Neurobiology of Pavlovian fear conditioning

P2X7 receptors modulate acquisition of cue fear extinction and contextual background memory generalization in male mice

The purinergic P2X7 receptors (P2X7R) are activated by adenosine triphosphate (ATP) in several brain regions, particularly those involved with emotional control and the regulation of fear-related memories. Here, we investigate the role of P2X7R in fear learning memory, specifically in the acquisition and consolidation phases of the cued fear conditioning paradigm. C57Bl/6 wildtype (WT) male mice that received a single i.p. injection of the selective P2X7R antagonist A438079 prior the conditioning session showed generalization of cued fear memory and impaired fear extinction recall in the test session, while those treated prior the extinction session exhibited a similar behavior profile accompanied by resistance in the extinction learning. However, no effects were observed when this drug was administered immediately after the conditioning, extinction, or before the test session. Our results with P2X7R knockout (P2X7 KO) mice showed a behavioral profile that mirrored the collective effects observed across all pharmacological treatment conditions. This suggests that the P2X7R KO model effectively replicates the behavioral changes induced by the pharmacological interventions, demonstrating that we have successfully isolated the role of P2X7R in the fear and extinction phases of memory. These findings highlight the role of P2X7R in the acquisition and recall of extinction memory and supports P2X7R as a promising candidate for controlling abnormal fear processing, with potential applications for stress exposure-related disorders such as post-traumatic stress disorder (PTSD).

Sex differences in prelimbic cortex calcium dynamics during stress and fear learning

In recent years, research has progressively increased the importance of considering sex differences in stress and fear memory studies. Many studies have traditionally focused on male subjects, potentially overlooking critical differences with females. Emerging evidence suggests that males and females can exhibit distinct behavioral and neurophysiological responses to stress and fear conditioning. These differences may be attributable to variations in hormone levels, brain structure, and neural circuitry, particularly in regions such as the prefrontal cortex (PFC). In the present study, we explored sex differences in prelimbic cortex (PL) calcium activity in animals submitted to immobilization stress (IMO), fear conditioning (FC), and fear extinction (FE). While no significant sex differences were found in behavioral responses, we did observe differences in several PL calcium activity parameters. To determine whether these results were related to behaviors beyond stress and fear memory, we conducted correlation studies between the movement of the animals and PL activity during IMO and freezing behavior during FC and FE. Our findings revealed a clear correlation between PL calcium activity with movement during stress exposure and freezing behavior, with no sex differences observed in these correlations. These results suggest a significant role for the PL in movement and locomotion, in addition to its involvement in fear-related processes. The inclusion of both female and male subjects is crucial for studies like this to fully understand the role of the PFC and other brain areas in stress and fear responses. Recognizing sex differences enhances our comprehension of brain function and can lead to more personalized and effective approaches in the study and treatment of stress and fear-related conditions.

Biased information routing through the basolateral amygdala, altered valence processing, and impaired affective states associated with psychiatric illnesses

Accumulating evidence supports a role for altered circuit function in impaired valence processing and altered affective states as a core feature of psychiatric illnesses. We review the circuit mechanisms underlying normal valence processing and highlight evidence supporting altered function of the basolateral amygdala (BLA), valence processing and affective states across psychiatric illnesses. The mechanisms controlling network activity which governs valence processing will be reviewed in the context of potential pathophysiological mechanisms mediating circuit dysfunction and impaired valence processing in psychiatric illnesses. Finally, we review emerging data demonstrating experience-dependent, biased information routing through the BLA promoting negative valence processing and discuss the potential relevance to impaired affective states and psychiatric illnesses.

HCN channel-dependent presynaptic potentiation at LA-BA synapses is required for fear memory formation

Previously, we demonstrated that auditory fear conditioning produces presynaptic potentiation at lateral to basal amygdala (LA-BA) synapses, which occludes high-frequency stimulation (HFS)-induced ex-vivo LTP. We also found that the HFS-induced ex-vivo LTP requires presynaptic hyperpolarization-activated cyclic nucleotide-gated (HCN) channel activity. In this study, we investigated whether HCN channels are necessary for auditory fear conditioning in vivo. Our results show that ZD7288, an HCN channel blocker, reduced synaptic transmission and decreased the paired pulse ratio (PPR) only in slices from rats that underwent auditory fear conditioning, but not from naïve rats. This indicates that fear conditioning involves HCN channel-dependent presynaptic potentiation at LA-BA synapses. Importantly, injecting ZD7288 into the basal amygdala (BA) before auditory fear conditioning significantly impaired long-term fear memory formation. Since HCN channel activity is necessary for LTP at LA-BA synapses but not at cortico-BA, cortico-LA, or thalamo-LA synapses, HCN channel-dependent presynaptic potentiation at LA-BA synapses appears to be crucial for auditory fear conditioning.

Role of the Nucleus Accumbens in Signaled Avoidance Actions

Animals, humans included, navigate their environments guided by sensory cues, responding adaptively to potential dangers and rewards. Avoidance behaviors serve as adaptive strategies in the face of signaled threats, but the neural mechanisms orchestrating these behaviors remain elusive. Current circuit models of avoidance behaviors indicate that the nucleus accumbens (NAc) in the ventral striatum plays a key role in signaled avoidance behaviors, but the nature of this engagement is unclear. Evolving perspectives propose the NAc as a pivotal hub for action selection, integrating cognitive and affective information to heighten the efficiency of both appetitive and aversive motivated behaviors. To unravel the engagement of the NAc during active and passive avoidance, we used calcium imaging fiber photometry to examine NAc GABAergic neuron activity in ad libitum moving mice performing avoidance behaviors. We then probed the functional significance of NAc neurons using optogenetics and genetically targeted or electrolytic lesions. We found that NAc neurons code contraversive orienting movements and avoidance actions. However, direct optogenetic inhibition or lesions of NAc neurons did not impair active or passive avoidance behaviors, challenging the notion of their purported pivotal role in adaptive avoidance. The findings emphasize that while the NAc encodes avoidance movements, it is not required for avoidance behaviors, highlighting the distinction between behavior encoding or representation and mediation or generation.

Memory engram stability and flexibility

Many studies have shown that memories are encoded in sparse neural ensembles distributed across the brain. During the post-encoding period, often during sleep, many of the cells that were active during encoding are reactivated, supporting consolidation of this memory. During memory recall, many of the same cells that were active during encoding and reactivated during consolidation are reactivated during recall. These ensembles of cells have been referred to as the memory engram cells, stably representing a specific memory. However, recent studies question the rigidity of the "stable memory engram." Here we review the past literature of how episodic-like memories are encoded, consolidated, and recalled. We also highlight more recent studies (as well as some older literature) that suggest that these stable memories and their representations are much more dynamic and flexible than previously thought. We highlight some of these processes, including memory updating, reconsolidation, forgetting, schema learning, memory-linking, and representational drift.

Anxiety and the brain: Neuropeptides as emerging factors

Anxiety disorders are characterized by intense feelings of worry and fear, which can significantly interfere with daily functioning. Current treatment options primarily include selective serotonin reuptake inhibitors, benzodiazepines, non-benzodiazepine anxiolytics, gabapentinoids, and beta-blockers. Neuropeptides have shown an important role in the regulation of complex behaviours, such as psychopathology and anxiety-related reactions. Neuropeptides have a great deal of promise to advance our understanding of and ability to help people with anxiety disorders. This review focuses on the expanding role of neuropeptides in anxiety management, particularly examining the impact of substance P, neuropeptide Y, corticotropin-releasing hormone, arginine-vasopressin, pituitary adenylate cyclase-activating polypeptide, and cholecystokinin. Furthermore, the paper discusses the neuropeptides that are becoming more and more recognized for their impact on anxiety-related reactions and their potential as therapeutic targets.

