Sex-dependent changes in anxiety, memory, and monoamines following one week of stress

Individual differences in the boldness of female zebrafish are associated with alterations in serotonin function

One of the most prevalent axes of behavioral variation in both humans and animals is risk taking, where individuals that are more willing to take risk are characterized as bold while those that are more reserved are regarded as shy. Brain monoamines (i.e. serotonin, dopamine and noradrenaline) have been found to play a role in a variety of behaviors related to risk taking. Using zebrafish, we investigated whether there was a relationship between monoamine function and boldness behavior during exploration of a novel tank. We found a correlation between serotonin metabolism (5-HIAA:5-HT ratio) and boldness during the initial exposure to the tank in female animals. The DOPAC:DA ratio correlated with boldness behavior on the third day in male fish. There was no relationship between boldness and noradrenaline. To probe differences in serotonergic function in bold and shy fish, we administered a selective serotonin reuptake inhibitor, escitalopram, and assessed exploratory behavior. We found that escitalopram had opposing effects on thigmotaxis in bold and shy female animals: the drug caused bold fish to spend more time near the center of the tank and shy fish spent more time near the periphery. Taken together, our findings indicate that variation in serotonergic function has sex-specific contributions to individual differences in risk-taking behavior.

Basolateral amygdala neuropeptide Y system modulates binge ethanol consumption

Neuropeptide Y (NPY) signaling regulation of corticolimbic communication is known to modulate binge-like ethanol consumption in rodents. In this work we sought to assess the impact of intra-BLA NPY system modulation on binge-like ethanol intake and to assess the role of the NPY1R+ projection from the BLA to the mPFC in this behavior. We used "drinking-in-the-dark" (DID) procedures in C57BL6J mice to address these questions. First, the impact of intra-BLA administration of NPY on binge-like ethanol intake was assessed. Next, the impact of repeated cycles of DID intake on NPY1R expression in the BLA was assessed with use of immunohistochemistry (IHC). Finally, chemogenetic inhibition of BLA→mPFC NPY1R+ projections was assessed to determine if limbic communication with the mPFC was specifically involved in binge-like ethanol intake. Importantly, as both the BLA and NPY system are sexually dimorphic, both sexes were assessed in these studies. Intra-BLA NPY dose-dependently decreased binge-like ethanol intake in males only. Repeated DID reduced NPY1R expression in the BLA of both sexes. Silencing of BLA→mPFC NPY1R+ neurons significantly reduced binge-like ethanol intake in both sexes in a dose-dependent manner. We provide novel evidence that (1) intra-BLA NPY reduces binge-like ethanol intake in males; (2) binge-like ethanol intake reduces NPY1R levels in the BLA; and (3) chemogenetic inhibition of BLA→mPFC NPY1R+ neurons blunts binge-like drinking in male and female mice. These observations provide the first direct evidence that NPY signaling in the BLA, and specifically BLA communication with the mPFC, modulates binge-like ethanol consumption.

Nrf2 activation rescues stress-induced depression-like behaviour and inflammatory responses in male but not female rats

BACKGROUND

Major depressive disorder (MDD) is a recurring affective disorder that is two times more prevalent in females than males. Evidence supports immune system dysfunction as a major contributing factor to MDD, notably in a sexually dimorphic manner. Nuclear factor erythroid 2-related factor 2 (Nrf2), a regulator of antioxidant signalling during inflammation, is dysregulated in many chronic inflammatory disorders; however, its role in depression and the associated sex differences have yet to be explored. Here, we investigated the sex-specific antidepressant and immunomodulatory effects of the potent Nrf2 activator dimethyl fumarate (DMF), as well as the associated gene expression profiles.

METHODS

Male and female rats were treated with vehicle or DMF (25 mg/kg) whilst subjected to 8 weeks of chronic unpredictable stress. The effect of DMF treatment on stress-induced depression- and anxiety-like behaviours, as well as deficits in recognition and spatial learning and memory were then assessed. Sex differences in hippocampal (HIP) microglial activation and gene expression response were also evaluated.

RESULTS

DMF treatment during stress exposure had antidepressant effects in male but not female rats, with no anxiolytic effects in either sex. Recognition learning and memory and spatial learning and memory were impaired in chronically stressed males and females, respectively, and DMF treatment rescued these deficits. DMF treatment also prevented stress-induced HIP microglial activation in males. Conversely, females displayed no HIP microglial activation associated with stress exposure. Last, chronic stress elicited sex-specific alterations in HIP gene expression, many of which were normalized in animals treated with DMF. Of note, most of the differentially expressed genes in males normalized by DMF were related to antioxidant, inflammatory or immune responses.

CONCLUSIONS

Collectively, these findings support a greater role of immune processes in males than females in a rodent model of depression. This suggests that pharmacotherapies that target Nrf2 have the potential to be an effective sex-specific treatment for depression.

Individual differences in the boldness of female zebrafish are associated with alterations in serotonin function

One of the most prevalent axes of behavioral variation in both humans and animals is risk taking, where individuals that are more willing to take risk are characterized as bold while those that are more reserved as shy. Brain monoamines (i.e., serotonin, dopamine, and norepinephrine) have been found to play a role in a variety of behaviors related to risk taking. Genetic variation related to monoamine function have also been linked to personality in both humans and animals. Using zebrafish, we investigated the relationship between monoamine function and boldness behavior during exploration of a novel tank. We found a sex-specific correlation between serotonin metabolism (5-HIAA:5-HT ratio) and boldness that was limited to female animals; there were no relationships between boldness and dopamine or norepinephrine. To probe differences in serotonergic function, we administered a serotonin reuptake inhibitor, escitalopram, to bold and shy fish, and assessed their exploratory behavior. We found that escitalopram had opposing effects on thigmotaxis in female animals with bold fish spending more time near the center of the tank and shy fish spent more time near the periphery. Taken together, our findings suggest that variation in serotonergic function makes sex-specific contributions to individual differences in risk taking behavior.

Chronic restraint stress and social transfer of stress produce tactile allodynia mediated by the HMGB1/TNFα/TNFR1 pathway in female and male rats

Previous studies have shown the relevance of high mobility group box 1 protein (HMGB1) and tumor necrosis factor α (TNFα) in nerve or tissue injury-induced nociception. However, the role of these proteins in chronic stress and social transfer of stress (STS)-induced dysfunctional pain is not entirely known. The aim of this study was to determine the participation of the spinal HMGB1-TNFα signaling pathway and TNFα receptor 1 (TNFR1) in rats subjected to chronic restraint stress (CRS) and STS. Non-stressed female and male rats in contact with CRS rats increased sniffing behavior of the anogenital area, behavior related to STS. Rats subjected to CRS and STS reduced 50 % withdrawal threshold and reached the value of tactile allodynia after 21 days of stress. Rats return to the basal withdrawal threshold after 30 days without stress and return to allodynia values in only 5 days of stress sessions (priming). Female and male rats subjected to 28 days of CRS or STS were intrathecal injected with glycyrrhizin (inhibitor of HMGB1), thalidomide (inhibitor of the TNFα synthesis), and R7050 (TNFR1 antagonist), in all the cases, an antiallodynic effect was observed. Rats under CRS or STS enhanced HMGB1 and TNFR1 protein expression in DRG and dorsal spinal cord. Data suggest that the spinal HMGB1/TNFα/TNFR1 signaling pathway plays a relevant role in the maintenance of CRS and STS-induced nociceptive hypersensitivity in rats. These proteins could be helpful in developing pain treatments for fibromyalgia in humans.

Adulthood effects of developmental exercise in rats

Exercise is known to promote efficient function of stress circuitry. The developing brain is malleable and thus exercise during adolescence could potentially exert lasting beneficial effects on the stress response that would be detectable in adulthood. The current study determined whether adolescent wheel running was associated with reduced stress response in adulthood, 6 weeks after cessation of exercise. Male and female adolescent rats voluntarily ran for 6 weeks and then were sedentary for 6 weeks prior to 10 days of chronic restraint stress in adulthood. Fecal corticosterone levels were measured during stress, and escape from the restraint tube was assessed on the final day as a proxy for depressive-like behavior. Anxiety-like behavior was measured 24 h later with the elevated plus maze and locomotor behaviors with the open field. Brain and body measurements were taken immediately following behavioral testing. Developmental exercise and adulthood stress both exerted independent effects on physiological and behavioral outcomes in adulthood. Exercise history increased the odds ratio of escape from restraint stress in males, but did not influence other stress-induced behaviors. In summary, exercise early in life exerted lasting effects, but did not substantially alter the adulthood response to restraint stress.

