The chronic despair model represents a suitable model for the induction of a chronic depressive-like state in mice in order to better understand the path of physiology of mood disorders and to evaluate potential antidepressive treatment interventions. Repetitive stress is used to induce a depressive-like state, following the idea that in humans depression is induced by chronic rather than acute stress. Since the effects of CDM induction are stable for up to four weeks, it is not only possible to investigate rapid acting antidepressants, but also substances with a delayed onset like most conventional antidepressants.
Before inducing the chronic depressive-like state, visually observe the mice for abnormalities including signs of biting or barbering. Exclude the whole cage from the experimental series if an animal shows signs of injury. To perform paired parametric tests afterward, make sure that each mouse is clearly marked or labeled.
Additionally weigh each mouse before starting the experiment. To perform a forced swim session, fill up a beaker or cylinder with room temperature water to a height of at least 20 centimeters from the bottom, leaving a minimum of 10 centimeters between the water surface and the upper border of the vessel. Then gently transfer the mouse into the water and set a timer for 10 minutes.
To prevent the mouse from drowning, continuously observe the mouse while it is in the water from a position where it cannot see the experimenter. After 10 minutes, take the mouse out of the water by simply grabbing its tail. Gently dry the mouse with a paper towel, and put it either under a heating light or on a heating mat.
Perform the forced swim session on each mouse every day for five consecutive days. After five swim sessions, transfer the animals back to their home cages and allow them to rest for at least two days. To use immobility time as read out method, record each swim session using a camera.
Then watch the offline video recordings and use a stopwatch to measure the total time spent by the mouse in the three different behavioral conditions during the swim test, struggling, swimming and immobility. The animal is considered to be struggling when it actively tries to escape from the threatening situation. This involves pawing the side of the cylinder with head oriented toward the wall and moving all limbs.
The water surface is typically slightly turbulent. The animal is considered to be swimming when it moves at least both hide paws and travels a distance through the water. It actively searches for a way out but does not lift its paws above the water surface or try to overcome the glass wall of the vessel.
In this condition animals frequently turn around or move in circles. Lastly, the animal is considered to be in the immobility stage. If it keeps still in a freezing-like position and does not move at all, or only moves the tail, or the forepaws to keep its head above the water surface, no distance is actively traveled except for passive floating, and no directed movement of the front paws is observed.
In the induction phase of the chronic despair model, during the first swim session, mice usually show a mean immobility time between 190 and 230 seconds, which then constantly increases with every additional swim session. The immobility time measured on day five remains stable over four weeks indicating stable behavioral despair. Antidepressive interventions, such as imipramine, sleep deprivation and ketamine, significantly reduce the immobility time.
While sleep deprivation combined with a recovery sleep does not show a significant change of the depressive-like phenotype. Since each treatment group has different mean values of immobility time on day five, different treatments should be compared using normalized mean values instead of absolute values. If immobility time remains unchanged during all five days, the applied stress could not change the behavior relevantly, and thus these mice cannot be used to evaluate treatment effects.
Other readout methods such as tail suspension test, nose poke sucrose preference test, and assessment of long term potentiation during the patch clamp technique can be used to describe a broader view of the behavioral despair of the animals. Measurement of the immobility time is an important part of the chronic despair model, but it should never be the only readout method. Multiple other assessments like the tail suspension test or sucrose preference test should be used.
