Feeding, hunger, satiety and serotonin in invertebrates

Abstract

The serotonergic modulation of feeding behaviour has been intensively studied in several invertebrate groups, including Arthropoda, Annelida, Nematoda and Mollusca. These studies offer comparative information on feeding regulation across divergent phyla and also provide general insights into the neural control of feeding. Specifically, model invertebrates are ideal for parsing feeding behaviour into component parts and examining the underlying mechanisms at the levels of biochemical pathways, single cells and identified neural circuitry. Research has found that serotonin is crucial during certain phases of feeding behaviour, especially movements directly underlying food intake, but inessential during other phases. In addition, while the serotonin system can be manipulated systemically in many animals, invertebrate model organisms also allow manipulations at the level of single cells and molecules, revealing limited and precise serotonergic actions. The latter highlight the importance of local versus global modulatory effects of serotonin, a potentially significant consideration for drug and pesticide design.

1. Introduction

Feeding is a complex behaviour common to all animals, and regulated by environmental circumstances and internal physiological factors. To survive, animals must integrate external and internal cues, and behave in ways that maintain energy homeostasis and ensure intake of correct nutrients. This integration relies on numerous neurotransmitters and neurohormones, including the biogenic amines serotonin, dopamine, norepinephrine (in vertebrates), and tyramine and octopamine (in invertebrates). Serotonin (5-HT), one of the most ancient and widely distributed neurotransmitters, has been especially well studied in the context of feeding behaviour, and the serotonergic system is the focus of drugs that seek to treat human eating disorders. In invertebrates, understanding the control of feeding behaviour is also of great importance. Many species directly harm human health by acting as parasites and disease vectors and many other species critically impact global agriculture, either by destroying crops and livestock or serving as essential pollinators. In addition, Drosophila melanogaster and Caenorhabditis elegans are highly advantageous model organisms used to gain new understanding of processes central to biology and neuroscience.

In both vertebrates and invertebrates, 5-HT-containing cells are limited in number, but have axons that project widely, allowing 5-HT to have multiple effects in the central nervous system and periphery [1]. These effects are mediated by an array of 5-HT receptors which in mammals include 14 subtypes organized into seven families [2]. Despite the evolutionary distance between mammals and invertebrates, the serotonergic system shows conservation at the molecular level, and identified invertebrate 5-HT receptors are homologous to mammalian subtypes in the 5-HT₁, 5-HT₂, 5-HT₄, 5-HT₆ and 5-HT₇ families [3]. Functionally, 5-HT serves as a neurotransmitter and neurohormone in both vertebrates and invertebrates, and regulates many of the same processes, though the nature of the regulation may vary. In the case of food-related processes, elevated 5-HT is generally thought to suppress feeding behaviour and serve as a satiety signal in mammals [4–6]. 5-HT has been reported to have opposite effects in invertebrates, serving to promote feeding behaviour [1,4,7]. However, as discussed below, the latter conclusion does not apply to all invertebrate species or all phases of feeding.

While 5-HT is often characterized as having global effects on feeding, searching for and consuming food is not a single behaviour, but a sequence of discrete actions each of which can possess its own neural underpinning. The number of actions and their sequence varies among species, but generally they may be categorized into two phases: appetitive and consummatory. Appetitive behaviour includes foraging, typically involving locomotion, food detection, and slowing or stopping of locomotion. Consummatory behaviour includes meal initiation and the ingestion of food. Ingestion is driven most directly by central pattern generator circuits that cause rhythmic movements of muscles that take in and swallow food. Both phases of feeding behaviour are related to satiety and hunger, complex motivational and behavioural states that are distinct from actual food intake. Satiety can be defined as a state that is a consequence of food intake and that inhibits eating behaviour for a period of time. Hunger is a state that exists prior to eating and that promotes food search and ingestion; a hungry animal is ‘ready to feed’ [8].

This paper examines the serotonergic control of feeding behaviour in four well-studied invertebrate phyla with the aim of elucidating the role of 5-HT in hunger, food intake and satiety. The review includes studies that examine behaviour following systemic administration of 5-HT and also those that investigate the actions of 5-HT in neural circuits and in limited subpopulations of neurons. In focusing on behaviour, this review does not cover all 5-HT-regulated physiological processes related to food intake, digestion, and metabolism. Also, decades of research have produced an expansive and detailed literature on the neural control of invertebrate feeding, and review articles cited in each section provide additional information on feeding behaviour in each phylum.

