Individual corticosterone response to intermittent swim stress predicts a shift in economic demand for ethanol from pre- stress to post-stress in male rats.

This study investigated the relationship between stress exposure and subsequent ethanol use, focusing on individual differences among male rats. We combined operant self-administration with behavioral economics to assess how intermittent swim stress affects ethanol consumption. This approach allowed for a nuanced analysis of the transition from regular ethanol intake to stress-induced escalation in economic demand. Results showed a consistent rise in ethanol demand post-stress among subjects, irrespective of exposure to actual swim stress or a sham procedure. This increase may result from a two-week abstinence or an inherent rise in demand over time. Significantly, we identified a direct link between post-stress corticosterone levels and the demand for ethanol, considering baseline levels. This correlation was particularly pronounced when examining the shifts in both corticosterone levels and demand for ethanol post-stress. However, neither post-stress corticosterone levels nor their change over time correlated significantly with changes in ethanol demand following a forced swim test that was administered 24 hours after the intermittent swim stress test. This suggests potential context-specific or stressor-specific effects. Importantly, pre-stress ethanol demand did not significantly predict the corticosterone response to stress, indicating that high ethanol-demand rats do not inherently exhibit heightened stress sensitivity. Our research brings to light the complex interplay between stress and ethanol consumption, highlighting the critical role of individual differences in this relationship. This research introduces a nuanced perspective, underscoring the need for future studies in the realm of stress and substance use to give greater consideration to individual variability.

Individual Vulnerability to Stress Is Associated With Increased Demand for Intravenous Heroin Self-administration in Rats.

Opioid use is a widespread epidemic, and traumatic stress exposure is a critical risk factor in opioid use and relapse. There is a significant gap in our understanding of how stress contributes to heroin use, and there are limited studies investigating individual differences underlying stress reactivity and subsequent stress-induced heroin self-administration. We hypothesized that greater individual vulnerability to stress would predict higher demand for heroin self-administration in a within-subjects rodent model of stress and heroin use comorbidity. Male rats were exposed to inescapable intermittent swim stress (ISS) and individual biological (corticosterone) or behavioral [open field, social exploration, and forced swim tests (FSTs)] measures were assessed before and after the stress episode. Individual demand for self-administered heroin (0.05 mg/kg/infusion; 12-h sessions) was assessed using a behavioral economics approach followed by extinction and reinstatement tests triggered by stress re-exposure, non-contingent cue presentations, and yohimbine (0, 1.0, or 2.5 mg/kg). We found that behavioral, biological, and a combination of behavioral and biological markers sampled prior to and after the stress episode that occurred weeks before the access to heroin self-administration predicted the magnitude of individual demand for heroin. Non-contingent presentation of cues, that were previously associated with heroin, reinstated heroin seeking in extinction. For the first time, we show that individual biological response to an ecologically relevant stressor in combination with associated behavioral markers can be used to predict subsequent economic demand for heroin.

Two models of inescapable stress increase tph2 mRNA expression in the anxiety-related dorsomedial part of the dorsal raphe nucleus.

Expression of TPH2, the rate-limiting enzyme for brain serotonin synthesis, is elevated in the dorsal raphe nucleus (DR) of depressed suicide victims. One hypothesis is that this increase in TPH2 expression is stress-induced. Here, we used an established animal model to address whether exposure to an acute stressor, inescapable tail shock (IS), increases tph2 mRNA and Tph2 protein expression, and if IS sensitizes the DR to a subsequent, heterotypic stressor. In Experiment 1, we measured tph2 mRNA expression 4 h after IS or home cage (HC) control conditions in male rats, using in situ hybridization histochemistry. In Experiment 2, we measured Tph2 protein expression 12 h or 24 h after IS using western blot. In Experiment 3, we measured tph2 mRNA expression following IS on Day 1, and cold swim stress (10 min, 15 °C) on Day 2. Inescapable tail shock was sufficient to increase tph2 mRNA expression 4 h and 28 h later, selectively in the dorsomedial DR (caudal aspect of the dorsal DR, cDRD; an area just rostral to the caudal DR, DRC) and increased Tph2 protein expression in the DRD (rostral and caudal aspects of the dorsal DR combined) 24 h later. Cold swim increased tph2 mRNA expression in the dorsomedial DR (cDRD) 4 h later. These effects were associated with increased immobility during cold swim, elevated plasma corticosterone, and a proinflammatory plasma cytokine milieu (increased interleukin (IL)-6, decreased IL-10). Our data demonstrate that two models of inescapable stress, IS and cold swim, increase tph2 mRNA expression selectively in the anxiety-related dorsomedial DR (cDRD).

