Diverging neural pathways assemble a behavioural state from separable features in anxiety

Evidence for involvement of a limbic paraventricular hypothalamic inhibitory network in hypothalamic-pituitary-adrenal axis adaptations to repeated stress

Emotional stressors activate a stereotyped set of limbic forebrain cell groups implicated in constraining stress-induced hypothalamic-pituitary-adrenal (HPA) axis activation by inhibiting hypophysiotropic neurons in the paraventricular hypothalamic nucleus (PVH). We previously identified a circumscribed, anterior part of the bed nuclei of the stria terminalis (aBST) that houses stress-sensitive, PVH-projecting, γ-aminobutyric acid (GABA)-ergic neurons as representing a site of convergence of stress-inhibitory influences originating from medial prefrontal and hippocampal cortices. Here we investigate whether exaggerated HPA axis responses associated with chronic variable stress (CVS; daily exposure to different stressors at unpredictable times over 14 days, followed by restraint stress on day 15) and diminished HPA output seen following repeated (14 days) restraint-stress exposure are associated with differential engagement of the limbic modulatory network. Relative to acutely restrained rats, animals subjected to CVS showed the expected increase (sensitization) in HPA responses and diminished levels of activation (Fos) of GABAergic neurons and glutamic acid decarboxylase (GAD) mRNA expression in the aBST. By contrast, repeated restraint stress produced habituation in HPA responses, maintained levels of activation of GABAergic neurons, and increased GAD expression in the aBST. aBST-projecting neurons in limbic sites implicated in HPA axis inhibition tended to show diminished activational responses in both repeated-stress paradigms, with the exception of the paraventricular thalamic nucleus, in which responsiveness was maintained in repeatedly restrained animals. The results are consistent with the view that differential engagement of HPA inhibitory mechanisms in the aBST may contribute to alterations in HPA axis responses to emotional stress in sensitization and habituation paradigms.

Closed-loop and activity-guided optogenetic control

Advances in optical manipulation and observation of neural activity have set the stage for widespread implementation of closed-loop and activity-guided optical control of neural circuit dynamics. Closing the loop optogenetically (i.e., basing optogenetic stimulation on simultaneously observed dynamics in a principled way) is a powerful strategy for causal investigation of neural circuitry. In particular, observing and feeding back the effects of circuit interventions on physiologically relevant timescales is valuable for directly testing whether inferred models of dynamics, connectivity, and causation are accurate in vivo. Here we highlight technical and theoretical foundations as well as recent advances and opportunities in this area, and we review in detail the known caveats and limitations of optogenetic experimentation in the context of addressing these challenges with closed-loop optogenetic control in behaving animals.

Long-range neural synchrony in behavior

Long-range synchrony between distant brain regions accompanies multiple forms of behavior. This review compares and contrasts the methods by which long-range synchrony is evaluated in both humans and model animals. Three examples of behaviorally relevant long-range synchrony are discussed in detail: gamma-frequency synchrony during visual perception, hippocampal-prefrontal synchrony during working memory, and prefrontal-amygdala synchrony during anxiety. Implications for circuit mechanism, translation, and clinical relevance are discussed.

Extending the amygdala in theories of threat processing

The central extended amygdala is an evolutionarily conserved set of interconnected brain regions that play an important role in threat processing to promote survival. Two core components of the central extended amygdala, the central nucleus of the amygdala (Ce) and the lateral bed nucleus of the stria terminalis (BST) are highly similar regions that serve complimentary roles by integrating fear- and anxiety-relevant information. Survival depends on the ability of the central extended amygdala to rapidly integrate and respond to threats that vary in their immediacy, proximity, and characteristics. Future studies will benefit from understanding alterations in central extended amygdala function in relation to stress-related psychopathology.

The CRH1 antagonist GSK561679 increases human fear but not anxiety as assessed by startle

Fear to predictable threat and anxiety to unpredictable threat reflect distinct processes mediated by different brain structures, the central nucleus of the amygdala and the bed nucleus of the stria terminalis (BNST), respectively. This study tested the hypothesis that the corticotropin-releasing factor (CRF1) antagonist GSK561679 differentially reduces anxiety but increases fear in humans. A total of 31 healthy females received each of four treatments: placebo, 50 mg GSK561679 (low-GSK), 400 mg GSK561679 (high-GSK), and 1 mg alprazolam in a crossover design. Participants were exposed to three conditions during each of the four treatments. The three conditions included one in which predictable aversive shocks were signaled by a cue, a second during which shocks were administered unpredictably, and a third condition without shock. Fear and anxiety were assessed using the acoustic startle reflex. High-GSK had no effect on startle potentiation during unpredictable threat (anxiety) but increased startle potentiation during the predictable condition (fear). Low-GSK did not affect startle potentiation across conditions. Consistent with previous findings, alprazolam reduced startle potentiation during unpredictable threat but not during predictable threat. The increased fear by high-GSK replicates animal findings and suggests a lift of the inhibitory effect of the BNST on the amygdala by the CRF1 antagonist.

Intranasally applied neuropeptide S shifts a high-anxiety electrophysiological endophenotype in the ventral hippocampus towards a "normal"-anxiety one

The neurobiological basis of pathological anxiety and the improvement of its pharmacological treatment are a matter of intensive investigation. Here, using electrophysiological techniques in brain slices from animals of the high anxiety-related behavior (HAB) and normal anxiety-related behavior (NAB) mouse model, we show that basal neurotransmission at ventral hippocampal CA3-CA1 synapses is weaker in HAB compared to NAB mice. We further demonstrate that paired-pulse facilitation (PPF) and long-term potentiation (LTP) at these synapses are more pronounced in slices from HAB animals. Based on previous findings, we also examined whether intranasal delivery of neuropeptide S (NPS), which increasingly emerges as a potential novel treatment option for anxiety symptoms occurring in a variety of diseases like anxiety disorders, posttraumatic stress disorder, and major depression, impacts on the high-anxiety electrophysiological endophenotype in HAB mice. Strikingly, we detected enhanced basal neurotransmission and reduced PPF and LTP in slices from NPS-treated HAB animals. Collectively, our study uncovers a multifaceted high-anxiety neurophysiological endophenotype in the murine ventral hippocampus and provides the first evidence that an intranasally applied neuropeptide can shift such an endophenotype in an anxiety-regulating brain structure towards a “normal”-anxiety one.

