Hippocampal hyperexcitability underlies enhanced fear memories in TgNTRK3, a panic disorder mouse model

The amygdala NT3-TrkC pathway underlies inter-individual differences in fear extinction and related synaptic plasticity

Fear-related pathologies are among the most prevalent psychiatric conditions, having inappropriate learned fear and resistance to extinction as cardinal features. Exposure therapy represents a promising therapeutic approach, the efficiency of which depends on inter-individual variation in fear extinction learning, which neurobiological basis is unknown. We characterized a model of extinction learning, whereby fear-conditioned mice were categorized as extinction (EXT)-success or EXT-failure, according to their inherent ability to extinguish fear. In the lateral amygdala, GluN2A-containing NMDAR are required for LTP and stabilization of fear memories, while GluN2B-containing NMDAR are required for LTD and fear extinction. EXT-success mice showed attenuated LTP, strong LTD and higher levels of synaptic GluN2B, while EXT-failure mice showed strong LTP, no LTD and higher levels of synaptic GluN2A. Neurotrophin 3 (NT3) infusion in the lateral amygdala was sufficient to rescue extinction deficits in EXT-failure mice. Mechanistically, activation of tropomyosin receptor kinase C (TrkC) with NT3 in EXT-failure slices attenuated lateral amygdala LTP, in a GluN2B-dependent manner. Conversely, blocking endogenous NT3-TrkC signaling with TrkC-Fc chimera in EXT-success slices strengthened lateral amygdala LTP. Our data support a key role for the NT3-TrkC system in inter-individual differences in fear extinction in rodents, through modulation of amygdalar NMDAR composition and synaptic plasticity.

Neurotrophin-3 into the insular cortex strengthens conditioned taste aversion memory

Memory consolidation is an essential process of long-term memory formation. Neurotrophins have been suggested as key regulators of activity dependent changes in the synaptic efficacy and morphology, which are considered the downstream mechanisms of memory consolidation. The neurotrophin 3 (NT-3), a member of the neurotrophin family, and its high affinity receptor TrkC, are widely expressed in the insular cortex (IC), a region with a critical role in the consolidation of the conditioned taste aversion (CTA) paradigm, in which an animal associates a novel taste with nausea. Nevertheless, the role of this neurotrophin in the cognitive processes that the IC mediates remains unexamined. To answer whether NT-3 is involved in memory consolidation at the IC, adult male Wistar rats were administered with NT-3 or NT-3 in combination with the Trk receptors inhibitor K252a into the IC, immediately after CTA acquisition under two different conditions: a strong-CTA (0.2 M lithium chloride i.p.) or a weak-CTA (0.1 M lithium chloride i.p.). Our results show that NT-3 strengthens the memory trace of CTA, transforming a weak conditioning into a strong one, in a Trk-dependent manner. The present evidence suggests that NT-3 has a key role in the consolidation process of an aversive memory in a neocortical region.

TrkC Intracellular Signalling in the Brain Fear Network During the Formation of a Contextual Fear Memory

Learned fear is orchestrated by a brain fear network that comprises the amygdala, hippocampus and the medial prefrontal cortex. Synaptic plasticity within this network is critical for the formation of proper fear memories. Known for their role in the promotion of synaptic plasticity, neurotrophins position as obvious candidates in the regulation of fear processes. Indeed, recent evidence from our laboratory and others associates dysregulated signalling through neurotrophin-3 and its receptor TrkC with the pathophysiology of anxiety and fear-related disorders. Here, we put wild-type C57Bl/6J mice through a contextual fear conditioning paradigm in order to characterize TrkC activation and expression in the main brain regions involved in (learned) fear - amygdala, hippocampus, and prefrontal cortex - during the formation of a fear memory. We report an overall decreased activation of TrkC in the fear network during fear consolidation and reconsolidation. During reconsolidation, hippocampal TrkC downregulation was accompanied by a decrease in the expression and activation of Erk, a critical signalling pathway in fear conditioning. Moreover, we did not find evidence that the observed decrease of TrkC activation was caused by altered expression of dominant negative form of TrkC, neurotrophin-3, or the PTP1B phosphatase. Our results indicate hippocampal TrkC inactivation through Erk signalling as a potential mechanism in the regulation of contextual fear memory formation.

