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

Chronic Unpredictable Mild Stress Model of Depression: Possible Sources of Poor Reproducibility and Latent Variables

Major depressive disorder (MDD) is one of the most common mood disorders worldwide. A lack of understanding of the exact neurobiological mechanisms of depression complicates the search for new effective drugs. Animal models are an important tool in the search for new approaches to the treatment of this disorder. All animal models of depression have certain advantages and disadvantages. We often hear that the main drawback of the chronic unpredictable mild stress (CUMS) model of depression is its poor reproducibility, but rarely does anyone try to find the real causes and sources of such poor reproducibility. Analyzing the articles available in the PubMed database, we tried to identify the factors that may be the sources of the poor reproducibility of CUMS. Among such factors, there may be chronic sleep deprivation, painful stressors, social stress, the difference in sex and age of animals, different stress susceptibility of different animal strains, handling quality, habituation to stressful factors, various combinations of physical and psychological stressors in the CUMS protocol, the influence of olfactory and auditory stimuli on animals, as well as the possible influence of various other factors that are rarely taken into account by researchers. We assume that careful inspection of these factors will increase the reproducibility of the CUMS model between laboratories and allow to make the interpretation of the obtained results and their comparison between laboratories to be more adequate.

Graphene oxide prevents lateral amygdala dysfunctional synaptic plasticity and reverts long lasting anxiety behavior in rats

Engineered small graphene oxide (s-GO) sheets were previously shown to reversibly down-regulate glutamatergic synapses in the hippocampus of juvenile rats, disclosing an unexpected translational potential of these nanomaterials to target selective synapses in vivo. Synapses are anatomical specializations acting in the Central Nervous System (CNS) as functional interfaces among neurons. Dynamic changes in synaptic function, named synaptic plasticity, are crucial to learning and memory. More recently, pathological mechanisms involving dysfunctional synaptic plasticity were implicated in several brain diseases, from dementia to anxiety disorders. Hyper-excitability of glutamatergic neurons in the lateral nucleus of the amygdala complex (LA) is substantially involved in the storage of aversive memory induced by stressful events enabling post-traumatic stress disorder (PTSD). Here we translated in PTSD animal model the ability of s-GO, when stereotaxically administered to hamper LA glutamatergic transmission and to prevent the behavioral response featured in long-term aversive memory. We propose that s-GO, by interference with glutamatergic plasticity, impair LA-dependent memory retrieval related to PTSD.

Acute exposure of rats to a severe stressor alters the circadian pattern of corticosterone and sensitizes to a novel stressor: Relationship to pre-stress individual differences in resting corticosterone levels

Traumatic events have been proposed to be associated with hypo-activity of the hypothalamic-pituitary-adrenal (HPA) axis, but data in animal models exposed to severe stressors are controversial and have important methodological concerns. Individual differences in resting or stress levels of corticosterone might explain some of the inconsistencies. We then studied this issue in male rats exposed to 2 h immobilization on boards (IMO), a severe stressor. Thirty-six rats were blood sampled under resting conditions four times a day on three non-consecutive days. Then, they were assigned to control (n = 14) or IMO (n = 22) to study the HPA response to IMO, the stressor-induced alterations in the circadian pattern of corticosterone (CPCORT), and the behavioral and HPA responsiveness to an open-field. Individual differences in pre-IMO resting corticosterone were inconsistent, but averaging data markedly improved consistency. The CPCORT was markedly altered on day 1 post-IMO (higher trough and lower peak levels), less altered on day 3 and apparently normal on day 7. Importantly, when rats were classified in low and high resting corticosterone groups (LCORT and HCORT, respectively), on the basis of the area under the curve (AUC) of the averaged pre-IMO data, AUC differences between LCORT and HCORT groups were maintained in controls but disappeared in IMO rats during the post-IMO week. Open-field hypo-activity and corticosterone sensitization were similar in LCORT and HCORT groups nine days after IMO. A single IMO exposure causes long-lasting HPA alterations, some of them dependent on pre-stress resting corticosterone levels, with no evidence for post-IMO resting corticosterone hypo-activity.

Differential antipredatory responses in the tuco-tuco (Ctenomys talarum) in relation to endogenous and exogenous changes in glucocorticoids

Glucocorticoids participate in the behavioral and physiological responses generated under stressful circumstances coming from different sources-physical and/or psychological. In mammals, the increases of these hormones are mediated by the activation of the hypothalamic-pituitary-adrenal axis. This response occurs after exposure to novel and unpredictable situations that lead to the loss of homeostasis, for example, a direct encounter with predators or their cues. However, the relationship between the physiological and behavioral responses is still a complex issue in vertebrates. We evaluate the effects of an experimental manipulation of glucocorticoid levels on the generation of the behavioral and physiological response to stress by predation in the subterranean rodent C. talarum. We found that when tuco-tucos encountered predator cues-fur odor, and largely, immobilization-they responded physiologically by secreting cortisol. This response was accompanied by an associated behavioral response. However, when the increase in plasma cortisol originated exogenously by the injection of cortisol, a behavioral response was not observed. Finally, inhibition of glucocorticoids' synthesis was effective in weakening the behavioral effects produced by immobilization. In conclusion, in tuco-tucos, predator cues act as stress factors that trigger differential increases in plasma cortisol and a behavioral response associated with the appearance of anxiety states.

