A temporal shift in the circuits mediating retrieval of fear memory

The role of carbonic anhydrases in extinction of contextual fear memory

Carbonic anhydrases (CAs; EC 4.2.1.1) are metalloenzymes present in mammals with 16 isoforms that differ in terms of catalytic activity as well as cellular and tissue distribution. CAs catalyze the conversion of CO₂ to bicarbonate and protons and are involved in various physiological processes, including learning and memory. Here we report that the integrity of CA activity in the brain is necessary for the consolidation of fear extinction memory. We found that systemic administration of acetazolamide, a CA inhibitor, immediately after the extinction session dose-dependently impaired the consolidation of fear extinction memory of rats trained in contextual fear conditioning. d-phenylalanine, a CA activator, displayed an opposite action, whereas C18, a membrane-impermeable CA inhibitor that is unable to reach the brain tissue, had no effect. Simultaneous administration of acetazolamide fully prevented the procognitive effects of d-phenylalanine. Whereas d-phenylalanine potentiated extinction, acetazolamide impaired extinction also when infused locally into the ventromedial prefrontal cortex, basolateral amygdala, or hippocampal CA1 region. No effects were observed when acetazolamide or d-phenylalanine was infused locally into the substantia nigra pars compacta. Moreover, systemic administration of acetazolamide immediately after the extinction training session modulated c-Fos expression on a retention test in the ventromedial prefrontal cortex of rats trained in contextual fear conditioning. These findings reveal that the engagement of CAs in some brain regions is essential for providing the brain with the resilience necessary to ensure the consolidation of extinction of emotionally salient events.

Excitation of prefrontal cortical neurons during conditioning enhances fear memory formation

Animals can remember a situation associated with an aversive event. Contextual fear memory is initially encoded and consolidated in the hippocampus and gradually consolidated in multiple brain regions over time, including the medial prefrontal cortex (PFC). However, it is not fully understood how PFC neurons contribute to contextual fear memory formation during learning. In the present study, neuronal activity was increased in PFC neurons utilizing the pharmacogenetic hM3Dq-system in male mice. We show that fear expression and memory formation are enhanced by increasing neuronal activity in PFC during conditioning phase. Previous studies showed that the activation of hM3Dq receptor in a subset of amygdala neurons enhanced fear memory formation and biased which neurons are allocated to a memory trace, in which immediate early gene c-fos was preferentially expressed following memory retrieval in these pre-activated neurons. In this study, hM3Dq activation in PFC could not change the probability of c-fos expression in pre-activated neurons flowing memory retrieval. Instead, the number c-fos positive neurons following memory retrieval was significantly increased in the basolateral amygdala. Our results suggest that neuronal activity in PFC at the time of learning modulates fear memory formation and downstream cellular activity at an early phase.

nNOS-expressing neurons in the vmPFC transform pPVT-derived chronic pain signals into anxiety behaviors

Anxiety is common in patients suffering from chronic pain. Here, we report anxiety-like behaviors in mouse models of chronic pain and reveal that nNOS-expressing neurons in ventromedial prefrontal cortex (vmPFC) are essential for pain-induced anxiety but not algesia, using optogenetic and chemogenetic strategies. Additionally, we determined that excitatory projections from the posterior subregion of paraventricular thalamic nucleus (pPVT) provide a neuronal input that drives the activation of vmPFC nNOS-expressing neurons in our chronic pain models. Our results suggest that the pain signal becomes an anxiety signal after activation of vmPFC nNOS-expressing neurons, which causes subsequent release of nitric oxide (NO). Finally, we show that the downstream molecular mechanisms of NO likely involve enhanced glutamate transmission in vmPFC CaMKIIα-expressing neurons through S-nitrosylation-induced AMPAR trafficking. Overall, our data suggest that pPVT excitatory neurons drive chronic pain-induced anxiety through activation of vmPFC nNOS-expressing neurons, resulting in NO-mediated AMPAR trafficking in vmPFC pyramidal neurons.

Chronic pain usually induces anxiety. Here, the authors report that vmPFC nNOS-expressing neurons are activated by excitatory inputs from pPVT during chronic pain and subsequently induce anxiety-like behaviors in mice through promoting AMPAR trafficking.

Principles of operation of a cerebellar learning circuit

We provide behavioral evidence using monkey smooth pursuit eye movements for four principles of cerebellar learning. Using a circuit-level model of the cerebellum, we link behavioral data to learning's neural implementation. The four principles are: (1) early, fast, acquisition driven by climbing fiber inputs to the cerebellar cortex, with poor retention; (2) learned responses of Purkinje cells guide transfer of learning from the cerebellar cortex to the deep cerebellar nucleus, with excellent retention; (3) functionally different neural signals are subject to learning in the cerebellar cortex versus the deep cerebellar nuclei; and (4) negative feedback from the cerebellum to the inferior olive reduces the magnitude of the teaching signal in climbing fibers and limits learning. Our circuit-level model, based on these four principles, explains behavioral data obtained by strategically manipulating the signals responsible for acquisition and recall of direction learning in smooth pursuit eye movements across multiple timescales.

Associative and plastic thalamic signaling to the lateral amygdala controls fear behavior

Decades of research support the idea that associations between a conditioned stimulus (CS) and an unconditioned stimulus (US) are encoded in the lateral amygdala (LA) during fear learning. However, direct proof for the sources of CS and US information is lacking. Definitive evidence of the LA as the primary site for cue association is also missing. Here, we show that calretinin (Calr)-expressing neurons of the lateral thalamus (Calr⁺LT neurons) convey the association of fast CS (tone) and US (foot shock) signals upstream from the LA in mice. Calr⁺LT input shapes a short-latency sensory-evoked activation pattern of the amygdala via both feedforward excitation and inhibition. Optogenetic silencing of Calr⁺LT input to the LA prevents auditory fear conditioning. Notably, fear conditioning drives plasticity in Calr⁺LT neurons, which is required for appropriate cue and contextual fear memory retrieval. Collectively, our results demonstrate that Calr⁺LT neurons provide integrated CS-US representations to the LA that support the formation of aversive memories.

Off-Target Influences of Arch-Mediated Axon Terminal Inhibition on Network Activity and Behavior

Archaerhodopsin (ArchT)-mediated photoinhibition of axon terminals is commonly used to test the involvement of specific long-range neural projections in behavior. Although sustained activation of this opsin in axon terminals has the unintended consequence of enhancing spontaneous vesicle release, it is unclear whether this desynchronized signaling is consequential for ArchT’s behavioral effects. Here, we compare axon terminal and cell body photoinhibition of nucleus accumbens (NAc) afferents to test the utility of these approaches for uncovering pathway-specific contributions of neural circuits to behavior. First, in brain slice recordings we confirmed that ArchT photoinhibition of glutamatergic axons reduces evoked synaptic currents and increases spontaneous transmitter release. A further consequence was increased interneuron activity, which served to broadly suppress glutamate input via presynaptic GABA_B receptors. In vivo, axon terminal photoinhibition increased feeding and reward-seeking behavior irrespective of the afferent pathway targeted. These behavioral effects are comparable to those obtained with broad inhibition of NAc neurons. In contrast, cell body inhibition of excitatory NAc afferents revealed a pathway-specific contribution of thalamic input to feeding behavior and amygdala input to reward-seeking under extinction conditions. These findings underscore the off-target behavioral consequences of ArchT-mediated axon terminal inhibition while highlighting cell body inhibition as a valuable alternative for pathway-specific optogenetic silencing.

