Social instability in female rodents as a model of stress related disorders: A systematic review

Multiple integrated social stress induces depressive-like behavioral and neural adaptations in female C57BL/6J mice

Despite women representing most of those affected by major depression, preclinical studies have focused almost exclusively on male subjects, partially due to a lack of ideal animal paradigms. As the persistent need regarding the sex balance of neuroscience research and female-specific pathology of mental disorders surges, the establishment of natural etiology-based and systematically validated animal paradigms for depression with female subjects becomes an urgent scientific problem. This study aims to establish, characterize, and validate a "Multiple Integrated Social Stress (MISS)" model of depression in female C57BL/6J mice by manipulating and integrating daily social stressors that females are experiencing. Female C57BL/6J mice randomly experienced social competition failure in tube test, modified vicarious social defeat stress, unescapable overcrowding stress followed by social isolation on each day, for ten consecutive days. Compared with their controls, female MISS mice exhibited a relatively decreased preference for social interaction and sucrose, along with increased immobility in the tail suspension test, which could last for at least one month. These MISS mice also exhibited increased levels of blood serum corticosterone, interleukin-6 L and 1β. In the pharmacological experiment, MISS-induced dysfunctions in social interaction, sucrose preference, and tail suspension tests were amended by systematically administrating a single dose of sub-anesthetic ketamine, a rapid-onset antidepressant. Compared with controls, MISS females exhibited decreased c-Fos activation in their anterior cingulate cortex, prefrontal cortex, nucleus accumbens and some other depression-related brain regions. Furthermore, 24 h after the last exposure to the paradigm, MISS mice demonstrated a decreased center zone time in the open field test and decreased open arm time in the elevated plus-maze test, indicating anxiety-like behavioral phenotypes. Interestingly, MISS mice developed an excessive nesting ability, suggesting a likely behavioral phenotype of obsessive-compulsive disorder. These data showed that the MISS paradigm was sufficient to generate pathological profiles in female mice to mimic core symptoms, serum biochemistry and neural adaptations of depression in clinical patients. The present study offers a multiple integrated natural etiology-based animal model tool for studying female stress susceptibility.

Repeated witness social stress causes cardiomyocyte contractile impairment and intracellular Ca2+ derangement in female rats

The impact of psychosocial stressors on cardiovascular health in women is of growing interest in both the popular and scientific literature. Rodent models are useful for providing direct experimental evidence of the adverse cardiovascular consequences of psychosocial stressors, yet studies in females are scarce. Here, we investigated the effects of repeated exposure to witness social defeat stress (WS) on cardiomyocyte contractile function and intracellular Ca2+ homeostasis in young adult wild-type Groningen female rats. Female rats bore witness to an aggressive social defeat episode between two males for nine consecutive days or were exposed to a control procedure. Stress-related behaviors were assessed during the first and last WS/control exposure. Twenty-four hours after the last exposure, plasma corticosterone levels were measured, and cardiomyocytes were isolated for analyses of contractile properties and Ca2+ transients, and expression levels of proteins involved in intracellular Ca2+dynamics. The results show an impairment of the intrinsic cardiac mechanical properties and prolonged intracellular Ca2+decay in WS female rats showing social stress-related behavioral (larger amounts of burying behavior) and neuroendocrine (elevated plasma corticosterone levels) phenotypes. Further, the results implicate alterations in the sarcoplasmic reticulum Ca2+-ATPase/phospholamban complex in the contractile defects described in cardiomyocytes of WS female rats. In conclusion, this study highlights the utility of the WS model as an ethologically relevant social stressor for investigating pathophysiological processes that occur in the heart of female subjects and may increase vulnerability to social stress-related cardiovascular risk.

The role of sociability in social instability stress: Behavioral, neuroendocrine and monoaminergic effects

