Glucocorticoid receptors are localized to dendritic spines and influence local actin signaling

Differential effects of acute exercise on emotional memory in men and women

Exercise may change emotional memory, which is associated with the induction of mental disorders such as depression and anxiety. This effect of exercise may be influenced by exercise-induced cortisol release. Depending on sex, cortisol exerts differential effects on emotional memory consolidation. However, whether acute exercise and exercise-induced cortisol release have sex-dependent effects on emotional memory has not been established. Therefore, first, we aimed to determine the effects of acute exercise on emotional memory, separately for men and women, in a within-subjects design. Second, we aimed to examine whether the effects of acute exercise on emotional memory are related to the effects of exercise-induced cortisol release, separately for men and women. Sixteen healthy men and 15 healthy women were presented with positive and negative emotional images, followed by either rest or a vigorous-intensity cycling exercise condition using a within-subjects design on separate days. Salivary cortisol was measured before presenting the emotional images presentation and 20 min after each intervention. Emotional memory was assessed two days later. Vigorous-intensity exercise decreased emotional memory in women, whereas there was no change in men after rest or exercise. Cortisol levels increased after exercise intervention in both men and women, although there was no association between cortisol levels and emotional memory. These findings demonstrate that the effect of a single bout of vigorous-intensity exercise on emotional memory differs between men and women and is associated with decreased emotional memory in women.

In vitro modeling of the neurobiological effects of glucocorticoids: A review

Hypothalamic-pituitary adrenal (HPA)axis dysregulation has long been implicated in stress-related disorders such as major depression and post-traumatic stress disorder. Glucocorticoids (GCs) are released from the adrenal glands as a result of HPA-axis activation. The release of GCs is implicated with several neurobiological changes that are associated with negative consequences of chronic stress and the onset and course of psychiatric disorders. Investigating the underlying neurobiological effects of GCs may help to better understand the pathophysiology of stress-related psychiatric disorders. GCs impact a plethora of neuronal processes at the genetic, epigenetic, cellular, and molecular levels. Given the scarcity and difficulty in accessing human brain samples, 2D and 3D in vitro neuronal cultures are becoming increasingly useful in studying GC effects. In this review, we provide an overview of in vitro studies investigating the effects of GCs on key neuronal processes such as proliferation and survival of progenitor cells, neurogenesis, synaptic plasticity, neuronal activity, inflammation, genetic vulnerability, and epigenetic alterations. Finally, we discuss the challenges in the field and offer suggestions for improving the use of in vitro models to investigate GC effects.

Stress vulnerability shapes disruption of motor cortical neuroplasticity

Chronic stress is a major cause of neuropsychiatric conditions such as depression. Stress vulnerability varies individually in mice and humans, measured by behavioral changes. In contrast to affective symptoms, motor retardation as a consequence of stress is not well understood. We repeatedly imaged dendritic spines of the motor cortex in Thy1-GFP M mice before and after chronic social defeat stress. Susceptible and resilient phenotypes were discriminated by symptom load and their motor learning abilities were assessed by a gross and fine motor task. Stress phenotypes presented individual short- and long-term changes in the hypothalamic-pituitary-adrenal axis as well as distinct patterns of altered motor learning. Importantly, stress was generally accompanied by a marked reduction of spine density in the motor cortex and spine dynamics depended on the stress phenotype. We found astrogliosis and altered microglia morphology along with increased microglia-neuron interaction in the motor cortex of susceptible mice. In cerebrospinal fluid, proteomic fingerprints link the behavioral changes and structural alterations in the brain to neurodegenerative disorders and dysregulated synaptic homeostasis. Our work emphasizes the importance of synaptic integrity and the risk of neurodegeneration within depression as a threat to brain health.

Ketamine, but not fluoxetine, rapidly rescues corticosterone-induced impairments on glucocorticoid receptor and dendritic branching in the hippocampus of mice

Although numerous studies have investigated the mechanisms underlying the fast and sustained antidepressant-like effects of ketamine, the contribution of the glucocorticoid receptor (GR) and dendritic branching remodeling to its responses remain to be fully established. This study investigated the ability of a single administration of ketamine to modulate the GR and dendritic branching remodeling and complexity in the hippocampus of mice subjected to chronic corticosterone (CORT) administration. CORT was administered for 21 days, followed by a single administration of ketamine (1 mg ∕kg, i.p.) or fluoxetine (10 mg ∕kg, p.o., conventional antidepressant) in mice. On 22^nd, 24 h after the treatments, GR immunocontent in the hippocampus was analyzed by western blotting, while the dendritic arborization and dendrite length in the ventral and dorsal dentate gyrus (DG) of the hippocampus was analyzed by Sholl analysis. Chronic CORT administration downregulated hippocampal GR immunocontent, but this alteration was completely reversed by a single administration of ketamine, but not fluoxetine. Moreover, CORT administration significantly decreased dendritic branching in the dorsal and ventral DG areas and caused a mild decrease in dendrite length in both regions. Ketamine, but not fluoxetine, reversed CORT-induced dendritic branching loss in the ventral and dorsal DG areas, regions associated with mood regulation and cognitive functions, respectively. This study provides novel evidence that a single administration of ketamine, but not fluoxetine, rescued the impairments on GR and dendritic branching in the hippocampus of mice subjected to chronic CORT administration, effects that may be associated with its rapid antidepressant response.

Deletion of hippocampal Glucocorticoid receptors unveils sex-biased microRNA expression and neuronal morphology alterations in mice

Sex differences in the brain have prompted many researchers to investigate the underlying molecular actors, such as the glucocorticoid receptor (GR). This nuclear receptor controls gene expression, including microRNAs (miRNAs), in non-neuronal cells. Here, we investigated sex-biased effects of GR on hippocampal miRNA expression and neuronal morphology by generating a neuron-specific GR knockout mouse (Emx1-Nr3c1^−/−). The levels of 578 mature miRNAs were assessed using NanoString technology and, in contrast to males, female Emx1-Nr3c1^−/− mice showed a substantially higher number of differentially expressed miRNAs, confirming a sex-biased effect of GR ablation. Based on bioinformatic analyses we identified several transcription factors potentially involved in miRNA regulation. Functional enrichment analyses of the miRNA-mRNA interactions revealed pathways related to neuronal arborization and both spine morphology and density in both sexes. Two recognized regulators of dendritic morphology, CAMKII-α and GSK-3β, increased their protein levels by GR ablation in female mice hippocampus, without changes in males. Additionally, sex-specific effects of GR deletion were observed on CA1 neuronal arborization and dendritic spine features. For instance, a reduced density of mushroom spines in apical dendrites was evidenced only in females, while a decreased length in basal dendrites was noted only in males. However, length and arborization of apical dendrites were reduced by GR ablation irrespective of the sex. Overall, our study provides new insights into the sex-biased GR actions, especially in terms of miRNAs expression and neuronal morphology in the hippocampus.

Brain mechanisms of HPA axis regulation: neurocircuitry and feedback in context Richard Kvetnansky lecture

Regulation of stress reactivity is a fundamental priority of all organisms. Stress responses are critical for survival, yet can also cause physical and psychological damage. This review provides a synopsis of brain mechanisms designed to control physiological responses to stress, focusing primarily on glucocorticoid secretion via the hypothalamo-pituitary-adrenocortical (HPA) axis. The literature provides strong support for multi-faceted control of HPA axis responses, involving both direct and indirect actions at paraventricular nucleus (PVN) corticotropin releasing hormone neurons driving the secretory cascade. The PVN is directly excited by afferents from brainstem and hypothalamic circuits, likely relaying information on homeostatic challenge. Amygdala subnuclei drive HPA axis responses indirectly via disinhibition, mediated by GABAergic relays onto PVN-projecting neurons in the hypothalamus and bed nucleus of the stria terminalis (BST). Inhibition of stressor-evoked HPA axis responses is mediated by an elaborate network of glucocorticoid receptor (GR)-containing circuits, providing a distributed negative feedback signal that inhibits PVN neurons. Prefrontal and hippocampal neurons play a major role in HPA axis inhibition, again mediated by hypothalamic and BST GABAergic relays to the PVN. The complexity of the regulatory process suggests that information on stressors is integrated across functional disparate brain circuits prior to accessing the PVN, with regions such as the BST in prime position to relay contextual information provided by these sources into appropriate HPA activation. Dysregulation of the HPA in disease is likely a product of inappropriate checks and balances between excitatory and inhibitory inputs ultimately impacting PVN output.

