Developmental consequences of behavioral inhibition: a model in rhesus monkeys (Macaca mulatta)

Rhesus infant nervous temperament predicts peri-adolescent central amygdala metabolism & behavioral inhibition measured by a machine-learning approach

Anxiety disorders affect millions of people worldwide and impair health, happiness, and productivity on a massive scale. Developmental research points to a connection between early-life behavioral inhibition and the eventual development of these disorders. Our group has previously shown that measures of behavioral inhibition in young rhesus monkeys (Macaca mulatta) predict anxiety-like behavior later in life. In recent years, clinical and basic researchers have implicated the central extended amygdala (EAc)-a neuroanatomical concept that includes the central nucleus of the amygdala (Ce) and the bed nucleus of the stria terminalis (BST)-as a key neural substrate for the expression of anxious and inhibited behavior. An improved understanding of how early-life behavioral inhibition relates to an increased lifetime risk of anxiety disorders-and how this relationship is mediated by alterations in the EAc-could lead to improved treatments and preventive strategies. In this study, we explored the relationships between infant behavioral inhibition and peri-adolescent defensive behavior and brain metabolism in 18 female rhesus monkeys. We coupled a mildly threatening behavioral assay with concurrent multimodal neuroimaging, and related those findings to various measures of infant temperament. To score the behavioral assay, we developed and validated UC-Freeze, a semi-automated machine-learning (ML) tool that uses unsupervised clustering to quantify freezing. Consistent with previous work, we found that heightened Ce metabolism predicted elevated defensive behavior (i.e., more freezing) in the presence of an unfamiliar human intruder. Although we found no link between infant-inhibited temperament and peri-adolescent EAc metabolism or defensive behavior, we did identify infant nervous temperament as a significant predictor of peri-adolescent defensive behavior. Our findings suggest a connection between infant nervous temperament and the eventual development of anxiety and depressive disorders. Moreover, our approach highlights the potential for ML tools to augment existing behavioral neuroscience methods.

Behavioral inhibition in a translational nonhuman primate model: A pilot study of Kagan's behavioral inhibition paradigm modified for use in infant rhesus macaques (Macaca mulatta)

Behavioral inhibition (BI), a temperamental trait first described by Jerome Kagan, is characterized by wariness to unfamiliar persons and novel situations. BI is a moderately stable trait, with biological and genetic underpinnings. Kagan's methodology for assessing BI is widely used in humans. Although this paradigm could be readily translated for use in nonhuman primates, thereby increasing generalizability from nonhuman primates to humans and fortifying evidence that BI is evolutionarily conserved, researchers have not done so. To address this, this study utilized a modified version of Kagan's paradigm to assess behaviors and biological markers of BI in nonhuman primates. Over the first 5 weeks of life, nursery-reared rhesus monkeys (Macaca mulatta; N = 12) were rated using the standardized Infant Behavior Assessment Scale for nonhuman primates on measures related to BI (consolability, irritability, struggle, and predominant state). Three months later, behavioral assessments were made in relation to a novel playroom, an unfamiliar peer, and a variety of attention-grabbing, unfamiliar stimuli, followed by the introduction of a human stranger. Behaviors from Kagan's studies of BI in toddlers (freezing, exploration, and latency to approach) and physiological measures related to BI (heart rate) were assessed. Random effects models showed that subjects rated high in temperamental BI spent less time exploring the environment and socializing with peers and more time freezing (an indication of anxiety in rhesus monkeys). These findings suggest that Kagan's paradigm is readily adapted for use in nonhuman primates and support the utility of rhesus monkeys as translational models for assessing the causes and consequences of human BI. (PsycInfo Database Record (c) 2024 APA, all rights reserved).

Relationship of behavioral inhibition to separation anxiety in a sample (N = 377) of adult individuals with mood and anxiety disorders

BACKGROUND

Behavioral Inhibition (BI) is an early temperamental trait characterized by shyness, withdrawal, avoidance, uneasiness, and fear of unfamiliar situations, people, objects, and events. The DSM-5 refers to behavioral inhibition as a temperamental factor related to neurodevelopmental conditions in childhood, including attention deficit hyperactivity disorder, selective mutism, and specific phobias; and to its influence on adult anxiety disorders including social anxiety disorder, agoraphobia, and generalized anxiety disorder, but, interestingly, not separation anxiety disorder (SAD). However, there are phenomenological overlaps between BI and SAD. We aimed to explore whether there is a correlation between BI as an early temperamental trait and childhood or adult separation anxiety disorder.

