Anxiolytic-like effect induced by chronic stress is reversed by naloxone pretreatment

Antistress Action of Melanocortin Derivatives Associated with Correction of Gene Expression Patterns in the Hippocampus of Male Rats Following Acute Stress

Natural melanocortins (MCs) have been used in the successful development of drugs with neuroprotective properties. Here, we studied the behavioral effects and molecular genetic mechanisms of two synthetic MC derivatives-ACTH(4-7)PGP (Semax) and ACTH(6-9)PGP under normal and acute restraint stress (ARS) conditions. Administration of Semax or ACTH(6-9)PGP (100 μg/kg) to rats 30 min before ARS attenuated ARS-induced behavioral alterations. Using high-throughput RNA sequencing (RNA-Seq), we identified 1359 differentially expressed genes (DEGs) in the hippocampus of vehicle-treated rats subjected to ARS, using a cutoff of >1.5 fold change and adjusted p-value (Padj) < 0.05, in samples collected 4.5 h after the ARS. Semax administration produced > 1500 DEGs, whereas ACTH(6-9)PGP administration led to <400 DEGs at 4.5 h after ARS. Nevertheless, ~250 overlapping DEGs were identified, and expression of these DEGs was changed unidirectionally by both peptides under ARS conditions. Modulation of the expression of genes associated with biogenesis, translation of RNA, DNA replication, and immune and nervous system function was produced by both peptides. Furthermore, both peptides upregulated the expression levels of many genes that displayed decreased expression after ARS, and vice versa, the MC peptides downregulated the expression levels of genes that were upregulated by ARS. Consequently, the antistress action of MC peptides may be associated with a correction of gene expression patterns that are disrupted during ARS.

An improved light dark box test by using a real-time video tracking system

In the behavioral science field, many of the oldest tests have still most frequently been used almost in the same way for decades. The subjective influence of human observer and the large inter-observer and interlab differences are substantial among these tests. This necessitates the possibility of using technological innovations for behavioral science to obtain new parameters, results and insights as well. The light-dark box (LDB) test is a characteristic tool used to assess anxiety in rodents. A complete behavioral analysis (including both anxiety and locomotion parameters) is not possible by performing traditional LDB test protocol, as it lacks the usage of a real-time video recording of the test. In the current report, we describe an improved approach to conduct LDB test using a real-time video tracking system.

Effect of chronic unpredictable stress on mice with developmental under-expression of the Ahi1 gene: behavioral manifestations and neurobiological correlates

The Abelson helper integration site 1 (Ahi1) gene plays a pivotal role in brain development and is associated with genetic susceptibility to schizophrenia, and other neuropsychiatric disorders. Translational research in genetically modified mice may reveal the neurobiological mechanisms of such associations. Previous studies of mice heterozygous for Ahi1 knockout (Ahi1+/-) revealed an attenuated anxiety response on various relevant paradigms, in the context of a normal glucocorticoid response to caffeine and pentylenetetrazole. Resting-state fMRI showed decreased amygdalar connectivity with various limbic brain regions and altered network topology. However, it was not clear from previous studies whether stress-hyporesponsiveness reflected resilience or, conversely, a cognitive-emotional deficit. The present studies were designed to investigate the response of Ahi1+/- mice to chronic unpredictable stress (CUS) applied over 9 weeks. Wild type (Ahi1+/+) mice were significantly affected by CUS, manifesting decreased sucrose preference (p < 0.05); reduced anxiety on the elevated plus maze and light dark box and decreased thigmotaxis in the open field (p < 0.01 0.05); decreased hyperthermic response to acute stress (p < 0.05); attenuated contextual fear conditioning (p < 0.01) and increased neurogenesis (p < 0.05). In contrast, Ahi1+/- mice were indifferent to the effects of CUS assessed with the same parameters. Our findings suggest that Ahi1 under-expression during neurodevelopment, as manifested by Ahi1+/- mice, renders these mice stress hyporesponsive. Ahi1 deficiency during development may attenuate the perception and/or integration of environmental stressors as a result of impaired corticolimbic connectivity or aberrant functional wiring. These neural mechanisms may provide initial clues as to the role Ahi1 in schizophrenia and other neuropsychiatric disorders.

Differential effects of chronic stress in young-adult and old female mice: cognitive-behavioral manifestations and neurobiological correlates

Stress-related psychopathology is highly prevalent among elderly individuals and is associated with detrimental effects on mood, appetite and cognition. Conversely, under certain circumstances repeated mild-to-moderate stressors have been shown to enhance cognitive performance in rodents and exert stress-inoculating effects in humans. As most stress-related favorable outcomes have been reported in adolescence and young-adulthood, this apparent disparity could result from fundamental differences in how aging organisms respond to stress. Furthermore, given prominent age-related alterations in sex hormones, the effect of chronic stress in aging females remains a highly relevant yet little studied issue. In the present study, female C57BL/6 mice aged 3 (young-adult) and 20-23 (old) months were subjected to 8 weeks of chronic unpredictable stress (CUS). Behavioral outcomes were measured during the last 3 weeks of the CUS protocol, followed by brain dissection for histological and molecular end points. We found that in young-adult female mice, CUS resulted in decreased anxiety-like behavior and enhanced cognitive performance, whereas in old female mice it led to weight loss, dysregulated locomotion and memory impairment. These phenotypes were paralleled by differential changes in the expression of hypothalamic insulin and melanocortin-4 receptors and were consistent with an age-dependent reduction in the dynamic range of stress-related changes in the hippocampal transcriptome. Supported by an integrated microRNA (miRNA)-mRNA expression analysis, the present study proposes that, when confronted with ongoing stress, neuroprotective mechanisms involving the upregulation of neurogenesis, Wnt signaling and miR-375 can be harnessed more effectively during young-adulthood. Conversely, we suggest that aging alters the pattern of immune activation elicited by stress. Ultimately, interventions that modulate these processes could reduce the burden of stress-related psychopathology in late life.

Long-term ethanol self-administration induces ΔFosB in male and female adolescent, but not in adult, Wistar rats

Early-onset ethanol consumption predicts later development of alcohol use disorders. Age-related differences in reactivity to ethanol's effects may underlie this effect. Adolescent rats are more sensitive and less sensitive than adults to the appetitive and aversive behavioral effects of ethanol, respectively, and more sensitive to the neurotoxic effects of experimenter-administered binge doses of ethanol. However, less is known about age-related differences in the neural consequences of self-administered ethanol. ΔFosB is a transcription factor that accumulates after chronic drug exposure and serves as a molecular marker of neural plasticity associated with the transition to addiction. We analyzed the impact of chronic (18 two-bottle choice intake sessions spread across 42days, session length: 18h) ethanol [or only vehicle (control group)] self-administration during adolescence or adulthood on the induction of ΔFosB in several brain areas, anxiety-like behavior, and ethanol-induced locomotor activity and conditioned place preference (CPP) in Wistar rats. Adolescent rats exhibited a progressive escalation of ethanol intake and preference, whereas adult rats exhibited a stable pattern of ingestion. Few behavioral differences in the open field or light-dark test were observed after the intake test. Furthermore, ethanol self-administration did not promote the expression of ethanol-induced CPP. There were, however, large age-related differences in the neural consequences of ethanol drinking: a significantly greater number of ethanol-induced ΔFosB-positive cells was found in adolescents vs. adults in the prelimbic cortex, dorsolateral striatum, nucleus accumbens core and shell, and central amygdala nucleus capsular and basolateral amygdala, with sex-related differences found at central amygdala. This greater ethanol-induced ΔFosB induction may represent yet another age-related difference in the sensitivity to ethanol that may put adolescents at higher risk for problematic ethanol use.

