Opposing roles of primate areas 25 and 32 and their putative rodent homologs in the regulation of negative emotion

Comparative basolateral amygdala connectomics reveals dissociable single-neuron projection patterns to frontal cortex in macaques and mice

Basolateral amygdala (BLA) is a key hub for affect in the brain,1,2,3 and dysfunction within this area contributes to a host of psychiatric disorders.4,5 BLA is extensively and reciprocally interconnected with frontal cortex,6,7,8,9 and some aspects of its function are evolutionarily conserved across rodents, anthropoid primates, and humans.10 Neuron density in BLA is substantially lower in primates compared to murine rodents,11 and frontal cortex (FC) is dramatically expanded in primates, particularly the more anterior granular and dysgranular areas.12,13,14 Yet, how these anatomical differences influence the projection patterns of single BLA neurons to frontal cortex across rodents and primates is unknown. Using a barcoded connectomic approach, we assessed the single BLA neuron connections to frontal cortex in mice and macaques. We found that BLA neurons are more likely to project to multiple distinct parts of FC in mice than in macaques. Further, while single BLA neuron projections to nucleus accumbens were similarly organized in mice and macaques, BLA-FC connections differed substantially. Notably, BLA connections to subcallosal anterior cingulate cortex (scACC) in macaques were least likely to branch to other medial frontal cortex areas compared to perigenual ACC (pgACC). This pattern of connections was reversed in the mouse homologues of these areas, infralimbic and prelimbic cortex (IL and PL), mirroring functional differences between rodents and non-human primates. Taken together, these results indicate that BLA connections to FC are not linearly scaled from mice to macaques and instead the organization of single-neuron BLA connections is distinct between these species.

Subcallosal area 25: Its responsivity to the stress hormone cortisol and its opposing effects on appetitive motivation in marmosets

Aberrant activity in caudal subcallosal anterior cingulate cortex (scACC) is implicated in depression and anxiety symptomatology, with its normalisation a putative biomarker of successful treatment response. The function of scACC in emotion processing and mental health is not fully understood despite its known influence on stress-mediated processes through its rich expression of mineralocorticoid and glucocorticoid receptors. Here we examine the causal interaction between area 25 within scACC (scACC-25) and the stress hormone, cortisol, in the context of anhedonia and anxiety-like behaviour. In addition, the overall role of scACC-25 in hedonic capacity and motivation is investigated under transient pharmacological inactivation and overactivation. The results suggest that a local increase of cortisol in scACC-25 shows a rapid induction of anticipatory anhedonia and increased responsiveness to uncertain threat. Separate inactivation and overactivation of scACC-25 increased and decreased motivation and hedonic capacity, respectively, likely through different underlying mechanisms. Together, these data show that area scACC-25 has a causal role in consummatory and motivational behaviour and produces rapid responses to the stress hormone cortisol, that mediates anhedonia and anxiety-like behaviour.

Stress and inflammation target dorsolateral prefrontal cortex function: Neural mechanisms underlying weakened cognitive control

Most mental disorders involve dysfunction of the dorsolateral prefrontal cortex (dlPFC), a recently evolved brain region that subserves working memory, abstraction and the thoughtful regulation of attention, action and emotion. For example, schizophrenia, depression, long-COVID and Alzheimer's disease are all associated with dlPFC dysfunction, with neuropathology often focused in layer III. The dlPFC has extensive top-down projections: e.g. to the posterior association cortices to regulate attention, and the subgenual cingulate cortex via the rostral and medial PFC to regulate emotional responses. However, the dlPFC is particularly dependent on arousal state, and is very vulnerable to stress and inflammation, which are etiological and/or exacerbating factors in most mental disorders. The cellular mechanisms by which stress and inflammation impact the dlPFC are a topic of current research, and are summarized in this review. For example, the layer III dlPFC circuits generating working memory-related neuronal firing have unusual neurotransmission, depending on NMDAR and nicotinic-α7R actions that are blocked under inflammatory conditions by kynurenic acid. These circuits also have unusual neuromodulation, with the molecular machinery to magnify calcium signaling in spines needed to support persistent firing, which must be tightly regulated to prevent toxic calcium actions. Stress rapidly weakens layer III connectivity by driving feedforward calcium-cAMP opening of potassium channels on spines. This is regulated by postsynaptic noradrenergic α2A-AR and mGluR3 signaling, but dysregulated by inflammation and/or chronic stress exposure, contributing to spine loss. Treatments that strengthen dlPFC, via pharmacological (the α2A-AR agonist, guanfacine) or rTMS manipulation, provide a rational basis for therapy.

The emotion paradox in the aging body and brain

With age, parasympathetic activity decreases, while sympathetic activity increases. Thus, the typical older adult has low heart rate variability (HRV) and high noradrenaline levels. Younger adults with this physiological profile tend to be unhappy and stressed. Yet, with age, emotional experience tends to improve. Why does older adults' emotional well-being not suffer as their HRV decreases? To address this apparent paradox, I present the autonomic compensation model. In this model, failing organs, the initial phases of Alzheimer's pathology, and other age-related diseases trigger noradrenergic hyperactivity. To compensate, older brains increase autonomic regulatory activity in the pregenual prefrontal cortex (PFC). Age-related declines in nerve conduction reduce the ability of the pregenual PFC to reduce hyperactive noradrenergic activity and increase peripheral HRV. But these pregenual PFC autonomic compensation efforts have a significant impact in the brain, where they bias processing in favor of stimuli that tend to increase parasympathetic activity (e.g., stimuli that increase feelings of safety) and against stimuli that tend to increase sympathetic activity (e.g., threatening stimuli). In summary, the autonomic compensation model posits that age-related chronic sympathetic/noradrenergic hyperactivity stimulates regulatory attempts that have the side effect of enhancing emotional well-being.

