The protective effects of voluntary exercise against the behavioral consequences of uncontrollable stress persist despite an increase in anxiety following forced cessation of exercise

The protective effects of voluntary exercise against the behavioral consequences of uncontrollable stress persist despite an increase in anxiety following forced cessation of exercise 

Behav Brain Res. 2012 August 1; 233(2): 314–321. doi:10.1016/j.bbr.2012.05.017. 

Benjamin N. Greenwood1,3,*, Alice B. Loughridge1, Nouara Sadaoui1, John P. Christianson2,3, and Monika Fleshner1,3
1Department of Integrative Physiology, University of Colorado-Boulder 2Department of Psychology and Neuroscience, University of Colorado-Boulder 3Center for Neuroscience, University of Colorado-Boulder 

Physical activity is protective against stress-induced effects and reduces the chance of developing anxiety- and depression-like behaviors. These stress-induced behaviors often include many sequelae. The current study focused primarily on three: social avoidance; shock-elicited fear, and; deficits in escape learning. Additionally, these behaviors and the stress that causes them are shown to coincide with an increase in corticosterone. However, it is unknown how long the protective effects of exercise persist after ending exercise.

The first goal of the study investigated whether or not six weeks of running inhibited the anxiety-linked behaviors. The second goal sought to determine whether or not the protective factors lasted up to 25 days after the forced cessation of exercise. Two experiments were completed to explore these questions. The animals in each of the studies were 6 to 7-week-old male, Fischer F344 rats. They were group houses in cages of 6. The rats each underwent 6 weeks of either running or sedentary conditions. However, each rat’s individual running distance was not accounted for during this time. It would be interesting to see whether the distance ran each day for each rat impacted their results. Additionally, it would be interesting to see if the stressor of being singly-housed would impact the rat running behavior or if running itself would be protective over their development of stress.

Rats either remained inactive or began the “uncontrollable stress” protocol, outlined below, at different times post cessation of exercise—the next day, 4 days, 14 days, or 24 days later. Social exploration began 24 hours after the uncontrollable stress protocol was administered. Shock-elicited fear and shuttle box escape—both of which were used to measure fear conditioning and escape learning—began immediately after the social exploration protocol.

Uncontrollable stress was induced through tail shocks due to its success in producing behavioral results that are similar to the stress-related behaviors observed in humans. The rats were either a part of the “No Stress” group or the “Stress” group that would receive 100, 5s, 1.5 mA tail shocks while restrained in a Plexiglass tube. “No Stress” rats would not experience this protocol at all.

Social exploration was tested at baseline (post-5 weeks of running or sedentary environment) and 1 week afterward (at 6 weeks of running or sedentary environment). Each individual rat would be placed in a clean, separate cage that would have a juvenile male rat introduced 1 hour after habituation. Exploratory behaviors were observed for 3 minutes. After the baseline measurements, the rats would be returned to their home cage. 1 week after baseline measurements, the same procedure would occur 24 hours after the rats underwent the uncontrollable “Stress” protocol. After the final social exploration, the rats either underwent the second experiment (the shock-elicited freezing and escape behavior protocol) or were sacrificed right away to have their corticosterone levels measured. Their corticosterone levels were measured against naïve rats—rats that had not undergone the social exploration protocol.

After the social exploration experiment, the rats were placed in a shuttle box that gave foot shocks on one half and was “safe” on the other. Shock-elicited freezing and escape behavior were used to assess escape learning. Additionally, exaggerated fear—presented as freezing rather than escaping when presented with shock cues—was used as a measure of anxiety-like behavior. While in the shuttle box, the rats underwent a fixed ratio of foot shocks that could be escaped by crossing to the other side of the box. This was followed by a 20-minute post-shock “freezing observation.” The rats then underwent a second fixed ratio of foot shocks. Their “escape latency” was measured before the end of the trial.

Exercise successfully reduced social aversion in rats within the social exploration protocol. Additionally, exercise prevented the increase in corticosterone levels post-social exploration. Therefore, the study concludes that the rats were protected against the stress induced by tail shocks prior to interacting with a novel rat. The rats felt more at-ease to interact with the novel rat and did not experience the stress that coincides with the protocol. 

Interestingly, stress from the tail shock protocol reduced cage crossings during the social exploration protocol regardless of prior exercise. These results suggest that all rats, including the exercise rats, were anxious within their new environment and did not “feel” comfortable enough to explore. This anxiety-like behavior was enhanced the longer the rat stopped wheel running so exercise could be protective in that regard. Therefore, Greenwood et. al conclude that exercise protected the rats from developing the behaviors associated with stress despite having an increase in anxiety (associated with the reduced number in cage crossings). The protective measures disappeared sometime between 15 and 25 days after exercise cessation, suggesting that exercise can protect against the behavioral developments for quite some time.

Only the sedentary rats showed stress potentiated, shock-elicited freezing. The running rats were “protected” from the behaviors associated with stress. However, the cessation of exercise itself did lead to the shock-elicited freezing. That is, the rats who were a part of the “No Stress” group still froze when presented with the cues associated with foot shocks. The freezing occurred as a function of time, affecting the rats who stopped running for 15 and 25 days more so than the 0- and 5-day rats. The anxiety-like behavior of exaggerated fear still emerged, which suggest that the exercise cessation itself is stressful to the rats.

Previous wheel running experience was associated with a removal of the shuttle box escape deficit that is seen in sedentary, stressed animals. Prior exercise exposure did not affect the first fixed ratio latency, which makes sense because no rat would have has learned to associate the keys with the foot shocks yet. Interestingly, the exercised rats were more likely to remember the cues during the second fixed ratio and escaped the foot shocks while the sedentary animals were slower to move to the other side. The sedentary animals are suggested to have impaired fear learning without the protective factors of exercise. These effects were present at 15-days exercise cessation and no longer present at 25 days. Therefore, the protective factors lasted well past the cessation of exercise.

The current study suggests that 6-weeks of running wheel leads to resistance to stress and the associated behavioral sequelae. These protective measures are shown to persist between 15 to 25 days after ending exercise. While the rats still showed signs of anxiety-like behaviors, there were still reductions in the consequences of stress and reductions in the corticosterone response. Therefore, the rats still develop symptoms similar to anxiety, but it does not lead to the negative behaviors that normally arise. Future studies could look into the neuronal plasticity that lead to these behavioral changes in rats and what changes in the brain to allow these behaviors to persist over time. Additionally, researchers should investigate the stress that could arise from the cessation of exercise as this adds a confounding factor in other studies.

-LivInLaVida