Timing matters: the interval between acute stressors within chronic mild stress modifies behavioral and physiologic stress responses in male rats

Chronic mild stress can lead to negative health outcomes. Frequency, duration, and intensity of acute stressors can affect health-related processes. We tested whether the temporal pattern of daily acute stressors (clustered or dispersed across the day) affects depression-related physiology. We used a rodent model to keep stressor frequency, duration, and intensity constant, and experimentally manipulated the temporal pattern of acute stressors delivered during the active phase of the day. Adult male Sprague-Dawley rats were exposed to one of three chronic mild stress groups: Clustered: stressors that occurred within 1 hour of each other (n = 21), Dispersed: stressors that were spread out across the active phase (n = 21), and Control: no stressors presented (n = 21). Acute mild stressors included noise, strobe lights, novel cage, cage tilt, wet bedding, and water immersion. Depression-related outcomes included: sucrose preference, body weight, circulating glucocorticoid (corticosterone) concentration after a novel acute stressor and during basal morning and evening times, and endotoxin-induced circulating interleukin-6 concentrations. Compared to control rats, those in the Clustered group gained less weight, consumed less sucrose, had a blunted acute corticosterone response, and an accentuated acute interleukin-6 response. Rats in the Dispersed group had an attenuated corticosterone decline during the active period and after an acute stressor compared to the Control group. During a chronic mild stress experience, the temporal distribution of daily acute stressors affected health-related physiologic processes. Regular exposure to daily stressors in rapid succession may predict more depression-related symptoms, whereas exposure to stressors dispersed throughout the day may predict diminished glucocorticoid negative feedback.

Chasing as a model of psychogenic stress: characterization of physiological and behavioral responses

Being chased by a predator or a dominant conspecific can induce significant stress. However, only a limited number of laboratory studies have employed chasing by itself as a stressor. In this study, we developed a novel stress paradigm in which rats were chased by a fast-moving object in an inescapable maze. In Experiment 1, defensive behaviors and stress hormone changes induced by chasing stress were measured. During the chasing stress, the chasing-stress group (n = 9) froze and emitted 22-kHz ultrasonic vocalizations (USVs), but the no-chasing control group (n = 10) did not. Plasma corticosterone levels significantly increased following the chasing and were comparable to those of the restraint-stress group (n = 6). In Experiment 2, the long-lasting memory of the chasing event was tested after three weeks. The chasing-stress group (n = 15) showed higher levels of freezing and USV than the no-chasing group (n = 14) when they were presented with the tone associated with the object's chasing action. Subsequently, the rats were subjected to Pavlovian threat conditioning with a tone as a conditioned stimulus and footshock as an unconditioned stimulus. The chasing-stress group showed higher levels of freezing and USV during the conditioning session than the no-chasing group, indicating sensitized defensive reactions in a different threat situation. Taken together, the current results suggest that chasing stress can induce long-lasting memory and sensitization of defensive responses to a new aversive event as well as immediate, significant stress responses.

Stress-induced sensitization: the hypothalamic-pituitary-adrenal axis and beyond

Exposure to certain acute and chronic stressors results in an immediate behavioral and physiological response to the situation followed by a period of days when cross-sensitization to further novel stressors is observed. Cross-sensitization affects to different behavioral and physiological systems, more particularly to the hypothalamus-pituitary-adrenal (HPA) axis. It appears that the nature of the initial (triggering) stressor plays a major role, HPA cross-sensitization being more widely observed with systemic or high-intensity emotional stressors. Less important appears to be the nature of the novel (challenging) stressor, although HPA cross-sensitization is better observed with short duration (5-15 min) challenging stressors. In some studies with acute immune stressors, HPA sensitization appears to develop over time (incubation), but most results indicate a strong initial sensitization that progressively declines over the days. Sensitization can affect other physiological system (i.e. plasma catecholamines, brain monoamines), but it is not a general phenomenon. When studied concurrently, behavioral sensitization appears to persist longer than that of the HPA axis, a finding of interest regarding long-term consequences of traumatic stress. In many cases, behavioral and physiological consequences of prior stress can only be observed following imposition of a new stressor, suggesting long-term latent effects of the initial exposure.

Sex differences in the long-lasting effects of a single exposure to immobilization stress in rats

In male rats, a single exposure to a severe stressor such as immobilization (IMO) results in marked activation of the HPA axis and reduction of body weight gain. In addition, the HPA response to the same (homotypic) stressor is reduced, whereas the response to a different (heterotypic) stressor is enhanced for days. Although sex differences in the responsiveness of the HPA axis have been described, there are few studies about the influence of sex on long-lasting effects of stress. Thus, we have compared the consequences of a single exposure to IMO in male and female rats. Females showed a similar ACTH response to the first IMO associated with higher corticosterone, but they were more resistant than males to stress-induced loss of body weight. Unstressed females showed higher resting levels of ACTH and corticosterone, but they did not show the increase in the resting levels of HPA hormones observed in males on the day after IMO. During exposure to a different stressor (open-field) two days after IMO, enhanced corticosterone response and hypoactivity was observed in males, but not in females. Finally, a second exposure to IMO 8 days after the first one resulted in a reduction of the HPA response and of the negative impact on body weight as compared to the first exposure, and this protective effect was greater in females. In sum, IMO-exposed females showed a greater reduction of the response to a second IMO and appear to be more resistant than males to some of the negative impacts of IMO.

Behavioral and neuroendocrine consequences of juvenile stress combined with adult immobilization in male rats

Exposure to stress during childhood and adolescence increases vulnerability to developing several psychopathologies in adulthood and alters the activity of the hypothalamic-pituitary-adrenal (HPA) axis, the prototypical stress system. Rodent models of juvenile stress appear to support this hypothesis because juvenile stress can result in reduced activity/exploration and enhanced anxiety, although results are not always consistent. Moreover, an in-depth characterization of changes in the HPA axis is lacking. In the present study, the long-lasting effects of juvenile stress on adult behavior and HPA function were evaluated in male rats. The juvenile stress consisted of a combination of stressors (cat odor, forced swim and footshock) during postnatal days 23-28. Juvenile stress reduced the maximum amplitude of the adrenocorticotropic hormone (ACTH) levels (reduced peak at lights off), without affecting the circadian corticosterone rhythm, but other aspects of the HPA function (negative glucocorticoid feedback, responsiveness to further stressors and brain gene expression of corticotrophin-releasing hormone and corticosteroid receptors) remained unaltered. The behavioral effects of juvenile stress itself at adulthood were modest (decreased activity in the circular corridor) with no evidence of enhanced anxiety. Imposition of an acute severe stressor (immobilization on boards, IMO) did not increase anxiety in control animals, as evaluated one week later in the elevated-plus maze (EPM), but it potentiated the acoustic startle response (ASR). However, acute IMO did enhance anxiety in the EPM, in juvenile stressed rats, thereby suggesting that juvenile stress sensitizes rats to the effects of additional stressors.