Extensive literature has reported a link between social stress and mental health. In this complex relationship, individual strategies for coping with social stress are thought to have a possible modulating effect, with sociability being a key factor. Despite the higher incidence of affective disorders in females and sex-related neurochemical differences, female populations have been understudied. The aim of the present study was, therefore, to analyze the behavioral, neuroendocrine, and neurochemical effects of stress in female OF1 mice, paying special attention to social connectedness (female mice with high vs low sociability). To this end, subjects were exposed to the Chronic Social Instability Stress (CSIS) model for four weeks. Although female mice exposed to CSIS had increased arousal, there was no evidence of depressive-like behavior. Neither did exposure to CSIS affect corticosterone levels, although it did increase the MR/GR ratio by decreasing GR expression. Female mice exposed to CSIS had higher noradrenaline and dopamine levels in the hippocampus and striatum respectively, with a lower monoaminergic turnover, resulting in an increased arousal. CSIS increased serotonin levels in both the hippocampus and striatum. Similarly, CSIS was found to reduce kynurenic acid, 3-HK, and IDO and iNOS enzyme levels in the hippocampus. Interestingly, the observed decrease in IDO synthesis and the increased serotonin and dopamine levels in the striatum were only found in subjects with high sociability. These highly sociable female mice also had significantly lower levels of noradrenaline in the striatum after CSIS application. Overall, our model has produced neuroendocrine and neurochemical but not behavioral changes, so it has not allowed us to study sociability in depth. Therefore, a model that induces both molecular and behavioral phenotypes should be applied to determine the role of sociability.

Stress-induced changes of the cholinergic circuitry promote retrieval-based generalization of aversive memories

Generalization, the process of applying knowledge acquired in one context to other contexts, often drives the expression of similar behaviors in related situations. At the cellular level, generalization is thought to depend on the activity of overlapping neurons that represent shared features between contexts (general representations). Using contextual fear conditioning in mice, we demonstrate that generalization can also occur in response to stress and result from reactivation of specific, rather than general context representations. We found that generalization emerges during memory retrieval, along with stress-induced abnormalities of septohippocampal oscillatory activity and acetylcholine release, which are typically found in negative affective states. In hippocampal neurons that represent aversive memories and drive generalization, cholinergic septohippocampal afferents contributed to a unique reactivation pattern of cFos, Npas4, and repressor element-1 silencing transcription factor (REST). Together, these findings suggest that generalization can be triggered by perceptually dissimilar but valence-congruent memories of specific aversive experiences. Through promoting the reactivation of such memories and their interference with ongoing behavior, abnormal cholinergic signaling could underlie maladaptive cognitive and behavioral generalization linked to negative affective states.

Chronic social instability stress down-regulates IL-10 and up-regulates CX3CR1 in tumor-bearing and non-tumor-bearing female mice

Extensive literature has reported a link between stress and tumor progression, and between both of these factors and mental health. Despite the higher incidence of affective disorders in females and the neurochemical differences according to sex, female populations have been understudied. The aim of this study was therefore to analyze the effect of stress on tumor development in female OF1 mice. For this purpose, subjects were inoculated with B16F10 melanoma cells and exposed to the Chronic Social Instability Stress (CSIS) model. Behavioral, neurochemical and neuroendocrine parameters were analyzed. Female mice exposed to CSIS exhibited reduced body weight and increased arousal, but there was no evidence of depressive behavior or anxiety. Exposure to CSIS did not affect either corticosterone levels or tumor development, although it did provoke an imbalance in cerebral inflammatory cytokines, decreasing IL-10 expression (IL-6/IL-10 and TNF-α/IL-10); chemokines, increasing CX3CR1 expression (CX3CL1/CX3CR1); and glucocorticoid receptors, decreasing GR expression (MR/GR). In contrast, tumor development did not alter body weight and, although it did alter behavior, it did so to a much lesser extent. Tumor inoculation did not affect corticosterone levels, but increased the MR/GR ratio in the hippocampus and provoked an imbalance in cerebral inflammatory cytokines and chemokines, although differently from stress. These results underscore the need for experimental approaches that allow us to take sex differences into account when exploring this issue, since these results appear to indicate that the female response to stress is mediated by mechanisms different from those often proposed in relation to male mice.

Under or Absent Reporting of Light Stimuli in Testing of Anxiety-Like Behaviors in Rodents: The Need for Standardization

