Two antagonistic gustatory receptor neurons responding to sweet-salty and bitter taste inDrosophila

Electrophysiological Characterization of Responses from Gustatory Receptor Neurons of sensilla chaetica in the Moth Heliothis virescens

Discrimination of edible and noxious food is crucial for survival in all organisms. We have studied the physiology of the gustatory receptor neurons (GRNs) in contact chemosensilla (insect gustatory organs) located on the antennae of the moth Heliothis virescens, emphasizing putative phagostimulants and deterrents. Sucrose and the 2 bitter substances quinine and sinigrin elicited responses in a larger proportion of GRNs than inositol, KCl, NaCl, and ethanol, and the firing thresholds were lowest for sucrose and quinine. Variations in GRN composition in individual sensilla occurred without any specific patterns to indicate specific sensillum types. Separate neurons showed excitatory responses to sucrose and the 2 bitter substances quinine and sinigrin, implying that the moth might be able to discriminate bitter substances in addition to separating phagostimulants and deterrents. Besides being detected by separate receptors on the moth antennae, the bitter tastants were shown to have an inhibitory effect on phagostimulatory GRNs. Sucrose was highly appetitive in behavioral studies of proboscis extension, whereas quinine had a nonappetitive effect in the moths.

An inhibitory sex pheromone tastes bitter for Drosophila males

Sexual behavior requires animals to distinguish between the sexes and to respond appropriately to each of them. In Drosophila melanogaster, as in many insects, cuticular hydrocarbons are thought to be involved in sex recognition and in mating behavior, but there is no direct neuronal evidence of their pheromonal effect. Using behavioral and electrophysiological measures of responses to natural and synthetic compounds, we show that Z-7-tricosene, a Drosophila male cuticular hydrocarbon, acts as a sex pheromone and inhibits male-male courtship. These data provide the first direct demonstration that an insect cuticular hydrocarbon is detected as a sex pheromone. Intriguingly, we show that a particular type of gustatory neurons of the labial palps respond both to Z-7-tricosene and to bitter stimuli. Cross-adaptation between Z-7-tricosene and bitter stimuli further indicates that these two very different substances are processed by the same neural pathways. Furthermore, the two substances induced similar behavioral responses both in courtship and feeding tests. We conclude that the inhibitory pheromone tastes bitter to the fly.

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.

Taste and pheromone perception in the fruit fly Drosophila melanogaster

Taste is an essential sense for detection of nutrient-rich food and avoidance of toxic substances. The Drosophila melanogaster gustatory system provides an excellent model to study taste perception and taste-elicited behaviors. "The fly" is unique in the animal kingdom with regard to available experimental tools, which include a wide repertoire of molecular-genetic analyses (i.e., efficient production of transgenics and gene knockouts), elegant behavioral assays, and the possibility to conduct electrophysiological investigations. In addition, fruit flies, like humans, recognize sugars as a food source, but avoid bitter tasting substances that are often toxic to insects and mammals alike. This paper will present recent research progress in the field of taste and contact pheromone perception in the fruit fly. First, we shall describe the anatomical properties of the Drosophila gustatory system and survey the family of taste receptors to provide an appropriate background. We shall then review taste and pheromone perception mainly from a molecular genetic perspective that includes behavioral, electrophysiological and imaging analyses of wild type flies and flies with genetically manipulated taste cells. Finally, we shall provide an outlook of taste research in this elegant model system for the next few years.

