Research on Research

Prospects & Overviews

Hunger and thirst interact to regulate ingestive behavior in flies and mammals

Nicholas Jourjine�

In animals, nervous systems regulate the ingestion of food

and water in a manner that reflects internal metabolic need.

While the coordination of these two ingestive behaviors is

essential for homeostasis, it has been unclear how internal

signals of hunger and thirst interact to effectively coordinate

foodandwateringestion.Inthelastyear,workininsectsand

mammalshasbeguntoelucidatesomeoftheseinteractions.

As reviewed here, these studies have identified novel

molecular and neural mechanisms that coordinate the

regulation of food and water ingestion behaviors. These

mechanisms include peptide signals that modulate neural

circuitsforboththirstandhunger,neuronsthatregulateboth

food and water ingestion, and neurons that integrate

sensory information about both food and water in the

external world. These studies argue that a deeper under-

standing of hunger and thirst will require closer examination

of how these two biological drives interact.

Keywords:.animal behavior; Drosophila; homeostasis; hunger; ingestion; neural circuits; thirst

Introduction

“The desires of the body are few: relief from cold, hunger, and thirst.” – Roger Bacon, Opus Majus

Animal behavior is constrained by the metabolic needs of cells, and few needs are more fundamental than those of food and water. While some exceptional animals can

survive for extended periods without either of these, most must ingest both on a daily basis. Remarkably, animal nervous systems regulate ingestion in a manner that precisely reflects internal metabolic need. This regulation is observed in animals with diverse nervous system organizations and evolutionary histories, suggesting shared principles of nervous system function. Nonetheless, how animal nervous systems regulate ingestion in a manner that reflects internal need remains an open and fundamental question.

Two key links between ingestion and metabolic need are the subjective experiences of hunger and thirst. As the medieval scientist Roger Bacon makes clear in the quote above, these experiences are among the most basic facts of life. Nonetheless, the biological mechanisms underlying hunger and thirst remain largely mysterious. For example, which aspects of nervous system function give rise to these experiences? What explains the powerful influence they have on our behavior? Entire fields of behavioral neuroscience and psychology have been devoted to addressing these questions, and they have made key conceptual contributions to our understanding of hunger and thirst [1]. One such contribution is the notion that hunger and thirst are, like sexual arousal, aggression, and fear, biological “drives,” internal motiva- tional states that cause behaviors oriented toward a specific goal. A long history of theoretical models has elaborated on the concept of drive to explain how hunger and thirst control behavior. For example, some have proposed that a key property of drives is to gradually increase in intensity until an animal performs specific actions [2], while others have posited various positive or negative interactions among drives [3].

Despite the conceptual contributions that these models have made, experimental tools to directly test whether and how neural circuits utilize them have been lacking. As a result, fundamental questions remain about the relationship between internal metabolic need, biological drives, and behavior. Historically, these questions have been addressed by studying specific food or water ingestion behaviors in isolation, with the assumption that mechanisms underlying these behaviors are analogous but ultimately separable. While this approach has led to fundamental insights, it has generally not addressed how neural circuits for food and water ingestion might interact. In the last year, however, work in insects and

DOI 10.1002/bies.201600261

Department of Molecular and Cell Biology, University of California Berkeley, Berkeley, CA, USA

Corresponding author: Nicholas Jourjine E-mail: [email protected]

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mammals has identified novel intersection points between mechanisms regulating hunger and thirst, and these studies have provided insight into the problem of how nervous systems coordinate distinct behaviors to satisfy multiple internal needs. Here, I review recent progress toward understanding the neural regulation of hunger and thirst by focusing on three questions: which aspects of internal physiology signal food and water deprivation to the nervous system? What are the identities of the neurons that detect these signals to generate the “drive” associated with hunger and thirst? And finally, how does activity in these neurons regulate behavior? As I argue below, the answers to these questions suggest that a deeper understanding of hunger and thirst will require examining how these biological drives interact to regulate behavior.

What are the internal signals of food and water deprivation?

Internal signals of food deprivation

A key way animals measure metabolic need is by detecting molecules that participate directly in metabolism. In animals, glucose is the starting point for cellular metabolism, and an essential function of food ingestion is to supply this sugar to cells foruseinthebiochemicalpathwaysthatproduceATP.Inaddition, blood glucose levels correlate with the sensation of hunger in humans [4]. These observations led to the hypothesis that nervous systems directly measure glucose abundance as a signal of metabolic need and hunger [4]. This hypothesis has been supported by the observation that a small number of neurons in both mammalian and insect brains are sensitive to circulating glucose [4–6], although how these neurons regulate behavior remainsunclear.Inaddition,neuronsintheDrosophilabrainsense fructose, another sugar that can be used in cellular metabolism, and this appears to be important both for promoting feeding in starved flies and suppressing feeding in satiated flies [7]. Insects and mammals have also been proposed to directly sense another important building block of cellular metabolism: amino acids. Interestingly, the same protein kinase – GCN2 – has been implicated in amino acid sensing in mammals and insects, although its role in mammals has been debated [8–11]. Thus, mammals and insects sense internal metabolic need by detecting multiplemoleculesthatdirectlyparticipateincellularmetabolism.

Internal nutrient sensing is an elegant way to directly link feedingwiththemetabolicneedforfood.Itisperhapssurprising, then, that most of the signals proposed to regulate feeding are in fact peptides that do not participate directly in metabolic processes.Thefirstofthesetobediscoveredwascholecystokinin (CCK), a 58 amino acid peptide secreted by the gut. Exogenous CCK is sufficienttorestrictmeal size in rats, arguing that CCK isa satiety factor [12]. Since this discovery, many other peptide hormones have beenproposed to regulate feeding in insects and mammals (Table 1). The sheer number of these peptides is daunting. However, this diversity is partly explained by the observation that different intensities of hunger, and different components of feeding behavior (e.g. initiation and termina- tion), are often regulated by different peptide systems [13–15]. For example, two key peptide regulators of feeding,ghrelin, and

leptin, function by exerting opposing effects on hunger: ghrelin promotes it while leptin suppresses it [16, 17].

Thus, insects and mammals use multiple direct signals (molecules that participate in metabolism) and indirect signals (peptide hormones) to sense the internal need for food. Understanding how these diverse, time varying signals are integrated by the nervous system to produce appropriate ingestive behaviors is an important avenue for future research.

Internal signals of water deprivation

Water is a fundamental requirement for all biological processes, and, as a result, most land animals cannot survive for more than a few days without it. Powerful regulatory mechanisms must therefore exist to ensure that water ingestion occurs on a regular basis. Although the details of these mechanisms differ from those described above in the context of hunger, a conserved theme is that the nervous system integrates multiple direct and indirect signals to measure the internal need for water.