Behavioral neuroscience tests such as the Light/Dark Test, the Open Field Test, the Elevated Plus Maze Test, and the Three Chamber Social Interaction Test have become both essential and widely used behavioral tests for transgenic and pre-clinical models for drug screening and testing. However, as fast as the field has evolved and the contemporaneous involvement of technology, little assessment of the literature has been done to ensure that these behavioral neuroscience tests that are crucial to pre-clinical testing have well-controlled ethological motivation by the use of lighting (i.e., Lux). In the present review paper, N = 420 manuscripts were examined from 2015 to 2019 as a sample set (i.e., n = ~20–22 publications per year) and it was found that only a meager n = 50 publications (i.e., 11.9% of the publications sampled) met the criteria for proper anxiogenic and anxiolytic Lux reported. These findings illustrate a serious concern that behavioral neuroscience papers are not being vetted properly at the journal review level and are being released into the literature and public domain making it difficult to assess the quality of the science being reported. This creates a real need for standardizing the use of Lux in all publications on behavioral neuroscience techniques within the field to ensure that contributions are meaningful, avoid unnecessary duplication, and ultimately would serve to create a more efficient process within the pre-clinical screening/testing for drugs that serve as anxiolytic compounds that would prove more useful than what prior decades of work have produced. It is suggested that improving the standardization of the use and reporting of Lux in behavioral neuroscience tests and the standardization of peer-review processes overseeing the proper documentation of these methodological approaches in manuscripts could serve to advance pre-clinical testing for effective anxiolytic drugs. This report serves to highlight this concern and proposes strategies to proactively remedy them as the field moves forward for decades to come.

Ginsenoside Rh2 administration produces crucial antidepressant-like effects in a CUMS-induced mice model of depression

INTRODUCTION

The most striking feature of depression is sadness and a loss of interest in activities, which represents a major cause of disability globally. Therefore, it is necessary to identify novel antidepressants for clinical practice. Ginsenoside Rh2 (Rh2) is one of the major bioactive ginsenosides that can be extracted from Panax ginseng and has been demonstrated to improve both memory and learning. The purpose of this study was to provide broad insight into the mechanisms underlying depression and gain greater insights into antidepressant therapy.

METHODS

In this study, we first established an effective and feasible depression animal model of chronic unpredictable mild stress (CUMS) and behavioral testing was evaluated by the forced swim test (FST), the tail suspension test (TST) and the sucrose preference test. Following pretreatment with Rh2 (10 and 20 mg/kg), the immobility time of mice was reduced without affecting locomotor activity in both the FST and TST. Western blotting and immunofluorescence were used to investigate the activation of the hippocampal BDNF signaling pathway and hippocampal neurogenesis.

RESULTS

Different concentrations of Rh2 significantly reduced depressive-like symptoms in CUMS-induced mice and downregulated the effects of the BDNF signaling cascade and neurogenesis in the hippocampus. Furthermore, the administration of K252a completely prevented the antidepressant-like activity of Rh2 in mice.

CONCLUSION

The results indicated that Rh2 possesses the antidepression action via the positive regulation of the BDNF-TrkB pathway.

Antidepressant-like activity of gestational administration of vitamin D is suppressed by prenatal overexposure to dexamethasone in female Wistar rats

Overexposure to glucocorticoids during gestation can lead to long-term mental disorders. Given the higher prevalence of depression in females, we investigated whether late gestational administration of dexamethasone could generate a depressive-like phenotype in the adult female offspring and if vitamin D could have a neuroprotective effect in this context. Pregnant rats received vitamin D (VitD, 500 IU/day) or vehicle (CTL) during gestation. Other pregnant rats received dexamethasone (Dex 0.1 mg/kg/ - 14th to the 19th gestational day) or dexamethasone + vitamin D (DexVitD). The offspring were tested for anhedonia (sucrose preference) and depressive-like behavior (forced swimming test) at postnatal months (PNM) 3, 6 and 12. Components of the serotonergic system, as well as glucocorticoids' receptors, were evaluated in the dorsal raphe nucleus at PNM 6 and 12. Prenatal vitamin D and dexamethasone increased sucrose preference at PNM 12. Prenatal vitamin D had an antidepressant-like effect at PNM 3 in rats overexposed to dexamethasone. However, at PNM 12, this effect was blunted in the DexVitD group. Prenatal dexamethasone reduced the protein content of SERT, TPH, and 5-HT_1A receptors in the dorsal raphe nucleus at 6 but not at 12 PNM. The glucocorticoids' receptors expression was similar in all groups. We concluded that prenatal overexposure to dexamethasone does not change emotional behaviors in females, but it blunts the antidepressant-like effect of gestational vitamin D in an age-dependent manner. The antidepressant-like activity of vitamin D in the offspring was not related either to alterations of the serotonergic system or the glucocorticoids' receptors expression in the dorsal raphe nucleus.

Sex Differences in Anxiety and Depression: What Can (and Cannot) Preclinical Studies Tell Us?

