Modulation of synaptic plasticity by stress hormone associates with plastic alteration of synaptic NMDA receptor in the adult hippocampus

Altered excitatory and decreased inhibitory transmission in the prefrontal cortex of male mice with early developmental disruption to the ventral hippocampus

Ventral hippocampal (vHPC)-prefrontal cortical (PFC) pathway dysfunction is a core neuroimaging feature of schizophrenia. However, mechanisms underlying impaired connectivity within this pathway remain poorly understood. The vHPC has direct projections to the PFC that help shape its maturation. Here, we wanted to investigate the effects of early developmental vHPC perturbations on long-term functional PFC organization. Using whole-cell recordings to assess PFC cellular activity in transgenic male mouse lines, we show early developmental disconnection of vHPC inputs, by excitotoxic lesion or cell-specific ablations, impairs pyramidal cell firing output and produces a persistent increase in excitatory and decrease in inhibitory synaptic inputs onto pyramidal cells. We show this effect is specific to excitatory vHPC projection cell ablation. We further identify PV-interneurons as a source of deficit in inhibitory transmission. We find PV-interneurons are reduced in density, show a reduced ability to sustain high-frequency firing, and show deficits in excitatory inputs that emerge over time. We additionally show differences in vulnerabilities to early developmental vHPC disconnection, wherein PFC PV-interneurons but not pyramidal cells show deficits in NMDA receptor-mediated current. Our results highlight mechanisms by which the PFC adapts to early developmental vHPC perturbations, providing insights into schizophrenia circuit pathology.

Non-genomic steroid signaling through the mineralocorticoid receptor: Involvement of a membrane-associated receptor?

Corticosteroid receptors in the mammalian brain mediate genomic as well as non-genomic actions. Although receptors mediating genomic actions were already cloned 35 years ago, it remains unclear whether the same molecules are responsible for the non-genomic actions or that the latter involve a separate class of receptors. Here we focus on one type of corticosteroid receptors, i.e. the mineralocorticoid receptor (MR). We summarize some of the known properties and the current insight in the localization of the MR in peripheral cells and neurons, especially in relation to non-genomic signaling. Previous studies from our own and other labs provided evidence that MRs mediating non-genomic actions are identical to the ones involved in genomic signaling, but may be translocated to the plasma cell membrane instead of the nucleus. With fixed cell imaging and live cell imaging techniques we tried to visualize these presumed membrane-associated MRs, using antibodies or overexpression of MR-GFP in COS7 and hippocampal cultured neurons. Despite the physiological evidence for MR location in or close to the cell membrane, we could not convincingly visualize membrane localization of endogenous MRs or GFP-MR molecules. However, we did find punctae of labeled antibodies intracellularly, which might indicate transactivating spots of MR near the membrane. We also found some evidence for trafficking of MR via beta-arrestins. In beta-arrestin knockout mice, we didn't observe metaplasticity in the basolateral amygdala anymore, indicating that internalization of MRs could play a role during corticosterone activation. Furthermore, we speculate that membrane-associated MRs could act indirectly via activating other membrane located structures like e.g. GPER and/or receptor tyrosine kinases.

Interaction Between Glucocorticoid Receptors and FKBP5 in Regulating Neurotransmission of the Hippocampus

FK501 binding protein 51 (FKBP5) is a stress response prolyl isomerase that inhibits the translocation of the glucocorticoid receptor (GR) heterocomplex to the nucleus. Previous studies have shown that the expression levels of FKBP5 are positively correlated with psychiatric disorders, including depression and post-traumatic stress disorder. In rodents, FKBP5 deletion in the brain leads to be resilient to stress-induced depression. The hippocampus is known to be one of the primary locations mediating stress responses in the brain by providing negative feedback signals to the hypothalamus-pituitaryadrenal gland axis. Therefore, we aimed to investigate the role of FKBP5 and its interaction with GRs in the hippocampus. We observed that FKBP5 deletion in the hippocampus resulted in a minimal change in synaptic transmission. In the hippocampus, GR activation alters the release probability in inhibitory synapses as well as the postsynaptic contribution of glutamate receptors in excitatory synapses; however, no such alterations were induced in the absence of FKBP5. FKBP5 deficiency causes insensitivity to activated GRs in the hippocampus suggesting that FKBP5 mediates synaptic changes caused by GR activation. Our study provides electrophysiological evidence of stress resilience observed in FKBP5-deficient mice.

Opposing Changes in Synaptic and Extrasynaptic N-Methyl-D-Aspartate Receptor Function in Response to Acute and Chronic Restraint Stress

A pertinent mechanism by which stress impacts learning and memory is through stress-induced plastic changes in glutamatergic transmission in the hippocampus. For instance, acute stress has been shown to alter the expression, binding, and function of the ionotropic glutamate N-methyl-D-aspartate receptor (NMDAR). However, the consequences of chronic stress, which could lead to various stress-related brain disorders, on NMDAR function remain unclear. While most studies on NMDARs focused on these receptors in synapses (synaptic NMDARs or sNMDARs), emerging findings have revealed functional roles of NMDARs outside synapses (extrasynaptic NMDARs or exNMDARs) that are distinct from those of sNMDARs. Using a restraint stress paradigm in adult rats, the objective of the current study is to examine whether sNMDARs and exNMDARs in the hippocampus are differentially regulated by acute and chronic stress. We examined sNMDAR and exNMDAR function in dorsal CA1 hippocampal neurons from brain slices of adult rats that were acutely (1 episode) or chronically (21 daily episodes) stressed by restraint (30 min). We found that acute stress increases sNMDAR but suppresses exNMDAR function. Surprisingly, we only observed a reduction in exNMDAR function after chronic stress. Taken together, our findings suggest that sNMDARs and exNMDARs may be differentially regulated by acute and chronic stress. Most importantly, the observed suppression in exNMDAR function by both acute and chronic stress implies crucial but overlooked roles of hippocampal exNMDARs in stress-related disorders.

