Amygdala inhibitory circuits and the control of fear memory

The effect of transcranial direct current and magnetic stimulation on fear extinction and return of fear: A meta-analysis and systematic review

BACKGROUND

We conducted a meta-analysis and qualitative review on the randomized controlled trials investigating the effects of transcranial direct current stimulation and transcranial magnetic stimulation on fear extinction and the return of fear in non-primate animals and humans.

METHODS

The meta-analysis was conducted by searching PubMed, Web of science, PsycINFO, and Cochrane Library and extracting fear response in the active and sham groups in the randomized controlled trials. The pooled effect size was quantified by Hedges' g using a three-level meta-analytic model in R.

RESULTS

We identified 18 articles on the tDCS effect and 5 articles on the TMS effect, with 466 animal subjects and 621 human subjects. Our findings show that tDCS of the prefrontal cortex significantly inhibit fear retrieval in animal models (Hedges' g = -0.50). In human studies, TMS targeting the dorsolateral/ventromedial prefrontal cortex has an inhibiting effect on the return of fear (Hedges' g = -0.24).

LIMITATIONS

The limited number of studies and the heterogeneous designs of the selected studies made cross-study and cross-species comparison difficult.

CONCLUSIONS

Our findings shed light on the optimal non-invasive brain stimulation protocols for targeting the neural circuitry of threat extinction in humans.

Deciphering the role of brainstem glycinergic neurons during startle and prepulse inhibition

Prepulse inhibition (PPI) of the auditory startle response, a key measure of sensorimotor gating, diminishes with age and is impaired in various neurological conditions. While PPI deficits are often associated with cognitive impairments, their reversal is routinely used in experimental systems for antipsychotic drug screening. Yet, the cellular and circuit-level mechanisms of PPI remain unclear, even under non-pathological conditions. We recently showed that brainstem neurons located in the caudal pontine reticular nucleus (PnC) expressing the glycine transporter type 2 (GlyT2±) receive inputs from the central nucleus of the amygdala (CeA) and contribute to PPI but via an uncharted pathway. Here, using tract-tracing, immunohistochemistry and in vitro optogenetic manipulations coupled to field electrophysiological recordings, we reveal the neuroanatomical distribution of GlyT2± PnC neurons and PnC-projecting CeA glutamatergic neurons and we provide mechanistic insights on how these glutamatergic inputs suppress auditory neurotransmission in PnC sections. Additionally, in vivo experiments using GlyT2-Cre mice confirm that optogenetic activation of GlyT2± PnC neurons enhances PPI and is sufficient to induce PPI in young mice, emphasizing their role. However, in older mice, PPI decline is not further influenced by inhibiting GlyT2± neurons. This study highlights the importance of GlyT2± PnC neurons in PPI and underscores their diminished activity in age-related PPI decline.

Amygdala neuronal dyshomeostasis via 5-HT receptors mediates mood and cognitive defects in Alzheimer's disease

Behavioral changes or neuropsychiatric symptoms (NPSs) are common features in dementia and are associated with accelerated cognitive impairment and earlier deaths. However, how NPSs are intertwined with cognitive decline remains elusive. In this study, we identify that the basolateral amygdala (BLA) is a key brain region that is associated with mood disorders and memory decline in the AD course. During the process from pre- to post-onset in AD, the dysfunction of parvalbumin (PV) interneurons and pyramidal neurons in the amygdala leads to hyperactivity of pyramidal neurons in the basal state and insensitivity to external stimuli. We further demonstrate that serotonin (5-HT) receptors in distinct neurons synergistically regulate the BLA microcircuit of AD rather than 5-HT levels, in which both restrained inhibitory inputs by excessive 5-HT1AR signaling in PV interneurons and depolarized pyramidal neurons via upregulated 5-HT2AR contribute to aberrant neuronal hyperactivity. Downregulation of these two 5-HT receptors simultaneously enables neurons to resist β-amyloid peptides (Aβ) neurotoxicity and ameliorates the mood and cognitive defects. Therefore, our study reveals a crucial role of 5-HT receptors for regulating neuronal homeostasis in AD pathogenesis, and this would provide early intervention and potential targets for AD cognitive decline.

The impact of the left inferior frontal gyrus on fear extinction: A transcranial direct current stimulation study

INTRODUCTION

Fear extinction is a fundamental component of exposure-based therapies for anxiety-related disorders. The renewal of fear in a different context after extinction highlights the importance of contextual factors. In this study, we aimed to investigate the causal role of the left inferior frontal gyrus (LiFG) in the context-dependency of fear extinction learning via administration of transcranial direct current stimulation (tDCS) over this area.

METHODS

180 healthy subjects were assigned to 9 groups: 3 tDCS conditions (anodal, cathodal, and sham) × 3 context combinations (AAA, ABA, and ABB). The fear conditioning/extinction task was conducted over three consecutive days: acquisition, extinction learning, and extinction recall. tDCS (2 mA, 10min) was administered during the extinction learning phase over the LiFG via a 4-electrode montage. Skin conductance response (SCR) data and self-report assessments were collected.

RESULTS

During the extinction learning phase, groups with excitability-enhancing anodal tDCS showed a significantly higher fear response to the threat cues compared to cathodal and sham stimulation conditions, irrespective of contextual factors. This effect was stable until the extinction recall phase. Additionally, excitability-reducing cathodal tDCS caused a significant decrease of the response difference between the threat and safety cues during the extinction recall phase. The self-report assessments showed no significant differences between the conditions throughout the experiment.

CONCLUSION

Independent of the context, excitability enhancement of the LiFG did impair fear extinction, and led to preservation of fear memory. In contrast, excitability reduction of this area enhanced fear extinction retention. These findings imply that the LiFG plays a role in the fear extinction network, which seems to be however context-independent.

Neuromodulation of inhibitory synaptic transmission in the basolateral amygdala during fear and anxiety

The basolateral amygdala plays pivotal roles in the regulation of fear and anxiety and these processes are profoundly modulated by different neuromodulatory systems that are recruited during emotional arousal. Recent studies suggest activities of BLA interneurons and inhibitory synaptic transmission in BLA principal cells are regulated by neuromodulators to influence the output and oscillatory network states of the BLA, and ultimately the behavioral expression of fear and anxiety. In this review, we first summarize a cellular mechanism of stress-induced anxiogenesis mediated by the interaction of glucocorticoid and endocannabinoid signaling at inhibitory synapses in the BLA. Then we discuss cell type-specific activity patterns induced by neuromodulators converging on the Gq signaling pathway in BLA perisomatic parvalbumin-expressing (PV) and cholecystokinin-expressing (CCK) basket cells and their effects on BLA network oscillations and fear learning.

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.

Minimizing Variability in Developmental Fear Studies in Mice: Toward Improved Replicability in the Field

In rodents, the first weeks of postnatal life feature remarkable changes in fear memory acquisition, retention, extinction, and discrimination. Early development is also marked by profound changes in brain circuits underlying fear memory processing, with heightened sensitivity to environmental influences and stress, providing a powerful model to study the intersection between brain structure, function, and the impacts of stress. Nevertheless, difficulties related to breeding and housing young rodents, preweaning manipulations, and potential increased variability within that population pose considerable challenges to developmental fear research. Here we discuss several factors that may promote variability in studies examining fear conditioning in young rodents and provide recommendations to increase replicability. We focus primarily on experimental conditions, design, and analysis of rodent fear data, with an emphasis on mouse studies. The convergence of anatomical, synaptic, physiological, and behavioral changes during early life may increase variability, but careful practice and transparency in reporting may improve rigor and consensus in the field. © 2024 The Authors. Current Protocols published by Wiley Periodicals LLC.