A Taste Receptor Required for the Caffeine Response In Vivo

Caffeine is a methylxanthine present in the coffee tree, tea plant, and other naturally occurring sources and is among the most commonly consumed drugs worldwide. Whereas the pharmacological action of caffeine has been studied extensively, relatively little is known concerning the molecular mechanism through which this substance is detected as a bitter compound. Unlike most tastants, which are detected through cell-surface G protein-coupled receptors, it has been proposed that caffeine and related methylxanthines activate taste-receptor cells through inhibition of a cyclic nucleotide phosphodiesterase (PDE) . Here, we show that the gustatory receptor Gr66a is expressed in the dendrites of Drosophila gustatory receptor neurons and is essential for the caffeine response. In a behavioral assay, the aversion to caffeine was specifically disrupted in flies missing Gr66a. Caffeine-induced action potentials were also eliminated, as was the response to theophylline, the methylxanthine in tea. The Gr66a mutant exhibited normal tastant-induced action potentials upon presentation of theobromine, a methylxanthine in cocoa. Given that theobromine and caffeine inhibit PDEs with equal potencies , these data further support the role of Gr66a rather than a PDE in mediating the caffeine response. Gr66a is the first gustatory receptor shown to be essential for caffeine-induced behavior and activity of gustatory receptor cells in vivo.

Gsalpha is involved in sugar perception in Drosophila melanogaster

In Drosophila melanogaster, gustatory receptor genes (Grs) encode G-protein-coupled receptors (GPCRs) in gustatory receptor neurons (GRNs) and some olfactory receptor neurons. One of the Gr genes, Gr5a, encodes a sugar receptor that is expressed in a subset of GRNs and has been most extensively studied both molecularly and physiologically, but the G-protein alpha subunit (Galpha) that is coupled to this sugar receptor remains unknown. Here, we propose that Gs is the Galpha that is responsible for Gr5a-mediated sugar-taste transduction, based on the following findings: First, immunoreactivities against Gs were detected in a subset of GRNs including all Gr5a-expressing neurons. Second, trehalose-intake is reduced in flies heterozygous for null mutations in DGsalpha, a homolog of mammalian Gs, and trehalose-induced electrical activities in sugar-sensitive GRNs were depressed in those flies. Furthermore, expression of wild-type DGsalpha in sugar-sensitive GRNs in heterozygotic DGsalpha mutant flies rescued those impairments. Third, expression of double-stranded RNA for DGsalpha in sugar-sensitive GRNs depressed both behavioral and electrophysiological responses to trehalose. Together, these findings indicate that DGsalpha is involved in trehalose perception. We suggest that sugar-taste signals are processed through the Gsalpha-mediating signal transduction pathway in sugar-sensitive GRNs in Drosophila.

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

Water perception is important for insects, because they are particularly vulnerable to water loss because their body size is small. In Drosophila, gustatory receptor neurons are located at the base of the taste sensilla on the labellum, tarsi, and wing margins. One of the gustatory receptor neurons in typical sensilla is known to respond to water. To reveal the neural mechanisms of water perception in Drosophila, it is necessary to identify water receptor neurons and their projection patterns. We used a Gal4 enhancer trap strain in which GAL4 is expressed in a single gustatory receptor neuron in each sensillum on the labellum. We investigated the function of these neurons by expressing the upstream activating sequence transgenes, shibire(ts1), tetanus toxin light chain, or diphtheria toxin A chain. Results from the proboscis extension reflex test and electrophysiological recordings indicated that the GAL4-expressing neurons respond to water. We show here that the water receptor neurons project to a specific region in the subesophageal ganglion, thus revealing the water taste sensory map in Drosophila.

Insect odor and taste receptors

Insect odor and taste receptors are highly sensitive detectors of food, mates, and oviposition sites. Following the identification of the first insect odor and taste receptors in Drosophila melanogaster, these receptors were identified in a number of other insects, including the malaria vector mosquito Anopheles gambiae; the silk moth, Bombyx mori; and the tobacco budworm, Heliothis virescens. The chemical specificities of many of the D. melanogaster receptors, as well as a few of the A. gambiae and B. mori receptors, have now been determined either by analysis of deletion mutants or by ectopic expression in in vivo or heterologous expression systems. Here we discuss recent advances in our understanding of the molecular and cellular basis of odor and taste coding in insects.

