Mismatches in resident and stranger serotonin transporter genotypes lead to escalated aggression, and the target for aggression is mediated by sex differences in male and female rhesus monkeys (Macaca mulatta).

A variety of studies show that the s-allele of the serotonin transporter genotype (5-HTT) is related to aggression. However, influences of sex and 5-HTT genotype of both subject and opponent have not received as much attention in aggression research. Using a nonhuman primate model, the present study explores differences in rates of aggression exhibited by 201 group-housed male and female rhesus monkeys (Macaca mulatta; 122 females; 79 males) exposed to an unfamiliar age- and sex-matched stranger while in the presence of other same-sex members of their social group. The study also assesses whether the rates of aggression increase when the home-cage resident, the unfamiliar stimulus animal, or both possess the short (s) allele of the 5-HTT. Results showed that, when compared to females, males exhibited higher rates of physical aggression toward the stranger, and when both the male resident and the male stranger possessed the s-allele, rates of physical aggression toward the stranger increased five-fold. Resident females also engaged in higher rates of physical aggression when they possessed the s-allele, although unlike the males, their physical aggression was directed toward familiar same-sex members of their social group. The findings of this study indicate that rates of physical aggression are modulated by 5-HTT resident and stranger suggest a role of sexual competition in the phenotype of the 5-HTT genotype. Importantly, when two males with impulse deficits, as a function of the s-allele, are placed together, rates of violence exhibited by the dyad escalate substantially.

Central nervous system monoamine metabolite response to alcohol exposure is associated with future alcohol intake in a nonhuman primate model (Macaca mulatta).

It is widely held that the central monoamine neurotransmitters modulate alcohol intake. Few studies, however, directly assess the relationship between baseline and alcohol-induced monoamine turnover, as well as the change from baseline, as predictors of alcohol intake. Using a nonhuman primate model, this study investigates baseline, alcohol-induced and alcohol-induced change in monoamine activity and their relationship with alcohol intake. Alcohol-naïve, adolescent rhesus macaques (Macaca mulatta, N = 114) were administered a standardized intravenous bolus of alcohol solution (16.8%, v/v) on two occasions, approximately 1 month apart. One month prior to and 1 h following each alcohol infusion, cisternal cerebrospinal fluid (CSF) was obtained and assayed for monoamine metabolite concentrations. Approximately 6-7 months later, subjects were allowed unfettered access to an aspartame-sweetened alcohol solution (8.4%, v/v) for 1 h/day, 5 days/week, over 5-7 weeks. Results showed strong positive correlations between baseline and post-infusion CSF monoamine metabolite concentrations, indicating a trait-like response. Low baseline and post-infusion serotonin and dopamine metabolite concentrations and a smaller change in serotonin and dopamine metabolites from one infusion to the next were associated with higher alcohol intake. Low baseline and post-infusion norepinephrine metabolite concentrations predicted high alcohol intake, but unlike the other monoamines, a greater change in norepinephrine metabolite concentrations from one infusion to the next was associated with higher alcohol intake. These findings suggest that individual differences in naturally occurring and alcohol-induced monoamine activity, as well as the change between exposures, are important modulators of initial alcohol consumption and may play a role in the risk for excessive alcohol intake.

Parental genetic contributions to neonatal temperament in a nonhuman primate (Macaca mulatta) model.

Temperament is an individual's nature and is widely believed to have a heritable foundation. Few studies, however, have evaluated paternal and maternal contributions to the triadic dimensions of temperament. Rhesus monkeys are widely utilized to model genetic contributions to human development due to their close genetic-relatedness and common temperament structure, providing a powerful translational model for investigating paternal and maternal genetic influences on temperament. The temperament of rhesus monkey infants born to 19 different sires and 50 different dams was assessed during the first month of life by comparing the temperament of paternal or maternal half-siblings reared with their mothers in species-normative conditions or reared in a neonatal nursery. Factor scores from three dimensions of temperament were obtained (Orienting/Regulation, Negative Affectivity, and Surgency/Extraversion) and ANOVAs were used to assess genetic effects. For paternal half-siblings, results showed a statistically significant paternal contribution to Orienting/Regulation, Negative Affectivity, and Surgency/Extraversion factor scores. For maternal half-siblings, results showed a statistically significant contribution to Orienting/Regulation factor scores. When parsed by early rearing condition, results showed a paternal contribution Orienting/Regulation, Negative Affectivity, and Surgency/Extraversion scores for paternal half-siblings reared in the neonatal nursery, while there was only a paternal contribution to Surgency/Extraversion for paternal half-siblings reared by their mothers. There was only a maternal contribution to Orienting/Regulation for maternal half-siblings reared by their mothers. These results show that paternal and maternal contributions to temperament vary by environmental context, and that mothers may environmentally buffer their infants from paternal contributions to their temperament.