METHODS

The study was conducted in 377 consecutive adults (mean age 40.2±12.4 years) outpatients with anxiety and mood disorders as the principal diagnosis, grouped on the presence/absence of a DSM-5 diagnosis of childhood or adult separation anxiety disorder. Separation anxiety was assessed by the Structured Clinical Interview for Separation Anxiety (SCI-SAS) and the Adult Separation Anxiety Checklist (ASA27). Behavioral inhibition was assessed by the Retrospective Self-Report of Inhibition (RSRI).

RESULTS

The four comparison groups included: 1) 168 patients without childhood or adult SAD, 2) 81 with adult SAD, 3) 97 with both adult SAD and childhood SAD, and 4) 31 with childhood SAD only. The group with both adult and childhood SAD had the highest scores on RSRI total and sub-scale scores. Both groups with adult SAD had significantly higher RSRI scores than the group with only childhood SAD or without SAD. Significant bivariate correlations were found between ASA-27 scores and RSRI scores. Correlations between RSRI scores and measures of anxiety and depressive symptoms were significantly weaker than those on the ASA-27. Regression analyses showed a significant predictive value of RSRI scores on ASA-27 total score, but not of age of onset of SAD.

CONCLUSIONS

BI has an onset in the very first years of life and may represent a potential developmental endophenotype for later anxiety disorders. Our findings indicate that BI and separation anxiety are connected in individuals with affective and anxiety disorders. This may have important clinical and therapeutic implications for preventive interventions.

The relationship of maternal rank, 5-HTTLPR genotype, and MAOA-LPR genotype to temperament in infant rhesus monkeys (Macaca mulatta)

Temperament is a construct whose manifestations are quantifiable from an early age, and whose origins have been proposed as "biological." Our goal was to determine whether maternal rank and infant genotype are associated with five measures of temperament in 3- to 4-month old rhesus monkeys (Macaca mulatta), all of whom were born and reared by their mothers in large, outdoor, half-acre cages. Maternal rank was defined as the proportion of animals outranked by each female, and the two genes of interest to us were monoamine oxidase and serotonin transporter, both of which are polymorphic in their promoter regions (MAOA-LPR and 5-HTTLPR, respectively), with one allele of each gene considered a "plasticity" allele, conferring increased sensitivity to environmental events. Our large sample size (n = 2014-3140) enabled us to examine the effects of individual genotypes rather than combining genotypes as is often done. Rank was positively associated with Confident temperament, but only for animals with the 5-repeat allele for MAOA-LPR. Rank had no other effect on temperament. In contrast, genotype had many different effects, with 5-HTTLPR associated with behavioral inhibition, and MAOA-LPR associated with ratings-based measures of temperament. We also examined the joint effect of the two genotypes and found some evidence for a dose-response: animals with the plasticity alleles for both genes were more likely to be behaviorally inhibited. Our results suggest phenotypic differences between animals possessing alleles for MAOA-LPR that show functional equivalence based on in vitro tests, and our data for 5-HTTLPR revealed differences between short/short homozygotes and long/short heterozygotes, strongly suggesting that combining genotypes for statistical analysis should be avoided if possible. Our analysis also provides evidence of sex differences in temperament, and, to our knowledge, the only evidence of differences in temperament based on specific pathogen-free status. We suggest several directions for future research.

Stress-induced plasma cortisol concentrations in infancy are associated with later parenting behaviors in female rhesus macaques (Macaca mulatta)