Dnmt3a1 regulates hippocampus-dependent memory via the downstream target Nrp1

Epigenetic factors are well-established players in memory formation. Specifically, DNA methylation is necessary for the formation of long-term memory in multiple brain regions including the hippocampus. Despite the demonstrated role of DNA methyltransferases (Dnmts) in memory formation, it is unclear whether individual Dnmts have unique or redundant functions in long-term memory formation. Furthermore, the downstream processes controlled by Dnmts during memory consolidation have not been investigated. In this study, we demonstrated that Dnmt3a1, the predominant Dnmt in the adult brain, is required for long-term spatial object recognition and contextual fear memory. Using RNA sequencing, we identified an activity-regulated Dnmt3a1-dependent genomic program in which several genes were associated with functional and structural plasticity. Furthermore, we found that some of the identified genes are selectively dependent on Dnmt3a1, but not its isoform Dnmt3a2. Specifically, we identified Neuropilin 1 (Nrp1) as a downstream target of Dnmt3a1 and further demonstrated the involvement of Nrp1 in hippocampus-dependent memory formation. Importantly, we found that Dnmt3a1 regulates hippocampus-dependent memory via Nrp1. In contrast, Nrp1 overexpression did not rescue memory impairments triggered by reduced Dnmt3a2 levels. Taken together, our study uncovered a Dnmt3a-isoform-specific mechanism in memory formation, identified a novel regulator of memory, and further highlighted the complex and highly regulated functions of distinct epigenetic regulators in brain function.

Reactivation-dependent transfer of fear memory between contexts requires M1 muscarinic receptor stimulation in dorsal hippocampus of male rats

Memory updating is essential for integrating new information into existing representations. However, this process could become maladaptive in conditions like post-traumatic stress disorder (PTSD), when fear memories generalize to neutral contexts. Previously, we have shown that contextual fear memory malleability in rats requires activation of M1 muscarinic acetylcholine receptors in the dorsal hippocampus. Here, we investigated the involvement of this mechanism in the transfer of contextual fear memories to other contexts using a novel fear memory updating paradigm. Following brief reexposure to a previously fear conditioned context, male rats (n = 8-10/group) were placed into a neutral context to evaluate the transfer of fear memory. We also infused the selective M1 receptor antagonist pirenzepine into the dorsal hippocampus before memory reactivation to try to block this effect. Results support the hypothesis that fear memory can be updated with novel contextual information, but only if rats are reexposed to the originally trained context relatively recently before the neutral context; evidence for transfer was not seen if the fear memory reactivation was omitted or if it occurred 6 h before neutral context exposure. The transferred fear persisted for 4 weeks, and the effect was blocked by M1 antagonism. These findings strongly suggest that fear transfer requires reactivation and destabilization of the original fear memory. The novel preclinical model introduced here, and its implication of muscarinic receptors in this process, could therefore inform therapeutic strategies for PTSD and similar conditions.

Eye tracking is more sensitive than skin conductance response in detecting mild environmental stimuli

The skin conductance (SC) and eye tracking data are two potential arousal-related psychophysiological signals that can serve as the interoceptive unconditioned response to aversive stimuli (e.g. electric shocks). The current research investigates the sensitivity of these signals in detecting mild electric shock by decoding the hidden arousal and interoceptive awareness (IA) states. While well-established frameworks exist to decode the arousal state from the SC signal, there is a lack of a systematic approach that decodes the IA state from pupillometry and eye gaze measurements. We extract the physiological-based features from eye tracking data to recover the IA-related neural activity. Employing a Bayesian filtering framework, we decode the IA state in fear conditioning and extinction experiments where mild electric shock is used. We independently decode the underlying arousal state using binary and marked point process (MPP) observations derived from concurrently collected SC data. Eight of 11 subjects present a significantly (P-value < 0.001 ) higher IA state in trials that were always accompanied by electric shock ( CS + US + ) compared to trials that were never accompanied by electric shock ( CS - ). According to the decoded SC-based arousal state, only five (binary observation) and four (MPP observation) subjects present a significantly higher arousal state in CS + US + trials than CS - trials. In conclusion, the decoded hidden brain state from eye tracking data better agrees with the presented mild stimuli. Tracking IA state from eye tracking data can lead to the development of contactless monitors for neuropsychiatric and neurodegenerative disorders.

Frontal Norepinephrine Represents a Threat Prediction Error Under Uncertainty

BACKGROUND

To adapt to threats in the environment, animals must predict them and engage in defensive behavior. While the representation of a prediction error signal for reward has been linked to dopamine, a neuromodulatory prediction error for aversive learning has not been identified.

METHODS

We measured and manipulated norepinephrine release during threat learning using optogenetics and a novel fluorescent norepinephrine sensor.

RESULTS

We found that norepinephrine response to conditioned stimuli reflects aversive memory strength. When delays between auditory stimuli and footshock are introduced, norepinephrine acts as a prediction error signal. However, temporal difference prediction errors do not fully explain norepinephrine dynamics. To explain noradrenergic signaling, we used an updated reinforcement learning model with uncertainty about time and found that it explained norepinephrine dynamics across learning and variations in temporal and auditory task structure.

CONCLUSIONS

Norepinephrine thus combines cognitive and affective information into a predictive signal and links time with the anticipation of danger.

Periaqueductal gray activates antipredatory neural responses in the amygdala of foraging rats

Pavlovian fear conditioning research suggests that the interaction between the dorsal periaqueductal gray (dPAG) and basolateral amygdala (BLA) acts as a prediction error mechanism in the formation of associative fear memories. However, their roles in responding to naturalistic predatory threats, characterized by less explicit cues and the absence of reiterative trial-and-error learning events, remain unexplored. In this study, we conducted single-unit recordings in rats during an 'approach food-avoid predator' task, focusing on the responsiveness of dPAG and BLA neurons to a rapidly approaching robot predator. Optogenetic stimulation of the dPAG triggered fleeing behaviors and increased BLA activity in naive rats. Notably, BLA neurons activated by dPAG stimulation displayed immediate responses to the robot, demonstrating heightened synchronous activity compared to BLA neurons that did not respond to dPAG stimulation. Additionally, the use of anterograde and retrograde tracer injections into the dPAG and BLA, respectively, coupled with c-Fos activation in response to predatory threats, indicates that the midline thalamus may play an intermediary role in innate antipredatory-defensive functioning.

Haploinsufficiency of GABAA Receptor-Associated Clptm1 Enhances Phasic and Tonic Inhibitory Neurotransmission, Suppresses Excitatory Synaptic Plasticity, and Impairs Memory

The mechanisms utilized by neurons to regulate the efficacy of phasic and tonic inhibition and their impacts on synaptic plasticity and behavior are incompletely understood. Cleft lip and palate transmembrane protein 1 (Clptm1) is a membrane-spanning protein that interacts with multiple γ-aminobutyric acid type A receptor (GABAAR) subunits, trapping them in the endoplasmic reticulum and Golgi network. Overexpression and knock-down studies suggest that Clptm1 modulates GABAAR-mediated phasic inhibition and tonic inhibition as well as activity-induced inhibitory synaptic homeostasis in cultured hippocampal neurons. To investigate the role of Clptm1 in the modulation of GABAARs in vivo, we generated Clptm1 knock-out (KO) mice. Here, we show that genetic KO of Clptm1 elevated phasic and tonic inhibitory transmission in both male and female heterozygous mice. Although basal excitatory synaptic transmission was not affected, Clptm1 haploinsufficiency significantly blocked high-frequency stimulation-induced long-term potentiation (LTP) in hippocampal CA3→CA1 synapses. In the hippocampus-dependent contextual fear-conditioning behavior task, both male and female Clptm1 heterozygous KO mice exhibited impairment in contextual fear memory. In addition, LTP and contextual fear memory were rescued by application of L-655,708, a negative allosteric modulator of the extrasynaptic GABAAR α5 subunit. These results suggest that haploinsufficiency of Clptm1 contributes to cognitive deficits through altered synaptic transmission and plasticity by elevation of inhibitory neurotransmission, with tonic inhibition playing a major role.

