Astrocytic NMDA Receptors in the Basolateral Amygdala Contribute to Facilitation of Fear Extinction

Effects of anxiety induced by conditioned fear on the expression of NMDA receptors and synaptic plasticity in the rat BLA

NMDA receptors (NMDAR) are vital in CNS activities such as anxiety, memory, and cognition, and the neurobiological mechanisms behind anxiety disorders are exceedingly complicated. The "glutamic acid theory" posits that glutamate excitotoxicity is a key pathophysiological mechanism in anxiety disorders. However, the exact mechanism by which conditioned fear contributes to anxiety disorders remains unknown.Based on the conditioned fear-induced anxiety disorder model, this work aims to investigate changes in NMDAR and related proteins throughout the acquisition and expression of fear memory, as well as the impact on synaptic structural and functional plasticity. Injecting the NMDA receptor endogenous agonist D-Serine (50 μg/μL) and the noncompetitive antagonist MK-801 (1 μg/μL) into the lateral ventricle of the conditioned fear model rats, as well as conducting behavioral observations, show that NMDAR are closely involved in the development of conditioned fear-induced anxiety. Model rats showed significant changes in glutamate (Glu) and γ-aminobutyric acid (GABA) levels in the amygdala (BLA), as well as expression of NMDAR and downstream BDNF/TrkB signaling pathway components. At the same time, model rats exhibited synaptic and neuronal injury, aberrant long-term potentiation (LTP), and decreased expression of essential synaptic proteins SYP and PSD-95. In conclusion, our study demonstrates that NMDAR and synaptic plasticity play a critical role in the development of conditioned fear-induced anxiety, serving as an important reference for understanding the neurobiological underpinnings of anxiety disorders and providing insights into their treatment and new possible targets.

The Role of Basolateral Amygdala and Medial Prefrontal Cortex in Fear: A Systematic Review

Fear is a primary adaptive response to potential threats. It triggers a complex cascade of physiological, cognitive, and behavioral changes that prepare an organism to cope with dangerous situations. The basolateral amygdala (BLA) and medial prefrontal cortex (mPFC) are both linked to adaptation, the generation of strong emotions, and decision-making. In this systematic review, we aimed to analyze recent studies of the connections between the BLA and mPFC in the context of their neuroanatomy, cellular composition, micro-circuitry, and involvement in fear. Utilizing the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) 2020 guidelines, our search strategy involved scouring articles from PubMed (National Center for Biotechnology Information, Bethesda, Maryland), Google Scholar (Google, Mountain View, CA), and Science Direct (Elsevier, Amsterdam, Netherlands) databases covering the last decade (2014-2024). Thirty-two rigorously evaluated studies formed the essence of our review. Review findings revealed complex bidirectional connectivity between BLA and mPFC, with distinct roles for different subregions. The rostral BLA primarily projects to the prelimbic cortex, while the caudal BLA connects with the infralimbic cortex. These circuits show specialized cellular composition, with BLA containing principal excitatory neurons and GABAergic interneurons, while mPFC exhibits layer-specific synaptic connections. Fear processing involves dynamic interactions between these regions, with the prelimbic cortex promoting fear expression and the infralimbic cortex facilitating extinction. The analysis showed that astrocytic signaling and N-methyl-D-aspartate (NMDA) receptor activation are essential in the process of both fear memory formation and its extinction. There was evidence that dysregulation of specific circuits is associated with the pathophysiology of several other psychiatric disorders, such as post-traumatic stress disorder (PTSD), anxiety disorders, and schizophrenia. This review clarifies that the BLA-mPFC circuitry is critical in perceiving fear and its regulation. The results highlight the importance of the interactions between brain regions and the types of cells in each region to respond appropriately to fear and its extinction. Uncovering such type of dysregulation further helps to understand the mechanisms of fear-associated disorders and may suggest further treatment options. Future research should focus on cellular plasticity mechanisms, translational applications, and the influence of individual factors on fear processing to develop more effective treatments for psychiatric conditions involving fear dysregulation.