Modelling posttraumatic stress disorders in animals

Animal models of posttraumatic stress disorder are useful tools to reveal the neurobiological basis of the vulnerability to traumatic events, and to develop new treatment strategies, as well as predicting treatment response contributing to personalized medicine approach. Different models have different construct, face and predictive validity and they model different symptoms of the disease. The most prevalent models are the single prolonged stress, electric foot-shock and predator odor. Freezing as 're-experiencing' in cluster B and startle as 'arousal' in cluster E according to DSM-5 are the most frequently studied parameters; however, several other symptoms related to mood, cognitive and social skills are part of the examinations. Beside behavioral characteristics, symptoms of exaggerated sympathetic activity and hypothalamic-pituitary-adrenocortical axis as well as signs of sleep disturbances are also warranted. Test battery rather than a single test is required to describe a model properly and the results should be interpreted in a comprehensive way, e.g. creating a z-score. Research is shifting to study larger populations and identifying the features of the resilient and vulnerable individuals, which cannot be easily done in humans. Incorporation of the "three hit theory" in animal models may lead to a better animal model of vulnerability and resilience. As women are twice as vulnerable as men, more emphasize should be taken to include female animals. Moreover, hypothesis free testing and big data analysis may help to identify an array of biomarkers instead of a single variable for identification of vulnerability and for the purpose of personalized medicine.

Brain c-fos expression patterns induced by emotional stressors differing in nature and intensity

Regardless of its particular nature, emotional stressors appear to elicit a widespread and roughly similar brain activation pattern as evaluated by c-fos expression. However, their behavioral and physiological consequences may strongly differ. Here we addressed in adult male rats the contribution of the intensity and the particular nature of stressors by comparing, in a set of brain areas, the number of c-fos expressing neurons in response to open-field, cat odor or immobilization on boards (IMO). These are qualitatively different stressors that are known to differ in terms of intensity, as evaluated by biological markers. In the present study, plasma levels of the adrenocorticotropic hormone (ACTH) demonstrated that intensity increases in the following order: open-field, cat odor and IMO. Four different c-fos activation patterns emerged among all areas studied: (i) positive relationship with intensity (posterior-dorsal medial amygdala, dorsomedial hypothalamus, lateral septum ventral and paraventricular nucleus of the hypothalamus), (ii) negative relationship with intensity (cingulate cortex 1, posterior insular cortex, dorsal striatum, nucleus accumbens and some subdivisions of the hippocampal formation); (iii) activation not dependent on the intensity of the stressor (prelimbic and infralimbic cortex and lateral and basolateral amygdala); and (iv) activation specifically associated with cat odor (ventromedial amygdala and ventromedial hypothalamus). Histone 3 phosphorylation at serine 10, another neuronal activation marker, corroborated c-fos results. Summarizing, deepest analysis of the brain activation pattern elicit by emotional stressor indicated that, in spite of activating similar areas, each stressor possess their own brain activation signature, mediated mainly by qualitative aspects but also by intensity.

Sex-dependent impact of early-life stress and adult immobilization in the attribution of incentive salience in rats

Early life stress (ELS) induces long-term effects in later functioning and interacts with further exposure to other stressors in adulthood to shape our responsiveness to reward-related cues. The attribution of incentive salience to food-related cues may be modulated by previous and current exposures to stressors in a sex-dependent manner. We hypothesized from human data that exposure to a traumatic (severe) adult stressor will decrease the attribution of incentive salience to reward-associated cues, especially in females, because these effects are modulated by previous ELS. To study these factors in Long-Evans rats, we used as an ELS model of restriction of nesting material and concurrently evaluated maternal care. In adulthood, the offspring of both sexes were exposed to acute immobilization (IMO), and several days after, a Pavlovian conditioning procedure was used to assess the incentive salience of food-related cues. Some rats developed more attraction to the cue predictive of reward (sign-tracking) and others were attracted to the location of the reward itself, the food-magazine (goal-tracking). Several dopaminergic markers were evaluated by in situ hybridization. The results showed that ELS increased maternal care and decreased body weight gain (only in females). Regarding incentive salience, in absolute control animals, females presented slightly greater sign-tracking behavior than males. Non-ELS male rats exposed to IMO showed a bias towards goal-tracking, whereas in females, IMO produced a bias towards sign-tracking. Animals of both sexes not exposed to IMO displayed an intermediate phenotype. ELS in IMO-treated females was able to reduce sign-tracking and decrease tyrosine hydroxylase expression in the ventral tegmental area and dopamine D1 receptor expression in the accumbens shell. Although the predicted greater decrease in females in sign-tracking after IMO exposure was not corroborated by the data, the results highlight the idea that sex is an important factor in the study of the long-term impact of early and adult stressors.

Developmental expression of anxiety and depressive behaviours after prenatal predator exposure and early life homecage enhancement

Stressful events during gestation can have sex-specific effects on brain and behaviour, and may contribute to some of the differences observed in adult stress responding and psychopathology. We investigated the impact of a novel repeated prenatal psychological stress (prenatal predator exposure - PPS) during the last week of gestation in rats on offspring behaviours related to social interaction (play behaviour), open field test (OFT), forced swim test (FST) and sucrose preference test (SP) during the juvenile period and in adulthood. We further examined the role of postnatal environmental, using an enhanced housing condition (EHC), to prevent/rescue any changes. Some effects on anxiety, anhedonia, and stress-related coping behaviours (e.g., OFT, SP and OFT) did not emerge until adulthood. PPS increased OFT anxiety behaviours in adult males, and some OFT and SP behaviours in adult females. Contrary to this, EHC had few independent effects; most were apparent only when combined with PPS. In keeping with age-group differences, juvenile behaviours did not necessarily predict the same adult behaviours although juvenile OFT rearing and freezing, and juvenile FST immobility did predict adult FST immobility and sucrose preference, suggesting that some aspects of depressive behaviours may emerge early and predict adult vulnerability or coping behaviours. Together, these results suggest an important, though complex, role for early life psychological stressors and early life behaviours in creating an adult vulnerability to anxiety or depressive disorders and that environmental factors further modulate the effects of the prenatal stressors.

