Gender differences in acute and chronic stress in Wistar Kyoto (WKY) rats

Unmoving and uninflamed: Characterizing neuroinflammatory dysfunction in the Wistar-Kyoto rat model of depression

Reductionistic research on depressive disorders has been hampered by the limitations of animal models. Recently, it has been hypothesized that neuroinflammation is a key player in depressive disorders. The Wistar-Kyoto (WKY) rat is an often-used animal model of depression, but no information so far exists on its neuroinflammatory profile. As such, we compared male young adult WKY rats to Wistar (WS) controls, with regard to both behavioral performance and brain levels of key neuroinflammatory markers. We first assessed anxiety- and depression-like behaviors in a battery consisting of the Elevated Plus Maze (EPM), the Novelty Suppressed Feeding (NSFT), Open Field (OFT), Social Interaction (SIT), Forced Swim (FST), Sucrose Preference (SPT), and Splash tests (ST). We found that WKY rats displayed increased NSFT feeding latency, decreased OFT center zone permanence, decreased EPM open arm permanence, decreased SIT interaction time, and increased immobility in the FST. However, WKY rats also evidenced marked hypolocomotion, which is likely to confound performance in such tests. Interestingly, WKY rats performed similarly, or even above, to WS levels in the SPT and ST, in which altered locomotion is not a significant confound. In a separate cohort, we assessed prefrontal cortex (PFC), hippocampus and amygdala levels of markers of astrocytic (GFAP, S100A10) and microglial (Iba1, CD86, Ym1) activation status, as well as of three key proinflammatory cytokines (IL-1β, IL-6, TNF-α). There were no significant differences between strains in any of these markers, in any of the regions assessed. Overall, results highlight that behavioral data obtained with WKY rats as a model of depression must be carefully interpreted, considering the marked locomotor activity deficits displayed. Furthermore, our data suggest that, despite WKY rats replicating many depression-associated neurobiological alterations, as shown by others, this is not the case for neuroinflammation-related alterations, thus representing a novel limitation of this model.

Behavioural characteristics and sex differences of a treatment-resistant depression model: Chronic mild stress in the Wistar-Kyoto rat

Depression affects 20% of the general population and is a leading cause of disability worldwide, with a strong female prevalence. Current pharmacotherapies have significant limitations, and one third of patients are unresponsive. Male Wistar-Kyoto rats exposed to chronic mild stress (CMS) were recently proposed as a model to study antidepressant resistance. However, sex differences and interindividual vulnerability to stress are yet unexplored in this model. We aimed to investigate these in the context of the behavioural impact of CMS in the sucrose preference test, elevated plus maze (EPM), forced swim test (FST), open field test and daily locomotor activity rhythms, in male and female WKY rats exposed or not to a 4-week CMS protocol. CMS-exposed animals were clustered through K-means into subgroups based on the EPM and FST results. In both sexes, one subgroup behaved similarly to non-stressed animals and was labelled stress-non vulnerable; the second exhibited less open arms exploration in the EPM and higher immobility in the FST and was named stress-vulnerable. Vulnerable males presented phase delay in daily locomotor activity following CMS, but no significant rhythm could be determined in females. CMS-exposed males of both groups showed hyperlocomotion in reaction to novelty and slower weight gain through the course of CMS, while CMS-exposed females showed smaller sucrose intake. Unexpectedly, CMS did not affect sucrose preference. Our findings strengthen the view that in models of psychiatric pathologies based on stress exposure it is important to consider the effect of sex and to differentiate the non vulnerable and vulnerable subpopulations.

Current Insights into the Use of Probiotics and Fatty Acids in Alleviating Depression

(1) Background: Depression is the most prevalent psychiatric symptom present among individuals of all ages and backgrounds, impacting an estimated 300 million people globally. Therefore, it demands a significant amount of attention when it comes to managing depression. A growing amount of data reveal that probiotics and fatty acids could be beneficial to depression. However, the opposing position maintains that they have no influence on depression. A network meta-analyses of existing datasets aid in the estimation of comparative efficacy as well as in achieving an understanding of the relative merits of different therapies. The purpose of this study was to investigate the current evidence for probiotic or fatty acid depression therapy and to establish a practical alternative for depression patients using a meta-analysis and metagenomic data from a Wistar-Kyoto (WKY) depressed rat model. (2) Methods: Probiotic data were obtained from seven randomized controlled trial studies (n = 394), and fatty acid data were obtained from 24 randomized controlled trial studies (n = 1876). Meanwhile, a metagenomics analysis of data on animal gut flora was also applied to validate the preceding evidence. (3) Results: The fatty acid studies were separated into three sections based on the duration of probiotic delivery: ≤8 weeks, 9-12 weeks, and >12 weeks. The results were as follows: for ≤8 weeks, MD = -1.65 (95% CI: -2.96--0.15), p = 0.01; for 9-12 weeks, MD = -2.22 (95% CI: -3.03--1.22), p < 0.001; for >12 weeks, MD = -1.23 (95% CI: -2.85-0.39), p = 0.14. Regarding the probiotics, the meta-analysis revealed MD = -2.19 (95% CI: -3.38--2.43), p < 0.001. The research presented herein illustrates that probiotics and fatty acids may successfully lower depression scores. Additionally, the probiotics were drastically reduced in the WKY rats. (4) Conclusions: According to the data, a depression intervention utilizing probiotics outperformed the control, implying that the use of probiotics and fatty acids may be a successful strategy for depression treatment.

