Behavioral Tracking of Possibility for Exercise in Rats

Abstract

Motivation to do sports and physical exercise greatly differs among individuals, with some exercising daily, some once a month, and others not at all. Despite knowing that exercise improves and maintains our health, regular exercise is difficult for people who dislike exercising. It remains unclear why preference for sports and exercise differs among individuals or what innate and environmental factors are involved. To clarify this, we aimed to demonstrate whether physical exercise is rewarding for organisms. We placed four 4-week-old Long-Evans rats in an experimental cage comprising three rooms: one with a wheel (ExR), one is a locked wheel (NoExR), and one in the middle (PreR, connecting ExR and NoExR). Rats could freely move and enter these rooms for 15 min/day. The position of ExR and NoExR was fixed for several weeks. Therefore, rats could learn the association between room position (left or right) and possibility for exercise (ExR or NoExR) over several days (Figure-1 left; 5 days/weeks). To quantify preference for exercise, a conditioned place preference (CPP) test was performed every 4-5 days. In the CPP test, wheels in both rooms were locked to exclude time spent exercise on the wheel. Furthermore, we reversed the association between room position and exercise possibility after animals showed a bias to either ExR or NoExR. Two rats stayed longer in ExR than in NoExR, and they tracked the position of ExR after reversal of room position and possibility for exercise (Figure-1 right). In contrast, the other two rats did not exhibit tracking of possibility for exercise. These results suggest that even among rats, preference for exercise differs among individuals. Subsequently, we aim to elucidate the neuronal mechanisms underlying motivation to exercise and its individual differences. Previous studies have shown that both voluntary and forced exercise induce c-Fos expression as a marker of neuronal activation in the nigrostriatal pathway, including the midbrain dopamine system (containing known reward circuitry). However, considering that the dopamine system is known to be activated not only by reward but also by motor execution per se, it is difficult to dissociate which of these (reward or motor execution) induces c-Fos expression after exercise. To address this problem, we are now developing a new conditioning task to extend the results of this study.