Maternal separation in mice leads to anxiety-like/aggressive behavior and increases immunoreactivity for glutamic acid decarboxylase and parvalbumin in the adolescence ventral hippocampus

It has been reported that stressful events in early life influence behavior in adulthood and are associated with different psychiatric disorders, such as major depression, post-traumatic stress disorder, bipolar disorder, and anxiety disorder. Maternal separation (MS) is a representative animal model for reproducing childhood stress. It is used as an animal model for depression, and has well-known effects, such as increasing anxiety behavior and causing abnormalities in the hypothalamic-pituitary-adrenal (HPA) axis. This study investigated the effect of MS on anxiety or aggression-like behavior and the number of GABAergic neurons in the hippocampus. Mice were separated from their dams for four hours per day for 19 d from postnatal day two. Elevated plus maze (EPM) test, resident-intruder (RI) test, and counted glutamic acid decarboxylase 67 (GAD67) or parvalbumin (PV) positive cells in the hippocampus were executed using immunohistochemistry. The maternal segregation group exhibited increased anxiety and aggression in the EPM test and the RI test. GAD67-positive neurons were increased in the hippocampal regions we observed: dentate gyrus (DG), CA3, CA1, subiculum, presubiculum, and parasubiculum. PV-positive neurons were increased in the DG, CA3, presubiculum, and parasubiculum. Consistent with behavioral changes, corticosterone was increased in the MS group, suggesting that the behavioral changes induced by MS were expressed through the effect on the HPA axis. Altogether, MS alters anxiety and aggression levels, possibly through alteration of cytoarchitecture and output of the ventral hippocampus that induces the dysfunction of the HPA axis.

Anxiety and hippocampal neuronal activity: Relationship and potential mechanisms

The hippocampus has been implicated in modulating anxiety. It interacts with a variety of brain regions, both cortical and subcortical areas regulating emotion and stress responses, including prefrontal cortex, amygdala, hypothalamus, and the nucleus accumbens, to adjust anxiety levels in response to a variety of stressful conditions. Growing evidence indicates that anxiety is associated with increased neuronal excitability in the hippocampus, and alterations in local regulation of hippocampal excitability have been suggested to underlie behavioral disruptions characteristic of certain anxiety disorders. Furthermore, studies have shown that some anxiolytics can treat anxiety by altering the excitability and plasticity of hippocampal neurons. Hence, identifying cellular and molecular mechanisms and neural circuits that regulate hippocampal excitability in anxiety may be beneficial for developing targeted interventions for treatment of anxiety disorders particularly for the treatment-resistant cases. We first briefly review a role of the hippocampus in fear. We then review the evidence indicating a relationship between the hippocampal activity and fear/anxiety and discuss some possible mechanisms underlying stress-induced hippocampal excitability and anxiety-related behavior.

TrkA-cholinergic signaling modulates fear encoding and extinction learning in PTSD-like behavior

Recent studies have suggested that the use of cognitive enhancers as adjuncts to exposure-based therapy in individuals suffering from post-traumatic stress disorder (PTSD) may be beneficial. Brain cholinergic signaling through basal forebrain projections to the hippocampus is an established pathway mediating fear response and cognitive flexibility. Here we employed a genetic strategy to enhance cholinergic activity through increased signaling of the NGF receptor TrkA. This strategy leads to increased levels of the marker of cholinergic activation, acetylcholine synthesizing enzyme choline acetyltransferase, in forebrain cholinergic regions and their projection areas such as the hippocampus. Mice with increased cholinergic activity do not display any neurobehavioral abnormalities except a selective attenuation of fear response and lower fear expression in extinction trials. Reduction in fear response is rescued by the GABA antagonist picrotoxin in mutant mice, and, in wild-type mice, is mimicked by the GABA agonist midazolam suggesting that GABA can modulate cholinergic functions on fear circuitries. Importantly, mutant mice also show a reduction in fear processing under stress conditions in a single prolonged stress (SPS) model of PTSD-like behavior, and augmentation of cholinergic signaling by the drug donepezil in wild-type mice promotes extinction learning in a similar SPS model of PTSD-like behavior. Donepezil is already in clinical use for the treatment of dementia suggesting a new translational application of this drug for improving exposure-based psychotherapy in PTSD patients.

Network pharmacology study on the mechanism of Qiangzhifang in the treatment of panic disorder

Background

Panic disorder (PD) is a kind of mental illness characterized by the symptom of recurring panic attacks. Qiangzhifang (QZF) is a novel decoction developed by Professor Zhaojun Yan based on a unique system of syndrome differentiation and clinical experience. It has achieved remarkable results after long-term clinical practice, but its mechanism of action is still unclear. This study aims to use network pharmacology and molecular docking to explore the mechanism of QZF in the treatment of PD.

