Behavioral and neurobiological characteristics of social stress versus depression in nonhuman primates

Quality of Life Assessment: The Value of Longitudinal Data in Making the End-of-Life Care Decision for a Macaque (Macaca Silenus/Macaca Nemestrina)

Assessing quality of life in animals is an art as much as a science. Despite the use of questionnaires and keeper reports which consider several aspects of well-being, the process often remains subjective. Keepers have unique insights, and anecdotal observations can be enhanced with objective data. We combined the art and science of assessments in this study on a geriatric macaque (1.0 lion-tailed/pig-tailed (Macaca silenus/macaca nemestrina) hybrid), using historic data to inform management decisions. Following the unexpected death of his cage mate, his activity and engagement with keepers decreased, and new concerning behaviors presented. While the zoo worked to identify new social opportunities, we used these data to develop a plan to improve his quality of life (e.g., increase training sessions, enrichment, social interactions). After intense implementation, we saw a significant increase in activity level and engagement with keepers; the frequency of unexpected behaviors suggesting a lower quality of life, however, increased over time. Our data allowed us to objectively compare changes in behavior, enabling the zoo to make the most informed animal management decision possible. [Figure: see text].

Evaluating depression- and anxiety-like behaviors in non-human primates

Depression and anxiety are some of the most prevalent and debilitating mental health conditions in humans. They can present on their own or as co-morbidities with other disorders. Like humans, non-human primates (NHPs) can develop depression- and anxiety-like signs. Here, we first define human depression and anxiety, examine equivalent species-specific behaviors in NHPs, and consider models and current methods to identify and evaluate these behaviors. We also discuss knowledge gaps, as well as the importance of evaluating the co-occurrence of depression- and anxiety-like behaviors in animal models of human disease. Lastly, we consider ethical challenges in depression and anxiety research on NHPs in order to ultimately advance the understanding and the personalized treatment of these disorders.

The use of gaze to study cognition: limitations, solutions, and applications to animal welfare

The study of gaze responses, typically using looking time paradigms, has become a popular approach to improving our understanding of cognitive processes in non-verbal individuals. Our interpretation of data derived from these paradigms, however, is constrained by how we conceptually and methodologically approach these problems. In this perspective paper, we outline the application of gaze studies in comparative cognitive and behavioral research and highlight current limitations in the interpretation of commonly used paradigms. Further, we propose potential solutions, including improvements to current experimental approaches, as well as broad-scale benefits of technology and collaboration. Finally, we outline the potential benefits of studying gaze responses from an animal welfare perspective. We advocate the implementation of these proposals across the field of animal behavior and cognition to aid experimental validity, and further advance our knowledge on a variety of cognitive processes and welfare outcomes.

Automated pose estimation in primates

Understanding the behavior of primates is important for primatology, for psychology, and for biology more broadly. It is also important for biomedicine, where primates are an important model organism, and whose behavior is often an important variable of interest. Our ability to rigorously quantify behavior has, however, long been limited. On one hand, we can rigorously quantify low-information measures like preference, looking time, and reaction time; on the other, we can use more gestalt measures like behavioral categories tracked via ethogram, but at high cost and with high variability. Recent technological advances have led to a major revolution in behavioral measurement that offers affordable and scalable rigor. Specifically, digital video cameras and automated pose tracking software can provide measures of full-body position (i.e., pose) of primates over time (i.e., behavior) with high spatial and temporal resolution. Pose-tracking technology in turn can be used to infer behavioral states, such as eating, sleeping, and mating. We call this technological approach behavioral imaging. In this review, we situate the behavioral imaging revolution in the history of the study of behavior, argue for investment in and development of analytical and research techniques that can profit from the advent of the era of big behavior, and propose that primate centers and zoos will take on a more central role in relevant fields of research than they have in the past.

Psychosocial stress increases risk for type 2 diabetes in female cynomolgus macaques consuming a western diet

Chronic psychosocial stress is associated with increased risk of many chronic diseases including type 2 diabetes mellitus. However, it is difficult to establish a causal relationship between stress and diabetes in human studies because stressors often are self-reported and may be distant in time from metabolic consequences. Macaques are useful models of the effects of chronic psychosocial stress on health and may develop obesity and diabetes similar to human beings. Thus, we studied the relationships between social subordination stress - a well-validated psychological stressor in macaques - and body composition and carbohydrate metabolism in socially housed, middle-aged female cynomolgus monkeys (Macaca fascicularis; n = 42). Following an 8-week baseline phase, the monkeys were fed a Western diet for 36 months (about equivalent to 10 human years). Social status was determined based on the outcomes of agonistic interactions (X¯= 33.3 observation hours/monkey). Phenotypes collected included plasma cortisol, body composition, circulating markers of glucose metabolism, activity levels, and heart rate variability measured as RMSSD (root of mean square of successive differences) and SDDN (standard deviation of beat to beat interval) after 1.5- and 3-years on diet. Mixed model analyses of variance revealed that aggression received, submissions sent, and cortisol were higher, and RMSSD and SDNN were lower in subordinates than dominants (social status: p < 0.05). After 3 years of Western diet consumption, fasting triglyceride, glucose and insulin concentrations, calculated insulin resistance (HOMA-IR), body weight and body fat mass increased in all animals (time: all p's < 0.05); however, the increase in fasting glucose and HOMA-IR was significantly greater in subordinates than dominants (time x social status: p's < 0.05). Impaired glucose metabolism, (glucose > 100 mg/dl) incidence was significantly higher in subordinates (23%) than dominants (0%) (Fisher's exact test, p < 0.05). These findings suggest that chronic psychosocial stress, on a Western diet background, significantly increases type 2 diabetes risk in middle-aged female primates.

Sleep Disturbances and Depression Are Co-morbid Conditions: Insights From Animal Models, Especially Non-human Primate Model

The incidence rates of depression are increasing year by year. As one of the main clinical manifestations of depression, sleep disorder is often the first complication. This complication may increase the severity of depression and lead to poor prognosis in patients. In the past decades, there have been many methods used to evaluate sleep disorders, such as polysomnography and electroencephalogram, actigraphy, and videography. A large number of rodents and non-human primate models have reproduced the symptoms of depression, which also show sleep disorders. The purpose of this review is to examine and discuss the relationship between sleep disorders and depression. To this end, we evaluated the prevalence, clinical features, phenotypic analysis, and pathophysiological brain mechanisms of depression-related sleep disturbances. We also emphasized the current situation, significance, and insights from animal models of depression, which would provide a better understanding for the pathophysiological mechanisms between sleep disturbance and depression.