The sex-dependent response to psychosocial stress and ischaemic heart disease

Stress is an important risk factor for modern chronic diseases, with distinct influences in males and females. The sex specificity of the mammalian stress response contributes to the sex-dependent development and impacts of coronary artery disease (CAD). Compared to men, women appear to have greater susceptibility to chronic forms of psychosocial stress, extending beyond an increased incidence of mood disorders to include a 2- to 4-fold higher risk of stress-dependent myocardial infarction in women, and up to 10-fold higher risk of Takotsubo syndrome-a stress-dependent coronary-myocardial disorder most prevalent in post-menopausal women. Sex differences arise at all levels of the stress response: from initial perception of stress to behavioural, cognitive, and affective responses and longer-term disease outcomes. These fundamental differences involve interactions between chromosomal and gonadal determinants, (mal)adaptive epigenetic modulation across the lifespan (particularly in early life), and the extrinsic influences of socio-cultural, economic, and environmental factors. Pre-clinical investigations of biological mechanisms support distinct early life programming and a heightened corticolimbic-noradrenaline-neuroinflammatory reactivity in females vs. males, among implicated determinants of the chronic stress response. Unravelling the intrinsic molecular, cellular and systems biological basis of these differences, and their interactions with external lifestyle/socio-cultural determinants, can guide preventative and therapeutic strategies to better target coronary heart disease in a tailored sex-specific manner.

Chronic restraint stress produces sex-specific behavioral and molecular outcomes in the dorsal and ventral rat hippocampus

Stress-related disorders display differences at multiple levels according to sex. While most studies have been conducted in male rodents, less is known about comparable outcomes in females. In this study, we found that the chronic restraint stress model (2.5 h/day for 14 days) triggers different somatic responses in male and female adult rats. Chronic restraint produced a loss in sucrose preference and novel location preference in male rats. However, chronic restraint failed to produce loss of sucrose preference in females, while it improved spatial performance. We then characterized the molecular responses associated with these behaviors in the hippocampus, comparing the dorsal and ventral poles. Notably, sex- and hippocampal pole-specific transcriptional signatures were observed, along with a significant concordance between the female ventral and male dorsal profiles. Functional enrichment analysis revealed both shared and specific terms associated with each pole and sex. By looking into signaling pathways that were associated with these terms, we found an ample array of sex differences in the dorsal and, to a lesser extent, in the ventral hippocampus. These differences were mainly present in synaptic TrkB signaling, Akt pathway, and glutamatergic receptors. Unexpectedly, the effects of stress on these pathways were rather minimal and mostly dissociated from the sex-specific behavioral outcomes. Our study suggests that female rats are resilient and males susceptible to the restraint stress exposure in the sucrose preference and object location tests, while the activity of canonical signaling pathways is primarily determined by sex rather than stress in the dorsal and ventral hippocampus.

The Wistar-Kyoto rat model of endogenous depression: A tool for exploring treatment resistance with an urgent need to focus on sex differences

