Mechanisms for acute stress-induced enhancement of glutamatergic transmission and working memory

An emerging role for microglia in stress‐effects on memory

Stressful experiences evoke, among others, a rapid increase in brain (nor)epinephrine (NE) levels and a slower increase in glucocorticoid hormones (GCs) in the brain. Microglia are key regulators of neuronal function and contain receptors for NE and GCs. These brain cells may therefore potentially be involved in modulating stress effects on neuronal function and learning and memory. In this review, we discuss that stress induces (1) an increase in microglial numbers as well as (2) a shift toward a pro‐inflammatory profile. These microglia have (3) impaired crosstalk with neurons and (4) disrupted glutamate signaling. Moreover, microglial immune responses after stress (5) alter the kynurenine pathway through metabolites that impair glutamatergic transmission. All these effects could be involved in the impairments in memory and in synaptic plasticity caused by (prolonged) stress, implicating microglia as a potential novel target in stress‐related memory impairments.

Chronic restraint stress impairs cognition via modulating HDAC2 expression

Background

To investigate the effects of chronic restraint stress on cognition and the probable molecular mechanism in mice.

Methods

In the current work, a restraining tube was used as a way to induce chronic stress in mice. The protein levels were determined with ELISA and western blot. A series of behavior tests, including the Morris water maze, elevated plus maze, open field test, and novel object recognition test, were also performed to examine the anxiety and the ability of learning and memory. Moreover, murine neuroblastoma N2a cells were used to confirm the findings from mice under chronic stress.

Results

Decreased synaptic functions were impaired in chronic stress with the downregulation of PSD95, GluR-1, the neurotrophic factor BDNF, and immediate-onset genes Arc and Egr. Chronic restraint decreased the histone acetylation level in hippocampal neurons while HDAC2 was increased and was co-localized with glucocorticoid receptors. Moreover, chronic stress inhibited the PI3K/AKT signaling pathway and induced energy metabolism dysfunctions.

Conclusion

This work examining the elevated levels of HDAC2 in the hippocampus may provide new insights and targets for drug development for treating many neurodegenerative diseases.

Oligophrenin-1 moderates behavioral responses to stress by regulating parvalbumin interneuron activity in the medial prefrontal cortex

Ample evidence indicates that individuals with intellectual disability (ID) are at increased risk of developing stress-related behavioral problems and mood disorders, yet a mechanistic explanation for such a link remains largely elusive. Here, we focused on characterizing the syndromic ID gene oligophrenin-1 (OPHN1). We find that Ophn1 deficiency in mice markedly enhances helpless/depressive-like behavior in the face of repeated/uncontrollable stress. Strikingly, Ophn1 deletion exclusively in parvalbumin (PV) interneurons in the prelimbic medial prefrontal cortex (PL-mPFC) is sufficient to induce helplessness. This behavioral phenotype is mediated by a diminished excitatory drive onto Ophn1-deficient PL-mPFC PV interneurons, leading to hyperactivity in this region. Importantly, suppressing neuronal activity or RhoA/Rho-kinase signaling in the PL-mPFC reverses helpless behavior. Our results identify OPHN1 as a critical regulator of adaptive behavioral responses to stress and shed light onto the mechanistic links among OPHN1 genetic deficits, mPFC circuit dysfunction, and abnormalities in stress-related behaviors.

SGK1 knockdown in the medial prefrontal cortex reduces resistance to stress-induced memory impairment

Down-regulation of serum and glucocorticoid-regulated kinase1 (SGK1) expression has been reported in the postmortem prefrontal cortex (PFC) of subjects with post-traumatic stress disorder. Furthermore, experimental treatments that reduce SGK1 function in the medial prefrontal cortex (mPFC) cause depressive-like behaviors and synaptic dysfunction. Therefore, we examined the effect of SGK1 down-regulation in the mPFC on resistance to stress-induced cognitive impairment. Rats with viral-mediated knockdown of SGK1 in the mPFC were subjected to either a brief 20-min restraint plus 20 intermittent tail shocks or a prolonged 60-min restraint plus 60 intermittent tail shocks, after which their performance in an object recognition task was assessed. Recognition memory remained intact in control rats following the brief stress, but was impaired in rats with SGK1 knockdown in the mPFC. Prolonged stress impaired recognition memory in both control rats and rats with SGK1 knockdown. Our findings indicate that altered mPFC SGK1 signaling is a potential mechanism for resistance to stress-induced cognitive impairment.

Sexual dimorphic effects of restraint stress on prefrontal cortical function are mediated by glucocorticoid receptor activation

Stress, a major regulator and precipitating factor of cognitive and emotional disorders, differentially manifests between males and females. Our aim was to investigate the mechanisms underlying the sexual dimorphic effects of acute restraint stress (RS) on males and females on the function of the prefrontal cortex (PFC). Adult male and female mice were subjected to RS or left in their home-cage (NR), and then tested in the light-dark test followed by the temporal order object recognition (TOR) task. Female mice exhibited increased anxiety-like levels, whereas male mice only showed deficits in the TOR task. When the behavioural tests were conducted 24 hr following restraint stress (RS24), only the reduced performance in the TOR task in male mice persisted. In a different cohort, evoked field excitatory postsynaptic potentials (fEPSPs) were recorded in layer II of acute PFC slices, immediately or 24 hr after RS. Long-term potentiation (LTP) was significantly reduced in RS and RS24 male, but not female, compared with their respective NR group. LTP in PFC slices incubated with corticosterone showed significantly reduced LTP only in males. To determine whether glucocorticoid signalling is implicated in the RS-induced behavioural effects, a different cohort of mice was administered mifepristone, a corticosterone receptor antagonist. Mifepristone administration 1 hr before RS prevented the effects of RS on the TOR task in males, but not anxiety. In conclusion, RS has differential effects on recency memory and anxiety, in males and females, which are partly mediated by the effects of corticosterone signalling on synaptic plasticity.

Glucocorticoids, metabolism and brain activity

The review integrates different experimental approaches including biochemistry, c-Fos expression, microdialysis (glutamate, GABA, noradrenaline and serotonin), electrophysiology and fMRI to better understand the effect of elevated level of glucocorticoids on the brain activity and metabolism. The available data indicate that glucocorticoids alter the dynamics of neuronal activity leading to context-specific changes including both excitation and inhibition and these effects are expected to support the task-related responses. Glucocorticoids also lead to diversification of available sources of energy due to elevated levels of glucose, lactate, pyruvate, mannose and hydroxybutyrate (ketone bodies), which can be used to fuel brain, and facilitate storage and utilization of brain carbohydrate reserves formed by glycogen. However, the mismatch between carbohydrate supply and utilization that is most likely to occur in situations not requiring energy-consuming activities lead to metabolic stress due to elevated brain levels of glucose. Excessive doses of glucocorticoids also impair the production of energy (ATP) and mitochondrial oxidation. Therefore, glucocorticoids have both adaptive and maladaptive effects consistently with the concept of allostatic load and overload.

Experimental Therapeutics in Treatment-Resistant Major Depressive Disorder

Treatment-Resistant Depression (TRD) patients remain a challenging sub-division of patients with Major Depressive Disorder (MDD) in day to day clinical practice. As with any diagnostic condition, comprehensive evaluation, exclusion of other psychiatric conditions, assessment of co-morbid medical and psychiatric illnesses and psychosocial stressors are the keys to appropriate diagnosis and subsequent management. There are various management options available for the treatment of MDD, however, about 30% of the patients fail to achieve full remission of symptoms despite multiple trials and belong to this sub-category of MDD. This article brings forth discussion of other non-conventional medication and non-medication treatment alternatives that merit exploration of their efficacy in TRD. Many of the proposed novel medications and other treatment modalities such as Deep Brain Stimulation, exercise, yoga are already used for other medical and psychiatric disorders and have some evidence suggesting their potential benefits in TRD in conjunction with conventional medications or even as monotherapy. Nevertheless, more research is needed in this direction to establish effectiveness.