Mediterranean diet, stress resilience, and aging in nonhuman primates

Persistent psychological stress increases the risk of many chronic diseases of aging. Little progress has been made to effectively reduce stress responses or mitigate stress effects suggesting a need for better understanding of factors that influence stress responses. Limited evidence suggests that diet may be a factor in modifying the effects of stress. However, long-term studies of diet effects on stress reactive systems are not available, and controlled randomized clinical trials are difficult and costly. Here we report the outcomes of a controlled, randomized preclinical trial of the effects of long-term consumption (31 months, ~ equivalent to 9 human years) of Western versus Mediterranean - like diets on behavioral and physiological responses to acute (brief social separation) and chronic (social subordination) psychosocial stress in 38 adult, socially-housed, female cynomolgus macaques. Compared to animals fed a Western diet, those fed the Mediterranean diet exhibited enhanced stress resilience as indicated by lower sympathetic activity, brisker and more overt heart rate responses to acute stress, more rapid recovery, and lower cortisol responses to acute psychological stress and adrenocorticotropin (ACTH) challenge. Furthermore, age-related increases in sympathetic activity and cortisol responses to stress were delayed by the Mediterranean diet. Population level diet modification in humans has been shown to be feasible. Our findings suggest that population-wide adoption of a Mediterranean-like diet pattern may provide a cost-effective intervention on psychological stress and promote healthy aging with the potential for widespread efficacy.

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There is no population level treatment to reduce stress and associated disease.

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Mediterranean diet reduced sympathetic activity.

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Mediterranean diet reduced cortisol response to acute stress and to ACTH challenge.

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Mediterranean diet delayed age-related increases in sympathetic activity and cortisol responses to stress.

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These results suggest a dietary strategy to increase stress resilience.

Reverse translation of major depressive disorder symptoms: A framework for the behavioural phenotyping of putative biomarkers

BACKGROUND

Reverse translating putative biomarkers of depression from patients to animals is complex because Major Depressive Disorder (MDD) is a highly heterogenous condition. This review proposes an approach to reverse translation based on relating relevant bio-behavioural functions in laboratory rodents to MDD symptoms.

METHODS

This systematic review outlines symptom clusters assessed by psychometric tests of MDD and antidepressant treatment response including the Montgomery-Åsberg Depression Rating Scale, the Hamilton Depression Rating Scale, and the Beck Depression Inventory. Symptoms were related to relevant behavioural assays in laboratory rodents.

RESULTS

The resulting battery of tests includes passive coping, anxiety-like behaviours, sleep, caloric intake, cognition, psychomotor functions, hedonic reactivity and aversive learning. These assays are discussed alongside relevant clinical symptoms of MDD, providing a framework through which reverse translation of a biomarker can be interpreted.

LIMITATIONS

Certain aspects of MDD may not be quantified by tests in laboratory rodents, and their biological significance may not always be of clinical relevance.

CONCLUSIONS

Using this reverse translation approach, it is possible to clarify the functional significance of a putative biomarker in rodents and hence translate its contribution to specific clinical symptoms, or clusters of symptoms.

Increased produce enrichment reduces trauma in socially-housed captive rhesus macaques (Macaca mulatta)

Due to primate adaptations for sociality, captive rhesus macaques have optimal welfare and utility as a biomedical model when they can be maintained in outdoor social groups. As a despotic species; however, aggression can result in costly injuries and may result in temporary or permanent removal of specific individuals from social housing. Enrichment items, such as toys, climbing structures, and foraging material, are employed to keep captive animals occupied. We hypothesized that produce enrichment that requires more processing to extract may reduce socially-derived injuries by keeping animals occupied. We tested the effects of additional weekly produce (corn-in-husk, whole melon, or whole squash) on trauma incidence in an outdoor social group of rhesus macaques across two distinct seasons (mating and birthing seasons) at the California National Primate Research Center. Aggression and status behavioral data, food resource use and proximity, and trauma incidence were collected over two 16-week periods, with eight control and treatment conditions alternating biweekly. Mixed-effects regression modeling was used to determine the best predictors of trauma risk and severe aggression at the group level and at an individual level. We found that food resource use was an important predictor of trauma risk at both group and individual levels; greater use of food resources reduced trauma risk. Produce enrichment did not; however, reduce severe aggression. We suggest that other captive social groups of rhesus macaques with high levels of trauma may benefit from supplemental produce enrichment that increases animal engagement with food resources.

Effects of Transportation and Relocation on Immunologic Measures in Cynomolgus Macaques (Macaca fascicularis)

NHP are a small, but critical, portion of the animals studied in research laboratories. Many NHP are imported or raised at one facility and subsequently moved to another facility for research purposes. To improve our understanding of the effects of transportation and relocation on the NHP immune system, to minimize potential confounds associated with relocation, and to maximize study validity, we examined the phenotype and function of PBMC in cynomolgus macaques (Macaca fascicularis) that were transported approximately 200 miles by road from one facility to another. We evaluated the phenotype of lymphocyte subsets through flow cytometry, mitogen-specific immune responses of PBMC in vitro, and plasma levels of circulating cytokines before transportation, at approximately 24 h after arrival (day 2), and after 30 d of acclimation. Analyses of blood samples revealed that the CD3+ and CD4+ T-cell counts increased significantly, whereas NK+, NKT, and CD14+ CD16+ nonclassical monocyte subsets were decreased significantly on day 2 after relocation compared with baseline. We also noted significantly increased immune cell function as indicated by mitogen-specific proliferative responses and by IFNγ levels on day 2 compared with baseline. After 30 d of acclimation, peripheral blood CD4+ T-cells and monocyte counts were higher than baseline, whereas B-cell numbers were lower. The mitogen-induced responses to LPS and IFNγ production after stimulation with pokeweed mitogen or phytohemagglutinin remained significantly different from baseline. In conclusion, the effects of transportation and relocation on immune parameters in cynomolgus monkeys are significant and do not fully return to baseline values even after 30 d of acclimation.

Could Greater Time Spent Displaying Waking Inactivity in the Home Environment Be a Marker for a Depression-Like State in the Domestic Dog?

Dogs exposed to aversive events can become inactive and unresponsive and are commonly referred to as being "depressed", but this association remains to be tested. We investigated whether shelter dogs spending greater time inactive "awake but motionless" (ABM) in their home-pen show anhedonia (the core reduction of pleasure reported in depression), as tested by reduced interest in, and consumption of, palatable food (KongTM test). We also explored whether dogs being qualitatively perceived by experts as disinterested in the food would spend greater time ABM (experts blind to actual inactivity levels). Following sample size estimations and qualitative behaviour analysis (n = 14 pilot dogs), forty-three dogs (6 shelters, 22F:21M) were included in the main study. Dogs relinquished by their owners spent more time ABM than strays or legal cases (F = 8.09, p = 0.032). One significant positive association was found between the KongTM measure for average length of KongTM bout and ABM, when length of stay in the shelter was accounted for as a confounder (F = 3.66, p = 0.035). Time spent ABM also correlated with scores for "depressed" and "bored" in the qualitative results, indirectly suggesting that experts associate greater waking inactivity with negative emotional states. The hypothesis that ABM reflects a depression-like syndrome is not supported; we discuss how results might tentatively support a "boredom-like" state and further research directions.