Acquisition and generalization of emotional and neural responses to faces associated with negative and positive feedback behaviours

Faces can acquire emotional meaning by learning to associate individuals with specific behaviors. Here, we investigated emotional evaluation and brain activations toward faces of persons who had given negative or positive evaluations to others. Furthermore, we investigated how emotional evaluations and brain activation generalize to perceptually similar faces. Valence ratings indicated learning and generalization effects for both positive and negative faces. Brain activation, measured with functional magnetic resonance imaging (fMRI), showed significantly increased activation in the fusiform gyrus (FG) to negatively associated faces but not positively associated ones. Remarkably, brain activation in FG to faces to which emotional meaning (negative and positive) was successfully generalized was decreased compared to neutral faces. This suggests that the emotional relevance of faces is not simply associated with increased brain activation in visual areas. While, at least for negative conditions, faces paired with negative feedback behavior are related to potentiated brain responses, the opposite is seen for perceptually very similar faces despite generalized emotional responses.

γ-Secretase Modulator BPN15606 Reduced Aβ42 and Aβ40 and Countered Alzheimer-Related Pathologies in a Mouse Model of Down Syndrome

OBJECTIVES

Due to increased gene dose for the amyloid precursor protein (APP), elderly adults with Down syndrome (DS) are at a markedly increased risk of Alzheimer's disease (AD), known as DS-AD. How the increased APP gene dose acts and which APP products are responsible for DS-AD is not well understood, thus limiting strategies to target pathogenesis. As one approach to address this question, we used a novel class of γ-secretase modulators that promote γ-site cleavages by the γ-secretase complex, resulting in lower levels of the Aβ42 and Aβ40 peptides.

METHODS

Ts65Dn mice, which serve as a model of DS, were treated via oral gavage with 10 mg/kg/weekday of BPN15606 (a potent and novel pyridazine-containing γ-secretase modulators). Treatment started at 3 months-of-age and lasted for 4 months.

RESULTS

Demonstrating successful target engagement, treatment with BPN15606 significantly decreased levels of Aβ40 and Aβ42 in the cortex and hippocampus; it had no effect on full-length APP or its C-terminal fragments in either 2 N or Ts65Dn mice. Importantly, the levels of total amyloid-β were not impacted, pointing to BPN15606-mediated enhancement of processivity of γ-secretase. Additionally, BPN15606 rescued hyperactivation of Rab5, a protein responsible for regulating endosome function, and normalized neurotrophin signaling deficits. BPN15606 treatment also normalized the levels of synaptic proteins and tau phosphorylation, while reducing astrocytosis and microgliosis, and countering cognitive deficits.

INTERPRETATION

Our findings point to the involvement of increased levels of Aβ42 and/or Aβ40 in contributing to several molecular and cognitive traits associated with DS-AD. They speak to increased dosage of the APP gene acting through heightened levels of Aβ42 and/or Aβ40 as supporting pathogenesis. These findings further the interest in the potential use of γ-secretase modulators for treating and possibly preventing AD in individuals with DS. ANN NEUROL 2024;96:390-404.

Oscillatory correlates of threat imminence during virtual navigation

The Predatory Imminence Continuum Theory proposes that defensive behaviors depend on the proximity of a threat. While the neural mechanisms underlying this proposal are well studied in animal models, it remains poorly understood in humans. To address this issue, we recorded EEG from 24 (15 female) young adults engaged in a first-person virtual reality Risk-Reward interaction task. On each trial, participants were placed in a virtual room and presented with either a threat or reward conditioned stimulus (CS) in the same room location (proximal) or different room location (distal). Behaviorally, all participants learned to avoid the threat-CS, with most using the optimal behavior to actively avoid the proximal threat-CS (88% accuracy) and passively avoid the distal threat-CS (69% accuracy). Similarly, participants learned to actively approach the distal reward-CS (82% accuracy) and to remain passive to the proximal reward-CS (72% accuracy). At an electrophysiological level, we observed a general increase in theta power (4-8 Hz) over the right posterior channel P8 across all conditions, with the proximal threat-CS evoking the largest theta response. By contrast, distal cues induced two bursts of gamma (30-60 Hz) power over midline-parietal channel Pz (200 msec post-cue) and right frontal channel Fp2 (300 msec post-cue). Interestingly, the first burst of gamma power was sensitive to the distal threat-CS and the second burst at channel Fp2 was sensitive to the distal reward-CS. Together, these findings demonstrate that oscillatory processes differentiate between the spatial proximity information during threat and reward encoding, likely optimizing the selection of the appropriate behavioral response.

A multi-modal assessment of fear conditioning in adolescent anorexia nervosa

OBJECTIVE

Anorexia nervosa (AN) is a pernicious psychiatric disorder which is principally characterized by a fear of weight gain. Notwithstanding the centrality of fear in the psychopathology of AN, controlled assessments of negative valence systems are lacking. Herein we assess fear conditioning in adolescent females with AN.

METHOD

Adolescent girls (Mage = 14.6 years, ±1.57) with DSM-5 diagnoses of AN (N = 25) and age-matched control girls (Mage = 14.8 years, ±1.46) with no DSM-5 diagnoses (N = 25) completed structured clinical interviews and participated in a classical three-phase Pavlovian fear conditioning paradigm. Participants with comorbid anxiety disorders were excluded. Skin conductance response (SCR) was measured, alongside self-reported fear, valence, and fear expectancy ratings.

RESULTS

Both groups demonstrated significant differential acquisition across all four measures. Regarding group comparisons, no differences emerged for self-reported fear, valence, and fear expectancy ratings during acquisition, although for SCR, those with AN demonstrated reduced physiological arousal relative to controls. Both groups demonstrated significant differential extinction for unconditioned stimuli (US) expectancy, self-report fear, and self-report valence. No statistically significant group differences were evident during extinction to the conditioned stimuli (CS)+, on any outcome measure. However, controls reported more positive valence to the CS- than those with AN.

CONCLUSIONS

Contrary to our hypotheses, our preliminary assessment did not find support for elevated fear responding among adolescent girls with AN with regards to fear acquisition or extinction. These data suggest that AN in adolescent girls may not be associated with a heightened propensity to acquire fear, but conversely, may suggest that exposure treatments for AN may be helpful, since extinction learning is intact in AN.

PUBLIC SIGNIFICANCE

AN is characterized by fear-related symptoms, including food and weight-related fear, and behavioral avoidance, yet controlled studies assessing fear learning are limited. Our preliminary assessment of adolescent AN indicates no abnormalities in fear learning among adolescents with AN. These findings may inform existing mechanistic models of AN psychopathology, and the development of exposure-based treatments for AN.

Periaqueductal gray activates antipredatory neural responses in the amygdala of foraging rats

Pavlovian fear conditioning research suggests that the interaction between the dorsal periaqueductal gray (dPAG) and basolateral amygdala (BLA) acts as a prediction error mechanism in the formation of associative fear memories. However, their roles in responding to naturalistic predatory threats, characterized by less explicit cues and the absence of reiterative trial-and-error learning events, remain unexplored. In this study, we conducted single-unit recordings in rats during an 'approach food-avoid predator' task, focusing on the responsiveness of dPAG and BLA neurons to a rapidly approaching robot predator. Optogenetic stimulation of the dPAG triggered fleeing behaviors and increased BLA activity in naive rats. Notably, BLA neurons activated by dPAG stimulation displayed immediate responses to the robot, demonstrating heightened synchronous activity compared to BLA neurons that did not respond to dPAG stimulation. Additionally, the use of anterograde and retrograde tracer injections into the dPAG and BLA, respectively, coupled with c-Fos activation in response to predatory threats, indicates that the midline thalamus may play an intermediary role in innate antipredatory defensive functioning.