Glucocorticoid impairs mitochondrial quality control in neurons

Neurons are particularly vulnerable to mitochondrial dysfunction due to high energy demand and an inability to proliferate. Therefore, dysfunctional mitochondria cause various neuropathologies. Mitochondrial damage induces maintenance pathways to repair or eliminate damaged organelles. This mitochondrial quality control (MQC) system maintains appropriate morphology, localization, and removal/replacement of mitochondria to sustain brain homeostasis and counter progression of neurological disorders. Glucocorticoid release is an essential response to stressors for adaptation; however, it often culminates in maladaptation if neurons are exposed to chronic and severe stress. Long-term exposure to high levels of glucocorticoids induces mitochondrial dysfunction via genomic and nongenomic mechanisms. Glucocorticoids induce abnormal mitochondrial morphology and dysregulate fusion and fission. Moreover, mitochondrial trafficking is arrested by glucocorticoids and dysfunctional mitochondria are subsequently accumulated around the soma. These alterations lead to energy deficiency, particularly for synaptic transmission that requires large amounts of energy. Glucocorticoids also impair mitochondrial clearance by preventing mitophagy of damaged organelle and suppress mitochondrial biogenesis, resulting in the reduced number of healthy mitochondria. Failure to maintain MQC degrades brain function and contributes to neurodegenerative diseases, including Alzheimer's disease, Parkinson's disease, and Huntington's disease. However, mechanisms of glucocorticoid action on the regulation of MQC during chronic stress conditions are not well understood. The present review discusses pathways involved in the impairment of MQC and the clinical significance of high glucocorticoid blood levels for neurodegenerative diseases.

Chronic adolescent stress causes sustained impairment of cognitive flexibility and hippocampal synaptic strength in female rats

Females that experience chronic stress during development, particularly adolescence, are the most vulnerable group to stress-induced disease. While considerable attention has been devoted to stress-induced manifestation of anxiety, depression, and PTSD, evidence indicates that a history of chronic stress is also a risk factor for cognitive decline and dementia - with females again in a higher risk group. This interplay between sex and stress history indicates specific mechanisms drive neural dysfunction across the lifespan. The presence of sex and stress steroid receptors in the hippocampus provides a point of influence for these variables to drive changes in cognitive function. Here, we used a rodent model of chronic adolescent stress (CAS) to determine the extent to which CAS modifies glutamatergic signaling resulting in cognitive dysfunction. Male and female Wistar rats born in-house remained non-stressed (NS), unmanipulated aside from standard cage cleaning, or were exposed to either physical restraint (60 min) or social defeat (CAS) each day (6 trials each), along with social isolation, throughout the adolescent period (PND 35-47). Cognition was assessed in adult (PND 80-130) male and female rats (n = 10-12) using the Barnes Maze task and the Attention Set-Shift task. Whole hippocampi were extracted from a second cohort of male and female rats (NS and CAS; n = 9-10) and processed for RNA sequencing. Brain tissue from the first cohort (n = 6) was processed for density of glutamatergic synaptic markers (GluA1, NMDA1a, and synaptophysin) or whole-cell patch clamping (n = 4) to determine glutamatergic activity in the hippocampus. Females with a history of chronic stress had shorter latencies to locate the goal box than NS controls during acquisition learning but showed an increased latency to locate the new goal box during reversal learning. This reversal deficit persisted across domains as females with a history of stress required more trials to reach criterion during the reversal phases of the Attention Set-Shift task compared to controls. Ovariectomy resulted in greater performance variability overall during reversal learning with CAS females showing worse performance. Males showed no effects of CAS history on learning or memory performance. Bioinformatic prediction using gene ontology categorization indicated that in females, postsynaptic membrane gene clusters, specifically genes related to glutamatergic synapse remodeling, were enriched with a history of stress. Structural analysis indicated that CAS did not alter glutamate receptor density in females. However, functionally, CAS females had a decreased AMPA/NMDA-dependent current ratio compared to controls indicating a weakening in synaptic strength in the hippocampus. Males showed only a slight change in density of NMDA1a labeling in the CA3 region with a history of stress. The data observed here suggest that females are at risk for impaired cognitive flexibility following a history of adolescent stress, possibly driven by changes in glutamatergic signaling.

Interaction of chronic intermittent ethanol and repeated stress on structural and functional plasticity in the mouse medial prefrontal cortex

Stress is a risk factor that plays a considerable role in the development and maintenance of alcohol (ethanol) abuse and relapse. Preclinical studies examining ethanol-stress interactions have demonstrated elevated ethanol drinking, cognitive deficits, and negative affective behaviors in mice. However, the neural adaptations in prefrontal cortical regions that drive these aberrant behaviors produced by ethanol-stress interactions are unknown. In this study, male C57BL/6J mice were exposed to chronic intermittent ethanol (CIE) and repeated forced swim stress (FSS). After two cycles of CIE x FSS, brain slices containing the prelimbic (PrL) and infralimbic (IfL) cortex were prepared for analysis of adaptations in dendritic spines and synaptic plasticity. In the PrL cortex, total spine density was increased in mice exposed to CIE. Immediately following induction of long-term potentiation (LTP), the fEPSP slope was increased in the PrL of CIE x FSS treated mice, indicative of a presynaptic adaptation on post-tetanic potentiation (PTP). In the IfL cortex, CIE exposure regardless of FSS experience resulted in an increase in spine density. FSS alone or when combined with CIE exposure increased PTP following LTP induction. Repeated FSS episodes increased IfL cortical paired-pulse facilitation, a second measure of presynaptic plasticity. In summary, CIE exposure resulted in structural adaptations while repeated stress exposure drove metaplastic changes in presynaptic function, demonstrating distinct morphological and functional changes in PrL and IfL cortical neurons. Thus, the structural and functional adaptations may be one mechanism underlying the development of excessive drinking and cognitive deficits associated with ethanol-stress interactions.

Corticosterone as a Potential Confounding Factor in Delineating Mechanisms Underlying Ketamine's Rapid Antidepressant Actions

Recent research into the rapid antidepressant effect of subanesthetic doses of ketamine have identified a series of relevant protein cascades activated within hours of administration. Prior to, or concurrent with, these activation cascades, ketamine treatment generates dissociative and psychotomimetic side effects along with an increase in circulating glucocorticoids. In rats, we observed an over 3-fold increase in corticosterone levels in both serum and brain tissue, within an hour of administration of low dose ketamine (10 mg/kg), but not with (2R, 6R)-hydroxynorketamine (HNK) (10 mg/kg), a ketamine metabolite shown to produce antidepressant-like action in rodents without inducing immediate side-effects. Hippocampal tissue from ketamine, but not HNK, injected animals displayed a significant increase in the expression of sgk1, a downstream effector of glucocorticoid receptor signaling. To examine the role conscious sensation of ketamine's side effects plays in the release of corticosterone, we assessed serum corticosterone levels after ketamine administration while under isoflurane anesthesia. Under anesthesia, ketamine failed to increase circulating corticosterone levels relative to saline controls. Concurrent with its antidepressant effects, ketamine generates a release of glucocorticoids potentially linked to disturbing cognitive side effects and the activation of distinct molecular pathways which should be considered when attempting to delineate the molecular mechanisms of its antidepressant function.

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.