Banting memorial lecture 2013. A life in balance: wandering the pathways of control

‘To keep in equilibrium’, one of the Oxford English Dictionary’s many definitions of balance, is a desirable target for anylife, but has special meaning for the life of a person with diabetes. Achieving balance—between hypo- and hyperglycaemia; between energy intake and energy consumption; between insulin action and insulin secretion; between attention to diabetes and attention to everything else—remains challenging, but progress has been made over the last three decades, both in our understanding of how nature achieves balance and in the tools we have to try to reproduce the actions of nature in disease states. In particular, the role of the brain in controlling diabetes, from glucose sensing to decision making, has been investigated. Physiological and neuro-imaging studies are finally being translated into patient benefit, with the aim of improving, as Dr Banting put it, the provision of ‘energy for the economic burdens of life’.

Role of hypothalamic corticotropin-releasing factor in mediating alcohol-induced activation of the rat hypothalamic-pituitary-adrenal axis

Alcohol stimulates the hypothalamic-pituitary-adrenal (HPA) axis through brain-based mechanisms in which endogenous corticotropin-releasing factor (CRF) plays a major role. This review first discusses the evidence for this role, as well as the possible importance of intermediates such as vasopressin, nitric oxide and catecholamines. We then illustrate the long-term influence exerted by alcohol on the HPA axis, such as the ability of a first exposure to this drug during adolescence, to permanently blunt neuroendocrine responses to subsequent exposure of the drug. In view of the role played by CRF in addiction, it is likely that a better understanding of the mechanisms through which this drug stimulates the HPA axis may lead to the development of new therapies used in the treatment of alcohol abuse, including clinically relevant CRF antagonists.

Comparison of the effects of single and daily repeated immobilization stress on resting activity and heterotypic sensitization of the hypothalamic-pituitary-adrenal axis

Acute exposure to severe stressors causes marked activation of the hypothalamic-pituitary-adrenal (HPA) axis that is reflected on the day after higher resting levels of HPA hormones and sensitization of the HPA response to novel (heterotypic) stressors. However, whether a single exposure to a severe stressor or daily repeated exposure to the same (homotypic) stressor modifies these responses to the same extent has not been studied. In this experiment, we studied this issue in adult male Sprague-Dawley rats daily exposed for seven days to a severe stressor such as immobilization on boards (IMO). A first exposure to 1 h IMO resulted in a marked activation of the HPA axis as reflected in plasma levels of adrenocorticotropic hormone (ACTH) and corticosterone, and such activation was significantly reduced after the seventh IMO. On the day after the first IMO, higher resting levels of ACTH and corticosterone and sensitization of their responses to a short exposure to an open-field (OF) were observed, together with a marked hypoactivity in this environment. Repeated exposure to IMO partially reduced hypoactivity, the increase in resting levels of HPA hormones and the ACTH responsiveness to the OF on the day after the last exposure to IMO. In contrast, corticosterone response was gradually increased, suggesting partial dissociation from ACTH. These results indicate that daily repeated exposure to the same stressor partially reduced the HPA response to the homotypic stressor as well as the sensitization of HPA axis activity observed the day after chronic stress cessation.

Effects of stressors in adolescence on learning and memory in rodent models

This article is part of a Special Issue "Puberty and Adolescence". Learning and memory is affected by a myriad of factors, including exposure to stressors and the corresponding rise in circulating glucocorticoids. Nevertheless, the effects of stressors depend on the sex, species, the type of stressor used, the duration of exposure, as well as the developmental time-point in which stressors are experienced. Effects of stress in adolescence, however, have received less attention than other developmental periods. In adolescence, the hypothalamic-pituitary-adrenal axis and brain regions involved in learning and memory, which also richly express corticosteroid receptors, are continuing to develop, and thus the effects of stress exposures would be expected to differ from those in adulthood. We conclude from a review of the available literature in animal models that hippocampal function is particularly sensitive to adolescent stressors, and the effects tend to be most evident several weeks after the exposure, suggesting stressors alter the developmental trajectory of the hippocampus.

Repeatedly stressed rats have enhanced vulnerability to amygdala kindling epileptogenesis

Psychiatric disorders associated with elevated stress levels, such as depression, are present in many epilepsy patients, including those with mesial Temporal Lobe Epilepsy (mTLE). Evidence suggests that these psychiatric disorders can predate the onset of epilepsy, suggesting a causal/contributory role. Prolonged exposure to elevated corticosterone, used as a model of chronic stress/depression, accelerates limbic epileptogenesis in the amygdala kindling model. The current study examined whether exposure to repeated stress could similarly accelerate experimental epileptogenesis. Female adult non-epileptic Wistar rats were implanted with a bipolar electrode into the left amygdala, and were randomly assigned into stressed (n=18) or non-stressed (n=19) groups. Rats underwent conventional amygdala kindling (two electrical stimulations per day) until 5 Class V seizures had been experienced ('the fully kindled state'). Stressed rats were exposed to 30min restraint immediately prior to each kindling stimulation, whereas non-stressed rats received control handling. Restraint stress increased circulating corticosterone levels (pre-stress: 122±17ng/ml; post-stress: 632±33ng/ml), with no habituation observed over the experiment. Stressed rats reached the 'fully kindled state' in significantly fewer stimulations than non-stressed rats (21±1 vs 33±3 stimulations; p=0.022; ANOVA), indicative of a vulnerability to epileptogenesis. Further, seizure durations were significantly longer in stressed rats (p<0.001; ANOVA). These data demonstrate that exposure to repeated experimental stress accelerates the development of limbic epileptogenesis, an effect which may be related to elevated corticosterone levels. This may have implications for understanding the effects of chronic stress and depression in disease onset and progression of mTLE in humans.

Acute stress-induced sensitization of the pituitary-adrenal response to heterotypic stressors: independence of glucocorticoid release and activation of CRH1 receptors

A single exposure to some severe stressors causes sensitization of the hypothalamic-pituitary-adrenal (HPA) response to novel stressors. However, the putative factors involved in stress-induced sensitization are not known. In the present work we studied in adult male rats the possible role of glucocorticoids and CRH type 1 receptor (CRH-R1), using an inhibitor of glucocorticoid synthesis (metyrapone, MET), the glucocorticoid receptor (GR) antagonist RU38486 (mifepristone) and the non-peptide CRH-R1 antagonist R121919. In a first experiment we demonstrated with different doses of MET (40-150 mg/kg) that the highest dose acted as a pharmacological stressor greatly increasing ACTH release and altering the normal circadian pattern of HPA hormones, but no dose affected ACTH responsiveness to a novel environment as assessed 3 days after drug administration. In a second experiment, we found that MET, at a dose (75 mg/kg) that blocked the corticosterone response to immobilization (IMO), did not alter IMO-induced ACTH sensitization. Finally, neither the GR nor the CRH-R1 antagonists blocked IMO-induced ACTH sensitization on the day after IMO. Thus, a high dose of MET, in contrast to IMO, was unable to sensitize the HPA response to a novel environment despite the huge activation of the HPA axis caused by the drug. Neither a moderate dose of MET that markedly reduced corticosterone response to IMO, nor the blockade of GR or CRH-R1 receptors was able to alter stress-induced HPA sensitization. Therefore, stress-induced sensitization is not the mere consequence of a marked HPA activation and does not involve activation of glucocorticoid or CRH-R1 receptors.

