Predictability of chronic intermittent stress: effects on sympathetic-adrenal medullary responses of laboratory rats

This experiment involved an examination of sympathetic-adrenal medullary responses of laboratory rats following exposure to chronic intermittent stress. Animals were assigned at random to one of three groups: (i) controls, handled briefly each day; (ii) restraint stress (RS), restrained for 30 min per day; or (iii) variable stress (VS), exposure to restraint, cold swim, or intermittent footshock during one of five time periods each day. On the 26th day, rats were prepared with chronic tail artery catheters for remote sampling of blood and direct measurement of mean arterial pressure and heart rate. On Day 28, rats of each group were exposed to 30 min of restraint stress and timed blood samples were collected and later analyzed for content of norepinephrine (NE) and epinephrine (EPI). VS rats gained significantly less body weight compared to control and RS rats. Basal measures of blood pressure and heart rate and of plasma NE and EPI were comparable for rats of the three groups. The plasma catecholamine responses to restraint stress on Day 28 were significantly reduced in RS and VS rats compared to first-time stressed controls. These findings suggest that predictability of the type of stressor and the time of its occurrence does not influence the pattern of diminished sympathetic-adrenal medullary responses of animals exposed to chronic intermittent stress.     

Testosterone prevents synaptic loss in the perineal motoneuron pool in the spinal cord in male rats exposed to chronic stress

Chronic stress is known to induce disorders of reproductive neuroendocrine functions. Motoneurons of the spinal nucleus of the bulbocavernosus (SNB) in male rats play an important role in copulatory behavior. In the present study, it was examined whether chronic stress would alter synaptic organization of the SNB motoneurons and whether androgen would modify the changes under chronic stress. Five male rats were under restraint stress for 5 days per week for 3 weeks, and five males implanted subcutaneously with Silastic capsules containing testosterone were also exposed to stress. Five males served as unstressed controls. After 3 weeks of restraint stress, cholera toxin-horseradish peroxidase (CT-HRP) was injected into the bulbocavernosus muscles and animals were killed 2 days later. The spinal cords containing the SNB were dissected, processed with a modified tetramethylbenzidine (TMB) method for visualization of retrogradely transported CT-HRP, and examined ultrastructurally. Neuronal structures apposing the membranes of 150 SNB motoneurons (total for three groups) were analyzed by measuring the percentage of somatic membranes covered by synaptic contacts. The mean percentage of somatic membranes covered by synapses in males exposed to chronic stress was significantly less than that in controls or stressed males treated with testosterone. Size and number of synaptic contacts per unit length of somatic membranes in males exposed to stress were also significantly less than those in controls or stressed males treated with testosterone. There was no significant difference in any of the parameters between controls and stressed males treated with testosterone. Changes in plasma levels of testosterone showed the same profile as changes in the synaptic contacts. These results suggest that the SNB motoneurons of male rats exposed to chronic stress retain a considerable synaptic plasticity in response to androgen, and that androgen treatment can rescue the SNB system in male rats when under chronic restraint stress.     

Long-term effects of footshock and social defeat on anxiety-like behaviours in rats: Relationships to pre-stressor plasma corticosterone concentration

We compared the consequences of two stressors, 'unnatural' inescapable footshocks (IFSs) and 'natural' social defeat (SD), on behaviours typically sensitive to stress [sucrose preference, open field (OF), elevated plus maze (EPM) and acoustic startle responses (ASRs)] and the association with pre-stressor plasma corticosterone concentration. After initial blood sampling, rats (n?=?20 per group) were exposed to either 10 IFSs (1?mA intensity, 5?s duration each) or to 1?h SD (defeat by an aggressive resident male rat and further exposure but separated in a small cage) or to control procedures (handling). Rats were tested once for ASR (day 19), while the other behavioural tests were applied once weekly for 3 weeks. Both stress groups showed short-lasting lowered sucrose preference, and in the EPM they showed shorter total distance moved, shorter distance moved on open arms and less time on open arms compared to controls. In the OF test, IFS rats showed shorter total distance moved up to 2 weeks after stress. The SD group showed shorter total distance moved in the OF, which was only significant 2 weeks after stress. Low pre-stressor plasma corticosterone concentration was only associated with defecation (IFS rats) and latency to enter open arms in the EPM (all low corticosterone subgroups, n?=?10 per subgroup). SD rats with high initial plasma corticosterone concentration showed enhanced ASR compared to the other subgroups with high initial plasma corticosterone concentration (n?=?9 per subgroup). The results indicate that footshock and SD, while generally leading to an increase in anxiety behaviours, represent qualitatively different stressors.     

