Corticosteroid-induced neural remodeling predicts behavioral vulnerability and resilience

Cocaine disrupts action flexibility via glucocorticoid receptors

Many addictive drugs increase stress hormone levels. They also alter the propensity of organisms to prospectively select actions based on long-term consequences. We hypothesized that cocaine causes inflexible action by increasing circulating stress hormone levels, activating the glucocorticoid receptor (GR). We trained mice to generate two nose pokes for food and then required them to update action-consequence associations when one response was no longer reinforced. Cocaine delivered in adolescence or adulthood impaired the capacity of mice to update action strategies, and inhibiting CORT synthesis rescued action flexibility. Next, we reduced Nr3c1, encoding GR, in the orbitofrontal cortex (OFC), a region of the brain responsible for interlacing new information into established routines. Nr3c1 silencing preserved action flexibility and dendritic spine abundance on excitatory neurons, despite cocaine. Spines are often considered substrates for learning and memory, leading to the discovery that cocaine degrades the representation of new action memories, obstructing action flexibility.

Neuronal Ensembles in the Amygdala Allow Social Information to Motivate Later Decisions

Social experiences carry tremendous weight in our decision-making, even when social partners are not present. To determine mechanisms, we trained female mice to respond for two food reinforcers. Then, one food was paired with a novel conspecific. Mice later favored the conspecific-associated food, even in the absence of the conspecific. Chemogenetically silencing projections from the prelimbic subregion (PL) of the medial prefrontal cortex to the basolateral amygdala (BLA) obstructed this preference while leaving social discrimination intact, indicating that these projections are necessary for socially driven choice. Further, mice that performed the task had greater densities of dendritic spines on excitatory BLA neurons relative to mice that did not. We next induced chemogenetic receptors in cells active during social interactions-when mice were encoding information that impacted later behavior. BLA neurons stimulated by social experience were necessary for mice to later favor rewards associated with social conspecifics but not make other choices. This profile contrasted with that of PL neurons stimulated by social experience, which were necessary for choice behavior in social and nonsocial contexts alike. The PL may convey a generalized signal allowing mice to favor particular rewards, while units in the BLA process more specialized information, together supporting choice motivated by social information.

Socioeconomic Disparities in Hypothalamic-Pituitary-Adrenal Axis Regulation and Prefrontal Cortical Structure

Socioeconomic disadvantage during childhood predicts an increased risk for mental health problems across the life span. Socioeconomic disadvantage shapes multiple aspects of children's proximal environments and increases exposure to chronic stressors. Drawing from multiple literatures, we propose that childhood socioeconomic disadvantage may lead to adaptive changes in the regulation of stress response systems including the hypothalamic-pituitary-adrenal (HPA) axis. These changes, in turn, affect the development of prefrontal cortical (PFC) circuitry responsible for top-down control over cognitive and emotional processes. Translational findings indicate that chronic stress reduces dendritic complexity and spine density in the medial PFC and anterior cingulate cortex, in part through altered HPA axis regulation. Socioeconomic disadvantage has frequently been associated with reduced gray matter in the dorsolateral and ventrolateral PFC and anterior cingulate cortex and lower fractional anisotropy in the superior longitudinal fasciculus, cingulum bundle, and uncinate fasciculus during middle childhood and adolescence. Evidence of socioeconomic disparities in hair cortisol concentrations in children has accumulated, although null findings have been reported. Coupled with links between cortisol levels and reduced gray matter in the PFC and anterior cingulate cortex, these results support mechanistic roles for the HPA axis and these PFC circuits. Future longitudinal studies should simultaneously consider multiple dimensions of proximal factors, including cognitive stimulation, while focusing on epigenetic processes and genetic moderators to elucidate how socioeconomic context may influence the HPA axis and PFC circuitry involved in cognitive and emotional control. These findings, which point to modifiable factors, can be harnessed to inform policy and more effective prevention strategies.

Elevated Expression of HSP72 in the Prefrontal Cortex and Hippocampus of Rats Subjected to Chronic Mild Stress and Treated with Imipramine

The HSP70 and HSP90 family members belong to molecular chaperones that exhibit protective functions during the cellular response to stressful agents. We investigated whether the exposure of rats to chronic mild stress (CMS), a validated model of depression, affects the expression of HSP70 and HSP90 in the prefrontal cortex (PFC), hippocampus (HIP) and thalamus (Thal). Male Wistar rats were exposed to CMS for 3 or 8 weeks. The antidepressant imipramine (IMI, 10 mg/kg, i.p., daily) was introduced in the last five weeks of the long-term CMS procedure. Depressive-like behavior was verified by the sucrose consumption test. The expression of mRNA and protein was quantified by real-time PCR and Western blot, respectively. In the 8-week CMS model, stress alone elevated HSP72 and HSP90B mRNA expression in the HIP. HSP72 mRNA was increased in the PFC and HIP of rats not responding to IMI treatment vs. IMI responders. The CMS exposure increased HSP72 protein expression in the cytosolic fraction of the PFC and HIP, and this effect was diminished by IMI treatment. Our results suggest that elevated levels of HSP72 may serve as an important indicator of neuronal stress reactions accompanying depression pathology and could be a potential target for antidepressant strategy.

Cocaine and habit training cause dendritic spine rearrangement in the prelimbic cortex

Successfully navigating dynamic environments requires organisms to learn the consequences of their actions. The prelimbic prefrontal cortex (PL) formulates action-consequence memories and is modulated by addictive drugs like cocaine. We trained mice to obtain food rewards and then unexpectedly withheld reinforcement, triggering new action-consequence memory. New memory was disrupted by cocaine when delivered immediately following non-reinforcement, but not when delayed, suggesting that cocaine disrupted memory consolidation. Cocaine also rapidly inactivated cofilin, a primary regulator of the neuronal actin cytoskeleton. This observation led to the discovery that cocaine also within the time of memory consolidation elevated dendritic spine elimination and blunted spine formation rates on excitatory PL neurons, culminating in thin-type spine attrition. Training drug-naive mice to utilize inflexible response strategies also eliminated thin-type dendritic spines. Thus, cocaine may disrupt action-consequence memory, at least in part, by recapitulating neurobiological sequalae occurring in the formation of inflexible habits.

The Aversive Lens: Stress effects on the prefrontal-cingulate cortical pathways that regulate emotion

ARNSTEN, A.F.T., M.K.P. Joyce and A.C. Roberts. The Aversive Lens: Stress effects on the prefrontal-cingulate cortical pathways that regulate emotion. NEUROSCI BIOBEHAV REV XXX-XXX, 2022. The symptoms of major-depressive-disorder include psychic pain and anhedonia, i.e. seeing the world through an "aversive lens". The neurobiology underlying this shift in worldview is emerging. Here these data are reviewed, focusing on how activation of subgenual cingulate (BA25) induces an "aversive lens", and how higher prefrontal cortical (PFC) areas (BA46/10/32) provide top-down regulation of BA25 but are weakened by excessive dopamine and norepinephrine release during stress exposure, and dendritic spine loss with chronic stress exposure. These changes may generate an attractor state, which maintains the brain under the control of BA25, requiring medication or neuromodulatory treatments to return connectivity to a more flexible state. In line with this hypothesis, effective anti-depressant treatments reduce the activity of BA25 and restore top-down regulation by higher circuits, e.g. as seen with SSRI medications, ketamine, deep brain stimulation of BA25, or rTMS to strengthen dorsolateral PFC. This research has special relevance in an era of chronic stress caused by the COVID19 pandemic, political unrest and threat of climate change.

