The effect of opioids on the signalling of the µ-opioid receptor and some variants

Opioid analgesics are the most effective medications used for the treatment of moderate to severe nociceptive pain. However, their clinical utility is limited by their adverse effects and prevalence of tolerance and dependence on chronic use. Moreover, the degree of pain relief varies in individuals and studies suggest genetic variation as one of the cause. In this project, we focus on comparing the effect of pethidine, O-desmethyl tramadol and TRV 130 on signalling of µ-opioid receptor (MOR) and some common polymorphisms of MOR (A118G and C17T). In addition, we sought to compare the efficacy of these opioids with enkephalin analog, DAMGO and commonly studied opioid-morphine in K channel activation assay in AtT20 transfected cells. FLIPR membrane potential assay is used to determine the change in membrane potential caused by different opioids. The highest concentration tested for pethidine and O-desmethyl tramadol showed a reduced inhibition of forskolin response in CHO cells expressing C17T as compared to WT (P<0.05) whereas no significant difference was observed in A118G. TRV 130 showed a significantly reduced inhibition of forskolin response (P<0.05) in the CHO cells expressing A118G and C17T than in WT. Pethidine, )-desmethyl tramadol and TRV 130 activated K channel in AtT20 hMOR cells with pEC50 of 5.7 ± 0.1, 6.3 ± 0.1, 8.1 ± 0.1 respectively, as compared to morphine (7.1 ± 0.1) and DAMGO (8.3 ± 0.1). Depleting MOR with an irreversible antagonist, ß-CNA, allowed us to calculate a measure of transduction efficiency (tau), which elates receptor occupancy to effect, forboth control and depleted cases for DAMGO (67.6, 7.1), morphine (11.1, 1.5) and TRV 130 (5.5, 0.6). demonstrating the lower efficacy of TRV 130 compared to morphine. All the opioids under study appeared to be lower efficacy agonist than morphine, and their efficacy may be further reduced at common MOR polymorphisms. It remains to be established whether the findings of this project, yields similar clinical effects.