The Tryptophan and Kynurenine Pathway Involved in the Development of Immune-Related Diseases

The tryptophan and kynurenine pathway is well-known to play an important role in nervous, endocrine, and immune systems, as well as in the development of inflammatory diseases. It has been documented that some kynurenine metabolites are considered to have anti-oxidative, anti-inflammatory, and/or neuroprotective properties. Importantly, many of these kynurenine metabolites may possess immune-regulatory properties that could alleviate the inflammation response. The abnormal activation of the tryptophan and kynurenine pathway might be involved in the pathophysiological process of various immune-related diseases, such as inflammatory bowel disease, cardiovascular disease, osteoporosis, and/or polycystic ovary syndrome. Interestingly, kynurenine metabolites may be involved in the brain memory system and/or intricate immunity via the modulation of glial function. In the further deliberation of this concept with engram, the roles of gut microbiota could lead to the development of remarkable treatments for the prevention of and/or the therapeutics for various intractable immune-related diseases.

Chronic Restraint Stress Alters Rat Behavior Depending on Sex and Duration of Stress

Chronic restraint stress (CRS) can have different behavioral effects depending on variables associated with the stressor and the organism. This study aimed to verify the effect of the interaction between sex and duration of the CRS protocol in rats. Sprague-Dawley rats were divided by sex, intervention (CRS; control), and CRS duration (11 days; 22 days). Rats exposed to CRS showed better spatial learning than controls in the Morris water maze test, regardless of sex and stress duration. Males exposed to CRS for 11 days showed a higher rate of behaviors associated with anxiety than males exposed to 22 days of CRS at the elevated plus maze test, but the same was not observed in females. The weight gain of animals exposed to stress decreased in the first 11 days, showing a recovery from day 11 to day 22 of intervention. No effects of CRS were observed on behaviors associated with depression in the sucrose preference test. The results suggest habituation to the protocol, with a progressive decrease in the harmful effects of stress on and maintenance of the beneficial effects. It is possible that females are more resistant to the harmful effects of CRS on anxiety.

Sex differences in cognition following variations in endocrine status

Spatial memory, mediated primarily by the hippocampus, is responsible for orientation in space and retrieval of information regarding location of objects and places in an animal's environment. Since the hippocampus is dense with steroid hormone receptors and is capable of robust neuroplasticity, it is not surprising that changes in spatial memory performance occur following a variety of endocrine alterations. Here, we review cognitive changes in both spatial and nonspatial memory tasks following manipulations of the hypothalamic-pituitary-adrenal and gonadal axes and after exposure to endocrine disruptors in rodents. Chronic stress impairs male performance on numerous behavioral cognitive tasks and enhances or does not impact female cognitive function. Sex-dependent changes in cognition following stress are influenced by both organizational and activational effects of estrogen and vary depending on the developmental age of the stress exposure, but responses to gonadal hormones in adulthood are more similar than different in the sexes. Also discussed are possible underlying neural mechanisms for these steroid hormone-dependent, cognitive effects. Bisphenol A (BPA), an endocrine disruptor, given at low levels during adolescent development, impairs spatial memory in adolescent male and female rats and object recognition memory in adulthood. BPA's negative effects on memory may be mediated through alterations in dendritic spine density in areas that mediate these cognitive tasks. In summary, this review discusses the evidence that endocrine status of an animal (presence or absence of stress hormones, gonadal hormones, or endocrine disruptors) impacts cognitive function and, at times, in a sex-specific manner.

Object recognition tasks in rats: Does sex matter?

Novelty recognition tasks based on object exploration are frequently used for the evaluation of cognitive abilities and investigation of neurobiological and molecular aspects of memory in rodents. This is an interesting approach because variations of the object recognition tasks focus on different aspects of the memory events such as novelty, location, context, and combinations of these elements. Nevertheless, as in most animal neuroscience research, female subjects are underrepresented in object recognition studies. When studies include females, the particularities of this sex are not always considered. For example, appropriate controls for manipulations conducted exclusively in females (such as estrous cycle verification) are not included. In addition, interpretation of data is often based on standardizations conducted with male subjects. Despite that, females are frequently reported as deficient and unable to adequately perform some memory tests. Thus, our study aims to review studies that describe similarities and differences between male and female performances in the different variations of object recognition tasks. In summary, although females are commonly described with deficits and the articles emphasize sex differences, most published data reveal similar performances when sexes are compared.

Sex differences in cognitive flexibility are driven by the estrous cycle and stress-dependent

Stress is associated with psychiatric disorders such as post-traumatic stress disorder, major depressive disorder, anxiety disorders, and panic disorders. Women are more likely to be diagnosed with these stress-related psychiatric disorders than men. A key phenotype in stress-related psychiatric disorders is impairment in cognitive flexibility, which is the ability to develop new strategies to respond to different patterns in the environment. Because gonadal hormones can contribute to sex differences in response to stress, it is important to consider where females are in their cycle when exposed to stress and cognitive flexibility testing. Moreover, identifying neural correlates involved in cognitive flexibility could not only build our understanding of the biological mechanisms behind this crucial skill but also leads to more targeted treatments for psychiatric disorders. Although previous studies have separately examined sex differences in cognitive flexibility, stress effects on cognitive flexibility, and the effect of gonadal hormones on cognitive flexibility, many of the findings were inconsistent, and the role of the estrous cycle in stress-induced impacts on cognitive flexibility is still unknown. This study explored potential sex differences in cognitive flexibility using an operant strategy shifting-paradigm after either control conditions or restraint stress in freely cycling female and male rats (with estrous cycle tracking in the female rats). In addition, we examined potential neural correlates for any sex differences observed. In short, we found that stress impaired certain aspects of cognitive flexibility and that there were sex differences in cognitive flexibility that were driven by the estrous cycle. Specifically, stress increased latency to first press and trials to criterion in particular tasks. The female rats demonstrated more omissions and perseverative errors than the male rats; the sex differences were mostly driven by proestrus female rats. Interestingly, the number of orexinergic neurons was higher in proestrus female rats than in the male rats under control conditions. Moreover, orexin neural count was positively correlated with number of perseverative errors made in cognitive flexibility testing. In sum, there are sex differences in cognitive flexibility that are driven by the estrous cycle and are stress-dependent, and orexin neurons may underlie some of the sex differences observed.

Investigating the role of environmental enrichment initiated in adolescence against the detrimental effects of chronic unpredictable stress in adulthood: Sex-specific differences in behavioral and neuroendocrinological findings

Environmental Enrichment (EE) improves cognitive function and enhances brain plasticity, while chronic stress increases emotionality, impairs learning and memory, and has adverse effects on brain anatomy and biochemistry. We explored the beneficial role of environmental enrichment initiated in adolescence against the negative outcomes of Chronic Unpredictable Stress (CUS) during adulthood on emotional behavior, cognitive function, as well as somatic and neuroendocrine markers in both sexes. Adolescent Wistar rats housed either in enriched or standard housing conditions for 10 weeks. On postnatal day 66, a subgroup from each housing condition was daily exposed to a 4-week stress protocol. Following stress, adult rats underwent behavioral testing to evaluate anxiety, exploration/locomotor activity, depressive-like behavior and spatial learning/memory. Upon completion of behavioral testing, animals were exposed to a 10-m stressful event to test the neuroendocrine response to acute stress. CUS decreased body weight gain and increased adrenal weight. Some stress-induced behavioral adverse effects were sex-specific since learning impairments were limited to males while depressive-like behavior to females. EE housing protected against CUS-related behavioral deficits and body weight loss. Exposure to CUS affected the neuroendocrine response of males to acute stress as revealed by the increased corticosterone levels. Our findings highlight the significant role of EE in adolescence as a protective factor against the negative effects of stress and underline the importance of inclusion of both sexes in animal studies.