Regulation of corticotropin-releasing hormone neuronal network activity by noradrenergic stress signals

Noradrenaline is a neurotransmitter released in response to homeostatic challenge and activates the hypothalamic-pituitary-adrenal axis via stimulation of corticotropin-releasing hormone (CRH) neurons. Here we investigated the mechanism through which noradrenaline regulates activity within the CRH neuronal network. Using a combination of in vitro GCaMP6f Ca²⁺ imaging and electrophysiology, we show that noradrenaline induces a robust increase in excitability in a proportion of CRH neurons with many neurons displaying a bursting mode of activity. Noradrenaline-induced activation required α₁ -adrenoceptors and L-type voltage-gated Ca²⁺ channels, but not GABA/glutamate synaptic transmission or sodium action potentials. Exposure of mice to elevated corticosterone levels was able to suppress noradrenaline-induced activation. These results provide further insight into the mechanisms by which noradrenaline regulates CRH neural network activity and hence stress responses. KEY POINTS: GCaMP6f Ca²⁺ imaging and on-cell patch-clamp recordings reveal that corticotropin-releasing hormone neurons are activated by noradrenaline with many neurons displaying a bursting mode of activity. Noradrenaline-induced activation requires α₁ -adrenoceptors. Noradrenaline-induced Ca²⁺ elevations persist after blocking GABA_A , AMPA, NMDA receptors and voltage-gated Na⁺ channels. Noradrenaline-induced Ca²⁺ elevations require L-type voltage-gated Ca²⁺ channels. Corticosterone suppresses noradrenaline-induced excitation.

Cell adhesion presence during adolescence controls the architecture of projection-defined prefrontal cortical neurons and reward-related action strategies later in life

Adolescent brain development is characterized by neuronal remodeling in the prefrontal cortex; relationships with behavior are largely undefined. Integrins are cell adhesion factors that link the extracellular matrix with intracellular actin cytoskeleton. We find that β1-integrin presence in the prelimbic prefrontal cortex (PL) during adolescence, but not adulthood, is necessary for mice to select actions based on reward likelihood and value. As such, adult mice that lacked β1-integrin during adolescence failed to modify response strategies when rewards lost value or failed to be delivered. This pattern suggests that β1-integrin-mediated neuronal development is necessary for PL function in adulthood. We next visualized adolescent PL neurons, including those receiving input from the basolateral amygdala (BLA) - thought to signal salience - and projecting to the dorsomedial striatum (DMS) - the striatal output by which the PL controls goal-seeking behavior. Firstly, we found that these projection-defined neurons had a distinct morphology relative to general layer V PL neurons. Secondly, β1-integrin loss triggered the overexpression of stubby-type dendritic spines at the expense of mature spines, including on projection-defined neurons. This phenotype was not observed when β1-integrins were silenced before or after adolescence. Altogether, our experiments localize β1-integrin-mediated cell adhesion within a developing di-synaptic circuit coordinating adaptive action.

Escalated (Dependent) Oxycodone Self-Administration Is Associated with Cognitive Impairment and Transcriptional Evidence of Neurodegeneration in Human Immunodeficiency Virus (HIV) Transgenic Rats

Substance use disorder is associated with accelerated disease progression in people with human immunodeficiency virus (HIV; PWH). Problem opioid use, including high-dose opioid therapy, prescription drug misuse, and opioid abuse, is high and increasing in the PWH population. Oxycodone is a broadly prescribed opioid in both the general population and PWH. Here, we allowed HIV transgenic (Tg) rats and wildtype (WT) littermates to intravenously self-administer oxycodone under short-access (ShA) conditions, which led to moderate, stable, "recreational"-like levels of drug intake, or under long-access (LgA) conditions, which led to escalated (dependent) drug intake. HIV Tg rats with histories of oxycodone self-administration under LgA conditions exhibited significant impairment in memory performance in the novel object recognition (NOR) paradigm. RNA-sequencing expression profiling of the medial prefrontal cortex (mPFC) in HIV Tg rats that self-administered oxycodone under ShA conditions exhibited greater transcriptional evidence of inflammation than WT rats that self-administered oxycodone under the same conditions. HIV Tg rats that self-administered oxycodone under LgA conditions exhibited transcriptional evidence of an increase in neuronal injury and neurodegeneration compared with WT rats under the same conditions. Gene expression analysis indicated that glucocorticoid-dependent adaptations contributed to the gene expression effects of oxycodone self-administration. Overall, the present results indicate that a history of opioid intake promotes neuroinflammation and glucocorticoid dysregulation, and excessive opioid intake is associated with neurotoxicity and cognitive impairment in HIV Tg rats.

Environmental certainty influences the neural systems regulating responses to threat and stress

Flexible calibration of threat responding in accordance with the environment is an adaptive process that allows an animal to avoid harm while also maintaining engagement of other goal-directed actions. This calibration process, referred to as threat response regulation, requires an animal to calculate the probability that a given encounter will result in a threat so they can respond accordingly. Here we review the neural correlates of two highly studied forms of threat response suppression: extinction and safety conditioning. We focus on how relative levels of certainty or uncertainty in the surrounding environment alter the acquisition and application of these processes. We also discuss evidence indicating altered threat response regulation following stress exposure, including enhanced fear conditioning, and disrupted extinction and safety conditioning. To conclude, we discuss research using an animal model of coping that examines the impact of stressor controllability on threat responding, highlighting the potential for previous experiences with control, or other forms of coping, to protect against the effects of future adversity.

The role of the vagus nerve in fibromyalgia syndrome

Fibromyalgia (FM) syndrome is a common illness characterized by chronic widespread pain, sleep problems, fatigue, and cognitive difficulties. Dysfunctional neurotransmitter systems that influence the body's endogenous stress response systems are thought to underlie many of the major FM-related symptoms. A model of FM pathogenesis suggests biological and psychosocial variables interact to influence the genetic predisposition, but the precise mechanisms remain unclear. The Polyvagal Theory provides a theoretical framework from which to investigate potential biological mechanisms. The vagus nerve (VN) has anti-inflammatory properties via its afferent and efferent fibers. A low vagal tone (as assessed by low heart rate variability), has been observed in painful and inflammatory diseases, including FM, while the ventral branch of the VN is linked to emotional expression and social engagement. These anti-inflammatory and psychological (limbic system) properties of the VN may possess therapeutic potential in treating FM. This review paper summarizes the scientific literature regarding the potential role of the VN in transducing and/or therapeutically managing FM signs and symptoms.

Connecting the Neurobiology of Developmental Brain Injury: Neuronal Arborisation as a Regulator of Dysfunction and Potential Therapeutic Target

Neurodevelopmental disorders can derive from a complex combination of genetic variation and environmental pressures on key developmental processes. Despite this complex aetiology, and the equally complex array of syndromes and conditions diagnosed under the heading of neurodevelopmental disorder, there are parallels in the neuropathology of these conditions that suggest overlapping mechanisms of cellular injury and dysfunction. Neuronal arborisation is a process of dendrite and axon extension that is essential for the connectivity between neurons that underlies normal brain function. Disrupted arborisation and synapse formation are commonly reported in neurodevelopmental disorders. Here, we summarise the evidence for disrupted neuronal arborisation in these conditions, focusing primarily on the cortex and hippocampus. In addition, we explore the developmentally specific mechanisms by which neuronal arborisation is regulated. Finally, we discuss key regulators of neuronal arborisation that could link to neurodevelopmental disease and the potential for pharmacological modification of arborisation and the formation of synaptic connections that may provide therapeutic benefit in the future.