Cingulate microstimulation induces negative decision-making via reduced top-down influence on primate fronto-cingulo-striatal network

The dorsolateral prefrontal cortex (dlPFC) is crucial for regulation of emotion that is known to aid prevention of depression. The broader fronto-cingulo-striatal (FCS) network, including cognitive dlPFC and limbic cingulo-striatal regions, has been associated with a negative evaluation bias often seen in depression. The mechanism by which dlPFC regulates the limbic system remains largely unclear. Here we have successfully induced a negative bias in decision-making in female primates performing a conflict decision-making task, by directly microstimulating the subgenual cingulate cortex while simultaneously recording FCS local field potentials (LFPs). The artificially induced negative bias in decision-making was associated with a significant decrease in functional connectivity from cognitive to limbic FCS regions, represented by a reduction in Granger causality in beta-range LFPs from the dlPFC to the other regions. The loss of top-down directional influence from cognitive to limbic regions, we suggest, could underlie negative biases in decision-making as observed in depressive states.

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.

Comparative basolateral amygdala connectomics reveals dissociable single-neuron projection patterns to frontal cortex in macaques and mice

The basolateral amygdala (BLA) projects to the frontal cortex (FC) in both rodents and primates, but the comparative organization of single-neuron BLA-FC projections is unknown. Using a barcoded connectomic approach, we found that BLA neurons are more likely to project to multiple distinct parts of FC in mice than in macaques. Further, while single BLA neuron projections to nucleus accumbens are similarly organized in mice and macaques, BLA-FC connections differ.

Single basolateral amygdala neurons in macaques exhibit distinct connectional motifs with frontal cortex

Basolateral amygdala (BLA) projects widely across the macaque frontal cortex, and amygdalo-frontal projections are critical for appropriate emotional responding and decision making. While it is appreciated that single BLA neurons branch and project to multiple areas in frontal cortex, the organization and frequency of this branching has yet to be fully characterized. Here, we determined the projection patterns of more than 3,000 macaque BLA neurons. We found that one-third of BLA neurons had two or more distinct projection targets in frontal cortex and subcortical structures. The patterns of single BLA neuron projections to multiple areas were organized into repeating motifs that targeted distinct sets of areas in medial and ventral frontal cortex, indicative of separable BLA networks. Our findings begin to reveal the rich structure of single-neuron connections in the non-human primate brain, providing a neuroanatomical basis for the role of BLA in coordinating brain-wide responses to valent stimuli.

A new perspective on HIV: effects of HIV on brain-heart axis

The human immunodeficiency virus (HIV) infection can cause damage to multiple systems within the body, and the interaction among these various organ systems means that pathological changes in one system can have repercussions on the functions of other systems. However, the current focus of treatment and research on HIV predominantly centers around individual systems without considering the comprehensive relationship among them. The central nervous system (CNS) and cardiovascular system play crucial roles in supporting human life, and their functions are closely intertwined. In this review, we examine the effects of HIV on the CNS, the resulting impact on the cardiovascular system, and the direct damage caused by HIV to the cardiovascular system to provide new perspectives on HIV treatment.

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.

High-throughput sequencing of macaque basolateral amygdala projections reveals dissociable connectional motifs with frontal cortex

The basolateral amygdala (BLA) projects widely across the macaque frontal cortex1-4, and amygdalo-frontal projections are critical for optimal emotional responding5 and decision-making6. Yet, little is known about the single-neuron architecture of these projections: namely, whether single BLA neurons project to multiple parts of the frontal cortex. Here, we use MAPseq7 to determine the projection patterns of over 3000 macaque BLA neurons. We found that one-third of BLA neurons have two or more distinct targets in parts of frontal cortex and of subcortical structures. Further, we reveal non-random structure within these branching patterns such that neurons with four targets are more frequently observed than those with two or three, indicative of widespread networks. Consequently, these multi-target single neurons form distinct networks within medial and ventral frontal cortex consistent with their known functions in regulating mood and decision-making. Additionally, we show that branching patterns of single neurons shape functional networks in the brain as assessed by fMRI-based functional connectivity. These results provide a neuroanatomical basis for the role of the BLA in coordinating brain-wide responses to valent stimuli8 and highlight the importance of high-resolution neuroanatomical data for understanding functional networks in the brain.

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.

The ventromedial prefrontal cortex and emotion regulation: lost in translation?

Neuroimaging studies implicate the ventromedial prefrontal cortex (vmPFC) in a wide range of emotional and cognitive functions, and changes in activity within vmPFC have been linked to the aetiology and successful treatment of depression. However, this is a large, structurally heterogeneous region and the extent to which this structural heterogeneity reflects functional heterogeneity remains unclear. Causal studies in animals should help address this question but attempts to map findings from vmPFC studies in rodents onto human imaging studies highlight cross-species discrepancies between structural homology and functional analogy. Bridging this gap, recent studies in marmosets - a species of new world monkey in which the overall organization of vmPFC is more like humans than that of rodents - have revealed that over-activation of the caudal subcallosal region of vmPFC, area 25, but not neighbouring area 32, heightens reactivity to negatively valenced stimuli whilst blunting responsivity to positively valenced stimuli. These co-occurring states resemble those seen in depressed patients, which are associated with increased activity in caudal subcallosal regions. In contrast, only reward blunting but not heightening of threat reactivity is seen following over-activation of the structurally homologous region in rodents. To further advance understanding of the role of vmPFC in the aetiology and treatment of depression, future work should focus on the behaviourally specific networks by which vmPFC regions have their effects, together with characterization of cross-species similarities and differences in function.

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.

Depression induced by low-frequency repetitive transcranial magnetic stimulation to ventral medial frontal cortex in monkeys

The medial frontal cortex (MFC), especially its ventral part, has long been of great interest with respect to the pathology of mood disorders. A number of human brain imaging studies have demonstrated the abnormalities of this brain region in patients with mood disorders, however, whether it is critically and causally involved in the pathogenesis of such disorders remains to be fully elucidated. In this study, we examined how the suppression of neural activity in the ventral region of the MFC (vMFC) affects the behavioral and physiological states of monkeys by using repetitive transcranial magnetic stimulation (rTMS). By using low-frequency rTMS (LF-rTMS) as an inhibitory intervention, we found that LF-rTMS targeting the vMFC temporarily induced a depression-like state in monkeys, which was characterized by a reduced movement activity level, impaired sociability, and decreased motivation level, as well as increased plasma cortisol level. On the other hand, no such significant changes in behavioral and physiological states were observed when targeting the other MFC regions, dorsal or posterior. We further found that the administration of an antidepressant agent, ketamine, ameliorated the abnormal behavioral and physiological states induced by the LF-rTMS intervention. These findings causally indicate the involvement of the vMFC in the regulation of mood and the validity of the LF-rTMS-induced dysfunction of the vMFC as a nonhuman primate model of depression.