Androgens Enhance Recognition Memory and Dendritic Spine Density in the Hippocampus and Prefrontal Cortex of Ovariectomized Female Rats

Estrogen replacement has been repeatedly shown to enhance memory and increase dendritic spine density in the hippocampus and prefrontal cortex of ovariectomized (OVX) female rats. Given the potential deleterious effects of chronic estrogen administration, the present study assessed cognitive function using recognition memory tasks and measured dendritic spine density in the CA1 region of the hippocampus and medial prefrontal cortex after subchronic androgen replacement to adult OVX female rats. All androgens enhanced recognition memory in OVX rats, but object placement (OP) and object recognition (OR) results differed. Only testosterone enhanced OR. Testosterone had no effect on OP while dehydroepiandrosterone (DHEA), dihydrotestosterone (DHT) and androstenedione (AD) enhanced OP. Dendritic spine density was increased by both TP and DHEA in both brain areas (DHT and AD were not tested). Lastly, we used the aromatase inhibitor, letrozole, to discriminate between potential androgenic and estrogenic effects of androgens on behavior. Letrozole alone did not alter recognition memory in OVX rats and did not block the effects of either TP or DHEA on recognition memory suggesting that effects were mediated via androgenic mechanisms. The present results expand previous information on gonadal hormone actions and show that, in addition to estrogens, androgens also improve memory and increase spine density in brains of OVX female rats. While requiring further investigation, these observations provide a basis for therapeutic interventions in the treatment of menopausal, age or disease related memory loss.

Stressed rats fail to exhibit avoidance reactions to innately aversive social calls

Disruptions in amygdalar function, a brain area involved in encoding emotionally salient information, has been implicated in stress-related affective disorders. Earlier animal studies on the behavioral consequences of stress-induced abnormalities in the amygdala focused on learned behaviors using fear conditioning paradigms. If and how stress affects unconditioned, innate fear responses to ethologically natural aversive stimuli remains unexplored. Hence, we subjected rats to aversive ultrasonic vocalization calls emitted on one end of a linear track. Unstressed control rats exhibited a robust avoidance response by spending more time away from the source of the playback calls. Unexpectedly, prior exposure to chronic immobilization stress prevented this avoidance reaction, rather than enhancing it. Further, this stress-induced impairment extended to other innately aversive stimuli, such as white noise and electric shock in an inhibitory avoidance task. However, conditioned fear responses were enhanced by the same stress. Inactivation of the basolateral amygdala (BLA) in control rats prevented this avoidance reaction evoked by the playback. Consistent with this, analysis of the immediate early gene cFos revealed higher activity in the BLA of control, but not stressed rats, after exposure to the playback. Further, in vivo recordings in freely behaving control rats exposed to playback showed enhanced theta activity in the BLA, which also was absent in stressed rats. These findings offer a new framework for studying stress-induced alterations in amygdala-dependent maladaptive responses to more naturally threatening and emotionally relevant social stimuli. The divergent impact of stress on defensive responses--impaired avoidance responses together with increased conditioned fear--also has important implications for models of learned helplessness and depression.

Behavioral alterations induced by intermittent ethanol intake and noise exposure in adolescent rats

Alcohol intake and exposure to noise are common activities of human adolescents performed in entertainment contexts worldwide that can induce behavioral disturbances. Therefore, the aim of the present work was to investigate in an experimental model of adolescent animals whether noise exposure and intermittent ethanol intake, when present individually or sequentially, might be able to modify different behaviors. Adolescent Wistar rats of both sexes were subjected to voluntary intermittent ethanol intake for 1 week followed by exposure to noise for 2 h and tested in a battery of behavioral tasks. Data show that males exposed to noise experienced a deficit in associative memory (AM), increase in anxiety-like behaviors (ALB) and altered reaction to novelty (RN) when compared with sham animals, whereas females also showed an increase in risk assessment behaviors (RAB) and a decrease in exploratory activity (EA). In contrast, ethanol intake induced an increase in RAB and RN in males and females, whereas females also showed a deficit in AM and EA as well as an increase in ALB. When ethanol was ingested before noise exposure, most parameters were counteracted both in male and females, but differed among sexes. In consequence, it could be hypothesized that an environmental acute stressor like noise might trigger a behavioral counteracting induced by a previous repeated exposure to a chemical agent such as ethanol, leading to a compensation of a non-adaptive behavior and reaching a better adjustment to the environment.

Suppression of pyramidal neuron G protein-gated inwardly rectifying K+ channel signaling impairs prelimbic cortical function and underlies stress-induced deficits in cognitive flexibility in male, but not female, mice

Imbalance in prefrontal cortical (PFC) pyramidal neuron excitation:inhibition is thought to underlie symptomologies shared across stress-related disorders and neuropsychiatric disease, including dysregulation of emotion and cognitive function. G protein-gated inwardly rectifying K+ (GIRK/Kir3) channels mediate excitability of medial PFC pyramidal neurons, however, the functional role of these channels in mPFC-dependent regulation of affect, cognition, and cortical dynamics is unknown. We used a viral-cre approach in male and female mice harboring a "floxed" version of the kcnj3 (Girk1) gene, to disrupt GIRK1-containing channel expression in pyramidal neurons within the prelimbic cortex (PrL). In males, loss of pyramidal GIRK1-dependent signaling differentially impacted measures of affect and impaired working memory and cognitive flexibility. Unexpectedly, ablation of PrL GIRK1-dependent signaling did not impact affect or cognition in female mice. Additional studies used a model of chronic unpredictable stress (CUS) to determine the impact on PrL GIRK-dependent signaling and cognitive function. CUS exposure in male mice produced deficits in cognition that paralleled a reduction in PrL pyramidal GIRK-dependent signaling akin to viral approaches whereas CUS exposure in female mice did not alter cognitive flexibility performance. Stress-induced behavioral deficits in male mice were rescued by systemic injection of a novel, GIRK1-selective agonist, ML297. In conclusion, GIRK1-dependent signaling in male mice, but not females, is critical for maintaining optimal PrL function and behavioral control. Disruption of this inhibition may underlie stress-related dysfunction of the PrL and represent a therapeutic target for treating stress-induced deficits in affect regulation and impaired cognition that reduce quality of life.

Differential chronic social stress models in male and female mice

Chronic stress creates an allostatic overload that may lead to mood disorders such as anxiety and depression. Modern causes of chronic stress in humans are mostly social in nature, relating to work and relationship stress. Research into neural and molecular mechanisms of vulnerability and resilience following chronic social stress (CSS) is ongoing and uses animal models to discover efficient prevention strategies and treatments. To date, most CSS studies have neglected the female sex and used male-focused aggression-based animal models such as chronic social defeat stress (CSDS). Accumulating evidence on sex differences suggests differences in the stress response, the prevalence of stress-related illness and in response to treatment, indicating that researchers should expand CSS investigation to include female-focused protocols alongside the popular CSDS protocols. Here, we describe a novel female mouse model of CSS and a parallel modified male mouse model of CSDS in C57BL/6 mice. These new models enable the investigation of vulnerability, coping and downstream effectors mediating short-term and long-term consequences of CSS in both sexes. Our data demonstrate differential effects on male and female mice during, soon after, and many weeks after CSS. Female mice are more prone to body weight loss during CSS and hyperactive anxious behaviour following CSS. Both sexes show reduced social interaction, but only stressed male mice show long-term changes in emotional memory and neuroendocrine function. We further discuss future avenues of research using these models to investigate mechanisms pertaining to sensitivity to CSS and treatment response profiles, in a sex-appropriate manner.

Effect of Cold Stress on Neurobehavioral and Physiological Parameters in Rats

Objective: Cold stress is an important current issue and implementing control strategies to limit its sometimes harmful effects is crucial. Cold is a common stressor that can occur in our work and our occupational or leisure time activities every day. There are substantial studies on the effects of chronic stress on memory and behavior, although, the cognitive changes and anxiety disorders that can occur after exposure to chronic intermittent cold stress are not completely characterized. Therefore, the present study was undertaken with an aim to investigate the effects of chronic intermittent cold stress on body weight, food intake and working memory, and to elucidate cold stress related anxiety disorders using cognitive and behavioral test batteries. Methods: We generated a cold stress model by exposing rats to chronic intermittent cold stress for 5 consecutive days and in order to test for the potential presence of sex differences, a comparable number of male and female rats were tested in the current study. Then, we measured the body weights, food intake and the adrenal glands weight. Working memory and recognition memory were assessed using the Y maze and the Novel Object Recognition (NOR) tasks. While, sex differences in the effects of chronic stress on behavior were evaluated by the elevated plus maze (EPM), open field maze (OF), and Marble burying (MB) tests. Results: We found that 2 h exposure to cold (4°C) resulted in an increase in the relative weight of the adrenal glands in male rats. Given the same chronic stress 5 days of cold exposure (2 h per day), increased weight gain in male rats, while females showed decreased food intake and no change in body weight. Both sexes successfully performed the Y maze and object recognition (OR) tasks, indicating intact spatial working memory performance and object recognition abilities in both male and female rats. In addition, we have shown that stress caused an increase in the level of anxiety in male rats. In contrast, the behavior of the female rats was not affected by cold exposure. Conclusion: Overall, the current results provide preliminary evidence that chronic intermittent cold stress model may not be an efficient stressor to female rats. Females exhibit resilience to cold exposure that causes an increase in the level of anxiety in male rats, which demonstrates that they are affected differently by stress and the gender is an important consideration in experimental design.