Methods

We used the Traditional Chinese Medicine Systems Pharmacology Database and Analysis Platform (TCMSP), a literature search, and Encyclopedia of Traditional Chinese Medicine (ETCM) to find active ingredients and targets of QZF. We searched for PD targets in GeneCards, Online Mendelian Inheritance in Man (OMIM), the Comparative Toxicogenomics Database (CTD), and DrugBank. We established a PD target database, constructed a protein-protein interaction (PPI) network, and performed Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analysis in order to screen possible pathways of action and analyze the mechanism.

Results

This study identified 84 effective components of QZF, 691 potential targets, 357 PD targets, and 97 intersectional targets. Enrichment analysis using the Database for Annotation, Visualization, and Integrated Discovery (DAVID) showed that QZF was associated with 118 biological processes (BPs), 18 cellular components (CCs), 35 molecular functions (MFs) [false discovery rate (FDR) <0.01], and 62 pathways (FDR <0.01). QZF mainly acts on its targets AKT1, FOS, and APP through active ingredients such as quercetin, β-sitosterol, 4-(4'-hydroxybenzyloxy)benzyl methyl ether, harmine, 1,7-dimethoxyxanthone, and 1-hydroxy-3,7-dimethoxyxanthone to regulate serotonin, gamma-aminobutyric acid (GABA), cyclic adenosine monophosphate (cAMP), and other signal pathways to treat PD.

Conclusions

Through network pharmacology and molecular docking technology, we predicted the possible mechanism of QZF in the treatment of PD, revealed the interaction targets and potential value of QZF, and provided a basis for its clinical application.

Smaller volume of right hippocampal CA2/3 in patients with panic disorder

The hippocampus is thought to play an important role in conveying contextual information to the amygdala as the neural basis of panic disorder (PD). Previous studies have revealed functional abnormalities in the hippocampus in patients with PD compared with healthy control subjects (HC), but no study has reported volume abnormalities in the hippocampus or evaluated minute structural changes in the hippocampus in such patients. We thus investigated volume abnormalities in the subfields of the hippocampus to better understand the neurobiological basis of PD. The hippocampus was extracted from structural brain magnetic resonance images obtained from 38 patients with PD and 38 HC and then segmented into six subfields. The relative volume of each subfield was compared between the two groups. The severity of symptoms was assessed using the Panic Disorder Severity Scale (PDSS) and social functioning was assessed using the Global Assessment of Functioning (GAF) scale. Our results revealed that patients with PD had a significantly smaller volume of the right cornu ammonis (CA) 2/3 hippocampal subfield compared with HC. No significant associations were found between the volumes of the right CA 2/3 and the PDSS or GAF scores in correlation analyses. In conclusion, CA2/3 is thought to be related to contextual memory function, and our results suggest that this particular hippocampal subfield plays a role in the development of PD symptoms.

Anticonvulsant and antipsychotic medications in the pharmacotherapy of panic disorder: a structured review

BACKGROUND

As the remission rate of panic disorder (PD) achieved with conventional pharmacotherapy ranges between 20% and 50%, alternative psychopharmacological strategies are needed. We aimed to firstly review data regarding use of antipsychotic and non-benzodiazepine anticonvulsant medication in PD patients with or without comorbidities; secondly, to review data concerning reduction of panic symptoms during treatment of another psychiatric disorder with the same medications; and thirdly, to examine reports of anticonvulsant- or antipsychotic-induced new-onset panic symptomatology.

METHODS

We performed a PubMed search (last day: 28 April 2020) of English-language studies only, combining psychopathological terms (e.g. 'panic disorder') and terms referring either to categories of psychotropic medications (e.g. 'anticonvulsants') or to specific drugs (e.g. 'carbamazepine'). All duplications were eliminated. All studies included in the review met certain inclusion/exclusion criteria. The level of evidence for the efficacy of each drug was defined according to widely accepted criteria.

