Early deprivation and behavioral and physiological responses to social separation/novelty in the marmoset

Reconsidering the nature of threat in infancy: Integrating animal and human studies on neurobiological effects of infant stress

Early life stress has been associated with elevated risk for later psychopathology. One mechanism that may contribute to such long-term risk is alterations in amygdala development, a brain region critical to stress responsivity. Yet effects of stress on the amygdala during human infancy, a period of particularly rapid brain development, remain largely unstudied. In order to model how early stressors may affect infant amygdala development, several discrepancies across the existing literatures on early life stress among rodents and early threat versus deprivation among older human children and adults need to be reconciled. We briefly review the key findings of each of these literatures. We then consider them in light of emerging findings from studies of human infants regarding relations among maternal caregiving, infant cortisol response, and infant amygdala volume. Finally, we advance a developmental salience model of how early threat may impact the rapidly developing infant brain, a model with the potential to integrate across these divergent literatures. Future work to assess the value of this model is also proposed.

Cross-sectional analysis reveals COVID-19 pandemic community lockdown was linked to dysregulated cortisol and salivary alpha amylase in children

The COVID-19 pandemic altered everyday life starting in March 2020. These alterations extended to the lives of children as their normal routines were disrupted by community lockdowns, online learning, limited in-person social contact, increased screen time, and reduced physical activity. Considerable research has investigated the physical health impact of COVID-19 infection, but far fewer studies have investigated the physiological impact of stressful pandemic-related changes to daily life, especially in children. The purpose of this study was to leverage an ongoing clinical trial to investigate physiological consequences associated with chronic stress of pandemic community lockdown on children. As a part of the clinical trial, children provided saliva samples. Saliva samples were analyzed for cortisol and salivary alpha amylase (sAA) content. This secondary cross-sectional analysis included 94 preadolescent children located within the Greater Boston, Massachusetts community. Children participated in the study either before, during, or following the pandemic community lockdown to form three groups for comparison. In response to chronic stress caused by the pandemic community lockdown, participants demonstrated dysregulation of fast-acting catecholamine response of the locus-coeruleus-norepinephrine system and slower-acting glucocorticoid response, resulting in an asymmetrical relationship of hypocortisolism (M = 0.78 ± 0.19 μg/mL, p < 0.001) paired with higher sAA (M = 12.73 ± 4.06 U/mL, p = 0.01). Results suggest that the abrupt COVID-19 disruption to daily life, including the stressful experience of community lockdown, had physiological effects on typically developing children. Further research is required to investigate mental health outcomes of children following the chronic stress of the pandemic community lockdown.

Early life social complexity shapes adult neural processing in the communal spiny mouse Acomys cahirinus

RATIONALE

Early life social rearing has profound consequences on offspring behavior and resilience. Yet, most studies examining early life development in rodents use species whose young are born immobile and do not produce complex social behavior until later in development. Furthermore, models of rearing under increased social complexity, rather than deprivation, are needed to provide alternative insight into the development of social neural circuitry.

OBJECTIVES

To understand precocial offspring social development, we manipulated early life social complexity in the communal spiny mouse Acomys cahirinus and assessed long-term consequences on offspring social behavior, exploration, and neural responses to novel social stimuli.

METHODS

Spiny mouse pups were raised in the presence or absence of a non-kin breeding group. Upon adulthood, subjects underwent social interaction tests, an open field test, and a novel object test. Subjects were then exposed to a novel conspecific and novel group and neural responses were quantified via immunohistochemical staining in brain regions associated with social behavior.

RESULTS

Early life social experience did not influence behavior in the test battery, but it did influence social processing. In animals exposed to non-kin during development, adult lateral septal neural responses toward a novel conspecific were weaker and hypothalamic neural responses toward a mixed-sex group were stronger.

CONCLUSIONS

Communal species may exhibit robust behavioral resilience to the early life social environment. But the early life environment can affect how novel social information is processed in the brain during adulthood, with long-term consequences that are likely to shape their behavioral trajectory.

Early life adversity in dogs produces altered physiological and behavioral responses during a social stress-buffering paradigm

Although owners can act as stress buffers for their dogs, whether dogs with poor early life histories with humans will respond similarly is unknown. We tested 45 dogs, 23 of which were rescued from adverse conditions, in a social paradigm in which a threatening stranger confronted them with either their owner or an unfamiliar human present. Salivary cortisol levels were assessed at three points, and the dogs' behavior and owners' responses to questionnaires were evaluated. Dogs from adverse backgrounds engaged in greater contact and exhibited more relaxed behaviors and social referencing when their owners were present. Dogs from the comparison group explored more when accompanied by their owners. Dogs from adverse backgrounds experienced greater decreases in cortisol levels from the first to third samples relative to dogs in the comparison group. Dogs from adverse backgrounds were also more likely to respond fearfully to a threatening stranger. Their owners rated them as having higher levels of stranger-directed fear, nonsocial fear, separation-related problems, attention seeking, and lower levels of chasing and trainability. These findings from this study suggest that early adverse environments may have lasting effects on dogs' social behavior.

The neural circuits of monogamous behavior

The interest in studying the neural circuits related to mating behavior and mate choice in monogamous species lies in the parallels found between human social structure and sexual behavior and that of other mammals that exhibit social monogamy, potentially expanding our understanding of human neurobiology and its underlying mechanisms. Extensive research has suggested that social monogamy, as opposed to non-monogamy in mammals, is a consequence of the neural encoding of sociosensory information from the sexual partner with an increased reward value. Thus, the reinforced value of the mate outweighs the reward value of mating with any other potential sexual partners. This mechanism reinforces the social relationship of a breeding pair, commonly defined as a pair bond. In addition to accentuated prosocial behaviors toward the partner, other characteristic behaviors may appear, such as territorial and partner guarding, selective aggression toward unfamiliar conspecifics, and biparental care. Concomitantly, social buffering and distress upon partner separation are also observed. The following work intends to overview and compare known neural and functional circuits that are related to mating and sexual behavior in monogamous mammals. We will particularly discuss reports on Cricetid rodents of the Microtus and Peromyscus genus, and New World primates (NWP), such as the Callicebinae subfamily of the titi monkey and the marmoset (Callithrix spp.). In addition, we will mention the main factors that modulate the neural circuits related to social monogamy and how that modulation may reflect phenotypic differences, ultimately creating the widely observed diversity in social behavior.

The marmoset as a model for investigating the neural basis of social cognition in health and disease

Social-cognitive processes facilitate the use of environmental cues to understand others, and to be understood by others. Animal models provide vital insights into the neural underpinning of social behaviours. To understand social cognition at even deeper behavioural, cognitive, neural, and molecular levels, we need to develop more representative study models, which allow testing of novel hypotheses using human-relevant cognitive tasks. Due to their cooperative breeding system and relatively small size, common marmosets (Callithrix jacchus) offer a promising translational model for such endeavours. In addition to having social behavioural patterns and group dynamics analogous to those of humans, marmosets have cortical brain areas relevant for the mechanistic analysis of human social cognition, albeit in simplified form. Thus, they are likely suitable animal models for deciphering the physiological processes, connectivity and molecular mechanisms supporting advanced cognitive functions. Here, we review findings emerging from marmoset social and behavioural studies, which have already provided significant insights into executive, motivational, social, and emotional dysfunction associated with neurological and psychiatric disorders.

