Metyrapone reveals that previous chronic stress differentially impairs hippocampal-dependent memory

Effects of the combination of chronic unpredictable stress and environmental enrichment on anxiety-like behavior assessed using the elevated plus maze in Swiss male mice: Hypothalamus-Pituitary-Adrenal Axis-mediated mechanisms

Environmental enrichment (EE) is a paradigm that offers the animal a plethora of stimuli, including physical, cognitive, sensory, and social enrichment. Exposure to EE can modulate both anxiety responses and plasma corticosterone. In this study, our objective was to explore how chronic unpredictable stress (CUS) impacts anxiety-related behaviors in male Swiss mice raised in EE conditions. Additionally, we investigated corticosterone and adrenocorticotropic hormone (ACTH) levels to assess the involvement of the hypothalamic-pituitary-adrenal (HPA) axis in mediating these responses. Mice were housed under either EE or standard housing conditions for 21 days. Afterward, they were exposed to 11 days of CUS while still reared in their distinct housing conditions, with half of the mice receiving daily pretreatment with the vehicle and the other half receiving daily metyrapone (MET) injections, an inhibitor of steroid synthesis, 30 mins before CUS exposure. Blood samples were obtained to assess plasma corticosterone and ACTH levels. The 11-day CUS protocol induced anxiety-like phenotype and elevated ACTH levels in EE mice. Chronic MET pretreatment prevented anxiety-like behavior in the EE-CUS groups, by mechanisms involving increased plasma corticosterone levels and decreased ACTH. These results suggest a role of the HPA axis in the mechanism underlying the anxiogenic phenotype induced by CUS in EE mice and shed light on the complex interplay between environmental factors, stress, and the HPA axis in anxiety regulation.

Reduced cortical volume of the default mode network in adolescents with generalized anxiety disorder

BACKGROUND

Widespread structural alterations have been shown to be implicated in individuals with generalized anxiety disorder (GAD). However, there have been inconsistent findings in cortical volume (CV) differences. Most structural neuroimaging studies looking at GAD used region-based approach with relatively small sample sizes, let alone be specific to adolescents with GAD. We believe this is the first study to look at CV measures using a network-based approach in a larger sample of adolescents with GAD. The goal of the current study was to focus on three different brain networks (i.e., Limbic, Frontoparietal, and Default Mode Network [DMN]) in adolescents with GAD.

METHOD

The study involved 81 adolescents with GAD and 112 typically developing (TD) comparison individuals matched on age (15.98 and 15.63 respective means), sex (42F/39M and 45F/67M), and IQ (101.90 and 103.94 respective means). Participants underwent structural MRI. Freesurfer was used to estimate CV (both network-specific and region-specific within networks) and region-specific sub-cortical volume measures. Multivariate analysis of covariance (MANCOVA; with sex, age, IQ, and intracranial volume [ICV] as potential covariates) was used to estimate group differences.

RESULTS

We found significantly lower CV for the DMN in adolescents with GAD, compared with TD individuals. Adolescents with GAD also showed significantly lower hemispheric mean CV of the default-mode regions (particularly the prefrontal and temporal regions) and the hippocampus, compared with TD individuals.

CONCLUSION

The current findings suggest structural alterations in adolescents with GAD. These structural alterations will need to be addressed when implementing and developing treatments for patients with GAD.

Neurogenic Interventions for Fear Memory via Modulation of the Hippocampal Function and Neural Circuits

Fear memory helps animals and humans avoid harm from certain stimuli and coordinate adaptive behavior. However, excessive consolidation of fear memory, caused by the dysfunction of cellular mechanisms and neural circuits in the brain, is responsible for post-traumatic stress disorder and anxiety-related disorders. Dysregulation of specific brain regions and neural circuits, particularly the hippocampus, amygdala, and medial prefrontal cortex, have been demonstrated in patients with these disorders. These regions are involved in learning, memory, consolidation, and extinction. These are also the brain regions where new neurons are generated and are crucial for memory formation and integration. Therefore, these three brain regions and neural circuits have contributed greatly to studies on neural plasticity and structural remodeling in patients with psychiatric disorders. In this review, we provide an understanding of fear memory and its underlying cellular mechanisms and describe how neural circuits are involved in fear memory. Additionally, we discuss therapeutic interventions for these disorders based on their proneurogenic efficacy and the neural circuits involved in fear memory.