Effect of chronic cold stress on intestinal epithelial cell proliferation and inflammation in rats

The present study evaluated the effect of chronic cold stress on intestinal epithelial cell proliferation and inflammation. Male Wistar rats were subjected to cold exposure for three weeks. At the end of the cold exposure, intestinal cell proliferation, luminal nitrite and protein levels, intestinal myeloperoxidase activity and mast cell numbers were evaluated. Severely compromised proliferation rate of the crypt-base cells was observed under chronic stress conditions. Cells isolated from stressed rats showed a decreased DNA content in villus and lower villus cell fractions and an increased DNA content in the crypt cells, as compared to controls. Chronic cold stress resulted in increased luminal nitrite, luminal protein levels, and intestinal myeloperoxidase activity. The number of mast cells was significantly elevated under chronic stress conditions. Chronic cold stress resulted in a compromised intestinal epithelial cell proliferation rate and induced inflammation in the rat small intestine, through the combined action of nitric oxide, neutrophils and mast cells.     

慢性束缚应激对大鼠脑内Fas、FasL含量的影响

为探讨慢性束缚应激对大鼠脑内不同部位Fas/FasL系统表达的不同影响。将24只雄性SD大鼠随机分为束缚应激组、装置对照组和正常对照组（n=8）。分别对三组大鼠给予相应的干预14天,用Western-blotting方法测定应激后大鼠大脑前脑皮质、内嗅皮质以及海马区域Fas、FasL蛋白含量。结果表明,应激后各脑区三组大鼠Fas含量差异具有统计学意义（p < 0.01）,束缚应激组明显高于正常对照组或装置对照组（p < 0.01）。在海马区域,三组大鼠FasL含量差异具有统计学意义（p < 0.01）,束缚应激组大鼠FasL蛋白水平显著高于正常对照组或装置对照组(p < 0.01)。三组大鼠自身大脑前脑皮质、内嗅皮质以及海马Fas含量的差异比较均无统计学意义( p > 0.05) 。束缚应激组大鼠海马的FasL含量高于前脑皮质和内嗅皮质(p < 0.05);正常对照组或装置对照组自身不同脑区间FasL含量的变化无统计学意义。提示慢性束缚应激能诱导大鼠脑内Fas/FasL系统表达水平的改变,对不同的脑区,其影响程度明显不同     

慢性应激对大鼠海马Akt/FKHRL1信号通路活性的影响

探讨慢性应激对大鼠海马Akt/FKHRL1信号通路活性的影响,及其应激源特异性。将24只雄性SD大鼠随机分为心理应激组、束缚应激组和正常对照组（n=8）。用Western-blotting方法测定28天应激后海马Akt、FKHRL1蛋白含量及其磷酸化水平。结果表明,应激后三组大鼠海马磷酸化Akt、FKHRL1含量差异有统计学意义（p < 0.01;p < 0.001）,束缚应激组低于正常对照组（p < 0.01）。提示慢性应激能导致海马磷酸化Akt、FKHRL1表达水平降低,但其影响程度与应激源特异性有关     

