TrkB modulates fear learning and amygdalar synaptic plasticity by specific docking sites

Comprehensive Bioinformatics Analysis Reveals Molecular Signatures and Potential Caloric Restriction Mimetics with Neuroprotective Effects: Validation in an In Vitro Stroke Model

Caloric restriction (CR) is a dietary intervention that reduces calorie intake without inducing malnutrition, demonstrating lifespan-extending effects in preclinical studies and some human trials, along with potential benefits in ameliorating age-related ailments. Caloric restriction mimetics (CRMs) are compounds mimicking CR effects, offering a potential therapeutic avenue for age-related diseases. This study explores the potential protective effects of CR on the brain neocortex (GSE11291) and the identification of CRMs using integrative bioinformatics and systems biology approaches. Our findings indicate that long-term CR activates cellular pathways improving mitochondrial function, enhancing antioxidant capacity, and reducing inflammation, potentially providing neuroprotection. The key signaling pathways enriched in our study include PPAR, mTOR, FoxO, AMPK, and Notch signaling pathways, which are crucial regulators of metabolism, cellular stress response, neuroprotection, and longevity. We identify key signaling molecules and molecular mechanisms associated with CR, including transcription factors, kinase regulators, and microRNAs linked to differentially expressed genes. Furthermore, potential CRMs such as rapamycin, replicating CR-related health benefits, are identified. Additionally, machine learning models were developed to classify small molecules based on their CNS activity and anti-inflammatory properties. As a proof of concept, we have demonstrated the ischemic neuroprotective effects of two top-ranked candidate reference molecules (CRMs) using the oxygen-glucose deprivation (OGD) model, an established in vitro stroke model. However, further investigations are essential to fully elucidate the therapeutic potential of these CRMs. In summary, our study suggests that long-term CR entails protective mechanisms preserving and safeguarding neuronal function, potentially impacting the treatment of age-related neurological diseases. Moreover, our findings contribute to the identification of potential genes and regulatory molecules involved in CR, along with potential CRMs, providing a promising foundation for future research in the field of neurological disorder treatment.

Effects of TrkB-related induced metaplasticity within the BLA on anxiety, extinction learning, and plasticity in BLA-modulated brain regions

BACKGROUND

Neuronal plasticity within the basolateral amygdala (BLA) is fundamental for fear learning. Metaplasticity, the regulation of plasticity states, has emerged as a key mechanism mediating the subsequent impact of emotional and stressful experiences. After mRNA knockdown of synaptic plasticity-related TrkB, we examined the impact of chronically altered activity in the rat BLA (induced metaplasticity) on anxiety-like behavior, fear memory-related behaviors, and neural plasticity in brain regions modulated by the BLA. These effects were investigated under both basal conditions and following exposure to acute trauma (UWT).

RESULTS

Under basal conditions, TrkB knockdown increased anxiety-like behavior and impaired extinction learning. TrkBKD also reduced LTP in the vSub-mPFC pathway but not in the dentate gyrus. Compared with those of control animals, acute trauma exposure led to increased anxiety-like behavior and impaired extinction learning in both the trauma-exposed group (CTR-UWT) and the trauma-exposed group on the background of TrkB knockdown (TrkBKD-UWT). However, the deficit in extinction learning was more pronounced in the TrkBKD-UWT group than in the CTR-UWT group. Accordingly, TrkBKD-UWT, but not CTR-UWT, resulted in impaired LTP in the vSub- mPFC pathway. Since LTP in this pathway is independent of BLA involvement, this result suggests that lasting intra-BLA-induced metaplasticity may also lead to transregional metaplasticity within the mPFC, as suggested previously.

CONCLUSIONS

Taken together, these findings reveal the dissociative involvement of BLA function, on the one hand, in anxiety, which is affected by the knockdown of TrkB, and, on the other hand, in extinction learning, which is more significantly affected by the combination of intra-BLA-induced metaplasticity and exposure to emotional trauma.

Adenosine and Cortical Plasticity

Brain plasticity is the ability of the nervous system to change its structure and functioning in response to experiences. These changes occur mainly at synaptic connections, and this plasticity is named synaptic plasticity. During postnatal development, environmental influences trigger changes in synaptic plasticity that will play a crucial role in the formation and refinement of brain circuits and their functions in adulthood. One of the greatest challenges of present neuroscience is to try to explain how synaptic connections change and cortical maps are formed and modified to generate the most suitable adaptive behavior after different external stimuli. Adenosine is emerging as a key player in these plastic changes at different brain areas. Here, we review the current knowledge of the mechanisms responsible for the induction and duration of synaptic plasticity at different postnatal brain development stages in which adenosine, probably released by astrocytes, directly participates in the induction of long-term synaptic plasticity and in the control of the duration of plasticity windows at different cortical synapses. In addition, we comment on the role of the different adenosine receptors in brain diseases and on the potential therapeutic effects of acting via adenosine receptors.

Advances in fear memory erasure and its neural mechanisms

Background

In nature, animals must learn to recognize danger signals and respond immediately to threats to improve their environmental adaptation. However, excessive fear responses can lead to diseases such as post-traumatic stress disorder, wherein traumatic events result in persistent traumatic memories. Therefore, erasing pathological fear memories in vivo is a crucial topic in neuroscience for understanding the nature of memories and treating clinically relevant diseases.

Main text

This article reviews recent studies on fear memory erasure, erasure of short- and long-term memory, fear memory erasure and neuroplasticity, the neural circuitry and molecular mechanisms of fear memory erasure, and the roles of engram cells and perineuronal nets in memory erasure.

Conclusion

Research on the mechanism of memory erasure is limited, and a plausible explanation for the essential difference between memory erasure and memory extinction still needs to be provided. Notably, this review may guide future studies on fear memory and its underlying molecular mechanisms, which may help to develop novel treatment strategies for post-traumatic stress disorder, anxiety, and other mental disorders.

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.

Astrocyte-mediated regulation of BLAWFS1 neurons alleviates risk-assessment deficits in DISC1-N mice

Assessing and responding to threats is vital in everyday life. Unfortunately, many mental illnesses involve impaired risk assessment, affecting patients, families, and society. The brain processes behind these behaviors are not well understood. We developed a transgenic mouse model (disrupted-in-schizophrenia 1 [DISC1]-N) with a disrupted avoidance response in risky settings. Our study utilized single-nucleus RNA sequencing and path-clamp coupling with real-time RT-PCR to uncover a previously undescribed group of glutamatergic neurons in the basolateral amygdala (BLA) marked by Wolfram syndrome 1 (WFS1) expression, whose activity is modulated by adjacent astrocytes. These neurons in DISC1-N mice exhibited diminished firing ability and impaired communication with the astrocytes. Remarkably, optogenetic activation of these astrocytes reinstated neuronal excitability via D-serine acting on BLAWFS1 neurons' NMDA receptors, leading to improved risk-assessment behavior in the DISC1-N mice. Our findings point to BLA astrocytes as a promising target for treating risk-assessment dysfunctions in mental disorders.