Hypergravity-induced malfunction was moderated by the regulation of NMDA receptors in the vestibular nucleus

Gravity alteration is one of the critical environmental factors in the space, causing various abnormal behaviors related with the malfunctioned vestibular system. Due to the high plastic responses in the central vestibular system, the behavioral failures were resolved in a short period of time (in approx. 72 h). However, the plastic neurotransmission underlying the functional recovery is still elusive. To understand the neurotransmitter-induced plasticity under hypergravity, the extracellular single neuronal recording and the immunohistochemistry were conducted in the vestibular nucleus (VN). The animals were grouped as control, 24-h, 72-h, and 15-day exposing to 4G-hypergravity, and each group had two subgroups based on the origins of neuronal responses, such as canal and otolith. The averaged firing rates in VN showed no significant difference in the subgroups (canal-related: p > 0.105, otolith-related: p > 0.138). Meanwhile, the number of NMDAr was significantly changed by the exposing duration to hypergravity. The NMDAr decreased in 24 h (p = 1.048 × 10^–9), and it was retrieved in 72 h and 15 days (p < 4.245 × 10^–5). Apparently, the reduction and the retrieval in the number of NMDAr were synchronized with the generation and recovery of the abnormal behaviors. Thus, the plasticity to resolve the hypergravity-induced malfunctional behaviors was conducted by regulating the number of NMDAr.

Methamphetamine pre-exposure induces steeper escalation of methamphetamine self-administration with consequent alterations in hippocampal glutamate AMPA receptor mRNAs

Methamphetamine use disorder (MUD) is often modeled using rodent self-administration (SA) experiments. Noncontingent injections of a drug given to rodents before self-administration training can increase drug SA. In the present study, we injected methamphetamine before putting rats through methamphetamine SA to investigate SA escalation. We also measured consequent changes in the expression of glutamate receptors in the hippocampus. Experimental groups included rats that received the methamphetamine injection prior to self-administration (MM) and those that received a prior saline injection before they underwent methamphetamine SA (SM). After SA training, rats also underwent tests of relapse potentials at one day and one month after withdrawal from methamphetamine SA. We used qPCR to identify potential changes in mRNA expression of AMPA, NMDA, and mGluR glutamate receptors. MM rats showed greater escalated methamphetamine intake in comparison to SM animals. There were no differences in incubation of methamphetamine craving between the two groups. In the hippocampus, MM rats showed decreased levels of GluA2 and GluA3 mRNAs in comparison to controls and of GluN2c mRNA in comparison to SM rats. In addition, SM rats had increased mGluR3 mRNA levels in comparison to control and MM rats. These data implicate hippocampal glutamate receptors in the longterm effects of methamphetamine. Further studies are necessary to identify the specific role that changes in the expression of these receptors might play in escalated intake of methamphetamine by human users.

The susceptibility to chronic social defeat stress is related to low hippocampal extrasynaptic NMDA receptor function

N-methyl-D-aspartate receptors (NMDARs) have been highly implicated in the pathogenesis and treatment of depression. While NMDARs can be found inside and outside glutamate synapses, it remains unclear if NMDARs at synaptic (sNMDAR) and extrasynaptic locations (exNMDAR) play different roles in the formation of depression-related behaviors. Using chronic social defeat stress (CSDS), an animal model for anxiety- and depression-related behaviors, we found that mice susceptible to CSDS exhibited low hippocampal exNMDAR function. Raising exNMDAR function by enhancing the release of glutamate from astrocytic cystine-glutamate antiporters or targeting extrasynaptic receptors with agonist-coated gold nanoparticles that cannot enter the synaptic cleft prevented social avoidance behavior in stressed mice. Interestingly, ketamine, which is a fast-acting antidepressant, exhibited stronger blockade to sNMDARs than to exNMDARs. These findings suggest that the susceptibility and resilience of mice toward CSDS is related to low and high exNMDAR function in the hippocampus, respectively. Enhancing exNMDAR function could be a novel treatment approach for mood and anxiety disorders.

Hippocampal Memory Recovery After Acute Stress: A Behavioral, Morphological and Molecular Study

Several studies have shown that a single exposure to stress may improve or impair learning and memory processes, depending on the timing in which the stress event occurs with relation to the acquisition phase. However, to date there is no information about the molecular changes that occur at the synapse during the stress-induced memory modification and after a recovery period. In particular, there are no studies that have evaluated—at the same time—the temporality of stress and stress recovery period in hippocampal short-term memory and the effects on dendritic spine morphology, along with variations in N-methyl-D-aspartate (NMDA) and α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor subunits. The aim of our study was to take a multidimensional approach to investigate concomitant behavioral, morphological and molecular changes induced by a single restraint stress exposure (2.5 h) and a recovery period of 6 and 24 h in rats. We found that acute stress elicited a reduced preference to explore an object placed in a novel position (a hippocampal-dependent task). These changes were accompanied by increased activity of LIM kinase I (LIMK; an actin-remodeling protein) and increased levels of NR2A subunits of NMDA receptors. After 6 h of recovery from stress, rats showed similar preference to explore an object placed in a novel or familiar position, but density of immature spines increased in secondary CA1 apical dendrites, along with a transient rise in GluA2 AMPA receptor subunits. After 24 h of recovery from stress, the animals showed a preference to explore an object placed in a novel position, which was accompanied by a normalization of NMDA and AMPA receptor subunits to control values. Our data suggest that acute stress produces reversible molecular and behavioral changes 24 h after stress, allowing a full reestablishment of hippocampal-related memory. Further studies need to be conducted to deepen our understanding of these changes and their reciprocal interactions.Adaptive stress responses are a promising avenue to develop interventions aiming at restoring hippocampal function impaired by repetitive stress exposure.

