Repeated Restraint Stress Research Articles

Repeated restraint stress increases active coping behavior in lactating female mice

Repeated restraint stress increases active coping behavior in lactating female mice

Changes in mouse thymus after exposure to tube-restraint stress

The thymus is the primary lymphoid organ involved in the regulation of the immune and endocrine systems. It is particularly sensitive to various types of stress, which induce its atrophy. This study deals with the effect of repeated restraint stress on the weight, proliferation and apoptosis of the thymus in mice. During restraint, the animals were placed in 50-mL conical plastic tubes for 2 h every day for either 10 or 20 consecutive days. A significant reduction in thymus weight along with decreased cellularity and pronounced atrophy of the cortical part of the thymus was observed in animals exposed to repeated tube-restraint stress for 10 and 20 consecutive days. The observed changes in the thymus were the same, regardless of the number of days of exposure to stress. These findings provide a more comprehensive view of repeated tube-restraint, with special emphasis on its duration on stress-induced thymus atrophy. The presented findings could serve as a basis for further studies aimed at identifying the mechanisms responsible for the adaptive response of the thymus after repeated exposure to stress.

Habituation of the cardiovascular response to restraint stress is inhibited by exposure to other stressor stimuli and exercise training.

This study evaluated the effect of exposure to either a chronic variable stress (CVS) protocol or social isolation, as well as treadmill exercise training, in the habituation of the cardiovascular response upon repeated exposure to restraint stress in rats. The habituation of the corticosterone response to repeated restraint stress was also evaluated. For this, animals were subjected to either acute or 10 daily sessions of 60 min of restraint stress. CVS and social isolation protocols lasted for 10 consecutive days, whereas treadmill training was performed for 1 h per day, 5 days per week for 8 weeks. We observed that the increase in serum corticosterone was reduced during both the stress and the recovery period of the 10th session of restraint. Habituation of the cardiovascular response was identified in terms of a faster return of heart rate to baseline values during the recovery period of the 10th session of restraint. The increase in blood pressure and the decrease in tail skin temperature were similar at the 1st and 10th session of restraint. Exposure to CVS, social isolation or treadmill exercise training inhibited the habituation of the restraint-evoked tachycardia. Additionally, CVS increased the blood pressure response at the 10th session of restraint, whereas social isolation enhanced both the tachycardia during the first session and the drop in skin temperature at the 10th session of restraint. Taken together, these findings provide new evidence that pathologies evoked by stress might be related to impairment in the habituation process to homotypic stressors.

Interactions between pyridostigmine bromide and stress on glutamatergic neurochemistry: Insights from a rat model of Gulf War Illness

Pyridostigmine bromide (PB) was administered to soldiers during the first Gulf War as a prophylactic treatment to protect against toxicity in the event of exposure to nerve agents. Although originally thought to pose minimal risk to soldiers, epidemiological studies have since correlated PB administration with the development of a variety of symptoms, including cognitive dysfunction, termed Gulf War Illness (GWI). We previously demonstrated in a rodent model of GWI that central cholinergic responses were altered to various stimuli. In the current study we used in vivo microdialysis to examine how combinations of PB and repeated restraint stress (RRS) altered extracellular glutamate levels in response to an innate immune challenge (lipopolysaccharide; LPS) and an immobilization stress challenge in the prefrontal cortex (PFC) and hippocampus. There were four groups in this study: vehicle non-stressed control (Veh-NSC), vehicle-stressed (Veh-RRS), PB-NSC, and PB-RRS. While LPS decreased glutamate levels in PB-treated rats relative to vehicle-treated rats in the PFC, PB and stress interacted to attenuate LPS-induced decreases in hippocampal glutamate levels. Although immobilization stress increased glutamate in the PFC, glutamate levels in PB-NSC rats failed to recover in the post-stress period relative to vehicle-treated rats. In the hippocampus, PB-stressed rats failed to exhibit habituation of the glutamate response to immobilization stress relative to vehicle-stressed rats. Collectively, these results indicate that PB and stress interacted to produce brain-region specific effects on glutamate neurochemistry, providing insight into the potential mechanisms underlying interactions between the immune system and persistent cognitive dysfunction in veterans with GWI.