Structural and functional variations in the prefrontal cortex are associated with learning in pre-adolescent common marmosets (Callithrix jacchus)

There is substantial evidence linking the prefrontal cortex (PFC) to a variety of cognitive abilities, with adolescence being a critical period in its development. In the current study, we investigated the neural basis of differences in learning in pre-adolescent common marmosets. At 8 months old, marmosets were given anatomical and resting state MRI scans (n=24). At 9 months old, association learning and inhibitory control was tested using a 'go/no go' visual discrimination (VD) task. Marmosets were grouped into 'learners' (n=12) and 'non-learners' (n=12), and associations between cognitive performance and sub-regional PFC volumes, as well as PFC connectivity patterns, were investigated. 'Learners' had significantly (p<0.05) larger volumes of areas 11, 25, 47 and 32 than 'non-learners', although 'non-learners' had significantly larger volumes of areas 24a and 8v than 'learners'. There was also a significant correlation between average % correct responses to the 'punished' stimulus and volume of area 47. Further, 'non-learners' had significantly greater global PFC connections, as well as significantly greater numbers of connections between the PFC and basal ganglia, cerebellum and hippocampus, compared to 'non-learners'. These results suggest that larger sub-regions of the orbitofrontal cortex and ventromedial PFC, as well more refined PFC connectivity patterns to other brain regions associated with learning, may be important in successful response inhibition. This study therefore offers new information on the neurodevelopment of individual differences in cognition during pre-adolescence in non-human primates.

A pragmatic reevaluation of the efficacy of nonhuman primate optogenetics for psychiatry

Translational neuroscience is committed to generating discoveries in the laboratory that ultimately can improve human lives. Optogenetics has received considerable attention because of its demonstrated promise in rodent brains to manipulate cells and circuits. In a recent report, Tremblay et al. [28] introduce an open resource detailing optogenetic studies of the nonhuman primate (NHP) brain and make robust claims about the translatability of the technology. We propose that their quantitative (e.g. a 91% success rate) and theoretical claims are questionable because the data were analyzed at a level relevant to the rodent but not NHP brain. Injections were clustered within a few monkeys in a few studies in a few brain regions, and their definitions of success were not clearly relevant to human neuropsychiatric disease. A reanalysis of the data with a modified definition of success that included a behavioral and biological effect revealed a 62.5% success rate that was lower when considering only strong outcomes (53.1%). This calls into question the current efficacy of optogenetic techniques in the NHP brain and suggests that we are a long way from being able to leverage them in 'the service of patients with neurological or psychiatric conditions' as the Tremblay report claims.

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.

Neuromodulation of prefrontal cortex cognitive function in primates: the powerful roles of monoamines and acetylcholine

The primate prefrontal cortex (PFC) subserves our highest order cognitive operations, and yet is tremendously dependent on a precise neurochemical environment for proper functioning. Depletion of noradrenaline and dopamine, or of acetylcholine from the dorsolateral PFC (dlPFC), is as devastating as removing the cortex itself, and serotonergic influences are also critical to proper functioning of the orbital and medial PFC. Most neuromodulators have a narrow inverted U dose response, which coordinates arousal state with cognitive state, and contributes to cognitive deficits with fatigue or uncontrollable stress. Studies in monkeys have revealed the molecular signaling mechanisms that govern the generation and modulation of mental representations by the dlPFC, allowing dynamic regulation of network strength, a process that requires tight regulation to prevent toxic actions, e.g., as occurs with advanced age. Brain imaging studies in humans have observed drug and genotype influences on a range of cognitive tasks and on PFC circuit functional connectivity, e.g., showing that catecholamines stabilize representations in a baseline-dependent manner. Research in monkeys has already led to new treatments for cognitive disorders in humans, encouraging future research in this important field.

Foraging with the frontal cortex: A cross-species evaluation of reward-guided behavior

Efficient foraging is essential to survival and depends on frontal cortex in mammals. Because of its role in psychiatric disorders, frontal cortex and its contributions to reward procurement have been studied extensively in both rodents and non-human primates. How frontal cortex of these animal models compares is a source of intense debate. Here we argue that translating findings from rodents to non-human primates requires an appreciation of both the niche in which each animal forages as well as the similarities in frontal cortex anatomy and function. Consequently, we highlight similarities and differences in behavior and anatomy, before focusing on points of convergence in how parts of frontal cortex contribute to distinct aspects of foraging in rats and macaques, more specifically. In doing so, our aim is to emphasize where translation of frontal cortex function between species is clearer, where there is divergence, and where future work should focus. We finish by highlighting aspects of foraging for which have received less attention but we believe are critical to uncovering how frontal cortex promotes survival in each species.

Comparative connectomics of the primate social brain

Social interaction is thought to provide a selection pressure for human intelligence, yet little is known about its neurobiological basis and evolution throughout the primate lineage. Recent advances in neuroimaging have enabled whole brain investigation of brain structure, function, and connectivity in humans and non-human primates (NHPs), leading to a nascent field of comparative connectomics. However, linking social behavior to brain organization across the primates remains challenging. Here, we review the current understanding of the macroscale neural mechanisms of social behaviors from the viewpoint of system neuroscience. We first demonstrate an association between the number of cortical neurons and the size of social groups across primates, suggesting a link between neural information-processing capacity and social capabilities. Moreover, by capitalizing on recent advances in species-harmonized functional MRI, we demonstrate that portions of the mirror neuron system and default-mode networks, which are thought to be important for representation of the other's actions and sense of self, respectively, exhibit similarities in functional organization in macaque monkeys and humans, suggesting possible homologies. With respect to these two networks, we describe recent developments in the neurobiology of social perception, joint attention, personality and social complexity. Together, the Human Connectome Project (HCP)-style comparative neuroimaging, hyperscanning, behavioral, and other multi-modal investigations are expected to yield important insights into the evolutionary foundations of human social behavior.