Ultrasonic vocalizations during intermittent swim stress forecasts resilience in a subsequent juvenile social exploration test of anxiety.

Current behavioral paradigms of stress resilience traditionally employ forms of prior manipulation or subsequent testing. Recent work has reported adult rat ultrasonic vocalizations (USVs) emitted during intermittent swim stress (ISS) may serve as a predictor of resilience. ISS-induced USVs predicted resilience on several endpoints of behavioral depression and may be considered a forecast of innate resilience. However, a potential problem for these previous findings is the lack of generalizability to other contexts, because both the stress induction and post-stress testing occur in water. The current study tests the generalizability of USVs as a predictor of stress resilience in a non-water-based post-test, the juvenile social exploration test of anxiety. The results provide further support that USVs emitted during ISS predict resilience to depression- and anxiety-like behaviors. Extensions of this work to examine the neurobiology of innate resilience associated with ISS-induced USVs are discussed with comparisons to extant models of learned resilience.

Learned stressor resistance requires extracellular signal-regulated kinase in the prefrontal cortex.

Behaviorally controllable stressors confer protection from the neurochemical and behavioral consequences of future uncontrollable stressors, a phenomenon termed "behavioral immunization". Recent data implicate protein synthesis within the ventromedial prefrontal cortex (mPFC) as critical to behavioral immunization. Adult, male Sprague-Dawley rats were exposed to a series of controllable tailshocks and 1 week later to uncontrollable tailshocks, followed 24 h later by social exploration and shuttlebox escape tests. To test the involvement of N-methyl-D-aspartate receptors (NMDARs) and the extracellular signal-regulated kinase (ERK) cascade in behavioral immunization, either D-AP5 or the MEK inhibitor U0126 was injected to the prelimbic (PL) or infralimbic (IL) mPFC prior to controllable stress exposure. Phosphorylated ERK and P70S6K, regulators of transcription and translation, were quantified by Western blot or immunohistochemistry after controllable or uncontrollable tailshocks. Prior controllable stress prevented the social exploration and shuttlebox performance deficits caused by the later uncontrollable stressor, and this effect was blocked by injections of D-AP5 into mPFC. A significant increase in phosphorylated ERK1 and ERK2, but not P70S6K, occurred within the PL and IL in rats exposed to controllable stress, but not to uncontrollable stress. However, U0126 only prevented behavioral immunization when injected to the PL. We provide evidence that NMDAR and ERK dependent signaling within the PL region is required for behavioral immunization, a learned form of stressor resistance.

Ultrasonic vocalizations during intermittent swim stress forecasts resilience in subsequent forced swim and spatial learning tests.

The examination of stress resilience has substantially increased in recent years. However, current paradigms require multiple behavioral procedures, which themselves may serve as secondary stressors. Therefore, a novel predictor of stress resilience is needed to advance the field. Ultrasonic vocalizations (USVs) have been observed as a behavioral correlate of stress in various rodent species. It was recently reported that rats that emitted ultrasonic vocalizations during intermittent swim stress (ISS) later showed resilience when tested on an instrumental swim escape test. In the current study, we extend this earlier observation on two additional behavioral endpoints. Rats were subjected to ISS, and USVs were recorded. Twenty-four hours later, behavioral performance was evaluated in either the forced swim test or Morris water maze. Rats that emitted ultrasonic vocalizations were resilient to the effects of ISS as indicated by performance similar to controls on both measures. These results extend the original findings that ISS-induced USVs are associated with resilience and are related to subsequent aversively motivated behavior. Such a non-invasive forecast of stress responsivity will allow future work to utilize USVs to examine the neural correlates of initial stress resistance/resilience, thereby eliminating potential confounds of further behavioral testing. Future studies can utilize USVs to target potentially unappreciated neural systems to provide novel pharmacotherapeutic strategies for treatment-resistant depression.