HPA Axis Interactions with Behavioral Systems

Perhaps the most salient behaviors that individuals engage in involve the avoidance of aversive experiences and the pursuit of pleasurable experiences. Engagement in these behaviors is regulated to a significant extent by an individual's hormonal milieu. For example, glucocorticoid hormones are produced by the hypothalamic-pituitary-adrenocortical (HPA) axis, and influence most aspects of behavior. In turn, many behaviors can influence HPA axis activity. These bidirectional interactions not only coordinate an individual's physiological and behavioral states to each other, but can also tune them to environmental conditions thereby optimizing survival. The present review details the influence of the HPA axis on many types of behavior, including appetitively-motivated behaviors (e.g., food intake and drug use), aversively-motivated behaviors (e.g., anxiety-related and depressive-like) and cognitive behaviors (e.g., learning and memory). Conversely, the manuscript also describes how engaging in various behaviors influences HPA axis activity. Our current understanding of the neuronal and/or hormonal mechanisms that underlie these interactions is also summarized. © 2016 American Physiological Society. Compr Physiol 6:1897-1934, 2016.

Ultrasound of alternating frequencies and variable emotional impact evokes depressive syndrome in mice and rats

Emotional stress is primarily triggered by the cognitive processing of negative input; it is regarded as a serious pathogenetic factor of depression that is challenging to model in animals. While available stress paradigms achieve considerable face and construct validity in modelling depressive disorders, broader use of naturalistic stressors instead of the more prevalent models with artificial challenges inducing physical discomfort or pain may substantially contribute to the development of novel antidepressants. Here, we investigated whether a 3-week exposure of Wistar rats and Balb/c mice to unpredictably alternating frequencies of ultrasound between the ranges of 20-25 and 25-45kHz, which are known to correspond with an emotionally negative and with a neutral emotional state, respectively, for small rodents in nature, can induce behavioural and molecular depressive-like changes. Both rats and mice displayed decreased sucrose preference, elevated "despair" behaviour in a swim test, reduced locomotion and social exploration. Rats showed an increased expression of SERT and 5-HT2A receptor, a decreased expression of 5-HT1A receptor in the prefrontal cortex and hippocampus, diminished BDNF on gene and protein levels in the hippocampus. Fluoxetine, administered to rats at the dose of 10mg/kg, largely precluded behavioural depressive-like changes. Thus, the here applied paradigm of emotional stress is generating an experimental depressive state in rodents, which is not related to any physical stressors or pain. In essence, this ultrasound stress model, besides enhancing animal welfare, is likely to provide improved validity in the modelling of clinical depression and may help advance translational research and drug discovery for this disorder.

Age-related effects of chronic restraint stress on ethanol drinking, ethanol-induced sedation, and on basal and stress-induced anxiety response

Adolescents are sensitive to the anxiolytic effect of ethanol, and evidence suggests that they may be more sensitive to stress than adults. Relatively little is known, however, about age-related differences in stress modulation of ethanol drinking or stress modulation of ethanol-induced sedation and hypnosis. We observed that chronic restraint stress transiently exacerbated free-choice ethanol drinking in adolescent, but not in adult, rats. Restraint stress altered exploration patterns of a light-dark box apparatus in adolescents and adults. Stressed animals spent significantly more time in the white area of the maze and made significantly more transfers between compartments than their non-stressed peers. Behavioral response to acute stress, on the other hand, was modulated by prior restraint stress only in adults. Adolescents, unlike adults, exhibited ethanol-induced motor stimulation in an open field. Stress increased the duration of loss of the righting reflex after a high ethanol dose, yet this effect was similar at both ages. Ethanol-induced sleep time was much higher in adult than in adolescent rats, yet stress diminished ethanol-induced sleep time only in adults. The study indicates age-related differences that may increase the risk for initiation and escalation in alcohol drinking.

Cross stress adaptation: Phenomenon of interactions between homotypic and heterotypic stressors

Individuals have an inherent capacity to cope with stressors in the form of stress adaptation. Apart from stress adaptation there is another well documented phenomenon known as cross stress adaptation. In this, there is a reduction in stress responsiveness to a novel stressor (in which the adapted organism had never encountered previously) in previously exposed organisms with another stressor given in either continuous or intermittent. However, regarding the existence of cross stress adaptation, there are mixed reports revealing that the positive cross stress adaptation exists between altitude and heat stress; swim and inescapable shock stress, hypoxia and cold stress, psychosocial stressor and exercise. However, there are other reports which reveal the non-existence of cross adaptation between forced swim and noise stress and cold and immobilized stress. The exact mechanisms responsible for cross stress adaptation are not defined and need to be investigated.

Noise stress changes mRNA expressions of corticotropin-releasing hormone, its receptors in amygdala, and anxiety-related behaviors

Noise is a psychological, environmental stressor that activates limbic sites in the brain. Limbic sites such as the amygdala and the amygdaloid corticotropin-releasing hormone (CRH) system play an important role in integrating stress response. We investigated the association between noise exposures, CRH-related molecules in the amygdala, and behavioral alterations. In total 54 Sprague-Dawley rats were divided into the following three groups: Control (CON), acute noise exposure (ANE), and chronic noise exposure (CNE). The ANE group was exposed to 100 dB white noise only once in 4 h and the CNE group was exposed to the same for 4 h per day for 30 days. Expression profiles of CRH and its receptors CRH-R1 and CRH-R2 were analyzed by quantitative real-time polymerase chain reaction (qPCR). The same stress procedure was applied to the ANE and CNE groups for behavior testing. The anxiety responses of the animals after acute and chronic stress exposure were measured in the defensive withdrawal test. CNE upregulated CRH and CRH-R1 mRNA levels but downregulated CRH-R2 mRNA levels. ANE led to a decrease in both CRH-R1 and CRH-R2 expression. In the defensive withdrawal test, while the ANE increased, CNE reduced anxiety-like behaviors. The present study shows that the exposure of rats to white noise (100 dB) leads to behavioral alterations and molecule-specific changes in the CRH system. Behavioral alterations can be related to these molecular changes in the amygdala.

Gender-dependent effect on nociceptive response induced by chronic variable stress

It has previously been reported that exposure to repeated restraint stress induces hyperalgesia in male rats, an effect that was not observed in females. The aim of the present study was to investigate the effects of chronic variable stress over 40days on nociception threshold indexed by tail-flick latency in male and female adult rats. The results showed different behavior in chronically stressed animals when compared to the control group: male rats showed a decrease in tail-flick latency while females presented an increase in this parameter. For female rats this effect was independent of the phase of the estrous cycle. Several sources of data indicate that behavioral and physiological responses to stress are sexually dimorphic, including in nociception, and the estrous cycle appears to be a factor that influences opioid analgesia in female. These effects are modulated by the strain and conditions of nociception assay. Additional studies concerning the mechanisms involved in the hyperalgesic response in males and the differences on nociceptive response in females chronically exposed to stress are needed.

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.

