Prefrontal Regulation of Threat-Elicited Behaviors: A Pathway to Translation

A shared threat-anticipation circuit is dynamically engaged at different moments by certain and uncertain threat

Temporal dynamics play a central role in models of emotion: "fear" is widely conceptualized as a phasic response to certain-and-imminent danger, whereas "anxiety" is a sustained response to uncertain-or-distal harm. Yet the underlying neurobiology remains contentious. Leveraging a translationally relevant fMRI paradigm and theory-driven modeling approach, we demonstrate that certain- and uncertain-threat anticipation recruit a shared circuit that encompasses the central extended amygdala (EAc), periaqueductal gray, midcingulate, and anterior insula. This circuit exhibits persistently elevated activation when threat is uncertain and distal, and transient bursts of activation just before certain encounters with threat. Although there is widespread agreement that the EAc plays a critical role in orchestrating responses to threat, confusion persists about the respective contributions of its major subdivisions, the bed nucleus of the stria terminalis (BST) and central nucleus of the amygdala (Ce). Here we used anatomical regions-of-interest to demonstrate that the BST and Ce exhibit statistically indistinguishable threat dynamics, with moderate-to-strong Bayesian evidence for the null hypothesis. Both regions exhibited activation dynamics that run counter to popular models, with the Ce showing sustained responses to uncertain-and-distal threat and the BST showing phasic responses to certain-and-imminent threat. For many scientists, feelings are the hallmark of fear and anxiety. Here we used an independently validated brain 'signature' to covertly decode the moment-by-moment dynamics of anticipatory distress for the first time. Results mirrored the dynamics of neural activation. These observations provide fresh insights into the neurobiology of threat-elicited emotions and set the stage for more ambitious clinical and mechanistic research.

Orbitofrontal and Prelimbic Cortices Serve Complementary Roles in Adapting Reward Seeking to Learned Anxiety

BACKGROUND

Anxiety is a common symptom of several mental health disorders and adversely affects motivated behaviors. Anxiety can emerge from associating risk of future harm while engaged in goal-guided actions. Using a recently developed behavioral paradigm to model this aspect of anxiety, we investigated the role of 2 cortical subregions, the prelimbic medial frontal cortex (PL) and lateral orbitofrontal cortex (lOFC), which have been implicated in anxiety and outcome expectation, in flexible representation of actions associated with harm risk.

METHODS

A seek-take reward-guided instrumental task design was used to train animals (N = 8) to associate the seek action with a variable risk of punishment. After learning, animals underwent extinction training for this association. Fiber photometry was used to measure and compare neuronal activity in the PL and lOFC during learning and extinction.

RESULTS

Animals increased action suppression in response to punishment contingencies. This increase dissipated after extinction training. These behavioral changes were associated with region-specific changes in neuronal activity. PL neuronal activity preferentially adapted to the threat of punishment, whereas lOFC activity adapted to safe aspects of the task. Moreover, correlated activity between these regions was suppressed during actions associated with harm risk, suggesting that these regions may guide behavior independently under anxiety.

CONCLUSIONS

These findings suggest that the PL and lOFC serve distinct but complementary roles in the representation of learned anxiety. This dissociation may provide a mechanism to explain how overlapping cortical systems are implicated in reward-guided action execution during anxiety.

Multiple faces of anxiety: a frontal lobe perspective

Marked dysregulation of the human prefrontal cortex (PFC) and anterior cingulate cortex (ACC) characterises a variety of anxiety disorders, and its amelioration is a key feature of treatment success. Overall treatment response, however, is highly variable, and about a third of patients are resistant to treatment. In this review we hypothesise that a major contributor to this variation in treatment response are the multiple faces of anxiety induced by distinct forms of frontal cortex dysregulation. Comparison of findings from humans and non-human primates reveals marked similarity in the functional organisation of threat regulation across the frontal lobes. This organisation is discussed in relation to the 'predatory imminence continuum' model of threat and the differential engagement of executive functions at the core of both emotion generation and regulation strategies.

Pathophysiology in cortico-amygdala circuits and excessive aversion processing: the role of oligodendrocytes and myelination

Stress-related psychiatric illnesses, such as major depressive disorder, anxiety and post-traumatic stress disorder, present with alterations in emotional processing, including excessive processing of negative/aversive stimuli and events. The bidirectional human/primate brain circuit comprising anterior cingulate cortex and amygdala is of fundamental importance in processing emotional stimuli, and in rodents the medial prefrontal cortex-amygdala circuit is to some extent analogous in structure and function. Here, we assess the comparative evidence for: (i) Anterior cingulate/medial prefrontal cortex<->amygdala bidirectional neural circuits as major contributors to aversive stimulus processing; (ii) Structural and functional changes in anterior cingulate cortex<->amygdala circuit associated with excessive aversion processing in stress-related neuropsychiatric disorders, and in medial prefrontal cortex<->amygdala circuit in rodent models of chronic stress-induced increased aversion reactivity; and (iii) Altered status of oligodendrocytes and their oligodendrocyte lineage cells and myelination in anterior cingulate/medial prefrontal cortex<->amygdala circuits in stress-related neuropsychiatric disorders and stress models. The comparative evidence from humans and rodents is that their respective anterior cingulate/medial prefrontal cortex<->amygdala circuits are integral to adaptive aversion processing. However, at the sub-regional level, the anterior cingulate/medial prefrontal cortex structure-function analogy is incomplete, and differences as well as similarities need to be taken into account. Structure-function imaging studies demonstrate that these neural circuits are altered in both human stress-related neuropsychiatric disorders and rodent models of stress-induced increased aversion processing. In both cases, the changes include altered white matter integrity, albeit the current evidence indicates that this is decreased in humans and increased in rodent models. At the cellular-molecular level, in both humans and rodents, the current evidence is that stress disorders do present with changes in oligodendrocyte lineage, oligodendrocytes and/or myelin in these neural circuits, but these changes are often discordant between and even within species. Nonetheless, by integrating the current comparative evidence, this review provides a timely insight into this field and should function to inform future studies-human, monkey and rodent-to ascertain whether or not the oligodendrocyte lineage and myelination are causally involved in the pathophysiology of stress-related neuropsychiatric disorders.