2. Arthropoda

In the phylum Arthropoda, studies on 5-HT and feeding have focused on insects. Many studies have systemically altered 5-HT actions using pharmacological agents or genetic manipulations and determined if feeding behaviour and/or food intake was affected positively or negatively. Long & Murdock [9] reported that serotonin injections in blowflies (Phormia regina) reduced sucrose consumption, whereas octopaminergic drugs increased consumption. The drug dl-fenfluramine, which suppresses appetite in mammals by elevating synaptic serotonin, decreased blowfly proboscis extension to sucrose, though it had no effect on meal size [10]. In the flesh fly Neobellieria bullata, injections of 5-HT decreased the time spent feeding and amount of food consumed [11]. In aphids (Myzus persicae), Kaufmann et al. [12] found that injected 5-HT suppressed feeding and had synergistic effects when combined with pymetrozine, an insecticide which kills plant-sucking insects by feeding suppression. 5-HT and pymetrozine also acted synergistically on neural tissue in locusts (Locusta migratoria) and 5-HT antagonists blocked the actions of pymetrozine, suggesting that the feeding-disruptive effects of the insecticide are mediated by 5-HT pathways [12]. 5-HT injections decreased carbohydrate intake and overall food consumption in the cockroach Rhyparobia madera [13] and decreased protein intake in P. regina [14]. Orally administered 5-HT depressed food intake in ants (Camponotus mus) by reducing the volume of sucrose solution ingested with each contraction of the feeding pump apparatus [15]. In honeybees (Apis mellifera), 5-HT inhibited the proboscis extension response (PER) [16] and direct application of 5-HT to the brain suppressed food intake [17]. Blocking 5-HT reuptake decreased feeding behaviour in fed, but not in food-deprived mosquito (Aedes aegypti) larvae [18]. Consistent with an association between 5-HT and satiety, elevated hemolymph 5-HT occurred following sucrose feeding in N. bullata [11] and blood feeding in A. aegypti [19] and Rhodnius prolixus [20].

The above studies generally support the idea that elevated 5-HT acts to suppress feeding in insects. However, systemic effects are complicated and affected by type of food consumed [13], nutritional state [18], trophic effects [21,22] and anatomical site of 5-HT injection [17]. In addition, some data indicate that 5-HT depletion suppresses food intake. In R. prolixus, injection of the neurotoxin 5,7-DHT, which depleted 5-HT in peripheral but not central neurons, led to decreased blood intake [23]. Depletion of 5-HT in the mosquito Aedes triseriatus did not alter meal-seeking behaviour, but resulted in reduced feeding success and lower blood intake [24].

In Drosophila, genetic and molecular tools, including the Gal4-UAS system, give researchers precise control of gene expression, allowing the actions of neurotransmitters to be manipulated both systemically and in targeted cells. Many studies have examined the neural control of feeding in Drosophila, and it is clear that 5-HT acts within a complex array of neuromodulators and neuropeptides [25–28]. Systemically, the actions of 5-HT can be regulated by altering synthesis, release, reuptake and receptor properties. For example, Neckameyer [22] altered 5-HT levels systemically by acute inhibition of neuronal serotonin synthesis via knockdown of Drosophila tryptophan hydroxylase (DTRH) and observed an increased feeding rate in larvae; elevated 5-HT levels via induced expression of DTRH and oral administration of 5-hydroxytryptophan (5-HTP) depressed feeding. Using a similar approach, Pooryasin & Fiala [29] found that acute, global release of 5-HT decreased feeding and food intake in adult flies, and Eriksson et al. [30] reported that suppression of 5-HT release in all 5-HT neurons caused flies to increase food intake. In contrast to these results which suggest an anorexigenic effect of 5-HT, Majeed et al. [31] found that pharmacological and genetic manipulations of 5-HT activity produced inconsistent effects on larvae feeding behaviour, and Gasque et al. [32] reported that the 5-HT receptor antagonist methiothepin acted via the 5-HT_2A receptor to depress larval feeding behaviour and food intake. Also, activation of 5-HT neurons enhanced the cycle frequency of motor programmes for feeding and increased food intake in larvae [33]. While this effect occurred when all 5-HT neurons were activated, it was dependent on four neuronal clusters located in the subesophageal zone [34].