Depression-like state behavioural outputs may confer beneficial outcomes in risky environments

Recent theories in evolutionary medicine have suggested that behavioural outputs associated with depression-like states (DLS) could be an adaptation to unpredictable and precarious situations. In animal models, DLS are often linked to diverse and unpredictable stressors or adverse experiences. Theoretically, there are a range of potential fitness benefits associated with behavioural inhibition (typical to DLS), as opposed to more active/aggressive responses to adverse or uncontrollable events. This stance of evolutionary medicine has to our knowledge not been tested empirically. Here we address a possible key benefit of behavioural inhibition in a comparative model for social stress (territorial rainbow trout). By treating fish with the fast-acting antidepressant ketamine, we reversed the behavioural inhibition (i.e. stimulated an increase in activity level) in subordinate fish. During confrontation with a previously unfamiliar larger, aggressive and dominant individual, this increase in activity led to higher amounts of received aggression compared to sham-treated subordinates. This suggests that the behavioural inhibition characterizing animal models of DLS is indeed an effective coping strategy that reduces the risk of injuries in vulnerable social situations.

What is animal happiness?

Today, we see a growing concern for the quality of life of nonhuman animals and an accompanying call for viable means of assessing how well animals thrive. Past research focused on minimizing negatives such as stress, while more recent endeavors strive to promote positives such as happiness. But what is animal happiness? Although often mentioned, the term lacks a clear definition. With recent advances in the study of animal emotion, current interest into positive rather than negative experiences, and the call for captive and domesticated animals to have good lives, the time is ripe to examine the concept of animal happiness. We draw from the human and animal literature to delineate a concept of animal happiness and propose how to assess it. We argue that animal happiness depends on how an individual feels generally-that is, a typical level of affect.

Ketamine and pharmacological imaging: use of functional magnetic resonance imaging to evaluate mechanisms of action

Ketamine has been used as a pharmacological model for schizophrenia as subanesthetic infusions have been shown to produce temporary schizophrenia-like symptoms in healthy humans. More recently, ketamine has emerged as a potential treatment for multiple psychiatric disorders, including treatment-resistant depression and suicidal ideation. However, the mechanisms underlying both the psychotomimetic and the therapeutic effects of ketamine remain poorly understood. This review provides an overview of what is known of the neural mechanisms underlying the effects of ketamine and details what functional MRI studies have yielded at a systems level focused on brain circuitry. Multiple analytic approaches show that ketamine exerts robust and consistent effects at the whole-brain level. These effects are highly conserved across human and nonhuman primates, validating the use of nonhuman primate models for further investigations with ketamine. Regional analysis of brain functional connectivity suggests that the therapeutic potential of ketamine may be derived from a strengthening of executive control circuitry, making it an intriguing candidate for the treatment of drug abuse. There are still important questions about the mechanism of action and the therapeutic potential of ketamine that can be addressed using appropriate functional neuroimaging techniques.

Effect of Chronic Social Stress on Prenatal Transfer of Antitetanus Immunity in Captive Breeding Rhesus Macaques (Macaca mulatta)

Because tetanus can cause significant morbidity and mortality in NHP, colonywide vaccination with tetanus toxoid is recommended for outdoor breeding colonies of rhesus macaques, with primary immunizations commonly given to infants at 6 mo of age followed by booster vaccines every 10 y. Maternal antibodies are thought to offer protective immunity to infants younger than 6 mo. However, historical colony data from the Yerkes National Primate Research Center show a higher incidence of tetanus among infants (≤ 6 mo old) born to subordinate dams. Whether this higher incidence of infantile tetanus is due to a higher incidence of trauma among subordinate animals or is a stress-induced impairment of maternal antibody protection is unknown. Studies in other NHP species suggest that chronic exposure to social stressors interferes with the receptor-mediated transplacental transfer of IgG. Therefore, the primary aim of this study was to determine whether chronic stress associated with social subordination impairs prenatal transfer of antitetanus immunity in breeding female rhesus macaques. Subjects included 26 high- and 26 low-ranking adult female rhesus macaques that were nearly 5 or 10 y after their initial immunization and their nonimmunized infants. We hypothesized that infants born to subordinate dams that were nearly 10 y after immunization would have the lowest infant-to-dam antibody ratios and thus would be at greatest risk for infection. Results revealed no significant intergroup differences in infant antitetanus IgG levels. However, infant-to-dam IgG ratios against tetanus were significantly lower among subordinate animals compared with dominant macaques, after accounting for the number of years since the dam's initial vaccination. In addition, higher maternal hair cortisol levels predicted lower infantto-dam tetanus toxoid IgG ratios. Together, these findings suggest that chronic social stress in female rhesus macaques may hamper the prenatal transfer of antitetanus immunity to offspring.

Long-term sertraline treatment and depression effects on carotid artery atherosclerosis in premenopausal female primates

OBJECTIVE

Atherosclerosis developed during premenopausal years predicts postmenopausal atherosclerosis burden. Selective serotonin reuptake inhibitor (SSRI) antidepressants, recently approved for hot flushes, have been associated with increased ischemic stroke risk in several observational studies; however, effects on carotid artery atherosclerosis, a strong predictor of future vascular events, are unknown.

METHODS

The effects of chronic administration of a commonly prescribed SSRI, sertraline HCl, on atherosclerosis in the carotid artery was assessed in a placebo-controlled, longitudinal, randomized study of premeonopausal depressed and nondepressed cynomolgus monkeys (Macaca fascicularis; n = 42). Physiologic and behavioral phenotypes were evaluated at baseline and after 18 months of oral sertraline (20 mg/kg, n = 21) or placebo (n = 21). Carotid artery atherosclerosis was measured post mortem via histomorphometry.

RESULTS

Atherosclerosis extent in the right common carotid artery, on average, was 60% greater in sertraline-treated depressed monkeys compared with all other groups (P = 0.028). The results of linear regression analyses suggested that sertraline and depression effects on atherosclerosis were not mediated by their effects on behavioral and physiological risk factors.

CONCLUSIONS

These findings suggest that chronic SSRI treatment is associated with the progression of carotid artery atherosclerosis, which may increase the risk for future vascular events, particularly in depressed women. The underlying mechanism remains to be determined, but does not appear to be related to SSRI effects on traditional cardiovascular risk factors.

Behavioral depression is associated with increased vagally mediated heart rate variability in adult female cynomolgus monkeys (Macaca fascicularis)

INTRODUCTION

Depressive symptoms (DS) in humans are associated with decreased resting state vagal activity, but sex seems to moderate this association. Recently, in human females DS have been associated with greater or similar cardiac vagal activity compared to men in both, clinical and non-clinical samples. A previously validated animal model of behavioral depression was used in the present study to investigate the association of DS and cardiac vagal activity in non-human primates.

METHODS

The root mean square of successive differences between adjacent heart beats (RMSSD) was used as an indicator of vagally-mediated heart rate variability in 24h heart rate recordings collected via telemetry in 42 adult female cynomolgus monkeys (Macaca fascicularis). Hierarchical regression models were used to estimate differences in RMSSD comparing monkeys with and without DS. To capture circadian variation patterns of RMSSD, additional quadratic, cubic and quartic terms of hour were added.

RESULTS

Monkeys showing behavioral DS had higher overall 24-h RMSSD. The interaction term of daytime with DS and polynomials of hour contributed significantly to the variance across models.

CONCLUSIONS

This is the first study investigating the association of DS and 24h cardiac vagal control in female non-human primates. Results replicate existing human studies showing higher cardiac vagal control in behavioral depressed vs. non-depressed female monkeys.