Slitrk4 is required for the development of inhibitory neurons in the fear memory circuit of the lateral amygdala

The Slitrk family consists of six synaptic adhesion molecules, some of which are associated with neuropsychiatric disorders. In this study, we aimed to investigate the physiological role of Slitrk4 by analyzing Slitrk4 knockout (KO) mice. The Slitrk4 protein was widely detected in the brain and was abundant in the olfactory bulb and amygdala. In a systematic behavioral analysis, male Slitrk4 KO mice exhibited an enhanced fear memory acquisition in a cued test for classical fear conditioning, and social behavior deficits in reciprocal social interaction tests. In an electrophysiological analysis using amygdala slices, Slitrk4 KO mice showed enhanced long-term potentiation in the thalamo-amygdala afferents and reduced feedback inhibition. In the molecular marker analysis of Slitrk4 KO brains, the number of calretinin (CR)-positive interneurons was decreased in the anterior part of the lateral amygdala nuclei at the adult stage. In in vitro experiments for neuronal differentiation, Slitrk4-deficient embryonic stem cells were defective in inducing GABAergic interneurons with an altered response to sonic hedgehog signaling activation that was involved in the generation of GABAergic interneuron subsets. These results indicate that Slitrk4 function is related to the development of inhibitory neurons in the fear memory circuit and would contribute to a better understanding of osttraumatic stress disorder, in which an altered expression of Slitrk4 has been reported.

Chronic Administration of Δ9-Tetrahydrocannabinol Alters Brain Glucose Uptake and Improves Waiting Impulsivity in the Rat

Introduction: Δ9-Tetrahydrocannabinol (THC) acts as an agonist at cannabinoid receptors. Its chronic intake affects many behaviors, including cognitive processes. The aims of this study in rats are to assess the chronic effects of THC on impulsivity and on regional brain glucose uptake. Materials and Methods: For the determination of "waiting impulsivity," a total of 20 male Lister Hooded rats were trained to perform a reaction time task, followed by a baseline test of impulsivity and baseline glucose uptake measurements with [18F]-fluoro-2-deoxy-D-glucose and positron emission tomography (PET). Then, 10 rats each received 3 mg/kg THC or vehicle injected intraperitoneally daily for 21 days. Subsequently, a second behavioral test and PET measurements were performed, and blood THC concentrations were determined. Analyses of variance of brain regions of the impulsivity network with the parameter "standardized uptake value" regarding glucose uptake and correlation analyses of the collected parameters were carried out. Discussion: After chronic THC treatment, decreased glucose uptake (p-values <0.05) was found in cingulate cortex, hippocampus, amygdala, thalamus, and cerebellar cortex, as compared with vehicle-treated rats. The number of correct no-go responses (increased waiting time) significantly increased (p<0.05) in THC-treated rats. Furthermore, correct no-go responses correlated positively and strongly with the THC blood concentrations (Spearman's ρ=0.79, p<0.01). Conclusion: These findings reflect a specific reduction in impulsive behavior after chronic THC treatment, showing a functionally relevant influence of THC on "waiting impulsivity" with reduced selective glucose uptake at the same time.

Activation of the CXCR4 Receptor by Chemokine CXCL12 Increases the Excitability of Neurons in the Rat Central Amygdala

Primarily regarded as immune proteins, chemokines are emerging as a family of molecules serving neuromodulatory functions in the developing and adult brain. Among them, CXCL12 is constitutively and widely expressed in the CNS, where it was shown to act on cellular, synaptic, network, and behavioral levels. Its receptor, CXCR4, is abundant in the amygdala, a brain structure involved in pathophysiology of anxiety disorders. Dysregulation of CXCL12/CXCR4 signaling has been implicated in anxiety-related behaviors. Here we demonstrate that exogenous CXCL12 at 2 nM but not at 5 nM increased neuronal excitability in the lateral division of the rat central amygdala (CeL) which was evident in the Late-Firing but not Regular-Spiking neurons. These effects were blocked by AMD3100, a CXCR4 antagonist. Moreover, CXCL12 increased the excitability of the neurons of the basolateral amygdala (BLA) that is known to project to the CeL. However, CXCL12 increased neither the spontaneous excitatory nor spontaneous inhibitory synaptic transmission in the CeL. In summary, the data reveal specific activation of Late-Firing CeL cells along with BLA neurons by CXCL12 and suggest that this chemokine may alter information processing by the amygdala that likely contributes to anxiety and fear conditioning.

Functional neuroanatomy of basal forebrain projections to the basolateral amygdala: Transmitters, receptors, and neuronal subpopulations

The projections of the basal forebrain (BF) to the hippocampus and neocortex have been extensively studied and shown to be important for higher cognitive functions, including attention, learning, and memory. Much less is known about the BF projections to the basolateral nuclear complex of the amygdala (BNC), although the cholinergic innervation of this region by the BF is actually far more robust than that of cortical areas. This review will focus on light and electron microscopic tract-tracing and immunohistochemical (IHC) studies, many of which were published in the last decade, that have analyzed the relationship of BF inputs and their receptors to specific neuronal subtypes in the BNC in order to better understand the anatomical substrates of BF-BNC circuitry. The results indicate that BF inputs to the BNC mainly target the basolateral nucleus of the BNC (BL) and arise from cholinergic, GABAergic, and perhaps glutamatergic BF neurons. Cholinergic inputs mainly target dendrites and spines of pyramidal neurons (PNs) that express muscarinic receptors (MRs). MRs are also expressed by cholinergic axons, as well as cortical and thalamic axons that synapse with PN dendrites and spines. BF GABAergic axons to the BL also express MRs and mainly target BL interneurons that contain parvalbumin. It is suggested that BF-BL circuitry could be very important for generating rhythmic oscillations known to be critical for emotional learning. BF cholinergic inputs to the BNC might also contribute to memory formation by activating M1 receptors located on PN dendritic shafts and spines that also express NMDA receptors.

Amygdala electrical stimulation for operant conditioning in rat navigation

There have been several attempts to navigate the locomotion of animals by neuromodulation. The most common method is animal training with electrical brain stimulation for directional cues and rewards; the basic principle is to activate dopamine-mediated neural reward pathways such as the medial forebrain bundle (MFB) when the animal correctly follows the external commands. In this study, the amygdala, which is the brain region responsible for fear modulation, was targeted for punishment training. The brain regions of MFB, amygdala, and barrel cortex were electrically stimulated for reward, punishment, and directional cues, respectively. Electrical stimulation was applied to the amygdala of rats when they failed to follow directional commands. First, two different amygdala regions, i.e., basolateral amygdala (BLA) and central amygdala (CeA), were stimulated and compared in terms of behavior responses, success and correction rates for training, and gene expression for learning and memory. Then, the training was performed in three groups: group R (MFB stimulation for reward), group P (BLA stimulation for punishment), and group RP (both MFB and BLA stimulation for reward and punishment). In group P, after the training, RNA sequencing was conducted to detect gene expression and demonstrate the effect of punishment learning. Group P showed higher success rates than group R, and group RP exhibited the most effective locomotion control among the three groups. Gene expression results imply that BLA stimulation can be more effective as a punishment in the learning process than CeA stimulation. We developed a new method to navigate rat locomotion behaviors by applying amygdala stimulation.

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.

A combinatory genetic strategy for targeting neurogliaform neurons in the mouse basolateral amygdala

The mouse basolateral amygdala (BLA) contains various GABAergic interneuron subpopulations, which have distinctive roles in the neuronal microcircuit controlling numerous behavioral functions. In mice, roughly 15% of the BLA GABAergic interneurons express neuropeptide Y (NPY), a reasonably characteristic marker for neurogliaform cells (NGFCs) in cortical-like brain structures. However, genetically labeled putative NPY-expressing interneurons in the BLA yield a mixture of interneuron subtypes besides NGFCs. Thus, selective molecular markers are lacking for genetically accessing NGFCs in the BLA. Here, we validated the NGFC-specific labeling with a molecular marker, neuron-derived neurotrophic factor (NDNF), in the mouse BLA, as such specificity has been demonstrated in the neocortex and hippocampus. We characterized genetically defined NDNF-expressing (NDNF+) GABAergic interneurons in the mouse BLA by combining the Ndnf-IRES2-dgCre-D transgenic mouse line with viral labeling, immunohistochemical staining, and in vitro electrophysiology. We found that BLA NDNF+ GABAergic cells mainly expressed NGFC neurochemical markers NPY and reelin (Reln) and exhibited small round soma and dense axonal arborization. Whole-cell patch clamp recordings indicated that most NDNF+ interneurons showed late spiking and moderate firing adaptation. Moreover, ∼81% of BLA NDNF+ cells generated retroaxonal action potential after current injections or optogenetic stimulations, frequently developing into persistent barrage firing. Optogenetic activation of the BLA NDNF+ cell population yielded both GABAA- and GABAB receptor-mediated currents onto BLA pyramidal neurons (PNs). We demonstrate a combinatory strategy combining the NDNF-cre mouse line with viral transfection to specifically target adult mouse BLA NGFCs and further explore their functional and behavioral roles.