The variable effects of systemic 5-HT are unsurprising in the light of studies on targeted neurons. For example, feeding behaviour was promoted by activating a subset of 5-HT neurons in each hemisphere of the fly brain, suggesting that these few neurons encode hunger [35]. However, the same study found that global activation of 5-HT neurons suppressed feeding in both sated and starved flies. Similarly, Pooryasin & Fiala [29] demonstrated that simultaneous activation of most 5-HT neurons suppressed arousal-like behaviour, including locomotion, feeding and mating. They further determined that activity in just two pairs of neurons was sufficient to inhibit locomotion and that the same neurons modulated some, but not all, components of mating behaviour. These particular neurons did not affect feeding at all, indicating that inhibition of arousal was behaviour-specific and controlled by local serotonergic subsystems. Xu et al. [36] found that the innate attraction of flies to ethanol could be suppressed by acute global elevation of 5-HT levels via oral 5-HTP, but the examination of targeted cells revealed much more nuanced effects of 5-HT. Blockade of 5-HT reuptake in four brain neurons inhibited attraction to ethanol, whereas blockade of reuptake in two neurons, the deutocerebral (CSD) cells, counteracted this inhibition by enhancing sensory input to antennal lobes and promoting ethanol attraction. In a final example, mildly starved flies displayed a preference for protein consumption found to be dependent upon 5-HT signalling via the 5-HT_2A receptor [37]. This effect was highly specific since 5-HT did not mediate other food preferences or protein preference in fed animals.

Studies such as these give tantalizing hints of intricacies in the serotonergic modulation of feeding and highlight the importance of local networks in regulating discrete aspects of food-related behaviour. The characterization of 5-HT as a global organizer of hunger or satiety in insects is premature and may ultimately not be applicable. Additional complexity is apparent in studies describing 5-HT interactions with olfaction [38], food-related learning and memory [39], nutrient choice [40] and other neuromodulatory systems that contribute to the control of feeding [25,26]. Clearly much remains to be learned about 5-HT interactions with other neurochemicals and behaviours in feeding regulation. Also, general conclusions about arthropods are limited by a lack of information on species other than insects. Especially in crustaceans, many studies describe the importance of 5-HT in behaviour, and the crustacean stomatogastric nervous system is a premier model for explicating the cellular basis of neuromodulator function [41]. However, studies have not specifically addressed the role of 5-HT in mediating feeding behaviour, hunger and satiety in non-insect arthropods.

3. Annelida

Feeding in the leech is a classic model for understanding serotonergic effects on behaviour. The relatively simple and accessible leech nervous system has allowed the cellular basis of feeding and other behaviour to be examined in great detail [42–45]. Most studies have focused on Hirudo medicinalis or Hirudo verbana, species that consume 8–10 times their own weight in blood at a single meal and can remain satiated for a year or more. Numerous experiments conducted in the 1980s demonstrated that 5-HT-mediated a sequence of feeding-related behaviours and physiological processes, including orientating and swimming towards a food source, locating a warm surface, ceasing swimming, biting the skin of a mammalian host, secreting saliva and mucus, pumping blood through the pharynx into the gut, softening the body wall, suppressing tactile stimuli that might disrupt food intake, and sensing body wall distention eventually leading to meal termination [46].

Appetitive behaviours and food intake occurred when 5-HT was bath applied or released in the periphery from endogenous sources. The paired large lateral (LL) cells located in the subesophageal ganglion and the huge Retzius cells, a pair of which occur in each segmental ganglia, were found to contain high levels of 5-HT and identified as most directly involved in releasing 5-HT to initiate food search and intake. Moreover, food-relevant sensory input from thermal or chemoreceptors on the mouthparts excited the Retzius neurons [47], and feeding caused depletion of 5-HT within these cells [42]. Satiety and anorexic behaviour occurred when these cells were inhibited by input from mechanoreceptors that detect body wall distention [48] and behavioural effects of distention were mimicked by 5-HT depletion via application of the toxin 5,7-DHT [49]. The latter indicates that satiation and persistent satiety is associated with reduced 5-HT activity and hence the amine does not signal satiety in the usual sense of being associated with elevated 5-HT levels or activity [4,5].

