The forced swim test (FST) is one of the most commonly used animal models for assessing antidepressant-like behavior. The forced swim test involves the scoring of active movements such as swimming and climbing vs. passive immobile behavior while swimming in a cylinder from which there is no escape. A wide range of antidepressant treatments have been shown consistently to reduce the amount of immobility time while increasing active escape behaviors.
The forced swim test is a simple and commonly used task that evaluates the anti-depressive properties of drugs and compounds in small mammals. This task is based on the fact that an animal will try to escape a stressful situation or stimulus. In this test, animals are placed in a tank of water, in which they cannot reach the bottom or escape the sides. Initially, the animals struggle in the deep water, but after several minutes, they adopt an immobility posture in which they float and make small movements only to keep their head above water. An animal that stops swimming behaviors early in the test is thought to be feeling hopelessness and is comparable to a human experiencing depressive symptoms. This failure to attempt escape has been called “behavioral despair” and is prevented in animals treated with anti-depressant drugs.
This test was first used and described by Roger D. Porsolt in the late 1970s in several publications (Porsolt et al. 1977a, Porsolt et al. 1977b, Porsolt et al. 1978). Porsolt and his colleagues showed for the first time that rodents could be used as models of depressive states and could be sensitive to anti-depressant compounds. When doses of anti-depressants were administered prior to the forced swim test, Porsolt saw a decrease in behavior despair and immobility of the animals (Porsolt et al. 1977a). In his later publication, Porsolt compared how different strains of mice behave in this test, with and without the administration of anti-depressive drugs (Porsolt et al. 1978). Since these initial papers, the forced swim test has become a commonly used test for evaluation anti-depressive drugs and anti-depressive properties of new compounds.
The apparatus used for the Forced Swim Test consists of a cylindrical tank, the height of which ranges from 25-50 cm and the diameter of which ranges from 10-30 cm to accommodate mice, rats, and small primates. The tank is filled with clean, room temperature water to a depth that prevents the animals from touching the bottom of the tank with their paws or tails. The height of the tank above the water level should be tall enough to prevent animals from escaping. Dividers can be used within the tank to allow multiple animals to be tested at once without seeing the others.
A mounted video camera is used to record the experiments from above. Live scoring is not recommended, especially when testing multiple animals at once, since it will be difficult to time the behaviors of individual animals. Live scoring also allows for inaccuracies and variances that will be avoided by closely scoring the recorded experiment. Tanks with backlighting are available to increase quality of recorded experiments.
A white noise generator should be used to mask sudden loud noises that might startle the animals and affect their behavior within the tank.
Evaluation of Depressive-like States in Small Mammals
The purpose of the forced swim test protocol is to assess the depressive state of animals, in an anti-depressant intervention vs. sham control group, by observing their behaviors when placed in a stressful environment where escape is impossible. Typically animals will exhibit struggling behaviors during the first couple minutes of the test, but then, as they reach a state of hopelessness, will become immobile and float making only small movements to keep their head above water. For this type of test, hopelessness is defined as the point when the animal realizes that escape is impossible, gives up, and stops struggling movements. Numerous studies have shown that the time spent in an immobile state is reduced with the treatment of anti-depressants (Porsolt et al. 1977a, Porsolt et al. 1977b, Porsolt et al. 1987, Detke et al. 1997, Cryan et al. 2004, Cryan et al. 2005, Petit-Demouliere et al. 2005).
While a six-minute testing period is often used, testing durations can range from 4 to 20 minutes, depending on the small mammal species. The entire test is recorded and generally a shorter portion of the test is analyzed by measuring the exact durations of specific behaviors. Of the typical six-minute test, usually the last four minutes are analyzed. During the first two minutes, the animals are typically very active in their behaviors and their initial movements can mask any potential effects of the treatment. It is only once control mice reach a state of hopelessness and behavior despair that the effects of treatment can be observed.
The forced swim test has proven extremely useful for studying the anti-depressive properties of drugs such as norepinephrine selective reuptake inhibitors (NSRIs) and selective serotonin reuptake inhibitors (SSRIs). Control animals are generally treated with a stimulus of motor activity, such as caffeine or apomorphine. These animals tend to exhibit increased climbing and swimming behaviors for the entire six-minute test, regardless of behavior despair.