Excitation-Inhibition Imbalance Leads to Alteration of Neuronal Coherence and Neurovascular Coupling under Acute Stress

A single stressful event can cause morphologic and functional changes in neurons and even malfunction of vascular systems, which can lead to acute stress disorder or post-traumatic stress disorder. However, there is a lack of evidence regarding how acute stress impacts neuronal activity, the concurrent vascular response, and the relationship between these two factors, which is defined as neurovascular coupling. Here, using in vivo two-photon imaging, we found that NMDA-evoked calcium transients of excitatory neurons were impaired and that vasodilation of penetrating arterioles was concomitantly disrupted in acutely stressed male mice. Furthermore, acute stress altered the relationship between excitatory neuronal calcium coherence and vascular responses. By measuring NMDA-evoked excitatory and inhibitory neuronal calcium activity in acute brain slices, we confirmed that neuronal coherence both between excitatory neurons and between excitatory and inhibitory neurons was reduced by acute stress but restored by blockade of glucocorticoid receptor signaling. Furthermore, the ratio of sEPSCs to sIPSCs was altered by acute stress, suggesting that the excitation-inhibition balance was disrupted by acute stress. In summary, in vivo, ex vivo, and whole-cell recording studies demonstrate that acute stress modifies excitatory-inhibitory neuronal coherence, disrupts the excitation-inhibition balance, and causes consequent neurovascular coupling changes, providing critical insights into the neural mechanism of stress-induced disorders.

SIGNIFICANCE STATEMENT Acute stress can cause pathologic conditions, such as acute stress disorder and post-traumatic stress disorder, by affecting the functions of neurons and blood vessels. However, investigations into the impacts of acute stress on neurovascular coupling, the tight connection between local neural activity and subsequent blood flow changes, are lacking. Through investigations at the in vivo, ex vivo, and whole-cell recording levels, we found that acute stress alters the NMDA-evoked vascular response, impairs the function and coherence of excitatory and inhibitory neurons, and disrupts the excitatory and inhibitory balance. These novel findings provide insights into the relevance of the excitatory-inhibitory balance, neuronal coherence, and neurovascular coupling to stress-induced disorders.

Anti-Stress Properties of Atypical Antipsychotics

Stress exposure represents a major environmental risk factor for schizophrenia and other psychiatric disorders, as it plays a pivotal role in the etiology as well as in the manifestation of disease symptomatology. It may be inferred that pharmacological treatments must be able to modulate the behavioral, functional, and molecular alterations produced by stress exposure to achieve significant clinical outcomes. This review aims at examining existing clinical and preclinical evidence that supports the ability of atypical antipsychotic drugs (AAPDs) to modulate stress-related alterations. Indeed, while the pharmacodynamic differences between AAPDs have been extensively characterized, less is known on their ability to regulate downstream mechanisms that are critical for functional recovery and patient stabilization. We will discuss stress-related mechanisms, spanning from neuroendocrine function to inflammation and neuronal plasticity, which are relevant for the manifestation of schizophrenic symptomatology, and we will discuss if and how AAPDs may interfere with such mechanisms. Considering the impact of stress in everyday life, we believe that a better understanding of the potential effects of AAPDs on stress-related mechanisms may provide novel and important insights for improving therapeutic strategies aimed at promoting coping mechanisms and enhancing the quality of life of patients affected by psychiatric disorders.

Circadian Modulation of Neurons and Astrocytes Controls Synaptic Plasticity in Hippocampal Area CA1

Most animal species operate according to a 24-h period set by the suprachiasmatic nucleus (SCN) of the hypothalamus. The rhythmic activity of the SCN modulates hippocampal-dependent memory, but the molecular and cellular mechanisms that account for this effect remain largely unknown. Here, we identify cell-type-specific structural and functional changes that occur with circadian rhythmicity in neurons and astrocytes in hippocampal area CA1. Pyramidal neurons change the surface expression of NMDA receptors. Astrocytes change their proximity to synapses. Together, these phenomena alter glutamate clearance, receptor activation, and integration of temporally clustered excitatory synaptic inputs, ultimately shaping hippocampal-dependent learning in vivo. We identify corticosterone as a key contributor to changes in synaptic strength. These findings highlight important mechanisms through which neurons and astrocytes modify the molecular composition and structure of the synaptic environment, contribute to the local storage of information in the hippocampus, and alter the temporal dynamics of cognitive processing.

Cumulative Effects of Social Stress on Reward-Guided Actions and Prefrontal Cortical Activity

BACKGROUND

When exposed to chronic social stress, animals display behavioral changes that are relevant to depressive-like phenotypes. However, the cascading relationship between incremental stress exposure and neural dysfunctions over time remains incompletely understood.

METHODS

We characterized the longitudinal effect of social defeat on goal-directed actions and prefrontal cortical activity in mice using a novel head-fixed sucrose preference task and two-photon calcium imaging.

RESULTS

Behaviorally, stress-induced loss of reward sensitivity intensifies over days. Motivational anhedonia, the failure to translate positive reinforcements into future actions, requires multiple sessions of stress exposure to become fully established. For neural activity, individual layer 2/3 pyramidal neurons in the cingulate and medial secondary motor subregions of the medial prefrontal cortex have heterogeneous responses to stress. Changes in ensemble activity differ significantly between susceptible and resilient mice after the first defeat session and continue to diverge following successive stress episodes before reaching persistent abnormal levels.

CONCLUSIONS

Collectively, these results demonstrate that the cumulative impact of an ethologically relevant stress can be observed at the level of cellular activity of individual prefrontal neurons. The distinct neural responses associated with resilience versus susceptibility suggests the hypothesis that the negative impact of social stress is neutralized in resilient animals, in part through an adaptive reorganization of prefrontal cortical activity.

Amelioration of autism-like social deficits by targeting histone methyltransferases EHMT1/2 in Shank3-deficient mice

Many of the genes disrupted in autism are identified as histone-modifying enzymes and chromatin remodelers, most prominently those that mediate histone methylation/demethylation. However, the role of histone methylation enzymes in the pathophysiology and treatment of autism remains unknown. To address this, we used mouse models of haploinsufficiency of the Shank3 gene (a highly penetrant monogenic autism risk factor), which exhibits prominent autism-like social deficits. We found that histone methyltransferases EHMT1 and EHMT2, as well as histone lysine 9 dimethylation (specifically catalyzed by EHMT1/2), were selectively increased in the prefrontal cortex (PFC) of Shank3-deficient mice and autistic human postmortem brains. Treatment with the EHMT1/2 inhibitor UNC0642 or knockdown of EHMT1/2 in PFC induced a robust rescue of autism-like social deficits in Shank3-deficient mice, and restored NMDAR-mediated synaptic function. Activity-regulated cytoskeleton-associated protein (Arc) was identified as one of the causal factors underlying the rescuing effects of UNC0642 on NMDAR function and social behaviors in Shank3-deficient mice. UNC0642 treatment also restored a large set of genes involved in neural signaling in PFC of Shank3-deficient mice. These results suggest that targeting histone methylation enzymes to adjust gene expression and ameliorate synaptic defects could be a potential therapeutic strategy for autism.

Facilitation of dopamine-dependent long-term potentiation in the medial prefrontal cortex of male rats follows the behavioral effects of stress

The effect of stress on animal behavior and brain activity has been attracting growing attention in the last decades. Stress dramatically affects several aspects of animal behavior, including motivation and cognitive functioning, and has been used to model human pathologies such as post-traumatic stress disorder. A key question is whether stress alters the plastic potential of synaptic circuits. In this work, we evaluated if stress affects dopamine (DA)-dependent synaptic plasticity in the medial prefrontal cortex (mPFC). On male adolescent rats, we characterized anxiety- and depressive-like behaviors using behavioral testing before and after exposure to a mild stress (elevated platform, EP). After the behavioral protocols, we investigated DA-dependent long-term potentiation (DA-LTP) and depression (DA-LTD) on acute slices of mPFC and evaluated the activation of DA-producing brain regions by western and dot blot analysis. We show that exposure to the EP stress enhances DA-LTP and that desipramine (DMI) treatment abolishes this effect. We also found that DA-LTD is not affected by EP stress unless when this is followed by DMI treatment. In addition, EP stress reduces anxiety, an effect abolished by both DMI and ketamine, while motivation is promoted by previous exposure to EP stress independently of pharmacological treatments. Finally, this form of stress reduces the expression of the early gene cFOS in the ventral tegmental area. These findings support the idea that mild stressors can promote synaptic plasticity in PFC through a dopaminergic mechanism, an effect that might increase the sensitivity of mPFC to subsequent stressful experiences.

