Multi-group multi-time point confirmatory factor analysis of the triadic structure of temperament: A nonhuman primate model

Attempts to describe the latent structure of human infant temperament have led some to suggest the existence of three major dimensions. An earlier exploratory factor analysis (EFA) supported a triadic structure of temperament in week-old rhesus monkey infants, paralleling the structure in human infants. This study sought to confirm the latent triadic structure of temperament across the first month of life in a larger sample of rhesus monkey infants (N = 668), reared by their mothers or in a neonatal nursery. A weekly behavioral assessment was obtained during the first month of life using a subset of items from the widely utilized Infant Behavioral Assessment Scale (IBAS), an instrument designed to measure temperament in infant monkeys. Using the latent constructs proposed by the earlier EFA (Orienting/Regulation, Negative Affectivity, Surgency/Extraversion), multi-group, multi-time point confirmatory factor analyses were conducted to confirm the latent temperament structure across rearing groups at each time point (weeks 1-4). Results confirm and extend those of the earlier EFA: latent Orienting/Regulation,  Negative Affectivity, and Surgency/Extraversion constructs were present across the rearing groups at each time point, with the IBAS items consistently loading onto the latent factors to a similar degree across rearing groups at each time point. These findings suggest foundational evolutionary roots for the triadic structure of human infant temperament, but that its behavioral manifestations vary across maturation and rearing condition. Similarities in latent temperament structure in humans and a representative nonhuman primate highlights the potential for utilizing translational nonhuman primate models to increase understanding of human temperament.

Environmental Control, Social Context, and Individual Differences in Behavioral and Cortisol Responses to Novelty in Infant Rhesus Monkeys

The effects of appetitive controllability on behavioral and cortisol reactivity to novelty in 12 infant rhesus monkeys were studied. Surrogate-peer-reared infants had homecage access to food treats contingently via lever pressing ("master") or noncontingently ("yoked") for 12 weeks from postnatal month 2. Masters lever-pressed more, but did not differ in baseline cortisol. At month 5, infants were exposed to a novel environment in social groups and individually. Masters were significantly more active and exhibited significantly lower cortisol reactivity to the novel environment, but only in the individual context. Also, individual differences in operant behavior were positively correlated with behavioral activity and negatively correlated with cortisol reactivity to the novel environment. The results reveal context-specific benefits of contingent stimulation in infancy.

Maternal absence and stability of individual differences in CSF 5-HIAA concentrations in rhesus monkey infants

OBJECTIVE

Early life events often lead to deficits in CNS serotonin function, which underlie a number of reoccurring psychopathological disorders. Studies using rhesus macaques have demonstrated that early maternal deprivation reduces CNS serotonin turnover, as measured by cisternal CSF 5-HIAA concentrations. In addition, individual differences in CSF 5-HIAA remain stable from the first year of life through adulthood. The purpose of this study was to assess 1) the impact of rearing environment on the early development (<6 months of age) of the serotonin system, and 2) at what stage of early development individual differences in CSF 5-HIAA concentrations stabilize.

METHOD

The subjects were 256 infant rhesus macaques reared in three different conditions (mother-reared, peer-reared, and surrogate/peer-reared). Cisternal CSF was obtained at 14, 30, 60, 90, 120, and 150 days of age.

RESULTS

No differences in CSF 5-HIAA concentrations were observed between peer only- and surrogate/peer-reared infants, and these groups combined exhibited lower 5-HIAA concentrations than mother-reared infants throughout early development. CSF 5-HIAA concentrations declined with increasing age regardless of rearing condition. Within each rearing condition, individual differences in CSF 5-HIAA concentrations remained stable from 14 to 150 days of age.

CONCLUSIONS

Early maternal deprivation reduces CNS serotonin turnover, and individual differences in CSF 5-HIAA concentrations are trait-like and appear to stabilize in infancy.

Monoamine oxidase A gene promoter variation and rearing experience influences aggressive behavior in rhesus monkeys

BACKGROUND

Allelic variation of the monoamine oxidase A (MAOA) gene has been implicated in conduct disorder and antisocial, aggressive behavior in humans when associated with early adverse experiences. We tested the hypothesis that a repeat polymorphism in the rhesus macaque MAOA gene promoter region influences aggressive behavior in male subjects.

METHODS

Forty-five unrelated male monkeys raised with or without their mothers were tested for competitive and social group aggression. Functional activity of the MAOA gene promoter polymorphism was determined and genotypes scored for assessing genetic and environmental influences on aggression.

RESULTS

Transcription of the MAOA gene in rhesus monkeys is modulated by an orthologous polymorphism (rhMAOA-LPR) in its upstream regulatory region. High- and low-activity alleles of the rhMAOA-LPR show a genotype x environment interaction effect on aggressive behavior, such that mother-reared male monkeys with the low-activity-associated allele had higher aggression scores.

CONCLUSIONS

These results suggest that the behavioral expression of allelic variation in MAOA activity is sensitive to social experiences early in development and that its functional outcome might depend on social context.

Social withdrawal behaviors in nonhuman primates and changes in neuroendocrine and monoamine concentrations during a separation paradigm

This study investigated relationships between withdrawal behaviors in rhesus macaques and changes in monoamine metabolite and endocrine concentrations during repeated psychosocial stress. Rhesus monkeys (N = 71) experienced maternal separation in which four separations took place during four consecutive weeks. Behavioral observations were made, as well as plasma concentrations of cortisol and cerebrospinal fluid concentrations of the serotonin, dopamine, and norepinephrine metabolites were obtained. Animals were assigned to high, moderate, and low withdrawal groups, defined using baseline durations of withdrawal behaviors. Highly withdrawn animals showed less reduction than nonwithdrawn animals in serotonin metabolite concentrations over repeated separations. Highly withdrawn macaques also failed to significantly reduce cortisol concentrations across separation weeks. More adaptation in central serotonin functioning and cortisol concentrations was seen in nonwithdrawn primates than in highly withdrawn primates; these findings have implications for increased risk of developing anxiety disorders in highly inhibited children.

