The context specificity of anxiety responses induced by chronic psychosocial stress in rats: a shift from anxiety to social phobia?

Chronic unpredictable restraint stress increases hippocampal pro-inflammatory cytokines and decreases motivated behavior in rats

Most chronic stress protocols are too laborious or do not abide by the two main characteristics of the stress concept: uncontrollability and unpredictability. The goal of this study was to establish a simple and reliable model of chronic stress, while maintaining the main features of the concept. Animals were exposed to chronic movement restraint with variable duration (2, 4 or 6 h, in an unpredictable schedule) for 3 weeks and assessed in several physiological and behavioral readouts known to reflect chronic stress states. Body weight, levels of plasma corticosterone, hippocampal pro-and anti-inflammatory cytokines, anxiety-like (novelty suppressed feeding and elevated plus maze) and motivated behaviors (sucrose negative contrast test and forced swim test) were evaluated three days after the end of the chronic protocol. Stressed animals had a lower body weight gain, higher levels of cytokines in the hippocampus, reduced suppression of a low concentration sucrose solution and increased immobility in the forced swim test. Based on these data, we suggest that chronic movement restraint with variable duration may be a suitable and simple protocol for the study of changes induced by chronic stress and for the testing of possible treatments relevant to psychiatry.

Positive Affect as a Buffer between Chronic Stress and Symptom Severity of Emotional Disorders

Research has demonstrated that stressors play a critical role in the development of generalized anxiety disorder (GAD), social anxiety disorder (SAD), and major depressive disorder (MDD). Separately, deficits in positive affect (PA) have been identified in GAD, SAD, and MDD. While previous research has linked the buffering effects of PA in chronic illness, such effects have yet to be investigated for chronic stressors and emotional disorder-related symptom severity. The purpose of the present study was to examine PA as a moderator of chronic interpersonal and non-interpersonal stress on GAD, SAD, and MDD symptom severity. Using a multilevel statistical approach with a sample of adolescents and young adults (N=463), PA was found to significantly moderate the relationship between chronic interpersonal stress and symptom severity for MDD and SAD. Findings suggest that in times of chronic interpersonal stress, higher PA may serve as a buffer from development of SAD and MDD symptoms.

Evaluation of the neutrophil:lymphocyte ratio as an indicator of chronic distress in the laboratory mouse

When evaluating the effect of husbandry and biomethodologies on the well-being of laboratory mice, it is critical to utilize measurements that allow the distinguishing of acute stress from chronic stress. One of the most common measurements of stress in laboratory animals is the corticosterone assessment. However, while this measurement provides a highly accurate reflection of the animal's response to acute stressors, its interpretation is more prone to error when evaluating the effect of chronic stress. This study evaluated the use of the neutrophil:lymphocyte (NE:LY) ratio as an assessment of chronic stress in male and female C57Bl/6N mice as compared to serum corticosterone. One group of mice was exposed to mild daily stressors for 7 days, while the control group was handled with normal husbandry. The NE:LY ratio and serum corticosterone levels were significantly elevated in the chronically stressed mice, though a significant increase in corticosterone was only significant in males when compared by sex. The chronically stressed mice also demonstrated significantly fewer entries into the open arms and less time spent in the open arms of the elevated plus maze, suggesting that the mild daily stressors had induced a state of distress. The findings of this study confirm that the NE:LY ratio is a valid measurement for chronic stress in the laboratory mouse. However, these assays do not distinguish between distress or eustress, so behavioral and physiological assessments should always be included to determine a complete assessment of the well-being of the mouse.

Modeling consequences of prolonged strong unpredictable stress in zebrafish: Complex effects on behavior and physiology

