Hydrophobic solution functions as a multifaceted mosquito repellent by enhancing chemical transfer, altering object tracking, and forming aversive memory.

Developing a safe and potent repellent of mosquitoes applicable to human skins is an effective measure against the spread of mosquito-borne diseases. Recently, we have identified that hydrophobic solutions such as low viscosity polydimethylsiloxane (L-PDMS) spread on a human skin prevent mosquitoes from staying on and biting it. This is likely due to the ability of L-PDMS in wetting mosquito legs and exerting a capillary force from which the mosquitoes attempt to escape. Here we show three additional functions of L-PDMS that can contribute to repel Aedes albopictus, by combining physicochemical analysis and behavioral assays in both an arm cage and a virtual flight arena. First, L-PDMS, when mixed with topical repellents and applied on a human skin, enhances the effect of topical repellents in reducing mosquito bites by efficiently transferring them to mosquito legs upon contact. Second, L-PDMS applied to mosquito tarsi compromises visual object tracking during flight, exerting an influence outlasting the contact. Finally, L-PDMS applied to mosquito tarsi acts as an aversive reinforcer in associative learning, making mosquitoes avoid the conditioned odor. These results uncover a multifaceted potential of L-PDMS in altering a sequence of mosquito behaviors from biting a human skin, visual object tracking following takeoff, to the response to an odor linked with L-PDMS.

Dopaminergic neurons dynamically update sensory values during olfactory maneuver.

Dopaminergic neurons (DANs) drive associative learning to update the value of sensory cues, but their contribution to the assessment of sensory values outside the context of association remains largely unexplored. Here, we show in Drosophila that DANs in the mushroom body encode the innate value of odors and constantly update the current value by inducing plasticity during olfactory maneuver. Our connectome-based network model linking all the way from the olfactory neurons to DANs reproduces the characteristics of DAN responses, proposing a concrete circuit mechanism for computation. Downstream of DANs, odors alone induce value- and dopamine-dependent changes in the activity of mushroom body output neurons, which store the current value of odors. Consistent with this neural plasticity, specific sets of DANs bidirectionally modulate flies' steering in a virtual olfactory environment. Thus, the DAN circuit known for discrete, associative learning also continuously updates odor values in a nonassociative manner.

Surfactants alter mosquito's flight and physical condition.

Mosquitoes carry lethal pathogens for humans and hundreds of thousands of people are killed by mosquito-borne diseases every year. Therefore, controlling mosquitoes is essential to protect the lives of people around the world. Insecticides are highly effective in controlling mosquitoes and have been used extensively worldwide. However, they have potentially harmful effects on biodiversity and environment, and some mosquitoes are resistant to insecticide ingredients and survive upon their application. Therefore, there is a demand for a method to control mosquitoes without using conventional insecticide ingredients. Here, we used Aedes albopictus to test whether solutions with low surface tension, particularly surfactant solutions can alter mosquito behavior by spreading over the hydrophobic cuticle of mosquitoes. We found that solutions with low surface tension indeed attached to mosquitoes flying or resting on the wall, and made them fall. In addition, solutions with yet lower surface tension covered the mosquito surface more quickly and widely, knocking down or killing mosquitoes. These results suggest that surfactants such as sodium dioctyl sulfosuccinate can be used to alter mosquito behavior without relying on conventional insecticides.

Central organization of a high-dimensional odor space.

Animals recognize groups and mixtures of odors as a unitary object. This ability is crucial to generalize known odors to newly encountered ones despite variations. However, it remains largely unknown how multitudes of odors are represented and organized in the higher brain regions to support odor object recognition. Here we discuss recent advances uncovering the population odor responses in the rodent piriform cortex and the Drosophila mushroom body, and highlight the emerging principles on the organization, mechanism, stereotypy, and experience-dependence of central odor representations.

An electrically coupled pioneer circuit enables motor development via proprioceptive feedback in Drosophila embryos.

Precocious movements are widely seen in embryos of various animal species. Whether such movements via proprioceptive feedback play instructive roles in motor development or are a mere reflection of activities in immature motor circuits is a long-standing question. Here we image the emerging motor activities in Drosophila embryos that lack proprioceptive feedback and show that proprioceptive experience is essential for the development of locomotor central pattern generators (CPGs). Downstream of proprioceptive inputs, we identify a pioneer premotor circuit composed of two pairs of segmental interneurons, whose gap-junctional transmission requires proprioceptive experience and plays a crucial role in CPG formation. The circuit autonomously generates rhythmic plateau potentials via IP3-mediated Ca2+ release from internal stores, which contribute to muscle contractions and hence produce proprioceptive feedback. Our findings demonstrate the importance of self-generated movements in instructing motor development and identify the cells, circuit, and physiology at the core of this proprioceptive feedback.

Synthesis of Conserved Odor Object Representations in a Random, Divergent-Convergent Network.