Vocal and physical phenotypes of calsyntenin2 knockout mouse pups model early-life symptoms of the autism spectrum disorder

This study discovered a novel acoustic phenotype in Calsyntenin2 deficient knockout (Clstn2-KO) pups in the neurodevelopment period of 5-9 postnatal days (PND 5-9). The narrowband ultrasonic calls (nUSVs) were less complex (mostly one-note, shorter in duration and higher in peak frequency) in Clsnt2-KO than in wild-type (WT) C57BL/6 J pups. The wideband ultrasonic calls (wUSVs) were produced substantially more often by Clstn2-KO than WT pups. The clicks were longer in duration and higher in peak frequency and power quartiles in Clstn2-KO pups. The elevated discomfort due to additional two-minute maternal separation coupled with experimenter's touch, resulted in significantly higher call rates of both nUSVs and clicks in pups of both genotypes and sexes compared to the previous two-minute maternal separation, whereas the call rate of wUSVs was not affected. In Clstn2-KO pups, the prevalence of emission of wUSVs retained at both sex and both degrees of discomfort, thus providing a reliable quantitative acoustic indicator for this genetic line. Besides the acoustic differences, we also detected the increased head-to-body ratio in Clstn2-KO pups. Altogether, this study demonstrated that lack of such synaptic adhesion protein as calsyntenin2 affects neurodevelopment of vocalization in a mouse as a model organism.

Natural and Experimental Evidence Drives Marmosets for Research on Psychiatric Disorders Related to Stress

Knowledge of the behavioral ecology of marmosets carried out in their natural habitat associated with the advent of a non-invasive technique for measuring steroid hormones in feces has made a significant contribution to understanding their social relationships and sexual strategies. These studies showed that they are mainly monogamous, live in relatively stable social groups according to a social hierarchy in which females compete and males cooperate, and form social bonds similar to humans, which makes this species a potential animal model to study disorders related to social stress. In addition, laboratory studies observed the expression of behaviors similar to those in nature and deepened the descriptions of their social and reproductive strategies. They also characterized their responses to the challenge using behavioral, cognitive, physiological, and genetic approaches that were sexually dimorphic and influenced by age and social context. These findings, added to some advantages which indicate good adaptation to captivity and the benefits of the birth of twins, small size, and life cycle in comparison to primates of the Old World, led to their use as animal models for validating psychiatric diseases such as major depression. Juvenile marmosets have recently been used to develop a depression model and to test a psychedelic brew called Ayahuasca from the Amazon rainforest as an alternative treatment for major depression, for which positive results have been found which encourage further studies in adolescents. Therefore, we will review the experimental evidence obtained so far and discuss the extension of the marmoset as an animal model for depression.

Early social deprivation does not affect cortisol response to acute and chronic stress in zebrafish

Social isolation is a well-established technique for inducing early adversity but, in rodent models, the need of parental care makes it difficult to distinguish the effects of social deprivation from the consequences of nutritional deficiencies. Zebrafish do not require parental care, allowing separation of social deprivation from nutritional deprivation, and have emerged as a promising model to study ontogeny of normal and pathological behaviors relevant for human neuropsychological disorders. Previous reports of life-long isolation in zebrafish showed some consistency with mammalian literature, depicting later social deficits and locomotor hyperactivity. However, unlike reports of higher anxiety and stress behavior in isolated rodents and primates, behavioral responses were tapered in isolated fish. To examine whether life-long developmental isolation has a dampening effect on zebrafish endocrine stress response, we applied stressors to zebrafish siblings that were either isolated or socialized, and compared their whole-body cortisol levels with non-stressed control siblings kept in low-housing densities. Utilizing previously validated paradigms (exposure to novel tank and unpredictable chronic stress), we exposed separate groups (n = 9-14, mixed-sex) of social and isolated zebrafish to acute and chronic stressors and measured their cortisol levels. A univariate ANOVA and post-hoc Tukey's HSD tests confirmed that compared to socially raised control fish, developmental isolation did not increase baseline cortisol levels in zebrafish. Additionally, compared to the non-stressed condition, application of both acute and chronic stressors significantly increased cortisol levels in isolated fish and, to a similar degree, to socially raised fish. Our findings suggest that zebrafish isolation studies may help separate effects of social deprivation from nonsocial aspects of early adversity. These studies further substantiate the use of developmental isolation in zebrafish, particularly with acute and chronic stress paradigms, for modeling neuropsychological disorders.LAY SUMMARYA difficult childhood can make humans react more frequently or severely to later stress and modeling this effect in animals can help explain how and why early stress affects subsequent mental and physical health. Early social isolation does not affect later response to stressful situations in adult zebrafish, providing us with a model of psychiatric disorders that allows separation of effects of poor physical environments (lacking food, shelter, etc.) from poor social environments (lack of appropriate socialization).

Early social experience has life-long effects on baseline but not stress-induced cortisol levels in a cooperatively breeding fish

In cooperatively breeding cichlid fish, the early social environment has lifelong effects on the offspring's behaviour, life-history trajectories and brain gene expression. Here, we asked whether the presence or absence of parents and subordinate helpers during early life also shapes fluctuating levels of cortisol, the major stress hormone in the cichlid Neolamprologus pulcher. To non-invasively characterize baseline and stress-induced cortisol levels, we adapted the 'static' holding-water method often used to collect waterborne steroid hormones in aquatic organisms by including a flow-through system allowing for repeated sampling without handling of the experimental subjects. We used 8-year-old N. pulcher either raised with (+F) or without (-F) parents and helpers in early life. We found that N. pulcher have a peak of their circadian cortisol cycle in the early morning, and that they habituated to the experimental procedure after four days. Therefore, we sampled the experimental fish in the afternoon after four days of habituation. -F fish had significantly lower baseline cortisol levels, whereas stress-induced cortisol levels did not differ between treatments. Thus, we show that the early social environment has life-long effects on aspects of the physiological stress system of the Hypothalamic-Pituitary-Interrenal (HPI) axis. We discuss how these differences in physiological state may have contributed to the specialization in different social and life-history trajectories of this species.

The Relevance of Operant Behavior in Conceptualizing the Psychological Well-Being of Captive Animals

The term "psychological well-being" is used in reference to husbandry with animals in human care settings such as research, agriculture, and zoos. This article seeks to clarify and conceptualize the term based upon two approaches that draw from several bodies of literature: the experimental analysis of behavior, experimental psychology, animal welfare and husbandry, farm animal behavior, zoo husbandry, and ethology. One approach focuses on the presence of problem behavior such as stereotypies, depressive-like behavior, and aggression, and emphasizes the conditions under which aberrant behavior in animals under human care occurs. The second approach examines what might be considered wellness by emphasizing opportunities to engage with its environment, or the absence of such opportunities, even if problematic behavior is not exhibited. Here, access to an interactive environment is relatively limited so opportunities for operant (voluntary) behavior could be considered. Designing for operant behavior provides opportunities for variability in both behavior and outcomes. Operant behavior also provides control over the environment, a characteristic that has been a core assumption of well-being. The importance of interactions with one's environment is especially evident in observations that animals prefer opportunities to work for items necessary for sustenance, such as food, over having them delivered freely. These considerations raise the importance of operant behavior to psychological well-being, especially as benefits to animals under human care.