The compelling evidence assembled by Lent and his colleagues was supported by many additional studies and led to the conclusion that 5-HT was both necessary and sufficient to orchestrate all phases of feeding behaviour from food search to prolonged satiety [42]. However, 5-HT necessarily exists within a matrix of additional neurotransmitters and neuropeptides that may contribute significantly to feeding regulation. For example, Wilson et al. [50] found that Retzius and Leydig neurons were both active during food search and biting behaviours, but that firing in Retzius cells decreased directly after blood ingestion began and remained low throughout consumption. Leydig neurons, on the other hand, increased firing with the onset of consumption and activity remained high throughout the meal. Leydig neurons contain the peptide myomodulin, but not serotonin [51], suggesting that this peptide may contribute to the maintenance of food intake during a feeding bout. Mesce et al. [52] reported that the LL cells which were previously thought to provide direct serotonergic control of the mouthparts instead innervated the five ganglia of the stomatogastric nervous system (STN). The STN, hypothesized to contain the central pattern generators (CPGs) that drive rhythmic biting and blood pumping, adds a layer of circuitry to the feeding system that may be modulated by dopamine [52] and the peptide FMRFamide [53] in addition to 5-HT.

Despite extensive information on 5-HT modulation of individual neurons and circuits, most data on behaviour derive from systemic manipulations of the amine and relatively little is known about local effects of 5-HT on the phases of feeding behaviour. Yet local effects as well as ancillary neuromodulators must be critical in determining how 5-HT is able to control a wide range of activities, some of which appear contradictory such as feeding and swimming that cannot occur simultaneously [43]. Specificity in 5-HT actions has been observed with applications of 5-HT to limited regions of the nervous system [54] or cells within a ganglion [55], but researchers have lacked the genetic tools needed for targeted manipulation of neuronal function in behaving animals. New technologies such as CRISPR and expanding information on the leech genome [56] should make molecular analyses and increasingly precise examination of 5-HT effects possible. Comparative information is also important to establish the generality of 5-HT effects and gain insight into the evolution of 5-HT modulation. In carnivorous leeches, 5-HT depletion was found to reduce food intake [57] and, compared to sanguivores, carnivores were more responsive to mechanical stimuli while feeding and more likely to abandon a meal when touched [44]. However, serotonergic effects on feeding have yet to be examined in the vast majority of leech species, and information is lacking on 5-HT and feeding in other members of the phylum.

4. Nematoda

Among nematodes, research has focused on C. elegans, an organism ideal for genetic and molecular dissection of the nervous system. With its 302 neurons, the tiny worm tackles the common challenges of life: foraging, feeding, avoiding harm and reproducing. The role of 5-HT in the C. elegans feeding has been intensively studied, unveiling molecular details that attest to the neural complexity underlying food-related behaviour [58–60]. Early experiments discovered that exogenous 5-HT elicited distinct behavioural changes, including suppression of locomotion, and stimulation of egg laying and pharyngeal pumping [61]. The latter movement underlies feeding in which bacteria are pumped into the pharynx, and it is coupled to isthmus peristalsis which transports bacteria to the posterior pharynx. 5-HT is not required for basal levels of pharyngeal pumping since it occurs in tph-1 mutants which cannot synthesize 5-HT; however, the amine is required for rapid and regular pumping when worms encounter food [62,63], and faster pumping generally results in increased food intake [64]. When food supply is low, worm locomotion (roaming) is high, whereas abundant food induces a slowing of locomotion (dwelling), a switch mediated by 5-HT in food-deprived worms [65]. Hence, 5-HT release regulates several behaviours that promote feeding, including changing locomotory state and increasing the rate and regularity of pharyngeal pumping.

Feeding movements are most directly controlled by the 20 neuron pharyngeal nervous system. Key cells are the MC and M3 motor neurons which drive the rhythmic movements of pharyngeal muscles and the M4 neuron which drives peristalsis in the posterior isthmus. These neurons alone are sufficient for production of feeding movements, but the rhythms are fine-tuned by other cells and molecules, including 5-HT and five 5-HT receptor subtypes. Endogenous 5-HT is supplied by the NSM and ADF neurons, both of which synthesize the amine and release it in response to food cues. The paired ADF cells are amphid chemosensory neurons which are activated by the presence of desirable bacteria in the environment. Knockdown of TPH-1 specifically in ADF neurons impairs food-induced fast pumping, and expression of TPH-1 is sufficient to rescue this response in tph-1 null mutants [66]. The NSM neurons, located in the pharyngeal nervous system, release 5-HT to upregulate pumping in response to external chemical cues [67] and contribute to slowing of locomotion in response to food ingestion [68]. These results indicate that an important function of 5-HT in C. elegans is not to signal hunger or promote food-seeking, but rather to detect the presence of bacteria and induce behaviours required for optimal food intake.