The most direct signals of internal water abundance are properties of blood, in particular its osmolarity and volume. Increasing blood osmolarity and decreasing volume are both potent stimulators of thirst in humans and water ingestion in other mammals. Conversely, decreasing blood osmolarity or increasing volume suppress thirst and water ingestion. In water deprived animals, these direct signals act on the nervous system and kidneys to initiate the renin-angiotensin system [18], a cascade of secondary peptides that regulate the kidneys and circulatory system to minimize water loss. This cascade also causes the secretion of the neuropeptide vasopressin (also known as anti-diuretic hormone). Like the other peptides in the renin-angiotensin system, vasopressin regulates water reabsorption in the kidneys. However, vasopressin is also sufficient to evoke intense water ingestion behavior, suggesting that it acts as a peptide signal of thirst [19, 20]. Interestingly, vasopressin release has also been shown to be induced by CCK, the gut peptide discussed above that has critical functions in regulating food ingestion [21]. While the behavioral relevance of this finding remains unclear, it suggests the possibility of complex interactions between neuropeptide systems underlying hunger and thirst.

Relative to mammals, insects have large surface area to volumeratiosandarethereforeatconstantriskoflosinginternal water through evaporation [22, 23]. For many insects, the only way to recover this lost water is to ingest it. Nonetheless, relatively little is known about the regulation of water ingestion behavior in invertebrates. Like mammals, it appears that hemolymph(insectblood)osmolarityandvolumeareimportant signals of water deprivation [23]. In addition, a handful of neuropeptides � such as leucokinin, insulin, capability (capa), and glycoprotein beta 3/5 (GPB-3/5) � have been shown to regulate water balance in the insect kidney [24–26]. Interest- ingly, insulin and leucokinin are also regulators of feeding behavior in Drosophila [24], suggesting cross-talk between peptide systems for food and water ingestion. How these peptides act on the nervous system to influence water ingestion behavior, and how their release is related to dehydration, remain important questions to be addressed.

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Table 1. Proposed signals of hunger and satiety in flies and mammals

Signal Organism studied Homologs? Proposed site of action References

Glucose Mouse Human Drosophila

n/a In mammals, multiple hypothalamic brain regions. In insects, the pars intercerebralis.

[4, 67]

Fructose Drosophila n/a Pars intercerebralis [7] Amino acids Mouse

Drosophila

n/a In mammals, piriform cortex, although this has been

debated (see [11]). In Drosophila larvae, dopaminergic neurons.

[8, 9, 11]

Blood osmolarity Rat

Drosophila

n/a In mammals, circumventricular organs. In insects,

ISNs.

[40, 68]

Cholecystokinin

(CCK)

Mouse

Human

Drosulfakinin Multiple brain regions, including cortex and olfactory

bulb.

[12, 69–72]

Ghrelin Mouse Human

No invertebrate homologs

characterized.

AgRP neurons in the Arcuate Nucleus of the hypothalamus; subfornical organ.

[59, 73–75]

Leptin Mouse

Human

Unpaired-2 POMC neurons in the arcuate nucleus of the

hypothalamus.

[28, 76]

Insulin Mouse Human

Drosophila

Insulin-like peptides 1–8.

In insects, the pars intercerebralis. Broad expression of insulin receptors in the mammalian brain.

[24, 72, 75, 77]

Glucagon-like peptide 1

Mouse No invertebrate homologs

characterized.

Vagus nerve; nucleus of the solitary tract. [78–81]

Corticotropin

releasing hormone (CRH)

Mouse

Human

Diuretic hormone

44

Ventromedial hypothalamus. [82–84]

Urocortin Mouse No invertebrate

homologs characterized.

Urocortin belongs to the CRH family and binds CRH

receptors.

[85]

Agouti related protein

Mouse No invertebrate homologs characterized.

Arcuate nucleus [86, 87]

Neuropeptide Y Mouse No invertebrate homologs

characterized.

NPY-receptor is highly expressed in NPY/AgRP neurons.

[88–91]

Peptide YY Mouse No invertebrate homologs

characterized.

Peptide YY binds neuropeptide Y receptor. [91]

Orexin (a.k.a. hypocretin)

Mouse No invertebrate homologs

characterized.

Ventromedial hypothalamus; Paraventricular hypothalamus

[92, 93]

Alpha-melanocyte

stimulating hormone

Mouse No invertebrate

homologs characterized.

Multiple sites of action including hypothalamus. [94–96]

Amylin Mouse No invertebrate

homologs characterized.

Multiple sites of action including circumventricular

organs.

[97–100]

Galanin Mouse Allatostatin A Multiple sites of action including hypothalamus. [96] Melanin- concentrating

hormone

Mouse No invertebrate homologs

characterized.

Multiple sites of action including hypothalamus. [94, 96]

Cocaine/

amphetamine- regulated transcript

Mouse No invertebrate

homologs characterized.

Receptors not yet identified. [96, 101]

Neuromedin U Rat Hugin Multiple sites of action including hypothalamus. [102, 103] Adipokinetic hormone (AKH)

Drosophila Glucagon ISNs [40, 104]

Drosulfakinin Drosophila Cholecystokinin Not yet characterized. [72] Allatostatin A Drosophila Galanin Not yet characterized. [41, 105]

Hugin Drosophila Neuromedin U Multiple sites of action including neuropeptide producing neurons.

[106, 107]

(Continued)

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Thirst in insects appears to exhibit some key similarities with thirst in mammals: it is caused by changes in blood (hemolymph) volume and osmolarity, it is likely heavily regulated by neuropeptides, and some of those peptides interact with peptide systems for food ingestion. However, there are also differences. For example, due to their flexible exoskeleton, dehydration in insects can be accompanied by a large reduction in blood volume. In addition, while loss of blood volume is a potent stimulator of thirst in mammals, most recent studies have specifically manipulated blood osmolarity [27–29]. An interesting question for future research will be to ask how thirst induced by changes in blood volume differs from thirst induced by changes in blood osmolarity. A second potential difference between insect and mammalian thirst systems is the well-documented distinction between need-based and need-free water ingestion in mammals. Specifically, water ingestion in mammals can be induced by changes in internal blood osmolarity and volume (need- based), or it can be independent of internal need (need-free). A recent example of such need-free ingestion is the observation that mice increase drinking prior to sleep to offset overnight water loss [20]. It will be interesting to see whether the distinction between need-free and need-based water ingestion also holds in insects.

Which neurons detect internal signals of food and water deprivation?

Neurons for sensing food deprivation

The nervous system orchestrates complex feeding behaviors in response to multiple signals of metabolic need. Which neurons detect these signals and by which mechanisms? An early approach to address this question was to lesion or electrically stimulate specific brain regions and ask what effect these manipulations had on food and water inges- tion [30–33]. While these experiments were difficult to interpret because lesions lacked cell-type specificity, many studies found that removing or stimulating neurons in the hypothalamus, a conserved mammalian forebrain region, could evoke intense, specific ingestive behaviors.