Noninvasive cardiac psychophysiology as a tool for translational science with marmosets

The importance of marmosets for comparative and translational science has grown in recent years because of their relatively rapid development, birth cohorts of twins, family social structure, and genetic tractability. Despite this, they remain understudied in investigations of affective processes. In this methodological note, we establish the validity of using noninvasive commercially available equipment to record cardiac physiology and compute indices of autonomic nervous system activity-a major component of affective processes. Specifically, we recorded electrocardiogram and impedance cardiogram, from which we derived heart rate, respiration rate, measures of high-frequency heart rate variability (indices of parasympathetic autonomic nervous system activity), and ventricular contractility (an index of sympathetic autonomic nervous system activity). Our methods produced physiologically plausible data, and further, animals with increased heart rates during testing were also more reactive to isolation from their social partner and presentation of novel objects, though no relationship was observed between reactivity and specific indices of parasympathetic or sympathetic nervous system activity.

Acute effects of ayahuasca in a juvenile non-human primate model of depression

OBJECTIVE

The incidence rate of major depression in adolescents reaches approximately 14%. This disorder is usually recurrent, without remission of symptoms even after pharmacological treatment, and persists throughout adult life. Since the effects of antidepressants take approximately 2 weeks to begin, new pharmacological therapies are under continuous exploration. Recent evidence suggests that psychedelics could produce rapid antidepressant effects. In this study, we evaluated the potential antidepressant effects of ayahuasca in a juvenile non-human primate model of depression.

METHODS

While living with their families, juvenile marmosets (8 males; 7 females) were observed on alternate days for four weeks during a baseline phase. This was followed by 8 weeks of an induced depressive state protocol, the social isolated context (IC), in which the animals were monitored in the first and last weeks. Subsequently, five males and four females were randomly selected for treatment, first with a single administration of saline vehicle (1.67 mL/300 g of body weight, via gavage), followed by a single dose of ayahuasca (1.67 mL/300 g of body weight, via gavage). Both phases lasted 1 week and the animals were monitored daily. A third week of sampling was called the tardive-pharmacological effects phase. In all phases the marmosets were assessed for behavior, fecal cortisol levels, and body weight.

RESULTS

After IC, the animals presented typical hypocortisolemia, but cortisol recovered to baseline levels 24 h after an acute dose of ayahuasca; this recovery was not observed in vehicle-treated animals. Additionally, in males, ayahuasca, but not the vehicle, reduced scratching, a stereotypic behavior, and increased feeding. Ayahuasca treatment also improved body weight to baseline levels in both sexes. The ayahuasca-induced behavioral response had long-term effects (14 days). Thus, in this translational juvenile animal model of depression, ayahuasca presented beneficial effects.

CONCLUSIONS

These results can contribute to the validation of ayahuasca as an antidepressant drug and encourage new studies on psychedelic drugs as a tool for treating mood disorders, including for adolescents with early-onset depression.

Stress, sensitive periods, and substance abuse

Research on the inter-relationship between drug abuse and social stress has primarily focused on the role of stress exposure during adulthood and more recently, adolescence. Adolescence is a time of heightened reward sensitivity, but it is also a time when earlier life experiences are expressed. Exposure to stress early in postnatal life is associated with an accelerated age of onset for drug use. Lifelong addiction is significantly greater if drug use is initiated during early adolescence. Understanding how developmental changes following stress exposure interact with sensitive periods to unfold over the course of maturation is integral to reducing their later impact on substance use. Arousal levels, gender/sex, inflammation, and the timing of stress exposure play a role in the vulnerability of these circuits. The current review focuses on how early postnatal stress impacts brain development during a sensitive period to increase externalizing and internalizing behaviors in adolescence that include social interactions (aggression; sexual activity), working memory impairment, and depression. How stress effects the developmental trajectories of brain circuits that are associated with addiction are discussed for both clinical and preclinical studies.

Defining predictive factors for reproductive output in captive common marmosets (Callithrix jacchus)

Common marmosets (Callithrix jacchus) demonstrate variations in reproductive output, not only in terms of total reproductive output during a lifetime but also in litter size per parturition. The present study explores factors, such as parents’ litter size, parturition number, maternal body weight at conception and maternal age, which may account for this variation. A retrospective analysis of clinical records of a captive breeding colony was conducted over a 9‐year period yielding reproductive summaries of 26 dams and 22 sires producing a total of 115 litters. Dams born from litters of  ≤2 (N = 20) more often produced litters of ≤2, whereas dams born from litters of >2 (N = 6) more often produced litters of >2 (p < 0.05). The dams’ maternal body weight at the time of conception had also a significant effect on subsequent litter size. In addition, the chance of triplets was higher after the second parturition. Maternal age, interbirth interval, and season of birth had no effect on litter size. Factors relating to the sire had a negligible effect on the size of the litter. Multivariate statistical modeling revealed that the dams’ original litter size, maternal bodyweight at conception and parturition number are determining factors for the number of babies per litter. This study identified factors determining marmoset litter size, some of which (maternal litter size) are novel to this study and were not reported previously. Further exploration of the potential role of maternal litter size as a determinant of the litter sizes produced by marmoset breeders is warranted.

Oxytocin regulates reunion affiliation with a pairmate following social separation in marmosets

While separation from significant social partners produces a host of neurobiological and behavioral perturbations, including behavioral distress and increased glucocorticoid production, positive social interactions upon reunion are critical for the reestablishment of normative relationship dynamics and the attenuation of the biobehavioral stress response. The hormone oxytocin has critical and pervasive roles in reproductive and behavioral processes across the lifespan, and plays a particularly prominent role in social bonding. In this study, we examined the extent that oxytocin modulates interactions with a pairmate following separation challenges that varied in both social context (isolation; separation) and duration (long; short), in marmosets. We demonstrated that the impact of pharmacological manipulations of the oxytocin system on the expression of affiliation upon reunion depended on both the context and duration of the separation challenge. Specifically, marmosets treated with an oxytocin antagonist spent less time in proximity with their pairmate upon reunion following a long-separation challenge. During the short-separation challenge, marmosets engaged in more social gaze when separated with an opposite-sex stranger, but not when separated with their mate. Furthermore, marmosets that received the most social gaze from opposite-sex strangers spent the most time in proximity with their long-term mate upon reunion. We also showed that marmosets treated with an OT agonist received increased levels of gaze from opposite-sex strangers, but not from their mate. Overall, these results suggest that marmosets are sensitive to the nature of the social interactions during separation, and subsequently alter their expression of affiliation upon reunion with their long-term mate. These findings further implicate oxytocin as a bond-enhancing molecule that regulates the reestablishment of normative levels of affiliation with a mate following separation, and add to the emerging literature that suggests the OT system underlies critical behavioral processes that contribute to the preservation of long-lasting social bonds.