Case report: Anxiety and depression as initial symptoms in a patient with acute hypoxia and patent foramen ovale

The prevalence of patent foramen ovale (PFO) is 15-35% among adults. The role of right-to-left shunting through the PFO, anxiety, depression, and hypoxemia in the systemic circulation remains poorly understood. Herein, we present the case of a 52-year-old woman with no heart or lung disease, who was admitted due to anxiety for 5 months and had symptom exacerbation with dizziness for 4 days and presented with cyanosis. She was noted to have acute hypoxemia, with an oxygen saturation of 94.48% on room air, and arterial blood gas showed an oxygen tension of 65.64 mmHg. Agitated saline contrast echocardiography showed right-to-left shunting due to PFO. Arteriovenous fistula, pneumonia, pulmonary embolism, pulmonary hypertension, congestion peripheral cyanosis, ischemic peripheral cyanosis, and methemoglobin were excluded. Additionally, the patient improved by taking Paroxetine, Oxazepam, and Olanzapine. Her oxygen tension returned to 90.42 mmHg, and her symptoms resolved. In the case of severe anxiety and depression, right-to-left shunting through the PFO may cause acute systemic hypoxemia via a flow-driven mechanism, occasionally manifesting as cyanosis. When anxiety improved, hypoxia also improved. Thus, the treatment of anxiety and depression seems effective in improving hypoxemia. Notably, this is a rare report, and we hope to draw the attention of psychosomatic specialists, psychiatrists, and clinicians to seek the relationship between anxiety appearing as acute stress and PFO. This may be a new therapeutic method for treating severe anxiety disorder.

Subgenual and Hippocampal Pathways in Amygdala Are Set to Balance Affect and Context Processing

The amygdala, hippocampus, and subgenual cortex area 25 (A25) are engaged in complex cognitive-emotional processes. Yet pathway interactions from hippocampus and A25 with postsynaptic sites in amygdala remain largely unknown. In rhesus monkeys of both sexes, we studied with neural tracers how pathways from A25 and hippocampus interface with excitatory and inhibitory microcircuits in amygdala at multiple scales. We found that both hippocampus and A25 innervate distinct as well as overlapping sites of the basolateral (BL) amygdalar nucleus. Unique hippocampal pathways heavily innervated the intrinsic paralaminar basolateral nucleus, which is associated with plasticity. In contrast, orbital A25 preferentially innervated another intrinsic network, the intercalated masses, an inhibitory reticulum that gates amygdalar autonomic output and inhibits fear-related behaviors. Finally, using high-resolution confocal and electron microscopy (EM), we found that among inhibitory postsynaptic targets in BL, both hippocampal and A25 pathways preferentially formed synapses with calretinin (CR) neurons, which are known for disinhibition and may enhance excitatory drive in the amygdala. Among other inhibitory postsynaptic sites, A25 pathways innervated the powerful parvalbumin (PV) neurons which may flexibly regulate the gain of neuronal assemblies in the BL that affect the internal state. In contrast, hippocampal pathways innervated calbindin (CB) inhibitory neurons, which modulate specific excitatory inputs for processing context and learning correct associations. Common and unique patterns of innervation in amygdala by hippocampus and A25 have implications for how complex cognitive and emotional processes may be selectively disrupted in psychiatric disorders.SIGNIFICANCE STATEMENT The hippocampus, subgenual A25, and amygdala are associated with learning, memory, and emotions. We found that A25 is poised to affect diverse amygdalar processes, from emotional expression to fear learning by innervating the basal complex and the intrinsic intercalated masses. Hippocampal pathways uniquely interacted with another intrinsic amygdalar nucleus which is associated with plasticity, suggesting flexible processing of signals in context for learning. In the basolateral (BL) amygdala, which has a role in fear learning, both hippocampal and A25 interacted preferentially with disinhibitory neurons, suggesting a boost in excitation. The two pathways diverged in innervating other classes of inhibitory neurons, suggesting circuit specificities that could become perturbed in psychiatric diseases.

Chemogenetics identifies separate area 25 brain circuits involved in anhedonia and anxiety in marmosets

Poor outcomes are common in individuals with anxiety and depression, and the brain circuits underlying symptoms and treatment responses remain elusive. To elucidate these neural circuits, experimental studies must specifically manipulate them, which is only possible in animals. Here, we used a chemogenetics strategy involving engineered designer receptors exclusively activated by designer drugs (DREADDs) to activate a region of the marmoset brain that is dysfunctional in human patients with major depressive disorder, called the subcallosal anterior cingulate cortex area 25 (scACC-25). Using this DREADDs system, we identified separate scACC-25 neural circuits that underlie specific components of anhedonia and anxiety in marmosets. Activation of the neural pathway connecting the scACC-25 to the nucleus accumbens (NAc) caused blunting of anticipatory arousal (a form of anhedonia) in marmosets in response to a reward-associated conditioned stimulus in an appetitive Pavlovian discrimination test. Separately, activation of the circuit between the scACC-25 and the amygdala increased a measure of anxiety (the threat response score) when marmosets were presented with an uncertain threat (human intruder test). Using the anhedonia data, we then showed that the fast-acting antidepressant ketamine when infused into the NAc of marmosets prevented anhedonia after scACC-25 activation for more than 1 week. These neurobiological findings provide targets that could contribute to the development of new treatment strategies.