Repeated restraint stress potentiates methylphenidate and modafinil-induced behavioral sensitization in rats.

Stress increases the susceptibility of drug abuse and drugs of abuse impair behavioral tolerance. It has been shown that stress exposure enhances the sensitivity to the reinforcing properties of drugs, augments locomotor sensitization effects of drugs of abuse and impairs behavioral tolerance. Previously, it has been shown that long-term administration of psychostimulants (Methylphenidate and Modafinil) induced locomotor sensitization effect that was more pronounced after 13days of drug administration and was greater at high dose. The present study is designed to investigate the relationship between restraint stress and psychostimulants (Methylphenidate and Modafinil) that induced sensitization. Methylphenidate (10mg/kg/day twice a day), modafinil (75mg/kg/day once daily), and saline (0.9% NaCl; 1ml/kg/day) were administered orally to treated and control animals. Rats were exposed to immobilization stress for 30days (until locomotor sensitization produced) to monitor any change in drug-induced behavioral sensitization. The motor activity was compared daily by using familiar environment of home cage and weekly by novel environment of open field. The results show that the methylphenidate and modafinil-induced locomotor sensitization is enhanced and impaired behavioral tolerance in repeated restrained rats. It shows that the psychostimulants like methylphenidate and modafinil produce greater locomotor sensitization in stressful environment, suggesting addictive effects of stress and psychostimulants (methylphenidate/modafinil) on dopaminergic neurotransmission. These finding may be helpful to develop potential pharmacotherapies for the patients with co-occurring depression and substance abuse/dependence disorder.

P.125 Samidorphan, an opioid receptor antagonist, mitigates olanzapine-induced metabolic dysfunction in female rats

We have earlier reported antidepressant-like effect of Cnestis ferruginea and its bioflavonoid constituent, amentoflavone in behavioural paradigms but its effects on neurochemical and neuroendocrine systems are yet to be elucidated. This study sought to investigate the effect of subchronic treatment of C. ferruginea (CF) on monoamines system, hypothalamo-pituitary adrenal axis and nitrosative/oxidative stresses. Male albino rats (150–200 g) randomly divided into seven groups; Group I: vehicle treated (0.2%v/v Tween 80 in normal saline (10 ml/kg; p.o.; unstressed), Group II: vehicle treated + restraint stress, Group III: imipramine (20 mg/kg; p.o.), Group IV–VI: CF (12.5, 50, or 100 mg/kg; p.o., respectively), Group VII: CF 6.25 + imipramine 5 mg/kg. One hour post-treatment, animals were subjected to 20 min restraint stress and 6 min, forced swim test (FST) for a period of 14 days. CF (12.5, 50 and 100 mg/kg) produced significant reduction in immobility time and an increase in climbing behaviour. CF attenuated repeated restraint stress × FST-induced serum corticosterone [F(6,28) = 5.45,P < 0.01]. Exposure of rats to FST + restraint stress paradigms produced significant (P < 0.05) increase in malondialdehyde and nitrite level, also reduced the glutathione level and superoxide dismutase activity which was reversed by subchronic treatment of rats with CF. Restraint stress × FST significantly decreased NA, DA and 5-HT concentrations, with increased DA and 5-HT turnover ratios in discrete brain regions which was ameliorated by CF or imipramine subchronic treatment. These results suggest that the antidepressant-like effect of C. ferruginea involved enhancement of monoamines and antioxidant as well as normalization of neuroendocrine systems.

