Update in the methodology of the chronic stress paradigm: internal control matters

To date, the reliability of induction of a depressive-like state using chronic stress models is confronted by many methodological limitations. We believe that the modifications to the stress paradigm in mice proposed herein allow some of these limitations to be overcome. Here, we discuss a variant of the standard stress paradigm, which results in anhedonia. This anhedonic state was defined by a decrease in sucrose preference that was not exhibited by all animals. As such, we propose the use of non-anhedonic, stressed mice as an internal control in experimental mouse models of depression. The application of an internal control for the effects of stress, along with optimized behavioural testing, can enable the analysis of biological correlates of stress-induced anhedonia versus the consequences of stress alone in a chronic-stress depression model. This is illustrated, for instance, by distinct physiological and molecular profiles in anhedonic and non-anhedonic groups subjected to stress. These results argue for the use of a subgroup of individuals who are negative for the induction of a depressive phenotype during experimental paradigms of depression as an internal control, for more refined modeling of this disorder in animals.     

Evaluation of the effects of chronic mild stressors on hedonic and physiological responses: sex and strain compared

The chronic mild stress (CMS) paradigm was developed in order to simulate in animals the symptom of anhedonia, a major feature of depression. Typically, changes in hedonic status are interpreted from a decrease in either intake or preference for a mild sucrose solution. Although the incidence of clinical depression is significantly higher in women than in men, the study of this disorder in most animal models of depression has been based on the responses of male rodents. The purpose of this study was to compare the effects of 6 weeks of CMS administration among male and female rats of two rat strains, Sprague-Dawley (SD) and Long Evans (LE), with respect to physiological (body, adrenal gland, and spleen weight) and biochemical (plasma corticosterone levels) indices of stress as well as evaluations of 1 and 24 h sucrose intake and preference. Estrous cycle was tracked throughout the study. Overall, our results indicate a slower rate of weight gain in animals, greater in males, exposed to the chronic stressor regime. Furthermore, CMS is shown to disrupt estrous cycling, predominantly in the Long Evans strain of rats. The main behavioral finding was a significant reduction in 24 h sucrose intake in female treated groups, which was not accompanied by alterations in preference. Corticosterone levels were elevated in CMS-treated animals relative to the singly housed control groups, but exposure to a subsequent stressor was not influenced by the stress history. Taken together, the effects of chronic stressor exposure are evident, based on physiological and biochemical indices, although none of the measures distinguished any striking gender specific reactions. The usefulness of sucrose intake or preference as behavioral indices of CMS-induced anhedonia in males and females is modest at best.     

Simulating the anhedonia symptom of depression in animals

One of the two core symptoms of depression is anhedonia, the loss of interest or pleasure in daily activities. Stressful life events are recognized as predisposing factors in the etiology of depression. Rats subjected to a chronic, mild, unpredictable stress regimen exhibit behavioral deficits consistent with a loss of responsiveness to reward, such as decreased sucrose consumption, decreased ability to associate rewards with a distinctive environment, and decreased sensitivity to rewarding electrical brain stimulation. Normal behavior is restored by chronic treatment with antidepressants or electroshocks. Chronically stressed animals also exhibit sleep abnormalities resembling those observed in depressed patients and recognized as biological markers of depression. Thus, stress-induced anhedonia in rats represents an original animal model of some aspects of human depression offering convergent elements of biological, symptomatological, etiological, and therapeutic validity. This simulation of depression may prove useful for better understanding of the pathophysiological mechanisms involved in depressive disorders.     

Integration of animal behaviors under stresses with different time courses

We used animal models of ＂forced swim stress＂ and ＂chronic unpredictable stress＂, and tried to reveal whether a passive coping style of high flotation behavior in forced swim stress predicts an- hedonia behavior after chronic unpredictable stress, and whether the dopamine system regulates floating and anhedonia behaviors. Our results confirmed that depression-prone rats use ＂floating behavior＂ as a coping strategy in forced swim stress and more readily suffer from anhedonia during chronic unpredictable stress. Intraperitoneal injection or nucleus accumbens microin- jection of the dopamine 2/3 receptor subtype agonist ropinirole reduced floating behaviors in depression-prone animals, but increased sucrose preference in rats showing anhedonia. These data indicate that floating behavior is a defensive mode that is preferred by susceptible individ- uals under conditions of acute stress. Simultaneously, these animals more readily experienced anhedonia under long-term stress; that is, they were more readily affected by depression. Our results suggest that dopamine 2/3 receptor subtypes in the nucleus accumbens play an important role in floating behaviors and anhedonia.     

