When the conversation of leading mortality factors come up, topics of smoking, and drinking come up. Heart attacks and cancer are talked about too. The idea of physical inactivity is overlooked, even though it happens to be the 4th leading cause of mortality. In fact, 80 percent of adolescents are unable to meet the recommended amount of physical inactivity. As a society, approaches focusing on infrastructure or health monitoring devices, and even education have been implemented however it doesn’t account for an important fact of why people are physically inactive: stress exposure.
In Dr. Trevor Buhr’s research talk, he explored this neurobiological relationship between stress exposure and physical inactivity. He highlighted that individuals with PTSD are three times less likely to be physically active and showed there was a correlation of how people with PTSD or a history with stress are overweight. His research compared the voluntary physical activity after an acclimation period of rats who were stressed (tail shocked) to non-stressed rats. The results between a 5 day acclimation period (of no activity allowed) versus an 8 day period where it was found that stress induced rats ran less. After this experiment was done, they looked at the Medium spiny neurons in the striatum to see how the brain was physically altered after the stress. The results indicated that stress alters plasticity genes like Ube3a and Zeb2 which affect AMPA receptors. Additionally, in the non stressed rats, the dopamine system indicated that running was affiliated with a reward. This system was essentially fried in the shocked rats, disassociating running with reward. In conclusion, stress transforms physical activity from a rewarding behavior to unrewarding, making rats unmotivated to run.
A research article published in 2024 called, “Stressful Life Events and Reward Processing in Adults: Moderation by Depression and Anhedonia” tested if stress-related deficits in the brains reward system seen in animals would also occur in humans who were exposed to daily stressors (since previous work focused on lab). Life events in the past 6 months from 107 adults who were either currently depressed, past-depressed or never depressed were analyzed. Then, others rated each event in the past 6 months of the adult’s lives for how severe it was and if that stressor was independent of the individual’s control. Lastly, reward processing was measured through “probabilistic reward task,” where a singular response was rewarded 3 times as much as another. Anhedonia, reduced pleasure in things you would normally enjoy,was also measured. In currently depressed people, more uncontrollable stressors had poor reward learning(worse at picking up which behavior was rewarded). Past depressed and never depressed have no significant negative relationship. Within the depressed participants only adults with high anhedonia have that negative relationship and worse reward learning. The results indicate that depression and symptoms of anhedonia make people vulnerable to impaired reward learning after experiencing uncontrollable stressful events.
Dr. Buhr’s findings from the rat study show that having uncontrollable stress disrupts the dopamine-reward pathway in rodent brains. This can’t directly prove if this pathway occurs in the human brain and how it would look like at different human stressors. The 2024 study looks at everyday uncontrollable stressors in humans and how that impairs reward processing, directly expanding on Buhr’s conclusions. This study also shows the same diminishing of intrinsic reward that occurred in Buhr’s experiments. Additionally, the human data measured that defects in reward learning occur mostly in those who are currently experiencing high levels of anhedonia rather than those who have experienced high stressors, like the rats experiencing shocks in the past. These results combined could explain how physical inactivity after high stress weakens the brain's reward system so voluntarily doing something like exercise just stops having worth. So, when we take approaches to physical inactivity by implementing infrastructure and education, assuming people are only inactive because they are uneducated or don’t have access to workout, it undermines what could actually be occurring in people’s brains. If stress makes exercise feel unrewarding then addressing any stress-related reward disruption could potentially improve the physical inactivity epidemic we have.
Buhr, T., Reed, C., Wee, O., Lee, J., Yuan, L., Fleshner, M., Valentine, R., & Clark, P. “The Persistence of Stress-Induced Physical Inactivity in Rats: An Investigation of Central Monoamine Neurotransmitters and Skeletal Muscle Oxidative Stress.” Frontiers in Behavioral Neuroscience. 2023.
Min, Sung, Mazurka, Raegan, Pizzagalli, Diego A., Whitton, Alexis E., Milev, Roumen V., Bagby, R. Michael, Kennedy, Sidney H., Harkness, Kate L., Stressful Life Events and Reward Processing in Adults: Moderation by Depression and Anhedonia, Depression and Anxiety, 2024, 8853631, 13 pages, 2024. https://doi.org/10.1155/2024/8853631
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