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.

Anterior Cingulate Cortex Ablation Disrupts Affective Vigor and Vigilance

Despite many observations of anterior cingulate cortex (ACC) activity related to cognition and affect in humans and nonhuman animals, little is known about the causal role of the ACC in psychological processes. Here, we investigate the causal role of the ACC in affective responding to threat in rhesus monkeys (Macaca mulatta), a species with an ACC largely homologous to humans in structure and connectivity. Male adult monkeys received bilateral ibotenate axon-sparing lesions to the ACC (sulcus and gyrus of areas 24, 32, and 25) and were tested in two classic tasks of monkey threat processing: the human intruder and object responsiveness tasks. Monkeys with ACC lesions did not significantly differ from controls in their overall mean reactivity toward threatening or novel stimuli. However, while control monkeys maintained their reactivity across test days, monkeys with ACC lesions reduced their reactivity toward stimuli as days advanced. Critically, this attenuated reactivity was found even when the stimuli presented each day were novel, suggesting that ACC lesions did not simply cause accelerated adaptation to stimuli as they became less novel over repeated presentations. Rather, these results imply that the primate ACC is necessary for maintaining appropriate affective responses toward potentially harmful and/or novel stimuli. These findings therefore have implications for mood disorders in which responding to threat and novelty is disrupted.SIGNIFICANCE STATEMENT Decades of research in humans and nonhuman animals have investigated the role of the anterior cingulate cortex in a huge number and variety of psychological processes spanning cognition and affect, as well as in psychological and neurologic diseases. The structure is broadly implicated in psychological processes and mental and neurologic health, yet its causal role in these processes has largely gone untested, particularly in primates. Here we demonstrate that when anterior cingulate cortex is completely eliminated, rhesus monkeys are initially responsive to threats, but these responses attenuate rather than persist, resembling a pattern of behavior commonly seen in patients diagnosed with mood disorders.

Differential Effects of the Inactivation of Anterior and Posterior Orbitofrontal Cortex on Affective Responses to Proximal and Distal Threat, and Reward Anticipation in the Common Marmoset

Structural and functional abnormalities of the orbitofrontal cortex (OFC) have been implicated in affective disorders that manifest anxiety-related symptoms. However, research into the functions of primate OFC has predominantly focused on reward-oriented rather than threat-oriented responses. To redress this imbalance, the present study performed a comprehensive analysis of the independent role of 2 distinct subregions of the central OFC (anterior area 11; aOFC and posterior area 13; pOFC) in the processing of distal and proximal threat. Temporary inactivation of both aOFC and pOFC heightened responses to distal threat in the form of an unknown human, but not to proximal threat assessed in a discriminative Pavlovian conditioning task. Inactivation of the aOFC, however, did unexpectedly blunt conditioned threat responses, although the effect was not valence-specific, as conditioned appetitive responses were similarly blunted and appeared restricted to a discriminative version of the task (when both CS⁻ and CS⁺ are present within a session). Inactivation of the pOFC did not affect conditioned responses to either proximal threat or reward and basal cardiovascular activity was unaffected by manipulations of activity in either subregion. The results highlight the contribution of aOFC and pOFC to regulation of responses to more distal uncertain but not proximal, certain threat and reveal their opposing contribution to that of the immediately adjacent medial OFC, area 14.

Chronic Stress Weakens Connectivity in the Prefrontal Cortex: Architectural and Molecular Changes

Chronic exposure to uncontrollable stress causes loss of spines and dendrites in the prefrontal cortex (PFC), a recently evolved brain region that provides top-down regulation of thought, action, and emotion. PFC neurons generate top-down goals through recurrent excitatory connections on spines. This persistent firing is the foundation for higher cognition, including working memory, and abstract thought. However, exposure to acute uncontrollable stress drives high levels of catecholamine release in the PFC, which activates feedforward calcium-cAMP signaling pathways to open nearby potassium channels, rapidly weakening synaptic connectivity to reduce persistent firing. Chronic stress exposures can further exacerbate these signaling events leading to loss of spines and resulting in marked cognitive impairment. In this review, we discuss how stress signaling mechanisms can lead to spine loss, including changes to BDNF-mTORC1 signaling, calcium homeostasis, actin dynamics, and mitochondrial actions that engage glial removal of spines through inflammatory signaling. Stress signaling events may be amplified in PFC spines due to cAMP magnification of internal calcium release. As PFC dendritic spine loss is a feature of many cognitive disorders, understanding how stress affects the structure and function of the PFC will help to inform strategies for treatment and prevention.

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.

Medial prefrontal cortex (A32 and A25) projections in the common marmoset: a subcortical anterograde study

This study was aimed at establishing the subcorticals substrates of the cognitive and visceromotor circuits of the A32 and A25 cortices of the medial prefrontal cortex and their projections and interactions with subcortical complexes in the common marmoset monkey (Callithrix jacchus). The study was primarily restricted to the nuclei of the diencephalon and amygdala. The common marmoset is a neotropical primate of the new world, and the absence of telencephalic gyrus favors the mapping of neuronal fibers. The biotinylated dextran amine was employed as an anterograde tracer. There was an evident pattern of rostrocaudal distribution of fibers within the subcortical nuclei, with medial orientation. Considering this distribution, fibers originating from the A25 cortex were found to be more clustered in the diencephalon and amygdala than those originating in the A32 cortex. Most areas of the amygdala received fibers from both cortices. In the diencephalon, all regions received projections from the A32, while the A25 fibers were restricted to the thalamus, hypothalamus, and epithalamus at different densities. Precise deposits of neuronal tracers provided here may significantly contribute to expand our understanding of specific connectivity among the medial prefrontal cortex with limbic regions and diencephalic areas, key elements to the viscerocognitive process.