RESULTS

In treatment-resistant PD, beneficial effects have been reported after treatment (mostly augmentation therapy) with a range of anticonvulsant (carbamazepine, gabapentin, lamotrigine, levetiracetam, oxcarbamazepine, valproate, vigabatrin, tiagabine) and antipsychotic (aripiprazole, olanzapine, risperidone, sulpiride) medications: overall, most medications appear generally well tolerated. Additionally, bipolar patients receiving valproate or quetiapine-XR (but not risperidone or ziprasidone) demonstrated reductions of comorbid panic-related symptoms. There are case reports of new-onset panic symptoms associated with clozapine, haloperidol, olanzapine and topiramate, in patients with conditions other than PD. The small-to-modest sample size, the lack of control groups and the open-label and short-term nature of most of the reviewed studies hinder definitive conclusions regarding either the short-term and long-term efficacy of antipsychotic and anticonvulsant medications or their potential long-term side effects.

CONCLUSION

Some atypical antipsychotic and anticonvulsant medications may have a role in the treatment of some PD patients, mostly when more conventional approaches have not been successful, but the quality of supporting evidence is limited.

Is neurotrophin-3 (NT-3): a potential therapeutic target for depression and anxiety?

Introduction: Neurotrophin-3 (NT-3) is thought to play a role in the neurobiological processes implicated in mood and anxiety disorders. NT-3 is a potential pharmacological target for mood disorders because of its effects on monoamine neurotransmitters, regulation of synaptic plasticity and neurogenesis, brain-derived neurotrophic factor (BDNF) signaling boosting, and modulation of the hypothalamic-pituitary-adrenal (HPA) axis. The mechanisms underlying NT-3 anxiolytic properties are less clear and require further exploration and definition. Areas covered: The evidence that supports NT-3 as a pharmacological target for anxiety and mood disorders is presented and this is followed by a reflection on the quandaries, stumbling blocks, and future perspectives for this novel target. Expert opinion: There is evidence for miRNAs being key post-transcriptional regulators of neurotrophin-3 receptor gene (NTRK3) in anxiety disorders; however, the anxiolytic properties of NT-3 need further examination and delineation. Moreover, NT-3 expression by non-neuronal cells and its role in brain circuits that participate in anxiety and mood disorders require further scrutiny. Further work is vital before progression into clinical trials can be realized.

Expression of Pea3 protein subfamily members in hippocampus and potential regulation following neuronal stimulation

Pea3 proteins belong to a subfamily of the E-twentysix (ETS) domain superfamily of transcription factors, which play various roles during development. Polyoma Enhancer-Activator 3 (Pea3) proteins Pea3, ERM and Er81 are particularly involved in tissues with branching morphogenesis, including kidney, lung, mammary gland and nervous system development. A recent transcriptomic study on novel targets of Pea3 transcription factor revealed various axon guidance and nervous system development related targets, supporting a role of Pea3 proteins in motor neuron connectivity, as well as novel targets in signaling pathways involved in synaptic plasticity. This study focuses on the expression of Pea3 family members in hippocampal neurons, and regulation of putative Pea3 targets in Pea3-overexpressing cell lines and following induction of long-term potentiation or seizure in vivo. We show that Pea3 proteins are expressed in hippocampus in both neuronal and non-neuronal cells, and that Pea3 represses Elk-1 but activates Prkca and Nrcam expression in hippocampal cell lines. We also show that mRNA and protein levels of Pea3 family members are differentially regulated in the dentate gyrus and CA1 region upon MECS stimulation, but not upon LTP induction.

Adolescent stress increases depression-like behaviors and alters the excitatory-inhibitory balance in aged mice

Background:

Depression affects approximately 5% of elderly people and its etiology might be related to chronic stress exposure during neurodevelopmental periods. In this study, we examined the effects of adolescent chronic social stress in aged mice on depressive behaviors and the excitatory-inhibitory (E/I) balance in stress-sensitive regions of the brain.

Methods:

Sixty-four adolescent, male C57BL/6 mice were randomly assigned to either the 7-week (from post-natal days 29 to 77) social instability stress (stress group, n = 32) or normal housing conditions (control group, n = 32). At 15 months of age, 16 mice were randomly selected from each group for a series of behavioral tests, including two depression-related tasks (the sucrose preference test and the tail suspension test). Three days following the last behavioral test, eight mice were randomly selected from each group for immunohistochemical analyses to measure the cell density of parvalbumin (PV⁺)- and calretinin (CR⁺)-positive gamma-aminobutyric-acid (GABA)ergic inhibitory inter-neurons, and the expression levels of vesicular transporters of glutamate-1 (VGluT1) and vesicular GABA transporter (VGAT) in three stress-sensitive regions of the brain (the medial pre-frontal cortex [mPFC], hippocampus, and amygdala).