NMDA Receptor Subunits Change after Synaptic Plasticity Induction and Learning and Memory Acquisition

NMDA ionotropic glutamate receptors (NMDARs) are crucial in activity-dependent synaptic changes and in learning and memory. NMDARs are composed of two GluN1 essential subunits and two regulatory subunits which define their pharmacological and physiological profile. In CNS structures involved in cognitive functions as the hippocampus and prefrontal cortex, GluN2A and GluN2B are major regulatory subunits; their expression is dynamic and tightly regulated, but little is known about specific changes after plasticity induction or memory acquisition. Data strongly suggest that following appropriate stimulation, there is a rapid increase in surface GluN2A-NMDAR at the postsynapses, attributed to lateral receptor mobilization from adjacent locations. Whenever synaptic plasticity is induced or memory is consolidated, more GluN2A-NMDARs are assembled likely using GluN2A from a local translation and GluN1 from local ER. Later on, NMDARs are mobilized from other pools, and there are de novo syntheses at the neuron soma. Changes in GluN1 or NMDAR levels induced by synaptic plasticity and by spatial memory formation seem to occur in different waves of NMDAR transport/expression/degradation, with a net increase at the postsynaptic side and a rise in expression at both the spine and neuronal soma. This review aims to put together that information and the proposed hypotheses.

Interactions between stress and physical activity on Alzheimer's disease pathology

Physical activity and stress are both environmental modifiers of Alzheimer's disease (AD) risk. Animal studies of physical activity in AD models have largely reported positive results, however benefits are not always observed in either cognitive or pathological outcomes and inconsistencies among findings remain. Studies using forced exercise may increase stress and mitigate some of the benefit of physical activity in AD models, while voluntary exercise regimens may not achieve optimal intensity to provide robust benefit. We evaluated the findings of studies of voluntary and forced exercise regimens in AD mouse models to determine the influence of stress, or the intensity of exercise needed to outweigh the negative effects of stress on AD measures. In addition, we show that chronic physical activity in a mouse model of AD can prevent the effects of acute restraint stress on Aβ levels in the hippocampus. Stress and physical activity have many overlapping and divergent effects on the body and some of the possible mechanisms through which physical activity may protect against stress-induced risk factors for AD are discussed. While the physiological effects of acute stress and acute exercise overlap, chronic effects of physical activity appear to directly oppose the effects of chronic stress on risk factors for AD. Further study is needed to identify optimal parameters for intensity, duration and frequency of physical activity to counterbalance effects of stress on the development and progression of AD.

Early life stress determines the effects of glucocorticoids and stress on hippocampal function: Electrophysiological and behavioral evidence respectively

Exposure to early-life adversity may program brain function to prepare individuals for adaptation to matching environmental contexts. In this study we tested this hypothesis in more detail by examining the effects of early-life stress - induced by raising offspring with limited nesting and bedding material from postnatal days 2-9 - in various behavioral tasks and on synaptic function in adult mice. Early-life stress impaired adult performance in the hippocampal dependent low-arousing object-in-context recognition memory task. This effect was absent when animals were exposed to a single stressor before training. Early-life stress did not alter high-arousing context and auditory fear conditioning. Early-life stress-induced behavioral modifications were not associated with alterations in the dendritic architecture of hippocampal CA1 pyramidal neurons or principal neurons of the basolateral amygdala. However, early-life stress reduced the ratio of NMDA to AMPA receptor-mediated excitatory postsynaptic currents and glutamate release probability specifically in hippocampal CA1 neurons, but not in the basolateral amygdala. These ex vivo effects in the hippocampus were abolished by acute glucocorticoid treatment. Our findings support that early-life stress can hamper object-in-context learning via pre- and postsynaptic mechanisms that affect hippocampal function but these effects are counteracted by acute stress or elevated glucocorticoid levels.

Morphological and functional changes in the preweaning basolateral amygdala induced by early chronic stress associate with anxiety and fear behavior in adult male, but not female rats

Suboptimal maternal care is a form of chronic early-life stress (ELS) and a risk factor for mental illness and behavioral impairments throughout the life span. The amygdala, particularly the basolateral amygdala (BLA), exhibits exquisite sensitivity to ELS and could promote dysregulation of stress reactivity and anxiety-related disorders. While ELS has profound impacts on the adult or adolescent amygdala, less is known regarding the sensitivity of the preweaning BLA to ELS. We employed a naturalistic rodent model of chronic ELS that limits the amount of bedding/nesting material (LB) available to the mother between postnatal day (PND) 1-9 and examined the morphological and functional effects in the preweaning BLA on PND10 and 18-22. BLA neurons displayed dendritic hypertrophy and increased spine numbers in male, but not female, LB pups already by PND10 and BLA volume tended to increase after LB exposure in preweaning rats, suggesting an accelerated and long-lasting recruitment of the amygdala. Morphological changes seen in male LB pups were paralleled with increased evoked synaptic responses recorded from BLA neurons in vitro, suggesting enhanced excitatory inputs to these neurons. Interestingly, morphological and functional changes in the preweaning BLA were not associated with basal hypercorticosteronemia or enhanced stress responsiveness in LB pups, perhaps due to a differential sensitivity of the neuroendocrine stress axis to the effects of LB exposure. Early changes in the synaptic organization and excitability of the neonatal amygdala might contribute to the increased anxiety-like and fear behavior observed in adulthood, specifically in male offspring.

Stress hormone rapidly tunes synaptic NMDA receptor through membrane dynamics and mineralocorticoid signalling

Stress hormones, such as corticosteroids, modulate the transmission of hippocampal glutamatergic synapses and NMDA receptor (NMDAR)-dependent synaptic plasticity, favouring salient behavioural responses to the environment. The corticosterone-induced synaptic adaptations partly rely on changes in NMDAR signalling, although the cellular pathway underlying this effect remains elusive. Here, we demonstrate, using single molecule imaging and electrophysiological approaches in hippocampal neurons, that corticosterone specifically controls GluN2B-NMDAR surface dynamics and synaptic content through mineralocorticoid signalling. Strikingly, extracellular corticosterone was sufficient to increase the trapping of GluN2B-NMDAR within synapses. Functionally, corticosterone-induced potentiation of AMPA receptor content in synapses required the changes in NMDAR surface dynamics. These high-resolution imaging data unveiled that, in hippocampal networks, corticosterone is a natural, potent, fast and specific regulator of GluN2B-NMDAR membrane trafficking, tuning NMDAR-dependent synaptic adaptations.