Animals are capable of recognizing mixtures and groups of odors as a unitary object. However, how odor object representations are generated in the brain remains elusive. Here, we investigate sensory transformation between the primary olfactory center and its downstream region, the mushroom body (MB), in Drosophila and show that clustered representations for mixtures and groups of odors emerge in the MB at the population and single-cell levels. Decoding analyses demonstrate that neurons selective for mixtures and groups enhance odor generalization. Responses of these neurons and those selective for individual odors all emerge in an experimentally well-constrained model implementing divergent-convergent, random connectivity between the primary center and the MB. Furthermore, we found that relative odor representations are conserved across animals despite this random connectivity. Our results show that the generation of distinct representations for individual odors and groups and mixtures of odors in the MB can be understood in a unified computational and mechanistic framework.

A Multi-regional Network Encoding Heading and Steering Maneuvers in Drosophila.

An internal sense of heading direction is computed from various cues, including steering maneuvers of the animal. Although neurons encoding heading and steering have been found in multiple brain regions, it is unclear whether and how they are organized into neural circuits. Here we show that, in flying Drosophila, heading and turning behaviors are encoded by population dynamics of specific cell types connecting the subregions of the central complex (CX), a brain structure implicated in navigation. Columnar neurons in the fan-shaped body (FB) of the CX exhibit circular dynamics that multiplex information about turning behavior and heading. These dynamics are coordinated with those in the ellipsoid body, another CX subregion containing a heading representation, although only FB neurons flip turn preference depending on the visual environment. Thus, the navigational system spans multiple subregions of the CX, where specific cell types show coordinated but distinct context-dependent dynamics.

Nanopore Formation in the Cuticle of an Insect Olfactory Sensillum.

Nanometer-level patterned surface structures form the basis of biological functions, including superhydrophobicity, structural coloration, and light absorption [1-3]. In insects, the cuticle overlying the olfactory sensilla has multiple small (50- to 200-nm diameter) pores [4-8], which are supposed to function as a filter that admits odorant molecules, while preventing the entry of larger airborne particles and limiting water loss. However, the cellular processes underlying the patterning of extracellular matrices into functional nano-structures remain unknown. Here, we show that cuticular nanopores in Drosophila olfactory sensilla originate from a curved ultrathin film that is formed in the outermost envelope layer of the cuticle and secreted from specialized protrusions in the plasma membrane of the hair forming (trichogen) cell. The envelope curvature coincides with plasma membrane undulations associated with endocytic structures. The gore-tex/Osiris23 gene encodes an endosomal protein that is essential for envelope curvature, nanopore formation, and odor receptivity and is expressed specifically in developing olfactory trichogen cells. The 24-member Osiris gene family is expressed in cuticle-secreting cells and is found only in insect genomes. These results reveal an essential requirement for nanopores for odor reception and identify Osiris genes as a platform for investigating the evolution of surface nano-fabrication in insects.

Olfactory Landmark-Based Communication in Interacting Drosophila.

To communicate with conspecifics, animals deploy various strategies to release pheromones, chemical signals modulating social and sexual behaviors [1-5]. Importantly, a single pheromone induces different behaviors depending on the context and exposure dynamics [6-8]. Therefore, to comprehend the ethological role of pheromones, it is essential to characterize how neurons in the recipients respond to temporally and spatially fluctuating chemical signals emitted by donors during natural interactions. In Drosophila melanogaster, the male pheromone 11-cis-vaccenyl acetate (cVA) [9] activates specific olfactory receptor neurons (ORNs) [10, 11] to regulate diverse social and sexual behaviors in recipients [12-15]. Physicochemical analyses have identified this chemical on an animal's body [16, 17] and in its local environment [18, 19]. However, because these methods are imprecise in capturing spatiotemporal dynamics, it is poorly understood how individual pheromone cues are released, detected, and interpreted by recipients. Here, we developed a system based on bioluminescence to monitor neural activity in freely interacting Drosophila, and investigated the active detection and perception of the naturally emitted cVA. Unexpectedly, neurons specifically tuned to cVA did not exhibit significant activity during physical interactions between males, and instead responded strongly to olfactory landmarks deposited by males. These landmarks mediated attraction through Or67d receptors and allured both sexes to the marked region. Importantly, the landmarks remained attractive even when a pair of flies was engaged in courtship behavior. In contrast, female deposits did not affect the exploration pattern of either sex. Thus, Drosophila use pheromone marking to remotely signal their sexual identity and to enhance social interactions.

Divergent Connectivity of Homologous Command-like Neurons Mediates Segment-Specific Touch Responses in Drosophila.

Animals adaptively respond to a tactile stimulus by choosing an ethologically relevant behavior depending on the location of the stimuli. Here, we investigate how somatosensory inputs on different body segments are linked to distinct motor outputs in Drosophila larvae. Larvae escape by backward locomotion when touched on the head, while they crawl forward when touched on the tail. We identify a class of segmentally repeated second-order somatosensory interneurons, that we named Wave, whose activation in anterior and posterior segments elicit backward and forward locomotion, respectively. Anterior and posterior Wave neurons extend their dendrites in opposite directions to receive somatosensory inputs from the head and tail, respectively. Downstream of anterior Wave neurons, we identify premotor circuits including the neuron A03a5, which together with Wave, is necessary for the backward locomotion touch response. Thus, Wave neurons match their receptive field to appropriate motor programs by participating in different circuits in different segments.