Stress, memory, and implications for major depression

The stress response comprises a phylogenetically conserved set of cognitive, physiological, and behavioral responses that evolved as a survival strategy. In this context, the memory of stressful events would be adaptive as it could avoid re-exposure to an adverse event, otherwise the event would be facilitated in positively stressful or non-distressful conditions. However, the interaction between stress and memory comprises complex responses, some of them which are not yet completely understood, and which depend on several factors such as the memory system that is recruited, the nature and duration of the stressful event, as well as the timing in which this interaction takes place. In this narrative review, we briefly discuss the mechanisms of the stress response, the main memory systems, and its neural correlates. Then, we show how stress, through the action of its biochemical mediators, influences memory systems and mnemonic processes. Finally, we make use of major depressive disorder to explore the possible implications of non-adaptive interactions between stress and memory to psychiatric disorders, as well as possible roles for memory studies in the field of psychiatry.

Impact of chronic variable stress on neuroendocrine hypothalamus and pituitary in male and female C57BL/6J mice

Chronic stress exerts multiple negative effects on the physiology and health of an individual. In the present study, we examined hypothalamic, pituitary and endocrine responses to 14 days of chronic variable stress (CVS) in male and female C57BL/6J mice. In both sexes, CVS induced a significant decrease in body weight and enhanced the acute corticosterone stress response, which was accompanied by a reduction in thymus weight only in females. However, single-point blood measurements of basal prolactin, thyroid-stimulating hormone, luteinising hormone, growth hormone and corticosterone levels taken at the end of the CVS were not different from those of controls. Similarly, pituitary mRNA expression of Fshb, Lhb, Prl and Gh was unchanged by CVS, although Pomc and Tsh were significantly elevated. Within the adrenal medulla, mRNA for Th, Vip and Gal were elevated following CVS. Avp transcript levels within the paraventricular nucleus of the hypothalamus were increased by CVS; however, levels of Gnrh1, Crh, Oxt, Sst, Trh, Ghrh, Th and Kiss1 remained unchanged. Oestrous cycles were lengthened slightly by CVS and ovarian histology revealed a reduction in the number of preovulatory follicles and corpora lutea. Taken together, these observations indicate that 14 days of CVS induces an up-regulation of the neuroendocrine stress axis and creates a mild disruption of female reproductive function. However, the lack of changes in other neuroendocrine axes controlling anterior and posterior pituitary secretion suggest that most neuroendocrine axes are relatively resilient to CVS.

Neurogenic hypothesis of positive psychology in stress-induced depression: Adult hippocampal neurogenesis, neuroinflammation, and stress resilience

Stress is an important risk factor for depression. Emerging evidence supports the hypothesis that stress-mediated neuroinflammation destroys brain function and leads to anxiety-like and depression-like behaviors. Previous studies of stress-induced depression have mainly focused on pathological damage; however, the rise of positive psychology has attracted the interest of many researchers in environmental enrichment to promote stress resilience. The hippocampus is one of the most severely damaged brain regions in stress-induced depression. In addition, the hippocampus is one of the most unique regions in the brain, as new neurons are produced in the adult hippocampus, a process known as adult hippocampal neurogenesis (AHN). AHN is an important core component of the neurogenic hypothesis and has also become a major innovative breakthrough in positive psychology, in which environmental enrichment mediates stress resilience. Neuroinflammation, by activating microglia and releasing some proinflammatory cytokines, is increasingly shown to be one of the key determinant pathophysiological factors that negatively affects AHNand cognitive reserve. AHN is mainly related to remodeling stress response mechanisms, such as memory clearing, emotional control, and pattern separation, suggesting that a correlation may exist between neuroinflammation and AHN in stress resilience. Therefore, we summarized the previous research results to systematically expound on the relationship between AHN, stress resilience, and neuroinflammation. We hope this neurogenic hypothesis of positive psychology in stress-induced depression will provide a new perspective for the study of depression and antidepressant therapy.

Long term effects of peripubertal stress on excitatory and inhibitory circuits in the prefrontal cortex of male and female mice

The impact of stressful events is especially important during early life, because certain cortical regions, especially the prefrontal cortex (PFC), are still developing. Consequently, aversive experiences that occur during the peripubertal period can cause long-term alterations in neural connectivity, physiology and related behaviors. Although sex influences the stress response and women are more likely to develop stress-related psychiatric disorders, knowledge about the effects of stress on females is still limited. In order to analyze the long-term effects of peripubertal stress (PPS) on the excitatory and inhibitory circuitry of the adult PFC, and whether these effects are sex-dependent, we applied an unpredictable chronic PPS protocol based on psychogenic stressors. Using two strains of transgenic mice with specific fluorescent cell reporters, we studied male and diestrus females to know how PPS affects the structure and connectivity of parvalbumin expressing (PV+) interneurons and pyramidal neurons. We also studied the expression of molecules related to excitatory and inhibitory neurotransmission, as well as alterations in the expression of plasticity-related molecules. The structure of pyramidal neurons was differentially affected by PPS in male and female mice: while the former had a decreased dendritic spine density, the latter displayed an increase in this parameter. PPS affected the density of puncta expressing excitatory and inhibitory synaptic markers exclusively in the female mPFC. Similarly, only in female mice we observed an increased complexity of the dendritic tree of PV+ neurons. Regarding the perisomatic innervation on pyramidal and PV + neurons by basket cells, we found a significant increase in the density of puncta in stressed animals, with interesting differences between the sexes and the type of basket cell analyzed. Finally, the PPS protocol also altered the total number of somata expressing the polysialylated form of the neural cell adhesion molecule (PSA-NCAM) when we analyzed both sexes together. These results highlight the strong programming effects of aversive experiences during early life for the establishment of cortical circuitry and the special impact of these stressful events on females.

The stressed orbitofrontal cortex

Stressor exposure causes dendritic remodeling on excitatory neurons in multiple regions of the brain, including the orbitofrontal cortex (OFC). Additionally, stressor and exogenous stress hormone exposure impair cognitive functions that are dependent on the OFC. For this Special Issue on the OFC, we summarize current literature regarding how stress-prenatal, postnatal, and even inter-generational-affects OFC neuron structure in rodents. We discuss dendrite structure, dendritic spines, and gene expression. We aim to provide a focused resource for those interested in how stressors impact this heterogeneous brain region. (PsycInfo Database Record (c) 2021 APA, all rights reserved).

Repeated victorious and defeat experiences induce similar apical dendritic spine remodeling in CA1 hippocampus of rats

In this study, apical dendritic spine density of neurons in hippocampal, amygdalar and prefrontal cortical areas was compared in rats that were repeatedly winning or losing social conflicts. Territorial male wild-type Groningen (WTG) rats were allowed multiple daily attacks (>20 times) on intruder males in the resident-intruder paradigm. Frequent winning experiences are known to facilitate uncontrolled aggressive behavior reflected in aggressive attacks on anesthetized males which was also observed in the winners in this study. Both winners and losers were socially housed during the experiments; winners with females to stimulate territorial behavior, and losers with two other losing male rats. Twenty-four hours after the last social encounter, brains from experienced residential winners and repeatedly defeated intruder rats were collected and neuronal morphology in selected brain regions was studied via Golgi-Cox staining. Results indicate that spine density in the apical dendrites of the hippocampal CA1 reduced similarly in both winners and losers. In addition, winners showed increased spine densities at the proximal segments (20-30 μm) of the basolateral amygdala neurons and losers tended to show a decreased spine density at the more proximal segments of the infralimbic region of prefrontal cortex neurons. No effect of winning and losing was observed in the medial amygdala. The atrophic effect of repeated defeats in hippocampal and prefrontal regions was anticipated despite the fact that social housing of the repeatedly losing intruder males may have played a protective role. The reduction of hippocampal spine density in the winners seems surprising but supports previous findings in hierarchical dominant males in rat colonies. The dominants showed even greater shrinkage of the apical dendritic arbors of hippocampal CA3 pyramidal neurons compared to the stressed subordinates.