Intermittent swim stress causes Morris water maze performance deficits in a massed-learning trial procedure that are exacerbated by reboxetine.

Various animal models of depression have been used to seek a greater understanding of stress-related disorders. However, there is still a great need for research in this area, as many unanswered questions remain. Therefore, we sought to employ a novel animal model of depression known as intermittent swim stress (ISS). In this model, the animal experiences 100 trials of cold water swim stress. ISS has already shown subsequent immobility in the forced swim test (FST), deficits in instrumental and spatial (spaced-trial procedure), and responsiveness to norepinephrine. We are now examining how this will translate in the Morris water maze for rats in a massed-learning trial procedure, and further assessing ISS sensitivity toward norepinephrine selective anti-depressant drugs. The results indicated no difference in cued learning when the platform was visible in the water maze, but a hidden platform task revealed poorer spatial learning for ISS-exposed rats versus controls. In terms of spatial memory, there was a notable ISS-induced deficit 1h after the learning trials, regardless of performance on the previous platform task. Interestingly, the administration of reboxetine interfered with the spatial learning and memory trials for both ISS and CC groups. As a result, ISS exposure compromised spatial learning and memory in the Morris water maze, and norepinephrine does not appear to be a mediator of this deficit. The results demonstrate a key difference in the effects of reboxetine in a massed- vs. spaced-learning trial procedure in the Morris water maze following ISS exposure.

Sex differences associated with intermittent swim stress.

Various animal models of depression have been used to seek a greater understanding of stress-related disorders. However, there is still a great need for novel research in this area, as many individuals suffering from depression are resistant to current treatment methods. Women have a higher rate of depression, highlighting the need to investigate mechanisms of sex differences. Therefore, we employed a new animal model to assess symptoms of depression, known as intermittent swim stress (ISS). In this model, the animal experiences 100 trials of cold water swim stress. ISS has already been shown to cause signs of behavioral depression in males, but has yet to be assessed in females. Following ISS exposure, we looked at sex differences in the Morris water maze and forced swim test. The results indicated a spatial learning effect only in the hidden platform task between male and female controls, and stressed and control males. A consistent spatial memory effect was only seen for males exposed to ISS. In the forced swim test, both sexes exposed to ISS exhibited greater immobility, and the same males and females also showed attenuated climbing and swimming, respectively. The sex differences could be due to different neural substrates for males and females. The goal of this study was to provide the first behavioral examination of sex differences following ISS exposure, so the stage of estrous cycle was not assessed for the females. This is a necessary future direction for subsequent experiments. The current article highlights the importance of sex differences in response to stress.

Resilience in shock and swim stress models of depression.

Experimental models of depression often entail exposing a rodent to a stressor and subsequently characterizing changes in learning and anhedonia, which may reflect symptoms of human depression. Importantly, not all people, and not all laboratory rats, exposed to stressors develop depressed behavior; these "resilient" individuals are the focus of our review. Herein we describe research from the "learned helplessness" and "intermittent swim stress" (ISS) models of depression in which rats that were allowed to control the offset of the aversive stimulus with a behavioral response, and in a subset of rats that were not allowed to control the stressor that appeared to be behaviorally and neurochemically similar to rats that were either naive to stress or had controllability over the stressor. For example, rats exposed to inescapable tailshock, but do not develop learned helplessness, exhibit altered sensitivity to the behavioral effects of GABAA receptor antagonists and reduced in vitro benzodiazepine receptor ligand binding. This pattern suggested that resilience might involve activation of an endogenous benzodiazepine-like compound, possibly an allostatic modulator of the GABAA receptor like allopregnanolone. From the ISS model, we have observed in resilient rats protection from stressor-induced glucocorticoid increases and immune activation. In order to identify the neural mediators of these correlates of resilience, non-invasive measures are needed to predict the resilient or vulnerable phenotype prior to analysis of neural endpoints. To this end, we found that ultrasonic vocalizations (USVs) appear to predict the resilient phenotype in the ISS paradigm. We propose that combining non-invasive predictive measures, such as USVs with biological endpoint measures, will facilitate future research into the neural correlates of resilience.

Anxiogenic effects of brief swim stress are sensitive to stress history.

