Selecting an appropriate stress model of depression in rodents

Abstract

Despite the well-established functions of neurotransmitters and their receptors in depression studies, the aetiology of depression remains unknown. Further research into the field of animal studies is required in order to facilitate a more comprehensive understanding of the underlying mechanisms that contribute to the development of depression. While the potential of animal behaviour to elucidate the molecular underpinnings of depression remains to be elucidated, the establishment of animal models can facilitate the identification of analogous pathogenic pathways through the application of rigorous methodologies. Animal models that are suitable for simulating the illness state of human depression can be utilised to investigate the pathophysiology of depression and the development of novel antidepressant medications. Currently, there is an absence of an optimal animal model that can fully replicate the pathogenic pathways of human depression, which limits future research in this field. It is evident that stress constitutes the primary catalyst for the onset of depressive states, a phenomenon that has been observed in both human and animal subjects. From this standpoint, animal models of stress-induced depression should be better equipped to simulate the onset process of human depression. This study offers a comprehensive summary and analysis of the most frequently employed rodent models of depression, with a view to providing a more diverse range of models and resources for animal studies in the field of depression research.

Key Words: Stress model; Rodent animals model; Stressor; Depression; Animal model of depression

Core Tip: Although there has been extensive research on depression through animal models, it is undeniable that it is challenging to directly and comprehensively find stress-related depression animal models in the literature. This is because existing studies mostly lump together stress, medication, genetic, and surgical factors in summarizing depression models without clear differentiation. Based on this, we have conducted the first relatively comprehensive summary of depression models caused by stress.

INTRODUCTION

Depression is a prevalent mood disorder, characterised by substantial and enduring feelings of sadness and anhedonia[1]. The prevalence of depression is a significant global health concern, with a high incidence, disability rate, and disease burden[2]. It is estimated that approximately 322 million individuals worldwide are affected by depression[3]. According to the predictions made by the World Health Organisation, depression is set to become the leading cause of disease burden by the year 2030[4]. According to the 2020 China National Nutrition and Chronic Disease Survey Report released at a Chinese government press conference, “the prevalence of depression in China reached 2.1% in 2019”. The use of antidepressant drugs has also been increasing in various countries[5]. Approximately 10% of the global population is affected by depression[6]. In a similar vein, the estimated depression rate in 2021 has increased sevenfold from 2017[7]. Depression has been shown to have a detrimental effect on patients’ mental and physical health, as well as having a considerable impact on their quality of life. The condition also imposes significant economic and medical burdens on families and society as a whole[8]. Despite the existence of numerous treatment options for depression, approximately one-third of patients exhibit an absence of response to antidepressant medication or psychotherapy, a condition referred to as treatment-resistant depression[9]. This may reflect the fact that our current understanding of the precise neurobiological mechanisms associated with depression is limited[10], or our diagnosis of depression is based solely on behavioral symptoms, and the medications used to treat these symptoms are not fully applicable to the underlying disease pathogenesis[11]. Consequently, it is imperative to prioritize the identification of the underlying mechanisms of depression at this stage in order to develop effective treatment strategies.

Despite the established roles of neurotransmitters and their receptors and transporters, neurotrophic factors, the neuroendocrine system, the inflammation hypothesis, and others in the research of depression, the pathogenesis of depression is still unclear. More than 20 years ago, it was determined that stress and mental disease are related[12]. Stressors that are brief or mild to moderate have been shown to improve neurological health and guarantee appropriate brain function. Rodent models have demonstrated that chronic stress, particularly psychological stressors at the social level, is a significant risk factor for the development of depression in humans[13-15]. Stressors play an important role in the pathogenesis of depression, and in most cases, severe or chronic stressors are detrimental, and these disrupt the brain’s ability to maintain a normal stress response, ultimately leading to depression[16-18]. As demonstrated in the relevant literature, considerable and brief periods of stress have been shown to enhance cognitive function in rats through the promotion of stem cell differentiation into neuronal cells[19].

Peripheral tissues (such as blood) are readily accessible yet prove challenging to utilise as objective indicators in patients with depression, as they yield relatively limited biological information[20]. A greater proportion of tissue samples are derived from post-mortem examinations of suicidal depression patients (including brain tissue)[21]. Nevertheless, ethical concerns persist in human depression research. However, limitations in the origins of the samples, including variations in antidepressant usage and inconsistent time intervals for autopsy sample collection, constrain the value of such studies[22]. Given the disparity between animal cognitive abilities and human higher-order cognitive and emotional capacities, simulating human depression in animal models presents significant challenges[23]. Despite the present uncertainty surrounding the capacity of animal behaviour to elucidate the biological underpinnings of depression, the development of animal models of depression remains a viable approach to delineate analogous pathogenic mechanisms[24], provided that suitable methodologies are employed. Suitable animal models can simulate the disease state of human depression and be used for the study of the pathogenesis of depression and the development of new antidepressant drugs. Further experimentation on animals is required to facilitate a more comprehensive understanding of the mechanisms that underpin the onset of depression. However, human depression presents with a variety of clinical manifestations, such as self-doubt and suicidal tendencies, which are difficult to accurately detect in animal models. Currently, there is an absence of an ideal animal model that can completely simulate the pathogenic mechanisms of human depression, which hinders further research in this area.

In order to circumvent the ethical challenges inherent in human studies of depression, researchers may employ animal models as a substitute. Furthermore, they have the capacity to amass a substantial number of sample sizes, thus enabling multiple iterations to evaluate the viability of hypotheses and medicines. As is evident in the current literature, the most commonly used animals for modelling depression include rats, mice, non-human primates, chickens and tree shrews[25,26]. Non-human primates have a high degree of overlap with humans in terms of their emotional central nervous system[27], but they are expensive with limited sample sizes, thus less commonly used. Currently, rodents, including mice and rats, are the main animals used for modeling depression[28]. Stress is a primary factor that induces depression in humans and rodents. Animal models of depression induced by stress modalities are the classic animal models of depression. These models outline the core behavioural features of depression and, more ideally, mimic the pathological mechanisms of human depression. The goals of this review are to outline the most popular rodent stress models of depression; and go above their advantages and drawbacks. The ultimate goal is to give researchers the appropriate animal models to further our understanding of depression studies.

