Chronic unpredictable stress impairs long-term potentiation in rat hippocampal CA1 area and dentate gyrus in vitro

Chronic stress induces a selective decrease in AMPA receptor-mediated synaptic excitation at hippocampal temporoammonic-CA1 synapses

Chronic stress promotes depression, but how it disrupts cognition and mood remains unknown. Chronic stress causes atrophy of pyramidal cell dendrites in the hippocampus and cortex in human and animal models, and a depressive-like behavioral state. We now test the hypothesis that excitatory temporoammonic (TA) synapses in the distal dendrites of CA1 pyramidal cells in rats are altered by chronic unpredictable stress (CUS) and restored by chronic antidepressant treatment, in conjunction with the behavioral consequences of CUS. We observed a decrease in AMPAR-mediated excitation at TA-CA1 synapses, but not Schaffer collateral-CA1 synapses, after CUS, with a corresponding layer-specific decrease in GluA1 expression. Both changes were reversed by chronic fluoxetine. CUS also disrupted long-term memory consolidation in the Morris water maze, a function of TA-CA1 synapses. The decreases in TA-CA1 AMPAR-mediated excitation and performance in the consolidation test were correlated positively with decreases in sucrose preference, a measure of anhedonia. We conclude that chronic stress selectively decreases AMPAR number and function at specific synapses and suggest that this underlies various depressive endophenotypes. Our findings provide evidence that glutamatergic dysfunction is an underlying cause of depression and that current first-line antidepressant drugs act by restoring excitatory synaptic strength. Our findings suggest novel therapeutic targets for this debilitating disease.

Unpredictable neonatal stress enhances adult anxiety and alters amygdala gene expression related to serotonin and GABA

Anxiety-related disorders are among the most common psychiatric illnesses, thought to have both genetic and environmental causes. Early-life trauma, such as abuse from a caregiver, can be predictable or unpredictable, each resulting in increased prevalence and severity of a unique set of disorders. In this study, we examined the influence of early unpredictable trauma on both the behavioral expression of adult anxiety and gene expression within the amygdala. Neonatal rats were exposed to unpaired odor-shock conditioning for 5 days, which produces deficits in adult behavior and amygdala dysfunction. In adulthood, we used the Light/Dark box test to measure anxiety-related behaviors, measuring the latency to enter the lit area and quantified urination and defecation. The amygdala was then dissected and a microarray analysis was performed to examine changes in gene expression. Animals that had received early unpredictable trauma displayed significantly longer latencies to enter the lit area and more defecation and urination. The microarray analysis revealed over-represented genes related to learning and memory, synaptic transmission and trans-membrane transport. Gene ontology and pathway analysis identified highly represented disease states related to anxiety phenotypes, including social anxiety, obsessive-compulsive disorders, post-traumatic stress disorder and bipolar disorder. Addiction-related genes were also overrepresented in this analysis. Unpredictable shock during early development increased anxiety-like behaviors in adulthood with concomitant changes in genes related to neurotransmission, resulting in gene expression patterns similar to anxiety-related psychiatric disorders.

Stress in the adult rat exacerbates muscle pain induced by early-life stress

BACKGROUND

Early-life stress and exposure to stressful stimuli play a major role in the development of chronic widespread pain in adults. However, how they interact in chronic pain syndromes remains unclear.

METHODS

Dams and neonatal litters were submitted to a restriction of nesting material (neonatal limited bedding [NLB]) for 1 week. As adults, these rats were exposed to a painless sound stress protocol. The involvement of sympathoadrenal catecholamines interleukin 6 (IL-6) and tumor necrosis factor alpha (TNFα) in nociception was evaluated through behavioral and enzyme-linked immunosorbent assays, surgical interventions, and intrathecal antisense treatments.

RESULTS

Adult NLB rats exhibited mild muscle hyperalgesia, which was markedly aggravated by sound stress (peaking 15 days after exposure). Adrenal medullectomy did not modify hyperalgesia in NLB rats but prevented its aggravation by sound stress. Sustained administration of epinephrine to NLB rats mimicked sound stress effect. Intrathecal treatment with antisense directed to IL-6 receptor subunit gp130 (gp130), but not to tumor necrosis factor receptor type 1 (TNFR1), inhibited hyperalgesia in NLB rats. However, antisense against either gp130 or TNFR1 inhibited sound stress-induced enhancement of hyperalgesia. Compared with control rats, NLB rats exhibit increased plasma levels of IL-6 but decreased levels of TNFα, whereas sound stress increases IL-6 plasma levels in control rats but not in NLB rats.

CONCLUSIONS

Early-life stress induces a persistent elevation of IL-6, hyperalgesia, and susceptibility to chronic muscle pain, which is unveiled by exposure to stress in adults. This probably depends on an interaction between adrenal catecholamines and proinflammatory cytokines acting at muscle nociceptor level.

Adolescent chronic mild stress alters hippocampal CB1 receptor-mediated excitatory neurotransmission and plasticity

Endocannabinoids (eCBs) are involved in the stress response and alterations in eCB signaling may contribute to the etiology of mood disorders. Exposure to chronic mild stress (CMS), a model of depression, produces downregulation of the cannabinoid 1 (CB1) receptor in the hippocampus of male rats. However, it is unknown how this stress-induced change in CB1 levels affects eCB-mediated neurotransmission. In vitro, field potential recordings from CMS-exposed (21-days) rats were performed to assess the effects of stress on eCB-regulated glutamatergic neurotransmission in/on hippocampal area CA1. We observed that application of the CB1 agonist, WIN 55,212-5 (1 μM), in stress animals resulted in a ∼135% increase in excitatory neurotransmission, whereas CB1 activation in non-stress animals leads to a ∼30% decrease. However, during blockade of GABA(A) neurotransmission with picrotoxin, CB1 activation yielded a ∼35% decrease in stress animals. These findings indicate that CMS does not directly affect glutamatergic neurotransmission. Rather, CMS sensitizes CB1 function on GABAergic terminals, leading to less inhibition and an increase in excitatory neurotransmission. This finding is reinforced in that induction of weak long-term-potentiation (LTP) is enhanced in CMS-exposed animals compared to controls and this enhancement is CB1-dependent. Lastly, we observed that the LTP-blocking property of WIN 55,212-5 shifts from being glutamate-dependent in non-stress animals to being GABA-dependent in stress animals. These results effectively demonstrate that CMS significantly alters hippocampal eCB-mediated neurotransmission and synaptic plasticity.