In recent years, the gender perspective in scientific research and sex differences in biological studies on emotional disorders have become increasingly important. However, sex bias in basic research on anxiety and depression is still far from being covered. This review addresses the study of sex differences in the field of anxiety and depression using animal models that consider this issue so far. What can preclinical studies tell us and what are their main limitations? First, we describe the behavioral tests most frequently used in preclinical research to assess depressive-like and anxiety-like behaviors in rodents. Then, we analyze the main findings, strengths, and weaknesses of rodent models of anxiety and depression, dividing them into three main categories: sex chromosome complement-biased sex differences; gonadal hormone-biased sex differences; environmental-biased sex differences. Regardless of the animal model used, none can reproduce all the characteristics of such complex and multifactorial pathologies as anxiety and depressive disorders; however, each animal model contributes to elucidating the bases that underlie these disorders. The importance is highlighted of considering sex differences in the responses that emerge from each model.

Stress-induced generalization of negative memories is mediated by an extended hippocampal circuit

Memories of negative experiences exert important control of behavior in the face of actual or anticipated threat. Sometimes, however, this control extends to non-threatening situations, a phenomenon known as overgeneralization of negative memories. Overgeneralization is a reliable cognitive phenotype of major depressive disorder, generalized anxiety disorder, and post-traumatic stress disorder. We therefore sought to develop an animal model to study stress-induced generalization of negative memories (SIG) and determine its dependence on the episodic-like memory circuit. We found that male and female mice, which were trained to differentiate a threatening from neutral context, exhibited robust SIG in response to subsequent social stress. Using chemogenetic circuit manipulations during memory retrieval, we demonstrated that both excitatory afferents to the dorsal hippocampus (DH) from the ventral tegmental area (VTA), and excitatory efferents from the DH to the retrosplenial cortex (RSC) contribute to SIG. Based on the known roles of these projections, we suggest that (1) by targeting subcortical VTA circuits that provide valence signals to the DH, stress prioritizes the retrieval of negative over neutral memories, and (2) by forwarding such information to the RSC, stress engages cortical mechanisms that support the retrieval of general relative to specific memory features. Altogether, these results suggest that various components of the extended hippocampal circuit can serve as treatment targets for memory overgeneralization.

The underestimated sex: a review on female animal models of depression

Major depression (MD) is the most common psychiatric disorder, predicted to affect around 264 million people worldwide. Although the etiology of depression remains elusive, the interplay between genetics and environmental factors, such as early life events, stress, exposure to drugs and health problems appears to underlie its development. Whereas depression is twice more prevalent in women than in men, most preclinical studies are performed in male rodents. In fact, females' physiology and reproductive experience are associated with changes to brain, behavior and endocrine profiles that may influence both stress, an important precipitating factor for depression, and response to treatment. These specificities emphasize the need to choose the most suitable models and readouts in order to better understand the pathophysiological mechanisms of depression in females. With this review, we aim to provide an overview of female animal models of depression highlighting the major differences between models, regarding behavioral, physiological, and molecular readouts, but also the major gaps in research, attending to the role of etiological factors, protocol variability and sex.

A Glimpse Into the Sexual Dimorphisms in Major Depressive Disorder Through Epigenetic Studies

Depression is an umbrella term used to describe a mood disorder with a broad spectrum of symptoms including a persistent feeling of sadness, loss of interest, and deficits in social behavior. Epigenetic research bridges the environmental and genetic landscape and has the potential to exponentially improve our understanding of such a complex disorder. Depression is also a sexually dimorphic disorder and variations exist within epigenetic modification sites between sexes. These sex-specific mediators may impact behavioral symptomology and could serve as therapeutic targets for treatments to improve behavioral deficits. This mini review will focus on the social behavior perspective of depression and specifically explore the sexually different epigenetic modifications on depression.

The social instability stress paradigm in rat and mouse: A systematic review of protocols, limitations, and recommendations

BACKGROUND

Social stress is an important environmental risk factor for the development of psychiatric disorders, including depression and anxiety disorders. Social stress paradigms are commonly used in rats and mice to gain insight into the pathogenesis of these disorders. The social instability stress (SIS) paradigm entails frequent (up to several times a week) introduction of one or multiple unfamiliar same-sex home-cage partners. The subsequent recurring formation of a new social hierarchy results in chronic and unpredictable physical and social stress.

PURPOSE

We compare and discuss the stress-related behavioral and physiological impact of SIS protocols in rat and mouse, and address limitations due to protocol variability. We further provide practical recommendations to optimize reproducibility of SIS protocols.

