Genetic predisposition and the development of posttraumatic stress disorder in an animal model

Maladaptation of dentate gyrus mossy cells mediates contextual discrimination deficit after traumatic stress

Fear overgeneralization is a maladaptive response to traumatic stress that is associated with the inability to discriminate between threat and safety contexts, a hallmark feature of post-traumatic stress disorder (PTSD). However, the neural mechanisms underlying this deficit remain unclear. Here, we show that traumatic stress exposure impairs contextual discrimination between threat and safety contexts in the learned helplessness (LH) model. Mossy cells (MCs) in the dorsal hippocampus are suppressed in response to traumatic stress. Bidirectional manipulation of MC activity in the LH model reveals that MC inhibition is causally linked to impaired contextual discrimination. Mechanistically, MC inhibition increases the number of active granule cells in a given context, significantly overlapping context-specific ensembles. Our study demonstrates that maladaptive inhibition of MCs after traumatic stress is a substantial mechanism underlying fear overgeneralization with contextual discrimination deficit, suggesting a potential therapeutic target for cognitive symptoms of PTSD.

Brain Environment Interactions: Stress, Posttraumatic Stress Disorder, and the Need for a Postmortem Brain Collection

Stress, especially the extreme stress of traumatic events, can alter both neurobiology and behavior. Such extreme environmental situations provide a useful model for understanding environmental influences on human biology and behavior. This paper will review some of the evidence of brain alterations that occur with exposure to environmental stress. This will include recent studies using neuroimaging and will address the need for histological confirmation of imaging study results. We will review the current scientific approaches to understanding brain environment interactions, and then make the case for the collection and study of postmortem brain tissue for the advancement of our understanding of the effects of environment on the brain.Creating a brain tissue collection specifically for the investigation of the effects of extreme environmental stressors fills a gap in the current research; it will provide another of the important pieces to the puzzle that constitutes the scientific investigation of negative effects of environmental exposures. Such a resource will facilitate new discoveries related to the psychiatric illnesses of acute stress disorder and posttraumatic stress disorder, and can enable scientists to correlate structural and functional imaging findings with tissue abnormalities, which is essential to validate the results of recent imaging studies.

Animal models of PTSD: a challenge to be met

Recent years have seen increased interest in psychopathologies related to trauma exposure. Specifically, there has been a growing awareness to posttraumatic stress disorder (PTSD) in part due to terrorism, climate change-associated natural disasters, the global refugee crisis, and increased violence in overpopulated urban areas. However, notwithstanding the increased awareness to the disorder, the increasing number of patients, and the devastating impact on the lives of patients and their families, the efficacy of available treatments remains limited and highly unsatisfactory. A major scientific effort is therefore devoted to unravel the neural mechanisms underlying PTSD with the aim of paving the way to developing novel or improved treatment approaches and drugs to treat PTSD. One of the major scientific tools used to gain insight into understanding physiological and neuronal mechanisms underlying diseases and for treatment development is the use of animal models of human diseases. While much progress has been made using these models in understanding mechanisms of conditioned fear and fear memory, the gained knowledge has not yet led to better treatment options for PTSD patients. This poor translational outcome has already led some scientists and pharmaceutical companies, who do not in general hold opinions against animal models, to propose that those models should be abandoned. Here, we critically examine aspects of animal models of PTSD that may have contributed to the relative lack of translatability, including the focus on the exposure to trauma, overlooking individual and sex differences, and the contribution of risk factors. Based on findings from recent years, we propose research-based modifications that we believe are required in order to overcome some of the shortcomings of previous practice. These modifications include the usage of animal models of PTSD which incorporate risk factors and of the behavioral profiling analysis of individuals in a sample. These modifications are aimed to address factors such as individual predisposition and resilience, thus taking into consideration the fact that only a fraction of individuals exposed to trauma develop PTSD. We suggest that with an appropriate shift of practice, animal models are not only a valuable tool to enhance our understanding of fear and memory processes, but could serve as effective platforms for understanding PTSD, for PTSD drug development and drug testing.

Logical fallacies in animal model research

Background

Animal models of human behavioural deficits involve conducting experiments on animals with the hope of gaining new knowledge that can be applied to humans. This paper aims to address risks, biases, and fallacies associated with drawing conclusions when conducting experiments on animals, with focus on animal models of mental illness.

Conclusions

Researchers using animal models are susceptible to a fallacy known as false analogy, where inferences based on assumptions of similarities between animals and humans can potentially lead to an incorrect conclusion. There is also a risk of false positive results when evaluating the validity of a putative animal model, particularly if the experiment is not conducted double-blind. It is further argued that animal model experiments are reconstructions of human experiments, and not replications per se, because the animals cannot follow instructions. This leads to an experimental setup that is altered to accommodate the animals, and typically involves a smaller sample size than a human experiment. Researchers on animal models of human behaviour should increase focus on mechanistic validity in order to ensure that the underlying causal mechanisms driving the behaviour are the same, as relying on face validity makes the model susceptible to logical fallacies and a higher risk of Type 1 errors. We discuss measures to reduce bias and risk of making logical fallacies in animal research, and provide a guideline that researchers can follow to increase the rigour of their experiments.

Systems biology approach to understanding post-traumatic stress disorder

This review paper presents known biological facts about PTSD and the mathematical/systems biology tools used to understand the underpinning molecular principles.