Revisiting the flip side: Long-term depression of synaptic efficacy in the hippocampus

Synaptic plasticity is widely regarded as a putative biological substrate for learning and memory processes. While both decreases and increases in synaptic strength are seen as playing a role in learning and memory, long-term depression (LTD) of synaptic efficacy has received far less attention than its counterpart long-term potentiation (LTP). Never-the-less, LTD at synapses can play an important role in increasing computational flexibility in neural networks. In addition, like learning and memory processes, the magnitude of LTD can be modulated by factors that include stress and sex hormones, neurotrophic support, learning environments, and age. Examining how these factors modulate hippocampal LTD can provide the means to better elucidate the molecular underpinnings of learning and memory processes. This is in turn will enhance our appreciation of how both increases and decreases in synaptic plasticity can play a role in different neurodevelopmental and neurodegenerative conditions.

d-Aspartic acid ameliorates painful and neuropsychiatric changes and reduces β-amyloid Aβ1-42 peptide in a long lasting model of neuropathic pain

Depressive symptoms and other neuropsychiatric dysfunctions are common in neurodegenerative disorders, including chronic pain and dementia. A correlation between the β-amyloid protein accumulation and the development of depression has been suggested, however the underlying mechanisms are unknown. d-Aspartate (d-Asp) is a free d-amino acid found in the mammalian brain and involved in neurological and psychiatric processes, such as cognition and affective disorders. In this study we have investigated the effects of a repeated treatment with d-Asp in a long-lasting (12 months) model of neuropathic pain, the spared nerve injury (SNI), in mice. Specifically, we evaluated i) the pain sensitivity and related emotional/cognitive dysfunctions induced by SNI, ii) possible changes in the β-amyloid protein accumulation in specific brain regions involved in pain mechanisms ii) possible changes in steroids level in neuropathic animals with or without d-Asp in the same brain areas. SNI mice showed an increase of the insoluble form of Aβ_1-42 at hippocampal level and displayed cognitive impairments, stereotypical and depressive-like behaviours. d-Asp treatment reduced abnormal behaviours and normalized the β-amyloid protein expression. Moreover, d-Asp dramatically increased steroids level measured in the prefrontal cortex and in the hippocampus. Our findings provide new insights into pain mechanisms and suggest a possible role of β-amyloid protein in neuropsychiatric dysfunctions associated with chronic pain.

The kynurenine pathway and the brain: Challenges, controversies and promises

Research on the neurobiology of the kynurenine pathway has suffered years of relative obscurity because tryptophan degradation, and its involvement in both physiology and major brain diseases, was viewed almost exclusively through the lens of the well-established metabolite serotonin. With increasing recognition that kynurenine and its metabolites can affect and even control a variety of classic neurotransmitter systems directly and indirectly, interest is expanding rapidly. Moreover, kynurenine pathway metabolism itself is modulated in conditions such as infection and stress, which are known to induce major changes in well-being and behaviour, so that kynurenines may be instrumental in the etiology of psychiatric and neurological disorders. It is therefore likely that the near future will not only witness the discovery of additional physiological and pathological roles for brain kynurenines, but also ever-increasing interest in drug development based on these roles. In particular, targeting the kynurenine pathway with new specific agents may make it possible to prevent disease by appropriate pharmacological or genetic manipulations. The following overview focuses on areas of kynurenine research which are either controversial, of major potential therapeutic interest, or just beginning to receive the degree of attention which will clarify their relevance to neurobiology and medicine. It also highlights technical issues so that investigators entering the field, and new research initiatives, are not misdirected by inappropriate experimental approaches or incorrect interpretations at this time of skyrocketing interest in the subject matter. This article is part of the Special Issue entitled 'The Kynurenine Pathway in Health and Disease'.

NMDA receptor antagonist prevents cell death in the hippocampal dentate gyrus induced by hyponatremia accompanying adrenal insufficiency in rats

Selective apoptosis of granule cells in the hippocampal dentate gyrus (DG) of rats with bilateral adrenalectomy (ADX) and in patients who died of adrenal insufficiency has been reported. Although adrenal insufficiency is a common disease and is usually associated with hyponatremia, its effect on the central nervous system and in apoptosis in the hippocampus remain to be elucidated. Using rat models to represent clinical hyponatremia accompanying adrenal insufficiency, we show that reduced serum [Na⁺] was associated with selective apoptosis in the DG. Nine days after ADX, apoptotic cells were observed in the DG of rats whose serum [Na⁺] was <125mEq/L (moderate hyponatremia), but rarely in those whose serum [Na⁺] was ≥125mEq/L or in normonatremic rats. Although all hyponatremic ADX rats survived following treatment with corticosterone and saline started 7days after ADX when apoptosis had not yet occurred, selective apoptosis on day 9 was not prevented in moderately hyponatremic rats. Interestingly, treatment with memantine, a noncompetitive NMDAR antagonist, prevented the selective apoptosis in the DG in moderately hyponatremic, ADX rats, and improved electrophysiological dysfunction, including impaired basal synaptic transmission and long-term potentiation at the entorhinal cortex-DG synapses. These results demonstrated that in adrenal insufficient rats, hyponatremia was associated with apoptosis in the DG, and that memantine prevented the apoptosis and improved cell function. Our data imply the importance of assessing the possibility of neurological impairments after treatment with CORT in patients with moderate or severe hyponatremia accompanying adrenal insufficiency and that memantine may represent a beneficial therapeutic strategy to prevent neurological impairments in such patients.