METHODS FOR PREPARING ANIMAL MODELS OF DEPRESSION INDUCED BY STRESS

Stress is a significant contributing factor to the development of depression in humans and rodents. The capacity of an organism to effectively manage stress and restore homeostasis is contingent on its ability to deploy effective coping strategies. Depression induced by stressor is characterized by inadequate or maladaptive coping strategies, which can be corrected by antidepressant drugs. Therefore, we briefly review a series of depression models induced by stressor as an operational approach.

Behavioral despair model

The protocol involves the administration of the forced swimming test (FST) and tail suspension test (TST) in rodents, which are classified as acute stress models. In both models, rodents initially display positive behaviours, such as swimming or struggling. However, these behaviours eventually progress to a terminal negative behaviour, characterised by intermittent immobility[29]. Because this behavior may be rectified or improved by antidepressants, the resting state is also regarded to be a depression-like condition and is extensively employed in depression models.

FST: The FST involves placing a rat/mouse in a confined, inescapable, smooth glass cylinder for swimming training. After multiple failed attempts to escape, the rat/mouse gives up struggling and floats on the surface of the water, resulting in a “despair” state (Figure 1A)[29,30]. Researchers have utilised the immobility time of rats and mice as a quantitative metric to assess the emergence or exacerbation of depressive symptoms. Consequently, this model is frequently employed to assess the efficacy of the depression model. The merits of this model are manifold; it is compatible with both rats and mice, straightforward to operate, and boasts high sensitivity and a high success rate. However, it is important to note that the capacity to elicit a single behavioural stress response is limited, and significant variations in stress responses have been observed among different strains of mice[31]. The model exhibits a propensity for false-positive reactions, and it has been demonstrated that certain stimulants, including amphetamine, can concomitantly reduce the immobility time of rats. Moreover, the model is vulnerable to the influences of water temperature, water depth, and the surrounding environment. A plethora of studies have indicated that forced swimming can compromise the immune system of rats[32].

Figure 1 Behavioral despair model. A: Forced swimming test. In the swim training scenario, the closed glass cylinder provides no escape route for the rat/mouse. Following multiple unsuccessful attempts to break free, the rodent eventually relinquishes the struggle and instead floats on the water’s surface, reflecting a state of desperation; B: Suspension test. The mouse tail suspension test involves hanging a rat upside down by its tail, which creates an abnormal posture. In response, the rat will exhibit struggling behaviors in an attempt to overcome this unfamiliar position. After numerous unsuccessful endeavors, the mice may display intermittent immobility, akin to a state of “behavioral desperation”.

TST: The mouse TST is a method where a mouse is hung by its tail and suspended upside down. The mouse will exhibit struggling behavior to overcome the abnormal posture. After multiple failed attempts, the mouse will exhibit intermittent immobility, similar to a “behavioral despair (BD)” state (Figure 1B). The model is distinguished by its rapidity, convenience, sensitivity, and high success rate. It has been demonstrated that this can reflect the stress-induced hypothermia in mice and can be used for the high-sensitivity screening of anti-depressant drugs, as well as for judging the success of depression model establishment. However, it should be noted that this model is only applicable to mice, and there are strain differences[33]. Studies have shown that female mice are more sensitive than males[34], and the model also shows a certain proportion of false positives when used after central stimulants.

The model is distinguished by its rapidity, convenience, sensitivity, and high success rate. It has been demonstrated that this can reflect the stress-induced hypothermia in mice and can be used for the high-sensitivity screening of anti-depressant drugs, as well as for judging the success of depression model establishment. However, it should be noted that this model is only applicable to mice, and there are strain differences[33]. Nevertheless, such conclusions are based on backward inference results, which researchers might choose to interpret as depression-like or antidepressant-like effects based on a decrease or rise in the length of the resting immobility state, respectively, which may be vulnerable to interpretation bias[35]. Quiescence may also be conceptualised as an adaptive trait, insofar as it enables the animal to cope with the prevailing circumstances for a limited period without expending energy. The primary controversy surrounding this model pertains to the question of whether the immobility of animals is attributable to their own fatigue or depression. Additionally, there is some uncertainty regarding whether brief stress can induce a depressive state. A comparative study of 11 strains of animals[36] found that the effectiveness of the BD model depends heavily on the background strain of the test animal, with differences of more than ten times among different strains, and C57BL/6 mice and Wistar rats have small individual differences, making them the preferred animals for making the BD model. As a result, using BD to screen for potential novel therapeutic strategies (drugs) or to assess depression-like states in animal models is far from sufficient. Researchers should also consider other symptomatic manifestations of depression and be more rigorous in interpreting the results of these tests[35].

Learned helplessness model

The model proposed by Seligman and Beagley[37] in 1967 involves the exposure of experimental animals to inescapable electric shock stimuli over an extended period of time. This results in the animals experiencing intense frustration and manifesting a phenomenon known as behavioural despair (Figure 2A), in which they cease to escape and demonstrate a sense of hopelessness. Animals may forgo food-seeking or require a greater time period to locate sustenance, displaying a passive demeanor in subsequent learning activities, such as avoidance, diminished spontaneous activity, reduced appetite, weight loss, and decreased aggression. Stressed animals exhibit several major signs of clinical depression, which are extremely comparable to the symptoms frequently used to diagnose depression. The model was developed from the human perception of depression, where animals are unable to control negative events and thus they feel anxious and helpless, a phenomenon that has also been demonstrated in rodents[38]. The learned helplessness (LH) model evokes feelings of dread, inevitability and uncontrollability, which result in a depressed response. However, it also provides a method for improving learned behaviour. In order to elucidate the underlying processes of the LH model, researchers employed functional neuroimaging methods to investigate neurobiological abnormalities in animals. It is noteworthy that researchers have identified neurobiological changes analogous to those observed in LH model mice in both healthy individuals in an uncontrolled state and patients diagnosed with major depression, thereby substantiating the model’s validity[39-41].

Figure 2 Learned helplessness model/social defeat stress model. A: Learned helplessness model. This model involves subjecting experimental animals to prolonged periods of social isolation, where they are deprived of social interactions. This isolation can lead to intense frustration and induce a phenomenon called behavioral despair in the animals; B: Social defeat stress model. The resident-intruder test involves placing a male rodent into an environment where it encounters a dominant or older male rodent, leading to aggressive interactions. The invading rodent is typically attacked by the resident animal, creating a model that is commonly referred to as the resident-intruder test. This experimental setup can induce intense frustration and evoke a phenomenon known as behavioral despair in the intruder animal.

