Individual differences in the immobility behavior in juvenile and adult rats are associated with monoaminergic neurotransmission and with the expression of corticotropin-releasing factor receptor 1 in the nucleus accumbens

Variability in Behavioral Phenotypes after Forced Swimming-Induced Stress in Rats Is Associated with Expression of the Glucocorticoid Receptor, Nurr1, and IL-1β in the Hippocampus

Individual differences in coping with stress may determine either a vulnerable or resilient phenotype. Therefore, it is important to better understand the biology underlying the behavioral phenotype. We assessed whether individual behavioral phenotype to acute stress is related with the hippocampal expression of glucocorticoid receptor (GR), Nurr1, interleukin-1 beta (IL-1β) or brain-derived neurotrophic factor (BDNF). Wistar male rats were exposed to forced swimming for 15 min and sacrificed at different times. Behavioral response was analyzed, and it was compared with the gene and protein expression of GR, Nurr1, IL-1β and BDNF in the hippocampus for each time point. Behavioral phenotyping showed a group with high immobility (vulnerable) while another had low immobility (resilient). No significant differences were found in the Nurr1, IL-1β and BDNF mRNA levels between resilient and vulnerable rats at different recovery times except for Nr3c1 (gene for GR). However, exposure to stress caused significantly higher levels of GR, Nurr1 and IL-1β proteins of vulnerable compared to resilient rats. This variability of behavioral phenotypes is associated with a differential molecular response to stress that involves GR, Nurr1, and IL-1β as mediators in coping with stress. This contributes to identifying biomarkers of susceptibility to stress.

The forced swim test: Historical, conceptual and methodological considerations and its relationship with individual behavioral traits

The forced swim test (FST), developed by Porsolt and collaborators in 1977 to evaluate antidepressant (AD) treatments in rodents, has become extensively used for this purpose and to evaluate depression-like states. Despite its popularity, studies have raised important concerns regarding its theoretical and predictive validity. In my view and that of others, the FST mainly evaluates coping strategies in an inescapable situation. Although it is reasonable to assume that ADs act favoring active coping whereas negative affective states would favor passive coping, this does not mean that only ADs should enhance active coping or that a depression state has developed, respectively. Given its simplicity, proper interpretation of the FST behavior is critically dependent on how FST behavior relates to other behavioral traits. Unfortunately, this issue has been poorly discussed previously. Then, the present review, using a historical perspective, offers information needed to better understand the meaning and limitations of the FST, discusses critical methodological aspects and analyzes the relationship of FST behavior with classical behavioral traits in rodents.

Different effects of chronic social defeat on social behavior and the brain CRF system in adult male C57 mice with different susceptibilities

Chronic social defeat stress (CSDS) has been found to produce different impacts on anxiety-like behaviors, spatial cognitive function and memory in rodents with different susceptibilities. However, the impacts of chronic social defeat on social behaviors in adult male mice with different susceptibilities to social defeat and the underlying mechanisms in the brain remain unclear. In the present study, we found that ten days of social defeat reduced the tendency of susceptible adult male C57 mice to approach an unfamiliar individual and increased their avoidance of an unfamiliar CD-1 mouse but had no effects on resilient individuals. In addition, CSDS enhanced anxiety-like behavior in susceptible animals, but produced no effects in the resilient group. Meanwhile, CSDS increased the number of corticotropin-releasing factor (CRF)-positive neurons in the paraventricular nucleus of the hypothalamus and CRF-R2-positive neurons in the accumbens nucleus shell in both resilient and susceptible animals. CSDS increased the number of CRF-R1-positive neurons and CRF-R1 mRNA expression in the prelimbic cortex (PrL) and the number of CRF-R2-positive neurons in the basolateral amygdala, but reduced the number of CRF-R2-positive neurons and mRNA expression in the PrL in susceptible animals. Therefore, the different effects of CSDS on sociability and anxiety-like behavior in mice with different susceptibilities may be associated with region- and type-specific alterations in CRF receptor levels. These findings help us understand the underlying mechanism by which social stress affects emotion and social behavior and provides an important basis for the treatment of disorders of social and emotional behavior caused by social stress.

