Cellular identification of water gustatory receptor neurons and their central projection pattern in Drosophila

The matricellular protein Drosophila Cellular Communication Network Factor is required for synaptic transmission and female fertility

Within the extracellular matrix, matricellular proteins are dynamically expressed nonstructural proteins that interact with cell surface receptors, growth factors, and proteases, as well as with structural matrix proteins. The cellular communication network factors family of matricellular proteins serve regulatory roles to regulate cell function and are defined by their conserved multimodular organization. Here, we characterize the expression and neuronal requirement for the Drosophila cellular communication network factor family member. Drosophila cellular communication network factor is expressed in the nervous system throughout development including in subsets of monoamine-expressing neurons. Drosophila cellular communication network factor-expressing abdominal ganglion neurons innervate the ovaries and uterus and the loss of Drosophila cellular communication network factor results in reduced female fertility. In addition, Drosophila cellular communication network factor accumulates at the synaptic cleft and is required for neurotransmission at the larval neuromuscular junction. Analyzing the function of the single Drosophila cellular communication network factor family member will enhance our potential to understand how the microenvironment impacts neurotransmitter release in distinct cellular contexts and in response to activity.

Drosophila sensory receptors-a set of molecular Swiss Army Knives

Genetic approaches in the fruit fly, Drosophila melanogaster, have led to a major triumph in the field of sensory biology-the discovery of multiple large families of sensory receptors and channels. Some of these families, such as transient receptor potential channels, are conserved from animals ranging from worms to humans, while others, such as "gustatory receptors," "olfactory receptors," and "ionotropic receptors," are restricted to invertebrates. Prior to the identification of sensory receptors in flies, it was widely assumed that these proteins function in just one modality such as vision, smell, taste, hearing, and somatosensation, which includes thermosensation, light, and noxious mechanical touch. By employing a vast combination of genetic, behavioral, electrophysiological, and other approaches in flies, a major concept to emerge is that many sensory receptors are multitaskers. The earliest example of this idea was the discovery that individual transient receptor potential channels function in multiple senses. It is now clear that multitasking is exhibited by other large receptor families including gustatory receptors, ionotropic receptors, epithelial Na+ channels (also referred to as Pickpockets), and even opsins, which were formerly thought to function exclusively as light sensors. Genetic characterizations of these Drosophila receptors and the neurons that express them also reveal the mechanisms through which flies can accurately differentiate between different stimuli even when they activate the same receptor, as well as mechanisms of adaptation, amplification, and sensory integration. The insights gleaned from studies in flies have been highly influential in directing investigations in many other animal models.

Aedes aegypti Ir8a mutant female mosquitoes show increased attraction to standing water

The detection of water sources is crucial for insects such as mosquitoes to avoid desiccation and survive. In addition, mosquitoes use humidity cues to successfully navigate the environment to find a suitable oviposition site. Previous studies have implicated some members of the ionotropic receptor family in humidity sensing by Drosophila. Here, we investigate if IR8a co-receptor mediates water detection in Aedes aegypti mosquitoes. Using a simple behavioral assay, we examined the attraction of Ir8a mutant mosquitoes to standing water. Ir8a mutant mosquitoes were able to discriminate between traps containing water and those without as well as wild-type and heterozygous control females. Surprisingly, the female mutants were more robustly drawn to standing water than control mosquitoes. Further investigation revealed that the increased behavioral attraction to water is likely not mediated by a metabolic need or an activity defect.

A complex peripheral code for salt taste in Drosophila

Each taste modality is generally encoded by a single, molecularly defined, population of sensory cells. However, salt stimulates multiple taste pathways in mammals and insects, suggesting a more complex code for salt taste. Here, we examine salt coding in Drosophila. After creating a comprehensive molecular map comprised of five discrete sensory neuron classes across the fly labellum, we find that four are activated by salt: two exhibiting characteristics of ‘low salt’ cells, and two ‘high salt’ classes. Behaviorally, low salt attraction depends primarily on ‘sweet’ neurons, with additional input from neurons expressing the ionotropic receptor IR94e. High salt avoidance is mediated by ‘bitter’ neurons and a population of glutamatergic neurons expressing Ppk23. Interestingly, the impact of these glutamatergic neurons depends on prior salt consumption. These results support a complex model for salt coding in flies that combinatorially integrates inputs from across cell types to afford robust and flexible salt behaviors.

Salt is essential for our survival, but too much can kill us. Our taste system has therefore evolved two different pathways to help us maintain balance. Low concentrations (like the salt on our chips) activate a pathway that makes us want to eat. But high concentrations (like the salt in seawater) activate pathways that do the opposite.

The nervous system takes on the role of detecting salt and encoding the information in a way that the brain can use. One specific type of cell detects each of the four other tastes: sweet, bitter, sour, and umami. But salt, with its two sensing pathways, is the exception to this rule. Previous work has examined salt taste responses in flies, but the picture is incomplete.

In flies, one type of taste neuron uses a different signaling mechanism to the others, suggesting that it might play a special role. So here, Jaeger, Stanley et al. asked how fly sensory cells encode salt information for the brain, and what those unusual neurons are for.

Mapping the taste receptor neurons in the tongue-like structure of the fly, the proboscis, revealed that salt information is not restricted to one or two types of cell. In fact, all five types of neurons tested (covering more than 90% of all the taste neurons present in flies) responded to salt in some way. Of these, two ‘low salt’ cell types made the fly want to eat salt, and two ‘high salt’ cell types made the fly want to avoid it. One of these high salt cell types was the unusual taste neuron identified previously. Rather than always encoding high salt as 'bad', the message from this type of cell changed depending on the diet of the fly. Salt-deprived flies ignored the activity of that cell type altogether. This complex way of encoding taste allowed the fly to change its behavior depending on how much salt it needed.

This work opens new questions, like how do the fly's neuronal circuits process this complex salt code? And how do the ‘high salt’ cells achieve their negative effect only when the need for salt is low? Understanding more about this system could lead to a better understanding of why our own brains enjoy salty foods so much.