Hydration and beyond: neuropeptides as mediators of hydromineral balance, anxiety and stress-responsiveness

Challenges to body fluid homeostasis can have a profound impact on hypothalamic regulation of stress responsiveness. Deficiencies in blood volume or sodium concentration leads to the generation of neural and humoral signals relayed through the hindbrain and circumventricular organs that apprise the paraventricular nucleus of the hypothalamus (PVH) of hydromineral imbalance. Collectively, these neural and humoral signals converge onto PVH neurons, including those that express corticotrophin-releasing factor (CRF), oxytocin (OT), and vasopressin, to influence their activity and initiate compensatory responses that alleviate hydromineral imbalance. Interestingly, following exposure to perceived threats to homeostasis, select limbic brain regions mediate behavioral and physiological responses to psychogenic stressors, in part, by influencing activation of the same PVH neurons that are known to maintain body fluid homeostasis. Here, we review past and present research examining interactions between hypothalamic circuits regulating body fluid homeostasis and those mediating behavioral and physiological responses to psychogenic stress.

Optical neural interfaces

Genetically encoded optical actuators and indicators have changed the landscape of neuroscience, enabling targetable control and readout of specific components of intact neural circuits in behaving animals. Here, we review the development of optical neural interfaces, focusing on hardware designed for optical control of neural activity, integrated optical control and electrical readout, and optical readout of population and single-cell neural activity in freely moving mammals.

The ecology of human fear: survival optimization and the nervous system

We propose a Survival Optimization System (SOS) to account for the strategies that humans and other animals use to defend against recurring and novel threats. The SOS attempts to merge ecological models that define a repertoire of contextually relevant threat induced survival behaviors with contemporary approaches to human affective science. We first propose that the goal of the nervous system is to reduce surprise and optimize actions by (i) predicting the sensory landscape by simulating possible encounters with threat and selecting the appropriate pre-encounter action and (ii) prevention strategies in which the organism manufactures safe environments. When a potential threat is encountered the (iii) threat orienting system is engaged to determine whether the organism ignores the stimulus or switches into a process of (iv) threat assessment, where the organism monitors the stimulus, weighs the threat value, predicts the actions of the threat, searches for safety, and guides behavioral actions crucial to directed escape. When under imminent attack, (v) defensive systems evoke fast reflexive indirect escape behaviors (i.e., fight or flight). This cascade of responses to threat of increasing magnitude are underwritten by an interconnected neural architecture that extends from cortical and hippocampal circuits, to attention, action and threat systems including the amygdala, striatum, and hard-wired defensive systems in the midbrain. The SOS also includes a modulatory feature consisting of cognitive appraisal systems that flexibly guide perception, risk and action. Moreover, personal and vicarious threat encounters fine-tune avoidance behaviors via model-based learning, with higher organisms bridging data to reduce face-to-face encounters with predators. Our model attempts to unify the divergent field of human affective science, proposing a highly integrated nervous system that has evolved to increase the organism's chances of survival.

Principles of designing interpretable optogenetic behavior experiments

Over the last decade, there has been much excitement about the use of optogenetic tools to test whether specific cells, regions, and projection pathways are necessary or sufficient for initiating, sustaining, or altering behavior. However, the use of such tools can result in side effects that can complicate experimental design or interpretation. The presence of optogenetic proteins in cells, the effects of heat and light, and the activity of specific ions conducted by optogenetic proteins can result in cellular side effects. At the network level, activation or silencing of defined neural populations can alter the physiology of local or distant circuits, sometimes in undesired ways. We discuss how, in order to design interpretable behavioral experiments using optogenetics, one can understand, and control for, these potential confounds.

Ventromedial hypothalamic neurons control a defensive emotion state

Defensive behaviors reflect underlying emotion states, such as fear. The hypothalamus plays a role in such behaviors, but prevailing textbook views depict it as an effector of upstream emotion centers, such as the amygdala, rather than as an emotion center itself. We used optogenetic manipulations to probe the function of a specific hypothalamic cell type that mediates innate defensive responses. These neurons are sufficient to drive multiple defensive actions, and required for defensive behaviors in diverse contexts. The behavioral consequences of activating these neurons, moreover, exhibit properties characteristic of emotion states in general, including scalability, (negative) valence, generalization and persistence. Importantly, these neurons can also condition learned defensive behavior, further refuting long-standing claims that the hypothalamus is unable to support emotional learning and therefore is not an emotion center. These data indicate that the hypothalamus plays an integral role to instantiate emotion states, and is not simply a passive effector of upstream emotion centers.

DOI:http://dx.doi.org/10.7554/eLife.06633.001

Animals have evolved a large number of ‘defensive behaviors’ to deal with the threat of predators. Examples include reptiles camouflaging themselves to avoid discovery, fish and birds swarming to confuse predators, insects releasing toxic chemicals, and humans readying themselves to fight or flee.

In mammals, defensive behaviors are thought to be mediated by a region of the brain called the amygdala. This structure, which is known as the brain's ‘emotion center’, receives and processes information from the senses about impending threats. It then sends instructions on how to deal with these threats to other regions of the brain including the hypothalamus, which pass them on to the brain regions that control the behavioral, endocrine and involuntary responses of the mammal.

For many years it has been thought that the role of the hypothalamus is to serve simply as a relay for emotion states encoded in the amygdala, rather than as an emotion center itself. However, Kunwar et al. have now challenged this assumption with the aid of a technique called optogenetics, in which light is used to activate specific populations of genetically labeled neurons. When light was used to directly activate neurons within the ventromedial hypothalamus in awake mice, the animals instantly froze and/or fled, just as they would when faced with a predator. Given that the optical stimulation had completely bypassed the amygdala, this suggested that the hypothalamus must be capable of generating this defensive response without any input from the amygdala.

The freezing and fleeing responses resembled the responses to a predator in a number of key ways. Mice chose to avoid areas of their cage in which they had received the stimulation, suggesting that—like a predator—these areas induced an unpleasant emotional state, perhaps akin to anxiety or fear. Freezing and fleeing persisted for several seconds after the stimulation had stopped, just as freezing and fleeing responses to predators do not immediately cease after the threat has gone. And finally, destroying the neurons targeted by the stimulation made mice less likely to avoid one of their main predators, the rat. It also made the animals less anxious.