Cortisol and polycystic ovarian syndrome - a systematic search and meta-analysis of case-control studies

BACKGROUND

Polycystic ovarian syndrome (PCOS) has become an emerging disorder affecting women of reproductive age group. Its intricate presentation of signs and symptoms makes it a disease of interest to research. While there are varied hypotheses related to its cause and pathogenesis, role of stress in this disease is yet to be grounded. There is emerging body of evidence on cortisol and PCOS, although it is currently equivocal.

METHODS

Medline, Embase, Pubmed, Science Direct, Google Scholar, and Scopus were searched from March 1985 to March 2020 using MeSH terms. After dual quality assessments and data abstraction, the final articles were included for meta-analysis.

RESULTS

Forty-one studies qualified for the analysis. Pooled meta-analysis showed that the level of cortisol was significantly higher in PCOS when compared to healthy controls (standard mean difference [SMD] = 0.83, 95% confidence interval [CI] = 0.42-1.23) with highly significant heterogeneity (I² = 94%). Subgroup analysis done based on type of sample stated high effect size for blood cortisol levels (SMD = 0.9, 95%CI = 0.32; 1.51) compared to overall effect.

CONCLUSIONS

This systematic review and meta-analysis on cortisol and PCOS have helped in generating evidence regarding the role of cortisol in the pathogenesis of PCOS and the use of cortisol estimation as a potential stress marker in PCOS.

Chronic unpredictable intermittent restraint stress disrupts spatial memory in male, but not female rats

Chronic stress leads to sex-dependent outcomes on spatial memory by producing deficits in males, but not in females. Recently it was reported that compared to daily restraint, intermittent restraint (IR) produced more robust stress and anxiety responses in male rats. Whether IR would be sufficiently robust to impair hippocampal-dependent spatial memory in both male and female rats was investigated. IR involved mixing restraint with non-restraint days over weeks before assessing spatial memory and anxiety profile on the radial arm water maze, object placement, novel object recognition, Y-maze, open field and novelty suppressed feeding. Experiments 1 and 2 used Sprague-Dawley male rats only and determined that IR for 6 h/d (IR6), but not 2 h/d, impaired spatial memory and that task order was important. In experiment 3, IR6 was extended for 6wks before spatial memory testing commenced using both sexes. Unexpectedly, an extended IR6 paradigm failed to impair spatial memory in either sex, suggesting that by 6wks IR6 may have become predictable. In experiment 4, an unpredictable IR (UIR) paradigm was implemented, in which restraint duration (30 or 60-min) combined with orbital shaking, time of day, and the days off from UIR were varied. UIR impaired spatial memory in males, but not in females. Together with other reports, these findings support the interpretation that chronic stress negatively impairs hippocampal-dependent function in males, but not in females. We interpret these findings to show that females are more resilient to chronic stress than are males as it pertains to spatial ability.

Vitamin A deficiency impairs contextual fear memory in rats: Abnormalities in the glucocorticoid pathway

Vitamin A and its active metabolite, retinoic acid (RA), play a key role in the maintenance of cognitive functions in the adult brain. Depletion of RA using the vitamin A deficiency (VAD) model in Wistar rats leads to spatial memory deficits in relation to elevated intrahippocampal basal corticosterone (CORT) levels and increased hippocampal 11β-hydroxysteroid dehydrogenase type 1 (11β-HSD1) activity. All of these effects are normalised by vitamin A supplementation. However, it is unknown whether vitamin A status also modulates contextual fear conditioning (CFC) in a glucocorticoid-associated fear memory task dependent on the functional integrity of the hippocampus. In the present study, we investigated the impact of VAD and vitamin A supplementation in adult male rats on fear memory processing, plasma CORT levels, hippocampal retinoid receptors and 11β-HSD1 expression following a novelty-induced stress. We also examined whether vitamin A supplementation or a single injection of UE2316, a selective 11β-HSD1 inhibitor, known to modulate local glucocorticoid levels, had any beneficial effects on contextual fear memory and biochemical parameters in VAD rats. We provide evidence that VAD rats exhibit a decreased fear conditioning response during training with a poor contextual fear memory 24 hours later. These VAD-induced cognitive impairments are associated with elevated plasma CORT levels under basal conditions, as well as following a stressful event, with saturated CORT release, altered hippocampal retinoid receptors and 11β-HSD1 expression. Vitamin A supplementation normalises VAD-induced fear conditioning training deficits and all biochemical effects, although it cannot prevent fear memory deficits. Moreover, a single injection of UE2316 not only impairs contextual fear memory, but also reduces plasma CORT levels, regardless of the vitamin A status and decreases slightly hippocampal 11β-HSD1 activity in VAD rats following stress. The present study highlights the importance of vitamin A status with respect to modulating fear memory conditioning in relation to plasma CORT levels and hippocampal 11β-HSD1.