Effects of Dexamethasone on Remodeling of the Hippocampal Synaptic Filamentous Actin Cytoskeleton in a Model of Pilocarpine-induced Status Epilepticus

The filamentous actin (F-actin) cytoskeleton is progressively damaged after status epilepticus (SE), which is related to delayed neuronal death, aberrant recurrent circuits and epileptogenesis. Glucocorticoids regulate dendritic spine remodeling by acting on glucocorticoid receptors and the dynamics of the F-actin cytoskeleton. Our previous study showed that administration of dexamethasone (DEX) in the latent period of the pilocarpine epileptic model reduces damage to the hippocampal filamentous actin cytoskeleton and the loss of hippocampal neurons and aids in maintaining the synaptic structures, but it is not sufficient to stop epileptogenesis. In this work, we focused on the role of glucocorticoids in regulating the hippocampal F-actin cytoskeleton during SE. We examined the abundance of synaptic F-actin, analyzed the hippocampal F-actin/G-actin (F/G) ratio and pCofilin, and evaluated the number of hippocampal neurons and pre/postsynaptic markers in pilocarpine-induced status epilepticus mice with or without administration of dexamethasone (DEX). We found that the latency of Stage 3 seizures increased, the mortality decreased, the damage to the synaptic F-actin cytoskeleton in the hippocampal subfields was significantly attenuated, and a greater number of postsynaptic structures were retained in the hippocampal subfields after treatment with DEX. These results indicate that treatment with dexamethasone stabilizes the synaptic F-actin cytoskeleton and reduces the damage to the brain due to SE. This approach is expected to be beneficial in alleviating delayed neuron damage and the process of epileptogenesis.

Cognitive dysfunction in mice lacking proper glucocorticoid receptor dimerization

Stress is a major risk factor for depression and anxiety. One of the effects of stress is the (over-) activation of the hypothalamic-pituitary-adrenal (HPA) axis and the release of stress hormones such as glucocorticoids (GCs). Chronically increased stress hormone levels have been shown to have detrimental effects on neuronal networks by inhibiting neurotrophic processes particularly in the hippocampus proper. Centrally, GCs modulate metabolic as well as behavioural processes by activating two classes of corticoid receptors, high-affinity mineralocorticoid receptors (MR) and low-affinity glucocorticoid receptors (GR). Upon activation, GR can modulate gene transcription either as a monomeric protein, or as a dimer interacting directly with DNA. GR can also modulate cellular processes via non-genomic mechanisms, for example via a GPCR-protein interaction. We evaluated the behavioral phenotype in mice with a targeted mutation in the GR in a FVB/NJ background. In GRdim/dim mice, GR proteins form poor homodimers, while the GR monomer remains intact. We evaluated the effect of poor GR dimerization on hippocampus-dependent cognition as well as on exploration and emotional behavior under baseline and chronically increased stress hormone levels. We found that GRdim/dim mice did not behave differently from GRwt/wt littermates under baseline conditions. However, after chronic elevation of stress hormone levels, GRdim/dim mice displayed a significant impairment in hippocampus-dependent memory compared to GRwt/wt mice, which correlated with differential expression of hippocampal Bdnf/TrkB and Fkbp5.

Dexamethasone ameliorates the damage of hippocampal filamentous actin cytoskeleton but is not sufficient to cease epileptogenesis in pilocarpine induced epileptic mice

Rogressive deconstruction of filament actin (F-actin) in hippocampal neurons in the epileptic brain have been associated with epileptogenesis. Previous clinical studies suggest that glucocorticoids treatment plays beneficial roles in refractory epilepsy. Glucocorticoids treatment affects dendritic spine morphology by regulating local glucocorticoid receptors and F-actin cytoskeleton dynamics. However, how glucocorticoids regulate epileptogenesis by controlling F-actin cytoskeleton is not clear yet. Here we study the function of glucocorticoids in epileptogenesis by examining F-actin abundance, hippocampal neuron number, and synaptic markers in pilocarpine-induced epileptic mice in the presence or absence of dexamethasone (DEX) treatment. We found that spontaneous seizure duration was significantly reduced; F-actin damage in hippocampal subfields was remarkably attenuated; loss of pyramidal cells was dramatically decreased; more intact synaptic structures indicated by pre- and postsynaptic markers were preserved in multiple hippocampal regions after DEX treatment. However, the number of ZNT3 positive particles in the molecular layer in the hippocampus of pilocarpine epileptic mice was not altered after DEX treatment. Although not sufficient to cease epileptogenesis, our results suggest that dexamethasone treatment ameliorates the damage of epileptic brain by stabilizing F-actin cytoskeleton in the pilocarpine epileptic mice.

Prefrontal cortical trkB, glucocorticoids, and their interactions in stress and developmental contexts

The tropomyosin/tyrosine receptor kinase B (trkB) and glucocorticoid receptor (GR) regulate neuron structure and function and the hormonal stress response. Meanwhile, disruption of trkB and GR activity (e.g., by chronic stress) can perturb neuronal morphology in cortico-limbic regions implicated in stressor-related illnesses like depression. Further, several of the short- and long-term neurobehavioral consequences of stress depend on the developmental timing and context of stressor exposure. We review how the levels and activities of trkB and GR in the prefrontal cortex (PFC) change during development, interact, are modulated by stress, and are implicated in depression. We review evidence that trkB- and GR-mediated signaling events impact the density and morphology of dendritic spines, the primary sites of excitatory synapses in the brain, highlighting effects in adolescents when possible. Finally, we review the role of neurotrophin and glucocorticoid systems in stress-related metaplasticity. We argue that better understanding the long-term effects of developmental stressors on PFC trkB, GR, and related factors may yield insights into risk for chronic, remitting depression and related neuropsychiatric illnesses.

Mineralocorticoid Antagonist Improves Glucocorticoid Receptor Signaling and Dexamethasone Analgesia in an Animal Model of Low Back Pain

Low back pain, a leading cause of disability, is commonly treated by epidural steroid injections that target the anti-inflammatory glucocorticoid receptor (GR). However, their efficacy has been controversial. All currently used epidural steroids also activate the pro-inflammatory mineralocorticoid receptor (MR) with significant potency. Local inflammation of the dorsal root ganglia (DRG), a rat model of low back pain, was used. This model causes static and dynamic mechanical allodynia, cold allodynia and guarding behavior (a measure of spontaneous pain), and activates the MR, with pro-nociceptive effects. In this study, effects of local Dexamethasone (DEX; a glucocorticoid used in epidural injections), and eplerenone (EPL; a second generation, more selective MR antagonist) applied to the DRG at the time of inflammation were examined. Mechanical and spontaneous pain behaviors were more effectively reduced by the combination of DEX and EPL than by either alone. The combination of steroids was particularly more effective than DEX alone or the model alone (3-fold improvement for mechanical allodynia) at later times (day 14). Immunohistochemical analysis of the GR in the DRG showed that the receptor was expressed in neurons of all size classes, and in non-neuronal cells including satellite glia. The GR immunoreactivity was downregulated by DRG inflammation (48%) starting on day 1, consistent with the reduction of GR (57%) observed by Western blot, when compared to control animals. On day 14, the combination of DEX and EPL resulted in rescue of GR immunoreactivity that was not seen with DEX alone, and was more effective in reducing a marker for satellite glia activation/neuroinflammation. The results suggest that EPL may enhance the effectiveness of clinically used epidural steroid injections, in part by enhancing the availability of the GR. Thus, the glucocorticoid-mineralocorticoid interactions may limit the effectiveness of epidural steroids through the regulation of the GR in the DRG.