Animals exposed to highly stressful uncontrolled events develop a depression model defined by etiology and symptoms. LH has become an effective example to examine the pathophysiology of depression due to its similarities to human depression[42]. Most symptoms of LH can be treated with multiple rapid antidepressant drugs. LH has the advantage of simulating the symptoms of depression comprehensively and is often used to screen antidepressant drugs and to study the pathogenesis of depression. It can also evaluate the depressive behavior of animals with mutations in depression-susceptibility genes and is the source of many theories in the pathophysiology of depression[43]. C1q (classical complement pathway promoter) gene knockout mice are more likely to develop learning helplessness. Nevertheless, the prevailing consensus in the field is that a control group is usually required for the purpose of conducting a comparison between animals that are predisposed to depression and those that demonstrate a greater degree of adaptability. The majority of experimental animals in this model exhibit a return to normal behaviour within a few days, the duration of their depressive behaviours is relatively brief, and the observation time is correspondingly reduced. This finding suggests that the model does not possess a long-term effect, and consequently, it is not appropriate for screening purposes related to chronic antidepressants. Furthermore, there are discrepancies in the propensity for depression among different rodent strains[44]. Another disadvantage of this model is that it requires specialized equipment, such as a shuttle box, and complex conditions.

The concept of LH in depression has been the subject of considerable controversy. At present, there is no evidence to suggest that the depression and despair symptoms of depressed patients are caused by LH. In the case of healthy individuals, the development of LH is not observed in circumstances where escape is not possible, and avoidance behaviours may be attributable to fear rather than LH[45]. Furthermore, not all rodents exhibit helplessness, and in the case of highly stressful uncontrollable events, only a susceptibility to LH (increased depression-like behavior) can be observed in male rats, which is not expressed in females, implying that sex differences in depression cannot yet be observed utilizing LH models[46-48]. The LH model is also associated with certain limitations in terms of its predictive validity. This is evidenced by the observation that certain antidepressant medication therapies can produce rapid effects that do not result in the amelioration of clinical symptoms in individuals diagnosed with depression. This finding stands in contrast to the model’s animal-based validity[41].

Social defeat stress model

Social stress is strongly linked to the development of depression and other psychopathologies in humans and has recently gained prominence as a serious public health issue[49]. Social defeat stress (SDS) has been extensively researched in humans, and mental disorders are prevalent among individuals who have been subjected to bullying. Adults who were subjected to bullying during their childhood are twice as likely as the general population to attempt suicide later in life[50]. Within the social environment delineated by the SDS model, excessive competition has been demonstrated to engender heightened susceptibility to stress in individuals. Animals exhibit the same behaviour. It is an inevitable consequence of survival competition in the context of animal social conflict that the quality of life for animals is diminished. This model induces psychological stress on the weaker individual by instigating conflict within the same species. Researchers place a male rodent into an environment dominated by a combative or older male rodent, where the invading animal is attacked by the resident animal. As a result, this model is also known as the resident-intruder test (Figure 2B). After repeating the process multiple times, the experimental animals eventually exhibit a lack of pleasure and symptoms of anxiety[51]. Additionally, the animals show decreased spontaneous activity, increased defensive behavior, changes in circadian rhythms, and impaired immune function[52].

The majority of individuals exposed to stressful circumstances demonstrate resilience, and the same stress does not invariably result in depression among different individuals[53]. This phenomenon can be attributed to the inherent variability in the human capacity to cope with stress, a notion that has been substantiated in the SDS model of C57BL/6J mice[54]. In the SDS model, this requires the presence of a vulnerable mouse that exhibits social avoidance. Furthermore, identifying neurobiological alterations linked with active stress coping, as found in these resilient individuals, will aid in the development of innovative and effective antidepressant medicines[55,56]. Conversely, thirty percent of animals that adapt positively in the face of stress, threat or extreme adversity do not demonstrate depression-like behaviours related to resilience[57]. The SDS model employs social conflict as a stressor, thereby inducing emotional and psychological distress that culminates in symptoms reminiscent of depression. Rodents have been observed to demonstrate an increased degree of vulnerability to social stress, manifesting endocrine disruption symptoms analogous to those observed in depressed patients[58]. Furthermore, SDS has been demonstrated to facilitate the identification of molecular mechanisms capable of instigating enduring alterations in phenotype[59].

The advantage of this model is that it simulates social attributes in human chronic stress, producing multiple organic and functional changes in experimental animals that can be used for long-term screening of antidepressant drugs. This model has been frequently utilized to elicit depression-like behavior and to investigate the molecular processes underlying depression[60,61]. Nevertheless, this model concomitantly instigates depressive and anxiety behaviours, rendering it unsuitable for the exclusive study of depressive behaviour. However, it is well-suited for the investigation of the underlying mechanisms of mixed features of both depressive behaviours[62]. The current model is only relevant to male mice since female rodents are less aggressive and performing this process on them is difficult[63], and another reason is that the tested subjects are only adult animals.

Chronic social isolation model

Whilst female and immature mice exhibit reduced levels of aggression, the creation of SDS models in these groups is more challenging. Furthermore, the effective induction of depressive-like behaviour is also more difficult. In human societies, females have been observed to be more susceptible to psychosocial stress[63]. Recent research has demonstrated that social failure replacement procedures can produce depressive-like behaviour in female rats[36]. According to studies, depression in older persons is strongly linked to grieving and spouse loss in later life[64]. People are born with social skills, but loneliness can make them less social and make it harder for them to meet their social demands. Social isolation is closely related to the start of depression[65], and it can result in a loss of both high-quality and quantity social contacts with others at the level of the individual’s social environment[66]. Researchers have utilised the chronic social isolation (CSI) model to simulate rat isolation on the grounds that rodents possess social characteristics in addition to their other characteristics (Figure 3). In addition to its capacity to reduce learning memory capacity, social isolation, a prevalent social stressor, has been demonstrated to induce alterations in brain physiology, neurochemistry, and neurobiology[67,68]. This, in turn, has been shown to precipitate a wide range of deleterious behaviours, including depression and anxiety[69,70]. Young rats that are kept alone and denied socialization and engagement with their peers from the moment they wean from breastfeeding until adulthood (about 24-52 days) exhibit decreased social interaction and social avoidance as adults[71]. Female prairie voles have been observed to manifest symptoms consistent with depression, as indicated by a decline in their level of enjoyment, when subjected to protracted social isolation. Furthermore, these animals exhibit a heightened response to perceived threats, as evidenced by their stronger neuroendocrine reactions to invaders[72].

