The effect of μ-opioid receptor polymorphisms on receptor signalling systems

There is significant variation in individual response to opioid drugs, one cause of which is likely to be polymorphisms on the opioid receptors themselves. The μ-opioid receptor (MOPr) is the primary site of action for most analgesic opioids. Previous studies have identified a number of naturally occurring single nucleotide polymorphisms (SNPs) in the coding region of MOPr. The A118G SNP (N40D), present at allelic frequencies ranging from 10 – 50%, has been associated with diverse phenotypic effects as well as differences in receptor signalling in vitro, with little consistency between studies. Few studies have examined the consequences of other MOPr polymorphisms on receptor function, or potential ligand and pathway specific effects. In this study, the relative potency and efficacy of a range of clinically important and/or structurally distinct opioid ligands were assessed in Chinese hamster ovary (CHO) cells and mouse pituitary neuroblastoma (AtT-20) cells stably transfected with human wild type MOPr and 5 naturally occurring MOPr variants, N40D, A6V, L85I, R260H and R265H. MOPr surface expression levels were similar for all variants examined. MOPr activation was measured using a membrane-potential assay of adenylyl cyclase (AC) inhibition, a whole-cell ELISA of extracellular signal-regulated kinase 1/2 (ERK1/2) phosphorylation, and a membrane-potential assay of G protein-activated potassium channels (GIRKs). In cells expressing MOPr-N40D, buprenorphine inhibition of AC and stimulation of ERK1/2 was significantly reduced, with GIRK activation unaffected. In cells expressing MOPR-C17T, buprenorphine signalling was abolished, with a loss of potency of morphine and other ligands. AC inhibition via non-morphinan opioids was enhanced at L85I, while a significant loss of potency for many opioids was observed at R260H. There were minor alterations in the signalling profile of R265H. These results suggest that MOPr SNPs have the potential to significantly alter receptor function, and may contribute to the individual variability in response to opioid analgesics observed clinically.