Behavioral, neuroendocrine and serotonergic consequences of single social defeat and repeated fluoxetine pretreatment in the Lewis rat strain

Validity of mental and physical stress models

Different stress models are employed to enhance our understanding of the underlying mechanisms and explore potential interventions. However, the utility of these models remains a critical concern, as their validities may be limited by the complexity of stress processes. Literature review revealed that both mental and physical stress models possess reasonable construct and criterion validities, respectively reflected in psychometrically assessed stress ratings and in activation of the sympathoadrenal system and the hypothalamic-pituitary-adrenal axis. The findings are less robust, though, in the pharmacological perturbations' domain, including such agents as adenosine or dobutamine. Likewise, stress models' convergent- and discriminant validity vary depending on the stressors' nature. Stress models share similarities, but also have important differences regarding their validities. Specific traits defined by the nature of the stressor stimulus should be taken into consideration when selecting stress models. Doing so can personalize prevention and treatment of stress-related antecedents, its acute processing, and chronic sequelae. Further work is warranted to refine stress models' validity and customize them so they commensurate diverse populations and circumstances.

Psychedelic-Induced Serotonin 2A Receptor Downregulation Does Not Predict Swim Stress Coping in Mice

Serotoninergic psychedelics such as psilocybin have been reported to elicit a long-lasting reduction in depressive symptoms. Although the main target for serotoninergic psychedelics, serotonin type 2A receptor (5-HT_2A), has been established, the possible mechanism of the antidepressant action of psychedelics remains unknown. Using the mouse forced swim test model, we examined whether the administration of the synthetic serotoninergic psychedelic 2,5-dimethoxy-4-iodoamphetamine (DOI) would modulate 5-HT_2A receptor levels in the medial prefrontal cortex (mPFC) and revert stress-induced changes in behavior. Mice subjected to swim stress developed a passive stress-coping strategy when tested in the forced swim test 6 days later. This change in behavior was not associated with the hypothesized increase in 5-HT_2A receptor-dependent head twitch behaviors or consistent changes in 5-HT_2A receptor levels in the mPFC. When DOI was administered 1 day before the forced swim test, a low dose (0.2 mg/kg i.p.) unexpectedly increased immobility while a high dose (2 mg/kg i.p.) had no significant effect on immobility. Nevertheless, DOI evoked a dose-dependent decrease in 5-HT_2A levels in the mPFC of mice previously exposed to swim stress. Our findings do not support the hypothesis that the downregulation of 5-HT_2A receptors in the mPFC contributes to the antidepressant-like properties of serotoninergic psychedelics.

Serotonin 5-HT1B receptors mediate the antidepressant- and anxiolytic-like effects of ventromedial prefrontal cortex deep brain stimulation in a mouse model of social defeat

BACKGROUND

Deep brain stimulation (DBS) delivered to the ventromedial prefrontal cortex (vmPFC) induces antidepressant- and anxiolytic-like responses in various animal models. Electrophysiology and neurochemical studies suggest that these effects may be dependent, at least in part, on the serotonergic system. In rodents, vmPFC DBS reduces raphe cell firing and increases serotonin (5-HT) release and the expression of serotonergic receptors in different brain regions.

METHODS

We examined whether the behavioural responses of chronic vmPFC DBS are mediated by 5-HT_1A or 5-HT_1B receptors through a series of experiments. First, we delivered stimulation to mice undergoing chronic social defeat stress (CSDS), followed by a battery of behavioural tests. Second, we measured the expression of 5-HT_1A and 5-HT_1B receptors in different brain regions with western blot. Finally, we conducted pharmacological experiments to mitigate the behavioural effects of DBS using the 5-HT_1A antagonist, WAY-100635, or the 5-HT_1B antagonist, GR-127935.

RESULTS

We found that chronic DBS delivered to stressed animals reduced the latency to feed in the novelty suppressed feeding test (NSF) and immobility in the forced swim test (FST). Though no significant changes were observed in receptor expression, 5-HT_1B levels in DBS-treated animals were found to be non-significantly increased in the vmPFC, hippocampus, and nucleus accumbens and reduced in the raphe compared to non-stimulated controls. Finally, while animals given vmPFC stimulation along with WAY-100635 still presented significant responses in the NSF and FST, these were mitigated following GR-127935 administration.

CONCLUSIONS

The antidepressant- and anxiolytic-like effects of DBS in rodents may be partially mediated by 5-HT1_B receptors.

Challenges in the use of animal models and perspectives for a translational view of stress and psychopathologies

The neurobiology and development of treatments for stress-related neuropsychiatric disorders rely heavily on animal models. However, the complexity of these disorders makes it difficult to model them entirely, so only specific features of human psychopathology are emulated and these models should be used with great caution. Importantly, the effects of stress depend on multiple factors, like duration, context of exposure, and individual variability. Here we present a review on pre-clinical studies of stress-related disorders, especially those developed to model posttraumatic stress disorder, major depression, and anxiety. Animal models provide relevant evidence of the underpinnings of these disorders, as long as face, construct, and predictive validities are fulfilled. The translational challenges faced by scholars include reductionism and anthropomorphic/anthropocentric interpretation of the results instead of a more naturalistic and evolutionary understanding of animal behavior that must be overcome to offer a meaningful model. Other limitations are low statistical power of analysis, poor evaluation of individual variability, sex differences, and possible conflicting effects of stressors depending on specific windows in the lifespan.

Acute administration of fluoxetine increases social avoidance and risk assessment behaviors in a sex- and social stress-dependent manner in Syrian hamsters (Mesocricetus auratus)

Most studies investigating the effects of acute administration of selective serotonin reuptake inhibitors (SSRI) on responses to social stress have been conducted with males. This is despite the fact that SSRIs remain the primary pharmacotherapy for social stress-related disorders for both sexes and that the prevalence of these disorders is twofold higher in women than in men. To determine whether acute treatment with the SSRI, fluoxetine, alters behavioral responses to social defeat stress in a sex- or social stress-dependent manner, male and female Syrian hamsters were subjected to one of three social defeat conditions: no defeat (placed into an empty resident aggressor (RA) cage), a single defeat by one RA for 15 min, or three consecutive defeats using different RAs for 5 min each. The day following social defeat, subjects were infused with either vehicle or fluoxetine (20 mg/kg, I.P.) 2 h prior to a 5 min social avoidance test. Overall, we found that fluoxetine increased social vigilance regardless of sex or defeat condition. We also found that fluoxetine affected social avoidance in a sex by stress intensity interaction, such that fluoxetine increased avoidance in no defeat males and in males defeated once but significantly increased avoidance in females only after three defeats. These data suggest that treatment with an SSRI could initially exacerbate the effects of social stress in both sexes. These data also emphasize the importance of including sex as a biological variable when investigating the efficacy of pharmacotherapy for stress-related disorders.

