Identification of neurons responsible for feeding behavior in the Drosophila brain

Drosophila melanogaster feeds mainly on rotten fruits,which contain many kinds of sugar.Thus,the sense of sweet taste has evolved to serve as a dominant regulator and driver of feeding behavior.Although several sugar receptors have been described,it remains poorly understood how the sensory input is transformed into an appetitive behavior.Here,we used a neural silencing approach to screen brain circuits,and identified neurons labeled by three Gal4 lines that modulate Drosophila feeding behavior.These three Gal4 lines labeled neurons mainly in the suboesophageal ganglia(SOG),which is considered to be the fly’s primary taste center.When we blocked the activity of these neurons,flies decreased their sugar consumption significantly.In contrast,activation of these neurons resulted in enhanced feeding behavior and increased food consumption not only towards sugar,but to an array of food sources.Moreover,upon neuronal activation,the flies demonstrated feeding behavior even in the absence of food,which suggests that neuronal activation can replace food as a stimulus for feeding behavior.These findings indicate that these Gal4-labeled neurons,which function downstream of sensory neurons and regulate feeding behavior towards different food sources is necessary in Drosophila feeding control.     

Candidate codes in the gustatory system of caterpillars

Larvae of tobacco hornworms offer unique opportunities to relate the electrophysiological output of identified chemosensory neurons to specific behavioral responses. Larvae can discriminate among three preferred plants with only eight functioning gustatory receptors. They can be induced to prefer any one of the plants, and these preferences can be reversed. All eight neurons respond to each plant sap. Two fire too infrequently to permit detailed analysis. Analyses of the remaining six show that all electrophysiological responses consist of phasic and tonic components. Only the "salt best" cell fires during the phasic period. Temporal analysis of the spike train during this period shows that tomato and tobacco could be distinguished from Jerusalem cherry but not from each other by a rate code. Measurements of behavioral response times together with the nonspecificity of this with respect of food plants, unacceptable plants, and sodium chloride eliminate a phasic period rate code as a probable mechanism for complex discrimination. Events occurring in the tonic period, when all cells are firing, suggest a major role for this period. Analyses of variance in the interval frequencies of the large and medium spikes suggest that a variance code could allow discrimination among the three plants as long as both cells were firing at the same time. Evidence has been found for temporal patterning in the tonic response of the "salt best" cell to Jerusalem cherry but is absent elsewhere. The most likely basis for coding the difference between each of the three plants is across- fiber patterning in which the relative rates of firing and the variances of all the sensory neurons in the tonic phase are critical.     

Female contact activates male-specific interneurons that trigger stereotypic courtship behavior in Drosophila

We determined the cellular substrate for male courtship behavior by quasinatural and artificial stimulation of brain neurons. Activation of fruitless (fru)-expressing neurons via stimulation of thermosensitive dTrpA1 channels induced an entire series of courtship acts in male Drosophila placed alone without any courting target. By reducing the number of neurons expressing dTrpA1 by MARCM, we demonstrated that the initiation of courtship behavior is significantly correlated with the activation of the transmidline P1 interneurons, the descending P2b interneurons, or both, indicating that these interneurons trigger courtship. Using an experimental paradigm in which a tethered male can be stimulated to initiate courtship by touching his foreleg tarsus to a female's abdomen, we found that P1 neurites of tethered males showed a transient Ca(2+) rise after tarsal stimulation with the female-associated sensory cues. These observations strongly suggest that P1 neurons are the prime components of the neural circuitry that initiates male courtship.     

Allocrine modulation of feeding behavior by the Sex Peptide of Drosophila

Mating elicits a dramatic reprogramming of female behavior in numerous insect species. In Drosophila, this postmating response (PMR) comprises increased egg-laying rate and reduced sexual receptivity and is controlled by the products of the male accessory glands, a family of approximately 80 small peptides transferred in the male seminal fluid . Here, we show that copulation strongly stimulates female food intake. Remarkably, this change is abolished if the males lack a single, small seminal protein, the Sex Peptide (SP). Ectopic expression of SP in virgin females mimics the effect of mating on feeding behavior, demonstrating that SP is the main agent controlling this behavioral paradigm. Our observations identify enhanced feeding behavior as a novel component of the Drosophila PMR and suggest that SP represents a molecular link between energy acquisition and reproductive investment.     