The WAG/Rij strain: a genetic animal model of absence epilepsy with comorbidity of depression [corrected

A great number of clinical observations show a relationship between epilepsy and depression. Idiopathic generalized epilepsy, including absence epilepsy, has a genetic basis. The review provides evidence that WAG/Rij rats can be regarded as a valid genetic animal model of absence epilepsy with comorbidity of depression. WAG/Rij rats, originally developed as an animal model of human absence epilepsy, share many EEG and behavioral characteristics resembling absence epilepsy in humans, including the similarity of action of various antiepileptic drugs. Behavioral studies indicate that WAG/Rij rats exhibit depression-like symptoms: decreased investigative activity in the open field test, increased immobility in the forced swimming test, and decreased sucrose consumption and preference (anhedonia). In addition, WAG/Rij rats adopt passive strategies in stressful situations, express some cognitive disturbances (reduced long-term memory), helplessness, and submissiveness, inability to make choice and overcome obstacles, which are typical for depressed patients. Elevated anxiety is not a characteristic (specific) feature of WAG/Rij rats; it is a characteristic for only a sub-strain of WAG/Rij rats susceptible to audiogenic seizures. Interestingly, WAG/Rij rats display a hyper-response to amphetamine similar to anhedonic depressed patients. WAG/Rij rats are sensitive only to chronic, but not acute, antidepressant treatments, suggesting that WAG/Rij rats fulfill a criterion of predictive validity for a putative animal model of depression. However, more and different antidepressant drugs still await evaluation. Depression-like behavioral symptoms in WAG/Rij rats are evident at baseline conditions, not exclusively after stress. Experiments with foot-shock stress do not point towards higher stress sensitivity at both behavioral and hormonal levels. However, freezing behavior (coping deficits) and blunted response of 5HT in the frontal cortex to uncontrollable sound stress, increased c-fos expression in the terminal regions of the meso-cortico-limbic brain systems and greater DA response of the mesolimbic system to forced swim stress suggest that WAG/Rij rats are vulnerable to some, but not to all types of stressors. We propose that genetic absence epileptic WAG/Rij rats have behavioral depression-like symptoms, are vulnerable to stress and might represent a model of chronic low-grade depression (dysthymia). Both 5HT and DAergic abnormalities detected in the brain of WAG/Rij rats are involved in modulation of vulnerability to stress and provocation of behavioral depression-like symptoms. The same neurotransmitter systems modulate SWDs as well. Recent studies suggest that the occurrence and repetition of absence seizures are a precipitant of depression-like behavior. Whether the neurochemical changes are primary to depression-like behavioral alterations remains to be determined. In conclusion, the WAG/Rij rats can be considered as a genetic animal model for absence epilepsy with comorbidity of dysthymia. This model can be used to investigate etiology, pathogenic mechanisms and treatment of a psychiatric comorbidity, such as depression in absence epilepsy, to reveal putative genes contributing to comorbid depressive disorder, and to screen novel psychotropic drugs with a selective and/or complex (dual) action on both pathologies.     

Early exposure to chronic variable stress facilitates the occurrence of anhedonia and enhanced emotional reactions to novel stressors: reversal by naltrexone pretreatment

The present research studied the influence of an early chronic variable stress (CVS) paradigm - an animal model of depression - on behavioral responses to subsequent environmental challenges suggested to model anhedonia and emotional reactions such as anxiety and fear. In order to explore a potential involvement of an endogenous opiate mechanism - presumably activated during CVS exposure - in the development of such behavioral reactions, in all experiments rats were administered naltrexone (NAL, 2 mg/kg, i.p.) or vehicle (VH) prior to each daily stressor of the CVS procedure. Animals were exposed to CVS and 1 week later tested for sucrose preference (1%) in a free choice paradigm after the presentation or not of a 90-min restraint period. Only CVS treated animals that were later exposed to restraint showed a reduction of sucrose preference, this reduction was absent when CVS rats were pretreated previously with NAL. Moreover, CVS rats were one week later tested on the elevated plus maze (EPM) and in their conditioned and unconditioned freezing response to a single shock session. Early chronic stress resulted in an anxiogenic behavior in the EPM and in an enhanced conditioned and unconditioned freezing which were all abolished by NAL pretreatment. These behavioral findings suggest that the potential activation of an endogenous opiate mechanism during CVS participates in the development of anhedonia and exaggerated emotional reactions in response to subsequent stressful experiences.     