Figure 3 Chronic social isolation model/chronic restraint stress model. A: Chronic social isolation model. This model involves subjecting experimental animals to long-term social isolation, depriving them of social interactions. This prolonged social isolation can lead to intense frustration and induce the animals to exhibit behavioral despair; B: Chronic restraint stress model. In this model, animals are chronically restrained, restricting their movements and limiting their normal behavioral expressions. This chronic restraint stress can also result in intense frustration and evoke behavioral despair in the animals.

There is conflicting evidence, despite the majority of research’ assertions that social isolation throughout adolescence causes long-lasting problems like mood dysregulation. Adult male C57BL/6 mice have been demonstrated to become more aggressive when they are socially isolated[73], and both Kunming breed (KM) and BALB/c mice experience cognitive deficits and an increase in aggression as a result. However, BALB/c mice demonstrated worse spatial/non-spatial memory and less aggression than KM mice under the same age and isolation time conditions[74], indicating that KM mice are better suited for the CSI model than BALB/c mice. It has been shown in some research that social isolation decreases physical activity in mice[75], but it has also been shown in experiments with isolated rats[76] that social isolation enhances hyperactivity and exploratory behavior in mice[77]. The consequences of early social isolation have been demonstrated to include alterations in neuroanatomical development, as evidenced by changes in the medial prefrontal cortex[78], decreased dendritic spine density in the pyramidal cells of the hippocampus[79], decreased synapses in the dentate gyrus[80], and decreased neurogenesis[81]. The aforementioned alterations have been linked to decreased activity of the dopamine and 5-hydroxytryptamine systems, altered hypothalamic-pituitary-adrenal (HPA) axis activity, and anomalies in brain neurochemical and neuroendocrine function in the vomeronasal nucleus[82], amygdala[83], hypothalamus[84], and hippocampus[85]. The chronic administration of antidepressants has been demonstrated to effectively alleviate symptoms of anxiety and pleasure deficit associated with social isolation in adults[86]. Acute fluoxetine treatment corrected the impaired social interaction behavior that male pups exhibited as a result of social isolation, which is interestingly how this was also verified in an animal model[87]. This provides the framework for researching how the CSI model and depression and medications are related.

In the CSI model, young rats who are isolated between 22 and 35 days after birth experience more negative effects on their social and behavioral growth[88]. Both rats and mice may be employed in age-related studies using the CSI model, though this model is unsuitable for gender-related investigations. We contend that the CSI model elicits more pronounced behavioural differences in rodents, potentially attributable to the combined effects of social isolation duration, species variation, animal sex or developmental stage, and the behavioural tasks administered.

Chronic unpredictable mild stress model

Human depression is a complex pathological result of various physiological, psychological, and social factors. Therefore, both the BD model and LH model are based on a single stimulus factor, and cannot simulate the process of human depression well. Katz et al[89] designed the chronic unpredictable stress model (CUS) model, which is more similar to the onset of human depression. In the course of their research, scientists subject animals to a range of potent stimuli over an extended period. These stimuli may include, but are not limited to, sudden noises, intense light exposure, elevated temperatures, ice water immersion, tail suspension, electric shocks, and water and food deprivation (Figure 4). Researchers need to repeat various stimuli 2-3 times without any regularity. By simulating multiple factors, researchers have constructed the CUS model, which can better simulate human depression. This model effectively avoids the animal adaptation phenomenon caused by the repetition of a single stimulus factor and is a classic model for simulating depression under conditions of chronic stress in humans. It can help researchers simulate and discover related risk factors for depression. The CUS model has also been used to verify the efficacy of new antidepressants and to study gender differences in rodent animals under stress conditions. However, the practical implementation of this model necessitates a substantial investment of effort and a protracted timeframe, and is vulnerable to the impact of the surrounding environment. The concurrent administration of multiple potent stressors to experimental animals has been demonstrated to result in a high mortality rate, a finding that does not accurately reflect the real-world phenomenon of human depression.

Figure 4 Chronic unpredictable stress model. The chronic unpredictable stress model is a widely used model in animal research. In this model, animals are exposed to a series of unpredictable stressors, which may include noise, changes in lighting, and adverse temperatures. These stressors vary in time and intensity, simulating the unpredictable stressors that occur in real life.

The stress stimulus in the CUS model is too strong and does not completely match the actual situation of human depression. Therefore, Willner improved and designed the chronic unpredictable mild stress (CUMS) model based on Katz’s work. Willner adjusted the stress stimulus intensity for experimental animals in the model and introduced the “sucrose preference” test to verify the success of the CUMS model[90]. The current preponderance of CUMS as the most widely utilised animal model of depression is attributable to the establishment of said model through the simulation of the long-term chronic low-intensity stress experienced by humans in daily life[91]. During a certain period of time, researchers expose rodents to a series of repeated unpredictable mild stimuli, which can be divided into two categories: Changes in the rodent’s surroundings (wet bedding, removal of bedding, tilted cages, reversed light-dark cycle, shaking cages, etc.) and physical stimuli (food deprivation, water deprivation, electric shock, tail clamping, odor, cold water swimming, etc.) (Figure 5)[92]. In the context of CUMS, rodents have been observed to manifest depression-like behaviour, characterised by a decline in sensitivity to reward reflex activity. For instance, rodents demonstrate a reduced sucrose preference, as indicated by lower ingestion of sucrose solutions, and exhibit symptoms such as weight loss and reduced appetite. It has been demonstrated that severe or chronic psychological stress, occasioned by protracted overstimulation, has the potential to induce depression and suicidal tendencies in experimental animals. Rodents typically exhibit functional impairments after 3 to 5 weeks of stimulation, resulting in persistent behavioural, neuroimmunological and neuroendocrine changes, which ultimately manifest as depression[93].

Figure 5 Chronic unpredictable mild stress model. In this model, animals are continuously exposed to multiple mild stressors that may represent ongoing small challenges in daily life. These stressors may include short-term mild electric shocks, wetting of bedding, tilting cage position, removing bedding, tail clipping, fasting, or water deprivation. Compared to the chronic unpredictable stress model, the stressors in this model are milder and more difficult to predict.