Electroacupuncture Alleviates Depressive-like Behavior by Modulating the Expression of P2X7/NLRP3/IL-1β of Prefrontal Cortex and Liver in Rats Exposed to Chronic Unpredictable Mild Stress

Depression is a complex clinical disorder associated with poor outcomes. Electroacupuncture (EA) has been demonstrated to have an important role in both clinical and pre-clinical depression investigations. Evidence has suggested that the P2X7 receptor (P2X7R), NLRP3, and IL-1β play an important role in depressive disorder. Our study is aimed at exploring the role of EA in alleviating depression-like behaviors in rats. We therefore investigated the effects of EA on the prefrontal cortex and liver of rats subjected to chronic unpredictable mild stress (CUMS) through behavior tests, transmission electron microscopy, Nissl staining, HE staining, immunohistochemistry and Western blotting. Five weeks after exposure to CUMS, Sprague-Dawley (SD) rats showed depression-like behavior. Three weeks after treatment with brilliant blue G (BBG) or EA, depressive symptoms were significantly improved. Liver cells and microglia showed regular morphology and orderly arrangement in the BBG and EA groups compared with the CUMS group. Here we show that EA downregulated P2X7R/NLRP3/IL-1β expression and relieved depression-like behavior. In summary, our findings demonstrated the efficacy of EA in alleviating depression-like behaviors induced by CUMS in rats. This suggests that EA may serve as an adjunctive therapy in clinical practice, and that P2X7R may be a promising target for EA intervention on the liver–brain axis in treatment of depression.

Cortical thickness and curvature abnormalities in patients with major depressive disorder with childhood maltreatment: Neural markers of vulnerability?

BACKGROUND

Childhood maltreatment has been related to various disadvantageous lifetime outcomes. However, the brain structural alterations that occur in major depressive disorder (MDD) patients with childhood maltreatment are incompletely investigated.

METHODS

We extensively explored the cortical abnormalities including cortical volume, surface area, thickness, sulcal depth, and curvature in maltreated MDD patients. Twoway ANOVA was performed to distinguish the effects of childhood maltreatment and depression on structural abnormalities. Partial correlation analysis was performed to explore the relationship between childhood maltreatment and cortical abnormalities. Moreover, we plotted the receiver operating characteristic curve to examine whether the observed cortical abnormalities could be used as neuro biomarkers to identify maltreated MDD patients.

RESULTS

We reach the following findings: (i) relative to MDD without childhood maltreatment, MDD patients with childhood maltreatment existed increased cortical curvature in inferior frontal gyrus; (ii) compared to HC without childhood maltreatment, decreased cortical thickness was observed in anterior cingulate cortex and medial prefrontal cortex in MDD patients with childhood maltreatment; (iii) we confirmed the inseparable relationship between cortical curvature alterations in inferior frontal gyrus as well as childhood maltreatment; (iv) cortical curvature abnormality in inferior frontal gyrus could be applied as neural biomarker for clinical identification of MDD patients with childhood maltreatment.

CONCLUSIONS

Childhood maltreatment have a significant effects on cortical thickness and curvature abnormalities involved in inferior frontal gyrus, anterior cingulate cortex and medial prefrontal cortex, constituting the vulnerability to depression.

The Aversive Lens: Stress effects on the prefrontal-cingulate cortical pathways that regulate emotion

ARNSTEN, A.F.T., M.K.P. Joyce and A.C. Roberts. The Aversive Lens: Stress effects on the prefrontal-cingulate cortical pathways that regulate emotion. NEUROSCI BIOBEHAV REV XXX-XXX, 2022. The symptoms of major-depressive-disorder include psychic pain and anhedonia, i.e. seeing the world through an "aversive lens". The neurobiology underlying this shift in worldview is emerging. Here these data are reviewed, focusing on how activation of subgenual cingulate (BA25) induces an "aversive lens", and how higher prefrontal cortical (PFC) areas (BA46/10/32) provide top-down regulation of BA25 but are weakened by excessive dopamine and norepinephrine release during stress exposure, and dendritic spine loss with chronic stress exposure. These changes may generate an attractor state, which maintains the brain under the control of BA25, requiring medication or neuromodulatory treatments to return connectivity to a more flexible state. In line with this hypothesis, effective anti-depressant treatments reduce the activity of BA25 and restore top-down regulation by higher circuits, e.g. as seen with SSRI medications, ketamine, deep brain stimulation of BA25, or rTMS to strengthen dorsolateral PFC. This research has special relevance in an era of chronic stress caused by the COVID19 pandemic, political unrest and threat of climate change.

To fight or not to fight: activation of the mPFC during decision to engage in aggressive behavior after ethanol consumption in a novel murine model

RATIONALE

Alcohol consumption is a common antecedent of aggressive behavior. The effects of alcohol on the decision to engage in aggression in preference over pro-social interaction are hypothesized to arise from augmented function within the medial prefrontal cortex (mPFC).