Altered emotionality, hippocampus-dependent performance and expression of NMDA receptor subunit mRNAs in chronically stressed mice

N-Methyl-D-aspartate receptor (NMDAR)-mediated neurotransmission in the hippocampus is implicated in cognitive and emotional disturbances during stress-related disorders. Here, using quantitative RT-PCR, we investigated the hippocampal expression of NR2A, NR2B and NR1 subunit mRNAs in a mouse stress paradigm that mimics clinically relevant conditions of simultaneously affected emotionality and hippocampus-dependent functions. A 2-week stress procedure, which comprised ethologically valid stressors, exposure to a rat and social defeat, was applied to male C57BL/6J mice. For predation stress, mice were introduced into transparent containers that were placed in a rat home cage during the night; social defeat was applied during the daytime using aggressive CD1 mice. This treatment impaired hippocampus-dependent performance during contextual fear conditioning. A correlation between this behavior and food displacement performance was demonstrated, suggesting that burrowing behavior is affected by the stress procedure and is hippocampus-dependent. Stressed mice (n = 22) showed behavioral invigoration and anomalous anxiolytic-like profiles in the O-maze and brightly illuminated open field, unaltered short-term memory in the step-down avoidance task and enhanced aggressive traits, as compared to non-stressed mice (n = 10). Stressed mice showed increased basal serum corticosterone concentrations, hippocampal mRNA expression for the NR2A subunit of the NMDAR and in the NR2A/NR2B ratio; mRNA expression of NR2B and NR1 was unchanged. Thus, stress-induced aberrations in both hippocampal-dependent performance and emotional abnormalities are associated with alterations in hippocampal mRNA NR2A levels and the NR2A/NR2B ratio and not with mRNA expression of NR2B or NR1.

Binge ethanol intoxication heightens subsequent ethanol intake in adolescent, but not adult, rats

A question still to be answered is whether ethanol initiation has a greater effect on ethanol consumption if it occurs during adolescence than in adulthood. This study assessed the effect of ethanol initiation during adolescence or adulthood on voluntary ethanol consumption when animals were still within the same age range. Adolescent or adult rats were given 5, 2, or 0 ethanol exposures. The animals were tested for ethanol consumption through two-bottle choice tests, before undergoing a 1-week deprivation. A two-bottle assessment was conducted after the deprivation. Adolescents, but not adults, given two ethanol administrations during initiation exhibited significantly higher ethanol intake during the pre-deprivation period. These adolescents also exhibited a threefold increase in ethanol intake after 7 days of drug withdrawal, when compared with controls. These findings suggest that very brief experience with binge ethanol intoxication in adolescence, but not in adulthood, impacts later predisposition to drink.

Adrenocortical and behavioural response to chronic restraint stress in neurokinin-1 receptor knockout mice

Brain substance P and its receptor (neurokinin-1, NK1) have a widespread brain distribution and are involved in an important number of behavioural and physiological responses to emotional stimuli. However, the role of NK1 receptors in the consequences of exposure to chronic stress has not been explored. The present study focused on the role of these receptors in the hypothalamic-pituitary-adrenal (HPA) response to daily repeated restraint stress (evaluated by plasma corticosterone levels), as well as on the effect of this procedure on anxiety-like behaviour, spatial learning and memory in the Morris water maze (MWM), a hippocampus-dependent task. Adult null mutant NK1-/- mice, with a C57BL/6J background, and the corresponding wild-type mice showed similar resting corticosterone levels and, also, did not differ in corticosterone response to a first restraint. Nevertheless, adaptation to the repeated stressor was faster in NK1-/- mice. Chronic restraint modestly increased anxiety-like behaviour in the light-dark test, irrespective of genotype. Throughout the days of the MWM trials, NK1-/- mice showed a similar learning rate to that of wild-type mice, but had lower levels of thigmotaxis and showed a better retention in the probe trial. Chronic restraint stress did not affect these variables in either genotype. These results indicate that deletion of the NK1 receptor does not alter behavioural susceptibility to chronic repeated stress in mice, but accelerates adaptation of the HPA axis. In addition, deletion may result in lower levels of thigmotaxis and improved short-term spatial memory, perhaps reflecting a better learning strategy in the MWM.

Update in the methodology of the chronic stress paradigm: internal control matters

To date, the reliability of induction of a depressive-like state using chronic stress models is confronted by many methodological limitations. We believe that the modifications to the stress paradigm in mice proposed herein allow some of these limitations to be overcome. Here, we discuss a variant of the standard stress paradigm, which results in anhedonia. This anhedonic state was defined by a decrease in sucrose preference that was not exhibited by all animals. As such, we propose the use of non-anhedonic, stressed mice as an internal control in experimental mouse models of depression. The application of an internal control for the effects of stress, along with optimized behavioural testing, can enable the analysis of biological correlates of stress-induced anhedonia versus the consequences of stress alone in a chronic-stress depression model. This is illustrated, for instance, by distinct physiological and molecular profiles in anhedonic and non-anhedonic groups subjected to stress. These results argue for the use of a subgroup of individuals who are negative for the induction of a depressive phenotype during experimental paradigms of depression as an internal control, for more refined modeling of this disorder in animals.

Pharmacological investigations on adaptation in rats subjected to cold water immersion stress

The present study was designed to investigate whether adaptogenic factors may be transferred from stress adapted rats to naïve rats and to explore the nature of endogenous adaptogens by pharmacological modulation. The rats were subjected to cold water immersion stress by placing them individually in a tank of water (depth=15.5cm; temperature=16±2°C) for 5min. The rats were subjected to single episode of cold water immersion stress for acute stress; while for adaptation, the rats were subjected to repeated episodes of same stressor for 5 consecutive days. The plasma of stress adapted rats was administered to naïve rats before subjecting to acute stress. The stress related behavioral alterations were assessed using the actophotometer, the hole board, the open field and the social interaction tests. Acute stress with single episode of cold water immersion was associated with behavioral alterations. However, the behavioral alterations were significantly restored on subjecting repeated episodes of cold water immersion. Administration of plasma of stress adapted rats also attenuated acute stress associated behavioral alterations. Administration of naltrexone abolished the restoration of behavioral changes as a part of adaptive process in repeated stress subjected rats as well as the anti-stress effects of plasma of stress adapted rat. It may be concluded that opioids may be the potential endogenous adaptogens that tend to restore the homeostasis during repeated episodes of stress. Furthermore, the endogenous adaptogens may be transferred in the form of plasma from repeated stress subjected rats to the naïve rats to confer the anti-stress properties.

Effects of repeated restraint stress on platelet endothelial cell adhesion molecule-1 immunoreactivity and protein levels in the gerbil hippocampus after transient cerebral ischemia

Stress has long been known to be a causative factor of various disease states. In this study, we investigated the effects of repeated restraint stress on platelet endothelial cell adhesion molecule-1 (PECAM-1), a very important mediator in inflammation, immunoreactivity and protein levels as well as neuronal damage, in the gerbil hippocampus after 5 minutes of transient cerebral ischemia. Transient ischemia-induced neuronal death was shown in CA1 pyramidal cells 4 days after ischemia/reperfusion. However, repeated restraint stress protected neuronal death induced by ischemic damage. In the ischemia-group, PECAM-1 immunoreactivity and its protein levels were significantly increased in all the hippocampal subregions 4 days after ischemia/reperfusion. However, PECAM-1 immunoreactivity and its protein levels did not change significantly in the hippocampus of the stress-ischemia-group compared to the sham-groups. These results indicate that repeated restraint stress protects neuronal damage induced by transient cerebral ischemia, and this may be associated with maintenance of PECAM-1levels.

