Early Environments and Genetic Effects on Alcohol Abuse and Associated Behavior

OPRM1 C77G

OPRM1 C77G In humans, a functional variant in the OPRM1 gene (OPRM1 A118G) confers a 3.5-fold increase in the receptors affinity for -Endorphin. This variant has been associated with alcohol-induced euphoria and naltrexone response in a variety of paradigms. A functionally similar variant (OPRM1 C77G) is present in the rhesus macaque and influences ethanol-induced stimulation. We have examined the effects of this variant and find that, similar to the human variant, it influences alcohol-induced stimulation, a marker of euphorogenic alcohol actions (Barr et al, 2007) and naltrexone response (Chen et al, in prep). Endorphin decreases release of CRH from the hypothalamus and, as such, can modulate HPA axis function. We have examined whether HPA axis activity differs in rhesus macaques as a function of OPRM1 variation (Barr et al, in preparation). We found that cortisol responding to stress is blunted in rhesus infants carrying the 77 G allele, despite increased behavioral responding (Barr et al, 2008). In response to alcohol, individuals carrying the 77G allele exhibited lower cortisol levels following alcohol challenge. Carriers of the 77G allele also had lower HPA axis activity across the postpartum period. Our data are consistent with human studies that have demonstrated attenuated cortisol responses to stress among carriers of the OPRM1 118G allele, lending further support to the argument that the rhesus and human allelic variants produce functionally similar effects. Our results also suggest that OPRM1 variation may influence alcohol-induced and post-partum levels of HPA axis activity and, as such, may modify vulnerability to alcohol use disorders and post-partum depression. Because the endogenous opioid system influences sensitivity to natural rewards, OPRM1 may predict sensation-seeking and related dimensions of temperament. We have found that OPRM1 C77G increases reward sensitivity, as reflected in increased measures of infant attachment (Barr et al, 2008). We also have found that bold/exploratory responding to social intrusion are increased in adult animals carrying the G allele. Given that this variant increases both alcohol-induced stimulation and is associated with a bold temperament, we proposed that it could be one factor that underlies individual differences in alcohol-induced aggression. Although there are no effects of this variant on aggression during social challenge, we do find that it predicts highly aggressive behavior to threatening stimuli during periods of intoxication (Barr et al, in prep), a finding that may extend to humans as well. CRH The corticotropin releasing hormone (CRH) system influences stress reactivity, exploration and response to novelty. We examined the CRH locus for variation in the rhesus. Several functional variants were identified. One (-2232 C>G), which led to loss of corticosteroid sensitivity under nonstressed conditions, was associated with decreased CSF levels of CRH and increased baseline levels of ACTH. This variant was part of an ancient, extended haplotype, suggesting it to be maintained by selection. We found that rhesus macaques with this functional CRH haplotype were exploratory, bold and consumed alcohol under conditions used to assess risky alcohol drinking (Barr et al, 2008). Cladistic clustering of haplotypes determined that alternative haplotypes are present and differ in corticoisteroid sensitivity in both humans and rhesus. Our data suggest that CRH variation may influence risk for externalizing disorders in human subjects. Another functional variant that we identified in the CRH promoter (-248 C>T) led to increased induction of CRH during periods of stress (Barr et al, submitted). Consistent with this finding, endocrine and behavioral responses to stress were higher among carriers of this SNP, particularly among those previously exposed to stress in the form of peer rearing. This functional promoter variant also drives excessive alcohol consumption. Like rhesus, humans exhibit variation in the proximal promoter of the CRH gene. Our findings suggest that, among individuals with a stressful life history, CRH promoter variation could increase risk for stress-related disorders, including alcohol dependence. NPY Neuropeptide Y (NPY) is an anxiolytic peptide that is involved in stress responding and whose dysregulation drives stress-related and post-dependent drinking in rodents. We have investigated the functionality of a rhesus macaque SNP (rhNPY -1002T>G), located in a region that is orthologous to one demonstrated to be important to regulation of human NPY promoter activity (Lindell et al, in prep), and our findings suggest this to be a loss-of-function mutation. Because of the role of the NPY system in stress and alcohol response, we wanted to determine whether this variant influenced behavioral reactivity to stress and alcohol consumption. When we examined effects of rhNPY genotype on alcohol consumption, we observed interactions between genotype and both early stress and alcohol exposure. G/G subjects consumed more alcohol as a function of early stress and also exhibited an escalation in alcohol intake over the course of four weeks of alcohol testing. In humans, haplotype-driven diminution in NPY expression is predictive of amygdala response and emotional reactivity to stress. Our findings suggest that loss-of-function NPY variants may influence vulnerability to alcohol use in stress-exposed individuals and may also increase risk for alcohol-induced dysregulation of the NPY system, thereby increasing addiction vulnerability. Our findings may further implicate this system as a treatment target in selected individuals. Intermittent access studies Alcohol induces neuroadaptative changes, which result in allostatic shifts in both affect and reward threshold, and dysregulation of both the NPY and CRH systems are thought to be critically involved in driving post-dependent drinking. In rodents, periods of alcohol deprivation have been shown to induce high levels of consumption, sufficient to produce blood alcohol levels (100-200 mg%) required for transitioning to the addicted state. We have instituted the use of an intermittent access paradigm to induce escalated alcohol intake in macaques, with the goal of developing a model that has predictive validity for evaluating compounds being developed treating alcoholism. We have tested several cohorts of male rhesus macaques using an every-other-day-access paradigm and find that their levels of consumption increase under this schedule, sufficient to produce blood alcohol levels in the 250-300 mg% range (Schwandt, Lindell et al, in prep). We are currently investigating whether a loss-of-function variant in the NPY promoter results in more rapid escalations in consumption among animals tested using this paradigm. Studies performed in rodents have shown that a history of alcohol dependence recruits the CRH system, resulting in increased behavioral sensitivity to stress, accompanied by increased alcohol seeking and consumption. The limitation of these studies is that primates exhibit markedly different patterns of CRH and CRH receptor expression from rodents and, while CRH-R1 antagonists are in development for the treatment of alcoholism, they have not been tested in a nonhuman primate model. We have shown that CRH-R1 blockade reduces behavioral responses to stress in rhesus macaques (Chen et al., in prep). We are currently performing studies to examine whether animals in which alcohol consumption is induced using an intermittent access paradigm are more sensitive to CRH receptor blockade (Schwandt et al, in progress), suggesting an upregulated CRH system drives drinking in primates.