Causal Evidence for Induction of Pessimistic Decision-Making in Primates by the Network of Frontal Cortex and Striosomes

Clinical studies have shown that patients with anxiety disorders exhibited coactivation of limbic cortices and basal ganglia, which together form a large-scale brain network. The mechanisms by which such a large-scale network could induce or modulate anxiety-like states are largely unknown. This article reviews our experimental program in macaques demonstrating a causal involvement of local striatal and frontal cortical sites in inducing pessimistic decision-making that underlies anxiety. Where relevant, we related these findings to the wider literature. To identify such sites, we have made a series of methodologic advances, including the combination of causal evidence for behavioral modification of pessimistic decisions with viral tracing methods. Critically, we introduced a version of the classic approach-avoidance (Ap-Av) conflict task, modified for use in non-human primates. We performed microstimulation of limbic-related cortical regions and the striatum, focusing on the pregenual anterior cingulate cortex (pACC), the caudal orbitofrontal cortex (cOFC), and the caudate nucleus (CN). Microstimulation of localized sites within these regions induced pessimistic decision-making by the monkeys, supporting the idea that the focal activation of these regions could induce an anxiety-like state, which subsequently influences decision-making. We further performed combined microstimulation and tract-tracing experiments by injecting anterograde viral tracers into focal regions, at which microstimulation induced increased avoidance. We found that effective stimulation sites in both pACC and cOFC zones projected preferentially to striosomes in the anterior striatum. Experiments in rodents have shown that the striosomes in the anterior striatum project directly to the dopamine-containing cells in the substantia nigra, and we have found evidence for a functional connection between striosomes and the lateral habenular region in which responses to reward are inhibitory. We present here further evidence for network interactions: we show that the pACC and cOFC project to common structures, including not only the anterior parts of the striosome compartment but also the tail of the CN, the subgenual ACC, the amygdala, and the thalamus. Together, our findings suggest that networks having pACC and cOFC as nodes share similar features in their connectivity patterns. We here hypothesize, based on these results, that the brain sites related to pessimistic judgment are mediated by a large-scale brain network that regulates dopaminergic functions and includes striosomes and striosome-projecting cortical regions.

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

Altered Posterior Cerebellar Lobule Connectivity With Perigenual Anterior Cingulate Cortex in Women With Primary Dysmenorrhea

Objectives: This study aimed to investigate the potential connectivity mechanism between the cerebellum and anterior cingulate cortex (ACC) and the cerebellar structure in primary dysmenorrhea (PDM).

Methods: We applied the spatially unbiased infratentorial template (SUIT) of the cerebellum to obtain anatomical details of cerebellar lobules, upon which the functional connectivity (FC) between the cerebellar lobules and ACC subregions was analyzed and the gray matter (GM) volume of cerebellar lobules was measured by using voxel-based morphometry (VBM) in 35 PDM females and 38 age-matched healthy females. The potential relationship between the altered FC or GM volume and clinical information was also evaluated in PDM females.

Results: PDM females showed higher connectivity between the left perigenual ACC (pACC) and lobule vermis_VI, between the left pACC and left lobule IX, and between right pACC and right cerebellar lobule VIIb than did the healthy controls. Compared with healthy controls, no altered GM volume was found in PDM females. No significant correlation was found between altered cerebellum–ACC FC and the clinical variables in the PDM females.

Conclusion: PDM females have abnormal posterior cerebellar connectivity with pACC but no abnormal structural changes. ACC–cerebellar circuit disturbances might be involved in the PDM females.

Translating Fear Circuitry: Amygdala Projections to Subgenual and Perigenual Anterior Cingulate in the Macaque

Rodent fear-learning models posit that amygdala–infralimbic connections facilitate extinction while amygdala–prelimbic prefrontal connections mediate fear expression. Analogous amygdala–prefrontal circuitry between rodents and primates is not established. Using paired small volumes of neural tracers injected into the perigenual anterior cingulate cortex (pgACC; areas 24b and 32; a potential homologue to rodent prelimbic cortex) and subgenual anterior cingulate cortex (sgACC, areas 25 and 14c; a potential homologue to rodent infralimbic cortex) in a single hemisphere, we mapped amygdala projections to the pgACC and sgACC within single subjects. All injections resulted in dense retrograde labeling specifically within the intermediate division of the basal nucleus (Bi) and the magnocellular division of the accessory basal nucleus (ABmc). Areal analysis revealed a bias for connectivity with the sgACC, with the ABmc showing a greater bias than the Bi. Double fluorescence analysis revealed that sgACC and pgACC projections were intermingled within the Bi and ABmc, where a proportion were double labeled. We conclude that amygdala inputs to the ACC largely originate from the Bi and ABmc, preferentially connect to the sgACC, and that a subset collaterally project to both sgACC and pgACC. These findings advance our understanding of fear extinction and fear expression circuitry across species.

The anterior cingulate cortex as a key locus of ketamine's antidepressant action

The subdivisions of the anterior cingulate cortex (ACC) - including subgenual, perigenual and dorsal zones - are implicated in the etiology, pathogenesis and treatment of major depression. We review an emerging body of evidence which suggests that changes in ACC activity are critically important in mediating the antidepressant effects of ketamine, the prototypical member of an emerging class of rapidly acting antidepressants. Infusions of ketamine induce acute (over minutes) and post-acute (over hours to days) modulations in subgenual and perigenual activity, and importantly, these changes can correlate with antidepressant efficacy. The subgenual and dorsal zones of the ACC have been specifically implicated in ketamine's anti-anhedonic effects. We emphasize the synergistic relationship between neuroimaging studies in humans and brain manipulations in animals to understand the causal relationship between changes in brain activity and therapeutic efficacy. We conclude with circuit-based perspectives on ketamine's action: first, related to ACC function in a central network mediating affective pain, and second, related to its role as the anterior node of the default mode network.