Projection-Targeted Photopharmacology Reveals Distinct Anxiolytic Roles for Presynaptic mGluR2 in Prefrontal- and Insula-Amygdala Synapses

Dissecting how membrane receptors regulate neural circuit function is critical for deciphering basic principles of neuromodulation and mechanisms of therapeutic drug action. Classical pharmacological and genetic approaches are not well-equipped to untangle the roles of specific receptor populations, especially in long-range projections which coordinate communication between brain regions. Here we use viral tracing, electrophysiological, optogenetic, and photopharmacological approaches to determine how presynaptic metabotropic glutamate receptor 2 (mGluR2) activation in the basolateral amygdala (BLA) alters anxiety-related behavior. We find that mGluR2-expressing neurons from the ventromedial prefrontal cortex (vmPFC) and posterior insular cortex (pIC) preferentially target distinct cell types and subregions of the BLA to regulate different forms of avoidant behavior. Using projection-specific photopharmacological activation, we find that mGluR2-mediated presynaptic inhibition of vmPFC-BLA, but not pIC-BLA, connections can produce long-lasting decreases in spatial avoidance. In contrast, presynaptic inhibition of pIC-BLA connections decreased social avoidance, novelty-induced hypophagia, and increased exploratory behavior without impairing working memory, establishing this projection as a novel target for the treatment of anxiety disorders. Overall, this work reveals new aspects of BLA neuromodulation with therapeutic implications while establishing a powerful approach for optical mapping of drug action via photopharmacology.

Mechanisms of alcohol influence on fear conditioning: a computational model

A connection between stress-related illnesses and alcohol use disorders is extensively documented. Fear conditioning is a standard procedure used to study stress learning and links it to the activation of amygdala circuitry. However, the connection between the changes in amygdala circuit and function induced by alcohol and fear conditioning is not well established. We introduce a computational model to test the mechanistic relationship between amygdala functional and circuit adaptations during fear conditioning and the impact of acute vs. repeated alcohol exposure. In accordance with experiments, both acute and prior repeated alcohol decreases speed and robustness of fear extinction in our simulations. The model predicts that, first, the delay in fear extinction in alcohol is mostly induced by greater activation of the basolateral amygdala (BLA) after fear acquisition due to alcohol-induced modulation of synaptic weights. Second, both acute and prior repeated alcohol shifts the amygdala network away from the robust extinction regime by inhibiting the activity in the central amygdala (CeA). Third, our model predicts that fear memories formed in acute or after chronic alcohol are more connected to the context. Thus, the model suggests how circuit changes induced by alcohol may affect fear behaviors and provides a framework for investigating the involvement of multiple neuromodulators in this neuroadaptive process.

Single-nucleus genomics in outbred rats with divergent cocaine addiction-like behaviors reveals changes in amygdala GABAergic inhibition

The amygdala processes positive and negative valence and contributes to addiction, but the cell-type-specific gene regulatory programs involved are unknown. We generated an atlas of single-nucleus gene expression and chromatin accessibility in the amygdala of outbred rats with high and low cocaine addiction-like behaviors following prolonged abstinence. Differentially expressed genes between the high and low groups were enriched for energy metabolism across cell types. Rats with high addiction index (AI) showed increased relapse-like behaviors and GABAergic transmission in the amygdala. Both phenotypes were reversed by pharmacological inhibition of the glyoxalase 1 enzyme, which metabolizes methylglyoxal-a GABAA receptor agonist produced by glycolysis. Differences in chromatin accessibility between high and low AI rats implicated pioneer transcription factors in the basic helix-loop-helix, FOX, SOX and activator protein 1 families. We observed opposite regulation of chromatin accessibility across many cell types. Most notably, excitatory neurons had greater accessibility in high AI rats and inhibitory neurons had greater accessibility in low AI rats.

DNMT1 Mediates Chronic Pain-Related Depression by Inhibiting GABAergic Neuronal Activation in the Central Amygdala

BACKGROUND

Chronic pain can induce depressive emotion. DNA methyltransferases (DNMTs) have been shown to be involved in the development of chronic pain and depression. However, the role and mechanism of DNMTs in chronic pain-induced depression are not well understood.

METHODS

In well-established spared nerve injury (SNI)-induced chronic pain-related depression models, the expression of DNMTs and the functional roles and underlying mechanisms of DNMT1 in central amygdala (CeA) GABAergic (gamma-aminobutyric acidergic) neurons were investigated using molecular, pharmacological, electrophysiological, optogenetic, and chemogenetic techniques and behavioral tests.

RESULTS

DNMT1, but not DNMT3a or DNMT3b, was upregulated in the CeA of rats with SNI-induced chronic pain-depression. Inhibition of DNMT1 by 5-Aza or viral knockdown of DNMT1 in GABAergic neurons in the CeA effectively ameliorated the depression-like behaviors induced by chronic pain. The DNMT1 action was associated with methylation at the CpG-rich Gad1 promoter and GAD67 downregulation, leading to a decrease of GABAergic neuronal activity. Optogenetic activation of GABAergic neurons in the CeA improved SNI-induced depression-like behaviors. Moreover, optogenetic or chemogenetic inhibition of GABAergic neurons in the CeA reversed DNMT1 knockdown-induced improvement of depression-like behaviors in SNI mice.

CONCLUSIONS

Our findings suggest that DNMT1 is involved in the development of chronic pain-related depression by epigenetic repression of GAD67, leading to the inhibition of GABAergic neuronal activation. This study indicates that DNMT1 could be a potential target for the treatment of chronic pain-related depression.

A dual-pathway architecture enables chronic stress to promote habit formation

Chronic stress can change how we learn and, thus, how we make decisions by promoting the formation of inflexible, potentially maladaptive, habits. Here we investigated the neuronal circuit mechanisms that enable this. Using a multifaceted approach in male and female mice, we reveal a dual pathway, amygdala-striatal, neuronal circuit architecture by which a recent history of chronic stress shapes learning to disrupt flexible goal-directed behavior in favor of inflexible habits. Chronic stress inhibits activity of basolateral amygdala projections to the dorsomedial striatum to impede the action-outcome learning that supports flexible, goal-directed decisions. Stress also increases activity in direct central amygdala projections to the dorsomedial striatum to promote the formation of rigid, inflexible habits. Thus, stress exerts opposing effects on two amygdala-striatal pathways to promote premature habit formation. These data provide neuronal circuit insights into how chronic stress shapes learning and decision making, and help understand how stress can lead to the disrupted decision making and pathological habits that characterize substance use disorders and other psychiatric conditions.

Synaptic and transcriptomic features of cortical and amygdala pyramidal neurons predict inefficient fear extinction

Fear-related disorders arise from inefficient fear extinction and have immeasurable social and economic costs. Here, we characterize mouse phenotypes that spontaneously show fear-independent behavioral traits predicting adaptive or maladaptive fear extinction. We find that, already before fear conditioning, specific morphological, electrophysiological, and transcriptomic patterns of cortical and amygdala pyramidal neurons predispose to fear-related disorders. Finally, by using an optogenetic approach, we show the possibility to rescue inefficient fear extinction by activating infralimbic pyramidal neurons and to impair fear extinction by activating prelimbic pyramidal neurons.