Major depressive disorder (MDD) is one of the leading causes of years lived with disability and contributor to the burden of disease worldwide. The incidence of MDD has increased by ~20% in the last decade. Currently antidepressant drugs such as the popular selective serotonin reuptake inhibitors (SSRIs) are the leading form of pharmaceutical intervention for the treatment of MDD. SSRIs however, are inefficient in ameliorating depressive symptoms in ~50% of patients and exhibit a prolonged latency of efficacy. Due to the burden of disease, there is an increasing need to understand the neurobiology underpinning MDD and to discover effective treatment strategies. Endogenous models of MDD, such as the Wistar-Kyoto (WKY) rat provide a valuable tool for investigating the pathophysiology of MDD. The WKY rat displays behavioural and neurobiological phenotypes similar to that observed in clinical cases of MDD, as well as resistance to common antidepressants. Specifically, the WKY strain exhibits increased anxiety- and depressive-like behaviours, as well as alterations in Hypothalamic Pituitary Adrenal (HPA) axis, serotonergic, dopaminergic and neurotrophic systems with emerging studies suggesting an involvement of neuroinflammation. More recent investigations have shown evidence for reduced cortical and hippocampal volumes and altered glutamatergic signalling in the WKY strain. Given the growing interest in therapeutics targeting the glutamatergic system, the WKY strain presents itself as a potentially useful tool for screening novel antidepressant drugs and their efficacy against treatment resistant depression. However, despite the sexual dimorphism present in the pathophysiology and aetiology of MDD, sex differences in the WKY model are rarely investigated, with most studies focusing on males. Accordingly, this review highlights what is known regarding sex differences and where further research is needed. Whilst acknowledging that investigation into a range of depression models is required to fully elucidate the underlying mechanisms of MDD, here we review the WKY strain, and its relevance to the clinic.

Individual differences in inflammatory and oxidative mechanisms of stress-related mood disorders

Emotional stress leads to the development of peripheral disorders and is recognized as a modifiable risk factor for psychiatric disorders, particularly depression and anxiety. However, not all individuals develop the negative consequences of emotional stress due to different stress coping strategies and resilience to stressful stimuli. In this review, we discuss individual differences in coping styles and the potential mechanisms that contribute to individual vulnerability to stress, such as parameters of the immune system and oxidative state. Initial differences in inflammatory and oxidative processes determine resistance to stress and stress-related disorders via the alteration of neurotransmitter content in the brain and biological fluids. Differences in coping styles may serve as possible predictors of resistance to stress and stress-related disorders, even before stressful conditions. The investigation of natural variabilities in stress resilience may allow the development of new methods for preventive medicine and the personalized treatment of stress-related conditions.

Metabolome and microbiome profiling of a stress-sensitive rat model of gut-brain axis dysfunction

Stress negatively impacts gut and brain health. Individual differences in response to stress have been linked to genetic and environmental factors and more recently, a role for the gut microbiota in the regulation of stress-related changes has been demonstrated. However, the mechanisms by which these factors influence each other are poorly understood, and there are currently no established robust biomarkers of stress susceptibility. To determine the metabolic and microbial signatures underpinning physiological stress responses, we compared stress-sensitive Wistar Kyoto (WKY) rats to the normo-anxious Sprague Dawley (SD) strain. Here we report that acute stress-induced strain-specific changes in brain lipid metabolites were a prominent feature in WKY rats. The relative abundance of Lactococcus correlated with the relative proportions of many brain lipids. In contrast, plasma lipids were significantly elevated in response to stress in SD rats, but not in WKY rats. Supporting these findings, we found that the greatest difference between the SD and WKY microbiomes were the predicted relative abundance of microbial genes involved in lipid and energy metabolism. Our results provide potential insights for developing novel biomarkers of stress vulnerability, some of which appear genotype specific.

Stressor during Early Adolescence in Hyperreactive Female Wistar Kyoto Rats Induces a 'Double Hit' Manifested by Variation in Neurobehaviors and Brain Monoamines

Stress is an additive factor in the development of depressive-like profiles that mainly onsets during adolescence. However, effects of early post-weaning stress on developing brain neurochemical pathways in inducing anxiety- and depressive-like profiles in vulnerable females have not been extensively studied. The Wistar Kyoto (WKY) rat, a putative model of adolescent depression and stress-sensitivity could elucidate the pathophysiology of stress-related depression in vulnerability. Through such an approach, links between inherent risk for predisposition to depression and homotypic stress, as in a 'double hit' would unravel endocrine regulation, interference in developing neural pathways and neurobehaviors. Here, early adolescent WKY female rats were subjected to 1-h physical restraint over 7 days followed by neurobehavioral testing in the elevated plus maze (EPM) and forced swim test (FST). The stressor's effectiveness was assayed by plasma corticosterone (CORT) and altered functioning in depression-implicated brain areas by assaying monoamines/metabolites. Homotypic stress induced an anxiolytic-like response in the EPM with learned helplessness and reduced struggling behavior in FST. Significant elevation in CORT levels (p < 0.05) indicated an upregulated HPA axis. Medial prefrontal cortex, a still maturing brain area, exhibited increased serotonin (5-HT) metabolite (p < 0.01) and turnover rates (p < 0.01) indicative of altered/maladaptive serotonergic functioning. Nucleus accumbens (p < 0.05) and dorsal striatum (p < 0.01) also depicted increased 5-HT metabolite, with the latter also demonstrating reduced Dopamine turnover (p < 0.01) as a result of homotypic stress. Hence, female WKY rats could constitute a diathesis-stress model to study underlying mechanisms of stress-related depression.