Internal amino acid state modulates yeast taste neurons to support protein homeostasis in Drosophila

To optimize fitness, animals must dynamically match food choices to their current needs. For drosophilids, yeast fulfills most dietary protein and micronutrient requirements. While several yeast metabolites activate known gustatory receptor neurons (GRNs) in Drosophila melanogaster, the chemosensory channels mediating yeast feeding remain unknown. Here we identify a class of proboscis GRNs required for yeast intake. Within this class, taste peg GRNs are specifically required to sustain yeast feeding. Sensillar GRNs, however, mediate feeding initiation. Furthermore, the response of yeast GRNs, but not sweet GRNs, is enhanced following deprivation from amino acids, providing a potential basis for protein-specific appetite. Although nutritional and reproductive states synergistically increase yeast appetite, reproductive state acts independently of nutritional state, modulating processing downstream of GRNs. Together, these results suggest that different internal states act at distinct levels of a dedicated gustatory circuit to elicit nutrient-specific appetites towards a complex, ecologically relevant protein source.

When animals run low on a certain nutrient, they change their behavior to seek out and feed on foods rich in that missing element. For example, fruit flies lacking sugar will look for and eat more sweet food; if they need proteins, they will instead favor yeast, flies’ principal source of proteins.

In fruit flies, certain neurons on the insects’ tongue (or proboscis) are dedicated only to taste. These cells are divided in groups specialized for a type of nutrient – for instance some of them only react to sugar. Taste neurons sense food and help coordinate how much and for how long the animals will feed. However, despite how important proteins are for flies, the neurons dedicated to tasting yeast had yet to be identified.

Here, Steck, Walker et al. report discovering a new set of taste neurons in fruit flies, which are activated by a unique combination of molecules present in yeast. Crucially, without these neurons being active, the insects can no longer adjust their diet to eat more yeast when they are deprived of proteins. The activity of these cells is also regulated by internal levels of nutrients derived from proteins.

The yeast-specific taste neurons are present in two areas on the fly’s proboscis, which is used like a straw when feeding. The two sets of cells have different roles in the consumption of yeast. The first group, which is located at the extremity of the proboscis, helps flies detect and start consuming the resource. The second group, which is on the inner surface of the proboscis, influences whether the insects keep feeding. If one of these groups of neurons is deactivated, flies continue to eat yeast as normal, showing that the system is redundant. However, if both sets are turned off artificially, the insects stop favoring yeast even when they are in need of proteins.

Steck, Walker et al. show how the animals’ internal states also influence the activity of these neurons. When the insects are deprived of molecules that are only found in proteins, these newly discovered neurons are primed to react more strongly when they are exposed to yeast. This potentially makes flies eat more yeast, and as a result consume more proteins.

Many biological systems in flies are similar in other insects and even humans. If this is the case for these taste neurons, fruit flies could be a good model to study how pests such as locusts and mosquitoes are attracted to the proteins in crops and blood, but also how humans make decisions about food.

Gustatory processing and taste memory in Drosophila

Taste allows animals to discriminate the value and potential toxicity of food prior to ingestion. Many tastants elicit an innate attractive or avoidance response that is modifiable with nutritional state and prior experience. A powerful genetic tool kit, well-characterized gustatory system, and standardized behavioral assays make the fruit fly, Drosophila melanogaster, an excellent system for investigating taste processing and memory. Recent studies have used this system to identify the neural basis for acquired taste preference. These studies have revealed a role for dopamine-mediated plasticity of the mushroom bodies that modulate the threshold of response to appetitive tastants. The identification of neural circuitry regulating taste memory provides a system to study the genetic and physiological processes that govern plasticity within a defined memory circuit.

A simple high throughput assay to evaluate water consumption in the fruit fly

Water intake is essential for survival and thus under strong regulation. Here, we describe a simple high throughput system to monitor water intake over time in Drosophila. The design of the assay involves dehydrating fly food and then adding water back separately so flies either eat or drink. Water consumption is then evaluated by weighing the water vessel and comparing this back to an evaporation control. Our system is high throughput, does not require animals to be artificially dehydrated, and is simple both in design and implementation. Initial characterisation of homeostatic water consumption shows high reproducibility between biological replicates in a variety of experimental conditions. Water consumption was dependent on ambient temperature and humidity and was equal between sexes when corrected for mass. By combining this system with the Drosophila genetics tools, we could confirm a role for ppk28 and DopR1 in promoting water consumption, and through functional investigation of RNAseq data from dehydrated animals, we found DopR1 expression in the mushroom body was sufficient to drive consumption and enhance water taste sensitivity. Together, we provide a simple high throughput water consumption assay that can be used to dissect the cellular and molecular machinery regulating water homeostasis in Drosophila.

Pharyngeal stimulation with sugar triggers local searching behavior in Drosophila

Foraging behavior is essential for all organisms to find food containing nutritional chemicals. A hungry Drosophila melanogaster fly performs local searching behavior after drinking a small amount of sugar solution. Using video tracking, we examined how the searching behavior is regulated in D. melanogaster We found that a small amount of highly concentrated sugar solution induced a long-lasting searching behavior. After the intake of sugar solution, a fly moved around in circles and repeatedly returned to the position where the sugar droplet had been placed. The non-nutritious sugar d-arabinose, but not the non-sweet nutritious sugar d-sorbitol, was effective in inducing the behavior, indicating that sweet sensation is essential. Furthermore, pox-neuro mutant flies, which have no external taste bristles, showed local searching behavior, suggesting the involvement of the pharyngeal taste organ. Experimental activation of pharyngeal sugar-sensitive gustatory receptor neurons by capsaicin using the GAL4/UAS system induced local searching behavior. In contrast, inhibition of pharyngeal sugar-responsive gustatory receptor neurons abolished the searching behavior. Together, our results indicate that, in Drosophila, the pharyngeal taste-receptor neurons trigger searching behavior immediately after ingestion.