Selective Enhancement of the Interneuron Network and Gamma-Band Power via GluN2C/GluN2D NMDA Receptor Potentiation

N-methyl-D-aspartate receptors (NMDARs) comprise a family of ligand-gated ionotropic glutamate receptors that mediate a slow, calcium-permeable component to excitatory neurotransmission. The GluN2D subunit is enriched in GABAergic inhibitory interneurons in cortical tissue. Diminished levels of GABAergic inhibition contribute to multiple neuropsychiatric conditions, suggesting that enhancing inhibition may have therapeutic utility, thus making GluN2D modulation an attractive drug target. Here, we describe the actions of a GluN2C/GluN2D-selective positive allosteric modulator (PAM), (+)-EU1180-453, which has improved drug-like properties such as increased aqueous solubility compared to the first-in-class GluN2C/GluN2D-selective prototypical PAM (+)-CIQ. (+)-EU1180-453 doubles the NMDAR response at lower concentrations (< 10 μM) compared to (+)-CIQ, and produces a greater degree of maximal potentiation at 30 μM. Using in vitro electrophysiological recordings, we show that (+)-EU1180-453 potentiates triheteromeric NMDARs containing at least one GluN2C or GluN2D subunit, and is active at both exon5-lacking and exon5-containing GluN1 splice variants. (+)-EU1180-453 increases glutamate efficacy for GluN2C/GluN2D-containing NMDARs by both prolonging the deactivation time and potentiating the peak response amplitude. We show that (+)-EU1180-453 selectively increases synaptic NMDAR-mediated charge transfer onto P11-15 CA1 stratum radiatum hippocampal interneurons, but is without effect on CA1 pyramidal cells. This increased charge transfer enhances inhibitory output from GABAergic interneurons onto CA1 pyramidal cells in a GluN2D-dependent manner. (+)-EU1180-453 also shifts excitatory-to-inhibitory coupling towards increased inhibition and produces enhanced gamma band power from carbachol-induced field potential oscillations in hippocampal slices. Thus, (+)-EU1180-453 can enhance overall circuit inhibition, which could prove therapeutically useful for the treatment of anxiety, depression, schizophrenia, and other neuropsychiatric disorders.

Significance Statement

Interneuron dysfunction and diminished GABAergic inhibition in neocortical and hippocampal circuits remains a prominent molecular hypothesis for neuropsychiatric diseases including anxiety, depression, and schizophrenia. Pharmacological agents that boost GABA receptor function have shown utility in various forms of depression and treating symptoms of schizophrenia. Cortical GABAergic interneurons, unlike their excitatory pyramidal cell counterparts, are enriched for the GluN2D subunit of the NMDA receptor. Thus, GluN2D subunit-selective modulation could be a useful therapeutic tool to enhance local inhibition, improving the prognosis for neuropsychiatric diseases for which interneuron dysfunction is prominent and causal to circuit aberration.

EphrinB2 in excitatory neurons and astrocytes in the basolateral amygdala controls long-term fear memory formation

EphrinB2 regulates synaptic transmission and morphology however its role in memory formation is unknown. Here we show that deleting ephrinB2 from excitatory neurons in the basolateral amygdala (BLA) of male mice impairs long-term (LTM), but not short-term (STM), fear memory formation. Deleting ephrinB2 from astrocytes in the BLA impairs fear LTM but not STM. Removing ephrinB2 from astrocytes in the BLA reduces the level of the excitatory amino acid transporter 1 (EAAT1) in these cells. Inhibiting EAAT1 activity in the BLA during fear conditioning, by its specific inhibitor UCPH-101, impairs fear LTM showing that EAAT1 in the BLA is needed for fear LTM formation. The administration of ephrinB2 into the BLA during fear conditioning training enhances fear LTM. Moreover, ephrinB2 increases the ability of fear conditioning to activate cells in the BLA as detected by c-Fos labeling. EphrinB2 therefore determines the threshold for fear memory formation. In contrast to mature neurons, we show that ephrinB2 in neural stem cells (NSCs) is not needed for fear LTM. Our study shows that ephrinB2 in the BLA determines the strength of long-term memory consolidation.