Inhibiting tumor necrosis factor-α before amyloidosis prevents synaptic deficits in an Alzheimer's disease model

Deficits in synaptic structure and function are likely to underlie cognitive impairments in Alzheimer's disease. While synaptic deficits are commonly found in animal models of amyloidosis, it is unclear how amyloid pathology may impair synaptic functions. In some amyloid mouse models of Alzheimer's disease, however, synaptic deficits are preceded by hyperexcitability of glutamate synapses. In the amyloid transgenic mouse model TgCRND8, we therefore investigated whether early enhancement of glutamatergic transmission was responsible for development of later synaptic deficits. Hippocampi from 1-month-old TgCRND8 mice revealed increased basal transmission and plasticity of glutamate synapses that was related to increased levels of tumor necrosis factor α (TNFα). Treating these 1-month-old mice for 4 weeks with the TNFα inhibitor XPro1595 prevented synaptic deficits otherwise apparent at the age of 6 months. In this mouse model at least, reversing the hyperexcitability of glutamate synapses via TNFα blockade before the onset of amyloid plaque formation prevented later synaptic deficits.

Early Growth Response 1 (Egr-1) Regulates N-Methyl-d-aspartate Receptor (NMDAR)-dependent Transcription of PSD-95 and α-Amino-3-hydroxy-5-methyl-4-isoxazole Propionic Acid Receptor (AMPAR) Trafficking in Hippocampal Primary Neurons

The N-methyl-d-aspartate receptor (NMDAR) controls synaptic plasticity and memory function and is one of the major inducers of transcription factor Egr-1 in the hippocampus. However, how Egr-1 mediates the NMDAR signal in neurons has remained unclear. Here, we show that the hippocampus of mice lacking Egr-1 displays electrophysiology properties and ultrastructure that are similar to mice overexpressing PSD-95, a major scaffolding protein of postsynaptic density involved in synapse formation, synaptic plasticity, and synaptic targeting of AMPA receptors (AMPARs), which mediate the vast majority of excitatory transmission in the CNS. We demonstrate that Egr-1 is a transcription repressor of the PSD-95 gene and is recruited to the PSD-95 promoter in response to NMDAR activation. Knockdown of Egr-1 in rat hippocampal primary neurons blocks NMDAR-induced PSD-95 down-regulation and AMPAR endocytosis. Likewise, overexpression of Egr-1 in rat hippocampal primary neurons causes reduction in PSD-95 protein level and promotes AMPAR endocytosis. Our data indicate that Egr-1 is involved in NMDAR-mediated PSD-95 down-regulation and AMPAR endocytosis, a process important in the expression of long term depression.

Lack of presynaptic interaction between glucocorticoid and CB1 cannabinoid receptors in GABA- and glutamatergic terminals in the frontal cortex of laboratory rodents

Corticosteroid and endocannabinoid actions converge on prefrontocortical circuits associated with neuropsychiatric illnesses. Corticosteroids can also modulate forebrain synapses by using endocannabinoid effector systems. Here, we determined whether corticosteroids can modulate transmitter release directly in the frontal cortex and, in doing so, whether they affect presynaptic CB1 cannabinoid receptor- (CB1R) mediated neuromodulation. By Western blotting of purified subcellular fractions of the rat frontal cortex, we found glucocorticoid receptors (GcRs) and CB1Rs enriched in isolated frontocortical nerve terminals (synaptosomes). CB1Rs were predominantly presynaptically located while GcRs showed preference for the post-synaptic fraction. Additional confocal microscopy analysis of cortical and hippocampal regions revealed vesicular GABA transporter-positive and vesicular glutamate transporter 1-positive nerve terminals endowed with CB1R immunoreactivity, apposing GcR-positive post-synaptic compartments. In functional transmitter release assay, corticosteroids, corticosterone (0.1-10 microM) and dexamethasone (0.1-10 microM) did not significantly affect the evoked release of [(3)H]GABA and [(14)C]glutamate in superfused synaptosomes, isolated from both rats and mice. In contrast, the synthetic cannabinoid, WIN55212-2 (1 microM) diminished the release of both [(3)H]GABA and [(14)C]glutamate, evoked with various depolarization paradigms. This effect of WIN55212-2 was abolished by the CB1R neutral antagonist, O-2050 (1 microM), and was absent in the CB1R KO mice. CB2R-selective agonists did not affect the release of either neurotransmitter. The lack of robust presynaptic neuromodulation by corticosteroids was unchanged upon either CB1R activation or genetic inactivation. Altogether, corticosteroids are unlikely to exert direct non-genomic presynaptic neuromodulation in the frontal cortex, but they may do so indirectly, via the stimulation of trans-synaptic endocannabinoid signaling.

Dicholine succinate, the neuronal insulin sensitizer, normalizes behavior, REM sleep, hippocampal pGSK3 beta and mRNAs of NMDA receptor subunits in mouse models of depression

Central insulin receptor-mediated signaling is attracting the growing attention of researchers because of rapidly accumulating evidence implicating it in the mechanisms of plasticity, stress response, and neuropsychiatric disorders including depression. Dicholine succinate (DS), a mitochondrial complex II substrate, was shown to enhance insulin-receptor mediated signaling in neurons and is regarded as a sensitizer of the neuronal insulin receptor. Compounds enhancing neuronal insulin receptor-mediated transmission exert an antidepressant-like effect in several pre-clinical paradigms of depression; similarly, such properties for DS were found with a stress-induced anhedonia model. Here, we additionally studied the effects of DS on several variables which were ameliorated by other insulin receptor sensitizers in mice. Pre-treatment with DS of chronically stressed C57BL6 mice rescued normal contextual fear conditioning, hippocampal gene expression of NMDA receptor subunit NR2A, the NR2A/NR2B ratio and increased REM sleep rebound after acute predation. In 18-month-old C57BL6 mice, a model of elderly depression, DS restored normal sucrose preference and activated the expression of neural plasticity factors in the hippocampus as shown by Illumina microarray. Finally, young naïve DS-treated C57BL6 mice had reduced depressive- and anxiety-like behaviors and, similarly to imipramine-treated mice, preserved hippocampal levels of the phosphorylated (inactive) form of GSK3 beta that was lowered by forced swimming in pharmacologically naïve animals. Thus, DS can ameliorate behavioral and molecular outcomes under a variety of stress- and depression-related conditions. This further highlights neuronal insulin signaling as a new factor of pathogenesis and a potential pharmacotherapy of affective pathologies.

