Effect of the A118G polymorphism on binding affinity, potency and agonist‐mediated endocytosis, desensitization, and resensitization of the human mu‐opioid receptor

Abstract

The most prevalent single-nucleotide polymorphism (SNP) A118G in the human l-opioid receptor gene predicts an amino acid change from an asparagine residue to an aspartatic residue in amino acid position 40. This N40D mutation, which has been implicated in the development of opioid addiction, was previously reported to result in an increased b-endorphin binding affinity and a decreased potency of morphine-6-glucuronide. Therefore, in the present study we have investigated whether this mutation might affect the binding affinity, potency, and/or the agonist-induced desensitization, internalization and resensitization of the human l-opioid receptor stably expressed in human embryonic kidney 293 cells. With the exception of a reduced expression level of N40D compared to human l-opioid receptor (hMOR) in HEK293 cells, our analyses revealed no marked functional differences to morphine or morphine-6-glucuronide. After prolonged treatment with morphine, morphine-6-glucuronide or b-endorphin both receptors showed similiar desensitization time courses. In addition, the receptor resensitization rates were nearly identical for both receptor types.

Genetic variability of the μ-opioid receptor influences intrathecal fentanyl analgesia requirements in laboring women ☆

“…While an increased binding affinity for β-endorphins was reported by Bond et al [4], this was not the case in their study for smaller endogenous opioid peptides or μ-preferring opioid agonists, or for the opioid antagonist, naloxone, nor was this finding replicated by others [2,3] or by the same group recently [26]. In fact, taken together, in vitro studies tend to suggest that the 304A/ G polymorphism is more likely to affect μOR function via alterations in expression, transduction systems or receptor trafficking rather than via altered receptor binding affinity [32].…”

“…An adenine to guanine substitution in gene OPRM1 1 that results in an asparagine residue replacing an aspartate 2 has been reported to occur at a minor allelic frequency of f(−) 0.10-0.30 [4,20,28]. There is major interest in this polymorphism due to its potential pharmacological [4] and physiological consequences [3,9,21,46]. In vitro, Bond et al determined that the presence of at least one 304G (called G118 in that study) allele increases binding affinity and potency of β-endorphin [4].…”

Altered Levels of Basal Cortisol in Healthy Subjects with a 118G Allele in Exon 1 of the Mu Opioid Receptor Gene

“…Further, a recent report on human receptors harvested from post-mortem brain tissue has found that some of the functional effects seen by Bond et al may be related to a decrease in mu opioid receptor mRNA expression in subjects with a variant 118G allele (Zhang et al, 2005). Other studies have also identified reduced receptor expression in transfected cell lines of receptors encoded by the 118G allele, a finding also noted in our ongoing studies (Beyer et al, 2004;Kroslak et al, 2003). The mu opioid receptor system is also involved, through tonic inhibition, in modulation of the stress responsive HPA axis and normal volunteers with a 118G allele have an increased HPA response (measured through levels of plasma cortisol) following antagonism of this inhibition with the opioid antagonist medication naloxone (Chong et al, 2006;Hernandez-Avila et al, 2003;Wand et al, 2002).…”

“…This A118G polymorphism (rs1799971) results in an asparagine to aspartic acid substitution at amino-acid position 40 and leads to increased receptor-binding affinity for the endogenous opioid beta-endorphin and increased potency of ion channel activation following beta-endorphin binding (Bond et al, 1998). Two other studies, one in transiently-expressing COS cells (Befort et al, 2001), the other in stable HEK293 cell lines (Beyer et al, 2004), did not replicate these findings. In our prior study (Bond et al, 1998), the variant and prototype receptors were stablytransfected AV-12 cells, which confer N-glycosylation.…”

Opioid pharmacogenetics of alcohol addiction

“…There was no change in binding affinity for alkaloid ligands. This result has not been confirmed in subsequent investigations (Beyer et al 2004;Ramchandani et al 2010). In one such study transfected HEK293 cells (a fibroblastoid cell type) were used (Beyer et al 2004), but the 118G allele did not differ in binding affinity for b-endorphin, compared to 118A.…”