Optimizing laboratory animal stress paradigms: The H-H* experimental design

Major advances in behavioral neuroscience have been facilitated by the development of consistent and highly reproducible experimental paradigms that have been widely adopted. In contrast, many different experimental approaches have been employed to expose laboratory mice and rats to acute versus chronic intermittent stress. An argument is advanced in this review that more consistent approaches to the design of chronic intermittent stress experiments would provide greater reproducibility of results across laboratories and greater reliability relating to various neural, endocrine, immune, genetic, and behavioral adaptations. As an example, the H-H* experimental design incorporates control, homotypic (H), and heterotypic (H*) groups and allows for comparisons across groups, where each animal is exposed to the same stressor, but that stressor has vastly different biological and behavioral effects depending upon each animal's prior stress history. Implementation of the H-H* experimental paradigm makes possible a delineation of transcriptional changes and neural, endocrine, and immune pathways that are activated in precisely defined stressor contexts.

The role of the hypothalamus-pituitary-adrenal/interrenal axis in mediating predator-avoidance trade-offs

Maintaining energy balance and reproducing are important for fitness, yet animals have evolved mechanisms by which the hypothalamus-pituitary-adrenal/interrenal (HPA/HPI) axis can shut these activities off. While HPA/HPI axis inhibition of feeding and reproduction may have evolved as a predator defense, to date there has been no review across taxa of the causal evidence for such a relationship. Here we review the literature on this topic by addressing evidence for three predictions: that exposure to predators decreases reproduction and feeding, that exposure to predators activates the HPA/HPI axis, and that predator-induced activation of the HPA/HPI axis inhibits foraging and reproduction. Weight of evidence indicates that exposure to predator cues inhibits several aspects of foraging and reproduction. While the evidence from fish and mammals supports the hypothesis that predator cues activate the HPA/HPI axis, the existing data in other vertebrate taxa are equivocal. A causal role for the HPA axis in predator-induced suppression of feeding and reproduction has not been demonstrated to date, although many studies report correlative relationships between HPA activity and reproduction and/or feeding. Manipulation of HPA/HPI axis signaling will be required in future studies to demonstrate direct mediation of predator-induced inhibition of feeding and reproduction. Understanding the circuitry linking sensory pathways to their control of the HPA/HPI axis also is needed. Finally, the role that fear and anxiety pathways play in the response of the HPA axis to predator cues is needed to better understand the role that predators have played in shaping anxiety related behaviors in all species, including humans.

Avoidance and contextual learning induced by a kairomone, a pheromone and a common odorant in female CD1 mice

Chemosignals mediate both intra- and inter-specific communication in most mammals. Pheromones elicit stereotyped reactions in conspecifics, whereas kairomones provoke a reaction in an allospecific animal. For instance, predator kairomones elicit anticipated defensive responses in preys. The aim of this work was to test the behavioral responses of female mice to two chemosignals: 2-heptanone (2-HP), a putative alarm pheromone, and 2,4,5-trimethylthiazoline (TMT), a fox-derived putative kairomone, widely used to investigate fear and anxiety in rodents. The banana-like odorant isoamyl acetate (IA), unlikely to act as a chemosignal, served as a control odorant. We first presented increasing amounts of these odorants in consecutive days, in a test box in which mice could explore or avoid them. Female mice avoided the highest amounts of all three compounds, with TMT and IA eliciting avoidance at lower amounts (3.8 pmol and 0.35 μmol, respectively) than 2-HP (35 μmol). All three compounds induced minimal effects in global locomotion and immobility in this set up. Further, mice detected 3.5 pmol of TMT and IA in a habituation–dishabituation test, so avoidance of IA started well beyond the detection threshold. Finally, both TMT and IA, but not 2-HP, induced conditioned place avoidance and increased immobility in the neutral compartment during a contextual memory test. These data suggest that intense odors can induce contextual learning irrespective of their putative biological significance. Our results support that synthetic predator-related compounds (like TMT) or other intense odorants are useful to investigate the neurobiological basis of emotional behaviors in rodents. Since intense odorants unlikely to act as chemosignals can elicit similar behavioral reactions than chemosignals, we stress the importance of using behavioral measures in combination with other physiological (e.g., hormonal levels) or neural measures (e.g., immediate early gene expression) to establish the ethological significance of odorants.

Time-dependent effects of rapamycin on consolidation of predator stress-induced hyperarousal

Previous studies have indicated that rapamycin, a potent inhibitor of the mammalian target of rapamycin (mTOR) pathway, blocks consolidation of shock-induced associative fear memories. Moreover, rapamycin's block of associative fear memories is time-dependent. It is unknown, however, if rapamycin blocks consolidation of predator stress-induced non-associative fear memories. Furthermore, the temporal pattern of mTOR activation following predator stress is unknown. Thus, the goal of the current studies was to determine if rapamycin blocks consolidation of predator stress-induced fear memories and if so, whether rapamycin's effect is time-dependent. Male rats were injected systemically with rapamycin at various time points following predator stress. Predator stress involves an acute, unprotected exposure of a rat to a cat, which causes long-lasting non-associative fear memories manifested as generalized hyperarousal and increased anxiety-like behaviour. We show that rapamycin injected immediately after predator stress blocked consolidation of stress-induced startle. However, rapamycin injected 9, 24 or 48h post predator stress potentiated stress-induced startle. Consistent with shock-induced associative fear memories, we show that mTOR signalling is essential for consolidation of predator stress-induced hyperarousal. However, unlike shock-induced fear memories, a second, persistent, late phase mTOR-dependent process following predator stress actually dampens startle. Consistent with previous findings, our data support the potential role for rapamycin in treatment of stress related disorders such as posttraumatic stress disorder. However, our data suggest timing of rapamycin administration is critical.