More recently, genetically defined neural populations have been identified in the hypothalamus whose activity influences feeding behavior [34–37]. Among these, neurons that express agouti related peptide (AgRP neurons) or pro-opiomelano- cortin (POMC neurons) were found to exert potent and opposing effects on food ingestion: AgRP neurons promote ingestion and are required for feeding, while POMC neurons inhibit feeding. Importantly, these neurons express receptors that make them directly sensitive to some of the internal signals of metabolic need discussed above. These include ghrelin, which promotes activity in AgRP neurons and causes feeding [16], and leptin, which promotes activity in POMC neurons to suppress feeding [17]. AgRP and POMC neurons therefore provide an important link between signals of internal metabolic state and neural circuits underlying ingestive behaviors. Understanding how they integrate these signals to regulate behavior is an important open question.

While insects lack a brain structure directly analogous to the hypothalamus, they do have regions that play functionally similar roles. One of these areas is the pars intercerebralis (PI), a dorsal brainregioncontainingmultipleneuroendocrinecells.PIneurons are the major site of insulin release in Drosophila, and neurons in and near the PI are directly sensitive to circulating glucose and fructose[5–7].Inaddition,anothergroupofPIneuronsisindirectly sensitive to the Drosophila ortholog of leptin, upd-2, which is released from adipose tissue and causes insulin release from the PI [38]. Mammalian leptin can rescue upd-2 mutant phenotypes in Drosophila,suggestinghomology,althoughtheeffectsofupd-2on feeding behavior in Drosophila are unclear [38]. Other neurons in the Drosophila brain have been identified that are sensitive to amino acid imbalances, insulin, and several neuropeptides (e.g. sNPF,AKH,andallatostatinA)[8,24,39–41].However,manymore Drosophila neuropeptides have been discovered than neurons sensitive to those peptides [42], arguing that neural sensors of metabolic need remain to be identified.

Neurons for sensing water deprivation

In the mammalian brain, neurons in the anterior wall of the third ventricle are directly sensitive to blood osmolarity, a key internal signal of thirst and hydration [43]. Recently, genetic approaches have made it possible to control the activity of

Table 1. (Continued)

Signal Organism studied Homologs? Proposed site of action References

Short

neuropeptide F

Drosophila No vertebrate

homologs characterized.

Multiple sites of action including subesophegeal zone. [39, 108]

Leucokinin Drosophila No vertebrate

homologs characterized.

Malpighian tubule, multiple central brain neurons

including fan shaped body.

[109]

Diuretic hormone 44

Drosophila CRH Pars intercerebralis [5]

CCH-amide-2 Drosophila No vertebrate

homologs characterized.

Not yet characterized. [110]

Unpaired-2 Drosophila Leptin GABAergic neurons innervating pars intercerebralis. [38]

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neurons in this region in a cell type specific manner [44]. These approaches have identified two populations of neurons in the subfornical organ (SFO) that have opposing effects on water ingestion (Fig. 1). One of these populations is marked by expression of the enzyme neuronal nitric oxide synthase (nNOS) and is sufficient to promote water ingestion. The other, marked by the vesicular GABA transporter (VGAT), inhibits water ingestion. nNOSþ SFO neurons are sensitive to a key internal signal of hydration state, blood osmolarity. Moreover, nNOSþ neurons increase activity in thirsty animals, thereby linking water ingestion behavior to a measure of internal water abundance [27].

nNOSþ and VGATþ SFO neurons are thus functionally analogous to AgRP and POMC neurons in the hypothalamus, which exert opposing effects on food ingestion. The discovery of these neural populations reveals an intriguing parallel between circuits regulating hunger and thirst: both utilize paired pathways in which each member of the pair exerts an opposing effect on behavior (Fig. 1). Whether this similarity is superficial, or if it reflects an adaptive circuit motif, will be an interesting area for future investigation.

Studies of thirst in insects have generally lagged behind those in mammals, due in part to the difficulty of lesioning and electrode stimulation in small brains. Early studies in blowflies and Drosophila showed that the nervous systems of these insects regulate both foraging for and ingestion of water [23, 45], although neurons that regulated these behaviors were not identified at the time. More recently, our lab has shown that the Drosophila gustatory system contains dedicated water-taste neurons, and identified the

molecular basis of water sensing in these neurons as the osmolarity-sensitive ion channel ppk-28 [46]. We have also identified four neurons in the central brain that sense internal water abundance via a different osmolarity-sensitive ion channel, nanchung, and that regulate water ingestion. Interestingly, nanchung belongs to the TRPV family, and members of this family have also been proposed to function as sensors of blood osmolarity in mammals [43, 47]. As I describe below, osmosensitive nanchung neurons also sense a key internal signal of food deprivation, making them an important intersection point for the regulation of food and water ingestion in Drosophila [40].

How do sensors of food and water deprivation regulate behavior?

Interoceptive neurons in insects and mammals detect multiple signals of metabolic need. As described above, these neurons are in many cases sufficient to rapidly regulate complex ingestive behaviors. What are the neural circuits through which they achieve this regulation?

One way to address this question is to ask which brain regions contain axonal projections of interoceptive neurons. Neural tracing strategies have, for example, been used to demonstrate connectivity between hunger promoting AgRP neurons and multiple brain regions. These include the paraventricular hypothalamus (PVH) and lateral hypothala- mus (LH), among others [48] (Fig. 2). Thirst promoting nNOSþ SFO neurons also send axonal projections to the PVH [27]. It will be interesting to test whether brain regions such as the PVH, which receive input from both hunger and thirst centers, function in integrating internal signals of food and water deprivation.

Understanding the anatomy of neural circuits is necessary for understanding how they regulate behavior, but it is not sufficient: knowledge of neural activity in those circuits is also required. Simultaneously monitoring neural activity in all brain regions that regulate food and water ingestion remains technically out of reach, but important insights have recently come from monitoring activity in interoceptive neurons that directly sense internal signals of food and water deprivation.

In mice, monitoring activity in AgRP neurons revealed that their activity gradually increases during starvation, consistent with the notion that they are sensing internal signals of food deprivation whose abundance correlates with starvation [49]. Surprisingly, however, when starved mice were presented with food, activity in AgRP neurons was reduced within seconds to levels observed in satiated mice, and this occurred prior to ingestion. This reduction appears to be caused by leptin receptor expressing inhibitory neurons in another region of the hypothalamus [50]. Rapid sensitivity to sensory information was also observed in thirst promoting nNOSþ SFO neurons [27]. As is the case with AgRP neurons, nNOSþ neurons gradually increased their activity level during water deprivation, consistent with their being sensitive to internal signals of water abundance. As a further similarity to AgRP neurons, this activity was rapidly reduced the moment mice taste water and before ingested water could have modified their internal state. nNosþ neurons differ from AgRP neurons

Figure 1. Circuit motifs for the regulation of food and water ingestion. Top: Cross sectional schematic of a mouse brain, showing relative locations of the subfornical organ (SFO) and the arcuate nucleus of the hypothalamus (Arc). Bottom: In the SFO, nNOSþ and VGATþ neurons have opposing effects on water ingestion. nNOSþ neurons promote water ingestion, while VGATþ neurons suppress this behavior. A similar circuit logic exists in the Arc to regulate food ingestion. Here, AgRPþ neurons promote food ingestion while POMCþ neurons suppress food ingestion.