While systemic effects of 5-HT appear consistent under standard laboratory conditions, they vary with external and internal circumstances, reflecting elaborate and shifting molecular events in the worm nervous system. For example, serotonergic neurons and their pre- and post-synaptic contacts create variable pumping rates in response to appealing and repellent odours [67], familiar and unfamiliar food [69], heat stress [70] and internal nutrient state [71]. As in other species, modulatory systems in addition to 5-HT contribute to food-related behaviour. Worms display a quiescent state following access to high-quality food or feeding after fasting in which they do not move or display pharyngeal pumping [72]. This state differs from roaming and dwelling (which includes pharyngeal pumping), and is thought to resemble post-prandial satiety in other species. Quiescent behaviour requires neuropeptide signalling through TGFβ and insulin pathways, and is dependent on nutritional status [73]. Switching among the three behavioural states of roaming, dwelling and quiescent, which presumably express changes in nutritional states from hunger to feeding to satiety, involves 5-HT and elaborate signalling among many additional cells and molecules.

Nematoda is a large phylum which includes free-living species that inhabit soil (like C. elegans) or water, and parasitic species. While differences in serotonergic cell number occur across groups [60,74], neuronal architecture among some species is conserved despite huge differences in size and ecology. Ascaris suum, an intestinal parasite that grows to be 30–40 cm long, has 298 neurons and a neuronal wiring pattern highly congruent with C. elegans [75]. The actions of 5-HT are likewise congruent in that the amine stimulates pharyngeal pumping [76]. 5-HT regulates feeding on bacteria and predation on other nematodes in Pristionchus pacificus [77], and activates rhythmic movements of the stylet in plant parasitic nematodes [60,78]. These studies indicate that 5-HT activates motor programmes that facilitate feeding in diverse nematodes, but less is known about mechanisms underlying hunger and satiety.

5. Mollusca

Much has been learned about the serotonergic modulation of feeding by studying gastropod molluscs, relatively large invertebrates with identifiable neurons and neural circuitry. Beginning in the 1970s, detailed electrophysiological and behavioural studies have described mechanisms of feeding in species from several genera including Aplysia, Lymnaea, Limax, Pleurobranchaea, Helisoma, Helix, Planorbis and Planorbarius [79–83]. Gastropod species display much variability in peripheral morphology and feeding mode, yet across many species homologous neurons and circuitry control feeding movements. The CPG and motor neurons directly responsible for feeding movements are located in the buccal ganglia and they produce rhythmic movements of the buccal muscles that bite and draw in food. The CPG receives input from many higher-order neurons in the cerebral ganglion, including bilateral giant serotonergic neurons. In all gastropods that have been examined, homologues of these giant 5-HT neurons are found, though they go by different names (e.g. Aplysia: MCC, metacerebral cell; Lymnaea: CGC, cerebral giant cell; Pleurobranchaea: MCG, metacerebral giant cell). They provide the major serotonergic innervation of the buccal CPG and peripheral feeding apparatus, including sensory neurons and musculature.

In intact gastropods, studies have found that giant 5-HT neuron activity and feeding behaviour are correlated. Kupfermann & Weiss [84] reported that the MCCs were silent in quiescent Aplysia californica, fired when mouthparts touched food, enhanced the rate and strength of food biting, and ceased to fire when animals became satiated and stopped eating. Lesions of the Aplysia MCCs reduced the rate of biting, though animals still displayed normal appetitive behaviours and food consumption [85]. In Lymnaea stagnalis, CGCs were also silent in quiescent animals and maximally active when feeding movements occurred [86]. At firing rates recorded in vivo, CGC activity did not initiate feeding, but enabled a full rhythm to occur and regulated its rate. Kemenes et al. [87] also found that depletion of 5-HT in intact Lymnaea increased the latency to respond to food and reduced the rate of feeding movements. As in Aplysia, appetitive behaviour such as exploratory movement in response to sucrose was not affected by 5-HT depletion. Extensive research on semi-intact, circuit and cellular level preparations generally supports these findings, demonstrating that giant cell stimulation or physiological levels of exogenous 5-HT act on neurons and muscles to facilitate buccal CPG output (e.g. [88,89]). Giant cell activity does not directly initiate or terminate CPG activity, suggesting that 5-HT functions as a modulator of feeding rhythms rather than a command element [81,83].