The chronic and mild unpredictable stimulus modality enables CUMS model animals to overcome stress-induced HPA axis stress habituation, which in this model concentrates on the lowering of reward sensitivity and the development of a lack of pleasure[94-97]. The animal’s plasma corticosterone reaction to the last stressor remains[96-98]. Acute stress stimulates the HPA axis, and elevated circulating glucocorticoids may result in prefrontal cortical and hippocampus shrinkage and apoptosis[52]. Cognitive abnormalities associated with depression have been shown to be alleviated by corticosterone release and downregulation of glucocorticoid receptor expression[99]. CUMS rats subjected to unexpected stress for three weeks before starting antidepressant therapy display depression-like behavior and synaptic plasticity impairments[100]. After three weeks, rats in the CUMS model acquire depression-like behavior, with changes in CA3-CA1 synaptic function and dendrites of CA1 and CA3 pyramidal neurons[101].

The CUMS model incorporates numerous stressors of moderate intensity and demonstrates significant effects following pre-treatment. The model under discussion is predicated on the hypothesis that stress induces depression[102]. The depression symptoms of most animals can be effectively treated with antidepressants, and the treatment time course and efficacy are extremely similar to clinical treatment. This model has face, predictive, and construct validity and is currently recognized as a classic model[103]. It is frequently utilised in the screening of antidepressants and possesses some degree of value in the study of the clinical mechanism of antidepressants and the pathophysiological mechanism of depression.

The CUMS model has been frequently employed as a rodent model of depression, and studies have identified strain variations in CUMS model animals. For instance, C57BL/6J mice have been observed to manifest heightened levels of anxiety and depression-like behaviours in comparison to ICR (CD-1) distant strains[104]. In contrast, C3H and CBA mice have been shown to be the least susceptible to the effects of CUMS, while BALB/c and C57BL/6 mice have been identified as the most sensitive[105]. In contrast, C57BL/6 mice do not exhibit symptoms of decreased fur status in CUMS[106], despite their aversion to commonly employed CUMS procedures, attributable to the presence of various transgenic strains[105]. Only C57BL/6 mice showed signs of depression and anxiety in the FST and novelty-suppressed feeding experiments after being exposed to CUMS[104]. In the CUMS model, sucrose preference was lowered more dramatically and consistently in Wistar rats than in Sprague Dawley (SD) rats[107]. Because SDS does not cause depression-like behavior in female mice, the CUMS model can be utilized in addition to SDS. Although there are variations in sensitivity and susceptibility in the CUMS model between male and female mice, this may be due to our present behavioral and neurobiological markers[108,109].

However, the actual operational workload of this model is large, the cycle is long, and the experimental conditions are complex. Moreover, there are difficulties in replicating the same experiment in a new experimental environment, and reliability is lacking. It should be noted that during modeling if the stress stimuli for the experimental group animals are too strong, the animals will develop tolerance or become agitated due to the stress. If the stimuli are too mild, the animals are not likely to exhibit depression-like behavior. Therefore, the intensity of the stimuli, the variability of the stress conditions, and the unpredictability are key factors for the successful establishment of the model.

Chronic restraint stress

Unfree prisoners and lonely seniors can develop depression due to social stress disorders. Therefore, researchers have designed the chronic restraint stress (CRS) model, in which rodents are repeatedly placed in a restraint tube for a certain period of time to limit their free movement, resulting in stress disorders similar to those in humans (Figure 3)[110]. Restraint is an inevitable paradigm, and restraint is a highly specific kind of stress that might operate as a mild stressor in CUMS. For a period of up to 21 days, researchers frequently confine animals in tiny tubes for 1-6 hours (usually no less than 2 hours)[111,112]. According to research, major behavioral changes usually occur after day 14 or day 21 when compared to controls. Eventually, the rodents display depression-like behaviors such as lack of pleasure, weight loss, reduced activity, and decreased appetite and sexual desire[113,114]. Studies have shown[115,116] that the pleasure-deprivation behavior induced by the continuous CRS model impairs rodent memory, reduces social abilities, increases cortisol secretion, and lowers body temperature. The CRS model has also been demonstrated to induce anxiety-like behaviour in rodents, resulting in alterations to the biological clock genes of model mice. However, the majority of rodent models of depression have been shown to respond positively to antidepressant treatment.

This model is easy to construct and, as a non-invasive stimulus, the CRS model has great similarities to human depression caused by stress disorders. It is commonly used to screen antidepressants and anti-anxiety drugs and is often used in conjunction with CUMS to create models. Studies have shown that the hippocampal and prefrontal cortex brain-derived neurotrophic factor levels and Na⁺, K⁺-ATPase activity in the CRS model mice are reduced, providing evidence for the “neurotrophic deficiency” hypothesis of depression[117].

The CRS model mice have been observed to exhibit elevated corticosteroid levels, as well as injury or atrophy of the CA3 pyramidal cells in the hippocampus[118]. Furthermore, alterations in apoptotic cell death have been documented[114,119]. After repeated stress, the HPA axis becomes desensitized, and when animals become accustomed to repeated exposure to homotypic restraint stressors, their plasma corticosterone levels decrease[98,120]. Short-term restraint has been demonstrated to enhance adaptive learning and memory functions in model animals[121], whereas prolonged restraint has been shown to induce dendritic atrophy and impaired memory in model animals[122]. Additionally, no significant increase in corticosterone levels has been observed after 21 days[96]. This repetitive homotypic constraint stressor produces stress that is more analogous to the steady, predictable stress that people feel on a daily basis[52]. The model has been found to elicit anxiety-like behaviour and lacks selectivity for antidepressant medication screening. SD rats are frequently utilised by researchers to induce CRS models of behavioural stress[52].

Maternal separation model

The impact on mental disease in later life is one component of traumatic experiences early in human life that can adversely alter human behavior in maturity[123], with victims of childhood abuse or parental neglect being significantly more likely to suffer from mood disorders[124]. Conversely, early life stressors have been demonstrated to elevate the probability of emotional disturbance in rodents during adulthood, culminating in protracted physiological and behavioural alterations[125]. For rodents, the maternal separation (MS) model belongs to a category of early life stressors. Researchers repeatedly separate the newborn mice from their mother, interrupting mother-infant interaction (Figure 6A), which leads to long-lasting changes in emotional and stress responses. The creation of MS models in birds, pigs, and guinea pigs is possible, but rats and mice are more stable. It is evident that a variety of laboratories employ MS for a range of separation times, extending from a few hours to several days. Research has demonstrated that this model primarily induces impairment to the mouse HPA axis, consequently engendering substantial alterations in neuroendocrine and behavioural responses. A substantial increase in serum corticosterone and adrenocorticotropic hormones, in addition to a notable alteration in glucocorticoid receptor expression within the hippocampus and prefrontal cortex of juvenile rats[126-128], has been observed to result in depression-like behaviour[129]. MS also affects the 5-hydroxytryptamine system and the expression of brain-derived neurotrophic factor (BDNF) in the hippocampus. The HPA axis activity and responsiveness to stress are modest in childhood and increase with age[130]. MS models have also shown that brief stress is good for animals[131], as illustrated by the fact that early life stress can reduce HPA activity and anxiety-like behavior in maturity.