慢性强迫游泳应激对大鼠情绪和脑细胞外信号调节激酶的影响

为探讨慢性强迫游泳应激对动物情绪和脑组织细胞外信号调节激酶（extracellular signal-regulated kinase, ERK1/2）的影响,动物情绪和脑组织ERK1/2之间的关系,将动物随机分为游泳应激组、装置对照组和控制组。分别对三组大鼠给予相应的干预14天, 然后进行行为观察,免疫印记法测定海马和前脑皮质ERK1/2水平。结果表明强迫游泳应激组和装置对照组都出现明显的情绪障碍。两组大鼠ERK1/2在前脑皮质的表达水平均显著升高,海马无显著变化。前脑皮质ERK2与糖精水摄入量呈显著负相关。提示慢性强迫游泳应激能够诱导大鼠的情绪障碍,提高ERK1/2在前脑皮质的表达水平,ERK1/2与情绪关系密切,可能是脑组织应激性情绪调节的重要生理机制。强迫游泳应激能够导致动物明显的抑郁反应,是比较理想的抑郁动物模型     

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

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

Influence of rearing conditions on voluntary ethanol intake and response to stress in rats

The effects of exposure to four environmental rearing conditions on subsequent voluntary ethanol intake and response to immobilization stress were examined. Male weanling rats were reared in an enriched environment, with a female partner, with a male partner, or individually, for 90 days. At 111 days of age, voluntary consumption of ethanol in increasing concentrations (3 to 9%, v/v) was assessed. Following the ethanol-exposure period, rats were randomly divided into stressed and nonstressed groups and exposed to 3 h of immobilization. Results indicated that the enriched animals consumed greater amounts of ethanol as compared to all other groups, suggesting that the enriched environment and not handling, housing conditions, or the presence of another male or female is responsible for the observed increase in ethanol drinking behavior. Ulcer data indicated that among environmentally enriched rats, ethanol attenuated stress ulcer development relative to their non-ethanol-exposed but stressed controls. In nonstressed enriched rats, ethanol alone exacerbated stomach damage. We suggest that environmental rearing conditions markedly influence the complex interaction between ethanol intake and the response to stress.     

Effect of chronic intermittent immobilization stress on Fos-like immunoreactivity in rat brain and adrenal medulla

The present study examined the influence of short- and long-term chronic intermittent immobilization stress throughout the brain and on the adrenal medulla of intact rats using Fos-like immunoreactivity (Fos-LI) as a marker of cellular activation. The effect of adreno-medullectomy on the central nervous system (CNS) response to chronic immobilization stress was also examined. It was found that control unoperated, unstressed rats had no Fos-LI cells in the brain or in the adrenal medulla. In intact rats, neither short term (1 week) nor long term (4 weeks) chronic intermittent immobilization stress produced significant increases in Fos-LI in the CNS compared with control animals. However, marked increase in the number of Fos-LI cells was observed in the adrenal medulla of animals stressed for 4 weeks compared with control, unstressed animals or those stressed for 1 or 2 weeks. In adreno-medullectomised rats, 4 weeks, but not 1 week, chronic immobilization stress produced significant increases in numbers of Fos-LI neurons in the paraventricular hypothalamic and supraoptic nuclei and the medial amygdala compared with intact animals stressed for a similar period of time. It is concluded that long term stress produces chronic Fos-LI in the adrenal medulla and that adreno-medullectomy increases the Fos response of the PVN, supraoptic nucleus and medial amygdala to long term stress.     

Environmental enrichment protects against the effects of chronic stress on cognitive and morphological measures of hippocampal integrity

Chronic stress has detrimental effects on hippocampal integrity, while environmental enrichment (EE) has beneficial effects when initiated early in development. In this study, we investigated whether EE initiated in adulthood would mitigate chronic stress effects on cognitive function and hippocampal neuronal architecture, when EE started one week before chronic stress began, or two weeks after chronic stress onset. Adult male Sprague Dawley rats were chronically restrained (6h/d) or assigned as non-stressed controls and subdivided into EE or non-EE housing. After restraint ended, rats were tested on a radial arm water maze (RAWM) for 2-d to assess spatial learning and memory. The first study showed that when EE began prior to 3-weeks of chronic stress, EE attenuated chronic stress-induced impairments in acquisition, which corresponded with the prevention of chronic stress-induced reductions in CA3 apical dendritic length. A second study showed that when EE began 2-weeks after the onset of a 5-week stress regimen, EE blocked chronic stress-induced impairments in acquisition and retention at 1-h and 24-h delays. RAWM performance corresponded with CA3 apical dendritic complexity. Moreover, rats in EE housing (control or stress) exhibited similar corticosterone profiles across weeks, which differed from the muted corticosterone response to restraint by the chronically stressed pair-housed rats. These data support the interpretation that chronic stress and EE may act on similar mechanisms within the hippocampus, and that manipulation of these factors may yield new directions for optimizing brain integrity and resilience under chronic stress or stress related neuropsychological disorders in the adult.     