Behavioral effects of SGK1 knockout in VTA and dopamine neurons

Drugs of abuse cause significant neuroadaptations within the ventral tegmental area (VTA), with alterations in gene expression tied to changes in reward behavior. Serum- and glucocorticoid-inducible kinase 1 (SGK1) transcription, catalytic activity, and phosphorylation are upregulated in the VTA by chronic cocaine or morphine treatment, positioning SGK1 as a critical mediator of reward behavior. Using transgenic mouse models, we investigated the effect of SGK1 knockout in the VTA and in dopamine (DA) neurons to evaluate the necessity of protein expression for natural and drug reward behaviors. SGK1 knockdown in the VTA did not impact reward behaviors. Given VTA cellular heterogeneity, we also investigated a DA neuron-specific SGK1 knockout (KO). DA SGK1 KO significantly decreased body weight of adult mice as well as increased general locomotor activity; however, reward behaviors were similarly unaltered. Given that SGK1 mutants virally overexpressed in the VTA are capable of altering drug-associated behavior, our current results suggest that changes in SGK1 protein signaling may be distinct from expression. This work yields novel information on the impact of SGK1 deletion, critical for understanding the role of SGK1 signaling in the central nervous system and evaluating SGK1 as a potential therapeutic target for treatment of substance use disorders.

Early life stress facilitates synapse premature unsilencing to enhance AMPA receptor function in the developing hippocampus

Chronic early life stress (ELS) increases vulnerability to psychopathologies and cognitive deficits in adulthood by disrupting the function of related neural circuits. However, whether this disruption emerges early in the developing brain remains largely unexplored. In the current study, using an established limited-bedding and nesting model of ELS in postnatal day (P)2-10 mice, we provide direct evidence that ELS caused early modification of hippocampal glutamatergic synapses in the developing brain. We demonstrated that ELS induced rapid enhancement of AMPA receptor function in hippocampal CA1 pyramidal neurons through a postsynaptic mechanism, and importantly, this was associated with premature unsilencing of NMDA receptor-only silent hippocampal synapses. These results suggest that potentiation of AMPAR function may represent an early mediator of ELS-induced alterations of neural networks in the developing brain and may potentially contribute to subsequent cognitive impairments later in life.NEW & NOTEWORTHY Early life stress (ELS) is known to increase the risk of later life cognitive deficits by disrupting neural circuit function. However, whether this disruption emerges early in the developing brain remains largely unexplored. The current study presents direct evidence that ELS prematurely unsilences hippocampal synapses to enhance AMPA receptor functions in a limited-bedding and nesting model, revealing an early mediator of ELS-induced neural circuit reorganizations.

Neonatal Tactile Stimulation Alters Behaviors in Heterozygous Serotonin Transporter Male Rats: Role of the Amygdala

The serotonin transporter (SERT) gene, especially the short allele of the human serotonin transporter linked polymorphic region (5-HTTLPR), has been associated with the development of stress-related neuropsychiatric disorders. In line, exposure to early life stress in SERT knockout animals contributes to anxiety- and depression-like behavior. However, there is a lack of investigation of how early-life exposure to beneficial stimuli, such as tactile stimulation (TS), affects later life behavior in these animals. In this study, we investigated the effect of TS on social, anxiety, and anhedonic behavior in heterozygous SERT knockouts rats and wild-type controls and its impact on gene expression in the basolateral amygdala. Heterozygous SERT^+/– rats were submitted to TS during postnatal days 8–14, for 10 min per day. In adulthood, rats were assessed for social and affective behavior. Besides, brain-derived neurotrophic factor (Bdnf) gene expression and its isoforms, components of glutamatergic and GABAergic systems as well as glucocorticoid-responsive genes were measured in the basolateral amygdala. We found that exposure to neonatal TS improved social and affective behavior in SERT^+/– animals compared to naïve SERT^+/– animals and was normalized to the level of naïve SERT^+/+ animals. At the molecular level, we observed that TS per se affected Bdnf, the glucocorticoid-responsive genes Nr4a1, Gadd45β, the co-chaperone Fkbp5 as well as glutamatergic and GABAergic gene expression markers including the enzyme Gad67, the vesicular GABA transporter, and the vesicular glutamate transporter genes. Our results suggest that exposure of SERT^+/– rats to neonatal TS can normalize their phenotype in adulthood and that TS per se alters the expression of plasticity and stress-related genes in the basolateral amygdala. These findings demonstrate the potential effect of a supportive stimulus in SERT rodents, which are more susceptible to develop psychiatric disorders.

What's Love Got to do with it: Role of oxytocin in trauma, attachment and resilience

Oxytocin (OT) is a neurohypophysial hormone and neuropeptide produced by the hypothalamus and released by the pituitary gland. It has multiple physiological roles including stimulation of parturition and lactation, and promotion of pro-adaptive social behaviors necessary for mammalian survival. OT interacts with one receptor subtype: the OT receptor (OTR) which, upon stimulation, triggers different intracellular signal transduction cascades to mediate its physiological actions. Preclinical studies show that OT regulates social behaviors such as pair bonding, recognition and social interaction. It also coordinates the activation of the hypothalamic-pituitary-adrenal (HPA) axis and the release of corticotrophin-releasing hormone. Further evidence suggests that OT plays an important role in regulating caloric intake and metabolism, and in maintaining electrolyte and cardiovascular homeostasis. OT is also involved in attenuating the neurophysiological and neurochemical effects of trauma on the brain and body by facilitating both physical attachment such as wound healing, and psychological/social attachment, thereby increasing resilience to subsequent traumatic events. Clinical trials have reported that intranasal administration of OT provides therapeutic benefits for patients diagnosed with traumatic stress-related diseases such as major depressive disorders and post-traumatic stress disorder. OT's therapeutic benefits may result from context-dependent interactions with key neural pathways (social, cognitive, and reward), neurotransmitters (dopamine, norepinephrine, serotonin, and endogenous opioids), and biomarkers (adrenocorticotropic hormone, cortisol, and dehydroepiandrosterone sulfate), that lead to a decrease in stress -associated behaviors, and facilitate post-traumatic growth, ultimately leading to increased resilience, through improved social cohesion and attachment. OT induced-augmentation of physical and cognitive resilience may play a significant role in both the prevention of, and improved clinical outcomes for, traumatic stress-related disorders following either acute or enduring traumatic experiences.

Are working memory and glutamate concentrations involved in early-life stress and severity of psychosis?

OBJECTIVE

Occurrences of early-life stress (ELS) are associated with the severity of psychotic symptoms and working memory (WM) deficits in patients with psychosis (PSY). This study investigated potential mediation roles of WM behavioral performance and glutamate concentrations in prefrontal brain regions on the association between ELS and psychotic symptom severity in PSY.

METHOD

Forty-seven patients with PSY (established schizophrenia, n = 30; bipolar disorder, n = 17) completed measures of psychotic symptom severity. In addition, data on ELS and WM performance were collected in both patients with PSY and healthy controls (HC; n = 41). Resting-state glutamate concentrations in the bilateral dorsolateral prefrontal cortex (DLPFC) and anterior cingulate cortex (ACC) were also assessed with proton magnetic resonance spectroscopy for both PSY and HC groups. t tests, analyses of variance, and regression analyses were utilized.

RESULTS

Participants with PSY reported significantly more ELS occurrences and showed poorer WM performance than HC. Furthermore, individuals with PSY displayed lower glutamate concentrations in the left DLPFC than HC. Neither ELS nor WM performance were predictive of severity of psychotic symptoms in participants with PSY. However, we found a significant negative correlation between glutamate concentrations in the left DLPFC and ELS occurrence in HC only.