Overall the results suggest that the hypothalamus may be more than simply a relay for the amygdala, and that ‘amygdala-centric’ views of emotion processing may need to be re-visited.

DOI:http://dx.doi.org/10.7554/eLife.06633.002

Collateral pathways from the ventromedial hypothalamus mediate defensive behaviors

The ventromedial hypothalamus (VMH) was thought to be essential for coping with threat, although its circuit mechanism remains unclear. To investigate this, we optogenetically activated steroidogenic factor 1 (SF1)-expressing neurons in the dorsomedial and central parts of the VMH (VMHdm/c), and observed a range of context-dependent somatomotor and autonomic responses resembling animals' natural defensive behaviors. By activating independent pathways emanating from the VMHdm/c, we demonstrated that VMHdm/c projection to the dorsolateral periaqueductal gray (dlPAG) induces inflexible immobility, while the VMHdm/c to anterior hypothalamic nucleus (AHN) pathway promotes avoidance. Consistent with the behavior changes induced by VMH to AHN pathway activation, direct activation of the AHN elicited avoidance and escape jumping, but not immobility. Retrograde tracing studies revealed that nearly 50% of PAG-projecting VMHdm/c neurons send collateral projection to the AHN and vice versa. Thus, VMHdm/c neurons employ a one-to-many wiring configuration to orchestrate multiple aspects of defensive behaviors.

Neuropeptide regulation of signaling and behavior in the BNST

Recent technical developments have transformed how neuroscientists can probe brain function. What was once thought to be difficult and perhaps impossible, stimulating a single set of long range inputs among many, is now relatively straight-forward using optogenetic approaches. This has provided an avalanche of data demonstrating causal roles for circuits in a variety of behaviors. However, despite the critical role that neuropeptide signaling plays in the regulation of behavior and physiology of the brain, there have been remarkably few studies demonstrating how peptide release is causally linked to behaviors. This is likely due to both the different time scale by which peptides act on and the modulatory nature of their actions. For example, while glutamate release can effectively transmit information between synapses in milliseconds, peptide release is potentially slower [See the excellent review by Van Den Pol on the time scales and mechanisms of release (van den Pol, 2012)] and it can only tune the existing signals via modulation. And while there have been some studies exploring mechanisms of release, it is still not as clearly known what is required for efficient peptide release. Furthermore, this analysis could be complicated by the fact that there are multiple peptides released, some of which may act in contrast. Despite these limitations, there are a number of groups making progress in this area. The goal of this review is to explore the role of peptide signaling in one specific structure, the bed nucleus of the stria terminalis, that has proven to be a fertile ground for peptide action.

From circuits to behaviour in the amygdala

The amygdala has long been associated with emotion and motivation, playing an essential part in processing both fearful and rewarding environmental stimuli. How can a single structure be crucial for such different functions? With recent technological advances that allow for causal investigations of specific neural circuit elements, we can now begin to map the complex anatomical connections of the amygdala onto behavioural function. Understanding how the amygdala contributes to a wide array of behaviours requires the study of distinct amygdala circuits.

GluN2B-containing NMDA receptors blockade rescues bidirectional synaptic plasticity in the bed nucleus of the stria terminalis of cocaine self-administering rats

Drugs of abuse have detrimental effects on homeostatic synaptic plasticity in the motivational brain network. Bidirectional plasticity at excitatory synapses helps keep neural circuits within a functional range to allow for behavioral flexibility. Therefore, impaired bidirectional plasticity of excitatory synapses may contribute to the behavioral hallmarks of addiction, yet this relationship remains unclear. Here we tracked excitatory synaptic strength in the oval bed nucleus of the stria terminalis (ovBNST) using whole-cell voltage-clamp recordings in brain slices from rats self-administering sucrose or cocaine. In the cocaine group, we measured both a persistent increase in AMPA to NMDA ratio (A:N) and slow decay time of NMDA currents throughout the self-administration period and after withdrawal from cocaine. In contrast, the sucrose group exhibited an early increase in A:N ratios (acquisition) that returned toward baseline values with continued self-administration (maintenance) and after withdrawal. The sucrose rats also displayed a decrease in NMDA current decay time with continued self-administration (maintenance), which normalized after withdrawal. Cocaine self-administering rats exhibited impairment in NMDA-dependent long-term depression (LTD) that could be rescued by GluN2B-containing NMDA receptor blockade. Sucrose self-administering rats demonstrated no impairment in NMDA-dependent LTD. During the maintenance period of self-administration, in vivo (daily intraperitoneally for 5 days) pharmacologic blockade of GluN2B-containing NMDA receptors did not reduce lever pressing for cocaine. However, in vivo GluN2B blockade did normalize A:N ratios in cocaine self-administrating rats, and dissociated the magnitude of ovBNST A:N ratios from drug-seeking behavior after protracted withdrawal. Altogether, our data demonstrate when and how bidirectional plasticity at ovBNST excitatory synapses becomes dysfunctional with cocaine self-administration and that NMDA-mediated potentiation of AMPA receptors in this region may be part of the neural circuits of drug relapse.

A whole-brain atlas of inputs to serotonergic neurons of the dorsal and median raphe nuclei

The serotonin system is proposed to regulate physiology and behavior and to underlie mood disorders; nevertheless, the circuitry controlling serotonergic neurons remains uncharacterized. We therefore generated a comprehensive whole-brain atlas defining the monosynaptic inputs onto forebrain-projecting serotonergic neurons of dorsal versus median raphe based on a genetically restricted transsynaptic retrograde tracing strategy. We identified discrete inputs onto serotonergic neurons from forebrain and brainstem neurons, with specific inputs from hypothalamus, cortex, basal ganglia, and midbrain, displaying a greater than anticipated complexity and diversity in cell-type-specific connectivity. We identified and functionally confirmed monosynaptic glutamatergic inputs from prefrontal cortex and lateral habenula onto serotonergic neurons as well as a direct GABAergic input from striatal projection neurons. In summary, our findings emphasize the role of hyperdirect inputs to serotonergic neurons. Cell-type-specific classification of connectivity patterns will allow for further functional analysis of the diverse but specific inputs that control serotonergic neurons during behavior.