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

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

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

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

Sex differences in the behavioural and hypothalamic-pituitary-adrenal response to contextual fear conditioning in rats

In recent years, special attention is being paid to sex differences in susceptibility to disease. In this regard, there is evidence that male rats present higher levels of both cued and contextual fear conditioning than females. However, little is known about the concomitant hypothalamic-pituitary-adrenal (HPA) axis response to those situations which are critical in emotional memories. Here, we studied the behavioural and HPA responses of male and female Wistar rats to context fear conditioning using electric footshock as the aversive stimulus. Fear-conditioned rats showed a much greater ACTH and corticosterone response than those merely exposed to the fear conditioning chamber without receiving shocks. Moreover, males presented higher levels of freezing whereas HPA axis response was greater in females. Accordingly, during the fear extinction tests, female rats consistently showed less freezing and higher extinction rate, but greater HPA activation than males. Exposure to an open-field resulted in lower activity/exploration in fear-conditioned males, but not in females, suggesting greater conditioned cognitive generalization in males than females. It can be concluded that important sex differences in fear conditioning are observed in both freezing and HPA activation, but the two sets of variables are affected in the opposite direction: enhanced behavioural impact in males, but enhanced HPA responsiveness in females. Thus, the role of sex differences on fear-related stimuli may depend on the variables chosen to evaluate it, the greater responsiveness of the HPA axis in females perhaps being an important factor to be further explored.

Active coping toward predatory stress is associated with lower corticosterone and progesterone plasma levels and decreased methylation in the medial amygdala vasopressin system

An active coping style displayed under stress - which involves proactive investigatory responses toward environmental threats - has been associated with reduced vulnerability to psychiatric illness. However, the neurobiological determinants of coping styles are not well understood. When rats are exposed to a naturalistic stressor (cat fur) in a group, some individuals in the group show robust active investigation of the stimulus while others show a passive response involving retreat, immobility and close aggregation with conspecifics. Here we explored endocrine and epigenetic correlates of these contrasting coping styles. Male Wistar rats (n=48) were exposed to cat fur in groups of 4 and the passive and active responders were identified and assessed for endocrine and epigenetic differences. Three days after the final cat fur exposure, active responders had substantially lower plasma levels of corticosterone and progesterone than passive responders. Plasma and testicular testosterone levels did not differ between active and passive responders. Active responders had markedly less methylation of the AVP CGCG promoter region located at base 4970 in the posterodorsal region of the medial amygdala but did not differ in the methylation status of the CCGG sequence located at base 2243. This is in agreement with prior research suggesting that AVP and progesterone act in opposition within the medial amygdala to modulate stress-related behaviors. The present study reports striking endocrine and epigenetic differences between active and passive responders, providing insight into potential systems involved in the manifestation of differing coping styles.

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

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

Long-term effects of chronic cocaine exposure throughout adolescence on anxiety and stress responsivity in a Wistar rat model

Adolescents display increased vulnerability to engage in drug experimentation. This is often considered a risk factor for later drug abuse. In this scenario, the permanent effects of cocaine exposure during adolescence on anxiety levels and stress responsivity, which may result in behavioral phenotypes prone to addiction, are now starting to be unveiled. Thus, the purpose of the present study was to evaluate the long-lasting effects of chronic cocaine administration during adolescence, on anxiety-like behavior and on stress response. Adolescent male Wistar rats were daily administered 45-mg cocaine/kg of body weight in three equal intraperitoneal doses with 1-h interval, from postnatal day (PND) 35 to 50. The effects of cocaine administration on anxiety levels, assessed in the Elevated Plus Maze (EPM), and on social stress response, assessed in the resident-intruder paradigm (R/I), were evaluated 10 days after withdrawal, when rats were reaching the adulthood. The underlying dopaminergic activity, and the corticosterone and testosterone levels were determined. Our results showed that cocaine induced long-lasting alterations in the hypothalamus-pituitary-adrenals (HPA) axis function and in testosterone levels. Such alterations resulted in significant and enduring changes in behavioral responses to environmental challenges, such as the EPM and R/I, including the evaluation of potential threats that may lead to high-risk behavior and low-benefit choices. This was further supported by an altered dopaminergic function in the amygdala and hippocampus. The present findings provide new insights into how the use of cocaine during adolescent development may modulate emotional behavior later in life. Compromised ability to recognize and deal with potential threats is an important risk factor to perpetuate compulsive drug seeking and relapse susceptibility.

Experimenter effects on behavioral test scores of eight inbred mouse strains under the influence of ethanol

Eight standard inbred mouse strains were evaluated for ethanol effects on a refined battery of behavioral tests in a study that was originally designed to assess the influence of rat odors in the colony on mouse behaviors. As part of the design of the study, two experimenters conducted the tests, and the study was carefully balanced so that equal numbers of mice in all groups and times of day were tested by each experimenter. A defect in airflow in the facility compromised the odor manipulation, and in fact the different odor exposure groups did not differ in their behaviors. The two experimenters, however, obtained markedly different results for three of the tests. Certain of the experimenter effects arose from the way they judged behaviors that were not automated and had to be rated by the experimenter, such as slips on the balance beam. Others were not evident prior to ethanol injection but had a major influence after the injection. For several measures, the experimenter effects were notably different for different inbred strains. Methods to evaluate and reduce the impact of experimenter effects in future research are discussed.