Alterations in the corticotropin-releasing hormone (CRH) neurocircuitry: Insights into post stroke functional impairments

Although it is well accepted that changes in the regulation of the hypothalamic-pituitary adrenal (HPA) axis may increase susceptibility to affective disorders in the general population, this link has been less examined in stroke patients. Yet, the bidirectional association between depression and cardiovascular disease is strong, and stress increases vulnerability to stroke. Corticotropin-releasing hormone (CRH) is the central stress hormone of the HPA axis pathway and acts by binding to CRH receptors (CRHR) 1 and 2, which are located in several stress-related brain regions. Evidence from clinical and animal studies suggests a role for CRH in the neurobiological basis of depression and ischemic brain injury. Given its importance in the regulation of the neuroendocrine, autonomic, and behavioral correlates of adaptation and maladaptation to stress, CRH is likely associated in the pathophysiology of post stroke emotional impairments. The goals of this review article are to examine the clinical and experimental data describing (1) that CRH regulates the molecular signaling brain circuit underlying anxiety- and depression-like behaviors, (2) the influence of CRH and other stress markers in the pathophysiology of post stroke emotional and cognitive impairments, and (3) context and site specific interactions of CRH and BDNF as a basis for the development of novel therapeutic targets. This review addresses how the production and release of the neuropeptide CRH within the various regions of the mesocorticolimbic system influences emotional and cognitive behaviors with a look into its role in psychiatric disorders post stroke.

Sensitivity of the prefrontal GABAergic system to chronic stress in male and female mice: Relevance for sex differences in stress-related disorders

Stress-induced modifications of the prefrontal cortex (PFC) are believed to contribute to the onset of mood disorders, such as depression and anxiety, which are more prevalent in women. In depression, the PFC is hypoactive; however the origin of this hypoactivity remains unclear. Possibly, stress could impact the prefrontal GABAergic inhibitory system that, as a result, impairs the functioning of downstream limbic structures controlling emotions. Preclinical evidence indicates that the female PFC is more sensitive to the effects of stress. These findings suggest that exposure to stress could lead to sex-specific alterations in prefrontal GABAergic signaling, which contribute to sex-specific abnormal functioning of limbic regions. These limbic changes could promote the onset of depressive and anxiety behaviors in a sex-specific manner, providing a possible mechanism mediating sex differences in the clinical presentation of stress-related mood disorders. We addressed this hypothesis using a mouse model of stress-induced depressive-like behaviors: the unpredictable chronic mild stress (UCMS) paradigm. We observed changes in prefrontal GABAergic signaling after exposure to UCMS most predominantly in females. Increased parvalbumin (PV) expression and decreased prefrontal neuronal activity were correlated in females with severe emotionality deficit following UCMS, and with altered activity of the amygdala. In males, small changes in emotionality following UCMS were associated with minor changes in prefrontal PV expression, and with hypoactivity of the nucleus accumbens. Our data suggest that prefrontal hypoactivity observed in stress-related mood disorders could result from stress-induced increases in PV expression, particularly in females. This increased vulnerability of the female prefrontal PV system to stress could underlie sex differences in the prevalence and symptomatology of stress-related mood disorders.

Water spray-induced grooming is negatively correlated with depressive behavior in the forced swimming test in rats

Rodents show grooming, a typical self-care behavior, under stress and non-stress conditions. Previous studies revealed that grooming under stress conditions such as the open-field test (OFT) or the elevated plus-maze test (EPM) is associated with anxiety, but the roles of grooming under non-stress conditions are not well understood. Here, we examined spray-induced grooming as a model of grooming under a non-stress condition to investigate the relationship between this grooming and depression-like behavior in the forced swim test (FST) and tail suspension test, and we compared spray-induced grooming with OFT- and EPM-induced grooming. The main finding was that the duration of spray-induced grooming, but not that of OFT/EPM-induced grooming, was negatively correlated with the duration of immobility in the FST, an index of depression-like behavior. The results suggest that spray-induced grooming is functionally different from the grooming in the OFT and EPM and is related to reduction of depressive behavior.