METHODS

We conducted a systematic review in accordance with the PRISMA statement in the following three databases: PubMed, Web of Science and Scopus. Our search strategy was not restricted to year of publication but was limited to articles in English that were published in peer-reviewed journals. Search terms included "social* instab*" AND ("animal" OR "rodent" OR "rat*" OR "mice" OR "mouse").

RESULTS

Thirty-three studies met our inclusion criteria. Fifteen articles used a SIS protocol in which the composition of two cage mates is altered daily for sixteen days (SIS16D). Eleven articles used a SIS protocol in which the composition of four cage mates is altered twice per week for 49 days (SIS49D). The remaining seven studies used SIS protocols that differed from these two protocols in experiment duration or cage mate quantity. Behavioral impact of SIS was primarily assessed by quantifying depressive-like, anxiety-like, social-, and cognitive behavior. Physiological impact of SIS was primarily assessed using metabolic parameters, hypothalamus-pituitary-adrenal axis activity, and the assessment of neurobiological parameters such as neuroplasticity and neurogenesis.

CONCLUSION

Both shorter and longer SIS protocols induce a wide range of stress-related behavioral and physiological impairments that are relevant for the pathophysiology of depression and anxiety disorders. To date, SIS16D has only been reported in rats, whereas SIS49D has only been reported in mice. Given this species-specific application as well as variability in reported SIS protocols, additional studies should determine whether SIS effects are protocol duration- or species-specific. We address several issues, including a lack of consistency in the used SIS protocols, and suggest practical, concrete improvements in design and reporting of SIS protocols to increase standardization and reproducibility of this etiologically relevant preclinical model of social stress.

Defining Valid Chronic Stress Models for Depression With Female Rodents

Women are twice as likely to experience depression than men, yet until recently, preclinical studies in rodents have focused almost exclusively on males. As interest in sex differences and sex-specific mechanisms of stress susceptibility increases, chronic stress models for inducing depression-relevant behavioral and physiological changes in male rodents are being applied to females, and several new models have emerged to include both males and females, yet not all models have been systematically validated in females. An increasing number of researchers seek to include female rodents in their experimental designs, asking the question "what is the ideal chronic stress model for depression in females?" We review criteria for assessing female model validity in light of key research questions and the fundamental distinction between studying sex differences and studying both sexes. In overviewing current models, we explore challenges inherent to establishing an ideal female chronic stress model, with particular emphasis on the need for standardization and adoption of validated behavioral tests sensitive to stress effects in females. Taken together, these considerations will empower female chronic stress models to provide a better understanding of stress susceptibility and allow the development of efficient sex-specific treatments.

Animal Models of Depression: What Can They Teach Us about the Human Disease?

Depression is apparently the most common psychiatric disease among the mood disorders affecting about 10% of the adult population. The etiology and pathogenesis of depression are still poorly understood. Hence, as for most human diseases, animal models can help us understand the pathogenesis of depression and, more importantly, may facilitate the search for therapy. In this review we first describe the more common tests used for the evaluation of depressive-like symptoms in rodents. Then we describe different models of depression and discuss their strengths and weaknesses. These models can be divided into several categories: genetic models, models induced by mental acute and chronic stressful situations caused by environmental manipulations (i.e., learned helplessness in rats/mice), models induced by changes in brain neuro-transmitters or by specific brain injuries and models induced by pharmacological tools. In spite of the fact that none of the models completely resembles human depression, most animal models are relevant since they mimic many of the features observed in the human situation and may serve as a powerful tool for the study of the etiology, pathogenesis and treatment of depression, especially since only few patients respond to acute treatment. Relevance increases by the fact that human depression also has different facets and many possible etiologies and therapies.

Similarities and dissimilarities in the effects of benzodiazepines and specific serotonin reuptake inhibitors (SSRIs) in the defensive marble burying test: A systematic review and meta-analysis