Enduring Effects of Paternal Deprivation in California Mice (Peromyscus californicus): Behavioral Dysfunction and Sex-Dependent Alterations in Hippocampal New Cell Survival

Early-life experiences with caregivers can significantly affect offspring development in human and non-human animals. While much of our knowledge of parent-offspring relationships stem from mother-offspring interactions, increasing evidence suggests interactions with the father are equally as important and can prevent social, behavioral, and neurological impairments that may appear early in life and have enduring consequences in adulthood. In the present study, we utilized the monogamous and biparental California mouse (Peromyscus californicus). California mouse fathers provide extensive offspring care and are essential for offspring survival. Non-sibling virgin male and female mice were randomly assigned to one of two experimental groups following the birth of their first litter: (1) biparental care: mate pairs remained with their offspring until weaning; or (2) paternal deprivation (PD): paternal males were permanently removed from their home cage on postnatal day (PND) 1. We assessed neonatal mortality rates, body weight, survival of adult born cells in the dentate gyrus of the hippocampus, and anxiety-like and passive stress-coping behaviors in male and female young adult offspring. While all biparentally-reared mice survived to weaning, PD resulted in a ~35% reduction in survival of offspring. Despite this reduction in survival to weaning, biparentally-reared and PD mice did not differ in body weight at weaning or into young adulthood. A sex-dependent effect of PD was observed on new cell survival in the dentate gyrus of the hippocampus, such that PD reduced cell survival in female, but not male, mice. While PD did not alter classic measures of anxiety-like behavior during the elevated plus maze task, exploratory behavior was reduced in PD mice. This observation was irrespective of sex. Additionally, PD increased some passive stress-coping behaviors (i.e., percent time spent immobile) during the forced swim task—an effect that was also not sex-dependent. Together, these findings demonstrate that, in a species where paternal care is not only important for offspring survival, PD can also contribute to altered structural and functional neuroplasticity of the hippocampus. The mechanisms contributing to the observed sex-dependent alterations in new cell survival in the dentate gyrus should be further investigated.

Common Marmosets: A Potential Translational Animal Model of Juvenile Depression

Major depression is a psychiatric disorder with high prevalence in the general population, with increasing expression in adolescence, about 14% in young people. Frequently, it presents as a chronic condition, showing no remission even after several pharmacological treatments and persisting in adult life. Therefore, distinct protocols and animal models have been developed to increase the understanding of this disease or search for new therapies. To this end, this study investigated the effects of chronic social isolation and the potential antidepressant action of nortriptyline in juvenile Callithrix jacchus males and females by monitoring fecal cortisol, body weight, and behavioral parameters and searching for biomarkers and a protocol for inducing depression. The purpose was to validate this species and protocol as a translational model of juvenile depression, addressing all domain criteria of validation: etiologic, face, functional, predictive, inter-relational, evolutionary, and population. In both sexes and both protocols (IDS and DPT), we observed a significant reduction in cortisol levels in the last phase of social isolation, concomitant with increases in autogrooming, stereotyped and anxiety behaviors, and the presence of anhedonia. The alterations induced by chronic social isolation are characteristic of the depressive state in non-human primates and/or in humans, and were reversed in large part by treatment with an antidepressant drug (nortriptyline). Therefore, these results indicate C. jacchus as a potential translational model of juvenile depression by addressing all criteria of validation.

Paternal deprivation affects social behaviors and neurochemical systems in the offspring of socially monogamous prairie voles

Early life experiences, particularly the experience with parents, are crucial to phenotypic outcomes in both humans and animals. Although the effects of maternal deprivation on offspring well-being have been studied, paternal deprivation (PD) has received little attention despite documented associations between father absence and children health problems in humans. In the present study, we utilized the socially monogamous prairie vole (Microtus ochrogaster), which displays male-female pair bonding and bi-parental care, to examine the effects of PD on adult behaviors and neurochemical expression in the hippocampus. Male and female subjects were randomly assigned into one of two experimental groups that grew up with both the mother and father (MF) or with the mother-only (MO, to generate PD experience). Our data show that MO subjects received less parental licking/grooming and carrying and were left alone in the nest more frequently than MF subjects. At adulthood (∼75days of age), MO subjects displayed increased social affiliation (SOA) toward a conspecific compared to MF subjects, but the two groups did not differ in social recognition (SOR) and anxiety-like behavior. Interestingly, MO subjects showed consistent increases in both gene and protein expression of the brain-derived neurotrophic factor (BDNF) and tropomyosin receptor kinase B (TrkB) as well as the levels of total histone 3 and histone 3 acetylation in the hippocampus compared to MF subjects. Further, PD experience increased glucocorticoid receptor beta (GRβ) protein expression in the hippocampus of females as well as increased corticotrophin receptor 2 (CRHR2) protein expression in the hippocampus of males, but decreased CRHR2 mRNA in both sexes. Together, our data suggest that PD has a long-lasting, behavior-specific effect on SOA and alters hippocampal neurochemical systems in the vole brain. The functional role of such altered neurochemical systems in social behaviors and the potential involvement of epigenetic events should be further studied.

Neurobehavioral assessment of maternal odor in developing rat pups: implications for social buffering

Social support can attenuate the behavioral and stress hormone response to threat, a phenomenon called social buffering. The mother's social buffering of the infant is one of the more robust examples; yet we understand little about the neurobiology. Using a rodent model, we explore the neurobiology of social buffering by assessing neural processing of the maternal odor, a major cue controlling social buffering in rat pups. We used pups before (postnatal day (PN) 7) and after (PN14, PN23) the functional emergence of social buffering. Pups were injected with 14C 2-deoxyglucose (2-DG) and presented with the maternal odor, a control preferred odor incapable of social buffering (acetophenone), or no odor. Brains were removed, processed for autoradiography and brain areas identified as important in adult social buffering were assessed, including the amygdala basolateral complex (Basolateral Amygdala [BLA]), medial prefrontal cortex (mPFC), and anterior cingulate cortex (ACC). Results suggest dramatic changes in the processing of maternal odor. PN7 pups show mPFC and ACC activation, although PN14 pups showed no activation of the mPFC, ACC, or BLA. All brain areas assessed were recruited by PN23. Additional analysis suggests substantial changes in functional connectivity across development. Together, these results imply complex nonlinear transitions in the neurobiology of social buffering in early life that may provide insight into the changing role of the mother in supporting social buffering.

Consequences of early adverse rearing experience(EARE) on development: insights from non-human primate studies

Early rearing experiences are important in one's whole life, whereas early adverse rearing experience(EARE) is usually related to various physical and mental disorders in later life. Although there were many studies on human and animals, regarding the effect of EARE on brain development, neuroendocrine systems, as well as the consequential mental disorders and behavioral abnormalities, the underlying mechanisms remain unclear. Due to the close genetic relationship and similarity in social organizations with humans, non-human primate(NHP) studies were performed for over 60 years. Various EARE models were developed to disrupt the early normal interactions between infants and mothers or peers. Those studies provided important insights of EARE induced effects on the physiological and behavioral systems of NHPs across life span, such as social behaviors(including disturbance behavior, social deficiency, sexual behavior, etc), learning and memory ability, brain structural and functional developments(including influences on neurons and glia cells, neuroendocrine systems, e.g., hypothalamic-pituitary-adrenal(HPA) axis, etc). In this review, the effects of EARE and the underlying epigenetic mechanisms were comprehensively summarized and the possibility of rehabilitation was discussed.