Functional neuroanatomy of monoaminergic systems in the basolateral nuclear complex of the amygdala: Neuronal targets, receptors, and circuits

This review discusses neuroanatomical aspects of the three main monoaminergic systems innervating the basolateral nuclear complex (BNC) of the amygdala (serotonergic, noradrenergic, and dopaminergic systems). It mainly focuses on immunohistochemical (IHC) and in situ hybridization (ISH) studies that have analyzed the relationship of specific monoaminergic inputs and their receptors to specific neuronal subtypes in the BNC in order to better understand the anatomical substrates of the monoaminergic modulation of BNC circuitry. First, light and electron microscopic IHC investigations identifying the main BNC neuronal subpopulations and characterizing their local circuitry, including connections with discrete PN compartments and other INs, are reviewed. Then, the relationships of each of the three monoaminergic systems to distinct PN and IN cell types, are examined in detail. For each system, the neuronal targets and their receptor expression are discussed. In addition, pertinent electrophysiological investigations are discussed. The last section of the review compares and contrasts various aspects of each of the three monoaminergic systems. It is concluded that the large number of different receptors, each with a distinct mode of action, expressed by distinct cell types with different connections and functions, should offer innumerable ways to subtlety regulate the activity of the BNC by therapeutic drugs in psychiatric diseases in which there are alterations of BNC monoaminergic modulatory systems, such as in anxiety disorders, depression, and drug addiction. It is suggested that an important area for future studies is to investigate how the three systems interact in concert at the neuronal and neuronal network levels.

Brain-wide neuron quantification toolkit reveals strong sexual dimorphism in the evolution of fear memory

Fear responses are functionally adaptive behaviors that are strengthened as memories. Indeed, detailed knowledge of the neural circuitry modulating fear memory could be the turning point for the comprehension of this emotion and its pathological states. A comprehensive understanding of the circuits mediating memory encoding, consolidation, and retrieval presents the fundamental technological challenge of analyzing activity in the entire brain with single-neuron resolution. In this context, we develop the brain-wide neuron quantification toolkit (BRANT) for mapping whole-brain neuronal activation at micron-scale resolution, combining tissue clearing, high-resolution light-sheet microscopy, and automated image analysis. The robustness and scalability of this method allow us to quantify the evolution of activity patterns across multiple phases of memory in mice. This approach highlights a strong sexual dimorphism in recruited circuits, which has no counterpart in the behavior. The methodology presented here paves the way for a comprehensive characterization of the evolution of fear memory.

Mouse maternal odontogenic infection with Porphyromonas gingivalis induces cognitive decline in offspring

Introduction

Porphyromonas gingivalis (P. gingivalis), a major periodontal pathogen, causes intrauterine infection/inflammation. Offspring exposed to intrauterine infection/inflammation have an increased risk of neurological disorders, regardless of gestational age. However, the relationship between maternal periodontitis and offspring functional/histological changes in the brain has not yet been elucidated.

Methods

In this study, we used a gestational mouse model to investigate the effects of maternal odontogenic infection of P. gingivalis on offspring behavior and brain tissue.

Results

The step-through passive avoidance test showed that the latency of the acquisition trial was significantly shorter in the P. gingivalis group (p < 0.05), but no difference in spontaneous motor/exploratory parameters by open-field test. P. gingivalis was diffusely distributed throughout the brain, especially in the hippocampus. In the hippocampus and amygdala, the numbers of neuron cells and cyclic adenosine monophosphate response element binding protein-positive cells were significantly reduced (p < 0.05), whereas the number of ionized calcium binding adapter protein 1-positive microglia was significantly increased (p < 0.05). In the hippocampus, the number of glial fibrillary acidic protein-positive astrocytes was also significantly increased (p < 0.05).

Discussion

The offspring of P. gingivalis-infected mothers have reduced cognitive function. Neurodegeneration/neuroinflammation in the hippocampus and amygdala may be caused by P. gingivalis infection, which is maternally transmitted. The importance of eliminating maternal P. gingivalis-odontogenic infection before or during gestation in maintenance healthy brain function in offspring should be addressed in near future.

Musical experience prior to traumatic exposure as a resilience factor: a conceptual analysis

Resilience mechanisms can be dynamically triggered throughout the lifecourse by resilience factors in order to prevent individuals from developing stress-related pathologies such as posttraumatic stress disorder (PTSD). Some interventional studies have suggested that listening to music and musical practice after experiencing a traumatic event decrease the intensity of PTSD, but surprisingly, no study to our knowledge has explored musical experience as a potential resilience factor before the potential occurrence of a traumatic event. In the present conceptual analysis, we sought to summarize what is known about the concept of resilience and how musical experience could trigger two key mechanisms altered in PTSD: emotion regulation and cognitive control. Our hypothesis is that the stimulation of these two mechanisms by musical experience during the pre-traumatic period could help protect against the symptoms of emotional dysregulation and intrusions present in PTSD. We then developed a new framework to guide future research aimed at isolating and investigating the protective role of musical experience regarding the development of PTSD in response to trauma. The clinical application of this type of research could be to develop pre-trauma training that promotes emotional regulation and cognitive control, aimed at populations at risk of developing PTSD such as healthcare workers, police officers, and military staffs.

The amygdala-ventral pallidum pathway contributes to a hypodopaminergic state in the ventral tegmental area during protracted abstinence from chronic cocaine

BACKGROUND AND PURPOSE

Incubation of craving, the progressive increase in drug seeking over the first weeks of abstinence, is associated with temporal changes during abstinence in the activity of several structures involved in drug-seeking behaviour. Decreases of dopamine (DA) release and DA neuronal activity (hypodopaminergic state) have been reported in the ventral tegmental area (VTA) during cocaine abstinence, but the mechanisms underlying these neuroadaptations are not well understood. We investigated the potential involvement of a VTA inhibiting circuit (basolateral amygdala [BLA]-ventral pallidum [VP] pathway) in the hypodopaminergic state associated with abstinence from chronic cocaine.

EXPERIMENTAL APPROACH

In a model of cocaine self-administration, we performed in vivo electrophysiological recordings of DA VTA neurons and BLA neurons from anaesthetised rats during early and protracted abstinence and evaluated the involvement of the BLA-VP pathway using a pharmacological approach.

KEY RESULTS

We found significant decreases in VTA DA population activity and significant increases in BLA activity after protracted but not after short-term abstinence from chronic cocaine. The decrease in VTA DA activity was restored by pharmacological inhibition of the activity of either the BLA or the VP, suggesting that these regions exert a negative influence on DA activity.

CONCLUSION AND IMPLICATIONS

Our study sheds new lights on neuroadaptations occurring during incubation of craving leading to relapse. In particular, we describe the involvement of the BLA-VP pathway in cocaine-induced decreases of DA activity in the VTA. This study adds important information about the specific brain network dysfunctions underlying hypodopaminergic activity during abstinence.

Hypocretin role in posttraumatic stress disorder-like behaviors induced by a novel stress protocol in mice

Introduction

Posttraumatic stress disorder (PTSD) is a psychiatric disorder developed in individuals who expose to traumatic events. These patients may experience symptoms, such as recurrent unwanted memory of the traumatic event, avoidance of reminders of the trauma, increased arousal, and cognitive difficulty. The hypocretinergic system originates from the lateral hypothalamic area (LHA) and projects diffusely to the whole brain, and hypocretin may be involved in the features of stress-related disorder, PTSD.

Methods

Our study aimed to investigate the role of basolateral amygdala (BLA) hypocretin signals in the pathophysiology of PTSD-like symptoms induced by the modified multiple-prolonged stress (MPS) protocol. The BLA, a brain region involved in fear-related behaviors, receives the hypocretin projections. In this study, TCS1102, a dual hypocretin receptor antagonist, was used to block the hypocretin signal in BLA.

Results

Our data indicated that the MPS protocol is a potential PTSD-like paradigm in mice. Meanwhile, the blockade of hypocretin signaling in the BLA relieved the MPS-induced fear response, and partially reduced PTSD-like anxiety behaviors performed by the open field test (OFT) and elevated plus maze (EPM) task.

Discussion

Our findings suggest that the hypocretinergic system is a potential therapeutic approach for PTSD treatment. With further research, the hypocretin-based medication can be a candidate for human PTSD treatment.