Stressors that are controllable not only protect an individual from the acute consequences of the stressor, but also the consequences of stressors that occur later. This phenomenon, termed "behavioral immunization", is studied in the rat by first administering tailshocks each of which can be terminated (escapable tailshock) by an instrumental wheel-turn response prior to exposure to a second stressor. Previous research has shown that exposure to escapable tailshock blocks the neurochemical and behavioral consequences of later inescapable tailshock or social defeat stress. Here we explored the generality of behavioral immunization by examining the impact of prior escapable tailshock on the behavioral consequences of cold swim stress. Exposure to a 5min cold-water (19°C) swim caused an anxiety-like reduction in social interaction that was dependent upon 5-HT2C receptor activation. Rats with prior exposure to escapable tailshock did not develop the swim-induced anxiety. Plasticity in the medial prefrontal cortex, a hypothetical neural mechanism underlying behavioral immunization, is discussed.

Coping in an intermittent swim stress paradigm compromises natural killer cell activity in rats.

The effects of intermittent swim stress and stressor controllability on natural killer cell activity (NKCA) was examined. Significant decreases in splenic NKCA were observed immediately post-stress, but only when the stress was controllable. Although decreased NKCA was also observed in yoked rats subjected to the same stressor, it failed to attain statistical significance. Previous results suggest these effects are not due to corticosterone. The results suggest a cost of coping on the acute, in vitro immune measure of NKCA.

Morris water maze performance deficit produced by intermittent swim stress is partially mediated by norepinephrine.

Intermittent swim stress (ISS) exposes a rat to cold water and the effects of the procedure produce detrimental results on activity measures 24h later. The ISS model can be used with the Morris water maze (MWM) to investigate the impact of stress on a spatial learning and memory task, known to involve the hippocampus. We investigated if the ISS model produced performance deficits in the MWM (experiments 1 and 2). We also investigated the role of norepinephrine by using an alpha-2 adrenergic agonist (i.e., clonidine) to exacerbate ISS-induced deficits (experiment 3), and using antidepressants (i.e., desipramine and reboxetine) that enhance the synaptic availability of norepinephrine to reduce ISS-induced deficits (experiments 4 and 5). Results indicated a main effect for stress in all experiments, with the exception of experiment 2, as ISS did induce performance deficits in the MWM. Clonidine enhanced ISS-induced deficits only in the learning trials, while desipramine and reboxetine reduced ISS-induced deficits in the learning trials. Additionally, only reboxetine reduced memory deficits in the MWM. These findings provide evidence that norepinephrine may act as a partial mediator of ISS-induced deficits in MWM performance.

Stress resilience and vulnerability: the association with rearing conditions, endocrine function, immunology, and anxious behavior.

BACKGROUND: The current study explored the underlying behavioral, endocrine, and immune markers of vulnerability to stress-induced depression, and the impact of rearing environments on adult functioning. METHOD: Adult Sprague-Dawley rats (n=195) were reared in either Maternal Separation (MS), Early Weaning and Isolation (EWI), or Non-Handled (NH) conditions. Anxiety behavior was assessed using the emergence test at mean postnatal day (PND) 60. Stress-induced depressive behavior was measured at mean PND 86 using an intermittent cold water swim stress and swim escape test (SET) paradigm. Immediately following the SET, and in a sample of naïve controls (N=31), trunk blood was collected to assay for serum corticosterone (CORT) and spleens were removed for determination of Concanavalin A (Con-A) stimulated T-cell proliferation. RESULTS: Stress vulnerable rats (top tertile of SET swim time) were characterised by increased anxiety-like behavior, greater post-stress CORT concentrations, and a significantly higher Con-A induced T-cell proliferative response compared to stress resilient rats (bottom tertile of SET swim time). The EWI rearing condition was a contributing factor in predicting total swim escape time, however MS was not. MS offspring did have double the basal level of CORT than NH offspring, suggestive of a hyperfunctioning HPA axis. CONCLUSION: The swim stress animal model enabled observation of stress vulnerability and resilience; results point towards the existence of distinct behavioral, endocrine, and immunological profiles of the vulnerable and resilient animal, which may have important implications for mental health and stress research.

Intermittent and continuous swim stress-induced behavioral depression: sensitivity to norepinephrine- and serotonin-selective antidepressants.