Cocaine and chronic stress exposure produce an additive increase in neuronal activity in the basolateral amygdala

Cocaine addiction is a chronic, relapsing disorder. Stress and cues related to cocaine are two common relapse triggers. We have recently shown that exposure to repeated restraint stress during early withdrawal accelerates the time-dependent intensification or "incubation" of cue-induced cocaine craving that occurs during the first month of withdrawal, although craving ultimately plateaus at the same level observed in controls. These data indicate that chronic stress exposure during early withdrawal may result in increased vulnerability to cue-induced relapse during this period. Previous studies have shown that chronic stress exposure in drug-naïve rats increases neuronal activity in the basolateral amygdala (BLA), a region critical for behavioral responses to stress. Given that glutamatergic projections from the BLA to the nucleus accumbens are critical for the incubation of cue-induced cocaine craving, we hypothesized that cocaine withdrawal and chronic stress exposure produce separate increases that additively increase BLA neuronal activity. To assess this, we conducted in vivo extracellular single-unit recordings from the BLA of anesthetized adult male rats following cocaine or saline self-administration (6 h/day for 10 days) and repeated restraint stress or control conditions on withdrawal days (WD) 6-14. Recordings were conducted from WD15 to WD20. Interestingly, cocaine exposure alone increased the spontaneous firing rate in the BLA to levels observed following chronic stress exposure in drug-naïve rats. Chronic stress exposure during cocaine withdrawal further increased firing rate. These studies may identify a potential mechanism by which both cocaine and chronic stress exposure drive cue-induced relapse vulnerability during abstinence.

Transgenerational effects of ancestral prenatal stress on the gut-brain axis

The effects of ancestral prenatal stress can propagate across generations to alter the well-being of directly and indirectly exposed descendants via epigenetic mechanisms. Prenatal stress has been shown to alter the function of the gut-brain axis, a bi-directional signaling pathway between the gut microbiome and the enteric and central nervous systems. There has been no study investigating the impact of remote prenatal stress in ancestors on the gut-microbiome connection. Here we investigated if exposure to transgenerational ancestral stress affects the gut-brain axis through changes in the microbiome and microbiota. A multigenerational rat cohort consisting of a F0, F1, F2, and F3 generation was utilized in this study. Pregnant dams in the F0 generation were exposed to repeated restraint stress and overnight social isolation from gestational days 12-18. Breeding of three successive generations occurred in the absence of gestational stress along with a lineage of yoked controls. Fecal collection occurred for males and female in each generation at the age of 30 days, 90 days, and 115 days. Fecal samples were analyzed using 1H-NMR spectroscopy to examine the metabolome. The data are being analysed using supervised and unsupervised machine learning approaches. The data are expected to reveal that the fecal metabolome is characteristically altered by ancestral prenatal stress in each generation, resulting in a biomarker signature that is linked to the behavioural phenotype. We predict changes in the gut metabolome and microbiome to be most significant in the F3 generation. These findings could lead to further understanding of intestinal dysbiosis and its impact on the brain, and sex-specific metabolic biomarkers that are predictive of stress-associated adverse health outcomes.

Repeated Restraint Stress Enhances Hepatic TFR2 Expression and Induces Hepatic Iron Accumulation in Rats.

Abnormal hepatic iron metabolism is detrimental to health. The objective of this study was to detect repeated restraint stress on liver iron metabolism in rats. Twenty-four male rats aged 7 weeks were randomly divided into 2 groups: control group (Con) and repeated restraint stress group (RS). Rats were subjected to 6 h of daily restraint stress for 14 consecutive days in the repeated restraint stress group. The results showed that repeated restraint stress exposure decreased growth performance including impaired final weight (P = 0.07), reducing average daily gain (P = 0.01), and average daily feed intake (P = 0.00) during the 14-day experimental period. Repeated restraint stress exposure did not affect hemoglobin content and plasma iron parameters except downregulated unsaturated iron-binding capacity (P = 0.04). Repeated restraint stress exposure inhibited liver development (P = 0.03) and induced liver iron accumulation (P = 0.05). In addition, repeated restraint stress downregulated the expression of transferrin (TF) and transferrin receptor 2 (TFR2) at the mRNA level (P < 0.01), but upregulated at the protein level (P = 0.03 for TF; P = 0.00 for TFR2). These results indicated that repeated restraint stress induces hepatic iron accumulation, which is closely related to higher expression of hepatic TFR2 protein in rats.