Age-Related Individual Behavioural Characteristics of Adult Wistar Rats

Simple Summary

Rats are considered adults from 2 to 5 months. During this period, they are used for experimentation in physiology and pharmacology. Adult rats, depending on their age, can be in a different physiological state, which can influence the results of experiments carried out on them. Despite this, age-related changes in adult rats have not yet been examined. Our results showed that as male and female rats progressed from 2 to 5 months of age there was a decrease in the level of motor and exploratory activities, and an increase in the level of anxiety-like behaviour. Age-related changes were dependent upon initial individual characteristics of behaviour. For example, animals that demonstrated high motor activity at 2 months become significantly less active by 5 months, and animals that showed a low level of anxiety at 2 months become more anxious by 5 months. Low-activity and high-anxiety rats did not show any significant age-related changes from 2 to 5 months of age. The results of this work should be taken into account when choosing the age of rats for conducting behavioural experiments.

Abstract

The aim of this work was to study age-related changes in the behaviour of adult Wistar rats using the open field (OF) and elevated plus maze (EPM) tests. Behavioural changes related to motor activity and anxiety were of particular interest. Results showed that as male and female rats progressed from 2 to 5 months of age, there was a decrease in the level of motor and exploratory activities and an increase in their level of anxiety. Age-related changes were dependent upon initial individual characteristics of behaviour. For example, animals that demonstrated high motor activity at 2 months become significantly less active by 5 months, and animals that showed a low level of anxiety at 2 months become more anxious by 5 months. Low-activity and high-anxiety rats did not show any significant age-related changes in OF and EPM tests from 2 to 5 months of age, except for a decrease in the number of rearings in the EPM. Thus, the behaviour of the same adult rat at 2 and 5 months of age is significantly different, which may lead to differences in the experimental results of physiological and pharmacological studies using adult animals of different ages.

Therapeutic effects of saffron extract on different memory types, anxiety, and hippocampal BDNF and TNF-α gene expressions in sub-chronically stressed rats

Objective: While stress reportedly impairs memory, saffron enhances it. This study investigated the therapeutic effects of saffron extract on different memory types, anxiety-like behavior, and expressions of BDNF and TNF-α genes in sub-chronically stressed rats.Methods: Rats were randomly assigned to control, restraint stress (6 h/day/7 days), two 7-days saffron treatments with 30 and 60 mg/kg, and two stress-saffron groups (30 and 60 mg/kg/7 post-stress days). Serum cortisol level and hippocampal BDNF and TNF-α gene expressions were measured. Open field, passive avoidance, novel object recognition, and object location tests were performed to assess anxiety-like behavior and avoidance as well as cognitive and spatial memories, respectively.Results: The low saffron dose in the sub-chronic stressed group led to a significant increase in passive avoidance latency from day 3 onward whereas this effect was observed after 7 days under the high-dose treatment that simultaneously led to a significant decline in serum cortisol level. While the low saffron dose led to a sharp drop in hippocampal TNF-α gene expression, the high dose significantly increased the hippocampal BDNF gene expression in the sub-chronic stress group. Finally, both saffron doses reduced anxiety in the stressed groups.Conclusion: Compared to the low saffron dose, the high dose had a latent but long-lasting impact. Cognitive and spatial memories remained unaffected by either stress or saffron treatment. In addition, only the high saffron dose reversed anxiety in the sub-chronically stressed group. These findings suggest that various doses of saffron act differently on different brain functions under sub-chronic stress conditions.Abbreviations: Brain derived neurotrophic factor (BDNF), tumor necrosis factor-α (TNF-α), hypothalamic-pituitary-adrenal axis (HPA), novel object recognition task (NORT), novel object location task (NOLT), open field test (OFT), passive avoidance (PA).

Chronic psychological stress and lower urinary tract symptoms

It is well established that lower urinary tract symptoms (LUTS), particularly urinary urgency and incontinence, cause stress and anxiety for patients. However, there is mounting evidence that the relationship between these two factors is bidirectional and that chronic psychological stress itself can result in the development of symptoms such as urinary frequency, urgency, incontinence, and pelvic pain. This review considers the evidence that such a relationship exists and reviews the literature from clinical and animal studies to identify some of the mechanisms that might be involved. Inflammatory responses induced by chronic stress appear to offer the strongest link to bladder dysfunction. There is overwhelming evidence, both in patients and animal models, for a release of pro-inflammatory cytokines and chemokines during periods of chronic stress. Furthermore, cytokines have been shown to cause bladder dysfunction and pain via actions in the central nervous system and locally in the bladder. In the brain and spinal cord, pro-inflammatory cytokines influence the regulation of micturition pathways by corticotropin-releasing factor (CRF) and its receptors, while peripherally cytokines affect bladder function, directly causing detrusor hypertrophy and afferent nerve hypersensitivity. There is little information on which treatments may have most benefit for stressed/anxious patients with LUTS, but animal studies suggest traditional drugs for overactive bladder (solifenacin, mirabegron) are more effective on LUTS than anxiolytic drugs (fluoxetine, imipramine). The preliminary preclinical data for CRF receptor antagonists is not consistent. A clearer understanding of the mechanisms involved in stress-induced LUTS should provide a basis for improved treatment of this condition.

Menopause, hormone therapy and cognition: maximizing translation from preclinical research

Menopause-associated and hormone-associated cognitive research has a rich history built from varied disciplines and species. This review discusses landmark rodent and human work addressing cognitive outcomes associated with varied experiences of menopause and hormone therapy. Critical variables in menopause and cognitive aging research are considered, including menopause etiology, background hormone milieu and parameters of exposure to estrogens and progestogens. Recent preclinical research has identified that menopause and ovarian hormone fluctuations across many neurobiological systems affect cognitive aging, mapping novel avenues for future research. Preclinical models provide insight into complex interdisciplinary relationships in a systematic and highly controlled fashion. We highlight that acknowledging the strengths and weaknesses for both preclinical and clinical research approaches is vital to accurate interpretation, optimal translation and the direction of future research. There is great value in collaboration and communication across preclinical and clinical realms, especially regarding reciprocal feedback of findings to advance preclinical models, improve experimental designs and enrich basic science translation to the clinic. In searching for biological mechanisms underlying the cognitive consequences of menopause and hormone therapies, it is noteworthy that clinical and preclinical scientists are grounded in the same fundamental goal of optimizing health outcomes for women across the lifespan.

An emerging role for microglia in stress‐effects on memory

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

Sex differences in the delayed impact of acute stress on the amygdala

There is accumulating evidence that stress triggers specific temporal patterns of morphological plasticity in the amygdala, a brain area that plays a pivotal role in the debilitating emotional symptoms of stress-related psychiatric disorders. Acute immobilization stress is known to cause a delayed increase in the density of dendritic spines on principal neurons in the basolateral amygdala (BLA) of rats. These neuronal changes are also accompanied by a delayed enhancement in anxiety-like behavior. However, these earlier studies used male rats, and the delayed behavioral and synaptic effects of acute stress on the BLA of female rats remain unexplored. Here, using whole-cell recordings in rat brain slices, we find that a single exposure to 2-h immobilization stress leads to an increase, 10 days later, in the frequency of miniature excitatory postsynaptic currents (mEPSCs) recorded from lateral amygdala (LA) principal neurons in male rats. Further, acute stress also causes a reduction in the frequency of miniature inhibitory postsynaptic currents (mIPSCs) in LA neurons 10 days after acute stress. In striking contrast, excitatory and inhibitory synaptic transmission in the LA of female rats does not exhibit any delayed change despite exposure to the same acute stress. Finally, we examined the functional impact of these contrasting synaptic changes at the behavioral level. Male rats exhibit a delayed increase in anxiety-like behavior on the elevated plus-maze 10 days after acute stress. However, the same stress does not lead to a delayed anxiogenic effect in female rats. Together, these results demonstrate that the delayed modulation of the balance of synaptic excitation and inhibition in the amygdala, as well as anxiety-like behavior, differ between males and females. These findings provide a framework, across biological scales, for analyzing how affective symptoms of stress disorders vary between the sexes.