Molecular mechanisms underlying sex and treatment-dependent differences in an animal model of cue-exposure therapy for cocaine relapse prevention

Environmental enrichment combined with the glycine transporter-1 inhibitor Org24598 (EE+ORG) during cocaine-cue extinction (EXT) inhibited reacquisition of 1.0 mg/kg cocaine self-administration in male but not female rats in a previous investigation. In this investigation, we determined if this treatment benefit in males required EXT training and ascertained the molecular basis for the observed sex difference in treatment efficacy. Nine groups of male rats trained to self-administer 1.0 mg/kg cocaine or receiving yoked-saline underwent EXT or NoEXT with or without EE and/or ORG. Next, they underwent reacquisition of cocaine self-administration or were sacrificed for molecular analysis of 9 protein targets indicative of neuroplasticity in four brain regions. Two groups of female rats trained to self-administer 1.0 mg/kg cocaine also underwent EXT with or without EE + ORG and were sacrificed for molecular analysis, as above. EE + ORG facilitated the rate of EXT learning in both sexes, and importantly, the therapeutic benefit of EE + ORG for inhibiting cocaine relapse required EXT training. Males were more sensitive than females to neuroplasticity-inducing effects of EE + ORG, which prevented reductions in total GluA1 and PSD95 proteins selectively in basolateral amygdala of male rats trained to self-administer cocaine and receiving EXT. Females were deficient in expression of multiple protein targets, especially after EE + ORG. These included total GluA1 and PSD95 proteins in basolateral amygdala, and total TrkB protein in basolateral amygdala, dorsal hippocampus, and ventromedial prefrontal cortex. Together, these results support the clinical view that sex-specific pharmacological and behavioral treatment approaches may be needed during cue exposure therapy to inhibit cocaine relapse.

Astrocytic transcriptional and epigenetic mechanisms of drug addiction

Addiction is a leading cause of disease burden worldwide and remains a challenge in current neuroscience research. Drug-induced lasting changes in gene expression are mediated by transcriptional and epigenetic regulation in the brain and are thought to underlie behavioral adaptations. Emerging evidence implicates astrocytes in regulating drug-seeking behaviors and demonstrates robust transcriptional response to several substances of abuse. This review focuses on the astrocytic transcriptional and epigenetic mechanisms of drug action.

Phytotherapy of abnormality of fear memory: A narrative review of mechanisms

It is generally believed that in post-traumatic stress disorder (PTSD), the high expression of fear memory is mainly determined by amygdala hyperactivity and hippocampus hypoactivity. In this review, we firstly updated the mechanisms of fear memory, and then searched the experimental evidence of phytotherapy for fear memory in the past five years. Based on the summary of those experimental studies, we further discussed the future research strategies of plant medicines, including the study of the mechanism of specific brain regions, the optimal time for the prevention and treatment of fear memory-related diseases such as PTSD, and the development of new drugs with active components of plant medicines. Accordingly, plant medicines play a clear role in improving fear memory abnormalities and have the drug development potential in the treatment of fear-related disorders.

GluN2D Subunit in Parvalbumin Interneurons Regulates Prefrontal Cortex Feedforward Inhibitory Circuit and Molecular Networks Relevant to Schizophrenia

BACKGROUND

Parvalbumin interneuron (PVI) activity synchronizes the medial prefrontal cortex circuit for normal cognitive function, and its impairment may contribute to schizophrenia (SZ). NMDA receptors in PVIs participate in these activities and form the basis for the NMDA receptor hypofunction hypothesis of SZ. However, the role of the GluN2D subunit, which is enriched in PVIs, in regulating molecular networks relevant to SZ is unknown.

METHODS

Using electrophysiology and a mouse model with conditional deletion of GluN2D from PVIs (PV-GluN2D knockout [KO]), we examined the cell excitability and neurotransmission in the medial prefrontal cortex. Histochemical, RNA sequencing analysis and immunoblotting were conducted to understand molecular mechanisms. Behavioral analysis was conducted to test cognitive function.

RESULTS

PVIs in the medial prefrontal cortex were found to express putative GluN1/2B/2D receptors. In a PV-GluN2D KO model, PVIs were hypoexcitable, whereas pyramidal neurons were hyperexcitable. Excitatory neurotransmission was higher in both cell types in PV-GluN2D KO, whereas inhibitory neurotransmission showed contrasting changes, which could be explained by reduced somatostatin interneuron projections and increased PVI projections. Genes associated with GABA (gamma-aminobutyric acid) synthesis, vesicular release, and uptake as well as those involved in formation of inhibitory synapses, specifically GluD1-Cbln4 and Nlgn2, and regulation of dopamine terminals were downregulated in PV-GluN2D KO. SZ susceptibility genes including Disc1, Nrg1, and ErbB4 and their downstream targets were also downregulated. Behaviorally, PV-GluN2D KO mice showed hyperactivity and anxiety behavior and deficits in short-term memory and cognitive flexibility.