Early life temperamental anxiety is associated with excessive alcohol intake in adolescence: A rhesus monkey (Macaca mulatta) model.

Teenage alcohol abuse is a major health concern, particularly because the majority of alcohol consumed by teenagers is via binge drinking, a known risk factor for increasing the likelihood for the development of future alcohol use disorders (AUDs). Identifying individuals at risk for excessive alcohol intake in adolescence is a step toward developing effective preventative measures and intervention programs. As adults with AUDs tend to self-medicate their anxiety with alcohol, this longitudinal study assesses the role of infant anxiety-like temperament in the development of adolescent alcohol abuse using a nonhuman primate model. From birth until they were 5 months of age, behaviors of 64 rhesus monkeys (Macaca mulatta) were coded twice a week using an objective mother-infant scoring system that included behaviors traditionally used to assess anxiety and fearfulness in rhesus monkeys. When subjects were four months old, plasma cortisol was obtained. When subjects were adolescents (Mage = 44.88 months), another plasma cortisol sample was obtained about one month prior to allowing them unfettered access to an 8.4% (v/v) aspartame-sweetened alcohol solution for one hour a day over five-to-seven weeks. Results showed that behavioral indications of anxiety-like temperament in infancy, including high levels of mother-infant mutual ventral contact, low levels of environmental exploration, and low levels of interactions with peers were predictive of high adolescent alcohol intake (ie, drinking to intoxication). Plasma cortisol levels in infancy were positively correlated with plasma cortisol in adolescence, and both were positively correlated with high adolescent alcohol intake. Our findings indicate that high levels of traditional anxiety-like behaviors measured in the context of mother-infant interactions, coupled with high infant and adolescent plasma cortisol, are associated with binge-like high alcohol intake in adolescence, suggesting that individuals at risk for developing an AUD later in life may be determined, at least in part, by assessing their physiological and behavioral propensity for anxiety early in life.

Neonatal temperament and neuromotor differences are predictive of adolescent alcohol intake in rhesus monkeys (Macaca mulatta).

Identifying predictors of teenage alcohol use disorder (AUDs) is a major health initiative, with studies suggesting that there are distinct personality-related traits that underlie patterns of alcohol intake. As temperament is biologically based, identifiable early in life, and stable across time, it is considered the foundation of personality. As such, we hypothesized that neonatal temperament traits would predict anxiety-mediated adolescent alcohol consumption. To test this, N = 145 rhesus macaque (Macaca mulatta) infants (14 days of age), reared in a neonatal nursery (n = 82) or in a control condition with their mothers (n = 63) were assessed with a widely used standardized nonhuman primate testing battery, the Infant Behavioral Assessment Scale (IBAS), modeled after the Brazelton Neonatal Assessment Scale, evaluating visual orienting, temperament, motor maturity and, more recently, sensory sensitivity. As adolescents (3-4 years of age), these same subjects were allowed unfettered access to a sweetened-alcohol solution for 1 hr/day, 4 days/week, over 5-7 weeks. Subjects were allowed to self-administer alcohol while housed alone (n = 70) or socially in their home cage (n = 55). Linear regressions showed that alcohol intake was predicted by neonatal orienting ability (ß = -.35; p = .01), state control (ß = -.19; p = .04), and motor maturity (ß = -.24; p = .01). Poor neonatal orienting, state control (ease of consolability), and motor maturity were associated with higher adolescent alcohol intake in rhesus monkeys. These findings suggest that neonatal temperament is predictive of patterns of adolescent alcohol intake. To the extent that these results generalize to humans, they provide evidence that early-life temperament and neurodevelopment may be important risk factors for adolescent AUDs and that the IBAS may be used as an assessment tool for identifying such risk.

