NMDA receptors in the CeA and BNST differentially regulate fear conditioning to predictable and unpredictable threats

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

Temporal dynamics play a central role in models of emotion: "fear" is widely conceptualized as a phasic response to certain-and-imminent danger, whereas "anxiety" is a sustained response to uncertain-or-distal harm. Yet the underlying human neurobiology remains contentious. Leveraging an ethnoracially diverse sample, translationally relevant paradigm, and theory-driven modeling approach, we demonstrate that certain and uncertain threat recruit a shared threat-anticipation circuit. This circuit exhibits persistently elevated activation when anticipating uncertain threat encounters and a transient burst of activation in the moments before certain encounters. For many scientists and clinicians, feelings are the defining feature of human fear and anxiety. Here we used an independently validated brain signature to covertly decode the momentary dynamics of anticipatory distress for the first time. Results mirrored the dynamics of neural activation. These observations provide fresh insights into the neurobiology of threat-elicited emotions and set the stage for more ambitious clinical and mechanistic research.

Adolescent social anxiety is associated with diminished discrimination of anticipated threat and safety in the bed nucleus of the stria terminalis

Social anxiety-which typically emerges in adolescence-lies on a continuum and, when extreme, can be devastating. Socially anxious individuals are prone to heightened fear, anxiety, and the avoidance of contexts associated with potential social scrutiny. Yet most neuroimaging research has focused on acute social threat. Much less attention has been devoted to understanding the neural systems recruited during the uncertain anticipation of potential encounters with social threat. Here we used a novel fMRI paradigm to probe the neural circuitry engaged during the anticipation and acute presentation of threatening faces and voices in a racially diverse sample of 66 adolescents selectively recruited to encompass a range of social anxiety and enriched for clinically significant levels of distress and impairment. Results demonstrated that adolescents with more severe social anxiety symptoms experience heightened distress when anticipating encounters with social threat, and reduced discrimination of uncertain social threat and safety in the bed nucleus of the stria terminalis (BST), a key division of the central extended amygdala (EAc). Although the EAc-including the BST and central nucleus of the amygdala-was robustly engaged by the acute presentation of threatening faces and voices, the degree of EAc engagement was unrelated to the severity of social anxiety. Together, these observations provide a neurobiologically grounded framework for conceptualizing adolescent social anxiety and set the stage for the kinds of prospective-longitudinal and mechanistic research that will be necessary to determine causation and, ultimately, to develop improved interventions for this often-debilitating illness.

Rapamycin attenuates reconsolidation of a backwards-conditioned aversive stimuli in female mice

RATIONALE

The mammalian target of rapamycin (mTOR) kinase is known to mediate consolidation and reconsolidation of aversive memories. Most studies in this area use a forward conditioning paradigm in which the conditioned stimulus (CS) precedes the unconditioned stimulus (US). Little is known, however, about the neurobiological underpinnings of backwards (BW) conditioning paradigms, particularly in female mice. In BW conditioning, the CS does not become directly associated with the US; it instead evokes conditioned fear by reactivating a memory of the conditioning context and indirectly retrieving a memory of the aversive US.

OBJECTIVES

We sought to examine BW conditioned fear memory processes in female mice. First, we examined whether freezing to a BW CS is mediated by fear to the conditioning context. Second, we tested whether blocking consolidation of a BW CS attenuated memory of the CS and conditioning context. Finally, we tested whether blocking reconsolidation of a BW CS attenuated memory of the conditioning context.

RESULTS

We show that conditioned freezing to a BW CS is mediated by fear to the conditioning context. Furthermore, rapamycin-an mTOR inhibitor, when given immediately following BW conditioning, impairs consolidation of both cued and contextual fear memory. Similarly, rapamycin given following retrieval of a BW CS blocks context recall. Rapamycin is acting on reconsolidation as CS retrieval is necessary to see the effects of rapamycin on context memory recall.

CONCLUSIONS

Our study provides novel evidence that indirect retrieval cues are sensitive to rapamycin in female mice. The capacity to indirectly reactivate memories and render them susceptible to disruption is critical in the translation of reconsolidation-based approaches to the clinic.