Is empathy necessary to comprehend the emotional faces? The empathic effect on attentional mechanisms (eye movements), cortical correlates (N200 event-related potentials) and facial behaviour (electromyography) in face processing

The present research explored the effect of social empathy on processing emotional facial expressions. Previous evidence suggested a close relationship between emotional empathy and both the ability to detect facial emotions and the attentional mechanisms involved. A multi-measure approach was adopted: we investigated the association between trait empathy (Balanced Emotional Empathy Scale) and individuals' performance (response times; RTs), attentional mechanisms (eye movements; number and duration of fixations), correlates of cortical activation (event-related potential (ERP) N200 component), and facial responsiveness (facial zygomatic and corrugator activity). Trait empathy was found to affect face detection performance (reduced RTs), attentional processes (more scanning eye movements in specific areas of interest), ERP salience effect (increased N200 amplitude), and electromyographic activity (more facial responses). A second important result was the demonstration of strong, direct correlations among these measures. We suggest that empathy may function as a social facilitator of the processes underlying the detection of facial emotion, and a general "facial response effect" is proposed to explain these results. We assumed that empathy influences cognitive and the facial responsiveness, such that empathic individuals are more skilful in processing facial emotion.

Optogenetic brain interfaces

The brain is a large network of interconnected neurons where each cell functions as a nonlinear processing element. Unraveling the mysteries of information processing in the complex networks of the brain requires versatile neurostimulation and imaging techniques. Optogenetics is a new stimulation method which allows the activity of neurons to be modulated by light. For this purpose, the cell-types of interest are genetically targeted to produce light-sensitive proteins. Once these proteins are expressed, neural activity can be controlled by exposing the cells to light of appropriate wavelengths. Optogenetics provides a unique combination of features, including multimodal control over neural function and genetic targeting of specific cell-types. Together, these versatile features combine to a powerful experimental approach, suitable for the study of the circuitry of psychiatric and neurological disorders. The advent of optogenetics was followed by extensive research aimed to produce new lines of light-sensitive proteins and to develop new technologies: for example, to control the distribution of light inside the brain tissue or to combine optogenetics with other modalities including electrophysiology, electrocorticography, nonlinear microscopy, and functional magnetic resonance imaging. In this paper, the authors review some of the recent advances in the field of optogenetics and related technologies and provide their vision for the future of the field.

Ventromedial prefrontal cortex damage alters resting blood flow to the bed nucleus of stria terminalis

The ventromedial prefrontal cortex (vmPFC) plays a key role in modulating emotional responses, yet the precise neural mechanisms underlying this function remain unclear. vmPFC interacts with a number of subcortical structures involved in affective processing, including the amygdala, hypothalamus, periaqueductal gray, ventral striatum, and bed nucleus of stria terminalis (BNST). While a previous study of non-human primates shows that vmPFC lesions reduce BNST activity and anxious behavior, no such causal evidence exists in humans. In this study, we used a novel application of magnetic resonance imaging (MRI) in neurosurgical patients with focal, bilateral vmPFC damage to determine whether vmPFC is indeed critical for modulating BNST function in humans. Relative to neurologically healthy subjects, who exhibited robust rest-state functional connectivity between vmPFC and BNST, the vmPFC lesion patients had significantly lower resting-state perfusion of the right BNST. No such perfusion differences were observed for the amygdala, striatum, hypothalamus, or periaqueductal gray. This study thus provides unique data on the relationship between vmPFC and BNST, suggesting that vmPFC serves to promote BNST activity in humans. This finding is relevant for neural circuitry models of mood and anxiety disorders.

Pathological circuit function underlying addiction and anxiety disorders

Current models of addiction and anxiety stem from the idea that aberrant function and remodeling of neural circuits cause the pathological behaviors. According to this hypothesis, a disease-defining experience (for example, drug reward or stress) would trigger specific forms of synaptic plasticity, which in susceptible subjects would become persistent and lead to the disease. While the notion of synaptic diseases has received much attention, no candidate disorder has been sufficiently investigated to yield new, rational therapies that could be tested in the clinic. Here we review the arguments in favor of abnormal neuronal plasticity underlying addiction and anxiety disorders, with a focus on the functional diversity of neurons that make up the circuits involved. We argue that future research must strive to obtain a comprehensive description of the relevant functional anatomy. This will allow identification of molecular mechanisms that govern the induction and expression of disease-relevant plasticity in identified neurons. To establish causality, one will have to test whether normalization of function can reverse pathological behavior. With these elements in hand, it will be possible to propose blueprints for manipulations to be tested in translational studies. The challenge is daunting, but new techniques, above all optogenetics, may enable decisive advances.

A translational neuroscience approach to understanding the development of social anxiety disorder and its pathophysiology

This review brings together recent research from molecular, neural circuit, animal model, and human studies to help understand the neurodevelopmental mechanisms underlying social anxiety disorder. Social anxiety disorder is common and debilitating, and it often leads to further psychopathology. Numerous studies have demonstrated that extremely behaviorally inhibited and temperamentally anxious young children are at marked risk of developing social anxiety disorder. Recent work in human and nonhuman primates has identified a distributed brain network that underlies early-life anxiety including the central nucleus of the amygdala, the anterior hippocampus, and the orbitofrontal cortex. Studies in nonhuman primates have demonstrated that alterations in this circuit are trait-like in that they are stable over time and across contexts. Notably, the components of this circuit are differentially influenced by heritable and environmental factors, and specific lesion studies have demonstrated a causal role for multiple components of the circuit. Molecular studies in rodents and primates point to disrupted neurodevelopmental and neuroplastic processes within critical components of the early-life dispositional anxiety neural circuit. The possibility of identifying an early-life at-risk phenotype, along with an understanding of its neurobiology, provides an unusual opportunity to conceptualize novel preventive intervention strategies aimed at reducing the suffering of anxious children and preventing them from developing further psychopathology.