Salt controls feeding decisions in a blood-sucking insect

Salts are necessary for maintaining homeostatic conditions within the body of all living organisms. Like with all essential nutrients, deficient or excessive ingestion of salts can result in adverse health effects. The taste system is a primary sensory modality that helps animals to make adequate feeding decisions in terms of salt consumption. In this work we show that sodium and potassium chloride salts modulate the feeding behavior of Rhodnius prolixus in a concentration-dependent manner. Feeding is only triggered by an optimal concentration of any of these salts (0.1-0.15M) and in presence of the phagostimulant ATP. Conversely, feeding solutions that do not contain salts or have a high-salt concentration (>0.3M) are not ingested by insects. Notably, we show that feeding decisions of insects cannot be explained as an osmotic effect, because they still feed over hyperosmotic solutions bearing the optimal salt concentration. Insects perceive optimal-salt, no-salt and high-salt solutions as different gustatory information, as revealed the electromyogram recordings of the cibarial pump. Moreover, because insects do a continuous gustatory monitoring of the incoming food during feeding, sudden changes beyond the optimal sodium concentration decrease and even inhibit feeding. The administration of amiloride, a sodium channel blocker, noticeably reduces the ingestion of the optimal sodium solution but not of the optimal potassium solution. Salt detection seems to occur at least through two salt receptors, one amiloride-sensitive and another amiloride-insensitive. Our results confirm the importance of the gustatory system in R. prolixus, showing the relevant role that salts play on their feeding decisions.

Mode of Action of the Natural Insecticide, Decaleside Involves Sodium Pump Inhibition

Decalesides are a new class of natural insecticides which are toxic to insects by contact via the tarsal gustatory chemosensilla. The symptoms of their toxicity to insects and the rapid knockdown effect suggest neurotoxic action, but the precise mode of action and the molecular targets for decaleside action are not known. We have presented experimental evidence for the involvement of sodium pump inhibition in the insecticidal action of decaleside in the cockroach and housefly. The knockdown effect of decaleside is concomitant with the in vivo inhibition of Na+, K+ -ATPase in the head and thorax. The lack of insecticidal action by experimental ablation of tarsi or blocking the tarsal sites with paraffin correlated with lack of inhibition of Na+- K+ ATPase in vivo. Maltotriose, a trisaccharide, partially rescued the toxic action of decaleside as well as inhibition of the enzyme, suggesting the possible involvement of gustatory sugar receptors. In vitro studies with crude insect enzyme preparation and purified porcine Na+, K+ -ATPase showed that decaleside competitively inhibited the enzyme involving the ATP binding site. Our study shows that the insecticidal action of decaleside via the tarsal gustatory sites is causally linked to the inhibition of sodium pump which represents a unique mode of action. The precise target(s) for decaleside in the tarsal chemosensilla and the pathway linked to inhibition of sodium pump and the insecticidal action remain to be understood.

Functional Gustatory Role of Chemoreceptors in Drosophila Wings

Neuroanatomical evidence argues for the presence of taste sensilla in Drosophila wings; however, the taste physiology of insect wings remains hypothetical, and a comprehensive link to mechanical functions, such as flight, wing flapping, and grooming, is lacking. Our data show that the sensilla of the Drosophila anterior wing margin respond to both sweet and bitter molecules through an increase in cytosolic Ca(2+) levels. Conversely, genetically modified flies presenting a wing-specific reduction in chemosensory cells show severe defects in both wing taste signaling and the exploratory guidance associated with chemodetection. In Drosophila, the chemodetection machinery includes mechanical grooming, which facilitates the contact between tastants and wing chemoreceptors, and the vibrations of flapping wings that nebulize volatile molecules as carboxylic acids. Together, these data demonstrate that the Drosophila wing chemosensory sensilla are a functional taste organ and that they may have a role in the exploration of ecological niches.

cGMP Signalling Mediates Water Sensation (Hydrosensation) and Hydrotaxis in Caenorhabditis elegans

Animals have developed the ability to sense the water content in their habitats, including hygrosensation (sensing humidity in the air) and hydrosensation (sensing the water content in other microenvironments), and they display preferences for specific water contents that influence their mating, reproduction and geographic distribution. We developed and employed four quantitative behavioural test paradigms to investigate the molecular and cellular mechanisms underlying sensing the water content in an agar substrate (hydrosensation) and hydrotaxis in Caenorhabditis elegans. By combining a reverse genetic screen with genetic manipulation, optogenetic neuronal manipulation and in vivo Ca²⁺ imaging, we demonstrate that adult worms avoid the wetter areas of agar plates and hypo-osmotic water droplets. We found that the cGMP signalling pathway in ciliated sensory neurons is involved in hydrosensation and hydrotaxis in Caenorhabditis elegans.

A gustatory second-order neuron that connects sucrose-sensitive primary neurons and a distinct region of the gnathal ganglion in the Drosophila brain

Although the gustatory system provides animals with sensory cues important for food choice and other critical behaviors, little is known about neural circuitry immediately following gustatory sensory neurons (GSNs). Here, we identify and characterize a bilateral pair of gustatory second-order neurons (G2Ns) in Drosophila. Previous studies identified GSNs that relay taste information to distinct subregions of the primary gustatory center (PGC) in the gnathal ganglia (GNG). To identify candidate G2Ns, we screened ∼5,000 GAL4 driver strains for lines that label neural fibers innervating the PGC. We then combined GRASP (GFP reconstitution across synaptic partners) with presynaptic labeling to visualize potential synaptic contacts between the dendrites of the candidate G2Ns and the axonal terminals of Gr5a-expressing GSNs, which are known to respond to sucrose. Results of the GRASP analysis, followed by a single-cell analysis by FLP-out recombination, revealed a pair of neurons that contact Gr5a axon terminals in both brain hemispheres and send axonal arborizations to a distinct region outside the PGC but within the GNG. To characterize the input and output branches, respectively, we expressed fluorescence-tagged acetylcholine receptor subunit (Dα7) and active-zone marker (Brp) in the G2Ns. We found that G2N input sites overlaid GRASP-labeled synaptic contacts to Gr5a neurons, while presynaptic sites were broadly distributed throughout the neurons' arborizations. GRASP analysis and further tests with the Syb-GRASP method suggested that the identified G2Ns receive synaptic inputs from Gr5a-expressing GSNs, but not Gr66a-expressing GSNs, which respond to caffeine. The identified G2Ns relay information from Gr5a-expressing GSNs to distinct regions in the GNG, and are distinct from other, recently identified gustatory projection neurons, which relay information about sugars to a brain region called the antennal mechanosensory and motor center (AMMC). Our findings suggest unexpected complexity for taste information processing in the first relay of the gustatory system.