Modulatory mechanisms of cortisol effects on emotional learning and memory: novel perspectives

It has long been known that cortisol affects learning and memory processes. Despite a wealth of research dedicated to cortisol effects on learning and memory, the strength or even directionality of the effects often vary. A number of the factors that alter cortisol's effects on learning and memory are well-known. For instance, effects of cortisol can be modulated by emotional arousal and the memory phase under study. Despite great advances in understanding factors that explain variability in cortisol's effects, additional modulators of cortisol effects on memory exist that are less widely acknowledged in current basic experimental research. The goal of the current review is to disseminate knowledge regarding less well-known modulators of cortisol effects on learning and memory. Since several models for the etiology of anxiety, such as post-traumatic stress disorder (PTSD), incorporate stress and the concomitant release of cortisol as important vulnerability factors, enhanced understanding of mechanisms by which cortisol exerts beneficial as opposed to detrimental effects on memory is very important. Further elucidation of the factors that modulate (or alter) cortisol's effects on memory will allow reconciliation of seemingly inconsistent findings in the basic and clinical literatures. The present review is based on a symposium as part of the 42nd International Society of Psychoneuroendocrinology Conference, New York, USA, that highlighted some of those modulators and their underlying mechanisms.

Previous history of chronic stress changes the transcriptional response to glucocorticoid challenge in the dentate gyrus region of the male rat hippocampus

Chronic stress is a risk factor for several neuropsychiatric diseases, such as depression and psychosis. In response to stress glucocorticoids (GCs) are secreted that bind to mineralocorticoid and glucocorticoid receptors, ligand-activated transcription factors that regulate the transcription of gene networks in the brain necessary for coping with stress, recovery, and adaptation. Chronic stress particularly affects the dentate gyrus (DG) subregion of the hippocampus, causing several functional and morphological changes with consequences for learning and memory, which are likely adaptive but at the same time make DG neurons more vulnerable to subsequent challenges. The aim of this study was to investigate the transcriptional response of DG neurons to a GC challenge in male rats previously exposed to chronic restraint stress (CRS). An intriguing finding of the current study was that having a history of CRS had profound consequences for the subsequent response to acute GC challenge, differentially affecting the expression of several hundreds of genes in the DG compared with challenged nonstressed control animals. This enduring effect of previous stress exposure suggests that epigenetic processes may be involved. In line with this, CRS indeed affected the expression of several genes involved in chromatin structure and epigenetic processes, including Asf1, Ash1l, Hist1h3f, and Tp63. The data presented here indicate that CRS alters the transcriptional response to a subsequent GC injection. We propose that this altered transcriptional potential forms part of the molecular mechanism underlying the enhanced vulnerability for stress-related disorders like depression caused by chronic stress.

Stress- and antidepressant treatment-induced modifications of 5-HT₇ receptor functions in the rat brain

This paper summarizes a series of electrophysiological studies aimed at finding the effects of the activation of 5-HT(7) receptors on neuronal excitability as well as on excitatory and inhibitory synaptic transmission in the hippocampus and in the frontal cortex of the rat. These studies demonstrated that 5-HT(7) receptors play an important role in the modulation of the activity of the hippocampal network by regulating the excitability of pyramidal cells of the CA1 area, as well as via their effect on GABA and glutamatergic transmission. The reactivity of 5-HT(7) receptors in the hippocampus is decreased by repeated administration of antidepressant drugs and increased by a prolonged high level of corticosterone. More importantly, administration of antidepressant drug, imipramine, prevents the occurrence of corticosterone-induced changes in the function of hippocampal 5-HT(7) receptors. It has also been found that the blockade of 5-HT(7) receptors by the selective antagonist SB 269970, lasting for a few days, causes similar changes to those observed after long-term administration of antidepressants. Thus, it seems that the pharmacological blockade of 5-HT(7) receptors produces faster effects compared to classic antidepressant drugs. A similarity between the changes in the glutamatergic transmission induced by the blockade of 5 HT7 receptors and those caused by repeated administration of the antidepressant drug, imipramine, has also been found in the frontal cortex. It has also been shown that the changes in glutamatergic transmission and the impairment of long-term synaptic plasticity in the frontal cortex of animals subjected to repeated restraint stress are reversed by the blockade of 5-HT(7) receptors. Overall, these studies, together with the data provided by other investigators, support the hypothesis that 5-HT(7) receptor antagonists may become a prototype of a new class of antidepressant drugs. Such compounds will not function by blocking 5-HT reuptake, as many of the currently used drugs, but through a direct interaction with the 5-HT(7) receptor. This type of action is highly selective and usually does not require the occurrence of adaptive changes in neuronal functions, thus allowing for a much quicker therapeutic effect.

Activation of mammalian target of rapamycin and synaptogenesis: role in the actions of rapid-acting antidepressants

Antidepressants that produce rapid and robust effects, particularly for severely ill patients, represent one of the largest unmet medical needs for the treatment of depression. Currently available drugs that modulate monoamine neurotransmission provide relief for only a subset of patients, and this minimal efficacy requires several weeks of chronic treatment. The recent discovery that the glutamatergic agent ketamine produces rapid antidepressant responses within hours has opened a new area of research to explore the molecular mechanisms through which ketamine produces these surprising responses. Clinical and preclinical findings have exposed some of the unique actions of ketamine and identified a cell-signaling pathway known as the mammalian target of rapamycin. Activation of mammalian target of rapamycin and increased synaptogenesis in the prefrontal cortex are crucial in mediating the antidepressant effects of ketamine. Importantly, the synaptic actions of ketamine allow rapid recovery from the insults produced by exposure to repeated stress that cause neuronal atrophy and loss of synaptic connections. In the following review, we explore some of the clinical and preclinical findings that have thrust ketamine to the forefront of rapid antidepressant research and unveiled some of its unique molecular and cellular actions.

Synaptic plasticity in depression: molecular, cellular and functional correlates

Synaptic plasticity confers environmental adaptability through modification of the connectivity between neurons and neuronal circuits. This is achieved through changes to synapse-associated signaling systems and supported by complementary changes to cellular morphology and metabolism within the tripartite synapse. Mounting evidence suggests region-specific changes to synaptic form and function occur as a result of chronic stress and in depression. Within subregions of the prefrontal cortex (PFC) and hippocampus structural and synapse-related findings seem consistent with a deficit in long-term potentiation (LTP) and facilitation of long-term depression (LTD), particularly at excitatory pyramidal synapses. Other brain regions are less well-studied; however the amygdala may feature a somewhat opposite synaptic pathology including reduced inhibitory tone. Changes to synaptic plasticity in stress and depression may correlate those to several signal transduction pathways (e.g. NOS-NO, cAMP-PKA, Ras-ERK, PI3K-Akt, GSK-3, mTOR and CREB) and upstream receptors (e.g. NMDAR, TrkB and p75NTR). Deficits in synaptic plasticity may further correlate disrupted brain redox and bioenergetics. Finally, at a functional level region-specific changes to synaptic plasticity in depression may relate to maladapted neurocircuitry and parallel reduced cognitive control over negative emotion.

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.