RATIONALE: Intermittent swim stress (ISS) produces deficits in swim escape learning and increases immobility in the forced swim test (FST). A previous attempt to reverse this immobility with the selective serotonin reuptake inhibitor (SSRI), fluoxetine (FLX), was unsuccessful, but the sensitivity of this immobility to other types of antidepressants is unknown. OBJECTIVES: In experiment 1, we evaluate the ability of the norepinephrine (NE) selective reuptake inhibitor (NSRI), desipramine (DES), to reverse the ISS-induced immobility in the FST compared to confined controls (CC), while in experiment 2, we test the efficacy of either the SSRI or NSRI to reverse the immobility produced by either ISS or continuous swim (CS)/FST. METHODS: Rats were exposed to their respective behavioral pretreatment (ISS, CS/FST, or CC) and were then injected with an antidepressant or saline solution 23.5, 5, and 1 h prior to the FST. RESULTS: In experiment 1, DES reduced immobility and increased the climbing behavior in the ISS group without altering these behaviors in the CC, while in experiment 2, the CS/FST-induced immobility was reduced by both antidepressants (i.e., FLX and DES), while the ISS-induced immobility was only affected by DES. CONCLUSIONS: These results suggest that the ISS-induced immobility is mediated through the NE system and may represent a model for atypical depression.

Swim stress-induced ultrasonic vocalizations forecast resilience in rats.

Ultrasonic vocalizations (USVs) have been observed in a number of rodent species. They occur under a variety of conditions, including aversive and stressful experiences. In the current study, we recorded USVs emitted by rats exposed to intermittent cold water swim (ICWS) stress and subsequently evaluated their performance in an instrumental swim escape test (SET). In the SET, rats exposed to ICWS fall into two categories, resilient or vulnerable, based on good or poor learning, respectively. Four of 16 rats exposed to ICWS emitted far more USVs during the stress procedure than the remaining 12. Interestingly, in the SET these USV-emitting rats appeared resilient with escape performance comparable to controls while on average the non-emitting rats failed to learn. This result demonstrates that USVs can serve as a predictor of stress resilience. USV screening during stress may serve as a novel and non-invasive strategy to predict subsequent stress reactivity and afford insight into the neural systems involved in resilience.

The immobility produced by intermittent swim stress is not mediated by serotonin.

Exposure to uncontrollable stressors such as intermittent swim stress (ISS) produces a behavioral syndrome that resembles behavioral depression including immobility in a Forced Swim Test (FST) and escape learning deficits. The results of previous studies suggest that stress causes a temporary sensitization of the brain serotonin (5-HT) system that is necessary and sufficient for producing behavioral depression. If this hypothesis is true in the ISS paradigm, then enhancing or inhibiting 5-HT transmission during stress should exacerbate or block the development of behavioral depression, respectively. The selective 5-HT uptake inhibitor fluoxetine (FLX) was administered prior to ISS or confinement; 24 h later the FST was used to detect behavioral immobility. ISS, but not FLX, significantly increased immobility in the FST. The purported 5-HT uptake enhancer tianeptine (TPT) was administered in place of FLX. Again ISS increased immobility in the FST, but TPT had no effect. These results suggested that 5-HT is not a critical mediator of ISS induced behavioral depression. However, some authors have raised concern that TPT does not act directly on 5-HT. Therefore, the 5-HT synthesis inhibitor, para-chlorophenylaline (PCPA) was administered to deplete central 5-HT before stress. PCPA did not alter immobility in the FST. Finally, a sub-chronic regimen of FLX given after ISS, but before the FST, was without effect on reversing the ISS-induced immobility. Taken together, these experiments indicate that ISS produces a significant behavioral depression manifested as increased immobility but offer no support of the hypothesis that 5-HT is a critical mediator of these effects.

Environmental and immune stressors enhance alcohol-induced motor ataxia in rat.