Relationships of depressive behavior and sertraline treatment with walking speed and activity in older female nonhuman primates

Depression is the most common mental health problem in aging persons and is a leading risk factor for physical disability, especially in women. Though antidepressant drugs such as serotonin reuptake inhibitors (SSRI) are commonly prescribed, epidemiological evidence reveals mixed effects of long-term antidepressant use on physical function and activity, possibly depending on depressive status. The purpose of this preclinical trial was to determine the relationships of depressive behavior and the potential for an SSRI treatment to modulate walking speed and activity patterns in older adult female cynomolgus monkeys (Macaca fascicularis). We evaluated the effects of depression and a commonly prescribed SSRI, sertraline HCl (20 mg/kg/day p.o.), on (a) walking speed, (b) accelerometry-derived activity (counts) and sedentariness (daytime 60-s sedentary epochs), and (c) observed locomotor and sedentary behaviors (% time) in adult female depressed and nondepressed monkeys (n = 42; 17.2 ± 1.8 years) during an 18 month pre-treatment phase and an 18 month treatment phase using a longitudinal, stratified placebo-control study design. Monkeys that were depressed prior to treatment (19/42) subsequently had slower walking speeds (F _D [1, 38] = 4.14; p ≤ 0.05) and tended to be more sedentary during the daytime (F _D [1, 38] = 3.63; p ≤ 0.06). Sertraline did not affect depressive behaviors, walking speed, accelerometry-derived physical activity or sedentariness, or time observed in total locomotor or sedentary behavior (all p > 0.10). This study provides the first experimental demonstration of relationships between nonhuman primate behavioral depression and walking speed, activity, and sedentariness and provides evidence for a lack of an effect of SSRI treatment on these phenotypes.

Pacing stereotypies in laboratory rhesus macaques: Implications for animal welfare and the validity of neuroscientific findings

Stereotypic behaviours are commonly observed in captive animals and are usually interpreted as a sign of poor welfare. Stereotypies have also been linked with brain abnormalities. However, stereotypies are a heterogeneous class of behaviours and mounting evidence indicates that different stereotypies can have different causes, and can be linked to different affective states. As a consequence, the implications of a specific stereotypy in a specific species cannot be safely inferred from evidence on other stereotypies or species. Here we review what is known about pacing behaviour in laboratory rhesus macaques, a common stereotypy in this species. Our review highlights the current lack of understanding of the causal factors underlying pacing behaviour. According to current knowledge, the welfare of pacing macaques could be either better, worse or equivalent to that of non-pacing individuals. It is also unclear whether pacing results from brain abnormalities. Since rhesus macaques are widely used as a model of healthy humans in neuroscience research, determining if pacing behaviour reflects an abnormal brain and/or poor welfare is urgent.

The impact of treatment with selective serotonin reuptake inhibitors on primate cardiovascular disease, behavior, and neuroanatomy

Selective serotonin reuptake inhibitor (SSRI) use is ubiquitous because they are widely prescribed for a number of disorders in addition to depression. Depression increases the risk of coronary heart disease (CHD). Hence, treating depression with SSRIs could reduce CHD risk. However, the effects of long term antidepressant treatment on CHD risk, as well as other aspects of health, remain poorly understood. Thus, we undertook an investigation of multisystem effects of SSRI treatment with a physiologically relevant dose in middle-aged adult female cynomolgus monkeys, a primate species shown to be a useful model of both depression and coronary and carotid artery atherosclerosis. Sertraline had no effect on depressive behavior, reduced anxious behavior, increased affiliation, reduced aggression, changed serotonin neurotransmission and volumes of neural areas critical to mood disorders, and exacerbated coronary and carotid atherosclerosis. These data suggest that a conservative approach to prescribing SSRIs for cardiovascular or other disorders for long periods may be warranted, and that further study is critical given the widespread use of these medications.

Repeated, Intermittent Social Defeat across the Entire Juvenile Period Resulted in Behavioral, Physiological, Hormonal, Immunological, and Neurochemical Alterations in Young Adult Male Golden Hamsters

The developing brain is vulnerable to social defeat during the juvenile period. As complements of human studies, animal models of social defeat provide a straightforward approach to investigating the functional and neurobiological consequences of social defeats. Taking advantage of agonist behavior and social defeat in male golden hamster, a set of 6 experiments was conducted to investigate the consequences at multiple levels in young adulthood resulting from repeated, intermittent social defeats or “social threats” across the entire juvenile period. Male hamsters at postnatal day 28 (P28) were randomly assigned to either the social defeat, “social threat”, or arena control group, and they correspondingly received a series of nine social interaction trials (i.e., either social defeat, “social threat”, or arena control conditions) from P33 to P66. At the behavioral level (Experiment 1), we found that repeated social defeats (but not “social threats”) significantly impacted locomotor activity in the familiar context and social interaction in the familiar/unfamiliar social contexts. At the physiological and hormonal levels (Experiments 2 and 3), repeated social defeat significantly enhanced the cortisol and norepinephrine concentrations in blood. Enlargement of the spleen was also found in the social defeat and “social threat” groups. At the immunological level (Experiment 4), the social defeat group showed lower levels of pro-inflammatory cytokines in the hypothalamus and hippocampus but higher concentration of IL-6 in the striatum compared to the other two groups. At the neurochemical level (Experiment 5), the socially defeated hamsters mainly displayed reductions of dopamine, dopamine metabolites, and 5-HT levels in the striatum and decreased level of 5-HT in the hippocampus. In Experiment 6, an increase in the spine density of hippocampal CA1 pyramidal neurons was specifically observed in the “social threat” group. Collectively, our findings indicate that repeated, intermittent social defeats throughout entire adolescence in hamsters impact their adult responses at multiple levels. Our results also suggest that the “social threat” group may serve as an appropriate control. This study further suggest that the alterations of behavioral responses and neurobiological functions in the body and brain might provide potential markers to measure the negative consequences of chronic social defeats.

Rhesus macaques (Macaca mulatta) with self-injurious behavior show less behavioral anxiety during the human intruder test

Self-injurious behavior (SIB) has been linked to anxiety in the human literature, but relatively few studies have explored this link in rhesus macaques. A widely used behavioral assessment of anxiety, the human intruder test (HIT), employs the mildly stressful stimulus of an unfamiliar experimenter to assess anxious behavior in macaques. The HIT was conducted on 59 (20 male) laboratory housed rhesus macaques, 30 with a record of SIB (10 male). If monkeys with SIB have a more anxious phenotype, they should show stronger reactions to the HIT. However, contrary to our predictions, monkeys with SIB did not show higher levels of anxious behavior compared to controls. They spent significantly less time showing anxious behavior and displayed little aggression in response to the stare of the intruder. SIB and control monkeys did not differ in a range score (number of unique behaviors expressed per phase); however, SIB monkeys had a lower change score (total number of behaviors expressed including repetitions) than controls. In general, monkeys that paced regardless of SIB status, showed a reduction in pacing when the intruder entered the room. Possible explanations for the failure of SIB monkeys to show increased anxiety in the HIT include greater exposure of SIB monkeys to unfamiliar humans because of their condition, evidence for a subtype of SIB which is not anxiety related, and/or the presence of comorbid depressive-like symptoms. Am. J. Primatol. 79:e22569, 2017. © 2016 Wiley Periodicals, Inc.

