We’ve all heard the traditional New Years Resolution of “consistently working out” and the subsequent mass cancellation of gym memberships mere months later. However, the lack of motivation to be physically active might not just be an issue of sheer will, but rather one that is impacted by one’s lifestyle and their environment.
Buhr et al. (2023) focused on how stress lowers motivation to participate in voluntary activity. His hypothesis was tested by giving groups of rats access to a running wheel and then exposing them to 0, 50, or 100 tail shocks. The shocks served as a stressor, and the wheel stayed locked until 36 hours after shock administration, when the rats were given free access and observed. The rats that were exposed to more shocks had persistent deficits in wheel running that lasted more than 40 days. This behavioral effect was limited to voluntary behavior specifically because all the animals recovered normally on forced exercise tasks such as a forced swim test within 72 hours. To examine how brain chemistry might explain this behavior, rats were sacrificed in order to compare differences in neurochemicals between each group. Within the prefrontal cortex, non-stressed rats had a higher rate of dopamine turnover, (how quickly dopamine is used and then replaced), which could mean that voluntary activity is more rewarding. Within the striatum, a region deep within the brain involved in movement, reward and decision making, the animals that were exposed to more shocks had lower levels of dopamine but similar levels of dopamine usage, which implies that stressed animals have a smaller reserve of dopamine to draw from. This biological deficit provides a potential explanation for why it is harder for stressed rats to stay motivated when performing voluntary tasks.
Friend et al. (2017, Cell Metabolism) examines a similar principle of how dopamine impacts movement and activity levels but this time through the lens of obesity. In the study mice were fed either a normal or high-fat diet for 18 weeks. High-fat mice started gaining weight after 2 weeks and then became less active after 4 weeks. Looking within the brains of these mice, the primary neurological difference was in the D2 dopamine receptors. Within obese mice there was less binding of D2 receptors despite similarities in the amount of dopamine and the dopamine-producing enzyme, tyrosine hydroxylase. Gene activity within the D2 receptor gene and the total amount of D2 protein was also similar among both groups of mice. Given the similarities in other receptors and the aforementioned levels of dopamine and enzymes, it was hypothesized that D2 receptors were less available or less functional rather than being in shorter supply within obese mice. Because the only difference was in the binding of dopamine to the receptors rather than the genetic activity or level of dopamine itself, the function of D2 is the most reasonable explanation for the differences in behavior. The overall firing rate of neurons within the striatum was normal within obese mice; however, fewer neurons turned on during movement with 19% of striatal neurons turning on in obese mice during movement in comparison to 58% in lean mice. Within indirect pathway medium spiny neurons (iMSNs), neurons that can act as a brake on movement, changes were made to further examine the effect of the D2 receptor. When D2 receptors were removed from iMSNs, lean mice became less active. When iMSNs were inhibited using DREADDs, obese mice became more active.
Both of these studies examine how dopamine affects the motivation to perform physical tasks but through different lenses, one focusing on dopamine and the other focusing on the broader pathway and the function of dopamine receptors themselves rather than just the neurotransmitter.
The fact that there are a variety of ways for dopamine to impact the desire to move shows how crucial a part it plays in our ability to interact with the world around us. Additionally, the biological backing for inactivity shows just how intensely one’s environment can impact their lives down to the most basic neuronal level. I would be interested to examine the role of dopamine through the linkage between stress and weight gain and see how the research done in both of these papers interacts to affect motivation and movement.
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