Early rearing history influences oxytocin receptor epigenetic regulation in rhesus macaques.

Adaptations to stress can occur through epigenetic processes and may be a conduit for informing offspring of environmental challenge. We employed ChIP-sequencing for H3K4me3 to examine effects of early maternal deprivation (peer-rearing, PR) in archived rhesus macaque hippocampal samples (male, n = 13). Focusing on genes with roles in stress response and behavior, we assessed the effects of rearing on H3K4me3 binding by ANOVA. We found decreased H3K4me3 binding at genes critical to behavioral stress response, the most robust being the oxytocin receptor gene OXTR, for which we observed a corresponding decrease in RNA expression. Based on this finding, we performed behavioral analyses to determine whether a gain-of-function nonsynonymous OXTR SNP interacted with early stress to influence relevant behavioral stress reactivity phenotypes (n = 194), revealing that this SNP partially rescued the PR phenotype. PR infants exhibited higher levels of separation anxiety and arousal in response to social separation, but infants carrying the alternative OXTR allele did not exhibit as great a separation response. These data indicate that the oxytocin system is involved in social-separation response and suggest that epigenetic down-modulation of OXTR could contribute to behavioral differences observed in PR animals. Epigenetic changes at OXTR may represent predictive adaptive responses that could impart readiness to respond to environmental challenge or maintain proximity to a caregiver but also contribute to behavioral pathology. Our data also demonstrate that OXTR polymorphism can permit animals to partially overcome the detrimental effects of early maternal deprivation, which could have translational implications for human psychiatric disorders.

Intergenerational effects of mother's early rearing experience on offspring treatment and socioemotional development.

This longitudinal study spans two generations of rhesus monkeys. First, the study investigates the effects of early rearing experiences on the maternal behavior of first-generation mothers (rates of premature infant rejection) and, second, the study investigates the effects of maternal rejection on the behavior of second-generation infants. Rhesus macaque mother-infant dyads (Macaca mulatta-N = 176) were observed twice weekly, with each session lasting 300 s. First-generation mothers were raised in one of three conditions: as mother-reared controls (MR; [n = 95]), in peer groups (PR; raised without adults but with constant access to three same-aged peers [n = 49]), or with an inanimate surrogate (SPR; raised with an inanimate fleece-covered, surrogate mother and limited daily peer-group interactions [n = 32]). Second-generation infants were all raised by their differentially reared mothers and statistically grouped into one of two groups: those that were rejected by their mothers beginning at a more-typical weaning age (controls), starting in the third month of life (n = 108), and those that were prematurely rejected, with mothers showing rejections before the third month of infant life (n = 68). Overall, PR mothers exhibited the highest rates of premature infant rejection, except for month 1 of infant life, when SPR mothers exhibited the highest rates of rejection. Intriguingly, after month 1, SPR mothers showed high rates of infant cradling and seldom rejected their infants. Independent of their mothers' early rearing environment, prematurely rejected infants displayed more aggression and passive vigilance, and were cradled and groomed less by their mothers, and there was evidence that the overall rates of rejection after the first 2 months of life had a cumulative negative effect on the developing infant. Post hoc analyses of plasma cortisol levels showed that the prematurely rejected infants had higher cortisol concentrations, suggesting a high level of stress in the prematurely rejected infants. These results suggest that maternal presence during infancy has long-term effects on a female's future maternal skills which, in turn, have intergenerational consequences for the socioemotional development of second-generation infants.

OPRM1 genotype interacts with serotonin system dysfunction to predict alcohol-heightened aggression in primates.