Prefrontal cortex projections to the nucleus reuniens suppress freezing following two-way signaled avoidance training

Signaled active avoidance (SAA) behavior requires the suppression of defensive reactions, such as freezing, that conflict with the avoidance response. The neural mechanisms of this inhibitory process are not well understood. Here, we demonstrate that ventromedial prefrontal cortex projections to the nucleus reuniens of the thalamus are recruited following SAA training to suppress freezing in rats. This projection may serve as a crucial common pathway for the inhibition of innate defensive reactions that interfere with proactive behavior, thus facilitating adaptive coping.

Space, Time, and Fear: Survival Computations along Defensive Circuits

Naturalistic observations show that decisions to avoid or escape predators occur at different spatiotemporal scales and that they are supported by different computations and neural circuits. At their extremes, proximal threats are addressed by a limited repertoire of reflexive and myopic actions, reflecting reduced decision and state spaces and model-free (MF) architectures. Conversely, distal threats allow increased information processing supported by model-based (MB) operations, including affective prospection, replay, and planning. However, MF and MB computations are often intertwined, and under conditions of safety the foundations for future effective reactive execution can be laid through MB instruction of MF control. Together, these computations are associated with distinct population codes embedded within a distributed defensive circuitry whose goal is to determine and realize the best policy.

Two genetically, anatomically and functionally distinct cell types segregate across anteroposterior axis of paraventricular thalamus

Unlike the sensory thalamus, studies on the functional organization of the midline and intralaminar nuclei are scarce, and this has hindered the establishment of conceptual models of the function of this brain region. We investigated the functional organization of the paraventricular nucleus of the thalamus (PVT), a midline thalamic structure that is increasingly being recognized as a critical node in the control of diverse processes such as arousal, stress, emotional memory and motivation, in mice. We identify two major classes of PVT neurons-termed type I and type II-that differ in terms of gene expression, anatomy and function. In addition, we demonstrate that type II neurons belong to a previously neglected class of PVT neurons that convey arousal-related information to corticothalamic neurons of the infralimbic cortex. Our results uncover the existence of an arousal-modulated thalamo-corticothalamic loop that links the PVT and the ventromedial prefrontal cortex.

Stress peptides sensitize fear circuitry to promote passive coping

Survival relies on optimizing behavioral responses through experience. Animals often react to acute stress by switching to passive behavioral responses when coping with environmental challenge. Despite recent advances in dissecting mammalian circuitry for Pavlovian fear, the neuronal basis underlying this form of non-Pavlovian anxiety-related behavioral plasticity remains poorly understood. Here, we report that aversive experience recruits the posterior paraventricular thalamus (PVT) and corticotropin-releasing hormone (CRH) and sensitizes a Pavlovian fear circuit to promote passive responding. Site-specific lesions and optogenetic manipulations reveal that PVT-to-central amygdala (CE) projections activate anxiogenic neuronal populations in the CE that release local CRH in response to acute stress. CRH potentiates basolateral (BLA)-CE connectivity and antagonizes inhibitory gating of CE output, a mechanism linked to Pavlovian fear, to facilitate the switch from active to passive behavior. Thus, PVT-amygdala fear circuitry uses inhibitory gating in the CE as a shared dynamic motif, but relies on different cellular mechanisms (postsynaptic long-term potentiation vs. presynaptic facilitation), to multiplex active/passive response bias in Pavlovian and non-Pavlovian behavioral plasticity. These results establish a framework promoting stress-induced passive responding, which might contribute to passive emotional coping seen in human fear- and anxiety-related disorders.

Prefrontal somatostatin interneurons encode fear memory

Theories stipulate that memories are encoded within networks of cortical projection neurons. Conversely, GABAergic interneurons are thought to function primarily to inhibit projection neurons and thereby impose network gain control, an important but purely modulatory role. Here we show in male mice that associative fear learning potentiates synaptic transmission and cue-specific activity of medial prefrontal cortex somatostatin (SST) interneurons and that activation of these cells controls both memory encoding and expression. Furthermore, the synaptic organization of SST and parvalbumin interneurons provides a potential circuit basis for SST interneuron-evoked disinhibition of medial prefrontal cortex output neurons and recruitment of remote brain regions associated with defensive behavior. These data suggest that, rather than constrain mnemonic processing, potentiation of SST interneuron activity represents an important causal mechanism for conditioned fear.

Making translation work: Harmonizing cross-species methodology in the behavioural neuroscience of Pavlovian fear conditioning

Translational neuroscience bridges insights from specific mechanisms in rodents to complex functions in humans and is key to advance our general understanding of central nervous function. A prime example of translational research is the study of cross-species mechanisms that underlie responding to learned threats, by employing Pavlovian fear conditioning protocols in rodents and humans. Hitherto, evidence for (and critique of) these cross-species comparisons in fear conditioning research was based on theoretical viewpoints. Here, we provide a perspective to substantiate these theoretical concepts with empirical considerations of cross-species methodology. This meta-research perspective is expected to foster cross-species comparability and reproducibility to ultimately facilitate successful transfer of results from basic science into clinical applications.

Neural dynamics of emotion and cognition: From trajectories to underlying neural geometry

How can we study, characterize, and understand the neural underpinnings of cognitive-emotional behaviors as inherently dynamic processes? In the past 50 years, Stephen Grossberg has developed a research program that embraces the themes of dynamics, decentralized computation, emergence, selection and competition, and autonomy. The present paper discusses how these principles can be heeded by experimental scientists to advance the understanding of the brain basis of behavior. It is suggested that a profitable way forward is to focus on investigating the dynamic multivariate structure of brain data. Accordingly, central research problems involve characterizing "neural trajectories" and the associated geometry of the underlying "neural space." Finally, it is argued that, at a time when the development of neurotechniques has reached a fever pitch, neuroscience needs to redirect its focus and invest comparable energy in the conceptual and theoretical dimensions of its research endeavor. Otherwise we run the risk of being able to measure "every atom" in the brain in a theoretical vacuum.

µ-opioid receptor-mediated downregulation of midline thalamic pathways to basal and central amygdala

Brain µ-opioid receptors (MOR) mediate reward and help coping with pain, social rejection, anxiety and depression. The dorsal midline thalamus (dMT) integrates visceral/emotional signals and biases behavior towards aversive or defensive states through projections to the amygdala. While a dense MOR expression in the dMT has been described, the exact cellular and synaptic mechanisms of µ-opioidergic modulation in the dMT-amygdala circuitry remain unresolved. Here, we hypothesized that MORs are important negative modulators of dMT-amygdala excitatory networks. Using retrograde tracers and targeted channelrhodopsin expression in combination with patch-clamp electrophysiology, we found that projections of dMT neurons onto both basal amygdala principal neurons (BA PN) and central amygdala (CeL) neurons are attenuated by stimulation of somatic or synaptic MORs. Importantly, dMT efferents to the amygdala drive feedforward excitation of centromedial amygdala neurons (CeM), which is dampened by MOR activation. This downregulation of excitatory activity in dMT-amygdala networks puts the µ-opioid system in a position to ameliorate aversive or defensive behavioral states associated with stress, withdrawal, physical pain or social rejection.

The rostroventral part of the thalamic reticular nucleus modulates fear extinction

The thalamus has been implicated in fear extinction, yet the role of the thalamic reticular nucleus (TRN) in this process remains unclear. Here, in mice, we show that the rostroventral part of the TRN (TRNrv) is critically involved in the extinction of tone-dependent fear memory. Optogenetic excitation of TRNrv neurons during extinction learning dramatically facilitated, whereas the inhibition disrupted, the fear extinction. Single unit recordings demonstrated that TRNrv neurons selectively respond to conditioned stimuli but not to neutral stimuli. TRNrv neurons suppressed the spiking activity of the medial part of the dorsal midline thalamus (dMTm), and a blockade of this inhibitory pathway disrupted fear extinction. Finally, we found that the suppression of dMTm projections to the central amygdala promotes fear extinction, and TRNrv neurons have direct connections to this pathway. Our results uncover a previously unknown function of the TRN and delineate the neural circuit for thalamic control of fear memory.