Chronic unilateral inhibition of GABA synthesis in the amygdala increases specificity of conditioned fear in a discriminative fear conditioning paradigm in rats

Neural activity in the amygdala is critical for fear learning. In anxiety disorder patients, bilateral hyperactivity of the amygdala can be observed. This hyperactivation is often associated with the facilitation of fear learning and/or over-generalization of conditioned fear. In contrast, hypoactivity of the amygdala, e.g. by pharmacological interventions, attenuates or blocks fear learning. To date, little is known about how neural excitability of the amygdala affects specificity or generalization of fear. Therefore, the present study utilized chronic inhibition of GABA synthesis in the amygdala to increase excitability and investigated the effect on the specificity of fear learning. In rats, unilateral cannulas aiming at the amygdala were implanted. The cannulas were connected to subcutaneously implanted osmotic mini pumps that delivered either the GABA synthesis inhibitor L-allylglycine or its inactive enantiomer D-allylglycine. Following one week of chronic GABA synthesis manipulation, the rats were submitted to a discriminative fear conditioning protocol. In addition, anxiety-like behavior in the light-dark box was measured. Our data show that chronic unilateral L-AG infusions into the amygdala improve the specificity of learned fear, support safety learning, and reduce fear generalization and anxiety. This data demonstrates that moderately increased amygdala excitability can be beneficial for the specificity of fear learning and highlights the potential application for therapeutic interventions.

Bimodal modulation of L1 interneuron activity in anterior cingulate cortex during fear conditioning

The anterior cingulate cortex (ACC) plays a crucial role in encoding, consolidating and retrieving memories related to emotionally salient experiences, such as aversive and rewarding events. Various studies have highlighted its importance for fear memory processing, but its circuit mechanisms are still poorly understood. Cortical layer 1 (L1) of the ACC might be a particularly important site of signal integration, since it is a major entry point for long-range inputs, which is tightly controlled by local inhibition. Many L1 interneurons express the ionotropic serotonin receptor 3a (5HT3aR), which has been implicated in post-traumatic stress disorder and in models of anxiety. Hence, unraveling the response dynamics of L1 interneurons and subtypes thereof during fear memory processing may provide important insights into the microcircuit organization regulating this process. Here, using 2-photon laser scanning microscopy of genetically encoded calcium indicators through microprisms in awake mice, we longitudinally monitored over days the activity of L1 interneurons in the ACC in a tone-cued fear conditioning paradigm. We observed that tones elicited responses in a substantial fraction of the imaged neurons, which were significantly modulated in a bidirectional manner after the tone was associated to an aversive stimulus. A subpopulation of these neurons, the neurogliaform cells (NGCs), displayed a net increase in tone-evoked responses following fear conditioning. Together, these results suggest that different subpopulations of L1 interneurons may exert distinct functions in the ACC circuitry regulating fear learning and memory.

Neuromedin B excites central lateral amygdala neurons and reduces cardiovascular output and fear-potentiated startle

Neuromedin B (NMB) and gastrin-releasing peptide (GRP) are the two mammalian analogs in the bombesin peptide family that exert a variety of actions including emotional processing, appetitive behaviors, cognition, and tumor growth. The bombesin-like peptides interact with three receptors: the NMB-preferring bombesin 1 (BB1) receptors, the GRP-preferring bombesin 2 (BB2) receptors and the orphan bombesin 3 (BB3) receptors. Whereas, injection of bombesin into the central amygdala reduces satiety and modulates blood pressure, the underlying cellular and molecular mechanisms have not been determined. As administration of bombesin induces the expression of Fos in the lateral nucleus of the central amygdala (CeL) which expresses BB1 receptors, we probed the effects of NMB on CeL neurons using in vitro and in vivo approaches. We showed that activation of the BB1 receptors increased action potential firing frequency recorded from CeL neurons via inhibition of the inwardly rectifying K+ (Kir) channels. Activities of phospholipase Cβ and protein kinase C were required, whereas intracellular Ca2+ release was unnecessary for BB1 receptor-elicited potentiation of neuronal excitability. Application of NMB directly into the CeA reduced blood pressure and heart rate and significantly reduced fear-potentiated startle. We may provide a cellular and molecular mechanism whereby bombesin-like peptides modulate anxiety and fear responses in the amygdala.

Neurons innervating both the central amygdala and the ventral tegmental area encode different emotional valences

Mammals are frequently exposed to various environmental stimuli, and to determine whether to approach or avoid these stimuli, the brain must assign emotional valence to them. Therefore, it is crucial to investigate the neural circuitry mechanisms involved in the mammalian brain's processing of emotional valence. Although the central amygdala (CeA) and the ventral tegmental area (VTA) individually encode different or even opposing emotional valences, it is unclear whether there are common upstream input neurons that innervate and control both these regions, and it is interesting to know what emotional valences of these common upstream neurons. In this study, we identify three major brain regions containing neurons that project to both the CeA and the VTA, including the posterior bed nucleus of the stria terminalis (pBNST), the pedunculopontine tegmental nucleus (PPTg), and the anterior part of the basomedial amygdala (BMA). We discover that these neural populations encode distinct emotional valences. Activating neurons in the pBNST produces positive valence, enabling mice to overcome their innate avoidance behavior. Conversely, activating neurons in the PPTg produces negative valence and induces anxiety-like behaviors in mice. Neuronal activity in the BMA, on the other hand, does not influence valence processing. Thus, our study has discovered three neural populations that project to both the CeA and the VTA and has revealed the distinct emotional valences these populations encode. These results provide new insights into the neurological mechanisms involved in emotional regulation.

Subgenual and Hippocampal Pathways in Amygdala Are Set to Balance Affect and Context Processing

The amygdala, hippocampus, and subgenual cortex area 25 (A25) are engaged in complex cognitive-emotional processes. Yet pathway interactions from hippocampus and A25 with postsynaptic sites in amygdala remain largely unknown. In rhesus monkeys of both sexes, we studied with neural tracers how pathways from A25 and hippocampus interface with excitatory and inhibitory microcircuits in amygdala at multiple scales. We found that both hippocampus and A25 innervate distinct as well as overlapping sites of the basolateral (BL) amygdalar nucleus. Unique hippocampal pathways heavily innervated the intrinsic paralaminar basolateral nucleus, which is associated with plasticity. In contrast, orbital A25 preferentially innervated another intrinsic network, the intercalated masses, an inhibitory reticulum that gates amygdalar autonomic output and inhibits fear-related behaviors. Finally, using high-resolution confocal and electron microscopy (EM), we found that among inhibitory postsynaptic targets in BL, both hippocampal and A25 pathways preferentially formed synapses with calretinin (CR) neurons, which are known for disinhibition and may enhance excitatory drive in the amygdala. Among other inhibitory postsynaptic sites, A25 pathways innervated the powerful parvalbumin (PV) neurons which may flexibly regulate the gain of neuronal assemblies in the BL that affect the internal state. In contrast, hippocampal pathways innervated calbindin (CB) inhibitory neurons, which modulate specific excitatory inputs for processing context and learning correct associations. Common and unique patterns of innervation in amygdala by hippocampus and A25 have implications for how complex cognitive and emotional processes may be selectively disrupted in psychiatric disorders.SIGNIFICANCE STATEMENT The hippocampus, subgenual A25, and amygdala are associated with learning, memory, and emotions. We found that A25 is poised to affect diverse amygdalar processes, from emotional expression to fear learning by innervating the basal complex and the intrinsic intercalated masses. Hippocampal pathways uniquely interacted with another intrinsic amygdalar nucleus which is associated with plasticity, suggesting flexible processing of signals in context for learning. In the basolateral (BL) amygdala, which has a role in fear learning, both hippocampal and A25 interacted preferentially with disinhibitory neurons, suggesting a boost in excitation. The two pathways diverged in innervating other classes of inhibitory neurons, suggesting circuit specificities that could become perturbed in psychiatric diseases.

