Animal models of depression

Introduction

Depression

Modeling depression in animals

Endophenotypes in animal model of depression

• 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.
• Behavioral despair: Behavioral 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 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 circadian rhythm and especially in the sleep architecture are often observed in depressed. In rodents, it is accessible via electroencephalography.
• 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

Antidepressant screening tests

Despair-based

• Forced-swimming test: The forced-swimming test is based on the observation that animals develop an immobile posture in an inescapable cylinder filled with water. In this test, immobility is interpreted as a passive stress-coping strategy or depression-like behavior. 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 acute antidepressants.
• Tail suspension test: The TST, also known as tail suspension test, shares a common 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 antidepressants are 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.

  Reward-based

• Sucrose preference: Rodents are born with an interest in sweet foods or solutions. Reduced preference for sweet solution in sucrose preference test represents anhedonia, while this reduction can be reversed by treatment with chronic antidepressants. This test can measure the affective state and motivation of subject rodents; however, further validation is needed for working as a model of depression.
• Intracranial self-stimulation: Intracranial self-stimulation can be utilized in rodents to understand how drugs affect the function of brain reward system. In this paradigm, in which the animal is trained to spin a wheel to receive a current through electrodes implanted in its own brain for rewarding hypothalamic stimulation. 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 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: 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: For the elevated plus maze test, the rodents are placed at the intersection of the four arms of the maze, 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: 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

Adult stress models

• Learned helplessness: The learned helplessness model, 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.
• Chronic mild stress: The chronic mild stress model is probably the most valid animal model of depression. It aims to model a chronic depressive-like state that develops gradually over time in response to stress, and they can provide more natural induction. CMS involves the exposure of animals to a series of mild and unpredictable stressors during at least 2 weeks. The model has been reported to result in long lasting changes of behavioral, neurochemical, neuroimmune, and 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.
• Social defeat stress: Social defeat stress is a chronic and recurring factor in the lives of virtually all higher animal species. Humans experiencing social defeat show increased symptoms of depression, loneliness, anxiety, social withdrawal and a loss of self-esteem. Since the majority of stress stimuli in humans that lead to psychopathological 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 territory of 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.

  Early life stress models

• Maternal deprivation: 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: The olfactory bulbectomy in rodents results in a disruption of the limbic-hypothalamic axis with the consequence of behavioral, neurochemical, neuroendocrine and neuroimmune changes, of which many resemble changes seen in depressed patients. It is still not clear how bulbectomy in animals actually relates to depression in humans. It might simply result from a high intensity of 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 : 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: Only few generated mutant lines can be regarded as depression models, for example, α2A adrenergic receptor knockout mice, glucocorticoid receptor heterozygous mice, and cAMP response element-binding protein overexpressing mice.
• Forward genetics: 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.