Morning cortisol levels and perceived stress in irregular shift workers compared with regular daytime workers

The 24/7 work environment and irregular shifts may markedly enhance the psychological pressure of media work. Changes in the hypothalamic-pituitary-adrenal axis reflect adaptation to stress. We analysed the correlation between subjective stress, sleep, salivary cortisol, and melatonin hormones among Finnish media workers with regular daytime work (RDW) and with irregular shift work (ISW) while controlling confounders. From 874 employees with regular daytime work or with irregular shift work, 70 employees from both groups were randomly selected. The final number of employees with a complete salivary cortisol profile was 66 in the RDW group and 65 in the ISW group. Five saliva samples were gathered from each subject before and during a working day. The salivary cortisol level of the sample taken 60 minutes after awakening (T1) was compared to the salivary cortisol level taken immediately after awakening (T0, T1/T0 ratio). The ratio was higher in the ISW group than in RDW group. Irregular shift work (P < 0.001), severe stress (P < 0.05), and less sleep (P < 0.05) were independently associated with an augmented cortisol response after awakening. A stressful work environment and irregular shift work enhance cortisol excretion after waking. In the long run, this may become detrimental to health.

Chronic stress in pregnant guinea pigs (Cavia aperea f. porcellus) attenuates long-term stress hormone levels and body weight gain, but not reproductive output

Stress, when extreme or chronic, can have a negative impact on health and survival of mammals. This is especially true for females during reproduction when self-maintenance and investment in offspring simultaneously challenge energy turnover. Therefore, we investigated the effects of repeated stress during early- and mid-gestation on the maternal stress axis, body weight gain and reproductive output. Female guinea pigs (Cavia aperea f. porcellus, n = 14) were either stressed (treatment: exposure to strobe light in an unfamiliar environment on gestational day -7, 0, 7, 14, 21, 28, 35, 42) or left completely undisturbed (control) throughout pregnancy. Females of both groups received the same respective diets, and reproductive parameters were evaluated upon parturition. Additionally, hormonal data were obtained from blood and feces. The stress exposure induced a significant increase in plasma cortisol concentrations during the afternoon. In contrast to this short-term response in plasma cortisol concentrations, we found no significant differences in the levels of cortisol metabolites in feces collected after stress exposure between groups and even significantly decreased levels of fecal cortisol metabolites on non-stress days over time in treatment females. Among treatment females, gain in body weight was attenuated over gestation and body weight was lower compared to control females during lactation, especially in cases of large litter sizes. No differences could be seen in the reproductive parameters. We conclude that repeated stress exposure with strobe light during early- and mid-gestation results in a down-regulation of the hypothalamic-pituitary-adrenal axis and lower weight gain in treatment females, but has no effect on reproductive output.

Vasopressin modulates hypothalamo-pituitary activity by paracrine action during acute and chronic immobilization stress in rats

The aim of the study was to examine changes in corticotropin releasing hormone (CRH), vasopressin (VP), adrenocorticotropin (ACTH) as well as VP3 receptor protein concentration in the hypothalamus and the pituitary of rats exposed to acute (3 h) and chronically repeated (3 h daily for 7 days) immobilization stress. Our results show that, unlike the increase in ACTH, there were no changes in serum VP despite the significant changes in this hormone and its receptor concentrations in the hypothalamus and pituitary (HP). This suggests that VP regulates HP activity by predominantly acting in a paracrine manner under the examined stress condition.

Different stress-related phenotypes of BALB/c mice from in-house or vendor: alterations of the sympathetic and HPA axis responsiveness

Background

Laboratory routine procedures such as handling, injection, gavage or transportation are stressful events which may influence physiological parameters of laboratory animals and may interfere with the interpretation of the experimental results. Here, we investigated if female BALB/c mice derived from in-house breeding and BALB/c mice from a vendor which were shipped during their juvenile life differ in their HPA axis activity and stress responsiveness in adulthood.

Results

We show that already transferring the home cage to another room is a stressful event which causes an increased HPA axis activation for at least 24 hours as well as a loss of circulating lymphocytes which normalizes during a few days after transportation. However and important for the interpretation of experimental data, commercially available strain-, age- and gender-matched animals that were shipped over-night showed elevated glucocorticoid levels for up to three weeks after shipment, indicating a heightened HPA axis activation and they gained less body weight during adolescence. Four weeks after shipment, these vendor-derived mice showed increased corticosterone levels at 45-min after intraperitoneal ACTH challenge but, unexpectedly, no acute stress-induced glucocorticoid release. Surprisingly, activation of monoaminergic pathways were identified to inhibit the central nervous HPA axis activation in the vendor-derived, shipped animals since depletion of monoamines by reserpine treatment could restore the stress-induced HPA axis response during acute stress.

Conclusions

In-house bred and vendor-derived BALB/c mice show a different stress-induced HPA axis response in adulthood which seems to be associated with different central monoaminergic pathway activity. The stress of shipment itself and/or differences in raising conditions, therefore, can cause the development of different stress response phenotypes which needs to be taken into account when interpreting experimental data.

Marked dissociation between hypothalamic-pituitary-adrenal activation and long-term behavioral effects in rats exposed to immobilization or cat odor

Exposure of rodents to cats or certain cat odors results in long-term behavioral effects reminiscent of enhanced anxiety that have been considered to model post-traumatic stress disorder. However, other severe stressors such as tail-shock or immobilization in wooden boards (IMO) appear to induce shorter lasting changes in anxiety. In addition, there are controversial results regarding the effects of urine/feces odors. In the present work, we studied in two experiments the relationship between the degree of stress experienced by the animals during exposure to IMO, urine odors or fur odors (as assessed by hypothalamic-pituitary-adrenal activation and plasma glucose) and the short- and long-term behavioral consequences. In the first experiment, rats were individually exposed for 15 min to a novel environment (white large cages) containing either clean cat litter (controls) or litter soiled by cats (urine odors). Half of the rats in each condition were left to freely explore the environment whereas the others were subjected to immobilization (IMO) within the cages. Although ACTH, corticosterone and glucose responses to IMO were much stronger than those to the white cages with clean litter or urine odors (which did not differ from each other), no effect of treatments on anxiety-like behavior in the elevated plus-maze (EPM) were found one week later. However, previous IMO exposure did cause sensitization of the ACTH response to the EPM. In the second experiment, the response to white large cages containing either no odor (controls), litter soiled by cats (urine odor) or a cloth impregnated with cat odor (fur odor) was compared. Urine and fur odors elicited similar ACTH and corticosterone responses that were higher than those of controls, but plasma glucose levels were slightly higher in rats exposed to fur odor. When compared to controls, activity was only diminished in the novel cages containing fur odor. Similarly, fur odor-exposed rats, but not those exposed to urine odor, showed signs of enhanced anxiety in the EPM seven days later, although the ACTH response to the EPM was similar in the three groups. The present data demonstrate: (a) a marked dissociation between the degree of ACTH, corticosterone and glucose responses to stressors and their long-term anxiety-like effects; (b) that the type of cat odor is critical in determining the short-term and long-term physiological and behavioral consequences of exposure; and (c) that plasma ACTH released during brief exposure to the EPM does not appear to reflect anxiety-like behavior.