Measuring behavior in mice with chronic stress depression paradigm

Many studies with chronic stress, a common depression paradigm, lead to inconsistent behavioral results. We are introducing a new model of stress-induced anhedonia, which provides more reproducible induction and behavioral measuring of depressive-like phenotype in mice. First, a 4-week stress procedure induces anhedonia, defined by decreased sucrose preference, in the majority of but not all C57BL/6 mice. The remaining 30-50% non-anhedonic animals are used as an internal control for stress effects that are unrelated to anhedonia. Next, a modified sucrose test enables the detection of inter-individual differences in mice. Moreover, testing under dimmed lighting precludes behavioral artifacts caused by hyperlocomotion, a major confounding factor in stressed mice. Finally, moderation of the stress load increases the reproducibility of anhedonia induction, which otherwise is difficult to provide because of inter-batch variability in laboratory mice. We believe that our new mouse model overcomes some major difficulties in measuring behavior with chronic stress depression models.

Chronic mild stress (CMS) in mice: of anhedonia, 'anomalous anxiolysis' and activity

Background

In a substantial proportion of depressed patients, stressful life events play a role in triggering the evolution of the illness. Exposure to stress has effects on different levels in laboratory animals as well and for the rat it has been shown that chronic mild stress (CMS) can cause antidepressant-reversible depressive-like effects. The adoption of the model to the mouse seems to be problematic, depending on the strain used and behavioural endpoint defined. Our aim was to evaluate the applicability of CMS to mice in order to induce behavioural alterations suggested to reflect depression-like symptoms.

Methodology/Principal Findings

A weekly CMS protocol was applied to male mice of different mouse strains (D2Ola, BL/6J and BL/6N) and its impact on stress-sensitive behavioural measures (anhedonia-, anxiety- and depression-related parameters) and body weight was assessed. Overnight illumination as commonly used stressor in CMS protocols was particularly investigated in terms of its effect on general activity and subsequently derived saccharin intake. CMS application yielded strain-dependent behavioural and physiological responses including ‘paradox’ anxiolytic-like effects. Overnight illumination was found to be sufficient to mimic anhedonic-like behaviour in BL/6J mice when being applied as sole stressor.

Conclusions/Significance

The CMS procedure induced some behavioural changes that are compatible with the common expectations, i.e. ‘anhedonic’ behaviour, but in parallel behavioural alterations were observed which would be described as ‘anomalous’ (e.g. decreased anxiety). The results suggest that a shift in the pattern of circadian activity has a particular high impact on the anhedonic profile. Changes in activity in response to novelty seem to drive the ‘anomalous’ behavioural alterations as well.

Changes in pain behavior induced by formalin, substance P, glutamate and pro-inflammatory cytokines in immobilization-induced stress mouse model

In the present study, we examined the change of pain behaviors induced by formalin injected subcutaneously (s.c.) into the hind paw, or substance P (SP), glutamate, and pro-inflammatory cytokines (TNF-alpha, IL-1beta, and IFN-gamma) injected intrathecally (i.t.) in the mouse immobilization stress model. The mouse was restrained either once for 1h or five times for 5 days (once/day). In the formalin test, a single immobilization stress attenuated pain behaviors (licking, biting or scratching) in the second phase, while it had no effect on the pain behaviors revealed during the first phase. In addition, repeated immobilization stress attenuated pain behaviors revealed during the second phase but not in the first phase. A single as well as repeated immobilization stress decreased pain behaviors induced by substance P i.t. injection, but there were no significant changes in the glutamate test. In the pro-inflammatory cytokine pain model, a single immobilization stress decreased the pain behaviors induced by TNF-alpha, IL-1beta administered i.t. but not by IFN-gamma administered i.t. Moreover, a mouse applied with repeated immobilization stress did not show any changes in pain behaviors elicited by pro-inflammatory cytokines (TNF-alpha, IL-1beta and IFN-gamma) compared to the control group. These results suggest that a single and repeated immobilization stress differentially affects such nociceptive processing induced by formalin, SP, glutamate and pro-inflammatory cytokines in different manners.

Previous stress facilitates fear memory, attenuates GABAergic inhibition, and increases synaptic plasticity in the rat basolateral amygdala

In experiments designed to investigate the relationship between stress and the acquisition of new fear memories, it was found that previous exposure to a restraint session increased fear conditioning in a contextual fear paradigm. Moreover, the infusion of bicuculline, a competitive antagonist of GABAA receptors, into the basolateral amygdala complex (BLA), but not into the central amygdaloid nucleus, induced the same behavioral effect. Pretreatment with midazolam (MDZ), a positive modulator of GABAA sites, prevented the facilitating influence on fear memory of both stress and GABAA receptor blockade in the BLA. These data suggest that facilitation of fear conditioning could be causally related to increased neuronal excitability attributable to depressed GABAergic inhibition in the BLA. To test this hypothesis, evoked potentials were studied in brain slices from stressed animals. Potentials evoked in the BLA by single stimuli applied to the external capsule showed multispike responses, suggestive of GABAergic disinhibition. These multiple responses were no longer evident after the slices were perfused with diazepam or if the stressed animals were pretreated with MDZ. In slices from stressed rats, paired-pulse inhibition (GABA dependent) was suppressed. Also, in stressed animals, long-term potentiation (LTP) was induced with a single train of high-frequency stimulation, which did not induce LTP in control rats. Moreover, MDZ pretreatment prevented the facilitating influence of stress on LTP induction. All of these findings support the hypothesis that previous stress attenuates inhibitory GABAergic control in the BLA, leading to neuronal hyperexcitability and increased plasticity that facilitates fear learning.

Stress-induced hyperlocomotion as a confounding factor in anxiety and depression models in mice

Chronic stress is broadly used to model anxiety and depression. However, in chronic stress models, anxiety- and depression-like behaviors might be masked by unspecific effects of stress. We tested whether chronic stress in mice can induce unspecific changes in locomotion, and whether these changes interfere with the measurement of anxiety and forced-swimming behaviors. Also, we studied these latter behaviors in relation to the duration of stress, the lighting conditions during testing, and after the injection of diazepam. We employed a 4-week chronic stress paradigm, adopted from a model of stress-induced anhedonia and a 1-week subchronic stress, both consisting of rat exposure, restraint stress and tail suspension. Chronically stressed mice, tested under bright and moderate illumination, exhibited 'anxiolytic-like' behavior along with prolonged swimming and hyperactivity. These behaviors were not detectable under weak illumination or after the injection of diazepam (0.25 mg/kg). Instead, normal locomotion, increased anxiety and inhibited swimming were revealed under these conditions. Thus, chronic stress can induce hyperlocomotion in mice, which is triggered by acute stressors such as light, and interferes with the evaluation of anxiety and forced swimming. One week of stress did not change locomotion and forced swimming, and increased anxiety irrespective of illumination applied during testing. Our data can possibly explain previously reported contradictions in the behavioral testing of mice with chronic stress models of anxiety and depression.