Sertraline inhibits increases in body fat and carbohydrate dysregulation in adult female cynomolgus monkeys

Selective serotonin reuptake inhibitor (SSRI) antidepressants are widely prescribed for depression and other disorders. SSRIs have become one of the most commonly used drugs in the United States, particularly by women. Acute effects on body composition and carbohydrate metabolism have been reported, but little is known regarding the effects of chronic SSRI use. We evaluated the effects of chronic administration of a commonly prescribed SSRI, sertraline HCl, on body weight and composition, fat distribution, carbohydrate metabolism, as well as activity, in adult female depressed and nondepressed cynomolgus monkeys (Macaca fascicularis; n=42) using a placebo-controlled, longitudinal, randomized study design. Phenotypes were evaluated prior to and after 18 months of oral sertraline (20mg/kg) or placebo. Over the 18 month treatment period, the placebo group experienced increases in body weight, body fat (visceral and subcutaneous) fasting insulin concentrations, and homeostasis model assessment of insulin resistance scores (HOMA-IR). Sertraline treatment prevented increases in body weight, fat, insulin, and HOMA-IR (all p<0.05), without significantly altering activity levels. Sertraline treatment altered adiponectin in an unusual way - reducing circulating adiponectin in depressed monkeys without affecting fat mass or body weight. Deleterious effects on adiponectin, a potentially insulin-sensitizing and atheroprotective protein, may result in adverse effects on cardiovascular health despite otherwise beneficial effects on body composition and carbohydrate metabolism.

Middle-range exploratory activity in adult rats suggests higher resilience to chronic social defeat

OBJECTIVE

Stressful life events play an important role in the aetiology of human mood disorders and are frequently modelled by chronic social defeat (SD) in rodents. Exploratory phenotype in rats is a stable trait that is likely related to inter-individual differences in reactivity to stress. The aim of the study was to confirm that low levels of exploratory activity (LE) are, in rodents, a risk factor for passive stress coping, and to clarify the role of medium (ME) and high (HE) exploratory disposition in the sensitivity to SD.

METHODS

We examined the effect of SD on male Wistar rats with LE, ME, and HE activity levels as measured in the exploration box. After SD, the rats were evaluated in social preference, elevated zero maze, and open-field tests. Brain tissue levels of monoamines were measured by high-performance liquid chromatography.

RESULTS

Rats submitted to SD exhibited lower weight gain, higher sucrose consumption, showed larger stress-induced hyperthermia, lower levels of homovanillic acid in the frontal cortex, and higher levels of noradrenaline in the amygdala and hippocampus. Open-field, elevated zero maze, and social preference tests revealed the interaction between stress and phenotype, as only LE-rats were further inhibited by SD. ME-rats exhibited the least reactivity to stress in terms of changes in body weight, stress-induced hyperthermia, and sucrose intake.

CONCLUSION

Both low and high novelty-related activity, especially the former, are associated with elevated sensitivity to social stress. This study shows that both tails of a behavioural dimension can produce stress-related vulnerability.

Neuroinflammation at the interface of depression and cardiovascular disease: Evidence from rodent models of social stress

A large body of evidence has emerged linking stressful experiences, particularly from one's social environment, with psychiatric disorders. However, vast individual differences emerge in susceptibility to developing stress-related pathology which may be due to distinct differences in the inflammatory response to social stress. Furthermore, depression is an independent risk factor for cardiovascular disease, another inflammatory-related disease, and results in increased mortality in depressed patients. This review is focused on discussing evidence for stress exposure resulting in persistent or sensitized inflammation in one individual while this response is lacking in others. Particular focus will be directed towards reviewing the literature underlying the impact that neuroinflammation has on neurotransmitters and neuropeptides that could be involved in the pathogenesis of comorbid depression and cardiovascular disease. Finally, the theme throughout the review will be to explore the notion that stress-induced inflammation is a key player in the high rate of comorbidity between psychosocial disorders and cardiovascular disease.

Brain serotonergic activation in growth-stunted farmed salmon: adaption versus pathology

Signalling systems activated under stress are highly conserved, suggesting adaptive effects of their function. Pathologies arising from continued activation of such systems may represent a mismatch between evolutionary programming and current environments. Here, we use Atlantic salmon (Salmo salar) in aquaculture as a model to explore this stance of evolutionary-based medicine, for which empirical evidence has been lacking. Growth-stunted (GS) farmed fish were characterized by elevated brain serotonergic activation, increased cortisol production and behavioural inhibition. We make the novel observation that the serotonergic system in GS fish is unresponsive to additional stressors, yet a cortisol response is maintained. The inability of the serotonergic system to respond to additional stress, while a cortisol response is present, probably leads to both imbalance in energy metabolism and attenuated neural plasticity. Hence, we propose that serotonin-mediated behavioural inhibition may have evolved in vertebrates to minimize stress exposure in vulnerable individuals.

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.

Animal models of major depression and their clinical implications

Major depressive disorder is a common, complex, and potentially life-threatening mental disorder that imposes a severe social and economic burden worldwide. Over the years, numerous animal models have been established to elucidate pathophysiology that underlies depression and to test novel antidepressant treatment strategies. Despite these substantial efforts, the animal models available currently are of limited utility for these purposes, probably because none of the models mimics this complex disorder fully. It is presumable that psychiatric illnesses, such as affective disorders, are related to the complexity of the human brain. Here, we summarize the animal models that are used most commonly for depression, and discuss their advantages and limitations. We discuss genetic models, including the recently developed optogenetic tools and the stress models, such as the social stress, chronic mild stress, learned helplessness, and early-life stress paradigms. Moreover, we summarize briefly the olfactory bulbectomy model, as well as models that are based on pharmacological manipulations and disruption of the circadian rhythm. Finally, we highlight common misinterpretations and often-neglected important issues in this field.

Functional differentiation of adult-born neurons along the septotemporal axis of the dentate gyrus

Over the past several decades, the proliferation and integration of adult-born neurons into existing hippocampal circuitry has been implicated in a wide range of behaviors, including novelty recognition, pattern separation, spatial learning, anxiety behaviors, and antidepressant response. In this review, we suggest that the diversity in behavioral requirements for new neurons may be partly caused by separate functional roles of individual neurogenic niches. Growing evidence shows that the hippocampal formation can be compartmentalized not only along the classic trisynaptic circuit, but also along a longitudinal septotemporal axis. We suggest that subpopulations of hippocampal adult-born neurons may be specialized for distinct mnemonic- or mood-related behavioral tasks. We will examine the literature supporting a functional and anatomical dissociation of the hippocampus along the longitudinal axis and discuss techniques to functionally dissect the roles of adult-born hippocampal neurons in these distinct subregions.

Why bother using non-human primate models of cognitive disorders in translational research?

Although everyone would agree that successful translation of therapeutic candidates for central nervous disorders should involve non-human primate (nhp) models of cognitive disorders, we are left with the paucity of publications reporting either the target validation or the actual preclinical testing in heuristic nhp models. In this review, we discuss the importance of nhps in translational research, highlighting the advances in technological/methodological approaches for 'bridging the gap' between preclinical and clinical experiments. In this process, we acknowledge that nhps remain a vital tool for the investigation of complex cognitive functions, given their resemblance to humans in aspects of behaviour, anatomy and physiology. The recent improvements made for a suitable nhp model in cognitive research, including new surrogates of disease and application of innovative methodological approaches, are continuous strides for reaching efficient translation for human benefit. This will ultimately aid the development of innovative treatments against the current and future threat of neurological and psychiatric disorders to the global population.

