Taken together, our data strongly support a causal role of the OPRM1
118G allele to confer a more vigorous DA response to alcohol in the ventral striatum. These findings may be related to observations of differential subjective alcohol effects as a function of OPRM1
A118G genotype in humans 12
, and of markedly increased psychomotor responses to alcohol in rhesus males carrying the functionally equivalent 77G variant 14
. Interestingly, we also found that subjective feelings of intoxication over time differed as a function of genotype. Carriers of the 118G allele showed signs of rapid acute tolerance, similar to that previously described in men at genetic risk for alcoholism 41
. The use of a reverse-translational approach allowed us to effectively isolate the influence of the human 118G variant from that of other polymorphisms with which it might be in LD.
Excessive activation of brain reward systems by alcohol in OPRM1
118G carriers might be expected to confer susceptibility for alcohol use disorders. Although we in fact found evidence in support of this notion in an ethnically homogenous Caucasian sample 42
, others have not. One study found that other polymorphisms within the same haplotype block, but not A118G, were associated with diagnoses of substance dependence 31
, while a meta-analysis of all available data for A118G was negative 43
. This apparent discrepancy closely parallels recent findings with the 5-HT transporter gene promoter length polymorphism (5-HTTLPR). A robust role of 5-HTTLPR variation has been found using an imaging based endophenotype that is closely related to negative affect 39,44
, and these findings are paralleled by experimental data in a non-human primate model 45,46
. Nevertheless, findings from 5-HTTLPR association studies in depression have not held up on meta-analysis 47,48
. The reasons for these discrepancies are presently unknown, much debated, and critical to resolve. One important possibility is that current diagnostic categories pool genetically heterogeneous phenocopies, as has specifically been proposed in depression 49
. If so, the use of biological endophenotypes, such as in the present study, can potentially aid the dissection of these populations into genetically and pathophysiologically more homogenous categories.
Irrespectively of its possible role as a susceptibility factor, excessive reward-related DA-response to alcohol in carriers of OPRM1
118G might be of pharmacogenetic importance in treatment of excessive alcohol use. Indeed, two studies have found that the presence of the 118G allele was associated with a therapeutic response to the opioid antagonist naltrexone 50,51
. Although one study failed to find such an effect 52
, recent non-human primate data provide strong evidence for this notion under closely controlled experimental conditions 53
. Our data are consistent with a selective role of endogenous opioids for alcohol reward in OPRM1
118G carriers, and provide a possible biological mechanism for the preferential naltrexone response in this population. However, the dissociation between objective measures of alcohol-induced DA-release and subjective reports of alcohol effects may argue against a simplistic view of alcohol-induced DA-release as an immediate mediator of drug reward. Instead, these data may be more compatible with a view of ventral striatal DA-activation as a signal predictive of alcohol reward, and perhaps involved in learning processes important for addiction 54
. Of note, the level of alcohol-induce DA-activation in our study was overall lower than previously reported 10
. A difference between the two studies is that alcohol administration was oral in the prior report, while we used intravenous administration. This may indicate that striatal DA response to alcohol is only in part driven by direct pharmacological actions of alcohol, while an additional component of the activation is evoked by smell or taste cues.
Our data demonstrate that the 118G allele, which encodes an amino acid substitution in the N-terminal extracellular loop of the receptor, confers a higher striatal DA-activation in response to alcohol regardless of whether it occurs in the context of other markers within its haplotype block or not. However, the molecular mechanism that mediates functional consequences of the 118G variant remains controversial. Initially described as a gain-of-function mutation due to increased affinity for β-End 11
118G has since been proposed to instead confer decreased expression in several systems, including human post-mortem brain tissue 55
, and a mouse model where a mutation was introduced in a position corresponding to the human A118G marker 56
. The latter model differs from that used in our study, in that it introduced the N → D substitution in the context of a murine N-terminal receptor protein sequence. In contrast, we generated mice expressing humanized receptors throughout this region by replacing all of exon 1, the main region of divergence between the two species (). Using this model, our present findings show that a key functional phenotype associated with the 118G variant, alcohol-induced striatal DA-release, is replicated across species in the absence of effects on receptor affinity, binding density, or signaling. A remaining possibility is altered oligomerization, a mechanism that is critical for opioid receptor desensitization, endocytosis, and re-insertion 57
A118G modifies a glycosylation site in the N-terminal extracellular loop of the receptor, and could thereby influence receptor dimerization and trafficking.
Functionally equivalent OPRM1
variants have evolved independently in two primate lineages, the rhesus macaque 13
and man 11
. In both species, minor allele carriers are more sensitive to disruption of social attachment bonds 58,59
. In rhesus, this is accompanied by bold - exploratory traits, while lower conscientiousness has been observed in human 118G carriers 13,58,60
. These traits may have been advantageous in the evolutionary history of the respective species, and recent studies suggest that the 118G allele has been under positive selection 61
. In contrast, the ability of these variants to modulate DA responses to alcohol is likely a coincidental evolutionary byproduct.
Identifying genetic sources of individual variation in complex behavioral phenotypes has proven challenging. Our findings support the notion that use of biological intermediate phenotypes and translational approaches can contribute important data to facilitate this difficult endeavor.