OBJECTIVE

In a newly developed procedure, we studied social decision-making in male C57BL/6 J mice based on preferentially seeking access to either sociosexual interactions with a female partner or the opportunity to attack an intruder male. While deciding to engage in aggressive vs. sociosexual behavior, corresponding neural activation was assessed via c-Fos immunoreactivity in cortical, amygdaloid and tegmental regions of interest. A further objective was to investigate how self-administered alcohol impacted social choice.

METHODS

During repeated confrontations with an intruder male in their home cage, experimental mice engaged in species-specific sequence of pursuit, threat, and attack behavior within < 2 min. Mice were then conditioned to respond at one of two separate illuminated operanda in an experimental chamber (octagon) attached to their home cage; completion of 10 responses (fixed ratio 10; FR10) was reinforced by access to either a female or a male intruder which were presented in the resident's home cage. Brains were harvested following choice between the concurrently available aggressive and sociosexual options and processed for c-Fos immunoreactivity across 10 brain regions. In two separate groups, mice were trained to rapidly self-administer ethanol prior to a social choice trial in order to examine the effects of alcohol on social choice, sociosexual, aggressive acts and postures, and concurrent c-Fos activity in the mPFC and limbic regions.

RESULTS AND DISCUSSION

Eight out of 65 mice consistently chose to engage in aggressive behavior in preference to sociosexual contact with a female when each outcome was concurrently available. Self-administered alcohol (experiment 1: 1.2 ± 0.02 g/kg; experiment 2: 0, 1.0, 1.5, and 1.8 g/kg) increased responding for the aggressive option in mice that previously opted predominantly for access to sociosexual interactions with the female. When choosing the aggressive, but not the sociosexual option, the prelimbic area of the mPFC revealed increased c-Fos activity, guiding future detailed inquiry into the neural mechanisms for aggressive choice.

Learning of probabilistic punishment as a model of anxiety produces changes in action but not punisher encoding in the dmPFC and VTA

Previously, we developed a novel model for anxiety during motivated behavior by training rats to perform a task where actions executed to obtain a reward were probabilistically punished and observed that after learning, neuronal activity in the ventral tegmental area (VTA) and dorsomedial prefrontal cortex (dmPFC) represent the relationship between action and punishment risk (Park and Moghaddam, 2017). Here, we used male and female rats to expand on the previous work by focusing on neural changes in the dmPFC and VTA that were associated with the learning of probabilistic punishment, and anxiolytic treatment with diazepam after learning. We find that adaptive neural responses of dmPFC and VTA during the learning of anxiogenic contingencies are independent from the punisher experience and occur primarily during the peri-action and reward period. Our results also identify peri-action ramping of VTA neural calcium activity, and VTA-dmPFC correlated activity, as potential markers for the anxiolytic properties of diazepam.

Higher-order brain regions show shifts in structural covariance in adolescent marmosets

Substantial progress has been made studying morphological changes in brain regions during adolescence, but less is known of network-level changes in their relationship. Here, we compare covariance networks constructed from the correlation of morphometric volumes across 135 brain regions of marmoset monkeys in early adolescence and adulthood. Substantial shifts are identified in the topology of structural covariance networks in the prefrontal cortex (PFC) and temporal lobe. PFC regions become more structurally differentiated and segregated within their own local network, hypothesized to reflect increased specialization after maturation. In contrast, temporal regions show increased inter-hemispheric covariances that may underlie the establishment of distributed networks. Regionally selective coupling of structural and maturational covariance is revealed, with relatively weak coupling in transmodal association areas. The latter may be a consequence of continued maturation within adulthood, but also environmental factors, for example, family size, affecting brain morphology. Advancing our understanding of how morphological relationships within higher-order brain areas mature in adolescence deepens our knowledge of the developing brain's organizing principles.

Prefrontal cortical circuits in anxiety and fear: an overview

Pathological anxiety is among the most difficult neuropsychiatric diseases to treat pharmacologically, and it represents a major societal problem. Studies have implicated structural changes within the prefrontal cortex (PFC) and functional changes in the communication of the PFC with distal brain structures in anxiety disorders. Treatments that affect the activity of the PFC, including cognitive therapies and transcranial magnetic stimulation, reverse anxiety- and fear-associated circuit abnormalities through mechanisms that remain largely unclear. While the subjective experience of a rodent cannot be precisely determined, rodent models hold great promise in dissecting well-conserved circuits. Newly developed genetic and viral tools and optogenetic and chemogenetic techniques have revealed the intricacies of neural circuits underlying anxiety and fear by allowing direct examination of hypotheses drawn from existing psychological concepts. This review focuses on studies that have used these circuit-based approaches to gain a more detailed, more comprehensive, and more integrated view on how the PFC governs anxiety and fear and orchestrates adaptive defensive behaviors to hopefully provide a roadmap for the future development of therapies for pathological anxiety.

Region-specific microRNA alterations in marmosets carrying SLC6A4 polymorphisms are associated with anxiety-like behavior

BACKGROUND

Psychiatric diseases such as depression and anxiety are multifactorial conditions, highly prevalent in western societies. Human studies have identified a number of high-risk genetic variants for these diseases. Among them, polymorphisms in the promoter region of the serotonin transporter gene (SLC6A4) have attracted much attention. However, due to the paucity of experimental models, molecular alterations induced by these genetic variants and how they correlate to behavioral deficits have not been examined. In this regard, marmosets have emerged as a powerful model in translational neuroscience to investigate molecular underpinnings of complex behaviors.

METHODS

Here, we took advantage of naturally occurring genetic polymorphisms in marmoset SLC6A4 gene that have been linked to anxiety-like behaviors. Using FACS-sorting, we profiled microRNA contents in different brain regions of genotyped and behaviorally-phenotyped marmosets.