Social deprivation stress is a triggering factor for the emergence of anxiety- and depression-like behaviours and leads to reduced brain BDNF levels in C57BL/6J mice

Stress is a main risk factor that can trigger psychiatric disorders, including anxiety and major depression. Neurotrophins, such as Brain-Derived Neurotrophic Factor (BDNF), have been identified as neuroendocrine effectors involved in the response to stress and in the neurobehavioural changes associated with depression. Aim of this paper was to study the relationship between neuroendocrine activation (circulating corticosterone and brain BDNF levels) and a wide array of depression- and anxiety-like behaviours (anhedonia, behavioural despair, generalised and social anxiety) resulting from exposure to chronic stress. To this end, 3-month-old C57BL/6J male mice were exposed to either chronic disruption of the social structure (SS), to a stable social structure (SG) or to social deprivation (SD), a condition lacking social stimuli. Results show that, despite not developing anhedonia (decreased preference for a sucrose solution), SD mice were characterised by increased emotionality and hypothalamic-pituitary-adrenal axis reactivity in addition to reduced BDNF levels. By contrast, SG and SS mice showed increased anhedonia accompanied by no alterations in the behavioural and neuroendocrine profile. The results here reported indicate that mice exposed to different social housing conditions use different behavioural strategies to cope with external challenges. In addition they suggest that social deprivation might represent a stressful condition triggering the emergence of both anxiety- and depression-like behaviours and clearly indicate BDNF as a main neurobiological variable mediating these responses.     

Inoculation of Bacillus Calmette-Guerin to mice induces an acute episode of sickness behavior followed by chronic depressive-like behavior

Although cytokine-induced sickness behavior is now well-established, the mechanisms by which chronic inflammation and depression are linked still remain elusive. Therefore this study aimed to develop a suitable model to identify the neurobiological basis of depressive-like behavior induced by chronic inflammation, independently of sickness behavior. We chose to measure the behavioral consequences of chronic inoculation of mice with Bacillus Calmette-Guerin (BCG), which has been shown to chronically activate both lung and brain indoleamine 2,3-dioxygenase (IDO), a tryptophan-catabolizing enzyme that mediates the occurrence of depressive-like behavior following acute innate immune system activation. BCG inoculation induced an acute episode of sickness (approximately 5 days) that was followed by development of delayed depressive-like behaviors lasting over several weeks. Transient body weight loss, reduction of motor activity and the febrile response to BCG were dissociated temporarily from a sustained increase in the duration of immobility in both forced swim and tail suspension tests, reduced voluntary wheel running and decreased preference for sucrose (a test of anhedonia). Moreover, we show that a distinct pattern of cytokine production and IDO activation parallels the transition from sickness to depression. Protracted depressive-like behavior, but not sickness behavior, was associated with sustained increase in plasma interferon-gamma and TNF-alpha concentrations and peripheral IDO activation. Together, these promising new data establish BCG inoculation of mice as a reliable rodent model of chronic inflammation-induced depressive-like behaviors that recapitulate many clinical observations and provide important clues about the neurobiological basis through which cytokines may have an impact on affective behaviors.     

Sucrose and quinine intake by maternally-deprived and control rhesus monkeys

Clinical depression is often characterized by a loss of interest or pleasure in formerly enjoyable activities. Analogs of anhedonia are established in rats, but the generality of this phenomenon to other species is unknown. Maternally-deprived rhesus macaques show a wide range of behavioral abnormalities that are reversed by chronic antidepressant treatment. We tested consumption by maternally deprived versus control macaques of sweetened (seven sucrose concentrations) or bitter water (four quinine concentrations) versus plain water to evaluate a non-human primate model of depression for signs of anhedonia. All monkeys consumed more sweetened than tap water, but maternally-deprived monkeys had a diminished preference for sweetened water than did controls. However, maternally deprived animals consumed more bitter water than did controls. Baseline fluid consumption did not differ. The data suggest that 'anhedonia' in animal models may be secondary to a generally attenuated responsiveness to stimuli, rather than a unitary reduction in responsiveness to the appetitive properties of stimuli. We conclude that maternally-deprived rhesus monkeys do not display gustatory signs of anhedonia, but rather of insensitivity to gustatory stimuli.     