Interaction between serotonin transporter gene variation and rearing condition in alcohol preference and consumption in female primates

BACKGROUND

Serotonin neurotransmission and limbic-hypothalamic-pituitary-adrenal (LHPA) axis hormones are thought to be involved in the reinforcement of alcohol intake and contribute to the risk for alcoholism. In humans and macaques, a promoter polymorphism that decreases transcription of the serotonin transporter gene is associated with anxiety and altered LHPA-axis responses to stress, and in female macaques, exposure to early-life stress alters LHPA-axis activation in response to alcohol. We wanted to determine whether serotonin transporter gene promoter variation (rh-5HTTLPR) and rearing condition would interact to influence alcohol preference in female rhesus macaques. Because of the involvement of stress and LHPA-axis activity in symptoms of withdrawal and relapse, we also wanted to determine whether serotonin transporter gene variation and rearing condition would influence changes in the patterns of alcohol consumption across a 6-week alcohol consumption paradigm.

METHODS

Female macaques were reared with their mothers in social groups (n = 18) or in peer-only groups (n = 14). As young adults, they were given access to an aspartame-sweetened 8.4% alcohol solution and vehicle for 1 hour per day, and volumes of consumption of alcoholic and nonalcoholic solutions were recorded. Serotonin transporter genotype (l/l and l/s) was determined using polymerase chain reaction followed by gel electrophoresis.

RESULTS

We found interactions between rearing condition and serotonin transporter genotype, such that l/s peer-reared females demonstrated higher levels of ethanol preference. We also found an effect of rearing condition on the percentage change in alcohol consumed during the 6 weeks as well as a phase by rearing interaction, such that peer-reared animals progressively increased their levels of consumption across the course of the study. This was especially evident for peer-reared females with the l/s rh5-HTTLPR genotype.

CONCLUSION

These data suggest a potential interaction between serotonin transporter gene variation and early experience in vulnerability to alcoholism.

Sexual dichotomy of an interaction between early adversity and the serotonin transporter gene promoter variant in rhesus macaques

A polymorphism in the human serotonin transporter gene promoter (5-HTTLPR) is associated with anxiety and increased risk for developing depression in the face of adversity. Here, we report that among infant rhesus macaques, an orthologous polymorphism (rh5-HTTLPR) interacts with adversity in the form of peer rearing to influence adrenocorticotropic hormone (ACTH) response to stress and, further, that this interaction is sexually dichotomous. ACTH responses to separation are higher in l/s than in l/l males. In females, however, it is only among those with a history of adversity that the s allele is associated with increased ACTH responses to stress. Of interest, peer-reared animals, in particular females carrying the s allele, also exhibit lower cortisol responses to stress, a pattern that has been recognized in association with certain stress-related neuropsychiatric disorders. By extension, our findings suggest the intriguing possibility that human females carrying the 5-HTTLPR s allele could be more vulnerable to the effects of early adversity. This interactive effect may underlie the increased incidence of certain stress-related disorders in women.

Early Experience and Sex Interact to Influence Limbic-Hypothalamic-Pituitary-Adrenal-Axis Function After Acute Alcohol Administration in Rhesus Macaques (Macaca mulatta)

BACKGROUND

Studies in rodents demonstrate sex differences in neuroendocrine stress axis activity after treatment with alcohol. In abstinent alcoholics, atypical depressives, and individuals with posttraumatic stress disorder, limbic-hypothalamic-pituitary-adrenal (LHPA)-axis activity is often blunted; among females in these patient populations, however, resistance to glucocorticoid feedback and increased pituitary reactivity is observed. Early parental loss is a major life stressor and is a risk factor for both affective disturbances and LHPA-axis abnormalities later in life. We wanted to determine whether sex and early life parental absence would interact to influence alcohol-induced alterations in LHPA-axis activity after exposure to ethanol in macaques.

METHODS

Animals were reared with their mothers in social groups (MR, n = 94) or without adults in peer-only groups (PR, n = 79). At 5 years of age, they received an intravenous infusion of alcohol (2-2.2 g/kg), and the effects of alcohol, sex, and rearing condition on ACTH and cortisol levels were analyzed by ANOVA.

RESULTS

Peer-reared females had higher ACTH levels than did PR males, MR females, and MR males after alcohol infusion. Alcohol-induced cortisol levels were not affected by sex and rearing condition.

CONCLUSIONS

These findings suggest that there are sex differences in glucocorticoid negative feedback, pituitary responsivity, or release of ACTH secretagogues among individuals exposed to early life stress and emphasize the importance of considering sex effects when studying LHPA-axis dysregulation in alcoholism and other stress-related neuropsychiatric disorders.

Rearing condition and rh5-HTTLPR interact to influence limbic-hypothalamic-pituitary-adrenal axis response to stress in infant macaques

BACKGROUND

In humans and macaques, a promoter polymorphism that decreases transcription of the serotonin transporter gene is associated with anxiety. Serotonin transporter gene disruption in rodents produces anxious animals with exaggerated limbic-hypothalamic-pituitary-adrenal (LHPA) responses to stress. We wanted to determine whether serotonin transporter gene promoter variation (rh-5HTTLPR) and rearing condition would interact to influence endocrine responses to stress in infant rhesus macaques.

