A New Method for Inducing a Depression-Like Behavior in Rats

The overall goal of this model is to establish depression contagion in rats. This method can help answer key questions in the behavioral field such as mechanism of action, prevention, containment, and treatment of contagious depression. The main advantage of this technique is that this is a new model for establishing contagious depression.

The implications of this technique extend towards therapy of depression because at the moment, there is no existing format for studying contagious depression. For the experimental procedure, select male Sprague Dawley rats with no overt pathology each weighing between 300 and 350 grams. House three rats per cage with chow and water available ad libitum.

Allow a minimum of two weeks of adaptation on a 12-hour light and dark cycle. Before initiation of the experiment, test all rats for the presence of depressive-like behaviors using a three-day sucrose preference test. To do this, place the rats in individual cages in the same room as the housing during the dark cycle.

Place a bottle of 100 milliliters of 1%sucrose solution in each cage for 24 hours and allow for adaptation. After 24 hours, remove the bottles and deprive the rats of food and water for 12 hours. After 12 hours, place two bottles, one containing 100 milliliters of 1%sucrose solution and the other 100 milliliters of tap water in each cage for four hours.

After four hours, record the volume in milliliters about the consumed sucrose solution and water and calculate the affinity to sucrose preference as shown. Eliminate rats that test positive for depression, then eliminate rats that were housed in the same cages as the depressed rats. If necessary, exclude additional rats at random to create the final groups.

Tag the rats using numbered stickers. To induce depression, subject the 60 rats in the chronic unpredictable stress exposed group to any two of the following seven stressors. In a random order for five consecutive weeks, use one stressor during the day and the other at night.

House the rats in larger groups by placing six rats instead of the usual three per cage for 18 hours. Place the rats in a cage tilted 45 degrees along the vertical axis for three hours. Food deprive the rats for 18 hours.

Deprive the rats of water for 18 hours exposing them to empty water bottles after one hour. Place the rats in a cage with wet bedding for eight hours during the night cycle. Expose the rats to continuous lighting for 24 hours in a reverse light-dark cycle twice per week.

Place the rats in a hot environment set at 40 degrees Celsius for five minutes during the night cycle. After exposure to the two chosen stressors, confirm the development of depression by performing the sucrose preference test. First, place two CUS exposed rats per cage in 30 different cages.

Then add one rat from the depression contagion group to each cage containing two CUS exposed rats and allow cohabitation for five weeks under standard conditions. After five weeks of cohabitation, subject all groups to the sucrose preference, open field, and forced-swim tests. Clean the apparatus with 5%alcohol prior to the introduction of each animal.

Place the rat at the corner of the open field facing the wall. Record the rat's behavior with a video camera for five minutes. Analyze post recordings of total travel distance, velocity in the central part of the field, and time spent in the central part of the field.

For habituation, place the rat in a glass cylinder containing water at room temperature for a 15-minute swimming session. Remove the rat from the cylinder, dry with paper towels, and place in a warm cage for 15 minutes before the returning the rat to the home cage. On the following day, videotape a five-minute swim session and analyze the recordings for immobility, climbing, and defecation.

After exposing rats to five weeks of CUS for induction of depression and then subsequently exposing healthy rats to the CUS exposed rats for a further five weeks, both sets of rats displayed depressive-like behaviors at the end as shown by reduced preference for sucrose. In the open field test rats in both the CUS exposed and depression contagion groups had decreased total travel distances and decreased mean velocities compared to the control group. The time spent at the center of the open field by the rats from the control group in the two experimental groups was non-significantly different.

Both CUS exposed and depression contagion rats showed extended immobility following the forced-swim test. However, a significantly extended time of inactivity was registered only in the CUS exposed group when compared with the control group. In another investigated forced-swim test parameter, climbing time, both groups of CUS exposed and contagion depression rats exhibited highly limited climbing properties when compared to the control group.

The assessment of the rate of defecation resulted in the registration of high quantities of feces for both experimental groups compared with control. While attempting this procedure, it's important to remember to plan ahead the random rotation of stressors during the chronic unpredictable stress model because of its high demand of manual labor. Following this procedure, other tests like elevated plus maze or mirror test can be performed in order to further investigate the behavioral changes following the depression.