The precise role of the thalamic reticular nucleus in fear is not understood. Here, the authors report that the rostroventral part of the reticular nucleus is involved in the extinction of tone conditioned fear memory through its inhibitory projections to the dorsal midline thalamus.

Single prolonged stress alters neural activation in the periacqueductal gray and midline thalamic nuclei during emotional learning and memory

Clinical and preclinical studies that have examined the neurobiology of persistent fear memory in posttraumatic stress disorder (PTSD) have focused on the medial prefrontal cortex, hippocampus, and amygdala. Sensory systems, the periaqueductal gray (PAG), and midline thalamic nuclei have been implicated in fear and extinction memory, but whether neural activity in these substrates is sensitive to traumatic stress (at baseline or during emotional learning and memory) remains unexplored. To address this, we used the single prolonged stress (SPS) model of traumatic stress. SPS and control rats were either subjected to fear conditioning (CS-fear) or presented with CSs alone (CS-only) during fear conditioning. All rats were then subjected to extinction training and testing. A subset of rats were euthanized after each behavioral stage and c-Fos and c-Jun used to measure neural activation in all substrates. SPS lowered c-Jun levels in the dorsomedial and lateral PAG at baseline, but the elevated c-Jun expression in the PAG during emotional learning and memory. SPS also altered c-Fos expression during fear and extinction learning/memory in midline thalamic nuclei. These findings suggest changes in neural function in the PAG and midline thalamic nuclei could contribute to persistent fear memory induced by traumatic stress. Interestingly, SPS effects were also observed in animals that never learned fear or extinction (i.e., CS-only). This raises the possibility that traumatic stress could have broader effects on the psychological function that are dependent on the PAG and midline thalamic nuclei.

Detection of a High-Turnover Serotonin Circuit in the Mouse Brain Using Mass Spectrometry Imaging

Monoamine neurotransmitters are released by specialized neurons regulating behavioral, motor, and cognitive functions. Although the localization of monoaminergic neurons in the brain is well known, the distribution and kinetics of monoamines remain unclear. Here, we generated a murine brain atlas of serotonin (5-HT), dopamine (DA), and norepinephrine (NE) levels using mass spectrometry imaging (MSI). We found several nuclei rich in both 5-HT and a catecholamine (DA or NE) and identified the paraventricular nucleus of the thalamus (PVT), where 5-HT and NE are co-localized. The analysis of 5-HT fluctuations in response to acute tryptophan depletion and infusion of isotope-labeled tryptophan in vivo revealed a close kinetic association between the raphe nuclei, PVT, and amygdala but not the other nuclei. Our findings imply the existence of a highly dynamic 5-HT-mediated raphe to PVT pathway that likely plays a role in the brain monoamine system.

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A murine brain atlas of monoamine (5-HT, DA, NE) levels was generated via MS imaging

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We identified several nuclei rich in both 5-HT and a catecholamine (DA or NE)

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The paraventricular nucleus of the thalamus (PVT) had high levels of 5-HT and NE

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The level of 5-HT in raphe to PVT pathway changed dynamically in response to blood Trp level

The paraventricular thalamus is a critical mediator of top-down control of cue-motivated behavior in rats

Cues in the environment can elicit complex emotional states, and thereby maladaptive behavior, as a function of their ascribed value. Here we capture individual variation in the propensity to attribute motivational value to reward-cues using the sign-tracker/goal-tracker animal model. Goal-trackers attribute predictive value to reward-cues, and sign-trackers attribute both predictive and incentive value. Using chemogenetics and microdialysis, we show that, in sign-trackers, stimulation of the neuronal pathway from the prelimbic cortex (PrL) to the paraventricular nucleus of the thalamus (PVT) decreases the incentive value of a reward-cue. In contrast, in goal-trackers, inhibition of the PrL-PVT pathway increases both the incentive value and dopamine levels in the nucleus accumbens shell. The PrL-PVT pathway, therefore, exerts top-down control over the dopamine-dependent process of incentive salience attribution. These results highlight PrL-PVT pathway as a potential target for treating psychopathologies associated with the attribution of excessive incentive value to reward-cues, including addiction.

Differential Alterations in Cortico-Amygdala Circuitry in Mice with Impaired Fear Extinction

129S1/SvImJ (S1) mice exhibit selective impairments in fear extinction, though the mechanisms underlying these impairments are not fully understood. The medial prefrontal cortex (mPFC) consists of the prelimbic cortex (PL) and infralimbic cortex (IL), which are known to be involved in fear conditioning and extinction, respectively. The PL and IL project to the basolateral amygdala (BLA) that also plays an important role in both mechanisms. In the present study, we utilized optogenetic and electrophysiological approaches to measure inhibitory/excitatory ratios (I/E ratios) in mPFC-BLA circuits of S1 and control C57BL/6 (B6) mice following fear conditioning and extinction. As suggested previously, PL inputs to the BLA became more excitatory after fear conditioning in B6 mice. S1 mice also exhibited strengthened PL-BLA circuit following fear conditioning. Interestingly, fear extinction restored PL-BLA circuit strength to levels comparable to the baseline in B6 mice. However, PL-BLA circuit strength remained abnormally high even after extinction in S1 mice. The IL-BLA circuit became more inhibitory in B6 mice after fear extinction, whereas extinction failed to change the excitability of the IL-BLA circuit in S1 mice. These data suggest that the fear extinction impairments observed in S1 mice may be due to constantly decreased I/E balance in the PL-BLA circuit and lack of changes in I/E balance in the IL-BLA circuit. This further suggests that investigation of both pathways is instrumental in developing more effective therapeutics for psychopathologies that involve impairments in fear extinction, such as chronic pain and posttraumatic stress disorder.

Paraventricular Thalamus Projection Neurons Integrate Cortical and Hypothalamic Signals for Cue-Reward Processing

The paraventricular thalamus (PVT) is an interface for brain reward circuits, with input signals arising from structures, such as prefrontal cortex and hypothalamus, that are broadcast to downstream limbic targets. However, the precise synaptic connectivity, activity, and function of PVT circuitry for reward processing are unclear. Here, using in vivo two-photon calcium imaging, we find that PVT neurons projecting to the nucleus accumbens (PVT-NAc) develop inhibitory responses to reward-predictive cues coding for both cue-reward associative information and behavior. The multiplexed activity in PVT-NAc neurons is directed by opposing activity patterns in prefrontal and lateral hypothalamic afferent axons. Further, we find that prefrontal cue encoding may maintain accurate cue-reward processing, as optogenetic disruption of this encoding induced long-lasting effects on downstream PVT-NAc cue responses and behavioral cue discrimination. Together, these data reveal that PVT-NAc neurons act as an interface for reward processing by integrating relevant inputs to accurately inform reward-seeking behavior.