The interaction between stress and exercise, and its impact on brain function

In response to acute adversity, emotional signals shift the body into a state that permits rapid detection, identification, and appropriate response to a potential threat. The stress response involves the release of a variety of substances, including neurotransmitters, neurotrophic factors, hormones, and cytokines, that enable the body to deal with the challenges of daily life. The subsequent activation of various physiological systems can be both protective and damaging to the individual, depending on timing, intensity, and duration of the stressor. Successful recovery from stressful challenges during early life leads to strengthening of synaptic connections in health-promoting neural networks and reduced vulnerability to subsequent stressors that can be protective in later life. In contrast, chronic intense uncontrollable stress can be pathogenic and lead to disorders such as depression, anxiety, hypertension, Alzheimer's disease, Parkinson's disease, and an increased toxic response to additional stressors such as traumatic brain injury and stroke. This review briefly explores the interaction between stress experienced at different stages of development and exercise later in life.

The three-hit concept of vulnerability and resilience: toward understanding adaptation to early-life adversity outcome

Stressful experiences during early-life can modulate the genetic programming of specific brain circuits underlying emotional and cognitive aspects of behavioral adaptation to stressful experiences later in life. Although this programming effect exerted by experience-related factors is an important determinant of mental health, its outcome depends on cognitive inputs and hence the valence an individual assigns to a given environmental context. From this perspective we will highlight, with studies in rodents, non-human primates and humans, the three-hit concept of vulnerability and resilience to stress-related mental disorders, which is based on gene-environment interactions during critical phases of perinatal and juvenile brain development. The three-hit (i.e., hit-1: genetic predisposition, hit-2: early-life environment, and hit-3: later-life environment) concept accommodates the cumulative stress hypothesis stating that in a given context vulnerability is enhanced when failure to cope with adversity accumulates. Alternatively, the concept also points to the individual's predictive adaptive capacity, which underlies the stress inoculation and match/mismatch hypotheses. The latter hypotheses propose that the experience of relatively mild early-life adversity prepares for the future and promotes resilience to similar challenges in later-life; when a mismatch occurs between early and later-life experience, coping is compromised and vulnerability is enhanced. The three-hit concept is fundamental for understanding how individuals can either be prepared for coping with life to come and remain resilient or are unable to do so and succumb to a stress-related mental disorder, under seemingly identical circumstances.

Prenatal immune activation interacts with stress and corticosterone exposure later in life to modulate N-methyl-D-aspartate receptor synaptic function and plasticity

Prenatal infection is an environmental risk factor for schizophrenia while later in life, stressful events have been associated with the onset and severity of psychosis. Recent findings on the impact of stress on the N-methyl-d-aspartate receptor (NMDAR), of which hypofunctioning is implicated in schizophrenia, suggest changes in stress-induced regulation of the glutamatergic system may be related to the pathogenesis of schizophrenia. Our study aimed to test whether prenatal immune activation could interact with stress at adolescence to alter NMDAR function. We used offspring from rat dams administered bacterial lipopolysaccharide (LPS) during pregnancy (gestational days 15 and 16), an animal model expressing schizophrenia-related behavioural phenotypes. Using electrophysiological techniques, we investigated effects of stress and the stress hormone corticosterone (Cort) on NMDAR-mediated synaptic function and long-term depression (LTD) in hippocampal CA1 slices from these adolescent (aged 28-39 d) male offspring. In prenatal LPS offspring, NMDAR-mediated synaptic function and LTD were reduced and abolished, respectively, compared to prenatal saline controls. Notably, in vivo stress and in vitro Cort treatment facilitated LTD in slices from prenatal LPS rats but not prenatal saline controls. Finally, Cort enhanced NMDAR-mediated synaptic function in slices from prenatal LPS rats only. We conclude that prenatal immune activation results in NMDAR hypofunction in the hippocampus of adolescent rats but also increases responsiveness of NMDAR-mediated synaptic function and LTD towards stress. Prenatal infection could confer susceptibility to schizophrenia through modification of hippocampal NMDAR function, with hypofunction in resting conditions and heightened responsiveness to stress, thus impacting the development of the disorder.

The inflammasome: pathways linking psychological stress, depression, and systemic illnesses

Stress is a common occurrence in everyday life and repeated or traumatic stress can be a precipitating factor for illnesses of the central nervous system, as well as peripheral organ systems. For example, severe or long-term psychological stress can not only induce depression, a leading illness worldwide, but can also cause psychosomatic diseases such as asthma and rheumatoid arthritis. Related key questions include how psychological stress influences both brain and peripheral systems, and what detection mechanisms underlie these effects? A clue is provided by the discovery of the pathways underlying the responses to host "danger" substances that cause systemic diseases, but can also contribute to depression. The inflammasome is a protein complex that can detect diverse danger signals and produce the accompanying immune-inflammatory reactions. Interestingly, the inflammasome can detect not only pathogen-associated molecules, but also cell damage-associated molecules such as ATP. Here, we propose a new inflammasome hypothesis of depression and related comorbid systemic illnesses. According to this hypothesis, the inflammasome is a central mediator by which psychological and physical stressors can contribute to the development of depression, and as well as a bridge to systemic diseases. This hypothesis includes an explanation for how psychological stress can influence systemic diseases, and conversely how systemic diseases can lead to psychiatric illnesses. The evidence suggests that the inflammasome may be a new target for the development of treatments for depression, as well as psychosomatic and somato-psycho diseases.