Chronic intranasal oxytocin reverses stress-induced social avoidance in female prairie voles

Social anxiety disorder (SAD) is a prevalent mental illness in both men and women, but current treatment approaches with selective serotonin reuptake inhibitors (SSRI) have limited success. The neuropeptide oxytocin (OXT) has become a therapeutic target due to its prosocial and anxiolytic effects. Nevertheless, no research has focused on the impact of chronic OXT treatment in animal models of SAD. Social defeat stress is an animal model of social conflict that reliably induces a social avoidance phenotype, reflecting symptoms observed in individuals suffering from SAD. Here, we used the socially monogamous prairie vole, which exhibits aggressive behavior in both sexes, to examine the effects of OXT and SSRI treatment following social defeat stress in males and females. Defeated voles became avoidant in unfamiliar social situations as early as one day after defeat experience, and this phenotype persisted for at least eight weeks. OXT receptor (OXTR) binding in mesocorticolimbic and paralimbic regions was reduced in defeated females during the eight-week recovery period. In males, serotonin 1A receptor binding was decreased in the basolateral amygdala and dorsal raphe nucleus starting at one week and four weeks post-defeat, respectively. Chronic intranasal treatment with OXT had a negative effect on sociability and mesolimbic OXTR binding in non-defeated females. However, chronic intranasal OXT promoted social engagement and increased mesolimbic OXTR binding in defeated females but not males. SSRI treatment led to only modest effects. This study identifies a sex-specific and stress-dependent function of intranasal OXT on mesolimbic OXTR and social behaviors.

Reverse translation of major depressive disorder symptoms: A framework for the behavioural phenotyping of putative biomarkers

BACKGROUND

Reverse translating putative biomarkers of depression from patients to animals is complex because Major Depressive Disorder (MDD) is a highly heterogenous condition. This review proposes an approach to reverse translation based on relating relevant bio-behavioural functions in laboratory rodents to MDD symptoms.

METHODS

This systematic review outlines symptom clusters assessed by psychometric tests of MDD and antidepressant treatment response including the Montgomery-Åsberg Depression Rating Scale, the Hamilton Depression Rating Scale, and the Beck Depression Inventory. Symptoms were related to relevant behavioural assays in laboratory rodents.

RESULTS

The resulting battery of tests includes passive coping, anxiety-like behaviours, sleep, caloric intake, cognition, psychomotor functions, hedonic reactivity and aversive learning. These assays are discussed alongside relevant clinical symptoms of MDD, providing a framework through which reverse translation of a biomarker can be interpreted.

LIMITATIONS

Certain aspects of MDD may not be quantified by tests in laboratory rodents, and their biological significance may not always be of clinical relevance.

CONCLUSIONS

Using this reverse translation approach, it is possible to clarify the functional significance of a putative biomarker in rodents and hence translate its contribution to specific clinical symptoms, or clusters of symptoms.

Presence of Mother Reduces Early-Life Social Stress: Linking the Alteration in Hypothalamic-Pituitary-Adrenal Axis and Serotonergic System

In this study, we examined whether the presence of mother suppresses early-life stressful social experience (SSE)-induced anxiety-like behavior and impairment of short-term memory later in life. On postnatal day (PND)-5, mothers with pups were grouped as follows: (i) control; (ii) maternal separation (MS); (iii) pups with mother experience the presence of a stranger (M+P-ST); and (iv) maternal separated pups experience the presence of a stranger (MSP-ST). Individuals were subjected to light-dark box and spontaneous alternation from PND-29 to 32. We observed that the MSP-ST group exhibits anxiety-like behavior and impairment in short-term memory. Further, SSE significantly elevated the adrenocorticotropic hormone, corticosterone and expression of glucocorticoid receptor (GR) in MSP-ST pups. Similarly, serotonin (5-hydroxytryptamine; 5-HT), dopamine, noradrenaline and expression of serotonin transporter levels were significantly elevated in MSP-ST pups. These observations suggest that during early postnatal days, the pups may recognize strangers by the sense of smell, and the presence of mother reduces the SSE-induced stress.

Effects of conditioned social fear on ethanol drinking and vice-versa in male mice

RATIONALE

Social anxiety disorder (SAD) is highly comorbid with alcohol use disorders, but the complex relationship between social fear and alcohol drinking is poorly understood due to the lack of specific animal models.

OBJECTIVES

We investigated whether social fear alters ethanol drinking under both stress-free and stress-inducing conditions and whether ethanol alleviates symptoms of social fear.

METHODS

We used the social fear conditioning (SFC) paradigm, an animal model with face and predictive validity to SAD, to induce specific social fear in male CD1 mice, i.e., without comorbid depression or anxiety-like behavior. Plasma corticosterone (CORT) levels were measured in conditioned (SFC⁺) and unconditioned (SFC^-) mice after exposure to non-social or social stimuli. Ethanol drinking was assessed in the two-bottle free-choice paradigm (1) for 16 days under stress-free conditions and (2) for 6 h after exposure to social stimuli. The effects of ethanol drinking and social fear on anxiety-like behavior and taste preference were tested in the elevated plus-maze and sucrose and quinine preference tests.

RESULTS

We show that exposure to social but not non-social stimuli leads to higher plasma CORT levels in SFC⁺ compared with SFC^- mice. We also show that social fear decreases voluntary ethanol consumption under stress-free conditions, but increases ethanol consumption after exposure to social stimuli. Ethanol drinking, on the other hand, reduces social fear without altering anxiety-like behavior, locomotor activity, and taste preference.

CONCLUSIONS

These results have important clinical connotations as they suggest that voluntary ethanol drinking might specifically reverse symptoms of social fear in a SAD-relevant animal model.

Protocol for a systematic review and meta-analysis of data from preclinical studies employing forced swimming test: an update

OBJECTIVE

Forced swimming test (FST) in rodents is a widely used behavioural test for screening antidepressants in preclinical research. Translational value of preclinical studies may be improved by appraisal of the quality of experimental design and risk of biases, which remains to be addressed for FST. The present protocol of a systematic review with meta-analysis aims to investigate the quality of preclinical studies employing FST to identify risks of bias in future publications. In addition, this protocol will help to determine the effect sizes (ES) for primary and secondary outcomes according to several aspects of the FST study design.

SEARCH STRATEGY SCREENING ANNOTATION DATA MANAGEMENT

Publications reporting studies testing different classes of antidepressants in FST will be collected from Medline, SCOPUS and Web of Science databases. A broad list of inclusion criteria will be applied excluding those studies whereby FST is used as a stressor or studies reporting data from co-treatments. For assessing the quality of the included publications, the quality checklist adapted by Collaborative Approach to Meta-Analysis and Review of Animal Data from Experimental Studies will be used. If the meta-analysis seems feasible, the ES and the 95% CI will be analysed. The heterogeneity between studies will be assessed by using the χ2statistic with n-1 degrees of freedom. Subgroup meta-analysis (meta-regression, and if necessary, stratified regression) will be performed when possible according to characteristics of study design and study quality to assess their impact on efficacy of the treatments. In addition, funnel plotting, Egger regression, and 'trim and fill' will be used to assess the risk of publication bias. Results of this protocol will help to create rational methodological guidelines for application of FST in rodents and improve the quality and translational value of preclinical research on antidepressant discovery.

REPORTING

A preliminary version of the present protocol has been preregistered with Systematic Review Facility (http://syrf.org.uk/). A preprint version of the current protocol has been registered with Open Science Framework (https://osf.io/9kxm4/). Results will be communicated in scientific meetings and peer-reviewed journals. We plan to conduct an anonymous and online survey within the scientific community to ask researchers about their perception of risk of bias and their experience with the publication of negative results.

