By Marcia-Ruth Ndege ‘21

Much is known about how the brain communicates needs such as thirst, hunger, pain, and fear. However, little is known about how the brain prioritizes such needs. In the context of pain, researchers must examine both acute and inflammatory pain. Acute pain is a reflexive response, while inflammatory pain is mediated by central mechanisms. In research funded by the University of Pennsylvania School of Arts and Sciences, the Whitehall Foundation, and the National Institutes of Health, Amber Alhadeff, a postdoctoral research fellow in the Betley Lab at UPenn, led a study to determine the specific neural circuits that regulate pain. Her goal was to discover a method by which to reduce inflammatory pain without affecting the sensors that regulate acute pain. Based on the prior knowledge that other physiological needs such as hunger affect the instinctive mechanisms that would otherwise prevent an individual from avoiding situations that could end in pain, the researchers hypothesized that the brain would prioritize the most important need.

Early results showed that hunger selectively blocked inflammatory phase pain responses. Following these findings, researchers induced inflammatory pain in mice by injecting their paws with formalin, a chemical known to induce acute and inflammatory pain. Among these mice, half were deprived of food for 24 hours. Next, researchers used a hot plate to induce thermal pain and mechanical pain on both sets of mice. They found that only the mice that had been fed beforehand exhibited a response to the thermal pain, the formalin injection, and the mechanical pain. The hungry mice, on the other hand, did not show a response to any of these stimuli. This showed that hunger is a powerful suppressor of inflammatory pain. To further their findings, the researchers began to study the neurons that produce AgRP and NPY (AgRP neurons). Inhibition of these neurons in hungry mice is known to reduce food intake, while activation increases food intake. Stimulating these neurons in mice reduced their responses to formalin injections but not to acute or thermal pain stimuli.

These findings clearly indicate that hunger and pain are somehow linked, and that hunger has an overarching effect on how an organism deals with inflammatory pain. It is important to note, however, that hunger does not have an effect on how an organism experiences acute pain. This indicates that while the brain may prioritize hunger over long-term pain, it will prioritize acute pain over hunger.

References

1. A. Alhadeff, et. al., A neural circuit for the suppression of pain by a competing need state. Cell 173, 140-152 (2018). doi: 10.1016/j.cell.2018.02.057.
2. Image retrieved from: https://pixabay.com/en/nerve-cell-neuron-brain-neurons-2213009/