Pyridostigmine bromide and stress interact to impact immune function, cholinergic neurochemistry and behavior in a rat model of Gulf War Illness

Gulf War Illness (GWI) is characterized by a constellation of symptoms that includes cognitive dysfunction. While the causes for GWI remain unknown, prophylactic use of the acetylcholinesterase inhibitor pyridostigmine bromide (PB) in combination with the stress of deployment has been proposed to be among the causes of the cognitive dysfunction in GWI. Mechanistically, clinical studies suggest that altered immune function may be an underlying factor in the neurochemical and neurobehavioral complications of GWI. Accordingly, the goal of this study was to determine how responses to an immune challenge (lipopolysaccharide; LPS) or stress impacts inflammation, acetylcholine (ACh) neurochemistry and behavior in an experimental model of GWI. Rats with a history of PB treatment exhibited potentiated increases in C-reactive protein levels in response to a submaximal LPS challenge compared to control rats, indicating that prior treatment with this cholinesterase inhibitor leads to exacerbated inflammatory responses to a subsequent immune challenge. ACh responses to LPS administration were decreased in the hippocampus, but not prefrontal cortex (PFC), in rats with a prior history of PB treatment or stress exposure. Additionally, ACh release in response to acute immobilization stress was attenuated in the PFC and hippocampus in these groups. These attenuated cholinergic responses were accompanied by impairments in contextual and cue-based fear learning. The results of this study suggest that stress and LPS challenges adversely affect central ACh neurochemistry in a rodent model of GWI and support the hypothesis that dysregulated immune responses are mechanistically linked to the neurological complications of GWI.

Sex differences in fear extinction

Despite the exponential increase in fear research during the last years, few studies have included female subjects in their design. The need to include females arises from the knowledge gap of mechanistic processes underlying the behavioral and neural differences observed in fear extinction. Moreover, the exact contribution of sex and hormones in relation to learning and behavior is still largely unknown. Insights from this field could be beneficial as fear-related disorders are twice as prevalent in women compared to men. Here, we review an up-to-date summary of animal and human studies in adulthood that report sex differences in fear extinction from a structural and functional approach. Furthermore, we describe how these factors could contribute to the observed sex differences in fear extinction during normal and pathological conditions.

Nucleus reuniens mediates the extinction of contextual fear conditioning

Learning and remembering the context in which events occur requires interactions between the hippocampus (HPC) and medial prefrontal cortex (mPFC). The nucleus reuniens (RE) is a ventral midline thalamic nucleus that coordinates activity in the mPFC and HPC and is involved in spatial and contextual memory. We recently found that the RE is critical for contextual fear conditioning in rats, a form of learning that involves interactions between the HPC and mPFC. Here we examined whether the RE mediates the extinction of contextual fear. After contextual fear conditioning, rats underwent an extinction procedure in which they were merely exposed to the conditioning context; freezing behavior during the extinction procedure and during a retrieval test 24 h later served as an index of conditioned fear. Muscimol inactivation of the RE prior to extinction impaired the acquisition of both short- and long-term extinction memories. Similarly, inactivation of the RE prior to the extinction retrieval test also impaired the expression of extinction; this effect was not state-dependent. Taken together, these results reveal that the extinction of contextual fear memories requires the RE, which is consistent with a broader role for the RE in forms of learning that require HPC-mPFC interactions.

Prefrontal cortex neuronal ensembles encoding fear drive fear expression during long-term memory retrieval

The prefrontal cortex is an important regulator of fear expression in humans and rodents. Specifically, the rodent prelimbic (PL) prefrontal cortex drives fear expression during both encoding and retrieval of fear memory. Neuronal ensembles have been proposed to function as memory encoding units, and their re-activation is thought to be necessary for memory retrieval and expression of conditioned behavior. However, it remains unclear whether PL cortex neuronal ensembles that encode fear memory contribute to long-term fear expression during memory retrieval. To address this, we employed a viral-mediated TRAP (Targeted Recombination in Active Population) technology to target PL cortex ensembles active during fear conditioning and expressed the inhibitory Gi-DREADD in fear-encoding ensembles. Male and female rats were trained to lever press for food and subjected to Pavlovian delay fear conditioning, then 28 days later, they underwent a fear memory retrieval test. Chemogenetic inhibition of TRAPed PL cortex ensembles reduced conditioned suppression of food seeking in females, but not males. Neither context nor tone freezing behavior was altered by this manipulation during the same retrieval test. Thus, fear-encoding ensembles in PL cortex drive long-term fear expression in a sex and fear modality dependent manner.

Occasion setting

Occasion setting refers to the ability of 1 stimulus, an occasion setter, to modulate the efficacy of the association between another, conditioned stimulus (CS) and an unconditioned stimulus (US) or reinforcer. Occasion setters and simple CSs are readily distinguished. For example, occasion setters are relatively immune to extinction and counterconditioning, and their combination and transfer functions differ substantially from those of simple CSs. Similarly, the acquisition of occasion setting is favored when stimuli are separated by longer intervals, by empty trace intervals, and are of different modalities, whereas the opposite conditions typically favor the acquisition of simple associations. Furthermore, the simple conditioning and occasion setting properties of a single stimulus can be independent, for example, that stimulus may simultaneously predict the occurrence of a reinforcer and indicate that another stimulus will not be reinforced. Many behavioral phenomena that are intractable to simple associative analysis are better understood within an occasion setting framework. Besides capturing the distinction between direct and modulatory control common to many arenas in neuroscience, occasion setting provides a model for the hierarchical organization of memory for events and event relations, and for contextual control more broadly. Although early lesion studies further differentiated between occasion setting and simple conditioning functions, little is known about the neurobiology of occasion setting. Modern techniques for precise manipulation and monitoring of neuronal activity in multiple brain regions are ideally suited for disentangling contributions of simple conditioning and occasion setting in associative learning. (PsycINFO Database Record (c) 2019 APA, all rights reserved).

Variability in response to severe stress: highly reactive rats exhibit changes in fear and anxiety-like behavior related to distinct neuronal co-activation patterns

There is an important individual variability in development of posttraumatic stress disorder (PTSD) and this feature needs to be better addressed in preclinical studies. Previously we showed that only rats that explored the context before a foot shock (delayed shock group) exhibited long-lasting behavioral changes. In this study the delayed shock group was segregated using the freezing response upon re-exposure to the shock-paired context and we investigated whether higher reactivity would be related to behavioral alterations and to activation of brain regions using Fos immunoreactivity. The latter allowed the analysis of co-activity patterns among brain regions within each group, by creating connectivity maps. High responder rats (HR) displayed heightened freezing response upon context re-exposure, anxiety-like behavior, impaired exploratory behavior and fear sensitization. Fos analysis showed that HR displayed a negative correlation between the medial prefrontal cortex and the ventral hippocampus (vHPC) after the first context re-exposure. After the second context re-exposure, HR displayed reduced Fos expression in vHPC CA1 area, whereas low responders (LR) showed increased Fos in the paraventricular nucleus of the thalamus. Pearson correlation analyses revealed positive associations between freezing and Fos in the dorsal the periaqueductal gray and vHPC after exposure to unfamiliar acoustic stimulus in a novel environment. Thus, assessment of individual variability allowed the identification of a subset of reactive animals that displayed behavioral modifications relevant to PTSD. Fos correlation and network analyses revealed co-activity patterns in HR rats that may point out to associations of brain areas relevant to the behavioral outcomes.

Embracing Complexity in Defensive Networks

The neural basis of defensive behaviors continues to attract much interest, not only because they are important for survival but also because their dysregulation may be at the origin of anxiety disorders. Recently, a dominant approach in the field has been the optogenetic manipulation of specific circuits or cell types within these circuits to dissect their role in different defensive behaviors. While the usefulness of optogenetics is unquestionable, we argue that this method, as currently applied, fosters an atomistic conceptualization of defensive behaviors, which hinders progress in understanding the integrated responses of nervous systems to threats. Instead, we advocate for a holistic approach to the problem, including observational study of natural behaviors and their neuronal correlates at multiple sites, coupled to the use of optogenetics, not to globally turn on or off neurons of interest, but to manipulate specific activity patterns hypothesized to regulate defensive behaviors.