What do we know about sex differences in depression: A review of animal models and potential mechanisms

Clinical studies have shown that women are more susceptible to depression than men. Sex differences in depression have been associated with social, cultural, as well as biological factors. In spite of extensive preclinical studies in animal models for depression that have been used for understanding the mechanisms of the disease as well as for new drug development, a substantive lack of attention on sex-specific phenotypes in depression might mask the effect of sex on the outcome. In this review article, we summarize findings on the influence of sex on behavior in the most commonly used animal models for depression. We also discuss the potential underlying mechanisms of such sex-dependent variation in the phenotype, particularly in the neuroendocrine system.

Hippocampal BDNF in physiological conditions and social isolation

Exposure of an organism to chronic psychosocial stress may affect brain-derived neurotrophic factor (BDNF) expression that has been implicated in the etiology of psychiatric disorders, such as depression. Given that depression in humans has been linked with social stress, the chronic social stress paradigms for modeling psychiatric disorders in animals have thus been developed. Chronic social isolation in animal models generally causes changes in hypothalamic-pituitary-adrenal axis functioning, associated with anxiety- and depressive-like behaviors. Also, this chronic stress causes downregulation of BDNF protein and mRNA in the hippocampus, a stress-sensitive brain region closely related to the pathophysiology of depression. In this review, we discuss the current knowledge regarding the structure, function, intracellular signaling, inter-individual differences and epigenetic regulation of BDNF in both physiological conditions and depression and changes in corticosterone levels, as a marker of stress response. Since BDNF levels are age dependent in humans and rodents, this review will also highlight the effects of adolescent and adult chronic social isolation models of both genders on the BDNF expression.

Immediate and delayed anxiety- and depression-like profiles in the adolescent Wistar-Kyoto rat model of endogenous depression following postweaning social isolation

In order to understand links that exist between inherited risk or predisposition, brain and behavioural development, endocrine regulation and social/environmental stimuli, animal models are crucial. The Wistar-Kyoto (WKY) rat has been shown to have validity as a model of adult and adolescent depression. While sex- and age-specific differences in some of the face, predictive and construct validities of the model such as depression-like behaviours have been established, anhedonia and anxiety using other induced anxiety paradigms such as elevated plus maze remain equivocal. First, post-weaning social isolation effects on inherent and induced anxiety behaviours were tested during two critical time periods, early- and mid-adolescence. Isolation induced immediate effects on novel environment-induced hyperactivity and anxiety-related behaviours. Adolescent WKYs demonstrated reduced 50-kHz ultrasonic vocalizations suggesting agoraphobia-like behaviours. Second, isolated rats, despite being subsequently social-/group-housed demonstrated longer lasting effects on social interaction measures and anhedonia. This establishes that the depression-like profile observed during early- and mid-adolescence persists into late adolescence and early adulthood in WKY. Further, that interventions at a later stage during adolescence may not be able to reverse early adolescent effects in the context of pre-disposition, thus highlighting the irreversibility of being double-hit during critical time periods of brain and behavioural development and maturation.

Antidepressant-like effects of buprenorphine in rats are strain dependent

The prevalence of major depressive disorder and the limited efficacy of conventional drug treatments provide significant impetus to develop novel and more rapidly acting antidepressants for individuals with treatment resistant forms of depression. The primary goal of these studies was to ascertain whether buprenorphine (BPN), a medically available drug with mixed effects at opioid receptors, was effective in behavioral tests using the Wistar Kyoto (WKY) rat strain, a rodent model of exaggerated depressive and anxiety behaviors that demonstrates resistance to certain antidepressants. As WKY rats are maintained by different sources, we assessed the behavioral effects of BPN using the modified rat forced swim test (FST) and the emergence test in WKY rat colonies obtained from different vendors. BPN dose-dependently reduced immobility and increased swimming behavior in the FST and reduced emergence latencies in two WKY lines (Charles River (WKY/NCrl) and Harlan laboratories (WKY/NHsd)) that also showed high baseline immobility in the FST. WKY rats from Taconic (WKY/NTac) did not show high baseline immobility in the FST or anxiety as had been previously reported, suggesting a drift in the phenotype of rats from this supplier. Furthermore, BPN did not reduce immobility in the FST or reduce latencies in the emergence test in WKY rats from Taconic. BPN also failed to produce antidepressant-like effects in Wistar and Sprague-Dawley rats. These results indicate a striking strain-selectivity for the effects of BPN, producing antidepressant and anxiolytic-like responses in WKY/NCrl and WKY/NHsd lines but not in the normosensitive control Wistar and Sprague-Dawley strains.