Synaptic Organization-Function Relationships of Amygdala Interneurons Supporting Associative Learning

Coordinated activity of basolateral amygdala (BLA) GABAergic interneurons (INs) and glutamatergic principal cells (PCs) is critical for associative learning, however the microcircuit organization-function relationships of distinct IN classes remain uncertain. Here, we show somatostatin (SOM) INs provide inhibition onto, and are excited by, local PCs, whereas vasoactive intestinal peptide (VIP) INs are driven by extrinsic afferents. Parvalbumin (PV) INs inhibit PCs and are activated by local and extrinsic inputs. Thus, SOM and VIP INs exhibit complementary roles in feedback and feedforward inhibition, respectively, while PV INs contribute to both microcircuit motifs. Functionally, each IN subtype reveals unique activity patterns across fear- and extinction learning with SOM and VIP INs showing most divergent characteristics, and PV INs display an intermediate phenotype parallelling synaptic data. Finally, SOM and PV INs dynamically track behavioral state transitions across learning. These data provide insight into the synaptic microcircuit organization-function relationships of distinct BLA IN classes.

A subiculum-hypothalamic pathway functions in dynamic threat detection and memory updating

Animals need to detect threats, initiate defensive responses, and, in parallel, remember where the threat occurred to avoid the possibility of re-encountering it. By probing animals capable of detecting and avoiding a shock-related threatening location, we were able to reveal a septo-hippocampal-hypothalamic circuit that is also engaged in ethological threats, including predatory and social threats. Photometry analysis focusing on the dorsal premammillary nucleus (PMd), a critical interface of this circuit, showed that in freely tested animals, the nucleus appears ideal to work as a threat detector to sense dynamic changes under threatening conditions as the animal approaches and avoids the threatening source. We also found that PMd chemogenetic silencing impaired defensive responses by causing a failure of threat detection rather than a direct influence on any behavioral responses and, at the same time, updated fear memory to a low-threat condition. Optogenetic silencing of the main PMd targets, namely the periaqueductal gray and anterior medial thalamus, showed that the projection to the periaqueductal gray influences both defensive responses and, to a lesser degree, contextual memory, whereas the projection to the anterior medial thalamus has a stronger influence on memory processes. Our results are important for understanding how animals deal with the threat imminence continuum, revealing a circuit that is engaged in threat detection and that, at the same time, serves to update the memory process to accommodate changes under threatening conditions.

Neuropsychobiology of fear-induced bradycardia in humans: progress and pitfalls

In the last century, the paradigm of fear conditioning has greatly evolved in a variety of scientific fields. The techniques, protocols, and analysis methods now most used have undergone a progressive development, theoretical and technological, improving the quality of scientific productions. Fear-induced bradycardia is among these techniques and represents the temporary deceleration of heart beats in response to negative outcomes. However, it has often been used as a secondary measure to assess defensive responding to threat, along other more popular techniques. In this review, we aim at paving the road for its employment as an additional tool in fear conditioning experiments in humans. After an overview of the studies carried out throughout the last century, we describe more recent evidence up to the most contemporary research insights. Lastly, we provide some guidelines concerning the best practices to adopt in human fear conditioning studies which aim to investigate fear-induced bradycardia.

Memory Trace for Fear Extinction: Fragile yet Reinforceable

Fear extinction is a biological process in which learned fear behavior diminishes without anticipated reinforcement, allowing the organism to re-adapt to ever-changing situations. Based on the behavioral hypothesis that extinction is new learning and forms an extinction memory, this new memory is more readily forgettable than the original fear memory. The brain's cellular and synaptic traces underpinning this inherently fragile yet reinforceable extinction memory remain unclear. Intriguing questions are about the whereabouts of the engram neurons that emerged during extinction learning and how they constitute a dynamically evolving functional construct that works in concert to store and express the extinction memory. In this review, we discuss recent advances in the engram circuits and their neural connectivity plasticity for fear extinction, aiming to establish a conceptual framework for understanding the dynamic competition between fear and extinction memories in adaptive control of conditioned fear responses.

Complementation testing identifies genes mediating effects at quantitative trait loci underlying fear-related behavior

Knowing the genes involved in quantitative traits provides an entry point to understanding the biological bases of behavior, but there are very few examples where the pathway from genetic locus to behavioral change is known. To explore the role of specific genes in fear behavior, we mapped three fear-related traits, tested fourteen genes at six quantitative trait loci (QTLs) by quantitative complementation, and identified six genes. Four genes, Lamp, Ptprd, Nptx2, and Sh3gl, have known roles in synapse function; the fifth, Psip1, was not previously implicated in behavior; and the sixth is a long non-coding RNA, 4933413L06Rik, of unknown function. Variation in transcriptome and epigenetic modalities occurred preferentially in excitatory neurons, suggesting that genetic variation is more permissible in excitatory than inhibitory neuronal circuits. Our results relieve a bottleneck in using genetic mapping of QTLs to uncover biology underlying behavior and prompt a reconsideration of expected relationships between genetic and functional variation.

The impact of doxycycline on human contextual fear memory

RATIONALE

 Previous work identified an attenuating effect of the matrix metalloproteinase (MMP) inhibitor doxycycline on fear memory consolidation. This may present a new mechanistic approach for the prevention of trauma-related disorders. However, so far, this has only been unambiguously demonstrated in a cued delay fear conditioning paradigm, in which a simple geometric cue predicted a temporally overlapping aversive outcome. This form of learning is mainly amygdala dependent. Psychological trauma often involves the encoding of contextual cues, which putatively necessitates partly different neural circuits including the hippocampus. The role of MMP signalling in the underlying neural pathways in humans is unknown.

METHODS

Here, we investigated the effect of doxycycline on configural fear conditioning in a double-blind placebo-controlled randomised trial with 100 (50 females) healthy human participants.

RESULTS

Our results show that participants successfully learned and retained, after 1 week, the context-shock association in both groups. We find no group difference in fear memory retention in either of our pre-registered outcome measures, startle eye-blink responses and pupil dilation. Contrary to expectations, we identified elevated fear-potentiated startle in the doxycycline group early in the recall test, compared to the placebo group.

CONCLUSION

Our results suggest that doxycycline does not substantially attenuate contextual fear memory. This might limit its potential for clinical application.

Impact of high-fat diet on cognitive behavior and central and systemic inflammation with aging and sex differences in mice

Aging and age-related diseases are associated with cellular stress, metabolic imbalance, oxidative stress, and neuroinflammation, accompanied by cognitive impairment. Lifestyle factors such as diet, sleep fragmentation, and stress can potentiate damaging cellular cascades and lead to an acceleration of brain aging and cognitive impairment. High-fat diet (HFD) has been associated with obesity, metabolic disorders like diabetes, and cardiovascular disease. HFD also induces neuroinflammation, impairs learning and memory, and may increase anxiety-like behavior. Effects of a HFD may also vary between sexes. The interaction between Age- and Sex- and Diet-related changes in neuroinflammation and cognitive function is an important and poorly understood area of research. This study was designed to examine the effects of HFD on neuroinflammation, behavior, and neurodegeneration in mice in the context of aging or sex differences. In a series of studies, young (2-3 months) or old (12-13 months) C57BL/6J male mice or young male and female C57Bl/6J mice were fed either a standard diet (SD) or a HFD for 5-6 months. Behavior was assessed in Activity Chamber, Y-maze, Novel Place Recognition, Novel Object Recognition, Elevated Plus Maze, Open Field, Morris Water Maze, and Fear Conditioning. Post-mortem analyses assessed a panel of inflammatory markers in the plasma and hippocampus. Additionally, proteomic analysis of the hypothalamus, neurodegeneration, neuroinflammation in the locus coeruleus, and neuroinflammation in the hippocampus were assessed in a subset of young and aged male mice. We show that HFD increased body weight and decreased locomotor activity across groups compared to control mice fed a SD. HFD altered anxiety-related exploratory behavior. HFD impaired spatial learning and recall in young male mice and impaired recall in cued fear conditioning in young and aged male mice, with no effects on spatial learning or fear conditioning in young female mice. Effects of Age and Sex were observed on neuroinflammatory cytokines, with only limited effects of HFD. HFD had a more significant impact on systemic inflammation in plasma across age and sex. Aged male mice had induction of microglial immunoreactivity in both the locus coeruleus (LC) and hippocampus an effect that HFD exacerbated in the hippocampal CA1 region. Proteomic analysis of the hypothalamus revealed changes in pathways related to metabolism and neurodegeneration with both aging and HFD in male mice. Our findings suggest that HFD induces widespread systemic inflammation and limited neuroinflammation. In addition, HFD alters exploratory behavior in male and female mice, and impairs learning and memory in male mice. These results provide valuable insight into the impact of diet on cognition and aging pathophysiology.