Conditioned inhibition of fear and reward in male and female rats

Stimuli in our environment are not always associated with an outcome. Some of these stimuli, depending on how they are presented, may gain inhibitory value or simply be ignored. If experienced in the presence of other cues predictive of appetitive or aversive outcomes, they typically gain inhibitory value and become predictive cues indicating the absence of appetitive or aversive outcomes. In this case, these cues are referred to as conditioned inhibitors. Here, male and female Long Evans rats underwent cue discrimination training where a reward cue was paired with sucrose, a fear cue with footshock, and an inhibitor cue resulted in neither sucrose or footshock. During a subsequent summation test for conditioned inhibition of fear and reward, the inhibitor cue was presented concurrently with the reward and fear cues without any outcome, intermixed with trials of reinforced reward and fear trials. Males showed significant conditioned inhibition of freezing, while females did not, which was not dependent on estrous. Both males and females showed significant conditioned inhibition of reward. During a retardation of fear acquisition test, the inhibitor was paired with footshock and both males and females showed delayed acquisition of fear. During a retardation of reward acquisition test, the inhibitor was paired with sucrose, and females showed delayed acquisition of reward, while males did not. In summary, males and females showed significant reward-fear-inhibitor cue discrimination, conditioned inhibition of reward, and retardation of fear acquisition. The main sex difference, which was not estrous-dependent, was the lack of conditioned inhibition of freezing in females. These data imply that while the inhibitor cue gained some inhibitory value in the females, the strength of this inhibitory value may not have been great enough to effectively downregulate freezing elicited by the fear cue.

Environmental certainty influences the neural systems regulating responses to threat and stress

Flexible calibration of threat responding in accordance with the environment is an adaptive process that allows an animal to avoid harm while also maintaining engagement of other goal-directed actions. This calibration process, referred to as threat response regulation, requires an animal to calculate the probability that a given encounter will result in a threat so they can respond accordingly. Here we review the neural correlates of two highly studied forms of threat response suppression: extinction and safety conditioning. We focus on how relative levels of certainty or uncertainty in the surrounding environment alter the acquisition and application of these processes. We also discuss evidence indicating altered threat response regulation following stress exposure, including enhanced fear conditioning, and disrupted extinction and safety conditioning. To conclude, we discuss research using an animal model of coping that examines the impact of stressor controllability on threat responding, highlighting the potential for previous experiences with control, or other forms of coping, to protect against the effects of future adversity.

Covert capture and attenuation of a hippocampus-dependent fear memory

Reconsolidation may be a viable therapeutic target to inhibit pathological fear memories. In the clinic, incidental or imaginal reminders are used for safe retrieval of traumatic memories of experiences that occurred elsewhere. However, it is unknown whether indirectly retrieved traumatic memories are sensitive to disruption. Here we used a backward (BW) conditioning procedure to indirectly retrieve and manipulate a hippocampus (HPC)-dependent contextual fear engram in male rats. We show that conditioned freezing to a BW conditioned stimulus (CS) is mediated by fear to the conditioning context, activates HPC ensembles that can be covertly captured and chemogenetically activated to drive fear, and is impaired by post-retrieval protein synthesis inhibition. These results reveal that indirectly retrieved contextual fear memories reactivate HPC ensembles and undergo protein synthesis-dependent reconsolidation. Clinical interventions that rely on indirect retrieval of traumatic memories, such as imaginal exposure, may open a window for editing or erasure of neural representations that drive pathological fear.

Behavioral and brain mechanisms mediating conditioned flight behavior in rats

Environmental contexts can inform animals of potential threats, though it is currently unknown how context biases the selection of defensive behavior. Here we investigated context-dependent flight responses with a Pavlovian serial-compound stimulus (SCS) paradigm that evokes freeze-to-flight transitions. Similar to previous work in mice, we show that male and female rats display context-dependent flight-like behavior in the SCS paradigm. Flight behavior was dependent on contextual fear insofar as it was only evoked in a shock-associated context and was reduced in the conditioning context after context extinction. Flight behavior was only expressed to white noise regardless of temporal order within the compound. Nonetheless, rats that received unpaired SCS trials did not show flight-like behavior to the SCS, indicating it is associative. Finally, we show that pharmacological inactivation of two brain regions critical to the expression of contextual fear, the central nucleus of the amygdala (CeA) and bed nucleus of the stria terminalis (BNST), attenuates both contextual fear and flight responses. All of these effects were similar in male and female rats. This work demonstrates that contextual fear can summate with cued and innate fear to drive a high fear state and transition from post-encounter to circa-strike defensive modes.

Anxiety and the Neurobiology of Temporally Uncertain Threat Anticipation

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