PET imaging of the mouse brain reveals a dynamic regulation of SERT density in a chronic stress model

The serotonin transporter (SERT, Slc6a4) plays an important role in the regulation of serotonergic neurotransmission and its aberrant expression has been linked to several psychiatric conditions. While SERT density has been proven to be amenable to in vivo quantitative evaluation by positron emission tomography (PET) in humans, this approach is in its infancy for rodents. Here we set out to evaluate the feasibility of using small-animal PET employing [¹¹C]DASB ([¹¹C]-3-amino-4-(2-dimethylaminomethyl-phenylsulfanyl)-benzonitrile) as a radiotracer to measure SERT density in designated areas of the mouse brain. Using Slc6a4^+/+, Slc6a4^+/-, and Slc6a4^-/- mice as a genetic model of different SERT expression levels, we showed the feasibility of SERT imaging in the mouse brain with [¹¹C]DASB-PET. The PET analysis was complemented by an evaluation of SERT protein expression using western blot, which revealed a highly significant correlation between in vivo and ex vivo measurements. [¹¹C]DASB-PET was then applied to the examination of dynamic changes of SERT levels in different brain areas in the chronic corticosterone mouse model of chronic stress. The observed significant reduction in SERT density in corticosterone-treated mice was independently validated by and correlated with western blot analysis. This is the first demonstration of a quantitative in vivo evaluation of SERT density in subregions of the mouse brain using [¹¹C]DASB-PET. The evidenced decrease in SERT density in response to chronic corticosterone treatment adds a new dimension to the complex involvement of SERT in the pathophysiology of stress-induced mental illnesses.

Social defeat stress before pregnancy induces depressive-like behaviours and cognitive deficits in adult male offspring: correlation with neurobiological changes

BACKGROUND

Epidemiological surveys and studies with animal models have established a relationship between maternal stress and affective disorders in their offspring. However, whether maternal depression before pregnancy influences behaviour and related neurobiological mechanisms in the offspring has not been studied.

RESULTS

A social defeat stress (SDS) maternal rat model was established using the resident-intruder paradigm with female specific pathogen-free Wistar rats and evaluated with behavioural tests. SDS maternal rats showed a significant reduction in sucrose preference and locomotor and exploratory activities after 4 weeks of stress. In the third week of the experiment, a reduction in body weight gain was observed in SDS animals. Sucrose preference, open field, the elevated-plus maze, light-dark box, object recognition, the Morris water maze, and forced swimming tests were performed using the 2-month-old male offspring of the female SDS rats. Offspring subjected to pre-gestational SDS displayed enhanced anxiety-like behaviours, reduced exploratory behaviours, reduced sucrose preference, and atypical despair behaviours. With regard to cognition, the offspring showed significant impairments in the retention phase of the object recognition test, but no effect was observed in the acquisition phase. These animals also showed impairments in recognition memory, as the discrimination index in the Morris water maze test in this group was significantly lower for both 1 h and 24 h memory retention compared to controls. Corticosterone, adrenocorticotropic hormone, and monoamine neurotransmitters levels were determined using enzyme immunoassays or radioimmunoassays in plasma, hypothalamus, left hippocampus, and left prefrontal cortex samples from the offspring of the SDS rats. These markers of hypothalamic-pituitary-adrenal axis responsiveness and the monoaminergic system were significantly altered in pre-gestationally stressed offspring. Brain-derived neurotrophic factor (BDNF), cyclic adenosine monophosphate response element binding protein (CREB), phosphorylated CREB (pCREB), and serotonin transporter (SERT) protein levels were evaluated using western blotting with right hippocampus and right prefrontal cortex samples. Expression levels of BDNF, pCREB, and SERT in the offspring were also altered in the hippocampus and in the prefrontal cortex; however, there was no effect on CREB.

CONCLUSION

We conclude that SDS before pregnancy might induce depressive-like behaviours, cognitive deficits, and neurobiological alterations in the offspring.

Social Defeat Stress, Sex, and Addiction-Like Behaviors

Social confrontation is a form of social interaction in animals where two conspecific individuals confront each other in dispute over territory, during the formation of hierarchies, and during breeding seasons. Typically, a social confrontation involves a prevailing individual and a yielding individual. The prevailing individual often exhibits aggressive postures and launches attacks, whereas the yielding individual often adopts postures of defeat. The yielding or defeated animals experience a phenomenon known as social defeat stress, in which they show exaggerated stress as well as autonomic and endocrine responses that cause impairment of both the brain and body. In laboratory settings, one can reliably generate social defeat stress by allowing a naïve (or already defeated) animal to intrude into a home cage in which its resident has already established a territory or is nursing. This resident-intruder paradigm has been widely used in both males and females to study mechanisms in the brain that underlie the stress responses. Stress has profound effects on drug reward for cocaine, methamphetamine, alcohol, and opioids. Particularly, previous experiences with social defeat can exaggerate subsequent addiction-like behaviors. The extent of these addiction-like behaviors depends on the intensity, duration, frequency, and intermittency of the confrontation episodes. This chapter describes four types of social defeat stress: acute, repeated, intermittent, and chronic. Specifically, it focuses on social defeat stress models used in laboratories to study individual, sex, and animal strain differences in addiction-like behaviors.

Maternal nicotine exposure has severe cross-generational effects on offspring behavior

Our previous studies showed that paternal nicotine exposure can lead to hyperactivity in the offspring. Nevertheless, the cross-generational effects of maternal and biparental nicotine exposure remain unclear. In this study, female and male mice were exposed respectively by nicotine before pregnancy. The maternal pre-pregnancy nicotine exposure led to depression-like behaviors in the F1 offspring. However, after biparental pre-pregnancy nicotine exposure, seventy percentage of the offspring exhibited a depressive phenotype while 20% were hyperactive, and the remaining exhibited no obvious abnormal behavior. The cross-generational effects appeared to be mediated via disruption of the balance between GSK3 and p-GKS3 by nicotine. These results suggested that pre-pregnancy nicotine exposure can induce alterations in the behavior of the offspring, and the cross-generational effects of maternal nicotine exposure were particularly serious.

Neurobiological Mechanisms of Stress Resilience and Implications for the Aged Population

BACKGROUND

Stress is a common reaction to an environmental adversity, but a dysregulation of the stress response can lead to psychiatric illnesses such as major depressive disorder (MDD), post-traumatic stress disorder (PTSD), and anxiety disorders. Yet, not all individuals exposed to stress will develop psychiatric disorders; those with enhanced stress resilience mechanisms have the ability to adapt successfully to stress without developing persistent psychopathology. Notably, the potential to enhance stress resilience in at-risk populations may prevent the onset of stress-induced psychiatric disorders. This novel idea has prompted a number of studies probing the mechanisms of stress resilience and how it can be manipulated.

METHODS

Here, we review the neurobiological factors underlying stress resilience, with particular focus on the serotoninergic (5-HT), glutamatergic, and γ-Aminobutyric acid (GABA) systems, as well as the hypothalamic-pituitary axis (HPA) in rodents and in humans. Finally, we discuss stress resiliency in the context of aging, as the likelihood of mood disorders increases in older adults.

RESULTS

Interestingly, increased resiliency has been shown to slow aging and improved overall health and quality of life. Research in the neurobiology of stress resilience, particularly throughout the aging process, is a nascent, yet, burgeoning field.

CONCLUSION

Overall, we consider the possible methods that may be used to induce resilient phenotypes, prophylactically in at-risk populations, such as in military personnel or in older MDD patients. Research in the mechanisms of stress resilience may not only elucidate novel targets for antidepressant treatments, but also provide novel insight about how to prevent these debilitating disorders from developing.