Long term sertraline effects on neural structures in depressed and nondepressed adult female nonhuman primates

BACKGROUND

Selective serotonin reuptake inhibitors (SSRIs) are widely prescribed for mood and other disorders. However, their neural effects are difficult to study due to patient compliance and drug history variability, and rarely studied in those prescribed SSRIs for non-mood disorders. Here we evaluated SSRI effects on neural volumetrics in depressed and nondepressed monkeys.

METHODS

42 socially-housed cynomolgus monkeys were randomized to treatment balanced on pretreatment depressive behavior and body weight. Monkeys were trained for oral administration of placebo or 20 mg/kg sertraline HCl daily for 18 months and depressive and anxious behavior recorded. Volumes of neural regions of interest in depression were measured in magnetic resonance images and analyzed by 2 (depressed, nondepressed)×2 (placebo, sertraline) ANOVA.

RESULTS

Sertraline reduced anxiety (p=0.04) but not depressive behavior (p=0.43). Left Brodmann's Area (BA) 32 was smaller in depressed than nondepressed monkeys (main effect of depression: p<0.05). Sertraline and depression status interacted to affect volumes of left Anterior Cingulate Cortex (ACC), left BA24, right hippocampus (HC), and right anterior HC (sertraline×depression interactions: all p's<0.05). In the Placebo group, depressed monkeys had smaller right anterior HC and left ACC than nondepressed monkeys. In nondepressed monkeys, sertraline reduced right HC volume, especially right anterior HC volume. In depressed monkeys sertraline increased left ACC volume. In nondepressed monkeys, sertraline reduced left BA24 volumes resulting in smaller BA24 volumes in nondepressed than sertraline-treated depressed monkeys.

CONCLUSIONS

These observations suggest that SSRIs may differentially affect neural structures in depressed and nondepressed individuals.

Serotonergic, brain volume and attentional correlates of trait anxiety in primates

Trait anxiety is a risk factor for the development and maintenance of affective disorders, and insights into the underlying brain mechanisms are vital for improving treatment and prevention strategies. Translational studies in non-human primates, where targeted neurochemical and genetic manipulations can be made, are critical in view of their close neuroanatomical similarity to humans in brain regions implicated in trait anxiety. Thus, we characterised the serotonergic and regional brain volume correlates of trait-like anxiety in the marmoset monkey. Low- and high-anxious animals were identified by behavioral responses to a human intruder (HI) that are known to be sensitive to anxiolytic drug treatment. Extracellular serotonin levels within the amygdala were measured with in vivo microdialysis, at baseline and in response to challenge with the selective serotonin reuptake inhibitor, citalopram. Regional brain volume was assessed by structural magnetic resonance imaging. Anxious individuals showed persistent, long-term fearful responses to both a HI and a model snake, alongside sustained attention (vigilance) to novel cues in a context associated with unpredictable threat. Neurally, high-anxious marmosets showed reduced amygdala serotonin levels, and smaller volumes in a closely connected prefrontal region, the dorsal anterior cingulate cortex. These findings highlight behavioral and neural similarities between trait-like anxiety in marmosets and humans, and set the stage for further investigation of the processes contributing to vulnerability and resilience to affective disorders.

Macaques exhibit a naturally-occurring depression similar to humans

Rodent models have dominated preclinical investigations into the mechanisms of depression. However, these models-which rely on subjecting individual rodents to physical stressors - do not realistically resemble the etiopathological development of depression, which occurs naturally in a social context. A non-human primate model that better reflects the social ethological aspects of depression would be more advantageous to investigating pathophysiological mechanisms and developing antidepressant therapeutics. Here, we describe and model a naturally-occurring depressive state in a non-human primate species, the cynomolgus monkey (Macaca fascicularis), in a realistic social ethological context and associate the depressed behavioral phenotype with significant serum metabolic perturbations. One to two subjects per stable social colony (17–22 subjects) manifested a depressive phenotype that may be attributed to psychosocial stress. In accordance with rodent and human studies, the serum metabolic phenotype of depressed and healthy subjects significantly differed, supporting the model's face validity. However, application of the fast-acting antidepressant ketamine failed to demonstrate predictive validity. This study proposes a non-human primate depression model in a realistic social ethological context that can better approximate the psychosocial stressors underlying depression.

Social inequalities in health in nonhuman primates

Overall health has been linked to socioeconomic status, with the gap between social strata increasing each year. Studying the impact of social position on health and biological functioning in nonhuman primates has allowed researchers to model the human condition while avoiding ethical complexities or other difficulties characteristic of human studies. Using female cynomolgus macaques (Macaca fascicularis), our lab has examined the link between social status and stress for 30 years. Female nonhuman primates are especially sensitive to social stressors which can deleteriously affect reproductive health, leading to harmful consequences to their overall health. Subordinates have lower progesterone concentrations during the luteal phase of menstrual cycle, which is indicative of absence or impairment of ovulation. Subordinate animals receive more aggression, less affiliative attention, and are more likely to exhibit depressive behaviors. They also express higher stress-related biomarkers such as increased heart rates and lower mean cortisol. While no differences in body weight between dominant and subordinate animals are observed, subordinates have lower bone density and more visceral fat than their dominant counterparts. The latter increases risk for developing inflammatory diseases. Differences are also observed in neurological and autonomic function. A growing body of data suggests that diet composition may amplify or diminish physiological stress responses which have deleterious effects on health. More experimental investigation of the health effects of diet pattern is needed to further elucidate these differences in an ongoing search to find realistic and long-term solutions to the declining health of individuals living across the ever widening socioeconomic spectrum.

Why primate models matter

Research involving nonhuman primates (NHPs) has played a vital role in many of the medical and scientific advances of the past century. NHPs are used because of their similarity to humans in physiology, neuroanatomy, reproduction, development, cognition, and social complexity-yet it is these very similarities that make the use of NHPs in biomedical research a considered decision. As primate researchers, we feel an obligation and responsibility to present the facts concerning why primates are used in various areas of biomedical research. Recent decisions in the United States, including the phasing out of chimpanzees in research by the National Institutes of Health and the pending closure of the New England Primate Research Center, illustrate to us the critical importance of conveying why continued research with primates is needed. Here, we review key areas in biomedicine where primate models have been, and continue to be, essential for advancing fundamental knowledge in biomedical and biological research.