METHODS

Animals were reared with their mothers (MR, n = 141) or in peer-only groups (PR, n = 67). At 6 months of age, adrenocorticotropic hormone (ACTH) and cortisol levels were determined at baseline and during separation stress. Serotonin transporter genotype (l/l and l/s) was determined with polymerase chain reaction followed by gel electrophoresis.

RESULTS

Cortisol levels increased during separation, and there was a main effect of rearing condition, with decreased cortisol levels among PR macaques. Animals with l/s rh5-HTTLPR genotypes had higher ACTH levels than did l/l animals. Adrenocorticotropic hormone levels increased during separation, and there was a separation x rearing x rh5-HTTLPR interaction, such that PR-l/s animals had higher ACTH levels during separation than did other animals studied.

CONCLUSIONS

These data demonstrate that serotonin transporter gene variation affects LHPA axis activity and that the influence of rh5-HTTLPR on hormonal responses during stress is modulated by early experience.

Neurobehavioral deficits in neonatal rhesus monkeys exposed to cocaine in utero

In spite of significant efforts, the neurobehavioral deficits in infants born from cocaine-abusing mothers have not been clearly defined. In the present study, we examined the presence of these abnormalities in a rhesus monkey model of prenatal cocaine exposure using a nonhuman primate adaptation of the Neonatal Behavioral Assessment Scale (NBAS). Pregnant monkeys (n = 14) received 10 mg/kg cocaine twice a day orally (in fruit treats) from the 40th through 102nd postconception days (PCD40-PCD102), which is the period of cerebral cortical neuronogenesis (approximately second trimester). The control consisted of pregnant monkeys (n = 14) receiving fruit treats only. The animals were allowed to deliver vaginally at term (approximately PCD165). The first testing session was conducted on PCD171 (within the first week after birth); the second testing session was conducted on PCD177 (within the second week after birth); the third test was conducted on PCD183 (within the third week after birth); and the fourth testing session was conducted on PCD189 (within the fourth week after birth). The prenatally cocaine-exposed infants showed deficits in orientation, state control, and motor maturity, which were detectable during the second, third, and fourth testing sessions. The same testing sessions also revealed a significant reduction in the time devoted to toy manipulation, which points to impaired attention. None of these abnormalities were seen during the first testing session. The first session, however, revealed increased tremulousness (one of the indicators of autonomic stability) in the prenatally cocaine-exposed infants. This impairment disappeared by the third testing session. The present findings demonstrate the potential of prenatal cocaine exposure to induce neurobehavioral deficits detectable by NBAS-like testing in primate infants.

The utility of the non-human primate; model for studying gene by environment interactions in behavioral research

Variation in the serotonin transporter gene-linked polymorphic region (5-HTTLPR) has been associated with anxiety and harm avoidance and is weakly associated with a number of neuropsychiatric disorders, including Type II alcoholism, which has a high rate of comorbidity with antisocial personality disorder. Studies have also demonstrated interactions between 5-HTLPR variation and environmental stress on the incidence of depression. As in humans, there is a serotonin transporter gene promoter length polymorphism in rhesus macaques that produces similar decreases in transcriptional efficiency. Macaques with histories of early-life stress have been shown to exhibit impulsive aggression, incompetent social behavior and increased behavioral and endocrine responsivity to stress. In this paper, we review studies performed previously in our lab and present preliminary data examining interactions between early rearing and serotonin transporter gene promoter variation on the incidences of play behavior and aggression in infant rhesus macaques. The data presented here highlight the importance of considering gene-environment interactions when studying childhood risk factors for aggression, anxiety and related neuropsychiatric disorders and support the use of the nonhuman primate for studing gene by environment interactions in behavioral research.

Serotonin transporter gene polymorphism, differential early rearing, and behavior in rhesus monkey neonates

A polymorphism in the serotonin (5-HT) transporter gene regulatory region (5-HTTLPR) is associated with measures of 5-HT transporter (5-HTT) expression and 5-HT-mediated behaviors in humans. An analogous length variation of the 5-HTTLPR has been reported in rhesus monkeys (rh5-HTTLPR). A retrospective association study was conducted on 115 rhesus macaque infants either homozygous for the long 5HTTLPR variant (l/l) or heterozygous for the short and long form (l/s). To assess contributions of genotype and early rearing environment, 36 mother-reared monkeys (l/l = 26, l/s = 10) and 79 nursery-reared monkeys (l/l = 54, l/s = 25) were assessed on days 7, 14, 21, and 30 of life on a standardized primate neurobehavioral test designed to measure orienting, motor maturity, reflex functioning, and temperament. Both mother-reared and nursery-reared heterozygote animals demonstrated increased affective responding relative to l/l homozygotes. Nursery-reared, but not mother-reared, l/s infants exhibited lower orientation scores than their l/l counterparts. These results demonstrate the contributions of rearing environment and genetic background, and their interaction, in a nonhuman primate model of behavioral development.

Serotonin transporter gene variation is associated with alcohol sensitivity in rhesus macaques exposed to early-life stress

BACKGROUND

Decreased sensitivity to alcohol has been demonstrated to be a predictor of alcoholism in humans, and variation in the gene-linked polymorphic region of the serotonin transporter (5-HTTLPR) is associated with the response to the motor-impairing effects of alcohol. In a nonhuman primate model of excessive alcohol intake, we have shown that decreased serotonin turnover is associated with both lower initial sensitivity to alcohol and higher prospective alcohol consumption using rhesus macaques. In addition, we have demonstrated that macaques separated from their mothers and reared in peer-only groups are more likely to consume alcohol as adults.