Stress and excitatory synapses: from health to disease

Individuals are exposed to stressful events in their daily life. The effects of stress on brain function ranges from highly adaptive to increasing the risk to develop psychopathology. For example, stressful experiences are remembered well which can be seen as a highly appropriate behavioral adaptation. On the other hand, stress is an important risk factor, in susceptible individuals, for depression and anxiety. An important question that remains to be addressed is how stress regulates brain function and what determines the threshold between adaptive and maladaptive responses. Excitatory synapses play a crucial role in synaptic transmission, synaptic plasticity and behavioral adaptation. In this review we discuss how brief and prolonged exposure to stress, in adulthood and early life, regulate the function of these synapses, and how these effects may contribute to behavioral adaptation and psychopathology.

Sex and regional differences in effects of chronic intermittent ethanol exposure on subsequent excitotoxic challenges in hippocampal slice cultures

The organotypic hippocampal slice culture technique was used to study how the effects of repeated ethanol withdrawal might differ between males and females at the cellular level, including potential modulation of subsequent insults. A chronic intermittent ethanol (CIE) exposure paradigm was employed, with 3 days of exposure followed by 24 h withdrawal for 3 cycles. Slices were next exposed to corticosterone (CORT) or pentylenetetrazol (PTZ) for 24 h then imaged for propidium iodide (PI) signal intensities. There were sex-selective responses in the CA1 region and dentate gyrus of the hippocampal slice cultures to treatment with CIE and/or CORT or PTZ. The 50 mM CIE alone generally did not increase the PI signal, but enhanced sensitivity to the toxic effects of CORT (particularly for females) and PTZ (particularly for males). In contrast, 100 mM CIE elicited a toxic response that was greater in females than males, and was exacerbated by exposure to PTZ. These data showed that hippocampal sexual dimorphism influences sensitivity to ethanol and other toxic chemicals even in an immature state. Low-dose CIE may attenuate harm from additional challenges in a hippocampal sex- and region-selective manner. These findings add to the growing evidence of important neurobiological sex differences in responses to chronic ethanol exposure and withdrawal.

Stable respiratory activity requires both P/Q-type and N-type voltage-gated calcium channels

P/Q-type voltage-gated calcium channels (Ca(v)2.1) play critical presynaptic and postsynaptic roles throughout the nervous system and have been implicated in a variety of neurological disorders. Here we report that mice with a genetic ablation of the Ca(v)2.1 pore-forming α(1A) subunit (α(1A)⁻/⁻) encoded by CACNA1a (Jun et al., 1999) suffer during postnatal development from increasing breathing disturbances that lead ultimately to death. Breathing abnormalities include decreased minute ventilation and a specific loss of sighs, which was associated with lung atelectasis. Similar respiratory alterations were preserved in the isolated in vitro brainstem slice preparation containing the pre-Bötzinger complex. The loss of Ca(v)2.1 was associated with an alteration in the functional dependency on N-type calcium channels (Ca(v)2.2). Blocking N-type calcium channels with conotoxin GVIA had only minor effects on respiratory activity in slices from control (CT) littermates, but abolished respiratory activity in all slices from α(1A)⁻/⁻ mice. The amplitude of evoked EPSPs was smaller in inspiratory neurons from α(1A)⁻/⁻ mice compared with CTs. Conotoxin GVIA abolished all EPSPs in inspiratory neurons from α(1A)⁻/⁻ mice, while the EPSP amplitude was reduced by only 30% in CT mice. Moreover, neuromodulation was significantly altered as muscarine abolished respiratory network activity in α(1A)⁻/⁻ mice but not in CT mice. We conclude that excitatory synaptic transmission dependent on N-type and P/Q-type calcium channels is required for stable breathing and sighing. In the absence of P/Q-type calcium channels, breathing, sighing, and neuromodulation are severely compromised, leading to early mortality.

NMDA receptor subunits in the adult rat hippocampus undergo similar changes after 5 minutes in an open field and after LTP induction

NMDA receptor subunits change during development and their synaptic expression is modified rapidly after synaptic plasticity induction in hippocampal slices. However, there is scarce information on subunits expression after synaptic plasticity induction or memory acquisition, particularly in adults. GluN1, GluN2A and GluN2B NMDA receptor subunits were assessed by western blot in 1) adult rats that had explored an open field (OF) for 5 minutes, a time sufficient to induce habituation, 2) mature rat hippocampal neuron cultures depolarized by KCl and 3) hippocampal slices from adult rats where long term potentiation (LTP) was induced by theta-burst stimulation (TBS). GluN1 and GluN2A, though not GluN2B, were significantly higher 70 minutes –but not 30 minutes- after a 5 minutes session in an OF. GluN1 and GluN2A total immunofluorescence and puncta in neurites increased in cultures, as evaluated 70 minutes after KCl stimulation. Similar changes were found in hippocampal slices 70 minutes after LTP induction. To start to explore underlying mechanisms, hippocampal slices were treated either with cycloheximide (a translation inhibitor) or actinomycin D (a transcription inhibitor) during electrophysiological assays. It was corroborated that translation was necessary for LTP induction and expression. The rise in GluN1 depends on transcription and translation, while the increase in GluN2A appears to mainly depend on translation, though a contribution of some remaining transcriptional activity during actinomycin D treatment could not be rouled out. LTP effective induction was required for the subunits to increase. Although in the three models same subunits suffered modifications in the same direction, within an apparently similar temporal course, further investigation is required to reveal if they are related processes and to find out whether they are causally related with synaptic plasticity, learning and memory.