Long-term neuroendocrine and behavioural effects of a single exposure to stress in adult animals

There is now considerable evidence for long-lasting sequels of stress. A single exposure to high intensity predominantly emotional stressors such as immobilisation in wooden-boards (IMO) induces long-term (days to weeks) desensitization of the hypothalamic-pituitary-adrenal (HPA) response to the same (homotypic) stressor, whereas the response to novel (heterotypic) stressors was enhanced. In addition, long-lasting changes in behaviour have been described after a single exposure to brief or more prolonged sessions of shocks, predator, predator odour, underwater stress or a combination of three stressors on 1 day. The most consistent changes are reduced entries into the open arms of the elevated plus-maze and enhanced acoustic startle response, both reflecting enhanced anxiety. However, it is unclear whether there is any relationship between the intensity of the stressors, as evaluated by the main physiological indexes of stress (e.g. HPA axis), the putative traumatic experience they represent and their long-term behavioural consequences. This is particularly critical when trying to model post-traumatic stress disorders (PTSD), which demands a great effort to validate such putative models.

Evaluation of effects of previous exposure to an acute stressor before testing for depression-like behaviours in mice

Test batteries are an essential and broadly used tool for behavioural phenotyping, especially with regard to mouse models of particular diseases, such as depression. Facing the problem of an often limited number of mutant animals, it therefore seems crucial to develop and optimise such test batteries in terms of an ideal throughput of subjects. This study aimed to characterize several common stressors, which are used for the investigation of depressive-like features with regard to their capability of each of them to affect performance in a subsequent behavioural test. Here we investigated swim-, restraint- and footshock-stress in male C57/BL6 mice, focusing on post-stress corticosterone elevations as well as potential effects on the behavioural level. The stressors increased circulating corticosterone levels when assessed 1 h after exposure. On the behavioural level, no test interactions could be detected, which suggests, that in general, combining these test conditions in experiments with a restricted availability of animals seems to be rather unproblematic.

Effects of perinatal oxycodone exposure on the response to CRH in late adolescent rats

We hypothesized that prenatal oxycodone exposure suppresses the Hypothalamic-Pituitary-Adrenal (HPA) response to stress in late adolescence. Dark Agouti rats were given either intravenous oxycodone or vehicle (controls, CON) daily from gestation day 8 until postnatal day (PD) 5. At PD 45, the male and female offspring received intravenously either ovine corticotropin releasing hormone (CRH) or saline. Plasma adrenocorticotropic hormone (ACTH) and corticosterone (CORT) levels were determined before, and 15, 30, and 60 min after injection. Prenatal oxycodone had no effect on baseline ACTH values; CRH elicited a greater ACTH response than saline. In males, prenatal oxycodone delayed and enhanced the peak ACTH response to CRH, but had no effect in females. The CORT response to CRH was not different between oxycodone and CON; however mean CORT levels in females were significantly higher than those in males at baseline and after stimulation. These results demonstrate that prenatal oxycodone increases pituitary response to CRH in late adolescent male rats, but not in females. The absence of an enhanced adrenal response in oxycodone-exposed males suggests either desensitization or maximal adrenal response to a high CRH dose. The mechanisms of postnatal sex-specific HPA dysregulation following prenatal oxycodone remain to be elucidated.

2006 Curt P. Richter award winner: Social influences on stress responses and health

Both positive and negative social interactions can modulate the hypothalamic-pituitary-adrenal (HPA) axis and influence recovery from injuries and illnesses, such as wounds, stroke, and cardiac arrest. Stress exacerbates neuronal death following stroke and cardiac arrest, and delays cutaneous wound healing, via a common mechanism involving stress-induced increases in corticosterone, acting on glucocorticoid receptors. In contrast, hamsters and mice that form social bonds are buffered against stress and heal cutaneous wounds more quickly than socially isolated animals, presumably because the physical contact experienced by the pairs releases oxytocin, which in turn suppresses the HPA axis and facilitates wound healing. Social housing also decreases stroke-induced neuronal death and improves functional recovery, but the mechanism appears to involve suppressing the inflammatory response that accompanies stroke, rather than alterations in HPA axis activity. An interaction between the HPA axis and immune system determines stroke outcome in neonatally manipulated mice that exhibit life-long dampening of the HPA axis. Taken together, these studies provide support for the detrimental effects of stress and identify potential mechanisms underlying the well-documented clinical observation that social support positively influences human health.

Hyperarousal does not depend on trauma-related contextual memory in an animal model of Posttraumatic Stress Disorder

Both classical fear conditioning and fear sensitization have been implicated in the development of Posttraumatic Stress Disorder (PTSD), but little is known about their interaction or interdependency. Therefore, we administered the NMDA receptor antagonist AP5 or vehicle bilaterally into the dorsal hippocampus of C57BL/6N mice 15 min before administration of an electric footshock. One month later mice were tested for their conditioned fear reaction to the shock context (associative fear memory) and for their sensitized fear response to a neutral tone in a new context (hyperarousal). AP5-treated animals exhibited only half as much conditioned, but equal amounts of sensitized fear, compared to vehicle-treated mice, demonstrating that hyperarousal does not depend on associative fear memory about the aversive encounter, but solely on sensitization induced by the inescapable footshock. This result points to the need for a 'desensitization' approach in PTSD therapy, as sensitization and classical conditioning seem to be independent processes in this psychiatric disorder.

Toward an animal model of posttraumatic stress disorder

Posttraumatic stress disorder (PTSD) belongs to the most frequent anxiety disorders. Despite a broad body of evidence concerning neurobiological correlates of this illness, the pathomechanisms of PTSD are still poorly understood. This illustrates the need to establish animal models of this disorder. Recently, PTSD model has become a somewhat fashionable term used in animal studies for almost every stress-induced behavioral alteration. Only few cases, however, reflect the human disorder closely enough to deserve this term. Systematic research requires valid animal modeling with clearly defined criteria. This article outlines and discusses criteria for prospective PTSD models, based on a theoretical framework that emphasizes the involvement of both associative and nonassociative memory processes in the development and maintenance of PTSD.

Establishing an agenda for translational research on PTSD

Animal research on brain mechanisms involved in psychiatric disorders presents an enormous challenge because it is impossible to precisely model symptoms of a human disorder in a rat or mouse. Nevertheless, there are uses for animal models as long as the limitations are recognized. Animal research related to posttraumatic stress disorder (PTSD) points to acute and chronic stressors, such as restraint or immobilization as being the most relevant stimuli to study how neural and endocrine systems are affected, both immediately and long term. Of particular relevance are the onset and duration of effects of stressors on brain areas subserving emotional memories, such as the amygdala, prefrontal cortex, and hippocampus. The hippocampus plays a role in memory and in vegetative functions of the body. The hippocampus receives input from the amygdala and its function in spatial memory is altered by amygdala activity. Repeated stress in the rat suppresses dentate gyrus neurogenesis and causes dendrites of hippocampal and medial prefrontal cortical neurons to shrink. Conversely, it causes basolateral amygdala neurons to increase in dendritic complexity and sprout new synapses. Repeated stress also increases fear and aggression, reduces spatial memory, and alters contextual fear conditioning. Antidepressants and mood stabilizers have diverse effects on these processes. New data indicate that a single stress episode can cause a delayed alteration in synapse formation in the basolateral amygdala without changing dendritic length and branching. Further studies are examining the structural changes in prefrontal cortex and hippocampus as a result of single traumatic stressors, which may reflect the functional interactions with the amygdala. Together with mechanistic studies of the role of adrenal glucocorticoids and catecholamines, these results may tell us how the brain is shaped by acute and repeated uncontrollable stress in ways that then can be investigated in human anxiety disorders.