METHOD

To examine the relationship between serotonin transporter genotype, early rearing experience, and initial sensitivity to alcohol, peer- and mother-reared, adolescent, alcohol-naive rhesus macaques (n = 123) were rated for intoxication after intravenous administration of ethanol (2.2 g/kg and 2.0 g/kg for males and females, respectively) during two testing periods. Serotonin transporter (rh5-HTTLPR) genotype was determined using polymerase chain reaction followed by gel electrophoresis, and data were analyzed using ANOVA and the Mann-Whitney U test.

RESULTS

Our analyses demonstrate an effect of serotonin transporter gene variation on ethanol sensitivity, such that animals homozygous for the l allele exhibited decreased sensitivity to the ataxic and sedating effects of alcohol. This effect remained after correction for blood ethanol concentrations and birth cohort. When animals were segregated according to rearing condition, serotonin transporter gene variation predicted intoxication scores among peer-reared animals.

CONCLUSIONS

As in some human reports, this study demonstrates a diminution in the response to alcohol in animals homozygous for the l rh5-HTTLPR allele. The phenotypic expression of this genotype in l/s animals, however, is environmentally dependent.

Early experience and serotonin transporter gene variation interact to influence primate CNS function

Nonhuman primates offer unique opportunities to study the effects of genes, environments, and their interaction, on physiology and complex behavior. We examined genotype and early environment contributions to CNS function in a large sample of rhesus monkeys. In humans, length variation of the serotonin (5-HT) transporter (5-HTT) gene-linked polymorphic region (5-HTTLPR) that results in allelic variation in 5-HTT expression is associated with decreased serotonergic function and 5-HT-mediated psychopathology. We report that an analogous variation of the gene's regulatory region in monkeys interacts with early experience to affect central 5-HT functioning. Monkeys with deleterious early rearing experiences were differentiated by genotype in cerebrospinal fluid concentrations of the 5-HT metabolite, 5-hydroxyindoleacetic acid, while monkeys reared normally were not. These findings demonstrate an environment-dependent effect of the rh5-HTTLPR genotype on CNS 5-HT function and suggest nonhuman primates may provide an important avenue for investigating gene/environment interactions using candidate genes for physiological and behavioral traits.

Fatty acid formula supplementation and neuromotor development in rhesus monkey neonates

Docosahexaenoic acid (DHA) is an omega-3 fatty acid that is highly concentrated in CNS tissues. Although breast milk contains the fatty acids DHA and arachidonic acid, infant formulas marketed in North America do not contain these nutrients. The potential deleterious effects of rearing infants with formulas devoid of these nutrients was assessed by comparing nursery-reared rhesus macaque infants (Macaca mulatta) fed standard formula with infants fed standard formula supplemented with physiologically relevant concentrations of DHA (1.0%) and arachidonic acid (1.0%). Neurobehavioral assessments were conducted on d 7, 14, 21, and 30 of life using blinded raters. The 30-min assessment consisted of 45 test items measuring orienting, temperament, reflex capabilities, and motor skills. Plasma concentrations of DHA in standard formula-fed infants were significantly lower than those fed supplemented formula or mother-raised (breast-fed) infants; however, infants fed the supplemented formula exhibited higher arachidonic acid levels than either mother-reared infants or infants fed standard formula. Infant monkeys fed the supplemented formula exhibited stronger orienting and motor skills than infants fed the standard formula, with the differences most pronounced during d 7 and 14. This pattern suggests an earlier maturation of specific visual and motor abilities in the supplemented infants. Supplementation did not affect measures of activity or state control, indicating no effect on temperament. These data support the assertion that preformed DHA and arachidonic acid in infant formulas are required for optimal development.

Feeding demand conditions and plasma cortisol in socially-housed squirrel monkey mother-infant dyads

Previous studies have demonstrated that experimentally altering the accessibility and availability of food can have profound impact on behavior and adrenocortical activity in nonhuman primate species. In this study, groups of mother-infant squirrel monkey dyads were housed in either high demand (HFD: 120% normal daily food intake provided), low demand (LFD: 600% normal daily food intake provided) or variable demand (VFD: alternating two-week blocks of low demand and high demand) conditions for 12 weeks. During the 12-week experimental foraging phase, animals in the HFD group exhibited prolonged and consistent cortisol elevations. The cortisol levels in the VFD group reflected the ambient demand condition, with higher levels exhibited during the high demand phases of the study, and lower values when the low demand condition was in effect. Overall, mothers were more affected by the experimental manipulation than were infants. The experimental condition did not affect the infants' response to a 24-h separation from their mothers. A suppression of cortisol levels, particularly in the HFD group, was observed upon resumption of ad-libitum feeding.

Characterization of oxalate oxidase and cell death in Al-sensitive and tolerant wheat roots

Several genes including oxalate oxidase (Oxo) are up-regulated in Triticum aestivum L. root tips exposed to Al. To better understand the function of Oxo during Al exposure, the protein level and enzyme activity were measured. The data indicate that both Oxo protein and activity are increased proportionally to the level of root growth inhibition (RGI). A high level of Oxo expression may result in excess H(2)O(2) production which could become toxic and induce cell death. However, the timing of H(2)O(2) production (observed after 24 h) indicates that it cannot be the primary cause of cell death first observed after 8 h. Moreover, at Al concentrations resulting in 50% RGI, we did not observe any cell death in the sensitive cultivar while a punctated pattern of death involving small groups of cells was found in the tolerant cultivar. This pattern was maintained for several days in the tolerant cultivar, suggesting the involvement of a cell death mechanism aimed at replacing epidermal cells intoxicated with Al while root growth is maintained. The accelerated epidermal cell turnover may represent a new detoxification mechanism helping to protect deeper cell layers of the meristematic and elongation zone essential for root growth.