Maternal Deprivation Influences Pup Ultrasonic Vocalizations of C57BL/6J Mice

Maternal deprivation (MD) is frequently used as an early life stress model in rodents to investigate behavioral and neurological responses under stressful conditions. However, the effect of MD on the early postnatal development of rodents, which is when multiple neural systems become established, is rarely investigated due to methodological limitations. Ultrasonic vocalizations (USVs) are one of the few responses produced by neonatal rodents that can be quantitatively analyzed, and the quantification of USVs is regarded as a novel approach to investigate possible alterations in the neurobehavioral and emotional development of infant rodents under stress. To investigate the effect of MD on pup mice, we subjected C57BL/6J mice to MD and recorded the USVs of pups on postnatal days 1, 3, 7, 8, and 14. To determine whether the effect of MD on USVs was acute or cumulative, pre- and post-separation USV groups were included; sex differences in pup USV emission were also investigated. Our results suggest that (i) USV activity was high on postnatal days 3-8; (ii) the MD effect on USVs was acute, and a cumulative effect was not found; (iii) the MD mice vocalized more and longer than the controls at a lower frequency, and the effect was closely related to age; and (iv) female pups were more susceptible than males to the effect of MD on USV number and duration between postnatal days 3-8.

Early adverse experience and substance addiction: dopamine, oxytocin, and glucocorticoid pathways

Substance addiction may follow a chronic, relapsing course and critically undermine the physical and psychological well-being of the affected individual and the social units of which the individual is a member. Despite the public health burden associated with substance addiction, treatment options remain suboptimal, with relapses often seen. The present review synthesizes growing insights from animal and human research to shed light upon developmental and neurobiological pathways that may increase susceptibility to addiction. We examine the dopamine system, the oxytocin system, and the glucocorticoid system, as they are particularly relevant to substance addiction. Our aim is to delineate how early adverse experience may induce long-lasting alterations in each of these systems at molecular, neuroendocrine, and behavioral levels and ultimately lead to heightened vulnerability to substance addiction. We further discuss how substance addiction in adulthood may increase the risk of suboptimal caregiving for the next generation, perpetuating the intergenerational cycle of early adverse experiences and addiction.

Perceived social isolation, evolutionary fitness and health outcomes: a lifespan approach

Sociality permeates each of the fundamental motives of human existence and plays a critical role in evolutionary fitness across the lifespan. Evidence for this thesis draws from research linking deficits in social relationship--as indexed by perceived social isolation (i.e. loneliness)--with adverse health and fitness consequences at each developmental stage of life. Outcomes include depression, poor sleep quality, impaired executive function, accelerated cognitive decline, unfavourable cardiovascular function, impaired immunity, altered hypothalamic pituitary-adrenocortical activity, a pro-inflammatory gene expression profile and earlier mortality. Gaps in this research are summarized with suggestions for future research. In addition, we argue that a better understanding of naturally occurring variation in loneliness, and its physiological and psychological underpinnings, in non-human species may be a valuable direction to better understand the persistence of a 'lonely' phenotype in social species, and its consequences for health and fitness.

Physiological and behavioral responses to routine procedures in captive common marmosets (Callithrix jacchus)

The effect of routine captive procedures on the welfare of species used as experimental models in biomedical research is of great interest, since stress may alter the generalization and interpretation of results. This study investigated behavioral and endocrine (fecal cortisol) reactivity patterns in common marmoset (Callithrix jacchus) adult males (N = 10) and females (N = 9) subjected to three types of routine procedures in captivity: (1) moving to a same-sized cage (P1), to a smaller cage (P2), and (2) first-time pair formation (P3). Sexually dimorphic cortisol responses were detected in animals submitted to a physical environmental stressor (cage change). Females showed an increased response throughout P1, in relation to baseline (BP) cortisol, and a trend during P2. Males increased cortisol only during P2. On the other hand, males and females showed a similar endocrine response when management involved social challenge (pair formation), with both sexes increasing cortisol levels, but females exhibited a more intense and longer-lasting cortisol increase. Males and females exhibited similar behavioral responses to cage change, except for autogrooming, with males decreasing this behavior in P1. Only females demonstrated a significantly higher increase in piloerection frequency than that of males during the pair formation phase. These endocrine and behavioral changes must be taken into account when interpreting research data that involve these types of procedures. Further studies on the impacts of routine colony management are required to devise and include protocols in official husbandry guidelines.

Early-life Social Adversity and Developmental Processes in Nonhuman Primates

Most primate species produce offspring that are altricial and highly dependent upon caregivers. As a consequence, a host of developmental trajectories can be dramatically altered by variation in early experiences. We review the impact of early social experiences (in both experimental models and natural contexts) on developmental profiles in three species of nonhuman primates: marmosets, squirrel monkeys, and macaques. Graded exposure to early-life social adversity (ELSA) produces short- to long-term effects on multiple developmental outcomes, including affect, social behavior, cognitive and attentional processes, and in the neural substrates that underlie these sociobehavioral traits.

Endocrine and Cognitive Adaptations to Cope with Stress in Immature Common Marmosets (Callithrix jacchus): Sex and Age Matter

Phenotypic sex differences in primates are associated with body differentiation during the early stages of life, expressed in both physiological and behavioral features. Hormones seem to play a pivotal role in creating a range of responses to meet environmental and social demands, resulting in better reactions to cope with challenges to survival and reproduction. Steroid hormones actively participate in neuroplasticity and steroids from both gonads and neurons seem to be involved in behavioral modulation in primates. Indirect evidence suggests the participation of sexual steroids in dimorphism of the stress response in common marmosets. This species is an important experimental model in psychiatry, and we found a dual profile for cortisol in the transition from juvenile to subadult, with females showing higher levels. Immature males and females at 6 and 9 months of age moved alone from the family group to a new cage, over a 21-day period, expressed distinct patterns of cortisol variation with respect to range and duration of response. Additional evidence showed that at 12 months of age, males and females buffered the hypothalamic-pituitary-adrenal axis during chronic stress. Moreover, chronic stressed juvenile marmoset males showed better cognitive performance in working memory tests and motivation when compared to those submitted to short-term stress living in family groups. Thus, as cortisol profile seems to be sexually dimorphic before adulthood, age and sex are critical variables to consider in approaches that require immature marmosets in their experimental protocols. Moreover, available cognitive tests should be scrutinized to allow better investigation of cognitive traits in this species.

Social buffering of stress responses in nonhuman primates: Maternal regulation of the development of emotional regulatory brain circuits

Social buffering, the phenomenon by which the presence of a familiar individual reduces or even eliminates stress- and fear-induced responses, exists in different animal species and has been examined in the context of the mother-infant relationship, in addition to adults. Although it is a well-known effect, the biological mechanisms that underlie it as well as its developmental impact are not well understood. Here, we provide a review of evidence of social and maternal buffering of stress reactivity in nonhuman primates, and some data from our group suggesting that when the mother-infant relationship is disrupted, maternal buffering is impaired. This evidence underscores the critical role that maternal care plays for proper regulation and development of emotional and stress responses of primate infants. Disruptions of the parent-infant bond constitute early adverse experiences associated with increased risk for psychopathology. We will focus on infant maltreatment, a devastating experience not only for humans, but for nonhuman primates as well. Taking advantage of this naturalistic animal model of adverse maternal caregiving, we have shown that competent maternal care is critical for the development of healthy attachment, social behavior, and emotional and stress regulation, as well as of the neural circuits underlying these functions.