Differential Activation States of Direct Pathway Striatal Output Neurons during l-DOPA-Induced Dyskinesia Development

l-DOPA-induced dyskinesia (LID) is a debilitating motor side effect arising from chronic dopamine (DA) replacement therapy with l-DOPA for the treatment of Parkinson's disease. LID is associated with supersensitivity of striatal dopaminergic signaling and fluctuations in synaptic DA following each l-DOPA dose, shrinking the therapeutic window. The heterogeneous composition of the striatum, including subpopulations of medium spiny output neurons (MSNs), interneurons, and supporting cells, complicates the identification of cell(s) underlying LID. We used single-nucleus RNA sequencing (snRNA-seq) to establish a comprehensive striatal transcriptional profile during LID development. Male hemiparkinsonian mice were treated with vehicle or l-DOPA for 1, 5, or 10 d, and striatal nuclei were processed for snRNA-seq. Analyses indicated a limited population of DA D1 receptor-expressing MSNs (D1-MSNs) formed three subclusters in response to l-DOPA treatment and expressed cellular markers of activation. These activated D1-MSNs display similar transcriptional changes previously associated with LID; however, their prevalence and transcriptional behavior were differentially influenced by l-DOPA experience. Differentially expressed genes indicated acute upregulation of plasticity-related transcription factors and mitogen-activated protein kinase signaling, while repeated l-DOPA-induced synaptic remodeling, learning and memory, and transforming growth factor-β (TGF-β) signaling genes. Notably, repeated l-DOPA sensitized Inhba, an activin subunit of the TGF-β superfamily, in activated D1-MSNs, and its pharmacological inhibition impaired LID development, suggesting that activin signaling may play an essential role in LID. These data suggest distinct subsets of D1-MSNs become differentially l-DOPA-responsive due to aberrant induction of molecular mechanisms necessary for neuronal entrainment, similar to processes underlying hippocampal learning and memory.

Brain-derived neurotrophic factor scales presynaptic calcium transients to modulate excitatory neurotransmission

Brain-derived neurotrophic factor (BDNF) plays a critical role in synaptic physiology, as well as mechanisms underlying various neuropsychiatric diseases and their treatment. Despite its clear physiological role and disease relevance, BDNF's function at the presynaptic terminal, a fundamental unit of neurotransmission, remains poorly understood. In this study, we evaluated single synapse dynamics using optical imaging techniques in hippocampal cell cultures. We find that exogenous BDNF selectively increases evoked excitatory neurotransmission without affecting spontaneous neurotransmission. However, acutely blocking endogenous BDNF has no effect on evoked or spontaneous release, demonstrating that different approaches to studying BDNF may yield different results. When we suppressed BDNF-Tropomyosin receptor kinase B (TrkB) activity chronically over a period of days to weeks using a mouse line enabling conditional knockout of TrkB, we found that evoked glutamate release was significantly decreased while spontaneous release remained unchanged. Moreover, chronic blockade of BDNF-TrkB activity selectively downscales evoked calcium transients without affecting spontaneous calcium events. Via pharmacological blockade by voltage-gated calcium channel (VGCC) selective blockers, we found that the changes in evoked calcium transients are mediated by the P/Q subtype of VGCCs. These results suggest that BDNF-TrkB activity increases presynaptic VGCC activity to selectively increase evoked glutamate release.

Childhood urbanicity is associated with emotional episodic memory-related striatal function and common variation in NTRK2

BACKGROUND

Childhoods in urban or rural environments may differentially affect the risk of neuropsychiatric disorders, possibly through memory processing and neural response to emotional stimuli. Genetic factors may not only influence individuals' choices of residence but also modulate how the living environment affects responses to episodic memory.

METHODS

We investigated the effects of childhood urbanicity on episodic memory in 410 adults (discovery sample) and 72 adults (replication sample) with comparable socioeconomic statuses in Beijing, China, distinguishing between those with rural backgrounds (resided in rural areas before age 12 and relocated to urban areas at or after age 12) and urban backgrounds (resided in cities before age 12). We examined the effect of childhood urbanicity on brain function across encoding and retrieval sessions using an fMRI episodic memory paradigm involving the processing of neutral or aversive pictures. Moreover, genetic association analyses were conducted to understand the potential genetic underpinnings that might contribute to memory processing and neural mechanisms influenced by early-life urban or rural environments.

RESULTS

Episodic memory retrieval accuracy for more difficult neutral stimuli was similar between those with urban and rural childhoods, whereas aversive stimuli elicited higher retrieval accuracy in the urban group (P = 0.023). For aversive stimuli, subjects with urban childhood had relatively decreased engagement of the striatum at encoding and decreased engagement of the hippocampus at retrieval. This more efficient striatal encoding of aversive stimuli in those with urban childhoods was associated with common variation in neurotrophic tyrosine kinase receptor type 2 (NTRK2) (right striatum: P = 1.58×10-6). These findings were confirmed in the replication sample.

CONCLUSIONS

We suggest that this differential striatal processing of aversive stimuli observed in individuals with urban or rural childhoods may represent mechanisms by which childhood urbanicity may affect brain circuits, heightening behavioral responses to negative stressors associated with urban environments. NTRK2-associated neural processes in the striatum may play a role in these processes.

Cellular mechanisms of contextual fear memory reconsolidation: Role of hippocampal SFKs, TrkB receptors and GluN2B-containing NMDA receptors

Memories are stored into long-term representations through a process that depends on protein synthesis. However, a consolidated memory is not static and inflexible and can be reactivated under certain circumstances, the retrieval is able to reactivate memories and destabilize them engaging a process of restabilization known as reconsolidation. Although the molecular mechanisms that mediate fear memory reconsolidation are not entirely known, so here we investigated the molecular mechanisms in the hippocampus involved in contextual fear conditioning memory (CFC) reconsolidation in male Wistar rats. We demonstrated that the blockade of Src family kinases (SFKs), GluN2B-containing NMDA receptors and TrkB receptors (TrkBR) in the CA1 region of the hippocampus immediately after the reactivation session impaired contextual fear memory reconsolidation. These impairments were blocked by the neurotrophin BDNF and the NMDAR agonist, D-Serine. Considering that the study of the link between synaptic proteins is crucial for understanding memory processes, targeting the reconsolidation process may provide new ways of disrupting maladaptive memories, such as those seen in post-traumatic stress disorder. Here we provide new insights into the cellular mechanisms involved in contextual fear memory reconsolidation, demonstrating that SFKs, GluN2B-containing NMDAR, and TrkBR are necessary for the reconsolidation process. Our findings suggest a link between BDNF and SFKs and GluN2B-containing NMDAR as well as a link between NMDAR and SFKs and TrkBR in fear memory reconsolidation. These preliminary pharmacological findings provide new evidence of the mechanisms involved in the reconsolidation of fear memory and have the potential to contribute to the development of treatments for psychiatric disorders involving maladaptive memories.

"NO" Time in Fear Response: Possible Implication of Nitric-Oxide-Related Mechanisms in PTSD

Post-traumatic stress disorder (PTSD) is a psychiatric condition characterized by persistent fear responses and altered neurotransmitter functioning due to traumatic experiences. Stress predominantly affects glutamate, a neurotransmitter crucial for synaptic plasticity and memory formation. Activation of the N-Methyl-D-Aspartate glutamate receptors (NMDAR) can trigger the formation of a complex comprising postsynaptic density protein-95 (PSD95), the neuronal nitric oxide synthase (nNOS), and its adaptor protein (NOS1AP). This complex is pivotal in activating nNOS and nitric oxide (NO) production, which, in turn, activates downstream pathways that modulate neuronal signaling, including synaptic plasticity/transmission, inflammation, and cell death. The involvement of nNOS and NOS1AP in the susceptibility of PTSD and its comorbidities has been widely shown. Therefore, understanding the interplay between stress, fear, and NO is essential for comprehending the maintenance and progression of PTSD, since NO is involved in fear acquisition and extinction processes. Moreover, NO induces post-translational modifications (PTMs), including S-nitrosylation and nitration, which alter protein function and structure for intracellular signaling. Although evidence suggests that NO influences synaptic plasticity and memory processing, the specific role of PTMs in the pathophysiology of PTSD remains unclear. This review highlights pathways modulated by NO that could be relevant to stress and PTSD.

Neuronal types in the mouse amygdala and their transcriptional response to fear conditioning

The amygdala is a brain region primarily associated with emotional response. The use of genetic markers and single-cell transcriptomics can provide insights into behavior-associated cell state changes. Here we present a detailed cell-type taxonomy of the adult mouse amygdala during fear learning and memory consolidation. We perform single-cell RNA sequencing on naïve and fear-conditioned mice, identify 130 neuronal cell types and validate their spatial distributions. A subset of all neuronal types is transcriptionally responsive to fear learning and memory retrieval. The activated engram cells upregulate activity-response genes and coordinate the expression of genes associated with neurite outgrowth, synaptic signaling, plasticity and development. We identify known and previously undescribed candidate genes responsive to fear learning. Our molecular atlas may be used to generate hypotheses to unveil the neuron types and neural circuits regulating the emotional component of learning and memory.