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in that only tasting water, and not seeing or expecting it, is sufficient to alter activity. Nonetheless these findings argue that neurons that sense internal signals of food and water deprivation are also highly sensitive to sensory information about the presence of food and water in the external world.

Work in Drosophila has uncovered a second theme: modulation of sensory systems by internal signals of deprivation. In the taste system, Inagaki et al. have shown that multiple neuromodulators that signal hunger also alter activity in gustatory neurons [51, 52]. For example, they have shown that starvation increases dopamine release onto sugar gustatory neurons, and this increases the sensitivity of these neurons to sugar taste, thereby making ingestion more likely [51]. Conversely, neuropeptides released in starved flies reduce the magnitude of bitter responses in bitter sensing gustatory neurons, thus reducing the threshold for ingesting unpalatable � but potentially nutritious � foods [52]. Interestingly, Gordon and coworkers have recently identified an independent mechanism for achieving the same effect [53]. This involves a novel class of central neurons, OA-VL, that release the neuromodulator octopamine onto bitter sensory neurons, potentiating their sensitivity to bitter substances. OA-VL neurons are inhibited during starvation, resulting in a decrease in sensitivity of bitter gustatory neurons and a reduction of the threshold for ingesting food. Modulation of sensory systems by internal state has also been observed in mammals: in mice, starvation-dependent increases in a class of neuromodulators, the endocannabinoids, potentiate olfac- tory sensitivity to food odors, and this potentiation is sufficient to increase feeding [54].

Thus, interoceptive neurons that detect internal signals of nutrient deprivation can modulate activity in sensory systems that detect signals of nutrients in the external world. Conversely, these sensory systems can also rapidly modulate activity in interoceptive neurons. Rapid feedback between sensors of an animal’s internal state and the external world might therefore be an important feature of neural circuits regulating ingestion. However, the larger question of how

nervous systems use information from internal sensors of food and water depriva- tion to produce appropriate ingestive behav- iors remains largely unanswered. To address this question, it will be important to monitor activity in the large-scale neural circuits connecting interoceptive neurons to sensory and motor systems.

Coordinated regulation of food and water ingestion

Most studies have examined food or water ingestion independently. In many ways, this approach is justified: eating and drinking are qualitatively different behaviors, and hunger and thirst are distinct internal experiences. However, it is also clear that regulatory mechanisms underlying these behaviors in- teract in humans and other animals [55–57]. For example, recent studies in mice have

shownthat ghrelin,a keyinternalsignalofhunger, iscapableof modulating water ingestion behavior [58], and that the subfornical organ, a key regulator of thirst, contains neurons that are sensitive to multiple signals of hunger [59–61].

While these findings are intriguing, the nature of cross talk between systems regulating food and water ingestion has remained elusive. However, three studies within the last several months [27, 29, 40] have shed light on this cross- talk by identifying novel mechanisms coupling the regulation of food and water ingestion. These studies argue that understanding these behaviors will require a deeper examination of how the regulatory systems underlying them interact.

Coordination of food and water ingestion in Drosophila

Two behavioral screens were recently carried out in our lab with the goal of identifying neurons that regulate sugar and water ingestion behavior, respectively. Surprisingly, the same four neurons were identified in both of these screens [40]. These neurons reside in the subesophegeal zone (SEZ) of the fly brain, a key region for taste information processing and feeding regulation, and for this reason we named them ISNs (Interoceptive SEZ Neurons). ISN activity has a striking effect on ingestive behavior: experimentally increasing activity not only promotes food ingestion, but also restricts water ingestion. Silencing neural activity in ISNs has the opposite effect.

ISNs are not directly regulated by the taste of sugar or water. Instead, ISN activity is regulated by two signals of internal physiology: insect glucagon (adipokinetic hormone, AKH), a peptide signal of food deprivation, and hemolymph osmolarity, a signal of water deprivation. Underlying this sensitivity is the fact that ISNs co-express a G protein coupled receptor that binds AKH, and the TRPV family member nanchung, which is sensitive to extracellular osmolarity [62].

Figure 2. Neural connectivity among regions involved in food and water ingestion in the mouse brain. Axonal projections that link brain areas implicated in food and water ingestion are highlighted in red. SFO, subfornical organ; Arc, arcuate nucleus; PVH, paraventricular hypothalamus; LH, lateral hypothalamus; PG, periaqueductal grey; OVLT, organum vasculosum of the lamina terminalis; MnPO, median preoptic nucleus; SON, supraoptic nucleus; aBNST, anterior subdivisions of the bed nucleus of the stria terminalis.

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Thus, by expressing molecular sensors of both AKH and osmolarity, ISNs integrate internal signals of both hunger and thirst.

Why might this integration be adaptive? In the wild, animals often experience internal signals of both food and water deprivation and must balance these needs to make appropriate decisions about what to ingest. To address how ISN activity responded to concurrent signals of food and water deprivation, we applied AKH to ISNs in extracellular solutions designed to resemble the hemolymph osmolarity of either a thirsty fly (high osmolarity) or a water satiated fly (low osmolarity). We found that in high osmolarity solutions reflecting thirst, responses to the hunger signal AKH were strongly diminished. This result suggests that signals of water deprivation can gate ISN responses to signals of food deprivation. Consistent with this hypothesis, we found that thirst suppresses feeding in wild-type flies. Thus, coupling hunger and thirst signals via ISNs may allow flies to prioritize ingestion of water, which is more essential for survival, over ingestion of food.

Coordination of food and water ingestion in mammals

Three recent studies have identified a similar pattern of regulation in mice. First, Knight and coworkers have directly observed neural activity in thirst promoting nNOSþ SFO neurons [27]. As described above (in the section How do sensors of food and water deprivation regulate behavior?), they found that activity in these neurons increased both in thirsty mice and in mice whose blood osmolarity was experimentally raised, consistent with the function of these neurons in promoting water ingestion. Surprisingly, how- ever, they also observed that activity in these thirst promoting neurons increased immediately upon the initia- tion of food ingestion. Similar findings have been described by Andermann and coworkers in vasopressin secreting neurons of the posterior pituitary. Vasopressin neurons regulate blood osmolarity by releasing vasopressin in high osmolarity states. Vasopressin then acts on the kidneys and brain to conserve water and stimulate thirst. Andermann and coworkers observed that in high osmolarity states, neural activity in vasopressin secreting neurons rapidly decreases upon presentation of water or water-predicting cues. Conversely, neural activity rapidly increases upon initiation of food ingestion (but not cues predicting food) [28]. Thus, thirst regulatory neurons in two distinct brain regions are acutely sensitive to sensory information about both food and water.

Why should neurons that regulate water ingestion be sensitive to food? One answer comes from the observation that water and food both alter a key internal signal for thirst � blood osmolarity � and they do this in an opposing manner: water ingestion causes blood osmolarity to decrease; food ingestion causes it to increase. Thus, sensory modulation of thirst neurons by food reflects the fact that internal signals of food and water deprivation do not vary independently. Specifically, ingestion of food alters internal signals of both thirst and hunger.