Preclinical studies of stress, extinction, and prefrontal cortex: intriguing leads and pressing questions

BACKGROUND

Stress is associated with cognitive and emotional dysfunction, and increases risk for a variety of psychological disorders, including depression and posttraumatic stress disorder. 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. Extinction of conditioned fear provides an excellent model system for examining how stress-induced changes in corticolimbic structure and function are related to stress-induced changes in neural function and behavior, as the neural circuitry underlying this behavior is well characterized.

OBJECTIVES

This review examines how acute and chronic stress influences extinction and describes how stress alters the structure and function of the medial prefrontal cortex, a potential neural substrate for these effects. In addition, we identify important unanswered questions about how stress-induced change in prefrontal cortex may mediate extinction deficits and avenues for future research.

KEY FINDINGS

A substantial body of work demonstrates deficits in extinction after either acute or chronic stress. A separate and substantial literature demonstrates stress-induced neuronal remodeling in medial prefrontal cortex, along with several key neurohormonal contributors to this remodeling, and there is substantial overlap in prefrontal mechanisms underlying extinction and the mechanisms implicated in stress-induced dysfunction of-and neuronal remodeling in-medial prefrontal cortex. However, data directly examining the contribution of changes in prefrontal structure and function to stress-induced extinction deficits is currently lacking.

CONCLUSIONS

Understanding how stress influences extinction and its neural substrates as well as individual differences in this effect will elucidate potential avenues for novel interventions for stress-sensitive disorders characterized by deficits in extinction.

Possible Link Between Stress-related Factors and Altered Body Composition in Women with Polycystic Ovarian Syndrome

Background

Stress is an invisible factor affecting modern day living and is strongly associated with many disease pathogenesis including polycystic ovarian syndrome (PCOS) in women. PCOS is the most frequent endocrinological disorder that affects women of reproductive age, leading to metabolic dysfunction and body composition alterations. Salivary amylase and cortisol are major stress mediators that have been implicated in PCOS. However, their role in altering body composition in PCOS is yet to be deciphered.

Aim

The present study aimed at understanding the relation between stress-associated factors and alterations in body composition among PCOS patients.

Design

This study enrolled a total of 100 patients (PCOS) and 60 age-matched controls. The female patients were of ages between 13 and 30 years.

Materials and Methods

Standard assay kits were used to evaluate the α-amylase activity and cortisol level in saliva. The participants were chosen on the basis of the Rotterdam American Society for Reproductive Medicine/European Society of Human Reproduction criteria. Saliva was collected from each participant as per the protocol of Salimetrics, USA.

Statistical Analysis

Statistical analysis was performed using SPSS version 20 for Windows. The quantitative variables are described as mean ± standard deviation. P < 0.05 was considered significant.

Results

Increased salivary cortisol level and α-amylase activity were seen in the PCOS population as compared to age-matched controls suggesting patients a sustained stress scenario in their system. Moreover, overweight PCOS participants reflected higher amylase activity than the lean patients participants. Pulse rate, body mass index (BMI), visceral adiposity, and waist-hip ratio (WHR) was considerably higher in the PCOS patients participants compared to controls. A significant correlation could be drawn between the α-amylase activity and BMI or WHR, respectively, among PCOS patients. These observations indicate a strong link between the stress marker and alterations in the body composition parameters of PCOS patients participants.

Conclusion

Higher prevalence of stress in PCOS patients participants has a critical role in their altered body composition.

The different roles of glucocorticoids in the hippocampus and hypothalamus in chronic stress-induced HPA axis hyperactivity

Hypothalamus-pituitary-adrenal (HPA) hyperactivity is observed in many patients suffering from depression and the mechanism underling the dysfunction of HPA axis is not well understood. Chronic stress has a causal relationship with the hyperactivity of HPA axis. Stress induces the over-synthesis of glucocorticoids, which will arrive at all the body containing the brain. It is still complicated whether glucocorticoids account for chronic stress-induced HPA axis hyperactivity and in which part of the brain the glucocorticoids account for chronic stress-induced HPA axis hyperactivity. Here, we demonstrated that glucocorticoids were indispensable and sufficient for chronic stress-induced hyperactivity of HPA axis. Although acute glucocorticoids elevation in the hippocampus and hypothalamus exerted a negative regulation of HPA axis, we found that chronic glucocorticoids elevation in the hippocampus but not in the hypothalamus accounted for chronic stress-induced hyperactivity of HPA axis. Chronic glucocorticoids exposure in the hypothalamus still exerted a negative regulation of HPA axis activity. More importantly, we found mineralocorticoid receptor (MR) - neuronal nitric oxide synthesis enzyme (nNOS) - nitric oxide (NO) pathway mediated the different roles of glucocorticoids in the hippocampus and hypothalamus in regulating HPA axis activity. This study suggests that the glucocorticoids in the hippocampus play an important role in the development of HPA axis hyperactivity and the glucocorticoids in the hypothalamus can't induce hyperactivity of HPA axis, revealing new insights into understanding the mechanism of depression.