Axonal iron transport in the brain modulates anxiety-related behaviors.

Iron is essential for a broad range of biochemical processes in the brain, but the mechanisms of iron metabolism in the brain remain elusive. Here we show that iron functionally translocates among brain regions along specific axonal projections. We identified two pathways for iron transport in the brain: a pathway from ventral hippocampus (vHip) to medial prefrontal cortex (mPFC) to substantia nigra; and a pathway from thalamus (Tha) to amygdala (AMG) to mPFC. While vHip-mPFC transport modulates anxiety-related behaviors, impairment of Tha-AMG-mPFC transport did not. Moreover, vHip-mPFC iron transport is necessary for the behavioral effects of diazepam, a well-known anxiolytic drug. By contrast, genetic or pharmacological promotion of vHip-mPFC transport produced anxiolytic-like effects and restored anxiety-like behaviors induced by repeated restraint stress. Taken together, these findings provide key insights into iron metabolism in the brain and identify the mechanisms underlying iron transport in the brain as a potential target for development of novel anxiety treatments.

Pharmacological Effects of Yokukansankachimpihange on Urinary Catecholamine in Restraint-stressed Mice

Herbal medicines, acupuncture and moxibustion are often used for unidentified complaints. It is well known that catecholamine secreted by the sympatho-adrenal medullary system primarily functions to increase cardiac output and raise glucose levels in the blood during acute stress. In the present study, the effects of yokukansankachimpihange (YKSKCH, a Kampo medicine) on urinary catecholamine in mice that were repeatedly stressed by restraining were examined. Restraint stress (240 min/d×3 d×3 cycles, daytime: 12:00-16:00) induced a marked increase in noradrenaline (NA) and adrenaline (A) levels in the urine. Oral administration of YKSKCH (750 mg/kg of body weight) significantly inhibited the increase in urinary NA and A levels in mice after repeated restraint stress. In addition, the NA/dopamine (physical stress) and A/dopamine (mental stress) ratios were lower in the 750 mg/kg YKSKCH-treated group than in the control group. The tail suspension test was also performed and locomotor activity was investigated. Oral administration of YKSKCH at 750 mg/kg significantly reduced the immobility time, which was longer in mice after repeated restraint stress. Furthermore, oral administration of YKSKCH at 750 mg/kg increased locomotor activity, which was lower in mice after repeated restraint stress. These results suggest that YKSKCH has positive effects on mental and physical stress after repeated restraint stress, without reducing locomotor activity.

Maternal high fat diet programs hypothalamic-pituitary-adrenal function in adult rat offspring

Abstract Maternal environmental factors such as diet have profound effects on offspring development and later health. The hypothalamic-pituitary-adrenal (HPA) axis is an important stress neuroendocrine system that is subject to programming by early life challenges. The present study was further to investigate whether maternal high fat diet (HFD) exposure during rat pregnancy and lactation can alter the HPA axis activity in adult male offspring. We observed that maternal HFD consumption exerted long-term effects on the basal activity of the HPA axis in adult offspring, with increased mean plasma corticosterone levels that result from elevated steroid pulse frequence and pulse amplitude. More importantly, maternal HFD offspring displayed enhanced corticosterone responses to restraint (1 h) and lipopolysaccharide (25 μg/kg, iv) but not insulin-induced hypoglycemia (0.3U/kg, iv) stress, suggesting a stressor-specific effect of maternal diet on the hyperresponsiveness of the HPA axis to stress. Additionally, maternal HFD exposure markedly attenuated the habituation of HPA responses to repeated restraint stress. These findings demonstrate that perinatal HFD exposure has a potent and long-lasting influence on development of neuroendocrine regulatory mechanisms. Maternal HFD consumption significantly increased basal corticotropin-releasing factor (CRF) mRNA expression in the paraventricular nucleus; nevertheless, similar increments in CRF mRNA levels following restraint were observed between maternal HFD offspring and control rats. Furthermore, the medial and central nuclei of amygdala played a pivotal role in maternal HFD-induced sensitization of the HPA response to psychological and systemic stress, respectively, suggesting that different neural pathways may mediate maternal HFD-induced HPA hyperresponsivity to different types of stressors. Take together, the long-term effects of maternal HFD challenge on the central regulation of the HPA axis, therefore, expose the adult offspring to greater HPA function throughout lifespan, in stressor-specific and region-specific manner.