Although the notion that alcohol promotes violence is widespread, not all individuals are aggressive while intoxicated. Genetic variation could be a contributing factor to individual differences in alcohol-heightened aggression. The present study examines the effects of OPRM1C77G genotype on responses to threat in rhesus macaques under normal conditions and following alcohol administration. Prior studies have shown that a low CSF level of 5-HIAA is a trait marker for individuals prone to escalated aggression. We wanted to examine whether the predictive value for this marker on aggression was moderated by OPRM1 genotype. Animals were administered alcohol (BAC 100-200 mg%), were provoked by a human intruder, and aggressive responses were recorded. Factor analysis was performed to generate aggressive response factors, which were then used as dependent variables for ANOVA, with OPRM1 genotype and CSF 5-HIAA as independent variables. Factor analysis generated three factors ('Threatening', 'Distance Decreasing' and 'High Intensity'). We found that High Intensity aggression was increased among carriers of the OPRM1 G allele, especially among individuals with low CSF levels of 5-HIAA. Aggression in the non-intoxicated state was predicted by 5-HIAA, but not by genotype. This study demonstrates that OPRM1 genotype predicts alcohol-heightened aggression in rhesus macaques with low CSF levels of 5-HIAA. Because OPRM1 variation predicts similar effects on alcohol response and behavior in humans and macaques, this study could suggest a role for OPRM1 genotype in alcohol-heightened aggression in humans. If so, it may be that compounds that block this receptor could reduce alcohol-associated violence in selected patient populations.

The rhesus macaque is three times as diverse but more closely equivalent in damaging coding variation as compared to the human.

BACKGROUND: As a model organism in biomedicine, the rhesus macaque (Macaca mulatta) is the most widely used nonhuman primate. Although a draft genome sequence was completed in 2007, there has been no systematic genome-wide comparison of genetic variation of this species to humans. Comparative analysis of functional and nonfunctional diversity in this highly abundant and adaptable non-human primate could inform its use as a model for human biology, and could reveal how variation in population history and size alters patterns and levels of sequence variation in primates. RESULTS: We sequenced the mRNA transcriptome and H3K4me3-marked DNA regions in hippocampus from 14 humans and 14 rhesus macaques. Using equivalent methodology and sampling spaces, we identified 462,802 macaque SNPs, most of which were novel and disproportionately located in the functionally important genomic regions we had targeted in the sequencing. At least one SNP was identified in each of 16,797 annotated macaque genes. Accuracy of macaque SNP identification was conservatively estimated to be >90%. Comparative analyses using SNPs equivalently identified in the two species revealed that rhesus macaque has approximately three times higher SNP density and average nucleotide diversity as compared to the human. Based on this level of diversity, the effective population size of the rhesus macaque is approximately 80,000 which contrasts with an effective population size of less than 10,000 for humans. Across five categories of genomic regions, intergenic regions had the highest SNP density and average nucleotide diversity and CDS (coding sequences) the lowest, in both humans and macaques. Although there are more coding SNPs (cSNPs) per individual in macaques than in humans, the ratio of dN/dS is significantly lower in the macaque. Furthermore, the number of damaging nonsynonymous cSNPs (have damaging effects on protein functions from PolyPhen-2 prediction) in the macaque is more closely equivalent to that of the human. CONCLUSIONS: This large panel of newly identified macaque SNPs enriched for functionally significant regions considerably expands our knowledge of genetic variation in the rhesus macaque. Comparative analysis reveals that this widespread, highly adaptable species is approximately three times as diverse as the human but more closely equivalent in damaging variation.

The serotonin transporter gene linked polymorphic region is associated with the behavioral response to repeated stress exposure in infant rhesus macaques.

The short allele of the serotonin transporter linked polymorphic region (5-HTTLPR) moderates the effects of stress on vulnerability to mood and anxiety disorders. The mechanism by which this occurs may relate to differential sensitivity to stressful life events. Here we explored whether 5-HTTLPR and sex affected behavioral responses to repeated maternal separation in infant rhesus macaques. Behaviors were collected during the acute (Day 1) and the chronic (Days 2-4) phases of the separation, and the effects of duration of separation (acute vs. chronic), genotype (long/long vs. short allele), and sex (male vs. female) on behavioral responses were analyzed across four successive separations. Males increased their levels of locomotion with repeated maternal separation, whereas females exhibited an increase in frequency of self-directed behavior, a measure of "depression-like" behavior. The short-allele predicted increased environmental exploration, particularly during the chronic phase of social separation, indicative of higher arousal. In addition, the short-allele carriers were more likely to increase their levels of self-directed behavior during the chronic phase of separation, as a function of repeated exposures. These findings suggest that the short allele may increase reactivity to repeated, chronic stressors, leaving them more vulnerable to affective psychopathology, with females particularly vulnerable.

