Dear Hafiz,

Indeed, open-field test is carried out to show the locomotor activity of rodents before Forced Swimming Test (FST) to show that despite the normal locomotion of animals they are immobile in the FST test. I added some reviews that are useful to better understanding the immobility behavior in the FST. Under acute unpredictable and unavoidable stressful conditions, depressed people get despair and hopeless and, in fact, FST mimics similar conditions for rodents in which immobility time is considered as a depressive-like behavior. The Tail Suspension Test is similar to FST as well. Also, Climbing behavior in FST reflects the activity of adrenergic system and swimming is associated with serotonergic system activity.

Cryan, John F., and Andrew Holmes. "The ascent of mouse: advances in modelling human depression and anxiety." Nature reviews Drug discovery 4.9 (2005): 775-790.

Cryan, John F., Rita J. Valentino, and Irwin Lucki. "Assessing substrates underlying the behavioral effects of antidepressants using the modified rat forced swimming test." Neuroscience & Biobehavioral Reviews 29.4 (2005): 547-569.

Slattery, David A., and John F. Cryan. "Using the rat forced swim test to assess antidepressant-like activity in rodents." Nature protocols 7.6 (2012): 1009-1014.

I guess that the question was done because most if not all the the test for assessing depression require a motor response to the rodent.

So to be sure that you are really evaluating a depressive phenotype not due to a locomotor impairment you should demonstrate that you can treat depression (i.e. improving rodents score in your tests with an antidepressant ) without affecting locomotor response.Vice versa you may show that improving locomotor response the score won't change.

By Porsolt's test of Behavioural despair.