Stress deficits in reward behaviour are associated with and replicated by dysregulated amygdala-nucleus accumbens pathway function in mice

Reduced reward interest/learning and reward-to-effort valuation are distinct, common symptoms in neuropsychiatric disorders for which chronic stress is a major aetiological factor. Glutamate neurons in basal amygdala (BA) project to various regions including nucleus accumbens (NAc). The BA-NAc neural pathway is activated by reward and aversion, with many neurons being monovalent. In adult male mice, chronic social stress (CSS) leads to reduced discriminative reward learning (DRL) associated with decreased BA-NAc activity, and to reduced reward-to-effort valuation (REV) associated, in contrast, with increased BA-NAc activity. Chronic tetanus toxin BA-NAc inhibition replicates the CSS-DRL effect and causes a mild REV reduction, whilst chronic DREADDs BA-NAc activation replicates the CSS effect on REV without affecting DRL. This study provides evidence that stress disruption of reward processing involves the BA-NAc neural pathway; the bi-directional effects implicate opposite activity changes in reward (learning) neurons and aversion (effort) neurons in the BA-NAc pathway following chronic stress.

Selective Disruption of Perineuronal Nets in Mice Lacking Crtl1 is Sufficient to Make Fear Memories Susceptible to Erasure

The ability to store, retrieve, and extinguish memories of adverse experiences is an essential skill for animals' survival. The cellular and molecular factors that underlie such processes are only partially known. Using chondroitinase ABC treatment targeting chondroitin sulfate proteoglycans (CSPGs), previous studies showed that the maturation of the extracellular matrix makes fear memory resistant to deletion. Mice lacking the cartilage link protein Crtl1 (Crtl1-KO mice) display normal CSPG levels but impaired CSPG condensation in perineuronal nets (PNNs). Thus, we asked whether the presence of PNNs in the adult brain is responsible for the appearance of persistent fear memories by investigating fear extinction in Crtl1-KO mice. We found that mutant mice displayed fear memory erasure after an extinction protocol as revealed by analysis of freezing and pupil dynamics. Fear memory erasure did not depend on passive loss of retention; moreover, we demonstrated that, after extinction training, conditioned Crtl1-KO mice display no neural activation in the amygdala (Zif268 staining) in comparison to control animals. Taken together, our findings suggest that the aggregation of CSPGs into PNNs regulates the boundaries of the critical period for fear extinction.

Expanding the canon: An inclusive neurobiology of thalamic and subthalamic fear circuits

Appropriate expression of fear in the face of threats in the environment is essential for survival. The sustained expression of fear in the absence of threat signals is a central pathological feature of trauma- and anxiety-related disorders. Our understanding of the neural circuitry that controls fear inhibition coalesces around the amygdala, hippocampus, and prefrontal cortex. By discussing thalamic and sub-thalamic influences on fear-related learning and expression in this review, we suggest a more inclusive neurobiological framework that expands our canonical view of fear. First, we visit how fear-related learning and expression is influenced by the aforementioned canonical brain regions. Next, we review emerging data that shed light on new roles for thalamic and subthalamic nuclei in fear-related learning and expression. Then, we highlight how these neuroanatomical hubs can modulate fear via integration of sensory and salient stimuli, gating information flow and calibrating behavioral responses, as well as maintaining and updating memory representations. Finally, we propose that the presence of this thalamic and sub-thalamic neuroanatomy in parallel with the tripartite prefrontal cortex-amygdala-hippocampus circuit allows for dynamic modulation of information based on interoceptive and exteroceptive signals. This article is part of the Special Issue on "Fear, Anxiety and PTSD".

From circuits to behavior: Amygdala dysfunction in fragile X syndrome

Fragile X syndrome (FXS) is a neurodevelopmental disorder caused by a repeat expansion mutation in the promotor region of the FMR1 gene resulting in transcriptional silencing and loss of function of fragile X messenger ribonucleoprotein 1 protein (FMRP). FMRP has a well-defined role in the early development of the brain. Thus, loss of the FMRP has well-known consequences for normal cellular and synaptic development leading to a variety of neuropsychiatric disorders including an increased prevalence of amygdala-based disorders. Despite our detailed understanding of the pathophysiology of FXS, the precise cellular and circuit-level underpinnings of amygdala-based disorders is incompletely understood. In this review, we discuss the development of the amygdala, the role of neuromodulation in the critical period plasticity, and recent advances in our understanding of how synaptic and circuit-level changes in the basolateral amygdala contribute to the behavioral manifestations seen in FXS.

Infralimbic Projections to the Nucleus Accumbens Shell and Amygdala Regulate the Encoding of Cocaine Extinction Learning

Prior evidence indicates that the infralimbic cortex (IL) mediates the ongoing inhibition of cocaine seeking following self-administration and extinction training in rats, specifically through projections to the nucleus accumbens shell (NAshell). Our own data indicate that IL activity immediately following an unreinforced lever press is critical for encoding the extinction contingencies in such procedures. Whether extinction encoding requires activity in the IL exclusively or also activity in its outputs, such as those to the NAshell and amygdala, is unknown. To address this issue, we used a closed-loop optogenetic approach in female and male Sprague Dawley rats to silence IL-NAshell or IL-amygdala activity following an unreinforced lever press during extinction training. Optical illumination (20 s) was given either immediately after a lever press or following a 20 s delay. IL-NAshell inhibition immediately following an unreinforced lever press increased lever pressing during extinction training and impaired retention of extinction learning, as assessed during subsequent extinction sessions without optical inhibition. Likewise, IL-amygdala inhibition given in the same manner impaired extinction retention during sessions without inhibition. Control experiments indicate that critical encoding of extinction learning does not require activity in these pathways beyond the initial 20 s post-lever press period, as delayed IL-NAshell and IL-amygdala inhibition had no effect on extinction learning. These results suggest that a larger network extending from the IL to the NAshell and amygdala is involved in encoding extinction contingencies following cocaine self-administration.SIGNIFICANCE STATEMENT Infralimbic cortex (IL) activity following an unreinforced lever press during extinction learning encodes the extinction of cocaine-seeking behavior. However, the larger circuitry controlling such encoding has not been investigated. Using closed-loop optogenetic pathway targeting, we found that inhibition of IL projections to the nucleus accumbens shell and to the amygdala impaired the extinction of cocaine seeking. Importantly, these effects were only observed when activity was disrupted during the first 20 s post-lever press and not when given following a 20 s delay. These findings suggest that successful cocaine extinction encoding requires activity across a larger circuit beyond simply inputs to the IL.

Second-order fear conditioning involves formation of competing stimulus-danger and stimulus-safety associations

How do animals process experiences that provide contradictory information? The present study addressed this question using second-order fear conditioning in rats. In second-order conditioning, rats are conditioned to fear a stimulus, S1, through its pairings with foot-shock (stage 1); and some days later, a second stimulus, S2, through its pairings with the already-conditioned S1 (stage 2). However, as foot-shock is never presented during conditioning to S2, we hypothesized that S2 simultaneously encodes 2 contradictory associations: one that drives fear to S2 (S2-danger) and another that reflects the absence of the expected unconditioned stimulus and partially masks that fear (e.g. S2-safety). We tested this hypothesis by manipulating the substrates of danger and safety learning in the brain (using a chemogenetic approach) and assessing the consequences for second-order fear to S2. Critically, silencing activity in the basolateral amygdala (important for danger learning) reduced fear to S2, whereas silencing activity in the infralimbic cortex (important for safety learning) enhanced fear to S2. These bidirectional changes are consistent with our hypothesis that second-order fear conditioning involves the formation of competing S2-danger and S2-safety associations. More generally, they show that a single set of experiences can produce contradictory associations and that the brain resolves the contradiction by encoding these associations in distinct brain regions.

