Inactivation of olfactory sensilla of a single morphological type differentially affects the response of Drosophila to odors

Continuous odor profile monitoring to study olfactory navigation in small animals

Olfactory navigation is observed across species and plays a crucial role in locating resources for survival. In the laboratory, understanding the behavioral strategies and neural circuits underlying odor-taxis requires a detailed understanding of the animal's sensory environment. For small model organisms like Caenorhabditis elegans and larval Drosophila melanogaster, controlling and measuring the odor environment experienced by the animal can be challenging, especially for airborne odors, which are subject to subtle effects from airflow, temperature variation, and from the odor's adhesion, adsorption, or reemission. Here, we present a method to control and measure airborne odor concentration in an arena compatible with an agar substrate. Our method allows continuous controlling and monitoring of the odor profile while imaging animal behavior. We construct stationary chemical landscapes in an odor flow chamber through spatially patterned odorized air. The odor concentration is measured with a spatially distributed array of digital gas sensors. Careful placement of the sensors allows the odor concentration across the arena to be continuously inferred in space and monitored through time. We use this approach to measure the odor concentration that each animal experiences as it undergoes chemotaxis behavior and report chemotaxis strategies for C. elegans and D. melanogaster larvae populations as they navigate spatial odor landscapes.

The 40-Year Mystery of Insect Odorant-Binding Proteins

The survival of insects depends on their ability to detect molecules present in their environment. Odorant-binding proteins (OBPs) form a family of proteins involved in chemoreception. While OBPs were initially found in olfactory appendages, recently these proteins were discovered in other chemosensory and non-chemosensory organs. OBPs can bind, solubilize and transport hydrophobic stimuli to chemoreceptors across the aqueous sensilla lymph. In addition to this broadly accepted "transporter role", OBPs can also buffer sudden changes in odorant levels and are involved in hygro-reception. The physiological roles of OBPs expressed in other body tissues, such as mouthparts, pheromone glands, reproductive organs, digestive tract and venom glands, remain to be investigated. This review provides an updated panorama on the varied structural aspects, binding properties, tissue expression and functional roles of insect OBPs.

Morphological Characterization of the Antennal Sensilla of the Afrotropical Sand Fly, Phlebotomus duboscqi (Diptera: Psychodidae)

We investigated by scanning electron microscopy the morphology, distribution, and abundance of antennal sensilla of females Phlebotomus duboscqi sand fly, an important vector of zoonotic cutaneous leishmaniasis at Afrotropical region. Thirteen well-differentiated sensilla were identified, among six types of cuticular sensilla. The probable function of these sensillary types is discussed in relation to their external structure and distribution. Five sensillary types were classified as olfactory sensilla, as they have specific morphological characters of sensilla with this function. Number and distribution of sensilla significantly differed between antennal segments. The results of the present work, besides corroborating in the expansion of the morphological and ultrastructural knowledge of P. duboscqi, can foment future electrophysiological studies for the development of volatile semiochemicals, to be used as attractants in traps for monitoring and selective vector control of this sand fly.

Central peptidergic modulation of peripheral olfactory responses

Background

Animal olfactory systems detect volatile environmental chemicals and integrate this information to direct the discovery of food and mates as well as danger avoidance. Rather than remaining constant, olfactory response thresholds are modulated by internal and external cues to adapt odor-guided behaviors to changing conditions.

Results

Here, we show in Drosophila melanogaster that neuropeptide F (NPF) modulates the responses of a specific population of antennal olfactory sensory neurons (OSNs) to food-derived odors. We show that knock-down of NPF in NPF neurons specifically reduces the responses of the ab3A neurons to ethyl butyrate, a volatile ester found in apples and other fruits. Knock-down of the NPF receptor (NPFR) in the ab3A neuron reduces their responses and disrupts the ability of the flies to locate food. We also identify a sexual dimorphism in ab3A responsiveness: ab3A neurons in females immediately post-eclosion are less responsive to ethyl butyrate than those of both age-matched males and older females. Not only does this change correlate with brain NPF levels, but also NPFR mutants show no such sexual dimorphism. Finally, by way of mechanism, we show that mutation of NPFR seems to cause intracellular clustering of OR22a, the odorant receptor expressed in the ab3A neurons.

Conclusions

Interestingly, this modulation of the peripheral odorant responsiveness of the ab3A neurons by NPF is distinct from the modulation of presynaptic gain in the ab3A neurons previously observed with the similarly named but distinct neuropeptide sNPF. Rather than affecting the strength of the output at the level of the first synapse in the antennal lobe, NPF-NPFR signaling may affect the process of odorant detection itself by causing intracellular OR clustering.

Electronic supplementary material

The online version of this article (doi:10.1186/s12915-017-0374-6) contains supplementary material, which is available to authorized users.