Relation between corticosterone and fear-related behavior in mice selectively bred for high or low alcohol preference

Blunted cortisol responses to stress or trauma have been linked with genetic (familial) risk for both alcoholism and post-traumatic stress disorder (PTSD). Mouse lines selectively bred for high (HAP) or low (LAP) alcohol preference may be a relevant model of genetic risk for co-morbid alcoholism and PTSD in humans. HAP mice show greater fear-potentiated startle (FPS), a model used to study PTSD, than LAP mice. The relation between corticosterone (CORT) and FPS behavior was explored in four experiments. Naïve male and female HAP2 and LAP2 mice received fear-conditioning or control treatments, and CORT levels were measured before and immediately after fear-conditioning or FPS testing. In two other experiments, HAP2 mice received CORT (1.0, 5.0 or 10.0 mg/kg) or a glucocorticoid receptor antagonist (mifepristone; 25.0 and 50.0 mg/kg) 30 minutes before fear conditioning. HAP2 mice exposed to fear conditioning and to control foot shock exposures showed lower CORT after the fear-conditioning and FPS testing sessions than LAP2 mice. A trend toward higher FPS was seen in HAP2 mice pretreated with 10.0 mg/kg CORT, and CORT levels were the lowest in this group, suggesting negative feedback inhibition of CORT release. Mifepristone did not alter FPS. Overall, these results are consistent with data in humans and rodents indicating that lower cortisol/CORT levels after stress are associated with PTSD/PTSD-like behavior. These findings in HAP2 and LAP2 mice suggest that a blunted CORT response to stress may be a biological marker for greater susceptibility to develop PTSD in individuals with increased genetic risk for alcoholism.

Immune status influences fear and anxiety responses in mice after acute stress exposure

Significant evidence suggests that exposure to traumatic and/or acute stress in both mice and humans results in compromised immune function that in turn may affect associated brain processes. Additionally, recent studies in mouse models of immune deficiency have suggested that adaptive immunity may play a role during traumatic stress exposure and that impairments in lymphocyte function may contribute to increased susceptibility to various psychogenic stressors. However, rodent studies on the relationship between maladaptive stress responses and lymphocyte deficiency have been complicated by the fact that genetic manipulations in these models may also result in changes in CNS function due to the expression of targeted genes in tissues other than lymphocytes, including the brain. To address these issues we utilized mice with a deletion of recombination-activating gene 2 (Rag2), which has no confirmed expression in the CNS; thus, its loss should result in the absence of mature lymphocytes without altering CNS function directly. Stress responsiveness of immune deficient Rag2(-/-) mice on a BALB/c background was evaluated in three different paradigms: predator odor exposure (POE), fear conditioning (FC) and learned helplessness (LH). These models are often used to study different aspects of stress responsiveness after the exposure to an acute stressor. In addition, immunoblot analysis was used to assess hippocampal BDNF expression under both stressed and non-stressed conditions. Subsequent to POE, Rag2(-/-) mice exhibited a reduced acoustic startle response compared to BALB/c mice; no significant differences in behavior were observed in either FC or LH. Furthermore, analysis of hippocampal BDNF indicated that Rag2(-/-) mice have elevated levels of the mature form of BDNF compared to BALB/c mice. Results from our studies suggest that the absence of mature lymphocytes is associated with increased resilience to stress exposure in the POE and does not affect behavioral responses in the FC and LH paradigms. These findings indicate that lymphocytes play a specific role in stress responsiveness dependent upon the type, nature and intensity of the stressor.

Negative learning bias is associated with risk aversion in a genetic animal model of depression

The lateral habenula (LHb) is activated by aversive stimuli and the omission of reward, inhibited by rewarding stimuli and is hyperactive in helpless rats—an animal model of depression. Here we test the hypothesis that congenital learned helpless (cLH) rats are more sensitive to decreases in reward size and/or less sensitive to increases in reward than wild-type (WT) control rats. Consistent with the hypothesis, we found that cLH rats were slower to switch preference between two responses after a small upshift in reward size on one of the responses but faster to switch their preference after a small downshift in reward size. cLH rats were also more risk-averse than WT rats—they chose a response delivering a constant amount of reward (“safe” response) more often than a response delivering a variable amount of reward (“risky” response) compared to WT rats. Interestingly, the level of bias toward negative events was associated with the rat's level of risk aversion when compared across individual rats. cLH rats also showed impaired appetitive Pavlovian conditioning but more accurate responding in a two-choice sensory discrimination task. These results are consistent with a negative learning bias and risk aversion in cLH rats, suggesting abnormal processing of rewarding and aversive events in the LHb of cLH rats.

Long-lasting marked inhibition of periaqueductal gray-evoked defensive behaviors in inescapably-shocked rats

Clinical evidence suggests that depression and trauma predispose the subject to panic. Accordingly, here we examined the late effects of uncontrollable stress, a presumptive model of depression and/or traumatic disorder, on panic-like behaviors evoked by electrical stimulation of the dorsal periaqueductal gray (DPAG). Changes in anxiety and depression were also assessed in the elevated plus-maze (EPM) and forced-swimming test (FST), respectively. Rats with electrodes in the DPAG were subjected to a 7-day shuttle-box one-way escape yoked training with foot-shocks either escapable (ES) or inescapable (IS). The day after the end of one-way escape training, rats were trained in a two-way escape novel task (test-session) to ascertain the effectiveness of uncontrollable stress. DPAG stimulations were carried out in an open field, both before the escape training and 2 and 7 days after it, and EPM and FST were performed on the 8th and 10th days afterwards, respectively. Controls were either trained with fictive shocks (FS) or subjected to intracranial stimulations only. Although the ES rats performed significantly better than the IS group in the two-way escape task, groups did not differ with respect to either the anxiety or depression scores. Unexpectedly, however, IS rats showed a marked attenuation of DPAG-evoked freezing and flight behaviors relative to both the ES and FS groups, 2 and 7 days after one-way escape training. The conjoint inhibition of passive (freezing) and active (flight) defensive behaviors suggests that IS inhibits a DPAG in-built motivational system that may be implicated in depressed patients' difficulties in coping with daily-life stress.