Early life stress and serotonin transporter gene variation interact to affect the transcription of the glucocorticoid and mineralocorticoid receptors, and the co-chaperone FKBP5, in the adult rat brain

The short allelic variant of the serotonin transporter (5-HTT) promoter-linked polymorphic region (5-HTTLPR) has been associated with the etiology of major depression by interaction with early life stress (ELS). A frequently observed endophenotype in depression is the abnormal regulation of levels of stress hormones such as glucocorticoids. It is hypothesized that altered central glucocorticoid influence on stress-related behavior and memory processes could underlie the depressogenic interaction of 5-HTTLPR and ELS. One possible mechanism could be the altered expression of the genes encoding the glucocorticoid and mineralocorticoid receptors (GR, MR) and their inhibitory regulator FK506-binding protein 51 (FKBP5) in stress-related forebrain areas. To test this notion, we exposed heterozygous (5-HTT^+/−) and homozygous (5-HTT^−/−) serotonin transporter knockout rats and their wildtype littermates (5-HTT^+/+) to daily 3 h maternal separations from postnatal day 2 to 14. In the medial prefrontal cortex (mPFC) and hippocampus of the adult male offspring, we found that GR, MR, and FKBP5 mRNA levels were affected by ELS × 5-HTT genotype interaction. Specifically, 5-HTT^+/+ rats exposed to ELS showed decreased GR and FKBP5 mRNA in the dorsal and ventral mPFC, respectively. In contrast, 5-HTT^+/− rats showed increased MR mRNA levels in the hippocampus and 5-HTT^−/− rats showed increased FKBP5 mRNA in the ventral mPFC after ELS exposure. These findings indicate that 5-HTT genotype determines the specific adaptation of GR, MR, and FKBP5 expression in response to early life adversity. Therefore, altered extra-hypothalamic glucocorticoid signaling should be considered to play a role in the depressogenic interaction of ELS and 5-HTTLPR.

The bed nucleus of the stria terminalis is critical for anxiety-related behavior evoked by CO2 and acidosis

Carbon dioxide (CO2) inhalation lowers brain pH and induces anxiety, fear, and panic responses in humans. In mice, CO2 produces freezing and avoidance behavior that has been suggested to depend on the amygdala. However, a recent study in humans with bilateral amygdala lesions revealed that CO2 can trigger fear and panic even in the absence of amygdalae, suggesting the importance of extra-amygdalar brain structures. Because the bed nucleus of the stria terminalis (BNST) contributes to fear- and anxiety-related behaviors and expresses acid-sensing ion channel-1A (ASIC1A), we hypothesized that the BNST plays an important role in CO2-evoked fear-related behaviors in mice. We found that BNST lesions decreased both CO2-evoked freezing and CO2-conditioned place avoidance. In addition, we found that CO2 inhalation caused BNST acidosis and that acidosis was sufficient to depolarize BNST neurons and induce freezing behavior; both responses depended on ASIC1A. Finally, disrupting Asic1a specifically in the BNST reduced CO2-evoked freezing, whereas virus-vector-mediated expression of ASIC1A in the BNST of Asic1a(-/-) and Asic1a(+/+) mice increased CO2-evoked freezing. Together, these findings identify the BNST as an extra-amygdalar fear circuit structure important in CO2-evoked fear-related behavior.

Illuminating circuitry relevant to psychiatric disorders with optogenetics

The brain's remarkable capacity to generate cognition and behavior is mediated by an extraordinarily complex set of neural interactions that remain largely mysterious. This complexity poses a significant challenge in developing therapeutic interventions to ameliorate psychiatric disease. Accordingly, few new classes of drugs have been made available for patients with mental illness since the 1950s. Optogenetics offers the ability to selectively manipulate individual neural circuit elements that underlie disease-relevant behaviors and is currently accelerating the pace of preclinical research into neurobiological mechanisms of disease. In this review, we highlight recent findings from studies that employ optogenetic approaches to gain insight into normal and aberrant brain function relevant to mental illness. Emerging data from these efforts offers an exquisitely detailed picture of disease-relevant neural circuits in action, and hints at the potential of optogenetics to open up entirely new avenues in the treatment of psychiatric disorders.

Light and chemical control of neuronal circuits: possible applications in neurotherapy

Millions of people worldwide suffer from diseases that result from a failure of central pathways to regulate behavioral and physiological processes. Advances in genetics and pharmacology have already allowed us to appreciate that rather than this dysregulation being systemic throughout the brain, it is usually rooted in specific subsets of dysfunctional cells within discrete neurological circuits. This article discusses the advent of opto- and chemogenetic tools and how they are providing the means to dissect these circuits with a degree of temporal and spatial sensitivity not previously possible. We also highlight the potential applications for treating disease and the key developments likely to have the greatest impact over the next 5 years.

Optogenetic insights on the relationship between anxiety-related behaviors and social deficits

Many psychiatric illnesses are characterized by deficits in the social domain. For example, there is a high rate of co-morbidity between autism spectrum disorders and anxiety disorders. However, the common neural circuit mechanisms by which social deficits and other psychiatric disease states, such as anxiety, are co-expressed remains unclear. Here, we review optogenetic investigations of neural circuits in animal models of anxiety-related behaviors and social behaviors and discuss the important role of the amygdala in mediating aspects of these behaviors. In particular, we focus on recent evidence that projections from the basolateral amygdala (BLA) to the ventral hippocampus (vHPC) modulate anxiety-related behaviors and also alter social interaction. Understanding how this circuit influences both social behavior and anxiety may provide a mechanistic explanation for the pathogenesis of social anxiety disorder, as well as the prevalence of patients co-diagnosed with autism spectrum disorders and anxiety disorders. Furthermore, elucidating how circuits that modulate social behavior also mediate other complex emotional states will lead to a better understanding of the underlying mechanisms by which social deficits are expressed in psychiatric disease.

Circuit-wide structural and functional measures predict ventromedial prefrontal cortex fear generalization: implications for generalized anxiety disorder

The ventromedial prefrontal cortex (vmPFC) plays a critical role in a number of evaluative processes, including risk assessment. Impaired discrimination between threat and safety is considered a hallmark of clinical anxiety. Here, we investigated the circuit-wide structural and functional mechanisms underlying vmPFC threat-safety assessment in humans. We tested patients with generalized anxiety disorder (GAD; n = 32, female) and healthy controls (n = 25, age-matched female) on a task that assessed the generalization of conditioned threat during fMRI scanning. The task consisted of seven rectangles of graded widths presented on a screen; only the midsize one was paired with mild electric shock [conditioned stimulus (CS)], while the others, safety cues, systematically varied in width by ±20, 40, and 60% [generalization stimuli (GS)] compared with the CS. We derived an index reflecting vmPFC functioning from the BOLD reactivity on a continuum of threat (CS) to safety (GS least similar to CS); patients with GAD showed less discrimination between threat and safety cues, compared with healthy controls (Greenberg et al., 2013b). Using structural, functional (i.e., resting-state), and diffusion MRI, we measured vmPFC thickness, vmPFC functional connectivity, and vmPFC structural connectivity within the corticolimbic systems. The results demonstrate that all three factors predict individual variability of vmPFC threat assessment in an independent fashion. Moreover, these neural features are also linked to GAD, most likely via an vmPFC fear generalization. Our results strongly suggest that vmPFC threat processing is closely associated with broader corticolimbic circuit anomalies, which may synergistically contribute to clinical anxiety.