Adolescent stress during, but not after, pubertal onset impairs indices of prepulse inhibition in adult rats

Exposure to stress during adolescence is a risk factor for developing several psychiatric disorders, many of which involve prefrontal cortex (PFC) dysfunction. The human PFC and analogous rodent medial prefrontal cortex (mPFC) continue to mature functionally and anatomically during adolescence, and some of these maturational events coincide with pubertal onset. As developing brain regions are more susceptible to the negative effects of stress, this may make puberty especially vulnerable. To test this, we exposed male and female rats to isolation and restraint stress during the onset of puberty or during the post-pubertal period of adolescence. In young adulthood, both stressed groups and an unstressed control group underwent testing on a battery of tasks to assess emotional and cognitive behaviors, and the volume of the mPFC was quantified postmortem. Factor analysis revealed only subjects stressed peri-pubertally showed a long-term deficiency compared to controls in prepulse inhibition. Additionally, both sexes showed volumetric mPFC decreases following adolescent stress, and these losses were most pronounced in females. Our findings suggest that pubertal onset may be a vulnerable window wherein adolescents are most susceptible to the negative consequences of stress exposure. Furthermore, it highlights the importance of accounting for pubertal status when studying adolescents.

How stress physically re-shapes the brain: Impact on brain cell shapes, numbers and connections in psychiatric disorders

Severe stress is among the most robust risk factors for the development of psychiatric disorders. Imaging studies indicate that life stress is integral to shaping the human brain, especially regions involved in processing the stress response. Although this is likely underpinned by changes to the cytoarchitecture of cellular networks in the brain, we are yet to clearly understand how these define a role for stress in human psychopathology. In this review, we consolidate evidence of macro-structural morphometric changes and the cellular mechanisms that likely underlie them. Focusing on stress-sensitive regions of the brain, we illustrate how stress throughout life may lead to persistent remodelling of the both neurons and glia in cellular networks and how these may lead to psychopathology. We support that greater translation of cellular alterations to human cohorts will support parsing the psychological sequalae of severe stress and improve our understanding of how stress shapes the human brain. This will remain a critical step for improving treatment interventions and prevention outcomes.

Impact of subchronic variable stress on ovariectomy and dendritic spine density in prefrontal cortex in mice

Major depressive disorder affects both women and men but females are two times more susceptible to the incidence of depression. The majority of stress models used male rodents, whereas fewer studies included females. Volumetric reductions have been reported in brain areas critical for the stress response, such as prefrontal cortex, and remodeling of dendritic spines has been proposed as an underlying factor. The aim of this study was to determine the effects of subchronic variable stress (SCVS) in males, sham, and specifically in ovariectomized (OVX) female mice. Here, we used 6 days SCVS model to induce depressive-like behavior, and only the OVX female mice showed the depressive-like behavior, while males showed resilient type behavior. Only OVX female mice showed significant increase in dendritic spine density in SCVS paradigm. Overall, this study suggests that (1) the effects SCVS produced on the behavior of males and OVX females and (2) SCVS may induce rapid and sustained changes of PL neurons, which highlights the importance of gonadal hormones in studying depression.

Hurricane Irma induces divergent behavioral and hormonal impacts on an urban and forest population of invasive Anolis lizards: evidence for an urban resilience hypothesis

Hurricanes can have both profound short-term effects on animal populations and serve as long-term drivers of evolutionary change. Animals inhabiting varying habitats may differ in their response to hurricane impacts. Increasing evidence suggests that animals from urban areas exhibit different behavioral and physiological traits compared to rural counterparts, including attenuated hormonal stress responses and a lowered propensity for flight behavior. A unique opportunity was presented when Hurricane Irma hit Florida on 10 September 2017 and interrupted a study of invasive brown anoles (Anolis sagrei) at an urban and a forest. Using data collected before and after Hurricane Irma, we documented that forest anoles exhibited a greater avoidance of people and more male territorial behavior for a longer period of time following the hurricane. Post-hurricane both populations increased corticosterone concentrations post-capture stress, but urban anoles recovered 2 weeks faster than forest conspecifics. A dexamethasone suppression experiment suggested that these population differences were the result of forest anoles having a less effective negative feedback regulating corticosterone secretion. In the brain, forest anoles had higher corticosterone concentrations within the amygdala and parts of the cortex associated with stress than urban lizards. One explanation may be Hurricane Irma brought flooding and debris that altered the landscape leading to behavioral instability, and urban lizards already exhibited ecological adjustments that permitted a more rapid recovery (i.e. the ‘urban resilience’ hypothesis). Testing if urban animals are more resilient to natural disasters can inform conservationists interested in understanding their role in facilitating invasive species expansion and what their increasing presence may indicate for animal populations.

Erxian decoction, a famous Chinese medicine formula, antagonizes corticosterone-induced injury in PC12 cells, and improves depression-like behaviours in mice

Context: In folk medicine, erxian decoction (EXD) is used to treat perimenopausal syndrome in women. It is also used clinically to treat depression, but the mechanism remains unknown.Objectives: To investigate the neuroprotective effect of EXD, and its antidepressant potential.Materials and methods: ICR mice were treated with EXD (0.5, 1.5 and 4.5 g/kg i.g.) and fluoxetine (6.0 mg/kg i.g.) for 10 days. On day 10 of the treatment, depression-like behaviour was induced by reserpine (2.5 mg/kg injected i.p.), and after 24 h of reserpine administration, it was assessed using the tail suspension and forced swimming tests. MTT assay, lactate dehydrogenase test, flow cytometry analysis, Hoechst staining and western blotting were used to assess the apoptosis of PC12 cells. Apoptosis proteins and neurotransmitter were tested in vitro and in vivo, respectively.Results: MTT assay results showed corticosterone prevented cell growth, but EXD at concentrations of 100, 200 and 400 μg/mL restored cell viability (EC₅₀: 204.016 μg/mL). EXD decreased lactate dehydrogenase leakage from 63.48 to 43.60 U/L, and upregulated expression of Bcl-2 while the expression of Bax, caspase-3 and caspase-8 were decreased in vivo and in vitro. Moreover, EXD improved depression-like behaviour in mice, and 4.5 g/kg EXD treatment increased the secretion of serotonin, dopamine and norepinephrine by 67.44, 28.12 and 42.12 pg/mg, respectively, in hypothalamus compared to that of reserpine group.Discussion and conclusions: EXD demonstrated neuroprotective effects and improved depression-like behaviour in mice. Further research should be focussed on the mechanism of the active components in EXD.