Anxiolytic effect of exercise against repeated restraint stress through 5-HT2A-mediated suppression of the adenosine A2A receptor in the basolateral amygdala

Anxiolytic effect of exercise against repeated restraint stress through 5-HT2A-mediated suppression of the adenosine A2A receptor in the basolateral amygdala

Disruptive effects of repeated stress on basolateral amygdala neurons and fear behavior across the estrous cycle in rats

Stress is a precipitating factor in depression and anxiety disorders. Patients with these disorders often show amygdala abnormalities. The basolateral amygdala (BLA) is integral in mood and emotion, and is sensitive to stress. While much is known about effects of stress on BLA neuron activity and morphology in males, less is known in females. We tested whether repeated stress exerts distinct effects on BLA in vivo neuronal activity and morphology of Golgi-stained BLA neurons [lateral (LAT) and basal (BA) nuclei] in adult female rats. Repeated restraint stress increased BLA neuronal firing and caused hypertrophy of BLA neurons in males, while it decreased LAT and BA neuronal firing and caused hypotrophy of neurons in the LAT of females. BLA neuronal activity and function, such as fear conditioning, shifts across the estrous cycle. Repeated stress disrupted this pattern of BLA activity and fear expression over the estrous cycle. The disruptive effects of stress on the pattern of BLA function across estrous may produce behavior that is non-optimal for a specific phase of the estrous cycle. The contrasting effects of stress may contribute to sex differences in the effects of stress on mood and psychiatric disorders.

Sex- and Stress-Dependent Effects on Dendritic Morphology and Spine Densities in Putative Orexin Neurons

We recently found that non-stressed female rats have higher basal prepro-orexin expression and activation of orexinergic neurons compared to non-stressed males, which lead to impaired habituation to repeated restraint stress at the behavioral, neural, and endocrine level. Here, we extended our study of sex differences in the orexin system by examining spine densities and dendritic morphology in putative orexin neurons in adult male and female rats that were exposed to 5 consecutive days of 30-min restraint. Analysis of spine distribution and density indicated that putative orexinergic neurons in control non-stressed females had significantly more dendritic spines than those in control males, and the majority of these were mushroom spines. This morphological finding may suggest more excitatory input onto orexin neurons in female rats. As orexin neurons are known to promote the hypothalamic–pituitary–adrenal response, this morphological change in orexin neurons could underlie the impaired habituation to repeated stress in female rats. Dendritic complexity did not differ between non-stressed males and females, however repeated restraint stress decreased total dendritic length, nodes, and branching primarily in males. Thus, reduced dendritic complexity of putative orexinergic neurons is observed in males but not in females after 5days of repeated restraint stress. This morphological change might be reflective of decreased orexin system function, which may allow males to habituate more fully to repeated restraint than females. These results extend our understanding of the role of orexin neurons in regulating habituation and demonstrate changes in putative orexin cell morphology and spines that may underlie sex differences in habituation.

The AT1 Receptor Antagonist Losartan Does Not Affect Depressive-Like State and Memory Impairment Evoked by Chronic Stressors in Rats.