Anatomically and functionally distinct locus coeruleus efferents mediate opposing effects on anxiety-like behavior

The locus coeruleus (LC) is a critical node in the stress response, and its activation has been shown to promote hypervigilance and anxiety-like behavior. This noradrenergic nucleus has historically been considered homogeneous with highly divergent neurons that operate en masse to collectively affect central nervous system function and behavioral state. However, in recent years, LC has been identified as a heterogeneous structure whose neurons innervate discrete terminal fields and contribute to distinct aspects of behavior. We have previously shown that in late adolescent male rats, an acute traumatic stressor, simultaneous physical restraint and exposure to predator odor, preferentially induces c-Fos expression in a subset of dorsal LC neurons and persistently increases anxiety-like behavior. To investigate how these neurons respond to and contribute to the behavioral response to stress, we used a combination of retrograde tracing, whole-cell patch clamp electrophysiology, and chemogenetics. Here we show that LC neurons innervating the central nucleus of the amygdala (CeA) and medial prefrontal cortex (mPFC) undergo distinct electrophysiological changes in response to stressor exposure and have opposing roles in mediating anxiety-like behavior. While neurons innervating CeA become more excitable in response to stress and promote anxiety-like behavior, those innervating mPFC become less excitable and appear to promote exploration. These findings show that LC neurons innervating distinct terminal fields have unique physiological responses to particular stimuli. Furthermore, these observations advance the understanding of the LC as a complex and heterogeneous structure whose neurons maintain unique roles in various forms of behavior.

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Locus coeruleus-central amygdala projections are hyperactive one week after stress.

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Locus coeruleus-prefrontal cortex projections are hypoactive one week after stress.

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Chemogenetic manipulation of each pathway distinctly affects anxiety-like behavior.

Effects of chronic stress on reinstatement of palatable food seeking: Sex differences and relationship to trait anxiety

Previous research in our lab has established a causal role for chronic stress exposure in subsequent increases in relapse-like behaviors in male rats with a history of palatable food self-administration. Given that many of the neurobehavioral consequences of stress are sex dependent, we aimed to determine whether sex differences exist with regard to the effects of chronic stress on relapse. Additionally, because high trait anxiety confers vulnerability to stress-related disorders, we examined whether individual differences in trait anxiety were related to differences in relapse-like behavior after chronic stress exposure. Following elevated plus maze testing for classification into high- or low-anxiety phenotypes, male and female rats responded for highly palatable food pellets. During subsequent extinction training, stress was manipulated (0 or 90 min restraint/day for 7 days). Rats were then tested for cue- and pellet priming-induced reinstatement of palatable food seeking. Results showed that female rats displayed higher levels of responding during cue-induced reinstatement tests compared to males, and that a history of chronic stress caused an attenuation of cue-induced reinstatement in female, but not male, rats. Regarding pellet priming-induced reinstatement, there was a three-way interaction such that neither stress history nor anxiety phenotype was related to reinstatement in females, but a history of stress in males caused increased and decreased responding in low- and high-anxiety rats, respectively. These results suggest that biological sex and trait anxiety level may help to explain differences in vulnerability to relapse among individuals exposed to chronic stress. Such information may be useful in designing more personalized and effective treatments for obesity and eating disorders.

A long-term cyclic plus tonic regimen of 17β-estradiol improves the ability to handle a high spatial working memory load in ovariectomized middle-aged female rats

The influence of estrogens on modifying cognition has been extensively studied, revealing that a wide array of factors can significantly impact cognition, including, but not limited to, subject age, estrogen exposure duration, administration mode, estrogen formulation, stress history, and progestogen presence. Less known is whether long-term, extended exposure to estrogens would benefit or otherwise impact cognition. The present study examined the effects of 17β-estradiol (E2) exposure for seven months, beginning in late adulthood and continuing into middle age, using a regimen of cyclic exposure (bi-monthly subcutaneous injection of 10 μg E2), or Cyclic+Tonic exposure (bi-monthly subcutaneous injection of 10 μg E2 + Silastic capsules of E2) in ovariectomized female Fischer-344-CDF rats. Subjects were tested on a battery of learning and memory tasks. All groups learned the water radial-arm maze (WRAM) and Morris water maze tasks in a similar fashion, regardless of hormone treatment regimen. In the asymptotic phase of the WRAM, rats administered a Cyclic+Tonic E2 regimen showed enhanced performance when working memory was taxed compared to Vehicle and Cyclic E2 groups. Assessment of spatial memory on object placement and object recognition was not possible due to insufficient exploration of objects; however, the Cyclic+Tonic group showed increased total time spent exploring all objects compared to Vehicle-treated animals. Overall, these data demonstrate that long-term Cyclic+Tonic E2 exposure can result in some long-term cognitive benefits, at least in the spatial working memory domain, in a surgically menopausal rat model.

Prior stress followed by a novel stress challenge results in sex-specific deficits in behavioral flexibility and changes in gene expression in rat medial prefrontal cortex

Chronic stress leads to sex-specific changes in the structure and function of rat medial prefrontal cortex (mPFC). Little is known about whether these effects persist following the cessation of chronic stress, or how these initial effects may impact responses to future stressors. Here we examined attentional set-shifting in male and female rats following chronic restraint stress, a post-chronic stress rest period, and an acute novel stress challenge. Chronic stress resulted in a reversible impairment in extradimensional set-shifting in males, but had no effect on attentional set-shifting in females. Surprisingly, chronically stressed female, but not male, rats had impaired extradimensional set-shifting following a novel stress challenge. Alterations in the balance of excitation and inhibition of mPFC have been implicated in behavioral deficits following chronic stress. Thus, in a separate group of rats, we examined changes in the expression of genes related to glutamatergic (NR1, NR2A, NR2B, GluR1) and GABAergic (Gad67, parvalbumin, somatostatin) neurotransmission in mPFC after acute and chronic stress, rest, and their combination. Stress significantly altered the expression of NR1, GluR1, Gad67, and parvalbumin. Notably, the pattern of stress effects on NR1, Gad67, and parvalbumin expression differed between males and females. In males, these genes were upregulated following the post-chronic stress rest period, while minimal changes were found in females. In contrast, both males and females had greater GluR1 expression following a rest period. These findings suggest that chronic stress leads to sex-specific stress adaptation mechanisms that may contribute to sex differences in response to subsequent stress exposure.

Female rats are resistant to the long-lasting neurobehavioral changes induced by adolescent stress exposure

Stress during adolescence is a risk factor for neuropsychiatric diseases, including schizophrenia. We recently observed that peripubertal male rats exposed to a combination of daily footshock plus restraint stress exhibited schizophrenia-like changes. However, numerous studies have shown sex differences in neuropsychiatric diseases as well as on the impact of coping with stress. Thus, we decided to evaluate, in adolescent female rats, the impact of different stressors (restraint stress [RS], footshock [FS], or the combination of FS and RS [FS+RS]) on social interaction (3-chamber social approach test/SAT), anxiety responses (elevated plus-maze/EPM), cognitive function (novel object recognition test/NOR), and dopamine (DA) system responsivity by evaluating locomotor response to amphetamine and in vivo extracellular single unit recordings of DA neurons in the ventral tegmental area (VTA) in adulthood. The impact of FS+RS during early adulthood was also investigated. Adolescent stress had no impact on social behavior, anxiety, cognition and locomotor response to amphetamine. In addition, adolescent stress did not induce long-lasting changes in VTA DA system activity. However, a decrease in the firing rate of VTA DA neurons was found 1-2 weeks post-adolescent stress. Similar to adolescent stress, adult stress did not induce long-lasting behavioral deficits and changes in VTA DA system activity, but FS+RS decreased VTA DA neuron population activity 1-2 weeks post-adult stress. Our results are consistent with previous studies showing that female rodents appear to be more resilient to developmental stress-induced persistent changes than males and may contribute to the delayed onset and lesser severity of schizophrenia in females.