Impact of social status and antidepressant treatment on neurogenesis in the baboon hippocampus

Adult hippocampal neurogenesis is critically implicated in rodent models of stress and anxiety as well as behavioral effects of antidepressants. Whereas similar factors such as psychiatric disorder and antidepressant administration are correlated with hippocampal volume in humans, the relationship between these factors and adult neurogenesis is less well understood. To better bridge the gap between rodent and human physiology, we examined the numbers of proliferating neural precursors and immature cells in the hippocampal dentate gyrus (DG) as well as in vivo magnetic resonance imaging (MRI)-estimated whole hippocampal volume in eight socially dominant- or subordinate-like (SL) baboons administered the antidepressant fluoxetine or vehicle. SL baboons had lower numbers of proliferating cells and immature neurons than socially dominant-like baboons. Fluoxetine treatment was associated with a larger whole hippocampal volume but surprisingly resulted in lower numbers of immature neurons. These findings are the first to indicate that adult neurogenesis in the baboon hippocampal DG may be functionally relevant in the context of social stress and mechanisms of antidepressant action.

Depressive-like behavioral profiles in captive-bred single- and socially-housed rhesus and cynomolgus macaques: a species comparison

BACKGROUND

To unravel the causes of major depressive disorder (MDD), the third leading cause of disease burden around the world, ethological animal models have recently been proposed. Our previous studies highlighted a depressive-like profile among single- and socially-housed farm-bred cynomolgus macaques. Although phylogenetically close, cynomolgus and rhesus macaques, the two most commonly used macaque species in biomedical research, differ on several levels such as patterns of aggression, reconciliation, temperament, or dominance styles. The question of whether one captive macaque species was more vulnerable than another in the development of a pathological profile reminiscent of MDD symptoms was explored.

METHODS

Behavioral data (including body postures, orientations, gaze directions, inter-individual distances, and locations in the cage) were collected in farming conditions. Using an unbiased validated ethological scan-sampling method, followed by multiple correspondence and hierarchical clustering analyses, 40 single- and 35 socially-housed rhesus macaques were assessed. Independently, for each housing condition, inter-species comparisons were made with previously acquired data on farm-bred cynomolgus monkeys.

RESULTS

Consistent with our previous studies, we found depressive-like characteristics (e.g., inactivity, low level of investigation and maintenance, long time spent inactive while facing the wall) among single- and socially-housed rhesus macaques. Species-specificities were reported in non-depressive time budgets and in the prevalence of the pathological profiles.

CONCLUSIONS

Our results suggest that rhesus may be more vulnerable to developing a despair-like state than cynomolgus macaques, both in single- and in social-housing conditions. Therefore, rhesus macaques are more suitable for use as a "spontaneous" model of depressive disorders.

Effect of depression and sertraline treatment on cardiac function in female nonhuman primates

OBJECTIVE

Depression is a proposed risk factor for heart failure based largely on epidemiological data; few experimental data addressing this hypothesis are available.

METHODS

Depression was evaluated in relation to cardiac structural and functional phenotypes assessed by transthoracic echocardiography in 42 adult female cynomolgus monkeys that consumed a Western-like diet for 3 years. Half of the monkeys were treated with sertraline HCl for 18 months, and depressive behavior was assessed for 12 months before echocardiography.

RESULTS

Depressed monkeys (the 19/42 with depressive behavior rates above the mean rate) had higher heart rates (HRs; 171 [4.1[ versus 152 [6.1]) and smaller body surface area (0.13 [0.003] versus 0.15 [0.004]), left ventricular (LV) end-systolic dimension (0.75 [0.05] versus 0.89 [0.04]), LV systolic (0.76 [0.08] versus 1.2 [0.11]) and diastolic (2.4 [0.23] versus 3.4 [0.26]) volumes, and left atrial volumes (1.15 [0.14] versus 1.75 [0.12]; p values < .05). Doppler profiles of depressed monkeys indicated greater myocardial relaxation (higher e' and higher e'/a' ratio) and lower filling pressures (lower E/e') compared to nondepressed monkeys (p values < .05). Although sertraline treatment reduced HR (150 [5.8] versus 171 [4.8]) and modestly increased chamber dimensions (LV end-systolic dimension: 0.91 [0.05] versus 0.74 [0.03]; LV end-diastolic dimension, body surface area adjusted 1.69 [0.05] versus 1.47 [0.06]; p values < .05), it did not overtly affect systolic or diastolic function (p values > .10).

CONCLUSIONS

These data suggest that behavioral depression in female primates is accompanied by differences in cardiac function, although not in ways classically associated with subclinical heart failure. Selective serotonin reuptakes show promise in supporting heart function by reducing HR and perhaps improving LV filling; however, further investigation is needed.

Altered expression of glial and synaptic markers in the anterior hippocampus of behaviorally depressed female monkeys

The anterior hippocampus is associated with emotional functioning and hippocampal volume is reduced in depression. We reported reduced neuropil volume and number of glia in the dentate gyrus (DG) and cornu ammonis (CA)1 of the anterior hippocampus in behaviorally depressed adult female cynomolgus macaques. To determine the biochemical correlates of morphometric and behavioral differences between behaviorally depressed and nondepressed adult female monkeys, glial and synaptic transcripts and protein levels were assessed in the DG, CA3 and CA1 of the anterior hippocampus. Glial fibrillary acidic protein (GFAP) was increased whereas spinophilin and postsynaptic density (PSD)-95 protein were decreased in the CA1 of depressed monkeys. GFAP was reciprocally related to spinophilin and PSD-95 protein in the CA1. Gene expression of GFAP paralleled the protein changes observed in the CA1 and was inversely related to serum estradiol levels in depressed monkeys. These results suggest that behavioral depression in female primates is accompanied by astrocytic and synaptic protein alterations in the CA1. Moreover, these findings indicate a potential role for estrogen in modulating astrocyte-mediated impairments in synaptic plasticity.

Models of stress in nonhuman primates and their relevance for human psychopathology and endocrine dysfunction

Stressful life events have been linked to the onset of severe psychopathology and endocrine dysfunction in many patients. Moreover, vulnerability to the later development of such disorders can be increased by stress or adversity during development (e.g., childhood neglect, abuse, or trauma). This review discusses the methodological features and results of various models of stress in nonhuman primates in the context of their potential relevance for human psychopathology and endocrine dysfunction, particularly mood disorders and dysregulation of the hypothalamic-pituitary-adrenocortical (HPA) system. Such models have typically examined the effects of stress on the animals' behavior, endocrine function (primarily the HPA and hypothalamic-pituitary-gonadal systems), and, in some cases, immune status. Manipulations such as relocation and/or removal of an animal from its current social group or, alternatively, formation of a new social group can have adverse effects on all of these outcome measures that may be either transient or more persistent depending on the species, sex, and other experimental conditions. Social primates may also experience significant stress associated with their rank in the group's dominance hierarchy. Finally, stress during prenatal development or during the early postnatal period may have long-lasting neurobiological and endocrine effects that manifest in an altered ability to cope behaviorally and physiologically with later challenges. Whereas early exposure to severe stress usually results in deficient coping abilities, certain kinds of milder stressors can promote subsequent resilience in the animal. We conclude that studies of stress in nonhuman primates can model many features of stress exposure in human populations and that such studies can play a valuable role in helping to elucidate the mechanisms underlying the role of stress in human psychopathology and endocrine dysfunction.

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.

Cell number and neuropil alterations in subregions of the anterior hippocampus in a female monkey model of depression

BACKGROUND

The anterior hippocampus is associated with emotional functioning and hippocampal volume is reduced in depression. More women are clinically depressed than men, yet the depressed female brain is little studied. We reported reduced anterior hippocampal volume in behaviorally depressed adult female cynomolgus macaques; the mechanisms contributing to that reduction are unknown. The present study represents the first systematic morphological investigation of the entire hippocampus in depressed female primates.