As a further demonstration of the interconnectedness of thirst and hunger, Knight and coworkers tested the effect of silencing nNOSþ SFO neurons on water and food ingestion. As expected for a thirst promoting neuron, silencing restricted water consumption. However, silencing also caused mice to increase food ingestion. This increase appears to occur because silencing nNOSþ SFO neurons inhibits dehydration induced anorexia, a behavior that limits feeding in thirsty mice. Thus, neurons that sense internal signals of water deprivation are sensitive not only tothetasteofwater,butalsothetasteoffood,andaresufficientto oppositely regulate ingestion of water and food.

Manipulating activity in a population of neurons originally identified in the context of thirst affects both water and food ingestion. Does manipulating a population of neurons originally identified in the context of hunger do the same? Recent work by Krashes and coworkers [29] addressed this question by optogenetically activating hunger promoting AgRP neurons and asking what effect this had on a panel of behaviors. As observed in several previous studies, activation of AgRP neurons was sufficient to promote feeding. However, it also greatly reduced water ingestion, suggesting competition between the two biologi- cal drives � hunger and thirst � underlying these behaviors. Taken together, these studies argue that, in flies and mammals, interoceptive neurons detecting inter- nal signals of food and water deprivation also regulate ingestion of food and water in a reciprocal manner: neurons that promote the ingestion of food suppress the ingestion of water and vice versa (Fig. 3).

Krashes and coworkers also observed that AgRP activation suppressed behaviors related to anxiety, innate fear, and social interactions. This raises a fundamental question: does inhibition of thirst by hunger (or vice versa) reflect a mechanism by which all behaviors other than feeding are non-specifically suppressed during hunger states, or does it reflect a mechanism by which neural circuits for hunger and thirst specifically interact with one another? Non-specific suppression of behaviors that an animal is not currently engaged in is a well-documented and important principle by which nervous systems select behavioral outputs [63]. It is also consistent with the observation that AgRP induced hunger inhibits not just water ingestion (and, presumably, thirst), but also behav- iors associated with anxiety and fear. However, these two possibilities are not mutually exclusive. The evidence reviewed above from flies and mice also suggests specific mechanisms by which hunger and thirst interact: in mammals, neurons regulating water ingestion are sensitive to the presence of both food and water in the external world, and in Drosophila the same neurons sense internal signals of both hunger and thirst. Thus, there exists evidence that hunger and thirst may interact by both specific and non- specific mechanisms. Understanding the neural circuits underlying these interactions, and how they regulate behavior, will be an exciting avenue for future research.

A potentially informative example of such interactions comes from recent studies of aggression and sexual arousal. These biological drives underlie fighting and mating behav- iors, respectively, and the choice between them is a fundamental decision animals must make when encountering

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another member of the same species. Surprisingly, the neural circuits that regulate fighting and mating have recently been shown to share key populations of neurons in Drosophila and mammals [64]. These neurons are sufficient to influence both aggression and sexual arousal, arguing that aggression and arousal are directly coupled by activity of a single population of neurons. This regulatory logic is reminiscent of ISNs, whose activity couples thirst and hunger in Drosophila. These observations suggest that nervous systems might coordinate pairs of opposing biological drives by coupling their regulation to common neural populations. How activity in a single population of neurons can concurrently regulate mutually exclusive behaviors, and to what extent this principle can be generalized to other behaviors, are exciting questions for future research.

Conserved mechanisms or superficial similarities?

Hunger and thirst systems in flies and mammals share many features, such as internal signals that promote eating and drinking, the essential role of neuropeptides, and the antagonistic interactions between hunger and thirst drives outlined above. However, there are also important differ- ences in ingestive behavior between these animal lineages. One crucial difference is that flies, while capable of eating solid food [65], usually consume sugars that have been dissolved in water. An important question is whether the

different ingestion strategies of flies and mammals have driven the evolution of different circuits for thirst and hunger, or whether some circuit mechanisms have been conserved. In particular, it remains to be seen whether the convergence of hunger and thirst signals onto the same neural population (the ISNs) is unique to flies, or whether this convergence is also an important regulator of mamma- lian thirst and hunger. In support of the latter possibility, some mammalian neurons appear to integrate hunger and thirst signals in a manner similar to ISNs. For example, a subset of SFO neurons have been shown to be sensitive to circulating glucose and ghrelin, hunger signals, as well as extracellular osmolarity, a thirst signal [59, 60, 66]. However, it is also clear that other functionally distinct neural populations, such as AgRP/POMC neurons in the arcuate nucleus, and nNOS/VGAT neurons in the SFO, detect signals of thirst and hunger separately.

The extent to which neural circuits for hunger and thirst are conserved between mice and flies is currently an open question. Nonetheless, the need for food and water is an ancient physiological constraint that has shaped the evolution of neural circuits in both of these lineages. Continued investigation into hunger and thirst in these animal models should therefore provide insights into how different nervous systems solve the basic problem of providing food and water to internal metabolic processes.

Conclusions and outlook

One of the most remarkable properties of animal nervous systems is their ability to satisfy diverse internal metabolic needs by coordinating multiple ingestive behaviors. Two important insights have emerged from recent studies of this ability. First, neurons that sense internal signals of food and water deprivation are rapidly modulated by sensory information about food and water in the external world. Thus, information about internal metabolic need and information about food and water in the external world are integrated directly by the same neural populations. Second, neural mechanisms exist in both mammals and insects that drive antagonistic interactions between hunger and thirst: in mammals, neurons that are sufficient to promote food ingestion also suppress water ingestion and vice versa. In Drosophila, a single class of neurons, the ISNs, is sensitive to internal signals of both food and water deprivation, and is sufficient to oppositely regulate ingestion of both sugar and water. Understanding the logic of neural circuits that mediate this antagonism is an exciting area for future research.

A central question in neuroscience is how nervous systems integrate information about internal state with sensory information about the external world to select appropriate behaviors. By identifying neural circuits that are sensitive to both internal state and the external world, studies of hunger and thirst are in a position to begin addressing this question in unprecedented depth. Future studies in insects and mammals should provide insights into how nervous systems coordinate diverse behaviors to satisfy diverse metabolic needs.

Figure 3. Neurons with opposing effects on food and water ingestion in Drosophila and mammals. In Drosophila, ISNs concur- rently sense internal signals of both thirst (hemolymph osmolarity) and hunger (AKH) via the AKH receptor AKHR and the osmolarity sensor Nanchung (Nan). Activity in ISNs is sufficient both to promote food ingestion and inhibit water ingestion. In mammals, distinct populations in the subfornical organ (nNOSþ neurons) and arcuate nucleus of the hypothalamus (AgRPþ neurons) are specifically sensitive to internal signals of thirst and hunger, respectively. However, each of these populations is sufficient to influence both food and water ingestion, and they do this by promoting one of these behaviors and suppressing the other. Other neurons in the mammalian brain may concurrently sense signals of thirst and hunger [59].

