Animal models of depression
Animal models of depression are research tools used to investigate
symptomatology and pathophysiology of depressive illness or used to screen novel antidepressants. These models provide insights into molecular, genetic, and epigenetic factors associated with depression. Criteria for valid animal models include face, construct, and predictive validity. Endophenotypes, such as anhedonia, behavioral despair, changes in appetite, neuroanatomical alterations, neuroendocrine disturbances, alterations in sleep architecture, and anxiety-related behaviors, are evaluated in these models. Antidepressant screening tests are employed to assess the effects of genetic, pharmacological, or environmental manipulations. Stress models, including learned helplessness, chronic mild stress, and social defeat stress
, simulate the impact of stressors on depression. Early life stress models, psychostimulant withdrawal models, olfactory bulbectomy, and genetically engineered mice contribute to a comprehensive understanding of depression's etiology and potential therapeutic interventions.
Introduction
Depression
Symptoms include low mood and aversion to activity. Depressed people may also feel sad, anxious, empty, hopeless, worried, helpless, worthless, guilty, irritable, hurt, or restless. They may lose interest in activities that once were pleasurable, experience loss of appetite or overeating, have problems concentrating, remembering details, or making decisions, and may contemplate or attempt suicide. Insomnia, excessive sleeping, fatigue, loss of energy, or aches, pains, or digestive problems that are resistant to treatment may also be present.[1]
About one in six people in the U.S. will succumb to depression at some point during their life span,[2] and according to the World Health Organization, depression is projected to reach second place as leading contributor to the global burden of disease by the year 2020.[3] The effects of current antidepressant drugs are often significantly delayed, with improvements beginning around 3–6 weeks after treatment is started. Despite the clinical success of many antidepressant drugs, such as tricyclic antidepressants (TCAs), monoamine oxidase inhibitors (MAOIs), and serotonin reuptake inhibitors (SRIs), many individuals' symptoms are not adequately alleviated by medication alone, and other methods of treatment may be recommended.[4] [5]
Modeling depression in animals
It is difficult to develop an animal model that perfectly reproduces the
symptoms of depression in patients. Is generic that 3 standards may be used to evaluate the reliability of an animal version of depression: the phenomenological or morphological appearances (face validity), a comparable etiology (assemble validity), and healing similarities (predictive validity) Many animals lack self-consciousness, self-reflection, and consideration; moreover, hallmarks of the disorder such as depressed mood, low self-esteem or suicidality are hardly accessible in non-humans. However, depression, as other mental disorders, consists of endophenotypes[6] that can be reproduced independently and evaluated in animals. An ideal animal model offers an opportunity to understand molecular, genetic, and epigenetic factors that may lead to depression. By using animal models, the underlying molecular alterations and the causal relationship between genetic or environmental alterations and depression can be examined, which would afford a better insight into pathology of depression. In addition, animal models of depression are indispensable for identifying novel therapies for depression.[7]
Endophenotypes in animal model of depression
The following endophenotypes have been described:[6]
- Anhedonia: The loss of interest is a core symptom of depression. Anhedonia in rodents can be assessed by sucrose preference or by intracranial self-stimulation.
- despair might be assessed with tests such as the forced-swimming test or the tail suspension test.
- Changes in appetite or weight gain: Depression is often associated with changes in appetite or weight gain, which is easily measured in rodents.
- Neuroanatomy: Depressed subjects display decreased hippocampal volume and rodents exposed to chronic stress or excess glucocorticoids exhibit similar signs of hippocampal loss of neurons and dendritic atrophy.
- Neuroendocrine disturbances: Disturbances of the hypothalamic–pituitary–adrenal axis (HPA) are one of the most consistent symptoms in major depression. The functionality of the HPA can be assessed by dexamethasone suppression test .
- Alterations in sleep architecture: Disturbances in the sleep architecture are often observed in depressed. In rodents, it is accessible via electroencephalography(EEG).
- Anxiety-related behavior: Anxiety is a symptom with high prevalence in depression. Therefore, animal models of depression often display altered anxiety-related behavior.
Criteria for valid animal models of depression
An appropriate animal model of human depression should fulfill the following
symptom
of depression.
Antidepressant screening tests
Antidepressant screening tests, not like the models which can be defined as an [organism] or a particular state of an organism that reproduces aspects of human
environmental
manipulation.
Despair-based
- cylinder filled with water. In this test, immobility is interpreted as a passive stress-coping strategy or depression-like behavior (behavioral despair). After antidepressant administration, the animals will actively perform escape-directed behaviors with longer duration than animals with control saline treatment. FST is the most widely used tool in depression research, more specifically as a screen for acuteantidepressants.
- The advantages of FST are that it is low-costing and is a fast and reliable tool, easy to handle and has proven its reliability across monoamineantidepressants is uncertain
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- theoretical basis and behavioral measure with the FST. In the TST, mice are suspended by their tails using adhesive tape to a horizontal bar for a certain couple of minutes, and the time of immobility is recorded. Typically, the suspended rodents perform immediately escape-like behaviors, followed by developing an immobile posture. If antidepressantsare given prior to the test, the subjects will be engaged in escape-directed behaviors for longer periods of time than after saline treatment, exhibiting a decrease in duration of immobility.