A Decision Architecture for Safety Computations

Accurately estimating safety is critical to pursuing nondefensive survival behaviors. However, little attention has been paid to how the human brain computes safety. We conceptualize a model that consists of two components: (i) threat-oriented evaluations that focus on threat value, imminence, and predictability; and (ii) self-oriented evaluations that focus on the agent's experience, strategies, and ability to control the situation. Our model points to the dynamic interaction between these two components as a mechanism of safety estimation. Based on a growing body of human literature, we hypothesize that distinct regions of the ventromedial prefrontal cortex (vmPFC) respond to threat and safety to facilitate survival decisions. We suggest safety is not an inverse of danger, but reflects independent computations that mediate defensive circuits and behaviors.

NMDAR Neurotransmission Needed for Persistent Neuronal Firing: Potential Roles in Mental Disorders

The dorsolateral prefrontal cortex (dlPFC) generates the mental representations that are the foundation of abstract thought, and provides top-down regulation of emotion through projections to the medial PFC and cingulate cortices. Physiological recordings from dlPFC Delay cells have shown that the generation of mental representations during working memory relies on NMDAR neurotransmission, with surprisingly little contribution from AMPAR. Systemic administration of low "antidepressant" doses of the NMDAR antagonist, ketamine, erodes these representations and reduces dlPFC Delay cell firing. In contrast to the dlPFC, V1 neuronal firing to visual stimuli depends on AMPAR, with much less contribution from NMDAR. Similarly, neurons in the dlPFC that respond to sensory events (cue cells, response feedback cells) rely on AMPAR, and systemic ketamine increases their firing. Insults to NMDAR transmission, and the impaired ability for dlPFC to generate mental representations, may contribute to cognitive deficits in schizophrenia, e.g., from genetic insults that weaken NMDAR transmission, or from blockade of NMDAR by kynurenic acid. Elevated levels of kynurenic acid in dlPFC may also contribute to cognitive deficits in other disorders with pronounced neuroinflammation (e.g., Alzheimer's disease), or peripheral infections where kynurenine can enter brain (e.g., delirium from sepsis, "brain fog" in COVID19). Much less is known about NMDAR actions in the primate cingulate cortices. However, NMDAR neurotransmission appears to process the affective and visceral responses to pain and other aversive experiences mediated by the cingulate cortices, which may contribute to sustained alterations in mood state. We hypothesize that the very rapid, antidepressant effects of intranasal ketamine may involve the disruption of NMDAR-generated aversive mood states by the anterior and subgenual cingulate cortices, providing a "foot in the door" to allow the subsequent return of top-down regulation by higher PFC areas. Thus, the detrimental vs. therapeutic effects of NMDAR blockade may be circuit dependent.

M1 receptors interacting with NMDAR enhance delay-related neuronal firing and improve working memory performance

The recurrent excitatory circuits in dlPFC underlying working memory are known to require activation of glutamatergic NMDA receptors (NMDAR). The neurons in these circuits also rely on acetylcholine to maintain persistent activity, with evidence for actions at both nicotinic α7 receptors and muscarinic M1 receptors (M1R). It is known that nicotinic α7 receptors interact with NMDAR in these circuits, but the interactions between M1R and NMDAR on dlPFC neuronal activity are unknown. Here, we investigated whether M1Rs contribute to the permissive effects of ACh in dlPFC circuitry underlying working memory via interactions with NMDA receptors. We tested interactions between M1Rs and NMDARs in vivo on single neuron activity in rhesus macaques performing a working memory task, as well as on working memory behavior in rodents following infusion of M1R and NMDAR compounds into mPFC. We report that M1R antagonists block the enhancing effects of NMDA application, consistent with M1R permissive actions. Conversely, M1R positive allosteric modulators prevented the detrimental effects of NMDAR blockade in single neurons in dlPFC and on working memory performance in rodents. These data support an interaction between M1R and NMDARs in working memory circuitry in both primates and rats, and suggest M1Rs contribute to the permissive actions of ACh in primate dlPFC. These results are consistent with recent data suggesting that M1R agonists may be helpful in the treatment of schizophrenia, a cognitive disorder associated with NMDAR dysfunction.

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Working memory-related persistent firing in primate prefrontal cortex relies on NMDAR.

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Unlike classic circuits, NMDAR transmission requires permissive acetylcholine actions.

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Muscarinic M1R blockade prevents the excitatory effects of NMDA on neuronal firing.

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M1R stimulation averts the harmful effects of NMDAR blockade on cell firing and memory.

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.

Anterior Cingulate Cortex and the Control of Dynamic Behavior in Primates

The brain mechanism for controlling continuous behavior in dynamic contexts must mediate action selection and learning across many timescales, responding differentially to the level of environmental uncertainty and volatility. In this review, we argue that a part of the frontal cortex known as the anterior cingulate cortex (ACC) is particularly well suited for this function. First, the ACC is interconnected with prefrontal, parietal, and subcortical regions involved in valuation and action selection. Second, the ACC integrates diverse, behaviorally relevant information across multiple timescales, producing output signals that temporally encapsulate decision and learning processes and encode high-dimensional information about the value and uncertainty of future outcomes and subsequent behaviors. Third, the ACC signals behaviorally relevant information flexibly, displaying the capacity to represent information about current and future states in a valence-, context-, task- and action-specific manner. Fourth, the ACC dynamically controls instrumental- and non-instrumental information seeking behaviors to resolve uncertainty about future outcomes. We review electrophysiological and circuit disruption studies in primates to develop this point, discuss its relationship to novel therapeutics for neuropsychiatric disorders in humans, and conclude by relating ongoing research in primates to studies of medial frontal cortical regions in rodents.