CONCLUSION

In individuals with PSY, the current study found no evidence that the association between ELS and psychotic symptoms is mediated by WM performance or prefrontal glutamate concentrations. In HC, the association between ELS experience and glutamate concentrations may indicate a neurometabolite effect of ELS that is independent of an illness effect in psychosis.

Chronic Stress-induced Behaviors Correlate with Exacerbated Acute Stress-induced Cingulate Cortex and Ventral Hippocampus Activation

Altered activity of corticolimbic brain regions is a hallmark of stress-related illnesses, including mood disorders, neurodegenerative diseases, and substance abuse disorders. Acute stress adaptively recruits brain region-specific functions for coping, while sustained activation under chronic stress may overwhelm feedback mechanisms and lead to pathological cellular and behavioral responses. The neural mechanisms underlying dysregulated stress responses and how they contribute to behavioral deficits are poorly characterized. Here, we tested whether prior exposure to chronic restraint stress (CRS) or unpredictable chronic mild stress (UCMS) in mice could alter functional response to acute stress and whether these changes are associated with chronic stress-induced behavioral deficits. More specifically, we assessed acute stress-induced functional activation indexed by c-Fos+ cell counts in 24 stress- and mood-related brain regions, and determined if changes in functional activation were linked to chronic stress-induced behavioral impairments, summarized across dimensions through principal component analysis (PCA). Results indicated that CRS and UCMS led to convergent physiological and anxiety-like deficits, whereas working and short-term memory were impaired only in UCMS mice. CRS and UCMS exposure exacerbated functional activation by acute stress in anterior cingulate cortex (ACC) area 24b and ventral hippocampal (vHPC) CA1, CA3, and subiculum. In dysregulated brain regions, levels of functional activation were positively correlated with principal components reflecting variance across behavioral deficits relevant to stress-related disorders. Our data supports an association between a dysregulated stress response, altered functional corticolimbic excitation/inhibition balance, and the expression of maladaptive behaviors.

The stressed brain: regional and stress-related corticosterone and stress-regulated gene expression in the adult zebra finch (Taeniopygia guttata)

Glucocorticoids (CORT) are well-known as important regulators of behaviour and cognition at basal levels and under stress. However, the precise mechanisms governing CORT action and functional outcomes of this action in the brain remain unclear, particularly in model systems other than rodents. In the present study, we investigated the dynamics of CORT regulation in the zebra finch, an important model system for vocal learning, neuroplasticity and cognition. We tested the hypothesis that CORT is locally regulated in the zebra finch brain by quantifying regional and stress-related variation in total CORT across brain regions. In addition, we used an ex vivo slice culture system to test whether CORT regulates target gene expression uniquely in discrete regions of the brain. We documented a robust increase in brain CORT across regions after 30 minutes of restraint stress but, interestingly, baseline and stress-induced CORT levels varied between regions. In addition, CORT treatment of brain slice cultures differentially affected expression of three CORT target genes: it up-regulated expression of FKBP5 in most regions and SGK1 in the hypothalamus only, whereas GILZ was unaffected by CORT treatment across all brain regions investigated. The specific mechanisms producing regional variation in CORT and CORT-dependent downstream gene expression remain unknown, although these data provide additional support for the hypothesis that the songbird brain employs regulatory mechanisms that result in precise control over the influence of CORT on glucocorticoid-sensitive neural circuits.

Serum- and glucocorticoid-inducible kinase 1 activity reduces dendritic spines in dorsal hippocampus

The hippocampus has a well-known role in mediating learning and memory, and its function can be directly regulated by both stress and glucocorticoid receptor activation. Hippocampal contributions to learning are thought to be dependent on changes in the plasticity of synapses within specific subregions, and these functional changes are accompanied by morphological changes in the number and shape of dendritic spines, the physical correlates of these glutamatergic synapses. Serum- and glucocorticoid-inducible kinase 1 (SGK1) regulates dendritic spine morphology in the prefrontal cortex, and modulation of SGK1 expression in mouse hippocampus regulates learning. However, the role of SGK1 in dendritic spine morphology within the CA1 and dentate gyrus regions of the hippocampus are unknown. Thus, herpes simplex viral vectors expressing GFP and various SGK1 constructs, including wild type SGK1, a catalytically inactive version of SGK1 (K127Q), and a phospho-defective version of SGK1 (S78A), were infused into the hippocampus of adult mice and confocal fluorescent microscopy was used to visualize dendritic spines. We show that increasing expression of SGK1 in the dentate gyrus increased the total number of spines, driven primarily by an increase in mushroom spines, while decreasing SGK1 activity (K127Q) in the CA1 region increased the total number of dendritic spines, driven by a significant increase in mushroom and stubby spines. The differential effects of SGK1 in these regions may be mediated by the interactions of SGK1 with multiple pathways required for spine formation and stability. As the formation of mature synapses is a crucial component of learning and memory, this indicates that SGK1 is a potential target in the pathway underlying stress-associated changes in cognition and memory.

Ketamina, un nuevo agente terapéutico para la depresión

Major depressive disorder affects about one in every 10 people in Mexico and is one of the first 5 causes of disability worldwide. Current treatment options are limited and only act upon some factors associated in its physiopathology. Moreover, the effects on depression are not immediate, which is a great limitation in obtaining a benefit over disability caused by this disorder and impedes a rapid action in the scenario of suicidality. Recently, ketamine (an anesthetic) has shown to have antidepressant properties by acting in the glutamate neurotransmission system (while no other current treatment acts on this level). It offers benefits in depressive symptoms in a matter of hours and has proven to be useful in patients that do not benefit from current therapeutic options. Recently, it has been approved for the treatment of depression. However, there are still many questions about its antidepressant mechanisms of action, safety, side effects, among others. Key words: Depression; antidepressants; ketamine.

Glucocorticoid and β-adrenergic regulation of hippocampal dendritic spines

Glucocorticoid hormones are particularly potent with respect to enhancing memory formation. Notably, this occurs in close synergy with arousal (i.e., when norepinephrine levels are enhanced). In the present study, we examined whether glucocorticoid and norepinephrine hormones regulate the number of spines in hippocampal primary neurons. We report that brief administration of corticosterone or the β-adrenergic receptor agonist isoproterenol alone increases spine number. This effect becomes particularly prominent when corticosterone and isoproterenol are administered together. In parallel, corticosterone and isoproterenol alone increased the amplitude of miniature excitatory postsynaptic currents, an effect that is not amplified when both hormones are administered together. The effects of co-application of corticosterone and isoproterenol on spines could be prevented by blocking the glucocorticoid receptor antagonist RU486. Taken together, both corticosterone and β-adrenergic receptor activation increase spine number, and they exert additive effects on spine number for which activation of glucocorticoid receptors is permissive.

Modulation of Distinct Intrinsic Resting State Brain Networks by Acute Exercise Bouts of Differing Intensity

Acute exercise bouts alter resting state functional connectivity (rs-FC) within cognitive, sensorimotor, and affective networks, but it remains unknown how these effects are influenced by exercise intensity.

Twenty-five male athletes underwent individual fitness assessments using an incremental treadmill test. On separate days, they performed ‘low’ (35% below lactate threshold) and ‘high’ (20% above lactate threshold) intensity exercise bouts of 30 min. Rs-fMRI and Positive and Negative Affect Scale (PANAS) were acquired before and after each exercise bout. Networks of interest were extracted from twenty-two participants (3 dropouts). Pre-to-post changes and between conditions effects were evaluated using FSL’s randomise by applying repeated measures ANOVA. Results were reported at p < 0.05, corrected for multiple comparisons using threshold free cluster enhancement.