Systemic Administration of Alcohol to Adult Rats Inhibits Leydig Cell Activity: Time Course of Effect and Role of Nitric Oxide

BACKGROUND

Alcohol has been shown to interfere with testosterone (T) release from Leydig cells. However, the mechanisms responsible for this phenomenon, which may include decreased activity of the luteinizing hormone-releasing hormone (LHRH)-LH axis, as well as a direct influence of the drug on the testes, are not fully understood. In this work, we investigated the influence of alcohol, administered intragastrically (i.g.) or delivered via vapors, on Leydig cell activity and T release. Leydig cell function was studied by measuring changes in the levels of the steroidogenic proteins steroidogenic acute regulatory (StAR), the peripheral-type benzodiazepine receptor (PBR), and the cytochrome P450 side-chain cleavage enzyme (P450scc). Testosterone release was studied under basal conditions or in response to human chorionic gonadotropin (hCG). Finally, to identify potential factors mediating the influence of alcohol, we measured the testicular variant of the neuronal nitric oxide (NO) synthase (NOS), TnNOS, in semipurified Leydig cells.

METHODS

Adult male Sprague-Dawley rats were either injected with alcohol i.g. once or exposed to alcohol vapors (4 h/d) for 1 or 5 days. Controls received the vehicle (i.g. model) or were kept in boxes through which no vapors were circulated. Following these treatments, one series of experiments was devoted to investigate Leydig cell responsiveness by measuring plasma T levels before or after the intravenous injection of hCG (1 U/kg). In another series of experiments, we used semipurified Leydig cell preparations to measure StAR, PBR, P450scc, and TnNOS by Western blot analysis.

RESULTS

In the i.g. model, the T response to hCG was blunted for 12 hours following alcohol injection, but showed a rebound at 48 hours. Levels of StAR protein and of PBR, but not of P450scc, were significantly decreased within 10 minutes of drug administration. While StAR then remained depressed for 24 hours, PBR values were variable over this time course. By 48 hours, StAR, PBR, and P450scc levels had increased above control values. Both StAR and PBR levels showed correlations with plasma T levels. In the alcohol vapor models, both regimens of the drug also significantly depressed StAR and PBR protein concentrations, blunted the T response to hCG, and did not alter P450scc. Finally, we observed that alcohol delivered i.g. or via vapors up-regulated TnNOS levels in Leydig cells, but that blockade of NO formation failed to restore a normal T response to hCG.

CONCLUSIONS

Collectively, these results suggest that (a) the ability of Leydig cells to release T does not show a simple correlation with changes in StAR, PBR, and P450scc levels; (b) the time course of the alcohol-induced changes were protein-specific; and (c) despite the ability of alcohol to stimulate TnNOS expression, NO does not appear to mediate the inhibitory influence of this drug on testicular steroidogenesis in the models that we studied.

An integrative science approach: Value added in stress research

Integrative approaches to understanding complex health issues can transcend disciplinary and knowledge boundaries and provide opportunities to view phenomena from diverse perspectives. These broad approaches to understanding phenomena of interest to nursing might provide new directions for nursing research and be a requisite for delivering safe, responsible, and holistic nursing care. The relationship between stress and illness is a strong example of a field of study that can be understood more fully from an integrative perspective. The potential of an integrative approach to contribute to improvements in human health and well-being outweigh historical biases that have been associated with an integrative science approach.

Stress in Autoimmune Disease Models

The release of endogenous glucocorticoids is critical in regulating the severity of disease activity in patients with inflammatory conditions such as rheumatoid arthritis (RA). Blocking cortisol production results in a flare-up in disease activity in RA patients, and surgical removal of the adrenals in patients with Cushing's disease has been reported to exacerbate autoimmune disease. In adjuvant-induced arthritis (AA; a rat model of RA), there is an activation of the hypothalamo-pituitary-adrenal (HPA) axis associated with the development of inflammation. In addition, there are profound changes in peptides within the paraventricular nucleus, which are responsible for regulating the HPA axis. These changes have profound implications on the ability of AA rats to respond to acute stress. Understanding the regulation of the HPA axis in health and disease holds out the promise of targeted therapy to alleviate inflammatory conditions. This article will consider the impact of stress on an individual and his or her susceptibility to inflammation. We wish to question the idea that stress is "all bad." As we shall see, exposure to a single acute stressor can alter the phenotype of the rat to change it from being susceptible to resistant in autoimmune disease models. This alteration in susceptibility takes days to manifest itself, but can last for weeks, suggesting beneficial effects of exposure to an acute stressor.

Influence of reactivity to novelty and anxiety on hypothalamic–pituitary–adrenal and prolactin responses to two different novel environments in adult male rats

Since stressor-induced activation of the hypothalamic-pituitary-adrenal (HPA) axis is involved in some stress-related pathologies, much attention has been paid in laboratory animals to the study of the relationship between endocrine, particularly HPA, responsiveness to stressors and other individual characteristics, such as reactivity to novelty and fear/anxiety. In the present study, adult male rats were classified as high or low reactive to novelty (HR versus LR), as a function of the horizontal activity displayed during 30 min in a circular corridor, and as high or low anxiety (HA versus LA) as a function of the time spent in the open arms of the elevated plus-maze. Then, the behavioural and hormonal response to two distinct novel environments (the hole-board and the light-dark) was assessed in the same subjects, using a counterbalanced design. Plasma prolactin, ACTH and corticosterone responses to the hole-board were higher than to the light-dark, a good correlation between the two tests being found for each hormone. Whereas the hormonal response to the novel environments was not affected by anxiety, HR rats showed a consistently higher HPA response than LR rats when the criteria to classify the animals were the activity during the first 15 min in the circular corridor, but not when the activity during the second 15 min was considered. Neither trait affected prolactin response. The present results demonstrate a good within-individual consistency of the endocrine response to novel environments and support the hypothesis of a higher HPA response to stressors for HR versus LR rats. In contrast, no contribution of fear/anxiety to endocrine responsiveness was observed.

Pretreatment with footshock alters some effects of subsequent organophosphate exposure

Pre-exposure to non-chemical stressors may alter a subject's vulnerability to chemical stressors. We found recently that rats given a subtoxic dose of chlorfenvinphos (CVP), an organophosphorus pesticide, develop behavioral hyposensitivity to amphetamine (AMPH). The present experiments were performed in order to find out whether pre-exposure to a non-chemical stressor several days prior to CVP exposure could influence this effect of the pesticide. In experiment 1 adult male Wistar rats were subjected once to either a short, 5 min (SFS) or long, 20 min (LFS) series of unavoidable footshocks (FS). Twenty-four hours or 14 days after the FS, their open field behavior was tested before and after a test dose of AMPH (0.5 mg/kg. i.p). In experiment 2, the rats were subjected to LFS and 14 days later they were injected intraperitoneally with CVP (1.0 mg/kg) or vehicle (corn oil). In both experiments, serum corticosterone (CORT) levels were determined in separate groups of rats in order to assess the magnitude of the stress response induced by the applied stressors. It was found that: (i) the rise in serum CORT concentration after SFS or LFS was similar in magnitude, while that following LFS was more persistent; (ii) exposure to LFS, but not to SFS, resulted in a decreased response to AMPH on day 14 after the experience; (iii) in rats not pretreated with LFS, CVP exposure resulted in a profound increase in serum CORT concentration. In LFS pretreated rats, however, this effect was significantly reduced; (iv) three weeks after the exposure to CVP, the psychomotor response to AMPH was diminished in control rats but was normal in LFS pretreated animals. The results indicate that pretreatment with a non-chemical stressor may protect the rat against at least some of the effects of an organophosphate pesticide.