Line- and sex-dependent effects of juvenile stress on contextual fear- and anxiety-related behavior in high- and low-alcohol-preferring mouse lines

Juvenile stress (JS) is a known risk factor for the development of alcohol use disorder (AUD) and post-traumatic stress disorder (PTSD), both of which are frequently co-morbid. Data suggest there may be common, genetically-influenced biological responses to stress that contribute to the development of both AUD and PTSD. The present study investigated the impact of JS on contextual fear learning and extinction, as well as corticosterone (CORT) responses before and after JS, before and after contextual fear conditioning (CFC), and after fear extinction in male and female high-alcohol-preferring (HAP2) and low-alcohol-preferring (LAP2) mouse lines. We also measured unconditioned anxiety-related behavior in the light-dark-transition test before CFC. HAP2 and LAP2 mice did not differ in fear acquisition, but HAP2 mice showed faster fear extinction compared to LAP2 mice. No effects of JS were seen in HAP2 mice, whereas in LAP2 mice, JS reduced fear acquisition in males and facilitated fear extinction in females. Females showed greater fear-related behavior relative to males, regardless of subgroup. HAP2 males demonstrated more anxiolytic-like responses than LAP2 males and LAP2 females demonstrated more anxiolytic-like responses than LAP2 males in the light-dark transition test. HAP2 and LAP2 mice did not differ in CORT during the juvenile stage; however, adult LAP2 mice showed greater CORT levels than HAP2 mice at baseline and after CFC and extinction testing. These findings build upon prior work in these unique mouse lines that differ in genetic propensity toward alcohol preference and provide new information regarding contextual fear learning and extinction mechanisms theorized to contribute to co-morbid AUD and PTSD.

Distinct electrophysiological properties of long-range GABAergic and glutamatergic neurons from the lateral amygdala to the auditory cortex of the mouse

Differentiating between auditory signals of various emotional significance plays a crucial role in an individual's ability to thrive and excel in social interactions and in survival. Multiple approaches, including anatomical studies, electrophysiological investigations, imaging techniques, optogenetics and chemogenetics, have confirmed that the auditory cortex (AC) impacts fear-related behaviours driven by auditory stimuli by conveying auditory information to the lateral amygdala (LA) through long-range excitatory glutamatergic and GABAergic connections. In addition, the LA provides glutamatergic projections to the AC which are important to fear memory expression and are modified by associative fear learning. Here we test the hypothesis that the LA also sends long-range direct inhibitory inputs to the cortex. To address this fundamental question, we used anatomical and electrophysiological approaches, allowing us to directly assess the nature of GABAergic inputs from the LA to the AC in the mouse. Our findings elucidate the existence of a long-range inhibitory pathway from the LA to the AC (LAC) via parvalbumin-expressing (LAC-Parv) and somatostatin-expressing (LAC-SOM) neurons. This research identifies distinct electrophysiological properties for genetically defined long-range GABAergic neurons involved in the communication between the LA and the cortex (LAC-Parv inhibitory projections → AC neurons; LAC-Som inhibitory projections → AC neurons) within the lateral amygdala cortical network. KEY POINTS: The mouse auditory cortex receives inputs from the lateral amygdala. Retrograde viral tracing techniques allowed us to identify two previously undescribed lateral amygdala to auditory cortex (LAC) GABAergic projecting neurons. Extensive electrophysiological, morphological and anatomical characterization of LAC neurons is provided here, demonstrating key differences in the three populations. This study paves the way for a better understanding of the growing complexity of the cortico-amygdala-cortico circuit.

The blink reflex and its modulation - Part 1: Physiological mechanisms

The blink reflex (BR) is a protective eye-closure reflex mediated by brainstem circuits. The BR is usually evoked by electrical supraorbital nerve stimulation but can be elicited by a variety of sensory modalities. It has a long history in clinical neurophysiology practice. Less is known, however, about the many ways to modulate the BR. Various neurophysiological techniques can be applied to examine different aspects of afferent and efferent BR modulation. In this line, classical conditioning, prepulse and paired-pulse stimulation, and BR elicitation by self-stimulation may serve to investigate various aspects of brainstem connectivity. The BR may be used as a tool to quantify top-down modulation based on implicit assessment of the value of blinking in a given situation, e.g., depending on changes in stimulus location and probability of occurrence. Understanding the role of non-nociceptive and nociceptive fibers in eliciting a BR is important to get insight into the underlying neural circuitry. Finally, the use of BRs and other brainstem reflexes under general anesthesia may help to advance our knowledge of the brainstem in areas not amenable in awake intact humans. This review summarizes talks held by the Brainstem Special Interest Group of the International Federation of Clinical Neurophysiology at the International Congress of Clinical Neurophysiology 2022 in Geneva, Switzerland, and provides a state-of-the-art overview of the physiology of BR modulation. Understanding the principles of BR modulation is fundamental for a valid and thoughtful clinical application (reviewed in part 2) (Gunduz et al., submitted).

Basal forebrain cholinergic systems as circuits through which traumatic stress disrupts emotional memory regulation

Contextual and spatial systems facilitate changes in emotional memory regulation brought on by traumatic stress. Cholinergic basal forebrain (chBF) neurons provide input to contextual/spatial systems and although chBF neurons are important for emotional memory, it is unknown how they contribute to the traumatic stress effects on emotional memory. Clusters of chBF neurons that project to the prefrontal cortex (PFC) modulate fear conditioned suppression and passive avoidance, while clusters of chBF neurons that project to the hippocampus (Hipp) and PFC (i.e. cholinergic medial septum and diagonal bands of Broca (chMS/DBB neurons) are critical for fear extinction. Interestingly, neither Hipp nor PFC projecting chMS/DBB neurons are critical for fear extinction. The retrosplenial cortex (RSC) is a contextual/spatial memory system that receives input from chMS/DBB neurons, but whether this chMS/DBB-RSC circuit facilitates traumatic stress effects on emotional memory remain unexplored. Traumatic stress leads to neuroinflammation and the buildup of reactive oxygen species. These two molecular processes may converge to disrupt chBF circuits enhancing the impact of traumatic stress on emotional memory.

Excess neonatal testosterone causes male-specific social and fear memory deficits in wild-type mice

Neurodevelopmental disorders (ND) disproportionately affect males compared to females, and Autism Spectrum Disorder (ASD) in particular exhibits a 4:1 male bias. The biological mechanisms of this female protection or male susceptibility have not been identified. There is some evidence to suggest that fetal/neonatal gonadal hormones, which play pivotal roles in many aspects of development, may contribute. Here, we investigate the role of testosterone administration during a critical period of development, and its effects on social approach and fear learning in C57BL/6J wildtype mice. Male, but not female mice treated with testosterone on the day of birth (PN0) exhibited deficits in both social behavior and contextual fear conditioning, whereas mice treated with the same dose of testosterone on postnatal day 18 (PN18) did not display such impairments. Testosterone administration did not induce anxiogenic effects or lead to changes in body weight compared to the vehicle-treated group. These impairmeants are relevant to ND and may help identify novel treatment targets.

Fear in action: Fear conditioning and alleviation through body movements

Fear memories enhance survival especially when the memories guide defensive movements to minimize harm. Accordingly, fear memories and body movements have tight relationships in animals: Fear memory acquisition results in adapting reactive defense movements, while training active defense movements reduces fear memory. However, evidence in humans is scarce because their movements are typically suppressed in experiments. Here, we tracked adult participants' body motions while they underwent ecologically valid fear conditioning in a 3D virtual space. First, with body motion tracking, we revealed that distinct spatiotemporal body movement patterns emerge through fear conditioning. Second, subsequent training to actively avoid threats with naturalistic defensive actions led to a long-term (24 h) reduction of physiological and embodied conditioned responses, while extinction or vicarious training only transiently reduced the responses. Together, our results highlight the role of body movements in human fear memory and its intervention.