The hippocampus and prefrontal cortex are differentially involved in serial memory retrieval in non-stress and stress conditions

We previously showed that 24 h after learning, mice significantly remembered the first (D1) but not the second (D2) discrimination in a serial spatial task and that an acute stress delivered 5 min before the test phase reversed this memory retrieval pattern.A first experiment evaluated the effects of dorsal hippocampus (HPC) or prefrontal cortex (PFC) lesions, these two brain areas being well-known for their involvement in serial and spatial memory processes. For this purpose, six independent groups of mice were used: non-lesioned (controls), PFC or HPC-lesioned animals, submitted or not to an acute stress (electric footshocks; 0.9 mA). Results show that (i) non-stressed controls as well as PFC-lesioned mice (stressed or not) remembered D1 but not D2; (ii) stressed controls and HPC-lesioned mice (stressed or not) remembered D2 but not D1; (iii) stress significantly increased plasma corticosterone in controls and PFC-lesioned mice, but not in HPC-lesioned mice which already showed a significant plasma corticosterone increase in non-stressed condition.Since data from this first experiment showed that stress inhibited the hippocampal-dependent D1 memory retrieval, a second experiment evaluated the behavioral effect of intrahippocampal corticosterone injection in non-stressed mice. Results show that intrahippocampal corticosterone injection induced a reversal of serial memory retrieval pattern similar to that induced by acute stress.Overall, our study shows that (i) in non-stress condition, the emergence of D1 is HPC-dependent; (ii) in stress condition, the emergence of D2 requires the PFC integrity; moreover, intrahippocampal corticosterone injection mimicked the effects of stress in the CSD task.     

Substance P is involved in terminating the hypothalamo- pituitary-adrenal axis response to acute stress through centrally located neurokinin-1 receptors

The neurokinin substance P (SP) has been previously shown to inhibit basal hypothalamo-pituitary-adrenal (HPA) axis activity. This study was designed to investigate the effects of central injection of the specific neurokinin-1 receptor antagonist RP67580 on the HPA axis response to acute restraint stress. In non-restrained rats injected with RP67580, plasma ACTH and corticosterone levels were elevated at 30 and 60 min compared to rats injected with vehicle, but there were no differences between vehicle and RP67580 groups at 4h. In restrained rats injected with vehicle, plasma ACTH and corticosterone levels were significantly elevated at 30 min and 60 min following initiation of the stress but had returned to basal levels at 4h. In restrained rats injected icv with RP67580, plasma corticosterone and ACTH levels were significantly elevated at 30 min and 60 min, with no significant differences compared to the restraint stressed vehicle-injected group. However, in the RP67580-injected group, corticosterone and ACTH levels remained significantly elevated at 4h following onset of restraint compared to those in the restraint stressed vehicle-injected group. Corticotrophin-releasing factor mRNA levels in the parvocellular subdivision of the paraventricular nucleus of the hypothalamus and POMC mRNA levels in the anterior pituitary were significantly increased in the stressed group 4h following injection with RP67580 compared to the stressed group injected with vehicle alone. These data show that endogenous SP does not inhibit the initial magnitude of the HPA axis response to restraint stress, but does act through neurokinin-1 receptors at a central level to reduce the duration of the response to stress. This suggests that SP may be an important central agent controlling the transition between acute and chronic stress.     