Severe childhood and adulthood stress associates with neocortical layer-specific reductions of mature spines in psychiatric disorders

Severe stress exposure causes the loss of dendritic spines on cortical pyramidal neurons and induces psychiatric-like symptoms in rodent models. These effects are strongest following early-life stress and are most persistent on apical dendrites. However, the long-term impacts and temporal effects of stress exposure on the human brain remain poorly understood. Using a novel postmortem cohort of psychiatric cases with severe stress experienced in childhood, adulthood, or no severe stress, and matched controls, we aimed to determine the impact of stress timing on pyramidal neuron structure in the human orbitofrontal cortex (OFC). We performed Golgi Cox staining and manually measured the morphology and density of over 22,000 dendritic spines on layer-specific pyramidal neuron apical dendrites. We also quantified glucocorticoid receptor mRNA and protein as a marker of stress dysregulation. Both childhood and adulthood stress were associated with large reductions in mature mushroom spine density (up to 56% loss) in both the superficial (II/III) and deeper layers (V) of the OFC. However, childhood stress caused more substantial reductions to both total and mature mushroom spines. No difference in glucocorticoid receptor mRNA and protein were seen between groups, although both negatively correlated with total spine density within the whole cohort. These findings indicate that severe stress, especially when experienced during childhood, persistently affects the fine morphological properties of neurons in the human OFC. This may impact on cell connectivity in this brain area, and at least partly explain the social and emotional symptoms that originate in the OFC in psychiatric disorders.

Morphological Responses of Excitatory Prelimbic and Orbitofrontal Cortical Neurons to Excess Corticosterone in Adolescence and Acute Stress in Adulthood

Considerable evidence indicates that chronic stress and excess glucocorticoids induce neuronal remodeling in prefrontal cortical (PFC) regions. Adolescence is also characterized by a structural reorganization of PFC neurons, yet interactions between stress- and age-related structural plasticity are still being determined. We quantified dendritic spine densities on apical dendrites of excitatory neurons in the medial prefrontal cortex, prelimbic subregion (PL). Densities decreased across adolescent development, as expected, and spine volume increased. Unexpectedly, exposure to excess corticosterone (CORT) throughout adolescence did not cause additional dendritic spine loss detectable in adulthood. As a positive control dendrite population expected to be sensitive to CORT, we imaged neurons in the orbitofrontal cortex (OFC), confirming CORT-induced dendritic spine attrition on basal arbors of layer V neurons. We next assessed the effects of acute, mild stress in adulthood: On PL neurons, an acute stressor increased the density of mature, mushroom-shaped spines. Meanwhile, on OFC neurons, dendritic spine volumes and lengths were lower in mice exposed to both CORT and an acute stressor (also referred to as a "double hit"). In sum, prolonged exposure to excess glucocorticoids during adolescence can have morphological and also metaplastic consequences, but they are not global. Anatomical considerations are discussed.

Infrared Thermography Reveals Sex-Specific Responses to Stress in Mice

Psychogenic hyperthermia is a stress-related condition reported mostly in women. Neuroendocrine responses to stress in females differ from those in males, and these differences cannot be explained solely based on hypothalamic-pituitary-adrenal (HPA) axis activity. Here, we used infrared (IR) thermographic imaging to record changes in cutaneous temperature following two types of stressful experiences in female and male mice. Mice were exposed to either single-session restraint stress or vertical exploration (rearing) deprivation and were monitored for exploratory activity and IR surface thermal changes. Females displayed higher rearing activity than males during the dark phase of the light cycle. Both sexes showed similar plasma corticosterone (CORT) responses after a challenge with restraint and rearing deprivation. However, only females responded to rearing deprivation with increased cutaneous temperature in the head and back, and a reduced thermal response in the tail. Circulating CORT levels were not correlated with the thermal variations. These findings, for the first time, provide evidence for sex-specific cutaneous thermal responses to short-term stress in mice following transient vertical-activity deprivation that may mimic clinical psychogenic hyperthermia.

Evidence for Similar Prefrontal Structural and Functional Alterations in Male and Female Rats Following Chronic Stress or Glucocorticoid Exposure

Previous work of ours and others has documented regressive changes in neuronal architecture and function in the medial prefrontal cortex (mPFC) of male rats following chronic stress. As recent focus has shifted toward understanding whether chronic stress effects on mPFC are sexually dimorphic, here we undertake a comprehensive analysis to address this issue. First, we show that chronic variable stress (14-day daily exposure to different challenges) resulted in a comparable degree of adrenocortical hyperactivity, working memory impairment, and dendritic spine loss in mPFC pyramidal neurons in both sexes. Next, exposure of female rats to 21-day regimen of corticosterone resulted in a similar pattern of mPFC dendritic spine attrition and increase in spine volume. Finally, we examined the effects of another widely used regimen, chronic restraint stress (CRS, 21-day of daily 6-h restraint), on dendritic spine changes in mPFC in both sexes. CRS resulted in response decrements in adrenocortical output (habituation), and induced a pattern of consistent, but less widespread, dendritic spine loss similar to the foregoing challenges. Our data suggest that chronic stress or glucocorticoid exposure induces a relatively undifferentiated pattern of structural and functional alterations in mPFC in both males and females.

The hypothalamic-pituitary-adrenal axis as a substrate for stress resilience: Interactions with the circadian clock

Stress, primarily processed via the hypothalamic-pituitary-adrenal (HPA) axis, engages biological pathways throughout the brain and body which promote adaptation and survival to changing environmental demands. Adaptation to environmental challenges is compromised when these pathways are no longer functioning optimally. The physiological and behavioral mechanisms through which HPA axis function influences stress adaptation and resilience are not fully elucidated. Our understanding of stress biology and disease must take into account the complex interactions between the endocrine system, neural circuits, and behavioral coping strategies. In addition, further consideration must be taken concerning influences of other aspects of physiology, including the circadian clock which is critical for regulation of daily changes in HPA activity. While adding a layer of complexity, it also offers targets for intervention. Understanding the role of HPA function in mediating these diverse biological responses will lead to important insights about how to bolster successful stress adaptation and promote stress resilience.

Effects of stress on the structure and function of the medial prefrontal cortex: Insights from animal models

Stress alters both cognitive and emotional function, and increases risk for a variety of psychological disorders, such as depression and posttraumatic stress disorder. The prefrontal cortex is critical for executive function and emotion regulation, is a target for stress hormones, and is implicated in many stress-influenced psychological disorders. Therefore, understanding how stress-induced changes in the structure and function of the prefrontal cortex are related to stress-induced changes in behavior may elucidate some of the mechanisms contributing to stress-sensitive disorders. This review focuses on data from rodent models to describe the effects of chronic stress on behaviors mediated by the medial prefrontal cortex, the effects of chronic stress on the morphology and physiology of the medial prefrontal cortex, mechanisms that may mediate these effects, and evidence for sex differences in the effects of stress on the prefrontal cortex. Understanding how stress influences prefrontal cortex and behaviors mediated by it, as well as sex differences in this effect, will elucidate potential avenues for novel interventions for stress-sensitive disorders characterized by deficits in executive function and emotion regulation.

Glucocorticoid-sensitive ventral hippocampal-orbitofrontal cortical connections support goal-directed action - Curt Richter Award Paper 2019

In an ever-changing and often ambiguous environment, organisms must use previously learned associations between antecedents and outcomes to predict future associations and make optimal choices. Chronic stress can impair one's ability to flexibly adjust behaviors when environmental contingencies change, particularly in cases of early-life stress. In mice, exposure to elevated levels of the primary stress hormone, corticosterone (CORT), during early adolescence is sufficient to impair response-outcome decision making later in life, biasing response strategies towards inflexible habits. Nevertheless, neurobiological mechanisms are still being defined. Here, we report that exposure to excess CORT in adolescence causes a loss of dendritic spines on excitatory pyramidal neurons in the lateral, but not medial, orbital prefrontal cortex (loPFC) of mice, and spine loss correlates with the severity of habit biases in adulthood. Excess CORT also reduces the presence of ventral hippocampal (vHC) axon terminals in the loPFC. To identify functional consequences, we inactivated vHC→loPFC projections in typical healthy mice during a period when mice must update response-outcome expectations to optimally acquire food reinforcers. Inactivation impaired the animals' subsequent ability to sustainably choose actions based on likely outcomes, causing them to defer to habit-based response strategies. Thus, vHC→loPFC projections are necessary for response-outcome expectancy updating and a target of excess glucocorticoids during early-life development. Their degradation is likely involved in long-term biases towards habit-based behaviors following glucocorticoid excess in adolescence.