Describing some behavioural animal models of anxiety and their mechanistics with special reference to oxidative stress and oxytocin relevance

It is now generally accepted that animal studies are playing an important role in the understanding of anxiety disorders, since they contribute to the current knowledge regarding the mechanisms and possible therapeutic approaches in anxiety. In the present review we will detail some essential aspects of behavioral animal models of anxiety related to social defeat paradigm, elevated plus maze, elevated zero or T maze, light/dark box, social interaction test or tests based on predator models, considering the latest theories and methodological approaches in this area of research, as well as our previous studies focusing on anxiety manifestations in a variety of species including rats, zebrafish, dogs and pigs. Moreover, in this context, we will focus on the recent theories concerning oxidative stress, as well as importance of oxytocin administration (especially the intranasal route). This could be important considering that these two factors are currently being investigated as possible mechanisms (oxidative stress status) and related therapeutic target (both intranasal oxytocin and antioxidants) in the pathology of the anxiety disorders.

Dominance behaviour in a non-aggressive flatfish, Senegalese sole (Solea senegalensis) and brain mRNA abundance of selected transcripts

Dominance is defined as the preferential access to limited resources. The present study aimed to characterise dominance in a non-aggressive flatfish species, the Senegalese sole (Solea senegalensis) by 1) identifying dominance categories and associated behaviours and 2) linking dominance categories (dominant and subordinate) with the abundance of selected mRNA transcripts in the brain. Early juveniles (n = 74, 37 pairs) were subjected to a dyadic dominance test, related to feeding, and once behavioural phenotypes had been described the abundance of ten selected mRNAs related to dominance and aggressiveness was measured in the brain. Late juveniles were subjected to two dyadic dominance tests (n = 34, 17 pairs), related to feeding and territoriality and one group test (n = 24, 4 groups of 6 fish). Sole feeding first were categorized as dominant and sole feeding second or not feeding as subordinate. Three social behaviours (i. "Resting the head" on another fish, ii. "Approaching" another fish, iii. "Swimming above another" fish) were associated with dominance of feeding. Two other variables (i. Total time occupying the preferred area during the last 2 hours of the 24 h test, ii. Organisms occupying the preferred area when the test ended) were representative of dominance in the place preference test. In all tests, dominant fish compared to subordinate fish displayed a significantly higher number of the behaviours "Rest the head" and "Approaches". Moreover, dominant sole dominated the sand at the end of the test, and in the group test dominated the area close to the feed delivery point before feed was delivered. The mRNA abundance of the selected mRNAs related to neurogenesis (nrd2) and neuroplasticity (c-fos) in dominant sole compared to subordinate were significantly different. This is the first study to characterise dominance categories with associated behaviours and mRNA abundance in Senegalese sole and provides tools to study dominance related problems in feeding and reproduction in aquaculture.

Repeated restraint stress lowers the threshold for response to third ventricle CRF administration

Rats and mice exposed to repeated stress or a single severe stress exhibit a sustained increase in energetic, endocrine, and behavioral response to subsequent novel mild stress. This study tested whether the hyper-responsiveness was due to a lowered threshold of response to corticotropin releasing factor (CRF) or an exaggerated response to a standard dose of CRF. Male Sprague-Dawley rats were subjected to 3h of restraint on each of 3 consecutive days (RRS) or were non-restrained controls. RRS caused a temporary hypophagia but a sustained reduction in body weight. Eight days after the end of restraint, rats received increasing third ventricle doses of CRF (0-3.0μg). The lowest dose of CRF (0.25μg) increased corticosterone release in RRS, but not control rats. Higher doses caused the same stimulation of corticosterone in the two groups of rats. Fifteen days after the end of restraint, rats were food deprived during the light period and received increasing third ventricle doses of CRF at the start of the dark period. The lowest dose of CRF inhibited food intake during the first hour following infusion in RRS, but not control rats. All other doses of CRF inhibited food intake to the same degree in both RRS and control rats. The lowered threshold of response to central CRF is consistent with the chronic hyper-responsiveness to CRF and mild stress in RRS rats during the post-restraint period.

Anxiety and depression with neurogenesis defects in exchange protein directly activated by cAMP 2-deficient mice are ameliorated by a selective serotonin reuptake inhibitor, Prozac

Intracellular cAMP and serotonin are important modulators of anxiety and depression. Fluoxetine, a selective serotonin reuptake inhibitor (SSRI) also known as Prozac, is widely used against depression, potentially by activating cAMP response element-binding protein (CREB) and increasing brain-derived neurotrophic factor (BDNF) through protein kinase A (PKA). However, the role of Epac1 and Epac2 (Rap guanine nucleotide exchange factors, RAPGEF3 and RAPGEF4, respectively) as potential downstream targets of SSRI/cAMP in mood regulations is not yet clear. Here, we investigated the phenotypes of Epac1 (Epac1(-/-)) or Epac2 (Epac2(-/-)) knockout mice by comparing them with their wild-type counterparts. Surprisingly, Epac2(-/-) mice exhibited a wide range of mood disorders, including anxiety and depression with learning and memory deficits in contextual and cued fear-conditioning tests without affecting Epac1 expression or PKA activity. Interestingly, rs17746510, one of the three single-nucleotide polymorphisms (SNPs) in RAPGEF4 associated with cognitive decline in Chinese Alzheimer's disease (AD) patients, was significantly correlated with apathy and mood disturbance, whereas no significant association was observed between RAPGEF3 SNPs and the risk of AD or neuropsychiatric inventory scores. To further determine the detailed role of Epac2 in SSRI/serotonin/cAMP-involved mood disorders, we treated Epac2(-/-) mice with a SSRI, Prozac. The alteration in open field behavior and impaired hippocampal cell proliferation in Epac2(-/-) mice were alleviated by Prozac. Taken together, Epac2 gene polymorphism is a putative risk factor for mood disorders in AD patients in part by affecting the hippocampal neurogenesis.

Mild Traumatic Brain Injury with Social Defeat Stress Alters Anxiety, Contextual Fear Extinction, and Limbic Monoamines in Adult Rats

Mild traumatic brain injury (mTBI) produces symptoms similar to those typifying posttraumatic stress disorder (PTSD) in humans. We sought to determine whether a rodent model of stress concurrent with mTBI produces characteristics of PTSD such as impaired contextual fear extinction, while also examining concurrent alterations to limbic monoamine activity in brain regions relevant to fear and anxiety states. Male rats were exposed to social stress or control conditions immediately prior to mTBI induction, and 6 days later were tested either for anxiety-like behavior using the elevated plus maze (EPM), or for contextual fear conditioning and extinction. Brains were collected 24 h after EPM testing, and tissue from various limbic regions analyzed for content of monoamines, their precursors and metabolites using HPLC with electrochemical detection. Either social defeat or mTBI alone decreased time spent in open arms of the EPM, indicating greater anxiety-like behavior. However, this effect was enhanced by the combination of treatments. Further, rats exposed to both social defeat and mTBI exhibited greater freezing within extinction sessions compared to all other groups, suggesting impaired contextual fear extinction. Social defeat combined with mTBI also had greater effects on limbic monoamines than either insult alone, particularly with respect to serotonergic effects associated with anxiety and fear learning. The results suggest social stress concurrent with mTBI produces provides a relevant animal model for studying the prevention and treatment of post-concussive psychobiological outcomes.

