Infections are an omnipresent threat to all species. Thus, effective adaptive responses to resist and tolerate infections are essential for survival. Beyond the immune response, animals also induce a set of pleiotropic responses including anorexia, adipsia, lethargy, and changes in temperature, collectively termed sickness, during infections. While these responses have been shown to be adaptive for animal survival during infection and have been attributed to the central nervous system, the underlying control mechanisms for this response have not been elucidated. The goal of this work was to quantitatively characterize the many phenotypes associated with infection-induced sickness and to identify neural substrates responsible for the induction and maintenance of these sickness phenotypes. To accomplish this, we use of a set of unbiased methodologies to show that a specific subpopulation of neurons in the brainstem can control the diverse responses to bacterial endotoxin (lipopolysaccharide, LPS) which potently induces sickness. We first thoroughly characterized the multimodal sickness response using high resolution measures of behavior, autonomic function, and metabolism. We then performed whole brain activity mapping to reveal that subsets of neurons in the nucleus of the solitary tract (NTS) and area postrema (AP) acutely express FOS after LPS treatment. To causally show that these activated neurons were inducers of the sickness, we showed that subsequent reactivation of these specific neurons in Fos2A-iCreERT2 (TRAP2) mice replicates the behavioral and thermal component of sickness. In addition, inhibition of these same LPS-activated neurons diminished all of the behavioral responses to LPS. For further mechanistic delineation of the neurons involved in this response, we utilized singlenuclei RNA-sequencing of the NTS/AP to identify the LPS activated neural populations. Molecular profiling identified multiple neuronal types activated during LPS-induced sickness, with the greatest enrichment of Adcyap1+ neurons. We found that activation of these Adcyap1+ neurons in the NTS/AP fully recapitulates the responses elicited by LPS. Furthermore, inhibition of these neurons significantly diminished the anorexia, adipsia, and locomotor cessation seen after LPS injection. In aggregate, these studies map the pleiotropic effects of LPS to a neural population that is both necessary and sufficient for canonical elements of the sickness response, thus establishing a critical link between the brain and the response to infection.

A thesis presented to the faculty of The Rockefeller University in partial fulfillment of the requirements for the degree of Doctor of Philosophy