Acute and repeated restraint stress influences cellular damage in rat hippocampal slices exposed to oxygen and glucose deprivation

Several studies have shown that high corticosteroid hormone levels increase neuronal vulnerability. Here we evaluate the consequences of in vivo acute or repeated restraint stress on cellular viability in rat hippocampal slices suffering an in vitro model of ischemia. Cellular injury was quantified by measuring lactate dehydrogenase (LDH) and neuron-specific enolase released into the medium. Acute stress did not affect cellular death when oxygen and glucose deprivation (OGD) was applied both immediately or 24h after restraint. The exposure to OGD, followed by reoxygenation, resulted in increased LDH in the medium. Repeated stress potentiated the effect of OGD both, on LDH and neuron-specific enolase released to the medium. There was no effect of repeated stress on the release of S100B, an astrocytic protein. Additionally, no effect of repeated stress was observed on glutamate uptake by the tissue. These results suggest that repeated stress increases the vulnerability of hippocampal cells to an in vitro model of ischemia, potentiating cellular damage, and that the cells damaged by the exposure to repeated stress+OGD are mostly neurons. The uptake of glutamate was not observed to participate in the mechanisms responsible for rendering the neurons more susceptible to ischemic damage after repeated stress.

Repeated Restraint Stress Reduces Opioid Receptor Binding in Different Rat CNS Structures

Different effects of exposure to acute or to repeated stress have been observed upon the nociceptive response in rats. In the present study, we repeatedly submitted Wistar rats to restraint for 40 days, a treatment known to induce an increase in the nociceptive response in the tail-flick test. Afterwards, the effect of repeated restraint stress on the density of opioid receptors in rat spinal cord, frontal cortex, and hippocampus was investigated. Results showed that repeatedly stressed rats displayed a significant decrease in opioid receptors density in all structures studied; cortex (141.3 +/- 5.7 for control and 103.3 +/- 15.9 for stressed rats), hippocampus (92.4 +/- 7.2 for control and 64.8 +/- 7.7 for stressed rats), and spinal cord (122.2 +/- 12.8 for control and 79.7 +/- 9.7 for stressed rats). These findings suggest opioid mediation of the altered responses observed in these repeatedly-stressed animals, although the participation of non-opioid mechanisms in this phenomenon cannot be ruled out.

Repeated restraint stress alters hippocampal glutamate uptake and release in the rat

Glutamatergic mechanisms are thought to be involved in stress-induced changes of brain function, especially in the hippocampus. We hypothesized that alterations caused by the hormonal changes associated with chronic and acute stress may affect glutamate uptake and release from hippocampal synaptosomes in Wistar rats. It was found that [3H]glutamate uptake and release by hippocampal nerve endings, when measured 24 h after 1 h of acute restraint, presented no significant difference. The exposure to repeated restraint stress for 40 days increased neuronal presynaptic [3H]glutamate uptake as well as basal and K+-stimulated glutamate release when measured 24 h after the last stress session. Chronic treatment also caused a significant decrease in [3H]glutamate binding to hippocampal membranes. We suggest that changes in the glutamatergic system are likely to take part in the mechanisms involved in nervous system plasticity following repeated stress exposure.

Taste modulation of nociception differently affects chronically stressed rats

Stress responses cover a wide range of physiological changes, including alterations in the perception of and response to pain. Animals submitted to repeated stress present altered nociception and this effect is part of this process of adaptation; in addition pleasant and unpleasant experiences with tastes and odors have been shown to affect distinct behavioral aspects, such as pain perception. The aim of the present study is to verify the responses of repeatedly stressed rats (1 h of daily immobilization during 40 days) to pleasant and unpleasant tastes on nociception, when compared to control animals. An increase in the tail-flick latency (TFL) was observed 5 min after exposure to a sweet taste in the control group, whereas no effect was observed in chronically stressed animals. When submitted to an unpleasant taste (5% acetic acid), the chronically stressed group presented an increase in TFL, whereas no effect was observed in the control group. In conclusion, chronically stressed animals present different nociceptive responses to sweet and acid tastes; although control animals suitably respond to a sweet stimulus, stressed animals seem to be more apt to react to the unpleasant stimulus.

Region-specific effects of acute and repeated restraint stress on the phosphorylation of mitogen-activated protein kinases

The mitogen-activated protein kinases (MAPKs) are a family of signal transduction mediators that regulate a host of cellular activities, including cell growth and proliferation, and differentiation and survival, via sequential phosphorylation and activation of a cassette of three protein kinases. MAPKs are also recruited when the brain undergoes synaptic plasticity and remodeling (e.g., during induction of long-term potentiation, learning and memory consolidation). The activities of some of these kinases are altered in response to various acute stimuli such as ischemic insult, visceral pain and electroconvulsive shock. In the present study we used immunoblotting techniques to examine the effects of acute and repeated restraint stress on the phosphorylation state of three MAPKs, the extracellular signal-regulated kinase Erk1/2, c-Jun-N-terminal kinase/stress-activated protein kinase (JNK/SAPK) and p38 MAPK, in different brain regions. A single exposure to 30 min of restraint stress-elevated phospho-Erk1/2 (P-Erk1/2) levels in all three brain regions examined (hippocampus, medial prefrontal cortex and cingulate cortex), but did not alter the phosphorylation pattern of the other two MAPKs in any region. In marked contrast, exposure to restraint for 11 days (30 min/day) reduced the levels of all three MAPKs, but only in the prefrontal cortex. The results are compared to the reported effects of acute and chronic stress on other biochemical and functional measures.

Long-lasting delayed hyperalgesia after chronic restraint stress in rats-effect of morphine administration

Different effects upon the nociceptive response have been observed with exposure to acute and chronic stress in rats. In the present study we repeatedly submitted rats to restraint for 40 days, inducing hyperalgesia using the tail-flick test. A new session of acute stress was applied at the end of 40 days period, and the chronically-stressed animals demonstrated analgesia after forced swimming, but not after restraint. The effect of stress interruption for 14 or 28 days on the nociceptive threshold was then investigated. The basal tail-flick latency remained decreased for at least 28 days (hyperalgesic effect). Following the periods of suspension, the animals were submitted to new session of acute restraint, and stress-induced analgesia was observed only after 28 days of stress interruption. Thus, the mechanisms involved in the long-lasting hyperalgesia presented in this study are not exactly the same as those responsible for the analgesia induced by acute stressors. After 40 days of chronic stress treatment, morphine was injected i.p. (1.0, 5.0 mg/kg or saline). The repeatedly stressed rats displayed decreased morphine effects on nociception compared to unstressed controls. The tolerance of the response to morphine agrees with previous studies suggesting that chronic restraint stress could modify the activity of opioid systems.

Influence of acute or repeated restraint stress on morphine-induced locomotion: involvement of dopamine, opioid and glutamate receptors

The development of restraint stress-induced sensitization to the locomotor stimulating effect of morphine (2 mg/kg i.p.) was investigated. In experiment 1, both a single restraint session (2 h) and a repeated restraint stress (2 h per day for 7 days), similarly enhanced the effects of morphine on motor activity. In experiment 2, we observed that this sensitization was prevented by administration of both D(1) and D(2) dopaminergic antagonist [SCH-23390 (0.5 mg/kg i.p.) and (+/-)-sulpiride (60 mg/kg i.p.)] 10 min prior to the stress session. In experiment 3, we showed that an opioid antagonist pretreatment [naltrexone (1 mg/kg i.p.) 10 min prior to stress session, suppressed the stress-induced sensitization after morphine administration. In experiment 4, pretreatment with a non-competitive antagonist of the N-methyl-D-aspartate (NMDA) type of glutamate receptors [(+)-MK-801 (0.1 mg/kg i.p.)], 30 min prior to the acute restraint session, prevented the development of sensitization to morphine. All these results suggest that: (1) sensitization to morphine on stimulating locomotor effect does not depend on the length of exposure to stress (acute vs. repeated); (2) stimulation of both D(1) and D(2) dopaminergic receptors is necessary for the development of restraint stress-induced sensitization to morphine; (3) an opioid system is also involved in this sensitization process; and (4) the stimulation of glutamatergic NMDA receptors is involved in this acute restraint-induced effect.