Metaplastic regulation of the median raphe nucleus via serotonin 5-HT1A receptor on hippocampal synaptic plasticity is associated with gender-specific emotional expression in rats

Gender differences in psychiatric disorders are considered to be associated with the serotonergic (5-HTergic) system; however the underlying mechanisms have not been clearly elucidated. In this study, possible involvement of the median raphe nucleus (MRN)-hippocampus 5-HTergic system in gender-specific emotional regulation was investigated, focusing on synaptic plasticity in rats. A behavioral study using a contextual fear conditioning (CFC) paradigm showed that the females exhibited low anxiety-like behavior. Extracellular 5-HT levels in the hippocampus were increased by CFC only in the males. Long-term potentiation (LTP) in the hippocampal CA1 field was suppressed after CFC in the males, which was mimicked by the synaptic response to MRN electrical stimulation. In the MRN, 5-HT immunoreactive cells significantly increased in the females compared with those in the males. Pretreatment with the 5-HT1A receptor agonists tandospirone (10 mg/kg, i.p.) and 8-OH DPAT (3 mg/kg, i.p.) significantly suppressed LTP induction in the males. Synaptic responses to CFC and 5-HT1A receptor interventions were not observed in the females. These results suggest that the metaplastic 5-HTergic mechanism via 5-HT1A receptors in the MRN-hippocampus pathway is a key component for gender-specific emotional regulation and may be a cause of psychiatric disorders associated with vulnerability or resistance to emotional stress.

Chronic social defeat up-regulates expression of the serotonin transporter in rat dorsal raphe nucleus and projection regions in a glucocorticoid-dependent manner

Chronic stress and dysfunction of the serotonergic system in the brain have been considered two of the major risks for development of depression. In this study, adult Fischer 344 rats were subjected to a regimen of chronic social defeat (CSD). To mimic stressful conditions, some rats were not exposed to CSD, but instead treated with corticosterone (CORT) in oral solution while maintained in their home cage. Protein levels of the serotonin transporter (SERT) in the dorsal raphe nucleus (DRN), hippocampus, frontal cortex, and amygdala were examined by Western blotting or immunofluorescence staining. The results showed that CSD up-regulated SERT protein levels in the DRN, hippocampus, frontal cortex, and amygdala regions. This up-regulation was abolished or prevented by adrenalectomy, or treatment with antagonists of corticosteroid receptors mifepristone and spironolactone, alone or in combination. Similarly, up-regulated SERT protein levels in these brain regions were also observed in rats treated with oral CORT ingestion, which was analogously prevented by treatment with mifepristone and spironolactone. Furthermore, both CSD- and CORT-induced up-regulation of SERT protein levels in the DRN and three brain regions were attenuated by simultaneous treatment with fluoxetine, an antidepressant that specifically inhibits serotonin reuptake. The results indicate that up-regulation in SERT protein levels in the DRN and forebrain limbic structures caused by CSD regimen was mainly motivated by CORT through corticosteroid receptors. The present findings demonstrate that chronic stress is closely correlated with the serotonergic system by acting on the regulation of the SERT expression in the DRN and its projection regions, which may contribute to the development of depression.

Adverse early life experience and social stress during adulthood interact to increase serotonin transporter mRNA expression

Anxiety disorders, depression and animal models of vulnerability to a depression-like syndrome have been associated with dysregulation of serotonergic systems in the brain. To evaluate the effects of early life experience, adverse experiences during adulthood, and potential interactions between these factors on serotonin transporter (slc6a4) mRNA expression, we investigated in rats the effects of maternal separation (180 min/day from days 2 to 14 of life; MS180), neonatal handing (15 min/day from days 2 to 14 of life; MS15), or normal animal facility rearing (AFR) control conditions with or without subsequent exposure to adult social defeat on slc6a4 mRNA expression in the dorsal raphe nucleus (DR) and caudal linear nucleus. At the level of specific subdivisions of the DR, there were no differences in slc6a4 mRNA expression between MS15 and AFR rats. Among rats exposed to a novel cage control condition, increased slc6a4 mRNA expression was observed in the dorsal part of the DR in MS180 rats, relative to AFR control rats. In contrast, MS180 rats exposed to social defeat as adults had increased slc6a4 mRNA expression throughout the DR compared to both MS15 and AFR controls. Social defeat increased slc6a4 mRNA expression, but only in MS180 rats and only in the "lateral wings" of the DR. Overall these data demonstrate that early life experience and stressful experience during adulthood interact to determine slc6a4 mRNA expression. These data support the hypothesis that early life experience and major stressful life events contribute to dysregulation of serotonergic systems in stress-related neuropsychiatric disorders.

