Transcriptional Profiling of Primate Central Nucleus of the Amygdala Neurons to Understand the Molecular Underpinnings of Early-Life Anxious Temperament

Transcriptomic diversity of amygdalar subdivisions across humans and nonhuman primates

The amygdaloid complex mediates learning, memory, and emotions. Understanding the cellular and anatomical features that are specialized in the amygdala of primates versus other vertebrates requires a systematic, anatomically-resolved molecular analysis of constituent cell populations. We analyzed five nuclear subdivisions of the primate amygdala with single-nucleus RNA sequencing in macaques, baboons, and humans to examine gene expression profiles for excitatory and inhibitory neurons and confirmed our results with single-molecule FISH analysis. We identified distinct subtypes of FOXP2 + interneurons in the intercalated cell masses and protein-kinase C-δ interneurons in the central nucleus. We also establish that glutamatergic, pyramidal-like neurons are transcriptionally specialized within the basal, lateral, or accessory basal nuclei. Understanding the molecular heterogeneity of anatomically-resolved amygdalar neuron types provides a cellular framework for improving existing models of how amygdalar neural circuits contribute to cognition and mental health in humans by using nonhuman primates as a translational bridge.

Understanding the heterogeneity of anxiety using a translational neuroscience approach

Anxiety disorders affect millions of people worldwide and present a challenge in neuroscience research because of their substantial heterogeneity in clinical presentation. While a great deal of progress has been made in understanding the neurobiology of fear and anxiety, these insights have not led to effective treatments. Understanding the relationship between phenotypic heterogeneity and the underlying biology is a critical first step in solving this problem. We show translation, reverse translation, and computational modeling can contribute to a refined, cross-species understanding of fear and anxiety as well as anxiety disorders. More specifically, we outline how animal models can be leveraged to develop testable hypotheses in humans by using targeted, cross-species approaches and ethologically informed behavioral paradigms. We discuss reverse translational approaches that can guide and prioritize animal research in nontraditional research species. Finally, we advocate for the use of computational models to harmonize cross-species and cross-methodology research into anxiety. Together, this translational neuroscience approach will help to bridge the widening gap between how we currently conceptualize and diagnose anxiety disorders, as well as aid in the discovery of better treatments for these conditions.

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.

Brain-Region-Specific Genes Form the Major Pathways Featuring Their Basic Functional Role: Their Implication in Animal Chronic Stress Model

The analysis of RNA-Sec data from murine bulk tissue samples taken from five brain regions associated with behavior and stress response was conducted. The focus was on the most contrasting brain region-specific genes (BRSG) sets in terms of their expression rates. These BRSGs are identified as genes with a distinct outlying (high) expression rate in a specific region compared to others used in the study. The analysis suggested that BRSG sets form non-randomly connected compact gene networks, which correspond to the major neuron-mediated functional processes or pathways in each brain region. The number of BRSGs and the connection rate were found to depend on the heterogeneity and coordinated firing rate of neuron types in each brain region. The most connected pathways, along with the highest BRSG number, were observed in the Striatum, referred to as Medium Spiny Neurons (MSNs), which make up 95% of neurons and exhibit synchronous firing upon dopamine influx. However, the Ventral Tegmental Area/Medial Raphe Nucleus (VTA/MRN) regions, although primarily composed of monoaminergic neurons, do not fire synchronously, leading to a smaller BRSG number. The Hippocampus (HPC) region, on the other hand, displays significant neuronal heterogeneity, with glutamatergic neurons being the most numerous and synchronized. Interestingly, the two monoaminergic regions involved in the study displayed a common BRSG subnetwork architecture, emphasizing their proximity in terms of axonal throughput specifics and high-energy metabolism rates. This finding suggests the concerted evolution of monoaminergic neurons, leading to unique adaptations at the genic repertoire scale. With BRSG sets, we were able to highlight the contrasting features of the three groups: control, depressive, and aggressive mice in the animal chronic stress model. Specifically, we observed a decrease in serotonergic turnover in both the depressed and aggressive groups, while dopaminergic emission was high in both groups. There was also a notable absence of dopaminoceptive receptors on the postsynaptic membranes in the striatum in the depressed group. Additionally, we confirmed that neurogenesis BRSGs are specific to HPC, with the aggressive group showing attenuated neurogenesis rates compared to the control/depressive groups. We also confirmed that immune-competent cells like microglia and astrocytes play a crucial role in depressed phenotypes, including mitophagy-related gene Prkcd. Based on this analysis, we propose the use of BRSG sets as a suitable framework for evaluating case-control group-wise assessments of specific brain region gene pathway responses.