NKCC1 Deficiency in Forming Hippocampal Circuits Triggers Neurodevelopmental Disorder: Role of BDNF-TrkB Signalling

The time-sensitive GABA shift from excitatory to inhibitory is critical in early neural circuits development and depends upon developmentally regulated expression of cation-chloride cotransporters NKCC1 and KCC2. NKCC1, encoded by the SLC12A2 gene, regulates neuronal Cl⁻ homeostasis by chloride import working opposite KCC2. The high NKCC1/KCC2 expression ratio decreases in early neural development contributing to GABA shift. Human SLC12A2 loss-of-function mutations were recently associated with a multisystem disorder affecting neural development. However, the multisystem phenotype of rodent Nkcc1 knockout models makes neurodevelopment challenging to study. Brain-Derived Neurotrophic Factor (BDNF)-NTRK2/TrkB signalling controls KCC2 expression during neural development, but its impact on NKCC1 is still controversial. Here, we discuss recent evidence supporting BDNF-TrkB signalling controlling Nkcc1 expression and the GABA shift during hippocampal circuit formation. Namely, specific deletion of Ntrk2/Trkb from immature mouse hippocampal dentate granule cells (DGCs) affects their integration and maturation in the hippocampal circuitry and reduces Nkcc1 expression in their target region, the CA3 principal cells, leading to premature GABA shift, ultimately influencing the establishment of functional hippocampal circuitry and animal behaviour in adulthood. Thus, immature DGCs emerge as a potential therapeutic target as GABAergic transmission is vital for specific neural progenitors generating dentate neurogenesis in early development and the mature brain.

Implications of Phosphoinositide 3-Kinase-Akt (PI3K-Akt) Pathway in the Pathogenesis of Alzheimer's Disease

Alzheimer's disease (AD) is the foremost type of dementia that afflicts considerable morbidity and mortality in aged population. Several transcription molecules, pathways, and molecular mechanisms such as oxidative stress, inflammation, autophagy, and immune system interact in a multifaceted way that disrupt physiological processes (cell growth, differentiation, survival, lipid and energy metabolism, endocytosis) leading to apoptosis, tauopathy, β-amyloidopathy, neuron, and synapse loss, which play an important role in AD pathophysiology. Despite of stupendous advancements in pathogenic mechanisms, treatment of AD is still a nightmare in the field of medicine. There is compelling urgency to find not only symptomatic but effective disease-modifying therapies. Recently, phosphoinositide 3-kinase (PI3K) and Akt are identified as a pathway triggered by diverse stimuli, including insulin, growth factors, cytokines, and cellular stress, that link amyloid-β, neurofibrillary tangles, and brain atrophy. The present review aims to explore and analyze the role of PI3K-Akt pathway in AD and agents which may modulate Akt and have therapeutic prospects in AD. The literature was researched using keywords "PI3K-Akt" and "Alzheimer's disease" from PubMed, Web of Science, Bentham, Science Direct, Springer Nature, Scopus, and Google Scholar databases including books. Articles published from 1992 to 2021 were prioritized and analyzed for their strengths and limitations, and most appropriate ones were selected for the purpose of review. PI3K-Akt pathway regulates various biological processes such as cell proliferation, motility, growth, survival, and metabolic functions, and inhibits many neurotoxic mechanisms. Furthermore, experimental data indicate that PI3K-Akt signaling might be an important therapeutic target in treatment of AD.

A Modified Two-Way Active Avoidance Test for Combined Contextual and Auditory Instrumental Conditioning

Available two-way active avoidance paradigms do not provide contextual testing, likely due to challenges in performing repetitive trials of context exposure. To incorporate contextual conditioning in the two-way shuttle box, we contextually modified one of the chambers of a standard two-chamber rat shuttle box with visual cues consisting of objects and black and white stripe patterns. During the 5 training days, electrical foot shocks were delivered every 10 s in the contextually modified chamber but were signaled by a tone in the plain chamber. Shuttling between chambers prevented an incoming foot shock (avoidance) or aborted an ongoing one (escape). During contextual retention testing, rats were allowed to freely roam in the box. During auditory retention testing, visual cues were removed, and tone-signaled shocks were delivered in both chambers. Avoidance gradually replaced escape or freezing behaviors reaching 80% on the last training day in both chambers. Rats spent twice more time in the plain chamber during contextual retention testing and had 90% avoidance rates during auditory retention testing. Our modified test successfully assesses both auditory and contextual two-way active avoidance. By efficiently expanding its array of outcomes, our novel test will complement standard two-way active avoidance in mechanistic studies and will improve its applications in translational research.

Brain-Derived Neurotrophic Factor/Tropomyosin Receptor Kinase B Signaling Controls Excitability and Long-Term Depression in Oval Nucleus of the BNST

Dysregulation of proteins involved in synaptic plasticity is associated with pathologies in the CNS, including psychiatric disorders. The bed nucleus of the stria terminalis (BNST), a brain region of the extended amygdala circuit, has been identified as the critical hub responsible for fear responses related to stress coping and pathologic systems states. Here, we report that one particular nucleus, the oval nucleus of the BNST (ovBNST), is rich in brain-derived neurotrophic factor (BDNF) and tropomyosin receptor kinase B (TrkB) receptor. Whole-cell patch-clamp recordings of neurons from male mouse ovBNST in vitro showed that the BDNF/TrkB interaction causes a hyperpolarizing shift of the membrane potential from resting value, mediated by an inwardly rectifying potassium current, resulting in reduced neuronal excitability in all major types of ovBNST neurons. Furthermore, BDNF/TrkB signaling mediated long-term depression (LTD) at postsynaptic sites in ovBNST neurons. LTD of ovBNST neurons was prevented by a BDNF scavenger or in the presence of TrkB inhibitors, indicating the contribution to LTD induction. Our data identify BDNF/TrkB signaling as a critical regulator of synaptic activity in ovBNST, which acts at postsynaptic sites to dampen excitability at short and long time scales. Given the central role of ovBNST in mediating maladaptive behaviors associated with stress exposure, our findings suggest a synaptic entry point of the BDNF/TrkB system for adaptation to stressful environmental encounters.

Neurotrophin signalling in amygdala-dependent cued fear learning

The amygdala is a central hub for fear learning assessed by Pavlovian fear conditioning. Indeed, the prevailing hypothesis that learning and memory are mediated by changes in synaptic strength was shown most convincingly at thalamic and cortical afferents to the lateral amygdala. The neurotrophin brain-derived neurotrophic factor (BDNF) is known to regulate synaptic plasticity and memory formation in many areas of the mammalian brain including the amygdala, where BDNF signalling via tropomyosin-related kinase B (TrkB) receptors is prominently involved in fear learning. This review updates the current understanding of BDNF/TrkB signalling in the amygdala related to fear learning and extinction. In addition, actions of proBDNF/p75NTR and NGF/TrkA as well as NT-3/TrkC signalling in the amygdala are introduced.

Immature Dentate Granule Cells Require Ntrk2/Trkb for the Formation of Functional Hippocampal Circuitry

Early in brain development, impaired neuronal signaling during time-sensitive windows triggers the onset of neurodevelopmental disorders. GABA, through its depolarizing and excitatory actions, drives early developmental events including neuronal circuit formation and refinement. BDNF/TrkB signaling cooperates with GABA actions. How these developmental processes influence the formation of neural circuits and affect adult brain function is unknown. Here, we show that early deletion of Ntrk2/Trkb from immature mouse hippocampal dentate granule cells (DGCs) affects the integration and maturation of newly formed DGCs in the hippocampal circuitry and drives a premature shift from depolarizing to hyperpolarizing GABAergic actions in the target of DGCs, the CA3 principal cells of the hippocampus, by reducing the expression of the cation-chloride importer Nkcc1. These changes lead to the disruption of early synchronized neuronal activity at the network level and impaired morphological maturation of CA3 pyramidal neurons, ultimately contributing to altered adult hippocampal synaptic plasticity and cognitive processes.