Motivational effects mu- and kappa-opioid agonists following acute and chronic restraint stress: involvement of dopamine D(1) and D(2) receptors

The influence of both acute and chronic restraint stress on the rewarding properties of morphine (1, 2 or 3 mg/kg i.p.) and the aversive effects of naloxone (0.5 mg/kg i.p. x3 or 1.0 mg/kg i.p.) or bremazocine (0.4 mg/kg i.p.) was investigated. An acute (2 h) but not chronic restraint (2 h daily for 7 days) enhanced the morphine place preference, and elicited a place aversion with a subthreshold dose of bremazocine. This enhancing effect on the reinforcing properties induced by the drugs was prevented by either R(+)-SCH-23390 hydrochloride (R(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H3-benzazepine, 30 microg/kg i.p.) or (+/-)-sulpiride (60 mg/kg i.p.), 10-20 min prior to the stress session. Naltrexone pretreatment (1 mg/kg i.p.) abolished the stress effect on morphine place preference but not that on bremazocine aversion. Instead, nor-BNI (30 microg/3 microl i.c.v.) abolished the stress's effects on bremazocine aversion, but did not modify those on morphine preference. These results show that: (1) acute stress enhanced the morphine and bremazocine conditioned reinforcing effects meanwhile chronic stress did not modify them; (2) the stimulation of D(1) and D(2) dopamine receptors is necessary for the development of restraint stress-induced sensitization to the conditioned reinforcing effects of drugs; and (3) the stimulation mu/delta- and kappa-opioid receptors seems to be differentially involved.

A dopaminergic mechanism is involved in the 'anxiogenic-like' response induced by chronic amphetamine treatment: a behavioral and neurochemical study

The purpose of this study was to examine the influence of chronic d-amphetamine (AMPH) treatment (2 mg/kg i.p., for 9 consecutive days) on behavioral and neurochemical responses to a subsequent exposure - 4 days after the last AMPH injection--to the elevated plus-maze (EPM), as well as to determine the involvement of a dopaminergic mechanism in that influence. Results showed that chronic AMPH treatment induced an 'anxiogenic-like' response when animals were evaluated in the EPM test. Pretreatment with either haloperidol (HAL, 1 mg/kg i.p., 20 min prior to each injection) or SCH-23390 (0.1 mg/kg i.p., 10 min prior to each injection) completely abolished the chronic AMPH-induced 'anxiogenic-like' effect displayed in the EPM test. However, sulpiride pretreatment (60 mg/kg i.p., 10 min prior to each AMPH injection) did not modify such effect. In addition, rats treated with AMPH and subsequently exposed to the EPM, showed a decrease in the maximal GABA-stimulated chloride uptake in cortical microsacs. HAL pretreatment restored the maximal chloride uptake induced by chronic AMPH. Altogether, these results suggest that: (1) previous exposure to chronic AMPH treatment induces an increased emotional response following a conflict situation, (2) dopamine D(1) receptors are mainly involved in chronic AMPH-induced changes in the behavior displayed in EPM test, and (3) an interaction between GABAergic and dopaminergic mechanisms may be implicated in neurochemical and behavioral changes induced by chronic AMPH treatment.

The anxiolytic effect of acute ethanol or diazepam exposure is unaltered in mu-opioid receptor knockout mice

Previous researchers demonstrate an opioidergic involvement in the anxiolytic and rewarding actions of ethanol and diazepam. Therefore, to further characterize the role of the opioid system in the anxiolytic action of ethanol and diazepam, normal (C57BL/6J), hybrid (B6129F1) and mu-opioid receptor knockout mice were given i.p. ethanol (0, 1.0 or 1.6 g/kg) or diazepam (1.5 mg/kg). The anxiolytic properties of these agents were then tested in the elevated plus-maze. Additional ethanol-treated mu-opioid receptor knockout mice (1 g/kg) were pretreated with the kappa-opioid receptor antagonist nor-BNI (0 or 3 mg/kg) to assess the involvement of kappa-opioid activity in ethanol's anxiolytic actions. The anxiolytic action of ethanol and diazepam in the mu-opioid receptor knockout mouse did not differ from the effects obtained in normal mice and pretreatment with nor-BNI did not significantly attenuate ethanol's actions in mu-opioid receptor knockout mice. Thus, the anxiolytic actions of ethanol and diazepam appear to be independent of opioid system activity in the mu-opioid receptor knockout mouse.

Diazepam fails to potentiate GABA-induced chloride uptake and to produce anxiolytic-like action in aged rats

The pharmacological response to benzodiazepines has been demonstrated to be different in aged individuals in comparison to adults. We studied the age-dependent changes in some of the in vitro and behavioral effects of diazepam in aged (24 months old) rats, comparing them to adults (3 months old). We evaluated the in vitro gamma-aminobutyric acid (GABA)-induced 36Cl- uptake and the diazepam potentiation of GABA-stimulated 36Cl- uptake in microsacs from cerebral cortex of both groups of animals. We found no differences in the GABA-stimulated 36Cl- uptake between adult and aged animals, and diazepam failed to potentiate GABA-induced 36Cl- flux in the aged cortical microsacs. We also examined the effect of 0.03-10 mg of diazepam on locomotor activity in an open-field test and the anxiolytic-like action of diazepam in doses ranging from 0.03 to 1 in a dark-light transition test. We observed no anxiolytic-like action of the drug in the dark-light transition test in the aged rats, while there was a shift to the left in the diminution of locomotor activity evaluated by the open-field test. We conclude that the pharmacodynamic changes observed in cortical GABA(A) receptors in aged rats could partially explain the lack of anxiolytic-like action but not the oversedation evidenced in this group of animals.

The effects of acute and chronic stress on motor and sensory performance in male Lewis rats

Any behavioral testing induces stress to some degree. A meaningful interpretation of behavioral results can be difficult if stress, caused by handling or the testing situation, modifies the experimental outcome. Especially for neurological animal models, it is important to know how stress affects motor and sensory performance. Therefore, we investigated the effects of varying degrees of stress on several motor and sensory tasks that are frequently used to assess functional recovery after lesion-induced impairments in adult rats. Acute, subchronic, and chronic stress impaired ladder walking and prolonged the duration of grasping a bar. Stress also altered walking patterns by increasing the base of support and foot rotation and reducing stride length. Furthermore, chronic stress induced hypersensitivity to painful stimuli, but did not significantly influence the latency to remove sticky papers from the hindpaws (sticky paper test). In the light--dark (L/D) test, stress reduced the latency to enter the dark compartment and enhanced the number of transitions supporting that cold swim stress modifies the animal's level of anxiety. These data point towards a critical influence of acute or chronic stress on motor control and sensory performance of rats, suggesting that stress might be a critical intervening variable of the outcome of behavioral tests.