Drosophila TRPA channel painless inhibits male-male courtship behavior through modulating olfactory sensation

The Drosophila melanogaster TRPA family member painless, expressed in a subset of multidendritic neurons embeding in the larval epidermis, is necessary for larval nociception of noxious heat or mechanical stimuli. However, the function of painless in adult flies remains largely unknown. Here we report that mutation of painless leads to a defect in male-male courtship behavior and alteration in olfaction sensitivity in adult flies. Specific downregulation of the expression of the Painless protein in the olfactory projection neurons (PNs) of the antennal lobes (ALs) resulted in a phenotype resembling that found in painless mutant flies, whereas overexpression of Painless in PNs of painless mutant males suppressed male-male courtship behavior. The downregulation of Painless exclusively during adulthood also resulted in male-male courtship behavior. In addition, mutation of the painless gene in flies caused changes in olfaction, suggesting a role for this gene in olfactory processing. These results indicate that functions of painless in the adult central nervous system of Drosophila include modulation of olfactory processing and inhibition of male-male courtship behavior.     

Mechanisms of lactic acid gustatory attraction in Drosophila

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Neuropeptides in interneurons of the insect brain

A large number of neuropeptides has been identified in the brain of insects. At least 35 neuropeptide precursor genes have been characterized in Drosophila melanogaster, some of which encode multiple peptides. Additional neuropeptides have been found in other insect species. With a few notable exceptions, most of the neuropeptides have been demonstrated in brain interneurons of various types. The products of each neuropeptide precursor seem to be co-expressed, and each precursor displays a unique neuronal distribution pattern. Commonly, each type of neuropeptide is localized to a relatively small number of neurons. We describe the distribution of neuropeptides in brain interneurons of a few well-studied insect species. Emphasis has been placed upon interneurons innervating specific brain areas, such as the optic lobes, accessory medulla, antennal lobes, central body, and mushroom bodies. The functional roles of some neuropeptides and their receptors have been investigated in D. melanogaster by molecular genetics techniques. In addition, behavioral and electrophysiological assays have addressed neuropeptide functions in the cockroach Leucophaea maderae. Thus, the involvement of brain neuropeptides in circadian clock function, olfactory processing, various aspects of feeding behavior, and learning and memory are highlighted in this review. Studies so far indicate that neuropeptides can play a multitude of functional roles in the brain and that even single neuropeptides are likely to be multifunctional.The original research in the authors’ laboratories was supported by DFG grants HO 950/14 and 950/16 (U.H.) and Swedish Research Council grant VR 621-2004-3715 (D.R.N).     

Candidate glutamatergic neurons in the visual system of Drosophila

The visual system of Drosophila contains approximately 60,000 neurons that are organized in parallel, retinotopically arranged columns. A large number of these neurons have been characterized in great anatomical detail. However, studies providing direct evidence for synaptic signaling and the neurotransmitter used by individual neurons are relatively sparse. Here we present a first layout of neurons in the Drosophila visual system that likely release glutamate as their major neurotransmitter. We identified 33 different types of neurons of the lamina, medulla, lobula and lobula plate. Based on the previous Golgi-staining analysis, the identified neurons are further classified into 16 major subgroups representing lamina monopolar (L), transmedullary (Tm), transmedullary Y (TmY), Y, medulla intrinsic (Mi, Mt, Pm, Dm, Mi Am), bushy T (T), translobula plate (Tlp), lobula intrinsic (Lcn, Lt, Li), lobula plate tangential (LPTCs) and lobula plate intrinsic (LPi) cell types. In addition, we found 11 cell types that were not described by the previous Golgi analysis. This classification of candidate glutamatergic neurons fosters the future neurogenetic dissection of information processing in circuits of the fly visual system.     

Cerebral interneurons controlling fictive feeding in Limax maximus

Summary Initiation and modulation of fictive feeding by cerebral to buccal interneurons (CBs) was examined in an isolated CNS preparation of Limax maximus. Three CBs which are phasically active during fictive feeding, CB₁, CB₃ and CB₄, will reliably trigger bouts of fictive feeding when activated alone or in pairs. Another phasic CB, CB_EC, is not effective for triggering feeding. One CB which is tonically active during fictive feeding, CB_ST, drives fictive feeding in 50% of preparations when activated alone and enhances triggering of feeding when co-activated with phasic CBs. The metacerebral giant cell (MGC) was found to be capable of triggering fictive feeding in preparations with an intact subcerebral commissure. The MGC was especially effective at increasing the effectiveness of other CBs for initiation of feeding. Short high-frequency bursts of phasic CB or MGC action potentials are capable of resetting ongoing fictive feeding. Resetting effects of CB action potentials are relatively independent of the phase of the bite-cycle in which they are activated. CB₄ phase-advances the bite-cycle while the other phasic CBs phase-delay the bite cycle. Moderate frequency stimulation of CB₄ speeds up the bite rate while moderate frequency stimulation of CB₃ slows biting. All CBs, except the tonic CB, CB_DL, increase the intensity of buccal motor neuron bursting during feeding. The excitatory effects of phasic CBs and the tonic CB, CB_EPSP, on fictive feeding persist for many seconds after the offset of stimulation. CBs form both monosynaptic excitatory and monosynaptic inhibitory connections with different BG motor neurons.Abbreviations BG buccal ganglion - BR buccal root - CB cerebral-buccal interneuron - CBC cerebral-buccal connective - CPG central pattern generator - FB fast burster neuron - FMP feeding motor program - IBI interbite interval - MGC metacerebral giant cell     