Guanfacine's mechanism of action in treating prefrontal cortical disorders: Successful translation across species

The selective norepinephrine (NE) α2A-adrenoceptor (α2A-AR) agonist, guanfacine (Intuniv™), is FDA-approved for treating Attention Deficit Hyperactivity Disorder (ADHD) based on research in animals, a translational success story. Guanfacine is also widely used off-label in additional mental disorders that involve impaired functioning of the prefrontal cortex (PFC), including stress-related disorders such as substance abuse, schizotypic cognitive deficits, and traumatic brain injury. The PFC subserves high order cognitive and executive functions including working memory, abstract reasoning, insight and judgment, and top-down control of attention, action and emotion. These abilities arise from PFC microcircuits with extensive recurrent excitation through NMDAR synapses. There is powerful modulation of these synapses, where cAMP-PKA opening of nearby potassium (K+) channels can rapidly and dynamically alter synaptic strength to coordinate arousal state with cognitive state, e.g. to take PFC "offline" during uncontrollable stress. A variety of evidence shows that guanfacine acts within the PFC via post-synaptic α2A-AR on dendritic spines to inhibit cAMP-PKA-K+ channel signaling, thus strengthening network connectivity, enhancing PFC neuronal firing, and improving PFC cognitive functions. Although guanfacine's beneficial effects are present in rodent, they are especially evident in primates, where the PFC greatly expands and differentiates. In addition to therapeutic actions in PFC, stress-related disorders may also benefit from additional α2-AR actions, such as weakening plasticity in the amygdala, reducing NE release, and anti-inflammatory actions by deactivating microglia. Altogether, these NE α2-AR actions optimize top-down control by PFC networks, which may explain guanfacine's benefits in a variety of mental disorders.

The connections of the insular VEN area in great apes: A histologically-guided ex vivo diffusion tractography study

We mapped the connections of the insular von Economo neuron (VEN) area in ex vivo brains of a bonobo, an orangutan and two gorillas with high angular resolution diffusion MRI imaging acquired in 36 h imaging sessions for each brain. The apes died of natural causes without neurological disorders. The localization of the insular VEN area was based on cresyl violet-stained histological sections from each brain that were coregistered with structural and diffusion images from the same individuals. Diffusion MRI tractography showed that the insular VEN area is connected with olfactory, gustatory, visual and other sensory systems, as well as systems for the mediation of appetite, reward, aversion and motivation. The insular VEN area in apes is most strongly connected with frontopolar cortex, which could support their capacity to choose voluntarily among alternative courses of action particularly in exploring for food resources. The frontopolar cortex may also support their capacity to take note of potential resources for harvesting in the future (prospective memory). All of these faculties may support insight and volitional choice when contemplating courses of action as opposed to rule-based decision-making.

Stress and Fear: Clinical Implications for Providers and Patients (in the Time of COVID-19 and Beyond)

In light of the coronavirus disease 2019 pandemic, we explore the role of stress, fear, and the impact of positive and negative emotions on health and disease. We then introduce strategies to help mitigate stress within the health care team, and provide a rationale for their efficacy. Additionally, we identify strategies to optimize patient care and explain their heightened importance in today's environment.

Over-activation of primate subgenual cingulate cortex enhances the cardiovascular, behavioral and neural responses to threat

Stress-related disorders such as depression and anxiety are characterized by enhanced negative emotion and physiological dysfunction. Whilst elevated activity within area 25 of the subgenual anterior cingulate cortex (sgACC/25) has been implicated in these illnesses, it is unknown whether this over-activity is causal. By combining targeted intracerebral microinfusions with cardiovascular and behavioral monitoring in marmosets, we show that over-activation of sgACC/25 reduces vagal tone and heart rate variability, alters cortisol dynamics during stress and heightens reactivity to proximal and distal threat. ¹⁸F-FDG PET imaging shows these changes are accompanied by altered activity within a network of brain regions including the amygdala, hypothalamus and dorsolateral prefrontal cortex. Ketamine, shown to have rapid antidepressant effects, fails to reverse elevated arousal to distal threat contrary to the beneficial effects we have previously demonstrated on over-activation induced reward blunting, illustrating the symptom-specificity of its actions.

Alexander et al. causally implicate over-activity in primate subgenual cingulate in affective and cardiovascular dysfunction relevant to anxiety and depression. Over-activation led to elevated activity in a stress-related network whilst decreasing activity in higher-order prefrontal cognitive regions.

Serial Prefrontal Pathways Are Positioned to Balance Cognition and Emotion in Primates

The delicate balance among primate prefrontal networks is necessary for homeostasis and behavioral flexibility. Dorsolateral prefrontal cortex (dlPFC) is associated with cognition, while the most ventromedial subgenual cingulate area 25 (A25) is associated with emotion and emotional expression. Yet A25 is weakly connected with dlPFC, and it is unknown how the two regions communicate. In rhesus monkeys of both sexes, we investigated how these functionally distinct areas may interact through pregenual anterior cingulate area 32 (A32), which is strongly connected with both. We found that dlPFC innervated the deep layers of A32, while A32 innervated all layers of A25, mostly targeting spines of excitatory neurons. Approximately 20% of A32 terminations formed synapses on inhibitory neurons in A25, notably the powerful parvalbumin inhibitory neurons in the deep layers, and the disinhibitory calretinin neurons in the superficial layers. By innervating distinct inhibitory microenvironments in laminar compartments, A32 is positioned to tune activity in columns of A25. The circuitry of the sequential pathway indicates that when dlPFC is engaged, A32 can dampen A25 output through the parvalbumin inhibitory microsystem in the deep layers of A25. A32 thus may flexibly recruit or reduce activity in A25 to maintain emotional equilibrium, a process that is disrupted in depression. Moreover, pyramidal neurons in A25 had a heightened density of NMDARs, which are the targets of novel rapid-acting antidepressants. Pharmacologic antagonism of NMDARs in patients with depression may reduce excitability in A25, mimicking the effects of the neurotypical serial pathway identified here.SIGNIFICANCE STATEMENT The anterior cingulate is a critical hub in prefrontal networks through connections with functionally distinct areas. Dorsolateral and polar prefrontal areas that are associated with complex cognition are connected with the anterior cingulate in a pattern that allows them to indirectly control downstream activity from the anterior cingulate to the subgenual cingulate, which is associated with heightened activity and negative affect in depression. This set of pathways provides a circuit mechanism for emotional regulation, with the anterior cingulate playing a balancing role for integration of cognitive and emotional processes. Disruption of these pathways may perturb network function and the ability to regulate cognitive and affective processes based on context.