The serotonin transporter gene is a substrate for age and stress dependent epigenetic regulation in rhesus macaque brain: potential roles in genetic selection and gene × environment interactions.

In humans, it has been demonstrated that the serotonin transporter linked polymorphic region (5-HTTLPR) genotype moderates risk in the face of adversity. One mechanism by which stress could interact with genotype is via epigenetic modifications. We wanted to examine whether stress interacted with genotype to predict binding of a histone 3 protein trimethylated at lysine 3 (H3K4me3) that marks active promoters. The brains (N = 61) of male rhesus macaques that had been reared in the presence or absence of stress were archived and the hippocampusi dissected. Chromatin immunoprecipitation was performed with an antibody against H3K4me3 followed by sequencing on a SolexaG2A. The effects of age, genotype (5-HTTLPR long/long vs. short), and stress exposure (peer-reared vs. mother-reared) on levels of H3K4me3 binding were determined. We found effects of age and stress exposure. There was a decline in H3K4me3 from preadolescence to postadolescence and lower levels in peer-reared monkeys and no effects of genotype. When we controlled for age, however, we found that there were effects of 5-HTTLPR genotype and rearing condition on H3K4me3 binding. In a larger sample, we observed that cerebrospinal fluid 5-hydroxyindoleacetic acid levels were subject to interactive effects among age, rearing history, and genotype. Genes containing both genetic selection and epigenetic regulation may be particularly important in stress adaptation and development. We find evidence for selection at the solute carrier family C6 member 4 gene and observe epigenetic reorganization according to genotype, stress, and age. These data suggest that developmental stage may moderate effects of stress and serotonin transporter genotype in the emergence of alternative adaptation strategies and in the vulnerability to developmental or psychiatric disorders.

Functional CRH variation increases stress-induced alcohol consumption in primates.

Corticotropin-releasing factor (CRF), encoded by the CRH gene, is a key integrator of stress responses, and, as such, CRH gene variation may contribute to individual differences in susceptibility to stress-related pathology. In rhesus macaques, a single nucleotide polymorphism (SNP) is found within the CRH promoter (-248C--> T). Here, we assessed whether this variant influenced stress responding and, because increased CRF system activity drives alcohol drinking in rodents, we examined whether it predicted voluntary alcohol consumption as a function of prior stress exposure. Using a hypothalamic nuclear extract, we showed that the -248 T allele resulted in increased DNA protein interactions relative to the C allele. In vitro, the T allele resulted in CRH promoter activity that was higher following both stimulation with forskolin and treatment with dexamethasone. Endocrine and behavioral responses to social separation stress (release of ACTH and cortisol, and suppression of environmental exploration, respectively) were higher among carriers of the T allele, particularly among those exposed to early adversity in the form of peer rearing. We also found that T allele carriers with a history of early life adversity consumed more alcohol in a limited-access paradigm. Our data suggest that CRH promoter variation that confers increased stress reactivity increases the risk for alcohol use disorders in stress-exposed individuals.

Variation in the Mu-Opioid Receptor (OPRM1) and Offspring Sex Are Associated With Maternal Behavior in Rhesus Macaques (Macaca mulatta).

A µ-opioid receptor (OPRM1) single-nucleotide-polymorphism, found in both humans and rhesus macaques mediates the mother-infant attachment bond. Because mothers treat their sons and daughters differently, it is somewhat surprising that the role of infant sex has not been assessed in the context of a maternal-OPRM1-genotype-by-infant-sex interaction. The present study investigates the effect of maternal-OPRM1-genotype and infant sex on mother-infant behaviors. Over the first 6 months of offspring life, mother-infant behavioral data assessing attachment quality was collected twice weekly from a large number of rhesus monkey mother-infant pairs (N = 161 dyads; n = 64 female infants, n = 97 male infants). Mothers were genotyped for OPRM1 variation. Factor analysis of the observed behaviors showed two factors: Attachment (maternal-infant cradling, rejections, and infant approaches and leaves), and Maternal Restraints (mother restrains infant, preventing exploration). Further analyses showed a two-way, maternal-genotype-by-infant-sex interaction for both factors. For Attachment, mothers with the CC genotype cradled and restrained (Maternal Restraints) their female infants more and rejected them less, when compared to female infants of CG mothers. Perhaps as a consequence, female infants of CC genotype mothers approached and left their mothers less often, when compared to female infants of CG mothers, likely an indication that female infants from mothers with CG genotype play a greater role in maintaining the mother-infant bond than do female infants from CC genotype mothers. This finding may also indicate a more secure attachment in infants from CC genotype mothers. Unlike female infants, on average, the mother-infant relationship of dyads with a male infant was largely undifferentiated by maternal genotype. These findings suggest that, in contrast to female infants from CG mothers, CC mothers and their female infants appear to have a closer mother-infant relationship which may portend close life-long bonds, as mothers and female offspring remain together throughout life. Male offspring appear to have a more aloof mother-infant bond regardless of OPRM1-genotype. The results of this study indicate that maternal-OPRM1 variation mediates mother-infant attachment behaviors for female infants and has less effect for male infants. This suggests that offspring sex should be included in studies investigating the effect of maternal-OPRM1 genotype on the mother-infant attachment relationship.