Interactions of carbon dioxide and food odours in Drosophila: olfactory hedonics and sensory neuron properties

Behavioural responses of animals to volatiles in their environment are generally dependent on context. Most natural odours are mixtures of components that can each induce different behaviours when presented on their own. We have investigated how a complex of two olfactory stimuli is evaluated by Drosophila flies in a free-flying two-trap choice assay and how these stimuli are encoded in olfactory receptor neurons. We first observed that volatiles from apple cider vinegar attracted flies while carbon dioxide (CO2) was avoided, confirming their inherent positive and negative values. In contradiction with previous results obtained from walking flies in a four-field olfactometer, in the present assay the addition of CO2 to vinegar increased rather than decreased the attractiveness of vinegar. This effect was female-specific even though males and females responded similarly to CO2 and vinegar on their own. To test whether the female-specific behavioural response to the mixture correlated with a sexual dimorphism at the peripheral level we recorded from olfactory receptor neurons stimulated with vinegar, CO2 and their combination. Responses to vinegar were obtained from three neuron classes, two of them housed with the CO2-responsive neuron in ab1 sensilla. Sensitivity of these neurons to both CO2 and vinegar per se did not differ between males and females and responses from female neurons did not change when CO2 and vinegar were presented simultaneously. We also found that CO2-sensitive neurons are particularly well adapted to respond rapidly to small concentration changes irrespective of background CO2 levels. The ability to encode temporal properties of stimulations differs considerably between CO2- and vinegar-sensitive neurons. These properties may have important implications for in-flight navigation when rapid responses to fragmented odour plumes are crucial to locate odour sources. However, the flies' sex-specific response to the CO2-vinegar combination and the context-dependent hedonics most likely originate from central rather than peripheral processing.

The Antennal sensilla of Oxelytrum erythrurum (Blanchard) and Oxelytrum apicale (Brullé) (Coleoptera: Silphidae)

The typology and placement of antennal sensilla of the carrion beetles Oxelytrum erythrurum (Blanchard) and Oxelytrum apicalis (Brullé) (Coleoptera: Silphidae) were studied using scanning electron microscopy. Two types of sensilla chaetica, two types of sensilla trichodea, four types of sensilla basiconica, one type of sensilla coeloconica, and an unidentified type of sensillum were found in both species. Sensilla chaetica type 1 are found on the antennomeres proximal to antennal club (A1-A8); chaetica type 2 are found on the club (A9-A11). Sensilla trichodea are found on A9-A11; one type (T1) is found on the proximal portion of the club, the other type (T2) on the apical portion. Basiconica type 1 are found on the dorsal surface of A9-A11; they are much denser on the apical portion of the antennal club than on the proximal. In O. erythrurum, a nocturnal species of the Chaco-Pampean plain, T2 two are found on A10 and A11. In Oxelytrum apicale, a mountain species, probably diurnal, only A11 bears T2, but they are denser than in the other species. It is suggested that O. apicale depends more on contact chemoreception than O. erythrurum. The ventral surface of the antennal clubs shows no remarkable difference between species.

Spatial representation of odorant valence in an insect brain

Brains have to decide whether and how to respond to detected stimuli based on complex sensory input. The vinegar fly Drosophila melanogaster evaluates food sources based on olfactory cues. Here, we performed a behavioral screen using the vinegar fly and established the innate valence of 110 odorants. Our analysis of neuronal activation patterns evoked by attractive and aversive odorants suggests that even though the identity of odorants is coded by the set of activated receptors, the main representation of odorant valence is formed at the output level of the antennal lobe. The topographic clustering within the antennal lobe of valence-specific output neurons resembles a corresponding domain in the olfactory bulb of mice. The basal anatomical structure of the olfactory circuit between insects and vertebrates is known to be similar; our study suggests that the representation of odorant valence is as well.

Genetic variability and robustness of host odor preference in Drosophila melanogaster

Chemosensory stimuli play a crucial role for host selection in insects, including the fruit fly Drosophila melanogaster. Drosophila has been instrumental in unraveling the neurological basis of olfactory processing in insects. Basic knowledge regarding chemical ecology and thorough studies of olfactory preferences are still lacking to a great extent in D. melanogaster, however. We have characterized repeatable variation in olfactory preference between five classical D. melanogaster wild-type strains toward a large array of natural host odors and synthetic compounds. By recording the rate of attraction over up to 24 hr, we could compare stimuli varying in attractiveness and characterize phenotypic parameters on the basis of individual stimuli and the whole stimulus array. Behavioral differences between strains were predominantly due to variation in a single phenotypic parameter: their overall responsiveness toward optimal and suboptimal olfactory stimuli. These differences were not explained by variation in olfactory sensitivity, locomotory activity, or general vigor monitored by survival. Comparisons with three recently established wild-type strains indicated that a high behavioral threshold against accepting suboptimal olfactory stimuli is the characteristic phenotype of wild D. melanogaster.

