Viewing odors in the mushroom body of the fly

A toolbox for light control of Drosophila behaviors through Channelrhodopsin 2-mediated photoactivation of targeted neurons

In order to study the function of specific neural circuits, we generated UAS-Channelrhodopsin2 (ChR2) transgenic Drosophila and established a ChR2-based system that enables specific activation of targeted neurons in larval and adult fruit flies with blue light illumination, under the control of a newly designed light source that provides fully programmable stimulation patterns. We showed that stimulating selectively the nociceptor of larvae expressing ChR2 elicited light-induced 'pain' response, confined freely behaving larvae in defined area and directed larva migration along a preset route. In freely behaving adult flies, rapid photoactivation of targeted gustatory sensory neurons, dopaminergic modulatory neurons and motor neurons triggered the proboscis extension response, escaping reflex and changes in the locomotion pattern, respectively, with precise temporal control. This non-invasive method for remote control of animal behaviors also provides a potential tool for conducting 'gain of function' studies toward understanding how animal behaviors are controlled by neural activity.

Specific expression of olfactory binding protein in the aerial olfactory cavity of adult and developing Xenopus

Olfactory binding proteins (OBP), commonly associated with aerial olfaction, are found in the olfactory mucus of mammals but have never been identified in fish. It is still not clear whether the presence of OBP in aerial olfactory systems is due to phylogenetic or to functional differences linked to the adaptation of the olfactory system to an aerial environment. To test this alternative, the olfactory system of Xenopus offers a unique opportunity because it includes two olfactory cavities, one of which is thought to be devoted to aquatic olfaction and the other to aerial olfaction. We therefore purified and cloned OBPs in two Xenopus species. Xenopus laevis OBP (XlaeOBP) and Xenopus tropicalis OBP (XtroOBP) exhibit 158 and 160 amino acids, respectively, sharing 89 residues. cRNA probes allowed us to demonstrate that XlaeOBP and XtroOBP are expressed at the level of Bowman's gland specifically in the aerial olfactory cavity, as confirmed using anti-XlaeOBP antiserum. OBP mRNA transcription occurs early during metamorphosis, as early as stage 57. This is the first study to demonstrate that OBPs are exclusively present in the aerial chamber and are only expressed as the tadpole becomes an adult in species which possess both aquatic and aerial olfactory organs.

Design parameters of the fan-out phase of sensory systems

This paper focuses on the calculation of boundary values for the design parameters in the fan-out phase of the olfactory system of insects. Three main criteria are taken into account to determine the boundaries of the parameters: (i) information conservation, (ii) low energy costs and (iii) full involvement of all the neurons. These criteria serve to determine the structural parameters that produce a sufficient minimal response. Analytical calculations lead to a few general expressions which show how the main internal parameters can be obtained for any system with similar characteristics. We calculate the optimal threshold for coincidence detection, connectivity and output activity values that verify criteria (i), (ii) and (iii). The range of parameter values obtained by these calculations include those observed in the olfactory system of the locust.

Odor localization requires visual feedback during free flight in Drosophila melanogaster

Adult fruit flies follow attractive odors associated with food and oviposition sites through widely varied visual landscapes. To examine the interaction between olfactory and visual cues during search behavior, we recorded three-dimensional flight trajectories as individuals explored controlled sensory landscapes. When presented with the source of an attractive odor invisibly embedded in the floor of a 1 m arena, flies spend most of their time hovering back and forth over the source when flying within a randomly textured visual background but fail to localize the source when searching within a uniform white surround. To test whether flies are associating unique features of the visual background with the strength of odor cues, we flew them within arenas containing evenly spaced vertical stripes. Flies readily localized the odor when flying within visual landscapes lacking azimuthal landmarks provided that vertical edges were present. Flies failed to localize odor when flying within a background pattern consisting of horizontal stripes. These results suggest that, whereas flies do not require spatially unique visual patterns to localize an odor source, they do require visual feedback generated by vertical edges. Quantitative shifts in several components of flight behavior accompanied successful odor localization. Flies decrease flight altitude, turn more often and approach visually textured walls of the arena near an odor source. A simple model based on the statistics of flight behavior supports the hypothesis that a subtle influence on these behaviors is sufficient to lead a fly to its food.

Cellular bases of behavioral plasticity: establishing and modifying synaptic circuits in the Drosophila genetic system

Genetic malleability and amenability to behavioral assays make Drosophila an attractive model for dissecting the molecular mechanisms of complex behaviors, such as learning and memory. At a cellular level, Drosophila has contributed a wealth of information on the mechanisms regulating membrane excitability and synapse formation, function, and plasticity. Until recently, however, these studies have relied almost exclusively on analyses of the peripheral neuromuscular junction, with a smaller body of work on neurons grown in primary culture. These experimental systems are, by themselves, clearly inadequate for assessing neuronal function at the many levels necessary for an understanding of behavioral regulation. The pressing need is for access to physiologically relevant neuronal circuits as they develop and are modified throughout life. In the past few years, progress has been made in developing experimental approaches to examine functional properties of identified populations of Drosophila central neurons, both in cell culture and in vivo. This review focuses on these exciting developments, which promise to rapidly expand the frontiers of functional cellular neurobiology studies in Drosophila. We discuss here the technical advances that have begun to reveal the excitability and synaptic transmission properties of central neurons in flies, and discuss how these studies promise to substantially increase our understanding of neuronal mechanisms underlying behavioral plasticity.

Characterization of a chemosensory protein (ASP3c) from honeybee (Apis mellifera L.) as a brood pheromone carrier

Chemosensory proteins (CSPs) are ubiquitous soluble small proteins isolated from sensory organs of a wide range of insect species, which are believed to be involved in chemical communication. We report the cloning of a honeybee CSP gene called ASP3c, as well as the structural and functional characterization of the encoded protein. The protein was heterologously secreted by the yeast Pichia pastoris using the native signal peptide. ASP3c disulfide bonds were assigned after trypsinolysis followed by chromatography and mass spectrometry combined with microsequencing. The pairing (Cys(I)-Cys(II), Cys(III)-Cys(IV)) was found to be identical to that of Schistocerca gregaria CSPs, suggesting that this pattern occurs commonly throughout the insect CSPs. CD measurements revealed that ASP3c mainly consists of alpha-helices, like other insect CSPs. Gel filtration analysis showed that ASP3c is monomeric at neutral pH. Using ASA, a fluorescent fatty acid anthroyloxy analogue as a probe, ASP3c was shown to bind specifically to large fatty acids and ester derivatives, which are brood pheromone components, in the micromolar range. It was unable to bind tested general odorants and other tested pheromones (sexual and nonsexual). This is the first report on a natural pheromonal ligand bound by a recombinant CSP with a measured affinity constant.