Glial pathology in an animal model of depression: reversal of stress-induced cellular, metabolic and behavioral deficits by the glutamate-modulating drug riluzole

Growing evidence indicates that glia pathology and amino-acid neurotransmitter system abnormalities contribute to the pathophysiology and possibly the pathogenesis of major depressive disorder. This study investigates changes in glial function occurring in the rat prefrontal cortex (PFC) after chronic unpredictable stress (CUS), a rodent model of depression. Furthermore, we analyzed the effects of riluzole, a Food and Drug Administration-approved drug for the treatment of amyotrophic laterosclerosis, known to modulate glutamate release and facilate glutamate uptake, on CUS-induced glial dysfunction and depressive-like behaviors. We provide the first experimental evidence that chronic stress impairs cortical glial function. Animals exposed to CUS and showing behavioral deficits in sucrose preference and active avoidance exhibited significant decreases in 13C-acetate metabolism reflecting glial cell metabolism, and glial fibrillary associated protein (GFAP) mRNA expression in the PFC. The cellular, metabolic and behavioral alterations induced by CUS were reversed and/or blocked by chronic treatment with the glutamate-modulating drug riluzole. The beneficial effects of riluzole on CUS-induced anhedonia and helplessness demonstrate the antidepressant action of riluzole in rodents. Riluzole treatment also reversed CUS-induced reductions in glial metabolism and GFAP mRNA expression. Our results are consistent with recent open-label clinical trials showing the drug's effect in mood and anxiety disorders. This study provides further validation of hypothesis that glial dysfunction and disrupted amino-acid neurotransmission contribute to the pathophysiology of depression and that modulation of glutamate metabolism, uptake and/or release represent viable targets for antidepressant drug development.     

Anhedonia and motivational deficits in rats: impact of chronic social stress

Stress, especially chronic stress, is one of the most important factors responsible for precipitation of affective disorders in humans. The animal models commonly used in the investigation of stress effects are based mainly on powerful physical stressors. In the majority of cases, these models are not relevant to situations that human beings encounter in everyday life. In our study, an animal model for chronic social stress has been developed for rats using a resident-intruder paradigm. This paradigm is considered a model of social defeat or subordination, and therefore may mimic situations occurring in humans. Rats were subjected daily to subordination stress for a period of five weeks and, in parallel, tested with a battery of behavioural tests. Chronically stressed rats showed behavioural changes, including decreased motility and exploratory activity, increased immobility in a forced swim test, and reduced preference for sweet sucrose solution (anhedonia). Reduced locomotor and exploratory activity represents a loss of interest in new stimulating situations, implying a deficit in motivation. Increased immobility in the forced swim test indicates behavioural despair, a characteristic of depressive disorders. Decreased sucrose preference may indicate desensitisation of the brain reward mechanism. Since anhedonia is one of the core symptoms of depression in humans, our findings suggest that the rat chronic social stress model may be an appropriate model for depressive disorders.     

A new animal model of (chronic) depression induced by repeated and intermittent lipopolysaccharide administration for 4 months

Chronic activation of immune-inflammatory and oxidative and nitrosative stress (O&NS) pathways plays an important role in the pathophysiology of clinical depression. Increased IgA responses directed against LPS of gram-negative bacteria, indicating increased bacterial translocation, may be one of the drivers underpinning these pathways. There is a strong association between signs of bacterial translocation and chronicity of depression and O&NS, but not pro-inflammatory cytokines.The aims of the present study were to: (1) develop a new neurobehavioral model of (chronic) depression (anhedonic behavior) that may reflect chronic LPS stimulation and is associated with increased oxidative stress, and (2) to delineate the effects of fluoxetine on this new depression model.We established that in female mice repeated LPS injections once daily for 5 days (from 750 μg/kg to a maximal dose 1250 μg/kg; increasing doses for the first three days which were then gradually decreased on day 4 and 5) at a one-month interval and this repeated for 4 consecutive months induced chronic anhedonia (estimated by the preference to drink a 1% sucrose) lasting for at least 7 weeks. Chronic LPS administration significantly decreased thymus weight, proliferative activity of splenocytes, production of interferon (IFN)γ and interleukin-(IL)10, and increased superoxide and corticosterone production. Treatment with fluoxetine for 3 weeks abolished the neurobehavioral effects of LPS. The antidepressant effect of fluoxetine was accompanied by increased production of IL-10 and reduced superoxide and corticosterone production.Our results suggest that repeated intermittent LPS injections to female mice may be a useful model of chronic depression and in particular for the depressogenic effects of long standing activation of the toll-like receptor IV complex.     