Targeting Kruppel-like Factor 9 in Excitatory Neurons Protects against Chronic Stress-Induced Impairments in Dendritic Spines and Fear Responses

Stress exposure is associated with the pathogenesis of psychiatric disorders, including post-traumatic stress disorder (PTSD) and major depressive disorder (MDD). Here, we show in rodents that chronic stress exposure rapidly and transiently elevates hippocampal expression of Kruppel-like factor 9 (Klf9). Inducible genetic silencing of Klf9 expression in excitatory forebrain neurons in adulthood prior to, but not after, onset of stressor prevented chronic restraint stress (CRS)-induced potentiation of contextual fear acquisition in female mice and chronic corticosterone (CORT) exposure-induced fear generalization in male mice. Klf9 silencing prevented chronic CORT and CRS induced enlargement of dendritic spines in the ventral hippocampus of male and female mice, respectively. KLF9 mRNA density was increased in the anterior dentate gyrus of women, but not men, with more severe recent stressful life events and increased mortality. Thus, Klf9 functions as a stress-responsive transcription factor that mediates circuit and behavioral resilience in a sex-specific manner.

Social Isolation in Adolescence Disrupts Cortical Development and Goal-Dependent Decision-Making in Adulthood, Despite Social Reintegration

The social environment influences neurodevelopment. Investigations using rodents to study this phenomenon commonly isolate subjects, then assess neurobehavioral consequences while animals are still isolated. This approach precludes one from dissociating the effects of on-going versus prior isolation, hindering our complete understanding of the consequences of social experience during particular developmental periods. Here, we socially isolated adolescent mice from postnatal day (P)31 to P60, then re-housed them into social groups. We tested their ability to select actions based on expected outcomes using multiple reinforcer devaluation and instrumental contingency degradation techniques. Social isolation in adolescence (but not adulthood) weakened instrumental response updating, causing mice to defer to habit-like behaviors. Habit biases were associated with glucocorticoid insufficiency in adolescence, oligodendrocyte marker loss throughout cortico-striatal regions, and dendritic spine and synaptic marker excess in the adult orbitofrontal cortex (OFC). Artificial, chemogenetic stimulation of the ventrolateral OFC in typical, healthy mice recapitulated response biases following isolation, causing habit-like behaviors. Meanwhile, correcting dendritic architecture by inhibiting the cytoskeletal regulatory protein ROCK remedied instrumental response updating defects in socially isolated mice. Our findings suggest that adolescence is a critical period during which social experience optimizes one's ability to seek and attain goals later in life. Age-typical dendritic spine elimination appears to be an essential factor, and in its absence, organisms may defer to habit-based behaviors.

β1-Integrins in the Developing Orbitofrontal Cortex Are Necessary for Expectancy Updating in Mice

Navigating a changing environment requires associating stimuli and actions with their likely outcomes and modifying these associations when they change. These processes involve the orbitofrontal cortex (OFC). Although some molecular mediators have been identified, developmental factors are virtually unknown. We hypothesized that the cell adhesion factor β1-integrin is essential to OFC function, anticipating developmental windows during which β1-integrins might be more influential than others. We discovered that OFC-selective β1-integrin silencing before adolescence, but not later, impaired the ability of mice to extinguish conditioned fear and select actions based on their likely outcomes. Early-life knock-down also reduced the densities of dendritic spines, the primary sites of excitatory plasticity in the brain, and weakened sensitivity to cortical inputs. Notwithstanding these defects in male mice, females were resilient to OFC (but not hippocampal) β1-integrin loss. Existing literature suggests that resilience may be explained by estradiol-mediated transactivation of β1-integrins and tropomyosin receptor kinase B (trkB). Accordingly, we discovered that a trkB agonist administered during adolescence corrected reward-related decision making in β1-integrin-deficient males. In sum, developmental β1-integrins are indispensable for OFC function later in life.SIGNIFICANCE STATEMENT The orbitofrontal cortex (OFC) is a subregion of the frontal cortex that allows organisms to link behaviors and stimuli with anticipated outcomes, and to make predictions about the consequences of one's behavior. Aspects of OFC development are particularly prolonged, extending well into adolescence, likely optimizing organisms' abilities to prospectively calculate the consequences of their actions and select behaviors appropriately; these decision making strategies improve as young individuals mature into adulthood. Molecular factors are not, however, well understood. Our experiments reveal that a cell adhesion protein termed "β1-integrin" is necessary for OFC neuronal maturation and function. Importantly, β1-integrins operate during a critical period equivalent to early adolescence in humans to optimize the ability of organisms to update expectancies later in life.

Stress-elicited glucocorticoid receptor signaling upregulates TIGIT in innate-like invariant T lymphocytes

Stress is known to impede certain host defense mechanisms, including those governed by conventional T lymphocytes. However, whether innate-like T lymphocytes, such as invariant natural killer T (iNKT) and mucosa-associated invariant T (MAIT) cells, are impacted by stress is unclear. Herein, we report that prolonged psychological stress caused by physical confinement results in robust upregulation of T cell immunoreceptor with immunoglobulin and ITIM domains (TIGIT), an immune checkpoint receptor that controls antitumor and antiviral immune responses. Elevated TIGIT expression was found not only on NK and conventional T cells, but also on iNKT and MAIT cells. Stress-provoked TIGIT upregulation was reversed through treatment with the glucocorticoid receptor (GR) antagonist RU486, but not with 6-hydroxydopamine that induces chemical sympathectomy. A Cre/Lox gene targeting model in which GR was ablated in cells expressing Lck under its proximal promoter revealed that TIGIT upregulation in stressed animals stems from direct GR signaling in T and iNKT cells. In fact, long-term oral administration of exogenous corticosterone (CS) to wild-type C57BL/6 (B6) mice was sufficient to increase TIGIT expression levels on T and iNKT cells. In vitro treatment with CS also potently and selectively upregulated TIGIT, but not CTLA-4 or LAG-3, on mouse iNKT and MAIT hybridomas. These results were recapitulated using primary hepatic iNKT and MAIT cells from wild-type B6 and B6.MAIT^CAST mice, respectively. Subjecting B6.MAIT^CAST mice to physical restraint also raised the frequency of TIGIT⁺ cells among hepatic MAIT cells in a GR-dependent manner. Finally, we found that TIGIT is similarly upregulated in a chronic variable stress model in which animals are exposed to unpredictable heterotypic stressors without developing habituation. Taken together, our findings link, for the first time to our knowledge, GR signaling to TIGIT expression. We propose that glucocorticoid hormones dampen immune responses, in part, by enhancing TIGIT expression across multiple critical subsets of effector lymphocytes, including innate-like T cells. Therefore, TIGIT may constitute an attractive target in immune-enhancing interventions for sustained physiological stress.