Plasma cortisol is associated with handedness in infant rhesus monkeys

In this research we examined the relationship between plasma cortisol and handedness in infant rhesus macaques (Macaca mulatta). Specifically, we tested the hypothesis that stress functioning is related to hemispheric specialization and manifested in a positive correlation between cortisol levels and the frequency of right- versus left- hand use. We found a significant positive relationship between cortisol levels sampled at ages 1 and 3 months and lateral bias toward greater use of the right hand versus left hand sampled between ages 4 and 11 months. Further, we found a significant negative relationship between cortisol sampled at age 5 months and strength of lateral bias (independent of direction). These data suggest an early developmental influence of stress functioning on hemispheric specialization for manual control in infant monkeys.

Rearing condition and plasma cortisol in rhesus monkey infants

Previous studies comparing plasma cortisol levels in mother-reared and nursery-reared rhesus monkey infants under baseline and stress conditions have reported conflicting findings. Differences in subject age, procedures, and specific rearing history may account for many of the discrepant findings. In the present study, mother-reared infants from large social groups, peer-only reared animals, and infants reared with surrogates and limited peer contact were studied in different test conditions across the first 6 months of life. Infants were sampled under three conditions: following a neonatal assessment at Days 14 and 30, immediately upon capture on Day 60, and after 30-min isolation periods on Days 90, 120, and 150. Mother-reared infants exhibited higher cortisol levels on Days 14 and 30 than did both types of nursery-reared infants. In addition, Day 60 basal values of mother-reared infants were higher than those of both peer-reared and surrogate/peer-reared infants. However, on Days 90, 120, and 150, both mother-reared and peer-reared infants exhibited higher cortisol levels in response to separation and 30-min isolation than did the surrogate/peer-reared infants. These differences may reflect group-specific variations in physical environment, capture time, feeding regimen, or diurnal HPA axis rhythms.

Oral administration of a corticotropin-releasing hormone receptor antagonist significantly attenuates behavioral, neuroendocrine, and autonomic responses to stress in primates

We evaluated the effects of the lipophilic nonpeptide corticotropin-releasing hormone (CRH) type 1 receptor antagonist antalarmin on the behavioral, neuroendocrine, and autonomic components of the stress response in adult male rhesus macaques. After oral administration, significant antalarmin concentrations were detected in the systemic circulation and the cerebrospinal fluid by a mass spectrometry-gas chromatography assay developed specifically for this purpose. Pharmacokinetic and dose-response studies suggested that an oral dose of 20 mg/kg was optimal for behavioral and endocrine effects. We then administered this dose in a double-blind, placebo-controlled fashion to monkeys exposed to an intense social stressor: namely, placement of two unfamiliar males in adjacent cages separated only by a transparent Plexiglas screen. Antalarmin significantly inhibited a repertoire of behaviors associated with anxiety and fear such as body tremors, grimacing, teeth gnashing, urination, and defecation. In contrast, antalarmin increased exploratory and sexual behaviors that are normally suppressed during stress. Moreover, antalarmin significantly diminished the increases in cerebrospinal fluid CRH as well as the pituitary-adrenal, sympathetic, and adrenal medullary responses to stress. We conclude that CRH plays a broad role in the physiological responses to psychological stress in primates and that a CRH type 1 receptor antagonist may be of therapeutic value in human psychiatric, reproductive, and cardiovascular disorders associated with CRH system hyperactivity.

Rearing experiences and stress-induced plasma cortisol as early risk factors for excessive alcohol consumption in nonhuman primates

BACKGROUND

The purpose of this study was to assess the impact of early rearing and stress-induced rise of plasma cortisol collected during infancy as a biological predictors of adult alcohol consumption in nonhuman primates.

METHODS

Ninety-seven female and male rhesus macaques (Macaca mulatta) were investigated. They were reared for their first 6 months of life either without mothers or other adults but with constant access to same-aged peers (peer-reared), or as controls with their mothers (mother-reared). When subjects reached 6 months of age, they underwent a series of four sequential weeks of 4-day social separations. Blood was drawn 1 and 2 hr after initiation of the 4-day separation periods, and the plasma was assayed for plasma cortisol concentrations. When the subjects were young adults (approximately 50 months of age), they were tested for voluntary intake of alcohol for 1 hr per day, 4 days a week, during a period of 5 to 7 weeks under normal living conditions.

RESULTS

The social separation challenge increased infant plasma cortisol concentrations, with peer-reared subjects exhibiting higher stress-induced cortisol concentrations than mother-reared animals. Subjects that responded to the social separation challenge with high cortisol levels consumed significantly more alcohol per kilogram of body weight as adults than subjects with a low cortisol response to the separation challenge, regardless of rearing condition. In addition, male and peer-reared subjects consumed significantly more alcohol than female and mother-reared subjects, respectively.

CONCLUSIONS

These findings suggest that early rearing experiences, such as adult absence, and high plasma cortisol concentrations early in life after a social separation stressor, are useful psychobiological predictors of future high alcohol consumption among nonhuman primates.

Rhesus monkeys with late-onset hydrocephalus differ from non-impaired animals during neonatal neurobehavioral assessments: six-year retrospective analysis

BACKGROUND AND PURPOSE

A recent case study indicated that a hydrocephalic rhesus monkey had abnormal response patterns in a standardized neonatal primate assessment. We conducted a retrospective study to determine whether this assessment could also differentiate neonatal rhesus monkeys that appeared normal but developed signs of hydrocephalus later in life from neonates with normal development and no evidence of hydrocephalus.