New neurons restore structural and behavioral abnormalities in a rat model of PTSD

Post-traumatic stress disorder (PTSD) has been associated with anxiety, memory impairments, enhanced fear, and hippocampal volume loss, although the relationship between these changes remain unknown. Single-prolonged stress (SPS) is a model for PTSD combining three forms of stress (restraint, swim, and anesthesia) in a single session that results in prolonged behavioral effects. Using pharmacogenetic ablation of adult neurogenesis in rats, we investigated the role of new neurons in the hippocampus in the long-lasting structural and behavioral effects of SPS. Two weeks after SPS, stressed rats displayed increased anxiety-like behavior and decreased preference for objects in novel locations regardless of the presence or absence of new neurons. Chronic stress produced by daily restraint for 2 or 6 hr produced similar behavioral effects that were also independent of ongoing neurogenesis. At a longer recovery time point, 1 month after SPS, rats with intact neurogenesis had normalized, showing control levels of anxiety-like behavior. However, GFAP-TK rats, which lacked new neurons, continued to show elevated anxiety-like behavior and enhanced serum corticosterone response to anxiogenic experience. Volume loss in ventral CA1 region of the hippocampus paralleled increases in anxiety-like behavior, occurring in all rats exposed to SPS at the early time point and only rats lacking adult neurogenesis at the later time point. In chronic stress experiments, volume loss occurred broadly throughout the dentate gyrus and CA1 after 6-hr daily stress but was not apparent in any hippocampal subregion after 2-hr daily stress. No effect of SPS was seen on cell proliferation in the dentate gyrus, but the survival of young neurons born a week after stress was decreased. Together, these data suggest that new neurons are important for recovery of normal behavior and hippocampal structure following a strong acute stress and point to the ventral CA1 region as a potential key mediator of stress-induced anxiety-like behavior.

Sex differences in stress reactivity in arousal and attention systems

Women are more likely than men to suffer from psychiatric disorders with hyperarousal symptoms, including posttraumatic stress disorder (PTSD) and major depression. In contrast, women are less likely than men to be diagnosed with schizophrenia and attention deficit hyperactivity disorder (ADHD), which share attentional impairments as a feature. Stressful events exacerbate symptoms of the aforementioned disorders. Thus, researchers are examining whether sex differences in stress responses bias women and men towards different psychopathology. Here we review the preclinical literature suggesting that, compared to males, females are more vulnerable to stress-induced hyperarousal, while they are more resilient to stress-induced attention deficits. Specifically described are sex differences in receptors for the stress neuropeptide, corticotropin-releasing factor (CRF), that render the locus coeruleus arousal system of females more vulnerable to stress and less adaptable to CRF hypersecretion, a condition found in patients with PTSD and depression. Studies on the protective effects of ovarian hormones against CRF-induced deficits in sustained attention are also detailed. Importantly, we highlight how comparing males and females in preclinical studies can lead to the development of novel therapeutics to improve treatments for psychiatric disorders in both women and men.

Environmental enrichment prevents acute restraint stress-induced anxiety-related behavior but not changes in basolateral amygdala spine density

Previous studies showed that acute restraint stress or transient elevation of glucocorticoid (GC) stress hormones produces emergent changes in both anxiety behavior and dendritic branches in the basolateral amygdala complex (BLA) of rats. In this work, we demonstrate that exposure to environmental enrichment (EE) prevented stress-induced increases in anxiety (emerging 10 days post-stress) in adult rats without blocking stress-induced dendritic branch remodeling in the BLA nor stress-induced enhancement of GC serum levels.

Female mice are protected from space radiation-induced maladaptive responses

Interplanetary exploration will be humankind's most ambitious expedition and the journey required to do so, is as intimidating as it is intrepid. One major obstacle for successful deep space travel is the possible negative effects of galactic cosmic radiation (GCR) exposure. Here, we investigate for the first time how combined GCR impacts long-term behavioral and cellular responses in male and female mice. We find that a single exposure to simulated GCR induces long-term cognitive and behavioral deficits only in the male cohorts. GCR exposed male animals have diminished social interaction, increased anxiety-like phenotype and impaired recognition memory. Remarkably, we find that the female cohorts did not display any cognitive or behavioral deficits after GCR exposure. Mechanistically, the maladaptive behavioral responses observed only in the male cohorts correspond with microglia activation and synaptic loss in the hippocampus, a brain region involved in the cognitive domains reported here. Furthermore, we measured reductions in AMPA expressing synaptic terminals in the hippocampus. No changes in any of the molecular markers measured here are observed in the females. Taken together these findings suggest that GCR exposure can regulate microglia activity and alter synaptic architecture, which in turn leads to a range of cognitive alterations in a sex dependent manner. These results identify sex-dependent differences in behavioral and cognitive domains revealing promising cellular and molecular intervention targets to reduce GCR-induced chronic cognitive deficits thereby boosting chances of success for humans in deep space missions such as the upcoming Mars voyage.

Progesterone receptor expression in cajal-retzius cells of the developing rat dentate gyrus: Potential role in hippocampus-dependent memory

The development of medial temporal lobe circuits is critical for subsequent learning and memory functions later in life. The present study reports the expression of progesterone receptor (PR), a powerful transcription factor of the nuclear steroid receptor superfamily, in Cajal-Retzius cells of the molecular layer of the dentate gyrus of rats. PR was transiently expressed from the day of birth through postnatal day 21, but was absent thereafter. Although PR immunoreactive (PR-ir) cells did not clearly express typical markers of mature neurons, they possessed an ultrastructural morphology consistent with neurons. PRir cells did not express markers for GABAergic neurons, neuronal precursor cells, nor radial glia. However, virtually all PR cells co-expressed the calcium binding protein, calretinin, and the glycoprotein, reelin, both reliable markers for Cajal-Retzius neurons, a transient population of developmentally critical pioneer neurons that guide synaptogenesis of perforant path afferents and histogenesis of the dentate gyrus. Indeed, inhibition of PR activity during the first two weeks of life impaired adult performance on both the novel object recognition and object placement memory tasks, two behavioral tasks hypothesized to describe facets of episodic-like memory in rodents. These findings suggest that PR plays an unexplored and important role in the development of hippocampal circuitry and adult memory function.

Sex differences in cardiovascular, neuroendocrine and behavioral changes evoked by chronic stressors in rats

This study investigated the physiological, somatic and behavioral changes evoked by daily exposure to the same type of stressor (homotypic) or different aversive stressor stimuli (heterotypic) in male and female rats. For this, adult Wistar rats were subjected to a 10days regimen of repeated restraint stress (RRS, homotypic stressor) or chronic variable stress (CVS, heterotypic stressor). Effects evoked by CVS included: (i) adrenal hypertrophy and decreased body weight gain in male animals, (ii) a sympathetically-mediated increase in basal heart rate in males, and (iii) a rise in plasma corticosterone concentration and anxiogenic effects in female animals. The homotypic stressor RRS also induced an increase in plasma corticosterone and anxiogenic effects in females, decreased body weight gain in males and evoked a sympathetically-mediated increase in heart rate in both sexes. Changes in cardiovascular function and autonomic activity evoked by both stressors were followed by impairment of baroreflex activity in males, but not female animals. Both chronic stressors evoked changes in blood pressure responsiveness to vasoconstrictor and vasodilator agents in both sexes. Taken together, these results indicate that regardless of chronic stress regimen males are more vulnerable to somatic effects of chronic stressors, while females appear to be more susceptible to neuroendocrine and behavioral changes. Present findings also indicate that females are selectively vulnerable to cardiovascular and autonomic changes evoked by homotypic stressors. Nevertheless, homotypic and heterotypic stressors similarly affect cardiovascular function and autonomic activity in males.