Memory-Related Synaptic Plasticity Is Sexually Dimorphic in Rodent Hippocampus

Men are generally superior to women in remembering spatial relationships, whereas the reverse holds for semantic information, but the neurobiological bases for these differences are not understood. Here we describe striking sexual dimorphism in synaptic mechanisms of memory encoding in hippocampal field CA1, a region critical for spatial learning. Studies of acute hippocampal slices from adult rats and mice show that for excitatory Schaffer-commissural projections, the memory-related long-term potentiation (LTP) effect depends upon endogenous estrogen and membrane estrogen receptor α (ERα) in females but not in males; there was no evident involvement of nuclear ERα in females, or of ERβ or GPER1 (G-protein-coupled estrogen receptor 1) in either sex. Quantitative immunofluorescence showed that stimulation-induced activation of two LTP-related kinases (Src, ERK1/2), and of postsynaptic TrkB, required ERα in females only, and that postsynaptic ERα levels are higher in females than in males. Several downstream signaling events involved in LTP were comparable between the sexes. In contrast to endogenous estrogen effects, infused estradiol facilitated LTP and synaptic signaling in females via both ERα and ERβ. The estrogen dependence of LTP in females was associated with a higher threshold for both inducing potentiation and acquiring spatial information. These results indicate that the observed sexual dimorphism in hippocampal LTP reflects differences in synaptic kinase activation, including both a weaker association with NMDA receptors and a greater ERα-mediated kinase activation in response to locally produced estrogen in females. We propose that male/female differences in mechanisms and threshold for field CA1 LTP contribute to differences in encoding specific types of memories.SIGNIFICANCE STATEMENT There is good evidence for male/female differences in memory-related cognitive function, but the neurobiological basis for this sexual dimorphism is not understood. Here we describe sex differences in synaptic function in a brain area that is critical for learning spatial cues. Our results show that female rodents have higher synaptic levels of estrogen receptor α (ERα) and, in contrast to males, require membrane ERα for the activation of signaling kinases that support long-term potentiation (LTP), a form of synaptic plasticity thought to underlie learning. The additional requirement of estrogen signaling in females resulted in a higher threshold for both LTP and hippocampal field CA1-dependent spatial learning. These results describe a synaptic basis for sexual dimorphism in encoding spatial information.

Identification of an acute functional cross-talk between amyloid-β and glucocorticoid receptors at hippocampal excitatory synapses

Amyloid-β is a peptide released by synapses in physiological conditions and its pathological accumulation in brain structures necessary for memory processing represents a key toxic hallmark underlying Alzheimer's disease. The oligomeric form of Amyloid-β (Aβο) is now believed to represent the main Amyloid-β species affecting synapse function. Yet, the exact molecular mechanism by which Aβο modifies synapse function remains to be fully elucidated. There is accumulating evidence that glucocorticoid receptors (GRs) might participate in Aβο generation and activity in the brain. Here, we provide evidence for an acute functional cross-talk between Aβ and GRs at hippocampal excitatory synapses. Using live imaging and biochemical analysis of post-synaptic densities (PSD) in cultured hippocampal neurons, we show that synthetic Aβo (100 nM) increases GR levels in spines and PSD. Also, in these cultured neurons, blocking GRs with two different GR antagonists prevents Aβo-mediated PSD95 increase within the PSD. By analyzing long-term potentiation (LTP) and long-term depression (LTD) in ex vivo hippocampal slices after pharmacologically blocking GR, we also show that GR signaling is necessary for Aβo-mediated LTP impairment, but not Aβo-mediated LTD induction. The necessity of neuronal GRs for Aβo-mediated LTP was confirmed by genetically removing GRs in vivo from CA1 neurons using conditional GR mutant mice. These results indicate a tight functional interplay between GR and Aβ activities at excitatory synapses.

Selective activation of parvalbumin interneurons prevents stress-induced synapse loss and perceptual defects

Stress, a prevalent experience in modern society, is a major risk factor for many psychiatric disorders. Although sensorimotor abnormalities are often present in these disorders, little is known about how stress affects the sensory cortex. Combining behavioral analyses with in vivo synaptic imaging, we show that stressful experiences lead to progressive, clustered loss of dendritic spines along the apical dendrites of layer (L) 5 pyramidal neurons (PNs) in the mouse barrel cortex, and such spine loss closely associates with deteriorated performance in a whisker-dependent texture discrimination task. Furthermore, the activity of parvalbumin-expressing inhibitory interneurons (PV+ INs) decreases in the stressed mouse due to reduced excitability of these neurons. Importantly, both behavioral defects and structural changes of L5 PNs are prevented by selective pharmacogenetic activation of PV+INs in the barrel cortex during stress. Finally, stressed mice raised under environmental enrichment (EE) maintain normal activation of PV+ INs, normal texture discrimination, and L5 PN spine dynamics similar to unstressed EE mice. Our findings suggest that the PV+ inhibitory circuit is crucial for normal synaptic dynamics in the mouse barrel cortex and sensory function. Pharmacological, pharmacogenetic and environmental approaches to prevent stress-induced maladaptive behaviors and synaptic malfunctions converge on the regulation of PV+ IN activity, pointing to a potential therapeutic target for stress-related disorders.

MDMA-assisted psychotherapy for PTSD: Are memory reconsolidation and fear extinction underlying mechanisms?

MDMA-assisted psychotherapy for treatment of PTSD has recently progressed to Phase 3 clinical trials and received Breakthrough Therapy designation by the FDA. MDMA used as an adjunct during psychotherapy sessions has demonstrated effectiveness and acceptable safety in reducing PTSD symptoms in Phase 2 trials, with durable remission of PTSD diagnosis in 68% of participants. The underlying psychological and neurological mechanisms for the robust effects in mitigating PTSD are being investigated in animal models and in studies of healthy volunteers. This review explores the potential role of memory reconsolidation and fear extinction during MDMA-assisted psychotherapy. MDMA enhances release of monoamines (serotonin, norepinephrine, dopamine), hormones (oxytocin, cortisol), and other downstream signaling molecules (BDNF) to dynamically modulate emotional memory circuits. By reducing activation in brain regions implicated in the expression of fear- and anxiety-related behaviors, namely the amygdala and insula, and increasing connectivity between the amygdala and hippocampus, MDMA may allow for reprocessing of traumatic memories and emotional engagement with therapeutic processes. Based on the pharmacology of MDMA and the available translational literature of memory reconsolidation, fear learning, and PTSD, this review suggests a neurobiological rationale to explain, at least in part, the large effect sizes demonstrated for MDMA in treating PTSD.