Activation of brain-derived neurotrophic factor signaling in the basal forebrain reverses acute sleep deprivation-induced fear memory impairments

INTRODUCTION

The mechanisms underlying sleep deprivation-induced memory impairments and relevant compensatory signaling pathways remain elusive. We tested the hypothesis that increased brain-derived neurotrophic factor (BDNF) expression in the basal forebrain following acute sleep deprivation was a compensatory mechanism to maintain fear memory performance.

METHODS

Adult male Wistar rats were deprived of 6-hr total sleep from the beginning of the light cycle. The effects of sleep deprivation on BDNF protein expression and activation of downstream tropomyosin receptor kinase B (TrkB)/phospholipase C-γ1 (PLCγ1) signaling in the basal forebrain and fear memory consolidation were examined. BDNF or selective downstream TrkB receptor antagonist ANA-12 was further injected into the basal forebrain bilaterally to observe the changes in fear memory consolidation in response to modulation of the BDNF/TrkB signaling.

RESULTS

Six hours of sleep deprivation-induced both short- and long-term fear memory impairments. Increased BDNF protein expression and TrkB and PLCγ1 phosphorylation in the basal forebrain were observed after sleep deprivation. Microinjection of BDNF into the basal forebrain partly reversed fear memory deficits caused by sleep deprivation, which were accompanied by increased BDNF protein levels and TrkB/PLCγ1 activation. After ANA-12 microinjection, sleep deprivation-induced activation of the BDNF/TrkB pathway was inhibited and impairments of fear memory consolidation were further aggravated.

CONCLUSIONS

Acute sleep deprivation induces compensatory increase of BDNF expression in the basal forebrain. Microinjection of BDNF into the basal forebrain mitigates the fear memory impairments caused by sleep deprivation by activating TrkB/PLCγ1 signaling.

Linking Early Life Hypothalamic-Pituitary-Adrenal Axis Functioning, Brain Asymmetries, and Personality Traits in Dyslexia: An Informative Case Study

Developmental dyslexia (DD) is a multi-system disorder, combining influences of susceptibility genes and environmental factors. The causative interaction between specific genetic factors, brain regions, and personality/mental disorders, as well as specific learning disabilities, has been thoroughly investigated with regard to the approach of developing a multifaceted diagnostic procedure with an intervention strategy potential. In an attempt to add new translational evidence to the interconnection of the above factors in the occurrence of DD, we performed a combinatorial analysis of brain asymmetries, personality traits, cognitive and learning skills, and expression profiles of selected genes in an adult, early diagnosed with DD, and in his son of typical development. We focused on the expression of genes, based on the assumption that the regulation of transcription may be affected by genetic and epigenetic factors. The results highlighted a potential chain link between neuroplasticity-related as well as stress-related genes, such as BDNF, Sox4, mineralocorticoid receptor (MR), and GILZ, leftward asymmetries in the amygdala and selective cerebellum lobules, and tendencies for personality disorders and dyslexia. This correlation may reflect the presence of a specific neuro-epigenetic component of DD, ensuing from the continuous, multifaceted difficulties in the acquisition of cognitive and learning skills, which in turn may act as a fostering mechanism for the onset of long-term disorders. This is in line with recent findings demonstrating a dysfunction in processes supported by rapid neural adaptation in children and adults with dyslexia. Accordingly, the co-evaluation of all the above parameters may indicate a stress-related dyslexia endophenotype that should be carefully considered for a more integrated diagnosis and effective intervention.

The Relation Between Long-Term Synaptic Plasticity at Glutamatergic Synapses in the Amygdala and Fear Learning in Adult Heterozygous BDNF-Knockout Mice

Brain-derived neurotrophic factor (BDNF) heterozygous knockout mice (BDNF+/- mice) show fear learning deficits from 3 months of age onwards. Here, we addressed the question how this learning deficit correlates with altered long-term potentiation (LTP) in the cortical synaptic input to the lateral amygdala (LA) and at downstream intra-amygdala synapses in BDNF+/- mice. Our results reveal that the fear learning deficit in BDNF+/- mice was not paralleled by a loss of LTP, neither at cortical inputs to the LA nor at downstream intra-amygdala glutamatergic synapses. As we did observe early fear memory (30 min after training) in BDNF+/- mice while long-term memory (24 h post-training) was absent, the stable LTP in cortico-LA and downstream synapses is in line with the intact acquisition of fear memories. Ex vivo recordings in acute slices of fear-conditioned wildtype (WT) mice revealed that fear learning induces long-lasting changes at cortico-LA synapses that occluded generation of LTP 4 and 24 h after training. Overall, our data show that the intact LTP in the tested amygdala circuits is consistent with intact acquisition of fear memories in both WT and BDNF+/- mice. In addition, the lack of learning-induced long-term changes at cortico-LA synapses in BDNF+/- mice parallels the observed deficit in fear memory consolidation.

Haplotypes of coping behavior associated QTL regions reveal distinct transcript profiles in amygdala and hippocampus

Stress response and coping behavior in pigs are largely shaped by hypothalamic-pituitary-adrenal axis and sympatho-adrenomedullary system action. However, the dynamic interaction between amygdala and hippocampus crucially modulates the behavioral response towards significant emotional events. While this functional relationship is well documented, the molecular underpinnings still remain insufficiently understood. Our study used transcriptome profiling of porcine amygdala and hippocampus to identify molecular pathways that are differentially activated depending on the haplotype of a significantly coping behavior-associated region on pig chromosome 12 (SSC12). The pigs were classified into two groups based on the haplotype information of this QTL-region discovered in our previous genome-wide association study. Ten each of high- (HR) and low- (LR) reactive pigs (n = 20) were selected for differential gene expression analysis and weighted gene co-expression analysis with subsequent pathway analysis. Differentially expressed genes identified in the amygdala include SELL, CXCR7 and NTS, while TRAF3, PTGS2 and CFI were detected in the hippocampus indicating a role of neuroinflammation and immunological processes. Pathway analysis revealed IL-8 signaling, NF-κB signaling, glutamate and GABA metabolism, glucocorticoid receptor signaling and chemokine signaling in the amygdala and ephrin receptor signaling, as well as NF-κB signaling in the hippocampus. We discovered candidate genes in regions detected by genome-wide association study including ARRB2, ADRBK2, THRB, NEK7 and ACVR2B, which relate to dopaminergic and other monoaminergic neurotransmitter systems, neuroimmunomodulation, neuroinflammation and GABA-ergic neurotransmission. These findings provide insights into the molecular underpinning of divergent coping behavior and associated haplotypes in limbic forebrain system in pig.