Classical and novel approaches to the preclinical testing of anxiolytics: A critical evaluation

Over 80% of current anxiety studies employ one of the tests that were developed earlier than, or concurrently with the elevated plus-maze, i.e. before 1985. Considering 1985 as a historical reference point, we briefly review here 115 new tests and models of anxiety, the development of which was likely prompted by the poor predictive validity of classical tests as shown here by the comparison of preclinical and clinical findings with putative novel anxiolytics. The new approaches comprise major innovations to classical tests, the pre-test application of manipulations that mimic etiological factors of anxiety disorders, and entirely new approaches including anxiety disorder-specific tests. Thus, intensive test development over the last 27 years created a large pool of novel approaches. However, these are infrequently used and as such, their impact on anxiolytic drug development remains low. We suggest here that test/model development should step over the intensive phase when several new methods are proposed each year and should start selecting and establishing the methodologies that would successfully replace or complement classical tests. We propose here a novel strategy for improving the validity of anxiety testing that includes the retrospective analysis of the predictive validity of new procedures (as opposed to classical pharmacological validation), and a call for concerted international efforts at both the conceptual and practical levels. Similar endeavors proved recently successful with other psychiatric disorders.

Lasting consequences of traumatic events on behavioral and skeletal parameters in a mouse model for post-traumatic stress disorder (PTSD)

BACKGROUND

Post-traumatic stress disorder (PTSD) is an anxiety disorder that not only affects mental health, but may also affect bone health. However, there have been no studies to examine the direct relationship between PTSD and bone.

METHODOLOGY/PRINCIPAL FINDINGS

We employed electric shocks in mice to simulate traumatic events that cause PTSD. We also injected the anxiogenic drug FG-7142 prior to electric shocks. Electric shocks created lasting conditioned fear memory in all mice. In young mice, electric shocks elicited not only behavioral response but also skeletal response, and injection of FG-7142 appeared to increase both types of response. For example in behavioral response within the first week, mice shocked alone froze an average of 6.2 sec in 10 sec tests, and mice injected with FG-7142 froze 7.6 sec, both significantly different (P<0.05) from control mice, which only froze 1.3 sec. In skeletal response at week 2, shocks alone reduced 6% bone mineral content (BMC) in total body (P = 0.06), while shocks with FG-7142 injection reduced not only 11% BMC (P<0.05) but also 6% bone mineral density (BMD) (P<0.05). In addition, FG-7142 injection also caused significant reductions of BMC in specific bones such as femur, lumbar vertebra, and tibia at week 3. Strong negative correlations (R(2) = -0.56, P<0.05) and regression (y = 0.2527-0.0037 * x, P<0.01) between freezing behavior and total body BMC in young mice indicated that increased contextual PTSD-like behavior was associated with reduced bone mass acquisition.

CONCLUSIONS/SIGNIFICANCE

This is the first study to document evidence that traumatic events induce lasting consequences on both behavior and skeletal growth, and electric shocks coupled with injection of anxiogenic FG-7142 in young mice can be used as a model to study the effect of PTSD-like symptoms on bone development.

Overlapping neurobiology of learned helplessness and conditioned defeat: implications for PTSD and mood disorders

Exposure to traumatic events can increase the risk for major depressive disorder (MDD) as well as posttraumatic stress disorder (PTSD), and pharmacological treatments for these disorders often involve the modulation of serotonergic (5-HT) systems. Several behavioral paradigms in rodents produce changes in behavior that resemble symptoms of MDD and these behavioral changes are sensitive to antidepressant treatments. Here we review two animal models in which MDD-like behavioral changes are elicited by exposure to an acute traumatic event during adulthood, learned helplessness (LH) and conditioned defeat. In LH, exposure of rats to inescapable, but not escapable, tailshock produces a constellation of behavioral changes that include deficits in fight/flight responding and enhanced anxiety-like behavior. In conditioned defeat, exposure of Syrian hamsters to a social defeat by a more aggressive animal leads to a loss of territorial aggression and an increase in submissive and defensive behaviors in subsequent encounters with non-aggressive conspecifics. Investigations into the neural substrates that control LH and conditioned defeat revealed that increased 5-HT activity in the dorsal raphe nucleus (DRN) is critical for both models. Other key brain regions that regulate the acquisition and/or expression of behavior in these two paradigms include the basolateral amygdala (BLA), central nucleus of the amygdala (CeA) and bed nucleus of the stria terminalis (BNST). In this review, we compare and contrast the role of each of these neural structures in mediating LH and conditioned defeat, and discuss the relevance of these data in developing a better understanding of the mechanisms underlying trauma-related depression. This article is part of a Special Issue entitled 'Post-Traumatic Stress Disorder'.