FINDINGS

We revealed that marmosets bearing different SLC6A4 variants exhibit distinct microRNAs signatures in a region of the prefrontal cortex whose activity has been consistently altered in patients with depression/anxiety. We also identified Deleted in Colorectal Cancer (DCC), a gene previously linked to these diseases, as a downstream target of the differently expressed microRNAs. Significantly, we showed that levels of both microRNAs and DCC in this region were highly correlated to anxiety-like behaviors.

INTERPRETATION

Our findings establish links between genetic variants, molecular modifications in specific cortical regions and complex behavioral responses, providing new insights into gene-behavior relationships underlying human psychopathology.

FUNDING

This work was supported by France National Agency, NRJ Foundation, Celphedia and Fondation de France as well as the Wellcome Trust.

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

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

Understanding them to understand ourselves: The importance of NHP research for translational neuroscience

Studying higher brain function presents fundamental scientific challenges but has great potential for impactful translation to the clinic, supporting the needs of many patients suffering from conditions that relate to neuronal dysfunction. For many key questions relevant to human neurological conditions and clinical interventions, non-human primates (NHPs) remain the only suitable model organism and the only effective way to study the relationship between brain structure and function with the knowledge and tools currently available. Here we present three exemplary studies of current research yielding important findings that are directly translational to human clinical patients but which would be impossible without NHP studies. Our first example shows how studies of the NHP prefrontal cortex are leading to clinically relevant advances and potential new treatments for human neuropsychiatric disorders such as depression and anxiety. Our second example looks at the relevance of NHP research to our understanding of visual pathways and the visual cortex, leading to visual prostheses that offer treatments for otherwise blind patients. Finally, we consider recent advances in treatments leading to improved recovery of movement and motor control in stroke patients, resulting from our improved understanding of brain stem parallel pathways involved in movement in NHPs. The case for using NHPs in neuroscience research, and the direct benefits to human patients, is strong but has rarely been set out directly. This paper reviews three very different areas of neuroscience research, expressly highlighting the unique insights offered to each by NHP studies and their direct applicability to human clinical conditions.

Prefrontal cortex and depression

The prefrontal cortex (PFC) has emerged as one of the regions most consistently impaired in major depressive disorder (MDD). Although functional and structural PFC abnormalities have been reported in both individuals with current MDD as well as those at increased vulnerability to MDD, this information has not translated into better treatment and prevention strategies. Here, we argue that dissecting depressive phenotypes into biologically more tractable dimensions - negative processing biases, anhedonia, despair-like behavior (learned helplessness) - affords unique opportunities for integrating clinical findings with mechanistic evidence emerging from preclinical models relevant to depression, and thereby promises to improve our understanding of MDD. To this end, we review and integrate clinical and preclinical literature pertinent to these core phenotypes, while emphasizing a systems-level approach, treatment effects, and whether specific PFC abnormalities are causes or consequences of MDD. In addition, we discuss several key issues linked to cross-species translation, including functional brain homology across species, the importance of dissecting neural pathways underlying specific functional domains that can be fruitfully probed across species, and the experimental approaches that best ensure translatability. Future directions and clinical implications of this burgeoning literature are discussed.

Modelling behaviors relevant to brain disorders in the nonhuman primate: Are we there yet?

Recent years have seen a profound resurgence of activity with nonhuman primates (NHPs) to model human brain disorders. From marmosets to macaques, the study of NHP species offers a unique window into the function of primate-specific neural circuits that are impossible to examine in other models. Examining how these circuits manifest into the complex behaviors of primates, such as advanced cognitive and social functions, has provided enormous insights to date into the mechanisms underlying symptoms of numerous neurological and neuropsychiatric illnesses. With the recent optimization of modern techniques to manipulate and measure neural activity in vivo, such as optogenetics and calcium imaging, NHP research is more well-equipped than ever to probe the neural mechanisms underlying pathological behavior. However, methods for behavioral experimentation and analysis in NHPs have noticeably failed to keep pace with these advances. As behavior ultimately lies at the junction between preclinical findings and its translation to clinical outcomes for brain disorders, approaches to improve the integrity, reproducibility, and translatability of behavioral experiments in NHPs requires critical evaluation. In this review, we provide a unifying account of existing brain disorder models using NHPs, and provide insights into the present and emerging contributions of behavioral studies to the field.

Differential Contribution of Anterior and Posterior Midcingulate Subregions to Distal and Proximal Threat Reactivity in Marmosets

The midcingulate cortex (MCC) is associated with cognition and emotion regulation. Structural and correlational functional evidence suggests that rather than being homogenous, the MCC may have dissociable functions that can be mapped onto distinct subregions. In this study, we use the marmoset monkey to causally investigate the contributions of two proposed subregions of the MCC: the anterior and posterior midcingulate cortices (aMCC and pMCC) to behavioral and cardiovascular correlates of threat processing relevant to anxiety disorders. Transient inactivation of the aMCC decreased anxiety-like responses to a postencounter distal threat, namely an unfamiliar human intruder, while inactivation of the pMCC showed a mild but opposing effect. Furthermore, although inactivation of neither MCC subregions had any effect on basal cardiovascular activity, aMCC inactivation blunted the expression of both cardiovascular and behavioral conditioned responses to a predictable proximal threat (a rubber snake) during the extinction in a Pavlovian conditioning task, with pMCC inactivation having again an opposing effect, but primarily on the behavioral response. These findings suggest that the MCC is indeed functionally heterogeneous with regards to its role in threat processing, with aMCC providing a marked facilitative contribution to the expression of the emotional response to both proximal and distal threat.