METHODS

One-hundred eighty-two rhesus monkeys were assessed on postnatal days 7, 14, 21, and 30. As neonates, clinical signs of hydrocephalus or other illnesses were not evident in any animal. Six monkeys developed signs of hydrocephalus between 5 months and 5 years of age, and each received confirmed diagnoses of hydrocephalus at necropsy.

RESULTS

Compared with colony norms, the monkeys that developed hydrocephalus had diminished orientation abilities, more muscle tension, less behavioral evidence of distress, and more pronounced responses to some reflex-evoking stimuli, and difficulty in self-righting (day 7 only). Discriminant function analysis comparing the hydrocephalic animals with a matched control group provided a high probability of correct group assignment at days 7, 14, and 21.

CONCLUSIONS

Some as yet undetermined factor may predispose some monkeys to develop hydrocephalus, which may also be reflected in different scores on neurodevelopmental test items during early infancy.

Behavioral and physiological characteristics of Indian and Chinese-Indian hybrid rhesus macaque infants

Strain differences in temperament and physiology have been reported for several animal species, but nonhuman primates have not been well studied in this regard. We assessed behavioral and physiology in Chinese-Indian hybrid (n = 13) and Indian-origin (n = 29) nursery-reared rhesus monkey infants. Previous data indicate that Chinese-origin and Chinese-Indian hybrid rhesus exhibit more aggression directed toward humans and conspecifics and are more irritable in response to neonatal assessment procedures than are Indian-derived rhesus. In addition, in rhesus adults, low levels of cerebrospinal fluid 5-HIAA have been correlated with impulsivity, aggressive behavior, and diminished social competence. We therefore hypothesized that hybrid infants would exhibit more behavioral and adrenocortical reactivity in the home cage and during social separations, would be less sociable in their peer groups, and would exhibit lower CSF 5-HIAA levels than Indian-derived monkeys. Blood and cerebrospinal fluid samples were obtained on Days, 14, 30, 60, 90, 120, and 150 of life, and prior to and during social separations at 6 months of age. Behavioral observations were conducted in the home cage and during the separation condition. No differences in behavior were observed between hybrid and Indian-derived animals in the home cage. Indian-derived and hybrid infants exhibited diverging patterns of behavioral reactivity across the 4 weeks of the repetitive social-separation procedure, and during reunion periods. Although plasma cortisol levels were sensitive to the testing conditions, no group differences were observed. CSF 5-HIAA declined over time for all monkeys, and hybrid animals exhibited significantly lower 5-HIAA levels than Indian monkeys beginning at 6 months of age. These findings are consistent with the known behavioral and physiological characteristics of Chinese-origin adult rhesus.

Hand preference in infant rhesus macaques (Macaca mulatta)

In this research we examined hand preference in infant rhesus macaques (Macaca mulatta). The subjects were 20 Macaca mulatta, each aged between 4 and 11 months. We assessed hand preference using both a unimanual reaching task and a bimanual coordination task. In the unimanual reaching task, we presented subjects with raisins and noted which hand the animals used to retrieve the food. In the bimanual coordination task, we presented the same subjects with plastic tubes filled with raisin paste and noted which hand the animals used to hold the tubes and which hand the animals used to remove the food. We noted a population-level bias toward use of the left hand for both tasks. These results suggest early right hemisphere advantage for reaching and bimanual coordination in Macaca mulatta, although we acknowledge that this issue needs to be examined more directly through neuroimaging procedures such as Positron Emission Tomography (PET) or functional Magnetic Resonance Imaging (fMRI). We speculate that early right hemisphere advantage contributes to differential patterns of hand preference development for unimanual and bimanual action, and that the development of hand preference for bimanual coordination is related to the emergence of hemispheric specialization for processing species-specific vocalizations.

Behavioral and Physical Concomitants of Congenital Hydrocephalus in a Rhesus Macaque (Macaca mulatta) Neonate

A colony-born male rhesus monkey neonate was assigned to an ongoing protocol assessing behavioral and physiologic development in nursery-reared infants. It was sacrificed at day 20 because of poor weight gain, inability to self-feed, and generalized weakness. An asymmetric internal hydrocephalus was the only notable gross finding at necropsy. A retrospective analysis was performed comparing physical and behavioral measures for it with values for age-matched nursery-reared peers. Weight gain and formula intake lagged behind others in its cohort. The affected macaque spent more time in sleep and awake/quiet states and less time in an awake active state than did its peers. Behaviors during a neonatal temperament/reflex examination were indicative of high levels of irritability, poor muscular tone, nonexistent voluntary motor activity, and slower overall responding. In addition, asymmetric responses for some motor items were observed. However, vestibular/ocular reflexes appeared normal. The occasional observation of hydrocephalus in infants, as well as its prevalence throughout the primate order, indicates that hydrocephalus should be considered in the differential diagnosis for infants with feeding difficulties and motor dysfunction.

Tympanic temperature asymmetry and stress behavior in rhesus macaques and children

OBJECTIVES

To examine left-to-right tympanic membrane temperature asymmetries and their possible association with biobehavioral stress responses in rhesus macaques and children.

SUBJECTS AND DESIGN

Infrared tympanic membrane thermometry was completed bilaterally in 19 two-year-old rhesus macaques and 18 eight-year-old children in a cross-sectional, laboratory-based study. Unidirectional temperature gradients were calculated as the mean of two left-sided measurements minus the mean of two right-sided measurements. Biobehavioral stress responses were assessed in monkeys as agitated motor activity and adrenocortical activation after separation from the social group, and in children as parent-reported resilience to psychological stress and child behavior problems.