Reunion behavior after social separation is associated with enhanced HPA recovery in young marmoset monkeys

The relationships that offspring develop with caregivers can exert a powerful influence on behavior and physiology, including the hypothalamic-pituitary-adrenal (HPA) axis. In many mammalian species, offspring-caregiver relationships are largely limited to interactions with mother. Marmoset monkeys receive care in early life from multiple classes of caregivers in addition to the mother, including fathers and siblings. We evaluated whether affiliative social interactions with family members in marmosets were associated with differences in cortisol reactivity to a short-term social separation stressor, and whether these variations in affiliative interactions upon reunion predicted how well marmosets subsequently regulated HPA axis function after cessation of the stressor. Marmosets were separated from the family for 8h at three developmental time points (6-, 12-, and 18-months of age), and interactions of the separated marmoset with the family group were recorded during reunion. Urinary cortisol was measured prior to social separation, every 2h during the separation, and on the morning after separation. Heightened cortisol reactivity during social separation did not predict affiliative social behavior upon reunion but higher rates of grooming and play behavior predicted enhanced HPA regulation. Marmosets with higher rates of grooming and play with family members upon reunion had post-stress cortisol levels closer to preseparation baseline than marmosets with lower rates of affiliative reunion behavior. Combined with previous research showing the early programming effects of social interactions with caregivers, as well as the buffering effect of a close social partner during stress, the current study highlights the high degree of behavioral and HPA adaptability to social stressors across development in marmoset monkeys.

Gestational cortisol and social play shape development of marmosets' HPA functioning and behavioral responses to stressors

Both gestational cortisol exposure (GCE) and variability in postnatal environments can shape the later-life behavioral and endocrine outcomes of the hypothalamic-pituitary-adrenal (HPA) axis. We examined the influence of GCE and social play on HPA functioning in developing marmosets. Maternal urinary cortisol samples were collected across pregnancy to determine GCE for 28 marmoset offspring (19 litters). We administered a social separation stressor to offspring at 6, 12, and 18 months of age, during which we collected urinary cortisol samples and behavioral observations. Increased GCE was associated with increased basal cortisol levels and cortisol reactivity, but the strength of this relationship decreased across age. Increased social play was associated with decreased basal cortisol levels and a marginally greater reduction in cortisol reactivity as offspring aged, regardless of offspring GCE. Thus, GCE is associated with HPA functioning, but socially enriching postnatal environments can alter the effects associated with increased fetal exposure to glucocorticoids.

Social isolation disrupts hippocampal neurogenesis in young non-human primates

Social relationships are crucial for the development and maintenance of normal behavior in non-human primates. Animals that are raised in isolation develop abnormal patterns of behavior that persist even when they are later reunited with their parents. In rodents, social isolation is a stressful event and is associated with a decrease in hippocampal neurogenesis but considerably less is known about the effects of social isolation in non-human primates during the transition from adolescence to adulthood. To investigate how social isolation affects young marmosets, these were isolated from other members of the colony for 1 or 3 weeks and evaluated for alterations in their behavior and hippocampal cell proliferation. We found that anxiety-related behaviors like scent-marking and locomotor activity increased after social isolation when compared to baseline levels. In agreement, grooming—an indicative of attenuation of tension—was reduced among isolated marmosets. These results were consistent with increased cortisol levels after 1 and 3 weeks of isolation. After social isolation (1 or 3 weeks), reduced proliferation of neural cells in the subgranular zone of dentate granule cell layer was identified and a smaller proportion of BrdU-positive cells underwent neuronal fate (doublecortin labeling). Our data is consistent with the notion that social deprivation during the transition from adolescence to adulthood leads to stress and produces anxiety-like behaviors that in turn might affect neurogenesis and contribute to the deleterious consequences of prolonged stressful conditions.

Behavioral responses to social separation stressor change across development and are dynamically related to HPA activity in marmosets

Psychosocial stressors activate two distinct stress-response systems, a central, behavioral response, and a peripheral, endocrine response. Both behavioral and endocrine responses to stressors are subject to individual and developmental variables, but it is not known whether stressor induced behaviors are stable across development, and how they correspond with changes in the endocrine component of the stress response. We characterized the development and stability of behavioral responses to a mild psychosocial stressor in marmosets (Callithrix geoffroyi), and assessed the degree to which the behavioral and endocrine stress-response systems were co-activated. The behavioral response to stressors was stable within individuals, but only some stressor-induced behaviors changed as the monkeys developed. Overall, there was more variability in the development of behavioral responses compared to stress-induced endocrine profiles found previously [French et al., 2012. Horm Behav 61:196-203]. In young marmosets, only increased alarm calling was correlated with increased cortisol reactivity, and in older marmosets increased cage manipulations and motor activity were associated with poorer post-stressor cortisol regulation. Because these relationships were so few, we conclude that while the behavioral and endocrine systems follow a similar developmental trajectory, each system maintains a level of independence. Furthermore, the relationship between stressor-induced behaviors and HPA activity changes across development.

Nonhuman primate models of depression: effects of early experience and stress

Depression causes significant morbidity in the human population. The Diathesis-Stress/Two-Hit model of depression hypothesizes that stress interacts with underlying (probably genetic) predispositions to produce a central nervous system that is primed to express psychopathology when confronted with stressful experiences later in life. Nonhuman primate (NHP) studies have been extensively utilized to test this model. NHPs are especially useful for studying effects of early experience, because many aspects of NHP infancy are similar to humans, whereas development occurs at an accelerated rate and therefore allows for more rapid assessment of experimental variables. In addition, the ability to manipulate putative risk factors, including introducing experimental stress during development, allows inference of causality not possible with human studies. This manuscript reviews experimental paradigms that have been utilized to model early adverse experience in NHPs, including peer-rearing, maternal separation, and variable foraging. It also provides examples of how this model has been used to investigate the effects of early experience on later neurobiology, physiology, and behavior associated with depression. We conclude that the NHP offers an excellent model to research mechanisms contributing to the Diathesis-Stress/Two-Hit model of depression.

Quality of maternal and paternal care predicts later stress reactivity in the cooperatively-breeding marmoset (Callithrix geoffroyi)

Variation in the early postnatal social environment can have lasting effects on hypothalamic-pituitary-adrenal (HPA) axis stress responses. Both rats and macaque monkeys subjected to low quality or abusive maternal care during the early postnatal period have more pronounced HPA responses to environmental stressors throughout development and into adulthood compared to animals reared in higher quality early maternal environments. However, little is known about the relative contributions to HPA stress response styles in developing offspring in species in which offspring care is routinely provided by group members other than the mother, such as in cooperatively breeding mammals. Marmoset monkeys exhibit cooperative offspring rearing, with fathers and older siblings providing care in addition to that provided by the mother. We evaluated the effects of early maternal, paternal, and older sibling care on HPA responses to social separation across development in captive white-faced marmoset offspring (Callithrix geoffroyi). We monitored offspring care by mothers, fathers, and older siblings in marmosets for the first 60 days of life. Later in development, each marmoset experienced three standardized social separation/novelty exposure stressors at 6, 12, and 18 months of age. During separation, we collected urine samples and analyzed them via enzyme immunoassay for cortisol levels. Infants that received higher rates of rejections from the entire family group showed higher cortisol responses to social separation. This relationship was found when mothers, fathers, and older siblings, were analyzed separately as well. No differences in cortisol responses were found between offspring that received high and low rates of carrying or high and low rates of licking and grooming by any group member. In the cooperatively breeding marmoset, early social cues from multiple classes of caregivers may influence HPA stress responses throughout the lifespan.