Metabolic mapping of the effects of the antidepressant fluoxetine on the brains of congenitally helpless rats

Antidepressants require adaptive brain changes before efficacy is achieved, and they may impact the affectively disordered brain differently than the normal brain. We previously demonstrated metabolic disturbances in limbic and cortical regions of the congenitally helpless rat, a model of susceptibility to affective disorder, and we wished to test whether administration of fluoxetine would normalize these metabolic differences. Fluoxetine was chosen because it has become a first-line drug for the treatment of affective disorders. We hypothesized that fluoxetine antidepressant effects may be mediated by decreasing metabolism in the habenula and increasing metabolism in the ventral tegmental area. We measured the effects of fluoxetine on forced swim behavior and regional brain cytochrome oxidase activity in congenitally helpless rats treated for 2 weeks with fluoxetine (5mg/kg, i.p., daily). Fluoxetine reduced immobility in the forced swim test as anticipated, but congenitally helpless rats responded in an atypical manner, i.e., increasing climbing without affecting swimming. As hypothesized, fluoxetine reduced metabolism in the habenula and increased metabolism in the ventral tegmental area. In addition, fluoxetine reduced the metabolism of the hippocampal dentate gyrus and dorsomedial prefrontal cortex. This study provided the first detailed mapping of the regional brain effects of an antidepressant drug in congenitally helpless rats. All of the effects were consistent with previous studies that have metabolically mapped the effects of serotonergic antidepressants in the normal rat brain, and were in the predicted direction of metabolic normalization of the congenitally helpless rat for all affected brain regions except the prefrontal cortex.

Heightened muscle tension and diurnal hyper-vigilance following exposure to a social defeat-conditioned odor cue in rats

Patients with post-traumatic stress disorder (PTSD) exhibit exaggerated daytime muscle tension as well as nocturnal sleep disturbances. Yet, these physiological and behavioral features of the disorder are little studied in animal models of PTSD. Accordingly, the present studies were designed to assess alterations in muscle tension and diurnal hyper-vigilance resulting from exposure to a social defeat stressor paired with an olfactory stimulus, which was then used as a reminder of stressor exposure. In the first series of experiments, rats presented with an olfactory cue paired previously with a single social defeat exhibited a significant increase in muscle tension 4 weeks following defeat. In the second series of experiments, an olfactory cue paired previously with a single social defeat induced a significant increase in locomotor activity among quiescent rats 4 weeks following stressor exposure. The present results thus support the a priori hypotheses that novel physiological and behavioral hallmarks of PTSD can be documented in an animal model of the disorder and that the present overt signs of reactive hyper-vigilance can be triggered by reintroduction of an olfactory stimulus present at the time of initial trauma exposure.

Plasma transcortin influences endocrine and behavioral stress responses in mice

Glucocorticoids are released after hypothalamus-pituitary-adrenal axis stimulation by stress and act both in the periphery and in the brain to bring about adaptive responses that are essential for life. Dysregulation of the stress response can precipitate psychiatric diseases, in particular depression. Recent genetic studies have suggested that the glucocorticoid carrier transcortin, also called corticosteroid-binding globulin (CBG), may have an important role in stress response. We have investigated the effect of partial or total transcortin deficiency using transcortin knockout mice on hypothalamus-pituitary-adrenal axis functioning and regulation as well as on behaviors linked to anxiety and depression traits in animals. We show that CBG deficiency in mice results in markedly reduced total circulating corticosterone at rest and in response to stress. Interestingly, free corticosterone concentrations are normal at rest but present a reduced surge after stress in transcortin-deficient mice. No differences were detected between transcortin-deficient mice for anxiety-related traits. However, transcortin-deficient mice display increased immobility in the forced-swimming test and markedly enhanced learned helplessness after prolonged uncontrollable stress. The latter is associated with an approximately 30% decrease in circulating levels of free corticosterone as well as reduced Egr-1 mRNA expression in hippocampus in CBG-deficient mice. Additionally, transcortin-deficient mice show no sensitization to cocaine-induced locomotor responses, a well described corticosterone-dependent test. Thus, transcortin deficiency leads to insufficient glucocorticoid signaling and altered behavioral responses after stress. These findings uncover the critical role of plasma transcortin in providing an adequate endocrine and behavioral response to stress.

Treatment with escitalopram but not desipramine decreases escape latency times in a learned helplessness model using juvenile rats

RATIONALE

The pharmacological treatment of depression in children and adolescents is different from that of adults due to the lack of efficacy of certain antidepressants in the pediatric age group. Our current understanding of why these differences occur is very limited.

OBJECTIVES

To develop more effective treatments, a juvenile animal model of depression was tested to validate it as a possible model to specifically study pediatric depression.

MATERIALS AND METHODS

Procedures for use with juvenile rats at postnatal day (PND) 21 and 28 were adapted from the adult learned helplessness model in which, 24 h after exposure to inescapable stress, animals are unable to remove themselves from an easily escapable stressor. Rats were treated for 7 days with either the selective serotonin reuptake inhibitor escitalopram at 10 mg/kg or the tricyclic antidepressant desipramine at 3, 10, or 15 mg/kg to determine if treatment could decrease escape latency times.

RESULTS

Escitalopram treatment was effective at decreasing escape latency times in all ages tested. Desipramine treatment did not decrease escape latency times for PND 21 rats, but did decrease times for PND 28 and adult animals.