Temporal profiling of an acute stress-induced behavioral phenotype in mice and role of hippocampal DRR1

Understanding the neurobiological mechanisms underlying the response to an acute stressor may provide novel insights into successful stress-coping strategies. Acute behavioral stress-effects may be restricted to a specific time window early after stress-induction. However, existing behavioral test batteries typically span multiple days or even weeks, limiting the feasibility for a broad behavioral analysis following acute stress. Here, we designed a novel comprehensive behavioral test battery in male mice that assesses multiple behavioral dimensions within a sufficiently brief time window to capture acute stress-effects and its temporal profile. Using this battery, we investigated the behavioral impact of acute social defeat stress (ASD) early thereafter (ASD-early, ∼4 h), when circulating corticosterone levels were elevated, and late after stress-induction (ASD-late, ∼8 h), when corticosterone were returned to timed control levels. ASD-early, but not ASD-late, displayed hippocampal-dependent cognitive impairments in the Y-maze and in the spatial object recognition test. The actin-binding protein (ABP) Tumor suppressor down-regulated in renal cell carcinoma 1 (DRR1) has been described as resilience-promoting factor but the potential of DRR1 to curb stress-effects has not been investigated. Hippocampal DRR1 mRNA-expression was increased in ASD-early and ASD-late whereas DRR1-protein levels were increased only in ASD-late. We hypothesized that the absence of hippocampal DRR1 protein-upregulation in ASD-early caused the associated cognitive impairments. Hence, virus-mediated hippocampal DRR1-overexpression was induced as putative treatment, but cognitive deficits in ASD-early were not improved. We conclude that hippocampal DRR1-overexpression is insufficient to protect from the detrimental cognitive effects following acute social stress where perhaps a more global response in local actin dynamics, involving multiple stress-responsive ABPs that act synergistically, was warranted.

Rapid nongenomic modulation by neurosteroids of dendritic spines in the hippocampus: Androgen, oestrogen and corticosteroid

Memories are stored in synapses that consist of axon terminals and dendritic spines. Dendritic spines are postsynaptic structures of synapses and are essential for synaptic plasticity and cognition. Therefore, extensive investigations concerning the functions and structures of spines have been performed. Sex steroids and stress steroids have been shown to modulate hippocampal synapses. Although the rapid modulatory action of sex steroids on synapses has been studied in hippocampal neurones over several decades, the essential molecular mechanisms have not been fully understood. Here, a description of kinase-dependent signalling mechanisms is provided that can explain the rapid nongenomic modulation of dendritic spinogenesis in rat and mouse hippocampal slices by the application of sex steroids, including dihydrotestosterone, testosterone, oestradiol and progesterone. We also indicate the role of synaptic (classic) sex steroid receptors that trigger these rapid synaptic modulations. Moreover, we describe rapid nongenomic spine modulation by applying corticosterone, which is an acute stress model of the hippocampus. The explanations for the results obtained are mainly based on the optical imaging of dendritic spines. Comparisons are also performed with results obtained from other types of imaging, including electron microscopic imaging. Relationships between spine modulation and modulation of cognition are discussed. We recognise that most of rapid effects of exogenously applied oestrogen and androgen were observed in steroid-depleted conditions, including acute slices of the hippocampus, castrated male animals and ovariectomised female animals. Therefore, the previously observed effects can be considered as a type of recovery event, which may be essentially similar to hormone replacement therapy under hormone-decreased conditions. On the other hand, in gonadally intact young animals with high levels of endogenous sex hormones, further supplementation of sex hormones might not be effective, whereas the infusion of blockers for steroid receptors or kinases may be effective, with respect to suppressing sex hormone functions, thus providing useful information regarding molecular mechanisms.

Fat-brain connections: Adipocyte glucocorticoid control of stress and metabolism

Glucocorticoids act via multiple mechanisms to mobilize energy for maintenance and restoration of homeostasis. In adipose tissue, glucocorticoids can promote lipolysis and facilitate adipocyte differentiation/growth, serving both energy-mobilizing and restorative processes during negative energy balance. Recent data suggest that adipose-dependent feedback may also be involved in regulation of stress responses. Adipocyte glucocorticoid receptor (GR) deletion causes increased HPA axis stress reactivity, due to a loss of negative feedback signals into the CNS. The fat-to-brain signal may be mediated by neuronal mechanisms, release of adipokines or increased lipolysis. The ability of adipose GRs to inhibit psychogenic as well as metabolic stress responses suggests that (1) feedback regulation of the HPA axis occurs across multiple bodily compartments, and (2) fat tissue integrates psychogenic stress signals. These studies support a link between stress biology and energy metabolism, a connection that has clear relevance for numerous disease states and their comorbidities.

Corticosteroid-induced dendrite loss and behavioral deficiencies can be blocked by activation of Abl2/Arg kinase

Stressor exposure induces neuronal remodeling in specific brain regions. Given the persistence of stress-related illnesses, key next steps in determining the contributions of neural structure to mental health are to identify cell types that fail to recover from stressor exposure and to identify "trigger points" and molecular underpinnings of stress-related neural degeneration. We evaluated dendrite arbor structure on hippocampal CA1 pyramidal neurons before, during, and following prolonged exposure to one key mediator of the stress response - corticosterone (cortisol in humans). Basal dendrite arbors progressively simplified during a 3-week exposure period, and failed to recover when corticosterone was withdrawn. Corticosterone exposure decreased levels of the dendrite stabilization factor Abl2/Arg nonreceptor tyrosine kinase and phosphorylation of its substrates p190RhoGAP and cortactin within 11days, suggesting that disruption of Arg-mediated signaling may trigger dendrite arbor atrophy and, potentially, behavioral abnormalities resulting from corticosterone exposure. To test this, we administered the novel, bioactive Arg kinase activator, 5-(1,3-diaryl-1H-pyrazol-4-yl)hydantoin, 5-[3-(4-fluorophenyl)-1-phenyl-1H-pyrazol-4-yl]-2,4-imidazolidinedione (DPH), in conjunction with corticosterone. We found that repeated treatment corrected CA1 arbor structure, otherwise simplified by corticosterone. DPH also corrected corticosterone-induced errors in a hippocampal-dependent reversal learning task and anhedonic-like behavior. Thus, pharmacological compounds that target cytoskeletal regulators, rather than classical neurotransmitter systems, may interfere with stress-associated cognitive decline and mental health concerns.

Converging, Synergistic Actions of Multiple Stress Hormones Mediate Enduring Memory Impairments after Acute Simultaneous Stresses

Stress influences memory, an adaptive process crucial for survival. During stress, hippocampal synapses are bathed in a mixture of stress-released molecules, yet it is unknown whether or how these interact to mediate the effects of stress on memory. Here, we demonstrate novel synergistic actions of corticosterone and corticotropin-releasing hormone (CRH) on synaptic physiology and dendritic spine structure that mediate the profound effects of acute concurrent stresses on memory. Spatial memory in mice was impaired enduringly after acute concurrent stresses resulting from loss of synaptic potentiation associated with disrupted structure of synapse-bearing dendritic spines. Combined application of the stress hormones corticosterone and CRH recapitulated the physiological and structural defects provoked by acute stresses. Mechanistically, corticosterone and CRH, via their cognate receptors, acted synergistically on the spine-actin regulator RhoA, promoting its deactivation and degradation, respectively, and destabilizing spines. Accordingly, blocking the receptors of both hormones, but not each alone, rescued memory. Therefore, the synergistic actions of corticosterone and CRH at hippocampal synapses underlie memory impairments after concurrent and perhaps also single, severe acute stresses, with potential implications to spatial memory dysfunction in, for example, posttraumatic stress disorder.

SIGNIFICANCE STATEMENT

Stress influences memory, an adaptive process crucial for survival. During stress, adrenal corticosterone and hippocampal corticotropin-releasing hormone (CRH) permeate memory-forming hippocampal synapses, yet it is unknown whether (and how) these hormones interact to mediate effects of stress. Here, we demonstrate novel synergistic actions of corticosterone and CRH on hippocampal synaptic plasticity and spine structure that mediate the memory-disrupting effects of stress. Combined application of both hormones provoked synaptic function collapse and spine disruption. Mechanistically, corticosterone and CRH synergized at the spine-actin regulator RhoA, promoting its deactivation and degradation, respectively, and destabilizing spines. Notably, blocking both hormones, but not each alone, prevented the enduring memory problems after acute concurrent stresses. Therefore, synergistic actions of corticosterone and CRH underlie enduring memory impairments after concurrent acute stresses, which might be relevant to spatial memory deficits described in posttraumatic stress disorder.