Sex differences and chronic stress effects on the neural circuitry underlying fear conditioning and extinction

There are sex differences in the rates of many stress-sensitive psychological disorders such as posttraumatic stress disorder (PTSD). As medial prefrontal cortex and amygdala are implicated in many of these disorders, understanding differential stress effects in these regions may shed light on the mechanisms underlying sex-dependent expression of disorders like depression and anxiety. Prefrontal cortex and amygdala are key regions in the neural circuitry underlying fear conditioning and extinction, which thus has emerged as a useful model of stress influences on the neural circuitry underlying regulation of emotional behavior. This review outlines the current literature on sex differences and stress effects on dendritic morphology within medial prefrontal cortex and basolateral amygdala. Such structural differences and/or alterations can have important effects on fear conditioning and extinction, behaviors that are mediated by the basolateral amygdala and prefrontal cortex, respectively. Given the importance of extinction-based exposure therapy as a treatment for anxiety disorders such as PTSD, understanding the neural mechanisms by which stress differentially influences fear learning and extinction in males and females is an important goal for developing sex-appropriate interventions for stress-related disorders.

Making memories matter

This article reviews some of the neuroendocrine bases by which emotional events regulate brain mechanisms of learning and memory. In laboratory rodents, there is extensive evidence that epinephrine influences memory processing through an inverted-U relationship, at which moderate levels enhance and high levels impair memory. These effects are, in large part, mediated by increases in blood glucose levels subsequent to epinephrine release, which then provide support for the brain processes engaged by learning and memory. These brain processes include augmentation of neurotransmitter release and of energy metabolism, the latter apparently including a key role for astrocytic glycogen. In addition to up- and down-regulation of learning and memory in general, physiological concomitants of emotion and arousal can also switch the neural system that controls learning at a particular time, at once improving some attributes of learning and impairing others in a manner that results in a change in the strategy used to solve a problem.

Adult-onset hypothyroidism enhances fear memory and upregulates mineralocorticoid and glucocorticoid receptors in the amygdala

Hypothyroidism is the most common hormonal disease in adults, which is frequently accompanied by learning and memory impairments and emotional disorders. However, the deleterious effects of thyroid hormones deficiency on emotional memory are poorly understood and often underestimated. To evaluate the consequences of hypothyroidism on emotional learning and memory, we have performed a classical Pavlovian fear conditioning paradigm in euthyroid and adult-thyroidectomized Wistar rats. In this experimental model, learning acquisition was not impaired, fear memory was enhanced, memory extinction was delayed and spontaneous recovery of fear memory was exacerbated in hypothyroid rats. The potentiation of emotional memory under hypothyroidism was associated with an increase of corticosterone release after fear conditioning and with higher expression of glucocorticoid and mineralocorticoid receptors in the lateral and basolateral nuclei of the amygdala, nuclei that are critically involved in the circuitry of fear memory. Our results demonstrate for the first time that adult-onset hypothyroidism potentiates fear memory and also increases vulnerability to develop emotional memories. Furthermore, our findings suggest that enhanced corticosterone signaling in the amygdala is involved in the pathophysiological mechanisms of fear memory potentiation. Therefore, we recommend evaluating whether inappropriate regulation of fear in patients with post-traumatic stress and other mental disorders is associated with abnormal levels of thyroid hormones, especially those patients refractory to treatment.

