The open-field test is primarily a behavioral test to study the motor and fear behavior of rodents. The arena consists of a square Plexiglas box (50 centimeters x 50 centimeters) in which the mice are placed for five minutes. The distance, speed, and time spent in the different areas of the arena are measured. Since rodents usually avoid open areas to avoid being captured by predators, they stay as close as possible to the wall or in the corners of the arena. The amount of time animals stay in the center of the box can be used as a parameter for fear behavior. Particularly brave animals stay in the center more than particularly fearful animals.
However, the open-field arena also finds application in experiments in which memory or learning performance is studied. For example, the novel-object-recognition task examines the extent to which an animal remembers a familiar object that was already in the arena the day before.
The Elevated-Plus-Maze test is another fear test. The maze consists of two open arms and two closed arms. Since rodents hide from predators by instinct, they stay more in the closed arms. Here, the time animals spend on the open arms can be measured as a parameter for fear behavior. Brave animals spend more time on the open arms than fearful ones.
A behavioral test to examine exploration and fear behavior is the Novelty-Supressed-Feeding test. To keep the mice motivated, they are given no food for one day. However, water is always available to them without limitation. In the open-field arena, a food pellet is placed in the center during performance. This serves to bring the animals into conflict: The food is in the dangerous part of the arena, the center. This test measures, among other things, the time it takes the mice to actually start eating. The less fearful the mice are, the faster they start eating.
The Barnes Maze test tests spatial memory and learning. The maze consists of a circular platform with 20 holes of equal size distributed around its perimeter. Underneath one hole is an escape box. Due to the natural instinct to hide from predators, the mouse tries to find the box as quickly as possible. For four consecutive days, the mice are trained to find this box. On the fifth day, the box is removed and the mouse's behavior is studied: The faster the mouse finds the place where the box was located and the longer the mouse stays at this place, the better its spatial memory.
In order to get as comprehensive a picture as possible of depressive symptomatology in relation to serotonin, two established depression models are used in the research group: The Chronic Mild Stress Test and the Chronic Social Defeat Test. The combination of both models offers the possibility to investigate both the consequences of stress due to physical environmental influences and the consequences of social stress and its effect on depressive symptomatology and the influence of serotonin on these symptoms.
In the chronic mild stress test, the experimental animals are exposed to different mild physical stressors, such as wet, or dirty litter, crooked cages, or noise, on a daily basis for a fixed period of time (six weeks). This chronic stress leads to long-term depression-associated symptoms in the experimental mice, such as anhedonia or poorer grooming. Anhedonia occurs as a core symptom of depression in both mice and depressed humans. To experimentally determine anhedonia, the so-called sugar preference test is performed. Since anhedonia means the loss of pleasure, an anhedonic mouse no longer feels pleasure from eating something sweet. If a mouse with anhedonia is now offered both a drinking bottle with water and a drinking bottle with a sugar solution, this mouse no longer has a preference for the sugar water.
In addition to sugar preference, body weight and coat grooming, i.e., grooming behavior and coat condition, are examined, as these are also indicators of depressive symptoms.
The Chronic Social Defeat Test, on the other hand, is a behavioral test suitable for examining the social component of depressive symptomatology. People suffering from depression often withdraw from their social environment. This behavior also occurs after chronic social stress in mice and is considered a depressive symptom. In this experimental model, the experimental animal is exposed to social stress for ten days. Daily, the experimental mouse is confronted with a foreign mouse of an aggressive mouse strain (aggressor mouse). Contact is physical for ten minutes and sensory only for the rest of the day. The two mice can continue to interact and smell each other during this time, and the cages are separated only by a Plexiglas wall that has holes in it. This is repeated for ten consecutive days. On the 11th day, the social behavior of the experimental mouse is examined in the Social Interaction (SI) test. This test lasts five minutes and takes place in the Open Field Arena. Here, we measure how long the experimental mouse interacts with a mouse it does not know (interaction mouse). In this test, the interaction mouse is separated from the Open Field Arena by a Plexiglas wall that has holes in it. A depressed mouse would spend less time overall in social interaction compared to a healthy control animal. In this depression model, there are also resilient animals that do not show depressive symptomatology. Using this model, researchers can look very closely at what distinguishes resilient mice from mice that are prone to depression.
Mouse Gambling Task (MGT).
The Mouse Gambling Task (MGT) is a behavioral test in which the decision making of mice is analyzed. The starting concept for this test is the so-called Iowa Gambling Task, an experimental paradigm for humans in which subjects have to make decisions with high uncertainty on positive and negative outcomes.
For mice, this is implemented with different reward levels. As a reward, the researcher:s use a strawberry milkshake, which the mice are very happy to eat.
The mouse has a choice of four options. The choices differ in the amount of reward and the probability of receiving a reward. There are always two advantageous choices where the amount of reward is less but the probability of being rewarded is higher. The other two choices are unfavorable, where the amount of reward is higher but the probability of being rewarded is much lower. Based on the percentage of advantageous choices, the mice undergoing this test can be divided into different groups. Using this behavioral test, researchers can investigate how addictive behaviors (repeated risky choices) arise, for example, and how they might be influenced by serotonin.