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Acknowledgments I would like to thank the members of the Scott lab and Dr. Zachary Knight for their insightful comments on this manuscript.

The authors declare that they have no conflict of interest.

References

1. Berridge KC. 2004. Motivation concepts in behavioral neuroscience. Physiol Behav 81: 179–209.

2. Lorenz K, Leyhausen P. 1973. Motivation of Human and and Animal Behavior. New York: Van Nostrand Reinhold Company.

3. Tinbergen N. 1951. The Study of Instinct. Oxford: Clarendon Press. 4. Mayer J. 1996. Glucostatic mechanism of regulation of food intake.

Obes Res 4: 493–6. 5. Dus M, Lai JS, Gunapala KM, Min S, et al. 2015. Nutrient sensor in the

brain directs the action of the brain-gut axis in Drosophila. Neuron 87: 139–51.

6. Zhan YP, Liu L, Zhu Y. 2016. Taotie neurons regulate appetite in Drosophila. Nat Commun 7: 13633.

7. Miyamoto T, Slone J, Song X, Amrein H. 2012. A fructose receptor functions as a nutrient sensor in the Drosophila brain. Cell 151: 1113–25.

8. Bjordal M, Arquier N, Kniazeff J, Pin JP, et al. 2014. Sensing of amino acids in a dopaminergic circuitry promotes rejection of an incomplete diet in Drosophila. Cell 156: 510–21.

9. Hao S, Sharp JW, Ross-Inta CM, McDaniel BJ, et al. 2005. Uncharged tRNA and sensing of amino acid deficiency in mammalian piriform cortex. Science 307: 1776–8.

10. Maurin AC, Jousse C, Averous J, Parry L, et al. 2005. The GCN2 kinase biases feeding behavior to maintain amino acid homeostasis in omnivores. Cell Metab 1: 273–7.

11. Leib DE, Knight ZA. 2016. Rapid sensing of dietary amino acid deficiency does not require GCN2. Cell Rep 16: 2051–2.

12. Gibbs J, Young RC, Smith GP. 1973. Cholecystokinin elicits satiety in rats with open gastric fistulas. Nature 245: 323–5.

13. Barsh GS, Schwartz MW. 2002. Genetic approaches to studying energy balance: perception and integration. Nat Rev Genet 3: 589–600.

14. Schwartz MW, Woods SC, Porte D, Jr., Seeley RJ, et al. 2000. Central nervous system control of food intake. Nature 404: 661–71.

15. Woods SC, Seeley RJ, Porte D, Jr., Schwartz MW. 1998. Signals that regulate food intake and energy homeostasis. Science 280: 1378–83.

16. De Ambrogi M, Volpe S, Tamanini C. 2003. Ghrelin: central and peripheral effects of a novel peptydil hormone. Med Sci Monit 9: RA217–24.

17. Baskin DG, Hahn TM, Schwartz MW. 1999. Leptin sensitive neurons in the hypothalamus. Horm Metab Res 31: 345–50.

18. Rolls B, Rolls E. 1982. Thirst. New York: Cambridge University Press. 19. Verney EB. 1947. The antidiuretic hormone and the factors which

determine its release. Proc R Soc Lond B Biol Sci 135: 25–106.

20. Gizowski C, Zaelzer C, Bourque CW. 2016. Clock-driven vasopressin neurotransmission mediates anticipatory thirst prior to sleep. Nature 537: 685–8.

21. Haley GE, Flynn FW. 2008. Tachykinin neurokinin 3 receptor signaling in cholecystokinin-elicited release of oxytocin and vasopressin. Am J Physiol Regul Integr Comp Physiol 294: R1760–7.

22. Chapman RF. 2013. The Insects: Structure and Function. New York: Cambridge University Press.

23. Dethier VG. 1976. The Hungry Fly: A Physiological Study of the Behavior Associated With Feeding. London: Harvard University Press.

24. Liu Y, Luo J, Carlsson MA, N€assel DR. 2015. Serotonin and insulin- like peptides modulate leucokinin-producing neurons that affect feeding and water homeostasis in Drosophila. J Comp Neurol 523: 1840–63.

25. Terhzaz S, Cabrero P, Robben JH, Radford JC, et al. 2012. Mechanism and function of Drosophila capa GPCR: a desiccation stress-responsive receptor with functional homology to human neuro- medinU receptor. PLoS ONE 7: e29897.

26. Sellami A, Agricola HJ, Veenstra JA. 2011. Neuroendocrine cells in Drosophila melanogaster producing GPA2/GPB5, a hormone with homology to LH, FSH and TSH. Gen Comp Endocrinol 170: 582–8.

27. Zimmerman CA, Lin YC, Leib DE, Guo L, et al. 2016. Thirst neurons anticipate the homeostatic consequences of eating and drinking. Nature 537: 680–4.

28. Mandelblat-Cerf Y, Kim A, Burgess CR, Subramanian S, et al. 2017. Bidirectional anticipation of future osmotic challenges by vasopressin neurons. Neuron 93: 57–65.

29. Burnett CJ, Li C, Webber E, Tsaousidou E, et al. 2016. Hunger-driven motivational state competition. Neuron 92: 187–201.

30. Sternson SM. 2013. Hypothalamic survival circuits: blueprints for purposive behaviors. Neuron 77: 810–24.

31. Stanley BG, Ha LH, Spears LC, Dee MG, 2nd. 1993. Lateral hypothalamic injections of glutamate, kainic acid, D,L-alpha-amino- 3-hydroxy-5-methyl-isoxazole propionic acid or N-methyl-D- aspartic acid rapidly elicit intense transient eating in rats. Brain Res 613: 88–95.

32. Hetherington AW, Ranson SW. 1940. Hypothalamic lesions and adiposity in the rat. Anat Rec 78: 149–72.

33. Elmquist JK, Elias CF, Saper CB. 1999. From lesions to leptin: hypothalamic control of food intake and body weight. Neuron 22: 221–32.

34. Luquet S, Perez FA, Hnasko TS, Palmiter RD. 2005. NPY/AgRP neurons are essential for feeding in adult mice but can be ablated in neonates. Science 310: 683–5.

35. Aponte Y, Atasoy D, Sternson SM. 2011. AGRP neurons are sufficient to orchestrate feeding behavior rapidly and without training. Nat Neurosci 14: 351–5.

36. Krashes MJ, Koda S, Ye C, Rogan SC, et al. 2011. Rapid, reversible activation of AgRP neurons drives feeding behavior in mice. J Clin Invest 121: 1424–8.

37. Zhan C, Zhou J, Feng Q, Zhang JE, et al. 2013. Acute and long-term suppression of feeding behavior by POMC neurons in the brainstem and hypothalamus, respectively. J Neurosci 33: 3624–32.

38. Rajan A, Perrimon N. 2012. Drosophila cytokine unpaired 2 regulates physiological homeostasis by remotely controlling insulin secretion. Cell 151: 123–37.

39. Carlsson MA, Enell LE, N€assel DR. 2013. Distribution of short neuropeptide F and its receptor in neuronal circuits related to feeding in larval Drosophila. Cell Tissue Res 353: 511–23.