Dietary ethyl-eicosapentaenoic acid but not soybean oil reverses central interleukin-1-induced changes in behavior, corticosterone and immune response in rats

Omega (n)-3 and n-6 fatty acids are important membrane components of neurons and immune cells, and related to psychiatric and inflammatory diseases. Increased ratio of n-6/n-3 in the blood has been reported in depressed patients and in students following stress exposure. The n-3 fatty acid, eicosapentaenoic acid (ethyl-EPA) suppresses inflammation and has antidepressant properties. Interleukin (IL)-1beta can stimulate corticosterone secretion, induce anxiety and stress-like behavior and inflammatory responses. This study was to evaluate the effect of diets enriched with coconut oil, ethyl-EPA and soybean oil on central IL-1beta induced stress and anxiety-like behavior, induced changes in the concentration of prostaglandin (PG) E2 and corticosterone and the release of IL-10. Groups of rats were fed with either 5% coconut oil (as control diet), 0.2% EPA with 4.8% coconut oil or 1% EPA with 4% coconut oil and 5% soybean oil for 7 weeks. The central administration of IL-1beta induced sickness, stress and anxiety-like behavior as indicated by a reduction in body weight, decreased time spent, and the number of entries, into the open arms of the elevated plus maze and decreased exploration and entry into the central zone of the "open field" apparatus. IL-1beta also increased PGE2 and corticosterone concentrations and decreased the release of IL-10 from leucocytes. Food enriched with ethyl-EPA but not soybean oil, significantly attenuated most of these changes. These results demonstrate that ethyl-EPA has anti-inflammatory, anti-stress and anti-anxiety effects in rats.     

Effects of social defeat on acute cardiovascular response in salt-sensitive and salt-resistant rats

We studied the effects of social stress (SS) and a high salt diet on systolic blood pressure (SBP) and heart rate (HR): S/JR male rats (which exhibit marked elevations in SBP when placed on a high sodium diet) and R/JR male rats (which are resistant to the BP-elevating effects of a high sodium diet) were maintained on a low sodium diet (0.3% NaCl) or placed on a high sodium diet (8% NaCl). Within each dietary condition independent groups were either exposed to SS, by placement in the cage of a trained fighter male (Long-Evans breed) for 25 min, or exposed to no stress. The dietary regimen was imposed for 10 days with stress exposures on Days 1, 2, 3, 5, 7, and 9, with SBP and HR measured indirectly by tail plethysmography 3 min following exposure to SS. SS produced an acute decrease in SBP (20-30 mm Hg) in S/JR rats on the second and subsequent exposures, but did not affect HR. SS did not affect SBP of R/JR rats, but did produce a significant elevation of HR. Maintenance on the high sodium diet increased SBP in S/JR, but not R/JR, rats when it was measured on the eighth (no stress) day, but SS obscured the effects of diet on SBP on days when rats were stressed. Following exposure to attacks, defeated SS rats displayed an upright submissive posture relatively late during the first stress exposure when no change in SBP was observed after SS in S/JRs, but displayed the submissive posture immediately and with long duration on the second and subsequent exposures when a marked decrement in SBP was seen.     

Acute stress persistently enhances estrogen levels in the female rat

Here we tested whether exposure to either tailshock or swim stress alters ovarian hormone levels, estrogen and progesterone, in females and whether the effects are persistent. Adrenal hormone levels were also measured in males and females. Estradiol levels were elevated in unstressed females during proestrus relative to females in other stages of estrous, and exposure to the stressors enhanced estradiol beyond basal levels. For females stressed during diestrus 2, estradiol levels were elevated immediately after stressor cessation and up to 24 hrs. Exposure to tailshock, but not swim-stress, transiently enhanced progesterone in females stressed during the stage of proestrus and estrus. Glucocorticoid levels were elevated in response to both stressors and were supraelevated in females under both basal and stress conditions relative to males, particularly in blood from females exposed to acute swim stress. These results indicate that exposure to a relatively acute stressful event immediately and persistently enhances serum estradiol and are discussed in the context of reports that exposure to the same stressors immediately and persistently impairs associative learning in the female rat.     