Long-term effects of social stress on brain and behavior: a focus on hippocampal functioning

In order to study mechanisms involved in the etiology of human affective disorders, there is an abundant use of various animal models. Next to genetic factors that predispose for psychopathologies, environmental stress is playing an important role in the etiology of these mental diseases. Since the majority of stress stimuli in humans that lead to psychopathology are of social nature, the study of consequences of social stress in experimental animal models is very valuable. The present review focuses on one of these models that uses the resident-intruder paradigm. In particular the long-lasting effects of social defeat in rats will be evaluated. Data from our laboratory on the consequences of social defeat on emotional behavior, stress responsivity and serotonergic functionality are presented. Furthermore, we will go into detail on hippocampal functioning in socially stressed rats. Very recent results show that there is a differential effect of a brief double social defeat and repetitive social defeat stress on dendritic remodeling in hippocampal CA3 neurons and that this has repercussions on hippocampal LTP and LTD. Both the structural and electrophysiological changes of principal neurons in the hippocampal formation after defeat are discussed as to their relationship with the maintenance in cognitive performance that was observed in socially stressed rats. The results are indicative of a large dynamic range in the adaptive plasticity of the brain, allowing the animals to adapt behaviorally to the previously occurred stressful situation with the progression of time.

Effects of stress on inflammatory autoimmune disease: destructive or protective?

We have summarised evidence in the literature for modulatory effects of stress on inflammatory autoimmune disease. We find that overall there is strong evidence for such an interrelationship. Apparent discrepancies between groups and studies are probably due to differences in experimental design, whether longitudinal or retrospective. Other important variables are the specific effects of different types of stress and the intensity and timing of the stressor relative to onset of inflammation. We conclude that there is much of benefit to be learned from scientific study of stress, such as harnessing and rationalising of stressful experiences through self-expression in patients, or the identification of novel anti-inflammatory compounds activated by stress.

Long-term effects of a single exposure to stress in adult rats on behavior and hypothalamic–pituitary–adrenal responsiveness: comparison of two outbred rat strains

We have previously observed that a single exposure to immobilization (IMO), a severe stressor, caused long-term (days to weeks) desensitization of the response of the hypothalamic-pituitary-adrenal (HPA) axis to the homotypic stressor, with no changes in behavioral reactivity to novel environments. In contrast, other laboratories have reported that a single exposure to footshock induced a long-term sensitization of both HPA and behavioral responses to novel environments. To test whether these apparent discrepancies can be explained by the use of different stressors or different strains of rats, we studied in the present work the long-term effects of a single exposure to two different stressors (footshock or IMO) in two different strains of rats (Sprague-Dawley from Iffa-Credo and Wistar rats from Harlan). We found that both strains showed desensitization of the HPA response to the same (homotypic) stressor after a previous exposure to either shock or IMO. The long-term effects were higher after IMO than shock. No major changes in behavior in two novel environments (circular corridor, CC and elevated plus-maze, EPM) were observed after a single exposure to shock or IMO in neither strain, despite the fact that shocked rats showed a conditioned freezing response to the shock boxes. The present results demonstrate that long-term stress-induced desensitization of the HPA axis is a reliable phenomenon that can be observed with different stressors and strains. However, only behavioral changes related to shock-induced conditioned fear were found, which suggests that so far poorly characterized factors are determining the long-term behavioral consequences of a single exposure to stress.

A single exposure to severe stressors causes long-term desensitisation of the physiological response to the homotypic stressor

Although some laboratories have reported that a single session of stress is able to induce a long-lasting sensitisation of the hypothalamic-pituitary-adrenal (HPA) response to further exposures to stress, we have found that a single exposure to severe emotional (immobilisation, restraint or shock) or systemic (endotoxin) stressors reduces the responsiveness of the HPA to the same, but not to a novel (heterotypic), stressor, in which case a slight sensitisation was observed. Long-term desensitisation has been found to reduce not only secretion of peripheral HPA hormones (ACTH and corticosterone), but also to reduce responses of central components of the HPA axis (c-fos and CRF gene expression at the level of the paraventricular nucleus of the hypothalamus, PVN). In addition, desensitisation also applies to the impact of the stressor on food intake and, probably, to stress-induced hyperglycaemia. The development of long-term desensitisation of the HPA axis does not appear to be a universal consequence of exposure to severe stressors as it was not observed in response to insulin-induced hypoglycaemia. Whether or not the development of long-term effects of stress depend on the specific pathways activated by particular stressors remains to be tested. The observed desensitisation of the HPA axis in response to the homotypic stressor shows two special features which makes it difficult to be interpreted in terms of an habituation-like process: (a) the effect increased with time (days to weeks) elapsed between the first and second exposure to the stressor, suggesting a progressive maturational process; and (b) the stronger the stressor the greater the long-term desensitisation. Therefore, it is possible that desensitisation of the HPA axis is the sum of two different phenomena: long-term effects and habituation-like processes. The contribution of the former may be more relevant with severe stressors and longer inter-stress intervals, and that of the latter with mild stressors and repeated exposures. Long-term stress-induced changes may not take place at the level of the PVN itself, but in brain nuclei showing synaptic plasticity and putatively involved in the control of the HPA axis and other physiological responses. As for the precise areas involved, these remain to be characterized.

Adaptive and maladaptive psychobiological responses to severe psychological stress: implications for the discovery of novel pharmacotherapy

Post-traumatic stress disorder (PTSD) is one of the few DSM-IV diagnoses contingent upon a psychosocial stressor. In this context, there is an urgent need to acquire a better understanding of both the adaptive and maladaptive psychobiological responses to traumatic stress. Preclinical investigators have utilized a variety of animal models to identify the behavioral and neurobiological features of the organism's response to stress. However, given the complexity of the healthy and pathological human response to physiological and psychological stress, the extent to which the animal data is immediately transferable to human remains to be fully determined. This review draws upon preclinical and clinical literature to examine the transformation of an adaptive human stress response into a maladaptive and debilitating mental disorder. An integrative psychobiological model for PTSD is presented, linking psychological processes and behavioral patterns with current findings in neurocircuitry, neurochemistry and psychophysiology. The implications of this model for the discovery of novel pharmacological approaches to the treatment of severe psychological distress are discussed.