Cholinergic Signals from the CNS Regulate G-CSF-Mediated HSC Mobilization from Bone Marrow via a Glucocorticoid Signaling Relay

Hematopoietic stem cells (HSCs) are mobilized from niches in the bone marrow (BM) to the blood circulation by the cytokine granulocyte colony-stimulating factor (G-CSF) through complex mechanisms. Among these, signals from the sympathetic nervous system regulate HSC egress via its niche, but how the brain communicates with the BM remains largely unknown. Here we show that muscarinic receptor type-1 (Chrm1) signaling in the hypothalamus promotes G-CSF-elicited HSC mobilization via hormonal priming of the hypothalamic-pituitary-adrenal (HPA) axis. Blockade of Chrm1 in the CNS, but not the periphery, reduces HSC mobilization. Mobilization is impaired in Chrm1-∕- mice and rescued by parabiosis with wild-type mice, suggesting a relay by a blood-borne factor. We have identified the glucocorticoid (GC) hormones as critical for optimal mobilization. Physiological levels of corticosterone promote HSC migration via the GC receptor Nr3c1-dependent signaling and upregulation of actin-organizing molecules. These results uncover long-range regulation of HSC migration emerging from the brain.

Synaptic Plasticity, Metaplasticity and Depression

The development of a persistent depressive affective state has for some time been thought to result from persistent alterations in neurotransmitter-mediated synaptic transmission. While the identity of those transmitters has changed over the years, the literature has lacked mechanistic connections between the neurophysiological mechanisms they regulate, and how these mechanisms alter neuronal function, and, hence, affective homeostasis. This review will examine recent work that suggests that both long-term activity-dependent changes in synaptic strength (“plasticity”), and shifting set points for the ease of induction of future long-term changes (“metaplasticity”), may be critical to establishing and reversing a depressive behavioral state. Activity-dependent long-term synaptic plasticity involves both strengthening and weakening of synaptic connections associated with a dizzying array of neurochemical alterations that include synaptic insertion and removal of a number of subtypes of AMPA, NMDA and metabotropic glutamate receptors, changes in presynaptic glutamate release, and structural changes in dendritic spines. Cellular mechanisms of metaplasticity are far less well understood. Here, we will review the growing evidence that long-term synaptic changes in glutamatergic transmission, in brain regions that regulate mood, are key determinants of affective homeostasis and therapeutic targets with immense potential for drug development.

Cellular softening mediates leukocyte demargination and trafficking, thereby increasing clinical blood counts

Leukocytes normally marginate toward the vascular wall in large vessels and within the microvasculature. Reversal of this process, leukocyte demargination, leads to substantial increases in the clinical white blood cell and granulocyte count and is a well-documented effect of glucocorticoid and catecholamine hormones, although the underlying mechanisms remain unclear. Here we show that alterations in granulocyte mechanical properties are the driving force behind glucocorticoid- and catecholamine-induced demargination. First, we found that the proportions of granulocytes from healthy human subjects that traversed and demarginated from microfluidic models of capillary beds and veins, respectively, increased after the subjects ingested glucocorticoids. Also, we show that glucocorticoid and catecholamine exposure reorganizes cellular cortical actin, significantly reducing granulocyte stiffness, as measured with atomic force microscopy. Furthermore, using simple kinetic theory computational modeling, we found that this reduction in stiffness alone is sufficient to cause granulocyte demargination. Taken together, our findings reveal a biomechanical answer to an old hematologic question regarding how glucocorticoids and catecholamines cause leukocyte demargination. In addition, in a broader sense, we have discovered a temporally and energetically efficient mechanism in which the innate immune system can simply alter leukocyte stiffness to fine tune margination/demargination and therefore leukocyte trafficking in general. These observations have broad clinically relevant implications for the inflammatory process overall as well as hematopoietic stem cell mobilization and homing.

Toward Understanding How Early-Life Stress Reprograms Cognitive and Emotional Brain Networks

Vulnerability to emotional disorders including depression derives from interactions between genes and environment, especially during sensitive developmental periods. Adverse early-life experiences provoke the release and modify the expression of several stress mediators and neurotransmitters within specific brain regions. The interaction of these mediators with developing neurons and neuronal networks may lead to long-lasting structural and functional alterations associated with cognitive and emotional consequences. Although a vast body of work has linked quantitative and qualitative aspects of stress to adolescent and adult outcomes, a number of questions are unclear. What distinguishes 'normal' from pathologic or toxic stress? How are the effects of stress transformed into structural and functional changes in individual neurons and neuronal networks? Which ones are affected? We review these questions in the context of established and emerging studies. We introduce a novel concept regarding the origin of toxic early-life stress, stating that it may derive from specific patterns of environmental signals, especially those derived from the mother or caretaker. Fragmented and unpredictable patterns of maternal care behaviors induce a profound chronic stress. The aberrant patterns and rhythms of early-life sensory input might also directly and adversely influence the maturation of cognitive and emotional brain circuits, in analogy to visual and auditory brain systems. Thus, unpredictable, stress-provoking early-life experiences may influence adolescent cognitive and emotional outcomes by disrupting the maturation of the underlying brain networks. Comprehensive approaches and multiple levels of analysis are required to probe the protean consequences of early-life adversity on the developing brain. These involve integrated human and animal-model studies, and approaches ranging from in vivo imaging to novel neuroanatomical, molecular, epigenomic, and computational methodologies. Because early-life adversity is a powerful determinant of subsequent vulnerabilities to emotional and cognitive pathologies, understanding the underlying processes will have profound implications for the world's current and future children.

Short-term withdrawal from developmental exposure to cocaine activates the glucocorticoid receptor and alters spine dynamics

Although glucocorticoid receptors (GRs) contribute to the action of cocaine, their role following developmental exposure to the psychostimulant is still unknown. To address this issue, we exposed adolescent male rats to cocaine (20mg/kg/day) from post-natal day (PND) 28 to PND 42 and sacrificed them at PND 45 or 90. We studied the medial prefrontal cortex (mPFC), a brain region that is still developing during adolescence. In PND 45 rats we found enhanced GR transcription and translation as well as increased trafficking toward the nucleus of the receptor, with no alteration in plasma corticosterone levels. We also showed reduced expression of the GR co-chaperone FKBP51, that normally keeps the receptor in the cytoplasm, and increased expression of Src1, which cooperates in the activation of GR transcriptional activity, revealing that short withdrawal alters the finely tuned mechanisms regulating GR action. Since activation of GRs regulate dendritic spine morphology, we next investigated spine dynamics in cocaine-withdrawn rats. We found that PSD95, cofilin and F-actin, molecules regulating spine actin network, are reduced in the mPFC of PND 45 rats suggesting reduced spine density, confirmed by confocal imaging. Further, formation of filopodia, i.e. the inactive spines, is enhanced suggesting the formation of non-functional spines. Of note, no changes were found in molecules related to GR machinery or spine dynamics following long-term abstinence, i.e. in adult rats (PND 90). These findings demonstrate that short withdrawal promotes plastic changes in the developing brain via the dysregulation of the GR system and alterations in the spine network.

Role of Paraventricular Nucleus Glutamate Signaling in Regulation of HPA Axis Stress Responses

The hypothalamus-pituitary-adrenal (HPA) axis is the main neuroendocrine arm of the stress response, activation of which leads to the production of glucocorticoid hormones. Glucocorticoids are steroid hormones that are secreted from the adrenal cortex, and have a variety of effects on the body, including modulation of the immune system, suppression of reproductive hormones maintenance of blood glucose levels, and maintenance of blood pressure. Glutamate plays an important role in coordination of HPA axis output. There is strong evidence that glutamate drives HPA axis stress responses through excitatory signaling via ionotropic glutamate receptor signaling. However, glutamate signaling via kainate receptors and group I metabotropic receptors inhibit HPA drive, probably via presynaptic inhibitory mechanisms. Notably, kainate receptors are also localized in the median eminence, and appear to play an excitatory role in control of CRH release at the nerve terminals. Finally, glutamate innervation of the PVN undergoes neuroplastic changes under conditions of chronic stress, and may be involved in sensitization of HPA axis responses. Altogether, the data suggest that glutamate plays a complex role in excitation of CRH neurons, acting at multiple levels to both drive HPA axis responses and limit over-activation.