CONCLUSIONS

The learned helplessness model with PND 21 rats predicts the efficacy of escitalopram and the lack of efficacy of desipramine seen in the treatment of pediatric depression. These findings suggest that the use of PND 21 rats in a modified learned helplessness procedure may be a valuable model of human pediatric depression that can predict pediatric antidepressant efficacy and be used to study antidepressant mechanisms involved in pediatric depression.

Predatory stress induces hippocampal cell death by apoptosis in rats

It is commonly accepted that stress may induce brain damage, especially in hippocampal formation, but the mechanisms that cause such damage are not well understood. This paper investigated the impacts of predatory stress on the hippocampus using cat-exposure to rats. The rats were divided randomly into single stress, 2-, 4-week stress and control group. ISEL and electronic microscope were employed to confirm the occurrence of apoptosis in hippocampus. Our results showed that repeated predatory stresses may result in a pattern of intensive behavioral and endocrinal responses, and lead to the neuronal cell loss in hippocampus. ISEL results displayed that there are some ISEL-positive cells with characteristic apoptotic ultrastructural changes found most in hippocampal CA3 and CA1 in the repeated stressed rats. These results indicate that apoptosis may be one of the most important neuropathological mechanisms for cell loss or hippocampal atrophy induced by predatory stress. Meanwhile, significant positive correlation between serum cortisol level and the number of apoptotic cells in CA3 supports that excessive GCs due to predatory stress, is associated with hippocampal cell apoptosis.

Establishing an agenda for translational research on PTSD

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

Irt>t schedule controlled behavior in ‘learned-helpless’ rats: Effects from a cannabinoid agonist

Human depression is partly a congenital disorder. Aspects of the behavior accompanying depression can be magnified by genetic manipulation of bred animal species. Learned Helplessness (LH) is a trait-mark behavior that successfully breeds in rodents. Here, 'congenital' LH (cLH) rats were trained to recognize and respond to 12s long interval cues (irt>12s schedule). Rats compliant to an irt>t schedule will space responses evenly and respond rhythmically. Irt>t schedule derived data are plotted in histograms showing irt (interresponse time) frequencies. A pause response peak emerges, for outbred rats, at irt values approximating the minimum interval for reinforcement. cLH rats [n=9] complied poorly to schedule contingencies when diluent (vehicle) was injected before testing. Moderate and high dose injections of a CB 1 receptor selective agonist drug (AM 411), however, increased operant schedule compliance and normalized the cLH rats' irt>t histogram distributions. Performance indicators for cLH rats are presented alongside coordinate measures from a comparison group [n=5] of normally bred Sprague-Dawley (SD) rats. In both cLH and SD rats, treatment session histograms revealed shifts of the pause response peak not accompanied by a change in motor responsiveness. The irt>12s histogram shifts were absent when AM 411 dosages were arranged to follow pre-medication injections of a CB 1 receptor selective antagonist drug (AM 251). In short, AM 411 increased timing acuity in rats prone to behavioral despair but had opposite timing effects in normally bred SD rats.

Behavioral characteristics of rats predisposed to learned helplessness: reduced reward sensitivity, increased novelty seeking, and persistent fear memories

The congenitally helpless rat strain, which was selectively bred for increased susceptibility to learned helplessness, may model the predisposition to affective disorders, including depression and post-traumatic stress disorder. Other than the selected trait, the behavior of this strain is not well characterized. In this study, we assessed congenitally helpless rats on several behavioral tests. First, we assessed reward sensitivity by measuring their consumption of a 5% sucrose solution. Next, we assessed exploratory behavior and fearfulness in both a novel and familiar open field, and in a light-dark test. Finally, we assessed fear conditioning by exposing the animals to 4 tone-shock pairs on 1 day (acquisition) and then presenting 60 tones over the next 2 days (extinction). Compared to normal Sprague-Dawley controls, congenitally helpless rats showed less consumption of the sucrose solution and more exploratory behavior in the novel, but not the familiar, open fields. They also showed less fearfulness in the light-dark test, but more conditioned freezing to the tone predicting shock. Moreover, this freezing was resistant to extinction; congenitally helpless rats not only failed to show a fear decrement during extinction, but actually showed increased fear, a phenomenon termed "paradoxical enhancement." Thus, congenitally helpless rats appear to have a behavioral phenotype characterized by reduced sensitivity to reward, increased drive to explore novel environments, and increased propensity to form and maintain fear-associated memories. This behavioral phenotype is discussed as resembling the personality of humans vulnerable to post-traumatic stress disorder.

Behavioral correlates of differences in neural metabolic capacity

Cytochrome oxidase is a rate-limiting enzyme in oxidative phosphorylation, the major energy-synthesizing pathway used by the central nervous system, and cytochrome oxidase histochemistry has been extensively utilized to map changes in neural metabolism following experimental manipulations. However, the value of cytochrome oxidase activity in predicting behavior has not been analyzed. We argue that this endeavor is important because genetic composition and embryonic environment can engender differences in baseline neural metabolism in pertinent neural circuits, and these differences could represent differences in the degree to which specific behaviors are 'primed.' Here we review our studies in which differences in cytochrome oxidase activity and in behavior were studied in parallel. Using mammalian and reptilian models, we find that embryonic experiences that shape the propensity to display social behaviors also affect cytochrome oxidase activity in limbic brain areas, and elevated cytochrome oxidase activity in preoptic, hypothalamic, and amygdaloid nuclei correlates with heightened aggressive and sexual tendencies. Selective breeding regimes were used to create rodent genetic lines that differ in their susceptibility to display learned helplessness and in behavioral excitability. Differences in cytochrome oxidase activity in areas like the paraventricular hypothalamus, frontal cortex, habenula, septum, and hippocampus correlate with differences in susceptibility to display learned helplessness, and differences in activity in the dentate gyrus and perirhinal and posterior parietal cortex correlate with differences in hyperactivity. Thus, genetic and embryonic manipulations that engender specific behavioral differences produce specific neurometabolic profiles. We propose that knowledge of neurometabolic differences can yield valuable predictions about behavioral phenotype in other systems.