Current and Future Approaches to Pediatric Anxiety Disorder Treatment

This overview critically appraises the literature on the treatment of pediatric anxiety disorders. The two established treatments for these conditions comprise cognitive-behavioral therapy (CBT) and antidepressant medications. Many youths receiving these treatments fail to achieve remission, which creates a need for new treatments. After summarizing the literature on CBT and currently available medications, the authors describe research that lays a foundation for improvements in the treatment of pediatric anxiety disorders. This foundation leverages neuroscientific investigations, also described in the overview, which provide insights on mechanisms of successful treatment.

Molecular diversity and functional dynamics in the central amygdala

The central amygdala (CeA) is crucial in integrating sensory and associative information to mediate adaptive responses to emotional stimuli. Recent advances in genetic techniques like optogenetics and chemogenetics have deepened our understanding of distinct neuronal populations within the CeA, particularly those involved in fear learning and memory consolidation. However, challenges remain due to overlapping genetic markers complicating neuron identification. Furthermore, a comprehensive understanding of molecularly defined cell types and their projection patterns, which are essential for elucidating functional roles, is still developing. Recent advancements in transcriptomics are starting to bridge these gaps, offering new insights into the functional dynamics of CeA neurons. In this review, we provide an overview of the expanding genetic markers for amygdala research, encompassing recent developments and current trends. We also discuss how novel transcriptomic approaches are redefining cell types in the CeA and setting the stage for comprehensive functional studies.

Gene expression in the primate orbitofrontal cortex related to anxious temperament

Anxiety disorders are among the most prevalent psychiatric disorders, causing significant suffering and disability. Relative to other psychiatric disorders, anxiety disorders tend to emerge early in life, supporting the importance of developmental mechanisms in their emergence and maintenance. Behavioral inhibition (BI) is a temperament that emerges early in life and, when stable and extreme, is linked to an increased risk for the later development of anxiety disorders and other stress-related psychopathology. Understanding the neural systems and molecular mechanisms underlying this dispositional risk could provide insight into treatment targets for anxiety disorders. Nonhuman primates (NHPs) have an anxiety-related temperament, called anxious temperament (AT), that is remarkably similar to BI in humans, facilitating the design of highly translational models for studying the early risk for stress-related psychopathology. Because of the recent evolutionary divergence between humans and NHPs, many of the anxiety-related brain regions that contribute to psychopathology are highly similar in terms of their structure and function, particularly with respect to the prefrontal cortex. The orbitofrontal cortex plays a critical role in the flexible encoding and regulation of threat responses, in part through connections with subcortical structures like the amygdala. Here, we explore individual differences in the transcriptional profile of cells within the region, using laser capture microdissection and single nuclear sequencing, providing insight into the molecules underlying individual differences in AT-related function of the pOFC, with a particular focus on previously implicated cellular systems, including neurotrophins and glucocorticoid signaling.