Infection is now accepted as a stressor, consequently we sought to compare the short- and longer-term consequences of several environmental stressors versus an endotoxin challenge on alcohol-induced motor ataxia. The present set of studies examined the impact of intermittent electric shock (SHOCK), intermittent cold water swim (ICWS), or lipopolysaccharide (LPS) administration on the motor ataxic effects of an intraperitoneal (i.p.) injection of alcohol (ETOH). In Experiment 1 SHOCK, but not ICWS, enhanced the motor ataxic effects of ethanol at both 2 and 24 h post-stress. In Experiment 2 administration of LPS did not affect the motor ataxic effects of ETOH 4 h later, but enhanced the ataxic potency of ETOH 24 h later. The results indicate that certain environmental and immune stressors have the potential to alter the long-term behavioral reactivity to alcohol. These examples of stress-induced enhancement of the motor ataxic effects of ETOH may have important implications for the development of alcohol dependence.

Endocrine and immunological correlates of behaviorally identified swim stress resilient and vulnerable rats.

Animal models of stress-induced depression have identified a bimodal reactivity to stress, namely 'resilience' and 'vulnerability.' Possible corresponding differences in endocrine and immunological responses between these groups have not been delineated. Male Sprague-Dawley rats were divided into three groups: stress (n=25), confined controls (n=7), and home cage controls (n=7). Stress rats were exposed to 80, 5-s inescapable cold water swim trials (15 degrees C). Twenty-four hours later, the stress rats were tested on an instrumental swim escape test (SET) but now they had access to an omnidirectional lever that terminated the stress. Immediately after the SET, trunk blood was collected to assay for serum corticosterone (CORT), and spleens were removed and natural killer cell activity (NKCA) and concanavalin A (CON-A) induced lymphocyte proliferation determined. Subjects in the stress treatment group were divided into distinct 'resilient' and 'vulnerable' categories by a median split for average escape latencies across the last 25 trials of the SET. Stress rats secreted more CORT than controls and vulnerable rats secreted greater levels than resilient rats. NKCA was greatest in control rats, and was decreased in the stress rats although the resilient and the vulnerable groups did not differ. Conversely, CON-A-induced lymphocyte proliferation was greatest in stress rats, vulnerable rats exhibiting more proliferation than resilient rats, but both were greater than both control groups. Stress animals were hypothermic throughout the swim stress procedures but exhibited a stress-induced fever following the initial swim trials. The observed differences may have important predictive and theoretical utility for vulnerable and resilient profiles.

Impact of water temperature and stressor controllability on swim stress-induced changes in body temperature, serum corticosterone, and immobility in rats.

The present study compared the effects of three different water temperatures (20, 25, and 30 degrees C) and stressor controllability on several physiological and behavioral endpoints in an intermittent swim stress paradigm. The escape latency of rats in the 20 and 25 degrees C water was less than that observed for the 30 degrees C group. Both escape and yoked groups at 20 and 25 degrees C exhibited moderate to severe hypothermia following the swim stress session that returned to prestress levels 30-40 min post-stress. At 30 degrees C core body temperature (Tb) only decreased by 1 degree C for either swim group. Following swim, serum corticosterone (CORT) levels were significantly elevated in both escape and yoked groups in comparison to confined and home cage controls. The confined control group showed a significant elevation that was approximately halfway between the home cage control and the swim stress groups. At 30 degrees C, there was still a significant elevation of serum CORT in both swim groups in comparison to confined and home cage controls. Therefore, 30 degrees C appears to be the optimal water temperature to evaluate stress controllability effects in the current paradigm. In a final experiment, swim stressor controllability effects were examined in a 5 min forced swim test (FST) 24 h following the initial stress exposure. Rats exposed to yoked-inescapable swim stress at 30 degrees C exhibited more immobility than their escapable swim stress and confined counterparts, while the escape and confined controls did not differ. These results demonstrate that the behavioral deficits observed in the FST are attributable to the stress of inescapable swim and not swim stress per se.

Intermittent cold water swim stress increases immobility and interferes with escape performance in rat.

The behavioral consequences of intermittent, 5 s cold-water swims (15 degrees C) or confinement were assessed 24 h after stress in a 5 min forced swim test or an instrumental swim escape test (SET). The SET was conducted with temporal and instrumental parameters similar to the shock-motivated shuttle escape test. The tests detected significantly increased immobility in the forced swim test and increased latency to escape in the SET. These results extend previous findings with intermittent swim stress and provide evidence that intermittent swim stress produces behavioral deficits similar to other stress models. This new model may be a useful tool for exploring the physiological mechanisms underlying the stress response.