Behavioral stress alters corticolimbic microglia in a sex- and brain region-specific manner

Women are more susceptible to numerous stress-linked psychological disorders (e.g., depression) characterized by dysfunction of corticolimbic brain regions critical for emotion regulation and cognitive function. Although sparsely investigated, a number of studies indicate sex differences in stress effects on neuronal structure, function, and behaviors associated with these regions. We recently demonstrated a basal sex difference in- and differential effects of stress on- microglial activation in medial prefrontal cortex (mPFC). The resident immune cells of the brain, microglia are implicated in synaptic and dendritic plasticity, and cognitive-behavioral function. Here, we examined the effects of acute (3h/day, 1 day) and chronic (3h/day, 10 days) restraint stress on microglial density and morphology, as well as immune factor expression in orbitofrontal cortex (OFC), basolateral amygdala (BLA), and dorsal hippocampus (DHC) in male and female rats. Microglia were visualized, classified based on their morphology, and stereologically counted. Microglia-associated transcripts (CD40, iNOS, Arg1, CX3CL1, CX3CR1, CD200, and CD200R) were assessed in brain punches from each region. Expression of genes linked with cellular stress, neuroimmune state, and neuron-microglia communication varied between unstressed male and female rats in a region-specific manner. In OFC, chronic stress upregulated a wider variety of immune factors in females than in males. Acute stress increased microglia-associated transcripts in BLA in males, whereas chronic stress altered immune factor expression in BLA more broadly in females. In DHC, chronic stress increased immune factor expression in males but not females. Moreover, acute and chronic stress differentially affected microglial morphological activation state in male and female rats across all brain regions investigated. In males, chronic stress altered microglial activation in a pattern consistent with microglial involvement in stress-induced dendritic remodeling across OFC, BLA, and DHC. Together, these data suggest the potential for microglia-mediated sex differences in stress effects on neural structure, function, and behavior.

Sex differences in stress regulation of arousal and cognition

There are sex differences in the prevalence and presentation of many psychiatric disorders. For example, posttraumatic stress disorder (PTSD) and major depression are more common in women than men, and women with these disorders present with more hyperarousal symptoms than men. In contrast, attention deficit hyperactivity disorder (ADHD) and schizophrenia are more common in men than women, and men with these disorders have increased cognitive deficits compared to women. A shared feature of the aforementioned psychiatric disorders is the contribution of stressful events to their onset and/or severity. Here we propose that sex differences in stress responses bias females towards hyperarousal and males towards cognitive deficits. Evidence from clinical and preclinical studies is detailed. We also describe underlying neurobiological mechanisms. For example, sex differences in stress receptor signaling and trafficking in the locus coeruleus-arousal center are detailed. In learning circuits, evidence for sex differences in dendritic morphology is provided. Finally, we describe how evaluating sex-specific mechanisms for responding to stress in female and male rodents can lead to better treatments for stress-related psychiatric disorders.

Is Forced Swimming Immobility a Good Endpoint for Modeling Negative Symptoms of Schizophrenia? - Study of Sub-Anesthetic Ketamine Repeated Administration Effects

Immobility time in the forced swimming has been described as analogous to emotional blunting or apathy and has been used for characterizing schizophrenia animal models. Several clinical studies support the use of NMDA receptor antagonists to model schizophrenia in rodents. Some works describe the effects of ketamine on immobility behavior but there is variability in the experimental design used leading to controversial results. In this study, we evaluated the effects of repeated administration of ketamine sub-anesthetic doses in forced swimming, locomotion in response to novelty and novel object recognition, aiming a broader evaluation of the usefulness of this experimental approach for modeling schizophrenia in mice. Ketamine (30 mg/kg/day i.p. for 14 days) induced a not persistent decrease in immobility time, detected 24h but not 72h after treatment. This same administration protocol induced a deficit in novel object recognition. No change was observed in mice locomotion. Our results confirm that repeated administration of sub-anesthetic doses of ketamine is useful in modeling schizophrenia-related behavioral changes in mice. However, the immobility time during forced swimming does not seem to be a good endpoint to evaluate the modeling of negative symptoms in NMDAR antagonist animal models of schizophrenia.

Sex differences in psychotomimetic-induced behaviours in rats

Animal model studies using equal numbers of males and females are sparse in psychiatry research. Given the marked sex differences observed in psychiatric disorders, such as schizophrenia, using both males and females in research studies is an important requirement. Thus the aim of this study was to examine sex differences in psychotomimetic-induced behavioural deficits relevant to psychosis. We therefore compared the acute effect of amphetamine or phencyclidine on locomotor activity and prepulse inhibition in adult male and female Sprague-Dawley rats. The results of this study were that: (1) amphetamine-induced distance travelled was greater in female rats than in male rats, (2) phencyclidine-induced locomotor hyperactivity was similar in male and female rats; (3) there were no sex differences in amphetamine- or phencyclidine-induced disruption of prepulse inhibition; (4) male rats had an increased startle response after amphetamine. These findings suggest that sensitivity to amphetamine, but not phencyclidine, differs between male and female rats, and that this sex difference is selective to locomotor hyperactivity and startle, but not prepulse inhibition. This study used two widely-used, validated preclinical assays relevant to psychosis; the results of this study have implications for psychiatry research, particularly for disorders where marked sex differences in onset and symptomology are observed.

Sex differences in chronic stress effects on cognition in rodents

Chronic stress causes deleterious changes in physiological function in systems ranging from neural cells in culture to laboratory rodents, sub-human primates and humans. It is notable, however, that the vast majority of research in this area has been conducted in males. In this review, we provide information about chronic stress effects on cognition in female rodents and contrast it with responses in male rodents. In general, females show cognitive resilience to chronic stressors which impair male cognitive function using spatial tasks including the radial arm maze, radial arm water maze, Morris water maze, Y-maze and object placement. Moreover, stress often enhances female performance in some of these cognitive tasks. Memory in females is not affected by stress in non-spatial memory tasks like recognition memory and temporal order recognition memory while males show impaired memory following stress. We discuss possible bases for these sex-dependent differences including the use of different strategies by the sexes to solve cognitive tasks. Whether the sex differences result from changes in non-mnemonic factors is also considered. Sex-dependent differences in alcohol and drug influences on stress responses are also described. Finally, the role of neurally derived estradiol in driving sex differences and providing resilience to stress in females is shown. The importance of determining the nature and extent of sex differences in stress responses is that such differences may provide vital information for understanding why some stress related diseases have different incidence rates between the sexes and for developing novel therapeutic treatments.

Estradiol: Mediator of memories, spine density and cognitive resilience to stress in female rodents

Estradiol rapidly activates, within minutes, various physiological functions and behaviors including cognition in rodents. This review describes rapid effects of estradiol on hippocampal dependent learning and memory tasks in rodents. Mechanisms underlying the memory enhancements including the activation of signaling molecules and the enhancement of dendritic spinogenesis are briefly reviewed. In addition, the role of estradiol in the cognitive resilience to chronic stress exhibited only in females is discussed including contributions of ovarian as well as intra-hippocampally derived estrogens to this sex difference. Finally, speculations on possible physiologic functions for rapid mnemonic changes mediated by estrogens are made. Overall, the emergence of a novel and powerful mechanism for regulation of cognition by estradiol is described.

Acoustic signalling reflects personality in a social mammal

Social interactions among individuals are often mediated through acoustic signals. If acoustic signals are consistent and related to an individual's personality, these consistent individual differences in signalling may be an important driver in social interactions. However, few studies in non-human mammals have investigated the relationship between acoustic signalling and personality. Here we show that acoustic signalling rate is repeatable and strongly related to personality in a highly social mammal, the domestic pig (Sus scrofa domestica). Furthermore, acoustic signalling varied between environments of differing quality, with males from a poor-quality environment having a reduced vocalization rate compared with females and males from an enriched environment. Such differences may be mediated by personality with pigs from a poor-quality environment having more reactive and more extreme personality scores compared with pigs from an enriched environment. Our results add to the evidence that acoustic signalling reflects personality in a non-human mammal. Signals reflecting personalities may have far reaching consequences in shaping the evolution of social behaviours as acoustic communication forms an integral part of animal societies.