Molecular neurobiology of Drosophila taste

Drosophila is a powerful model in which to study the molecular and cellular basis of taste coding. Flies sense tastants via populations of taste neurons that are activated by compounds of distinct categories. The past few years have borne witness to studies that define the properties of taste neurons, identifying functionally distinct classes of sweet and bitter taste neurons that express unique subsets of gustatory receptor (Gr) genes, as well as water, salt, and pheromone sensing neurons that express members of the pickpocket (ppk) or ionotropic receptor (Ir) families. There has also been significant progress in terms of understanding how tastant information is processed and conveyed to higher brain centers, and modulated by prior dietary experience or starvation.

A novel assay reveals hygrotactic behavior in Drosophila

Humidity is one of the most important factors that determines the geographical distribution and survival of terrestrial animals. The ability to detect variation in humidity is conserved across many species. Here, we established a novel behavioral assay that revealed the thirsty Drosophila exhibits strong hygrotactic behavior, and it can locate water by detecting humidity gradient. In addition, exposure to high levels of moisture was sufficient to elicit proboscis extension reflex behavior in thirsty flies. Furthermore, we found that the third antennal segment was necessary for hygrotactic behavior in thirsty flies, while arista was required for the avoidance of moist air in hydrated flies. These results indicated that two types of hygroreceptor cells exist in Drosophila: one located in the third antennal segment that mediates hygrotactic behavior in thirst status, and the other located in arista which is responsible for the aversive behavior toward moist air in hydration status. Using a neural silencing screen, we demonstrated that synaptic output from the mushroom body α/β surface and posterior neurons was required for both hygrotactic behavior and moisture-aversive behavior.

Secondary taste neurons that convey sweet taste and starvation in the Drosophila brain

The gustatory system provides vital sensory information to determine feeding and appetitive learning behaviors. Very little is known, however, about higher-order gustatory circuits in the highly tractable model for neurobiology, Drosophila melanogaster. Here we report second-order sweet gustatory projection neurons (sGPNs) in the Drosophila brain using a powerful behavioral screen. Silencing neuronal activity reduces appetitive behaviors, whereas inducible activation results in food acceptance via proboscis extensions. sGPNs show functional connectivity with Gr5a(+) sweet taste neurons and are activated upon sucrose application to the labellum. By tracing sGPN axons, we identify the antennal mechanosensory and motor center (AMMC) as an immediate higher-order processing center for sweet taste. Interestingly, starvation increases sucrose sensitivity of the sGPNs in the AMMC, suggesting that hunger modulates the responsiveness of the secondary sweet taste relay. Together, our results provide a foundation for studying gustatory processing and its modulation by the internal nutrient state.

The Drosophila IR20a clade of ionotropic receptors are candidate taste and pheromone receptors

Insects use taste to evaluate food, hosts, and mates. Drosophila has many "orphan" taste neurons that express no known taste receptors. The Ionotropic Receptor (IR) superfamily is best known for its role in olfaction, but virtually nothing is known about a clade of ∼35 members, the IR20a clade. Here, a comprehensive analysis of this clade reveals expression in all taste organs of the fly. Some members are expressed in orphan taste neurons, whereas others are coexpressed with bitter- or sugar-sensing Gustatory receptor (Gr) genes. Analysis of the closely related IR52c and IR52d genes reveals signatures of adaptive evolution, roles in male mating behavior, and sexually dimorphic expression in neurons of the male foreleg, which contacts females during courtship. These neurons are activated by conspecific females and contact a neural circuit for sexual behavior. Together, these results greatly expand the repertoire of candidate taste and pheromone receptors in the fly.

Peripheral coding of taste

Five canonical tastes, bitter, sweet, umami (amino acid), salty, and sour (acid), are detected by animals as diverse as fruit flies and humans, consistent with a near-universal drive to consume fundamental nutrients and to avoid toxins or other harmful compounds. Surprisingly, despite this strong conservation of basic taste qualities between vertebrates and invertebrates, the receptors and signaling mechanisms that mediate taste in each are highly divergent. The identification over the last two decades of receptors and other molecules that mediate taste has led to stunning advances in our understanding of the basic mechanisms of transduction and coding of information by the gustatory systems of vertebrates and invertebrates. In this Review, we discuss recent advances in taste research, mainly from the fly and mammalian systems, and we highlight principles that are common across species, despite stark differences in receptor types.

The neuronal and molecular basis of quinine-dependent bitter taste signaling in Drosophila larvae

The sensation of bitter substances can alert an animal that a specific type of food is harmful and should not be consumed. However, not all bitter compounds are equally toxic and some may even be beneficial in certain contexts. Thus, taste systems in general may have a broader range of functions than just in alerting the animal. In this study we investigate bitter sensing and processing in Drosophila larvae using quinine, a substance perceived by humans as bitter. We show that behavioral choice, feeding, survival, and associative olfactory learning are all directly affected by quinine. On the cellular level, we show that 12 gustatory sensory receptor neurons that express both GR66a and GR33a are required for quinine-dependent choice and feeding behavior. Interestingly, these neurons are not necessary for quinine-dependent survival or associative learning. On the molecular receptor gene level, the GR33a receptor, but not GR66a, is required for quinine-dependent choice behavior. A screen for gustatory sensory receptor neurons that trigger quinine-dependent choice behavior revealed that a single GR97a receptor gene expressing neuron located in the peripheral terminal sense organ is partially necessary and sufficient. For the first time, we show that the elementary chemosensory system of the Drosophila larva can serve as a simple model to understand the neuronal basis of taste information processing on the single cell level with respect to different behavioral outputs.

Acid sensing by sweet and bitter taste neurons in Drosophila melanogaster

Drosophila melanogaster can taste various compounds and separate them into few basic categories such as sweet, bitter and salt taste. Here we investigate mechanisms underlying acid detection in Drosophila and report that the fly displays strong taste aversion to common carboxylic acids. We find that acid tastants act by the activation of a subset of bitter neurons and inhibition of sweet neurons. Bitter neurons begin to respond at pH 5 and show an increase in spike frequency as the extracellular pH drops, which does not rely on previously identified chemoreceptors. Notably, sweet neuron activity depends on the balance of sugar and acid tastant concentrations. This is independent of bitter neuron firing, and allows the fly to avoid acid-laced food sources even in the absence of functional bitter neurons. The two mechanisms may allow the fly to better evaluate the risk of ingesting acidic foods and modulate its feeding decisions accordingly.