Effects of β-carboline harmine on behavioral and physiological parameters observed in the chronic mild stress model: Further evidence of antidepressant properties

The chronic mild stress (CMS) model has been used as an animal model of depression which induces anhedonic behavior in rodents. The present study was aimed to evaluate the behavioral and physiological effects of administration of β-carboline harmine in rats exposed to CMS procedure. To this aim, after 40 days of exposure to CMS procedure, rats were treated with harmine (15 mg/kg/day) for 7 days. In this study, sweet food consumption, adrenal gland weight, adrenocorticotrophin hormone (ACTH) levels, and hippocampal brain-derived-neurotrophic factor (BDNF) protein levels were assessed. Our findings demonstrated that chronic stressful situations induced anhedonia, hypertrophy of adrenal gland weight, increase ACTH circulating levels in rats and increase BDNF protein levels. Interestingly, treatment with harmine reversed anhedonia, the increase of adrenal gland weight, normalized ACTH circulating levels and BDNF protein levels. Finally, these findings further support the hypothesis that harmine could be a new pharmacological tool for the treatment of depression.     

Gap junction dysfunction in the prefrontal cortex induces depressive-like behaviors in rats

Growing evidence has implicated glial anomalies in the pathophysiology of major depression disorder (MDD). Gap junctional communication is a main determinant of astrocytic function. However, it is unclear whether gap junction dysfunction is involved in MDD development. This study investigates changes in the function of astrocyte gap junction occurring in the rat prefrontal cortex (PFC) after chronic unpredictable stress (CUS), a rodent model of depression. Animals exposed to CUS and showing behavioral deficits in sucrose preference test (SPT) and novelty suppressed feeding test (NSFT) exhibited significant decreases in diffusion of gap junction channel-permeable dye and expression of connexin 43 (Cx43), a major component of astrocyte gap junction, and abnormal gap junctional ultrastructure in the PFC. Furthermore, we analyzed the effects of typical antidepressants fluoxetine and duloxetine and glucocorticoid receptor (GR) antagonist mifepristone on CUS-induced gap junctional dysfunction and depressive-like behaviors. The cellular and behavioral alterations induced by CUS were reversed and/or blocked by treatment with typical antidepressants or mifepristone, indicating that the mechanism of their antidepressant action may involve the amelioration of gap junction dysfunction and the cellular changes may be related to GR activation. We then investigated the effects of pharmacological gap junction blockade in the PFC on depressive-like behaviors. The results demonstrate that carbenoxolone (CBX) infusions induced anhedonia in SPT, and anxiety in NSFT, and Cx43 mimetic peptides Gap27 and Gap26 also induced anhedonia, a core symptom of depression. Together, this study supports the hypothesis that gap junction dysfunction contributes to the pathophysiology of depression.     

Chronic mild stress affects sucrose intake and sleep in rats

Depression in humans is associated with sleep abnormalities of three types: altered rapid eye movement (REM) sleep, fragmented sleep, and reduced delta sleep. In an animal model of depression, chronic exposure to mild stressors (CMS, e.g. periods of soiled cage, reversed light/dark cycle, grouped housing, food and/or water deprivation) causes behavioral and hormonal changes which, in humans, often are associated with depression. In the CMS model, a reduced sucrose intake has been defined as one of the core symptoms of depression, anhedonia, although this finding is not consistent among various laboratories. In the present study, we investigated if the CMS procedure, in our laboratory, would cause decreased sucrose intake and, also, give sleep changes similar to what is found in depressed patients.

Exposure to CMS decreased sucrose intake in our rats. The largest effect was obtained after 2 weeks of the stress protocol. CMS rats spent more time in REM sleep and showed more fragmented sleep compared to their baseline recording, while there were no changes in the control rats. Increased sleep fragmentation in CMS rats was particularly evident by increased number of arousals, and increased REM sleep and slow-wave-sleep-1 (SWS-1) episodes. The duration of sleep stage episodes was decreased. The amount of slow-wave-sleep-2 (SWS-2) was not decreased, however SWS-2 in percent of total SWS was reduced. Correlation analysis showed that animals that had less consumption of sucrose spent more time in REM sleep and had increased number of REM sleep episodes. In this study, CMS appears to be a model of depression.     