Hippocampal Interaction With Area 25, but not Area 32, Regulates Marmoset Approach-Avoidance Behavior

Affective disorders are associated with increased sensitivity to negative feedback that influences approach-avoidance decision making. Although neuroimaging studies of these disorders reveal dysregulation in primate cingulate areas 25 and 32 and the anterior hippocampus (aHipp), the causal involvement of these structures and their interaction in the primate brain is unknown. We therefore investigated the effects of localized pharmacological manipulations of areas 25 and 32 and/or the aHipp of the marmoset monkey on performance of an anxiolytic-sensitive instrumental decision-making task in which an approach-avoidance conflict is created by pairing a response with reward and punishment. During control infusions animals avoided punishment, but this bias was reduced by increasing glutamate release within the aHipp or area 32, and inactivation or 5-HT1a antagonism within area 25. Conversely, increasing glutamate release in area 25 enhanced punishment avoidance but, in contrast to previous reports, area 32 and aHipp inactivations had no effect. Simultaneous inactivation or 5-HT1a antagonism within area 25, but not area 32, abolished the reduced punishment avoidance seen after increasing aHipp glutamate. Besides providing causal evidence that these primate areas differentially regulate negative feedback sensitivity, this study links the decision-making deficits in affective disorders to aberrant aHipp-area 25 circuit activity.

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.

Divergence of rodent and primate medial frontal cortex functional connectivity

With the medial frontal cortex (MFC) centrally implicated in several major neuropsychiatric disorders, it is critical to understand the extent to which MFC organization is comparable between humans and animals commonly used in preclinical research (namely rodents and nonhuman primates). Although the cytoarchitectonic structure of the rodent MFC has mostly been conserved in humans, it is a long-standing question whether the structural analogies translate to functional analogies. Here, we probed this question using ultra high field fMRI data to compare rat, marmoset, and human MFC functional connectivity. First, we applied hierarchical clustering to intrinsically define the functional boundaries of the MFC in all three species, independent of cytoarchitectonic definitions. Then, we mapped the functional connectivity "fingerprints" of these regions with a number of different brain areas. Because rats do not share cytoarchitectonically defined regions of the lateral frontal cortex (LFC) with primates, the fingerprinting method also afforded the unique ability to compare the rat MFC and marmoset LFC, which have often been suggested to be functional analogs. The results demonstrated remarkably similar intrinsic functional organization of the MFC across the species, but clear differences between rodent and primate MFC whole-brain connectivity. Rat MFC patterns of connectivity showed greatest similarity with premotor regions in the marmoset, rather than dorsolateral prefrontal regions, which are often suggested to be functionally comparable. These results corroborate the viability of the marmoset as a preclinical model of human MFC dysfunction, and suggest divergence of functional connectivity between rats and primates in both the MFC and LFC.

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.

Stability or Plasticity? - A Hierarchical Allostatic Regulation Model of Medial Prefrontal Cortex Function for Social Valuation

The medial prefrontal cortex (mPFC) has long been recognized as the key component of the neurocircuitry involved in various social as well as non-social behaviors, however, little is known regarding the organizing principle of distinctive subregions in the mPFC that integrates a wide range of mPFC functions. The present study proposes a hierarchical model of mPFC functionality, where three functionally dissociable subregions, namely, the ventromedial prefrontal cortex (vmPFC), rostromedial prefrontal cortex (rmPFC), and dorsomedial prefrontal cortex (dmPFC), are differentially involved in computing values of decision-making. According to this model, the mPFC subregions interact with each other in such a way that more dorsal regions utilize additional external sensory information from environment to predict and prevent conflicts occurring in more ventral regions tuned to internal bodily signals, thereby exerting the hierarchically organized allostatic regulatory control over homeostatic reflexes. This model also emphasizes the role of the thalamic reticular nucleus (TRN) in arbitrating the transitions between different thalamo-cortical loops, detecting conflicts between competing options for decision-making, and in shifting flexibly between decision modes. The hierarchical architecture of the mPFC working in conjunction with the TRN may play a key role in adjusting the internal (bodily) needs to suit the constraints of external (environmental) variables better, thus effectively addressing the stability-plasticity dilemma.

Heart Rate Variability, Cue-Evoked Ventromedial Prefrontal Cortical Response, and Problem Alcohol Use in Adult Drinkers

BACKGROUND

Many studies employed cue exposure paradigms to investigate the neural processes underlying cue-elicited alcohol craving. Cue exposure elicits robust autonomic reactivity. However, whether or how cue-elicited autonomic response relates to the severity of alcohol misuse and the neural bases underlying the potential relationship remain unclear.

METHODS

We examined cue-related brain activations in association with heart rate variability, as indexed by the root mean square of the successive differences (RMSSD), during alcohol versus neutral cue blocks in 50 adult alcohol drinkers (24 men). Imaging and heart rate variability data were collected and processed with published routines. Mediation analyses were conducted to examine the interrelationship between regional activities, cue-elicited changes in RMSSD, and the severity of problem alcohol use, as assessed with the Alcohol Use Disorders Identification Test (AUDIT).

RESULTS

The results showed higher RMSSD during alcohol than during neutral cue exposures, with alcohol (vs. neutral) cue-evoked RMSSD positively correlated with AUDIT score. Further, alcohol (vs. neutral) cue-elicited activity in the ventromedial prefrontal cortex was negatively correlated both with increases in RMSSD and with the AUDIT score. Mediation analyses suggested that the RMSSD mediated the relationship between ventromedial prefrontal cortex cue activity and the AUDIT score.

CONCLUSIONS

These findings substantiate the neural correlates of the presumably parasympathetic response during alcohol cue exposure and the interrelationship among ventromedial prefrontal cortex activity, autonomic response, and problem alcohol use.