Variations in postnatal maternal care and the epigenetic regulation of metabotropic glutamate receptor 1 expression and hippocampal function in the rat

Variations in maternal care in the rat affect hippocampal morphology and function as well as performance on hippocampal-dependent tests of learning and memory in the offspring. Preliminary genome-wide analyses of gene transcription and DNA methylation of the molecular basis for such maternal effects suggested differences in the epigenetic state and transcriptional activity of the Grm1 gene in the rat as a function of maternal care. Grm1 encodes the type I metabotropic glutamate receptor (mGluR1), and we found increased mGluR1 mRNA and protein in hippocampus from the adult offspring of mothers showing an increased frequency of pup licking/grooming (i.e., high-LG mothers) that was associated with a decrease in the methylation of Grm1. ChIP assays showed increased levels of histone 3 lysine 9 acetylation and histone 3 lysine 4 trimethylation of Grm1 in hippocampus from the adult offspring of high-LG compared with low-LG mothers. These histone posttranslational modifications were highly correlated, and both associate inversely with DNA methylation and positively with transcription. Studies of mGluR1 function showed increased hippocampal mGluR1-induced long-term depression in the adult offspring of high-LG compared with low-LG mothers, as well as increased paired-pulse depression (PPD). PPD is an inhibitory feedback mechanism that prevents excessive glutamate release during high-frequency stimulation. The maternal effects on both long-term depression and PPD were eliminated by treatment with an mGluR1-selective antagonist. These findings suggest that variations in maternal care can influence hippocampal function and cognitive performance through the epigenetic regulation of genes implicated in glutamatergic synaptic signaling.

Maternal care influences hippocampal N-methyl-D-aspartate receptor function and dynamic regulation by corticosterone in adulthood

BACKGROUND

Variations in maternal care in the rat associate with robust differences in hippocampal development and synaptic plasticity in the offspring. Maternal care also influences pituitary-adrenal stress responses and corticosterone (CORT) regulation of hippocampal plasticity. N-methyl-D-aspartate receptors (NMDAR) regulate synaptic plasticity, and NMDAR function is modulated by stress and CORT. We hypothesized that altered NMDAR function underlies the interaction of maternal and stress effects on hippocampal synaptic plasticity.

METHODS

We used electrophysiology and western blot to examine NMDAR synaptic function/expression and NMDAR-dependent long-term potentiation (LTP) in adult offspring of mothers that varied in the frequency of pup licking/grooming (LG) (i.e., High or Low LG).

RESULTS

Basal NMDAR synaptic function was enhanced in the hippocampal dentate gyrus (DG) of adult Low LG offspring. Synaptic expression of NMDAR but not α-amino-3-hydroxy-methyl-4-isoxazole propionic acid receptors was also increased. Stress level CORT (100 nmol/L) rapidly (< 20 min) and robustly increased NMDAR function in High LG offspring, eliminating the maternal effect. Corticosterone did not affect NMDAR function in Low LG offspring. Bovine serum albumin-conjugated CORT reproduced the CORT effect in High LG offspring, implicating a membrane-bound corticosteroid receptor. NMDAR hyperfunction might impair synaptic plasticity. Partial NMDAR antagonism by low concentration DL-2-Amino-5-phosphonopentanoic acid rescued a basal LTP deficit in Low LG offspring and inhibited LTP in High LG offspring.

CONCLUSIONS

Low LG offspring exhibit basally elevated NMDAR function coupled with insensitivity to CORT modulation indicative of a chronic alteration of NMDAR function. Elevated NMDAR function in the hippocampus might underlie impaired LTP in Low LG offspring.

Opposing alterations in excitation and inhibition of layer 5 medial prefrontal cortex pyramidal neurons following neonatal ventral hippocampal lesion

Cognitive abnormalities in schizophrenia reflect deficits in prefrontal cortical function, which could be related to attrition of dendritic structures of prefrontal cortical neurons. Schizophrenia-related prefrontal deficits have been modeled in postpubertal neonatal ventral hippocampal lesioned (NVHL) rats, which displayed a loss of dendritic complexity and spines in layer 3 pyramidal neurons in the medial prefrontal cortex (mPFC). The influence of dendritic attrition on synaptic function and neuronal excitability in the mPFC remains poorly understood. Here, we performed electrophysiological recordings of layer 5 mPFC pyramidal neurons from postpubertal (postnatal 40-60 days) NVHL rats and sham-operated controls. We found that the dendritic length, complexity, and spine density of neurobiotin-labeled layer 5 mPFC pyramidal neurons in NVHL rats were significantly lower than those in sham-operated rats. However, the excitability of layer 5 mPFC pyramidal neurons remained unchanged after NVHL. We found no significant changes in the expression of vesicular glutamate and γ-aminobutyric acid transporters after NVHL. Intriguingly, NVHL increased the amplitude of action potential-independent miniature excitatory postsynaptic currents and decreased the frequency of miniature inhibitory postsynaptic currents. These opposing alterations in excitatory and inhibitory synapses, possibly shifting basal synaptic activity toward increased excitation, could be cellular substrates for mPFC functional deficits reported in NVHL rats.

Dynamic regulation of NMDAR function in the adult brain by the stress hormone corticosterone

Stress and corticosteroids dynamically modulate the expression of synaptic plasticity at glutamatergic synapses in the developed brain. Together with alpha-amino-3-hydroxy-methyl-4-isoxazole propionic acid receptors (AMPAR), N-methyl-D-aspartate receptors (NMDAR) are critical mediators of synaptic function and are essential for the induction of many forms of synaptic plasticity. Regulation of NMDAR function by cortisol/corticosterone (CORT) may be fundamental to the effects of stress on synaptic plasticity. Recent reports of the efficacy of NMDAR antagonists in treating certain stress-associated psychopathologies further highlight the importance of understanding the regulation of NMDAR function by CORT. Knowledge of how corticosteroids regulate NMDAR function within the adult brain is relatively sparse, perhaps due to a common belief that NMDAR function is stable in the adult brain. We review recent results from our laboratory and others demonstrating dynamic regulation of NMDAR function by CORT in the adult brain. In addition, we consider the issue of how differences in the early life environment may program differential sensitivity to modulation of NMDAR function by CORT and how this may influence synaptic function during stress. Findings from these studies demonstrate that NMDAR function in the adult hippocampus remains sensitive to even brief exposures to CORT and that the capacity for modulation of NMDAR may be programmed, in part, by the early life environment. Modulation of NMDAR function may contribute to dynamic regulation of synaptic plasticity and adaptation in the face of stress, however, enhanced NMDAR function may be implicated in mechanisms of stress-related psychopathologies including depression.