Individual variability in the recall of fear extinction is associated with phosphorylation of mitogen-activated protein kinase in the infralimbic cortex

RATIONALE

Although most individuals will be exposed to trauma at some point, only a small portion of individuals develops posttraumatic stress disorder (PTSD), suggesting there are factors which render some individuals particularly susceptible to the development of this disorder. One cardinal feature of PTSD is the failure to extinguish fear responses to cues that once signaled danger. Rodent studies of fear learning and extinction have provided insight into the neural mechanisms underlying extinction; however, most of these studies have focused on mechanisms involved in typical responses and fewer have identified mechanisms that distinguish animals that extinguish well versus those that do not extinguish their fear responses. Investigation of individual differences in fear extinction might help us better understand the susceptibility to and development of PTSD.

OBJECTIVES

In order to understand the neural mechanisms underlying such variation, we assessed phosphorylated mitogen-activated protein kinase (P-MAPK) levels in infralimbic cortex (IL), basolateral amygdala (BLA), and dorsal hippocampus in subsets of rats which exhibited good or poor recall of extinction.

RESULTS

We found a relationship between extinction recall and P-MAPK in the IL such that rats which had good extinction recall had higher levels of P-MAPK than those which had poor extinction recall. We also found that rats which had good extinction recall had higher levels of P-MAPK in the dorsal hippocampus than control rats.

CONCLUSIONS

Our findings suggest that individual differences in the recall of extinction learning can be explained by altered cell signaling in the IL.

Anterior Cingulate Cortex and Ventral Hippocampal Inputs to the Basolateral Amygdala Selectively Control Generalized Fear

A common symptom of anxiety disorders is the overgeneralization of fear across a broad range of contextual cues. We previously found that the ACC and ventral hippocampus (vHPC) regulate generalized fear. Here, we investigate the functional projections from the ACC and vHPC to the amygdala and their role in governing generalized fear in a preclinical rodent model. A chemogenetic approach (designer receptor exclusively activated by designer drugs) was used to inhibit glutamatergic projections from the ACC or vHPC that terminate within the BLA at recent (1 d) or remote (28 d) time points after contextually fear conditioning male mice. Inactivating ACC or vHPC projections to the BLA significantly reduced generalized fear to a novel, nonthreatening context but had no effect on fear to the training context. Further, our data indicate that the ACC-BLA circuit supports generalization in a time-independent manner. We also identified, for the first time, a strictly time-dependent role of the vHPC-BLA circuit in supporting remote generalized contextual fear. Dysfunctional signaling to the amygdala from the ACC or the HPC could underlie overgeneralized fear responses that are associated with anxiety disorders. Our findings demonstrate that the ACC and vHPC regulate fear expressed in novel, nonthreatening environments via projections to the BLA but do so as a result of training intensity or time, respectively.SIGNIFICANCE STATEMENT Anxiety disorders are characterized by a common symptom that promotes overgeneralization of fear in nonthreatening environments. Dysregulation of the amygdala, ACC, or hippocampus (HPC) has been hypothesized to contribute to increased fear associated with anxiety disorders. Our findings show that the ACC and HPC projections to the BLA regulate generalized fear in nonthreatening, environments. However, descending ACC projections control fear generalization independent of time, whereas HPC projections play a strictly time-dependent role in regulating generalized fear. Thus, dysfunctional ACC/HPC signaling to the BLA may be a predominant underlying mechanism of nonspecific fear associated with anxiety disorders. Our data have important implications for predictions made by theories about aging memories and interactions between the HPC and cortical regions.

Reactivation of Recall-Induced Neurons in the Infralimbic Cortex and the Basolateral Amygdala After Remote Fear Memory Attenuation

Whether the attenuation of traumatic memories is mediated through the suppression of the original memory trace of fear by a new memory trace of safety, or through an updating of the original fear trace towards safety has been a long-standing question at the interface of neuroscience and psychology. This matter is of particular importance for remote fear memories as they lie at the core of stress- and anxiety-related disorders. Recently, we have found that in the dentate gyrus, the effective attenuation of remote fear memories is accompanied by a reactivation of memory recall-induced neurons and that the continued activity of these neurons is critical for fear reduction. However, whether this also applies to other brain areas implicated in the storage of remote fear memories remains to be determined. Here, we show-by cellular compartment analysis of temporal activity using fluorescence in situ hybridization-that such reactivation also occurs in the basolateral amygdala and the infralimbic cortex, two brain areas known to be involved in fear memory attenuation. These results provide further experimental support for effective traumatic memory attenuation likely being mediated by an updating of the original fear trace towards safety.

Paraventricular Thalamus Controls Behavior during Motivational Conflict

Decision-making often involves motivational conflict because of the competing demands of approach and avoidance for a common resource: behavior. This conflict must be resolved as a necessary precursor for adaptive behavior. Here we show a role for the paraventricular thalamus (PVT) in behavioral control during motivational conflict. We used Pavlovian counterconditioning in male rats to establish a conditioned stimulus (CS) as a signal for reward (or danger) and then transformed the same CS into a signal for danger (or reward). After such training, the CS controls conflicting appetitive and aversive behaviors. To assess PVT involvement in conflict, we injected an adeno-associated virus (AAV) expressing the genetically encoded Ca²⁺ indicator GCaMP and used fiber photometry to record population PVT Ca²⁺ signals. We show distinct profiles of responsivity across the anterior-posterior axis of PVT during conflict, including an ordinal relationship between posterior PVT CS responses and behavior strength. To study the causal role of PVT in behavioral control during conflict, we injected AAV expressing the inhibitory hM4Di DREADD and determined the effects of chemogenetic PVT inhibition on behavior. We show that chemogenetic inhibition across the anterior-posterior axis of the PVT, but not anterior or posterior PVT alone, disrupts arbitration between appetitive and aversive behaviors when they are in conflict but has no effect when these behaviors are assessed in isolation. Together, our findings identify PVT as central to behavioral control during motivational conflict.SIGNIFICANCE STATEMENT Animals, including humans, approach attractive stimuli and avoid aversive ones. However, they frequently face conflict when the demands of approach and avoidance are incompatible. Resolution of this conflict is fundamental to adaptive behavior. Here we show a role for the paraventricular thalamus, a nucleus of the dorsal midline thalamus, in the arbitration of appetitive and aversive behavior during motivational conflict.

New viral-genetic mapping uncovers an enrichment of corticotropin-releasing hormone-expressing neuronal inputs to the nucleus accumbens from stress-related brain regions

Corticotropin-releasing hormone (CRH) is an essential, evolutionarily-conserved stress neuropeptide. In addition to hypothalamus, CRH is expressed in brain regions including amygdala and hippocampus where it plays crucial roles in modulating the function of circuits underlying emotion and cognition. CRH⁺ fibers are found in nucleus accumbens (NAc), where CRH modulates reward/motivation behaviors. CRH actions in NAc may vary by the individual's stress history, suggesting roles for CRH in neuroplasticity and adaptation of the reward circuitry. However, the origin and extent of CRH⁺ inputs to NAc are incompletely understood. We employed viral genetic approaches to map both global and CRH⁺ projection sources to NAc in mice. We injected into NAc variants of a new designer adeno-associated virus that permits robust retrograde access to NAc-afferent projection neurons. Cre-dependent viruses injected into CRH-Cre mice enabled selective mapping of CRH⁺ afferents. We employed anterograde AAV1-directed axonal tracing to verify NAc CRH⁺ fiber projections and established the identity of genetic reporter-labeled cells via validated antisera against native CRH. We quantified the relative contribution of CRH⁺ neurons to total NAc-directed projections. Combined retrograde and anterograde tracing identified the paraventricular nucleus of the thalamus, bed nucleus of stria terminalis, basolateral amygdala, and medial prefrontal cortex as principal sources of CRH⁺ projections to NAc. CRH⁺ NAc afferents were selectively enriched in NAc-projecting brain regions involved in diverse aspects of the sensing, processing and memory of emotionally salient events. These findings suggest multiple, complex potential roles for the molecularly-defined, CRH-dependent circuit in modulation of reward and motivation behaviors.