The present study investigated the effect of the treatment with the angiotensin II type 1 receptor (AT1) antagonist losartan in the depressive-like state and memory impairment evoked by exposure to either homotypic (i.e., repeated exposure to the same type of stressor) or heterotypic (i.e., exposure to different aversive stimuli) chronic stressors in rats. For this, male Wistar rats were subjected to a 10 days regimen of repeated restraint stress (RRS, homotypic stressor) or chronic variable stress (CVS, heterotypic stressor) while being concurrently treated daily with losartan (30 mg/kg/day, p.o.). Depressive-like state was evaluated by analysis of the alterations considered as markers of depression (decreased sucrose preference and body weight and coat state deterioration), whereas cognitive non-emotional performance was tested using the novel object recognition (NOR) test. Locomotor activity was also evaluated in the open field test. Both RRS and CVS impaired sucrose preference and caused coat state deterioration, whereas only CVS impaired body weight gain. Besides, RRS impaired short-term memory (but not long-term memory) in the NOR test. Neither depressive-like state nor memory impairment evoked by the chronic stressors was affected by the treatment with losartan. Nevertheless, CVS increased the locomotion, which was inhibited by losartan. Taken together, these results provide evidence that the chronic treatment with losartan does not affect the depressive-like state and memory impairment evoked by either homotypic or heterotypic chronic stress regimens in rats.

Exercise exerts an anxiolytic effect against repeated restraint stress through 5-HT2A-mediated suppression of the adenosine A2A receptor in the basolateral amygdala

Repeated or chronic stressful stimuli induce emotion- and mood-related abnormalities, such as anxiety and depression. Conversely, regular exercise exerts protective effects. Here, we found that exercise recovered anxiety-like behaviors, as measured using the open field and elevated plus maze tests in an anxiety mouse model. In addition to behavioral improvement, exercise enhanced the synaptic density of the 5-hydroxytryptamine 2A receptor (5-HT2AR), but not the 5-HT1AR in the basolateral amygdala (BLA) region in this mouse model. Furthermore, global treatment with a selective 5-HT2AR antagonist (MDL11930) generated an anxiety phenotype. Thus, synaptic recruitment of 5-HT2AR in BLA neurons may mediate the anxiolytic effects of exercise. The exercise regimen also reduced adenosine A2A receptor (A2AR)-mediated protein kinase A (PKA) activation, and the anxiolytic effect of the exercise was blunted by local activation of A2AR within the BLA using CGS21680, a selective A2AR agonist. Particularly, A2AR-mediated PKA activity was shown to be dependent on 5-HT2AR signaling in the BLA. These results imply that repeated stress upregulates A2AR-mediated adenosine signaling to facilitate PKA activation, whereas regular exercise inhibits A2AR function by increasing 5-HT2AR in the BLA. Accordingly, this integrated modulation of 5-HT and adenosine signaling, via 5-HT2AR and A2AR respectively, may be a mechanism underlying the anxiolytic effect of regular exercise.

Stress resilience evidenced by grooming behaviour and dopamine levels in male mice selected for high and low immobility using the tail suspension test.

Grooming behaviour has different functions on many species during development and can be observed and affected during periods of stress. By selecting male mice with high (HI) and low (LI) immobility traits in the tail suspension test, a screening for antidepressant drugs, we investigate how these phenotypes associated with grooming behaviour may be influenced by the effects of repeated restraint stress. For this we used the sucrose preference test and the splash test in a novel and a familiar cage performed before and after exposure to 2days of restraint stress. Animals were submitted to an additional day of restraint stress before the hypothalamus, prefrontal cortex and midbrain extraction for dopamine activity analysis. Corticosterone analysis was made in three distinct moments: without stress (prior first restraint session), immediately after the last restrain, and 1hr after the last restrain episode. Compared to LI group, HI animals exhibited an increased frequency and decreased time of grooming in the familiar cage. In the novel cage, stress increased frequency and time of grooming of HI animals compared to LI. Corticosterone levels were increased in HI animals after 3days of stress. Lower hypothalamic dopaminergic activity without stress and decreased hypothalamic dopaminergic activity immediately after stress in HI group were observed. The HI group displayed decreased prefrontal cortex dopaminergic activity and increased activity in the mesolimbic area. We proposed that through the influence of stress the two phenotypes manifested as a resilient (LI) and a not resilient (HI) trait in response to restraint stress.