Long-term behavioural alterations in female rats after a single intense footshock followed by situational reminders

Post-traumatic stress disorder (PTSD) affects a vulnerable sub-population of individuals exposed to a traumatic event. This psychopathology induces long-lasting hypothalamo-pituitary-adrenal (HPA) axis hypoactivity, hyperarousal and avoidance of trauma-like situation. PTSD also manifests a high co-morbidity with anxiety disorders. The aim of the present study was to characterise long-term biobehavioural alterations in female rats in an animal model of PTSD consisting in an intense footshock (2 mA, 10s) followed by three weekly situational reminders. This procedure induced several long-term alterations: increased anxiety behaviour, reduced time spent in an 'aversive-like' context, altered social behaviour and blunted corticosterone response to stress. These results demonstrate that exposure to an intense footshock associated with repeated situational reminders elicited long-term disturbances which lasted more than 1 month after the footshock administration. Our findings suggest that this paradigm could provide a useful animal model of PTSD.

Functional connectivity of dissociative responses in posttraumatic stress disorder: a functional magnetic resonance imaging investigation

BACKGROUND

The purpose of this study was to assess interregional brain activity covariations during traumatic script-driven imagery in subjects with posttraumatic stress disorder (PTSD).

METHODS

Functional magnetic resonance imaging and functional connectivity analyses were used to assess interregional brain activity covariations during script-driven imagery in PTSD subjects with a dissociative response, PTSD subjects with a flashback response, and healthy control subjects.

RESULTS

Significant between-group differences in functional connectivity were found. Comparing dissociated PTSD patients and control subjects' connectivity maps in the left ventrolateral thalamus (VLT) [-14, -16, 4] revealed that control subjects had higher covariations between activations in VLT and in the left superior frontal gyrus (Brodmann's area [BA] 10), right parahippocampal gyrus (BA 30), and right superior occipital gyrus (BA 19, 39), whereas greater covariation with VLT in dissociated PTSD subjects occurred in the right insula (BA 13, 34), left parietal lobe (BA 7), right middle frontal gyrus (BA 8), superior temporal gyrus (BA 38, 34), and right cuneus (BA 19). Comparing dissociated PTSD and flashback PTSD connectivity maps in the right cingulate gyrus [3, 16, 30] revealed that dissociated PTSD subjects had higher covariations between activations in this region and the left inferior frontal gyrus (BA 47).

CONCLUSIONS

Greater activation of neural networks involved in representing bodily states was seen in dissociated PTSD subjects than in non-PTSD control subjects. These findings might illuminate the mechanisms underlying distorted body perceptions often observed clinically during dissociative episodes.

Stress models of depression: Forming genetically vulnerable strains

Among the most useful models for depressive disorders are those, which involve a stress induced change in behaviour. Learned helplessness is one such model and is induced through exposure to uncontrollable and unpredictable aversive events. Learned helplessness as induced in rats using foot shock is well characterized and has good face validity and predictive validity as a model of depression, including alterations in HPA axis activity and REM sleep characteristic of depression. The data concerning the validity will be briefly reviewed. The model can also be used to look at the role of genetics through selective breeding. These studies will be reviewed and the utility of the genetic strains for understanding the interaction of stress and affect will be examined. A second model of depression using exposure to chronic stress also has high face and predictive validity. A new form of this approach, recently described, also is suitable for the examination of genetic factors leading to depressive like behaviour and this will be presented.

Biphasic effects of adrenal steroids on learned helplessness behavior induced by inescapable shock

Corticosterone (CS) has been shown to regulate behavior in the learned helplessness (LH) paradigm. Here we provide evidence for a U-shaped relationship between the increasing doses of CS administered and escape failures in the LH model. Replacement with CS (20-400 microg/ml in drinking water) in adrenalectomized (ADX) animals was utilized to examine how the selective activation of mineralocorticoid (MR) and glucocorticoid (GR) receptors is related to the behavioral impairments induced by inescapable shock (IS). Available MR and GR levels were determined in hippocampal cytosol by radioligand binding assays. Non-CS replaced ADX animals showed a high percentage of escape failures assessed 48 h after IS. A CS does of 100 microg/ml given to ADX animals markedly reduced escape failures and resulted in an almost total reduction of available MR associated with a partial reduction of GR. However, the administration of aldosterone (ALDO), a selective MR agonist, was not sufficient to restore normal coping behavior. Moreover, an important role for GR was further shown by means of the specific GR antagonist, RU 38486, which blocked the reduction of LH in ADX rats that were given 100 microg/ml CS. Higher doses of CS given to ADX rats reinstated the LH behavior, and SHAM rats that produced stress CS levels also produced LH behavior. The results indicate a U-shaped dose response function with both negligible and high CS levels being associated with LH behavior. Hence, along with a moderate reduction of available GR level in the cytosol, a large decrease in MR availability seems to be necessary to prevent the acquisition and expression of LH. However, very high reduction of available GR is associated with LH behavior.