40. Jourjine N, Mullaney BC, Mann K, Scott K. 2016. Coupled sensing of hunger and thirst signals balances sugar and water consumption. Cell 166: 855–66.

41. Hergarden AC, Tayler TD, Anderson DJ. 2012. Allatostatin-A neurons inhibit feeding behavior in adult Drosophila. Proc Natl Acad Sci USA 109: 3967–72.

42. Nassel DR, Winther AM. 2010. Drosophila neuropeptides in regulation of physiology and behavior. Prog Neurobiol 92: 42–104.

43. Bourque CW. 2008. Central mechanisms of osmosensation and systemic osmoregulation. Nat Rev Neurosci 9: 519–31.

44. Oka Y, Ye M, Zuker CS. 2015. Thirst driving and suppressing signals encoded by distinct neural populations in the brain. Nature 520: 349–52.

45. Perttunen V, Erkkila H. 1952. Humidity reaction in Drosophila melanogaster. Nature 169: 78.

46. Cameron P, Hiroi M, Ngai J, Scott K. 2010. The molecular basis for water taste in Drosophila. Nature 465: 91–5.

47. Liedtke W, Friedman JM. 2003. Abnormal osmotic regulation in trpv4-/- mice. Proc Natl Acad Sci USA 100: 13698–703.

48. Betley JN, Cao ZF, Ritola KD, Sternson SM. 2013. Parallel, redundant circuit organization for homeostatic control of feeding behavior. Cell 155: 1337–50.

49. Chen Y, Lin YC, Kuo TW, Knight ZA. 2015. Sensory detection of food rapidly modulates arcuate feeding circuits. Cell 160: 829–41.

50. Garfield AS, Shah BP, Burgess CR, Li MM, et al. 2016. Dynamic GABAergic afferent modulation of AgRP neurons. Nat Neurosci 19: 1628–35.

51. Inagaki HK, Ben-Tabou de-Leon S, Wong AM, Jagadish S, et al. 2012. Visualizing neuromodulation in vivo: TANGO-mapping of dopa- mine signaling reveals appetite control of sugar sensing. Cell 148: 583–95.

52. Inagaki HK, Panse KM, Anderson DJ. 2014. Independent, reciprocal neuromodulatory control of sweet and bitter taste sensitivity during starvation in Drosophila. Neuron 84: 806–20.

53. LeDue EE, Mann K, Koch E, Chu B, et al. 2016. Starvation- induced depotentiation of bitter taste in Drosophila. Curr Biol 26: 2854–61.

54. Soria-G�omez E, Bellocchio L, Reguero L, Lepousez G, et al. 2014. The endocannabinoid system controls food intake via olfactory processes. Nat Neurosci 17: 407–15.

....Prospects & Overviews N. Jourjine

1600261 (9 of 11)Bioessays 39, 5, 1600261, � 2017 WILEY Periodicals, Inc.

R e v ie w

e s s a y s

55. Hsiao S, Trankina F. 1969. Thirst-hunger interaction. I. Effects of body- fluid restoration on food and water intake in water-deprived rats. J Comp Physiol Psychol 69: 448–53.

56. Hsiao S, Smutz ER. 1976. Thirst-reducing and hunger-inducing effects of water and saline by stomach-tubing vs. drinking in rats. Psychobiol- ogy 4.

57. Thornton SN. 2010. Thirst and hydration: physiology and consequen- ces of dysfunction. Physiol Behav 100: 15–21.

58. Mietlicki EG, Nowak EL, Daniels D. 2009. The effect of ghrelin on water intake during dipsogenic conditions. Physiol Behav 96: 37–43.

59. Ferguson AV. 2014. Circumventricular organs: integrators of circulating signals controlling hydration, energy balance, and immune function. In De Luca LA, Jr., Menani JV, Johnson AK, eds; Neurobiology of Body Fluid Homeostasis: Transduction and Integration. Boca Raton (FL): CRC Press.

60. Medeiros N, Dai L, Ferguson AV. 2012. Glucose-responsive neurons in the subfornical organ of the rat-a novel site for direct CNS monitoring of circulating glucose. Neuroscience 201: 157–65.

61. Mimee A, Smith PM, Ferguson AV. 2013. Circumventricular organs: targets for integration of circulating fluid and energy balance signals?. Physiol Behav 121: 96–102.

62. Kim J, Chung YD, Park DY, Choi S, et al. 2003. A TRPV family ion channel required for hearing in Drosophila. Nature 424: 81–4.

63. Sherington C. 1906. The Integrative Action of the Nervous System. New York: Yale University Press.

64. Anderson DJ. 2016. Circuit modules linking internal states and social behaviour in flies and mice. Nat Rev Neurosci 17: 692–704.

65. Lin S, Owald D, Chandra V, Talbot C, et al. 2014. Neural correlates of water reward in thirsty Drosophila. Nat Neurosci 17: 1536–42.

66. Pulman KJ, Fry WM, Cottrell GT, Ferguson AV. 2006. The subfornical organ: a central target for circulating feeding signals. J Neurosci 26: 2022–30.

67. Steinbusch L, Labouebe G, Thorens B. 2015. Brain glucose sensing in homeostatic and hedonic regulation. Trends Endocrinol Metab 26: 455–66.

68. Krausz YGaM. 1969. Regulation of food and water intake in rats as related to plasma osmolarity and volume. Physiol Behav 4: 311–3.

69. Saito A, Sankaran H, Goldfine ID, Williams JA. 1980. Cholecystokinin receptors in the brain: characterization and distribution. Science 208: 1155–6.

70. Wennogle LP, Steel DJ, Petrack B. 1985. Characterization of central cholecystokinin receptors using a radioiodinated octapeptide probe. Life Sci 36: 1485–92.

71. Ma J, Dankulich-Nagrudny L, Lowe G. 2013. Cholecystokinin: an excitatory modulator of mitral/tufted cells in the mouse olfactory bulb. PLoS ONE 8: e64170.

72. Soderberg JA, Carlsson MA, Nassel DR. 2012. Insulin-producing cells in the Drosophila brain also express satiety-inducing cholecys- tokinin-like peptide, drosulfakinin. Front Endocrinol (Lausanne) 3: 109.

73. Kojima M, Hosoda H, Date Y, Nakazato M, et al. 1999. Ghrelin is a growth-hormone-releasing acylated peptide from stomach. Nature 402: 656–60.

74. Nakazato M, Murakami N, Date Y, Kojima M, et al. 2001. A role for ghrelin in the central regulation of feeding. Nature 409: 194–8.

75. Kim JD, Leyva S, Diano S. 2014. Hormonal regulation of the hypothalamic melanocortin system. Front Physiol 5: 480.

76. Halaas JL, Gajiwala KS, Maffei M, Cohen SL, et al. 1995. Weight- reducing effects of the plasma protein encoded by the obese gene. Science 269: 543–6.