An updated animal model capturing both the cognitive and emotional features of post-traumatic stress disorder (PTSD)

The new-released Diagnostic and Statistical Manual of Mental Disorders (DSM-5) defines post-traumatic stress disorder (PTSD) as a "trauma and stressor-related disorder". PTSD pathogenesis relies on paradoxical changes of emotional memory processing induced by the trauma exposure and associated with emotional dysfunction. Several animal models of PTSD have been validated and are currently used. Each one mimics a particular subset of the disorder with particular emphasis, mainly driven by the past classification of PTSD in the DSM-4, on the emotional features. In view of the recent update in the DSM-5, our aim was to develop, by using well-validated paradigms, a modified model of PTSD able to mimic at the same time both the cognitive and emotional features of the disease. We exposed male rats to either a piece of worn cat collar or to a series of inescapable footshocks paired with a PTSD risk factor, i.e., social isolation. Animals were subsequently re-exposed to the conditioned contexts at different time intervals in order to test memory retention for the stressors. In addition, footshock-exposed rats were tested in the elevated-plus-maze and social interaction tests. We found that rats exposed to a cat collar exhibited an acute fear response that did not lead to enduring memory retention. Conversely, footshock-exposed rats expressed a successful retention of the stressful experience at 1, 7, 14, 21 and 56 post-exposure days. Footshock-exposed rats displayed an anxious behavioral profile in the social interaction test and a significantly reduced locomotor activity in the elevated-plus-maze test. These dysfunctions were not observed when animals were socially housed, thus highlighting a social buffering effect in the development of the pathology. Our results underline the good validity of a footshock-based paradigm paired with social isolation as a PTSD animal model, able to mimic at the same time both some of the enduring cognitive and emotional facets of the pathology.

Behavioral stress induces regionally-distinct shifts of brain mineralocorticoid and glucocorticoid receptor levels

Mineralocorticoid and glucocorticoid receptors (MRs and GRs) mediate the impact of stress on brain function primarily by affecting gene transcription in the cell nucleus. In vitro studies using hippocampal neurons indicate that MRs and GRs translocate to the nucleus after binding to the stress hormone corticosterone, yet the in vivo temporal dynamics of MR and GR levels in other limbic regions critical for the stress response, however, are largely unknown. Rats underwent an elevated platform (EP) stress procedure and brain tissue was sampled from the amygdala (AMY), medial prefrontal cortex (mPFC), dorsal hippocampus and ventral hippocampus. By measuring MR and GR levels in the nuclear fraction from the tissue sampled, we observed striking shifts in the protein levels that varied by receptor, brain region and by the time after EP stress. These findings indicate that the subcellular trafficking of corticosteroid receptors display distinct temporal dynamics in different limbic regions after behavioral stress. These heterogeneous effects could underlie contrasting regional responses to stress within the brain, and they highlight the importance for systems-level analysis of stress responsivity.

Behavioral and physiological responses of female prairie voles (Microtus ochrogaster) to various stressful conditions

Stressful life events elicit hypothalamic-pituitary-adrenal (HPA) axis activation, which may alter psychological states or behavioral routines. Therefore, the current study focused on the HPA axis response to better understand such manifestations in female prairie voles (Microtus ochrogaster). In Experiment 1, females were stressed for 1 h via one of the four stressors: exposure to a novel environment, immobilization ("plastic mesh"), brief social defeat, or prolonged social defeat. Following a 30-min recovery, the females received a 5-min elevated plus maze (EPM) test and, subsequently, blood was collected to measure plasma corticosterone concentrations. Only immobilization stress induced an anxiety-like behavioral response in the EPM test and elevated plasma corticosterone levels compared to the control groups. Corticosterone concentrations were also significantly elevated following exposure to prolonged social defeat compared to the control conditions, but not after novel environment stress or short social defeat. In Experiment 2, females were exposed to immobilization stress over 1, 3, or 7 days in a daily (predictable; pIMO) or irregular (unpredictable; uIMO) schedule. The biobehavioral stress response in females exposed to pIMO for 3 or 7 days did not differ significantly from controls, suggesting these females habituated. By comparison, females exposed to uIMO over 3 or 7 days did not habituate behaviorally or physiologically, even producing augmented corticosterone levels. In both experiments, positive correlations were found between corticosterone levels and anxiety-like behaviors in the EPM test. Together, our data suggest that the stress response by female prairie voles is dependent on stress intensity, source, previous experience, and predictability. Furthermore, the HPA axis response, as evident by corticosterone levels, is associated with the impact that these factors have on behavioral routine.

Inhibition of mTOR kinase via rapamycin blocks persistent predator stress-induced hyperarousal

Traumatic, stressful life events are thought to trigger acquired anxiety disorders such as post-traumatic stress disorder (PTSD). Recent data suggests that the mammalian target of rapamycin (mTOR) plays a key role in the formation of traumatic memories. The predator stress paradigm allows us to determine whether mTOR mediates the formation of both context-dependent (associative) and context-independent (non-associative) fear memories. Predator stress involves an acute, unprotected exposure of a rat to a cat which causes long-lasting non-associative fear memories manifested as generalized hyperarousal and increased anxiety-like behavior. Here, we show that rapamycin, an mTOR inhibitor, attenuates predator stress-induced hyperarousal, lasting at least three weeks. In addition, rapamycin blocks a subset of anxiety-like behaviors as measured in the elevated plus maze and hole board. Furthermore, when re-exposed to the predator stress context, rapamycin-treated stressed rats showed increased activity compared to vehicle controls suggesting that rapamycin blocks predator stress-induced associative fear memory. Taken together with past research, our results indicate that mTOR regulation of protein translation is required for the formation of both associative and non-associative fear memories. Overall, these data suggest that mTOR activation may contribute to the development of acquired anxiety disorders such as PTSD.