CONCLUSIONS

These findings demonstrate that GluN2D in PVIs serves as a point of convergence of pathways involved in the regulation of GABAergic synapses relevant to SZ.

Update on neurobiological mechanisms of fear: illuminating the direction of mechanism exploration and treatment development of trauma and fear-related disorders

Fear refers to an adaptive response in the face of danger, and the formed fear memory acts as a warning when the individual faces a dangerous situation again, which is of great significance to the survival of humans and animals. Excessive fear response caused by abnormal fear memory can lead to neuropsychiatric disorders. Fear memory has been studied for a long time, which is of a certain guiding effect on the treatment of fear-related disorders. With continuous technological innovations, the study of fear has gradually shifted from the level of brain regions to deeper neural (micro) circuits between brain regions and even within single brain regions, as well as molecular mechanisms. This article briefly outlines the basic knowledge of fear memory and reviews the neurobiological mechanisms of fear extinction and relapse, which aims to provide new insights for future basic research on fear emotions and new ideas for treating trauma and fear-related disorders.

Basolateral Amygdala Astrocytes Are Engaged by the Acquisition and Expression of a Contextual Fear Memory

Astrocytes are key cellular regulators within the brain. The basolateral amygdala (BLA) is implicated in fear memory processing, yet most research has entirely focused on neuronal mechanisms, despite a significant body of work implicating astrocytes in learning and memory. In the present study, we used in vivo fiber photometry in C57BL/6J male mice to record from amygdalar astrocytes across fear learning, recall, and three separate periods of extinction. We found that BLA astrocytes robustly responded to foot shock during acquisition, their activity remained remarkably elevated across days in comparison to unshocked control animals, and their increased activity persisted throughout extinction. Further, we found that astrocytes responded to the initiation and termination of freezing bouts during contextual fear conditioning and recall, and this behavior-locked pattern of activity did not persist throughout the extinction sessions. Importantly, astrocytes do not display these changes while exploring a novel context, suggesting that these observations are specific to the original fear-associated environment. Chemogenetic inhibition of fear ensembles in the BLA did not affect freezing behavior or astrocytic calcium dynamics. Overall, our work presents a real-time role for amygdalar astrocytes in fear processing and provides new insight into the emerging role of these cells in cognition and behavior.SIGNIFICANCE STATEMENT We show that basolateral amygdala astrocytes are robustly responsive to negative experiences, like shock, and display changed calcium activity patterns through fear learning and memory. Additionally, astrocytic calcium responses become time locked to the initiation and termination of freezing behavior during fear learning and recall. We find that astrocytes display calcium dynamics unique to a fear-conditioned context, and chemogenetic inhibition of BLA fear ensembles does not have an impact on freezing behavior or calcium dynamics. These findings show that astrocytes play a key real-time role in fear learning and memory.

Chronic Gq activation of ventral hippocampal neurons and astrocytes differentially affects memory and behavior

Network dysfunction is implicated in numerous diseases and psychiatric disorders, and the hippocampus serves as a common origin for these abnormalities. To test the hypothesis that chronic modulation of neurons and astrocytes induces impairments in cognition, we activated the hM3D(Gq) pathway in CaMKII+ neurons or GFAP+ astrocytes within the ventral hippocampus across 3, 6, and 9 months. CaMKII-hM3Dq activation impaired fear extinction at 3 months and acquisition at 9 months. Both CaMKII-hM3Dq manipulation and aging had differential effects on anxiety and social interaction. GFAP-hM3Dq activation impacted fear memory at 6 and 9 months. GFAP-hM3Dq activation impacted anxiety in the open field only at the earliest time point. CaMKII-hM3Dq activation modified the number of microglia, while GFAP-hM3Dq activation impacted microglial morphological characteristics, but neither affected these measures in astrocytes. Overall, our study elucidates how distinct cell types can modify behavior through network dysfunction, while adding a more direct role for glia in modulating behavior.