While outside stress on the animals should be avoided for this test, it requires no or very little prior training of the animals. Initial struggle followed closely by behavior despair is a natural reaction of the animals placed in the forced swim apparatus.
To conduct the test, prepare the apparatus by adding the dividers, if using, and fill the tank with clean, room temperature water to the proper depth. Ensure that recording equipment is in position, is ready to record, and has available memory. Start the white noise machine, if using. Bring animals into the room, and allow and an acclimation period if necessary. Start the video recording before placing animals into the tank.
Holding the tail, carefully and slowly place the animals in the tank one at a time. Prevent the animal’s head from being submerged under water. Start a timer.
During the test, the researcher should ensure that the animals are able to maintain swimming and floating behaviors. An animal that dives or is not capable of swimming and floating should be removed from the tank.
After the six-minute test is complete, stop the recording and remove animals from the tank by their tails. Dry the animals carefully using towels and a heat lamp prior to returning to home cages.
Since its initial use, few modifications have been made to the forced swim test apparatus and protocol. Nearly a decade after Porsolt developed the test, Lucki and colleagues increased the depth of the water in the apparatus. They also began using a time-sampling technique when scoring the animals’ behavior movements (Lucki 1997). This allowed them to categorize escape-directed behaviors into two types: climbing or swimming. Making this distinction of behaviors, and the time spent doing them, allowed for a convenient method for quantifying escape behavior. This also allowed researchers to observe a difference in behaviors caused by NSRIs and SSRIs.
The data obtained from the forced swim test is generally visualized by showing the time an animal spent performing different behaviors. The amount of time an animal spent swimming, climbing, mobile, or immobile can be obtained by analyzing the experiment recording with a stopwatch. Alternatively, a time-sampling method of scoring can be used where the experiment is divided into five-second intervals and the animal’s activity during that interval is recorded. A general definition of mobility for this test includes any behavior other than those necessary to balance the body and keep the head above water (Cryan & Mombereau 2004). The time spent mobile and immobile can be easily graphed and compared across the intervention and sham control groups, as shown in the example graph below.
Using graphs similar to this to compare the amount of time different treatment groups spend immobile or mobile allows for easy visualization of the effect of the anti-depressant. Animals treated with saline should show increased immobility and decreased mobile movements. In contrast, those treated with an anti-depressant should show decreased immobility and increased swimming and climbing. Control animals treated with a stimulus of motor activity, such as caffeine, should show increased activity for the entire duration of the test. Generally, animal cohorts of 10-15 animals are sufficient to obtain p-values of <0.05 using a 1- or 2-way ANOVA, Dunnett’s post hoc, or Bonferroni’s post hoc tests (Can et al. 2012, Costa et al. 2013).
A major strength of the forced swim test is the availability of numerous studies that have validated its use for evaluating anti-depressant compounds. The forced swim test has been used extensively and researchers can easily compare their results to those of other groups. There is a large collection of data regarding the use of different animal strains and anti-depressant compounds. The availability of this data, along with the ease, reliability, and speed of this test make it essential for academic and industrial drug discovery.
Despite the fact that this test requires no prior training, the singular test places significant stress on the animals. While the environmental stress is necessary to induce behavior despair, repeated testing and exposure to other stressful stimuli may influence the resulting behaviors. The test models and observes a very specific type of stress-induced behavior, which is believed to be related to depressive human feelings, but it is important to remember that these behaviors are caused by extreme stress (Petit-Demouliere et al. 2005).
While the forced swim test forces animals into a state of hopelessness comparable to human depressive feelings, it is important to note that this test is not an equivalent of the entire spectrum of human depressive states. It remains unclear how similar human depression and the neurobiological mechanisms of behaviors seen in animal models are (Bourin et al. 2001). The observable outcome of this test is one-dimensional, and while it can determine if a compound has anti-depressant properties, it cannot determine the mechanisms by which that compound acts. The exception to this is seen in rats, where the difference between NSRIs and SSRIs can be observed (Detke et al. 1995). It is also possible that the tested drugs may affect overall activity levels of the animals, which could lead to unreliable results of the forced swim test. It is therefore important to control for overall activity levels using an additional test such as the open-field test.