Two populations of glucocorticoid receptor-binding sites in the male rat hippocampal genome

In the present study, genomic binding sites of glucocorticoid receptors (GR) were identified in vivo in the rat hippocampus applying chromatin immunoprecipitation followed by next-generation sequencing. We identified 2470 significant GR-binding sites (GBS) and were able to confirm GR binding to a random selection of these GBS covering a wide range of P values. Analysis of the genomic distribution of the significant GBS revealed a high prevalence of intragenic GBS. Gene ontology clusters involved in neuronal plasticity and other essential neuronal processes were overrepresented among the genes harboring a GBS or located in the vicinity of a GBS. Male adrenalectomized rats were challenged with increasing doses of the GR agonist corticosterone (CORT) ranging from 3 to 3000 μg/kg, resulting in clear differences in the GR-binding profile to individual GBS. Two groups of GBS could be distinguished: a low-CORT group that displayed GR binding across the full range of CORT concentrations, and a second high-CORT group that displayed significant GR binding only after administering the highest concentration of CORT. All validated GBS, in both the low-CORT and high-CORT groups, displayed mineralocorticoid receptor binding, which remained relatively constant from 30 μg/kg CORT upward. Motif analysis revealed that almost all GBS contained a glucocorticoid response element resembling the consensus motif in literature. In addition, motifs corresponding with new potential GR-interacting proteins were identified, such as zinc finger and BTB domain containing 3 (Zbtb3) and CUP (CG11181 gene product from transcript CG11181-RB), which may be involved in GR-dependent transactivation and transrepression, respectively. In conclusion, our results highlight the existence of 2 populations of GBS in the rat hippocampal genome.

Age-related cognitive impairments in mice with a conditional ablation of the neural cell adhesion molecule

Most of the mechanisms involved in neural plasticity support cognition, and aging has a considerable effect on some of these processes. The neural cell adhesion molecule (NCAM) of the immunoglobulin superfamily plays a pivotal role in structural and functional plasticity and is required to modulate cognitive and emotional behaviors. However, whether aging is associated with NCAM alterations that might contribute to age-related cognitive decline is not currently known. In this study, we determined whether conditional NCAM-deficient mice display increased vulnerability to age-related cognitive and emotional alterations. We assessed the NCAM expression levels in the hippocampus and medial prefrontal cortex (mPFC) and characterized the performance of adult and aged conditional NCAM-deficient mice and their age-matched wild-type littermates in a delayed matching-to-place test in the Morris water maze and a delayed reinforced alternation test in the T-maze. Although aging in wild-type mice is associated with an isoform-specific reduction of NCAM expression levels in the hippocampus and mPFC, these mice exhibited only mild impairments in working/episodic-like memory performance. However, aged conditional NCAM-deficient mice displayed pronounced impairments in both the delayed matching-to-place and the delayed reinforced alternation tests. Importantly, the deficits of aged NCAM-deficient mice in these working/episodic-like memory tasks could not be attributed to increased anxiety-like behaviors or to differences in locomotor activity. Taken together, these data indicate that reduced NCAM expression in the forebrain might be a critical factor for the occurrence of cognitive impairments during aging.

Clinical Safety, Pharmacokinetics, and Pharmacodynamics of the 11β-Hydroxysteroid Dehydrogenase Type 1 Inhibitor ABT-384 in Healthy Volunteers and Elderly Adults

ABT-384 is a potent and selective inhibitor of 11β-hydroxysteroid dehydrogenase type 1 (HSD-1), the enzyme that regenerates cortisol in several tissues. Two clinical studies of ABT-384 were undertaken to assess its safety, pharmacokinetics, target engagement, and pharmacologic effects in healthy subjects. Single doses from 1 to 240 mg, and multiple doses from 1 to 100 mg once daily for 7-14 days, were administered to healthy adults. Multiple doses from 10 to 100 mg once daily for 21 days were administered to elderly subjects. A total of 103 subjects received at least 1 dose of ABT-384. A maximum-tolerated dose was not defined in either study. The pharmacokinetic profiles of ABT-384 and its active metabolite support once daily dosing. Analysis of urine cortisol metabolites demonstrated full hepatic HSD-1 inhibition with regimens from 1 mg daily, and confirmed in vitro target selectivity. Pharmacologic effects included increases of adrenocorticotrophic hormone levels, cortisol production and androgen and estradiol levels. ABT-384 has a wide therapeutic index relative to full hepatic target engagement which is relevant for indications such as diabetes and metabolic syndrome. Its therapeutic index for other potential indications such as Alzheimer's disease remains to be established.

Novel insights into depression and antidepressants: a synergy between synaptogenesis and neurogenesis?

Major depressive disorder has been associated with manifold pathophysiological changes. These include metabolic abnormalities in discreet brain areas; modifications in the level of stress hormones, neurotransmitters, and neurotrophic factors; impaired spinogenesis and synaptogenesis in crucial brain areas, such as the prefrontal cortex and the hippocampus; and impaired neurogenesis in the hippocampus. Antidepressant therapy facilitates remission by reversing most of these disturbances, indicating that these dysfunctions may participate causally in depressive symptomatology. However, few attempts have been made to integrate these different pathophysiologies into one model. The present chapter endeavors (1) to review the extant literature in the field, with particular focus on the role of neurogenesis and synaptogenesis in depression; (2) and to suggest a possible interplay between these two processes, as well as, describe the ways by which improving both neurogenesis and synaptogenesis may enable effective recovery by acting on a larger neuronal network.

Glucocorticoids modulate the mTOR pathway in the hippocampus: differential effects depending on stress history

Glucocorticoid (GC) hormones, released by the adrenals in response to stress, are key regulators of neuronal plasticity. In the brain, the hippocampus is a major target of GC, with abundant expression of the GC receptor. GC differentially affect the hippocampal transcriptome and consequently neuronal plasticity in a subregion-specific manner, with consequences for hippocampal information flow and memory formation. Here, we show that GC directly affect the mammalian target of rapamycin (mTOR) signaling pathway, which plays a central role in translational control and has long-lasting effects on the plasticity of specific brain circuits. We demonstrate that regulators of the mTOR pathway, DNA damage-induced transcript (DDIT)4 and FK506-binding protein 51 are transcriptionally up-regulated by an acute GC challenge in the dentate gyrus (DG) subregion of the rat hippocampus, most likely via a GC-response element-driven mechanism. Furthermore, two other mTOR pathway members, the mTOR regulator DDIT4-like and the mTOR target DDIT3, are down-regulated by GC in the rat DG. Interestingly, the GC responsiveness of DDIT4 and DDIT3 was lost in animals with a recent history of chronic stress. Basal hippocampal mTOR protein levels were higher in animals exposed to chronic stress than in controls. Moreover, an acute GC challenge significantly reduced mTOR protein levels in the hippocampus of animals with a chronic stress history but not in unstressed controls. Based on these findings, we propose that direct regulation of the mTOR pathway by GC represents an important mechanism regulating neuronal plasticity in the rat DG, which changes after exposure to chronic stress.