Generalized extinction of fear memory depends on co-allocation of synaptic plasticity in dendrites

Memories can be modified by new experience in a specific or generalized manner. Changes in synaptic connections are crucial for memory storage, but it remains unknown how synaptic changes associated with different memories are distributed within neuronal circuits and how such distributions affect specific or generalized modification by novel experience. Here we show that fear conditioning with two different auditory stimuli (CS) and footshocks (US) induces dendritic spine elimination mainly on different dendritic branches of layer 5 pyramidal neurons in the mouse motor cortex. Subsequent fear extinction causes CS-specific spine formation and extinction of freezing behavior. In contrast, spine elimination induced by fear conditioning with >2 different CS-USs often co-exists on the same dendritic branches. Fear extinction induces CS-nonspecific spine formation and generalized fear extinction. Moreover, activation of somatostatin-expressing interneurons increases the occurrence of spine elimination induced by different CS-USs on the same dendritic branches and facilitates the generalization of fear extinction. These findings suggest that specific or generalized modification of existing memories by new experience depends on whether synaptic changes induced by previous experiences are segregated or co-exist at the level of individual dendritic branches.

2-AG-Mediated Control of GABAergic Signaling Is Impaired in a Model of Epilepsy

Repeated seizures result in a persistent maladaptation of endocannabinoid (eCB) signaling, mediated part by anandamide signaling deficiency in the basolateral amygdala (BLA) that manifests as aberrant synaptic function and altered emotional behavior. Here, we determined the effect of repeated seizures (kindling) on 2-arachidonoylglycerol (2-AG) signaling on GABA transmission by directly measuring tonic and phasic eCB-mediated retrograde signaling in an in vitro BLA slice preparation from male rats. We report that both activity-dependent and muscarinic acetylcholine receptor (mAChR)-mediated depression of GABA synaptic transmission was reduced following repeated seizure activity. These effects were recapitulated in sham rats by preincubating slices with the 2-AG synthesizing enzyme inhibitor DO34. Conversely, preincubating slices with the 2-AG degrading enzyme inhibitor KML29 rescued activity-dependent 2-AG signaling, but not mAChR-mediated synaptic depression, over GABA transmission in kindled rats. These effects were not attributable to a change in cannabinoid type 1 (CB1) receptor sensitivity or altered 2-AG tonic signaling since the application of the highly selective CB1 receptor agonist CP55,940 provoked a similar reduction in GABA synaptic activity in both sham and kindled rats, while no effect of either DO34 or of the CB1 inverse agonist AM251 was observed on frequency and amplitude of spontaneous IPSCs in either sham or kindled rats. Collectively, these data provide evidence that repeated amygdala seizures persistently alter phasic 2-AG-mediated retrograde signaling at BLA GABAergic synapses, probably by impairing stimulus-dependent 2-AG synthesis/release, which contributes to the enduring aberrant synaptic plasticity associated with seizure activity.SIGNIFICANCE STATEMENT The plastic reorganization of endocannabinoid (eCB) signaling after seizures and during epileptogenesis may contribute to the negative neurobiological consequences associated with seizure activity. Therefore, a deeper understanding of the molecular basis underlying the pathologic long-term eCB signaling remodeling following seizure activity will be crucial to the development of novel therapies for epilepsy that not only target seizure activity, but, most importantly, the epileptogenesis and the comorbid conditions associated with epilepsy.

Topographic representation of current and future threats in the mouse nociceptive amygdala

Adaptive behaviors arise from an integration of current sensory context and internal representations of past experiences. The central amygdala (CeA) is positioned as a key integrator of cognitive and affective signals, yet it remains unknown whether individual populations simultaneously carry current- and future-state representations. We find that a primary nociceptive population within the CeA of mice, defined by CGRP-receptor (Calcrl) expression, receives topographic sensory information, with spatially defined representations of internal and external stimuli. While Calcrl+ neurons in both the rostral and caudal CeA respond to noxious stimuli, rostral neurons promote locomotor responses to externally sourced threats, while caudal CeA Calcrl+ neurons are activated by internal threats and promote passive coping behaviors and associative valence coding. During associative fear learning, rostral CeA Calcrl+ neurons stably encode noxious stimulus occurrence, while caudal CeA Calcrl+ neurons acquire predictive responses. This arrangement supports valence-aligned representations of current and future threats for the generation of adaptive behaviors.

Endocannabinoid signaling in the central nervous system

It is hard to overestimate the influence of the endocannabinoid signaling (ECS) system on central nervous system (CNS) function. In the 40 years since cannabinoids were found to trigger specific cell signaling cascades, studies of the ECS system continue to cause amazement, surprise, and confusion! CB1 cannabinoid receptors are expressed widely in the CNS and regulate cell-cell communication via effects on the release of both neurotransmitters and gliotransmitters. CB2 cannabinoid receptors are difficult to detect in the CNS but seem to "punch above their weight" as compounds targeting these receptors have significant effects on inflammatory state and behavior. Positive and negative allosteric modulators for both receptors have been identified and examined in preclinical studies. Concentrations of the endocannabinoid ligands, N-arachidonoylethanolamine and 2-arachidonoylglycerol (2-AG), are regulated by a combination of enzymatic synthesis and degradation and inhibitors of these processes are available and making their way into clinical trials. Importantly, ECS regulates many essential brain functions, including regulation of reward, anxiety, inflammation, motor control, and cellular development. While the field is on the cusp of preclinical discoveries providing impactful clinical and therapeutic insights into many CNS disorders, there is still much to be learned about this remarkable and versatile modulatory system.

Sex differences in fear responses: Neural circuits

Women have increased vulnerability to PTSD and anxiety disorders compared to men. Understanding the neurobiological underpinnings of these disorders is critical for identifying risk factors and developing appropriate sex-specific interventions. Despite the clear clinical relevance of an examination of sex differences in fear responses, the vast majority of pre-clinical research on fear learning and memory formation has exclusively used male animals. This review highlights sex differences in context and cued fear conditioning, fear extinction and fear generalization with a focus on the neural circuits underlying these behaviors in rodents. There are mixed reports of behavioral sex differences in context and cued fear conditioning paradigms, which can depend upon the behavioral indices of fear. However, there is greater evidence of differential activation of the hippocampus, amygdalar nuclei and the prefrontal cortical regions in male and female rodents during context and cued fear conditioning. The bed nucleus of the stria terminalis (BNST), a sexually dimorphic structure, is of particular interest as it differentially contributes to fear responses in males and females. In addition, while the influence of the estrous cycle on different phases of fear conditioning is delineated, the clearest modulatory effect of estrogen is on fear extinction processes. Examining the variability in neural responses and behavior in both sexes should increase our understanding of how that variability contributes to the neurobiology of affective disorders. This article is part of the Special Issue on 'Fear, anxiety and PTSD'.

The potential role of the cholecystokinin system in declarative memory

As one of the most abundant neuropeptides in the central nervous system, cholecystokinin (CCK) has been suggested to be associated with higher brain functions, including learning and memory. In this review, we examined the potential role of the CCK system in declarative memory. First, we summarized behavioral studies that provide evidence for an important role of CCK in two forms of declarative memory-fear memory and spatial memory. Subsequently, we examined the electrophysiological studies that support the diverse roles of CCK-2 receptor activation in neocortical and hippocampal synaptic plasticity, and discussed the potential mechanisms that may be involved. Last but not least, we discussed whether the reported CCK-mediated synaptic plasticity can explain the strong influence of the CCK signaling system in neocortex and hippocampus dependent declarative memory. The available research supports the role of CCK-mediated synaptic plasticity in neocortex dependent declarative memory acquisition, but further study on the association between CCK-mediated synaptic plasticity and neocortex dependent declarative memory consolidation and retrieval is necessary. Although a direct link between CCK-mediated synaptic plasticity and hippocampus dependent declarative memory is missing, noticeable evidence from morphological, behavioral, and electrophysiological studies encourages further investigation regarding the potential role of CCK-dependent synaptic plasticity in hippocampus dependent declarative memory.

