Activity of brain stem glucagon neurons are modulated by energy state and encode sex and frequency-dependent negative valence and anxiety

The glucagon-like peptide 1 (GLP-1) system has emerged as an important drug target for the treatment of obesity and diabetes. Preclinical and clinical studies demonstrate that the activation of GLP-1 receptors (GLP-1Rs) directly in the brain through overexpression of GLP-1 or GLP-1R agonists produces potent anorexigenic effects, yet the behavioral role and modulation of the endogenous GLP-1 producing system in the brain by energy status is unclear. In this study, we examined the anatomical, physiological, and behavioral properties of preproglucagon-expressing neurons in the nucleus of the solitary tract, Gcg ^NTS neurons, which serve as the primary source of GLP-1 in the brain. Using transgenic laboratory mice, we observed no sex differences in the density and distribution of Gcg ^NTS neurons in male and female mice. Fos immunolabeling experiments show that Gcg ^NTS neurons are not significantly activated after intermittent access to palatable food, but the magnitude of Fos activation was linearly related to the amount of food intake in mice provided with ad libitum intermittent access to palatable food. Electrophysiological examination of Gcg ^NTS neurons revealed that these neurons show energy-status and sex-dependent changes in neuronal firing and intrinsic excitability. Twenty-four hour food deprivation produced a significant reduction in excitability and firing in male, but not female mice. We then used optogenetics to investigate the causal behavioral role of Gcg ^NTS neurons. High frequency optogenetic activation of Gcg ^NTS neurons using the red light-gated opsin ChrimsonR produced female-specific anxiety-like behavior and real-time place aversion. For feeding, we observed that reversible optogenetic stimulation at high frequencies produced a significant reduction in homeostatic refeeding that did not differ by sex. Using operant conditioning, we found that reversible optogenetic activation of Gcg ^NTS neurons at 20 Hz, but not 5, also reduces appetitive behavior. These data demonstrate that Gcg ^NTS neurons exert control over motivation and food-seeking behavior in addition to consumption.