The dilemmas of the gourmet fly: the molecular and neuronal mechanisms of feeding and nutrient decision making in Drosophila

To survive and successfully reproduce animals need to maintain a balanced intake of nutrients and energy. The nervous system of insects has evolved multiple mechanisms to regulate feeding behavior. When animals are faced with the choice to feed, several decisions must be made: whether or not to eat, how much to eat, what to eat, and when to eat. Using Drosophila melanogaster substantial progress has been achieved in understanding the neuronal and molecular mechanisms controlling feeding decisions. These feeding decisions are implemented in the nervous system on multiple levels, from alterations in the sensitivity of peripheral sensory organs to the modulation of memory systems. This review discusses methodologies developed in order to study insect feeding, the effects of neuropeptides and neuromodulators on feeding behavior, behavioral evidence supporting the existence of internal energy sensors, neuronal and molecular mechanisms controlling protein intake, and finally the regulation of feeding by circadian rhythms and sleep. From the discussed data a conceptual framework starts to emerge which aims to explain the molecular and neuronal processes maintaining the stability of the internal milieu.

Decaleside: a new class of natural insecticide targeting tarsal gustatory sites

Natural sources for novel insecticide molecules hold promise in view of their eco-friendly nature, selectivity, and mammalian safety. Recent progress in understanding the biology of insect olfaction and taste offers new strategies for developing selective pest control agents. We have isolated two natural insecticidal molecules from edible roots of Decalepis hamiltonii named Decalesides I and II, which are novel trisaccharides, highly toxic to household insect pests and stored-product insects. We have experimentally shown that insecticidal activity requires contact with tarsi on the legs but is not toxic orally. The insecticidal activity of molecules is lost by hydrolysis, and various sugars modify toxic response, showing that the insecticidal activity is via gustatory sites on the tarsi. Selective toxicity to insects by virtue of their gustatory site of action and the mammalian safety of the new insecticides is inherent in their chemical structure with 1-4 or 1-1 α linkage that is easily hydrolyzed by digestive enzymes of mammals. Decalesides represent a new chemical class of natural insecticides with a unique mode of action targeting tarsal chemosensory/gustatory system of insects.

Nutritional value-dependent and nutritional value-independent effects on Drosophila melanogaster larval behavior

Gustatory stimuli allow an organism not only to orient in its environment toward energy-rich food sources to maintain nutrition but also to avoid unpleasant or even poisonous substrates. For both mammals and insects, sugars-perceived as "sweet"-potentially predict nutritional benefit. Interestingly, even Drosophila adult flies are attracted to most high-potency sweeteners preferred by humans. However, the gustatory information of a sugar may be misleading as some sugars, although perceived as "sweet," cannot be metabolized. Accordingly, in adult Drosophila, a postingestive system that additionally evaluates the nutritional benefit of an ingested sugar has been shown to exist. By using a set of seven different sugars, which either offer (fructose, sucrose, glucose, maltodextrin, and sorbitol) or lack (xylose and arabinose) nutritional benefit, we show that Drosophila, at the larval stage, can perceive and evaluate sugars based on both nutrition-dependent and -independent qualities. In detail, we find that larval survival and feeding mainly depend on the nutritional value of a particular sugar. In contrast, larval choice behavior and learning are regulated in a more complex way by nutrition value-dependent and nutrition value-independent information. The simplicity of the larval neuronal circuits and their accessibility to genetic manipulation may ultimately allow one to identify the neuronal and molecular basis of the larval sugar perception systems described here behaviorally.

Simple ways to measure behavioral responses of Drosophila to stimuli and use of these methods to characterize a novel mutant

The behavioral responses of adult Drosophila fruit flies to a variety of sensory stimuli – light, volatile and non-volatile chemicals, temperature, humidity, gravity, and sound - have been measured by others previously. Some of those assays are rather complex; a review of them is presented in the Discussion. Our objective here has been to find out how to measure the behavior of adult Drosophila fruit flies by methods that are inexpensive and easy to carry out. These new assays have now been used here to characterize a novel mutant that fails to be attracted or repelled by a variety of sensory stimuli even though it is motile.

How to identify water from thickener aqueous solutions by touch

Water detection is one of the most crucial psychological processes for many animals. However, nobody knows the perception mechanism of water through our tactile sense. In the present study, we found that a characteristic frictional stimulus with large acceleration is one of the cues to differentiate water from water contaminated with thickener. When subjects applied small amounts of water to a glass plate, strong stick-slip phenomena with a friction force of 0.46 ± 0.30 N and a vertical force of 0.57 ± 0.36 N were observed at the skin surface, as shown in previous studies. Surprisingly, periodic shears with acceleration seven times greater than gravitational acceleration occurred during the application process. Finite-element analyses predicted that these strong stimuli could activate tactile receptors: Meissner's corpuscle and Pacinians. When such stimuli were applied to the fingertips by an ultrasonic vibrator, a water-like tactile texture was perceived by some subjects, even though no liquid was present between the fingertip and the vibrator surface. These findings could potentially be applied in the following areas: materials science, information technology, medical treatment and entertainment.

Identification of a novel gene, anorexia, regulating feeding activity via insulin signaling in Drosophila melanogaster

Feeding activities of animals, including insects, are influenced by various signals from the external environment, internal energy status, and physiological conditions. Full understanding of how such signals are integrated to regulate feeding activities has, however, been hampered by a lack of knowledge about the genes involved. Here, we identified an anorexic Drosophila melanogaster mutant (GS1189) in which the expression of a newly identified gene, Anorexia (Anox), is mutated. In Drosophila larvae, Anox encodes an acyl-CoA binding protein with an ankyrin repeat domain that is expressed in the cephalic chemosensory organs and various neurons in the central nervous system (CNS). Loss of its expression or disturbance of neural transmission in Anox-expressing cells decreased feeding activity. Conversely, overexpression of Anox in the CNS increased food intake. We further found that Anox regulates expression of the insulin receptor gene (dInR); overexpression and knockdown of Anox in the CNS, respectively, elevated and repressed dInR expression, which altered larval feeding activity in parallel with Anox expression levels. Anox mutant adults also showed significant repression of sugar-induced nerve responses and feeding potencies. Although Anox expression levels did not depend on the fasting and feeding states cycle, stressors such as high temperature and desiccation significantly repressed its expression levels. These results strongly suggest that Anox is essential for gustatory sensation and food intake of Drosophila through regulation of the insulin signaling activity that is directly regulated by internal nutrition status. Therefore, the mutant strain lacking Anox expression cannot enhance feeding potencies even under starvation.