Not all stressors are equal: behavioral and endocrine evidence for development of contextual fear conditioning after a single session of footshocks but not of immobilization

Exposure of animals to footshocks (FS) in absence of any specific cue results in the development of fear to the compartment where shocks were given (contextual fear conditioning), and this is usually evaluated by time spent freezing. However, the extent to which contextual fear conditioning always develops when animals are exposed to other stressors is not known. In the present work we firstly demonstrated, using freezing, that exposure of adult rats to a single session of FS resulted in short-term and long-term contextual fear conditioning (freezing) that was paralleled by increased hypothalamic-pituitary-adrenal (HPA) activation. In contrast, using a similar design, no HPA or behavioral evidence for such conditioning was found after exposure to immobilization on boards (IMO), despite this stressor being of similar severity as FS on the basis of standard physiological measures of stress, including HPA activation. In a final experiment we directly compared the exposure to the two stressors in the same type of context and tested for the development of conditioning to the context and to a specific cue for IMO (the board). We observed the expected high levels of freezing and the conditioned HPA activation after FS, but not after IMO, regardless of the presence of the board during testing. Therefore, it can be concluded that development of fear conditioning to context or particular cues, as evaluated by either behavioral or endocrine measures, appears to be dependent on the nature of the aversive stimuli, likely to be related to biologically preparedness to establish specific associations.

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

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

fMRI fingerprint of unconditioned fear-like behavior in rats exposed to trimethylthiazoline

Unconditioned fear plays an important yet poorly understood role in anxiety disorders, and only few neuroimaging studies have focused on evaluating the underlying neuronal mechanisms. In rodents the predator odor trimethylthiazoline (TMT), a synthetic component of fox feces, is commonly used to induce states of unconditioned fear. In this study, arterial spin labeling-based functional magnetic resonance imaging (fMRI) was applied to detect TMT-induced regional modulations of neuronal activity in Wistar rats. During TMT exposure the rats displayed increased freezing behavior and reduced exploration in the odor-associated area. Neuronal activity was selectively increased in the dorsal periaqueductal gray, superior colliculus and medial thalamus and reduced in the median raphe, locus coeruleus, nucleus accumbens shell, ventral tegmental area, ventral pallidum and entorhinal piriform cortex. This fMRI fingerprint involving distinct neuronal pathways was used to describe a schematic model of fear processing. Key brain areas known to underlie fear and anxiety-related autonomic and behavioral responses as well as centers of motivational processing were identified as being part of this functional circuitry of innate fear. Thus, preclinical fMRI studies based on unconditioned fear methods may provide a valuable translational approach to better characterize etiological and pathological processes underlying anxiety disorders.

Stress-induced enhancement of fear conditioning and sensitization facilitates extinction-resistant and habituation-resistant fear behaviors in a novel animal model of posttraumatic stress disorder

Posttraumatic stress disorder (PTSD) is characterized by stress-induced symptoms including exaggerated fear memories, hypervigilance and hyperarousal. However, we are unaware of an animal model that investigates these hallmarks of PTSD especially in relation to fear extinction and habituation. Therefore, to develop a valid animal model of PTSD, we exposed rats to different intensities of footshock stress to determine their effects on either auditory predator odor fear extinction or habituation of fear sensitization. In Experiment 1, rats were exposed to acute footshock stress (no shock control, 0.4 mA, or 0.8 mA) immediately prior to auditory fear conditioning training involving the pairing of auditory clicks with a cloth containing cat odor. When presented to the conditioned auditory clicks in the next 5 days of extinction testing conducted in a runway apparatus with a hide box, rats in the two shock groups engaged in higher levels of freezing and head out vigilance-like behavior from the hide box than the no shock control group. This increase in fear behavior during extinction testing was likely due to auditory activation of the conditioned fear state because Experiment 2 demonstrated that conditioned fear behavior was not broadly increased in the absence of the conditioned auditory stimulus. Experiment 3 was then conducted to determine whether acute exposure to stress induces a habituation resistant sensitized fear state. We found that rats exposed to 0.8 mA footshock stress and subsequently tested for 5 days in the runway hide box apparatus with presentations of nonassociative auditory clicks exhibited high initial levels of freezing, followed by head out behavior and culminating in the occurrence of locomotor hyperactivity. In addition, Experiment 4 indicated that without delivery of nonassociative auditory clicks, 0.8 mA footshock stressed rats did not exhibit robust increases in sensitized freezing and locomotor hyperactivity, albeit head out vigilance-like behavior continued to be observed. In summary, our animal model provides novel information on the effects of different intensities of footshock stress, auditory-predator odor fear conditioning, and their interactions on facilitating either extinction-resistant or habituation-resistant fear-related behavior. These results lay the foundation for exciting new investigations of the hallmarks of PTSD that include the stress-induced formation and persistence of traumatic memories and sensitized fear.