Stressful life events and cognitive decline: Sex differences in the Baltimore Epidemiologic Catchment Area Follow-Up Study

INTRODUCTION

The reasons why women are at higher risk than men for developing dementia are unclear. Although studies implicate sex differences in the effect of stress on cognitive functioning, whether stressful life events are associated with subsequent cognitive decline has received scant research attention.

METHODS

In Wave 3 (1993-1996) of the Baltimore Epidemiologic Catchment Area study, 337 men and 572 women (mean age = 47 years) reported recent (within the last year) and remote (from 1981 until 1 year ago) traumatic events (eg, combat) and stressful life events (eg, divorce/separation). At Waves 3 and 4 (2004-2005), they completed the Mini Mental State Examination (MMSE) and a word-list memory test. Multivariable models were used to examine the association between traumatic and stressful life events at Wave 3 and cognitive change by Wave 4.

RESULTS

A greater number of recent stressful life events at Wave 3, but not of more remote stressful events, was associated with greater verbal memory decline by Wave 4 in women but not in men. Stressful events were not associated with change in MMSE, and there were no associations between traumatic events occurring at any time and subsequent memory or MMSE decline in either sex.

CONCLUSIONS

Unlike men, middle-aged women with a greater number of recent stressful life events demonstrate memory decline over a decade later. Sex differences in cognitive vulnerability to stressful life events may underlie women's increased risk of memory impairment in late life, suggesting that stress reduction interventions may help prevent cognitive decline in women.

Reward-Related Expectations Trigger Dendritic Spine Plasticity in the Mouse Ventrolateral Orbitofrontal Cortex

An essential aspect of goal-directed decision-making is selecting actions based on anticipated consequences, a process that involves the orbitofrontal cortex (OFC) and potentially, the plasticity of dendritic spines in this region. To investigate this possibility, we trained male and female mice to nose poke for food reinforcers, or we delivered the same number of food reinforcers non-contingently to separate mice. We then decreased the likelihood of reinforcement for trained mice, requiring them to modify action-outcome expectations. In a separate experiment, we blocked action-outcome updating via chemogenetic inactivation of the OFC. In both cases, successfully selecting actions based on their likely consequences was associated with fewer immature, thin-shaped dendritic spines and a greater proportion of mature, mushroom-shaped spines in the ventrolateral OFC. This pattern was distinct from spine loss associated with aging, and we identified no effects on hippocampal CA1 neurons. Given that the OFC is involved in prospective calculations of likely outcomes, even when they are not observable, constraining spinogenesis while preserving mature spines may be important for solidifying durable expectations. To investigate causal relationships, we inhibited the RNA-binding protein fragile X mental retardation protein (encoded by Fmr1), which constrains dendritic spine turnover. Ventrolateral OFC-selective Fmr1 knockdown recapitulated the behavioral effects of inducible OFC inactivation (and lesions; also shown here), impairing action-outcome conditioning, and caused dendritic spine excess. Our findings suggest that a proper balance of dendritic spine plasticity within the OFC is necessary for one's ability to select actions based on anticipated consequences.SIGNIFICANCE STATEMENT Navigating a changing environment requires associating actions with their likely outcomes and updating these associations when they change. Dendritic spine plasticity is likely involved, yet relationships are unconfirmed. Using behavioral, chemogenetic, and viral-mediated gene silencing strategies and high-resolution microscopy, we find that modifying action-outcome expectations is associated with fewer immature spines and a greater proportion of mature spines in the ventrolateral orbitofrontal cortex (OFC). Given that the OFC is involved in prospectively calculating the likely outcomes of one's behavior, even when they are not observable, constraining spinogenesis while preserving mature spines may be important for maintaining durable expectations.

Transcriptome profiles of corticosterone-induced cytotoxicity reveals the involvement of neurite growth-related genes in depression

Corticosterone (CORT), the main HPA-axis glucocorticoid hormone in rodents, is involved in the regulation of animal stress responses. However, the neural mechanisms underlying the effects of corticosteroids on depression are yet to be elucidated. We found that fluoxetine reversed neurite growth inhibition induced by CORT in PC12 cells, a widely used model system for neurobiological and neurotoxicological studies. Transcriptome profiling showed that 1,609 genes were up-regulated, whereas 1,764 genes were down-regulated significantly in the CORT group in comparison with the Control group. Of them, the expression of 589 DEGs was reversed after fluoxetine treatment, and genes related to cell morphogenesis, neurite growth, and immune function were involved in the neuroprotective effect of fluoxetine against CORT. Furthermore, expression of neurite growth-related genes, such as such as Calpain 2 (Capn2), vesicle-associated membrane protein 7 (Vamp7) and C-type natriuretic peptide (Cnp), altered in a brain region- or treatment-specific manner in the animal models of depression. Therefore, the interaction between stress, glucocorticoids, and neurite growth inhibition may be a candidate pathophysiology underlying major depressive disorder (MDD), and the identification of Capn2, Vamp7 and Cnp might provide insight into treatment of MDD.

Resilience Is Associated With Larger Dentate Gyrus, While Suicide Decedents With Major Depressive Disorder Have Fewer Granule Neurons

BACKGROUND

Early life adversity (ELA) increases major depressive disorder (MDD) and suicide risk and potentially affects dentate gyrus (DG) plasticity. We reported smaller DG and fewer granular neurons (GNs) in MDD. ELA effects on DG plasticity in suicide decedents with MDD (MDDSui) and resilient subjects (ELA history without MDD or suicide) are unknown.

METHODS

We quantified neural progenitor cells (NPCs), GNs, glia, and DG volume in whole hippocampus postmortem in four groups of drug-free, neuropathology-free subjects (N = 52 total): psychological autopsy-defined MDDSui and control subjects with and without ELA (before 15 years of age).

RESULTS

ELA was associated with larger DG (p < .0001) and trending fewer NPCs (p = .0190) only in control subjects in whole DG, showing no effect on NPCs and DG volume in MDDSui. ELA exposure was associated with more GNs (p = .0003) and a trend for more glia (p = .0160) in whole DG in MDDSui and control subjects. MDDSui without ELA had fewer anterior and mid DG GNs (p < .0001), fewer anterior DG NPCs (p < .0001), and smaller whole DG volume (p = .0005) compared with control subjects without ELA. In MDDSui, lower Global Assessment Scale score correlated with fewer GNs and smaller DG.

CONCLUSIONS

Resilience to ELA involves a larger DG, perhaps related to more neurogenesis depleting NPCs, and because mature GNs and glia numbers do not differ in the resilient group, perhaps there are effects on process extension and synaptic load that can be examined in future studies. In MDDSui without ELA, smaller DG volume, with fewer GNs and NPCs, suggests less neurogenesis and/or more apoptosis and dendrite changes.

Sustained rescue of prefrontal circuit dysfunction by antidepressant-induced spine formation

The neurobiological mechanisms underlying the induction and remission of depressive episodes over time are not well understood. Through repeated longitudinal imaging of medial prefrontal microcircuits in the living brain, we found that prefrontal spinogenesis plays a critical role in sustaining specific antidepressant behavioral effects and maintaining long-term behavioral remission. Depression-related behavior was associated with targeted, branch-specific elimination of postsynaptic dendritic spines on prefrontal projection neurons. Antidepressant-dose ketamine reversed these effects by selectively rescuing eliminated spines and restoring coordinated activity in multicellular ensembles that predict motivated escape behavior. Prefrontal spinogenesis was required for the long-term maintenance of antidepressant effects on motivated escape behavior but not for their initial induction.