Combining brain perturbation and neuroimaging in non-human primates

Brain perturbation studies allow detailed causal inferences of behavioral and neural processes. Because the combination of brain perturbation methods and neural measurement techniques is inherently challenging, research in humans has predominantly focused on non-invasive, indirect brain perturbations, or neurological lesion studies. Non-human primates have been indispensable as a neurobiological system that is highly similar to humans while simultaneously being more experimentally tractable, allowing visualization of the functional and structural impact of systematic brain perturbation. This review considers the state of the art in non-human primate brain perturbation with a focus on approaches that can be combined with neuroimaging. We consider both non-reversible (lesions) and reversible or temporary perturbations such as electrical, pharmacological, optical, optogenetic, chemogenetic, pathway-selective, and ultrasound based interference methods. Method-specific considerations from the research and development community are offered to facilitate research in this field and support further innovations. We conclude by identifying novel avenues for further research and innovation and by highlighting the clinical translational potential of the methods.

Controlling one's world: Identification of sub-regions of primate PFC underlying goal-directed behavior

Impaired detection of causal relationships between actions and their outcomes can lead to maladaptive behavior. However, causal roles of specific prefrontal cortex (PFC) sub-regions and the caudate nucleus in mediating such relationships in primates are unclear. We inactivated and overactivated five PFC sub-regions, reversibly and pharmacologically: areas 24 (perigenual anterior cingulate cortex), 32 (medial PFC), 11 (anterior orbitofrontal cortex, OFC), 14 (rostral ventromedial PFC/medial OFC), and 14-25 (caudal ventromedial PFC) and the anteromedial caudate to examine their role in expressing learned action-outcome contingencies using a contingency degradation paradigm in marmoset monkeys. Area 24 or caudate inactivation impaired the response to contingency change, while area 11 inactivation enhanced it, and inactivation of areas 14, 32, or 14-25 had no effect. Overactivation of areas 11 and 24 impaired this response. These findings demonstrate the distinct roles of PFC sub-regions in goal-directed behavior and illuminate the candidate neurobehavioral substrates of psychiatric disorders, including obsessive-compulsive disorder.

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Monkey pgACC-24 is necessary for detecting causal control of actions over outcomes

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Its projection target in the caudate nucleus is also implicated

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Three other subregions of the ventromedial prefrontal cortex are not necessary

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Anterior OFC-11 may mediate Pavlovian influences on goal-directed behavior

Medial prefrontal cortex encoding of stress and anxiety

The prefrontal cortex (PFC) is involved in adaptive control of behavior and optimizing action selection. When an organism is experiencing an aversive event, such as a sustained state of anxiety or an overt experience of fear or stress, the mechanisms that govern PFC regulation of action selection may be critical for survival. A large body of literature has shown that acute aversive states influence the activity of PFC neurons and the release of neurotransmitters in this region. These states also result in long-term neurobiological changes in the PFC and expression of PFC-dependent motivated behaviors. The mechanism for how these changes lead to modifying action selection is only recently beginning to emerge. Here, we review animal and human studies into the neural mechanisms which may mediate the adaptive changes in the PFC that emerge during negative affective states. We then highlight recent advances in approaches for understanding how anxiety influences action selection and related cortical processes. We conclude by proposing that PFC neurons selectively influence action encoding during conditions where actions toward obtaining a reward or avoiding harm are executed under a fog of fear and anxiety.

Reward signaling by the rodent medial frontal cortex

The anatomical relevance and functional significance of medial parts of the rodent frontal cortex have been intensely debated over the modern history of neuroscience. Early studies emphasized common functions among medial frontal regions in rodents and the dorsolateral prefrontal cortex of primates. Behavioral tasks emphasized memory-guided performance and persistent neural activity as a marker of working memory. Over time, it became clear that long-standing concerns about cross-species homology were justified and the view emerged that rodents are useful for understanding medial parts of the frontal cortex in primates, and not the dorsolateral prefrontal cortex. Here, we summarize a series of studies on the rodent medial frontal cortex that began with an interest in studying working memory in the perigenual prelimbic area and ended up studying reward processing in the medial orbital region. Our experiments revealed a role for a 4-8Hz "theta" rhythm in tracking engagement in the consumption of rewarding fluids and denoting the value of a given reward. Evidence for a functional differentiation between the rostral and caudal medial frontal cortex and its relationship to other frontal cortical areas is also discussed with the hope of motivating future work on this part of the cerebral cortex.

Differential effects of GABAA receptor activation in the prelimbic and orbitofrontal cortices on anxiety

RATIONALE

The development of effective anxiety treatments has been hindered by limited understanding of the neurobiological mechanisms involved in anxiety regulation. Whilst gamma-aminobutyric acid (GABA) neurotransmission in the prefrontal cortex (PFC) is one mechanism consistently implicated in anxiety regulation, PFC subregions may contribute uniquely.

OBJECTIVES

The present study examined the effects of inactivating the PFC subregions of the prelimbic cortex (PrL) or orbitofrontal cortex (OFC) through GABA_A receptor (GABA_AR) activation, on anxiety behaviours in male Wistar rats.

METHODS

Sixty-six male Wistar rats were surgically implanted with bilateral cannulae into the PrL (n = 33) or the OFC (n = 33). Rats then received a microinjection of either the GABA_A receptor agonist muscimol or vehicle prior to each experiment, conducted 1 week apart. Measures of anxiety were examined using the elevated plus maze (EPM) and the emergence test (ET). The effect on locomotor activity (baseline or methamphetamine-induced) was also tested.

