly showed that AM251 reduces the β-endorphin production induced by AEA and naloxone abolished the fever induced by this cannabinoid suggesting that the anti-opioid effect of AM251 is also related to the endogenous opioids synthesis inhibition.…”