PANAS revealed a significant increase in positive mood after both exercise conditions. Significant effects were observed between conditions in the right affective and reward network (ARN), the right fronto parietal network (FPN) and the sensorimotor network (SMN). Pre-to-post comparisons after ‘low’ exercise intensity revealed a significant increase in rs-FC in the left and right FPN, while after ‘high’-intensity exercise rs-FC decreased in the SMN and the dorsal attention network (DAN) and increased in the left ARN.

Supporting recent findings, this study is the first to report distinct rs-FC alterations driven by exercise intensity: (i) Increased rs-FC in FPN may indicate beneficial functional plasticity for cognitive/attentional processing, (ii) increased rs-FC in ARN may be linked to endogenous opioid-mediated internal affective states. Finally, (iii) decreased rs-FC in the SMN may signify persistent motor fatigue. The distinct effects on rs-FC fit with theories of transient persistent network alterations after acute exercise bouts that are mediated by different exercise intensities and impact differentially on cognitive/attentional or affective responses.

Response and sample bridging in a primate short-term memory task

Freely-moving rodents can solve short-term memory (STM) tasks using "response bridging" strategies, relying on motor patterns instead of mnemonic functions. This limits the interpretational power of results yielded by some STM tasks in rodents. To determine whether head-fixed monkeys can employ parallel non-mnemonic strategies, we measured eye position and velocity of two head-fixed monkeys performing a delayed response reaching and grasping task. We found that eye position during the delay period was correlated with reach direction. Moreover, reach direction as well as grasp object could be predicted from eye kinematics during the delay. Both eye velocity and eye position contributed to the prediction of reach direction. These results show that motor signals carry sufficient information to allow monkeys to solve STM tasks without using any mnemonic functions. Thus, the potential of animals to solve STM tasks using motor patterns is more diverse than previously recognized.

"Braking" the Prefrontal Cortex: The Role of Glucocorticoids and Interneurons in Stress Adaptation and Pathology

The medial prefrontal cortex (mPFC) receives information regarding stimuli and appropriately orchestrates neurophysiological, autonomic, and behavioral responses to stress. The cellular and neurochemical heterogeneity of the mPFC and its projections are key to fine-tuning of stress responses and adaptation. Output of the mPFC is mediated by glutamatergic pyramidal neurons whose activity is coordinated by an intricate network of interneurons. Excitatory/inhibitory (E/I) balance in the mPFC is critical for appropriate responsiveness to stress, and E/I imbalance occurs in numerous neuropsychiatric disorders that co-occur with chronic stress. Moreover, there is mounting data suggesting that chronic stress may precipitate E/I imbalance. This review will provide information regarding the cellular and anatomical makeup of the mPFC and discuss the impact of acute and chronic stress in adulthood and early life on interneuron function, with implications for E/I balance affecting functional connectivity. Specifically, the review will highlight the importance of interneuron type, connectivity, and location (both layer- and subregion-specific). The discussion of local mPFC networks will focus on stress context, including stressor duration (acute vs. chronic) and timing (early life vs. adulthood), as these factors have significant implications for the interpretation of experiments and mPFC E/I balance. Indeed, interneurons appear to play a prominent role in prefrontal adaptation, and a better understanding of the interactions between stress and interneuron function may yield insight to the transition from adaptation to pathology. Ultimately, determining the mechanisms mediating adaptive versus pathologic plasticity will promote the development of novel treatments for neuropsychiatric disorders related to prefrontal E/I imbalance.

Quantitative analysis of Gria1, Gria2, Dlg1 and Dlg4 expression levels in hippocampus following forced swim stress in mice

AMPA receptors and interacting proteins are importantly involved in mediating stress-dependent plasticity. Previously we reported that GluA1-containing AMPA receptors and their interaction with PDZ-proteins are required for the experience-dependent expression of behavioral despair in the forced swim test. However, it is unclear if the expression of GluA1-containing AMPA receptors is affected by this type of behavior. Here we investigated in wild type mice, whether hippocampal gene or protein levels of GluA1 or associated PDZ proteins is altered following forced swim stress. We show that expression of Dlg4 (the gene coding for PSD-95) was strongly reduced after two days of forced swimming. In contrast, levels of Dlg1, Gria1, and Gria2 (coding for SAP97, GluA1, and GluA2 respectively) were not affected after one or two days of forced swimming. The changes in gene expression largely did not translate to the protein level. These findings indicate a limited acute effect of forced swim stress on the expression of the investigated targets and suggest that the acute involvement of GluA1-containing AMPA receptors tor forced swim behavior is a result of non-genomic mechanisms.

Striatal responsiveness to reward under threat-of-shock and working memory load: A preliminary study

INTRODUCTION

Reward and stress are important determinants of motivated behaviors. Striatal regions play a crucial role in both motivation and hedonic processes. So far, little is known on how cognitive effort interacts with stress to modulate reward processes. This study examines how cognitive effort (load) interacts with an unpredictable acute stressor (threat-of-shock) to modulate motivational and hedonic processes in healthy adults.

MATERIALS AND METHODS

A reward task, involving stress with unpredictable mild electric shocks, was conducted in 23 healthy adults aged 20-37 (mean age: 24.7 ± 0.9; 14 females) during functional magnetic resonance imaging (fMRI). Manipulation included the use of (a) monetary reward for reinforcement, (b) threat-of-shock as the stressor, and (c) a spatial working memory task with two levels of difficulty (low and high load) for cognitive load. Reward-related activation was investigated in a priori three regions of interest, the nucleus accumbens (NAcc), caudate nucleus, and putamen.

RESULTS

During anticipation, threat-of-shock or cognitive load did not affect striatal responsiveness to reward. Anticipated reward increased activation in the ventral and dorsal striatum. During feedback delivery, both threat-of-shock and cognitive effort modulated striatal activation. Higher working memory load blunted NAcc responsiveness to reward delivery, while stress strengthened caudate nucleus reactivity regardless reinforcement or load.

CONCLUSIONS

These findings provide initial evidence that both stress and cognitive load modulate striatal responsiveness during feedback delivery but not during anticipation in healthy adults. Of clinical importance, sustained stress exposure might go along with dysregulated arousal, increasing therefore the risk for the development of maladaptive incentive-triggered motivation. This study brings new insight that might help to build a framework to understand common stress-related disorders, given that these psychiatric disorders involve disturbances of the reward system, cognitive deficits, and abnormal stress reactivity.

Chemogenetic Activation of Prefrontal Cortex in Shank3-Deficient Mice Ameliorates Social Deficits, NMDAR Hypofunction, and Sgk2 Downregulation

Haploinsufficiency of the SHANK3 gene is causally linked to autism spectrum disorders (ASDs) in human genetic studies. Here we found that chemogenetic activation of pyramidal neurons in the prefrontal cortex (PFC) of Shank3-deficient mice with the hM3D (Gq) DREADD restored social preference behaviors and elevated glutamatergic synaptic function in PFC. Moreover, the expression of Sgk2 (serum- and glucocorticoid-inducible kinase 2), a member of the Sgk family, which plays a key role in regulating the membrane trafficking of glutamate receptors, was diminished by Shank3 deficiency and rescued by Gq DREADD activation of PFC. Blocking Sgk function in Shank3-deficient mice prevented Gq DREADD from rescuing social and synaptic deficits, whereas blocking Sgk function in wild-type mice led to the attenuation of PFC glutamatergic signaling and the induction of autism-like social deficits. These results have provided a potential circuit intervention and molecular target for autism treatment.