Results:

Behaviorally, compared with the control group, adolescent chronic stress increased depression-like behaviors as shown in decreased sucrose preference (54.96 ± 1.97% vs. 43.11 ± 2.85%, t₍₂₂₎ = 3.417, P = 0.003) and reduced latency to immobility in the tail suspension test (92.77 ± 25.08 s vs. 33.14 ± 5.95 s, t₍₂₅₎ = 2.394, P = 0.025), but did not affect anxiety-like behaviors and pre-pulse inhibition. At the neurobiologic level, adolescent stress down-regulated PV⁺, not CR⁺, inter-neuron density in the mPFC (F_{(1, 39)} = 19.30, P < 0.001), and hippocampus (F_{(1, 42)} = 5.823, P = 0.020) and altered the CR⁺, not PV⁺, inter-neuron density in the amygdala (F_{(1, 28)} = 23.16, P < 0.001). The VGluT1/VGAT ratio was decreased in all three regions (all F > 10.09, all P < 0.004), which suggests stress-induced hypoexcitability in these regions.

Conclusions:

Chronic stress during adolescence increased depression-like behaviors in aged mice, which may be associated with the E/I imbalance in stress-sensitive brain regions.

Transplanted interneurons improve memory precision after traumatic brain injury

Repair of the traumatically injured brain has been envisioned for decades, but regenerating new neurons at the site of brain injury has been challenging. We show GABAergic progenitors, derived from the embryonic medial ganglionic eminence, migrate long distances following transplantation into the hippocampus of adult mice with traumatic brain injury, functionally integrate as mature inhibitory interneurons and restore post-traumatic decreases in synaptic inhibition. Grafted animals had improvements in memory precision that were reversed by chemogenetic silencing of the transplanted neurons and a long-lasting reduction in spontaneous seizures. Our results reveal a striking ability of transplanted interneurons for incorporating into injured brain circuits, and this approach is a powerful therapeutic strategy for correcting post-traumatic memory and seizure disorders.

Fear Network Model in Panic Disorder: The Past and the Future

The core concept for pathophysiology in panic disorder (PD) is the fear network model (FNM). The alterations in FNM might be linked with disturbances in the autonomic nervous system (ANS), which is a common phenomenon in PD. The traditional FNM included the frontal and limbic regions, which were dysregulated in the feedback mechanism for cognitive control of frontal lobe over the primitive response of limbic system. The exaggerated responses of limbic system are also associated with dysregulation in the neurotransmitter system. The neuroimaging studies also corresponded to FNM concept. However, more extended areas of FNM have been discovered in recent imaging studies, such as sensory regions of occipital, parietal cortex and temporal cortex and insula. The insula might integrate the filtered sensory information via thalamus from the visuospatial and other sensory modalities related to occipital, parietal and temporal lobes. In this review article, the traditional and advanced FNM would be discussed. I would also focus on the current evidences of insula, temporal, parietal and occipital lobes in the pathophysiology. In addition, the white matter and functional connectome studies would be reviewed to support the concept of advanced FNM. An emerging dysregulation model of fronto-limbic-insula and temporooccipito-parietal areas might be revealed according to the combined results of recent neuroimaging studies. The future delineation of advanced FNM model can be beneficial from more extensive and advanced studies focusing on the additional sensory regions of occipital, parietal and temporal cortex to confirm the role of advanced FNM in the pathophysiology of PD.

Lost in translation: how to upgrade fear memory research

We address some of the current limitations of translational research in fear memory and suggest alternatives that might help to overcome them. Appropriate fear responses are adaptive, but disruption of healthy fear memory circuits can lead to anxiety and fear-based disorders. Stress is one of the main environmental factors that can disrupt memory circuits and constitutes as a key factor in the etiopathology of these psychiatric conditions. Current therapies for anxiety and fear-based disorders have limited success rate, revealing a clear need for an improved understanding of their neurobiological basis. Although animal models are excellent for dissecting fear memory circuits and have driven tremendous advances in the field, translation of these findings into the clinic has been limited so far. Animal models of stress-induced pathological fear combined with powerful cutting-edge techniques would help to improve the translational value of preclinical studies. We also encourage combining animal and human research, including psychiatric patients in order to find new pharmacological targets with real therapeutic potential that will improve the extrapolation of the findings. Finally, we highlight novel neuroimaging approaches that improve our understanding of anxiety and fear-based disorders.

