Hunger equals coping?
By Michelle Lim, Psychology, 2022
Why do we eat food? The most obvious answer is to survive. When we are feeling hungry, we eat food in response to the perception of required nutrition, as demanded by our hypothalami. When our brains think we are hungry, they display hormone signals to indicate an energy deficit, which increases activity of agouti-related protein (AgRP) neurons (starvation-sensitive neurons) located in the hypothalamus. Activation of this neuron motivates food consumption and active food-seeking behavior within minutes, whereas inhibition suppresses feeding behavior. But to what extent can AgRP neurons directly affect this mechanism?
A study published in Nature in 2015 by Betley et al. studied 22 mice to answer this question; half were food-restricted, and the rest were fed regularly. They were trained to press a lever to gain food pellets, with each successive round increasing the number of lever presses to get food. The food-restricted group consistently pressed the lever, while on the other hand, normally fed mice gradually refused as the lever-press rate increased. This indicates AgRP neuron activity influences food-seeking responses through a negative-valence signal. In other words, normally fed mice give up lever pressing because the nutrition gained is not capable of reducing the energy deficit that elevates the activity of AgRP neurons when the lever-press rate gets too high.
However, what if stress intervenes in this progress? In 2016, Jikomes et al. compared food-restricted mice and fed-mice in a sensory discrimination task. The mice were rewarded with food when they reacted to seeing a diagonal striped image, but if they responded to the wrong image or failed to leave after getting the reward, they got shocked. Interestingly, food-restricted mice worked to obtain the food reward but chose to stay near the food port, which led to a failure to avoid the additional shock that could have been prevented by moving to a safety platform. On the other hand, the fed-mice chose to remain on the safety platform and avoided the cued shock rather than working to obtain the food reward.
Even though there is no clear evidence that proves AgRP neurons inhibit response to stress, these studies are the initial breakthrough to understanding the associations between AgRP neurons on food-seeking and other learning behaviors. This could provide new insight into the comorbidity of eating and stress disorders.
Current Biology (2016). DOI: doi.org/10.1016/j.cub.2016.07.019
Nature (2015). DOI: doi.org/10.1038/nature14416