Glucocorticoids and resilience

All organisms endure frequent challenges to homeostasis, or stressors, that require adaptation. Depending on the individual, the context, and the magnitude of stress, this active adaptation can lead to behavioral susceptibility or resilience. The latter is an under-appreciated consequence of stress, as the damaging effects of chronic stress and chronically elevated glucocorticoids have received much more attention. We suggest here that neural pathways promoting resilience are initiated at the time of stress, and that they involve glucocorticoid signaling. By focusing on the neurobiology of resilience induction and the identification of vulnerable individuals, we may be able to intervene in the future at the time of stress to promote positive adaptation.

p190RhoGAPs, the ARHGAP35- and ARHGAP5-Encoded Proteins, in Health and Disease

Small guanosine triphosphatases (GTPases) gathered in the Rat sarcoma (Ras) superfamily represent a large family of proteins involved in several key cellular mechanisms. Within the Ras superfamily, the Ras homolog (Rho) family is specialized in the regulation of actin cytoskeleton-based mechanisms. These proteins switch between an active and an inactive state, resulting in subsequent inhibiting or activating downstream signals, leading finally to regulation of actin-based processes. The On/Off status of Rho GTPases implicates two subsets of regulators: GEFs (guanine nucleotide exchange factors), which favor the active GTP (guanosine triphosphate) status of the GTPase and GAPs (GTPase activating proteins), which inhibit the GTPase by enhancing the GTP hydrolysis. In humans, the 20 identified Rho GTPases are regulated by over 70 GAP proteins suggesting a complex, but well-defined, spatio-temporal implication of these GAPs. Among the quite large number of RhoGAPs, we focus on p190RhoGAP, which is known as the main negative regulator of RhoA, but not exclusively. Two isoforms, p190A and p190B, are encoded by ARHGAP35 and ARHGAP5 genes, respectively. We describe here the function of each of these isoforms in physiological processes and sum up findings on their role in pathological conditions such as neurological disorders and cancers.

Reciprocal interactions across and within multiple levels of monoamine and cortico-limbic systems in stress-induced depression: A systematic review

The monoamine hypothesis of depression, namely that the reduction in synaptic serotonin and dopamine levels causes depression, has prevailed in past decades. However, clinical and preclinical studies have identified various cortical and subcortical regions whose altered neural activities also regulate depressive-like behaviors, independently from the monoamine system. Our systematic review indicates that neural activities of specific brain regions and associated neural circuitries are adaptively altered after chronic stress in a specific direction, such that the neural activity in the infralimbic cortex, lateral habenula and amygdala is upregulated, whereas the neural activity in the prelimbic cortex, hippocampus and monoamine systems is downregulated. The altered neural activity dynamics between monoamine systems and cortico-limbic systems are reciprocally interwoven at multiple levels. Furthermore, depressive-like behaviors can be experimentally reversed by counteracting the altered neural activity of a specific neural circuitry at multiple brain regions, suggesting the importance of the reciprocally interwoven neural networks in regulating depressive-like behaviors. These results promise for reshaping altered neural activity dynamics as a therapeutic strategy for treating depression.

Maturational Changes in Prefrontal and Amygdala Circuits in Adolescence: Implications for Understanding Fear Inhibition during a Vulnerable Period of Development

Anxiety disorders that develop in adolescence represent a significant burden and are particularly challenging to treat, due in no small part to the high occurrence of relapse in this age group following exposure therapy. This pattern of persistent fear is preserved across species; relative to those younger and older, adolescents consistently show poorer extinction, a key process underpinning exposure therapy. This suggests that the neural processes underlying fear extinction are temporarily but profoundly compromised during adolescence. The formation, retrieval, and modification of fear- and extinction-associated memories are regulated by a forebrain network consisting of the prefrontal cortex (PFC), the amygdala, and the hippocampus. These regions undergo robust maturational changes in early life, with unique alterations in structure and function occurring throughout adolescence. In this review, we focus primarily on two of these regions-the PFC and the amygdala-and discuss how changes in plasticity, synaptic transmission, inhibition/excitation, and connectivity (including modulation by hippocampal afferents to the PFC) may contribute to transient deficits in extinction retention. We end with a brief consideration of how exposure to stress during this adolescent window of vulnerability can permanently disrupt neurodevelopment, leading to lasting impairments in pathways of emotional regulation.

Corticosterone mediated functional and structural plasticity in corticotropin-releasing hormone neurons

Corticosteroid stress hormones drive a multitude of adaptations in the brain. Hypothalamic corticotropin-releasing hormone (CRH) neurons control the circulating levels of corticosteroid stress hormones in the body and are themselves highly sensitive to corticosteroids. CRH neurons have been shown to undergo various adaptions in response to acute stress hormone elevations. However, their structural and physiological changes under chronically elevated corticosterone are less clear. To address this, we determined the structural and functional changes in CRH neurons in the paraventricular nucleus of the hypothalamus following 14 days of corticosterone treatment. We find that prolonged corticosterone elevation reduces CRH neuron intrinsic excitability as measured by summation of subthreshold postsynaptic depolarisations and spiking output. We find that under normal conditions, CRH neurons have a relatively compact and simple dendritic arbor, with a low density of somatic and dendritic spines. Interestingly, the axon originated from a proximal dendrite close to the soma in approximately half of the CRH neurons reconstructed. While prolonged elevation in corticosterone levels did not result in any changes to gross dendritic morphology, it induced a significant reduction in both somatic and dendritic spine density. Together these data reveal the morphological features of hypothalamic CRH neurons and highlight their capacity to undergo functional and morphological plasticity in response to chronic corticosterone elevations. This article is part of the Special Issue entitled 'Hypothalamic Control of Homeostasis'.

Chronic Stress Remodels Synapses in an Amygdala Circuit-Specific Manner

BACKGROUND

Chronic stress exposure increases the risk of developing various neuropsychiatric illnesses. The behavioral sequelae of stress correlate with dendritic hypertrophy and glutamate-related synaptic remodeling at basolateral amygdala projection neurons (BLA PNs). Yet, though BLA PNs are functionally heterogeneous with diverse corticolimbic targets, it remains unclear whether stress differentially impacts specific output circuits.

METHODS

Confocal imaging was used to reconstruct the morphology of mouse BLA PNs with the aid of retrograde tracing and biocytin staining. The synaptic activity in these neurons was measured with in vitro electrophysiology, and anxiety-like behavior of the mice was assessed with the elevated plus maze and open field test.

RESULTS

Chronic restraint stress (CRS) produced dendritic hypertrophy across mouse BLA PNs, regardless of whether they did (BLA→dorsomedial prefrontal cortex [dmPFC]) or did not (BLA↛dmPFC) target dmPFC. However, CRS increased the size of dendritic spine heads and the number of mature, mushroom-shaped spines only in BLA↛dmPFC PNs, sparing neighboring BLA→dmPFC PNs. Moreover, the excitatory glutamatergic transmission was also selectively increased in BLA↛dmPFC PNs, and this effect correlated with CRS-induced increases in anxiety-like behavior. Segregating BLA↛dmPFC PNs based on their targeting of ventral hippocampus (BLA→ventral hippocampus) or nucleus accumbens (BLA→nucleus accumbens) revealed that CRS increased spine density and glutamatergic signaling in BLA→ventral hippocampus PNs in a manner that correlated with anxiety-like behavior.

CONCLUSIONS

Chronic stress caused BLA PN neuronal remodeling with a previously unrecognized degree of circuit specificity, offering new insight into the pathophysiological basis of depression, anxiety disorders, and other stress-related conditions.