The Neurobiology of Panic: A Chronic Stress Disorder

Panic disorder is an often chronic and impairing human anxiety syndrome, which frequently results in serious psychiatric and medical comorbidities. Although, to date, there have been many advances in the diagnosis and treatment of panic disorder, its pathophysiology still remains to be elucidated. In this review, recent evidence for a neurobiological basis of panic disorder is reviewed with particular attention to risk factors such as genetic vulnerability, chronic stress, and temperament. In addition, neuroimaging data are reviewed which provides support for the concept of panic disorder as a fear network disorder. The potential impact of the National Institute of Mental Health Research Domain Criteria constructs of acute and chronic threats responses and their implications for the neurobiology of panic disorder are also discussed.

Infralimbic Neurotrophin-3 Infusion Rescues Fear Extinction Impairment in a Mouse Model of Pathological Fear

The inability to properly extinguish fear memories constitutes the foundation of several anxiety disorders, including panic disorder. Recent findings show that boosting prefrontal cortex synaptic plasticity potentiates fear extinction, suggesting that therapies that augment synaptic plasticity could prove useful in rescue of fear extinction impairments in this group of disorders. Previously, we reported that mice with selective deregulation of neurotrophic tyrosine kinase receptor, type 3 expression (TgNTRK3) exhibit increased fear memories accompanied by impaired extinction, congruent with an altered activation pattern of the amygdala-hippocampus-medial prefrontal cortex fear circuit. Here we explore the specific role of neurotrophin 3 and its cognate receptor in the medial prefrontal cortex, and its involvement in fear extinction in a pathological context. In this study we combined molecular, behavioral, in vivo pharmacology and ex vivo electrophysiological recordings in TgNTRK3 animals during contextual fear extinction processes. We show that neurotrophin 3 protein levels are increased upon contextual fear extinction in wild-type animals but not in TgNTRK3 mice, which present deficits in infralimbic long-term potentiation. Importantly, infusion of neurotrophin 3 to the medial prefrontal cortex of TgNTRK3 mice rescues contextual fear extinction and ex vivo local application improves medial prefrontal cortex synaptic plasticity. This effect is blocked by inhibition of extracellular signal-regulated kinase phosphorylation through peripheral administration of SL327, suggesting that rescue occurs via this pathway. Our results suggest that stimulating neurotrophin 3-dependent medial prefrontal cortex plasticity could restore contextual fear extinction deficit in pathological fear and could constitute an effective treatment for fear-related disorders.

Targeted exome sequencing for the identification of a protective variant against Internet gaming disorder at rs2229910 of neurotrophic tyrosine kinase receptor, type 3 (NTRK3): A pilot study

Background and aims Internet gaming disorder (IGD) has gained recognition as a potential new diagnosis in the fifth revision of the Diagnostic and Statistical Manual of Mental Disorders, but genetic evidence supporting this disorder remains scarce. Methods In this study, targeted exome sequencing was conducted in 30 IGD patients and 30 control subjects with a focus on genes linked to various neurotransmitters associated with substance and non-substance addictions, depression, and attention deficit hyperactivity disorder. Results rs2229910 of neurotrophic tyrosine kinase receptor, type 3 (NTRK3) was the only single nucleotide polymorphism (SNP) that exhibited a significantly different minor allele frequency in IGD subjects compared to controls (p = .01932), suggesting that this SNP has a protective effect against IGD (odds ratio = 0.1541). The presence of this potentially protective allele was also associated with less time spent on Internet gaming and lower scores on the Young's Internet Addiction Test and Korean Internet Addiction Proneness Scale for Adults. Conclusions The results of this first targeted exome sequencing study of IGD subjects indicate that rs2229910 of NTRK3 is a genetic variant that is significantly related to IGD. These findings may have significant implications for future research investigating the genetics of IGD and other behavioral addictions.

Hippocampal Theta Input to the Amygdala Shapes Feedforward Inhibition to Gate Heterosynaptic Plasticity

The dynamic interactions between hippocampus and amygdala are critical for emotional memory. Theta synchrony between these structures occurs during fear memory retrieval and may facilitate synaptic plasticity, but the cellular mechanisms are unknown. We report that interneurons of the mouse basal amygdala are activated during theta network activity or optogenetic stimulation of ventral CA1 pyramidal cell axons, whereas principal neurons are inhibited. Interneurons provide feedforward inhibition that transiently hyperpolarizes principal neurons. However, synaptic inhibition attenuates during theta frequency stimulation of ventral CA1 fibers, and this broadens excitatory postsynaptic potentials. These effects are mediated by GABA_B receptors and change in the Cl⁻ driving force. Pairing theta frequency stimulation of ventral CA1 fibers with coincident stimuli of the lateral amygdala induces long-term potentiation of lateral-basal amygdala excitatory synapses. Hence, feedforward inhibition, known to enforce temporal fidelity of excitatory inputs, dominates hippocampus-amygdala interactions to gate heterosynaptic plasticity.

