Starved animals often exhibit elevated locomotion, which has been speculated to partly resemble foraging behavior and facilitate food acquisition and energy intake. Despite its importance, the neural mechanism underlying this behavior remains unknown in any species. In this study we confirmed and extended previous findings that starvation induced locomotor activity in adult fruit flies Drosophila melanogaster. We also showed that starvationinduced hyperactivity was directed toward the localization and acquisition of food sources, because it could be suppressed upon the detection of food cues via both central nutrient-sensing and peripheral sweet-sensing mechanisms, via induction of food ingestion. We further found that octopamine, the insect counterpart of vertebrate norepinephrine, as well as the neurons expressing octopamine, were both necessary and sufficient for starvationinduced hyperactivity. Octopamine was not required for starvation-induced changes in feeding behaviors, suggesting independent regulations of energy intake behaviors upon starvation. Taken together, our results establish a quantitative behavioral paradigm to investigate the regulation of energy homeostasis by the CNS and identify a conserved neural substrate that links organismal metabolic state to a specific behavioral output.T he CNS plays an essential role in energy homeostasis (1). It actively monitors changes in the internal energy state and modulates an array of physiological and behavioral responses to enable energy homeostasis. Foraging behavior is critical for the localization and acquisition of food supply and hence energy homeostasis. It has been extensively documented both in ethological settings (2, 3) and under well-controlled laboratory conditions (4). Laboratory rodents with limited food access exhibit stereotypic food anticipatory activity (FAA) several hours before the mealtime, which is characterized by a steady increase in locomotion and other appetitive behaviors (5). The neural substrate that drives FAA still remains elusive (5, 6). Notably, the regulation of FAA seems to be dissociable from that of feeding behavior (7,8). These results hint at the presence of an independent and somewhat discrete regulatory mechanism of foraging behavior.Foraging behavior has also been extensively studied in invertebrate species such as the roundworm Caenorhabditis elegans (9) and fruit flies Drosophila melanogaster (10). Roundworm populations exhibit two naturally emerged foraging patterns: "solitary" worms disperse across the bacterial lawn, and "social" worms aggregate along the food edge and form clumps (9). This behavioral dimorphism is controlled by natural variations of the npr-1 (neuropeptide receptor resemblance) gene that encodes a receptor homologous to the receptor family of orexigenic neuropeptide Y in mammals (9). A comparable scenario has also been identified in larval fruit flies (10), with two distinct forms of foraging present in nature: "rover" and "sitter." On food sources, sitter but not rover reduces moving speed ...

24The function of the central nervous system to regulate food intake can be disrupted by 25 sustained metabolic challenges such as high-fat diet (HFD), which may contribute to various 26 53 Leibowitz, 1984). These neurons detect various neural and hormonal cues such as circulating 54 glucose and fatty acids, leptin, and ghrelin, and modulate energy intake and expenditure 55 accordingly (Belgardt et al., 2009). Upon the reduction of the internal energy state, NPY/AgRP 56 neurons are activated and exert a robust orexigenic effect (Belgardt et al., 2009). Genetic 57 ablation of NPY/AgRP neurons in neonatal mice completely abolishes food consumption 58 5 whereas acute activation of these neurons significantly enhances food consumption (Aponte et 59 al., 2011; Krashes et al., 2011). NPY/AgRP neurons also antagonize the function of POMC 60 neurons that plays a suppressive role on food consumption (Roseberry et al., 2004). Taken 61 together, these two groups of neurons, among other neuronal populations, work in synergy to 62 ensure a refined balance between energy intake and expenditure, and hence organismal 63 metabolism. 64 65 In spite of their critical roles, the function of the nervous system to accurately regulate appetite 66 and metabolism may be disrupted by sustained metabolic stress, resulting in eating disorders 67 and various metabolic diseases such as obesity and type 2 diabetes. Several lines of evidence 68 have begun to reveal the underlying neural mechanisms. For example, HFD increases the 69 intrinsic excitability of orexigenic NPY/AgRP neurons (Vernia et al., 2016), induces leptin 70 resistance (Mazor et al., 2018; Olofsson et al., 2013), and enhances their inhibitory 71 innervations with anorexigenic POMC neurons (Newton et al., 2013), altogether resulting in 72 hypersensitivity to starvation and increased food consumption. Interestingly, besides HFD, 73 other metabolic challenges, including maternal HFD, alcohol consumption, as well as aging, 74 also disrupt normal food intake via affecting the excitability and/or innervation of NPY/AgRP 75 neurons (Cains et al., 2017; Furedi et al., 2018; Rivera et al., 2015). All these interventions may 76 contribute to the onset and progression of metabolic disorders. 77 78 96 Reiter et al., 2001). Therefore, it offers a good model to characterize food-seeking behavior in 97 depth and provides insight into the regulation of energy intake and the pathogenesis of 98 metabolic disorders in more complex organisms such as rodents and human. 99 7 100Our previous work showed that fruit flies exhibited robust starvation-induced hyperactivity 101 that was directed towards the localization and acquisition of food sources, therefore resembling 102 an important aspect of food-seeking behavior upon starvation (Yang et al., 2015). We also 103 identified a small subset of OA neurons in the fly brain that specifically regulated 104 starvation-induced hyperactivity (Yu et al., 2016). Analogous to mammalian systems, a 105 number of neural and hormonal cues are involved in the sys...

Our findings in fruit flies characterized a group of sleep-promoting neurons surrounded by a group of wake-promoting neurons. The two groups of neurons are both cholinergic and use Go inhibitory signal to regulate sleep.