Local and regional heterogeneity underlying hippocampal modulation of cognition and mood

While the hippocampus has been classically studied for its role in learning and memory, there is significant support for a role of the HPC in regulating emotional behavior. Emerging research suggests these functions may be segregated along the dorsoventral axis of the HPC. In addition to this regional heterogeneity, within the HPC, the dentate gyrus is one of two areas in the adult brain where stem cells continuously give rise to new neurons. This process can influence and be modulated by the emotional state of the animal, suggesting that adult neurogenesis within the DG may contribute to psychiatric disorders and cognitive abilities. Yet, the exact mechanism by which these newborn neurons influence behavior remains unknown. Here, we will examine the contribution of hippocampal neurogenesis to the output of the HPC, and suggest that the role of neurogenesis may vary along the DV axis. Next, we will review literature indicating that anatomical connectivity varies along the DV axis of the HPC, and that this underlies the functional segregation along this axis. This analysis will allow us to synthesize novel hypotheses for the differential contribution of the HPC to cognition and mood.

Glutamatergic mechanisms associated with stress-induced amygdala excitability and anxiety-related behavior

The neural factors underlying individual differences in susceptibility to chronic stress remain poorly understood. Preclinical studies demonstrate that mouse strains vary greatly in anxiety-related responses to chronic stress in a manner paralleled by differential stress-induced changes in glutamatergic signaling in the basolateral amygdala (BLA). Previous work has also shown that alterations in the amygdala gene expression of the GluN1 NMDA and the GluK1 kainate receptors are associated with stress-induced alterations in anxiety-like behavior in the C57BL/6J mouse strain. Using in vivo behavioral pharmacological and ex vivo physiological approaches, the aim of the current study was to further elucidate changes in glutamate neurotransmission in the BLA caused by stress and to test the functional roles of GluN1 and GluK1 in mediating stress-related changes in behavior. Results showed that stress-induced alterations in anxiety-like behavior (light/dark exploration test) were absent following bilateral infusion of the GluK1 agonist ATPA into the BLA. Intra-BLA infusion of the competitive NMDA antagonist AP5 produced a generalized behavioral disinhibition/locomotor hyperactivity, irrespective of stress. Slice electrophysiological recordings showed that ATPA augmented BLA GABAergic neurotransmission and that stress increased the amplitude of network-dependent spontaneous excitatory postsynaptic currents and amplitude of GABAergic miniature inhibitory postsynaptic currents in BLA. These findings could indicate stress-induced BLA glutamatergic neuronal network hyperexcitability and a compensatory increase in GABAergic neurotransmission, suggesting that GluK1 agonism augmented GABAergic inhibition to prevent behavioral sequelae of stress. Current data could have implications for developing novel therapeutic approaches, including GluK1 agonists, for stress-related anxiety disorders.

BNST neurocircuitry in humans

Anxiety and addiction disorders are two of the most common mental disorders in the United States, and are typically chronic, disabling, and comorbid. Emerging evidence suggests the bed nucleus of the stria terminalis (BNST) mediates both anxiety and addiction through connections with other brain regions, including the amygdala and nucleus accumbens. Although BNST structural connections have been identified in rodents and a limited number of structural connections have been verified in non-human primates, BNST connections have yet to be described in humans. Neuroimaging is a powerful tool for identifying structural and functional circuits in vivo. In this study, we examined BNST structural and functional connectivity in a large sample of humans. The BNST showed structural and functional connections with multiple subcortical regions, including limbic, thalamic, and basal ganglia structures, confirming structural findings in rodents. We describe two novel connections in the human brain that have not been previously reported in rodents or non-human primates, including a structural connection with the temporal pole, and a functional connection with the paracingulate gyrus. The findings of this study provide a map of the BNST's structural and functional connectivity across the brain in healthy humans. In large part, the BNST neurocircuitry in humans is similar to the findings from rodents and non-human primates; however, several connections are unique to humans. Future explorations of BNST neurocircuitry in anxiety and addiction disorders have the potential to reveal novel mechanisms underlying these disabling psychiatric illnesses.

An update on the connections of the ventral mesencephalic dopaminergic complex

This review covers the intrinsic organization and afferent and efferent connections of the midbrain dopaminergic complex, comprising the substantia nigra, ventral tegmental area and retrorubral field, which house, respectively, the A9, A10 and A8 groups of nigrostriatal, mesolimbic and mesocortical dopaminergic neurons. In addition, A10dc (dorsal, caudal) and A10rv (rostroventral) extensions into, respectively, the ventrolateral periaqueductal gray and supramammillary nucleus are discussed. Associated intrinsic and extrinsic connections of the midbrain dopaminergic complex that utilize gamma-aminobutyric acid (GABA), glutamate and neuropeptides and various co-expressed combinations of these compounds are considered in conjunction with the dopamine-containing systems. A framework is provided for understanding the organization of massive afferent systems descending and ascending to the midbrain dopaminergic complex from the telencephalon and brainstem, respectively. Within the context of this framework, the basal ganglia direct and indirect output pathways are treated in some detail. Findings from rodent brain are briefly compared with those from primates, including humans. Recent literature is emphasized, including traditional experimental neuroanatomical and modern gene transfer and optogenetic studies. An attempt was made to provide sufficient background and cite a representative sampling of earlier primary papers and reviews so that people new to the field may find this to be a relatively comprehensive treatment of the subject.

Distributed circuits underlying anxiety

Anxiety is of paramount importance for animals, as it allows assessment of the environment while minimizing exposure to potential threats. Furthermore, anxiety disorders are highly prevalent. Consequently, the neural circuitry underlying anxiety has been a topic of great interest. In this mini review, we will discuss current views on anxiety circuits. We will focus on rodent anxiety paradigms, but we will also consider results from human neuroimaging and clinical studies. We briefly review studies demonstrating the central role that the amygdala and the bed nucleus of the stria terminals (BNST) play in modulating anxiety and present evidence showing how the bed nucleus uses different output pathways to influence specific features of anxiolysis. Lastly, we propose that several brain regions, such as the medial prefrontal cortex (mPFC) and the ventral hippocampus (vHPC), act in a coordinated fashion with the amygdala and BNST, forming a distributed network of interconnected structures that control anxiety both in rodents and humans.