Rats with congenital learned helplessness respond less to sucrose but show no deficits in activity or learning

Inbred rat strains for congenital learned helplessness (cLH) and for congenital resistance to learned helplessness (cNLH) were investigated as a model to study genetic predisposition to major depression. Congenitally helpless rats respond less to sucrose under a progressive ratio schedule. This is not confounded by locomotor hypoactivity: in contrast, cLH rats show a slight hyperactivity during the first 5 min of an open field test. cLH rats acquire operant responding to sucrose as readily as cNLH rats and exhibit normal memory acquisition and retrieval in the Morris water maze, thus ruling out general learning deficits as the cause of the decreased response to sucrose. Reduced total responses and reduced breaking points for sucrose in the cLH strain argue for anhedonia, which is an analogue to loss of pleasure essential for the diagnosis of major depressive episodes, and thus confirm the validity of congenitally learned helpless rats as a model of major depression.

Effect of neonatal handling and sex on basal and chronic stress-induced corticosterone and leptin secretion

Neonatal handling is an experimental paradigm for early experiences. It affects the programming of hypothalamo-pituitary-adrenal (HPA) axis function, known to be sexually dimorphic. Recently leptin, a hormone related to energy balance and secreted mainly by adipocytes, has been implicated in the stress response. We thus determined the effect of neonatal handling on plasma concentrations of corticosterone and leptin of male and female rats under basal conditions and after two consecutive chronic stressors: chronic forced swimming stress and chronic restraint. Handling resulted in lower basal corticosterone levels in both males and females and in a more efficient HPA response, with a large corticosterone surge following the first chronic stressor and a return to basal levels following the second. Handling also resulted in decreased plasma leptin concentrations in males, thus abolishing the sex difference in leptin levels. Furthermore, handling increased body weight while it decreased food intake under basal conditions. Food intake and body weight gain during chronic forced swimming was lower in handled than in non-handled males, while in females these parameters were not influenced by handling. In both males and females, handling resulted in decreased food intake and increased body weight loss during chronic restraint stress. Body weight loss during chronic restraint stress, which is considered an index of maladaptation and 'depression', was particularly pronounced in the handled females. Our results also showed that non-handled females had higher corticosterone and lower leptin levels than males under basal conditions and following each of the two chronic stressors. The present work suggests that early experiences, such as the mother-infant relationship, interact with endogenous factors, such as gonadal hormones, to determine the organism's response to stressful stimuli during adulthood.

Brain environment interactions: stress, posttraumatic stress disorder, and the need for a postmortem brain collection

Stress, especially the extreme stress of traumatic events, can alter both neurobiology and behavior. Such extreme environmental situations provide a useful model for understanding environmental influences on human biology and behavior. This paper will review some of the evidence of brain alterations that occur with exposure to environmental stress. This will include recent studies using neuroimaging and will address the need for histological confirmation of imaging study results. We will review the current scientific approaches to understanding brain environment interactions, and then make the case for the collection and study of postmortem brain tissue for the advancement of our understanding of the effects of environment on the brain. Creating a brain tissue collection specifically for the investigation of the effects of extreme environmental stressors fills a gap in the current research; it will provide another of the important pieces to the puzzle that constitutes the scientific investigation of negative effects of environmental exposures. Such a resource will facilitate new discoveries related to the psychiatric illnesses of acute stress disorder and posttraumatic stress disorder, and can enable scientists to correlate structural and functional imaging findings with tissue abnormalities, which is essential to validate the results of recent imaging studies.

Selectively bred Wistar-Kyoto rats: an animal model of depression and hyper-responsiveness to antidepressants

The Wistar-Kyoto (WKY) rat strain demonstrates endogenous hormonal and behavioral abnormalities that emulate many of those found in symptom-presenting depressive patients. Evidence suggests that the WKY strain may harbor heterogeneity not found in other inbred strains, including greater behavioral and genetic variability. We took advantage of this variability and selectively bred WKY for 'depressive' behavior using immobility in the forced swim test (FST) as a functional selector. Successive generations of selective breeding resulted in rats that exhibited the extremes of immobility in the FST: 'WKY most immobile' (WMI) and 'WKY least immobile' (WLI). Male WMI rats also showed significantly decreased activity in the open field test (OFT). Plasma corticosterone (CORT) response to restraint stress was significantly lower and less variable in WMI compared to WLI males. Subacute treatment of males with several classes of antidepressant had different effects on FST behavior in the two substrains. Both desipramine (10 mg/kg body weight), a tricyclic antidepressant, and phenelzine (7.5 mg/kg), a monoamine oxidase inhibitor, significantly and drastically decreased FST immobility in WMI. In contrast, WLI showed a limited response to these antidepressants. Neither substrain responded to fluoxetine (10 mg/kg), a selective serotonin reuptake inhibitor. These data show that selective breeding of WKY rats has resulted in two substrains with reduced variability and differing responsiveness to antidepressants, which represent a novel means to dissect the molecular mechanisms underlying depressive behavior.