Genetic predisposition to anxiety-related behavior predicts predator odor response

While rodents have a keen sense of smell and largely depend on olfactory cues for operating in their environment, most of the widely used tests to assess anxiety-related behavior largely ignore the olfactory system, being primarily based on fear of brightly lit, novel and open spaces. Here, we aimed at testing whether the genetic predisposition to anxiety predicts the predator odor response in mice. In the first experiment, using the 3-chamber avoidance test in CD-1 mice, trimethylthiazoline (TMT), a synthetic fox fecal odor, was shown to induce stronger behavioral and neuroendocrine effects than cat odor and butyric acid, respectively, and was therefore chosen as aversive odor for the following series of experiments. In this series, bidirectionally, selectively inbred CD-1 mice with either high (HAB), intermediate (NAB) or low (LAB) anxiety-related behavior responded differently to TMT, with HABs spending significantly less time than both NABs and LABs in the chamber harbouring the predator odor. Importantly, this result is not confounded by any deficit of the olfactory system, as LAB and NAB mice, while not or only moderately responding to TMT, responded to both the pleasant odor of female urine and the repugnant odor of butyric acid. Probably due to the strength of TMT, a similar increase in corticosterone levels upon predator odor exposure was observed in all three groups. Together, the results suggest that, dependent on the genetic predisposition to extremes in anxiety-related behavior, mice differentially interpret the odor of a potential predator, making this type of avoidance behavior highly predictable.

Glucocorticoids are required for extinction of predator stress-induced hyperarousal

BACKGROUND

The role of glucocorticoids in extinction of traumatic memories has not been fully characterized despite its potential as a therapeutic target for acquired posttraumatic stress disorder (PTSD). The predator stress paradigm allows us to determine whether glucocorticoids mediate the extinction of both context-dependent and context-independent fear memories.

METHODS

Male C57BL/6J mice were exposed to a predator (cat) then repeatedly exposed to the predator stress context in the absence of the cat. Context-dependent (associative) fear memory was assessed as suppression of activity during re-exposure to the predator stress context without the cat (extinction trials). Context-independent fear (non-associative) was assessed seven days after extinction trials using measures of hyperarousal and anxiety-like behaviours in environments unlike the predator stress context. To assess the role of glucocorticoids, mice were injected with metyrapone (50mg/kg) 90 min prior to extinction trials in predator stressed mice and context-dependent and context-independent fear memories were assessed. Finally, metyrapone-treated predator stressed mice were injected with corticosterone (5 or 10mg/kg) immediately following extinction trials and context-dependent and context-independent fear memories were assessed.

RESULTS

Repeated re-exposure to the predator stress context without the cat present extinguished context-dependent fear memory, and also reduced hyperarousal, a generalized, chronic PTSD-like symptom. We show that extinction of context-independent predator stress-induced hyperarousal is dependent on endogenous glucocorticoids during the extinction trials. Furthermore, the inhibition of extinction by metyrapone on startle amplitude was reduced by exogenous administration of corticosterone following extinction trials. Overall, these data implicate glucocorticoids in the extinction of hyperarousal, a core symptom of PTSD.

Behavioral and endocrine consequences of simultaneous exposure to two different stressors in rats: interaction or independence?

Although behavioral and endocrine consequences of acute exposure to stressors have been extensively studied, little is known about how simultaneous exposure to two different stressors interacts to induce short- and long-term effects. In the present experiment we studied this interaction in adult male rats exposed to cat fur odor (impregnated cloth) or immobilization on boards either separately or simultaneously. We reasoned that exposure to the odor of a potential predator while immobilized, may potentiate its negative consequences as compared to exposure to only one of the stressors. Exposure to cat odor elicited the expected reduction of activity and avoidance of the area where the impregnated cloth was located. The endocrine response (plasma levels of ACTH and corticosterone, as a measure of the hypothalamic-pituitary-adrenal axis, HPA) was markedly greater after immobilization than after cat fur odor and no additive effects were found by simultaneous exposure to both stressors. Cat odor, but not immobilization, increased anxiety-like behavior as evaluated in the elevated plus-maze 7 days after the stressors, with no evidence of enhanced HPA activation. In addition, cat odor exposure resulted in long-lasting (8 days later) fear conditioning to the box containing a clean cloth, which was reflected by hypoactivity, avoidance of the cloth area and enhanced HPA activation. All these effects were similarly observed in rats exposed simultaneously to cat odor and immobilization. In rats only exposed to immobilization, only some weak behavioral signs of fear conditioning were found, but HPA activation in response to the context paired to immobilization was enhanced to the same extent as in cat odor-exposed animals, supporting a certain degree of endocrine conditioning. The present results did not reveal important behavioral interactions between the two stressors when animals experienced both simultaneously, whereas some interactions were found regarding HPA activation. Theoretical implications are discussed.

Inescapable but not escapable stress leads to increased struggling behavior and basolateral amygdala c-fos gene expression in response to subsequent novel stress challenge

Control over an aversive experience can greatly impact the organism's response to subsequent stressors. We compared the effects of escapable (ES) and yoked inescapable (IS) electric tail shocks on the hypothalamic-pituitary-adrenal (HPA) axis hormonal (corticosterone and adrenocorticotropic hormone (ACTH)), neural (c-fos mRNA) and behavioral (struggling) response to subsequent restraint. We found that although the HPA axis response during restraint of both previously stressed groups were higher than stress-naïve rats and not different from each other, lack of control over the tailshock experience led to an increase in restraint-induced struggling behavior of the IS rats compared to both stress-naïve and ES rats. Additionally, c-fos expression in the basolateral amygdala was increased selectively in the IS group, and relative c-fos mRNA expression in the basolateral amygdala positively correlated with struggling behavior. Restraint-induced c-fos expression in the medial prefrontal cortex, a brain area critical for mediating some of the differential neurochemical and behavioral effects of ES and IS, was surprisingly similar in both ES and IS groups, lower than that of stress-naïve rats, and did not correlate with struggling behavior. Our findings indicate that basolateral amygdala activity may be connected with the differential effects of ES and IS on subsequent behavioral responses to restraint, without contributing to the concurrent HPA axis hormone response.