Systemic Cellular Activation Mapping of an Extinction-Impaired Animal Model

Fear extinction diminishes conditioned fear responses and impaired fear extinction has been reported to be related to anxiety disorders such as post-traumatic stress disorder (PTSD). We and others have reported that 129S1/SvImJ (129S1) strain of mice showed selective impairments in fear extinction following successful auditory or contextual fear conditioning. To investigate brain regions involved in the impaired fear extinction of 129S1 mice, we systemically analyzed c-Fos expression patterns before and after contextual fear conditioning and extinction. After fear conditioning, 129S1 mice showed significantly increased c-Fos expression in the medial division of the central amygdala (CEm), prelimbic (PL) cortex of the medial prefrontal cortex (mPFC), and dorsal CA3 of the hippocampus, compared to that of control C57BL/6 mice. Following fear extinction, 129S1 mice exhibited significantly more c-Fos-positive cells in the CEm, PL, and paraventricular nucleus of the thalamus (PVT) than did C57BL/6 mice. These results reveal the dynamic circuitry involved in different steps of fear memory formation and extinction, thus providing candidate brain regions to study the etiology and pathophysiology underlying impaired fear extinction.

A Disinhibitory Microcircuit Mediates Conditioned Social Fear in the Prefrontal Cortex

Fear behavior is under tight control of the prefrontal cortex, but the underlying microcircuit mechanism remains elusive. In particular, it is unclear how distinct subtypes of inhibitory interneurons (INs) within prefrontal cortex interact and contribute to fear expression. We employed a social fear conditioning paradigm and induced robust social fear in mice. We found that social fear is characterized by activation of dorsal medial prefrontal cortex (dmPFC) and is largely diminished by dmPFC inactivation. With a combination of in vivo electrophysiological recordings and fiber photometry together with cell-type-specific pharmacogenetics, we further demonstrated that somatostatin (SST) INs suppressed parvalbumin (PV) INs and disinhibited pyramidal cells and consequently enhanced dmPFC output to mediate social fear responses. These results reveal a previously unknown disinhibitory microcircuit in prefrontal cortex through interactions between IN subtypes and suggest that SST INs-mediated disinhibition represents an important circuit mechanism in gating social fear behavior.

The paraventricular thalamic nucleus: A key hub of neural circuits underlying drug addiction

Drug addiction is a chronic relapsing brain disease characterized by compulsive, out-of-control drug use and the appearance of negative somatic and emotional consequences when drug access is prevented. The limited efficacy of treatment urges researchers toward a deeper understanding of the neural mechanism of drug addiction. Brain circuits that regulate reward and motivation are considered to be the neural substrate of drug addiction. An increasing body of literature indicates that the paraventricular thalamic nucleus (PVT) could serve as a key node in the neurocircuits that control goal-directed behaviors. In this review, we summarize the anatomical and functional evidence that the PVT regulates drug-related behaviors. The PVT receives extensive inputs from the brainstem and hypothalamus, and is reciprocally connected with the limbic system. Neurons in the PVT are recruited by drug exposure as well as cues and context associated with drug taking. Pathway-specific perturbation studies have begun to decipher the precise role of PVT circuits in drug-related behaviors. We also highlight recent findings about the involvement of neural plasticity of the PVT pathways in drug addiction and provide perspectives on future studies.

Upregulation of Anandamide Hydrolysis in the Basolateral Complex of Amygdala Reduces Fear Memory Expression and Indices of Stress and Anxiety

Increased anandamide (AEA) signaling through inhibition of its catabolic enzyme fatty acid amide hydrolase (FAAH) in the basolateral complex of amygdala (BLA) is thought to buffer against the effects of stress and reduces behavioral signs of anxiety and fear. However, examining the role of AEA signaling in stress, anxiety, and fear through pharmacological depletion has been challenging due to the redundant complexity of its biosynthesis and the lack of a pharmacological synthesis inhibitor. We developed a herpes simplex viral vector to rapidly yet transiently overexpress FAAH specifically within the BLA to assess the impact of suppressing AEA signaling on stress, fear, and anxiety in male rats. Surprisingly, FAAH overexpression in BLA dampened stress-induced corticosterone release, reduced anxiety-like behaviors, and decreased conditioned fear expression. Interestingly, depleting AEA signaling in the BLA did not prevent fear conditioning itself or fear reinstatement. These effects were specific to the overexpression of FAAH because they were reversed by intra-BLA administration of an FAAH inhibitor. Moreover, the fear-suppressive effects of FAAH overexpression were also mitigated by intra-BLA administration of a low dose of a GABAA receptor antagonist, but not an NMDA/AMPA/kainate receptor antagonist, suggesting that they were mediated by an increase in GABAergic neurotransmission. Our data suggest that a permissive AEA tone within the BLA might gate GABA release and that loss of this tone through elevated AEA hydrolysis increases inhibition in the BLA, which in turn reduces stress, anxiety, and fear. These data provide new insights on the mechanisms by which amygdalar endocannabinoid signaling regulates emotional behavior.SIGNIFICANCE STATEMENT Amygdala endocannabinoid signaling is involved in the regulation of stress, anxiety, and fear. Our data indicate that viral-mediated augmentation of anandamide hydrolysis within the basolateral amygdala reduces behavioral indices of stress, anxiety, and conditioned fear expression. These same effects have been previously documented with inhibition of anandamide hydrolysis in the same brain region. Our results indicate that the ability of anandamide signaling to regulate emotional behavior is nonlinear and may involve actions at distinct neuronal populations, which could be influenced by the basal level of anandamide. Modulation of anandamide signaling is a current clinical therapeutic target for stress-related psychiatric illnesses, so these data underscore the importance of fully understanding the mechanisms by which anandamide signaling regulates amygdala-dependent changes in emotionality.

Neuroscience Research and Mentoring in Puerto Rico: What Succeeds in This Environment?

Twenty years ago, I arrived in Puerto Rico from New York City to establish a neuroscience laboratory and research program on extinction of conditioned fear. The lab's first research paper appeared in the Journal of Neuroscience (Quirk et al., 2000) and has been cited >900 times. The success of this project in Puerto Rico far surpassed my original expectations. Therefore, I thought it might be useful to identify the factors responsible for this success, with the hope of facilitating the development of laboratories in diverse settings. A description of our lab practices is interspersed with personal statements from trainees hailing from Puerto Rico and other parts of Latin America. Creating an effective research and training environment depends less on the director's personality and more on the proper practice of activities that foster intellectual growth, such as journal clubs, lab meetings, and philosophy of science retreats. On a personal level, this project has been enormously gratifying. The unique environment in Puerto Rico fostered my best work, and I am very happy to have established my laboratory here.