Anger Emotional Stress Influences VEGF/VEGFR2 and Its Induced PI3K/AKT/mTOR Signaling Pathway

Objective. We discuss the influence of anger emotional stress upon VEGF/VEGFR2 and its induced PI3K/AKT/mTOR signal pathway. Methods. We created a rat model of induced anger (anger-out and anger-in) emotional response using social isolation and resident-intruder paradigms and assessed changes in hippocampus' VEGF content, neuroplasticity, and the PI3K/AKT/mTOR signaling pathway. Results. The resident-intruder method successfully generated anger-out and anger-in models that differed significantly in composite aggression score, aggression incubation, open field behavior, sucrose preference, and weight gain. Anger emotional stress decreased synaptic connections and VEGFR2 expression. Anger emotional stress led to abnormal expression of VEGF/VEGFR2 mRNA and protein and disorderly expression of key factors in the PI3K/AKT/mTOR signal pathway. Fluoxetine administration ameliorated behavioral abnormalities and damage to hippocampal neurons caused by anger emotional stress, as well as abnormal expression of some proteins in VEGF/VEGFR2 and its induced PI3K/AKT/mTOR signal pathway. Conclusion. This research provides a detailed classification of anger emotion and verifies its influence upon VEGF and the VEGF-induced signaling pathway, thus providing circumstantial evidence of mechanisms by which anger emotion damages neurogenesis. As VEGFR2 can promote neurogenesis and vasculogenesis in the hippocampus and frontal lobe, these results suggest that anger emotional stress can result in decreased neurogenesis.

Ethological Evaluation of the Effects of Social Defeat Stress in Mice: Beyond the Social Interaction Ratio

In rodents, repeated exposure to unavoidable aggression followed by sustained sensory treat can lead to prolonged social aversion. The chronic social defeat stress model explores that phenomenon and it has been used as an animal model for human depression. However, some authors have questioned whether confounding effects may arise as the model also boosts anxiety-related behaviors. Despite its wide acceptance, most studies extract limited information from the behavior of the defeated animal. Often, the normalized occupancy around the social stimulus, the interaction zone, is taken as an index of depression. We hypothesized that this parameter is insufficient to fully characterize the behavioral consequences of this form of stress. Using an ethological approach, we showed that repeated social defeat delayed the expression of social investigation in long (10 min) sessions of social interaction. Also, the incidence of defensive behaviors, including stretched-attend posture and high speed retreats, was significantly higher in defeated mice in comparison to controls. Interestingly, a subpopulation of defeated mice showed recurrent and non-habituating stretched-attend posture and persistent flights during the entire session. Two indexes were created based on defensive behaviors to show that only recurrent flights correlates with sucrose intake. Together, the present study corroborates the idea that this model of social stress can precipitate a myriad of behaviors not readily disentangled. We propose that long sessions (>150 s) and detailed ethological evaluation during social interaction tests are necessary to provide enough information to correctly classify defeated animals in terms of resilience and susceptibility to social defeat stress.

Histologic and Biomechanical Evaluation of the Effects of Social Stress and the Antidepressant Fluoxetine on Tendon Healing in Rats

Impaired wound healing in humans under psychological stress and the positive effects of antidepressant drugs on wound healing were also shown in the literature. However, there are currently no studies regarding the effects of antidepressant drugs on tendon healing. The aim of this study was to compare tendon healing under normal conditions versus social stress. We also aimed to perform a histological and biomechanical analysis of the effects of the antidepressant drug fluoxetine on tendon healing. Sixty Sprague Dawley rats were divided into six groups. A social stress regimen was used to stress the rats. The use of fluoxetine in the social stress group yielded significantly better biomechanical results and the collagen organizations of the fluoxetine group were more similar to the normal tendon collagen organization. Fluoxetine seems to inhibit the negative effects of stress on tendon healing and seems to improve tendon healing.

LEVELS OF EVIDENCE

Level 5.

Pathogenesis of depression: Insights from human and rodent studies

Major depressive disorder (MDD) will affect one out of every five people in their lifetime and is the leading cause of disability worldwide. Nevertheless, mechanisms associated with the pathogenesis of MDD have yet to be completely understood and current treatments remain ineffective in a large subset of patients. In this review, we summarize the most recent discoveries and insights for which parallel findings have been obtained in human depressed subjects and rodent models of mood disorders in order to examine the potential etiology of depression. These mechanisms range from synaptic plasticity mechanisms to epigenetics and the immune system where there is strong evidence to support a functional role in the development of specific depression symptomology. Ultimately we conclude by discussing how novel therapeutic strategies targeting central and peripheral processes might ultimately aid in the development of effective new treatments for MDD and related stress disorders.

Social defeat as an animal model for depression

Depression is one of the most disabling medical conditions in the world today, yet its etiologies remain unclear and current treatments are not wholly effective. Animal models are a powerful tool to investigate possible causes and treatments for human diseases. We describe an animal model of social defeat as a possible model for human depression. We discuss the paradigm, behavioral correlates to depression, and potential underlying neurobiological mechanisms with an eye toward possible future therapies.

Chronic and acute effects of stress on energy balance: are there appropriate animal models?

Stress activates multiple neural and endocrine systems to allow an animal to respond to and survive in a threatening environment. The corticotropin-releasing factor system is a primary initiator of this integrated response, which includes activation of the sympathetic nervous system and the hypothalamic-pituitary-adrenal (HPA) axis. The energetic response to acute stress is determined by the nature and severity of the stressor, but a typical response to an acute stressor is inhibition of food intake, increased heat production, and increased activity with sustained changes in body weight, behavior, and HPA reactivity. The effect of chronic psychological stress is more variable. In humans, chronic stress may cause weight gain in restrained eaters who show increased HPA reactivity to acute stress. This phenotype is difficult to replicate in rodent models where chronic psychological stress is more likely to cause weight loss than weight gain. An exception may be hamsters subjected to repeated bouts of social defeat or foot shock, but the data are limited. Recent reports on the food intake and body composition of subordinate members of group-housed female monkeys indicate that these animals have a similar phenotype to human stress-induced eaters, but there are a limited number of investigators with access to the model. Few stress experiments focus on energy balance, but more information on the phenotype of both humans and animal models during and after exposure to acute or chronic stress may provide novel insight into mechanisms that normally control body weight.

To stress or not to stress: a question of models

Stress research is a rapidly evolving field that encompasses numerous disciplines ranging from neuroscience to metabolism. With many new researchers migrating into the field, navigating the hows and whys of specific research questions can sometimes be enigmatic given the availability of so many models in the stress field. Additionally, as with every field, there are many seemingly minor experimental details that can have dramatic influences on data interpretation, although many of these are unknown to those not familiar with the field. The aim of this overview is to provide some suggestions and points to guide researchers moving into the stress field and highlight relevant methodological points that they should consider when choosing a model for stress and deciding how to structure a study. We briefly provide a primer on the basics of endpoint measurements in the stress field, factors to consider when choosing a model for acute stress, the difference between repeated and chronic stress, and importantly, influencing variables that modulate endpoints of analysis in stress work.

Short-term fluoxetine treatment induces neuroendocrine and behavioral anxiogenic-like responses in adolescent male rats

Fluoxetine (FLX) is prescribed to treat depression and anxiety in adolescent patients. However, FLX has anxiogenic effects during the acute phase of treatment, and caution has been raised due to increased suicidal thinking and behavior. Herein, we sought to study in adolescent (35-day-old) male rats, the effects of short-term FLX treatment (10 mg/kg/day, i.p. for 3-4 days) on hypothalamic-pituitary-adrenal axis activity, serotonin (5-hidroxytriptamine, 5-HT) transporter (SERT) mRNA expression in the dorsal raphe nucleus (DRN), energy balance-related variables and behavioral profiles in the holeboard. Our results revealed that daily FLX administration increased plasma corticosterone (B) concentrations without affecting basal gene expression of corticotrophin releasing hormone in the hypothalamic paraventricular nucleus (PVN) nor of pro-opiomelanocortin in the anterior pituitary. However, FLX had significant effects increasing the mRNA expression of PVN arginine vasopressin (AVP) and reducing SERT mRNA levels in the dorsolateral subdivision of the DRN. In the holeboard, FLX-induced anxiety/emotionality-like behaviors. As expected, FLX treatment was endowed with anorectic effects and reduced body weight gain. Altogether, our study shows that short-term FLX treatment results in physiological, neuroendocrine and behavioral stress-like effects in adolescent male rats. More importantly, considering that the AVP- and 5-HTergic systems: (1) are intimately involved in regulation of the stress response; (2) are regulated by sex hormones and (3) are related to regulation of aggressive behaviors, our results highlight the potential significance of these systems mediating the anxiogenic/emotionality/stress-like responses of adolescent male rats to short-term FLX treatment.

