CB2 cannabinoid receptor antagonist SR144528 decreases mu-opioid receptor expression and activation in mouse brainstem: Role of CB2 receptor in pain

Formerly considered as an exclusively peripheral receptor, it is now accepted that CB2 cannabinoid receptor is also present in limited amounts and distinct locations in the brain of several animal species, including mice. However, the possible roles of CB2 receptors in the brain need to be clarified. The aim of our work was to Study the mu-opioid receptor (MOR) mRNA expression level and functional activity after acute in vivo and in vitro treatments with the endocannabinoid noladin ether (NE) and with the CB2 receptor antagonist SR144528 in brainstem of mice deficient in either CB1 or CB2 receptors. This study is based Oil Our previous observations that noladin ether (NE) produces decrease in the activity of MOR in forebrain and this attenuation call be antagonized by the CB2 cannabinoid antagonist SR144528, suggesting a CB2 receptor mediated effect. We U sed quantitative real-time PCR to examine the changes of MOR mRNA levels, [S-35]GTP gamma S binding assay to analyze the capability of mu-opioid agonist DAMGO to activate G-proteins and competition binding assays to directly measure the ligand binding to MOR ill mice brainstem. After acute NE administration no significant changes were observed oil MOR signaling. Nevertheless pretreatment of mice with SR144528 prior to the administration of NE significantly decreased MOR signaling Suggesting the involvement of SR144528 in mediating the effect of MOR. mRNA expression of MORs significantly decreased both in CB1 wild-type and CB1 knockout mice after a single injection of SR144528 at 0.1 mg/kg when compared to the vehicle treated controls. Consequently, MOR-mediated signaling was attenuated after acute in Vivo treatment with SR 144528 in both CB1 wild-type and CB1 knockout mice. In vitro addition of 1 mu M SR144528 Caused a decrease in the maximal stimulation of DAMGO in [S-35]GTP gamma S binding assays in CB2 wild-type brainstem membranes whereas no significant changes were observed in CB2 receptor knockouts. Radioligand binding competition studies showed that the noticed effect of SR 144528 oil MOR signaling is not mediated through MORs. Our data demonstrate that the SR144528 caused pronounced decrease in the activity of MOR is mediated via CB2 cannabinoid receptors. (C) 2008 Elsevier Ltd. All rights reserved.