Molecular and cellular organization of insect chemosensory neurons

Animals use their chemosensory systems to detect and discriminate among chemical cues in the environment. Remarkable progress has recently been made in our knowledge of the molecular and cellular basis of chemosensory perception in insects, based largely on studies in Drosophila. This progress has been possible due to the identification of gene families for olfactory and gustatory receptors, the use of electro-physiological recording techniques on sensory neurons, the multitude of genetic manipulations that are available in this species, and insights from several insect model systems. Recent studies show that the superfamily of chemoreceptor proteins represent the essential elements in chemosensory coding, endowing chemosensory neurons with their abilities to respond to specific sets of odorants, tastants or pheromones. Investigating how insects detect chemicals in their environment can show us how receptor protein structures relate to ligand binding, how nervous systems process complex information, and how chemosensory systems and genes evolve.

Chemosensory coding by neurons in the coeloconic sensilla of the Drosophila antenna

Odor coding is based on the diverse sensitivities and response properties of olfactory receptor neurons (ORNs). In the Drosophila antenna, ORNs are housed in three major morphological types of sensilla. Although investigation of the Drosophila olfactory system has been expanding rapidly, the ORNs in one of these types, the coeloconic sensilla, have been essentially unexplored. We define four functional types of coeloconic sensilla through extracellular physiological recordings. Each type contains at least two neurons, with a total of at least seven distinct ORN classes that vary remarkably in their breadth of tuning. Analysis of 315 odorant-ORN combinations reveals how these neurons sample odor space via both excitation and inhibition. We identify a class of neurons that is narrowly tuned to small amines, and we find humidity detectors that define a cellular basis for hygroreception in Drosophila. The temporal dynamics of responses vary widely, enhancing the potential for complexity in the odor code. Molecular and genetic analysis shows that a broadly tuned ORN, antennal coeloconic 3B (ac3B), requires the odor receptor gene Or35a for its response in vivo. The activity of ac3B is not required for the response of the other ORN within that sensillum, ac3A. The functional analysis presented here, revealing a combination of highly specialized neurons and a broadly tuned ORN, along with the ancient origin of coeloconic sensilla, suggests that the specificities of these ORNs may reflect basic needs of an ancestral insect.

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.

Or83b encodes a broadly expressed odorant receptor essential for Drosophila olfaction

Fruit flies are attracted by a diversity of odors that signal the presence of food, potential mates, or attractive egg-laying sites. Most Drosophila olfactory neurons express two types of odorant receptor genes: Or83b, a broadly expressed receptor of unknown function, and one or more members of a family of 61 selectively expressed receptors. While the conventional odorant receptors are highly divergent, Or83b is remarkably conserved between insect species. Two models could account for Or83b function: it could interact with specific odor stimuli independent of conventional odorant receptors, or it could act in concert with these receptors to mediate responses to all odors. Our results support the second model. Dendritic localization of conventional odorant receptors is abolished in Or83b mutants. Consistent with this cellular defect, the Or83b mutation disrupts behavioral and electrophysiological responses to many odorants. Or83b therefore encodes an atypical odorant receptor that plays an essential general role in olfaction.

Experimental studies of adult Drosophila chemosensory behaviour

Drosophila melanogaster is a powerful animal model to study the processes underlying behavioural responses to chemical cues. This paper provides a review of the important literature to present recent advances in our understanding of how gustatory and olfactory stimuli are perceived. An overview is given of the experimental procedures currently used to characterize the fly chemosensory behaviour. Since this species provides extremely useful genetic tools, a focus is made on those allowing to manipulate behaviour, and hence to understand its molecular and cellular bases. Such tools include single-gene mutants and the Gal4/UAS system. They can be combined with studies of the natural polymorphism of behavioural responses. Recent data obtained with these various approaches unravel some important aspects of taste and olfaction. These appear as rather complex processes, as revealed by results showing dose-dependence, plasticity and sexual dimorphism. Taken together, these results and the available tools open interesting perspectives for the years to come, in our attempts to make the link between genes and behaviour.

Blocking sensory inputs to identified antennal glomeruli selectively modifies odorant perception in Drosophila

Neural coding of sensory input is a major unsolved issue in neuroscience. Current experimental methods rely on neural activity recording or visualization following sensory stimulation. Most of them, however, do not include behavioral correlates on the actual perception by the animal. We present a novel approach to address olfaction and coding in adult Drosophila. Sensory input was selectively blocked in two subsets of sensory neurons that project to different, albeit overlapping, groups of central targets, by means of tetanus toxin expressed under the control of the yeast transcription factor Gal4. Glomeruli DL1, DL2, VM1, and VM4 were tested following stimulation with benzaldehyde, ethyl acetate, propionic acid, butanol, or acetone at various concentrations. The behavioral response was found to be modified in an odorant-specific and a concentration-dependent manner. Sensory input to DL2 and, to a minor extent, VM1 and/or VM4, appear to be required for benzaldehyde perception, while acetone is processed through DL1. None of these glomeruli, however, seem necessary for butanol perception. In addition, sexual differences were observed for some stimuli. These results demonstrate the behavioral relevance of odor representation as maps of glomerular activity generated in the antennal lobes following specific sensory input. The strategy used here should be useful to characterize olfactory coding, as new and selective Gal4 lines become available.