This protocol allows researchers to examine the effects of chronic stress on effortful responding in both male and female mice. An advantage of this technique is that both male and female mice can simultaneously undergo a validated chronic social defeat paradigm allowing reward behaviors to be directly compared. Historically, preclinical research into mood disorders has utilized behavioral tasks that involve avoidance of threatening environments.
However, reward and motivation tasks, such as effort-related choice, may offer more translational relevance. To prepare the vehicle solution, dissolve 3.375 grams of beta-cyclodextrin in 750 milliliters of tap water in a one liter screw top glass container. To prepare the corticosterone solution, dissolve 3.375 grams of beta-cyclodextrin in 750 milliliters of tap water, followed by the addition of 26.25 milligrams of corticosterone.
Dissolve the corticosterone in solution in an ultrasonic cleaner water bath for approximately 30 minutes at 40 kilohertz until the liquid demonstrates a clear appearance, then fill the water bottles of the vehicle and corticosterone mouse group with the appropriate solutions and record the volume of liquid consumed twice a week. To habituate the mice to the Y-maze, the day after food deprivation, place a large number of 20 milligram grain-based food pellets in the caps of two 50 milliliter centrifuge tubes and place one cap at the end of each arm of the Y-maze, then place a mouse in the start box of the Y-maze with the start box divider in place for a few seconds before removing the divider and allowing the mouse to explore the Y-maze for 15 minutes. After two days of Y-maze habituation, place for pellets in a cap for the high reward arm and place two pellets in a cap for the low reward arm.
For a high reward forced choice trial, block access to the low reward arm and place the mouse in the start box for a few seconds before removing the divider, allowing the mouse 60 seconds to enter the high reward arm to consume the available pellets. For a low reward forced choice trial, block access to the high reward arm. For free choice training, begin each session with one high and one low arm forced choice trial before completing 10 free choice trials.
Allow the mouse to select either maze arm. Once the mouse has traveled to the end of the arm to the cap of pellets, place an arm divider behind the mouse, locking in the animal until it has consumed the pellets. Record which arm is selected for all 10 free choice trials daily.
Once a mouse has selected the high reward arm for 7 out of 10 trials in a free choice training day, place a 10 centimeter barrier halfway down the high reward arm in the Y-maze and perform at least two high and low arm forced choice trials with the barrier in place. After completing all four trials, place the mouse in the start box and allow the mouse to select an arm for 10 free choice trials with the 10 centimeter barrier to the high reward arm in place. On the following day, habituate and test the mouse as demonstrated, but use a 15 centimeter barrier in the high reward arm.
The next day, test the mouse with a 20 centimeter barrier as demonstrated. For chronic nondiscriminatory social defeat stress testing, align the cages of CD1 males with C57BL/6J males and females with CD1 cages in the front and C57BL/6J cages adjacent to the CD1 male mice in the social defeat room. To start the trial, place one adult male and one adult female experimental C57BL/6J mouse into the home cage of each CD1 aggressor male for a five-minute social defeat session and record the attack latency and frequency of attack for both male and female experimental animals.
At the end of the session, transfer the male C57BL/6J mouse into the cage of co-housed CD1 males separated by a clear perforated plexiglass barrier. Separate the attacking CD1 and female C57BL/6 mice with a similar clear perforated plexiglass barrier. For a control session, place one control female mouse in the home cage of one control male C57BL/6 mouse.
After five minutes, place a clear perforated plexiglass divider between the animals. At the end of the first experiment, rotate the mice for the remaining nine defeat sessions, such that each C57BL/6 male and female pair is rotated one cage to the left to provide a new interaction with novel CD1 mice for each session. Chronic corticosterone and chronic nondiscriminatory social defeat stress mice both exhibit a reduced mean body weight compared to vehicle and control animals.
These mice also consumed less mean home cage lab chow. Vehicle and corticosterone-administered mice consume a similar volume of liquid over four weeks of treatment and three weeks of behavior testing. Chronic nondiscriminatory social defeat stress produces a maladaptive phenotype in susceptible mice compared to either resilient or control mice not exposed to chronic nondiscriminatory social defeat stress displaying a reduction in time spent in the interaction zone containing a novel CD1 mouse.
Both chronic corticosterone and chronic nondiscriminatory social defeat stress mice produce a shift in effortful responding when the barrier height increases to 15 and 20 centimeters. High and low reward arm latency with the 15 centimeter barrier is not impacted by corticosterone administration and is similar for both groups with both low and high reward arms. Importantly, if chronic corticosterone or chronic nondiscriminatory social defeat stress impairs learning of the Y-maze barrier task, these mice may fail to reach the criterion in free choice training sessions, impacting subsequent barrier result interpretation.
In addition, the social interaction task can be used to stratify the mice into resilient and susceptible populations, as well as by sex. When conducting the Y-maze barrier task, it is critical to record both the selected arm and the latency to reach the food pellet reward in each individual trial. Other important and translationallly relevant reward-related behaviors, such as outcome devaluation or progressive ratio, can be performed following the chronic corticosterone social defeat or chronic nondiscriminatory social defeat protocols.