Prior electrical stimulation of the inferior colliculus sensitizes rats to the stress of the elevated plus-maze test

Besides its primary function in the transmission of acoustic signals, the inferior colliculus (IC) is involved in conveying auditory information of aversive nature to higher cortical structures. In the field of anxiety research, one widely used animal model is the electrical stimulation of a given structure supposed to be involved in the neural circuitry underlying emotional behavior. Indeed, electrical stimulation of the inferior colliculus produces fear-like responses. Moreover, prior exposure to stressful events sensitizes rodents' responsivity to fearful stimuli when they are subsequently tested in the elevated plus-maze. Based on these evidence it seems to be important to know whether animals stimulated in the inferior colliculus would show a heightened behavioral responsivity to subsequent stressful events. To this end, we examined the temporal course of the effects of the electrical stimulation of this midbrain region (5, 10 and 15 min afterward) on the conventional and ethological measures of the behavior of rats exposed to the elevated plus-maze test. Prior electrical stimulation of the inferior colliculus produced an 'anxiogenic' profile in the elevated plus-maze, i.e. a significant reduction in the number of entries and time spent into the open arms. Still, previous electrical stimulation of the inferior colliculus caused a significant decrease in rearing, scanning, peeping out, head dipping and end-arm activity while increased immobility. All these changes occurred after 5 min of inferior colliculus electrical stimulation. Therefore, stimulation of the inferior colliculus causes a heightened responsivity to anxiogenic stimuli inherent to the elevated plus-maze test. These findings bring additional support to the proposed role of this midbrain structure in the elaboration of adaptive responses to stressful situations.

Effect of acute and chronic stress restraint on amphetamine-associated place preference: involvement of dopamine D(1) and D(2) receptors

The purpose of this study was to determine the D-amphetamine (1.0, 1. 5 and 2 mg/kg i.p.)-induced place preference in rats pre-exposed to acute or chronic restraint stress, using the conditioned place preference model. We also studied the involvement of opioid and dopamine mechanisms in the acute restraint stress-induced increase of D-amphetamine-induced place preference. A single restraint session (2 h) but not chronic restraint (2 h/day for 7 days) leading to adaptation to the stressor, enhanced the D-amphetamine-induced place preference. This enhancing effect was prevented by haloperidol administration (0.4 mg/kg i.p.), (+/-)-sulpiride (60 mg/kg i.p.) or R(+)-SCH-23390 hydrochloride (R(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4, 5-tetrahydro-1H-3-benzazepine hydrochloride, 30 microg/kg i.p.) 10-20 min prior to the acute restraint session. However, naltrexone pretreatment (1 or 2 mg/kg i.p.) failed to prevent the acute restraint-induced enhancement of D-amphetamine-induced place preference. These results suggest that: (1) the enhancement of D-amphetamine-induced place preference occurred after a single restraint stress but not following chronic restraint stress, (2) the stimulation of both dopamine D(1) and D(2) receptors is necessary for the development of single restraint stress-induced enhancement of D-amphetamine-induced place preference and (3) apparently, an opioid system is not involved in this acute restraint-induced effect.

Effect of repeated restraint stress on memory in different tasks

The present study investigated the effect of repeated stress applied to female rats on memory evaluated by three behavioral tasks: two-way shuttle avoidance, inhibitory avoidance and habituation to an open field. Repeated stress had different effects on rat behavior when different tasks were considered. In the two-way active avoidance test the stressed animals presented memory of the task, but their memory scores were impaired when compared to all other groups. In the habituation to the open field, only the control group showed a significant difference in the number of rearings between training and testing sessions, which is interpreted as an adequate memory of the task. In the handled and chronically stressed animals, on the other hand, no memory was observed, suggesting that even a very mild repeated stress would be enough to alter habituation to this task. The performance in the inhibitory avoidance task presented no significant differences between groups. The findings suggest that repeated restraint stress might induce cognitive impairments that are dependent on the task and on stress intensity.

Social stress of variable intensity: physiological and behavioral consequences

Stress effects in humans depend on stress type, intensity, and duration. Animal models of social stress serve as good ways to mimic stress experienced in humans. However, the available stress paradigms pay little attention to the relationship between the intensity and the type of social stressors. The aim of the present work is to study behavioral and endocrinological consequences of social stress by varying the intensity and type of agonistic social contacts. Subjects were exposed to the attacks of an experienced fighter resident rat once a day for 4 consecutive days. Mild versus strong effects were studied by varying the length of daily confrontations (30 min vs. 4 h). The type of social confrontations was varied by ceasing or maintaining sensory contacts among contestants between encounters. Endocrinological variables were measured on the 5th day. Anxiety was assessed by means of the elevated plus-maze. The stress state depended on the length of daily encounters: 30-min encounters did not, whereas 4-h encounters did result in weight loss and chronic elevation of plasma corticosterone. The type of contacts between subjects and dominants also affected the resulting stress state: adrenal hypertrophy was obtained only when contacts between contestants were maintained between encounters. Although the mildest stress procedure (30-min encounters on 4 consecutive days) did not affect endocrinological variables, it resulted in subtle behavioral modifications that changed the anxiety-related effects of additional acute stressors. Thus, anxiety-related behavioral changes resulting from repeated mild stressors may be hidden factors that can have long-term consequences on the development of anxiety-like behavioral deficits. Results outline the necessity of studying the effects of social stressors of different intensities and different types.

Acute effects of glucocorticoids: behavioral and pharmacological perspectives

There has been evidence since the early eighties that glucocorticoids, apart from their well known chronic effects, may have acute, short-term effects. However, a lack of understanding of the molecular mechanisms of action has hampered appreciation of these observations. Mounting evidence over the years has continued to confirm the early observations on a fast corticosterone control of acute behavioral responses. We summarize experimental data obtained mainly in rats but also in other species which show: (1) that glucocorticoid production is sufficiently quick to affect ongoing behavior; (2) that there exist molecular mechanisms that could conceivably explain the fast neuronal effects of glucocorticoids (although these are still insufficiently understood); (3) that glucocorticoids are able to stimulate a wide variety of behaviors within minutes; and (4) that acute glucocorticoid production (at least in the case of aggressive behavior) is linked to the achievement of the behavioral goal (winning). The achievement of the behavioral goal reduces glucocorticoid production. It is argued that glucocorticoids are regulatory factors having a well-defined behavioral role. Both the acute (stimulatory) effects and the chronic (inhibitory) effects are adaptive in nature. The acute control of behavior by corticosterone is a rather unknown process that deserves further investigation. The pharmacologic importance of the acute glucocorticoid response is that it may readily affect the action of pharmacologic agents. An interaction between acute glucocorticoid increases and noradrenergic treatments has been shown in the case of offensive and defensive agonistic behavior. Non-behavioral data demonstrate that acute increases in glucocorticoids may interfere with other neurotransmitter systems (e.g., with the 5HT system) as well. These observations show the importance of taking into account endocrine background and endocrine responsiveness in behavior pharmacological experiments.

