Noladin ether, a putative endocannabinoid, inhibits μ-opioid receptor activation via CB2 cannabinoid receptors

We examined the occurrence of possible changes in mRNA expression and the functional activity of opioid receptors after acute in vivo and in vitro treatment with the putative endogenous cannabinoid noladin ether. While noladin ether (NE) demonstrates agonist activity at CB1 cannabinoid receptors, recent data indicate that NE acts as a full agonist at CB2 cannabinoid receptors too. Considering the functional interactions between opioids and cannabinoids, it is of interest to examine whether NE affects the opioid system. To that end, we studied the influence of NE on μ-opioid receptor (MOR) mRNA expression and MOR mediated G-protein signaling. We used real-time PCR and [35S]GTPγS binding assays to examine the changes of MOR mRNA levels and the capability of the μ-opioid agonist peptide ([d-Ala2,NMePhe4,Gly5-ol]enkephalin (DAMGO) in activating regulatory G-proteins via MORs in forebrain membrane fractions of wild-type (w.t., CB1+/+) and CB1 receptor deficient transgenic mice (knockout, CB1−/−). We found, that the expression of MOR mRNAs significantly decreased both in CB1+/+ and CB1−/− forebrain after a single injection of NE at 1 mg/kg when compared to control. Consequently, MOR-mediated signaling is attenuated after acute in vivo treatment with NE in both CB1+/+ and CB1−/− mice. Inhibition on MOR mediated activation is observed after in vitro NE administration as well. Radioligand binding competition studies showed that the noticed effect of NE on MOR signaling is not mediated through MORs. Both in vivo and in vitro attenuations of NE can be antagonized by the CB2 selective antagonist SR144528. Taken together, our data suggest that the NE caused pronounced decrease in the activity of MOR is mediated via CB2 cannabinoid receptors.