Downregulation of growth hormone 1 gene in the cerebellum and prefrontal cortex of rats with depressive-like behavior

Depressive-like behaviors in animals are usually assessed by standardized behavioral tests such as the forced swimming test (FST). However, individual variation in test performance may obscure group differences and thereby hinder the discovery of genes responsible for depression. Few reports have shown the influence of individual variability in identifying the genes associated with depressive-like behaviors. In this study, we conducted microarray analysis to identify genes differentially expressed in the prefrontal cortex (PFC) and cerebellum of rats stratified by FST immobility ratio (% immobility in 5 min) into a control group [immobility ratio: −1 to +1 standard deviation (SD) from the mean] and a depressive group (immobility ratio: +1 to +2 SDs above the mean). Genes differentially expressed in both the cerebellum and PFC of the depressive group were Alas2, Gh1, Hba-a2, Hbb, Hbb-b1, Hbe2, LOC689064, Mrps10, Mybpc, Olf6415, and Pfkb1. Ingenuity Pathway Analysis identified Gh1 as a hub gene in the networks of differentially expressed genes in both brain regions. This study indicates that the depressive-like behavior may be related to the decrease of Gh1 expression in the cerebellum and PFC.

Immobility behavior during the forced swim test correlates with BNDF levels in the frontal cortex, but not with cognitive impairments

The forced swim test (FST) is widely used to evaluate the antidepressant-like activity of compounds and is sensitive to stimuli that cause depression-like behaviors in rodents. The immobility behavior observed during the test has been considered to represent behavioral despair. In addition, some studies suggest that the FST impairs rats' performance on cognitive tests, but these findings have rarely been explored. Thus, we investigated the effects of the FST on behavioral tests related to neuropsychiatric diseases that involve different cognitive components: novel object recognition (NOR), the object location test (OLT) and prepulse inhibition (PPI). Brain-derived neurotrophic factor (BDNF) levels in the frontal cortex and hippocampus were evaluated. The rats were forced to swim twice (15-min session followed by a 5-min session 24h later) and underwent cognitive tests 24h after the last swimming exposure. The FST impaired the rats' performance on the OLT and reduced the PPI and acoustic startle responses, whereas the NOR was not affected. The cognitive impairments were not correlated with an immobility behavior profile, but a significant negative correlation between the frontal BDNF levels and immobility behavior was identified. These findings suggest a protective role of BDNF against behavioral despair and demonstrate a deleterious effect of the FST on spatial memory and pre-attentive processes, which point to the FST as a tool to induce cognitive impairments analogous to those observed in depression and in other neuropsychiatric disorders.

Sex-specific enhanced behavioral toxicity induced by maternal exposure to a mixture of low dose endocrine-disrupting chemicals

Humans are increasingly and consistently exposed to a variety of endocrine disrupting chemicals (EDCs), chemicals that have been linked to neurobehavioral disorders such as ADHD and autism. Many of such EDCs have been shown to adversely influence brain mesocorticolimbic systems raising the potential for cumulative toxicity. As such, understanding the effects of developmental exposure to mixtures of EDCs is critical to public health protection. Consequently, this study compared the effects of a mixture of four EDCs to their effects alone to examine potential for enhanced toxicity, using behavioral domains and paradigms known to be mediated by mesocorticolimbic circuits (fixed interval (FI) schedule controlled behavior, novel object recognition memory and locomotor activity) in offspring of pregnant mice that had been exposed to vehicle or relatively low doses of four EDCs, atrazine (ATR - 10mg/kg), perfluorooctanoic acid (PFOA - 0.1mg/kg), bisphenol-A (BPA - 50 μg/kg), 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD - 0.25 μg/kg) alone or combined in a mixture (MIX), from gestational day 7 until weaning. EDC-treated males maintained significantly higher horizontal activity levels across three testing sessions, indicative of delayed habituation, whereas no effects were found in females. Statistically significant effects of MIX were seen in males, but not females, in the form of increased FI response rates, in contrast to reductions in response rate with ATR, BPA and TCDD, and reduced short term memory in the novel object recognition paradigm. MIX also reversed the typically lower neophobia levels of males compared to females. With respect to individual EDCs, TCDD produced notable increases in FI response rates in females, and PFOA significantly increased ambulatory locomotor activity in males. Collectively, these findings show the potential for enhanced behavioral effects of EDC mixtures in males and underscore the need for animal studies to fully investigate mixtures, including chemicals that converge on common physiological substrates to examine potential mechanisms of toxicity with full dose effect curves to assist in interpretations of relevant mechanisms.

