This protocol allows researchers to resistance train large cohorts of mice at a much lower cost compared to models that use commercially available running wheel equipment. The primary advantage of this resistance training model is that it is entirely voluntary, which reduces stress for the animals and time commitment for the researcher. This model can help better understand the cellular and molecular mechanisms that regulate muscle mass in response to exercise training.
By design, this loaded wheel running model is relatively simple to execute. However, it is still recommended that researchers perform pilot testing in the unique laboratory setting to estimate the running performance of mice prior to experimentation. Demonstrating this procedure will be PJ Koopmans, a graduate research assistant in my laboratory.
To set up the running wheel apparatus, glue a single one gram sensor magnet to the outer middle circumference of the running wheel and use this wheel for only the first week of wheel acclimation. Loaded wheel running requires two grams of load, hence glue two magnets of one gram side by side onto the outer circumference of the wheel. A tape can be used to hold the magnets in place until the glue firmly dries.
As the weeks pass, apply additional load in weeks 3, 4, 5, and 7 by placing another one gram magnet on top of either magnet already present. As these magnets firmly adhere to one another, no glue will be required. High loaded wheel running setup requires three sets of wheels.
The first set of wheels, which is required for week two only has a single 2.5 gram magnet glued onto the outer circumference of the wheel. The second set of wheels, which is required for week three only, has two 2.5 gram magnets glued side by side onto the outer circumference of the wheel. The third set of wheels required for four weeks and beyond has three 2.5 gram magnets glued side by side onto the outer circumference of the wheel.
Apply additional load for week six and eight by placing another 2.5 gram magnet on top of either of the magnets already present. Ensure that a fresh battery is inserted into the bike computer before the assembly. Then assemble the running wheels using a cage equipped with a digital bike computer to monitor time and distance traveled during exercise.
The average speed in kilometers per hour is derived arithmetically. Set the wheel size during initial bike computer programming and calculate per revolution distance by measuring the outer circumference of the running wheel. To ensure that all computer and sensor components are contained within a solid barrier outside the cage to prevent mice from chewing on the components, utilize the lid of an empty pipette tip box with a small rectangle cutout for the magnetic bike sensor and the main part of the box to hold the bike computer, and wire.
Drill two holes through the corners of the pipette tip box lid to secure the magnetic bike sensor and running wheel stand in place on the outside of the cage and insert the running wheel base upside down through the gaps in the cage lid but on top of the solid surface. Secure the wheel base and computer sensor to the top of the cage with hardware. Ensure that the bike computer sensor and the pipette tip box lid is directly above where the sensor magnet of the wheel is located and that the sensor magnet and computer sensor are spaced no more than one centimeter apart to allow the proper recording of wheel movement.
Attach the appropriate running wheel to the wheel base before placing the lid onto the cage, then securely placing the lid onto the cage. With the wheel hanging from the cage lid, ensure at least 2.5 centimeters of clearance from the cage floor. Then place a minimal amount of bedding material in the cage to ensure that the wheel spins freely but does not become impeded by the buildup of bedding.
As mice are an nocturnal species, most of their natural cage activity, including wheel running, will be performed during the dark hours of the light cycle. During the experiment, record this data from the bike computer at a consistent interval scheduled to ensure accurate activity monitoring. Individually house sedentary mice for nine weeks in a cage containing a locked running wheel to prevent any running.
Reduce load for the loaded wheel running and high loaded wheel running groups if necessary to ensure that mice continue to exercise for the entire nine a week protocol as per the loading schedule. During the study, mice were randomly assigned to one of three treatment groups, namely sedentary, loaded wheel running, or high loaded wheel running, and then completed their respective nine week protocol. After the one week of acclimation, there were no group or group by time differences in running distance or training volume.
Normalized soleus mass was 21.4%larger in the high loaded wheel running group than in the sedentary group despite no difference in fiber cross-sectional area. Although plantaris muscle mass and average fiber cross-sectional area did not displace statistically significant differences, there appears to be a shift in the the proportion of fibers with a larger cross-sectional area in the plantaris of high loaded wheel running compared to sedentary and loaded wheel running. There were no significant differences in twitch or peak force of the GPS complex between groups as measured by an NC2 muscle function test.
It is important to ensure the wheel spins freely within the cage and that the wheel sensor magnet is close to the bike computer sensor so wheel rotation is unimpeded and the bike computer accurately records running data. Following this procedure, researchers can perform subsequent analyses such as contractile function or immunohistochemical techniques to examine the various physiological responses further to exercise training.