- A major advantage of the TST is that it is simple and inexpensive. A major disadvantage of the TST is that it is restricted to mice and limited to strains that do not tend to climb their tail. Besides, like FST, TST is sensitive to monoamineantidepressants is uncertain.
Reward-based
- Sucrose preference:affective state and motivation of subject rodents; however, the face and construct validity of the sucrose preference test to measure depression-related behavior appears low.[14]
- Intracranial self-stimulation:hypothalamicstimulation. ICSS shares a common theoretical basis with the sucrose preference. Reduced preference for self-stimulating for reward cognition represents a loss of interest, fatigue and a loss of energy during depressive episodes, while this reduction can be reversed by treatment with antidepressants. Like sucrose preference test, ICSS can measure the affective state and motivation of subject rodents, and again, further validation is needed for working as a model of depression.
Anxiety-based
- Novelty-induced hypophagia:Hypophagia, one of the anxiety symptoms in rodents, is defined as the reduction in feeding in response to novelty, and it can be evoked by various novel features of the environment, including novel food, novel testing environment and novel food containers. Novelty-induced hypophagia (NIH) is a very recently developed test, which measures the latency and consumption of food in a novel unfamiliar environment. The test rather reflects the anxiolytic effects of antidepressants, and the response is seen only after chronic treatment with antidepressants rather than acute.
- Open field:[16] Rodents tend to avoid brightly illuminated areas, and this avoidance is interpreted as a symptom of anxiety. Open field is a bright enclosure and during the test rodents are placed in this arena thus forced to interact with a novel and bright environment. The movement of the experimental subject will be recorded in distance and pathway.
- Elevated plus maze:[16] For the elevated plus maze test, the rodents are placed at the intersection of the four arms of the maze (two open, two closed), facing an open arm. The number of entries and time spent in each arm is recorded and valid results are obtained in a single 5-minute testing session. An increase in the open-arm time is an index of anti-anxiety behavior of rodents.
- Dark/light box:[16] The dark/light box test is also based on the rodents' innate aversion to brightly illuminated areas and on the spontaneous exploratory behaviour of the animals. A natural conflict situation occurs when an animal is exposed to an unfamiliar environment or novel objects. The conflict is between the tendency to explore and the initial tendency to avoid the unfamiliar. The exploratory activity reflects the combined result of these tendencies in novel situations. The test apparatus of dark/light box consists of a dark compartment and an illuminated compartment. Drug-induced increases in behaviors in the white part of a two-compartment box are suggested as an index of anxiolytic activity.
- Open field test, elevated plus maze test, and dark/light box test can work as an antidepressant screen by measuring anxiety-related behavior as an accompanying endophenotype of depression. It is known that some antidepressants administration will cause a decrease behavior in these tests just like anxiolytics. However, the response to some antidepressants couldn't be detected. Besides, these tests each have their own problems. It is difficult to discriminate decreased anxiety-related avoidance from increased novelty-seeking in these tests.
Stress models
Certain types of human depression are precipitated by stressful life events, and vulnerable individuals experiencing these stressors may develop clinical depression. Consequently, the majority of animal models of depression are based on the exposure to various types of acute or chronic stressors.
Adult stress models
- Learned helplessness:[17][18][19][20][21] The learned helplessness model (LH), one of the well validated animal models, is the best replicated one. The rationale is that exposure to uncontrollable and stressful life events makes people feel like they are losing control, and this sometimes leads to depressive like behaviors. The model is based on the observation that animals also develop deficits in escape, cognitive and rewarded behaviors when they have been subjected to repeated unavoidable and uncontrollable shocks. LH is induced in one day or over several days of repeated inescapable stress by the treating of tail shock or foot shock in shuttle boxes. Helpless behavior is evaluated by analyzing the performance in an active escape test, such as the latency to press a lever or cross a door.
- An advantage of LH is that the cognitive and other behavioral outcomes seem to be correlated, thus helping to understand the depressive symptomatology in humans. Besides, this model can also be generally used to measure the escape performance of mice with different mutations, in which target genes of depression may affect the vulnerability to develop a depressive-like state. These excellent face and predictive validities make LH an interesting model to explore the pathophysiology of depression. The biggest disadvantage of LH is it requires very strong stressors to induce the behavioral phenotypes, which does raise ethical problems. Also, most of the symptoms do not persist long enough following cessation of the uncontrollable shock.