Chronic intermittent hypoxia enhances glycinergic inhibition in nucleus tractus solitarius

Chronic intermittent hypoxia (CIH), an animal model of sleep apnea, has been shown to alter the activity of second-order chemoreceptor neurons in the caudal nucleus of the solitary tract (cNTS). Although numerous studies have focused on excitatory plasticity, few studies have explored CIH-induced plasticity impacting inhibitory inputs to NTS neurons, and the roles of GABAergic and glycinergic inputs on heightened cNTS excitability following CIH are unknown. In addition, changes in astrocyte function may play a role in cNTS plasticity responses to CIH. This study tested the effects of a 7-day CIH protocol on miniature inhibitory postsynaptic currents (mIPSCs) in cNTS neurons receiving chemoreceptor afferents. Normoxia-treated rats primarily displayed GABA mIPSCs, whereas CIH-treated rats exhibited a shift toward combined GABA/glycine-mediated mIPSCs. CIH increased glycinergic mIPSC amplitude and area. This shift was not observed in dorsal motor nucleus of the vagus neurons or cNTS cells from females. Immunohistochemistry showed that strengthened glycinergic mIPSCs were associated with increased glycine receptor protein and were dependent on receptor trafficking in CIH-treated rats. In addition, CIH altered astrocyte morphology in the cNTS, and inactivation of astrocytes following CIH reduced glycine receptor-mediated mIPSC frequency and overall mIPSC amplitude. In cNTS, CIH produced changes in glycine signaling that appear to reflect increased trafficking of glycine receptors to the cell membrane. Increased glycine signaling in cNTS associated with CIH also appears to be dependent on astrocytes. Additional studies will be needed to determine how CIH influences glycine receptor expression and astrocyte function in cNTS.NEW & NOTEWORTHY Chronic intermittent hypoxia (CIH) has been used to mimic the hypoxemia associated with sleep apnea and determine how these hypoxemias influence neural function. The nucleus of the solitary tract is the main site for chemoreceptor input to the CNS, but how CIH influences NTS inhibition has not been determined. These studies show that CIH increases glycine-mediated miniature IPSCs through mechanisms that depend on protein trafficking and astrocyte activation.

From Synapses to Circuits, Astrocytes Regulate Behavior

Astrocytes are non-neuronal cells that regulate synapses, neuronal circuits, and behavior. Astrocytes ensheath neuronal synapses to form the tripartite synapse where astrocytes influence synapse formation, function, and plasticity. Beyond the synapse, recent research has revealed that astrocyte influences on the nervous system extend to the modulation of neuronal circuitry and behavior. Here we review recent findings on the active role of astrocytes in behavioral modulation with a focus on in vivo studies, primarily in mice. Using tools to acutely manipulate astrocytes, such as optogenetics or chemogenetics, studies reviewed here have demonstrated a causal role for astrocytes in sleep, memory, sensorimotor behaviors, feeding, fear, anxiety, and cognitive processes like attention and behavioral flexibility. Current tools and future directions for astrocyte-specific manipulation, including methods for probing astrocyte heterogeneity, are discussed. Understanding the contribution of astrocytes to neuronal circuit activity and organismal behavior will be critical toward understanding how nervous system function gives rise to behavior.

Discovery of (R)-2-amino-3-triazolpropanoic acid derivatives as NMDA receptor glycine site agonists with GluN2 subunit-specific activity

N-Methyl-d-aspartate (NMDA) receptors play critical roles in central nervous system function and are involved in variety of brain disorders. We previously developed a series of (R)-3-(5-furanyl)carboxamido-2-aminopropanoic acid glycine site agonists with pronounced variation in activity among NMDA receptor GluN1/2A-D subtypes. Here, a series of (R)-2-amino-3-triazolpropanoic acid analogues with a novel chemical scaffold is designed and their pharmacological properties are evaluated at NMDA receptor subtypes. We found that the triazole can function as a bioisostere for amide to produce glycine site agonists with variation in activity among NMDA receptor subtypes. Compounds 13g and 13i are full and partial agonists, respectively, at GluN1/2C and GluN1/2D with 3- to 7-fold preference in agonist potency for GluN1/2C-D over GluN1/2A-B subtypes. The agonist binding mode of these triazole analogues and the mechanisms by which the triazole ring can serve as a bioisostere for amide were further explored using molecular dynamics simulations. Thus, the novel (R)-2-amino-3-triazolpropanoic acid derivatives reveal insights to agonist binding at the GluN1 subunit of NMDA receptors and provide new opportunities for the design of glycine site agonists.