Inducing Physical Inactivity in Mice: Preventing Climbing and Reducing Cage Size Negatively Affect Physical Fitness and Body Composition

Abstract

Physical inactivity has emerged as an important and risk factor for cardiovascular and metabolic diseases, independent of levels of exercise engagement. Moreover, inactivity is associated with poor brain functioning. However, little data on the effects of physical inactivity on the brain is available and few methods are suitable to investigate this matter. We tested whether preventing lid climbing and reducing cage size could be used to model physical inactivity in mice. Sixty young adult C57Bl6 mice (10 weeks old) were divided over six groups with different housing conditions: in cages of three different sizes with lids that either allowed or prevented lid climbing. Housing under these conditions was maintained for a period of 19 weeks before the mice were killed for body composition analysis. Physical fitness tests performed around 5 and 10 weeks into the intervention revealed that motor coordination in the balance beam test was reduced by 30.65%, grip strength by 8.91% and muscle stamina in the inverted screen test by 70.37% in non-climbing mice as compared to climbing controls. Preventing climbing increased visceral fat mass by 17.31%, but did not reduce muscle mass. Neither preventing climbing nor reducing cage size affected anxiety assessed in the Open Field test and the Elevated Plus Maze. We did not find any negative effect of inactivity on spatial learning and memory in the novel object location test or working memory measured with the Y-maze Alternation test. The reduced physical fitness and increase in visceral fat mass show that our inactivity method models most effects of physical inactivity that are observed in experimental and observational studies in humans. Whereas established methods such as hindlimb unloading mimic many of the effects of bed rest, our novel method can be applied to study the effects of less extreme forms of physical inactivity (i.e., sedentary behavior) in various disease models including rodent models for brain diseases (i.e., stroke, Alzheimer’s disease).

Talking About Physical “Activity” or “Inactivity”? The Need of Accurate Activity Controlling in Exercise Studies in Rodents

“…A recent investigation revealed that preventing climbing and reducing cage sizes in mouse studies impact outcomes that are associated with physical inactivity, such as bodyweight and fat mass. These results underline the influence of housing conditions on movement behavior (Roemers et al, 2019 ).…”

Early-life mice housed in standard stocking density reduce the spontaneous physical activity and increase visceral fat deposition before reaching adulthood

“…In line with this, some studies show that adult rodents kept in small cages or even in housing conditions taken as standard exhibited reduced muscle enzyme activities as well as decreased muscle strength on an inclined plane test, 7 decreased motor coordination, 9 increased adiposity 10 and reduced aerobic capacity and spontaneous physical activity (SPA) 11 compared with their counterparts kept in cages with a larger area available per animal (low stocking density). Other models, such as those with climbing prevention, 9 hindlimb unloading 12,13 and immobilization cages 14 could be used for inducing physical inactivity, but they are more extreme in changing rodents’ life. While the existing animal housing studies are important to show that physical fitness and health outcomes of rodents may be compromised by cages with limited space (often taken as standard cages), they fail to provide a more complete and realistic picture of daily routines of rodents in their cages, especially in terms of physical activity.…”

“…A high stocking density is a situation that could lead to animals being physically inactive throughout their life span. In line with this, some studies show that adult rodents kept in small cages or even in housing conditions taken as standard exhibited reduced muscle enzyme activities as well as decreased muscle strength on an inclined plane test, 7 decreased motor coordination, 9 increased adiposity 10 and reduced aerobic capacity and spontaneous physical activity (SPA) 11 compared with their counterparts kept in cages with a larger area available per animal (low stocking density). Other models, such as those with climbing prevention, 9 hindlimb unloading 12,13 and immobilization cages 14 could be used for inducing physical inactivity, but they are more extreme in changing rodents' life.…”

The prevention of home-cage grid climbing affects muscle strength in mice

“…As mice exhibit climbing behavior throughout their lives 39 , 40 , muscle strength can be maintained by climbing the grid lid in the home cage. Climbing prevention in 10-week-old mice has been reported to result in decreased muscle strength in grip strength tests 41 . The observed decrease in muscle strength indicated that muscles were weakened by the prevention of grid climbing.…”