Preliminary evidence that sub-chronic citalopram triggers the re-evaluation of value in intimate partnerships

Depression frequently involves disrupted inter-personal relationships, while treatment with serotonergic anti-depressants can interfere with libido and sexual function. However, little is known about how serotonin activity influences appraisals of intimate partnerships. Learning more could help to specify how serotonergic mechanisms mediate social isolation in psychiatric illness. Forty-four healthy heterosexual adults, currently in romantic relationships, received 8 days treatment with the selective serotonin re-uptake inhibitor citalopram (N = 21; 10 male) or placebo (N = 23; 12 male). Participants viewed photographs of unknown, heterosexual couples and made a series of judgements about their relationships. Participants also indicated the importance of relationship features in their own close partnerships, and close partnerships generally. Citalopram reduced the rated quality of couples' physical relationships and the importance attributed to physical and intimate aspects of participants' own relationships. In contrast, citalopram also enhanced the evaluated worth of mutual trust in relationships. Amongst males, citalopram was associated with judgements of reduced turbulence and bickering in others' relationships, and increased male dominance. These data constitute preliminary evidence that enhancing serotonin activity modulates cognitions about sexual activity as part of a re-appraisal of sources of value within close intimate relationships, enhancing the judged importance of longer-term benefits of trust and shared experiences.

Effects of curcumin on learning and memory deficits, BDNF, and ERK protein expression in rats exposed to chronic unpredictable stress

Accumulating evidence suggests that cognitive processes, such as learning and memory, are affected in depression and antidepressant treatment may ameliorate cognitive impairments. Recent studies have shown that curcumin exhibits antidepressant-like effects. The aim of the present study was to determine whether curcumin administration influences chronic unpredictable stress (CUS)-induced cognitive deficits and explores underlying mechanisms. Male Wistar rats were subjected to CUS protocol for a period of 5 weeks to induce depression. The depressive-like behavior was tested using sucrose preference test, open field test and Morris water maze test. Effects of curcumin on brain-derived neurotrophic factor (BDNF) and extracellular signal-regulated kinase (ERK) levels in the hippocampus were also examined. Chronic treatment with curcumin significantly reversed the CUS-induced behavioral and cognitive parameters (reduced sucrose preference and impaired learning and memory function) in stressed rats. Additionally, CUS reduced hippocampal BDNF and ERK levels, while curcumin effectively reversed these alterations. Taken together, our results indicate that the antidepressant-like effects of curcumin in CUS rats are related to its aptitude to promote BDNF and ERK in the hippocampus.

Effects of stressors on the reinforcing efficacy of nicotine in adolescent and adult rats

RATIONALE AND OBJECTIVES

Stress increases drug intake. This depends on the stressor, drug, and aspect of drug seeking assessed. The objectives of these experiments done in adolescent and adult male rats were to (1) examine social defeat effects on acquisition of nicotine self-administration (SA) and the reinforcing efficacy of nicotine and (2) determine the effects of acute exposure to intermittent footshock (FS) or yohimbine on the reinforcing efficacy of nicotine.

METHODS

In experiment 1, rats received four defeat exposures prior to nicotine SA acquisition and progressive ratio (PR) SA sessions (30 μg/kg nicotine/infusion). Exposure to an olfactory cue previously paired with defeat was also tested on responding maintained by nicotine on the PR schedule. In experiments 2 and 3, the effects of FS (5 and 10 min) or yohimbine (0.625 and 1.25 mg/kg, i.p.) on PR responding for nicotine (15, 30, or 60 μg/kg/infusion) were assessed. Adolescents were aged PD34-36 and adults PD81-85 at the beginning of nicotine SA training.

RESULTS

Defeat did not affect nicotine SA acquisition. Prior exposure to defeat or a defeat-paired olfactory cue did not affect PR responding for nicotine. FS modestly decreased PR responding in adolescents at the middle nicotine infusion dose. Yohimbine increased PR responding independent of nicotine infusion dose and age.

CONCLUSIONS

Together with previous work with other drugs, our data indicate that the effects of stress on the reinforcing efficacy of nicotine are stressor- and drug-dependent. This suggests that there is heterogeneity among stressors on how they affect neuronal systems underlying drug intake.

Urine scent marking (USM): a novel test for depressive-like behavior and a predictor of stress resiliency in mice

Decreased interest in pleasurable stimuli including social withdrawal and reduced libido are some of the key symptomatic criteria for major depression, and thus assays that measure social and sexual behavior in rodents may be highly appropriate for modeling depressive states. Here we present a novel approach for validating rodent models of depression by assessing male urine scent marking (USM) made in consequence to a spot of urine from a proestrous female. USM is an ethologically important form of sexual communication expressed by males to attract females. The expression of this behavior is highly sensitive and adaptive to environmental cues and social status. We hypothesized that male USM behavior offers a naturalistic measure of social motivation that can be used to evaluate hedonic behaviors relevant to the study of mood disorders. We demonstrated that 1) adult male mice displayed a strong preference for marking proestrous female urine with a high degree of specificity, 2) exposure to chronic social defeat profoundly decreased USM whereas exposure to environmental enrichment increased USM, 3) the standard antidepressant fluoxetine reversed declines in USM induced by social defeat, 4) USM behavior closely correlated with other hedonic measures, and 5) USM scores in non-stressed mice predicted behavioral outcomes after defeat exposure such that mice displaying high preference for marking female urine prior to social defeat showed behavioral resiliency after social defeat. The findings indicate that the USM test is a sensitive, validated measure of psychosocial stress effects that has high predictive value for examination of stress resiliency and vulnerability and their neurobiological substrates.

The effect of short-term stress on serotonin gene expression in high and low resilient macaques

Female cynomolgus monkeys exhibit different degrees of reproductive dysfunction with moderate metabolic and psychosocial stress. When stressed with a paradigm of relocation and diet for 60 days, or 2 menstrual cycles, highly stress resilient monkeys continue to ovulate during both stress cycles (HSR); medium stress resilient monkeys ovulate once (MSR) and stress sensitive monkeys do not ovulate for the entire 60 days (SS). This study examines serotonin-related gene expression in monkeys with different sensitivity to stress and exposed to 5 days of moderate stress. Monkeys were first characterized as HSR, MSR or SS. After resumption of menstrual cycles, each monkey was re-stressed for 5 days in the early follicular phase. The expression of 3 genes pivotal to serotonin neural function was assessed in the 3 groups of monkeys (n=4-5/group). Tryptophan hydroxylase 2 (TPH2), the serotonin reuptake transporter (SERT), and the 5HT1A autoreceptor mRNAs expression were determined at 4 morphological levels of the dorsal raphe nucleus with in situ hybridization (ISH) using digoxigenin-incorporated riboprobes. In addition, cFos was examined with immunohistochemistry. Positive pixel area and/or cell number were measured. All data were analyzed with ANOVA (3 groups) and with a t-test (2 groups). After 5 days of stress, TPH2, SERT, 5HT1A and cFos were significantly lower in the SS group than the HSR group (p<0.05, all). This pattern of expression was the same as the pattern observed in the absence of stress in previous studies. Therefore, the ratio of the HSR/SS expression of each serotonergic gene was calculated in the presence and absence of stress. There was little or no difference in the ratio of HSR/SS gene expression in the presence or absence of stress. Moreover, cFos expression indicates that overall, cell activation in the dorsal raphe nucleus and periaquaductal gray is lower in SS than HSR animals. These data suggest that the serotonin system may set the sensitivity or resilience of the individual, but serotonin-related gene expression may not rapidly respond to moderate stress in nonhuman primates.