Hippocampal spine changes across the sleep-wake cycle: corticosterone and kinases

The corticosterone (CORT) level changes along the circadian rhythm. Hippocampus is sensitive to CORT, since glucocorticoid receptors are highly expressed. In rat hippocampus fixed in a living state every 3 h, we found that the dendritic spine density of CA1 pyramidal neurons increased upon waking (within 3 h), as compared with the spine density in the sleep state. Particularly, the large-head spines increased. The observed change in the spine density may be due to the change in the hippocampal CORT level, since the CORT level at awake state (∼30 nM) in cerebrospinal fluid was higher than that at sleep state (∼3 nM), as observed from our earlier study. In adrenalectomized (ADX) rats, such a wake-induced increase of the spine density disappeared. S.c. administration of CORT into ADX rats rescued the decreased spine density. By using isolated hippocampal slices, we found that the application of 30 nM CORT increased the spine density within 1 h and that the spine increase was mediated via PKA, PKC, ERK MAPK, and LIMK signaling pathways. These findings suggest that the moderately rapid increase of the spine density on waking might mainly be caused by the CORT-driven kinase networks.

The adaptive and maladaptive continuum of stress responses – a hippocampal perspective

Exposure to stressors elicits a spectrum of responses that span from potentially adaptive to maladaptive consequences at the structural, cellular and physiological level. These responses are particularly pronounced in the hippocampus where they also appear to influence hippocampal-dependent cognitive function and emotionality. The factors that influence the nature of stress-evoked consequences include the chronicity, severity, predictability and controllability of the stressors. In addition to adult-onset stress, early life stress also elicits a wide range of structural and functional responses, which often exhibit life-long persistence. However, the outcome of early stress exposure is often contingent on the environment experienced in adulthood, and could either aid in stress coping or could serve to enhance susceptibility to the negative consequences of adult stress. This review comprehensively examines the consequences of adult and early life stressors on the hippocampus, with a focus on their effects on neurogenesis, neuronal survival, structural and synaptic plasticity and hippocampal-dependent behaviors. Further, we discuss potential factors that may tip stress-evoked consequences from being potentially adaptive to largely maladaptive.

Membrane-initiated non-genomic signaling by estrogens in the hypothalamus: cross-talk with glucocorticoids with implications for behavior

The estrogen receptor and glucocorticoid receptor are members of the nuclear receptor superfamily that can signal using both non-genomic and genomic transcriptional modes. Though genomic modes of signaling have been well characterized and several behaviors attributed to this signaling mechanism, the physiological significance of non-genomic modes of signaling has not been well understood. This has partly been due to the controversy regarding the identity of the membrane ER (mER) or membrane GR (mGR) that may mediate rapid, non-genomic signaling and the downstream signaling cascades that may result as a consequence of steroid ligands binding the mER or the mGR. Both estrogens and glucocorticoids exert a number of actions on the hypothalamus, including feedback. This review focuses on the various candidates for the mER or mGR in the hypothalamus and the contribution of non-genomic signaling to classical hypothalamically driven behaviors and changes in neuronal morphology. It also attempts to categorize some of the possible functions of non-genomic signaling at both the cellular level and at the organismal level that are relevant for behavior, including some behaviors that are regulated by both estrogens and glucocorticoids in a potentially synergistic manner. Lastly, it attempts to show that steroid signaling via non-genomic modes may provide the organism with rapid behavioral responses to stimuli.

Early life inflammatory pain induces long-lasting deficits in hippocampal-dependent spatial memory in male and female rats

The present experiment tested the hypothesis that neonatal injury disrupts adult hippocampal functioning and that normal aging or chronic stress during adulthood, which are known to have a negative impact on hippocampal function, exacerbate these effects. Male and female Sprague-Dawley rats were given an intraplantar injection of the inflammatory agent carrageenan (1%) on the day of birth and their memory was tested in the hippocampal-dependent spatial water maze in adulthood and again in middle age. We found that neonatal injury impaired hippocampal-dependent memory in adulthood, that the effects of injury on memory were more pronounced in middle-aged male rats, and that chronic stress accelerated the onset of these memory deficits. Neonatal injury also decreased glucocorticoid receptor mRNA in the dorsal CA1 area of middle-aged rats, a brain region critical for spatial memory. Morphine administration at the time of injury completely reversed injury-induced memory deficits, but neonatal morphine treatments in the absence of injury produced significant memory impairments in adulthood. Collectively, these findings are consistent with our hypothesis that neonatal injury produces long-lasting disruption in adult hippocampal functioning.

Partial genetic deletion of neuregulin 1 modulates the effects of stress on sensorimotor gating, dendritic morphology, and HPA axis activity in adolescent mice

Stress has been linked to the pathogenesis of schizophrenia. Genetic variation in neuregulin 1 (NRG1) increases the risk of developing schizophrenia and may help predict which high-risk individuals will transition to psychosis. NRG1 also modulates sensorimotor gating, a schizophrenia endophenotype. We used an animal model to demonstrate that partial genetic deletion of Nrg1 interacts with stress to promote neurobehavioral deficits of relevance to schizophrenia. Nrg1 heterozygous (HET) mice displayed greater acute stress-induced anxiety-related behavior than wild-type (WT) mice. Repeated stress in adolescence disrupted the normal development of higher prepulse inhibition of startle selectively in Nrg1 HET mice but not in WT mice. Further, repeated stress increased dendritic spine density in pyramidal neurons of the medial prefrontal cortex (mPFC) selectively in Nrg1 HET mice. Partial genetic deletion of Nrg1 also modulated the adaptive response of the hypothalamic-pituitary-adrenal axis to repeated stress, with Nrg1 HET displaying a reduced repeated stress-induced level of plasma corticosterone than WT mice. Our results demonstrate that Nrg1 confers vulnerability to repeated stress-induced sensorimotor gating deficits, dendritic spine growth in the mPFC, and an abberant endocrine response in adolescence.

Novel aspects of glucocorticoid actions

Normal hypothalamic-pituitary-adrenal (HPA) axis activity leading to the rhythmic and episodic release of adrenal glucocorticoids (GCs) is essential for body homeostasis and survival during stress. Acting through specific intracellular receptors in the brain and periphery, GCs regulate behaviour, as well as metabolic, cardiovascular, immune and neuroendocrine activities. By contrast to chronic elevated levels, circadian and acute stress-induced increases in GCs are necessary for hippocampal neuronal survival and memory acquisition and consolidation, as a result of the inhibition of apoptosis, the facilitation of glutamatergic neurotransmission and the formation of excitatory synapses, and the induction of immediate early genes and dendritic spine formation. In addition to metabolic actions leading to increased energy availability, GCs have profound effects on feeding behaviour, mainly via the modulation of orexigenic and anorixegenic neuropeptides. Evidence is also emerging that, in addition to the recognised immune suppressive actions of GCs by counteracting adrenergic pro-inflammatory actions, circadian elevations have priming effects in the immune system, potentiating acute defensive responses. In addition, negative-feedback by GCs involves multiple mechanisms leading to limited HPA axis activation and prevention of the deleterious effects of excessive GC production. Adequate GC secretion to meet body demands is tightly regulated by a complex neural circuitry controlling hypothalamic corticotrophin-releasing hormone (CRH) and vasopressin secretion, which are the main regulators of pituitary adrenocorticotrophic hormone (ACTH). Rapid feedback mechanisms, likely involving nongenomic actions of GCs, mediate the immediate inhibition of hypothalamic CRH and ACTH secretion, whereas intermediate and delayed mechanisms mediated by genomic actions involve the modulation of limbic circuitry and peripheral metabolic messengers. Consistent with their key adaptive roles, HPA axis components are evolutionarily conserved, being present in the earliest vertebrates. An understanding of these basic mechanisms may lead to novel approaches for the development of diagnostic and therapeutic tools for disorders related to stress and alterations of GC secretion.