Chronic stress, cyclic 17β-estradiol, and daily handling influences on fear conditioning in the female rat

Chronic stress and estrogens alter many forebrain regions in female rats that affect cognition. In order to investigate how chronic stress and estrogens influence fear learning and memory, we ovariectomized (OVX) female Sprague-Dawley rats and repeatedly injected them (s.c.) with 17β-estradiol (E, 10 μg/250 g or sesame oil vehicle, VEH). Concurrently, rats were restrained for 6 h/d/21 d (STR) or left undisturbed (CON). Rats were then fear conditioned with 4 tone-footshock pairings and then after 1 h and 24 h delays, given 15 tone extinction trials. Regardless of E treatment, chronic stress (VEH, E) facilitated freezing to tone during acquisition and extinction following a 1h delay, but not during extinction after a 24 h delay. E did not influence freezing to tone during any phase of fear conditioning for either the control or chronically stressed rats, but did influence contextual conditioning that may have been carried predominately by the STR group. In the second experiment, we investigated "handling" influences on fear conditioning acquisition, given the disparate findings from the current study and previous work (Baran, Armstrong, Niren, & Conrad, 2010; Baran, Armstrong, Niren, Hanna, & Conrad, 2009). Female rats remained gonadally-intact since E did not influence tone fear conditioning. Indeed, brief daily handling (1-3 m/d/21 d) facilitated acquisition of fear conditioning in chronically stressed female rats, and either had no effect or slightly attenuated fear conditioning in controls. Thus, chronic stress impacts amygdala-mediated fear learning in both OVX- and gonadally-intact females as found previously in males, with handling significantly influencing these outcomes.

A critical review of chronic stress effects on spatial learning and memory

The purpose of this review is to evaluate the effects of chronic stress on hippocampal-dependent function, based primarily upon studies using young, adult male rodents and spatial navigation tasks. Despite this restriction, variability amongst the findings was evident and how or even whether chronic stress influenced spatial ability depended upon the type of task, the dependent variable measured and how the task was implemented, the type and duration of the stressors, housing conditions of the animals that include accessibility to food and cage mates, and duration from the end of the stress to the start of behavioral assessment. Nonetheless, patterns emerged as follows: For spatial memory, chronic stress impairs spatial reference memory and has transient effects on spatial working memory. For spatial learning, however, chronic stress effects appear to be task-specific: chronic stress impairs spatial learning on appetitively motivated tasks, such as the radial arm maze or holeboard, tasks that evoke relatively mild to low arousal components from fear. But under testing conditions that evoke moderate to strong arousal components from fear, such as during radial arm water maze testing, chronic stress appears to have minimal impairing effects or may even facilitate spatial learning. Chronic stress clearly impacts nearly every brain region and thus, how chronic stress alters hippocampal spatial ability likely depends upon the engagement of other brain structures during behavioral training and testing.

Chronic stress causes amygdala hyperexcitability in rodents

BACKGROUND

Chronic stress is a major health concern, often leading to depression, anxiety, or when severe enough, posttraumatic stress disorder. While many studies demonstrate that the amygdala is hyperresponsive in patients with these disorders, the cellular neurophysiological effects of chronic stress on the systems that underlie psychiatric disorders, such as the amygdala, are relatively unknown.

METHODS

In this study, we examined the effects of chronic stress on the activity and excitability of amygdala neurons in vivo in rats. We used in vivo intracellular recordings from single neurons of the lateral amygdala (LAT) to measure neuronal properties and determine the cellular mechanism for the effects of chronic stress on LAT neurons.

RESULTS

We found a mechanism for the effects of chronic stress on amygdala activity, specifically that chronic stress increased excitability of LAT pyramidal neurons recorded in vivo. This hyperexcitability was caused by a reduction of a regulatory influence during action potential firing, facilitating LAT neuronal activity. The effects of stress on excitability were occluded by agents that block calcium-activated potassium channels and reversed by pharmacological enhancement of calcium-activated potassium channels.

CONCLUSIONS

These data demonstrate a specific channelopathy that occurs in the amygdala after chronic stress. This enhanced excitability of amygdala neurons after chronic stress may explain the observed hyperresponsiveness of the amygdala in patients with posttraumatic stress disorder and may facilitate the emergence of depression or anxiety in other patients.

Chronic 17beta-estradiol or cholesterol prevents stress-induced hippocampal CA3 dendritic retraction in ovariectomized female rats: possible correspondence between CA1 spine properties and spatial acquisition