Neural architecture of the primary gustatory center of Drosophila melanogaster visualized with GAL4 and LexA enhancer-trap systems

Gustatory information is essential for animals to select edible foods and avoid poisons. Whereas mammals detect tastants with their taste receptor cells, which convey gustatory signals to the brain indirectly via the taste sensory neurons, insect gustatory receptor neurons (GRNs) send their axons directly to the primary gustatory center in the suboesophageal ganglion (SOG). In spite of this relatively simple architecture, the precise structure of the insect primary gustatory center has not been revealed in enough detail. To obtain comprehensive anatomical knowledge about this brain area, we screened the Drosophila melanogaster GAL4 enhancer-trap strains that visualize specific subsets of the gustatory neurons as well as putative mechanosensory neurons associated with the taste pegs. Terminals of these neurons form three branches in the SOG. To map the positions of their arborization areas precisely, we screened newly established LexA::VP16 enhancer-trap strains and obtained a driver line that labels a large subset of peripheral sensory neurons. By double-labeling specific and landmark neurons with GAL4 and LexA strains, we were able to distinguish 11 zones in the primary gustatory center, among which 5 zones were identified newly in this study. Arborization areas of various known GRNs on the labellum, oesophagus, and legs were also mapped in this framework. The putative mechanosensory neurons terminate exclusively in three zones of these areas, supporting the notion of segregated primary centers that are specialized for chemosensory and mechanosensory signals associated with gustatory sensation.

Water as an independent taste modality

To qualify as a "basic" taste quality or modality, defined as a group of chemicals that taste alike, three empirical benchmarks have commonly been used. The first is that a candidate group of tastants must have a dedicated transduction mechanism in the peripheral nervous system. The second is that the tastants evoke physiological responses in dedicated afferent taste nerves innervating the oropharyngeal cavity. Last, the taste stimuli evoke activity in central gustatory neurons, some of which may respond only to that group of tastants. Here we argue that water may also be an independent taste modality. This argument is based on the identification of a water dedicated transduction mechanism in the peripheral nervous system, water responsive fibers of the peripheral taste nerves and the observation of water responsive neurons in all gustatory regions within the central nervous system. We have described electrophysiological responses from single neurons in nucleus of the solitary tract (NTS) and parabrachial nucleus of the pons, respectively the first two central relay nuclei in the rodent brainstem, to water presented as a taste stimulus in anesthetized rats. Responses to water were in some cases as robust as responses to other taste qualities and sometimes occurred in the absence of responses to other tastants. Both excitatory and inhibitory responses were observed. Also, the temporal features of the water response resembled those of other taste responses. We argue that water may constitute an independent taste modality that is processed by dedicated neural channels at all levels of the gustatory neuraxis. Water-dedicated neurons in the brainstem may constitute key elements in the regulatory system for fluid in the body, i.e., thirst, and as part of the swallowing reflex circuitry.

Avoiding DEET through insect gustatory receptors

DEET is the most widely used insect repellent worldwide. In Drosophila olfactory receptor neurons (ORNs), DEET is detected through a mechanism employing the olfactory receptor, OR83b. However, it is controversial as to whether ORNs respond directly to DEET or whether DEET blocks the response to attractive odors. Here, we showed that DEET suppressed feeding behavior in Drosophila, and this effect was mediated by gustatory receptor neurons (GRNs). DEET was potent in suppressing feeding as <0.1% DEET elicited aversive behavior. Inhibition of feeding required multiple gustatory receptors (GRs) expressed in inhibitory GRNs. DEET stimulated action potentials in GRNs that respond to aversive compounds, and this response was lost in the Gr32a, Gr33a, and Gr66a mutants. Since 0.02% DEET elicited action potentials, we conclude that DEET directly activates of GRNs. We suggest that the effectiveness of DEET in pest control owes to its dual action in inducing avoidance simultaneously via GRNs and ORNs.

The amiloride-sensitive epithelial Na+ channel PPK28 is essential for drosophila gustatory water reception

Water sensation is a specific taste modality in the fruit fly. Water-induced hypoosmolarity activates specific gustatory receptor neurons; however, the molecular identity of the putative osmolarity sensor in these neurons remains unknown. We found that amiloride and its analogs specifically antagonized the response of water gustatory receptor neurons and the behavior of flies toward water stimulation. Deletion of the gene that encodes the amiloride-sensitive PPK28 channel, a DEG/eNaC (degenerin/epithelial sodium channel) family member, abolished the water-induced activity of water gustatory receptor neurons and greatly diminished the behavioral response of flies to water. Ectopic expression of the PPK28 channel in the bitter cells within the intermediate-type sensilla renders these sensilla responsive to water stimuli. Thus, the amiloride-sensitive PPK28 channel may serve as the osmolarity sensor for gustatory water reception in the fruit fly.

A housefly sensory-motor integration laboratory

Insects have many interesting behaviors that can be observed in an introductory biology laboratory setting. In the present article, we describe several reflexes using the housefly Musca domestica that can be used to introduce students to sensory and motor responses and encourage them to think about the underlying neural circuits and integration of sensory information that mediate the behaviors.