In Aplysia, the modulatory effects of MCC activity are limited in function and time. Food stimuli induce a general state of arousal, including excitation of MCCs. Should appealing food be available, MCC firing acts specifically to enable and potentiate the rhythmic movements that cause food intake, but different neurons control other aspects of food-induced arousal such as locomotion and head waving [81,85,90]. In Lymnaea, 5-HT also appears to act specifically on biting movements rather than feeding behaviour in general [87]. On a physiological level, many studies have shown that 5-HT acts with specificity on CPG neurons and buccal muscles, reflecting the precise actions of 5-HT on ion channels and second messenger pathways [91]. The timing of 5-HT modulation also displays specificity in that MCC activity declines after the onset of feeding and ongoing activity of the buccal CPG is sustained by neuropeptide release [88]. In other studies, researchers have described an array of modulators that act redundantly or sequentially with 5-HT to promote the initiation and continuation of gastropod feeding movements, including dopamine [92,93] and many neuropeptides [94].

Like other animals, gastropods display satiety after feeding during which they ignore or avoid food. In Pleurobranchaea californica, the level of 5-HT in MCGs is reduced fourfold in sated compared to hungry animals, a change that may contribute to food refusal by reducing 5-HT-dependent excitation of the buccal CPG [8]. If this occurs, 5-HT would not act as a typical satiety signal where elevated rather than diminished 5-HT activity induces feeding refusal. Termination of food intake in gastropods has also been shown to result from inhibition of the feeding CPG by mechanoreceptors that detect gut distention [1] and neuropeptide release that shifts the CPG from an ingestive to an egestive rhythm [95]. While 5-HT actions have not been ruled out, the amine does not appear to play a crucial role in feeding termination and satiety. It is also unclear if 5-HT acts to signal hunger if defined as a motivational state that exists prior to food detection and the onset of biting behaviour.

In the class Bivalvia, feeding-related movements are also regulated by 5-HT. Most bivalves use filter feeding in which food particles are captured in gills and transported by ciliary beating to the mouth. The cerebral and visceral ganglia supply the gill cilia with serotonergic innervation, and application of 5-HT to the gill has been shown to stimulate ciliary beating rates in many species, including oysters [96] and mussels [97]. These findings indicate that 5-HT modulation of rhythmic feeding movements may be widespread in the phylum, though information is not yet available for other molluscan classes.

6. Does 5-HT function in invertebrates to promote or suppress feeding behaviour?

The answer to this question depends on multiple factors, including how feeding behaviour is defined. While feeding is sometimes viewed as a single 5-HT-regulated behaviour, invertebrate systems reveal with great clarity that the neural control of appetitive and consummatory phases of feeding can be different, as can mechanisms that initiate and terminate feeding. In leeches, 5-HT release appears to control appetitive actions such as swimming, locating a host, and biting; however, Retzius cells cease to fire once consummatory feeding starts, suggesting that this phase is sustained by modulators other than 5-HT. By contrast, 5-HT stimulation of feeding in nematodes occurs primarily during the consummatory phase. 5-HT-containing neurons are activated by food detection and contribute to meal initiation by slowing locomotion and enhancing the rate of pharyngeal pumping. In gastropods, giant 5-HT neurons are likewise activated by food detection, and 5-HT serves to enable and potentiate activity in the feeding CPG. However, as in the leech, 5-HT giant cells cease to fire once consummatory feeding is under way, and hence the continuation of this phase, as well as appetitive behaviours prior to food intake, are not 5-HT-dependent. In insects, 5-HT stimulated consummatory feeding in Drosophila larvae by increasing the rate of pharyngeal pumping [34] and, in adult flies, excitation of specific 5-HT neurons induced food consumption [35]. The role of these 5-HT neurons in foraging is less certain since activation did not increase locomotion [35], a typical fly appetitive behaviour, but did contribute to yeast seeking by hungry flies in another study [98]. Appetitive behaviour was unaffected in mosquitos where depletion of 5-HT markedly reduced the ability to initiate and ingest a blood meal, while host-seeking activity was unimpaired [24]. These findings indicate that, rather than serving as an overall modulator of food-related behaviour, 5-HT is most important at specific phases of feeding when it does indeed promote food intake.

