Further studies of this polymorphism in rhesus macaques showed that carriers of the OPRM1 77G allele exhibited higher levels of attachment behavior in infancy, marked by persistent distress vocalizations in response to maternal separation and increased preference for maternal contact upon reunion (Barr et al., 2008). More recently, a study of naltrexone effects on alcohol preference suppression in rhesus macaques suggested that following naltrexone administration, carriers of the 77G allele significantly decreased their alcohol preference, whereas 77C homozygotes were unaffected by naltrexone treatment with regard to alcohol preference (Barr et al., 2010). This finding, which suggests that this polymorphism drives reward-dependent drinking behavior, was recently replicated in an independent sample of animals trained to self-administer alcohol and subsequently treated with naltrexone. Results revealed that animals with the G77G genotype showed greater naltrexone-induced reductions in alcohol self-administration as compared to those with C/C or C/G genotypes (Vallender et al., 2010). In short, studies of rhesus macaques have been consistent with the human findings, particularly those from the experimental studies. As with the rodent models, the nonhuman primate studies offer a unique opportunity for translational research on the molecular, pharmacological, and behavioral significance of this polymorphism in humans.
