When we feel uncomfortable, we tend to eat less, drink less, and exercise less. This is not unique to humans—most animals reduce these three behaviors when fighting infection.
Recently, a new study pinpoints clusters of neurons that control these responses (uncomfortable behavior). By provoking immune responses in mice, the researchers demonstrated that specific cell populations in the brainstem could effectively induce three suggestive uncomfortable behaviors. In addition, inhibition of these neurons attenuates each behavioral element of the uncomfortable response.
The findings, published in Nature, directly link inflammation to neural pathways that regulate behavior and provide in-depth insights into how the immune system interacts with the brain.
Professor Jeffrey M. Friedman of Rockefeller University said: "We are still in the early stages of trying to understand the role of the brain in infection. But with these results, we now have a unique opportunity to ask: what is your brain like when you are sick?"
Sickness behavior has been shown to play an important role in helping animals recover. Previous studies have confirmed that forcing animals to eat when they become ill significantly increases mortality. "These behavioral changes during infection are really important for survival," says lead author Anoj Ilanges, who was a graduate student in Friedman 's lab and is currently working at Howard Hughes Medical Institute.
However, it has been unclear how the brain coordinates this relationship, that is, it does not want to eat after the infection occurs and only wants to hide in bed. Thus, Friedman and Ilanges set out to map the brain regions behind the discomfort behavior of mice.
The team first exposed mice to lipopolysaccharide (LPS), a component of the bacterial cell wall that activates the immune system and effectively induces uncomfortable behavior. Shortly after LPS injection there was a surge of activity in the region of the dorsal vagal complex in the brainstem and these neurons expressed the neuropeptide ADCYAP1. To confirm that they found the right brain cells, the researchers then activated these neurons in healthy mice, and they found that the mice ate, drank, and walked less often. In contrast, the effect of LPS on these behaviors was significantly reduced when ADCYAP1 neurons were inactivated.
"We don't know if the same neurons or different neurons control each behavior," Friedman says. "We were surprised to find that a single neuronal population seemed to modulate every component of the malaise response."
However, the researchers were not surprised that this brainstem region was involved in regulating uncomfortable behavior. The dorsal vagal complex is one of the few crossroads in the central nervous system, where there is no blood-brain barrier and circulating factors in the blood can transmit information directly to the brain.
"This area has become a warning center for the brain, which transmits information about disgust or toxic substances and reduces the intake of related foods," Friedman said.
In the coming months, Friedman’s team at Rockefeller University intends to incorporate these findings into their overall goal of understanding the physiological signals and neural circuits that regulate eating behavior. They were particularly interested in why engineered bulimic mice stopped eating when infected with bacteria.
At the same time, Ilanges plans to investigate the role other areas of the brain play in responding to infection. "We looked at one area of the brain, but there were many other areas that were activated with the immune response," he said. "It opens a door to understand what the brain is doing overall during infection."