The Schizophrenia and Bipolar Disorder associated BRD1 gene is regulated upon chronic restraint stress

Recent genetic evidence has implicated the bromodomain containing 1 gene (BRD1) with brain development and susceptibility to Schizophrenia and Bipolar Disorder. The BRD1 protein, which is essential for acetylation of histone H3K14, is a putative regulator of transcription during brain development and in the mature CNS. However, several issues remain to be clarified for example regarding the regulation of the BRD1 gene upon environmental interventions. Chronic restraint stress (CRS) in rats represents an environmental method for induction of morphological and functional changes in the hippocampus and the prefrontal cortex. In order to investigate whether the expression of the rat Brd1 gene may be regulated during such conditions, Brd1 mRNA and protein levels in hippocampus and prefrontal cortex extracts from rats subjected to either 1/2 or 6h of CRS per day for 21days were measured. We found a significant 2-fold up-regulation of long exon 7 splice variants of the Brd1 gene (Brd1-L) in hippocampus in both groups of CRS rats compared to controls. Concomitantly, we found a similar up-regulation of the BRD1 protein. In prefrontal cortex, we found no significant differences in Brd1 mRNA or protein levels. As selective histone deacetylase (HDAC) inhibitors not only preserve stress-induced hyperacetylation of histone H3K14 but also have hippocampal-dependent antidepressant-like activity, we propose that BRD1 by its intrinsic acetylation activity towards histone H3K14 is a player in the regulatory processes underlying adaptation to stress in the mature CNS.

Stress sensitivity of childhood epilepsy is related to experienced negative life events

PURPOSE

To evaluate the effect of stress on seizure frequency in childhood epilepsy, and to assess possible differences between children in whom seizures are precipitated by stress and those in whom they are not.

METHODS

Parents or caregivers of children with active epilepsy (aged 2-16 years) were sent questionnaires on developmental and epilepsy characteristics, life-time stress exposure, and the effect of stressful periods and moments of acute stress on seizure frequency in their child. Further information was extracted from patient files.

KEY FINDINGS

Parents or caregivers of 153 children with a median age of 8.8 years responded to the questionnaires. Thirty-nine percent reported an increase in seizure frequency during periods of stress, with a median increase of 2.5 times the frequency compared to nonstressful periods. Thirty-seven percent reported that seizures were precipitated by acute stress, with stress being a precipitating factor in 33% (median value) of the seizures. Overall, 51% of the patients reported stress sensitivity of seizures. A higher number of negative life events experienced in total life was related to an increase in seizure frequency in stressful periods (odds ratio [OR] 1.3, p = 0.01) as well as to the precipitation of seizures by acute stress (OR 1.3, p = 0.02).

SIGNIFICANCE

Stress sensitivity is reported in half of the children with epilepsy. Results of this study suggest a relation between experienced negative life events and stress sensitivity of childhood epilepsy. One possible explanation could be that experiencing negative life events may cause a larger response to daily stressors, thereby increasing the likelihood to induce epileptic activity.

Steroid modulation of hippocampal plasticity: switching between cognitive and emotional memories

Several new observations have shifted the view of the hippocampus from a structure in charge of cognitive processes to a brain area that participates in the formation of emotional memories, in addition to its role in cognition. Specifically, while the dorsal hippocampus is involved in the processing of cognitive memories; the ventral sector is mainly associated with the control of behavioral inhibition, stress, and emotional memory. Stress is likely to cause this switch in control of hippocampal functions by modulating synaptic plasticity in the dorsal and ventral sectors of the hippocampus through the differential activation of mineralocorticosteroid or glucocorticosteroid receptors. Herein, we will review the effects of stress hormones on synaptic plasticity in the hippocampus and outline the outcomes on stress-related global functions of this structure. We propose that steroid hormones act as molecular switches: by changing the strength of synaptic connectivity in the hippocampus following stress, they regulate the routes by which the hippocampus is functionally linked to the rest of the brain. This hypothesis has profound implications for the pathophysiology of psychiatric disorders.

Repeated stress causes cognitive impairment by suppressing glutamate receptor expression and function in prefrontal cortex

Chronic stress could trigger maladaptive changes associated with stress-related mental disorders; however, the underlying mechanisms remain elusive. In this study, we found that exposing juvenile male rats to repeated stress significantly impaired the temporal order recognition memory, a cognitive process controlled by the prefrontal cortex (PFC). Concomitantly, significantly reduced AMPAR- and NMDAR-mediated synaptic transmission and glutamate receptor expression were found in PFC pyramidal neurons from repeatedly stressed animals. All these effects relied on activation of glucocorticoid receptors and the subsequent enhancement of ubiquitin/proteasome-mediated degradation of GluR1 and NR1 subunits, which was controlled by the E3 ubiquitin ligase Nedd4-1 and Fbx2, respectively. Inhibition of proteasomes or knockdown of Nedd4-1 and Fbx2 in PFC prevented the loss of glutamatergic responses and recognition memory in stressed animals. Our results suggest that repeated stress dampens PFC glutamatergic transmission by facilitating glutamate receptor turnover, which causes the detrimental effect on PFC-dependent cognitive processes.

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.

Candidate hippocampal biomarkers of susceptibility and resilience to stress in a rat model of depression

Susceptibility to stress plays a crucial role in the development of psychiatric disorders such as unipolar depression and post-traumatic stress disorder. In the present study the chronic mild stress rat model of depression was used to reveal stress-susceptible and stress-resilient rats. Large-scale proteomics was used to map hippocampal protein alterations in different stress states. Membrane proteins were successfully captured by two-phase separation and peptide based proteomics. Using iTRAQ labeling coupled with mass spectrometry, more than 2000 proteins were quantified and 73 proteins were found to be differentially expressed. Stress susceptibility was associated with increased expression of a sodium-channel protein (SCN9A) currently investigated as a potential antidepressant target. Differential protein profiling also indicated stress susceptibility to be associated with deficits in synaptic vesicle release involving SNCA, SYN-1, and AP-3. Our results indicate that increased oxidative phosphorylation (COX5A, NDUFB7, NDUFS8, COX5B, and UQCRB) within the hippocampal CA regions is part of a stress-protection mechanism.

Learning and memory alterations are associated with hippocampal N-acetylaspartate in a rat model of depression as measured by 1H-MRS

It is generally accepted that cognitive processes, such as learning and memory, are affected in depression. The present study used a rat model of depression, chronic unpredictable mild stress (CUMS), to determine whether hippocampal volume and neurochemical changes were involved in learning and memory alterations. A further aim was to determine whether these effects could be ameliorated by escitalopram treatment, as assessed with the non-invasive techniques of structural magnetic resonance imaging (MRI) and magnetic resonance spectroscopy (MRS). Our results demonstrated that CUMS had a dramatic influence on spatial cognitive performance in the Morris water maze task, and CUMS reduced the concentration of neuronal marker N-acetylaspartate (NAA) in the hippocampus. These effects could be significantly reversed by repeated administration of escitalopram. However, neither chronic stress nor escitalopram treatment influenced hippocampal volume. Of note, the learning and memory alterations of the rats were associated with right hippocampal NAA concentration. Our results indicate that in depression, NAA may be a more sensitive measure of cognitive function than hippocampal volume.