A nonhuman primate model of human non-suicidal self-injury: serotonin-transporter genotype-mediated typologies.

While non-suicidal self-injury (NSSI) occurs in the general population at a surprisingly high rate, with higher rates among certain clinical  populations, its etiology is not well-understood. Consequently, the DSM-5 lists NSSI as requiring further research. This study utilizes a translational model of naturally-occurring NSSI to assess the role of early parental neglect and variation in the serotonin transporter genotype (5-HTT) in the etiology of NSSI. Subjects (N = 161) were rhesus macaques (Macaca mulatta) reared in one of three conditions (mother-reared (MR), peer-reared (PR), or surrogate peer-reared (SPR)), and classified as NSSI (n = 18) or non-NSSI (n = 143). Subjects were genotyped for 5-HTT and their behaviors were recorded during an ecologically-meaningful, stress-evoking, intruder paradigm. Two weeks prior to testing, blood samples were obtained and assayed for plasma cortisol and adrenocorticotropic hormone (ACTH) concentrations. NSSI subjects were more likely to be SPR, paralleling human studies showing that individuals that exhibit NSSI tend to have experienced abuse or neglect early in life. Results also indicated that variation in the 5-HTT genotype differentiated the NSSI subjects. NSSI subjects that were homozygous for the L allele exhibited high plasma ACTH and high rates of stress-induced stereotypies; whereas NSSI subjects with the s allele exhibited impulsive behaviors, including frequently approaching the potentially dangerous intruder, high rates of aggressive vocal threats, and more activity. These results suggest that there may be different 5-HTT genotype-mediated NSSI typologies and that both early experiences and variation in the 5-HTT genotype may be important factors in understanding the etiology of NSSI.

Variation at the mu-opioid receptor gene (OPRM1) influences attachment behavior in infant primates.

In a variety of species, development of attachment to a caregiver is crucial for infant survival and partly mediated by the endogenous opioids. Functional mu-opioid receptor gene polymorphisms are present in humans (OPRM1 A118G) and rhesus macaques (OPRM1 C77G). We hypothesized that rhesus infants carrying a gain-of-function OPRM1 77G allele would experience increased reward during maternal contact and would, therefore, display increased measures of attachment. We collected behavioral data from rhesus macaques (n = 97) during early infancy and at 6 months of age, across four cycles of maternal separation (4 days) and reunion (3 days). Animals were genotyped for the OPRM1 C77G polymorphism, and the effects of this allele on attachment-related behaviors were analyzed. Infants carrying the G allele exhibited higher levels of attachment behavior during early infancy. During prolonged periods of maternal separation, although infant macaques homozygous for the C allele exhibited decreases in their levels of distress vocalization with repeated separation, this response persisted in G allele carriers. The OPRM1 77G allele also affected social preference during reunion. C/G infants spent increasing amounts of time in social contact with their mothers as a function of repeated separation and were less likely to interact with other individuals in the social group, a pattern not observed among infants with the C/C genotype. These findings suggest a role for OPRM1 variation in the expression of attachment behavior in human subjects, especially as a function of separation from the caregiver.

The Effects of At-Birth Adoption on Atypical Behavior and Anxiety: A Nonhuman Primate Model.