MicroRNA-137 regulates a glucocorticoid receptor-dependent signalling network: implications for the etiology of schizophrenia

BACKGROUND

Schizophrenia is a highly heritable neurodevelopmental disorder. A genetic variant of microRNA-137 (miR-137) has yielded significant genome-wide association with schizophrenia, suggesting that this miRNA plays a key role in its etiology. Therefore, a molecular network of interacting miR-137 targets may provide insights into the biological processes underlying schizophrenia.

METHODS

We first used bioinformatics tools to obtain and analyze predicted human and mouse miR-137 targets. We then determined miR-137 levels in rat barrel cortex after environmental enrichment (EE), a neuronal plasticity model that induces upregulation of several predicted miR-137 targets. Subsequently, expression changes of these predicted targets were examined through loss of miR-137 function experiments in rat cortical neurons. Finally, we conducted bioinformatics and literature analyses to examine the targets that were upregulated upon miR-137 downregulation.

RESULTS

Predicted human and mouse miR-137 targets were enriched in neuronal processes, such as axon guidance, neuritogenesis and neurotransmission. The miR-137 levels were significantly downregulated after EE, and we identified 5 novel miR-137 targets through loss of miR-137 function experiments. These targets fit into a glucocorticoid receptor-dependent signalling network that also includes 3 known miR-137 targets with genome-wide significant association with schizophrenia.

LIMITATIONS

The bioinformatics analyses involved predicted human and mouse miR-137 targets owing to lack of information on predicted rat miR-137 targets, whereas follow-up experiments were performed with rats. Furthermore, indirect effects in the loss of miR-137 function experiments cannot be excluded.

CONCLUSION

We have identified a miR-137-regulated protein network that contributes to our understanding of the molecular basis of schizophrenia and provides clues for future research into psychopharmacological treatments for schizophrenia.

Corticosterone treatment during adolescence induces down-regulation of reelin and NMDA receptor subunit GLUN2C expression only in male mice: implications for schizophrenia

Stress exposure during adolescence/early adulthood has been shown to increase the risk for psychiatric disorders such as schizophrenia. Reelin plays an essential role in brain development and its levels are decreased in schizophrenia. However, the relationship between stress exposure and reelin expression remains unclear. We therefore treated adolescent reelin heteroyzogous mice (HRM) and wild-type (WT) littermates with the stress hormone, corticosterone (CORT) in their drinking water (25 mg/l) for 3 wk. In adulthood, we measured levels of full-length (FL) reelin and the N-R6 and N-R2 cleavage fragments in the frontal cortex (FC) and dorsal (DH) and ventral (VH) hippocampus. As expected, levels of all reelin forms were approximately 50% lower in HRMs compared to WT. In male mice, CORT treatment significantly decreased FL and N-R2 expression in the FC and N-R2 and N-R6 levels in the DH. This reelin down-regulation was accompanied by significant reductions in downstream N-methyl-D-aspartate (NMDA) GluN2C subunit levels. There were no effects of CORT treatment in the VH of either of the sexes and only subtle changes in female DH. CORT-induced reelin and GluN2C down-regulation in males was not associated with changes in two GABAergic neuron markers, GAD67 and parvalbumin, or glucocorticoids receptors (GR). These results show that CORT treatment causes long-lasting and selective reductions of reelin form levels in male FC and DH accompanied by changes in NMDAR subunit composition. This sex-specific reelin down-regulation in regions implicated in schizophrenia could be involved in the effects of stress in this disease.

The effects of glucocorticoids on neuropathic pain: a review with emphasis on intrathecal methylprednisolone acetate delivery

Methylprednisolone acetate (MPA) has a long history of use in the treatment of sciatic pain and other neuropathic pain syndromes. In several of these syndromes, MPA is administered in the epidural space. On a limited basis, MPA has also been injected intrathecally in patients suffering from postherpetic neuralgia and complex regional pain syndrome. The reports on efficacy of intrathecal administration of MPA in neuropathic pain patients are contradictory, and safety is debated. In this review, we broadly consider mechanisms whereby glucocorticoids exert their action on spinal cascades relevant to the pain arising after nerve injury and inflammation. We then focus on the characteristics of the actions of MPA in pharmacokinetics, efficacy, and safety when administered in the intrathecal space.

Pharmacological reversion of sphingomyelin-induced dendritic spine anomalies in a Niemann Pick disease type A mouse model

Understanding the role of lipids in synapses and the aberrant molecular mechanisms causing the cognitive deficits that characterize most lipidosis is necessary to develop therapies for these diseases. Here we describe sphingomyelin (SM) as a key modulator of the dendritic spine actin cytoskeleton. We show that increased SM levels in neurons of acid sphingomyelinase knock out mice (ASMko), which mimic Niemann Pick disease type A (NPA), result in reduced spine number and size and low levels of filamentous actin. Mechanistically, SM accumulation decreases the levels of metabotropic glutamate receptors type I (mGluR1/5) at the synaptic membrane impairing membrane attachment and activity of RhoA and its effectors ROCK and ProfilinIIa. Pharmacological enhancement of the neutral sphingomyelinase rescues the aberrant molecular and morphological phenotypes in vitro and in vivo and improves motor and memory deficits in ASMko mice. Altogether, these data demonstrate the influence of SM and its catabolic enzymes in dendritic spine physiology and contribute to our understanding of the cognitive deficits of NPA patients, opening new perspectives for therapeutic interventions.

Subject Categories Genetics, Gene Therapy & Genetic Disease; Neuroscience

NMDA receptor activation and calpain contribute to disruption of dendritic spines by the stress neuropeptide CRH

The complex effects of stress on learning and memory are mediated, in part, by stress-induced changes in the composition and structure of excitatory synapses. In the hippocampus, the effects of stress involve several factors including glucocorticoids and the stress-released neuropeptide corticotropin-releasing hormone (CRH), which influence the integrity of dendritic spines and the structure and function of the excitatory synapses they carry. CRH, at nanomolar, presumed-stress levels, rapidly abolishes short-term synaptic plasticity and destroys dendritic spines, yet the mechanisms for these effects are not fully understood. Here we tested the hypothesis that glutamate receptor-mediated processes, which shape synaptic structure and function, are engaged by CRH and contribute to spine destabilization. In cultured rat hippocampal neurons, CRH application reduced dendritic spine density in a time- and dose-dependent manner, and this action depended on the CRH receptor type 1. CRH-mediated spine loss required network activity and the activation of NMDA, but not of AMPA receptors; indeed GluR1-containing dendritic spines were resistant to CRH. Downstream of NMDA receptors, the calcium-dependent enzyme, calpain, was recruited, resulting in the breakdown of spine actin-interacting proteins including spectrin. Pharmacological approaches demonstrated that calpain recruitment contributed critically to CRH-induced spine loss. In conclusion, the stress hormone CRH co-opts mechanisms that contribute to the plasticity and integrity of excitatory synapses, leading to selective loss of dendritic spines. This spine loss might function as an adaptive mechanism preventing the consequences of adverse memories associated with severe stress.