Lestaurtinib (CEP-701) modulates the effects of early life hypoxic seizures on cognitive and emotional behaviors in immature rats

Hypoxic encephalopathy of the newborn is a major cause of long-term neurological sequelae. We have previously shown that CEP-701 (lestaurtinib), a drug with an established safety profile in children, attenuates short-term hyperexcitability and tropomyosin-related kinase B (TrkB) receptor activation in a well-established rat model of early life hypoxic seizures (HS). Here, we investigated the potential long-term neuroprotective effects of a post-HS transient CEP-701 treatment. Following exposure to global hypoxia, 10 day old male Sprague-Dawley pups received CEP-701 or its vehicle and were sequentially subjected to the light-dark box test (LDT), forced swim test (FST), open field test (OFT), Morris water maze (MWM), and the modified active avoidance (MAAV) test between postnatal days 24 and 44 (P24-44). Spontaneous seizure activity was assessed by epidural cortical electroencephalography (EEG) between P50 and 100. Neuronal density and glial fibrillary acidic protein (GFAP) levels were evaluated on histological sections in the hippocampus, amygdala, and prefrontal cortex at P100. Vehicle-treated hypoxic rats exhibited significantly increased immobility in the FST compared with controls, and post-HS CEP-701 administration reversed this HS-induced depressive-like behavior (p < 0.05). In the MAAV test, CEP-701-treated hypoxic rats were slower at learning both context-cued and tone-signaled shock-avoidance behaviors (p < 0.05). All other behavioral outcomes were comparable, and no recurrent seizures, neuronal loss, or increase in GFAP levels were detected in any of the groups. We showed that early life HS predispose to long-lasting depressive-like behaviors, and that these are prevented by CEP-701, likely via TrkB modulation. Future mechanistically more specific studies will further investigate the potential role of TrkB signaling pathway modulation in achieving neuroprotection against neonatal HS, without causing neurodevelopmental adverse effects.

Neurotrophin and FGF Signaling Adapter Proteins, FRS2 and FRS3, Regulate Dentate Granule Cell Maturation and Excitatory Synaptogenesis

Dentate granule cells (DGCs) play important roles in cognitive processes. Knowledge about how growth factors such as FGFs and neurotrophins contribute to the maturation and synaptogenesis of DGCs is limited. Here, using brain-specific and germline mouse mutants we show that a module of neurotrophin and FGF signaling, the FGF Receptor Substrate (FRS) family of intracellular adapters, FRS2 and FRS3, are together required for postnatal brain development. In the hippocampus, FRS promotes dentate gyrus morphogenesis and DGC maturation during developmental neurogenesis, similar to previously published functions for both neurotrophins and FGFs. Consistent with a role in DGC maturation, two-photon imaging revealed that Frs2,3-double mutants have reduced numbers of dendritic branches and spines in DGCs. Functional analysis further showed that double-mutant mice exhibit fewer excitatory synaptic inputs onto DGCs. These observations reveal roles for FRS adapters in DGC maturation and synaptogenesis and suggest that FRS proteins may act as an important node for FGF and neurotrophin signaling in postnatal hippocampal development.

Pre-synaptic TrkB in basolateral amygdala neurons mediates BDNF signaling transmission in memory extinction

Brain-derived neurotrophic factor (BDNF) and its high affinity receptor, TrkB, play an essential role in memory extinction. Our previous work has shown that JIP3 (JNK interacted protein 3) mediates anterograde axonal transport of TrkB through the direct binding of its coiled-coil domain 1 (CC1) with TrkB. Here, we constructed a fluorescent CC1 and enhanced green fluorescent protein (EGFP) fused protein, CC1-EGFP, and found that CC1-EGFP could specifically interrupt TrkB anterograde axonal transport and its localization at the pre-synaptic site. Consistent with this, TrkB-mediated pre-synaptic vesicle release and retrograde axonal signaling transmission were disrupted by CC1-EGFP. Neuronal expression of CC1-EGFP in the basolateral amygdala (BLA) impaired fear memory extinction. And, it blocked BDNF in the BLA-induced enhancement of TrkB phosphorylation in the infralimbic prefrontal cortex (IL). Together, this study not only suggests that pre-synaptic TrkB in BLA neurons is necessary for memory extinction and contributes to the BDNF signaling transduction from the BLA to IL, but also provides CC1-EGFP as a novel tool to specifically regulate pre-synaptic TrkB expression in vitro and in vivo.

Neurobiology of local and intercellular BDNF signaling

Brain-derived neurotrophic factor (BDNF) is a member of the neurotrophin family of secreted proteins. Signaling cascades induced by BDNF and its receptor, the receptor tyrosine kinase TrkB, link neuronal growth and differentiation with synaptic plasticity. For this reason, interference with BDNF signaling has emerged as a promising strategy for potential treatments in psychiatric and neurological disorders. In many brain circuits, synaptically released BDNF is essential for structural and functional long-term potentiation, two prototypical cellular models of learning and memory formation. Recent studies have revealed an unexpected complexity in the synaptic communication of mature BDNF and its precursor proBDNF, not only between local pre- and postsynaptic neuronal targets but also with participation of glial cells. Here, we consider recent findings on local actions of the BDNF family of ligands at the synapse and discuss converging lines of evidence which emerge from per se conflicting results.

Mutation of the HERC 1 Ubiquitin Ligase Impairs Associative Learning in the Lateral Amygdala

Tambaleante (tbl/tbl) is a mutant mouse that carries a spontaneous Gly483Glu substitution in the HERC1 (HECT domain and RCC1 domain) E3 ubiquitin ligase protein (HERC1). The tbl/tbl mutant suffers an ataxic syndrome given the almost complete loss of cerebellar Purkinje cells during adult life. More recent analyses have identified alterations at neuromuscular junctions in these mice, as well as in other neurons of the central nervous system, such as motor neurons in the spinal cord, or pyramidal neurons in the hippocampal CA3 region and the neocortex. Accordingly, the effect of the tbl/tbl mutation apparently extends to other regions of the nervous system far from the cerebellum. As HERC1 mutations in humans have been correlated with intellectual impairment, we studied the effect of the tbl/tbl mutation on learning. Using a behavioral test, ex vivo electrophysiological recordings, immunohistochemistry, and Golgi method, we analyzed the associative learning in the lateral amygdala of the tbl/tbl mouse. The tbl/tbl mice perform worse than wild-type animals in the passive avoidance test, and histologically, the tbl/tbl mice have more immature forms of dendritic spines. In addition, LTP cannot be detected in these animals and their STP is dampened, as is their glutamatergic input to the lateral amygdala. Together, these data suggest that HERC1 is probably involved in regulating synaptic function in the amygdala. Indeed, these results indicate that the tbl/tbl mutation is a good model to analyze the effect of alterations to the ubiquitin-proteasome pathway on the synaptic mechanisms involved in learning and its defects.

Calbindin Knockout Alters Sex-Specific Regulation of Behavior and Gene Expression in Amygdala and Prefrontal Cortex

Calbindin-D(28K) (Calb1), a high-affinity calcium buffer/sensor, shows abundant expression in neurons and has been associated with a number of neurobehavioral diseases, many of which are sexually dimorphic in incidence. Behavioral and physiological end points are affected by experimental manipulations of calbindin levels, including disruption of spatial learning, hippocampal long-term potentiation, and circadian rhythms. In this study, we investigated novel aspects of calbindin function on social behavior, anxiety-like behavior, and fear conditioning in adult mice of both sexes by comparing wild-type to littermate Calb1 KO mice. Because Calb1 mRNA and protein are sexually dimorphic in some areas of the brain, we hypothesized that sex differences in behavioral responses of these behaviors would be eliminated or revealed in Calb1 KO mice. We also examined gene expression in the amygdala and prefrontal cortex, two areas of the brain intimately connected with limbic system control of the behaviors tested, in response to sex and genotype. Our results demonstrate that fear memory and social behavior are altered in male knockout mice, and Calb1 KO mice of both sexes show less anxiety. Moreover, gene expression studies of the amygdala and prefrontal cortex revealed several significant genotype and sex effects in genes related to brain-derived neurotrophic factor signaling, hormone receptors, histone deacetylases, and γ-aminobutyric acid signaling. Our findings are the first to directly link calbindin with affective and social behaviors in rodents; moreover, the results suggest that sex differences in calbindin protein influence behavior.