Sex differences in skilled movement in response to restraint stress and recovery from stress

Sex differences exist in skilled movement, and skilled motor performance is also influenced by stress. As shown for cognitive function, the effects of stress are usually characterized by considerable sexual dimorphism. The purpose of this study was to investigate sex differences in skilled motor function in response to stress. Male and female Long-Evans rats were trained and tested in skilled reaching and skilled walking tasks. Both groups of animals were then exposed to daily restraint stress for 15 days. Recovery from daily stress was assessed by comparing reaching performance at 10 min versus 60 min after restraint stress, and recovery from chronic stress was tested for 21 days after cessation of stress. Animals were tested daily in skilled reaching for the entire period. Observations showed that females performed significantly better than males during the stress period in terms of reaching success and number of attempts needed to grasp a food pellet. No difference between testing at 10 or 60 min after daily stress was found. Analysis of movement patterns and recovery from stress indicated that males and females use different strategies to overcome stress-induced motor disturbance. While male rats preferred to use original movement patterns, females tended to modify these patterns in order to increase reaching success. Modification of movement patterns in female rats was accompanied by a faster recovery in success rate after the cessation of stress. These results indicate sex differences in skilled reaching in response to stress, and in the recovery period after stress.

Wistar Kyoto rats exhibit reduced sucrose pellet reinforcement behavior and intravenous nicotine self-administration

A phenotype of heightened anxiety-like behavior is hypothesized to be associated with altered reinforcement behavior. To test this hypothesis, we studied patterns of sucrose pellet intake and intravenous nicotine self-administration in animals that exhibit anxiety-like behavior at baseline, Wistar Kyoto (WKY) rats, as compared to normal controls (Wistar rats). WKY rats exhibited significantly reduced sucrose pellet self-administration behavior as assessed by both fixed and progressive ratio schedules of reinforcement and exhibited significantly reduced self-administration of intravenous nicotine. On the basis of previously published findings, we hypothesize that altered mesolimbic dopamine responses, as well as heightened HPA axis functioning, may account for reduced nicotine self-administration and sucrose pellet reinforcement responding in WKY rats. These studies highlight the role of heightened anxiety-like behavior, resulting from the genetic background of the animal, in altering behavioral responses to reinforcing stimuli.

Stress-induced reductions of sensory reactivity in female rats depend on ovarian hormones and the application of a painful stressor

The current experiments occurred in the context of disputes in the literature concerning whether inescapable stress causes differential changes in sensory reactivity, which could lead to differences in many learning procedures. We tested rats for differences in sensitivity and responsivity to acoustic stimuli through the use of the acoustic startle response (ASR) 2 h after stressor exposure and ambulatory activity 24 h later in the open field. Stressed females showed reduced responsivity to acoustic stimuli with no apparent shift in stimulus sensitivity. Males did not show differences in either reactivity index following stressor exposure. Reduced responsivity did not occur if females had been OVX (OVX alone did not effect stimulus responsivity or sensitivity). All groups that experienced tailshock stress also had reduced open field activity 24 h later. Restraint for 2 h did not reduce stimulus responsivity in the ASR or open field activity in female rats. Acute reductions in ASRs after a painful stressor appear to be a feature specific to females, with an apparent role of ovarian hormones as a modulator of the effect. Possible hormone and/or immunological mechanisms of these sex-specific effects are discussed. Understanding the mechanisms of this stressor-induced reduction in sensory reactivity could advance our knowledge of how individual differences in ovarian hormone levels influence the physical and psychological processes by which females acutely respond and later recover from traumatic events.

Differences in serotonergic neurotransmission between rats displaying high or low anxiety/depression-like behaviour: effects of chronic paroxetine treatment