Bidirectional shift in the cornu ammonis 3 pyramidal dendritic organization following brief stress

The negative impact of chronic stress at the structure of apical dendrite branches of cornu ammonis 3 (CA3) pyramidal neurons is well established. However, there is no information available on the CA3 dendritic organization related to short-lasting stress, which suffices to produce long-term habituation or sensitization of anxiety behaviors and neuroendocrine responses. Here, we tested the effects evoked by brief stress on the arrangements of CA3 pyramidal neuron dendrites, and the activity-dependent properties of the commissural-associational (C/A) excitatory postsynaptic potentials (EPSPs). Adult male rats were socially defeated followed by 3 weeks without further treatment or as comparison exposed to a regimen of a social defeat every second day for the same time period. We assessed CA3 pyramidal neurons with somatic whole-cell recording and neurobiotin application in acute hippocampal slices. The results from morphometric analysis of post hoc reconstructions demonstrated that CA3 dendrites from repeatedly stressed rats were reduced in surface area and length selectively at the apical cone (70% of control, approximately 280 microm from the soma). Brief stress, however, produced a similar decrease in apical dendritic length (77% of control, approximately 400 microm from the soma), accompanied by an increased length (167% of control) and branch complexity at the basal cone. The structural changes of the dendrites significantly influenced signal propagation by shortening the onset latency of EPSPs and increasing input resistance (r=0.45, P<0.01), of which the first was significantly changed in repeatedly stressed animals. Both brief and repeated stress long-lastingly impaired long-term potentiation of C/A synapses to a similar degree (P<0.05). These data indicate that the geometric plasticity of CA3 dendrites is dissociated from repetition of aversive experiences. A double social conflict suffices to drive a dynamic reorganization, by site-selective elimination and de novo growth of dendrite branches over the course of weeks after the actual experience.

The hypothalamic–pituitary–adrenal and glucose responses to daily repeated immobilisation stress in rats: individual differences

It is accepted that there are important individual differences in the vulnerability to stress-induced pathologies, most of them associated to the hypothalamic-pituitary and sympatho-medullo-adrenal axes, the two prototypical stress-responsive systems. However, there are few studies specifically aimed at characterising individual differences in the physiological response to daily repeated stress in rats. In the present work, male rats were submitted to repeated immobilisation (IMO) stress (1 h daily for 13 days) and several samples were taken at specific days and time points. Animals only subjected to blood sampling procedure served as controls. Daily adrenocorticotropic-hormone (ACTH), corticosterone and glucose responses to immobilisation (that included the post-immobilisation period) progressively declined over the days. In addition, repeated immobilisation resulted in decreased relative thymus weight, increased relative adrenal weight, elevated corticotropin-releasing factor (CRF) mRNA levels in the hypothalamic paraventricular nucleus (PVN), and down-regulation of glucocorticoid receptor gene transcription in hippocampus CA1. However, only CRF mRNA levels in the paraventricular nucleus correlated with the ACTH (on day 1) and corticosterone responses (from day 4-13) to immobilisation. When the animals were classified in three groups on the basis of their plasma ACTH levels immediately after the first immobilisation, individual differences in the ACTH response progressively disappeared on successive exposures to the stressor, whereas those in corticosterone and glucose were more sustained. The present results suggest that there are individual differences in the physiological response to stress that tend to be reduced rather than accentuated by repeated exposure to the stressor. Nevertheless, this buffering effect of repeated stress was dependent on the particular variable studied.

Immunohistochemical changes induced by repeated footshock stress: revelations of gender-based differences

As a growing literature has proven, adverse experiences, particularly when severe and persistent, play a pivotal role in the development of neuronal dysfunctions and psychopathology. In the present study, the neurochemical changes induced by acute and repeated footshock exposure were investigated at the molecular and cellular level, using c-fos and phospho-ERK1/2 immunoreactivity and gene expression arrays. Marked gender-related differences were found following both acute and prolonged footshock exposure. Acute aversive conditioning resulted in significant immunohistochemical changes that might be critically involved in the modulation of fear-related responses, especially in males. Prolonged footshock exposure, on the contrary, was associated with sustained hypothalamic-pituitary-adrenal axis hyperactivity, differential gender-related patterns of cortical-limbic activity, and abnormal neuronal plasticity, especially in medial prefrontocortical regions. These data may provide additional insights into the understanding of the neural circuits underlying the effects of acute and repeated footshock exposure as well as clarify some of the mechanisms involved in the development of stress-related neuronal abnormalities.

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

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

Long-term effects of a single exposure to immobilization stress on the hypothalamic-pituitary-adrenal axis: transcriptional evidence for a progressive desensitization process

In the present work we have characterized the long-term influence of a single exposure to the stress of immobilization (IMO) on the hypothalamic-pituitary-adrenal (HPA) axis of adult rats. Rats without prior stress (control) and rats exposed to IMO for 2 h on day 1 (IMO+4wk) or on day 21 (IMO+1wk) were killed on day 28, either without stress (basal), immediately after IMO for 1 h (IMO), or 1 h after termination of IMO (post-IMO). IMO caused a strong activation of c-fos mRNA and corticotropin-releasing factor (CRF) and vasopressin (AVP) heteronuclear RNA (hnRNA) in the paraventricular nucleus of the hypothalamus in control rats; this activation was essentially maintained in the post-IMO period. The overall AVP hnRNA response to day 28 stress was not affected by prior stress. Post-IMO c-fos mRNA and CRF hnRNA levels were lower in previously stressed rats, as compared with controls. Whereas the effect of prior IMO on both peripheral HPA hormones and c-fos mRNA was maximal in IMO+1wk rats, the effect of prior stress on CRF hnRNA was only observed in IMO+4wk rats. The present data indicate that prior single IMO triggers a process of desensitization of the HPA responsiveness to IMO over the course of the following weeks. Although the various components of the HPA axis were modified in the same direction, a clear temporal dissociation was found among them, revealing the fine tuning of stress-induced activation of the HPA axis.

Behavioural changes after an acute stress: stressor and test types influences

Behavioural consequences of different acute stressors (30 min of restraint, 20 min of forced swim stress, 15 min of inescapable footshocks) applied at the beginning of the active period were assessed in using two behavioural tests: a 20 min light extinction test 24 h after the stressor exposure in order to explore the psychomotor ability and a 10 min open field session within the dark period 48 h after the stressor exposure to estimate the emotional status and the locomotor activity of the rat. Different behavioural responses were observed depending on the nature of the applied stressor. In the light extinction test, the footshock-stressed rats developed a very low activity independent on light conditions whereas the rats submitted to forced swim and restraint exhibited an activity level depending on the strain. Moreover, restrained rats had a higher transient activity than forced swim rats under light condition. In the open field test, none of the stressed rats did develop differences in behaviour. The efficacy of a 24 h recovery period on the behavioural response to an acute stressor exposure depends on the intensity of the applied stressor and the behavioural demands.

Neuroendocrinology of autoimmunity

The HPA axis is fundamental for long-term survival and protection from the ravages of autoimmune disease. Continuing investigations suggest that the hypothesis linking susceptibility to autoimmune disease and a hyporesponsive HPA axis is somewhat simplistic. Instead, data from a number of different human diseases and from preclinical studies in a variety of models have suggested a more complicated picture. Alterations in the diurnal rhythms of ACTH, cortisol, and immune parameters appear to be linked to severity of disease. The use of low doses of steroids timed to target disrupted diurnal immune system changes in patients with RA may reduce the unfortunate side effects of long-term steroid use. Studies in cancer patients have related alterations in diurnal cortisol to survival. Whether differences in individual cortisol profiles are predictive of a deterioration in symptoms of autoimmune disease remains to be established. Responsiveness of the HPA axis to subtle challenges such as the dexamethasone suppression test and the related dexamethasone-CRF test suggest that there are different sub-populations of patients with RA and MS and these may have confounded earlier, apparently contradictory, studies. These different responses may be related to the severity of the disease. That these HPA axis differences can be altered beneficially through the use of antidepressants, as has been shown in MS, may impact on future health care strategies. However, reports of negative developments in arthritis associated with SSRI use suggest that the SSRIs may be unsuitable under some circumstances. The link of behavioral differences to alterations in neurotransmitter changes associated with disease is intriguing and opens new avenues of research. These future studies will require input from neuroscientists, neuroendocrinologists, psychologists, and immunologists working with the clinical specialties already involved in treating patients with autoimmune disease. These multidisciplinary studies reflecting the increased importance of hormonal and neurotransmitter involvement with the immune system hold great promise for the future.