Video Abstract

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Theta stimulation of CA1 ventral hippocampal fibers activates amygdala interneurons

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Interneurons induce feedforward inhibition that hyperpolarizes principal neurons

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Theta-evoked inhibition attenuates to broaden excitation on principal neurons

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Feedforward inhibition gates heterosynaptic plasticity via GABA_B receptors

The GAD65 knock out mouse - a model for GABAergic processes in fear- and stress-induced psychopathology

The γ-amino butyric acid (GABA) synthetic enzyme glutamic acid decarboxylase (GAD)65 is critically involved in the activity-dependent regulation of GABAergic inhibition in the central nervous system. It is also required for the maturation of the GABAergic system during adolescence, a phase that is critical for the development of several neuropsychiatric diseases. Mice bearing a null mutation of the GAD65 gene develop hyperexcitability of the amygdala and hippocampus, and a phenotype of increased anxiety and pathological fear memory reminiscent of posttraumatic stress disorder. Although genetic association of GAD65 in human has not yet been reported, these findings are in line with observations of reduced GABAergic function in these brain regions of anxiety disorder patients. The particular value of GAD65(-/-) mice thus lies in modeling the effects of reduced GABAergic function in the mature nervous system. The expression of GAD65 and a second GAD isozyme, GAD67, are differentially regulated in response to stress in limbic brain areas suggesting that by controlling GABAergic inhibition these enzymes determine the vulnerability for the development of pathological anxiety and other stress-induced phenotypes. In fact, we could recently show that GAD65 haplodeficiency, which results in delayed postnatal increase of GABA levels, provides resilience to juvenile-stress-induced anxiety to GAD65(+/-) mice thus foiling the increased fear and anxiety in homozygous GAD65(-/-) mice.

Differential Effect of Neuropeptides on Excitatory Synaptic Transmission in Human Epileptic Hippocampus

Development of novel disease-modifying treatment strategies for neurological disorders, which at present have no cure, represents a major challenge for today's neurology. Translation of findings from animal models to humans represents an unresolved gap in most of the preclinical studies. Gene therapy is an evolving innovative approach that may prove useful for clinical applications. In animal models of temporal lobe epilepsy (TLE), gene therapy treatments based on viral vectors encoding NPY or galanin have been shown to effectively suppress seizures. However, how this translates to human TLE remains unknown. A unique possibility to validate these animal studies is provided by a surgical therapeutic approach, whereby resected epileptic tissue from temporal lobes of pharmacoresistant patients are available for neurophysiological studies in vitro. To test whether NPY and galanin have antiepileptic actions in human epileptic tissue as well, we applied these neuropeptides directly to human hippocampal slices in vitro. NPY strongly decreased stimulation-induced EPSPs in dentate gyrus and CA1 (up to 30 and 55%, respectively) via Y2 receptors, while galanin had no significant effect. Receptor autoradiographic binding revealed the presence of both NPY and galanin receptors, while functional receptor binding was only detected for NPY, suggesting that galanin receptor signaling may be impaired. These results underline the importance of validating findings from animal studies in human brain tissue, and advocate for NPY as a more appropriate candidate than galanin for future gene therapy trials in pharmacoresistant TLE patients.

From neural to genetic substrates of panic disorder: Insights from human and mouse studies

Fear is an ancestral emotion, an intrinsic defensive response present in every organism. Although fear is an evolutionarily advantageous emotion, under certain pathologies such as panic disorder it might become exaggerated and non-adaptive. Clinical and preclinical work pinpoints that changes in cognitive processes, such as perception and interpretation of environmental stimuli that rely on brain regions responsible for high-level function, are essential for the development of fear-related disorders. This review focuses on the involvement of cognitive function to fear circuitry disorders. Moreover, we address how animal models are contributing to understand the involvement of human candidate genes to pathological fear and helping achieve progress in this field. Multidisciplinary approaches that integrate human genetic findings with state of the art genetic mouse models will allow to elucidate the mechanisms underlying pathology and to develop new strategies for therapeutic targeting.