Feeding and food intake are typically considered in the context of hunger and satiety, motivational/behavioural states that are distinct from food consumption itself. In humans and other mammals, elevated serotonin is considered a hunger suppressant or a ‘satiety signal', a belief supporting the treatment of obesity with 5-HT-enhancing drugs [4,6,99]. Among invertebrates, 5-HT has been described as mediating either hunger or satiety, but the role of 5-HT in these processes is unclear. Since animals cannot describe feelings of hunger or satiety, motivational states must be inferred from behaviour. In the case of hunger, appetitive behaviour indicates that animals are ‘ready to feed', yet as noted above, appetitive behaviour appears to be mediated by non-5-HT mechanisms in some nematode, mollusk and insect species. Hunger may also be inferred from increased food intake, however, this may involve direct 5-HT effects on consummative motor programmes and hence is not unequivocal evidence for 5-HT-mediated hunger. Satiation and satiety are also distinct states expressed behaviourally as food refusal or avoidance. Also, post-feeding quiescence is believed to reflect satiety in mammals [100] and invertebrates [73,101]. As detailed above, 5-HT release occurs during feeding to modulate feeding CPGs and certain additional feeding-related processes, but definitive evidence that it causes meal termination and satiety is lacking. A role for 5-HT has not been ruled out, but thus far studies indicate that satiation and satiety are mediated mainly by neuropeptide networks and sensory mechanisms that detect gut distention.

In insects, systemic applications of exogenous 5-HT have yielded a variety of outcomes, including suppression of feeding in many cases. The inconsistent results may be due to actual species differences in 5-HT actions. Alternatively or in addition, responses to systemic 5-HT may reflect state-dependent factors and experimental conditions which can influence how events unfold in serotonergic and other interacting networks. State-dependence, including current activity and modulatory conditions in local neural networks, determines how networks respond to exogenous neurotransmitters at a particular point in time [102]. State-dependent effects may include those directly related to feeding since responses to neuromodulators can vary depending on current nutritional state [18]. Because feeding is regulated by numerous neurochemical networks in addition to 5-HT, and 5-HT modulates numerous behaviours and physiological processes in addition to feeding, the possibilities for state-dependent effects are nervous system wide.

Among the experimental conditions likely to affect behavioural outcomes, an important consideration is where and at what concentration 5-HT actually contacts receptors throughout the nervous system. Invertebrates possess multiple 5-HT receptor subtypes which differ in molecular structure, distribution and transduction mechanism [3,103]. Receptors may act antagonistically, for example, by activating or inhibiting cAMP production, and subtypes differ in their affinity for 5-HT. In Drosophila, Gasque et al. [32] determined that sensitivity to 5-HT varied by two orders of magnitude among five receptor subtypes expressed in HEK293 cells. The application of exogenous transmitter stimulates multiple receptor subtypes, and may induce non-specific effects on feeding such as malaise, lethargy or hyperactivity. These effects may not be obvious if food intake is the main or only outcome of 5-HT manipulations [100]. Also local effects of 5-HT probably determine behaviour and probably vary depending on how experimental protocols influence the sequence and strength of receptor subtype activation across the nervous system. While hidden when 5-HT is applied globally, these effects have been revealed with exquisite precision in Drosophila [29,35–37].

Effects of systemic neurotransmitter administration are inherently ambiguous, but they are important as a starting point for identifying neurochemicals that affect behaviour. They are also important as an ending point since much research returns to the systemic administration of chemicals to combat invertebrate pests whose feeding habits cause human diseases and extensive agricultural damage. Given the molecular tools available and relative simplicity of invertebrate nervous systems, conceivably research will eventually reveal the specific ways that serotonergic drugs act in the nervous system, even when they are applied globally. This task will be vastly more difficult in humans, though serotonergic drugs are widely administered and there is a great need to understand their actions. In the case of obesity, drugs have been approved to reduce appetite/induce satiety, but actual effects on food-related motivation and behaviour are confounded with other central nervous system effects and numerous peripheral effects on energy expenditure and metabolism [5,6,99]. While deemed clinically useful, drug effectiveness varies widely and even positive outcomes on weight loss are modest [104]. With the recent removal of the 5-HT_2C agonist Lorcaserin from the market, the search is on for new drugs to regulate food intake in humans. In this difficult work, information from invertebrate systems offers some insights into the nature of serotonergic regulation of feeding, indicating that while effects of serotonergic drugs may be diverse and widespread, effects specifically related to feeding behaviour may be limited and local.

Data accessibility

This article has no additional data.

Competing interests

I declare I have no competing interests.

Funding

This study was supported by Colgate Research Council.