Fear conditioning and extinction distinctively alter bidirectional synaptic plasticity within the amygdala of an animal model of post-traumatic stress disorder

Synaptic plasticity in the amygdala plays an essential role in the formation and inhibition of fear memory; however, this plasticity has mainly been studied in the lateral amygdala, making it largely uninvestigated in other subnuclei. Here, we investigated long-term potentiation (LTP) and long-term depression (LTD) in the basolateral amygdala (BLA) to the medial division of the central amygdala (CEm) synapses of juvenile C57BL/6N (B6) and 129S1/SvImJ (S1) mice. We found that in naïve B6 and S1 mice, LTP was not induced at the BLA to CEm synapses, whereas fear conditioning lowered the threshold for LTP induction in these synapses of both B6 and S1 mice. Interestingly, fear extinction disrupted the induction of LTP at the BLA to CEm synapses of B6 mice, whereas LTP was left intact in S1 mice. Both low-frequency stimulation (LFS) and modest LFS (mLFS) induced LTD in naïve B6 and S1 mice, suggesting that the BLA to CEm synapses express bidirectional plasticity. Fear conditioning disrupted both types of LTD induction selectively in S1 mice and LFS-LTD, presumably NMDAR-dependent LTD was partially recovered by fear extinction. However, mLFS-LTD which has been known to be endocannabinoid receptor 1 (CB1R)-dependent was not induced after fear extinction in both mouse strains. Our observations suggest that fear conditioning enhances LTP while fear extinction diminishes LTP at the BLA to the CEm synapses of B6 mice with successful extinction. Considering that S1 mice showed strong fear conditioning and impaired extinction, strong fear conditioning in the S1 strain may be related to disrupted LTD, and impaired extinction may be due to constant LTP and weak LFS-LTD at the BLA to CEm synapses. Our study contributes to the further understanding of the dynamics of synaptic potentiation and depression between the subnuclei of the amygdala in juvenile mice after fear conditioning and extinction.

The ventral hippocampus is activated in olfactory but not auditory threat memory

Hippocampal networks required for associative memory formation are involved in cue- and context-dependent threat conditioning. The hippocampus is functionally heterogeneous at its dorsal and ventral poles, and recent investigations have focused on the specific roles required from each sub-region for associative conditioning. Cumulative evidence suggests that contextual and emotional information is processed by the dorsal and ventral hippocampus, respectively. However, it is not well understood how these two divisions engage in threat conditioning with cues of different sensory modalities. Here, we compare the involvement of the dorsal and ventral hippocampus in two types of threat conditioning: olfactory and auditory. Our results suggest that the dorsal hippocampus encodes contextual information and is activated upon recall of an olfactory threat memory only if contextual cues are relevant to the threat. Overnight habituation to the context eliminates dorsal hippocampal activation, implying that this area does not directly support cue-dependent threat conditioning. The ventral hippocampus is activated upon recall of olfactory, but not auditory, threat memory regardless of habituation duration. Concurrent activation of the piriform cortex is consistent with its direct connection with the ventral hippocampus. Together, our study suggests a unique role of the ventral hippocampus in olfactory threat conditioning.

Molecular diversity and functional dynamics in the central amygdala

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

Neural circuits for the adaptive regulation of fear and extinction memory

The regulation of fear memories is critical for adaptive behaviors and dysregulation of these processes is implicated in trauma- and stress-related disorders. Treatments for these disorders include pharmacological interventions as well as exposure-based therapies, which rely upon extinction learning. Considerable attention has been directed toward elucidating the neural mechanisms underlying fear and extinction learning. In this review, we will discuss historic discoveries and emerging evidence on the neural mechanisms of the adaptive regulation of fear and extinction memories. We will focus on neural circuits regulating the acquisition and extinction of Pavlovian fear conditioning in rodent models, particularly the role of the medial prefrontal cortex and hippocampus in the contextual control of extinguished fear memories. We will also consider new work revealing an important role for the thalamic nucleus reuniens in the modulation of prefrontal-hippocampal interactions in extinction learning and memory. Finally, we will explore the effects of stress on this circuit and the clinical implications of these findings.

Fear Learning: An Evolving Picture for Plasticity at Synaptic Afferents to the Amygdala

Unraveling the neuronal mechanisms of fear learning might allow neuroscientists to make links between a learned behavior and the underlying plasticity at specific synaptic connections. In fear learning, an innocuous sensory event such as a tone (called the conditioned stimulus, CS) acquires an emotional value when paired with an aversive outcome (unconditioned stimulus, US). Here, we review earlier studies that have shown that synaptic plasticity at thalamic and cortical afferents to the lateral amygdala (LA) is critical for the formation of auditory-cued fear memories. Despite the early progress, it has remained unclear whether there are separate synaptic inputs that carry US information to the LA to act as a teaching signal for plasticity at CS-coding synapses. Recent findings have begun to fill this gap by showing, first, that thalamic and cortical auditory afferents can also carry US information; second, that the release of neuromodulators contributes to US-driven teaching signals; and third, that synaptic plasticity additionally happens at connections up- and downstream of the LA. Together, a picture emerges in which coordinated synaptic plasticity in serial and parallel circuits enables the formation of a finely regulated fear memory.

Activated somatostatin interneurons orchestrate memory microcircuits

Despite recent advancements in identifying engram cells, our understanding of their regulatory and functional mechanisms remains in its infancy. To provide mechanistic insight into engram cell functioning, we introduced a novel local microcircuit labeling technique that enables the labeling of intraregional synaptic connections. Utilizing this approach, we discovered a unique population of somatostatin (SOM) interneurons in the mouse basolateral amygdala (BLA). These neurons are activated during fear memory formation and exhibit a preference for forming synapses with excitatory engram neurons. Post-activation, these SOM neurons displayed varying excitability based on fear memory retrieval. Furthermore, when we modulated these SOM neurons chemogenetically, we observed changes in the expression of fear-related behaviors, both in a fear-associated context and in a novel setting. Our findings suggest that these activated SOM interneurons play a pivotal role in modulating engram cell activity. They influence the expression of fear-related behaviors through a mechanism that is dependent on memory cues.

Targeting Human Glucocorticoid Receptors in Fear Learning: A Multiscale Integrated Approach to Study Functional Connectivity

Fear extinction is a phenomenon that involves a gradual reduction in conditioned fear responses through repeated exposure to fear-inducing cues. Functional brain connectivity assessments, such as functional magnetic resonance imaging (fMRI), provide valuable insights into how brain regions communicate during these processes. Stress, a ubiquitous aspect of life, influences fear learning and extinction by changing the activity of the amygdala, prefrontal cortex, and hippocampus, leading to enhanced fear responses and/or impaired extinction. Glucocorticoid receptors (GRs) are key to the stress response and show a dual function in fear regulation: while they enhance the consolidation of fear memories, they also facilitate extinction. Accordingly, GR dysregulation is associated with anxiety and mood disorders. Recent advancements in cognitive neuroscience underscore the need for a comprehensive understanding that integrates perspectives from the molecular, cellular, and systems levels. In particular, neuropharmacology provides valuable insights into neurotransmitter and receptor systems, aiding the investigation of mechanisms underlying fear regulation and potential therapeutic targets. A notable player in this context is cortisol, a key stress hormone, which significantly influences both fear memory reconsolidation and extinction processes. Gaining a thorough understanding of these intricate interactions has implications in terms of addressing psychiatric disorders related to stress. This review sheds light on the complex interactions between cognitive processes, emotions, and their neural bases. In this endeavor, our aim is to reshape the comprehension of fear, stress, and their implications for emotional well-being, ultimately aiding in the development of therapeutic interventions.