Social stress models in depression research: what do they tell us?

Interest has recently surged in the use of social stress models, especially social defeat. Such interest lies both in the recognition that stressors of social origin play a major role in human psychopathologies and in the acknowledgement that natural and hence ethologically-based stress models have important translational value. The use of the most recent technology has allowed the recognition of the mechanisms through which social defeat might have enduring psychoneuroendocrine effects, especially social avoidance and anhedonia, two behaviours relevant to human depression. In view of the sensitivity of these behavioural outcomes to repeated antidepressant treatments, the social defeat model has been proposed as a possible animal model of depression. The present survey is aimed at examining the limits of such an interpretation and focuses on methodological aspects and on the relevance of social defeat to the study of anxiety-related pathologies.

Stress produces aversion and potentiates cocaine reward by releasing endogenous dynorphins in the ventral striatum to locally stimulate serotonin reuptake

Activation of the dynorphin/κ-opioid receptor (KOR) system by repeated stress exposure or agonist treatment produces place aversion, social avoidance, and reinstatement of extinguished cocaine place preference behaviors by stimulation of p38α MAPK, which subsequently causes the translocation of the serotonin transporter (SERT, SLC6A4) to the synaptic terminals of serotonergic neurons. In the present study we extend those findings by showing that stress-induced potentiation of cocaine conditioned place preference occurred by a similar mechanism. In addition, SERT knock-out mice did not show KOR-mediated aversion, and selective reexpression of SERT by lentiviral injection into the dorsal raphe restored the prodepressive effects of KOR activation. Kinetic analysis of several neurotransporters demonstrated that repeated swim stress exposure selectively increased the V(max) but not K(m) of SERT without affecting dopamine transport or the high-capacity, low-affinity transporters. Although the serotonergic neurons in the dorsal raphe project throughout the forebrain, a significant stress-induced increase in cell-surface SERT expression was only evident in the ventral striatum, and not in the dorsal striatum, hippocampus, prefrontal cortex, amygdala, or dorsal raphe. Stereotaxic microinjections of the long-lasting KOR antagonist norbinaltorphimine demonstrated that local KOR activation in the nucleus accumbens, but not dorsal raphe, mediated this stress-induced increase in ventral striatal surface SERT expression. Together, these results support the hypothesis that stress-induced activation of the dynorphin/KOR system produces a transient increase in serotonin transport locally in the ventral striatum that may underlie some of the adverse consequences of stress exposure, including the potentiation of the rewarding effects of cocaine.

Chronic social defeat up-regulates expression of the serotonin transporter in rat dorsal raphe nucleus and projection regions in a glucocorticoid-dependent manner

Chronic stress and dysfunction of the serotonergic system in the brain have been considered two of the major risks for development of depression. In this study, adult Fischer 344 rats were subjected to a regimen of chronic social defeat (CSD). To mimic stressful conditions, some rats were not exposed to CSD, but instead treated with corticosterone (CORT) in oral solution while maintained in their home cage. Protein levels of the serotonin transporter (SERT) in the dorsal raphe nucleus (DRN), hippocampus, frontal cortex, and amygdala were examined by Western blotting or immunofluorescence staining. The results showed that CSD up-regulated SERT protein levels in the DRN, hippocampus, frontal cortex, and amygdala regions. This up-regulation was abolished or prevented by adrenalectomy, or treatment with antagonists of corticosteroid receptors mifepristone and spironolactone, alone or in combination. Similarly, up-regulated SERT protein levels in these brain regions were also observed in rats treated with oral CORT ingestion, which was analogously prevented by treatment with mifepristone and spironolactone. Furthermore, both CSD- and CORT-induced up-regulation of SERT protein levels in the DRN and three brain regions were attenuated by simultaneous treatment with fluoxetine, an antidepressant that specifically inhibits serotonin reuptake. The results indicate that up-regulation in SERT protein levels in the DRN and forebrain limbic structures caused by CSD regimen was mainly motivated by CORT through corticosteroid receptors. The present findings demonstrate that chronic stress is closely correlated with the serotonergic system by acting on the regulation of the SERT expression in the DRN and its projection regions, which may contribute to the development of depression.

Fluoxetine prevents development of an early stress-related molecular signature in the rat infralimbic medial prefrontal cortex. Implications for depression?

Background

Psychological stress, particularly in chronic form, can lead to mood and cognitive dysfunction and is a major risk factor in the development of depressive states. How stress affects the brain to cause psychopathologies is incompletely understood. We sought to characterise potential depression related mechanisms by analysing gene expression and molecular pathways in the infralimbic medial prefrontal cortex (ILmPFC), following a repeated psychological stress paradigm. The ILmPFC is thought to be involved in the processing of emotionally contextual information and in orchestrating the related autonomic responses, and it is one of the brain regions implicated in both stress responses and depression.

Results

Genome-wide microarray analysis of gene expression showed sub-chronic restraint stress resulted predominantly in a reduction in transcripts 24 hours after the last stress episode, with 239 genes significantly decreased, while just 24 genes had increased transcript abundance. Molecular pathway analysis using DAVID identified 8 pathways that were significantly enriched in the differentially expressed gene list, with genes belonging to the brain-derived neurotrophic factor – neurotrophin receptor tyrosine kinase 2 (BDNF-Ntrk2) pathway most enriched. Of the three intracellular signalling pathways that are downstream of Ntrk2, real-time quantitative PCR confirmed that only the PI3K-AKT-GSK3B and MAPK/ERK pathways were affected by sub-chronic stress, with the PLCγ pathway unaffected. Interestingly, chronic antidepressant treatment with the selective serotonin reuptake inhibitor, fluoxetine, prevented the stress-induced Ntrk2 and PI3K pathway changes, but it had no effect on the MAPK/ERK pathway.

Conclusions

These findings indicate that abnormal BDNF-Ntrk2 signalling may manifest at a relatively early time point, and is consistent with a molecular signature of depression developing well before depression-like behaviours occur. Targeting this pathway prophylactically, particularly in depression-susceptible individuals, may be of therapeutic benefit.