Stress switches cannabinoid type-1 (CB1) receptor-dependent plasticity from LTD to LTP in the bed nucleus of the stria terminalis

The bed nucleus of the stria terminalis (BNST) exerts a coordinated modulation of the psychoneuroendocrine responses to stress. However, how acute stress impacts on BNST in vivo plasticity is a crucial question that still remains unanswered. Here, neurons from the anterior portion of the BNST (aBNST) were recorded in vivo during and after stimulation of their medial prefrontal cortical (mPFC) afferents. In C57BL/6N mice, a 1 h restraint stress induced a switch from long-term depression (LTD) to long-term potentiation (LTP) in the aBNST after a 10 Hz mPFC stimulation. This switch was independent from glucocorticoid receptor stimulation. Because the endocannabinoid system regulates aBNST activity, we next examined the role of cannabinoid type-1 receptors (CB1-Rs) in these changes. Mutant mice lacking CB1-Rs (CB1(-/-) mice) displayed a marked deficit in the ability to develop plasticity under control and stress conditions, compared with their wild-type littermates (CB1(+/+) mice). This difference was not accounted for by genetic differences in stress sensitivity, as revealed by Fos immunohistochemistry analyses. Local blockade of CB1-Rs in the aBNST and the use of mutant mice bearing a selective deletion of CB1-Rs in cortical glutamatergic neurons indicated that stress-elicited LTP involved CB1-Rs located on aBNST excitatory terminals. These results show that acute stress reverts LTD into LTP in the aBNST and that the endocannabinoid system plays a key role therein.

Psychiatric disturbances regulate the innate immune system in CSF of conscious mice

Environment may affect brain activity through cerebrospinal fluid (CSF) only if there are regulatory molecules or cascades in CSF that are sensitive to external stimuli. This study was designed to identify regulatory activity present in CSF, better elucidating environmental regulation of brain function. By using cannulation-based sequential CSF sampling coupled with mass spectrometry-based identification and quantification of proteins, we show that the naive mouse CSF harbors, among 22 other pathways, the innate immune system as a main pathway, which was downregulated and upregulated, respectively, by acute stressor (AS) and acute cocaine (AC) administrations. Among novel processes and molecular functions, AS also regulated schizophrenia-associated proteins. Furthermore, AC upregulated exosome-related proteins with a false discovery rate of 1.0 × 10(-)(16). These results suggest that psychiatric disturbances regulate the neuroimmune system and brain disorder-related proteins, presenting a sensitive approach to investigating extracellular mechanisms in conscious and various mouse models of psychiatric disorders.

Optogenetic and chemogenetic insights into the food addiction hypothesis

Obesity is clinically diagnosed by a simple formula based on the weight and height of a person (body mass index), but is associated with a host of other behavioral symptoms that are likely neurological in origin. In recent years, many scientists have asked whether similar behavioral and cognitive changes occur in drug addiction and obesity, lending many to discuss the potential for “food addiction”. Advances in understanding the circuitry underlying both feeding behaviors and drug addiction may allow us to consider this question from the viewpoint of neural circuits, to complement behavioral perspectives. Here, we review advances in understanding of these circuits and use them to consider whether drawing comparisons to drug addiction is helpful for understanding certain forms of obesity.

How the amygdala affects emotional memory by altering brain network properties

The amygdala has long been known to play a key role in supporting memory for emotionally arousing experiences. For example, classical fear conditioning depends on neural plasticity within this anterior medial temporal lobe region. Beneficial effects of emotional arousal on memory, however, are not restricted to simple associative learning. Our recollection of emotional experiences often includes rich representations of, e.g., spatiotemporal context, visceral states, and stimulus-response associations. Critically, such memory features are known to bear heavily on regions elsewhere in the brain. These observations led to the modulation account of amygdala function, which postulates that amygdala activation enhances memory consolidation by facilitating neural plasticity and information storage processes in its target regions. Rodent work in past decades has identified the most important brain regions and neurochemical processes involved in these modulatory actions, and neuropsychological and neuroimaging work in humans has produced a large body of convergent data. Importantly, recent methodological developments make it increasingly realistic to monitor neural interactions underlying such modulatory effects as they unfold. For instance, functional connectivity network modeling in humans has demonstrated how information exchanges between the amygdala and specific target regions occur within the context of large-scale neural network interactions. Furthermore, electrophysiological and optogenetic techniques in rodents are beginning to make it possible to quantify and even manipulate such interactions with millisecond precision. In this paper we will discuss that these developments will likely lead to an updated view of the amygdala as a critical nexus within large-scale networks supporting different aspects of memory processing for emotionally arousing experiences.

CGRP inhibits neurons of the bed nucleus of the stria terminalis: implications for the regulation of fear and anxiety

The bed nucleus of the stria terminalis (BNST) is thought to generate anxiety-like states via its projections to autonomic and neuroendocrine regulatory structures of the brain. However, because most BNST cells are GABAergic, they are expected to inhibit target neurons. In contrast with this, infusion of calcitonin gene-related peptide (CGRP) into BNST was reported to potentiate anxiety while activating BNST targets. The present study aimed to shed light on this paradox. The CGRP innervation of BNST originates in the pontine parabrachial nucleus and targets its anterolateral sector (BNST-AL). Thus, we investigated the effects of CGRP on BNST-AL neurons using patch recordings in vitro in male rats. CGRP did not alter the passive properties of BNST-AL cells but increased the amplitude of IPSPs evoked by stimulation of the stria terminalis (ST). However, IPSP paired-pulse ratios were unchanged by CGRP, and there was no correlation between IPSP potentiation and variance, suggesting that CGRP acts postsynaptically. Consistent with this, CGRP hyperpolarized the GABA-A reversal of BNST-AL cells. These results indicate that CGRP increases ST-evoked GABA-A IPSPs and hyperpolarizes their reversal potential through a postsynaptic change in Cl(-) homeostasis. Overall, our findings suggest that CGRP potentiates anxiety-like behaviors and increases neural activity in BNST targets, by inhibiting BNST-AL cells, supporting the conclusion that BNST-AL exerts anxiolytic effects.