Pharmacological Inactivation of the Bed Nucleus of the Stria Terminalis Increases Affiliative Social Behavior in Rhesus Macaques

The bed nucleus of the stria terminalis (BNST) has been implicated in a variety of social behaviors, including aggression, maternal care, mating behavior, and social interaction. Limited evidence from rodent studies suggests that activation of the BNST results in a decrease in social interaction between unfamiliar animals. The role of the BNST in social interaction in primates remains wholly unexamined. Nonhuman primates provide a valuable model for studying social behavior because of both their rich social repertoire and neural substrates of behavior with high translational relevance to humans. To test the hypothesis that the primate BNST is a critical modulator of social behavior, we performed intracerebral microinfusions of the GABAA agonist muscimol to transiently inactivate the BNST in male macaque monkeys. We measured changes in social interaction with a familiar same-sex conspecific. Inactivation of the BNST resulted in significant increase in total social contact. This effect was associated with an increase in passive contact and a significant decrease in locomotion. Other nonsocial behaviors (sitting passively alone, self-directed behaviors, and manipulation) were not impacted by BNST inactivation. As part of the "extended amygdala," the BNST is highly interconnected with the basolateral (BLA) and central (CeA) nuclei of the amygdala, both of which also play critical roles in regulating social interaction. The precise pattern of behavioral changes we observed following inactivation of the BNST partially overlaps with our prior reports in the BLA and CeA. Together, these data demonstrate that the BNST is part of a network regulating social behavior in primates.SIGNIFICANCE STATEMENT The bed nucleus of the stria terminalis (BNST) has a well-established role in anxiety behaviors, but its role in social behavior is poorly understood. No prior studies have evaluated the impact of BNST manipulations on social behavior in primates. We found that transient pharmacological inactivation of the BNST increased social behavior in pairs of macaque monkeys. These data suggest the BNST contributes to the brain networks regulating sociability.

Corticotropin Releasing Factor (CRF) Coexpression in GABAergic, Glutamatergic, and GABA/Glutamatergic Subpopulations in the Central Extended Amygdala and Ventral Pallidum of Young Male Primates

The central extended amygdala (CEA) and ventral pallidum (VP) are involved in diverse motivated behaviors based on rodent models. These structures are conserved, but expanded, in higher primates, including human. Corticotropin releasing factor (CRF), a canonical "stress molecule" associated with the CEA and VP circuitry across species, is dynamically regulated by stress and drugs of abuse and misuse. CRF's effects on circuits critically depend on its colocation with primary "fast" transmitters, making this crucial for understanding circuit effects. We surveyed the distribution and colocalization of CRF-, VGluT2- (vesicular glutamate transporter 2), and VGAT- (vesicular GABA transporter) mRNA in specific subregions of the CEA and VP in young male monkeys. Although CRF-containing neurons were clustered in the lateral central bed nucleus (BSTLcn), the majority were broadly dispersed throughout other CEA subregions, and the VP. CRF/VGAT-only neurons were highest in the BSTLcn, lateral central amygdala nucleus (CeLcn), and medial central amygdala nucleus (CeM) (74%, 73%, and 85%, respectively). In contrast, lower percentages of CRF/VGAT only neurons populated the sublenticular extended amygdala (SLEAc), ventrolateral bed nucleus (BSTLP), and VP (53%, 54%, 17%, respectively), which had higher complements of CRF/VGAT/VGluT2-labeled neurons (33%, 29%, 67%, respectively). Thus, the majority of CRF-neurons at the "poles" (BSTLcn and CeLcn/CeM) of the CEA are inhibitory, while the "extended" BSTLP and SLEAc subregions, and neighboring VP, have a more complex profile with admixtures of "multiplexed" excitatory CRF neurons. CRF's colocalization with its various fast transmitters is likely circuit-specific, and relevant for understanding CRF actions on specific target sites.SIGNIFICANCE STATEMENT The central extended amygdala (CEA) and ventral pallidum (VP) regulate multiple motivated behaviors through differential downstream projections. The stress neuropeptide corticotropin releasing factor (CRF) is enriched in the CEA, and is thought to "set the gain" through modulatory effects on coexpressed primary transmitters. Using protein and transcript assays in monkey, we found that CRF neurons are broadly and diffusely distributed in CEA and VP. CRF mRNA+ neurons colocalize with VGAT (GABA) and VGluT2 (glutamate) mRNAs in different proportions depending on subregion. CRF mRNA was also coexpressed in a subpopulation of VGAT/VGluT2 mRNA ("multiplexed") cells, which were most prominent in the VP and "pallidal"-like parts of the CEA. Heterogeneous CRF and fast transmitter coexpression across CEA/VP subregions implies circuit-specific effects.