RESULTS

Differential effects of brain region inactivation on anxiety-like behaviour were shown by measures in the EPM and ET; muscimol infused into the PrL-reduced anxiety-like behaviour, yet had no significant effect when infused into the OFC, compared with control treated rats. No effects on locomotor activity at baseline or following methamphetamine treatment were found.

CONCLUSIONS

This study highlights that activation of GABA_ARs specifically within the PrL, but not OFC, reduces anxiety behaviours in male rats. This suggests that activity of the PrL plays a more important role than the OFC in the neurobiological mechanisms of unconditioned anxiety and should be targeted for future therapies.

Damage to Orbitofrontal Areas 12 and 13, but Not Area 14, Results in Blunted Attention and Arousal to Socioemotional Stimuli in Rhesus Macaques

An earlier study in monkeys indicated that lesions to the mid-portion of the ventral orbitofrontal cortex (OFC), including Walker’s areas 11 and 13 (OFC11/13), altered the spontaneous scanning of still pictures of primate faces (neutral and emotional) and the modulation of arousal. Yet, these conclusions were limited by several shortcomings, including the lesion approach, use of static rather than dynamic stimuli, and manual data analyses. To confirm and extend these earlier findings, we compared attention and arousal to social and nonsocial scenes in three groups of rhesus macaques with restricted lesions to one of three OFC areas (OFC12, OFC13, or OFC14) and a sham-operated control group using eye-tracking to capture scanning patterns, focal attention and pupil size. Animals with damage to the lateral OFC areas (OFC12 and OFC13) showed decreased attention specifically to the eyes of negative (threatening) social stimuli and increased arousal (increased pupil diameter) to positive social scenes. In contrast, animals with damage to the ventromedial OFC area (OFC14) displayed no differences in attention or arousal in the presence of social stimuli compared to controls. These findings support the notion that areas of the lateral OFC are critical for directing attention and modulating arousal to emotional social cues. Together with the existence of face-selective neurons in these lateral OFC areas, the data suggest that the lateral OFC may set the stage for multidimensional information processing related to faces and emotion and may be involved in social judgments.

Anxiety and the Neurobiology of Temporally Uncertain Threat Anticipation

When extreme, anxiety-a state of distress and arousal prototypically evoked by uncertain danger-can be debilitating. Uncertain anticipation is a shared feature of situations that elicit signs and symptoms of anxiety across psychiatric disorders, species, and assays. Despite the profound significance of anxiety for human health and wellbeing, the neurobiology of uncertain-threat anticipation remains unsettled. Leveraging a paradigm adapted from animal research and optimized for fMRI signal decomposition, we examined the neural circuits engaged during the anticipation of temporally uncertain and certain threat in 99 men and women. Results revealed that the neural systems recruited by uncertain and certain threat anticipation are anatomically colocalized in frontocortical regions, extended amygdala, and periaqueductal gray. Comparison of the threat conditions demonstrated that this circuitry can be fractionated, with frontocortical regions showing relatively stronger engagement during the anticipation of uncertain threat, and the extended amygdala showing the reverse pattern. Although there is widespread agreement that the bed nucleus of the stria terminalis and dorsal amygdala-the two major subdivisions of the extended amygdala-play a critical role in orchestrating adaptive responses to potential danger, their precise contributions to human anxiety have remained contentious. Follow-up analyses demonstrated that these regions show statistically indistinguishable responses to temporally uncertain and certain threat anticipation. These observations provide a framework for conceptualizing anxiety and fear, for understanding the functional neuroanatomy of threat anticipation in humans, and for accelerating the development of more effective intervention strategies for pathological anxiety.SIGNIFICANCE STATEMENT Anxiety-an emotion prototypically associated with the anticipation of uncertain harm-has profound significance for public health, yet the underlying neurobiology remains unclear. Leveraging a novel neuroimaging paradigm in a relatively large sample, we identify a core circuit responsive to both uncertain and certain threat anticipation, and show that this circuitry can be fractionated into subdivisions with a bias for one kind of threat or the other. The extended amygdala occupies center stage in neuropsychiatric models of anxiety, but its functional architecture has remained contentious. Here we demonstrate that its major subdivisions show statistically indistinguishable responses to temporally uncertain and certain threat. Collectively, these observations indicate the need to revise how we think about the neurobiology of anxiety and fear.

Opportunities and limitations of genetically modified nonhuman primate models for neuroscience research

The recently developed new genome-editing technologies, such as the CRISPR/Cas system, have opened the door for generating genetically modified nonhuman primate (NHP) models for basic neuroscience and brain disorders research. The complex circuit formation and experience-dependent refinement of the human brain are very difficult to model in vitro, and thus require use of in vivo whole-animal models. For many neurodevelopmental and psychiatric disorders, abnormal circuit formation and refinement might be at the center of their pathophysiology. Importantly, many of the critical circuits and regional cell populations implicated in higher human cognitive function and in many psychiatric disorders are not present in lower mammalian brains, while these analogous areas are replicated in NHP brains. Indeed, neuropsychiatric disorders represent a tremendous health and economic burden globally. The emerging field of genetically modified NHP models has the potential to transform our study of higher brain function and dramatically facilitate the development of effective treatment for human brain disorders. In this paper, we discuss the importance of developing such models, the infrastructure and training needed to maximize the impact of such models, and ethical standards required for using these models.