Chronic stress may have different effects on hippocampal CA3 and CA1 neuronal morphology and function depending upon hormonal status, but rarely are manipulations of stress and gonadal steroids combined. Experiment 1 investigated the effects of chronic restraint and 17beta-estradiol replacement on CA3 and CA1 dendritic morphology and spatial learning in ovariectomized (OVX) female Sprague-Dawley rats. OVX rats were implanted with 25% 17beta-estradiol, 100% cholesterol, or blank silastic capsules and then chronically restrained (6h/d/21d) or kept in home cages. 17beta-Estradiol or cholesterol prevented stress-induced CA3 dendritic retraction, increased CA1 apical spine density, and altered CA1 spine shape. The combination of chronic stress and 17beta-estradiol facilitated water maze acquisition compared to chronic stress + blank implants and nonstressed controls + 17beta-estradiol. To further investigate the interaction between 17beta-estradiol and stress on hippocampal morphology, experiment 2 was conducted on gonadally intact, cycling female rats that were chronically restrained (6h/d/21d), and then euthanized at proestrus (high ovarian hormones) or estrus (low ovarian hormones). Cycling female rats failed to show chronic stress-induced CA3 dendritic retraction at either estrous phase. Chronic stress enhanced the ratio of CA1 basal spine heads to headless spines as found in experiment 1. In addition, proestrous rats displayed increased CA1 spine density regardless of stress history. These results show that 17beta-estradiol or cholesterol protect against chronic stress-induced CA3 dendritic retraction in females. These stress- and 17beta-estradiol-induced morphological changes may provide insight into how dendritic complexity and spine properties contribute to spatial ability.

Prefrontal cortex lesions and sex differences in fear extinction and perseveration

Electrolytic lesions of the medial prefrontal cortex (PFCX) were examined using fear conditioning to assess the recall of fear extinction and performance in the Y-maze, open field, and object location/recognition in male and female Sprague-Dawley rats. Rats were conditioned to seven tone/footshocks, followed by extinction after 1-h and 24-h delays, revealing PFCX effects and sex differences during all phases of fear conditioning. In male rats, PFCX impaired 24-h recall of fear extinction to tone, which required the 1-h delay extinction and was not attributed to nonassociative factors. In contrast, sham and PFCX females increased freezing to tone following a 24-h delay, whether or not 1-h delay tone extinction was presented. Moreover, PFCX females failed to extinguish to tone, contrasting to the robust extinction to tone that was observed for sham females, PFCX, and sham males. Also, sex differences were found during acquisition, with sham females acquiring fear conditioning slower than PFCX females. By the last tone-shock presentation, sham and PFCX females showed a slight but significant reduction in freezing to tone relative to those of sham and PFCX males. Of the other behavioral measures, PFCX females maintained exploration of a novel object during object recognition when sham females habituated. PFCX did not influence other behaviors in the remaining tasks. These findings show important sex differences in PFC function, with the PFC influencing the recall of fear extinction in males and contributing to the acquisition and maintenance of fear extinction memory in females, perhaps through altering perseveration.

Chronic stress- and sex-specific neuromorphological and functional changes in limbic structures

Chronic stress produces sex-specific neuromorphological changes in a variety of brain regions, which likely contribute to the gender differences observed in stress-related illnesses and cognitive ability. Here, we review the literature investigating the relationship between chronic stress and sex differences on brain plasticity and function, with an emphasis on morphological changes in dendritic arborization and spines in the hippocampus, prefrontal cortex, and amygdala. These brain structures are highly interconnected and sensitive to stress and gonadal hormones, and influence a variety of cognitive abilities. Although much less work has been published using female subjects than with male subjects, the findings suggest that the relationship between brain morphology and function is very different between the sexes. After reviewing the literature, we present a model showing how chronic stress influences the morphology of these brain regions and changes the dynamic of how these limbic structures interact with each other to produce altered behavioral outcomes in spatial ability, behavioral flexibility/executive function, and emotional arousal.

Chronic stress and sex differences on the recall of fear conditioning and extinction

Chronic stress effects and sex differences were examined on conditioned fear extinction. Male and female Sprague-Dawley rats were chronically stressed by restraint (6 h/d/21 d), conditioned to tone and footshock, followed by extinction after 1 h and 24 h delays. Chronic stress impaired the recall of fear extinction in males, as evidenced by high freezing to tone after the 24 h delay despite exposure to the previous 1 h delay extinction trials, and this effect was not due to ceiling effects from overtraining during conditioning. In contrast, chronic stress attenuated the recall of fear conditioning acquisition in females, regardless of exposure to the 1 h extinction exposure. Since freezing to tone was reinstated following unsignalled footshocks, the deficit in the stressed rats reflected impaired recall rather than impaired consolidation. Sex differences in fear conditioning and extinction were observed in nonstressed controls as well, with control females resisting extinction to tone. Analysis of contextual freezing showed that all groups (control, stress, male, female) increased freezing immediately after the first tone extinction trial, demonstrating contextual discrimination. These findings show that chronic stress and sex interact to influence fear conditioning, with chronic stress impairing the recall of delayed fear extinction in males to implicate the medial prefrontal cortex, disrupting the recall of the fear conditioning acquisition in females to implicate the amygdala, and nonstressed controls exhibiting sex differences in fear conditioning and extinction, which may involve the amygdala and/or corticosterone levels.