Neurosteroids and GABA(A) Receptor Interactions: A Focus on Stress

Since the pioneering discovery of the rapid CNS depressant actions of steroids by the "father of stress," Hans Seyle 70 years ago, brain-derived "neurosteroids" have emerged as powerful endogenous modulators of neuronal excitability. The majority of the intervening research has focused on a class of naturally occurring steroids that are metabolites of progesterone and deoxycorticosterone, which act in a non-genomic manner to selectively augment signals mediated by the main inhibitory receptor in the CNS, the GABA(A) receptor. Abnormal levels of such neurosteroids associate with a variety of neurological and psychiatric disorders, suggesting that they serve important physiological and pathophysiological roles. A compelling case can be made to implicate neurosteroids in stress-related disturbances. Here we will critically appraise how brain-derived neurosteroids may impact on the stress response to acute and chronic challenges, both pre- and postnatally through to adulthood. The pathological implications of such actions in the development of psychiatric disturbances will be discussed, with an emphasis on the therapeutic potential of neurosteroids for the treatment of stress-associated disorders.

Chronic stress exacerbates tau pathology, neurodegeneration, and cognitive performance through a corticotropin-releasing factor receptor-dependent mechanism in a transgenic mouse model of tauopathy

Because overactivation of the hypothalamic-pituitary-adrenal (HPA) axis occurs in Alzheimer's disease (AD), dysregulation of stress neuromediators may play a mechanistic role in the pathophysiology of AD. However, the effects of stress on tau phosphorylation are poorly understood, and the relationship between corticosterone and corticotropin-releasing factor (CRF) on both β-amyloid (Aβ) and tau pathology remain unclear. Therefore, we first established a model of chronic stress, which exacerbates Aβ accumulation in Tg2576 mice and then extended this stress paradigm to a tau transgenic mouse model with the P301S mutation (PS19) that displays tau hyperphosphorylation, insoluble tau inclusions and neurodegeneration. We show for the first time that both Tg2576 and PS19 mice demonstrate a heightened HPA stress profile in the unstressed state. In Tg2576 mice, 1 month of restraint/isolation (RI) stress increased Aβ levels, suppressed microglial activation, and worsened spatial and fear memory compared with nonstressed mice. In PS19 mice, RI stress promoted tau hyperphosphorylation, insoluble tau aggregation, neurodegeneration, and fear-memory impairments. These effects were not mimicked by chronic corticosterone administration but were prevented by pre-stress administration of a CRF receptor type 1 (CRF(1)) antagonist. The role for a CRF(1)-dependent mechanism was further supported by the finding that mice overexpressing CRF had increased hyperphosphorylated tau compared with wild-type littermates. Together, these results implicate HPA dysregulation in AD neuropathogenesis and suggest that prolonged stress may increase Aβ and tau hyperphosphorylation. These studies also implicate CRF in AD pathophysiology and suggest that pharmacological manipulation of this neuropeptide may be a potential therapeutic strategy for AD.

Chronic stress and antidepressant agomelatine induce region-specific changes in synapsin I expression in the rat brain

The antidepressant agomelatine acts as a melatonergic receptor (MT(1)/MT(2)) agonist and 5-HT(2C) receptor antagonist. Agomelatine has demonstrated efficacy in treating depression, but its neurobiological effects merit further investigation. Preclinical studies reported that agomelatine enhances adult hippocampal neurogenesis and increases expression of several neuroplasticity-associated molecules. Recently, we showed that agomelatine normalizes hippocampal neuronal activity and promotes neurogenesis in the stress-compromised brain. To characterize further the effects of this antidepressant in the stressed brain, here we investigated whether it induces changes in the expression of synapsin I (SynI), a regulator of synaptic transmission and plasticity. Adult male rats were subjected to daily footshock stress and agomelatine treatment for 3 weeks. Their brains were subsequently stained for total and phosphorylated SynI. Chronic footshock and agomelatine induced region-specific changes in SynI expression. Whereas chronic stress increased total SynI expression in all layers of the medial prefrontal cortex, agomelatine treatment abolished some of these effects. Furthermore, chronic agomelatine administration decreased total SynI expression in the hippocampal subregions of both stressed and nonstressed rats. Importantly, chronic stress decreased the fraction of phosphorylated SynI in all layers of the medial prefrontal cortex as well as selectively in the outer and middle molecular layers of the hippocampal dentate gyrus. These stress effects were at least partially abolished by agomelatine. Altogether, our data show that chronic stress and agomelatine treatment induce region-specific changes in SynI expression and its phosphorylation. Moreover, agomelatine partially counteracts the stress effects on SynI, suggesting a modulation of synaptic function by this antidepressant.

Perceived threat predicts the neural sequelae of combat stress

Exposure to severe stressors increases the risk for psychiatric disorders in vulnerable individuals, but can lead to positive outcomes for others. However, it remains unknown how severe stress affects neural functioning in humans and what factors mediate individual differences in the neural sequelae of stress. The amygdala is a key brain region involved in threat detection and fear regulation, and previous animal studies have suggested that stress sensitizes amygdala responsivity and reduces its regulation by the prefrontal cortex. In this study, we used a prospective design to investigate the consequences of severe stress in soldiers before and after deployment to a combat zone. We found that combat stress increased amygdala and insula reactivity to biologically salient stimuli across the group of combat-exposed individuals. In contrast, its influence on amygdala coupling with the insula and dorsal anterior cingulate cortex was dependent on perceived threat, rather than actual exposure, suggesting that threat appraisal affects interoceptive awareness and amygdala regulation. Our results demonstrate that combat stress has sustained consequences on neural responsivity, and suggest a key role for the appraisal of threat on an amygdala-centered neural network in the aftermath of severe stress.

Impact of glucocorticoids on brain function: relevance for mood disorders

Exposure to stressful situations activates two hormonal systems that help the organism to adapt. On the one hand stress hormones achieve adaptation by affecting peripheral organs, on the other hand by altering brain function such that appropriate behavioral strategies are selected for optimal performance at the short term, while relevant information is stored for reference in the future. In this chapter we describe how cellular effects induced by stress hormones--in particular by glucocorticoids--may contribute to the behavioral outcome after a single stressor. In addition to situations of acute stress, chronic uncontrollable and unpredictable stress also exerts profound effects on structure and function of limbic neurons. The impact of chronic stress is not a mere cumulative effect of what is seen after acute stress exposure. Dendritic trees are expanded in some regions but reduced in others. In general, cells are exposed to a higher calcium load upon depolarization, but show attenuated responses to serotonin. Synaptic strengthening is largely impaired. In this viewpoint we speculate how cellular effects after chronic stress may be maladaptive and could contribute to the development of psychopathology in genetically vulnerable individuals.