Brain interleukin-1 mediates chronic stress-induced depression in mice via adrenocortical activation and hippocampal neurogenesis suppression

Several lines of evidence implicate the pro-inflammatory cytokine interleukin-1 (IL-1) in the etiology and pathophysiology of major depression. To explore the role of IL-1 in chronic stress-induced depression and some of its underlying biological mechanisms, we used the chronic mild stress (CMS) model of depression. Mice subjected to CMS for 5 weeks exhibited depressive-like symptoms, including decreased sucrose preference, reduced social exploration and adrenocortical activation, concomitantly with increased IL-1 beta levels in the hippocampus. In contrast, mice with deletion of the IL-1 receptor type I (IL-1rKO) or mice with transgenic, brain-restricted overexpression of IL-1 receptor antagonist did not display CMS-induced behavioral or neuroendocrine changes. Similarly, whereas in wild-type (WT) mice CMS significantly reduced hippocampal neurogenesis, measured by incorporation of bromodeoxyuridine (BrdU) and by doublecortin immunohistochemistry, no such decrease was observed IL-1rKO mice. The blunting of the adrenocortical activation in IL-1rKO mice may play a causal role in their resistance to depression, because removal of endogenous glucocorticoids by adrenalectomy also abolished the depressive-like effects of CMS, whereas chronic administration of corticosterone for 4 weeks produced depressive symptoms and reduced neurogenesis in both WT and IL-1rKO mice. The effects of CMS on both behavioral depression and neurogenesis could be mimicked by exogenous subcutaneous administration of IL-1 beta via osmotic minipumps for 4 weeks. These findings indicate that elevation in brain IL-1 levels, which characterizes many medical conditions, is both necessary and sufficient for producing the high incidence of depression found in these conditions. Thus, procedures aimed at reducing brain IL-1 levels may have potent antidepressive actions.     

成纤维细胞生长因子2和成纤维细胞生长因子受体1蛋白在慢性应激抑郁大鼠海马中的表达

There is evidence that the expression of members of the fibroblast growth factor(FGF) protein family is altered in post-mortem brains of humans suffering from major depressive disorder.The present study examined whether the expression of fibroblast growth factor-2(FGF2) and fibroblast growth factor receptor-1(FGFR1) protein is altered following chronic stress in an animal model.Rats were exposed to 35 days of chronic unpredictable mild stress,and then tested using open-field and sucrose consumption tests.Compared with the control group,rats in the chronic stress group exhibited obvious depressive-like behaviors,including anhedonia,anxiety and decreased mobility.The results of western blot analysis and immunohistochemical analysis revealed a downregulation of the expression of FGF2 and FGFR1 in the hippocampus of rats,particularly in the CA1,CA3 and dentate gyrus.This decreased expression is in accord with the results of post-mortem studies in humans with major depressive disorder.These findings suggest that FGF2 and FGFR1 proteins participate in the pathophysiology of depressive-like behavior,and may play an important role in the mechanism of chronic stress-induced depression.     

Neurobiology of chronic mild stress: Parallels to major depression

The chronic mild (or unpredictable/variable) stress (CMS) model was developed as an animal model of depression more than 20 years ago. The foundation of this model was that following long-term exposure to a series of mild, but unpredictable stressors, animals would develop a state of impaired reward salience that was akin to the anhedonia observed in major depressive disorder. In the time since its inception, this model has also been used for a variety of studies examining neurobiological variables that are associated with depression, despite the fact that this model has never been critically examined to validate that the neurobiological changes induced by CMS are parallel to those documented in depressive disorder. The aim of the current review is to summarize the current state of knowledge regarding the effects of chronic mild stress on neurobiological variables, such as neurochemistry, neurochemical receptor expression and functionality, neurotrophin expression and cellular plasticity. These findings are then compared to those of clinical research examining common variables in populations with depressive disorders to determine if the changes observed following chronic mild stress are in fact consistent with those observed in major depression. We conclude that the chronic mild stress paradigm: (1) evokes an array of neurobiological changes that mirror those seen in depressive disorders and (2) may be a suitable tool to investigate novel systems that could be disturbed in depression, and thus aid in the development of novel targets for the treatment of depression.