10The nucleus of the solitary tract (NTS) is emerging as a major site of action for the appetite-suppressive 11 effects of leading pharmacotherapies currently investigated for the treatment of obesity. However, our 12 understanding of how NTS neurons regulate appetite remains incomplete. Here we used NTS nutrient 13 sensing as an entry point to characterize stimulus-defined neuronal ensembles engaged by the NTS to 14 produce physiological satiety. Using activity-dependent expression of genetically-encoded circuit 15 analysis tools, we found that NTS detection of leucine engages NTS prolactin-releasing peptide (PrRP) 16 neurons to inhibit AgRP neurons via a population of leptin-receptor-expressing neurons in the 17 dorsomedial hypothalamus. This circuit is necessary for the anorectic response to NTS leucine, the 18 appetite-suppressive effect of high protein diets, and the long-term control of energy balance. These 19 results extends the integrative capability of AgRP neurons to include brainstem nutrient sensing inputs. 21Keywords: nucleus of the solitary tract, appetite, obesity, AgRP neurons, hypothalamus, PrRP neurons, 22 metabolic diseases, circuit mapping 23 24 52 provides a neuroanatomical basis for the latter (Petrov et al., 1993), which could explain the ability of 53 high doses of the satiation hormones like CCK to recruit aversive circuits (Swerdlow et al., 1983), and/or 54 provide a mechanism for the synergistic feeding suppressive effects produced by the combination of 55 anorectic signals (Bhavsar et al., 1998; Blevins et al., 2009; Blouet & Schwartz, 2012). Addressing this 56 3 question with molecularly-defined circuit analysis tools is difficult because most identified NTS 57 neurochemical subsets are functionally heterogeneous, respond to multiple cues and project widely 58 throughout the neuraxis (D'Agostino et al., 2016; Rinaman, 2010). Instead, it may be possible to better 59 understand the functional organization of NTS feeding-regulatory circuits using functionally-defined 60 circuit mapping, which could be particularly insightful if subsets of NTS neurons are specialized in the 61 transmission of highly specific sensory cues and organized in a similar fashion as gustatory and vagal 62 sensory neurons (Bai et al., 2019; Williams et al., 2016). Applying such a strategy to signals able to 63 produce satiation or satiety without negative consequences may lead to important new understanding 64 of how to pharmacologically suppress appetite without undesirable side effects. 65We previously showed that NTS sensing of the branched-chain amino acid leucine not only 66 modulates the control of meal size but also rapidly suppresses hunger in fasted animals and increases 67 satiety without the production of conditioned taste aversion (Blouet & Schwartz, 2010; Cheng et al., 68 2020). Here, we used NTS leucine sensing as a functional entry point to investigate ascending neural 69 circuits engaged by NTS neurons to modulate hunger and satiety. In these experiments, Leucine is 70 injected into the NTS ...