Circuit dynamics of adaptive and maladaptive behaviour

The recent development of technologies for investigating specific components of intact biological systems has allowed elucidation of the neural circuitry underlying adaptive and maladaptive behaviours. Investigators are now able to observe and control, with high spatio-temporal resolution, structurally defined intact pathways along which electrical activity flows during and after the performance of complex behaviours. These investigations have revealed that control of projection-specific dynamics is well suited to modulating behavioural patterns that are relevant to a broad range of psychiatric diseases. Structural dynamics principles have emerged to provide diverse, unexpected and causal insights into the operation of intact and diseased nervous systems, linking form and function in the brain.

Control of stress-induced persistent anxiety by an extra-amygdala septohypothalamic circuit

The extended amygdala has dominated research on the neural circuitry of fear and anxiety, but the septohippocampal axis also plays an important role. The lateral septum (LS) is thought to suppress fear and anxiety through its outputs to the hypothalamus. However, this structure has not yet been dissected using modern tools. The type 2 CRF receptor (Crfr2) marks a subset of LS neurons whose functional connectivity we have investigated using optogenetics. Crfr2(+) cells include GABAergic projection neurons that connect with the anterior hypothalamus. Surprisingly, we find that these LS outputs enhance stress-induced behavioral measures of anxiety. Furthermore, transient activation of Crfr2(+) neurons promotes, while inhibition suppresses, persistent anxious behaviors. LS Crfr2(+) outputs also positively regulate circulating corticosteroid levels. These data identify a subset of LS projection neurons that promote, rather than suppress, stress-induced behavioral and endocrinological dimensions of persistent anxiety states and provide a cellular point of entry to LS circuitry.

Optogenetic control of cardiomyocytes via viral delivery

Optogenetics is an emerging technology for the manipulation and control of excitable tissues, such as the brain and heart. As this technique requires the genetic modification of cells in order to inscribe light sensitivity, for cardiac applications, here we describe the process through which neonatal rat ventricular myocytes are virally infected in vitro with channelrhodopsin-2 (ChR2). We also describe in detail the procedure for quantitatively determining the optimal viral dosage, including instructions for patterning gene expression in multicellular cardiomyocyte preparations (cardiac syncytia) to simulate potential in vivo transgene distributions. Finally, we address optical actuation of ChR2-transduced cells and means to measure their functional response to light.

Optogenetics in preclinical neuroscience and psychiatry research: recent insights and potential applications

There have been significant advances in the treatment of psychiatric disease in the last half century, but it is still unclear which neural circuits are ultimately responsible for specific disease states. Fortunately, technical limitations that have constrained this research have recently been mitigated by advances in research tools that facilitate circuit-based analyses. The most prominent of these tools is optogenetics, which refers to the use of genetically encoded, light-sensitive proteins that can be used to manipulate discrete neural circuits with temporal precision. Optogenetics has recently been used to examine the neural underpinnings of both psychiatric disease and symptom relief, and this research has rapidly identified novel therapeutic targets for what could be a new generation of rational drug development. As these and related methodologies for controlling neurons ultimately make their way into the clinic, circuit-based strategies for alleviating psychiatric symptoms could become a remarkably refined approach to disease treatment.

A blueprint for research on Shankopathies: a view from research on autism spectrum disorder

Autism spectrum disorders (ASD) are associated with mutations in a host of genes including a number that function in synaptic transmission. Phelan McDermid syndrome involves mutations in SHANK3 which encodes a protein that forms a scaffold for glutamate receptors at the synapse. SHANK3 is one of the genes that underpins the synaptic hypothesis for ASD. We discuss this hypothesis with a view to the broader context of ASD and with special emphasis on highly penetrant genetic disorders including Shankopathies. We propose a blueprint for near and longer-term goals for fundamental and translational research on Shankopathies.

Insertion of long interspersed element-1 in the Mitf gene is associated with altered neurobehavior of the black-eyed white Mitf(mi-bw) mouse

Microphthalmia-associated transcription factor (Mitf) is required for the differentiation of melanoblasts of the neural crest origin. The mouse homozygous for the black-eyed white (Mitf(mi-bw) ) allele is characterized by white-coat color and deafness with black eye, due to the loss of melanoblasts during embryonic development. The Mitf(mi-bw) allele carries an insertion of long interspersed element-1 (L1) in intron 3 of the Mitf gene, which may cause the deficiency of melanocyte-specific Mitf-M. Here, we show that the L1 insertion results in the generation of alternatively spliced Mitf-M mRNA species, such as Mitf-M mRNA lacking exon 3, exon 4 or both exons 3 and 4, each of which encodes Mitf-M protein with an internal deletion. Transient expression assays showed the loss of or reduction in function of each aberrant Mitf-M protein and the dominant negative effect of Mitf-M lacking exon 4 that encodes an activation domain. Thus, the L1 insertion may decrease the expression level of functional Mitf-M. Importantly, Mitf-M mRNA is expressed in the wild-type mouse brain, with the highest expression level in the hypothalamus. Likewise, aberrant Mitf-M mRNAs are expressed in the bw mouse brain. The bw mice show the altered neurobehavior under a stressful environment, suggesting the role of Mitf-M in sensory perception.

Fabrication and application of flexible, multimodal light-emitting devices for wireless optogenetics

The rise of optogenetics provides unique opportunities to advance materials and biomedical engineering, as well as fundamental understanding in neuroscience. This protocol describes the fabrication of optoelectronic devices for studying intact neural systems. Unlike optogenetic approaches that rely on rigid fiber optics tethered to external light sources, these novel devices carry wirelessly powered microscale, inorganic light-emitting diodes (μ-ILEDs) and multimodal sensors inside the brain. We describe the technical procedures for construction of these devices, their corresponding radiofrequency power scavengers and their implementation in vivo for experimental application. In total, the timeline of the procedure, including device fabrication, implantation and preparation to begin in vivo experimentation, can be completed in ~3-8 weeks. Implementation of these devices allows for chronic (tested for up to 6 months) wireless optogenetic manipulation of neural circuitry in animals navigating complex natural or home-cage environments, interacting socially, and experiencing other freely moving behaviors.