The central extended amygdala guides survival-relevant tradeoffs: Implications for understanding common psychiatric disorders

To thrive in challenging environments, individuals must pursue rewards while avoiding threats. Extensive studies in animals and humans have identified the central extended amygdala (EAc)-which includes the central nucleus of the amygdala (Ce) and bed nucleus of the stria terminalis (BST)-as a conserved substrate for defensive behavior. These studies suggest the EAc influences defensive responding and assembles fearful and anxious states. This has led to the proliferation of a view that the EAc is fundamentally a defensive substrate. Yet mechanistic work in animals has implicated the EAc in numerous appetitive and consummatory processes, yielding fresh insights into the microcircuitry of survival- and emotion-relevant response selection. Coupled with the EAc's centrality in a conserved network of brain regions that encode multisensory environmental and interoceptive information, these findings suggest a broader role for the EAc as an arbiter of survival- and emotion-relevant tradeoffs for action selection. Determining how the EAc optimizes these tradeoffs promises to improve our understanding of common psychiatric illnesses such as anxiety, depression, alcohol- and substance-use disorders, and anhedonia.

Putting Genetics to Work in the Psychiatric Clinic

The rapid progress in psychiatric genetics over the past 10 years, while exciting from a research perspective, has not yet had an impact on clinical practice. How will we really be able to put genetics to work in the psychiatric clinic? This overview will attempt to answer this question. A survey of widely used methods and major study designs highlights key findings that have emerged so far. These findings inform a broad conceptual model of how genetic risk may act to influence dimensions of psychopathology and clinical presentations. The overview concludes with highlights of some of the most clinically relevant findings to date and their implications for psychiatric practice in the near future.

Amygdala substructure volumes in Major Depressive Disorder

The role of the amygdala in the experience of emotional states and stress is well established. Connections from the amygdala to the hypothalamus activate the hypothalamic-pituitaryadrenal (HPA) axis and the cortisol response. Previous studies have failed to find consistent whole amygdala volume changes in Major Depressive Disorder (MDD), but differences may exist at the smaller substructural level of the amygdala nuclei. High-resolution T1 and T2-weighted-fluid-attenuated inversion recovery MRIs were compared between 80 patients with MDD and 83 healthy controls (HC) using the automated amygdala substructure module in FreeSurfer 6.0. Volumetric assessments were performed for individual nuclei and three anatomico-functional composite groups of nuclei. Salivary cortisol awakening response (CAR), as a measure of HPA responsivity, was measured in a subset of patients. The right medial nucleus volume was larger in MDD compared to HC (p = 0.002). Increased right-left volume ratios were found in MDD for the whole amygdala (p = 0.004), the laterobasal composite (p = 0.009) and in the central (p = 0.003) and medial (p = 0.014) nuclei. The CAR was not significantly different between MDD and HC. Within the MDD group the left corticoamygdaloid transition area was inversely correlated with the CAR, as measured by area under the curve (AUCg) (p ≤ 0.0001). In conclusion, our study found larger right medial nuclei volumes in MDD compared to HC and relatively increased right compared to left whole and substructure volume ratios in MDD. The results suggest that amygdala substructure volumes may be involved in the pathophysiology of depression.