Pregnanolone Glutamate, a Novel Use-Dependent NMDA Receptor Inhibitor, Exerts Antidepressant-Like Properties in Animal Models

A number of studies demonstrated a rapid onset of an antidepressant effect of non-competitive N-methyl-d-aspartic acid receptor (NMDAR) antagonists. Nonetheless, its therapeutic potential is rather limited, due to a high coincidence of negative side-effects. Therefore, the challenge seems to be in the development of NMDAR antagonists displaying antidepressant properties, and at the same time maintaining regular physiological function of the NMDAR. Previous results demonstrated that naturally occurring neurosteroid 3α5β-pregnanolone sulfate shows pronounced inhibitory action by a use-dependent mechanism on the tonically active NMDAR. The aim of the present experiments is to find out whether the treatment with pregnanolone 3αC derivatives affects behavioral response to chronic and acute stress in an animal model of depression. Adult male mice were used throughout the study. Repeated social defeat and forced swimming tests were used as animal models of depression. The effect of the drugs on the locomotor/exploratory activity in the open-field test was also tested together with an effect on anxiety in the elevated plus maze. Results showed that pregnanolone glutamate (PG) did not induce hyperlocomotion, whereas both dizocilpine and ketamine significantly increased spontaneous locomotor activity in the open field. In the elevated plus maze, PG displayed anxiolytic-like properties. In forced swimming, PG prolonged time to the first floating. Acute treatment of PG disinhibited suppressed locomotor activity in the repeatedly defeated group-housed mice. Aggressive behavior of isolated mice was reduced after the chronic 30-day administration of PG. PG showed antidepressant-like and anxiolytic-like properties in the used tests, with minimal side-effects. Since PG combines GABA_A receptor potentiation and use-dependent NMDAR inhibition, synthetic derivatives of neuroactive steroids present a promising strategy for the treatment of mood disorders.

Highlights: -

3α5β-pregnanolone glutamate (PG) is a use-dependent antagonist of NMDA receptors.

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We demonstrated that PG did not induce significant hyperlocomotion.

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We showed that PG displayed anxiolytic-like and antidepressant-like properties.

Individual differences in the forced swimming test and the effect of environmental enrichment: searching for an interaction

Animals with low and high immobility in the forced swimming test (FST) differ in a number of neurobehavioral factors. A growing body of evidence suggests that the exposure to enriched environments mediates a number of changes in the brain. Therefore, we studied if animals' individuality can somehow modulate the response to environmental stimuli. Male rats were classified according to their immobility time scores in the FST test session as animals with low, medium or high immobility. Then, rats from groups with low and high immobility were randomly distributed in two groups to be reared in different housing conditions (i.e., enriched and standard conditions) during 8weeks. Animals were subjected to the open field test (OFT) before and 6weeks after the start of housing protocol. Rats with high immobility in the FST also showed high ambulation and high rearing time in the first OFT. Such findings were not observed in the second OFT. Conversely, an effect of environmental enrichment was found in the second OFT where enriched animals showed lower ambulation and higher grooming time than the standard control group. Rats were sacrificed after the housing protocol and neurochemical content and/or gene expression were studied in three different brain regions: the prefrontal cortex, the hippocampus and the nucleus accumbens. Rats with low immobility showed significantly higher accumbal 5-HT levels than animals with high immobility, whereas no neurochemical differences were observed between enriched and standard animals. Regarding expression data, however, an effect of enrichment on accumbal corticotropin-releasing factor (CRF) and its receptor 1 (CRFR1) levels was observed, and such effect depended on immobility levels. Thus, our results not only allowed us to identify a number of differences between animals with low and high immobility or animals housed in standard and enriched conditions, but also suggested that animals' individuality modulated in some way the response to environmental stimuli.