Stress-induced dynamic routing of hippocampal connectivity: a hypothesis

Recent observations have caused a drastic shift in the conception of the hippocampus as a homogeneous structure that subserves cognitive functions, either spatial maps or short term episodic memory, to a structure that is associated with both cognitive and emotional functions. In fact, the assignment of cognitive functions to the hippocampus is restricted to its dorsal sector. In contrast, the ventral hippocampus (VH) appears to be associated with control of behavioral inhibition, stress and emotional memory, but not with strictly cognitive functions. Curiously, the VH but not the dorsal hippocampus (DH) is associated with the development of affective disorders. In line with these collective observations, we and others have found that the ability to evoke a sustained long term potentiation (LTP), a cellular correlate of learning and memory, is much lower in the VH compared to the DH. Strikingly, acute stress as well as direct exposure to corticosterone affect DH and VH in an opposite manner; causing facilitation of LTP in the VH and its suppression in the DH. This double dissociative action results from activation of different steroid receptor species in the DH and VH. Since the DH and VH differ in efferent connectivity, and since the strength of LTP can be considered as an indicator of strength of synaptic connectivity, these results suggest that stress regulates the routes by which the hippocampus is functionally linked to the rest of the brain such that under stress, the ventral route to the amygdala is enabled while the dorsal route to the neocortex is suppressed. This selective routing may underlie the complex outcome of stress on hippocampal and amygdala physiology and behavior.

Emerging roles of epigenetic mechanisms in the enduring effects of early-life stress and experience on learning and memory

Epigenetic mechanisms are involved in programming gene expression throughout development. In addition, they are key contributors to the processes by which early-life experience fine-tunes the expression levels of key neuronal genes, governing learning and memory throughout life. Here we describe the long-lasting, bi-directional effects of early-life experience on learning and memory. We discuss how enriched postnatal experience enduringly augments spatial learning, and how chronic early-life stress results in persistent and progressive deficits in the structure and function of hippocampal neurons. The existing and emerging roles of epigenetic mechanisms in these fundamental neuroplasticity phenomena are illustrated.

Adult hippocampal glucocorticoid receptor expression and dentate synaptic plasticity correlate with maternal care received by individuals early in life

Maternal care in mammals is the prevailing environmental influence during perinatal development. The adult rat offspring of mothers exhibiting increased levels of pup licking/grooming (LG; High LG mothers), compared to those reared by Low LG dams, show increased hippocampal glucocorticoid receptor expression, complex dendritic tree structure, and an enhanced capacity for synaptic potentiation. However, these data were derived from studies using the total amount of maternal care directed toward the entire litter, thus ignoring possible within-litter variation. We show that the amount of LG received by individual pups within a litter varies considerably. Therefore, we questioned if the amount of LG received by individual pups correlates with and thus putatively predicts later hippocampal structure and function. To this end, LG-scores were determined during the first postnatal week for all pups in 32 litters and correlated with neuroendocrine and hippocampal parameters in young-adulthood. Pup LG-score positively correlated with the glucocorticoid receptor mRNA expression in the adult hippocampus. Moreover, the ability to induce synaptic potentiation in the dentate gyrus in vitro was enhanced in animals with high LG-scores. Structural plasticity correlated less reliably with LG-scores early in life and differed between sexes. Male offspring with high LG-scores displayed fewer newborn neurons, higher brain derived neurotrophic factor expression and tended to have more complex granule cell dendritic trees. We conclude that even moderate variations in early life environment have a major impact on adult hippocampal function. This principle could provide a mechanistic basis for individual differences in susceptibility to psychopathology.

Cortisol's effects on hippocampal activation in depressed patients are related to alterations in memory formation

Many investigators have hypothesized that brain response to cortisol is altered in depression. However, neural activation in response to exogenously manipulated cortisol elevations has not yet been directly examined in depressed humans. Animal research shows that glucocorticoids have robust effects on hippocampal function, and can either enhance or suppress neuroplastic events in the hippocampus depending on a number of factors. We hypothesized that depressed individuals would show 1) altered hippocampal response to exogenous administration of cortisol, and 2) altered effects of cortisol on learning. In a repeated-measures design, 19 unmedicated depressed and 41 healthy individuals completed two fMRI scans. Fifteen mg oral hydrocortisone (i.e., cortisol) or placebo (order randomized and double-blind) was administered 1 h prior to encoding of emotional and neutral words during fMRI scans. Data analysis examined the effects of cortisol administration on 1) brain activation during encoding, and 2) subsequent free recall for words. Cortisol affected subsequent recall performance in depressed but not healthy individuals. We found alterations in hippocampal response to cortisol in depressed women, but not in depressed men (who showed altered response to cortisol in other regions, including subgenual prefrontal cortex). In both depressed men and women, cortisol's effects on hippocampal function were positively correlated with its effects on recall performance assessed days later. Our data provide evidence that in depressed compared to healthy women, cortisol's effects on hippocampal function are altered. Our data also show that in both depressed men and women, cortisol's effects on emotional memory formation and hippocampal function are related.

The transcriptional response to chronic stress and glucocorticoid receptor blockade in the hippocampal dentate gyrus

The dentate gyrus (DG) of the hippocampus plays a crucial role in learning and memory. This subregion is unique in its ability to generate new neurons throughout life and integrate these new neurons into the hippocampal circuitry. Neurogenesis has further been implicated in hippocampal plasticity and depression. Exposure to chronic stress affects DG function and morphology and suppresses neurogenesis and long-term potentiation (LTP) with consequences for cognition. Previous studies demonstrated that glucocorticoid receptor (GR) blockade by a brief treatment with the GR antagonist mifepristone (RU486) rapidly reverses the stress and glucocorticoid effects on neurogenesis. The molecular pathways underlying both the stress-induced effects and the RU486 effects on the DG are, however, largely unknown. The aim of this study was therefore (1) to investigate by microarray analysis which genes and pathways in the DG are sensitive to chronic stress and (2) to investigate to what extent blockade of GR can normalize these stress-induced effects on DG gene expression. Chronic stress exposure affected the expression of 90 genes in the DG (P < 0.01), with an overrepresentation of genes involved in brain development and morphogenesis and synaptic transmission. RU486 treatment of stressed animals affected expression of 107 genes; however, mostly different genes than those responding to stress. Interestingly, we found CREBBP to be normalized by RU486 treatment to levels observed in control animals, suggesting that CREB-signaling may play a central role in mediating the chronic stress effects on neurogenesis, LTP and calcium currents. The identified genetic pathways provide insight into the stress-induced adaptive plasticity of the hippocampal DG that is so central in learning and memory and will direct future studies on the functional outcome and modulation of these stress effects.

How might stress contribute to increased risk for schizophrenia in children with chromosome 22q11.2 deletion syndrome?

The most common human microdeletion occurs at chromosome 22q11.2. The associated syndrome (22q11.2DS) has a complex and variable phenotype with a high risk of schizophrenia. While the role of stress in the etiopathology of schizophrenia has been under investigation for over 30 years (Walker et al. 2008), the stress–diathesis model has yet to be investigated in children with 22q11.2DS. Children with 22q11.2DS face serious medical, behavioral, and socioemotional challenges from infancy into adulthood. Chronic stress elevates glucocorticoids, decreases immunocompetence, negatively impacts brain development and function, and is associated with psychiatric illness in adulthood. Drawing knowledge from the extant and well-developed anxiety and stress literature will provide invaluable insight into the complex etiopathology of schizophrenia in people with 22q11.2DS while suggesting possible early interventions. Childhood anxiety is treatable and stress coping skills can be developed thereby improving quality of life in the short-term and potentially mitigating the risk of developing psychosis.