Figure 6 Maternal separation model/sleep interruption model. A: Maternal separation model. The model involves separating the offspring from the mother for a certain period of time. This can be achieved by placing the offspring in a separate cage or exposing them to a different environment. The purpose of this model is to simulate the effects of maternal separation on the behavior, physiology, and development of the offspring and to study the impact of early environmental factors on the offspring; B: Sleep interruption model. The model, on the other hand, disrupts the normal sleep patterns of rodents. This can be done by periodically waking them up or exposing them to disruptive stimuli, preventing them from achieving deep and uninterrupted sleep. The objective of this model is to investigate the effects of sleep deprivation or sleep interruption on the health, cognition, and behavior of rodents.

Early life stress and DNA methylation can interact to cause changes in many neurobiological pathways, ultimately leading to behavioral abnormalities in rodents. Sustained hypomethylation of the arginine pressor protein gene in early life stress mice results in a considerable increase in arginine pressor protein expression[132]. However, in the MS model, rats and mice do not produce the same neurochemical results, with MS rats exhibiting sustained methylation changes in the BDNF promoter, which is accompanied by altered BDNF gene expression[133]. Subsequent phases of development and ecological factors in MS model animals yield a multitude of behavioural and neurochemical manifestations[134].

This model can increase susceptibility to other stressors in animals in adulthood (CUMS), effectively simulate stress environments with stable phenotypes, and provide a theoretical basis for the “hormone-stress” hypothesis of depression[135,136]. It is mainly used to study the pathological, physiological, and behavioral changes in adult mice caused by early stress. The disadvantage is that the experimental period is long and the result is susceptible to external influences. Varying separation times and sexes in different strains lead to significantly different modeling results, as female mice in this model exhibit greater sensitivity and susceptibility[137]. Furthermore, offspring stress vulnerability has been demonstrated to be associated with individual differences in maternal behaviour of the maternal generation[138].

Sleep interruption model

In clinical practice, depression patients commonly experience sleep problems, and the group with sleep problems has a significantly higher risk of developing depression or anxiety compared to those with normal sleep patterns[139,140]. A plethora of studies have identified the brain neural circuit basis for the relationship between depression and sleep quality, thus confirming the comorbid pathological mechanism between depression and sleep problems[141]. Consequently, a number of researchers have developed depression models in rodents by disturbing their sleep (Figure 6B). Various methods are used to ensure that the experimental animals sleep less than 4 hours in a 24-hour period to achieve sleep disruption. Chronic sleep deprivation and insomnia can act as external stressors and lead to depression, as seen by decreased hippocampus BDNF expression and disruption of frontal cortex BDNF expression, as well as lower serum BDNF expression levels and impaired circadian changes[142]. Currently, the main methods of sleep disruption include physical and pharmacological methods, and researchers can use physical sleep disruption methods such as intentional touching, platform over water, and forced locomotion, or pharmacological methods such as caffeine and ephedrine. The main rodent strains used for sleep disturbance are male Wistar rats and ICR mice. Acute rapid-eye-movement sleep loss can disrupt the cycle by repairing the hippocampus and perhaps restoring cortical and serum BDNF expression[142].

The sleep interruption model is a new type of depression animal model that exhibits both sleep disorders and depression characteristics, providing a model basis for studying the comorbid pathological mechanism between depression and sleep problems[143]. However, the intervention methods and standards for this model are not yet consistent, and its effectiveness still needs to be validated. In addition, there are also novel depression mouse models, such as the 24-hour confinement depression mouse model[144] and the dental clinic noise-induced depression mouse model[145].

SELECTION OF TYPES AND STRAINS FOR RODENT ANIMAL MODELS OF DEPRESSION

The ideal animal models for depression should meet two conditions: Firstly, the physiological and pathological manifestations of the model must be highly similar to those observed in human subjects. Secondly, the pathogenesis of the model must be similar to that of human depression, and the symptoms must be alleviated by antidepressant drugs used in humans[63]. Ideal animal models for depression should also include three principles: Face validity, construct validity, and predictive validity. Face validity refers to the degree of similarity between the animal model and human depressive symptoms; construct validity refers to the degree of similarity in causes (process) between the animal model and human depression; predictive validity refers to the degree of similarity between the animal model and humans in terms of sensitivity to antidepressant drugs or effectiveness of non-pharmacological interventions[146,147]. Animal models that meet these criteria can more accurately simulate the pathological mechanisms of human depression; therefore, their research results are more feasible and convincing.

Currently, over 50% of neuroscience experiments use rodent models, and for preparing animal models of depression, the main animals used are mice and rats[62]. C57BL/6 mice[24,148,149], BALB/c mice[150], and KM mice[151] are the most often utilized animals. In open-field investigations, C57BL/6 mice were shown to be more prone to display depression-like behaviors and to maintain a protracted psychological stress state than mice of other strains, making this strain particularly ideal for differential research of long-term depressed states[152].

C57BL/6 is an excellent strain in mice models of depression, with modest individual variations and great stability, and may be used to prepare stress-like models[153]. BALB/c mice were shown to be more prone to depression-like behavior when exposed to 22 °C water. In comparison to other models, BALB/c mice had qi deficiency-like symptoms as well as emotional stress alterations, which is particularly relevant to investigations involving the Disease Syndrome Combination Model of Traditional Chinese medicine[153]. Individual differences were evident in KM mice, and in the CUMS model, female KM mice exhibited emotional stress alterations in addition to impaired reproductive function, which is particularly suitable for examining the influence of sex differences on psychiatric investigations. Research findings indicate that male KM mice demonstrate superior retest reliability in the elevated cross-maze trial[154].