Ventromedial prefrontal area 14 provides opposing regulation of threat and reward-elicited responses in the common marmoset

The ventromedial prefrontal cortex (vmPFC) is a key brain structure implicated in mood and anxiety disorders, based primarily on evidence from correlational neuroimaging studies. Composed of a number of brain regions with distinct architecture and connectivity, dissecting its functional heterogeneity will provide key insights into the symptomatology of these disorders. Focusing on area 14, lying on the medial and orbital surfaces of the gyrus rectus, this study addresses a key question of causality. Do changes in area 14 activity induce changes in threat- and reward-elicited responses within the nonhuman primate, the common marmoset, similar to that seen in mood and anxiety disorders? Area 14 overactivation was found to induce heightened responsivity to uncertain, low-imminence threat while blunting cardiovascular and behavioral anticipatory arousal to high-value food reward. Conversely, inactivation enhanced the arousal to high-value reward cues while dampening the acquisition of cardiovascular and behavioral responses to a Pavlovian threat cue. Basal cardiovascular activity, including heart rate variability and sympathovagal balance, which are dysfunctional in mood and anxiety disorders, are insensitive to alterations in area 14 activity as is the extinction of conditioned threat responses. The distinct pattern of dysregulation compared to neighboring region area 25 highlights the heterogeneity of function within vmPFC and reveals how the effects of area 14 overactivation on positive and negative reactivity mirror symptoms of anhedonia and anxiety that are so often comorbid in mood disorders.

Associations between (sub) clinical stress- and anxiety symptoms in mentally healthy individuals and in major depression: a cross-sectional clinical study

BACKGROUND

Responses to stressful circumstances have psychological and physiological dimensions, and are related to anxiety symptoms and mental disorders such as depression. Nonetheless, the relationship between subclinical stress and anxiety symptoms is still elusive.

METHODS

To explore possible associations between stress and anxiety symptoms, patients with major depression (N = 77) and mentally healthy individuals of different age clusters and occupations (N = 412) were enrolled into the study. Stress was assessed with the new subclinical stress symptom questionnaire (SSQ-25). Anxiety was studied with the Beck Anxiety Inventory (BAI), mainly focusing on clinical anxiety, whilst anxiety as a personality trait was assessed with the trait aspect of the State Τrait Αnxiety Ιnventory Y (STAI Y). Statistical analyses included ANOVA, Scheffe test, linear regression models and a two-step cluster analysis using Log-Likelihood Distance measure and fixed number of two clusters.

RESULTS

Age, stress symptoms and BAI scores differed significantly between among groups (P < 0.001), whilst STAI Y scores did not. Stress levels were found to be related to clinical anxiety (P < 0.001), while neither group identity nor age exerted any influence on anxiety levels (P > 0.05). The two Step Cluster analysis classified 76 out of 77 participants with milder stress (subclinical) symptoms into the cluster with moderate anxiety, as indicated by BAI scores, and all individuals with more severe stress into the severe anxiety cluster.

CONCLUSIONS

The observed associations between stress and anxiety shed light on the interrelations between even very mild (subclinical) stress and anxiety symptoms and may point to the potential of mild stress to serve as a target for early interventions aiming to prevent anxiety morbidity.

Trait Anxiety Mediated by Amygdala Serotonin Transporter in the Common Marmoset

High trait anxiety is associated with altered activity across emotion regulation circuitry and a higher risk of developing anxiety disorders and depression. This circuitry is extensively modulated by serotonin. Here, to understand why some people may be more vulnerable to developing affective disorders, we investigated whether serotonin-related gene expression across the brain's emotion regulation circuitry may underlie individual differences in trait anxiety using the common marmoset (Callithrix jacchus, mixed sexes) as a model. First, we assessed the association of region-specific expression of the serotonin transporter (SLC6A4) and serotonin receptor (HTR1A, HTR2A, HTR2C) genes with anxiety-like behavior; and second, we investigated their causal role in two key features of the high trait anxious phenotype: high responsivity to anxiety-provoking stimuli and an exaggerated conditioned threat response. While the expression of the serotonin receptors did not show a significant relationship with anxiety-like behavior in any of the targeted brain regions, serotonin transporter expression, specifically within the right ventrolateral prefrontal cortex (vlPFC) and most strongly in the right amygdala, was associated positively with anxiety-like behavior. The causal relationship between amygdala serotonin levels and an animal's sensitivity to threat was confirmed via direct amygdala infusions of a selective serotonin reuptake inhibitor (SSRI), citalopram. Both anxiety-like behaviors, and conditioned threat-induced responses were reduced by the blockade of serotonin reuptake in the amygdala. Together, these findings provide evidence that high amygdala serotonin transporter expression contributes to the high trait anxious phenotype and suggest that reduction of threat reactivity by SSRIs may be mediated by their actions in the amygdala.SIGNIFICANCE STATEMENT Findings here contribute to our understanding of how the serotonin system underlies an individual's expression of threat-elicited negative emotions such as anxiety and fear within nonhuman primates. Exploration of serotonergic gene expression across brain regions implicated in emotion regulation revealed that serotonin transporter gene expression in the ventrolateral prefrontal cortex (vlPFC) and most strongly in the amygdala, but none of the serotonin receptor genes, were predictive of interindividual differences in anxiety-like behavior. Targeting of amygdala serotonin reuptake with selective serotonin reuptake inhibitors (SSRIs) confirmed the causal relationship between amygdala serotonin transporter and an animal's sensitivity to threat by reversing expression of two key features of the high trait-like anxiety phenotype: high responsivity to anxiety-provoking uncertain threat and responsivity to certain conditioned threat.