Chronic stress is the major pathogenetic factor of human anxiety and depression. Zebrafish (Danio rerio) have become a novel popular model species for neuroscience research and CNS drug discovery. The utility of zebrafish for mimicking human affective disorders is also rapidly growing. Here, we present a new zebrafish model of clinically relevant, prolonged unpredictable strong chronic stress (PUCS). The 5-week PUCS induced overt anxiety-like and motor retardation-like behaviors in adult zebrafish, also elevating whole-body cortisol and proinflammatory cytokines - interleukins IL-1β and IL-6. PUCS also elevated whole-body levels of the anti-inflammatory cytokine IL-10 and increased the density of dendritic spines in zebrafish telencephalic neurons. Chronic treatment of fish with an antidepressant fluoxetine (0.1mg/L for 8days) normalized their behavioral and endocrine phenotypes, as well as corrected stress-elevated IL-1β and IL-6 levels, similar to clinical and rodent data. The CNS expression of the bdnf gene, the two genes of its receptors (trkB, p75), and the gfap gene of glia biomarker, the glial fibrillary acidic protein, was unaltered in all three groups. However, PUCS elevated whole-body BDNF levels and the telencephalic dendritic spine density (which were corrected by fluoxetine), thereby somewhat differing from the effects of chronic stress in rodents. Together, these findings support zebrafish as a useful in-vivo model of chronic stress, also calling for further cross-species studies of both shared/overlapping and distinct neurobiological responses to chronic stress.

Chronic unpredictable stress regulates visceral adipocyte-mediated glucose metabolism and inflammatory circuits in male rats

Chronic psychological stress is a prominent risk factor involved in the pathogenesis of many complex diseases, including major depression, obesity, and type II diabetes. Visceral adipose tissue is a key endocrine organ involved in the regulation of insulin action and an important component in the development of insulin resistance. Here, we examined for the first time the changes on visceral adipose tissue physiology and on adipocyte‐associated insulin sensitivity and function after chronic unpredictable stress in rats. Male rats were subjected to chronic unpredictable stress for 35 days. Total body and visceral fat was measured. Cytokines and activated intracellular kinase levels were determined using high‐throughput multiplex assays. Adipocyte function was assessed via tritiated glucose uptake assay. Stressed rats showed no weight gain, and their fat/lean mass ratio increased dramatically compared to control animals. Stressed rats had significantly higher mesenteric fat content and epididymal fat pad weight and demonstrated reduced serum glucose clearing capacity following glucose challenge. Alterations in fat depot size were mainly due to changes in adipocyte numbers and not size. High‐throughput molecular screening in adipocytes isolated from stressed rats revealed activation of intracellular inflammatory, glucose metabolism, and MAPK networks compared to controls, as well as significantly reduced glucose uptake capacity in response to insulin stimulation. Our study identifies the adipocyte as a key regulator of the effects of chronic stress on insulin resistance, and glucose metabolism, with important ramifications in the pathophysiology of several stress‐related disease states.

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Our study identifies the adipocyte as a key regulator of the effects of chronic stress on insulin resistance, and glucose metabolism, with important ramifications in the pathophysiology of several stress‐related disease states.

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.

Repeated restraint stress exerts different impact on structure of neurons in the lateral and basal nuclei of the amygdala

Chronic stress exacerbates and can induce symptoms of depression and anxiety disorders. Chronic stress causes amygdala hyperactivity, which may contribute to these detrimental effects. One potential mechanism for amygdala hyperactivity is an increase of excitatory drive after stress. Excitatory inputs to the amygdala predominantly synapse upon dendritic spines, and repeated stress has been demonstrated to increase dendritic spines in the basolateral amygdala (BLA). However, the BLA is comprised of several nuclei, including the lateral nucleus (LAT) and the basal nucleus (BA), which exert functionally distinct roles in amygdala-dependent behaviors. Furthermore, while an increase of dendritic spines can impart significant functional ramifications, a shift of spine distribution can also exert significant impact. However, differences in the effects of repeated stress on LAT and BA have not been examined, nor differential effects on spine distribution. This study examined the effects of repeated restraint stress on dendritic structure of principal neurons from the LAT and BA in Golgi-stained tissue. This study found that repeated stress increased spine number in LAT and BA, but in very distinct patterns, with proximal increases in LAT neurons and non-proximal increases in BA neurons. Furthermore, repeated stress increased dendritic length in the BA, but not the LAT, leading to a global change of spine density in BA, but a focal change in LAT. These distinct effects of repeated stress in the LAT and BA may exert significant functional effects on fear behavior, and may underlie differences in the effects of repeated stress on acquisition, contextual modulation and extinction of fear behavior.

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.