- Chronic mild stress:neuroendocrinological variables resembling reward functions including decreased intracranial self-stimulation, reflecting anhedonia that is reversed by chronic but not acute antidepressant treatment. Of note, this CMS model can be used to screen and test potential antidepressant compounds and to develop new treatment strategies.[26]
- The advantages of this model are its good predictive validity (behavioral changes are reversed by chronic treatment with a wide variety of antidepressants), face validity (almost all demonstrable symptoms of depression have been reproduced), and construct validity (CMS causes a generalized decrease in responsiveness to rewards). However, there is a common practical difficulty in carrying out CMS experiments, which are labor intensive, demanding of space, and of long duration. Besides, the procedure can be difficult to be established and data can be hardly replicated.
- Social defeat stress:pathological changes are of social nature, SDS model have gained increasing attention as they might render useful to study certain endophenotypes of depression. During the stress period, the male rodent will be introduced into a different territoryof other cospecific males for each day as an intruder, that cause it to be investigated, attacked and defeated by the residents. The consequent behavior changes in the subject caused by SDS, like decreased social interaction or lack of interest, are similar to some parts of human depression, and behavioral treatment and antidepressants can reverse these changes in SDS model.
- Like CMS, SDS has good predictive validity (behavioral changes are reversed by chronic treatment with a wide variety of antidepressants), face validity (many symptoms of depression have been reproduced), and construct validity (causing a generalized decrease in responsiveness to rewards) and gives another validity that only chronic but not acute antidepressant administration can reverse the social aversion. One disadvantage of SDS model is the long duration. To apply SDS model for studying human depression, the period of it should last at least 20 days otherwise only anxiety symptoms could be induced. Worthy of note, only male rodents can be used for this model, since female rodents do not fight each other in a resident–intruder confrontation.
Early life stress models
Early
maternal
separation. All these treatments have been demonstrated to produce significant effects that last until adulthood.
- Maternal deprivation:[32] The maternal deprivation model is the most widely used early life stress model. This model manipulates the maternal separation of early life deprivation, in which pups are separated from the dam for 1–24 h per day during the first two postnatal weeks. Maternal separation results in increased anxiety- and depression-like behaviors and increased HPA response in adulthood.
Other models
- Olfactory bulbectomy:chronic stressor caused by chronic sensory deprivation. This model shows high predictive validity as it mimics the slow onset of antidepressant action reported in clinical studies, responding chronic but not subchronic antidepressant treatment with no response to other drugs. It is worth to be mentioned that, unlike stress-related models, the rat in lesion model represents an agitated, hyposerotonergic depression-related phenotype, rather than a retarded depression.
- Psychostimulant withdrawal (amphetamine, cocaine):[34] In humans, withdrawal from chronic psychostimulants generates symptoms that have strong behavioral and physiological parallels to depression. Therefore, the examination of the behavioral effects of drug withdrawal in rodents may provide insights into the underlying neurobiological mechanisms and aid in the development of animal models of depression that are sensitive to antidepressant agents. Following withdrawal from drugs such as amphetamine or cocaine, rodents display behavioral changes that are highly similar to some aspects of depression in humans, such as anhedonia, and behaviors opposite to those seen after treatment with antidepressant drugs.
- Genetically engineered mice:heterozygous mice, and cAMPresponse element-binding protein overexpressing mice.
- Forward genetics:[38] Forward genetics allows identifying relevant genes without any prior knowledge of genetic to the phenotype. Large scale random mutagenesis screens, like ENU, have resulted in a great number of mutants displaying depression or antidepressant-like behavior.
See also
- Animal testing
- Institutional Animal Care and Use Committees
- Pit of despair, an apparatus used for animal models of clinical depression
References
- ^ "Depression Basics". National Institute of Mental Health. Archived from the original on 2013-06-11. Retrieved 2013-05-07.
- PMID 15939837.
- S2CID 10556268.
- PMID 20651815.
- ^ Hao, Y., Ge, H., Sun, M., & Gao, Y. (2019). Selecting an Appropriate Animal Model of Depression. International journal of molecular sciences, 20(19), 4827. https://doi.org/10.3390/ijms20194827
- ^ PMID 15213704.
- ^ Hao, Yuanzhen, Huixiang Ge, Mengyun Sun, and Yun Gao. 2019. "Selecting an Appropriate Animal Model of Depression" International Journal of Molecular Sciences 20, no. 19: 4827. https://doi.org/10.3390/ijms20194827
- PMID 12122308.
- S2CID 5904832.
- S2CID 16130112.
- S2CID 1222576.
- S2CID 2758433.
- ^ ]
- ]
- S2CID 21949826.
- ^ S2CID 7947941.
- PMID 9438074.
- S2CID 33316951.
- )
- PMID 16338640.
- PMID 11522522.
- S2CID 22504035.
- S2CID 9078352.
- S2CID 36727320.
- S2CID 23269414.
- PMID 29197398.
- S2CID 26571582. Archived from the original(PDF) on 2018-07-28. Retrieved 2018-07-28.
- PMID 17956738.
- ]
- S2CID 24450372.
- PMID 12953293.
- PMID 11520931.
- S2CID 42450349.
- S2CID 21885931.
- PMID 11425914.
- PMID 15987954.
- PMID 11549750.
- S2CID 5985686.
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