c-fos mRNA induction in acute and chronic audiogenic stress: possible role of the orbitofrontal cortex in habituation

To study putative brain circuits involved in habituation to stress, rats were exposed daily (30 min for 15 days) to an environment in the presence (Chronic) or absence (Acute) of loud noise (105 dB sound pressure level--SPL A Scale). Behavioral and endocrine measures of stress were taken throughout this habituation period, and both measures displayed strong habituation in the Chronic group. All rats were killed immediately after the day 16 exposure, constituting an acute stressor for the Acute group, and regional brain activity was assessed using c-fos mRNA induction with in situ hybridization. Hearing damage could not easily explain these results because additional rats exposed to a similar stress protocol exhibited no changes in auditory brainstem evoked potentials. c-fos mRNA induction in the central auditory system was similar between the Acute and Chronic groups, particularly at lower auditory processing levels, also arguing against a simple reduction in auditory processing in the chronically stressed rats. However, c-fos mRNA expression was reduced in chronically, as compared to acutely, stressed rats in several regions previously implicated in audiogenic stress (lateral septum, bed nucleus of the stria terminalis, some preoptic areas, and the paraventricular hypothalamic nucleus). Interestingly, the orbitofrontal cortex was the only region displaying higher c-fos mRNA induction in the chronically as compared to acutely stressed rats. This region has connections to several stress-responsive areas and may thus be a critical region actively inhibiting stress.     

不同应激范式对大鼠行为和脑神经颗粒素含量的影响

为探讨慢性情绪应激、生理应激对大鼠旷场行为和脑神经颗粒素（Neurogranin,NG）含量的不同作用,以及NG含量变化与应激性行为效应之间的相互关系。分别以不确定性空瓶刺激和饮水剥夺,建立情绪应激和生理应激动物模型。将40只雄性SD大鼠随机分为情绪应激组（ES）、生理应激组（PS）、定时饮水组（C1）和正常对照组（C2）（n=10）。以旷场行为任务来评定大鼠应激后的行为变化,Western blotting方法测定海马和前脑皮层中的NG含量。结果表明：应激后四组大鼠海马的NG含量差异无显著性;ES组前脑皮层的NG含量低于C2组,差异具有显著性,p<0.01;PS组的前脑皮层NG含量也下降,但与C2组相比差异无显著性; 应激后ES组、PS组修饰行为多于C2组,差异具有显著性,分别为p<0.01,p<0.05;前脑皮层NG含量与修饰行为之间的相关达显著水平。提示慢性情绪和生理应激均能导致前脑皮层NG含量下降,修饰行为增加,情绪应激作用更显著。修饰行为可能是反映情绪状态的较敏感行为指标,前脑皮层NG水平可能是预测情绪应激所致焦虑或抑郁行为的较敏感生物学指标。     

Enduring deficits in contextual and auditory fear conditioning after adolescent, not adult, social instability stress in male rats

Adolescence is a time of developmental changes and reorganization in the brain and stress systems, thus, adolescents may be more vulnerable than adults to the effects of chronic mild stressors. Most studies, however, have not directly compared stress experienced in adolescence to the same stress experience in adulthood. In the present study, adolescent (n=46) and adult (n=48) male rats underwent 16 days of social instability stress (daily 1h isolation and change of cage partners) or were non-stress controls. Rats were then tested on the strength of acquired contextual and cued fear conditioning, as well as extinction learning, beginning either the day after the stress procedure or 3 weeks later. No difference was found among the groups during the Training Phase of conditioning. Irrespective of the time between the social stress experience and fear conditioning, rats stressed in adolescence had decreased context and cue memory, and cue generalization compared to control rats, as measured by the percentage of time spent freezing in tests. Social instability stress in adulthood had no effect on any measure of fear conditioning. The results support the hypothesis that adolescence is a time of heightened vulnerability to stressors.