Influence of the Peripheral Nervous System on Murine Osteoporotic Fracture Healing and Fracture-Induced Hyperalgesia

Osteoporotic fractures are often linked to persisting chronic pain and poor healing outcomes. Substance P (SP), α-calcitonin gene-related peptide (α-CGRP) and sympathetic neurotransmitters are involved in bone remodeling after trauma and nociceptive processes, e.g., fracture-induced hyperalgesia. We aimed to link sensory and sympathetic signaling to fracture healing and fracture-induced hyperalgesia under osteoporotic conditions. Externally stabilized femoral fractures were set 28 days after OVX in wild type (WT), α-CGRP- deficient (α-CGRP -/-), SP-deficient (Tac1-/-) and sympathectomized (SYX) mice. Functional MRI (fMRI) was performed two days before and five and 21 days post fracture, followed by µCT and biomechanical tests. Sympathectomy affected structural bone properties in the fracture callus whereas loss of sensory neurotransmitters affected trabecular structures in contralateral, non-fractured bones. Biomechanical properties were mostly similar in all groups. Both nociceptive and resting-state (RS) fMRI revealed significant baseline differences in functional connectivity (FC) between WT and neurotransmitter-deficient mice. The fracture-induced hyperalgesia modulated central nociception and had robust impact on RS FC in all groups. The changes demonstrated in RS FC in fMRI might potentially be used as a bone traumata-induced biomarker regarding fracture healing under pathophysiological musculoskeletal conditions. The findings are of clinical importance and relevance as they advance our understanding of pain during osteoporotic fracture healing and provide a potential imaging biomarker for fracture-related hyperalgesia and its temporal development. Overall, this may help to reduce the development of chronic pain after fracture thereby improving the treatment of osteoporotic fractures.

Amygdala Intercalated Cells: Gate Keepers and Conveyors of Internal State to the Circuits of Emotion

Generating adaptive behavioral responses to emotionally salient stimuli requires evaluation of complex associations between multiple sensations, the surrounding context, and current internal state. Neural circuits within the amygdala parse this emotional information, undergo synaptic plasticity to reflect learned associations, and evoke appropriate responses through their projections to the brain regions orchestrating these behaviors. Information flow within the amygdala is regulated by the intercalated cells (ITCs), which are densely packed clusters of GABAergic neurons that encircle the basolateral amygdala (BLA) and provide contextually relevant feedforward inhibition of amygdala nuclei, including the central and BLA. Emerging studies have begun to delineate the unique contribution of each ITC cluster and establish ITCs as key loci of plasticity in emotional learning. In this review, we summarize the known connectivity and function of individual ITC clusters and explore how different neuromodulators conveying internal state act via ITC gates to shape emotionally motivated behavior. We propose that the behavioral state-dependent function of ITCs, their unique genetic profile, and rich expression of neuromodulator receptors make them potential therapeutic targets for disorders, such as anxiety, schizophrenia spectrum, and addiction.

Constitutive 5-HT2C receptor knock-out facilitates fear extinction through altered activity of a dorsal raphe-bed nucleus of the stria terminalis pathway

Serotonin 2C receptors (5-HT2CRs) are widely distributed throughout the brain and are strongly implicated in the pathophysiology of anxiety disorders such as post-traumatic stress disorder (PTSD). Although in recent years, a considerable amount of evidence supports 5-HT2CRs facilitating effect on anxiety behavior, the involvement in learned fear responses and fear extinction is rather unexplored. Here, we used a 5-HT2CR knock-out mouse line (2CKO) to gain new insights into the involvement of 5-HT2CRs in the neuronal fear circuitry. Using a cued fear conditioning paradigm, our results revealed that global loss of 5-HT2CRs exclusively accelerates fear extinction, without affecting fear acquisition and fear expression. To investigate the neuronal substrates underlying the extinction enhancing effect, we mapped the immediate-early gene product cFos, a marker for neuronal activity, in the dorsal raphe nucleus (DRN), amygdala and bed nucleus of the stria terminalis (BNST). Surprisingly, besides extinction-associated changes, our results revealed alterations in neuronal activity even under basal home cage conditions in specific subregions of the DRN and the BNST in 2CKO mice. Neuronal activity in the dorsal BNST was shifted in an extinction-supporting direction due to 5-HT2CR knock-out. Finally, the assessment of DRN-BNST connectivity using antero- and retrograde tracing techniques uncovered a discrete serotonergic pathway projecting from the most caudal subregion of the DRN (DRC) to the anterodorsal portion of the BNST (BNSTad). This serotonergic DRC-BNSTad pathway showed increased neuronal activity in 2CKO mice. Thus, our results provide new insights for the fear extinction network by revealing a specific serotonergic DRC-BNSTad pathway underlying a 5-HT2CR-sensitive mechanism with high significance in the treatment of PTSD.

Lactobacillus reuteri normalizes altered fear memory in male Cntnap4 knockout mice

BACKGROUND

Autism spectrum disorder (ASD) is a common neurodevelopmental disease, characterized by deficits in social communication, restricted and repetitive behaviours, and impaired fear memory processing. Severe gastrointestinal dysfunction and altered gut microbiome have been reported in ASD patients and animal models. Contactin associated protein-like 4 (CNTNAP4) has been suggested to be a novel risk gene, though its role in ASD remains unelucidated.

METHODS

Cntnap4^-/- mice were generated to explore its role in ASD-related behavioural abnormalities. Electrophysiological recording was employed to examine GABAergic transmission in the basolateral amygdala (BLA) and prefrontal cortex. RNA-sequencing was performed to assess underlying mechanisms. 16S rDNA analysis was performed to explore changes in faecal microbial composition. Male Cntnap4^-/- mice were fed with Lactobacillus reuteri (L. reuteri) or faecal microbiota to evaluate the effects of microbiota supplementation on the impaired fear conditioning mediated by Cntnap4 deficiency.

FINDINGS

Male Cntnap4^-/- mice manifested deficiency in social behaviours and tone-cue fear conditioning. Notably, reduced GABAergic transmission and GABA receptor expression were found in the BLA but not the prefrontal cortex. In addition, gut Lactobacillus were less abundant in male Cntnap4^-/- mice, and L. reuteri treatment or faecal microbiota transplantation rescued abnormal tone-cued fear memory and improved local GABAergic transmission in the BLA of male Cntnap4^-/- mice.

INTERPRETATION

Cntnap4 shapes GABAergic transmission of amygdala and fear conditioning, and microbial intervention represents a promising therapy in ASD intervention.

FUNDING

National Natural Science Foundation of China, Science and Technology Planning Project of Guangzhou, Guangzhou Medical University, and China Postdoctoral Science Foundation.

Recombinant human erythropoietin induces neuroprotection, activates MAPK/CREB pathway, and rescues fear memory after traumatic brain injury with delayed hypoxemia in mice

Therapeutic interventions targeting secondary insults, such as delayed hypoxemia, provide a unique opportunity for treatment in severe traumatic brain injury (TBI). Erythropoietin (EPO) is a hypoxia-responsive cytokine with important roles in neurodevelopment, neuroprotection and neuromodulation. We hypothesized that recombinant human erythropoietin (rhEPO) administration would mitigate injury in a combined injury model of TBI and delayed hypoxemia. Utilizing a clinically relevant murine model of TBI and delayed hypoxemia, we characterized how ongoing rhEPO administration influenced neurogenesis, neuroprotection, synaptic density and, behavioral outcomes early after TBI, and the impact on long-lasting outcomes 6 months after injury. We employed novel object recognition (NOR) and fear conditioning to assess long-term memory. At 1-month post-injury, we observed a significant increase in cued-fear memory response in the rhEPO-injured mice compared with vehicle-injured mice. This was associated with neuroprotection and neurogenesis in the hippocampus and mitogen-activated protein kinase (MAPK)/cAMP response element-binding protein (CREB) signaling activation and increased of excitatory synaptic density in the amygdala. Early rhEPO treatment after injury reduced neurodegeneration and increased excitatory synaptic density in the hippocampus and amygdala at 6 months post-injury. However at 6 months post-injury (4 months after discontinuation of rhEPO), we did not observe changes in behavioral assessments nor MAPK/CREB pathway activation. In summary, these data demonstrate that ongoing rhEPO treatment initiated at a clinically feasible time point improves neurological, cognitive, and histological outcomes after TBI in the setting of secondary hypoxemic insults.