Electrophysiological and behavioural characterization of gustatory responses to antennal ‘bitter’ taste in honeybees

We combined behavioural and electrophysiological experiments to study whether bitter taste is perceived at the antennal level in honeybees, Apis mellifera. Our behavioural studies showed that neither quinine nor salicin delivered at one antenna at different concentrations induced a retraction of the proboscis once it was extended in response to 1 M sucrose solution delivered to the opposite antenna. Bees that extended massively their proboscis to 1 M sucrose responded only partially when stimulated with a mixture of 1 M sucrose and 100 mM quinine. The mixture of 1 m sucrose and 100 mM salicin had no such suppressive effect. No behavioural suppression was found for mixtures of salt solution and either bitter substance. Electrophysiological recordings of taste sensillae at the antennal tip revealed sensillae that responded specifically either to sucrose or salt solutions, but none responded to the bitter substances quinine and salicin at the different concentrations tested. The electrophysiological responses of sensillae to 15 mM sucrose solution were inhibited by a mixture of 15 mM sucrose and 0.1 mM quinine, but not by a mixture of 15 mM sucrose and 0.1 mM salicin. The responses of sensillae to 50 mM NaCl were reduced by a mixture of 50 mm NaCl and 1 mM quinine but not by a mixture of 50 mM NaCl and 1 mM salicin. We concluded that no receptor cells for the bitter substances tested, exist at the level of the antennal tip of the honeybee and that antennal bitter taste is not represented as a separate perceptual quality.

Insect chemoreception

Insect chemoreception is mediated by a large and diverse superfamily of seven-transmembrane domain receptors. These receptors were first identified in Drosophila, but have since been found in other insects, including mosquitoes and moths. Expression and functional analysis of these receptors have been used to identify receptor ligands and to map receptors to functional classes of neurons. Many receptors detect general odorants or tastants, whereas some detect pheromones. The non-canonical receptor Or83b, which is highly conserved across insect orders, dimerizes with odorant and pheromone receptors and is required for efficient localization of these proteins to dendrites of sensory neurons. These studies provide a foundation for understanding the molecular and cellular basis of olfactory and gustatory coding.

G-protein gamma subunit 1 is required for sugar reception in Drosophila

Though G-proteins have been implicated in the primary step of taste signal transduction, no direct demonstration has been done in insects. We show here that a G-protein gamma subunit, Ggamma1, is required for the signal transduction of sugar taste reception in Drosophila. The Ggamma1 gene is expressed mainly in one of the gustatory receptor neurons. Behavioral responses of the flies to sucrose were reduced by the targeted suppression of neural functions of Ggamma1-expressing cells using neural modulator genes such as the modified Shaker K+ channel (EKO), the tetanus toxin light chain or the shibire (shi(ts1)) gene. RNA interference targeting to the Ggamma1 gene reduced the amount of Ggamma1 mRNA and suppressed electrophysiological response of the sugar receptor neuron. We also demonstrated that responses to sugars were lowered in Ggamma1 null mutant, Ggamma1(N159). These results are consistent with the hypothesis that Ggamma1 participates in the signal transduction of sugar taste reception.

Gustatory Perception and Behavior in Drosophila melanogaster

The sense of taste is essential for the survival of virtually all animals. Considered a 'primitive sense' and present in the form of chemotaxis in many bacteria, taste is also a sense of sophistication in humans. Regardless, taste behavior is a crucial activity for the world's most abundant (insects) and most successful (mammals) inhabitants, providing a means of discrimination between nutrient-rich substrates, such as sugars and amino acids, from harmful, mostly bitter-tasting chemicals present in many plants. In this review, we present an update on progress in understanding taste perception in the model fruit fly Drosophila melanogaster. An introduction to the fly's taste system will be presented first, followed by a description of relevant behavioral assays developed to quantify taste perception at the organismal level and a short overview of electrophysiological studies performed on taste cells. The focal point will be the recent molecular-genetic investigations of the gustatory receptor (Gr) genes, which is complemented by a comparison between Drosophila and mammalian taste perception and transduction. Finally, we provide a perspective on the future of Drosophila taste research, including three specific proposals that seem uniquely applicable to this exquisite model system and cannot, at least currently, be pursued elsewhere.