Agent-based modeling of the central amygdala and pain using cell-type specific physiological parameters

The amygdala is a brain area involved in emotional regulation and pain. Over the course of the last 20 years, multiple researchers have studied sensory and motor connections within the amygdala in trying to understand the ultimate role of this structure in pain perception and descending control of pain. A number of investigators have been using cell-type specific manipulations to probe the underlying circuitry of the amygdala. As data have accumulated in this research space, we recognized a critical need for a single framework to integrate these data and evaluate emergent system-level responses. In this manuscript, we present an agent-based computational model of two distinct inhibitory neuron populations in the amygdala, those that express protein kinase C delta (PKCδ) and those that express somatostatin (SOM). We utilized a network of neural links to simulate connectivity and the transmission of inhibitory signals between neurons. Type-specific parameters describing the response of these neurons to noxious stimuli were estimated from published physiological and immunological data as well as our own wet-lab experiments. The model outputs an abstract measure of pain, which is calculated in terms of the cumulative pro-nociceptive and anti-nociceptive activity across neurons in both hemispheres of the amygdala. Results demonstrate the ability of the model to produce changes in pain that are consistent with published studies and highlight the importance of several model parameters. In particular, we found that the relative proportion of PKCδ and SOM neurons within each hemisphere is a key parameter in predicting pain and we explored model predictions for three possible values of this parameter. We compared model predictions of pain to data from our earlier behavioral studies and found areas of similarity as well as distinctions between the data sets. These differences, in particular, suggest a number of wet-lab experiments that could be done in the future.

In this manuscript, we present a computational modeling approach to understand and predict pain output from a part of the brain, the amygdala, involved in stress adaptation, emotional regulation, and pain. Over the last several years, a variety of groups have begun to dissect the specific cells that are responsible for the impact of amygdala activation on pain, which can include both increases and decreases in pain and pain-like output in animal models. It is helpful and necessary to use computational models to develop a framework to understand the amygdala as these wet lab techniques add to the complexity of our understanding of the brain structure. The model presented here was based on our recent published physiology experiments along with multiple examples of expression data. This model can be used to design future wet-lab experiments and can continue to be refined to help us evaluate the impact of the amygdala and similar limbic system structures in pain and other disease. Here we present the first computational model for amygdala signaling that includes physiological and histological properties of neurons and allows dynamic simulation of nociceptive signal propagation through the network.

Nonhuman Primate Models to Explore Mechanisms Underlying Early-Life Temperamental Anxiety

Anxiety disorders are among the most prevalent psychiatric disorders, causing significant suffering and disability. Behavioral inhibition is a temperament that is linked to an increased risk for the later development of anxiety disorders and other stress-related psychopathology, and understanding the neural systems underlying this dispositional risk could provide insight into novel treatment targets for anxiety disorders. Nonhuman primates (NHPs) have anxiety-related temperaments that are similar to those of humans with behavioral inhibition, facilitating the design of translational models related to human psychopathology. Characterization of our NHP model of behavioral inhibition, which we term anxious temperament (AT), reveals that it is trait-like. Exploration of the neural substrates of AT in NHPs has revealed a distributed neural circuit that is linked to individual differences in AT, which includes the dorsal amygdala. AT-related metabolism in the dorsal amygdala, including the central nucleus, is stable across time and can be detected even in safe contexts, suggesting that AT has trait-like neural signatures within the brain. The use of lesioning and novel chemogenetic methods allows for mechanistic perturbation of the amygdala to determine its causal contribution to AT. Studies characterizing the molecular bases for individual differences in AT in the dorsal amygdala, which take advantage of novel methods for probing cellular and molecular systems, suggest involvement of neurotrophic systems, which point to the importance of neuroplasticity in AT. These novel methods, when used in combination with translational NHP models such as AT, promise to provide insights into the brain systems underlying the early risk for anxiety disorder development.

Brain transcriptomics of nonhuman primates: A review

The brain is one of the most important and intricate organs in our bodies. Interpreting brain function and illustrating the changes and molecular mechanisms during physiological or pathological processes are essential but sometimes difficult to achieve. In addition to histology, ethology and pharmacology, the development of transcriptomics alleviates this condition by enabling high-throughput observation of the brain at various levels of anatomical specificity. Moreover, because human brain samples are scarce, the brains of nonhuman primates are important alternative models. Here in this review, we summarize the applications of transcriptomics in nonhuman primate brain studies, including investigations of brain development, aging, toxic effects and diseases. Overall, as a powerful tool with developmental potential, transcriptomics has been widely utilized in neuroscience.