OBJECTIVE: Adopted children tend to show an increased risk for a variety of psychopathological outcomes, even when adoption occurs at birth, which some suggest is a result of nonrandom assignment of adoptees and parents. This study uses a nonhuman primate model, in which adoptions were randomly assigned, to investigate the behavioral and physiological outcomes associated with at-birth adoption. METHOD: Immediately following birth, rhesus monkey infants were randomly assigned to be reared by either their biological mother (n = 113) or by an unrelated, lactating, adoptive mother (n = 34). At 6 months of age, infant behavior and physiology were assessed during a stressful series of mother-infant separations. Four years later, stress-related behaviors were measured following confrontation by an unfamiliar intruder, an ecologically meaningful stressor. RESULTS: When compared to infants reared by their biological mothers, adopted infants exhibited more behavioral withdrawal and higher plasma adrenocorticotropic hormone (ACTH) concentrations in response to separation. These behavioral differences persisted 4 years later during a stressful intruder challenge, with adoptees exhibiting more behavioral withdrawal, stereotypies, and impulsive approaches of the potentially aggressive intruder. CONCLUSION: Compared to infants reared by their biological mothers, adopted infants exhibited more behavioral inhibition, impulsivity, and higher ACTH concentrations, even when subjects were randomly assigned to be adopted or to remain with their biological mother. To the extent that these findings generalize to humans, they suggest that the overall risk for psychopathology in adopted individuals persists even after random assignment to adoption conditions.

Early Rearing Conditions Affect Monoamine Metabolite Levels During Baseline and Periods of Social Separation Stress: A Non-human Primate Model (Macaca mulatta).

A variety of studies show that parental absence early in life leads to deleterious effects on the developing CNS. This is thought to be largely because evolutionary-dependent stimuli are necessary for the appropriate postnatal development of the young brain, an effect sometimes termed the "experience-expectant brain," with parents providing the necessary input for normative synaptic connections to develop and appropriate neuronal survival to occur. Principal among CNS systems affected by parental input are the monoamine systems. In the present study, N = 434 rhesus monkeys (233 males, 201 females) were reared in one of two conditions: as mother-reared controls (MR; n = 269) or without adults with 24-h access to same-aged peers (PR; n = 165). When subjects were six-months-old, they underwent a separation paradigm involving 4, sequential, four-day social separations from their mothers or peers, with each separation followed by three-day reunions with their mothers or their peers. Prior to the separation paradigm, baseline cisternal CSF samples were obtained, as well as at the end of each the four social separations, and after final separation, during a recovery period. CSF was assayed for concentrations of monoamine metabolites and a blood sample was genotyped for the serotonin transporter (5-HTT) genotype. Replicating earlier landmark findings, PR subjects with the s allele exhibited lower baseline concentrations of the serotonin metabolite 5-hydroxyindoleacetic acid (5-HIAA), when compared to PR subjects homozygous for the L allele. MR subjects were undifferentiated by genotype. PR subjects exhibited lower CSF 5-HIAA concentrations during baseline, but higher CSF 5-HIAA during social separations, when compared to MR subjects. There were rearing effects for the dopamine metabolite homovanillic acid (HVA) and for the norepinephrine metabolite 3-methoxy-4-hydroxyphenylglycol (MHPG), with PR subjects showing higher HVA and lower MHPG when compared to MR subjects. These findings indicate that there are long-term deficits in the response of monoamines following early maternal absence. The results of this study confirm and extend earlier findings that early parental absence has deleterious consequences for the development of the monoamine systems, and that these consequences are modulated by the 5-HTT genotype.

Association of a functional polymorphism in the mu-opioid receptor gene with alcohol response and consumption in male rhesus macaques.