RESULTS

Significant asymmetry was detected in tympanic membrane temperatures in both monkey and child samples, with left-sided temperatures measuring slightly but significantly higher than those from the right tympanic membrane. Higher-magnitude left-to-right temperature gradients were associated with stress-related locomotion in macaques and with lower resilience and more behavior problems in children.

CONCLUSIONS

There are small but reliable asymmetries in the tympanic membrane temperatures of young human and nonhuman primates. Tympanic membrane temperature gradients reflect important individual differences in biologically derived responses to psychological stressors.

Stability of interindividual differences in serotonin function and its relationship to severe aggression and competent social behavior in rhesus macaque females

Few studies have investigated longitudinally interindividual stability of cerebrospinal fluid (CSF) 5-hydroxyindoleacetic acid (5-HIAA) concentrations in adult nonhuman primates across time and between baseline and stressful conditions. Furthermore, whereas studies with male macaques consistently have reported a significant, negative correlation between CSF 5-HIAA and rates of spontaneous aggression, wounding, and severe aggression, very few studies have examined this relationship in adult female nonhuman primates. Even fewer studies have investigated correlations between CSF 5-HIAA and competent social behavior, such as social dominance, in female monkeys. In the present study, two social groups of adult rhesus monkeys (Macaca mulatta) were formed by placing 16 females (aged 42 to 180 months, mean age: 68 months) in one of two indoor-outdoor enclosures with one or two adult males. CSF norepinephrine (NE), monoamine metabolites, and behavioral data were collected systematically over a 24-week period. In week 5 of the study, one additional female, not familiar to any of the other subjects, was added to each social group. Thereafter the groups were left undisturbed, and data characterizing spontaneous aggressive wounding and severe wound injuries in the females were collected for an additional year. The results showed that both group introduction and the addition of a new subject into each group resulted in increased monoamine turnover in the animals within the social groups. Interindividual differences in CSF concentrations of each of the monoamine metabolites and NE were highly stable from the baseline period to the stress samplings, and between stress samplings. Females with low CSF 5-HIAA exhibited higher rates of spontaneous aggressive wounding, and they were more likely to be removed from their social groups for aggressive wounding and/or treatment of injuries. CSF NE concentrations also were negatively correlated with rates of spontaneous aggression. In contrast, competitive aggression, i.e. noninjurious aggression used to maintain social dominance ranking, was not correlated with CSF 5-HIAA or NE. Females with above average CSF 5-HIAA prior to and following group formation were more likely to attain and maintain a high social dominance ranking within their social group than females with below average CSF 5-HIAA. The present findings indicate that CNS monoamine functioning in adult female rhesus macaques is traitlike, showing a high degree of interindividual stability across time and setting. These findings also suggest a role for serotonin in controlling impulses that regulate aggression and that competent social behavior among nonhuman primates may require average or above average serotonin functioning.

Salivary cortisol in nursery-reared rhesus monkeys: reactivity to peer interactions and altered circadian activity

Five nursery-reared rhesus (Macaca mulatta) infants were studied on 8 consecutive days over a period of 2 weeks when they ranged from 4 to 5.5 months. Saliva cortisol samples were obtained by giving each animal, while in its individual home cage, a 6-in. cotton dental roll pretreated with sugared fruit-drink crystals and dried. Subjects were allowed to mouth the cotton roll for 10 min during each collection period. Saliva was then expressed from the cotton and analyzed by radioimmune assay. Samples were collected at 0830, 1100, 1400, and 1630 hr on each of the 8 days. On four of the days, 2-hr peer socialization sessions were imposed between 1200 and 1400 hr. Significant time-of-day effects were obtained. Values tended to be low at 0830 hr, rising significantly to peak levels at 1100 hr, and then declining over the 1400-hr and 1630-hr sampling periods. All 4 subjects with sufficient samples had higher average cortisol concentrations when the 1400-hr sampling followed peer-socialization sessions, compared to no-socialization days. This difference was only marginally significant by paired t test, however. These results (a) support the feasibility of using noninvasive salivary sampling procedures with infant monkeys, and (b) indicate that salivary cortisol measures are sensitive to daily rhythms. Unexpectedly, the results also raise the possibility that nursery-reared infant rhesus macaques may be phase-delayed in their HPA rhythm, with the morning peak occurring several hours after rather than in the hour before lights-on and the morning feeding.

Development of iron deficiency anemia in infant rhesus macaques

Although previous research has documented the occurrence of an iron deficiency anemia in infant nonhuman primates, the temporal development of this condition has not been studied to our knowledge. We conducted a retrospective analysis of complete blood count data obtained from 56 rhesus macaque infants over a 5-month period. Fourteen infants were exclusively mother-reared, and 42 were nursery-reared. By 60 days of age, erythrocyte indices were lower in the mother-reared monkeys than in the nursery-reared animals. Between 90 to 150 days of age, an apparent iron deficiency anemia developed in the mother-reared infants only. This anemia was characterized as a microcytic, hypochromic anemia. The time of onset of this anemia was comparable developmentally to that observed in exclusively breast-fed human infants. At no time were these infants clinically or behaviorally affected by the anemia. Hematologic status of the mothers did not correlate with that of their infants. It could not be determined whether mother's parity was predictive of the development of anemia in the infant. These observed phenomena in rhesus monkeys may serve as a potential nonhuman primate model for the anemia that is observed in exclusively breast-fed human infants.