Regulation of histone acetylation in the hippocampus of chronically stressed rats: a potential role of sirtuins

The hippocampus is a brain region that is particularly susceptible to structural and functional changes in response to chronic stress. Recent literature has focused on changes in gene transcription mediated by post-translational modifications of histones in response to stressful stimuli. Chronic variable stress (CVS) is a rodent model that mimics certain symptoms of depression in humans. Given that stress exhibits distinct effects on the cells of the sub-regions of the hippocampus, we investigated changes in histone acetylation in the CA1, CA3, and dentate gyrus (DG) of the hippocampus in response to CVS. Western blotting revealed a significant decrease in acetylation of histone 4 (H4) at Lys12 in CA3 and DG of CVS animals compared to control animals. Furthermore, phospho-acetyl H3 (Lys9/Ser10) was also decreased in the CA3 and DG regions of the hippocampus of CVS animals. In addition, since histone deacetylases (HDACs) contribute to the acetylation state of histones, we investigated the effects of two HDAC inhibitors, sodium butyrate, a class I and II global HDAC inhibitor, and sirtinol, a class III sirtuin inhibitor, on acetylation of histone 3 (H3) and H4. Application of HDAC inhibitors to hippocampus slices from control and CVS animals revealed increased histone acetylation in CVS animals, suggesting that levels of histone deacetylation by HDACs were higher in the CVS animals compared to control animals. Interestingly, histone acetylation in response to sirtinol was selectively increased in the slices from the CVS animals, with very little effect of sirtuin inhibitors in slices from control animals. In addition, sirtuin activity was increased specifically in CA3 and DG of CVS animals. These results suggest a complex and regionally-specific pattern of changes in histone acetylation within the hippocampus which may contribute to stress-induced pathology.

Gene-environment interaction research and transgenic mouse models of Alzheimer's disease

The etiology of the sporadic form of Alzheimer's disease (AD) remains largely unknown. Recent evidence has suggested that gene-environment interactions (GxE) may play a crucial role in its development and progression. Whereas various susceptibility loci have been identified, like the apolipoprotein E4 allele, these cannot fully explain the increasing prevalence of AD observed with aging. In addition to such genetic risk factors, various environmental factors have been proposed to alter the risk of developing AD as well as to affect the rate of cognitive decline in AD patients. Nevertheless, aside from the independent effects of genetic and environmental risk factors, their synergistic participation in increasing the risk of developing AD has been sparsely investigated, even though evidence points towards such a direction. Advances in the genetic manipulation of mice, modeling various aspects of the AD pathology, have provided an excellent tool to dissect the effects of genes, environment, and their interactions. In this paper we present several environmental factors implicated in the etiology of AD that have been tested in transgenic animal models of the disease. The focus lies on the concept of GxE and its importance in a multifactorial disease like AD. Additionally, possible mediating mechanisms and future challenges are discussed.

Hippocampal volume deficits associated with exposure to psychological trauma and posttraumatic stress disorder in adults: a meta-analysis

Trauma exposure itself in the absence of posttraumatic stress disorder (PTSD) may be associated with hippocampal volume deficits. We meta-analytically compared hippocampal volumes in PTSD subjects, in trauma-exposed subjects without PTSD, and in trauma-unexposed subjects. Using the words and phrases PTSD, neuroimaging, hippocampus, brain, violence, trauma, abuse, rape, war, combat, accident, and disaster, we searched major computerized databases to obtain candidate studies through 2008 for inclusion. We identified 39 hippocampal volumetric studies in adults with PTSD compared to control groups consisting of either trauma-exposed controls without PTSD or trauma-unexposed controls, or both. We meta-analytically compared left, right, and total hippocampal volumes between 1) PTSD subjects and a trauma-unexposed group, 2) PTSD subjects and a trauma-exposed group without PTSD, and 3) a trauma-unexposed group and a trauma-exposed group without PTSD. Hippocampal volumes were smaller in the PTSD group and trauma-exposed group without PTSD compared to the trauma-unexposed group. Further, the right hippocampus was smaller in the PTSD group compared to the trauma-exposed group without PTSD. Additionally, the right hippocampus was larger than the left in the PTSD and trauma-unexposed groups but not in the trauma-exposed group without PTSD. Hippocampal volume reduction is associated with trauma exposure independent of PTSD diagnosis, albeit additional hippocampal reduction was found in PTSD compared to the trauma-exposed group without PTSD.

Learning as a model for neural plasticity in major depression

BACKGROUND

The neuroplasticity hypothesis of depression proposes that a dysfunction of neural plasticity-the basic ability of living organisms to adapt their neural function and structure to external and internal cues-might represent a final common pathway underlying the biological and clinical characteristics of the disorder. This study examined learning and memory as correlates of long-term synaptic plasticity in humans to further test the neuroplasticity hypothesis of depression.

METHODS

Learning in three tasks, for which memory consolidation has been shown to depend on local synaptic refinement in areas of interest (hippocampus-dependent declarative word-pair learning, amygdala-dependent fear conditioning, and primary-cortex-dependent visual texture discrimination), was assessed in 23 inpatients who met International Classification of Disease, 10th Revision, criteria for severe unipolar depression and 35 nondepressed comparison subjects.

RESULTS

Depressed subjects showed a significant deficit in declarative memory consolidation and enhanced fear acquisition as indicated by skin conductance responses to conditioned stimuli, in comparison with nondepressed subjects. Depressed subjects demonstrated impaired visual discrimination at baseline, not allowing for valid group comparisons of gradual improvement, the plasticity-dependent phase of the task.

CONCLUSIONS

The results of the study are consistent with the neuroplasticity hypothesis of depression, showing decreased synaptic plasticity in a dorsal executive network that comprises the hippocampus and elevated synaptic plasticity in a ventral emotional network that includes the amygdala in depression. Evaluation of further techniques aimed at modulating synaptic plasticity might prove useful for developing novel treatments for major depressive disorder.

Loss of limbic system-associated membrane protein leads to reduced hippocampal mineralocorticoid receptor expression, impaired synaptic plasticity, and spatial memory deficit

BACKGROUND

The limbic system-associated membrane protein (LAMP) promotes development of neurons of limbic origin. We have previously shown that genetic deletion of LAMP results in heightened reactivity to novelty and reduced anxiety-like behaviors in mice. Here, we demonstrate a critical role of LAMP in hippocampal-dependent synaptic physiology and behavior.

METHODS

We tested spatial memory performance, hippocampal synaptic plasticity, and stress-related modalities in Lsamp(-/-) mice and their littermate control mice.

RESULTS

Lsamp(-/-) mice exhibit a pronounced deficit in spatial memory acquisition and poorly sustained CA1 long-term potentiation. We found reduced expression of mineralocorticoid receptor (MR) transcripts in the hippocampus and reduction in the corticosterone-induced, MR-mediated nongenomic modulatory effects on CA1 synaptic transmission. Importantly, the impaired long-term potentiation in Lsamp(-/-) mice can be rescued by stress-like levels of corticosterone in a MR-dependent manner.