METHODS

Cellular determinants of hippocampal size were examined in subregions of anterior and posterior hippocampus in antidepressant-naïve, adult female monkeys characterized for behavioral depression and matched on variables that influence hippocampal size (n = 8 depressed, 8 nondepressed). Unbiased stereology was used to estimate neuronal and glial numbers, neuronal soma size, and regional and layer volumes.

RESULTS

Neuropil and cell layer volumes were reduced in cornu ammonis (CA)1 and dentate gyrus (DG) of the anterior but not the posterior hippocampus of depressed compared with nondepressed monkeys. Glial numbers were 30% lower in anterior CA1 and DG of depressed monkeys, with no differences observed in the posterior hippocampus. Granule neuron number tended toward a reduction in anterior DG; pyramidal neuron number was unchanged in any region. Size of pyramidal neurons and glial densities tended to be reduced throughout the whole hippocampus of depressed monkeys, whereas neuronal densities were unchanged.

CONCLUSIONS

The reduced size of the anterior hippocampus in depressed female monkeys appears to arise from alterations in numbers of glia and extent of neuropil, but not numbers of neurons, in CA1 and DG.

Birth origin differentially affects depressive-like behaviours: are captive-born cynomolgus monkeys more vulnerable to depression than their wild-born counterparts?

BACKGROUND

Adverse early-life experience might lead to the expression of abnormal behaviours in animals and the predisposition to psychiatric disorder (e.g. major depressive disorder) in Humans. Common breeding processes employ weaning and housing conditions different from what happens in the wild.

METHODS

The present study, therefore, investigated whether birth origin impacts the possible existence of spontaneous atypical/abnormal behaviours displayed by 40 captive-born and 40 wild-born socially-housed cynomolgus macaques in farming conditions using an unbiased ethological scan-sampling analysis followed by multifactorial correspondence and hierarchical clustering analyses.

RESULTS

We identified 10 distinct profiles (groups A to J) that significantly differed on several behaviours, body postures, body orientations, distances between individuals and locations in the cage. Data suggest that 4 captive-born and 1 wild-born animals (groups G and J) present depressive-like symptoms, unnatural early life events thereby increasing the risk of developing pathological symptoms. General differences were also highlighted between the captive- and wild-born populations, implying the expression of differential coping mechanisms in response to the same captive environment.

CONCLUSIONS

Birth origin thus impacts the development of atypical ethologically-defined behavioural profiles, reminiscent of certain depressive-like symptoms. The use of unbiased behavioural observations might allow the identification of animal models of human mental/behavioural disorders and their most appropriate control groups.

Behavioural profiles in captive-bred cynomolgus macaques: towards monkey models of mental disorders?

BACKGROUND

To date, experimental and preclinical studies on neuropsychiatric conditions have almost exclusively been performed in experimentally-induced animal models and have only rarely relied upon an ethological approach where animals have been observed in more naturalistic settings. The laboratory species of choice has been the rodent while the potential of more closely-related non-human primates have remained largely underexplored.

METHODS

The present study, therefore, aimed at investigating the possible existence of spontaneous atypical/abnormal behaviours displayed by 40 cynomolgus macaques in captive conditions using an unbiased ethological scan-sampling analysis followed by multifactorial correspondence analysis and a hierarchical clustering.

RESULTS

The study identified five distinct profiles (groups A to E) that significantly differed on several behaviours, body postures, body orientations, gaze directions and locations in the cage environment. We suggest that animals from the low n groups (D and E) present depressive-like and anxious-like symptoms, reminiscent of depressive and generalized anxiety disorders. Inter-individual differences were highlighted through unbiased ethological observations of spontaneous behaviours and associated parameters, although these were not associated with differences in plasma or cerebrospinal fluid levels of either stress-related hormones or monoamines, i.e. in accordance with the human situation.

CONCLUSIONS

No interventional behavioural testing was required to discriminate between 3 typical and 2 atypical ethologically-defined behavioural profiles, reminiscent of certain depressive-like and anxiety-like symptoms. The use of unbiased behavioural observations might, thus, allow the identification of animal models of human mental/behavioural disorders and their most appropriate control groups.

Social stress models in depression research: what do they tell us?

Interest has recently surged in the use of social stress models, especially social defeat. Such interest lies both in the recognition that stressors of social origin play a major role in human psychopathologies and in the acknowledgement that natural and hence ethologically-based stress models have important translational value. The use of the most recent technology has allowed the recognition of the mechanisms through which social defeat might have enduring psychoneuroendocrine effects, especially social avoidance and anhedonia, two behaviours relevant to human depression. In view of the sensitivity of these behavioural outcomes to repeated antidepressant treatments, the social defeat model has been proposed as a possible animal model of depression. The present survey is aimed at examining the limits of such an interpretation and focuses on methodological aspects and on the relevance of social defeat to the study of anxiety-related pathologies.

Are we getting closer to valid translational models for major depression?

Advances in characterizing the neuropathology and functional dysconnectivity of depression and promising trials with emerging circuit-targeted and fast-onset therapeutics are providing unprecedented opportunities to gain deeper insight into the neurobiology of this devastating and pervasive disorder. Because of practical and ethical limitations to dissecting these mechanisms in humans, continued progress will critically depend on our ability to emulate aspects of depressive symptomatology and treatment response in nonhuman organisms. Although various experimental models are currently available, they often draw skepticism from both clinicians and basic research scientists. We review recent progress and highlight some of the best leads to diversify and improve discovery end points for preclinical depression research.

Social subordination produces distinct stress-related phenotypes in female rhesus monkeys

Social subordination in female macaques is imposed by harassment and the threat of aggression and produces reduced control over one's social and physical environment and a dysregulation of the limbic-hypothalamic-pituitary-adrenal axis resembling that observed in people suffering from psychopathologies. These effects support the contention that this particular animal model is an ethologically relevant paradigm in which to investigate the etiology of stress-induced psychological illness related to women. Here, we sought to expand this model by performing a discriminate analysis (DA) on 33 variables within three domains; behavioral, metabolic/anthropomorphic, and neuroendocrine, collected from socially housed female rhesus monkeys in order to assess whether exposure to social subordination produces a distinct phenotype. A receiver operating characteristic (ROC) curve was also calculated to determine each domain's classification accuracy. DA found significant markers within each domain that differentiated dominant and subordinate females. Subordinate females received more aggression, showed more submissive behavior, and received less of affiliation from others than did dominant females. Metabolic differences included increased leptin, and reduced adiponectin in dominant compared to subordinate females. Dominant females exhibited increased sensitivity to hormonal stimulation with higher serum LH in response to estradiol, cortisol in response to ACTH, and increased glucocorticoid negative feedback. Serum oxytocin, CSF DOPAC and serum PACAP were all significantly higher in dominant females. ROC curve analysis accurately predicted social status in all three domains. Results suggest that socially house rhesus monkeys represent a cogent animal model in which to study the physiology and behavioral consequences of chronic psychosocial stress in humans.