77. Wu Q, Zhang Y, Xu J, Shen P. 2005. Regulation of hunger-driven behaviors by neural ribosomal S6 kinase in Drosophila. Proc Natl Acad Sci USA 102: 13289–94.

78. Flint A, Raben A, Astrup A, Holst JJ. 1998. Glucagon-like peptide 1 promotes satiety and suppresses energy intake in humans. J Clin Invest 101: 515–20.

79. Knudsen LB, Secher A, Hecksher-Sorensen J, Pyke C. 2016. Long- acting glucagon-like peptide-1 receptor agonists have direct access to and effects on pro-opiomelanocortin/cocaine- and amphetamine- stimulated transcript neurons in the mouse hypothalamus. J Diabetes Investig 7: 56–63.

80. Hayes MR, De Jonghe BC, Kanoski SE. 2010. Role of the glucagon- like-peptide-1 receptor in the control of energy balance. Physiol Behav 100: 503–10.

81. Williams EK, Chang RB, Strochlic DE, Umans BD, et al. 2016. Sensory neurons that detect stretch and nutrients in the digestive system. Cell 166: 209–21.

82. Heinrichs SC, Menzaghi F, Pich EM, Hauger RL, et al. 1993. Corticotropin-releasing factor in the paraventricular nucleus modulates feeding induced by neuropeptide Y. Brain Res 611: 18–24.

83. Chen P, Hover CV, Lindberg D, Li C. 2012. Central urocortin 3 and type 2 corticotropin-releasing factor receptor in the regulation of energy homeostasis: critical involvement of the ventromedial hypothalamus. Front Endocrinol (Lausanne) 3: 180.

84. Richard D, Lin Q, Timofeeva E. 2002. The corticotropin-releasing factor family of peptides and CRF receptors: their roles in the regulation of energy balance. Eur J Pharmacol 440: 189–97.

85. Spina M, Merlo-Pich E, Chan RK, Basso AM, et al. 1996. Appetite- suppressing effects of urocortin, a CRF-related neuropeptide. Science 273: 1561–4.

86. Graham M, Shutter JR, Sarmiento U, Sarosi I, et al. 1997. Overexpression of Agrt leads to obesity in transgenic mice. Nat Genet 17: 273–4.

87. Huszar D, Lynch CA, Fairchild-Huntress V, Dunmore JH, et al. 1997. Targeted disruption of the melanocortin-4 receptor results in obesity in mice. Cell 88: 131–41.

88. Tatemoto K, Carlquist M, Mutt V. 1982. Neuropeptide Y-a novel brain peptide with structural similarities to peptide YY and pancreatic polypeptide. Nature 296: 659–60.

89. Clark JT, Kalra PS, Crowley WR, Kalra SP. 1984. Neuropeptide Y and human pancreatic polypeptide stimulate feeding behavior in rats. Endocrinology 115: 427–9.

90. Sohn JW. 2015. Network of hypothalamic neurons that control appetite. BMB Rep 48: 229–33.

91. Batterham RL, Cowley MA, Small CJ, Herzog H, et al. 2002. Gut hormone PYY(3-36) physiologically inhibits food intake. Nature 418: 650–4.

92. Hara J, Beuckmann CT, Nambu T, Willie JT, et al. 2001. Genetic ablation of orexin neurons in mice results in narcolepsy, hypophagia, and obesity. Neuron 30: 345–54.

93. Sakurai T. 1999. Orexins and orexin receptors: implication in feeding behavior. Regul Pept 85: 25–30.

94. Ludwig DS, Mountjoy KG, Tatro JB, Gillette JA, et al. 1998. Melanin- concentrating hormone: a functional melanocortin antagonist in the hypothalamus. Am J Physiol 274: E627–33.

95. Eberle AN. 1988. The Melanotropins. Chemistry, Physiology, and Mechanism of Action. Basel: Karger.

96. Parker JA, Bloom SR. 2012. Hypothalamic neuropeptides and the regulation of appetite. Neuropharmacology 63: 18–30.

97. Li Z, Kelly L, Heiman M, Greengard P, et al. 2015. Hypothalamic amylin acts in concert with leptin to regulate food intake. Cell Metab 22: 1059–67.

98. Mietlicki-Baase EG, Olivos DR, Jeffrey BA, Hayes MR. 2015. Cooperative interaction between leptin and amylin signaling in the ventral tegmental area for the control of food intake. Am J Physiol Endocrinol Metab 308: E1116–22.

99. Morley JE, Flood JF. 1991. Amylin decreases food intake in mice. Peptides 12: 865–9.

100. Mietlicki-Baase EG, Hayes MR. 2014. Amylin activates distributed CNS nuclei to control energy balance. Physiol Behav 136: 39–46.

101. Kristensen P, Judge ME, Thim L, Ribel U, et al. 1998. Hypothalamic CART is a new anorectic peptide regulated by leptin. Nature 393: 72–6.

102. Bechtold DA, Ivanov TR, Luckman SM. 2009. Appetite-modifying actions of pro-neuromedin U-derived peptides. Am J Physiol Endocrinol Metab 297: E545–51.

103. Brighton PJ, Szekeres PG, Willars GB. 2004. Neuromedin U and its receptors: structure, function, and physiological roles. Pharmacol Rev 56: 231–48.

104. Lee G, Park JH. 2004. Hemolymph sugar homeostasis and starvation- induced hyperactivity affected by genetic manipulations of the adipokinetic hormone-encoding gene in Drosophila melanogaster. Genetics 167: 311–23.

105. Chen J, Reiher W, Hermann-Luibl C, Sellami A, et al. 2016. Allatostatin A signalling in Drosophila regulates feeding and sleep and is modulated by PDF. PLoS Genet 12: e1006346.

106. Bader R, Colomb J, Pankratz B, Schrock A, et al. 2007. Genetic dissection of neural circuit anatomy underlying feeding behavior in Drosophila: distinct classes of hugin-expressing neurons. J Comp Neurol 502: 848–56.

N. Jourjine Prospects & Overviews....

1600261 (10 of 11) Bioessays 39, 5, 1600261, � 2017 WILEY Periodicals, Inc.

R e v ie w

e s s a y s

107. Schlegel P, Texada MJ, Miroschnikow A, Schoofs A, et al. 2016. Synaptic transmission parallels neuromodulation in a central food- intake circuit. Elife 5: e16799.

108. Lee KS, You KH, Choo JK, Han YM, et al. 2004. Drosophila short neuropeptide F regulates food intake and body size. J Biol Chem 279: 50781–9.

109. Al-Anzi B, Armand E, Nagamei P, Olszewski M, et al. 2010. The leucokinin pathway and its neurons regulate meal size in Drosophila. Curr Biol 20: 969–78.

110. Ren GR, Hauser F, Rewitz KF, Kondo S, et al. 2015. CCHamide-2 is an orexigenic brain-gut peptide in Drosophila. PLoS One 10: e0133017.

....Prospects & Overviews N. Jourjine

1600261 (11 of 11)Bioessays 39, 5, 1600261, � 2017 WILEY Periodicals, Inc.

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