RESEARCH STRATEGY: Communication between the gut and the brain is essential for energy homeostasis, but how this communication is represented in the dynamics of hypothalamic feeding circuitry is unknown. Early studies of the gut-brain axis relied upon indirect measurements of the effects of nutritionally regulated peripheral signals on feeding circuitry. These studies led to a model in which the activity of key hypothalamic hunger neurons ? AgRP neurons ? fluctuates gradually as the animal's nutritional state changes. With the development of techniques to record the activity of genetically-defined neuronal populations in awake animals, the dynamics of AgRP neurons were recently observed in vivo for the first time. These studies revealed, contrary to the prevailing model, that AgRP neurons are inhibited rapidly when an animal sees or smells food, before it takes a single bite; however, food ingestion is required for maintenance of this inhibition. We have developed a tool combining in vivo monitoring of AgRP neuron dynamics with intragastric nutrient infusion to show for the first time that nutrient delivery to the gut, in the absence of the sensory stimuli normally associated with eating, is sufficient to inhibit AgRP neurons over a time-scale of minutes. This inhibition is independent of the macronutrient composition of the food but depends upon the number of calories ingested. The goal of this proposal is to determine the molecular and circuit-based mechanisms by which each macronutrient inhibits AgRP neurons. This will be accomplished across three aims: to identify the hormonal mediators responsible, to identify the nutrient sensors involved, and to dissect the pathway by which these signals reach AgRP neurons. CANDIDATE/ENVIRONMENT: Dr. Lisa Beutler is a senior fellow in the Division of Endocrinology at UCSF. She recently completed internal medicine residency at UCSF and an MD/PhD at the University of Washington, where she earned her PhD in Dr. Richard Palmiter's laboratory. She is finishing her fellowship research, which is the subject of a first-author publication in the journal Neuron, in Dr. Zachary Knight's laboratory at UCSF. Having gained expertise in in vivo neural recording and advanced rodent surgery, she now seeks to expand her expertise in the lab to include optical circuit dissection, single-cell resolution calcium imaging, and data analysis and programming skills prior to obtaining an independent position as an academic physician-scientist. CAREER DEVELOPMENT: This award will ensure that Dr. Beutler is able to launch her career as an independent investigator armed with a combination of experimental tools that both position her at the cutting edge of her field (calcium-based imaging techniques, optogenetics) and set her apart from others in the field (advanced rodent surgical techniques). Combined with her clinical training in endocrinology, this will make her uniquely poised to address questions that require detailed knowledge of both neural circuitry and peripheral metabolism. This award will also facilitate Dr. Beutler's acquisition of other professional skills required for independence including formal training in scientific writing, leadership, and management.
Gut-brain communication is critical for bodyweight and appetite regulation, and dysfunction of this axis causes obesity. Despite decades of study, it remains unknown how this communication is represented in the in vivo dynamics of hypothalamic feeding circuitry. We have developed a model to monitor the effects of nutrient delivery to the gut on the real time, in vivo dynamics of key hunger neurons in the hypothalamus, and propose using this model in conjunction with other, cutting-edge neuroscience techniques to dissect the mechanism of nutritional regulation of hunger neurons.