A common NTRK2 variant is associated with emotional arousal and brain white-matter integrity in healthy young subjects

Dysregulation of emotional arousal is observed in many psychiatric diseases such as schizophrenia, mood and anxiety disorders. The neurotrophic tyrosine kinase receptor type 2 gene (NTRK2) has been associated with these disorders. Here we investigated the relation between genetic variability of NTRK2 and emotional arousal in healthy young subjects in two independent samples (n1=1171; n2=707). In addition, diffusion tensor imaging (DTI) data in a subgroup of 342 participants were used to identify NTRK2-related white-matter structure differences. After correction for multiple testing, we identified a NTRK2 single nucleotide polymorphism associated with emotional arousal in both samples (n1: Pnominal=0.0003, Pcorrected=0.048; n2: Pnominal=0.0141, Pcorrected=0.036). DTI revealed significant, whole-brain corrected correlations between emotional arousal and brain white-matter mean diffusivity (MD), as well as significant, whole-brain corrected NTRK2 genotype-related differences in MD (PFWE<0.05). Our study demonstrates that genetic variability of NTRK2, a susceptibility gene for psychiatric disorders, is related to emotional arousal and-independently-to brain white-matter properties in healthy individuals.

Plasticity-related genes in brain development and amygdala-dependent learning

Learning about motivationally important stimuli involves plasticity in the amygdala, a temporal lobe structure. Amygdala-dependent learning involves a growing number of plasticity-related signaling pathways also implicated in brain development, suggesting that learning-related signaling in juveniles may simultaneously influence development. Here, we review the pleiotropic functions in nervous system development and amygdala-dependent learning of a signaling pathway that includes brain-derived neurotrophic factor (BDNF), extracellular signaling-related kinases (ERKs) and cyclic AMP-response element binding protein (CREB). Using these canonical, plasticity-related genes as an example, we discuss the intersection of learning-related and developmental plasticity in the immature amygdala, when aversive and appetitive learning may influence the developmental trajectory of amygdala function. We propose that learning-dependent activation of BDNF, ERK and CREB signaling in the immature amygdala exaggerates and accelerates neural development, promoting amygdala excitability and environmental sensitivity later in life.

Effect of preexposure on methylphenidate-induced taste avoidance and related BDNF/TrkB activity in the insular cortex of the rat

RATIONALE

Exogenous brain-derived neurotrophic factor (BDNF) in the insular cortex (IC) is known to influence conditioned taste avoidance (CTA) learning, but little is known of its endogenous role in the phenomenon. Preexposure to many abusable compounds attenuates their ability to induce CTA, thus providing a possible platform from which to further elucidate the endogenous role of IC BDNF in CTA.

OBJECTIVES

The role of IC BDNF in CTA learning was examined by assessing the effect of preexposure to methylphenidate (MPH) on MPH-induced CTA, followed by expression between preexposure groups of BDNF in the IC, central nucleus of the amygdala (CeA), and the nucleus accumbens (NAc).

METHODS

Following preexposure to MPH (18 mg/kg), CTAs induced by MPH (0, 10, 18, and 32 mg/kg) were assessed in adult male Sprague-Dawley rats (n = 64). In separate groups (n = 31), differences in BDNF and tropomyosin-related kinase receptor-B (TrkB) were assessed using Western blots following similar preexposure and conditioning procedures.

RESULTS

Preexposure to MPH significantly blunted MPH-CTA compared to preexposure to vehicle. Unexpectedly, there were no significant effects of MPH on BDNF activity following CTA, but animals preexposed to MPH exhibited decreased activity in the CeA and enhanced activity in the IC and NAc.

CONCLUSIONS

Preexposure to MPH attenuates its aversive effects on subsequent presentations, and BDNF's impact on CTA learning may be dependent upon its temporal relation to other CTA-related intracellular cascades.

Prenatal exposure to ultrasound affects learning and memory in young rats

Prenatal exposure to ultrasound may cause cognitive impairments in experimental animals; however, the exact mechanisms remain unknown. In this study, we exposed pregnant rats (or sham-exposed controls) to different intensities of ultrasound repeatedly on days 6, 12 and 18 of pregnancy for 4 min (3.5 MHz, spatial peak time average intensity = 7.6 mW/cm(2), mechanical index = 0.1, thermal index bone = 0.1: 4-min group) or 20 min (3.5 MHz, spatial peak time average intensity = 106 mW/cm(2), mechanical index = 1.4, thermal index bone = 1.0: 20-min group). The Morris water maze was used to assess learning and memory function in pups at 2 mo of age. Noticeable deficits in behavior occurred in the group exposed to ultrasound for 20 min. Using real-time polymerase chain reaction and Western blot, we also determined that both the mRNA and protein expression levels of hippocampal N-methyl-D-aspartate (NMDA) receptor units 1 (NR1) and 2B (NR2B) and brain-derived neurotrophic factor (BDNF) were significantly lower in pups exposed to ultrasound for 20 min than in controls. Furthermore, the morphology of the synapses in the hippocampus was partially damaged. Compared with the control group, the 4-min group had better spatial learning and memory abilities, as well as higher mRNA and protein levels of NR1, NR2B and BDNF. Our study suggests that high-intensity ultrasound irradiation can decrease learning and memory abilities by reducing the expression of NR1, NR2B and BDNF in the hippocampal regions and damaging the structure of synapses. In contrast, low-intensity ultrasound irradiation can enhance the learning and memory abilities of the offspring rats by increasing the expression of NR1, NR2B and BDNF receptor in the hippocampal regions.

BDNF deletion or TrkB impairment in amygdala inhibits both appetitive and aversive learning

Brain-derived neurotrophic factor (BDNF) is known to have an integral role in establishing stable memories after learning events. The neuroplasticity induced by Pavlovian fear conditioning has likewise been shown to rely on interactions between BDNF and its principal receptor, tyrosine kinase receptor B (TrkB), in the amygdala after training. Although the necessity of amygdala bdnf expression and TrkB activation for associative learning within aversive contexts has been explored, it is unclear to what extent this interaction is involved in appetitive learning. It is also unclear whether the noted increases in amygdala BDNF after fear conditioning are due to local gene transcription and translation or anterograde transmission from cortical regions. To address both of these questions, we used two lentiviral approaches in mice, using both fear conditioning and cocaine-conditioned place preference (CPP), during acquisition and extinction. First, we decreased expression of bdnf mRNA in the amygdala of homozygous floxed mice with a Cre-expressing virus. In a second set of studies, we infused a virus that expressed a dominant-negative TrkB isoform into the same region. These approaches significantly impaired consolidation of fear conditioning and cocaine-CPP, as well as extinction of CPP. Together, these data suggest that BDNF-TrkB signaling is critical for amygdala-dependent learning of both appetitive and aversive emotional memories.

BDNF and NT4 play interchangeable roles in gustatory development

A limited number of growth factors are capable of regulating numerous developmental processes, but how they accomplish this is unclear. The gustatory system is ideal for examining this issue because the neurotrophins brain-derived neurotrophic factor (BDNF) and neurotrophin-4 (NT4) have different developmental roles although both of them activate the same receptors, TrkB and p75. Here we first investigated whether the different roles of BDNF and NT4 are due to their differences in temporal and spatial expression patterns. Then, we asked whether or not these two neurotrophins exert their unique roles on the gustatory system by regulating different sets of downstream genes. By using Bdnf(Nt4/Nt4) mice, in which the coding region for BDNF is replaced with NT4, we examined whether the different functions of BDNF and NT4 are interchangeable during taste development. Our results demonstrated that NT4 could mediate most of the unique roles of BDNF during taste development. Specifically, caspase-3-mediated cell death, which was increased in the geniculate ganglion in Bdnf(-/-) mice, was rescued in Bdnf(Nt4/Nt4) mice. In BDNF knockout mice, tongue innervation was disrupted, and gustatory axons failed to reach their targets. However, disrupted innervation was rescued and target innervation is normal when NT4 replaced BDNF. Genome wide expression analyses revealed that BDNF and NT4 mutant mice exhibited different gene expression profiles in the gustatory (geniculate) ganglion. Compared to wild type, the expression of differentiation-, apoptosis- and axon guidance-related genes was changed in BDNF mutant mice, which is consistent with their different roles during taste development. However, replacement of BDNF by NT4 rescued these gene expression changes. These findings indicate that the functions of BDNF and NT4 in taste development are interchangeable. Spatial and temporal differences in BDNF and NT4 expression can regulate differential gene expression in vivo and determine their specific roles during development.