Common rat strains include Wistar Kyoto (WKY) rats[155], SD rats[156], Flinder Resistant Line (FSL) rats[157], and Wistar rats[152]. The WKY rats exhibited symptoms consistent with LH during modelling, extended periods of resting despair in forced swimming trials, and limited distances of activity in open area studies. These symptoms are indicative of depression. In contrast, WKY rats have been demonstrated to exhibit a heightened propensity for anxiety-related psychological distress, attributable to their elevated sensitivity to stress. Consequently, these rats have historically been a prevalent subject in anxiety-related research investigations[158,159]. SD rats and Wistar rats are favored subjects for clinical experimental research and mechanistic studies linked to learning memory impairment because they can better simulate the illness condition of individuals under stress, which causes depression in modern society. Non-spatial learning memory capacity is impaired more pronounced in SD rats, which have distinct depression-like features, whereas anxiety-like manifestations are less pronounced in SD rats, ensuring that the animals are modeled to appear more closely matched to the condition manifested by depressed patients[153]. FSL rats are distinguished by their sensitivity to a wide range of medications with diverse pharmacological processes; as a result, FSL rats can be utilized as a genetic animal model for depression[160]. In the context of liver depression and spleen deficiency, Wistar rats manifest variable degrees of blood hyperviscosity, a phenomenon that serves as a crucial indicator in the research of “qi stagnation and blood stasis” and “qi movement and blood movement” as outlined in the principles of Chinese medicine[161].

Longitudinally, strategies for constructing animal models of depression have demonstrably evolved from single stressors towards composite and chronic patterns. Early behavioural despair models (such as the FST and TST) and LH models, being operationally simple and short-term, are frequently employed for preliminary screening of antidepressants. However, their reliance on acute, high-intensity stress typically induces only singular behavioural alterations, making it difficult to replicate the multidimensional symptoms and chronic course commonly observed in human depression. As research has deepened, chronic stress models have gradually become the mainstream approach. For instance, the CUMS model employs prolonged, low-intensity, and variable stressors. It not only effectively induces core symptoms such as anhedonia but also induces alterations in neuroendocrine and immune systems. Possessing high ecological and predictive validity, it has become one of the most widely applied models in depression mechanism research and drug evaluation.

Comparative analysis reveals significant distinctions among models in terms of stress delivery, behavioural phenotypes, duration, and applicability. SDS effectively mimics depression-like behaviours induced by interpersonal conflict, particularly in male adult rodents, though its application to females remains limited. CSI, while gender-agnostic and better suited to simulating loneliness and social deprivation, exhibits substantial behavioural variability and lower standardisation. MS focuses on the impact of early-life stress on long-term depressive susceptibility, offering a unique avenue for studying developmental programming and epigenetic mechanisms, though it involves lengthy experimental cycles and is susceptible to external environmental interference. Furthermore, strain selection significantly influences model efficacy. For instance, C57BL/6 mice are particularly suited for differential studies of persistent depressive states, whereas BALB/c mice more readily exhibit anxiety-like and “deflated” behavioural traits. Thus, model design must holistically consider strain background alongside research objectives.

Currently, the application of composite models is increasingly becoming a crucial approach to enhance research reliability and validity. For instance, combining CUMS with CRS can simultaneously simulate chronic stress across psychological and physiological dimensions; applying CUMS post-MS in adulthood aids in revealing the regulatory mechanisms of early stress on adult stress responses. Such integrated strategies substantially mitigate the limitations of singular models, namely, their restricted perspectives and incomplete symptom replication, better mirroring the complex aetiology and clinical manifestations of human depression. Looking ahead, we must deepen investigations into model standardisation, mechanisms of gender differences, strain-specific foundations, and cross-model comparative validation. This will propel animal models of depression towards greater clinical translational value.

Mice possess a multitude of distinctive phenotypes and offer numerous advantages, including their small size, cost-effectiveness, and extensive resources for genetic modification. Researchers can obtain the mouse genome sequencing map from institutions or organisations, which facilitates the induction of mouse models using genetic engineering technology[162]. Rats are easy to construct models and are less susceptible to external environmental interference. Changes in body weight and behavioral observation data of rats are more obvious in experiments, making the effects and indicators of antidepressant drugs more referenceable. Rats may have more potential for study on emotional behavior and mental diseases if genetically altered models become more prevalent (Table 1)[163].

Table 1 Advantages and disadvantages of common depression model animal strains.

Strains	Advantages	Disadvantages
C57BL/6	Maintain a state of chronic psychological stress	Little spontaneous activity
	More likely to exhibit depressive states	Poor ability to explore novel environments
	Little individual variation and high stability
BALB/c	Lower social performance	Poor motherhood, low litter size, and high mortality of litters
	Better replicates some models of psychosomatic illness in humans	Relatively low aggressiveness
	Besides showing emotional stress changes, it also shows qi deficiency-like symptoms, which is suitable for building a combined Chinese medicine disease and evidence model
Kunming breed	Only one experiment is needed to achieve the observation effect and the experiment cost is low	High variation in growth and reproduction in the presence of inconsistent genetic backgrounds
	Can be used for experiments on mental differences between genders
	Good reproducibility of experiments
Wistar Kyoto	Susceptibility to learned helplessness	High reactivity to stress is more likely to lead to anxiety-based mental illness
	Long duration of resting despair in the forced swim experiment
	Short activity distances also appear in the open field experiment
Sprague Dawley	Significant impairment of non-spatial learning memory capacity	No obvious disadvantages
	A marked depression-like profile is accompanied by a less pronounced anxiety-like presentation
Flinder Resistant Line	Very sensitive to a wide range of drugs with different pharmacological mechanisms	Antidepressant-like behaviour can occur with long-term medication
Wistar	This is an important guideline for the study of the “qi stagnation and blood stasis” model in Chinese medicine	Habitat reproduction patterns are easily influenced by the external environment

FUTURE CHALLENGES

The development of rodent stress models for depression continues to face significant challenges, which constrain their translational relevance and scientific utility. While the integration of sensitive animal strains with integrated modelling approaches enhances the reliability and credibility of experimental outcomes, the complexity and operational feasibility of many composite models remain insufficient, limiting their widespread application. It is imperative that future research places a priority on the development of standardised, operationally feasible models that align with animal behavioural characteristics. This will ensure experimental viability while maintaining efficiency.

Notwithstanding the indisputable epidemiological data demonstrating that women exhibit twice the prevalence of depression compared to men, depressive models persistently rely on male animals. This methodological bias has the effect of limiting our understanding of female-specific pathophysiological mechanisms, as well as hindering the development of targeted therapeutic interventions. There is an urgent need to establish validated depressive models that are responsive to female-specific stressors, and to incorporate samples from both sexes routinely in preclinical research. It is recommended that future models incorporate systematic comparisons based on sex hormones and gender-related genetic backgrounds, with a particular focus on identifying and integrating depression-related biomarkers to enhance pathophysiological understanding.