CONCLUSIONS

Our study reveals a novel functional relationship between a cell adhesion molecule enriched in developing limbic circuits, glucocorticoid receptors, and cognitive functioning.

Chronic stress effects on hippocampal structure and synaptic function: relevance for depression and normalization by anti-glucocorticoid treatment

Exposure of an organism to environmental challenges activates two hormonal systems that help the organism to adapt. As part of this adaptational process, brain processes are changed such that appropriate behavioral strategies are selected that allow optimal performance at the short term, while relevant information is stored for the future. Over the past years it has become evident that chronic uncontrollable and unpredictable stress also exerts profound effects on structure and function of limbic neurons, but the impact of chronic stress is not a mere accumulation of repeated episodes of acute stress exposure. Dendritic trees are reduced in some regions but expanded in others, and cells are generally exposed to a higher calcium load upon depolarization. Synaptic strengthening is largely impaired. Neurotransmitter responses are also changed, e.g., responses to serotonin. We here discuss: (a) the main cellular effects after chronic stress with emphasis on the hippocampus, (b) how such effects could contribute to the development of psychopathology in genetically vulnerable individuals, and (c) their normalization by brief treatment with anti-glucocorticoids.

Correlated memory defects and hippocampal dendritic spine loss after acute stress involve corticotropin-releasing hormone signaling

Stress affects the hippocampus, a brain region crucial for memory. In rodents, acute stress may reduce density of dendritic spines, the location of postsynaptic elements of excitatory synapses, and impair long-term potentiation and memory. Steroid stress hormones and neurotransmitters have been implicated in the underlying mechanisms, but the role of corticotropin-releasing hormone (CRH), a hypothalamic hormone also released during stress within hippocampus, has not been elucidated. In addition, the causal relationship of spine loss and memory defects after acute stress is unclear. We used transgenic mice that expressed YFP in hippocampal neurons and found that a 5-h stress resulted in profound loss of learning and memory. This deficit was associated with selective disruption of long-term potentiation and of dendritic spine integrity in commissural/associational pathways of hippocampal area CA3. The degree of memory deficit in individual mice correlated significantly with the reduced density of area CA3 apical dendritic spines in the same mice. Moreover, administration of the CRH receptor type 1 (CRFR(1)) blocker NBI 30775 directly into the brain prevented the stress-induced spine loss and restored the stress-impaired cognitive functions. We conclude that acute, hours-long stress impairs learning and memory via mechanisms that disrupt the integrity of hippocampal dendritic spines. In addition, establishing the contribution of hippocampal CRH-CRFR(1) signaling to these processes highlights the complexity of the orchestrated mechanisms by which stress impacts hippocampal structure and function.

Dopamine receptor mechanisms mediate corticotropin-releasing factor-induced long-term potentiation in the rat amygdala following cocaine withdrawal

Corticotropin-releasing factor (CRF) in the amygdala is involved in stress responses. Moreover, dopaminergic neurotransmission in the brain reward system including the amygdala plays a significant role in the pathology of cocaine addiction. The present study analysed CRF-induced synaptic plasticity, its pharmacological sensitivity and interactions with the dopamine (DA) system in the basolateral to lateral capsula central amygdala (lcCeA) pathway after a 2-week withdrawal from repeated cocaine administration. A physiologically relevant CRF concentration (25 nm) induced long-term potentiation (LTP) that was enhanced after cocaine withdrawal. In saline-treated rats, CRF-induced LTP was mediated through N-methyl-d-aspartate (NMDA) receptors, L-type voltage-gated calcium channels (L-VGCCs) and CRF(1) receptors. However, in cocaine-withdrawn animals, activation of CRF(1) and CRF(2) receptors was found to enhance LTP. This enhanced CRF-induced LTP after cocaine withdrawal was mediated through endogenous activation of both D1- and D2-like receptors. Furthermore, expression of the D1 receptor (D1R) but not the D2R, D3R, D4R or D5R was significantly increased after cocaine withdrawal. CRF(1) but not CRF(2) protein expression was increased, suggesting that elevated levels of these proteins contributed to the enhancement of CRF-induced LTP during cocaine withdrawal. CRF interactions with the DA system in the amygdala may represent a fundamental neurochemical and cellular mechanism linking stress to cocaine-induced neuronal plasticity.

Anti-glucocorticoid gene therapy reverses the impairing effects of elevated corticosterone on spatial memory, hippocampal neuronal excitability, and synaptic plasticity

Moderate release of the major stress hormones, glucocorticoids (GCs), improves hippocampal function and memory. In contrast, excessive or prolonged elevations produce impairments. Enzymatic degradation and reformation of GCs help to maintain optimal levels within target tissues, including the brain. We hypothesized that expressing a GC-degrading enzyme in hippocampal neurons would attenuate the negative impact of an excessive elevation in GC levels on synaptic physiology and spatial memory. We tested this by expressing 11-beta-hydroxysteroid dehydrogenase (type II) in dentate gyrus granule cells during a 3 d GC treatment followed by examination of synaptic responses in hippocampal slices or spatial performance in the Morris water maze. In adrenalectomized rats with basal GC replacement, additional GC treatments for 3 d reduced synaptic strength and promoted the expression of long-term depression at medial perforant path synapses, increased granule cell and CA1 pyramidal cell excitability, and impaired spatial reference memory (without influencing learning). Expression of 11-beta-hydroxysteroid dehydrogenase (type II), mostly in mature dentate gyrus granule cells, reversed the effects of high GC levels on granule cell and pyramidal cell excitability, perforant path synaptic plasticity, and spatial memory. These data demonstrate the ability of neuroprotective gene expression limited to a specific cell population to both locally and trans-synaptically offset neurophysiological disruptions produced by prolonged increases in circulating stress hormones. This report supplies the first physiological explanation for previously demonstrated cognitive sparing by anti-stress gene therapy approaches and lends additional insight into the hippocampal processes that are important for memory.

Neurobiological constituents of active, passive, and variable coping strategies in rats: integration of regional brain neuropeptide Y levels and cardiovascular responses

Effective coping strategies build resilience against stress-induced pathology. In the current study, young male rats were categorized as active, passive, or variable copers by observing their responses to being gently restrained on their backs (i.e., the back-test). The rats were subsequently exposed to chronic unpredictable stress, which included several ethologically relevant stressors such as predator odors and calls, for approximately three weeks. During this time, the variable copers, defined as rats that demonstrated a variable as opposed to a rigid response to stress, exhibited more seemingly adaptive responsiveness in three successive forced swim tests than the more consistently responding passive and active copers. This behavioral flexibility was accompanied by increased neuropeptide Y-immunoreactivity in the bed nucleus of the stria terminalis (BNST) and the amygdala and increased fos-immunoreactivity in the BNST. Additionally, the alterations in fecal corticosteroid levels and cardiovascular measures (systolic blood pressure and tail blood volume) between baseline and stress conditions differed according to coping strategy. Factor analysis indicates that variable copers were characterized by a distinct cardiovascular and neural response to the stress exposure. These results suggest that this animal coping model may be useful in discerning the adaptive nature of particular response strategies in the face of environmental exigencies.