Disturbances in serotonergic neurotransmission have been suggested to be closely interlinked with hyperactivity of the hypothalamic-pituitary-adrenocortical (HPA) system, and are likely to be involved in the pathophysiology of anxiety disorders and major depression. We therefore investigated markers of serotonergic transmission and their modulation by chronic paroxetine in rats selectively bred for high (HAB) or low (LAB) anxiety-related behaviour, both under basal conditions and in response to emotional stress. Hippocampal serotonin 1 A (5-HT1A) receptor mRNA expression was reduced in HAB rats, whereas 5-HT concentrations in hippocampal microdialysates did not differ between HAB and LAB rats under basal conditions. In the hippocampus, overall expression of serotonin transporter binding sites was increased in HAB compared with LAB rats. Exposure to emotional stress failed to increase intrahippocampal 5-HT release in HAB rats whereas LAB rats displayed a physiological, albeit small rise. Chronic paroxetine treatment markedly increased the stress-induced rise in hippocampal 5-HT in HAB, but not LAB rats. This effect may be (at least in part) related to a greater down-regulation of hippocampal serotonin transporter binding sites by paroxetine in HABs compared with LABs, while 5-HT1A receptor expression remained unaffected in this brain area. The findings indicate reduced hippocampal serotonergic transmission in HAB rats as compared with LAB rats, which is evident both at the presynaptic (5-HT release) and the postsynaptic (5-HT1A receptor) level. Chronic paroxetine enhanced the presynaptic responsivity in HAB rats, but not LAB rats, pointing to a preferential efficacy of paroxetine in rats with enhanced anxiety/depression-related behaviour.

The role of palatable food and hunger as trigger factors in an animal model of stress induced binge eating

OBJECTIVE

Dieting and stress are etiological factors in eating disorders, and dieting strongly predicts stress-induced overeating in the nonclinical population. We developed an animal model of binge eating in sated rats that is evoked by stress, but only in rats with a history of caloric restriction and only if highly palatable food (HPF) is available after stress. This study investigated the effect of known binge triggers, a taste of HPF and of hunger, on this type of binge eating.

METHOD

Female rats were cycled through the R/S protocol but this time were given just a taste of HPF with ad lib regular chow. After another R/S cycle, rats were stressed during restriction (while hungry) and were given HPF and chow.

RESULTS

Although binge eating did not occur if only chow was available after stress, just a taste of HPF sufficed to increase chow intake to more than 160% (p < 0.001) of rats with a history of restriction only, stress-only, or neither. Hunger increased the proportion of chow consumed by both restricted groups, but stress magnified this hunger-induced overeating by increasing HPF intake to 137% of restriction-only rats (p < 0.001).

DISCUSSION

These effects suggest that binge eating in this model is motivated by reward, not metabolic need, and parallels observations of binge triggers described in clinical binge-eating disorders. This strengthens the validity of using this animal model to target the physiology and treatment of eating disorders preceded by dieting and stress.

Fearfulness and sex in F2 Roman rats: males display more fear though both sexes share the same fearfulness traits

The pattern of sex differences in a large sample (about 400 for each sex) of F2-generation rats, derived from inbred Roman high- and low-avoidance strains differing in fearfulness and brain functioning, was investigated. We obtained measures from responses to a battery of novel/threatening tests [open field (OF), plus maze (PM), hole board (HB), activity (A), and acoustic startle reflex (ASR)] as well as learned fear paradigms [classical fear conditioning (CFC) and shuttlebox avoidance conditioning (SAC)]. The results showed that almost all behaviors assessed fit with a pattern of unidirectional sex effects characterized by male rats as being more fearful than females: males defecated more than females in the OF, PM, HB, ASR, and CFC; ambulated less in the OF, PM, A, and SAC; showed more self-grooming in PM and HB; explored the open arms of the PM and the holes of the HB less; displayed enhanced ASR; and showed poorer performance in the SAC task. We applied two factor analyses to each sex showing that, in general, they shared a common three-factor structure: a Learned Fear Factor comprising SAC and CFC responding, a Fear of Heights/Open Spaces Factor with the highest loadings for open arm behavior in the PM, and an Emotional Reactivity Factor, mainly grouping defecations, ambulation, and self-grooming. These results indicate that the essential components of fearful behavior are similar for both sexes in an inbred but genetically heterogeneous population.

Coping strategies in male and female rats exposed to multiple stressors

Because of the pathogenic effects of chronic stress exposure, it is important to identify factors, such as effective coping strategies, that mitigate stress-induced pathology. Of interest in the present study was the consistency of behavioral responses across a diverse array of stressors. Sixteen male and 16 female Long-Evans rats were assigned to either a stress or control group. The stressed animals were subsequently exposed to a battery of ecologically relevant stressors (e.g., predator odor, novel stimuli, and immunological challenge) to determine trends in coping strategies. Blood was collected for corticosterone (CORT) assay and brains were harvested for assessment of fos immunoreactivity in the paraventricular hypothalamus (PVH) and central amygdala (CEA) following exposure to the final stressor of fox urine. A correlational analysis indicated that certain response strategies (e.g., latency to respond in different stress tests such as the open-field and novel item tests) persist across several behavioral tests, especially those tests involving exploratory components. A subsequent principal component factor analysis revealed the following four components: initiative to explore, low reactivity, variable reactivity, and high reactivity. Females exhibited higher recovery CORT levels than males; however, sex only affected one behavioral response measure (i.e., females demonstrated more attempts to climb the wall in the forced-swim test than their male counterparts). In conclusion, these results support the importance and prevalence of initiative to explore as a common factor in many stress tests; additionally, the principal component analysis indicated that physiological correlates of stress are more closely associated with more challenging environments and stimuli such as forced swimming, immunological challenges, and exposure to predator odors.