A further significant challenge pertains to the inadequate reproducibility of prevalent depression models. In these models, variations in stress protocols, environmental conditions, and behavioural test interpretations across laboratories frequently result in inconsistent outcomes. The implementation of standardised operating procedures, in conjunction with automated behavioural tracking and computational analysis techniques, has been demonstrated to significantly enhance reproducibility and reduce subjective bias. Moreover, the prevailing focus of contemporary research on susceptibility phenotypes has led to an inadequate exploration of the underlying mechanisms. It is recommended that future models seek to elucidate the molecular and neural circuitry mechanisms associated with adaptive responses through longitudinal designs and multi-omics approaches. Ethical considerations remain central to model development. The application of prolonged or severe stress regimens requires strict adherence to the 3Rs framework (replacement, reduction, refinement) and promotes innovation in alternative methods, such as in vitro neuronal systems, organoid models or computational simulations, to minimise animal use and suffering.

Finally, it is imperative for the field to broaden its scope to encompass understudied demographic groups, with a particular emphasis on adolescents and the elderly. The majority of extant models employ young adult animals, a strategy that is inadequate in capturing neurodevelopmental or ageing-related dimensions of depression. The development of models that are age-appropriate is imperative to enhance the clinical relevance across the lifespan. This includes models that address early-life stress, such as MS, and models that consider chronic comorbidity in geriatric depression. Addressing these multifaceted challenges will require coordinated, interdisciplinary efforts spanning neuroscience, psychiatry, computational biology and ethics. Prioritising the development of more nuanced, reproducible and human-relevant models is essential to advance our understanding of depression and accelerate the discovery of novel therapeutics.

CONCLUSION

In summary, the utilisation of animal models in the study of depression holds considerable significance for the development of novel antidepressant medications and the exploration of the underlying pathophysiology of the condition. At present, the majority of strategies for developing rodent models of depression are based on etiology and pathophysiology. Researchers have developed depression models to test their theories using acute and chronic stress, exogenous medication treatment, genetic manipulation, and nerve injury. In the majority of cases, stress models are utilised in the creation of rodent models of depression. The CUMS model has become one of the most well-known rodent depression models. However, the pathophysiology of depression is intricate, and any animal model is subject to limitations. It is important to note that a single model is capable of depicting only a limited number of depressive symptoms, and that the experimental results are susceptible to subjective preferences. This factor serves to increase the likelihood of false positive results (Table 2). Despite the limitations of the current animal models in fully replicating the complex symptoms of human depression, each model exhibits distinct advantages and disadvantages, as discussed in this review.

Table 2 Advantages and disadvantages of animal models of depression.

Depression modeling approaches	Advantages	Disadvantages
Behavioral despair model	Fast model construction	Strain differences
	High success rate of model construction	Cannot evaluate the etiologic mechanism
	Low expenditure and manpower requirements	Possibility of false positive
	High predictive validity	Single stimulus source
	Can be used for the first screening of broad-spectrum antidepressants	Excessive irritation
Learned helplessness model	High face and predictive validity	Need to set up control groups
	More comprehensive simulation of depression symptoms	The duration of depressive behavior is short
	High repeatability	Need special equipment and complex conditions
	Imitates neural circuit alterations of depression	Strain differences
	Can be used to study the pathogenesis of depression	Easily affected by subjective impacts
	Can be used to screen depression susceptibility strains and susceptibility genes
Social defeat stress model	Superior face, construct, and predictive validity	Can produce symptoms of depression and anxiety at the same time
	Simulate human social situations	Can be confused with anxiety
	Produce organic and functional changes	Not applicable to females
	It can be used for screening long-term chronic antidepressants	Longer modeling duration
	Depressive behavior lasts for a long time
Chronic social isolation model	Superior face, construct, and predictive validity	Strain differences
	Simulate human social situations	Large variation in model animal behaviour
	Produce organic and functional changes	Lack of uniformity in the length and standard of model preparation
	Not limited by animal gender
Chronic unpredictable stress model	Great face, construct, and predictive validity	Longer modeling duration
	Can avoid the adaptive phenomenon caused by the repetition of a single stimulus	Heavy workload, waste of resources and labor
	Measures anhedonia	High stimulation and high animal mortality
	Can help simulate and find the risk factors of depression	Susceptible to environmental influences
	Can verify the efficacy of new antidepressants
	Can explore the gender differences of rodents under stress conditions
Chronic unpredictable mild stress model	Superior face, construct, and predictive validity	Low repeatability
	Depressive behavior lasts for a long time	Longer modeling duration
	The stimulation intensity is mild	Heavy workload, waste of resources and labor
	No lasting and irreversible effects on animals	Different strains have different sensitivity to stimulation intensity
	More in line with animal ethical requirements
	Stimulus is unpredictable
	Can supplement problems that other models cannot carry out
	Best suited for combination with other types of models or multi-method combinations of the same type
Chronic restraint stress model	Simple operation and strong repeatability	Single form of stimulus
	Non-invasive stimulation	Animals are prone to adaptation
	Can be used for the screening of anti-depression and anti-anxiety drugs	Can be confused with anxiety
	Can be used in combination with chronic unpredictable mild stress
Maternal separation model	Can improve the susceptibility of animals to other stresses (chronic unpredictable mild stress) after adulthood	Easily affected by environmental impacts
	Can explore the effect of early stress on the pathophysiology of offspring after adulthood	Strain differences
	Emotional and stress responses have long-term effects	Sex difference, and female animals show better sensitivity
	Stable phenotype
Sleep interruption model	Have the dual characteristics of sleep disorder and depression	Model intervention methods and standards are not uniform
	Can be used to study the comorbidity mechanism of depression and sleep problems	Validity of the model needs to be verified

Despite the fact that studies have summarised and analysed models of depression[52], they have also examined potential links between these models and important genes[164]. Furthermore, findings about major depressive disorder from rodent models that have been confirmed in depressed human subjects have been reported, as well as potential links between dendritic spine remodelling and alterations in specific brain regions from an animal model perspective. However, studies that have employed animal models of depression using a stress-like methodology are still rare.

According to recent research, more research is still needed on the animal model of depression. The pathophysiology of depression may be explored and the development of antidepressant medications can be sped up in the future by mastering the characteristics of many model animals and improving and building the depression model. We believe that while determining which model of depression to use, researchers should evaluate the differences between them. Our analysis may be useful in informing these judgments. Furthermore, more in-depth research is required to find animal models of depression that are most similar to human depression.