Animal models of anxiety disorders

Animal models may be useful in investigating the fundamental mechanisms underlying psychiatric disorders, and may contribute to the development of new medications. A computerized literature search was used to collect studies on recently developed animal models for anxiety disorders. Particular cognitive-affective processes (eg, fear conditioning, control of stereotypic movements, social submissiveness, and trauma sensitization) may be particularly relevant to understanding specific anxiety disorders. Delineation of the phenomenology and psychobiology of these processes in animals leads to a range of useful models of these conditions. These models demonstrate varying degrees of face, construct, and predictive validity.

Congenitally learned helpless rats show abnormalities in intracellular signaling

BACKGROUND

Affective disorders and the drugs used to treat them lead to changes in intracellular signaling. We used a genetic animal model to investigate to what extent changes in intracellular signal transduction confer a vulnerability to mood or anxiety disorders.

METHODS

Levels of gene expression in a selectively bred strain of rats with a high vulnerability to develop congenitally learned helplessness (cLH), a strain highly resistant to the same behavior (cNLH) and outbred Sprague-Dawley (SD) control animals were compared using quantitative reverse transcription polymerase chain reaction.

RESULTS

Congenitally learned helpless animals had a 24%-30% reduced expression of the cyclic adenosine monophosphate response element binding protein messenger ribonucleic acid (mRNA) in the hippocampus and a 40%-41% increased level of the antiapoptotic protein bcl-2 mRNA in the prefrontal cortex compared to cNLH and SD rats. Other significant changes included changes in the expression levels of the alpha catalytic subunit of protein kinase A, glycogen synthase kinase 3beta, and protein kinase C epsilon.

CONCLUSIONS

Congenitally learned helpless animals show evidence of altered signal transduction and regulation of apoptosis compared to cNLH and SD control animals.

Opposite metabolic changes in the habenula and ventral tegmental area of a genetic model of helpless behavior

Congenitally helpless rats have been selectively bred to display an immediate helpless response to stress in order to model hereditary brain differences that contribute to depression vulnerability. Differences in regional brain metabolism between congenitally helpless and non-helpless rats were investigated using quantitative cytochrome oxidase histochemistry. The results indicated that congenitally helpless rats had 64-71% elevated metabolism in the habenula and a 25% elevation in the related interpeduncular nucleus. In contrast, helpless rats had 28% reduced metabolism in the ventral tegmental area (VTA) and 14-16% reductions in the basal ganglia and basolateral and central amygdala. The opposite metabolic changes in the habenula and ventral tegmental area may be especially important for determining the congenitally helpless rat's global pattern of brain activity, which resembles the metabolic activity pattern produced by dopamine antagonism.

Circuits and systems in stress. I. Preclinical studies

This paper reviews the preclinical literature related to the effects of stress on neurobiological and neuroendocrine systems. Preclinical studies of stress provide a comprehensive model for understanding neurobiological alterations in post-traumatic stress disorder (PTSD). The pathophysiology of stress reflects long-standing changes in biological stress response systems and in systems involved in stress responsivity, learning, and memory. The neural circuitry involved includes systems mediating hypothalamic-pituitary-adrenal (HPA) axis, norepinephrine (locus coeruleus), and benzodiazepine, serotonergic, dopaminergic, neuropeptide, and central amino acid systems. These systems interact with brain structures involved in memory, including hippocampus, amygdala, and prefrontal cortex. Stress responses are of vital importance in living organisms; however excessive and/or repeated stress can lead to long-lasting alterations in these circuits and systems involved in stress responsiveness. Intensity and duration of the stressor, and timing of the stressor in life, have strong impact in this respect.

5-HT(1B) mrna regulation in two animal models of altered stress reactivity

BACKGROUND

Acute stress has profound effects on serotonergic activity, but it is not known whether alterations in the serotonin system can predispose individuals to exaggerated stress responses. We examined the regulation of 5-HT(1B) and 5-HT(1A) mRNA in two rodent models of differential sensitivity to stress: congenital learned helplessness (cLH) and handling and maternal separation (HMS).

METHODS

5-HT(1B) and 5-HT(1A) mRNAs in brain tissue sections were quantitated by in situ hybridization from control, stress-sensitive, and stress-resistant male rats in the HMS model and stress-sensitive and stress-resistant rats (both males and females) in the cLH model. Dorsal raphe nucleus, striatum, and hippocampus were examined.

RESULTS

The main result was that dorsal raphe 5-HT(1B) mRNA was substantially elevated (63-73%) in male rats in the stress-resistant group of both models compared with stress-sensitive animals. 5-HT(1B) mRNA in female rats did not differ between groups in the cLH model. There were no differences in 5-HT(1A) mRNA between HMS groups.

CONCLUSIONS

These findings suggest that 5-HT(1B) autoreceptor regulation is altered in animals with diminished stress reactivity. These results suggest that 5-HT(1B) autoreceptors in unstressed and acutely stressed animals differ, indicating the importance of state versus trait changes in serotonin function in animal models of anxiety and depression.

The validity of animal models of predisposition to depression

Some animal models of depression, including the majority of the more recently introduced models, are better characterized as models of predisposition to depression. In the first part of this paper, we show that the basis for such a model could be either a procedure that increases the ease with which an analogue of major depression may be evoked, or a presentation analogous to dysthymia (chronic mild depression). We then consider how the concepts of predictive, face, and construct validity apply to such models. Next, we review the validity of the available models of predisposition to depression, which derive from genetics, genomics, developmental manipulations, and brain lesioning. Finally, we compare the performance of the different models, using a novel scoring system that formalizes the evaluation of animal models against each of the three sets of validation criteria.