What is the functional significance of chronic stress-induced CA3 dendritic retraction within the hippocampus?

Chronic stress produces consistent and reversible changes within the dendritic arbors of CA3 hippocampal neurons, characterized by decreased dendritic length and reduced branch number. This chronic stress-induced dendritic retraction has traditionally corresponded to hippocampus-dependent spatial memory deficits. However, anomalous findings have raised doubts as to whether a CA3 dendritic retraction is sufficient to compromise hippocampal function. The purpose of this review is to outline the mechanism underlying chronic stress-induced CA3 dendritic retraction and to explain why CA3 dendritic retraction has been thought to mediate spatial memory. The anomalous findings provide support for a modified hypothesis, in which chronic stress is proposed to induce CA3 dendritic retraction, which then disrupts hypothalamic-pituitary-adrenal axis activity, leading to dysregulated glucocorticoid release. The combination of hippocampal CA3 dendritic retraction and elevated glucocorticoid release contributes to impaired spatial memory. These findings are presented in the context of clinical conditions associated with elevated glucocorticoids.

Chronic stress enhances spatial memory in ovariectomized female rats despite CA3 dendritic retraction: possible involvement of CA1 neurons

Emerging data report sex differences in how the brain responds to chronic stress. Here, we investigated the effects of chronic restraint stress (6 h/day/21 days) on hippocampal morphology and function in ovariectomized female rats. Chronic restraint stress caused CA3 apical dendritic retraction in short- and long-shafted neurons, while it reduced basal dendritic arbors in long-shafted neurons only. Chronic restraint did not affect CA1 dendritic arborization, although it increased the proportion of CA1 spine heads compared with controls. Both stressed and control animals performed well on the Y-maze, a spatial memory task. However, chronic stress enhanced Y-maze performance compared with controls, which may reflect facilitated spatial memory or reduced habituation. Y-maze performance correlated with CA1 spine head proportion. This relationship suggests that spatial ability in females may be more tightly coupled with CA1 morphology, which may override the influence of CA3 dendritic retraction. Thus, this research provides additional evidence that CA3 morphology does not always parallel spatial memory.

Chronic stress leaves novelty-seeking behavior intact while impairing spatial recognition memory in the Y-maze

This experiment examined whether chronic stress disrupts novelty-seeking behavior under conditions that impair spatial memory. Rats were restrained for 6 h per day for 21 days, then tested in either a traditional spatial recognition Y-maze that requires extra-maze spatial cues to navigate or a version with salient intra-maze cues in addition to the extra-maze spatial cues. As previously shown, chronic restraint stress impaired performance on the spatial version of the Y-maze. However, chronically stressed rats performed well in the intra-maze cue version. The results indicate that the deficits in Y-maze performance following chronic stress are not attributed to neophobia, but likely reflect neurochemical and/or neurobiological changes underlying spatial memory ability.

Chronic stress enhances ibotenic acid-induced damage selectively within the hippocampal CA3 region of male, but not female rats

The purpose of this investigation was to assess the ability of the hippocampus to withstand a metabolic challenge following chronic stress. An N-methyl-d-aspartate receptor excitotoxin (ibotenic acid, IBO) was infused into the CA3 region of the hippocampus following a period of restraint for 6 h/day/21 days. Following the end of restraint when CA3 dendritic retraction persists (3 to 4 days), rats were infused with IBO (or vehicle) into the CA3 region of the hippocampus. Stressed male rats showed significantly more CA3 damage after IBO infusion relative to controls and the saline-infused side. Moreover, IBO-exacerbation of damage in males was not observed in the CA3 region 3 to 4 days after acute stress (6 h restraint), nor in the CA1 region after chronic stress. Females were also examined and chronic stress did not exacerbate IBO damage in the CA3 region. Overall, these results demonstrate that chronic stress compromises the ability of the hippocampus to withstand a metabolic challenge days after the chronic stress regimen has subsided in male rats. Whether the conditions surrounding CA3 dendritic retraction in females represents vulnerability is less clear and warrants further investigation.