Complementation testing identifies causal genes at quantitative trait loci underlying fear related behavior

Knowing the genes involved in quantitative traits provides a critical entry point to understanding the biological bases of behavior, but there are very few examples where the pathway from genetic locus to behavioral change is known. Here we address a key step towards that goal by deploying a test that directly queries whether a gene mediates the effect of a quantitative trait locus (QTL). To explore the role of specific genes in fear behavior, we mapped three fear-related traits, tested fourteen genes at six QTLs, and identified six genes. Four genes, Lsamp, Ptprd, Nptx2 and Sh3gl, have known roles in synapse function; the fifth gene, Psip1, is a transcriptional co-activator not previously implicated in behavior; the sixth is a long non-coding RNA 4933413L06Rik with no known function. Single nucleus transcriptomic and epigenetic analyses implicated excitatory neurons as likely mediating the genetic effects. Surprisingly, variation in transcriptome and epigenetic modalities between inbred strains occurred preferentially in excitatory neurons, suggesting that genetic variation is more permissible in excitatory than inhibitory neuronal circuits. Our results open a bottleneck in using genetic mapping of QTLs to find novel biology underlying behavior and prompt a reconsideration of expected relationships between genetic and functional variation.

Hippocampal Engrams and Contextual Memory

Memories are not formed in a vacuum and often include rich details about the time and place in which events occur. Contextual stimuli promote the retrieval of events that have previously occurred in the encoding context and limit the retrieval of context-inappropriate information. Contexts that are associated with traumatic or harmful events both directly elicit fear and serve as reminders of aversive events associated with trauma. It has long been appreciated that the hippocampus is involved in contextual learning and memory and is central to contextual fear conditioning. However, little is known about the underlying neuronal mechanisms underlying the encoding and retrieval of contextual fear memories. Recent advancements in neuronal labeling methods, including activity-dependent tagging of cellular ensembles encoding memory ("engrams"), provide unique insight into the neural substrates of memory in the hippocampus. Moreover, these methods allow for the selective manipulation of memory ensembles. Attenuating or erasing fear memories may have considerable therapeutic value for patients with post-traumatic stress disorder or other trauma- or stressor-related conditions. In this chapter, we review the role of the hippocampus in contextual fear conditioning in rodents and explore recent work implicating hippocampal ensembles in the encoding and retrieval of aversive memories.

The Stress Phenotyping Framework: A multidisciplinary biobehavioral approach for assessing and therapeutically targeting maladaptive stress physiology

Although dysregulated stress biology is becoming increasingly recognized as a key driver of lifelong disparities in chronic disease, we presently have no validated biomarkers of toxic stress physiology; no biological, behavioral, or cognitive treatments specifically focused on normalizing toxic stress processes; and no agreed-upon guidelines for treating stress in the clinic or evaluating the efficacy of interventions that seek to reduce toxic stress and improve human functioning. We address these critical issues by (a) systematically describing key systems and mechanisms that are dysregulated by stress; (b) summarizing indicators, biomarkers, and instruments for assessing stress response systems; and (c) highlighting therapeutic approaches that can be used to normalize stress-related biopsychosocial functioning. We also present a novel multidisciplinary Stress Phenotyping Framework that can bring stress researchers and clinicians one step closer to realizing the goal of using precision medicine-based approaches to prevent and treat stress-associated health problems.

Hippocampal Involvement in Safety Signal Learning Varies With Anxiety Among Healthy Adults

Background

Safety signal learning (SSL), based on conditioned inhibition of fear in the presence of learned safety, can effectively attenuate threat responses in animal models and humans. Difficulty regulating threat responses is a core feature of anxiety disorders, suggesting that SSL may provide a novel mechanism for fear reduction. Cross-species evidence suggests that SSL involves functional connectivity between the anterior hippocampus and the dorsal anterior cingulate cortex. However, the neural mechanisms supporting SSL have not been examined in relation to trait anxiety or while controlling for the effect of novelty.

Methods

Here, we investigated the neural mechanisms involved in SSL and associations with trait anxiety in a sample of 64 healthy (non-clinically anxious) adults (ages 18-30 years; 43 female, 21 male) using physiological, behavioral, and neuroimaging (functional magnetic resonance imaging) data collected during an SSL task.

Results

During SSL, compared with individuals with lower trait anxiety, individuals with higher trait anxiety showed less fear reduction as well as altered hippocampal activation and hippocampal-dorsal anterior cingulate cortex functional connectivity, and lower inferior frontal gyrus and ventrolateral prefrontal cortex activation. Importantly, the findings show that SSL reduces threat responding, across learning and over and above the effect of novelty, and involves hippocampal activation.

Conclusions

These findings provide new insights into the nature of SSL and suggest that there may be meaningful variation in SSL and related neural correlates as a function of trait anxiety, with implications for better understanding fear reduction and optimizing interventions for individuals with anxiety disorders.

Inhibition of fatty acid binding protein-5 in the basolateral amygdala induces anxiolytic effects and accelerates fear memory extinction

RATIONALE

The endocannabinoid (eCB) system critically controls anxiety and fear-related behaviours. Anandamide (AEA), a prominent eCB ligand, is a hydrophobic lipid that requires chaperone proteins such as Fatty Acid Binding Proteins (FABPs) for intracellular transport. Intracellular AEA transport is necessary for degradation, so blocking FABP activity increases AEA neurotransmission.

OBJECTIVE

To investigate the effects of a novel FABP5 inhibitor (SBFI-103) in the basolateral amygdala (BLA) on anxiety and fear memory.

METHODS

We infused SBFI-103 (0.5 μg-5 μg) to the BLA of adult male Sprague Dawley rats and ran various anxiety and fear memory behavioural assays, neurophysiological recordings, and localized molecular signaling analyses. We also co-infused SBFI-103 with the AEA inhibitor, LEI-401 (3 μg and 10 μg) to investigate the potential role of AEA in these phenomena.

RESULTS

Acute intra-BLA administration of SBFI-103 produced strong anxiolytic effects across multiple behavioural tests. Furthermore, animals exhibited acute and long-term accelerated associative fear memory extinction following intra-BLA FABP5 inhibition. In addition, BLA FABP5 inhibition induced strong modulatory effects on putative PFC pyramidal neurons along with significantly increased gamma oscillation power. Finally, we observed local BLA changes in the phosphorylation activity of various anxiety- and fear memory-related molecular biomarkers in the PI3K/Akt and MAPK/Erk signaling pathways. At all three levels of analyses, we found the functional effects of SBFI-103 depend on availability of the AEA ligand.

CONCLUSIONS

These findings demonstrate a novel intra-BLA FABP5 signaling mechanism regulating anxiety and fear memory behaviours, neuronal activity states, local anxiety-related molecular pathways, and functional AEA modulation.

Context-induced renewal of passive but not active coping behaviours in the shock-probe defensive burying task

Renewal is the return of extinguished responding after removal from the extinction context. Renewal has been extensively studied using classical aversive conditioning procedures that measure a passive freezing response to an aversive conditioned stimulus. However, coping responses to aversive stimuli are complex and can be reflected in passive and active behaviours. Using the shock-probe defensive burying task, we investigated whether different coping responses are susceptible to renewal. During conditioning, male, Long-Evans rats were placed into a specific context (Context A) where an electrified shock-probe delivered a 3 mA shock upon contact. During extinction, the shock-probe was unarmed in either the same (Context A) or a different context (Context B). Renewal of conditioned responses was assessed in the conditioning context (ABA) or in a novel context (ABC or AAB). Renewal of passive coping responses, indicated by an increased latency and a decreased duration of shock-probe contacts, was observed in all groups. However, renewal of passive coping, measured by increased time spent on the side of the chamber opposite the shock-probe, was only found in the ABA group. Renewal of active coping responses linked to defensive burying was not observed in any group. The present findings highlight the presence of multiple psychological processes underlying even basic forms of aversive conditioning and demonstrate the importance of assessing a broader set of behaviours to tease apart these different underlying mechanisms. The current findings suggest that passive coping responses may be more reliable indicators for assessing renewal than active coping behaviours associated with defensive burying.