Flying the fly: long-range flight behavior of Drosophila melanogaster to attractive odors

The fruit fly, Drosophila melanogaster Meigen (Diptera: Drosophilidae), is a model for how animals sense, discriminate, and respond to chemical signals. However, with D. melanogaster our knowledge of the behavioral activity of olfactory receptor ligands has relied largely on close-range attraction, rather than on long-range orientation behavior. We developed a flight assay to relate chemosensory perception to behavior. Headspace volatiles from vinegar attracted 62% of assayed flies during a 15-min experimental period. Flies responded irrespective of age, sex, and mating state, provided they had been starved. To identify behaviorally relevant chemicals from vinegar, we compared the responses to vinegar and synthetic chemicals. Stimuli were applied by a piezoelectric sprayer at known and constant release rates. Re-vaporized methanol extracts of Super Q-trapped vinegar volatiles attracted as many flies as vinegar. The main volatile component of vinegar, acetic acid, elicited significant attraction as a single compound. Two other vinegar volatiles, 2-phenyl ethanol and acetoin, produced a synergistic effect when added to acetic acid. Geosmin, a microbiological off-flavor, diminished attraction to vinegar. This wind tunnel assay based on a conspicuous and unambiguous behavioral response provides the necessary resolution for the investigation of physiologically and ecologically relevant odors and will become an essential tool for the functional analysis of the D. melanogaster olfactory system.

The molecular basis for water taste in Drosophila

The detection of water and the regulation of water intake are essential for animals to maintain proper osmotic homeostasis1. Drosophila and other insects have gustatory sensory neurons that mediate the recognition of external water sources2-4, but little is known about the underlying molecular mechanism for water taste detection. Here, we identify a member of the Degenerin/Epithelial Sodium Channel family5, ppk28, as an osmosensitive ion channel that mediates the cellular and behavioral response to water. We use molecular, cellular, calcium imaging and electrophysiological approaches to show that ppk28 is expressed in water-sensing neurons and loss of ppk28 abolishes water sensitivity. Moreover, ectopic expression of ppk28 confers water sensitivity to bitter-sensing gustatory neurons in the fly and sensitivity to hypo-osmotic solutions when expressed in heterologous cells. These studies link an osmosensitive ion channel to water taste detection and drinking behavior, providing the framework for examining the molecular basis for water detection in other animals.

Common sense about taste: from mammals to insects

The sense of taste is a specialized chemosensory system dedicated to the evaluation of food and drink. Despite the fact that vertebrates and insects have independently evolved distinct anatomic and molecular pathways for taste sensation, there are clear parallels in the organization and coding logic between the two systems. There is now persuasive evidence that tastant quality is mediated by labeled lines, whereby distinct and strictly segregated populations of taste receptor cells encode each of the taste qualities.

Plant insecticide L-canavanine repels Drosophila via the insect orphan GPCR DmX

An orphan G-protein-coupled gustatory receptor mediates detection of the plant poison L-canavanine in fruit flies.

For all animals, the taste sense is crucial to detect and avoid ingesting toxic molecules. Many toxins are synthesized by plants as a defense mechanism against insect predation. One example of such a natural toxic molecule is l-canavanine, a nonprotein amino acid found in the seeds of many legumes. Whether and how insects are informed that some plants contain l-canavanine remains to be elucidated. In insects, the taste sense relies on gustatory receptors forming the gustatory receptor (Gr) family. Gr proteins display highly divergent sequences, suggesting that they could cover the entire range of tastants. However, one cannot exclude the possibility of evolutionarily independent taste receptors. Here, we show that l-canavanine is not only toxic, but is also a repellent for Drosophila. Using a pharmacogenetic approach, we find that flies sense food containing this poison by the DmX receptor. DmXR is an insect orphan G-protein–coupled receptor that has partially diverged in its ligand binding pocket from the metabotropic glutamate receptor family. Blockade of DmXR function with an antagonist lowers the repulsive effect of l-canavanine. In addition, disruption of the DmXR encoding gene, called mangetout (mtt), suppresses the l-canavanine repellent effect. To avoid the ingestion of l-canavanine, DmXR expression is required in bitter-sensitive gustatory receptor neurons, where it triggers the premature retraction of the proboscis, thus leading to the end of food searching. These findings show that the DmX receptor, which does not belong to the Gr family, fulfills a gustatory function necessary to avoid eating a natural toxin.

Plants evolve to fend off the insects that attack them, often by synthesizing compounds toxic to insects. In turn, insects develop strategies to avoid these plants or resist their toxins. Some plant toxins are nonprotein amino acids. For example, seeds from numerous legumes contain high amounts of l-canavanine, a nonprotein amino acid that is structurally related to l-arginine and is highly toxic to most insects. How insects can detect l-canavanine remains to be elucidated. Using pharmacology, genetics, and behavioral approaches, we show that flies sense l-canavanine using the receptor DmX, an orphan G-protein–coupled receptor that has diverged in its ligand binding pocket from metabotropic glutamate receptors. Disruption of the DmXR gene, called mangetout (mtt), suppresses the l-canavanine repellent effect. DmXR is expressed and required in aversive gustatory receptor neurons, where it triggers the premature retraction of the proboscis, thus leading to the end of food searching. Our results indicate a mechanism by which some insects may detect and avoid a plant toxin.

Water taste transduction pathway is calcium dependent in Drosophila

In mammals, detection of osmolarity by the gustatory system was overlooked until recently. In insects, specific taste receptor neurons detect hypoosmotic stimuli and are commonly called "W" (water) cells. W cells are easy to access in vivo and represent a good model to study the transduction of hypoosmotic stimuli. Using pharmacological and genetic approaches in Drosophila, we show that tarsal W cell firing activity depends on the concentration of external calcium bathing the dendrite. This dependence was confirmed by the strong inhibition of W cell responses to hypoosmotic stimuli by lanthanum (IC(50) = 8 nM), an ion known to inhibit calcium-permeable channels. Downstream, the transduction pathway likely involves calmodulin because calmodulin antagonists such as W-7 (IC(50) = 2 microM) and fluphenazine (IC(50) = 30 microM) prevented the activation of the W cell by hypoosmotic stimuli. A protein kinase C (PKC) may also be involved as W cell responses were blocked by PKC inhibitors, chelerythrine (IC(50) = 20 microM) and staurosporine (IC(50) = 30 microM). It was also reduced when expressing an inhibitory pseudosubstrate of PKC in gustatory receptor neurons. In the rat, the transduction pathway underlying low osmolarity detection involves aquaporin and swelling-activated ion channels. Our study suggests that the transduction pathway of hypoosmotic stimuli in insects differs from mammals.