Hippocampal changes produced by overexpression of the human CHRNA5/A3/B4 gene cluster may underlie cognitive deficits rescued by nicotine in transgenic mice

Addiction involves long-lasting maladaptive changes including development of disruptive drug-stimuli associations. Nicotine-induced neuroplasticity underlies the development of tobacco addiction but also, in regions such as the hippocampus, the ability of this drug to enhance cognitive capabilities. Here, we propose that the genetic locus of susceptibility to nicotine addiction, the CHRNA5/A3/B4 gene cluster, encoding the α5, α3 and β4 subunits of the nicotinic acetylcholine receptors (nAChRs), may influence nicotine-induced neuroadaptations. We have used transgenic mice overexpressing the human cluster (TgCHRNA5/A3/B4) to investigate hippocampal structure and function in genetically susceptible individuals. TgCHRNA5/A3/B4 mice presented a marked reduction in the dendrite complexity of CA1 hippocampal pyramidal neurons along with an increased dendritic spine density. In addition, TgCHRNA5/A3/B4 exhibited increased VGLUT1/VGAT ratio in the CA1 region, suggesting an excitatory/inhibitory imbalance. These hippocampal alterations were accompanied by a significant impairment in short-term novelty recognition memory. Interestingly, chronic infusion of nicotine (3.25 mg/kg/d for 7 d) was able to rescue the reduced dendritic complexity, the excitatory/inhibitory imbalance and the cognitive impairment in TgCHRNA5/A3/B4. Our results suggest that chronic nicotine treatment may represent a compensatory strategy in individuals with altered expression of the CHRNA5/A3/B4 region.

Persistent pain facilitates response to morphine reward by downregulation of central amygdala GABAergic function

Opioid-based analgesics are widely used for treating chronic pain, but opioids are highly addictive when repeatedly used because of their strong rewarding effects. In recent years, abuse of prescription opioids has dramatically increased, including incidences of misuse of opioid drugs prescribed for pain control. Despite this issue in current clinical pain management, it remains unknown how pain influences the abuse liability of prescription opioids. Pain as aversive experience may affect opioid reward of positive emotion through common brain sites involved in emotion processing. In this study, on a rat model of chronic pain, we determined how persistent pain altered behavioral responses to morphine reward measured by the paradigm of unbiased conditioned place preference (CPP), focusing on GABAergic synaptic activity in neurons of the central nucleus of the amygdala (CeA), an important brain region for emotional processing of both pain and reward. We found that pain reduced the minimum number of morphine-conditioning sessions required for inducing CPP behavior. Both pain and morphine conditioning that elicited CPP inhibited GABA synaptic transmission in CeA neurons. Pharmacological activation of CeA GABAA receptors reduced the pain and inhibited CPP induced both by an effective dose of morphine and by a sub-threshold dose of morphine under pain condition. Furthermore, inhibition of CeA GABAA receptors mimicked the pain effect, rendering the sub-threshold dose of morphine effective in CPP induction. These findings suggest that pain facilitates behavioral responses to morphine reward by predisposing the inhibitory GABA function in the CeA circuitry involved in the behavior of opioid reward.

Heat exposure in female rats elicits abnormal fear expression and cellular changes in prefrontal cortex and hippocampus

Despite a twofold higher prevalence of fear-related disorders in women, the neurobiological factors that modulate and drive fear expression are rarely studied in female animals. Fear conditioning and extinction are useful tools for dissecting these mechanisms, and here we tested the effects of environmental manipulations - four days of exposure to 31°C temperatures in the animal housing facility - on fear learning and memory exclusively in female rats. We found that heat exposure disrupted freezing to tone during fear conditioning, and elicited enhanced freezing during extinction and extinction retrieval. We also performed immunohistochemistry for c-fos expression in the infralimbic (IL) and prelimbic (PL) regions of the prefrontal cortex during extinction retrieval, and found that heat exposure induced a switch from IL-dominated activity to PL-dominated activity. Finally, morphological analysis of spines in hippocampal CA3 neurons revealed an increase in spine head diameter in heat-exposed animals, which may partly underlie the persistent freezing observed in these animals. Together, our data show that heat exposure can induce changes at behavioral, physiological, and structural levels, and add to a woefully lacking body of literature on fear processes in female animals.