A single footshock causes long-lasting hypoactivity in unknown environments that is dependent on the development of contextual fear conditioning

Exposure to a single session of footshocks induces long-lasting inhibition of activity in unknown environments that markedly differ from the shock context. Interestingly, these effects are not necessarily associated to an enhanced anxiety and interpretation of this hypoactivity remains unclear. In the present experiment we further studied this phenomenon in male Sprague-Dawley rats. In a first experiment, a session of three shocks resulted in hypoactivity during exposure, 6-12days later, to three different unknown environments. This altered behaviour was not accompanied by a greater hypothalamic-pituitary-adrenal (HPA) activation, although greater HPA activation paralleling higher levels of freezing was observed in the shock context. In a second experiment we used a single shock and two procedures, one with pre-exposure to the context before the shock and another with immediate shock that did not induce contextual fear conditioning. Hypoactivity and a certain level of generalization of fear (freezing) to the unknown environments only appeared in the group that developed fear conditioning, but no evidence for enhanced anxiety in the elevated plus-maze was found in any group. The results suggest that if animals are able to associate an aversive experience with a distinct unknown environment, they would display more cautious behaviour in any unknown environment and such strategy persists despite repeated experience with different environments. This long-lasting cautious behaviour was not associated to greater HPA response to the unknown environment that was however observed in the shock context. The present findings raised some concerns about interpretation of long-lasting behavioural changes caused by brief stressors.

Ability of predator odour exposure to elicit conditioned versus sensitised post traumatic stress disorder-like behaviours, and forebrain deltaFosB expression, in rats

At present, exposure of a rodent to the odour of a predator is one of the most common animal models of post traumatic stress disorder (PTSD). Despite this, the model remains incompletely characterized, particularly in regard to within subject assessment of major PTSD-like behaviours. In an attempt to redress this situation, we have extensively characterized the two broad categories of behaviour that are considered to characterize PTSD, that is sensitized behaviours such as social withdrawal and hypervigilance and conditioned behaviours such as avoidance of trauma linked cues. Specifically, we determined the presence and duration of both conditioned and sensitized behaviours, in the same cohort of animals, after three exposures to predator odour. Conditioned fear was assessed on the basis of inhibition of locomotor activity upon return to context 2, 7, 14, 21, and 28 days after the last odour exposure session. To assess the impact on sensitization behaviours, we monitored acoustic startle responses and social interaction behaviour 4, 9, 16, 23, and 30 days after the last exposure session. In addition to examining the behavioural consequences associated with odour exposure, we also determined the key brain regions that were activated using DeltaFosB immunohistochemistry. Our results show that the two groups of behaviours thought to characterize PTSD (conditioned and sensitized) do not travel together in the predator odour model, with clear evidence of enduring changes in conditioned fear but little evidence of changes in social interaction or acoustic startle. With regard to associated patterns of activity in the brain, we observed that odour-exposed animals exhibited significantly higher numbers of FosB-positive nuclei in only the medial prefrontal cortex (mPFC), a finding that can be viewed as being consistent with the observed behavioural changes.

Do odors from different cats induce equivalent unconditioned and conditioned responses in rats?

Since cats are predators of rats and mice, interest in the consequences of exposure to cat-associated odors has increased in the last decade, particularly regarding the development of putative animal models of post-traumatic stress disorder (PTSD). Although in the literature there are some comments on the variability of the effects depending on the individual cat used, there are no reports on this subject. In the present study, we demonstrated, using male Sprague-Dawley rats and cloths impregnated with fur/skin odors from three different cats (one ovariectomized female and two intact males), that the unconditioned endocrine (release of corticosterone) and behavioral (inhibition of activity and avoidance of the cloth area) responses to the presence of the cat odors were statistically significant and similar among the three cats. The conditioned behavioral response to a clean cloth, studied 7 days after the initial exposure to odors, was also evident with the three cats, with minor differences among them. In contrast, only the fur odor from one of the male cats clearly increased anxiety-like behavior in the elevated plus-maze 6 days later. These data indicate that: (i) evaluation of unconditioned and conditioned responses to cat odors does not appear to strongly predict long-lasting increases in anxiety-like behavior; and, therefore, both types of responses are partially dissociated; and (ii) differences among cats mainly affected the induction of long-lasting changes in anxiety-like behavior. The ultimate reasons for these differences are not known, but their characterization is critical for a proper understanding of putative PTSD models.

Cat odor causes long-lasting contextual fear conditioning and increased pituitary-adrenal activation, without modifying anxiety

A single exposure to a cat or cat odors has been reported by some groups to induce contextual and auditory fear conditioning and long-lasting changes in anxiety-like behaviour, but there is no evidence for parallel changes in biological stress markers. In the present study we demonstrated in male rats that exposure to a novel environment containing a cloth impregnated with cat fur odor resulted in avoidance of the odor, lower levels of activity and higher pituitary-adrenal (PA) response as compared to those exposed to the novel environment containing a clean cloth, suggesting increased levels of stress in the former animals. When re-exposed 9 days later to the same environment with a clean cloth, previously cat fur exposed rats again showed avoidance of the cloth area and lower levels of activity, suggesting development of contextual fear conditioning, which again was associated with a higher PA activation. In contrast, unaltered both anxiety-like behaviour and PA responsiveness to an elevated plus-maze were found 7 days after cat odor exposure. It is concluded that: (i) PA activation is able to reflect both the stressful properties of cat fur odor and odor-induced contextual fear conditioning; (ii) development of cat odor-induced contextual fear conditioning is independent of the induction of long-lasting changes in anxiety-like behaviour; and (iii) greater PA activation during exposure to the odor context is not explained by non-specific sensitization of the PA axis caused by previous exposure to cat fur odor.