Time course of the physiological stress response to an acute stressor and its associations with the primacy and recency effect of the serial position curve

Whether stress affects memory depends on which stress pathway becomes activated and which specific memory system is involved. The activation of the sympathetic nervous system (SNS), leads to a release of catecholamines. The activation of the hypothalamic-pituitary-adrenal (HPA) axis, leads to a release of glucocorticoids. In thus study, it was investigated whether SNS and/or HPA axis activation are associated with long-term memory (LTM) and/or working memory (WM) performance in humans. Thirty-three participants underwent the socially evaluated cold-pressor test. Salivary alpha-amylase (sAA) was used as a marker for the activation of the SNS and cortisol as marker for HPA axis activation. Memory was assessed by means of word lists with 15 words each. The primacy effect (i.e., the correctly recalled words from the beginning of the lists) of the serial position curve was considered as indicator for LTM. The recency effect (i.e., the correctly recalled words from the end of the lists) were used as estimator for WM performance. In sAA responders, the recency effect and, therefore, WM performance increased immediately after the stressor. This was not found in sAA non-responders. In cortisol responders, the primacy effect and, thus, LTM performance decreased 20 minutes after the stressor. No change in LTM performance was found in cortisol non-responders. Our study supports the assumptions that 1) SNS activation is associated with WM processes via stimulation of the prefrontal cortex, and 2) HPA axis activation is associated with LTM processes through interactions with the hippocampus.

Acute Inescapable Stress Rapidly Increases Synaptic Energy Metabolism in Prefrontal Cortex and Alters Working Memory Performance

Brain energy metabolism actively regulates synaptic transmission and activity. We have previously shown that acute footshock (FS)-stress induces fast and long-lasting functional and morphological changes at excitatory synapses in prefrontal cortex (PFC). Here, we asked whether FS-stress increased energy metabolism in PFC, and modified related cognitive functions. Using positron emission tomography (PET), we found that FS-stress induced a redistribution of glucose metabolism in the brain, with relative decrease of [18F]FDG uptake in ventro-caudal regions and increase in dorso-rostral ones. Absolute [18F]FDG uptake was inversely correlated with serum corticosterone. Increased specific hexokinase activity was also measured in purified PFC synaptosomes (but not in total extract) of FS-stressed rats, which positively correlated with 2-Deoxy [3H] glucose uptake by synaptosomes. In line with increased synaptic energy demand, using an electron microscopy-based stereological approach, we found that acute stress induced a redistribution of mitochondria at excitatory synapses, together with an increase in their volume. The fast functional and metabolic activation of PFC induced by acute stress, was accompanied by rapid and sustained alterations of working memory performance in delayed response to T-maze test. Taken together, the present data suggest that acute stress increases energy consumption at PFC synaptic terminals and alters working memory.

Inhibition of EHMT1/2 rescues synaptic and cognitive functions for Alzheimer's disease

Epigenetic dysregulation, which leads to the alteration of gene expression in the brain, is suggested as one of the key pathophysiological bases of ageing and neurodegeneration. Here we found that, in the late-stage familial Alzheimer's disease (FAD) mouse model, repressive histone H3 dimethylation at lysine 9 (H3K9me2) and euchromatic histone methyltransferases EHMT1 and EHMT2 were significantly elevated in the prefrontal cortex, a key cognitive region affected in Alzheimer's disease. Elevated levels of H3K9me2 were also detected in the prefrontal cortex region of post-mortem tissues from human patients with Alzheimer's disease. Concomitantly, H3K9me2 at glutamate receptors was increased in prefrontal cortex of aged FAD mice, which was linked to the diminished transcription, expression and function of AMPA and NMDA receptors. Treatment of FAD mice with specific EHMT1/2 inhibitors reversed histone hyper-methylation and led to the recovery of glutamate receptor expression and excitatory synaptic function in prefrontal cortex and hippocampus. Chromatin immunoprecipitation-sequencing (ChIP-seq) data indicated that FAD mice exhibited genome-wide increase of H3K9me2 enrichment at genes involved in neuronal signalling (including glutamate receptors), which was reversed by EHMT1/2 inhibition. Moreover, the impaired recognition memory, working memory, and spatial memory in aged FAD mice were rescued by the treatment with EHMT1/2 inhibitors. These results suggest that disrupted epigenetic regulation of glutamate receptor transcription underlies the synaptic and cognitive deficits in Alzheimer's disease, and targeting histone methylation enzymes may represent a novel therapeutic strategy for this prevalent neurodegenerative disorder.

The Neurobiology and Pharmacotherapy of Posttraumatic Stress Disorder

New approaches to the neurobiology of posttraumatic stress disorder (PTSD) are needed to address the reported crisis in PTSD drug development. These new approaches may require the field to move beyond a narrow fear-based perspective, as fear-based medications have not yet demonstrated compelling efficacy. Antidepressants, particularly recent rapid-acting antidepressants, exert complex effects on brain function and structure that build on novel aspects of the biology of PTSD, including a role for stress-related synaptic dysconnectivity in the neurobiology and treatment of PTSD. Here, we integrate this perspective within a broader framework-in other words, a dual pathology model of ( a) stress-related synaptic loss arising from amino acid-based pathology and ( b) stress-related synaptic gain related to monoamine-based pathology. Then, we summarize the standard and experimental (e.g., ketamine) pharmacotherapeutic options for PTSD and discuss their putative mechanism of action and clinical efficacy.

Stress and Ketamine, Bimodal Influence on Cognitive Functions

The glutamate N-methyl-D-aspartate receptor (NMDAR) non-selective antagonist, ketamine, has been recently repurposed as a rapidly acting antidepressant, catalyzing the vigorous investigation of glutamate-signaling modulators as novel therapeutic agents for depressive disorders. Beneficial effects of this drug in the quick-acting treatment of depression are recognized. The long-term effects of ketamine have not been known, including the cognitive sphere. It is well acknowledged that prolonged exposure to stress induces depression and cognitive impairment. It seemed reasonable to ask how the long-term ketamine administration would affect stressed animals in the aspect of cognitive functions. In the current study we tested whether it is possible for ketamine, used in prolonged-regimen in rats, to alleviate stress-evoked memory deficits? Stressed (restraint 2 h daily for 21 days) and non-stressed rats (6-weeks-old) were treated with ketamine for 21 days and next subjected to a battery of behavioral tests: for the assessment of working and reference spatial memory (Morris water maze (MWM) and Barnes maze (BM)), stereotypy (stereotypy test - ST), locomotor functions (Open field - OF) and anxiety behavior (Elevated plus maze - EPM). Ketamine administration resulted in a significant stereotype behaviour in rats tested in ST. Stressed rats displayed a significant decline in the spatial working and reference memory. The effect of chronic ketamine administration depended on the type of test and differed between control rats and animals simultaneously exposed to chronic stress. However, in the MWM the impact was quite unequivocal, as we observed an improvement in spatial memory in stressed animals and a deterioration in non-stressed animals after ketamine administration. In the BM, the effect of ketamine changed in successive attempts, from favorable in the initial period to negative at the end of the test in the group of stressed animals and without a significant impact on control animals. We found no significant effects of ketamine on locomotor performance and on the level of anxiety. Taken together, these findings demonstrate that ketamine potently abolishes or prevents some kinds of stress-induced memory impairments and cognitive decline in rats, although in some circumstances, it could even increase damage to memory, especially in unstressed animals. It seems that the prolonged use of ketamine in the prevention of stress-induced memory declines can fulfill its role.

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.