Cholinergic interneurons in the feeding system of the pond snail Lymnaea stagnalis. II. N1 interneurons make cholinergic synapses with feeding motoneurons.

The N1 neurons are a population of interneurons active during the protraction phase of the feeding rhythm. All the N1 neurons are coupled by electrical synapses which persist in a high Mg/low Ca saline which blocks chemical synapses. Individual N1 spikes produce discrete electrotonic postsynaptic potentials (PSPS) in other N1 cells, but the coupling is not strong enough to ensure 1:1 firing. Bursts of N1 spikes generate compound PSPS in the feeding motoneurons. The sign (excitation or inhibition) of the N1 input corresponds with the synaptic barrage recorded during the protraction phase. Discrete PSPS are only resolved in a Hi-Di saline. Their variation in latency and number can be explained by variation in electrotonic propagation within the electrically coupled network of N1 cells. The excitatory postsynaptic potentials (ESPS) in the 1 cell are reduced by 0.5 mM antagonists hexamethonium (HMT), atropine (ATR), curare (d-TC) and by methylxylocholine (MeXCh), all of which block the excitatory cholinergic receptor (Elliott et al. (Phil. Trans. R. Soc. Lond. 336, 157-166 (Preceding paper.) (1992)). The 1 cell EPSPS were transiently blocked by phenyltrimethylammonium (PTMA), which is both an agonist and antagonist at the 1 cell excitatory acetylcholine (ACh) receptor (Elliott et al. 1992). The inhibitory postsynaptic potential (IPSP) in the 3 cell is blocked by bath applications of MeXCh and PTMA, which both abolish the response of the 3 cell to ACh (Elliott et. al. 1992). The effects of the cholinergic antagonists on the response of 4 cluster and 5 cells to N1 stimulation matches their response to ACh (Elliott et al. 1992). It is concluded that the population of N1 cells are multiaction, premotor cholinergic interneurons.     

Spatially restricted expression of candidate taste receptors in the Drosophila gustatory system

BACKGROUND: Taste is an important sensory modality in most animals. In Drosophila, taste is perceived by gustatory neurons located in sensilla distributed on several different appendages throughout the body of the animal. Here we show that the gustatory receptors are encoded by a family of at least 54 genes (Gr genes), most of which are expressed exclusively in a small subset of taste sensilla located in narrowly defined regions of the fly's body. RESULTS: BLAST searches with the predicted amino acid sequences of 6 7-transmembrane-receptor genes of unknown function and 20 previously identified, putative gustatory receptor genes led to the identification of a large gene family comprising at least 54 genes. We investigated the expression of eight genes by using a Gal4 reporter gene assay and found that five of them were expressed in the gustatory system of the fly. Four genes were expressed in 1%-4% of taste sensilla, located in well-defined regions of the proboscis, the legs, or both. The fifth gene was expressed in about 20% of taste sensilla in all major gustatory organs, including the taste bristles on the anterior wing margin. Axon-tracing experiments demonstrated that neurons expressing a given Gr gene project their axons to a spatially restricted domain of the subesophageal ganglion in the fly brain. CONCLUSIONS: Our findings suggest that each taste sensillum represents a discrete, functional unit expressing at least one Gr receptor and that most Gr genes are expressed in spatially restricted domains of the gustatory system. These observations imply the potential for high taste discrimination of the Drosophila brain.     

