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The FST is used to monitor depressive-like behavior and is based on the assumption that immobility reflects a measure of behavioral despair 3. The main advantages of this procedure lie in its relatively easy operation and fast results. Moreover, its sensitivity to a broad range of antidepressant drugs that makes it a suitable screening test is one of the most important features leading to its high predictive validity 29. Importantly, this test can also differentiate between drugs that are not aimed for the treatment of depression such as benzodiazepines, which have been shown to possess anti-anxiety effects 3,30. Another example of the value of the FST in the study of depression, which also contributes to its face validity, is the fact that models of predisposition to depression were found to influence the immobility behavior. For example, its use as a marker of depressive-like behavior in genetic research. Genetic animal models of depression have been successfully breeding rodents on the basis of their immobility in the FST 32, suggesting that immobility is determined by a heritable trait, although some strains have shown innately high immobility in the FST 33,34. Additionally, it is important to mention that genetic models that are not based on the performance in the FST were also found to be successful in providing valid models for studying depressive-like behavior 35,36. In addition to the genetic research, the immobility behavior in the FST has been shown to be influenced in several animal models that are linked with predisposition to depression, such as early exposure to stress 37,38, clinical diabetes 39,40 and acute drug withdrawal41.
Despite its appeal, reservations regarding its construct validity that cast doubt on it being a model for depression have also been raised. For example, the issue of chronic augmentation is problematic in this test because in real life patients need to be treated for at least several weeks before they experience any relief from their symptoms 42-44. That raises the question of whether immobility in the FST and depression share the same long-term adaptive changes in neuronal circuitry that underlie the effects of antidepressants in humans. With that said, it is important to mention that several recent studies have shown effects of antidepressants in this test following chronic treatment at much lower doses than those needed to induce effects after acute augmentation 24,45. Another much-discussed issue is the precise meaning of the immobility behavior as a measure that reflects the symptoms of depression 18,25. It is important to note that the immobility in these tests seems to be the result of an inability or reluctance to maintain effort rather than a generalized hypoactivity. This is of special importance due to the fact that patients suffering from depression show psychomotor impairments, particularly in those tests requiring the duration of effort 46. The active behaviors in this model (i.e. struggling and swimming) could potentially lead to escape and as a result decrease stress, whereas the passive behavior (i.e. immobility) may preserve energy while waiting for a possible escape. The animal’s choice of behavior varies and depends on numerous factors (e.g. pre-exposure, energy status, treatment, etc.). SSRIs have shown to postpone the transition from active to passive coping strategies, whereas factors that were found associated with depression accelerate this transition 18. Moreover, few questions have been raised regarding immobility as a learned process, meaning that the animal might learn that the best solution would be to be passive and wait to be removed from the water, what has been described as learned immobility 47-49. However, one might claim that this view leads to an anthropomorphic perspective of this model. In addition, negative correlation between longer immobility duration and stress hormones was not found 50,51. Furthermore, SSRIs were found to reduce immobility in a single test session following chronic administration in rats 52 or even following acute administration in mice 29 suggesting that where SSRIs are concerned learned immobility does not seem to play a role.
Another important notion is the role of the active behaviors during the FST. Although immobility is the behavior that is usually presented in articles, the other two measures have also been shown to be significant. Specifically, antidepressants that increase serotonergic neurotransmission led to longer swimming durations whereas those that were found to increase catacholaminergic neurotransmission led to longer struggling durations 18. This may help us to differentiate the neurochemical mechanisms underlying this behavior in different experiments. This was observed mainly in rats 53-56 and may be the cause that in our results exposure to the SSRI escitalopram resulted in the increase of the struggling but not the swimming behavior.
In addition, due to the fact that some antidepressants are known for reducing locomotor activity 18 and also that drugs such as psychomotor stimulants were found to reduce immobility in the FST 18,20,31 but are not effective for treating depression, it is recommended to preform locomotor activity tests in addition to the FST to rule out that the basal activity level is not the determining factor in this model. Moreover, due the aversiveness of the FST, it is important to take into account possible influences it might have on brain structure/function if brain analyses are to be carried out following this procedure. Also when preforming a number of behavioral tests, if the other paradigms are not considered stressful, it is recommended that the FST will be the last assay that is carried out.
Last, although in the past the scoring in this procedure was submitted to the possibility of bias by the experimenter, it is becoming more and more common to use designated software that eliminates this disadvantage 12,57,58.