ABSTRACT The taste of food is a critical factor that determines whether an organism will accept or reject a food source. In addition, the sensory perception of food taste changes significantly depending on the metabolic state of animals. Despite the significant progress in understanding the homeostatic biology of food intake in the mammalian brain, how metabolic states (e.g., how hungry an animal is) alter the perception of food taste at the level of specific neuronal circuits remains poorly understood. Here, we assembled a team of investigators that will use quantitative measurements of behavior, cutting edge 3P microendoscopy, and optogenetic manipulation of neural activity to study how taste representations in the brainstem are modulated with metabolic state, and how the state dependent activity of brainstem taste circuits contributes to the regulation of ingestive behaviors in mice. Specifically, this new team of investigators will probe the functions and the state dependent activity of two distinct populations of neurons, sugar sensitive calbindin 2- neurons (Calb2) and bitter sensitive somatostatin- neurons (Sst), in the rostral nucleus solitary tract (rNTS), the first central relay station of the taste pathway. In Aim 1 the investigators will develop and optimize a state of the art 3-photon (3P) microendoscope that will allow optical access to Calb2 and Sst neurons in the rNTS at cellular resolution. In Aim 2, the 3P microendoscope will be used to examine the taste responses of Calb2 and Sst neurons in hungry and sated mice to investigate whether sugar or bitter responses are modulated in response to changes in metabolic state. In Aim 3, the investigators will investigate whether acute changes in the hunger state of mice affect the taste representations in the rNTS by artificially modulating the activity of hunger promoting AgRP neurons in the hypothalamus. More specifically, they will record the activity of Calb2 or Sst neurons using the 3P microendoscope, while optogenetically activating or inhibiting AgRP neurons of sated or hungry mice that are offered a sugar solution or sugar plus bitter mixture. The proposed study will provide critical information for understanding a core mechanism of taste and metabolic state integration in the brainstem. It will also allow the team to investigate whether taste coding is modulated in the brainstem based on metabolic state before the information is sent to higher-order gustatory nuclei and to the insula cortex in a feed-forward fashion. Together, the proposed experiments will generate novel knowledge and provide necessary technical and conceptual advances to lay the groundwork for a subsequent Targeted BRAIN Circuits R01 with the same team of investigators aiming to further investigate how brainstem taste circuits communicate with other central gustatory nuclei in a metabolic state dependent manner to control ingestive behaviors in mice.

Project Narrative We propose to investigate the function of brainstem taste circuits in regulating state dependent taste coding and food intake behaviors. We will use a 3P microendoscope to capture the activity of brainstem neurons during ingestion of different tastants when mice are sated or hungry. We will also investigate how brainstem taste circuits change activity in response to optogenetic activation or inhibition of hypothalamic AgRP neurons, a key regulator of metabolic homeostasis and food intake.