A single lipopolysaccharide administration is sufficient to induce a long-term desensitization of the hypothalamic-pituitary-adrenal axis

We have previously shown that a single exposure of adult rats to a severe emotional stressor such as immobilization is able to exert a long-term desensitization of the response of the hypothalamic-pituitary-adrenal (HPA) axis to the same stimulus when applied days to weeks later. Surprisingly, the intensity of the effect increased with time elapsed between the two exposures, suggesting that we are dealing with a new type of stress-associated phenomenon. Taking into account the clinical importance of tolerance to endotoxin, in the present study we assessed whether a single exposure to an immunological stressor such as lipopolysaccharide can induce effects similar to those of immobilization. Rats injected with lipopolysaccharide (1 mg/kg) showed a reduction of the response of the corticotropin-releasing factor mRNA in the paraventricular nucleus of the hypothalamus after a new lipopolysaccharide injection 4, but not 2 weeks later. In an additional experiment using a different blood sampling procedure, adrenocorticotropin hormone, corticosterone and tumor necrosis factor-alpha responses were reduced approximately to the same extent by previous experience with lipopolysaccharide either 1 or 4 weeks before. Our data suggest that a previous single exposure to lipopolysaccharide induces a long-lasting tolerance of the HPA axis that likely involves some kind of learning-like brain plasticity.

Protective effect of prior acute immune challenge, but not footshock, on inflammation in the rat

Previous studies have revealed that a single exposure to an acute stress or acute immune stimulus can produce long-lasting changes in the activity and responsiveness of the hypothalamo-pituitary-adrenal (HPA) axis. The HPA axis is believed to be an important component in determining the susceptibility and severity of inflammation in autoimmune disease models such as adjuvant-induced arthritis (AA). In the present study we have tested the hypothesis that a single exposure to either footshock or lipopolysaccharide (LPS) 3 weeks prior to adjuvant injection can alter susceptibility to AA. Changes in HPA axis parameters were also determined. The results demonstrated that prior exposure to LPS conferred resistance to inflammation in AA, which was not related to a delay in onset of inflammation but rather an alteration in susceptibility. In contrast, prior exposure to the acute stress of footshock did not alter susceptibility. HPA axis parameters were increased in adjuvant-injected rats whether inflammation was present or not. These data suggest that prior exposure to acute immune stimuli, but not to acute footshock stress, may alter susceptibility to inflammation in the rat AA model. These changes in susceptibility do not appear to be solely mediated by increases in HPA axis activity, which were apparent in all AA groups irrespective of the presence of inflammation.

Post-stress recovery of pituitary-adrenal hormones and glucose, but not the response during exposure to the stressor, is a marker of stress intensity in highly stressful situations

Acute immobilization in male rats elicited the same ACTH, corticosterone and glucose response as foot shock when measured immediately after stress. However, post-stress recovery of plasma ACTH, corticosterone and glucose levels were delayed in immobilized versus shocked rats. Similarly, stress-induced anorexia was much greater in the former animals. All these data suggest that post-stress speed of recovery of some physiological variables is positively related to stressor intensity and could be used to evaluate it.

Is repeated exposure to immobilization needed to induce adaptation of the hypothalamic-pituitary-adrenal axis? Influence of adrenal factors

We have previously observed that a single exposure to a severe stressor such as immobilization in wooden boards (IMO) resulted in a faster return of plasma corticosterone (and to a lesser extent of ACTH) to basal activity when the rats were exposed again to the same stressor. In addition, the effect enhanced with time (days) elapsed between the two exposures. These data raised the question of to what extent adaptation of the hypothalamic-pituitary-adrenal (HPA) axis to repeated stress might be, at least partially, explained by the time elapsed between the two exposures rather than by daily repetition of the stressor. To answer this question and the role of glucocorticoids in the process, we studied the effects of single versus repeated exposure to IMO on the HPA response to the same stressor in both sham-operated and adrenalectomized rats maintained with corticosterone in their drinking saline (ADX+B). In sham rats, daily exposure to 20 min IMO for 9 days resulted in a decrease of the ACTH response to the stressor and a faster return of corticosterone to basal levels in the post-stress period. Similar effects were observed with a single session of IMO 8 days before. In ADX+B rats, a reduction of the ACTH response to the stressor was observed in repeated IMO rats but not in single IMO rats. The present results suggest that (i) in sham rats, a single exposure to IMO can induce a degree of adaptation of the HPA response to the same stressor applied days later that is very similar to that caused by repeated exposure to the situation; (ii) stress-induced release of glucocorticoids (or other adrenal factors) is not mandatory for the development of adaptation of the HPA axis to repeated stress, but may be involved in the long-term effects of a single exposure to stress.

A single administration of interleukin-1 or amphetamine induces long-lasting increases in evoked noradrenaline release in the hypothalamus and sensitization of ACTH and corticosterone responses in rats

Single administration of the cytokine interleukin-1beta (IL-1) or the psychostimulant amphetamine causes long-term sensitization of the hypothalamus pituitary adrenal (HPA) axis, i.e. enhanced adrenocorticotropine hormone (ACTH) and corticosterone responses weeks later. HPA responses to these stimuli involve activation of hypothalamic corticotropin-releasing hormone (CRH) neurons by noradrenergic projections to the paraventricular nucleus (PVN). In search of the underlying mechanisms, we studied the temporal pattern of HPA sensitization in relation to (1) the reactivity of noradrenergic projections to the PVN and (2) altered secretagogue production in hypothalamic CRH neurons. Single exposure to IL-1 or amphetamine induced cross-sensitization of ACTH and corticosterone responses 11 and 22 days later, but not after 42 days. Amphetamine-induced HPA sensitization was not accompanied by increased costorage of arginine vasopressin (AVP) in CRH terminals, as found previously after IL-1 pretreatment. The reactivity of noradrenergic terminals was assessed by measuring the electrically evoked release of [3H]-noradrenaline from superfused PVN slices. Single administration of amphetamine and IL-1 induced a long-lasting (up to 22 days) increase (up to 165%) of evoked noradrenaline release. This indicates that single exposure to psychostimulants or to cytokines can induce a long-lasting increase in stimulus-secretion coupling in brainstem noradrenergic neurons that innervate the PVN. This common, long-lasting functional change may underlie, at least in part, IL-1- and amphetamine-induced HPA cross-sensitization. In addition, increased AVP signalling by hypothalamic CRH neurons appears to play a role in IL-1-induced, but not in amphetamine-induced, HPA sensitization.