Estrogen in prefrontal cortex blocks stress-induced cognitive impairments in female rats

Animal and human studies have found that males and females show distinct stress responses. Recent studies suggest the contribution of estrogen in the brain to this sexual dimorphism. Repeated stress has been found to impair cognitive behaviors via suppressing glutamatergic transmission and glutamate receptor surface expression in pyramidal neurons of prefrontal cortex (PFC) in male rats. On the contrary, female rats exposed to the same stress paradigms show normal synaptic function and PFC-mediated cognition. The level of aromatase, the enzyme for the biosynthesis of estrogen, is significantly higher in the PFC of females than males. The stress-induced glutamatergic deficits and memory impairment are unmasked by blocking estrogen receptors or aromatase in females, suggesting a protective role of estrogen against the detrimental effects of repeated stress.

Associations between Prenatal Exposure to Black Carbon and Memory Domains in Urban Children: Modification by Sex and Prenatal Stress

Background

Whether fetal neurodevelopment is disrupted by traffic-related air pollution is uncertain. Animal studies suggest that chemical and non-chemical stressors interact to impact neurodevelopment, and that this association is further modified by sex.

Objectives

To examine associations between prenatal traffic-related black carbon exposure, prenatal stress, and sex with children’s memory and learning.

Methods

Analyses included N = 258 mother-child dyads enrolled in a Boston, Massachusetts pregnancy cohort. Black carbon exposure was estimated using a validated spatiotemporal land-use regression model. Prenatal stress was measured using the Crisis in Family Systems-Revised survey of negative life events. The Wide Range Assessment of Memory and Learning (WRAML2) was administered at age 6 years; outcomes included the General Memory Index and its component indices [Verbal, Visual, and Attention Concentration]. Relationships between black carbon and WRAML2 index scores were examined using multivariable-adjusted linear regression including effect modification by stress and sex.

Results

Mothers were primarily minorities (60% Hispanic, 26% Black); 67% had ≤12 years of education. The main effect for black carbon was not significant for any WRAML2 index; however, in stratified analyses, among boys with high exposure to prenatal stress, Attention Concentration Index scores were on average 9.5 points lower for those with high compared to low prenatal black carbon exposure (P_{3-way interaction} = 0.04).

Conclusion

The associations between prenatal exposure to black carbon and stress with children’s memory scores were stronger in boys than in girls. Studies assessing complex interactions may more fully characterize health risks and, in particular, identify vulnerable subgroups.

Administration of Lactobacillus helveticus NS8 improves behavioral, cognitive, and biochemical aberrations caused by chronic restraint stress

Increasing numbers of studies have suggested that the gut microbiota is involved in the pathophysiology of stress-related disorders. Chronic stress can cause behavioral, cognitive, biochemical, and gut microbiota aberrations. Gut bacteria can communicate with the host through the microbiota-gut-brain axis (which mainly includes the immune, neuroendocrine, and neural pathways) to influence brain and behavior. It is hypothesized that administration of probiotics can improve chronic-stress-induced depression. In order to examine this hypothesis, the chronic restraint stress depression model was established in this study. Adult specific pathogen free (SPF) Sprague-Dawley rats were subjected to 21 days of restraint stress followed by behavioral testing (including the sucrose preference test (SPT), elevated-plus maze test, open-field test (OFT), object recognition test (ORT), and object placement test (OPT)) and biochemical analysis. Supplemental Lactobacillus helveticus NS8 was provided every day during stress until the end of experiment, and selective serotonin reuptake inhibitor (SSRI) citalopram (CIT) served as a positive control. Results showed that L. helveticus NS8 improved chronic restraint stress-induced behavioral (anxiety and depression) and cognitive dysfunction, showing an effect similar to and better than that of CIT. L. helveticus NS8 also resulted in lower plasma corticosterone (CORT) and adrenocorticotropic hormone (ACTH) levels, higher plasma interleukin-10 (IL-10) levels, restored hippocampal serotonin (5-HT) and norepinephrine (NE) levels, and more hippocampal brain-derived neurotrophic factor (BDNF) mRNA expression than in chronic stress rats. Taken together, these results indicate an anti-depressant effect of L. helveticus NS8 in rats subjected to chronic restraint stress depression and that this effect could be due to the microbiota-gut-brain axis. They also suggest the therapeutic potential of L. helveticus NS8 in stress-related and possibly other kinds of depression.

What's wrong with my mouse cage? Methodological considerations for modeling lifestyle factors and gene-environment interactions in mice

The mechanistic understanding of lifestyle contributions to disease has been largely driven by work in laboratory rodent models using environmental interventions. These interventions show an array of methodologies and sometimes unclear collective conclusions, hampering clinical interpretations. Here we discuss environmental enrichment, exercise and stress interventions to illustrate how different protocols can affect the interpretations of environmental factors in disease. We use Huntington's disease (HD) as an example because its mouse models exhibit excellent validity and HD was the first genetic animal model in which environmental stimulation was found to be beneficial. We make a number of observations and recommendations. Firstly, environmental enrichment and voluntary exercise generally show benefits across laboratories and mouse models. However, the extent to which these environmental interventions have beneficial effects depends on parameters such as the structural complexity of the cage in the case of enrichment, the timing of the intervention and the nature of the control conditions. In particular, clinical interpretations should consider deprived control living conditions and the ethological relevance of the enrichment. Secondly, stress can have negative effects on the phenotype in mouse models of HD and other brain disorders. When modeling stress, the effects of more than one type of experimental stressor should be investigated due to the heterogeneity and complexity of stress responses. With stress in particular, but ideally in all studies, both sexes should be used and the randomized group sizes need to be sufficiently powered to detect any sex effects. Opportunities for clinical translation will be guided by the 'environmental construct validity' of the preclinical data, including the culmination of complementary protocols across multiple animal models. Environmental interventions in mouse models of HD provide illustrative examples of how valid preclinical studies can lead to conclusions relevant to clinical populations.

A critical evaluation of the activity-regulated cytoskeleton-associated protein (Arc/Arg3.1)'s putative role in regulating dendritic plasticity, cognitive processes, and mood in animal models of depression

Major depressive disorder (MDD) is primarily conceptualized as a mood disorder but cognitive dysfunction is also prevalent, and may limit the daily function of MDD patients. Current theories on MDD highlight disturbances in dendritic plasticity in its pathophysiology, which could conceivably play a role in the production of both MDD-related mood and cognitive symptoms. This paper attempts to review the accumulated knowledge on the basic biology of the activity-regulated cytoskeleton-associated protein (Arc or Arg3.1), its effects on neural plasticity, and how these may be related to mood or cognitive dysfunction in animal models of MDD. On a cellular level, Arc plays an important role in modulating dendritic spine density and remodeling. Arc also has a close, bidirectional relationship with postsynaptic glutamate neurotransmission, since it is stimulated by multiple glutamatergic receptor mechanisms but also modulates α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor internalization. The effects on AMPA receptor trafficking are likely related to Arc's ability to modulate phenomena such as long-term potentiation, long-term depression, and synaptic scaling, each of which are important for maintaining proper cognitive function. Chronic stress models of MDD in animals show suppressed Arc expression in the frontal cortex but elevation in the amygdala. Interestingly, cognitive tasks depending on the frontal cortex are generally impaired by chronic stress, while those depending on the amygdala are enhanced, and antidepressant treatments stimulate cortical Arc expression with a timeline that is reminiscent of the treatment efficacy lag observed in the clinic or in preclinical models. However, pharmacological treatments that stimulate regional Arc expression do not universally improve relevant cognitive functions, and this highlights a need to further refine our understanding of Arc on a subcellular and network level.

Actions and interactions of estradiol and glucocorticoids in cognition and the brain: Implications for aging women

Menopause involves dramatic declines in estradiol production and levels. Importantly, estradiol and the class of stress hormones known as glucocorticoids exert countervailing effects throughout the body, with estradiol exerting positive effects on the brain and cognition, glucocorticoids exerting negative effects on the brain and cognition, and estradiol able to mitigate negative effects of glucocorticoids. Although the effects of these hormones in isolation have been extensively studied, the effects of estradiol on the stress response and the neuroprotection offered against glucocorticoid exposure in humans are less well known. Here we review evidence suggesting that estradiol-related protection against glucocorticoids mitigates stress-induced interference with cognitive processes. Animal and human research indicates that estradiol-related mitigation of glucocorticoid damage and interference is one benefit of estradiol supplementation during peri-menopause or soon after menopause. The evidence for estradiol-related protection against glucocorticoids suggests that maintaining estradiol levels in post-menopausal women could protect them from stress-induced declines in neural and cognitive integrity.