Biobehavioral comparisons between adopted and nonadopted rhesus monkey infants

Differences between adopted and nonadopted infant rhesus monkeys were examined, as were differences between biological and foster mothers, in measures of infancy and postinfancy behaviors, maternal-infant interactions, and neuroendocrine and behavioral responses to separations. Newborns were experimentally allocated to continuous postnatal care by either their biological mothers (n = 9) or adoptive, nonbiological mothers (n = 7). Behavioral observations were completed during the neonatal period, during separations at 30 days and 5 months, and from 6 to 18 months of age, when animals were housed in a large social group. Maternal and infant responses to separation stress were assessed using measures of behavioral, adrenocortical, and growth hormone reactivity. Out of 84 possible comparisons, only six achieved statistical significance, a number compatible with the operation of chance. Negligible differences in behavioral and neuroendocrine endpoints were found between adopted and nonadopted mother-infant pairs. These findings lend additional credence to human studies finding no increase in the incidence or severity of mental disorders in adopted children.

Temperament differences between captive Indian and Chinese-Indian hybrid rhesus macaque neonates

Anecdotal evidence has accumulated from research and animal care personnel regarding the aggressive behavior reported in captive rhesus macaques originating in China. In this study, we compared neonatal temperament, activity, and neuromotor reflexes in 13 Chinese-Indian hybrid and 29 Indian-derived nursery-reared infants. Neonatal assessments were conducted on days 7, 14, 21, and 30, using a test based on the Brazelton Neonatal Assessment Scale developed for use in human newborns. Hybrid infants had lower scores for all items pertaining to orientation and ability to sustain attention. They were also rated as more temperamentally reactive and irritable than the Indian-derived infants. These results suggest that constitutional factors may underlie some of the behavioral differences observed in Chinese- and Indian-origin adults and that these qualities emerge very early in life.

Food search demand effort effects on behavior and cortisol in adult female squirrel monkeys

Laboratory-born, group-housed, ovariectomized adult female squirrel monkeys (Saimiri sciureus) were exposed to feeding conditions in which the availability and accessibility of food were altered. Both high- and variable-demand feeding conditions were utilized. The variable-demand condition required alternating periods of high effort (120% of normal daily intake presented) and low effort (600% of normal daily intake presented) to obtain food for 10-12 weeks. An additional group was exposed solely to the high-demand condition for 10 weeks. Blood samples were obtained weekly, and behavioral observations were conducted daily. In the variable-demand condition, plasma cortisol was elevated above baseline during the periods of high effort. For the constant high-demand group, cortisol was elevated for the duration of the experimental treatment. Contact with other animals, as well as a species-specific inactive posture, decreased as a result of exposure to high demand. Maintenance of body weight indicated that nutritional deprivation did not occur. The imposition of increased food-seeking efforts provides an ecologically relevant and noninvasive method of producing chronic stress in the squirrel monkey.

Paternal and maternal genetic and environmental contributions to cerebrospinal fluid monoamine metabolites in rhesus monkeys (Macaca mulatta)

BACKGROUND

To study genetic and environmental contributions to cerebrospinal fluid (CSF) monoamine concentrations, 55 young rhesus monkeys (Macaca mulatta) were reared apart from their 10 fathers to perform a paternal half-sibling analysis.

METHODS

To study maternal genetic contributions, 23 infants were reared with their mothers, 23 infants were removed from their mothers at birth and fostered to unrelated lactating female monkeys, and 24 infants were removed from their mothers at birth and reared with age-matched peers. When the monkeys reached age 6 months, CSF samples were obtained via cisternal puncture prior to and during a series of social separations.

RESULTS

When the results were statistically pooled according to the biological father, comparisons using analysis of variance indicated that both CSF 5-hydroxyindoleacetic acid (5-HIAA) and homovanillic acid (HVA) concentrations showed significant heritable (h2) effects (h2 > 0.5) for both sons and daughters, whereas 3-methoxy-4-hydroxyphenylglycol (MHPG) showed a nearly significant paternal genetic effect only for sons (h2 > 0.5). In addition, there were substantial maternal genetic influences on the young monkeys' CSF MHPG and 5-HIAA (h2 > 0.5) levels. Structural equation analyses indicated a maternal genetic contribution without a maternal environmental contribution to CSF 5-HIAA concentration; on the other hand, there was both a maternal genetic and environmental contribution to MHPG.

CONCLUSIONS

These findings suggest that a significant portion of the variance in the turnover of the monoamine neurotransmitters is determined by genetic mechanisms.

Psychosocial factors and immune senescence in the aged primate

With increasing age, old animals and humans show decreases in a number of immune responses indicative of the process of immune senescence. Our studies investigated whether social companionship, as a potentially positive psychological intervention, would increase lymphocyte proliferation and natural killer cell activity in the aged nonhuman primate. Contrary to our initial hypothesis, social stimulation resulted in decreased immune responses in old monkeys. With specific modifications of the housing conditions it was possible to prevent these decreases in immune responses from occurring, but social companionship still failed to enhance immune responses in the aged monkey.

Rearing paradigm in a nonhuman primate affects response to beta-CCE challenge

Two groups of socially housed rhesus monkeys were raised under conditions differing in the degree of control or mastery over appetitive stimuli (food, water, and treats) in the 1st year of life. At age 18-24 months, a benzodiazepine receptor inverse agonist, beta-carboline-3-carboxylic acid ethyl ester (beta-CCE), was administered to both social groups to investigate the effects of differential rearing on the response to a putative anxiogenic drug. In both groups beta-CCE was followed by behavioral activation with a profile suggesting increases in aggression rather than fear or anxiety. A group by drug analysis revealed that the increase in aggression was observed predominantly in the subjects reared with mastery during the 1st year. Plasma cortisol increased in both groups, but its increase was greater in the yoked subjects. These results suggest that early experience with controllability may have long term consequences which can be revealed by challenge with beta-CCE.