Influence of chronic corticosterone and glucocorticoid receptor antagonism in the amygdala on fear conditioning

Glucocorticoid receptor activation within the basolateral amygdala (BLA) during fear conditioning may mediate enhancement in rats chronically exposed to stress levels of corticosterone. Male Sprague-Dawley rats received corticosterone (400 microg/ml) in their drinking water (days 1-21), a manipulation that was previously shown to cause hippocampal CA3 dendritic retraction. Subsequently, rats were adapted to the fear conditioning chamber (day 22), then trained (day 23), and tested for conditioned fear to context and tone (day 25). Training consisted of two tone (20s) and footshock (500 ms, 0.25 mA) pairings. In Experiment 1, muscimol (4.4 nmol/0.5 microl/side), a GABAergic agonist, was microinfused to temporarily inactivate the BLA during training. Rats given chronic corticosterone showed enhanced freezing to context, but not tone, compared to vehicle-supplemented rats. Moreover, BLA inactivation impaired contextual and tone conditioning, regardless of corticosterone treatment. In Experiment 2, RU486 (0, 0.3, and 3.0 ng/0.2 microl/side) was infused on training day to antagonize glucocorticoid receptors in the BLA. Corticosterone treatment enhanced fear conditioning to context and tone when analyzed together, but not separately. Moreover, RU486 (3.0 ng/side) selectively exacerbated freezing to context in chronic corticosterone-exposed rats only, but failed to alter tone conditioning. Serum corticosterone levels were negatively correlated with contextual, not tone, conditioning. Altogether, these suggest that chronic corticosterone influences fear conditioning differently than chronic stress as shown previously. Moreover, chronic exposure to corticosteroids alters BLA functioning in a non-linear fashion and that contextual conditioning is influenced more than tone conditioning by chronic corticosterone and BLA glucocorticoid receptor stimulation.

Prior exposure to a single stress session facilitates subsequent contextual fear conditioning in rats. Evidence for a role of corticosterone

Previous studies showed that exposure of rats to chronic restraint stress for 21 days enhances subsequent contextual fear conditioning. Since recent evidence suggest that this effect is not dependent on stress-induced neurodegenerative processes, but to elevated training-elicited glucocorticoid release in chronically stressed animals, we aimed to explore here whether a single exposure to restraint stress, which is not expected to induce neuronal damage, would also affect contextual fear conditioning. We also questioned whether post-training corticosterone levels might be associated with any potential effect of stress on fear conditioning. Adult male Wistar rats were exposed to acute restraint stress for 2 h and, two days later, trained in the contextual fear conditioning task, under training conditions involving either moderate (0.4 mA shock) or high (1 mA shock) stress levels. The results showed that acute stress enhanced conditioned freezing at both training conditions, although data from the 1 mA shock intensity experiment only approached significance. Stressed animals were shown to display higher post-training corticosterone levels. Furthermore, the facilitating effect of prior stress was not evident when animals were trained in the hippocampal-independent auditory-cued conditioning task. Therefore, these findings support the idea that stress experiences preceding exposure to new types of stressors facilitate the development of contextual fear conditioning. They also indicate that not only repeated, but also a single exposure to aversive stimulation is sufficient to facilitate context-dependent fear conditioning, and suggest that increased glucocorticoid release at training might be implicated in the mechanisms mediating the memory facilitating effects induced by prior stress experiences.

Sex differences in spatial and non-spatial Y-maze performance after chronic stress

Chronic restraint is known to alter hippocampal CA3 dendritic morphology and spatial memory in male rats. The present study examined whether female rats, which exhibit different anatomical adaptations to chronic stress than those of males, would also show spatial memory impairments. Male and female Sprague-Dawley rats were restrained for 6 h/day for 21 days, a time frame previously demonstrated to cause hippocampal CA3 dendritic atrophy. The day after the last restraint session, rats were tested on a Y-maze, a habituation task that can be used to assess spatial memory. Chronic stress impaired Y-maze performance in both sexes without affecting levels of locomotion as measured by total arm entries in the first minute. However, Y-maze performance of stressed females improved in 2-5 min when chronically stressed males continued to show poor Y-maze performance. The enhanced Y-maze performance of chronically stressed females occurred when total arm entries were higher compared to the entries made by males. Therefore, correlations were performed between total arm entries and spatial memory in 1 and 2-5 min. In the first minute when control females demonstrated functional spatial memory, female controls with the lowest locomotor levels exhibited the best performance. The correlations for stressed females were not significant, and neither were the correlations for any group in 2-5 min. Overall, these results show important sex differences in response to chronic stress with females exhibiting an ability to recover quickly from deficits in Y-maze performance.