Administration of cholecystokinin sulphated octapeptide (CCK-8S) induces changes on rat amino acid tissue levels and on a behavioral test for anxiety

1. The effect of the intraperitoneal administration of cholecystokinin sulphated octapeptide (CCK-8S) (10 nmol/kg i.p.) on endogenous levels of several amino acids in five areas of the rat brain was analyzed. The olfactory bulb, hypothalamus, hippocampus, cerebral frontal cortex, and corpus striatum were evaluated. In addition, the effects of CCK-8S and PD 135,158 (1 mg/kg), a selective CCK(B) antagonist, on the performance of rats submitted to a dark/light transition test were also studied. 2. Upon administration of CCK-8S, the concentration of glutamate was reduced (27%) in the olfactory bulb. The same was observed when the levels of glycine (31%) or alanine (43%) were determined. No significant effects were produced by CCK-8S on cortical and hypothalamic levels. In the hippocampus, the concentration of both glutamate (27%) and taurine (29%) were reduced, whereas the levels of GABA in the striatum (29%) were increased. 3. After a single injection of CCK-8S, the time spent by the rats in the illuminated site of the dark/light transition test box, was not changed. On the contrary, the administration of PD 135,158 increased the time spent in the lighted compartment. 4. These results show that systemic administration of CCK-8S produced regional specific changes in brain amino acids, without producing any significant behavioral modification in the rat exposed to a dark/light box. In contrast, the selective CCKB receptor antagonist, PD 135,158, induces anxiolytic-like action in an animal model of anxiety.

Corticotropin-releasing factor and defensive withdrawal: inhibition of monoamine oxidase prevents habituation to chronic stress

There is growing evidence for a role of extrahypothalamic corticotropin-releasing factor (CRF) in the pathogenesis of anxiety. A modified form of the defensive withdrawal test was used to test the anxiogenic effects of acute administration of intracerebroventricular (1 microg, i.c.v.) CRF in adult male rats. Habituation to the mild stress of daily handling and subcutaneous (s.c.) saline injection over 2-6 weeks abolished the anxiogenic effects of exogenous CRF. At 6 weeks this habituation also resulted in attenuation of baseline withdrawal behavior. CRF receptor binding was significantly decreased in the amygdala of chronically handled animals and may have been responsible for this habituation phenomenon. Comparison of rats treated with the monoamine oxidase (MAO) inhibitor, phenelzine [3 mg/kg, s.c., daily for 2-6 weeks] to the saline-treated groups revealed a failure to habituate to the chronic handling, as the baseline withdrawal (after injection of artificial CSF) by the phenelzine-treated animals was not different from the baseline withdrawal by unhandled rats. In comparison to rats treated chronically with saline, phenelzine treatment enhanced the anxiogenic effect of CRF. In summary, habituation to a mild chronic stress decreased baseline defensive withdrawal. Intraventricular administration of CRF produced an anxiogenic response as measured in the defensive withdrawal test, which was lost through exposure to mild chronic stress. Two or 6 weeks of daily handling and SC saline injection caused a downregulation of CRF receptors in the amygdala, which could account for the behavioral habituation and the loss of CRF-induced defensive withdrawal. Phenelzine treatment concurrent with mild chronic stress prevented habituation and maintained the anxiogenic effect of CRF in spite of the downregulation of CRF receptors in the amygdala.

The effects of acute and repeated restraint stress on the nociceptive response in rats

The effects of acute and repeated restraint stress on nociception, as measured by the tail-flick latency, were studied in adult male and female rats. After the exposure to a single restraint session, both male and female rats presented an increased latency in the tail-flick test. On the other hand, chronically stressed females presented a performance similar to the control group, whereas chronically stressed male rats responded to restraint with a decrease in the tail-flick latency. This response could be determined by the chronic treatment itself or by the restraint done just before the measurement. Thus, the effect of chronic stress upon basal tail-flick latency was evaluated. In male rats, this latency was significantly decreased in the stressed animals compared with the control group. In female rats, no difference between those groups was observed. Therefore, the results suggest that: (a) acute restraint stress induces an analgesic response in both male and female rats, and (b) there is a gender-specific nociceptive response induced by repeated restraint stress with a hyperalgesic effect in response to stress only in males.

D1 and D2 dopamine and opiate receptors are involved in the restraint stress-induced sensitization to the psychostimulant effects of amphetamine

The time course of the restraint stress-induced sensitization to the stimulant effects of amphetamine (AMPH, 0.5 mg/kg IP) on locomotor activity was investigated for up to 8 days. In a series of separate experiments, the involvement of opioid and dopaminergic mechanisms in the development of acute restraint stress-induced behavioral sensitization were characterized. Both a single restraint session (2 h) and chronic restraint (2 h per day for 7 days) similarly potentiated the effects of AMPH on motor activity. This behavioral sensitization was prevented by the administration of naltrexone (2 mg/kg IP), haloperidol (1 mg/kg IP), sulpiride (60 mg/kg IP) or SCH23390 (0.5 mg/kg IP) 10-20 min prior to restraint. These results indicate that 1) the development of sensitization to amphetamine-induced effects on motor activity does not depend on the length of exposure to stress (acute or chronic). 2) the stimulation of both D1 and D2 dopaminergic receptors is necessary for the development of the restraint stress-induced sensitization to AMPH and 3) and opioid system is also implicated in this sensitization process.

Prior exposure to a brief restraint session facilitates the occurrence of fear in response to a conflict situation: behavioral and neurochemical correlates

The influence of two different stressors on the behavioral and neurochemical responses to a subsequent exposure to the elevated plus maze (EPM) was examined. Rats were submitted to either a 15-min restraint period or to a 15-min forced swimming test (FS) and one day later exposed to the EPM. Animals with early restraint exhibited a significant decrease in the percent time spent and in the number of entries on the open arms. In addition, restraint induced a reduction in the total number of entries. An identical behavior in the EPM was observed between unstressed rats and those exposed to a previous swimming experience. As a humoral index of stress, corticosterone (CS) secretion in response to each stressor was evaluated. A similar increase of CS release was observed following each aversive stimulus. Exposure to both restraint and EPM decreased the cortical chloride uptake following GABA stimulation. Similar values of chloride flux were obtained from animals submitted to either restraint but without subsequent exposure to the EPM, exposed only to the EPM, or without any manipulation (controls). These findings are discussed in terms of a facilitated behavioral and neurochemical response to a fearful situation following an early and brief restraint experience.

A review of the validity and variability of the elevated plus-maze as an animal model of anxiety

Despite or possibly by virtue of the fact that it is one of the most commonly used animal models of anxiety the Elevated Plus-Maze (EPM) results in a wide range of, often contradictory, results following pharmacological experiments. The responses from a questionnaire distributed to 65 groups that have published studies using the EPM in the past 3 years has, along with reference to published reports, enabled some conclusions regarding the influencing factors to be drawn. Some evidence for differential sensitivities between strains exists, with albino rats being more sensitive to the anxiolytic effects of 5-HT3 receptor antagonists and 5-HT1A receptor agonists than pigmented animals. Most important, however, is the manipulation of the animals prior to testing and the aversiveness of the test conditions themselves. Stressing animals before testing (e.g., by moving from holding to test room) or using more aversive test conditions (e.g., elevated light levels) increases sensitivity to potential anxiolytics. Animals that are habituated to gentle handling or tested in less aversive conditions (e.g., EPM with ledges) show reduced likelihood of anxiolytic responses with administration of 5-HT3 antagonists, 5-HT1A agonists, and benzodiazepines.