Stress, the HPA axis, and nonhuman primate well-being: A review

Numerous stressors are routinely encountered by wild-living primates (e.g., food scarcity, predation, aggressive interactions, and parasitism). Although many of these stressors are eliminated in laboratory environments, other stressors may be present in that access to space and social partners is often restricted. Stress affects many physiological systems including the hypothalamic-pituitary-adrenocortical (HPA) axis, which is the focus of this review. The glucocorticoid, cortisol, is the ultimate output of this system in nonhuman primates, and levels of this hormone are used as an index of stress. Researchers can measure cortisol from several sampling matrices that include blood, saliva, urine, faeces, and hair. A comparison of the advantages and disadvantages of each sampling matrix is provided to aid researchers in selecting an optimal strategy for their research. Stress and its relationship to welfare have been examined in nonhuman primates using two complimentary approaches: comparing baseline cortisol levels under different conditions, or determining the reactivity of the system through exposure to a stressor. Much of this work is focused on colony management practices and developmental models of abnormal behaviour. Certain colony practices are known to increase stress at least temporarily. Both blood sampling and relocation are examples of this effect, and efforts have been made to reduce some of the more stressful aspects of these procedures. In contrast, other colony management practices such as social housing and environmental enrichment are hypothesized to reduce stress. Testing this hypothesis by comparing baseline cortisol levels has not proved useful, probably due to "floor" effects; however, social buffering studies have shown the powerful role of social housing in mitigating reactions of nonhuman primates to stressful events. Models of abnormal behaviour come from two sources: experimentally induced alterations in early experience (e.g., nursery rearing), and the spontaneous development of behavioural pathology (e.g., self-injurious behaviour). Investigators have often assumed that abnormal behaviour is a marker for stress and thus such monkeys are predicted to have higher cortisol levels than controls. However, an emerging finding is that monkeys with abnormal behaviour are more likely to show a pattern of lowered cortisol concentrations which may reflect either an altered set point or a blunting of the stress response system. These findings parallel human clinical studies demonstrating that neuropsychiatric disorders may be associated with either increased or decreased activity of the HPA system, depending on the aetiology and manifestation of the disorder and their potential influence in provoking allostatic shifts in system functioning.

Social and emotional predictors of the tempo of puberty in female rhesus monkeys

A cascade of neuroendocrine events regulates the initiation and progression of female puberty. However, the factors that determine the timing of these events across individuals are still uncertain. While the consequences of puberty on subsequent emotional development and adult behavior have received significant attention, what is less understood are the social and environmental factors that actually alter the initiation and progression of puberty. In order to more fully understand what factors influence pubertal timing in females, the present study quantified social and emotional behavior; stress physiology; and growth and activity measures in juvenile female rhesus monkeys to determine what best predicts eventual puberty. Based on previous reports, we hypothesized that increased agonistic behavior resulting from subordinate status in their natal group, in combination with slowed growth, reduced prosocial behavior, and increased emotional reactivity would predict delayed puberty. The analyses were restricted to behavioral and physiological measures obtained prior to the onset of puberty, defined as menarche. Together, our findings indicate that higher rates of aggression but lower rates of submission received from group mates; slower weight gain; and greater emotional reactivity, evidenced by higher anxiety, distress and appeasing behaviors, and lower cortisol responsivity in response to a potentially threatening situation, predicts delayed puberty. Together the combination of these variables accounted for 58% of the variance in the age of menarche, 71% in age at first ovulation, and 45% in the duration of adolescent sterility. While early puberty may be more advantageous for the individual from a fertility standpoint, it presents significant health risks, including increased risk for a number of estrogen dependent cancers and as well as the emergence of mood disorders during adulthood. On the other hand, it is possible that increased emotional reactivity associated with delayed puberty could persist, increasing the risk for emotional dysregulation to socially challenging situations. The data argue for prospective studies that will determine how emotional reactivity shown to be important for pubertal timing is affected by early social experience and temperament, and how these stress-related variables contribute to body weight accumulation, affecting the neuroendocrine regulation of puberty.

The influence of sex and relatedness on stress response in common marmosets (Callithrix jacchus)

Research in stress physiology has demonstrated the benefits of receiving social support during stressful conditions. However, recent data have shown that the efficacy of social support in buffering physiological and behavioral responses to stressor agents depends on species, sex, and relatedness among animals. This study investigated whether different kinds of social support (presence of same sex related or nonrelated conspecifics) have the same effect on hormonal (fecal cortisol levels) and behavioral responses (agonistic: scent-marking and individual piloerection; anxiety: locomotion; tension-reducing: autogrooming, allogrooming, and body contact). We used adult male and female isosexual dyads of Callithrix jacchus, a small Neotropical primate from the Callitrichidae family, widely used in the study of stress and related diseases. Following a 28-day baseline phase, dyads faced three challenging situations (phase 1: dyads were moved together from the baseline cage to a similar new cage; phase 2: each dyad member was moved alone to a new cage; and phase 3: dyad members were reunited in the same baseline cage). Type of social support was found to influence the response to stressors differently for each sex. Related male dyads did not change their hormonal or behavioral profile over the three experimental phases, when compared to the baseline phase. For nonrelated male dyads, social support buffered hormonal but not behavioral response. For females, the social support offered by a related and nonrelated animal, does not seem to buffer the stress response, as shown by correlations between agonistic behaviors versus cortisol and locomotion during all three experimental phases and a significant increase in fecal cortisol levels during phases 2 and 3, when compared with baseline levels. The results only partially support the buffering model theory and corroborate other studies reporting that the benefits of social support during a period of crisis arise only when it is adaptive for that species.

Stress reactivity in young marmosets (Callithrix geoffroyi): ontogeny, stability, and lack of concordance among co-twins

Variation in response styles in the hypothalamic-pituitary-adrenal (HPA) axis are known to be predictors of short- and long-term health outcomes. The nature of HPA responses to stressors changes with developmental stage, and some components of the stress response exhibit long-term individual consistency (i.e., are trait-like) while others are transient or variable (i.e., state-like). Here we evaluated the response of marmoset monkeys (Callithrix geoffroyi) to a standardized social stressor (social separation and exposure to a novel environment) at three different stages of development: juvenile, subadult, and young adult. We monitored levels of urinary cortisol (CORT), and derived multiple measures of HPA activity: Baseline CORT, CORT reactivity, CORT Area Under the Curve (AUC), and CORT regulation. Juvenile marmosets exhibited the most dramatic stress response, had higher AUCs, and tended to show poorer regulation. While baseline CORT and CORT regulation were not consistent within an individual across age, CORT reactivity and measures of AUC were highly correlated across time; i.e., individuals with high stress reactivity and AUC as juveniles also had high measures as subadults and adults, and vice-versa. Marmoset co-twins did not exhibit similar patterns of stress reactivity. These data suggest that regardless of the source of variation in stress response styles in marmosets, individually-distinctive patterns are established by six months of age, and persist for at least a year throughout different phases of marmoset life history.

