The single nucleotide polymorphism 118A>G of the human -opioid receptor gene OPRM1, which leads to an exchange of the amino acid asparagine (N) to aspartic acid (D) at position 40 of the extracellular receptor region, alters the in vivo effects of opioids to different degrees in pain-processing brain regions. The most pronounced N40D effects were found in brain regions involved in the sensory processing of pain intensity. Using the -opioid receptor-specific agonist DAMGO, we analyzed the -opioid receptor signaling, expression, and binding affinity in human brain tissue sampled postmortem from the secondary somatosensory area (S II ) and from the ventral posterior part of the lateral thalamus, two regions involved in the sensory processing and transmission of nociceptive information. We show that the main effect of the N40D -opioid receptor variant is a reduction of the agonist-induced receptor signaling efficacy. In the S II region of homo-and heterozygous carriers of the variant 118G allele (n ‫؍‬ 18), DAMGO was only 62% as efficient (p ‫؍‬ 0.002) as in homozygous carriers of the wild-type 118A allele (n ‫؍‬ 15). In contrast, the number of [ 3 H]DAMGO binding sites was unaffected. Hence, the -opioid receptor G-protein coupling efficacy in S II of carriers of the 118G variant was only 58% as efficient as in homozygous carriers of the 118A allele (p < 0.001). The thalamus was unaffected by the OPRM1 118A>G SNP. In conclusion, we provide a molecular basis for the reduced clinical effects of opioid analgesics in carriers of -opioid receptor variant N40D.The human -opioid receptor variant N40D coded by the single nucleotide polymorphism (SNP) 2 118AϾG of the -opioid receptor gene, OPRM1 (dbSNP rs1799971; allelic frequency 8.2-17%) has been found to be associated with diminished opioid effects in experimental (1-5) and clinical (6 -9) settings. However, in vitro experiments trying to elucidate the underlying molecular mechanisms provided inconsistent results, falling short to support the comparatively strong clinical evidence of decreased opioid effects in carriers of the variant 118G allele.Among the reported molecular consequences of the 118AϾG polymorphism is a three times higher binding affinity of ␤-endorphin at the N40D -opioid receptors expressed in transfected Syrian hamster adenovirus-12-induced tumor cells (AV-12), whereas the affinity of other exogenous opioids was unaffected (10). In contrast, a leftward shift of the potencies of DAMGO and morphine-mediated Ca 2ϩ channel inhibition has been shown in N40D variant -opioid receptor expressed in rat sympathetic superior cervical ganglion (SCG) neurons (11). However, both findings of a higher ligand affinity and potency were not reproduced in three attempts using N40D variant -opioid receptors transfected Cercopithecus aethiops kidney cells (COS), human 293 embryonic kidney cells (HEK293), and again AV-12 cells (12-14). Moreover, they do not explain the decreased clinical opioid potency. As an alternative mechanism, decreased -opioid receptor expression caused...

Genetic and epigenetic mechanisms play important roles in protein expression, although at different levels. Genetic variations can alter CpG sites and thus influence the epigenetic regulation of mRNA expression, providing an increasingly recognized mechanism of functional consequences of genetic polymorphisms. One of those genetic effects is the association of reduced μ-opioid receptor expression with the functional genetic variant N40D (OPRM1 118A>G, rs1799971) that causes an amino acid exchange in the extracellular terminal of the μ-opioid receptor. We report that the nucleotide exchange at gene position +118 introduces a new CpG-methylation site into the OPRM1 DNA at position +117. This leads to an enhanced methylation of the OPRM1 DNA at this site and downstream. This epigenetic mechanism impedes μ-opioid receptor upregulation in brain tissue of Caucasian chronic opiate addicts, assessed postmortem. While in wild-type subjects, a reduced signalling efficiency associated with chronic heroin exposure was compensated by an increased receptor density, this upregulation was absent in carriers of the 118G receptor variant due to a diminished OPRM1 mRNA transcription. Thus, the OPRM1 118A>G SNP variant not only reduces µ-opioid receptor signalling efficiency, but, by a genetic-epigenetic interaction, reduces opioid receptor expression and therefore, depletes the opioid system of a compensatory reaction to chronic exposure. This demonstrates that a change in the genotype can cause a change in the epigenotype with major functional consequences.