CONTEXT: Innate differences in opioid neurotransmission are hypothesized to influence abuse liability of alcohol. In humans, a variant of the mu-opioid receptor gene (OPRM1A118G) increases receptor affinity, alcohol-induced euphoria, and risk for alcohol use disorders. OBJECTIVE: To determine whether a variant in the mu-opioid receptor gene (OPRM1C77G) that increases affinity of the receptor is associated with alcohol response and consumption in macaques. DESIGN: Young adult rhesus macaques (Macaca mulatta) were intravenously administered 2.0 to 2.1 g of ethanol per kilogram of body weight and assessed for alcohol response. Animals were later given simultaneous access to an aspartame-sweetened 8.4% (vol/vol) ethanol solution and a vehicle for 1 hour per day, 5 days a week, for a period of 6 weeks. Animals (N = 82) were genotyped for the OPRM1C77G polymorphism; the effects of the genotype on alcohol response and consumption were determined by analysis of variance, with sex included as a nominal independent variable. MAIN OUTCOME MEASURES: Alcohol response (ataxia, stimulation, and sedation), average alcohol consumption, the percentage of days during which an animal consumed alcohol at a level sufficient to produce intoxication (> or =0.67 g of alcohol per kilogram of body weight), and alcohol preference (calculated as 100 x {alcoholic solution/[alcoholic solution + nonalcoholic solution]}). RESULTS: Increased alcohol-induced stimulation was observed among male macaques carrying the OPRM1C77G allele. OPRM1C77G allele carriers consumed more ethanol and exhibited increased ethanol preference. Male carriers of the OPRM1C77G allele exhibited higher alcohol preference and consumption, and drank to intoxication more frequently than did C/C males. CONCLUSIONS: These findings demonstrate that the rhesus macaques' equivalent of the OPRM1A118G variant is associated with increased alcohol response, consumption, and preference. Our results reveal effects of the OPRM1C77G genotype to be male-restricted or more marked among male macaques. This is of interest, given the fact that early-onset type II alcoholism is more common among men and that, among addicted individuals, men are more responsive to mu-opioid receptor blockade.

Early maternal rejection affects the development of monoaminergic systems and adult abusive parenting in rhesus macaques (Macaca mulatta).

This study investigated the effects of early exposure to variable parenting style and infant abuse on cerebrospinal fluid (CSF) concentrations of monoamine metabolites and examined the role of monoaminergic function in the intergenerational transmission of infant abuse in rhesus monkeys (Macaca mulatta). Forty-three infants reared by their biological mothers and 15 infants that were cross-fostered at birth and reared by unrelated mothers were followed longitudinally through their first 3 years of life or longer. Approximately half of the infants were reared by abusive mothers and half by nonabusive controls. Abused infants did not differ from controls in CSF concentrations of 5-hydroxyindoleacetic acid (5-HIAA), homovanillic acid (HVA), or 3-methoxy-4-hydroxyphenylgycol (MHPG). Abused infants, however, were exposed to higher rates of maternal rejection, and highly rejected infants had lower CSF 5-HIAA and HVA than low-rejection infants. The abused females who became abusive mothers in adulthood had lower CSF 5-HIAA than the abused females who did not. A similar trend was also observed among the cross-fostered females, suggesting that low serotonergic function resulting from early exposure to high rates of maternal rejection plays a role in the intergenerational transmission of infant abuse.

Influence of parenting style on the offspring's behaviour and CSF monoamine metabolite levels in crossfostered and noncrossfostered female rhesus macaques.

We investigated the association between variation in parenting style and the offspring's behaviour and CSF monoamine metabolite (5-HIAA, HVA, and MHPG) levels in rhesus monkeys. Study subjects were 25 two-year-old females reared by their biological mothers and 15 same-aged females that were crossfostered at birth and reared by unrelated mothers. Subjects that were rejected more by their mothers in the first 6 months of life engaged in more solitary play and had lower CSF concentrations of 5-HIAA than subjects that were rejected less. The relation between these variables was generally similar in crossfostered and noncrossfostered females. CSF levels of 5-HIAA were negatively correlated with rates of scratching, a behavioural indicator of anxiety. These results suggest that that early exposure to high rates of maternal rejection can result in higher anxiety later in life, and that this effect may be mediated by serotonergic mechanisms. Variation in maternal protectiveness did not affect offspring behaviour and neither protectiveness nor rejection affected CSF levels of HVA and MHPG. CSF levels of MHPG, however, were negatively correlated with solitary play behaviour and avoidance of other individuals, suggesting that individuals with lower CSF MHPG were more fearful and socially phobic than those with higher CSF MHPG. Taken together, these findings suggest that individual differences in anxiety and fearfulness in young rhesus monkeys are accounted for, at least in part, by variation in CSF levels of monoamine metabolites, and that the development of brain monoamine systems, particularly serotonin, can be affected by early exposure to variable maternal behaviour.