Cytoskeletal protein translation and expression in the rat brain are stressor-dependent and region-specific

Stress is an integral component of life that can sometimes cause a critical overload, depending on the qualitative and quantitative natures of the stressors. The involvement of actin, the predominant component of dendritic integrity, is a plausible candidate factor in stress-induced neuronal cytoskeletal changes. The major aim of this study was to compare the effects of three different stress conditions on the transcription and translation of actin-related cytoskeletal genes in the rat brain. Male Wistar rats were exposed to one or other of the frequently used models of physical stress, i.e. electric foot shock stress (EFSS), forced swimming stress (FSS), or psychosocial stress (PSS) for periods of 3, 7, 14, or 21 days. The relative mRNA and protein expressions of β-actin, cofilin and mitogen-activated protein kinase 1 (MAPK-1) were determined by qRT- PCR and western blotting from hippocampus and frontal cortex samples. Stressor-specific alterations in both β-actin and cofilin expression levels were seen after stress. These alterations were most pronounced in response to EFSS, and exhibited a U-shaped time course. FSS led to a significant β-actin mRNA expression elevation in the hippocampus and the frontal cortex after 3 and 7 days, respectively, without any subsequent change. PSS did not cause any change in β-actin or cofilin mRNA or protein expression in the examined brain regions. EFSS, FSS and PSS had no effect on the expression of MAPK-1 mRNA at any tested time point. These findings indicate a very delicate, stress type-dependent regulation of neuronal cytoskeletal components in the rat hippocampus and frontal cortex.

Environmental enrichment reveals effects of genotype on hippocampal spine morphologies in the mouse model of Fragile X Syndrome

Fragile X Syndrome (FXS) and the Fmr1 knockout (KO) mouse model of this disorder exhibit abnormal dendritic spines in neocortex, but the degree of spine disturbances in hippocampus is not clear. The present studies tested if the mutation influences dendritic branching and spine measures for CA1 pyramidal cells in Fmr1 KO and wild-type (WT) mice provided standard or enriched environment (EE) housing. Automated measures from 3D reconstructions of green fluorescent protein (GFP)-labeled cells showed that spine head volumes were ∼ 40% lower in KOs when compared with WTs in both housing conditions. With standard housing, average spine length was greater in KOs versus WTs but there was no genotype difference in dendritic branching, numbers of spines, or spine length distribution. However, with EE rearing, significant effects of genotype emerged including greater dendritic branching in WTs, greater spine density in KOs, and greater numbers of short thin spines in KOs when compared with WTs. Thus, EE rearing revealed greater effects of the Fmr1 mutation on hippocampal pyramidal cell morphology than was evident with standard housing, suggesting that environmental enrichment allows for fuller appreciation of the impact of the mutation and better representation of abnormalities likely to be present in human FXS.

Glucocorticoid receptor regulation of action selection and prefrontal cortical dendritic spines

We recently reported that prolonged exposure to the glucocorticoid receptor (GR) ligand corticosterone impairs decision-making that is dependent on the predictive relationship between an action and its outcome (Gourley et al.; Proceedings of the National Academy of Sciences, 2012). Additionally, acute GR blockade, when paired with action-outcome conditioning, also blocks new learning. We then showed that dendritic spines in the prelimbic prefrontal cortex remodeled under both conditions. Nonetheless, the relationship between deep-layer dendritic spines and outcome-based decision-making remains opaque. We report here that a history of prolonged corticosterone exposure increases dendritic spine density in deep-layer prelimbic cortex. When spines are imaged simultaneously with corticosteroid exposure (i.e., without a washout period), dendritic spine densities are, however, reduced. Thus, the morphological response of deep-layer prelimbic cortical neurons to prolonged corticosteroid exposure may be quite dynamic, with spine elimination during a period of chronic exposure and spine proliferation during a subsequent washout period. We provide evidence, using a Rho-kinase inhibitor, that GR-mediated dendritic spine remodeling is causally related to complex decision-making. Finally, we conclude this report with evidence that a history of early-life (adolescent) GR blockade, unlike acute blockade in adulthood, enhances subsequent outcome-based decision-making. Together, our findings suggest that physiological levels of GR binding enable an organism to learn about the predictive relationship between an action and its outcome, but a history of GR blockade may, under some circumstances, also have beneficial consequences.

Are BDNF and glucocorticoid activities calibrated?

One hypothesis to account for the onset and severity of neurological disorders is the loss of trophic support. Indeed, changes in the levels and activities of brain-derived neurotrophic factor (BDNF) occur in numerous neurodegenerative and neuropsychiatric diseases. A deficit promotes vulnerability whereas a gain of function facilitates recovery by enhancing survival, synapse formation and synaptic plasticity. Implementation of 'BDNF therapies', however, faces numerous methodological and pharmacokinetic issues. Identifying BDNF mimetics that activate the BDNF receptor or downstream targets of BDNF signaling represent an alternative approach. One mechanism that shows great promise is to study the interplay of BDNF and glucocorticoid hormones, a major class of natural steroid secreted during stress reactions and in synchrony with circadian rhythms. While small amounts of glucocorticoids support normal brain function, excess stimulation by these steroid hormones precipitates stress-related affective disorders. To date, however, because of the paucity of knowledge of underlying cellular mechanisms, deleterious effects of glucocorticoids are not prevented following extreme stress. In the present review, we will discuss the complementary roles shared by BDNF and glucocorticoids in synaptic plasticity, and delineate possible signaling mechanisms mediating these effects.

Early life stress-induced histone acetylations correlate with activation of the synaptic plasticity genes Arc and Egr1 in the mouse hippocampus

Early life stress (ELS) programs the developing organism and influences the development of brain and behavior. We tested the hypothesis that ELS-induced histone acetylations might alter the expression of synaptic plasticity genes that are critically involved in the establishment of limbic brain circuits. Maternal separation (MS) from postnatal day 14-16 was applied as ELS and two immediate early genes underlying experience-induced synaptic plasticity, Arc and early growth response 1 (Egr1) were analyzed. We show here that repeated ELS induces a rapid increase of Arc and Egr1 in the mouse hippocampus. Furthermore, immunoblotting revealed that these changes are paralleled by histone modifications, reflected by increased acetylation levels of H3 and H4. Most importantly, using native Chromatin immunoprecipitation quantitative PCR (nChIP-qPCR), we show for the first time a correlation between elevated histone acetylation and increased Arc and Egr1 expression in response to ELS. These rapid epigenetic changes are paralleled by increases of dendritic complexity and spine number of hippocampal CA3 pyramidal neurons in ELS animals at weaning age. Our results are in line with our working hypothesis that ELS induces activation of synaptic plasticity genes, mediated by epigenetic mechanisms. These events are assumed to represent early steps in the adaption of neuronal networks to a stressful environment.

Brain-derived neurotrophic factor and glucocorticoids: reciprocal influence on the central nervous system

Brain-derived neurotrophic factor (BDNF) has multiple roles in the central nervous system (CNS), including maintaining cell survival and regulation of synaptic function. In CNS neurons, BDNF triggers activation of phospholipase Cγ (PLCγ), mitogen-activated protein/extracellular signal-regulated kinase (MAPK/ERK), and phosphoinositide 3-kinase (PI3K)/Akt pathways, influencing neuronal cells beneficially through these intracellular signaling cascades. There is evidence to suggest that decreased BDNF expression or function is related to the pathophysiology of brain diseases including psychiatric disorders. Additionally, glucocorticoids, which are critical stress hormones, also influence neuronal function in the CNS, and are putatively involved in the onset of depression when levels are abnormally high. In animal models of depression, changes in glucocorticoid levels, expression of glucocorticoid receptor (GR), and alterations in BDNF signaling are observed. Interestingly, several studies using in vivo and in vitro systems suggest that glucocorticoids interact with BDNF to ultimately affect CNS function. In the present review, we provide an overview of recent evidence concerning the interaction between BDNF and glucocorticoids.