From the stressed adolescent to the anxious and depressed adult: investigations in rodent models

Anxiety and depression are the most prevalent of the psychiatric disorders. The average age of onset of these disorders is in adolescence, and stressful experiences are recognized as an important pathway to such dysfunction. Until recently, however, most animal models of these disorders involved adult males. We provide a brief overview of anxiety and depression and the extent to which adolescent rodents are a valid model for their investigation, and briefly review the main measures of anxiety-like and depressive behaviour in rodents. The focus of the review is investigations in which adolescent rodents were exposed to chronic stressors, describing our research using social instability stress and that of other researchers using various social and non-social stressors. The evidence to date suggests stress in adolescence alters the trajectory of brain development, and particularly that of the hippocampus, increasing anxiety and depressive behaviour in adulthood.

Current animal models of anxiety, anxiety disorders, and anxiolytic drugs

PURPOSE OF REVIEW

The perception that 'classical' anxiety tests are deficient was formulated in the mid-1990s. Recent clinical trials also demonstrate that the predictive power of such tests is low, which emphasizes the need for developing models of higher translational value. Several novel models are proposed each year. Here, we investigate their impact on anxiolytic-related studies performed in 2010 and 2011.

RECENT FINDINGS

Here, we depict as 'classical' all the tests that were developed at the same time as or earlier than the elevated plus-maze test. No test equaled its success in the subsequent decades; therefore, we consider it the endpoint of the period when the methodological bases of current laboratory research were laid down. Fourteen classical tests were employed in the investigated period, which were used in more than 80% of studies. Concurrently, 36 'nonclassical' tests were used and six novel tests were also proposed. These accounted for fewer than 20% of studies. 'Classical' tests were often performed under unconventional conditions that putatively increased their translational value. Taken together, half of the studies involved at least one innovative step. Yet, the new procedures were infrequently used. Out of the 36 'nonclassical' tests, only 11 were used more than once, while the amendments to 'classical tests' were almost entirely laboratory specific.

SUMMARY

Our analysis shows that there is a large interest in performing anxiety research innovatively. However, efforts are highly divergent and result in large numbers of poorly validated and infrequently used novel approaches. Thus, models with increased translational value still need to be developed.

Male Wistar rats are more susceptible to lasting social anxiety than Wild-type Groningen rats following social defeat stress during adolescence

Adolescence is an important period for the development of adult social competences. Social stress during adolescence may contribute not only to an inadequate social development but also to adult vulnerability to social anxiety. There seems to be a clear individual differentiation, however, in the vulnerability to the long-term negative consequences of social stress. The current study further explores this individual vulnerability and is aimed at the influence of social stress during adolescence on adult social anxiety and its context specificity. Rats from different strains (Wistar and Wild-type Groningen rats) were exposed to the resident-intruder paradigm five times during 10 min each in the period between postnatal day 45 and 58. Three and 7 weeks later, the animals were re-exposed to the context in the presence of either a dominant male or an anestrous female behind a wire mesh screen. Wistar rats that were socially defeated spent less time exploring the social stimulus in comparison with socially defeated Wild-type rats and their non-defeated controls. We conclude that the stressed Wistar rat shows signs of generalized social anxiety indicating that the Wistar rat can be considered as a vulnerable phenotype to effects of adolescent social stress.

Vulnerability to nicotine abstinence-related social anxiety-like behavior: molecular correlates in neuropeptide Y, Y2 receptor and corticotropin releasing factor

An outbred rat model of the novelty-seeking phenotype is used to study nicotine vulnerability, where experimentally naïve rats were phenotype screened as high or low responders (HRs or LRs, ranking in the upper or lower one-third of the population respectively) based on locomotor activity displayed in a novel environment. Following nicotine training and abstinence, HR animals pre-trained with nicotine showed expression of locomotor sensitization to nicotine challenge along with enhanced social anxiety-like behavior in the social interaction test compared to saline pre-trained controls. HR rats also showed a downregulation in neuropeptide Y (NPY) mRNA levels in the medial nucleus of amygdala and the CA1 field of the hippocampus, an upregulation in Y2 mRNA levels in the CA3 field of the hippocampus, and an upregulation in the corticotropin releasing factor (CRF) mRNA levels in the central nucleus of the amygdala. These findings implicate dysregulations in the NPY-CRF systems in the HR hippocampus and amygdala associated with the emergence of social anxiety-like behavior, and a novel Y2R-mediated pathway in nicotine relapse.