Project Summary: Diabetes is a growing epidemic in the United States. Drugs that target glucagon- like peptide-1 (GLP-1) or its receptor (GLP-1R) are used to treat type 2 diabetes mellitus. In addition to the demonstrated therapeutic efficacy for diabetes, GLP-1 analogs decrease food intake. The effect on food intake is promising because of the clear association between diabetes and obesity. More recent studies, however, show that GLP-1R agonists also decrease water intake. This is particularly relevant for certain patient populations that are at high risk for both diabetes and dehydration. In older adults, for example, the incidence of diabetes is more than two times greater than it is in younger adults and dehydration is common and particularly problematic in the elderly. As such, knowing how GLP-1 affects fluid intake is essential to inform treatment decisions and help lead to future therapies that treat diabetes and reduce body weight, while preventing any complications related to decreased fluid intake. Studies from our laboratory suggest that GLP-1 can act in the brain to suppress fluid intake, but it remains unknown if endogenous GLP-1 is normally involved in the regulation of fluid intake. The studies in this proposal are designed to test the working model that GLP-1 does, indeed, play a role in the control of fluid intake and, more specifically, does so by acting as a satiety signal. The proposal describes experiments that test the following Specific Aims: 1) Does knockdown of GLP-1 or GLP-1R increase drinking behavior and is this effect the same in male and female rats? 2) What effect do challenges to body fluid homeostasis have on GLP-1-expressing cells in the CNS, particularly in the nucleus of the solitary tract (NTS)? 3) What is the effect of challenges to body fluid homeostasis on targets of NTS projections? These experiments use multiple approaches including viral-mediated knockdown, measures of gene expression, in vivo microdialysis, optogenetics, and refined analysis of behavior to test for changes in fluid intake, neural activity, gene expression, and release of GLP-1. These studies will improve our understanding of the control of fluid intake and could have a broader impact by improving our understanding of peptides that have divergent actions (e.g., used as hormone and neuromodulator). From a translational perspective, the experiments will provide greater insight regarding treatment options for patients with diabetes and could reveal information about the link between co-morbid disorders related to energy balance (e.g., obesity) and disorders related to fluid balance (e.g., hypertension).