Diazepam-induced decrease in anxiety-like behaviors of marmoset monkeys exposed to a novel open-field

Unfamiliar environments can be a source of stress, fear and anxiety for marmoset monkeys. In spite of existing data, the influence of putative anxiolytics on the effects of novel environments has yet to be tested in primates. Therefore, the behavior of adult black tufted-ear marmosets to a single brief (15 min) exposure to a novel environment was analyzed in the presence and absence of diazepam (DZP). Marmosets were pre-treated with vehicle (n=5) or diazepam (0.5 mg/kg, ip; n=5) and submitted to a 15 min free exploration trial within a rectangular open-field arena. DZP-treated subjects, compared to vehicle controls, demonstrated significantly lower rates of (phee) contact calls and exploration, while a higher scan duration. Sojourn time in the arena's central zone was also significantly higher in the former group and sedation was not observed. Thus, pre-treatment with the benzodiazepine DZP decreased several anxiety-related behaviors induced by subjecting the marmosets to a new environment. The results also indicate that, as with rodent subjects, the open-field may provide a useful simple paradigm for assessing anxiety-like behaviors in this primate and, as such, constitutes a unique opportunity for direct comparative studies between rodents and marmoset monkeys in terms of anxiety and/or sedation.

OPRM1 gene variation influences hypothalamic-pituitary-adrenal axis function in response to a variety of stressors in rhesus macaques

The endogenous opioid system is involved in modulating a number of behavioral and physiological systems, including the hypothalamic-pituitary-adrenal (HPA) axis. In humans, a functional variant in the OPRM1 gene (OPRM1 A118G) is associated with a number of outcomes, including attenuated HPA axis responses to stress. A nonsynonymous variant (OPRM1 C77G) in the rhesus macaque has been shown to have similar effects in vivo to the human variant. The current study investigated whether OPRM1 C77G influences HPA axis response to stress in rhesus macaques. We analyzed plasma adrenocorticotropic hormone (ACTH) and cortisol levels measured in response to three different stressors: (1) maternal separation in infant subjects at 6 months of age, (2) acute ethanol administration in adolescent subjects at 4 years of age, and (3) postpartum HPA axis function in adult rhesus macaque females. For the maternal separation paradigm, ACTH and cortisol levels were determined at baseline as well as peak levels during each of 4 consecutive separation episodes. For the acute ethanol administration paradigm, hormone levels were determined at baseline and again at 5 min, 10 min, and 60 min following the ethanol infusion. For postpartum sampling, hormone levels were determined at postpartum days 7, 14, 21, 30, 60, 90, 120, and 150. Infants carrying the 77G allele exhibited lower levels of cortisol across all 4 separation episodes. Furthermore, adolescents carrying the 77G allele exhibited lower cortisol levels at 5 and 10 min following acute ethanol administration. Adult females with prior reproductive experience and who carry the 77G allele exhibited lower cortisol levels across the postpartum period. No significant genotype effects were found for ACTH, although there were some trends for lower ACTH levels in 77G allele carriers. These data are consistent with human studies that have demonstrated attenuated cortisol responses to stress among carriers of the OPRM1 118G allele, lending further support to the argument that the rhesus and human allelic variants are functionally similar. Our results also suggest that OPRM1 variation may influence coping style, as well as alcohol-induced and postpartum levels of HPA axis activity and, as such, may modify vulnerability to alcohol use disorders and postpartum depression.

Maternal separation produces lasting changes in cortisol and behavior in rhesus monkeys

Maternal separation (MS), which can lead to hypothalamic pituitary adrenal axis dysfunction and behavioral abnormalities in rhesus monkeys, is frequently used to model early adversity. Whether this deleterious effect on monkeys is reversible by later experience is unknown. In this study, we assessed the basal hair cortisol in rhesus monkeys after 1.5 and 3 y of normal social life following an early separation. These results showed that peer-reared monkeys had significantly lower basal hair cortisol levels than the mother-reared monkeys at both years examined. The plasma cortisol was assessed in the monkeys after 1.5 y of normal social life, and the results indicated that the peak in the peer-reared cortisol response to acute stressors was substantially delayed. In addition, after 3 y of normal social life, abnormal behavioral patterns were identified in the peer-reared monkeys. They showed decreases in locomotion and initiated sitting together, as well as increases in stereotypical behaviors compared with the mother-reared monkeys. These results demonstrate that the deleterious effects of MS on rhesus monkeys cannot be compensated by a later normal social life, suggesting that the effects of MS are long-lasting and that the maternal-separated rhesus monkeys are a good animal model to study early adversity and to investigate the development of psychiatric disorders induced by exposure to early adversity.

Identifying key features of early stressful experiences that produce stress vulnerability and resilience in primates

This article examines the complex role of early stressful experiences in producing both vulnerability and resilience to later stress-related psychopathology in a variety of primate models of human development. Two types of models are reviewed: Parental Separation Models (e.g., isolate-rearing, peer-rearing, parental separations, and stress inoculation) and Maternal Behavior Models (e.g., foraging demands, variation in maternal style, and maternal abuse). Based on empirical evidence, it is argued that early life stress exposure does not increase adult vulnerability to stress-related psychopathology as a linear function, as is generally believed, but instead reflects a quadratic function. Features of early stress exposure including the type, duration, frequency, ecological validity, sensory modality, and developmental timing, within and between species, are identified to better understand how early stressful experiences alter neurobiological systems to produce such diverse developmental outcomes. This article concludes by identifying gaps in our current knowledge, providing directions for future research, and discussing the translational implications of these primate models for human development and psychopathology.

[Childhood maltreatment and adult psychopathology: pathways to hypothalamic-pituitary-adrenal axis dysfunction]

OBJECTIVE

The aim of this paper was to examine the relationship between childhood maltreatment and adult psychopathology, as reflected in hypothalamic-pituitary-adrenal axis dysfunction.

METHOD

A selective review of the relevant literature was undertaken in order to identify key and illustrative research findings.

RESULTS

There is now a substantial body of preclinical and clinical evidence derived from a variety of experimental paradigms showing how early-life stress is related to hypothalamic-pituitary-adrenal axis function and psychological state in adulthood, and how that relationship can be modulated by other factors.

DISCUSSION

The risk for adult psychopathology and hypothalamic-pituitary-adrenal axis dysfunction is related to a complex interaction among multiple experiential factors, as well as to susceptibility genes that interact with those factors. Although acute hypothalamic-pituitary-adrenal axis responses to stress are generally adaptive, excessive responses can lead to deleterious effects. Early-life stress alters hypothalamic-pituitary-adrenal axis function and behavior, but the pattern of hypothalamic-pituitary-adrenal dysfunction and psychological outcome in adulthood reflect both the characteristics of the stressor and other modifying factors.

CONCLUSION

Research to date has identified multiple determinants of the hypothalamic-pituitary-adrenal axis dysfunction seen in adults with a history of childhood maltreatment or other early-life stress. Further work is needed to establish whether hypothalamic-pituitary-adrenal axis abnormalities in this context can be used to develop risk endophenotypes for psychiatric and physical illnesses.

The clinical correlates of reported childhood sexual abuse: an association between age at trauma onset and severity of depression and PTSD in adults

This study investigated the relationship between the age of -self-reported sexual abuse occurrence and the development of post-traumatic stress disorder and/or depressive symptoms in adulthood. Subjects were evaluated for the presence of post-traumatic stress disorder and/or depressive symptoms as well as for a self-reported history of sexual abuse before the age of 18. Results found that relative risk of having severe post-traumatic stress disorder symptoms was 10 times higher in patients reporting sexual abuse after age 12 than in those reporting sexual abuse before age 12. Relative risk of having severe depressive symptoms was higher for those abused before the age of 12 than for those abused after the age of 12. Findings suggest that the impact of reported sexual abuse at different stages of development may lead to distinct psychiatric symptoms in adulthood.