BDNF-dependent consolidation of fear memories in the perirhinal cortex

In the recent years the perirhinal cortex (PRh) has been identified as a crucial brain area in fear learning. Since the neurotrophin brain-derived neurotrophic factor (BDNF) is an important mediator of synaptic plasticity and also crucially involved in memory consolidation of several learning paradigms, we analyzed now whether fear conditioning influences the expression of BDNF protein in the PRh. Here we observed a specific increase of BDNF protein 120 min after fear conditioning training. In order to test whether this increase of BDNF protein level is also required for the consolidation of the fear memory, we locally applied the Trk receptor inhibitor k252a into the PRh during this time window in a second series of experiments. By interfering with Trk-signaling during this critical time window, the formation of a long-term fear memory was completely blocked, indicated by a complete lack of fear potentiated startle 1 day later. In conclusion the present study further emphasizes the important role of the PRh in cued fear learning and identified BDNF as an important mediator for fear memory consolidation in the PRh.

Distribution of CaMKIIα expression in the brain in vivo, studied by CaMKIIα-GFP mice

To facilitate the study of the CaMKIIα function in vivo, a CaMKIIα-GFP transgenic mouse line was generated. Here, our goal is to provide the first neuroanatomical characterization of GFP expression in the CNS of this line of mouse. Overall, CaMKIIα-GFP expression is strong and highly heterogeneous, with the dentate gyrus of the hippocampus as the most abundantly expressed region. In the hippocampus, around 70% of granule and pyramidal neurons expressed strong GFP. In the neocortex, presumed pyramidal neurons were GFP positive: around 32% of layer II/III and 35% of layer VI neurons expressed GFP, and a lower expression rate was found in other layers. In the thalamus and hypothalamus, strong GFP signals were detected in the neuropil. GFP-positive cells were also found in many other regions such as the spinal trigeminal nucleus, cerebellum and basal ganglia. We further compared the GFP expression with specific antibody staining for CaMKIIα and GABA. We found that GFP+ neurons were mostly positive for CaMKIIα-IR throughout the brain, with some exceptions throughout the brain, especially in the deeper layers of neocortex. GFP and GABA-IR marked distinct neuronal populations in most brain regions with the exception of granule cells in the olfactory bulb, purkinje cells in the cerebellar, and some layer I cells in neocortex. In conclusion, GFP expression in the CaMKIIα-GFP mice is similar to the endogenous expression of CaMKIIα protein, thus these mice can be used in in vivo and in vitro physiological studies in which visualization of CaMKIIα- neuronal populations is required.

Cholinergic dysfunction alters synaptic integration between thalamostriatal and corticostriatal inputs in DYT1 dystonia

Projections from thalamic intralaminar nuclei convey sensory signals to striatal cholinergic interneurons. These neurons respond with a pause in their pacemaking activity, enabling synaptic integration with cortical inputs to medium spiny neurons (MSNs), thus playing a crucial role in motor function. In mice with the DYT1 dystonia mutation, stimulation of thalamostriatal axons, mimicking a response to salient events, evoked a shortened pause and triggered an abnormal spiking activity in interneurons. This altered pattern caused a significant rearrangement of the temporal sequence of synaptic activity mediated by M(1) and M(2) muscarinic receptors in MSNs, consisting of an increase in postsynaptic currents and a decrease of presynaptic inhibition, respectively. Consistent with a major role of acetylcholine, either lowering cholinergic tone or antagonizing postsynaptic M(1) muscarinic receptors normalized synaptic activity. Our data demonstrate an abnormal time window for synaptic integration between thalamostriatal and corticostriatal inputs, which might alter the action selection process, thereby predisposing DYT1 gene mutation carriers to develop dystonic movements.

Hippocampus-specific deletion of tissue plasminogen activator "tPA" in adult mice impairs depression- and anxiety-like behaviors

Anxiety and depression are multifactorial disorders that have become prominent health problems all over the world. Neurotrophic factors have emerged underlying pathogenesis of these diseases. Although a number of studies indicate that the hippocampus-brain-derived neurotrophic factor (BDNF) may be involved in these psychiatric illnesses, little is known about the molecular mediators of these disorders. In this study we further investigate the role of tissue plasminogen activator (tPA), a serine protease involved in pro-BDNF cleavage to BDNF, in depression and anxiety-like behaviors in adult mice. To address this issue, we investigated the effect of hippocampus tPA manipulation, using viral vectors, on anxiety- and depression-like behaviors, including the marble burying test (MBT), elevated plus maze (EPM), tail suspension test (TST), novelty suppressed feeding (NSF) and forced swim test (FST). Our results showed that tPA knock-down - using lentiviral vectors expressing specific short hairpin RNAs (LV-shRNA) - increased the number of buried marbles together with the digging time in the MBT and decreased the time spent in open the arms of an EPM. In addition, tPA-knock down in the hippocampus increased immobility in the FST and TST, and increased time to feed in the NSF test. These effects were reversed when tPA-over-expressing vectors (LV-tPA) were injected in the hippocampus. We also found that BDNF protein levels were elevated in the hippocampus of mice receiving tPA-expressing vectors. Together, our results imply that tPA manipulation may provide an effective therapeutic intervention for depression and anxiety disorders.

Epigenetic modulation of Homer1a transcription regulation in amygdala and hippocampus with pavlovian fear conditioning

The consolidation of conditioned fear involves upregulation of genes necessary for long-term memory formation. An important question remains as to whether this results in part from epigenetic regulation and chromatin modulation. We examined whether Homer1a, which is required for memory formation, is necessary for Pavlovian cued fear conditioning, whether it is downstream of BDNF-TrkB activation, and whether this pathway utilizes histone modifications for activity-dependent transcriptional regulation. We initially found that Homer1a knock-out mice exhibited deficits in cued fear conditioning (5 tone-shock presentations with 70 dB, 6 kHz tones and 0.5 s, 0.6 mA footshocks). We then demonstrated that: (1) Homer1a mRNA increases after fear conditioning in vivo within both amygdala and hippocampus of wild-type mice; (2) it increases after BDNF application to primary hippocampal and amygdala cultures in vitro; and (3) these increases are dependent on transcription and MAPK signaling. Furthermore, using chromatin immunoprecipitation we found that both in vitro and in vivo manipulations result in decreases in Homer1 promoter H3K9 methylation in amygdala cells but increases in Homer1 promoter H3 acetylation in hippocampal cells. However, no changes were observed in H4 acetylation or H3K27 dimethylation. Inhibition of histone deacetylation by sodium butyrate enhanced contextual but not cued fear conditioning and enhanced Homer1 H3 acetylation in the hippocampus. These data provide evidence for dynamic epigenetic regulation of Homer1a following BDNF-induced plasticity and during a BDNF-dependent learning process. Furthermore, upregulation of this gene may be regulated through distinct epigenetic modifications in the hippocampus and amygdala.

Kainate receptor-mediated depression of glutamatergic transmission involving protein kinase A in the lateral amygdala

Kainate receptors (KARs) have been described as modulators of synaptic transmission at different synapses. However, this role of KARs has not been well characterized in the amygdala. We have explored the effect of kainate receptor activation at the synapse established between fibers originating at medial geniculate nucleus and the principal cells in the lateral amygdala. We have observed an inhibition of evoked excitatory postsynaptic currents (eEPSCs) amplitude after a brief application of KARs agonists KA and ATPA. Paired-pulse recordings showed a clear pair pulse facilitation that was enhanced after KA or ATPA application. When postsynaptic cells were loaded with BAPTA, the depression of eEPSC amplitude observed after the perfusion of KAR agonists was not prevented. We have also observed that the inhibition of the eEPSCs by KARs agonists was prevented by protein kinase A but not by protein kinase C inhibitors. Taken together our results indicate that KARs present at this synapse are pre-synaptic and their activation mediate the inhibition of glutamate release through a mechanism that involves the activation of protein kinase A.