Motor control in a Drosophila taste circuit

Tastes elicit innate behaviors critical for directing animals to ingest nutritious substances and reject toxic compounds, but the neural basis of these behaviors is not understood. Here, we use a neural silencing screen to identify neurons required for a simple Drosophila taste behavior and characterize a neural population that controls a specific subprogram of this behavior. By silencing and activating subsets of the defined cell population, we identify the neurons involved in the taste behavior as a pair of motor neurons located in the subesophageal ganglion (SOG). The motor neurons are activated by sugar stimulation of gustatory neurons and inhibited by bitter compounds; however, experiments utilizing split-GFP detect no direct connections between the motor neurons and primary sensory neurons, indicating that further study will be necessary to elucidate the circuitry bridging these populations. Combined, these results provide a general strategy and a valuable starting point for future taste circuit analysis.

Smelling, tasting, learning: Drosophila as a study case

Understanding brain function is to account for how the sensory system is integrated with the organism's needs to organize behaviour. We review what is known about these processes with regard to chemosensation and chemosensory learning in Drosophila. We stress that taste and olfaction are organized rather differently. Given that, e.g., sugars are nutrients and should be eaten (irrespective of the kind of sugar) and that toxic substances should be avoided (regardless of the kind of death they eventually cause), tastants are classified into relatively few behavioural matters of concern. In contrast, what needs to be done in response to odours is less evolutionarily determined. Thus, discrimination ability is warranted between different kinds of olfactory input, as any difference between odours may potentially be or become important. Therefore, the olfactory system has a higher dimensionality than gustation, and allows for more sensory-motor flexibility to attach acquired behavioural 'meaning' to odours. We argue that, by and large, larval and adult Drosophila are similar in these kinds of architecture, and that additionally there are a number of similarities to vertebrates, in particular regarding the cellular architecture of the olfactory pathway, the functional slant of the taste and smell systems towards classification versus discrimination, respectively, and the higher plasticity of the olfactory sensory-motor system. From our point of view, the greatest gap in understanding smell and taste systems to date is not on the sensory side, where indeed impressive advances have been achieved; also, a satisfying account of associative odour-taste memory trace formation seems within reach. Rather, we lack an understanding as to how sensory and motor formats of processing are centrally integrated, and how adaptive motor patterns actually are selected. Such an understanding, we believe, will allow the analysis to be extended to the motivating factors of behaviour, eventually leading to a comprehensive account of those systems which make Drosophila do what Drosophila's got to do.

Chemical sensing in Drosophila

Chemical sensing begins when peripheral receptor proteins recognise specific environmental stimuli and translate them into spatial and temporal patterns of sensory neuron activity. The chemosensory system of the fruit fly, Drosophila melanogaster, has become a dominant model to understand this process, through its accessibility to a powerful combination of molecular, genetic and electrophysiological analysis. Recent results have revealed many surprises in the biology of peripheral chemosensation in Drosophila, including novel structural and signalling properties of the insect odorant receptors (ORs), combinatorial mechanisms of chemical recognition by the gustatory receptors (GRs), and the implication of Transient Receptor Potential (TRP) ion channels as a novel class of chemosensory receptors.

The detection of carbonation by the Drosophila gustatory system

There are five known taste modalities in humans: sweet, bitter, sour, salty and umami (the taste of monosodium glutamate). Although the fruitfly Drosophila melanogaster tastes sugars, salts and noxious chemicals, the nature and number of taste modalities in this organism is not clear. Previous studies have identified one taste cell population marked by the gustatory receptor gene Gr5a that detects sugars, and a second population marked by Gr66a that detects bitter compounds. Here we identify a novel taste modality in this insect: the taste of carbonated water. We use a combination of anatomical, calcium imaging and behavioural approaches to identify a population of taste neurons that detects CO2 and mediates taste acceptance behaviour. The taste of carbonation may allow Drosophila to detect and obtain nutrients from growing microorganisms. Whereas CO2 detection by the olfactory system mediates avoidance, CO2 detection by the gustatory system mediates acceptance behaviour, demonstrating that the context of CO2 determines appropriate behaviour. This work opens up the possibility that the taste of carbonation may also exist in other organisms.

Molecular Architecture of Smell and Taste inDrosophila

The chemical senses-smell and taste-allow animals to evaluate and distinguish valuable food resources from dangerous substances in the environment. The central mechanisms by which the brain recognizes and discriminates attractive and repulsive odorants and tastants, and makes behavioral decisions accordingly, are not well understood in any organism. Recent molecular and neuroanatomical advances in Drosophila have produced a nearly complete picture of the peripheral neuroanatomy and function of smell and taste in this insect. Neurophysiological experiments have begun to provide insight into the mechanisms by which these animals process chemosensory cues. Given the considerable anatomical and functional homology in smell and taste pathways in all higher animals, experimental approaches in Drosophila will likely provide broad insights into the problem of sensory coding. Here we provide a critical review of the recent literature in this field and comment on likely future directions.

20-hydroxyecdysone deters oviposition and larval feeding in the European grapevine moth, Lobesia botrana

European grapevine moth females (Lobesia botrana, Lepidoptera Tortricidae) select an oviposition site by tasting the host plant surface and then gluing a single egg on berries from grapes or from several other host plant species. In doing so, females should avoid ovipositing on plants that are detrimental to their progeny. Do they sense the same deterrent compounds as larvae, despite the fact that they do not have access to the same compartments of the plants? We tested this hypothesis with 20-hydroxyecdysone (20E), purified from Leuzea carthamoides. Phytoecdysteroids are usually found inside plant tissues and accessible to larvae in an aqueous phase, while adults would access them only through the epicuticular wax. We first confirmed that larvae avoid feeding on 20E and that they taste 20E with their lateral sensilla styloconica, at a threshold of 10(-6) M. Then, we tested whether adult females avoid ovipositing on glass spheres sprayed with 20E. When given a choice, females avoided laying eggs on a treated surface, at a threshold of 8 ng/cm(2). In addition, they deposited significantly fewer eggs in the presence of 20E. Presuming that legs play an important role in assessing the oviposition substrate, we assessed the sensitivity of their taste receptors. In females, 14 taste sensilla are located on the ventral side of the last tarsus of the prothoracic leg. One group of these sensilla house one neuron that is sensitive to 20E, with a detection threshold of about 10(-7) M. The same molecule is thus sensed both in larvae and adults of L. botrana where it respectively inhibits feeding and oviposition.