Chronic psychosocial stress results in sensitization of the HPA axis to acute heterotypic stressors despite a reduction of adrenal in vitro ACTH responsiveness

Although chronic psychosocial stress is often accompanied by changes in basal hypothalamo-pituitary-adrenal (HPA) axis activity, it is vital for a chronically-stressed organism to mount adequate glucocorticoid (GC) responses when exposed to acute challenges. The main aim of the present study was to test whether this is true or not for the chronic subordinate colony housing (CSC, 19 days) paradigm, an established and clinically relevant mouse model of chronic psychosocial stress. As shown previously, CSC mice are characterized by unaffected morning and decreased evening plasma corticosterone (CORT) levels despite enlarged adrenals, suggesting a maladaptive breakdown of adrenal functioning. Plasma CORT levels, determined by repeated blood sampling via jugular vein catheters, as well as relative right adrenal CORT content were increased in CSC compared with single-housed control (SHC) mice in response to acute elevated platform (EPF, 5min) exposure. However, in vitro stimulation of adrenal explants with physiological and pharmacological doses of ACTH revealed an attenuated responsiveness of both the left and right adrenal glands following CSC, despite mRNA and/or protein expression of melanocortin 2 receptor (Mc2r), Mc2r accessory protein (MRAP), and key enzymes of steroidogenesis were not down-regulated. Taken together, we show that chronic psychosocial stressor exposure impairs in vitro ACTH responsiveness of both the left and right adrenal glands, whereas it increases adrenal responsiveness to an acute heterotypic stressor in vivo. This suggests that an additional factor present during acute stressor exposure in vivo rescues left and right adrenal ACTH sensitivity, or itself acts as CORT secretagogue in chronically stressed CSC mice.

Neurogenetics of emotional reactivity to stress in animals

There is much evidence for the involvement of central monoaminergic systems, the key targets of stress, in the regulation of mood. Animal and human findings indicate that genetics play a role in the etiology of mood disorders, and so we selected divergent inbred rat strains according to their anxiety-related behaviors on exposure to novel environments. We compared these strains for psychoneuroendocrine response to stressors and/or antidepressants. Molecular genetic studies were also performed to localize the genomic regions associated with these strain-dependent anxiety profiles. We then examined human results indicating that allelic variations in the serotonin transporter (5-HTT) may play a role in the etiology of neuroticism and depression. Thus, we compared inbred rat strains for the 5-HTT, with regard to central and peripheral (platelet) protein expression and function, and the consequences of local application of a selective serotonin reuptake inhibitor (SSRI) on extracellular serotonin (5-HT) levels. Our results indicate that spontaneously hypertensive rats and Lewis rats (LEW) selectively diverge in terms of anxiety-related behaviors and that this divergence is located on chromosome 4. The use of social defeat in LEW and the analysis of its psychoneuroendocrine consequences strongly suggest that such a paradigm, which is sensitive to repeated SSRI treatment, models posttraumatic stress disorder. The Wistar-Kyoto rat may be an adequate model to study the consequences of a genetically driven hypersensitivity to stress and noradrenergic antidepressants. Our most recent findings show that the Fischer 344 and LEW strains differ in protein expression and function of hippocampal and platelet 5-HTT; the divergence in protein expression is not due to allelic variations in the gene-coding sequences and leads to marked differences in extracellular 5-HT levels under basal conditions or SSRI. These examples illustrate how the use of inbred rat strains may complement our knowledge on the genetics of behavior, in the same way as the use of transgenic mice.

The imposition of, but not the propensity for, social subordination impairs exploratory behaviors and general cognitive abilities

Imposed social subordination, such as that which accompanies physical defeat or alienation, has been associated with impaired cognitive function in both human and non-human animals. Here we examined whether domain-specific and/or domain-general learning abilities (c.f. general intelligence) are differentially influenced by the imposition of social subordination. Furthermore, we assessed whether the impact of subordination on cognitive abilities was the result of imposed subordination per se, or if it reflected deficits intrinsically expressed in subjects that are predisposed to subordination. Subordinate and dominant behaviors were assessed in two groups of CD-1 male mice. In one group (Imposed Stratification), social stratification was imposed (through persistent physical defeat in a colonized setting) prior to the determination of cognitive abilities, while in the second group (Innate Stratification), an assessment of social stratification was made after cognitive abilities had been quantified. Domain-specific learning abilities were measured as performance on individual learning tasks (odor discrimination, fear conditioning, spatial maze learning, passive avoidance, and egocentric navigation) while domain-general learning abilities were determined by subjects' aggregate performance across the battery of learning tasks. We observed that the imposition of subordination prior to cognitive testing decreased exploratory tendencies, moderately impaired performance on individual learning tasks, and severely impaired general cognitive performance. However, similar impairments were not observed in subjects with a predisposition toward a subordinate phenotype (but which had not experienced physical defeat at the time of cognitive testing). Mere colonization, regardless of outcome (i.e., stratification), was associated with an increase in stress-induced serum corticosterone (CORT) levels, and thus CORT elevations were not themselves adequate to explain the effects of imposed stratification on cognitive abilities. These findings indicate that absent the imposition of subordination, individuals with subordinate tendencies do not express learning impairments. This observation could have important ramifications for individuals in environments where social stratification is prevalent (e.g., schools or workplace settings).

Models for depression in drug screening and preclinical studies: Future directions

The basic consideration in the field of antidepressants is that tests to model depression do not exist, as depression etiopathology is unknown. So far, any kind of proposed model for depression needs to satisfy construct, face and predictive validities. In the present editorial, this idea is challenged, based on the fact that “old” methods can only reveal therapeutical “me-too” drugs and that there is no longer a need of therapeutical “me-too” drugs in the field of antidepressants. Since reduction in the number of antidepressant non-responders is a real medical need, the predictive validity of animal models will be challenged in the future, as the new methods should be based on antidepressant-insensitive animals. Moreover, antidepressants exert similar effects in depressed and non-depressed subjects, but mood normalization is only induced in depressed patients. This implies that the use of normal cells and animals only involves pharmacological rather than therapeutical actions of drugs. Therefore, the use of environmental-induced changes, in the hope that these can evidence antidepressant-insensitive animals, will predominantly be used in the future. In the choice of experimental settings, other factors need to be taken into consideration: (1) gender of animals, as depression affects females more than males, (2) natural rhythmicity in drug effects; (3) pharmacokinetics; and (4) possible biomarker(s) to be measured. There are no golden recipes to discover new antidepressants but the experimental long-term strategy should very clearly be declared before starting the experiments.

Stress and animal models of inflammatory bowel disease--an update on the role of the hypothalamo-pituitary-adrenal axis

Chronic psychosocial stress has been repeatedly shown in humans to be a risk factor for the development of several affective and somatic disorders, including inflammatory bowel diseases (IBD). There is also a large body of evidence from rodent studies indicating a link between stress and gastrointestinal dysfunction, resembling IBD in humans. Despite this knowledge, the detailed underlying neuroendocrine mechanisms are not sufficiently understood. This is due, in part, to a lack of appropriate animal models, as most commonly used rodent stress paradigms do not adequately resemble the human situation and/or do not cause the development of spontaneous colitis. Therefore, our knowledge regarding the link between stress and IBD is largely based on rodent models with low face and predictive validity, investigating the effects of unnatural stressors on chemically induced colitis. These studies have consistently reported that hypothalamo-pituitary-adrenal (HPA) axis activation during stressor exposure has an ameliorating effect on the severity of a chemically induced colitis. However, to show the biological importance of this finding, it needs to be replicated in animal models employing more clinically relevant stressors, themselves triggering the development of spontaneous colitis. Important in view of this, recent studies employing chronic/repeated psychosocial stressors were able to demonstrate that such stressors indeed cause the development of spontaneous colitis and, thus, represent promising tools to uncover the mechanisms underlying stress-induced development of IBD. Interestingly, in these models the development of spontaneous colitis was paralleled by decreased anti-inflammatory glucocorticoid (GC) signaling, whereas adrenalectomy (ADX) prior to stressor exposure prevented its development. These findings suggest a more complex role of the HPA axis in the development of spontaneous colitis. In the present review I summarize the available human and rodent data in order to provide a comprehensive understanding of the biphasic role of the HPA axis and/or the GC signaling during stressor exposure in terms of spontaneous colitis development.