Temporal evolution of cortical ensembles promoting remote memory retrieval

Memories of fearful events can last a lifetime. The prelimbic (PL) subregion of prefrontal cortex plays a critical role in fear memory retrieval over time. Most studies have focused on acquisition, consolidation, and retrieval of recent memories, but much less is known about the neural mechanisms of remote memory. Using a new knock-in mouse for activity-dependent genetic labeling (TRAP2), we demonstrate that neuronal ensembles in PL are dynamic. PL neurons TRAPed during later memory retrievals are more likely to be reactivated and make larger behavioral contributions to remote memory retrieval compared to those TRAPed during learning or early memory retrieval. PL activity during learning is required to initiate this time-dependent reorganization in PL ensembles underlying memory retrieval. Finally, while neurons TRAPed during earlier and later retrievals have similar broad projections throughout the brain, PL neurons TRAPed later have a stronger functional recruitment of cortical targets.

Data-driven criteria to assess fear remission and phenotypic variability of extinction in rats

Fear conditioning is widely employed to examine the mechanisms that underlie dysregulations of the fear system. Various manipulations are often used following fear acquisition to attenuate fear memories. In rodent studies, freezing is often the main output measure to quantify 'fear'. Here, we developed data-driven criteria for defining a standard benchmark that indicates remission from conditioned fear and for identifying subgroups with differential treatment responses. These analyses will enable a better understanding of individual differences in treatment responding.This article is part of a discussion meeting issue 'Of mice and mental health: facilitating dialogue between basic and clinical neuroscientists'.

The mysteries of remote memory

Long-lasting memories form the basis of our identity as individuals and lie central in shaping future behaviours that guide survival. Surprisingly, however, our current knowledge of how such memories are stored in the brain and retrieved, as well as the dynamics of the circuits involved, remains scarce despite seminal technical and experimental breakthroughs in recent years. Traditionally, it has been proposed that, over time, information initially learnt in the hippocampus is stored in distributed cortical networks. This process-the standard theory of memory consolidation-would stabilize the newly encoded information into a lasting memory, become independent of the hippocampus, and remain essentially unmodifiable throughout the lifetime of the individual. In recent years, several pieces of evidence have started to challenge this view and indicate that long-lasting memories might already ab ovo be encoded, and subsequently stored in distributed cortical networks, akin to the multiple trace theory of memory consolidation. In this review, we summarize these recent findings and attempt to identify the biologically plausible mechanisms based on which a contextual memory becomes remote by integrating different levels of analysis: from neural circuits to cell ensembles across synaptic remodelling and epigenetic modifications. From these studies, remote memory formation and maintenance appear to occur through a multi-trace, dynamic and integrative cellular process ranging from the synapse to the nucleus, and represent an exciting field of research primed to change quickly as new experimental evidence emerges.This article is part of a discussion meeting issue 'Of mice and mental health: facilitating dialogue between basic and clinical neuroscientists'.

Preclinical studies of stress, extinction, and prefrontal cortex: intriguing leads and pressing questions

BACKGROUND

Stress is associated with cognitive and emotional dysfunction, and increases risk for a variety of psychological disorders, including depression and posttraumatic stress disorder. Prefrontal cortex is critical for executive function and emotion regulation, is a target for stress hormones, and is implicated in many stress-influenced psychological disorders. Extinction of conditioned fear provides an excellent model system for examining how stress-induced changes in corticolimbic structure and function are related to stress-induced changes in neural function and behavior, as the neural circuitry underlying this behavior is well characterized.

OBJECTIVES

This review examines how acute and chronic stress influences extinction and describes how stress alters the structure and function of the medial prefrontal cortex, a potential neural substrate for these effects. In addition, we identify important unanswered questions about how stress-induced change in prefrontal cortex may mediate extinction deficits and avenues for future research.

KEY FINDINGS

A substantial body of work demonstrates deficits in extinction after either acute or chronic stress. A separate and substantial literature demonstrates stress-induced neuronal remodeling in medial prefrontal cortex, along with several key neurohormonal contributors to this remodeling, and there is substantial overlap in prefrontal mechanisms underlying extinction and the mechanisms implicated in stress-induced dysfunction of-and neuronal remodeling in-medial prefrontal cortex. However, data directly examining the contribution of changes in prefrontal structure and function to stress-induced extinction deficits is currently lacking.

CONCLUSIONS

Understanding how stress influences extinction and its neural substrates as well as individual differences in this effect will elucidate potential avenues for novel interventions for stress-sensitive disorders characterized by deficits in extinction.

Rodent models of impaired fear extinction

The measurement of Pavlovian forms of fear extinction offers a relatively simple behavioral preparation that is nonetheless tractable, from a translational perspective, as an approach to study mechanisms of exposure therapy and biological underpinnings of anxiety and trauma-related disorders such as post-traumatic stress disorder (PTSD). Deficient fear extinction is considered a robust clinical endophenotype for these disorders and, as such, has particular significance in the current "age of RDoC (research domain criteria)." Various rodent models of impaired extinction have thus been generated with the objective of approximating this clinical, relapse prone aberrant extinction learning. These models have helped to reveal neurobiological correlates of extinction circuitry failure, gene variants, and other mechanisms underlying deficient fear extinction. In addition, they are increasingly serving as tools to investigate ways to therapeutically overcome poor extinction to support long-term retention of extinction memory and thus protection against various forms of fear relapse; modeled in the laboratory by measuring spontaneous recovery, reinstatement and renewal of fear. In the current article, we review models of impaired extinction built around (1) experimentally induced brain region and neural circuit disruptions (2) spontaneously-arising and laboratory-induced genetic modifications, or (3) exposure to environmental insults, including stress, drugs of abuse, and unhealthy diet. Collectively, these models have been instrumental in advancing in our understanding of extinction failure and underlying susceptibilities at the neural, genetic, molecular, and neurochemical levels; generating renewed interest in developing novel, targeted and effective therapeutic treatments for anxiety and trauma-related disorders.

A cFos activation map of remote fear memory attenuation

RATIONALE

The experience of strong traumata leads to the formation of enduring fear memories that may degenerate into post-traumatic stress disorder. One of the most successful treatments for this condition consists of extinction training during which the repeated exposure to trauma-inducing stimuli in a safe environment results in an attenuation of the fearful component of trauma-related memories. While numerous studies have investigated the neural substrates of recent (e.g., 1-day-old) fear memory attenuation, much less is known about the neural networks mediating the attenuation of remote (e.g., 30-day-old) fear memories. Since extinction training becomes less effective when applied long after the original encoding of the traumatic memory, this represents an important gap in memory research.

OBJECTIVES

Here, we aimed to generate a comprehensive map of brain activation upon effective remote fear memory attenuation in the mouse.

METHODS

We developed an efficient extinction training paradigm for 1-month-old contextual fear memory attenuation and performed cFos immunohistochemistry and network connectivity analyses on a set of cortical, amygdalar, thalamic, and hippocampal regions.

RESULTS

Remote fear memory attenuation induced cFos in the prelimbic cortex, the basolateral amygdala, the nucleus reuniens of the thalamus, and the ventral fields of the hippocampal CA1 and CA3. All these structures were equally recruited by remote fear memory recall, but not by the recall of a familiar neutral context.

CONCLUSION

These results suggest that progressive fear attenuation mediated by repetitive exposure is accompanied by sustained neuronal activation and not reverted to a pre-conditioning brain state. These findings contribute to the identification of brain areas as targets for therapeutic approaches against traumatic memories.