Few studies have longitudinally assessed the relationship between infant stress reactivity and future parenting style. Studies show that stress-induced plasma cortisol concentrations are stable over development and that they can be utilized as a marker for stress reactivity. This study investigates the relationship between stress-induced plasma cortisol concentrations in infancy and later parenting behavior in a translational nonhuman primate model. We hypothesized that higher stress-induced cortisol levels in infancy would predict impairments in maternal behaviors in adulthood. Subjects were rhesus macaque females (N = 122; Macaca mulatta), assessed as infants and again as mothers. At 3-4 months of age, subjects underwent a standardized BioBehavioral Assessment during which blood samples were obtained and they were assessed for behaviorally inhibition. Approximately 7 years later, subjects were observed as they interacted with their own offspring for four 300-s sessions. Typical rhesus monkey mother-offspring behaviors were recorded, including approaches and leaves and maternal cradling. Results showed that subjects' stress-induced cortisol concentrations and whether they exhibited behavioral inhibition as infants predicted later maternal behavior, with high cortisol concentrations and behavioral inhibition predicting high rates of offspring approaches and leaves and low rates of maternal cradling. Results also showed that higher stress-induced cortisol concentrations in infancy predicted higher scores on the Brown Index, an indication that the subjects' offspring, rather than the subject themselves, initiated changes in proximity. Taken together, these results suggest that individuals that exhibit higher stress-induced cortisol concentrations and behavioral inhibition at 3-4 months of age are at risk for engaging in less sensitive parenting behaviors as adults. To the extent that these findings generalize to humans, they suggest an important link between stress-induced cortisol concentrations and behavioral inhibition in infancy and behavior later in life, such that early-life stress reactivity can serve as a marker for later parenting behavior.

Structural differences in the hippocampus and amygdala of behaviorally inhibited macaque monkeys

Behavioral inhibition is a temperamental disposition to react warily when confronted by unfamiliar people, objects, or events. Behaviorally inhibited children are at greater risk of developing anxiety disorders later in life. Previous studies reported that individuals with a history of childhood behavioral inhibition exhibit abnormal activity in the hippocampus and amygdala. However, few studies have investigated the structural differences that may underlie these functional abnormalities. In this exploratory study, we evaluated rhesus monkeys exhibiting a phenotype consistent with human behavioral inhibition. We performed quantitative neuroanatomical analyses that cannot be performed in humans including estimates of the volume and neuron number of distinct hippocampal regions and amygdala nuclei in behaviorally inhibited and control rhesus monkeys. Behaviorally inhibited monkeys had larger volumes of the rostral third of the hippocampal field CA3, smaller volumes of the rostral third of CA2, and smaller volumes of the accessory basal nucleus of the amygdala. Furthermore, behaviorally inhibited monkeys had fewer neurons in the rostral third of CA2. These structural differences may contribute to the functional abnormalities in the hippocampus and amygdala of behaviorally inhibited individuals. These structural findings in monkeys are consistent with a reduced modulation of amygdala activity via prefrontal cortex projections to the accessory basal nucleus. Given the putative roles of the amygdala in affective processing, CA3 in associative learning and CA2 in social memory, increased amygdala and CA3 activity, and diminished CA2 structure and function, may be associated with increased social anxiety and the heritability of behavioral inhibition. The findings from this exploratory study compel follow-up investigations with larger sample sizes and additional analyses to provide greater insight and more definitive answers regarding the neurobiological bases of behavioral inhibition.

Animal models of human mood

Humans' everyday experience of the world is influenced by our moods. Moods are consciously accessible affective states that extend over time that are characterized by their valence and arousal. They also likely have a long evolutionary heritage and serve as an important adaptive affective mechanism. When they become maladaptive or overly biased, pathological affective states such as depression can emerge. Despite the importance of moods for human experience, little is known about their causal neurobiological mechanisms. In humans, limitations related to methods and interpretations of the data prevent causal investigations into the origins of mood, highlighting the importance of animal models. Nonhuman primates that share key neuroanatomical, affective, and social features with humans will be essential to uncovering their foundation. Identifying and validating mood-like states in animals is, however, challenging not least because mood is a human construct requiring verbal communication. Here we outline a theoretical framework for animal models of human mood, drawing upon established psychological literature where it exists before reviewing the extant studies of non-human primate models of mood-like states.

Social inhibition in population-based and cardiac patient samples: Robustness of inhibition, sensitivity and withdrawal as distinct facets

OBJECTIVE

Behavioral inhibition plays a key role in animal stress research and developmental research in children. Therefore, we examined the robustness of our multifaceted model of adult social inhibition that comprises behavioral inhibition, interpersonal sensitivity, and social withdrawal components.

METHOD

A total of 899 adults completed the 15-item Social Inhibition Questionnaire (SIQ15) and measures of emotional distress. Confirmatory Factor Analysis (CFA), reliability estimates, and correlational and second-order factor analyses were used to examine the robustness of our model.