Mutations modulating the Argos-regulated signaling pathway in Drosophila eye development

Argos is a secreted protein that contains an EGF-like domain and acts as an inhibitor of Drosophila EGF receptor activation. To identify genes that function in the Argos-regulated signaling pathway, we performed a genetic screen for enhancers and suppressors of the eye phenotype caused by the overexpression of argos. As a result, new alleles of known genes encoding components of the EGF receptor pathway, such as Star, sprouty, bulge, and clown, were isolated. To study the role of clown in development, we examined the eye and wing phenotypes of the clown mutants in detail. In the eye discs of clown mutants, the pattern of neuronal differentiation was impaired, showing a phenotype similar to those caused by a gain-of-function EGF receptor mutation and overexpression of secreted Spitz, an activating ligand for the EGF receptor. There was also an increased number of pigment cells in the clown eyes. Epistatic analysis placed clown between argos and Ras1. In addition, we found that clown negatively regulated the development of wing veins. These results suggest that the clown gene product is important for the Argos-mediated inhibition of EGF receptor activation during the development of various tissues. In addition to the known genes, we identified six mutations of novel genes. Genetic characterization of these mutants suggested that they have distinct roles in cell differentiation and/or survival regulated by the EGF receptor pathway.     

Scratching for food: An original feeding behavior in an African flower breeding Drosophila

Drosophila suma is a flower breeding species widespread in the Afrotropical region. We describe an original and so far unique feeding behavior in that species. Strong black spines on the fore tarsus of both sexes are used for scratching the surface of the petals: the juice pouring out from the scratched cells is immediately ingested and this might be the only source of food. D. suma has been investigated on two kinds of flowers, Ipomoea and Crinum. In spite of the very different shape and color of these flowers, the feeding behavior of the flies is the same. Various morphological and anatomical adaptations are described, especially those related to laying a few very big eggs. The taxonomic position of D. suma has long remained and still is elusive. Further taxonomic and phylogenetic investigations would thus be needed to understand how adaptation to flower breeding has evolved.     

A temporal model of animal behavior based on a fractality in the feeding of Drosophila melanogaster

We present a new temporal model of animal behavior based on the ethological idea that the internal states of the individual essentially determine the behavior. The internal states, however, are conditioned by the external stimuli. This model, including environmental and internal parameters, predicts a fractal property of the behavior, that is, an inverse power law distribution of the duration. Being consistent with the model, we have found a fractal property of feeding in Drosophila melanogaster: The dwelling time of starved flies on food showed a clear inverse power law distribution. The dependence of the fractal dimension on the intensity of food stimuli has been observed, and the predicted change into an exponential distribution was proved.     

Acute differential sensitivity and role of the central nervous system in the feeding behavior of Drosophila melanogaster

Feeding behavior of the fruitfly, Drosophila melanogaster, isexamined quantitatively by determining the amount of fool intakecolorimetrically. The obtained differential sensitivity (Weberfraction l/l = 0.025) by the two-choice test is surprisinglyacute, compared with the previously reported values for insects.Besides, files also choose only the most stimulative prey inthree-choice tests and even a single fly, free from ‘gangingup’ exhibits a clear intake choice. Adaptation in thetest organ was experimentally proved not to play a leading rolein ingestion choice. The present experimental results supportan important role of the central nervous system in choice behavior.Although deprivation significantly enhances the amount of ingestion,it does not affect the choice itself under the present conditions,suggesting that the choice centre exists separately from theingestion center. Discrimination sensitivity depends considerablyon the food distribution and taste intensities paired.     

Re-patterning sleep architecture in Drosophila through gustatory perception and nutritional quality

Organisms perceive changes in their dietary environment and enact a suite of behavioral and metabolic adaptations that can impact motivational behavior, disease resistance, and longevity. However, the precise nature and mechanism of these dietary responses is not known. We have uncovered a novel link between dietary factors and sleep behavior in Drosophila melanogaster. Dietary sugar rapidly altered sleep behavior by modulating the number of sleep episodes during both the light and dark phase of the circadian period, independent of an intact circadian rhythm and without affecting total sleep, latency to sleep, or waking activity. The effect of sugar on sleep episode number was consistent with a change in arousal threshold for waking. Dietary protein had no significant effect on sleep or wakefulness. Gustatory perception of sugar was necessary and sufficient to increase the number of sleep episodes, and this effect was blocked by activation of bitter-sensing neurons. Further addition of sugar to the diet blocked the effects of sweet gustatory perception through a gustatory-independent mechanism. However, gustatory perception was not required for diet-induced fat accumulation, indicating that sleep and energy storage are mechanistically separable. We propose a two-component model where gustatory and metabolic cues interact to regulate sleep architecture in response to the quantity of sugar available from dietary sources. Reduced arousal threshold in response to low dietary availability may have evolved to provide increased responsiveness to cues associated with alternative nutrient-dense feeding sites. These results provide evidence that gustatory perception can alter arousal thresholds for sleep behavior in response to dietary cues and provide a mechanism by which organisms tune their behavior and physiology to environmental cues.