Coordination between Prefrontal Cortex Clock Gene Expression and Corticosterone Contributes to Enhanced Conditioned Fear Extinction Recall

Post-traumatic stress disorder (PTSD) is associated with impaired conditioned fear extinction learning, a ventromedial prefrontal cortex (vmPFC)-dependent process. PTSD is also associated with dysregulation of vmPFC, circadian, and glucocorticoid hormone function. Rats have rhythmic clock gene expression in the vmPFC that requires appropriate diurnal circulatory patterns of corticosterone (CORT), suggesting the presence of CORT-entrained intrinsic circadian clock function within the PFC. We examined the role of vmPFC clock gene expression and its interaction with CORT profiles in regulation of auditory conditioned fear extinction learning. Extinction learning and recall were examined in male rats trained and tested either in the night (active phase) or in the day (inactive phase). Using a viral vector strategy, Per1 and Per2 clock gene expression were selectively knocked down within the vmPFC. Circulating CORT profiles were manipulated via adrenalectomy (ADX) ± diurnal and acute CORT replacement. Rats trained and tested during the night exhibited superior conditioned fear extinction recall that was absent in rats that had knock-down of vmPFC clock gene expression. Similarly, the superior nighttime extinction recall was absent in ADX rats, but restored in ADX rats given a combination of a diurnal pattern of CORT and acute elevation of CORT during the postextinction training consolidation period. Thus, conditioned fear extinction learning is regulated in a diurnal fashion that requires normal vmPFC clock gene expression and a combination of circadian and training-associated CORT. Strategic manipulation of these factors may enhance the therapeutic outcome of conditioned fear extinction related treatments in the clinical setting.

Dynamic salience processing in paraventricular thalamus gates associative learning

The salience of behaviorally relevant stimuli is dynamic and influenced by internal state and external environment. Monitoring such changes is critical for effective learning and flexible behavior, but the neuronal substrate for tracking the dynamics of stimulus salience is obscure. We found that neurons in the paraventricular thalamus (PVT) are robustly activated by a variety of behaviorally relevant events, including novel ("unfamiliar") stimuli, reinforcing stimuli and their predicting cues, as well as omission of the expected reward. PVT responses are scaled with stimulus intensity and modulated by changes in homeostatic state or behavioral context. Inhibition of the PVT responses suppresses appetitive or aversive associative learning and reward extinction. Our findings demonstrate that the PVT gates associative learning by providing a dynamic representation of stimulus salience.

Midline thalamic inputs to the amygdala: Ultrastructure and synaptic targets

One of the main subcortical inputs to the basolateral nucleus of the amygdala (BL) originates from a group of dorsal thalamic nuclei located at or near the midline, mainly from the central medial (CMT), and paraventricular (PVT) nuclei. Although similarities among the responsiveness of BL, CMT, and PVT neurons to emotionally arousing stimuli suggest that these thalamic inputs exert a significant influence over BL activity, little is known about the synaptic relationships that mediate these effects. Thus, the present study used Phaseolus vulgaris-leucoagglutinin (PHAL) anterograde tracing and electron microscopy to shed light on the ultrastructural properties and synaptic targets of CMT and PVT axon terminals in the rat BL. Virtually all PHAL-positive CMT and PVT axon terminals formed asymmetric synapses. Although CMT and PVT axon terminals generally contacted dendritic spines, a substantial number ended on dendritic shafts. To determine whether these dendritic shafts belonged to principal or local-circuit cells, calcium/calmodulin-dependent protein kinase II (CAMKIIα) immunoreactivity was used as a selective marker of principal BL neurons. In most cases, dendritic shafts postsynaptic to PHAL-labeled CMT and PVT terminals were immunopositive for CaMKIIα. Overall, these results suggest that CMT and PVT inputs mostly target principal BL neurons such that when CMT or PVT neurons fire, little feed-forward inhibition counters their excitatory influence over principal cells. These results are consistent with the possibility that CMT and PVT inputs constitute major determinants of BL activity.

Dopamine tunes prefrontal outputs to orchestrate aversive processing

Decades of research suggest that the mesocortical dopamine system exerts powerful control over mPFC physiology and function. Indeed, dopamine signaling in the medial prefrontal cortex (mPFC) is implicated in a vast array of processes, including working memory, stimulus discrimination, stress responses, and emotional and behavioral control. Consequently, even slight perturbations within this delicate system result in profound disruptions of mPFC-mediated processes. Many neuropsychiatric disorders are associated with dysregulation of mesocortical dopamine, including schizophrenia, depression, attention deficit hyperactivity disorder, post-traumatic stress disorder, among others. Here, we review the anatomy and functions of the mesocortical dopamine system. In contrast to the canonical role of striatal dopamine in reward-related functions, recent work has revealed that mesocortical dopamine fine-tunes distinct efferent projection populations in a manner that biases subsequent behavior towards responding to stimuli associated with potentially aversive outcomes. We propose a framework wherein dopamine can serve as a signal for switching mPFC states by orchestrating how information is routed to the rest of the brain.

Optogenetic Long-Term Depression Induction in the PVT-CeL Circuitry Mediates Decreased Fear Memory

The dysregulation of fear learning and abnormal activities of cerebral networks may contribute to the etiologies of anxiety disorders. Although it has been proposed that decreased activity in the paraventricular nucleus of the thalamus (PVT) to the lateral central nucleus of amygdala (CeL) pathway could induce an attenuation of learned fear, no study has shown the effect of the direct optogenetic activation of PVT projecting CeL neurons in vivo on unconditioned fear-related behaviors or learned fear expression. The mechanisms that control the neuronal activity of the PVT-CeL pathway involved in anxiety are rare. Here, we found that CeL neurons have varied responses to optogenetic excitation of PVT terminals in the CeL: neurons with relative high excitability(~ 30%), neurons with relative low excitability(~ 60%), and neurons with no excitability (~ 10%). We next explored the role of the PVT-CeL pathway in unconditioned and conditioned fear-related behaviors by using optogenetics and anxiety assays in freely moving mice. We observed that temporally precise optogenetic activation of the CeL-projecting PVT neurons had no effect on unconditioned fear-related behaviors on the elevated plus maze test and the open field test. But optogenetic activation of the CeL-projecting PVT neurons increased conditioned fear expression. We then found that optogenetic long-term depression (LTD) induction in the CeL receiving PVT afferents effectively exerted a persistent attenuation of learned fear. The percentage of neurons with relative high excitability was decreased by the LTD induction, and the percentage of neurons with relative low excitability was increased by the LTD induction. Taking these results together, we identify that increased activity of the PVT-CeL pathway could lead to as excessive learned fear. The CeL neurons with relative high responses to the photo-stimulation of PVT afferents in the CeL may be the key neurons that regulate the output of learned fear expression. Our optogenetic LTD protocol may inspire the development of novel treatments for anxiety disorders involving deep brain stimulation to induce plasticity at relevant brain areas.

Prefrontal projections to the thalamic nucleus reuniens mediate fear extinction

The thalamic nucleus reuniens (RE) receives dense projections from the medial prefrontal cortex (mPFC), interconnects the mPFC and hippocampus, and may serve a pivotal role in regulating emotional learning and memory. Here we show that the RE and its mPFC afferents are critical for the extinction of Pavlovian fear memories in rats. Pharmacological inactivation of the RE during extinction learning or retrieval increases freezing to an extinguished conditioned stimulus (CS); renewal of fear outside the extinction context was unaffected. Suppression of fear in the extinction context is associated with an increase in c-fos expression and spike firing in RE neurons to the extinguished CS. The role for the RE in suppressing extinguished fear requires the mPFC, insofar as pharmacogenetically silencing mPFC to RE projections impairs the expression of extinction memory. These results reveal that mPFC-RE circuits inhibit the expression of fear, a function that is essential for adaptive emotional regulation.

Previous work has shown that the thalamic nucleus reuniens (RE) is involved in memory and emotion. Here the authors report that the RE and its inputs from the medial prefrontal cortex are indispensable for the top-down inhibition of fear memories after extinction.