A new animal model of binge eating: key synergistic role of past caloric restriction and stress

Dieting and stress are important in the etiology and maintenance of eating disorders, and dieting strongly predicts stress-induced overeating in humans. We hypothesized that caloric restriction and stress interact in a unique manner to promote binge eating. To test this hypothesis, a group of young female rats were cycled through a restriction period (4 days of 66% of control food intake) followed by 6 days of free feeding prior to being stressed by acute foot shock. After three of these cycles, the food intake of rats exposed only to restriction (R), or only to stress (S), did not differ from controls. However, R+S rats that were restricted and refed, despite normal body weight and food intake after free feeding, engaged in a powerful bout of hyperphagia when stressed (Experiment 1). The R + S effect was replicated in an older group of rats (Experiment 2). The hyperphagia was characteristically binge-like, it constituted a 40% selective increase in highly palatable (HP) food (P < .001) over a discrete period of time (within 24 h post-stress), and reflected feeding for reward (higher HP:chow ratio) over metabolic need as occurred after restriction (higher chow:HP ratio). Subsequent experiments revealed that binge eating did not occur if only chow was available (Experiment 3) or if restriction-refeeding (R-R) did not proximally precede stress (Experiment 4). Experiment 5 revealed that a history of R-R cycles followed by only one stress episode was sufficient to increase intake to 53% above controls as early as 2 h after stress (P < .001). This animal model of binge eating should facilitate investigations into the neurochemical changes induced by dieting and environmental stress to produce disordered eating and provide a preclinical tool to test preventive strategies and treatments more relevant to bulimia nervosa, multiple cases of binge eating disorder (BED) and binge-purge type anorexia nervosa.

Repeated experience of social defeats increases serotonin transporter and monoamine oxidase A mRNA levels in raphe nuclei of male mice

Serotonin transporter (SERT) and monoamine oxidase A (MAOA) mRNA levels in the raphe nuclei area of the midbrain were measured by the multiplex reverse transcription-polymerase chain reaction method in male mice with repeated experience of social victories (winners) and defeats (losers) in ten daily agonistic confrontations. Experiments revealed enhanced SERT and MAOA mRNA levels in the losers compared with the winners and controls. It has been supposed that SERT and MAOA genes are involved in enhancement of serotonin inactivation in response to the increase of serotonergic activity shown earlier in the losers. A positive correlation between MAOA and SERT mRNA levels in the raphe nuclei area of the midbrain was shown.

Alterations induced by gestational stress in brain morphology and behaviour of the offspring

Retrospective studies in humans suggest that chronic maternal stress during pregnancy, associated with raised plasma levels of CRH, ACTH and cortisol may increase the likelihood of preterm birth, developmental delays and behavioural abnormalities in the children. In adulthood, it may contribute to the significant association between the incidence of schizophrenia, increased left or mixed handedness, reduction in cerebral asymmetry and anomalies in brain morphology. Our studies and others have shown that prenatal stress in rats can mimic these developmental and behavioural alterations. These rats show a reduced propensity for social interaction, increased anxiety in intimidating or novel situations and a reduction in cerebral asymmetry and dopamine turnover, consistent with those in schizophrenic humans. Prenatally-stressed (PS) rats also show behaviour consistent with depression, including a phase-shift in their circadian rhythm for corticosterone, sleep abnormalities, a hedonic deficit and greater acquisition of learned helplessness under appropriate conditions. These behavioural abnormalities are associated with impaired regulation of the hypothalamic-pituitary-adrenal axis response to stress and increased CRH activity. PS males may show demasculinisation and feminisation of their sexual behaviour. The developmental and behavioural abnormalities in PS offspring could occur through sensitisation of the foetal brain by maternal stress hormones to the action of glucocorticoid and CRH and to neurotransmitters affected by them. This may have long-lasting consequences and could explain the precipitation of depressive symptoms or schizophrenia by psychosocial stress in later life. The character of the behavioural abnormalities probably depends on the timing of the maternal stress in relation to development of the particular neuronal systems.