Genome-wide association for fear conditioning in an advanced intercross mouse line

Fear conditioning (FC) may provide a useful model for some components of post-traumatic stress disorder (PTSD). We used a C57BL/6J × DBA/2J F(2) intercross (n = 620) and a C57BL/6J × DBA/2J F(8) advanced intercross line (n = 567) to fine-map quantitative trait loci (QTL) associated with FC. We conducted an integrated genome-wide association analysis in QTLRel and identified five highly significant QTL affecting freezing to context as well as four highly significant QTL associated with freezing to cue. The average percent decrease in QTL width between the F(2) and the integrated analysis was 59.2%. Next, we exploited bioinformatic sequence and expression data to identify candidate genes based on the existence of non-synonymous coding polymorphisms and/or expression QTLs. We identified numerous candidate genes that have been previously implicated in either fear learning in animal models (Bcl2, Btg2, Dbi, Gabr1b, Lypd1, Pam and Rgs14) or PTSD in humans (Gabra2, Oprm1 and Trkb); other identified genes may represent novel findings. The integration of F(2) and AIL data maintains the advantages of studying FC in model organisms while significantly improving resolution over previous approaches.

An experimental rat model of sporadic Alzheimer's disease and rescue of cognitive impairment with a neurotrophic peptide

Alzheimer's disease (AD) is multifactorial and, to date, no single cause of the sporadic form of this disease, which accounts for over 99% of the cases, has been established. In AD brain, protein phosphatase-2A (PP2A) activity is known to be compromised due to the cleavage and translocation of its potent endogenous inhibitor, I2PP2A, from the neuronal nucleus to the cytoplasm. Here, we show that adeno-associated virus vector-induced expression of the N-terminal I2NTF and C-terminal I2CTF halves of I2PP2A , also called SET, in brain reproduced key features of AD in Wistar rats. The I2NTF-CTF rats showed a decrease in brain PP2A activity, abnormal hyperphosphorylation and aggregation of tau, a loss of neuronal plasticity and impairment in spatial reference and working memories. To test whether early pharmacologic intervention with a neurotrophic molecule could rescue neurodegeneration and behavioral deficits, 2.5-month-old I2NTF-CTF rats and control littermates were treated for 40 days with Peptide 6, an 11-mer peptide corresponding to an active region of the ciliary neurotrophic factor. Peripheral administration of Peptide 6 rescued neurodegeneration and cognitive deficit in I2NTF-CTF animals by increasing dentate gyrus neurogenesis and mRNA level of brain derived neurotrophic factor. Moreover, Peptide 6-treated I2NTF-CTF rats showed a significant increase in dendritic and synaptic density as reflected by increased expression of synapsin I, synaptophysin and MAP2, especially in the pyramidal neurons of CA1 and CA3 of the hippocampus.

Postsynaptic BDNF signalling regulates long-term potentiation at thalamo-amygdala afferents

The neurotrophin brain-derived neurotrophic factor (BDNF) is known to regulate synaptic plasticity and memory formation in the hippocampus and the neocortex of the mammalian brain. In contrast, a role of BDNF in mediating synaptic plasticity and fear learning in the amygdala is just beginning to evolve. Using patch clamp recordings from projection neurons of the dorsal lateral amygdala (LA) in acute slices of mice, we now investigated the cellular mechanism of BDNF-mediated long-term potentiation (LTP) of excitatory postsynaptic currents (EPSCs) in the amygdala. LTP was elicited in cortical and thalamic synaptic inputs by pairing postsynaptic depolarisation with presynaptic stimulation. LTP in the cortico-amygdala pathway was not changed in heterozygous BDNF-knockout (BDNF(+/-)) mice. In contrast, pairing induced LTP in the thalamic input was abolished in BDNF(+/-) mice (BDNF(+/-): 104.0 ± 5.7% of initial EPSC values; WT: 132.5 ± 7.3%). Likewise, inhibition of BDNF/TrkB signalling with TrkB-IgGs as scavenger molecules for endogenous BDNF blocked LTP in wild-type mice in this pathway (TrkB-IgG: 102.7 ± 6.9% of initial EPSC values; control: 132.5 ± 8.7%). Inclusion of the tyrosine kinase inhibitor K252a in the pipette solution also prevented the induction of LTP in the thalamic pathway, indicating a postsynaptic site of action of BDNF in regulating LTP. Reduced BDNF levels in BDNF(+/-) mice did not affect intrinsic membrane properties of LA projection neurons. Likewise, presynaptic glutamate release, and postsynaptic membrane properties also remained unaffected in BDNF(+/-) mice. These data suggest a postsynaptic site of action of BDNF in mediating LTP selectively in the thalamic fear conditioning pathway.

Synergistic activation of dopamine D1 and TrkB receptors mediate gain control of synaptic plasticity in the basolateral amygdala

Fear memory formation is thought to require dopamine, brain-derived neurotrophic factor (BDNF) and zinc release in the basolateral amygdala (BLA), as well as the induction of long term potentiation (LTP) in BLA principal neurons. However, no study to date has shown any relationship between these processes in the BLA. Here, we have used in vitro whole-cell patch clamp recording from BLA principal neurons to investigate how dopamine, BDNF, and zinc release may interact to modulate the LTP induction in the BLA. LTP was induced by either theta burst stimulation (TBS) protocol or spaced 5 times high frequency stimulation (5xHFS). Significantly, both TBS and 5xHFS induced LTP was fully blocked by the dopamine D1 receptor antagonist, SCH23390. LTP induction was also blocked by the BDNF scavenger, TrkB-FC, the zinc chelator, DETC, as well as by an inhibitor of matrix metalloproteinases (MMPs), gallardin. Conversely, prior application of the dopamine reuptake inhibitor, GBR12783, or the D1 receptor agonist, SKF39393, induced robust and stable LTP in response to a sub-threshold HFS protocol (2xHFS), which does not normally induce LTP. Similarly, prior activation of TrkB receptors with either a TrkB receptor agonist, or BDNF, also reduced the threshold for LTP-induction, an effect that was blocked by the MEK inhibitor, but not by zinc chelation. Intriguingly, the TrkB receptor agonist-induced reduction of LTP threshold was fully blocked by prior application of SCH23390, and the reduction of LTP threshold induced by GBR12783 was blocked by prior application of TrkB-FC. Together, our results suggest a cellular mechanism whereby the threshold for LTP induction in BLA principal neurons is critically dependent on the level of dopamine in the extracellular milieu and the synergistic activation of postsynaptic D1 and TrkB receptors. Moreover, activation of TrkB receptors appears to be dependent on concurrent release of zinc and activation of MMPs.

Fear conditioning, synaptic plasticity and the amygdala: implications for posttraumatic stress disorder

Posttraumatic stress disorder (PTSD) is an anxiety disorder that can develop after a traumatic experience such as domestic violence, natural disasters or combat-related trauma. The cost of such disorders on society and the individual can be tremendous. In this article, we review how the neural circuitry implicated in PTSD in humans is related to the neural circuitry of fear. We then discuss how fear conditioning is a suitable model for studying the molecular mechanisms of the fear components that underlie PTSD, and the biology of fear conditioning with a particular focus on the brain-derived neurotrophic factor (BDNF)-tyrosine kinase B (TrkB), GABAergic and glutamatergic ligand-receptor systems. We then summarize how such approaches might help to inform our understanding of PTSD and other stress-related disorders and provide insight to new pharmacological avenues of treatment of PTSD.