Brain activation during fear extinction recall in unmedicated patients with obsessive-compulsive disorder

Specific brain activation patterns during fear conditioning and the recall of previously extinguished fear responses have been associated with obsessive-compulsive disorder (OCD). However, further replication studies are necessary. We measured skin-conductance response and blood oxygenation level-dependent responses in unmedicated adult patients with OCD (n = 27) and healthy participants (n = 22) submitted to a two-day fear-conditioning experiment comprising fear conditioning, extinction (day 1) and extinction recall (day 2). During conditioning, groups differed regarding the skin conductance reactivity to the aversive stimulus (shock) and regarding the activation of the right opercular cortex, insular cortex, putamen, and lingual gyrus in response to conditioned stimuli. During extinction recall, patients with OCD had higher responses to stimuli and smaller differences between responses to conditioned and neutral stimuli. For the entire sample, the higher the response delta between conditioned and neutral stimuli, the greater the dACC activation for the same contrast during early extinction recall. While activation of the dACC predicted the average difference between responses to stimuli for the entire sample, groups did not differ regarding the activation of the dACC during extinction recall. Larger unmedicated samples might be necessary to replicate the previous findings reported in patients with OCD.

Emotional dissociations in temporal associations: opposing effects of arousal on memory for details surrounding unpleasant events

Research targeting emotion's impact on relational episodic memory has largely focused on spatial aspects, but less is known about emotion's impact on memory for an event's temporal associations. The present research investigated this topic. Participants viewed a series of interspersed negative and neutral images with instructions to create stories linking successive images. Later, participants performed a surprise memory test, which measured temporal associations between pairs of consecutive pictures where one picture was negative and one was neutral. Analyses focused on how the order of negative and neutral images during encoding influenced retrieval accuracy. Converging results from a discovery study (N = 72) and pre-registered replication study (N = 150) revealed a "forward-favouring" effect of emotion in temporal memory encoding: Participants encoded associations between negative stimuli and subsequent neutral stimuli more strongly than associations between negative stimuli and preceding neutral stimuli. This finding may reflect a novel trade-off regarding emotion's effects on memory and is relevant for understanding affective disorders, as key clinical symptoms can be conceptualised as maladaptive memory retrieval of temporal details.

The Neurotransmission Basis of Post-Traumatic Stress Disorders by the Fear Conditioning Paradigm

Fear conditioning constitutes the best and most reproducible paradigm to study the neurobiological mechanisms underlying emotions. On the other hand, studies on the synaptic plasticity phenomena underlying fear conditioning present neural circuits enforcing this learning pattern related to post-traumatic stress disorder (PTSD). Notably, in both humans and the rodent model, fear conditioning and context rely on dependent neurocircuitry in the amygdala and prefrontal cortex, cingulate gyrus, and hippocampus. In this review, an overview of the role that classical neurotransmitters play in the contextual conditioning model of fear, and therefore in PTSD, was reported.

Optogenetic activation of dopamine D1 receptors in island cells of medial entorhinal cortex inhibits temporal association learning

A critical feature of episodic memory formation is to associate temporally segregated events as an episode, called temporal association learning. The medial entorhinal cortical-hippocampal (EC-HPC) networks is essential for temporal association learning. We have previously demonstrated that pyramidal cells in the medial EC (MEC) layer III project to the hippocampal CA1 pyramidal cells and are necessary for trace fear conditioning (TFC), which is an associative learning between tone and aversive shock with the temporal gap. On the other hand, Island cells in MECII, project to GABAergic neurons in hippocampal CA1, suppress the MECIII input into the CA1 pyramidal cells through the feed-forward inhibition, and inhibit TFC. However, it remains unknown about how Island cells activity is regulated during TFC. In this study, we report that dopamine D1 receptor is preferentially expressed in Island cells in the MEC. Optogenetic activation of dopamine D1 receptors in Island cells facilitate the Island cell activity and inhibited hippocampal CA1 pyramidal cell activity during TFC. The optogenetic activation caused the impairment of TFC memory recall without affecting contextual fear memory recall. These results suggest that dopamine D1 receptor in Island cells have a crucial role for the regulation of temporal association learning.

Behavioral and genetic architecture of fear conditioning and related phenotypes

Contextual fear conditioning is a form of Pavlovian learning during which an organism learns to fear previously neutral stimuli following their close temporal presentation with an aversive stimulus. In mouse models, freezing behavior is typically used to quantify learned fear. This dependent variable is the sum of multiple processes, including associative/configural learning, fear and anxiety, and general activity. To explore phenotypic constructs underlying contextual fear conditioning and correlated behaviors, as well as factors that may contribute to individual differences in learning and mental health, we tested BXD recombinant inbred strains previously found to show extreme contextual fear conditioning phenotypes and BXD parental strains, C57BL/6J and DBA/2J, in a series of tests including locomotor, anxiety, contextual/cued fear conditioning and non-associative hippocampus-dependent learning behaviors. Hippocampal expression of two previously identified candidate genes for contextual fear conditioning was also quantified. Behavioral and gene expression data were analyzed using exploratory factor analysis (EFA), which suggested five unique constructs representing activity/anxiety/exploration, associative fear learning, anxiety, post-shock freezing, and open field activity phenotypes. Associative fear learning and expression of one candidate gene, Hacd4, clusteredas a construct withinthefactor analysis. Post-shock freezingduring fear conditioning and expression of candidate gene Ptprd emerged as another unique construct, highlighting theindependenceof freezing after footshock from other fear conditioning variables in the current dataset.EFA results additionally suggest shared phenotypic variance in adaptive murine behaviors related to anxiety, general activity, and exploration. These findings inform understanding of fear learning and underlying biological mechanisms that may interact to produce individual differences in fear- and learning-related behaviors in mice.

Time-course analysis of frontal gene expression profiles in the rat model of posttraumatic stress disorder and a comparison with the conditioned fear model

Posttraumatic stress disorder (PTSD) is a complex disorder that involves physiological, emotional, and cognitive dysregulation that may occur after exposure to a life-threatening event. In contrast with the condition of learned fear with resilience to extinction, abnormal fear with impaired fear extinction and exaggeration are considered crucial factors for the pathological development of PTSD. The prefrontal cortex (mPFC) is considered a critical region of top-down control in fear regulation, which involves the modulation of fear expression and extinction. The pathological course of PTSD is usually chronic and persistent; a number of studies have indicated temporal progression in gene expression and phenotypes may be involved in PTSD pathology. In the current study, we use a well-established modified single-prolonged stress (SPS&FS) rat model to feature PTSD-like phenotypes and compared it with a footshock fear conditioning model (FS model); we collected the frontal tissue after extreme stress exposure or fear conditioning and extracted RNA for transcriptome-level gene sequencing. We compared the genetic profiling of the mPFC at early (<2 h after solely FS or SPS&FS exposure) and late (7 days after solely FS or SPS&FS exposure) stages in these two models. First, we identified temporal differences in the expressional patterns between these two models and found pathways such as protein synthesis factor eukaryotic initiation factor 2 (EIF2), transcription factor NF-E2-related factor 2 (NRF2)-mediated oxidative stress response, and acute phase responding signaling enriched in the early stage in both models with significant p-values. Furthermore, in the late stage, the sirtuin signaling pathway was enriched in both models; other pathways such as STAT3, cAMP, lipid metabolism, Gα signaling, and increased fear were especially enriched in the late stage of the SPS&FS model. However, pathways such as VDR/RXR, GP6, and PPAR signaling were activated significantly in the FS model's late stage. Last, the network analysis revealed the temporal dynamics of psychological disorder, the endocrine system, and also genes related to increased fear in the two models. This study could help elucidate the genetic temporal alteration and stage-specific pathways in these two models, as well as a better understanding of the transcriptome-level differences between them.