Trauma exposure acutely alters neural function during Pavlovian fear conditioning

Posttraumatic stress disorder (PTSD) is associated with dysfunction of the neural circuitry that supports fear learning and memory processes. However, much of what is known about neural dysfunction in PTSD is based on research in chronic PTSD populations. Less is known about neural function that supports fear learning acutely following trauma exposure. Determining the acute effects of trauma exposure on brain function would provide new insight into the neural processes that mediate the cognitive-affective dysfunction associated with PTSD. Therefore, the present study investigated neural activity that supports fear learning and memory processes in recently Trauma-Exposed (TE) and Non-Trauma-Exposed (NTE) participants. Participants completed a Pavlovian fear conditioning procedure during functional magnetic resonance imaging (fMRI). During fMRI, participants' threat expectancy was continuously monitored. NTE participants showed greater threat expectancy during warning than safety cues, while no difference was observed in the TE group. This finding suggests TE participants overgeneralized the fear association to the safety cue. Further, only the TE group showed a negative relationship between fMRI signal responses within dorsomedial prefrontal cortex (PFC) and threat expectancy during safety cues. These results suggest the dorsomedial PFC mediates overgeneralization of learned fear as an acute result of trauma exposure. Finally, neural activity within the PFC and inferior parietal lobule showed a negative relationship with PTSD symptom severity assessed three months posttrauma. Thus, neural activity measured acutely following trauma exposure predicted future PTSD symptom severity. The present findings elucidate the acute effects of trauma exposure on cognitive-affective function and provide new insight into the neural mechanisms of PTSD.

Glucocorticoid receptor signaling in astrocytes is required for aversive memory formation

Stress elicits the release of glucocorticoids (GCs) that regulate energy metabolism and play a role in emotional memory. Astrocytes express glucocorticoid receptors (GR), but their contribution to cognitive effects of GC's action in the brain is unknown. To address this question, we studied how astrocyte-specific elimination of GR affects animal behavior known to be regulated by stress. Mice with astrocyte-specific ablation of GR presented impaired aversive memory expression in two different paradigms of Pavlovian learning: contextual fear conditioning and conditioned place aversion. These mice also displayed compromised regulation of genes encoding key elements of the glucose metabolism pathway upon GR stimulation. In particular, we identified that the glial, but not the neuronal isoform of a crucial stress-response molecule, Sgk1, undergoes GR-dependent regulation in vivo and demonstrated the involvement of SGK1 in regulation of glucose uptake in astrocytes. Together, our results reveal astrocytes as a central element in GC-dependent formation of aversive memory and suggest their relevance for stress-induced alteration of brain glucose metabolism. Consequently, astrocytes should be considered as a cellular target of therapies of stress-induced brain diseases.

Neuronal nitric oxide synthase and affective disorders

Affective disorders including major depressive disorder (MDD), bipolar disorder (BPD), and general anxiety affect more than 10% of population in the world. Notably, neuronal nitric oxide synthase (nNOS), a downstream signal molecule of N-methyl-D-aspartate receptors (NMDARs) activation, is abundant in many regions of the brain such as the prefrontal cortex (PFC), hippocampus, amygdala, dorsal raphe nucleus (DRN), locus coeruleus (LC), and hypothalamus, which are closely associated with the pathophysiology of affective disorders. Decreased levels of the neurotransmitters including 5-hydroxytryptamine or serotonin (5-HT), noradrenalin (NA), and dopamine (DA) as well as hyperactivity of the hypothalamic-pituitary-adrenal (HPA) axis are common pathological changes of MDD, BPD, and anxiety. Increasing data suggests that nNOS in the hippocampus play a crucial role in the etiology of MDD whereas nNOS-related dysregulation of the nitrergic system in the LC is closely associated with the pathogenesis of BPD. Moreover, hippocampal nNOS is implicated in the role of serotonin receptor 1 A (5-HTR1 A) in modulating anxiety behaviors. Augment of nNOS and its carboxy-terminal PDZ ligand (CAPON) complex mediate stress-induced anxiety and disrupting the nNOS-CAPON interaction by small molecular drug generates anxiolytic effect. To date, however, the function of nNOS in affective disorders is not well reviewed. Here, we summarize works about nNOS and its signal mechanisms implicated in the pathophysiology of affective disorders. On the basis of this review, it is suggested that future research should more fully focus on the role of nNOS in the pathomechanism and treatment of affective disorders.

Effects of social defeat stress on dopamine D2 receptor isoforms and proteins involved in intracellular trafficking

Background

Chronic social defeat stress induces depression and anxiety-like behaviors in rodents and also responsible for differentiating defeated animals into stress susceptible and resilient groups. The present study investigated the effects of social defeat stress on a variety of behavioral parameters like social behavior, spatial learning and memory and anxiety like behaviors. Additionally, the levels of various dopaminergic markers, including the long and short form of the D2 receptor, and total and phosphorylated dopamine and cyclic adenosine 3′,5′-monophosphate regulated phosphoprotein-32, and proteins involved in intracellular trafficking were assessed in several key brain regions in young adult mice.

Methods

Mouse model of chronic social defeat was established by resident-intruder paradigm, and to evaluate the effect of chronic social defeat, mice were subjected to behavioral tests like spontaneous locomotor activity, elevated plus maze (EPM), social interaction and Morris water maze tests.

Results

Mice were divided into susceptible and unsusceptible groups after 10 days of social defeat stress. The susceptible group exhibited greater decreases in time spent in the open and closed arms compared to the control group on the EPM. In the social interaction test, the susceptible group showed greater increases in submissive and neutral behaviors and greater decreases in social behaviors relative to baseline compared to the control group. Furthermore, increased expression of D2L, D2S, Rab4, and G protein-coupled receptor associated sorting protein-1 was observed in the amygdala of the susceptible group compared to the control group.

Conclusion

These findings suggest that social defeat stress induce anxiety-like and altered social interacting behaviors, and changes in dopaminergic markers and intracellular trafficking-related proteins.

Electronic supplementary material

The online version of this article (10.1186/s12993-018-0148-5) contains supplementary material, which is available to authorized users.

The neurobiology of depression, ketamine and rapid-acting antidepressants: Is it glutamate inhibition or activation?

The discovery of the antidepressant effects of ketamine has opened a breakthrough opportunity to develop a truly novel class of safe, effective, and rapid-acting antidepressants (RAADs). In addition, the rapid and robust biological and behavioral effects of ketamine offered a unique opportunity to utilize the drug as a tool to thoroughly investigate the neurobiology of stress and depression in animals, and to develop sensitive and reproducible biomarkers in humans. The ketamine literature over the past two decades has considerably enriched our understanding of the mechanisms underlying chronic stress, depression, and RAADs. However, considering the complexity of the pharmacokinetics and in vivo pharmacodynamics of ketamine, several questions remain unanswered and, at times, even answered questions continue to be considered controversial or at least not fully understood. The current perspective paper summarizes our understanding of the neurobiology of depression, and the mechanisms of action of ketamine and other RAADs. The review focuses on the role of glutamate neurotransmission - reviewing the history of the "glutamate inhibition" and "glutamate activation" hypotheses, proposing a synaptic connectivity model of chronic stress pathology, and describing the mechanism of action of ketamine. It will also summarize the clinical efficacy findings of putative RAADs, present relevant human biomarker findings, and discuss current challenges and future directions.

Decoding the transcriptional programs activated by psychotropic drugs in the brain

Analysis of drug-induced gene expression in the brain has long held the promise of revealing the molecular mechanisms of drug actions as well as predicting their long-term clinical efficacy. However, despite some successes, this promise has yet to be fulfilled. Here, we present an overview of the current state of understanding of drug-induced gene expression in the brain and consider the obstacles to achieving a robust prediction of the properties of psychoactive compounds based on gene expression profiles. We begin with a comprehensive overview of the mechanisms controlling drug-inducible transcription and the complexity resulting from expression of noncoding RNAs and alternative gene isoforms. Particular interest is placed on studies that examine the associations within drug classes with regard to the effects on gene transcription, alterations in cell signaling and neuropharmacological drug properties. While the ability of gene expression signatures to distinguish specific clinical classes of psychotropic and addictive drugs remains unclear, some reports show that under specific constraints, drug properties can be predicted based on gene expression. Such signatures offer a simple and effective way to classify psychotropic drugs and screen novel psychoactive compounds. Finally, we note that the amount of data regarding molecular programs activated in the brain by drug treatment has grown exponentially in recent years and that future advances may therefore come in large part from integrating the currently available high-throughput data sets.