RESULTS

CFA (RMSEA = 0.052; NFI = 0.938; CFI = 0.957) and Cronbach's α estimates ≥0.87 confirmed the robustness of our multi-facet social inhibition model based on three correlated inhibition, sensitivity, and withdrawal factors in 560 adults from the general population and in 194 undergraduate students. Inhibition, sensitivity, and withdrawal were stable over time (3-month test-retest correlations ≥ 0.78), and were closely related to the Gest Behavioral Inhibition and PID-5 Withdrawal measures in a clinical sample of 145 cardiac patients. Of note, male cardiac patients reported more inhibition and withdrawal than female patients. Across samples, social inhibition was distinctly different from negative affectivity.

CONCLUSIONS

Our 3-facet model of inhibition, sensitivity and withdrawal was robust across samples, and may promote research on adult social inhibition in population-based and clinical studies.

The multidimensional nature of adult social inhibition: Inhibition, sensitivity and withdrawal facets of the SIQ15

BACKGROUND

Social inhibition may promote emotional problems in children, but little is known about this disposition in adults. Our research builds on a theory-based model to suggest that adult social inhibition involves distinct behavioral (inhibition), cognitive (sensitivity), and affective (withdrawal) characteristics.

METHODS

A total of 1385 adults completed measures of social inhibition, emotional distress, and social stress. Factor analyses, reliability estimates and regression analyses were used to examine the robustness of our model, and the validity of the 15-item Social Inhibition Questionnaire (SIQ15).

RESULTS

In Study 1 (N = 1180; M_age 46.9 years; 52% women), factor analysis confirmed that behavioral inhibition, interpersonal sensitivity, and social withdrawal reflected distinct facets of social inhibition. Next, we developed the SIQ15 that covers these facets with 5 items each; e.g. has difficulty making contact; expects negative reactions from others; keeps others at a distance. Study 2 (N = 209; M_age 20.3 years; 77% women) showed that the SIQ15 and its 5-item Inhibition, Sensitivity and Withdrawal facet scales were internally consistent (Cronbach's α between 0.86/0.94) and stable over time (test-retest between r = 0.73/0.78). The SIQ15 facets differentially predicted related inhibition (Behavioral Inhibition Scale), rumination (Penn State Worry Questionnaire) and withdrawal (Personality Inventory for DSM-5) scores at 6 months follow-up. Younger age and having no partner were associated with more social inhibition.

LIMITATIONS

Findings are based on self-report; experimental and prospective studies are needed to further validate our inhibition model.

CONCLUSIONS

Inhibition, sensitivity, and withdrawal are distinct manifestations of adult social inhibition that can be reliably assessed with the SIQ15. Research needs to examine how this multidimensional nature of social inhibition has an effect on stress, health, and wellbeing.

Environmental Enrichment in the 21st Century

More than a quarter of a century has elapsed since the Animal Welfare Act mandated that research facilities develop and follow a plan to promote the psychological well-being of captive primates. Since passage of this law, considerable effort and resources have been directed to designing environmental enrichment strategies in an effort to improve animal welfare. These plans typically consist of environmental enrichment and socialization efforts. While environmental enhancement has undergone a great deal of improvement in the past 25 years, it should be viewed as a continual work in progress, which takes advantage of emergent and future technologies. In this review, we discuss the objectives of the environmental enhancement plan along with relevant outcome measures, as well as ongoing challenges, costs, and benefits. We then review various enrichment strategies and assess their efficacy in meeting goals and objectives. Finally, we look forward to consider what the future might hold for environmental enrichment of nonhuman primates used in research.

Advances in nonhuman primate models of autism: Integrating neuroscience and behavior

Given the prevalence and societal impact of autism spectrum disorders (ASD), there is an urgent need to develop innovative preventative strategies and treatments to reduce the alarming number of cases and improve core symptoms for afflicted individuals. Translational efforts between clinical and preclinical research are needed to (i) identify and evaluate putative causes of ASD, (ii) determine the underlying neurobiological mechanisms, (iii) develop and test novel therapeutic approaches and (iv) ultimately translate basic research into safe and effective clinical practices. However, modeling a uniquely human brain disorder, such as ASD, will require sophisticated animal models that capitalize on unique advantages of diverse species including drosophila, zebra fish, mice, rats, and ultimately, species more closely related to humans, such as the nonhuman primate. Here we discuss the unique contributions of the rhesus monkey (Macaca mulatta) model to ongoing efforts to understand the neurobiology of the disorder, focusing on the convergence of brain and behavior outcome measures that parallel features of human ASD.