Case-control study of cognitive and communicative development in premature infants

Maternal environmental factors such as diet have profound effects on offspring development and later health. The hypothalamic-pituitary-adrenal (HPA) axis is an important stress neuroendocrine system that is subject to programming by early life challenges. The present study was further to investigate whether maternal high fat diet (HFD) exposure during rat pregnancy and lactation can alter the HPA axis activity in adult male offspring. We observed that maternal HFD consumption exerted long-term effects on the basal activity of the HPA axis in adult offspring, with increased mean plasma corticosterone levels that result from elevated steroid pulse frequence and pulse amplitude. More importantly, maternal HFD offspring displayed enhanced corticosterone responses to restraint (1 h) and lipopolysaccharide (25 μg/kg, iv) but not insulin-induced hypoglycemia (0.3U/kg, iv) stress, suggesting a stressor-specific effect of maternal diet on the hyperresponsiveness of the HPA axis to stress. Additionally, maternal HFD exposure markedly attenuated the habituation of HPA responses to repeated restraint stress. These findings demonstrate that perinatal HFD exposure has a potent and long-lasting influence on development of neuroendocrine regulatory mechanisms. Maternal HFD consumption significantly increased basal corticotropin-releasing factor (CRF) mRNA expression in the paraventricular nucleus; nevertheless, similar increments in CRF mRNA levels following restraint were observed between maternal HFD offspring and control rats. Furthermore, the medial and central nuclei of amygdala played a pivotal role in maternal HFD-induced sensitization of the HPA response to psychological and systemic stress, respectively, suggesting that different neural pathways may mediate maternal HFD-induced HPA hyperresponsivity to different types of stressors. Take together, the long-term effects of maternal HFD challenge on the central regulation of the HPA axis, therefore, expose the adult offspring to greater HPA function throughout lifespan, in stressor-specific and region-specific manner.

Effect of chronic stress on cardiovascular and ventilatory responses activated by both chemoreflex and baroreflex in rats.

Chronic stress results in physiological and somatic changes. It has been recognized as a risk factor for several types of cardiovascular dysfunction and changes in autonomic mechanisms, such as baroreflex and chemoreflex activity. However, the effects of different types of chronic stress on these mechanisms are still poorly understood. Therefore, in the present study, we investigated, in adult male rats, the effect of repeated restraint stress (RRS) or chronic variable stress (CVS) on baroreflex, chemoreflex and heart rate variability in a protocol of 14 days of stress sessions. Exposure to RRS and CVS indicated no changes in the basal level of either arterial pressure or heart rate. However, RRS and CVS were able to attenuate sympathovagal modulation and spontaneous baroreflex gain. Additionally, only RRS was able to increase the power of the low-frequency band of the systolic blood pressure spectrum, as well as the slope of linear regression of baroreflex bradycardic and tachycardic responses induced by vasoactive compounds. Additionally, our study is one of the first to show that exposure to RRS and CVS decreases the magnitude of the pressor response and potentiates respiratory responses to chemoreflex activation, which can trigger cardiovascular and respiratory pathologies. Furthermore, the basal respiratory parameters, such as minute ventilation and tidal volume, were significantly decreased by both protocols of chronic stress. However, only CVS increased the basal respiratory frequency. In this way, the findings of the present study demonstrate the impact of chronic stress in terms of not only depressive-like behavior but also alterations of the autonomic baroreflex responses and cardiocirculatory variability (systolic blood pressure and pulse interval).Our results provide evidence that chronic stress promotes autonomic dysregulation, and impairment of baroreflex, chemoreflex and heart rate variability.