One problem areas of animal models and tests for neuropsychiatric disorders is unclear reproducibility, including both internal and external validity. One way to examine external validity is with systematic reviews and meta-analyses, a standard practice in clinical research that is relatively neglected in preclinical research. Considering the need to evaluate the validity and reproducibility of frequently used animal models, this study presents a meta-analysis of the effects of prototypic benzodiazepines and specific serotonin reuptake inhibitors (SSRIs) in the mouse defensive marble burying test (MBT). These drug groups were selected because although they differ in their biological targets as well as in their clinical use, they are both commonly used for the treatment of anxiety disorders. A PubMed literature search was performed to identify studies that examined the effects of benzodiazepines (diazepam, alprazolam, chlordiazepoxide, clonazepam) or SSRIs (fluoxetine, citalopram, escitalopram, fluvoxamine, paroxetine) in the MBT in mice. For benzodiazepines, 73 experiments were included. Benzodiazepines effect size was 2.04 and Q statistics was 1959 with a significant correlation between dose and effect size (r = 0.31, p = 0.007). For SSRIs we identified 47 experiments. Effect size of SSRIs was 2.24 and Q statistics was 493.38. No correlation was found between dose and effect size (r = 0.23, p = 0.12). The current results support the external validity of the defensive marble burying test as a screening test for anxiolytic effects. However, these results indicate that significant attention should be given to the administration schedules of benzodiazepines and SSRIs.

Reward Circuitry and Motivational Deficits in Social Anxiety Disorder: What Can Be Learned From Mouse Models?

Social anxiety disorder (SAD) is a common and serious psychiatric condition that typically emerges during adolescence and persists into adulthood if left untreated. Prevailing interventions focus on modulating threat and arousal systems but produce only modest rates of remission. This gap in efficacy suggests that most mainstream treatment concepts do not sufficiently target core processes involved in the onset and maintenance of SAD. This idea has further driven the development of new theoretical models that target dopamine (DA)-driven reward circuitry and motivational deficits that appear to be systematically altered in SAD. Most of the available data linking systemic alterations in DA neurobiology to SAD in humans, although abundant, remains at the level of correlational evidence. Accordingly, the purpose of this brief review is to critically evaluate the relevance of experimental work in rodent models that link details of DA function to symptoms of social anxiety. We conclude that, despite certain systematic limitations inherent in animal models, these approaches provide useful insights into human biomarkers of social anxiety including that (1) adolescence may serve as a critical period for the convergence of neurobiological and environmental factors that modify future expectations about social reward through experience dependent changes in DA-ergic circuitry, (2) females may show unique susceptibility to social anxiety symptoms when encountering relational instability that influences DA-related neural processes, and (3) separate from fear and arousal systems, the functional neurobiology of central DA systems contribute uniquely to susceptibility and maintenance of anhedonic factors relevant to human models of SAD.

Revisiting the validity of the mouse tail suspension test: Systematic review and meta-analysis of the effects of prototypic antidepressants

Animal models in neuropsychiatric research need validation. One way to address external validity is systematic reviews and meta-analyses. The present study presents a meta-analysis of the effects of antidepressants in the mouse tail suspension test (TST). A PubMed search identified studies that examined imipramine and fluoxetine effects in the TST. Inclusion criteria were testing in the light phase; trial duration was six minutes; immobility time scored 6 or (last) 4 min; adult mice; acute intraperitoneal (IP) administration. Effect sizes (ES) were estimated using Cohen's d, heterogeneity of ES with Cochran's Q test, correlations between dose and ES with Pearson's correlation and differences between strains with Analysis of variance. Results show that antidepressants decrease immobility time in the TST and a correlation between drug dose and ES but no effects of strain. We suggest that the TST is a valid tool to quantitatively, consistently and reproducibly capture the immobility-reducing aspects of fluoxetine and imipramine and that the lack of strain effects is due to small number of experiments in many of the strains.

Animal models of major depression: drawbacks and challenges

Major depression is a leading contributor to the global burden of disease. This situation is mainly related to the chronicity and/or recurrence of the disorder, and to poor response to antidepressant therapy. Progress in this area requires valid animal models. Current models are based either on manipulating the environment to which rodents are exposed (during the developmental period or adulthood) or biological underpinnings (i.e. gene deletion or overexpression of candidate genes, targeted lesions of brain areas, optogenetic control of specific neuronal populations, etc.). These manipulations can alter specific behavioural and biological outcomes that can be related to different symptomatic and pathophysiological dimensions of major depression. However, animal models of major depression display substantial shortcomings that contribute to the lack of innovative pharmacological approaches in recent decades and which hamper our capabilities to investigate treatment-resistant depression. Here, we discuss the validity of these models, review putative models of treatment-resistant depression, major depression subtypes and recurrent depression. Furthermore, we identify future challenges regarding new paradigms such as those proposing dimensional rather than categorical approaches to depression.