Parallel encoding of recent visual experience and self-motion during navigation in Drosophila.

Animal navigation requires multiple types of information for decisions on directional heading. We identified neural processing channels that encode multiple cues during navigational decision-making in Drosophila melanogaster. In a flight simulator, we found that flies made directional choices on the basis of the location of a recently presented landmark. This experience-guided navigation was impaired by silencing neurons in the bulb (BU), a region in the central brain. Two-photon calcium imaging during flight revealed that the dorsal part of the BU encodes the location of a recent landmark, whereas the ventral part of the BU tracks self-motion reflecting turns. Photolabeling-based circuit tracing indicated that these functional compartments of the BU constitute adjacent, yet distinct, anatomical pathways that both enter the navigation center. Thus, the fly's navigation system organizes multiple types of information in parallel channels, which may compactly transmit signals without interference for decision-making during flight.

Origins of Cell-Type-Specific Olfactory Processing in the Drosophila Mushroom Body Circuit.

How cell-type-specific physiological properties shape neuronal functions in a circuit remains poorly understood. We addressed this issue in the Drosophila mushroom body (MB), a higher olfactory circuit, where neurons belonging to distinct glomeruli in the antennal lobe feed excitation to three types of intrinsic neurons, α/ß, α'/ß', and γ Kenyon cells (KCs). Two-photon optogenetics and intracellular recording revealed that whereas glomerular inputs add similarly in all KCs, spikes were generated most readily in α'/ß' KCs. This cell type was also the most competent in recruiting GABAergic inhibition fed back by anterior paired lateral neuron, which responded to odors either locally within a lobe or globally across all lobes depending on the strength of stimuli. Notably, as predicted from these physiological properties, α'/ß' KCs had the highest odor detection speed, sensitivity, and discriminability. This enhanced discrimination required proper GABAergic inhibition. These results link cell-type-specific mechanisms and functions in the MB circuit.

Decoding of Context-Dependent Olfactory Behavior in Drosophila.

Odor information is encoded in the activity of a population of glomeruli in the primary olfactory center. However, how this information is decoded in the brain remains elusive. Here, we address this question in Drosophila by combining neuronal imaging and tracking of innate behavioral responses. We find that the behavior is accurately predicted by a model summing normalized glomerular responses, in which each glomerulus contributes a specific, small amount to odor preference. This model is further supported by targeted manipulations of glomerular input, which biased the behavior. Additionally, we observe that relative odor preference changes and can even switch depending on the context, an effect correctly predicted by our normalization model. Our results indicate that olfactory information is decoded from the pooled activity of a glomerular repertoire and demonstrate the ability of the olfactory system to adapt to the statistics of its environment.

Functional Differences between Global Pre- and Postsynaptic Inhibition in the Drosophila Olfactory Circuit.

The Drosophila antennal lobe is subdivided into multiple glomeruli, each of which represents a unique olfactory information processing channel. In each glomerulus, feedforward input from olfactory receptor neurons (ORNs) is transformed into activity of projection neurons (PNs), which represent the output. Recent investigations have indicated that lateral presynaptic inhibitory input from other glomeruli controls the gain of this transformation. Here, we address why this gain control acts "pre"-synaptically rather than "post"-synaptically. Postsynaptic inhibition could work similarly to presynaptic inhibition with regard to regulating the firing rates of PNs depending on the stimulus intensity. We investigate the differences between pre- and postsynaptic gain control in terms of odor discriminability by simulating a network model of the Drosophila antennal lobe with experimental data. We first demonstrate that only presynaptic inhibition can reproduce the type of gain control observed in experiments. We next show that presynaptic inhibition decorrelates PN responses whereas postsynaptic inhibition does not. Due to this effect, presynaptic gain control enhances the accuracy of odor discrimination by a linear decoder while its postsynaptic counterpart only diminishes it. Our results provide the reason gain control operates "pre"-synaptically but not "post"-synaptically in the Drosophila antennal lobe.

Cell death triggers olfactory circuit plasticity via glial signaling in Drosophila.

The Drosophila antennal lobe is organized into glomerular compartments, where olfactory receptor neurons synapse onto projection neurons. Projection neuron dendrites also receive input from local neurons, which interconnect glomeruli. In this study, we investigated how activity in this circuit changes over time when sensory afferents are chronically removed in vivo. In the normal circuit, excitatory connections between glomeruli are weak. However, after we chronically severed receptor neuron axons projecting to a subset of glomeruli, we found that odor-evoked lateral excitatory input to deafferented projection neurons was potentiated severalfold. This was caused, at least in part, by strengthened electrical coupling from excitatory local neurons onto projection neurons, as well as increased activity in excitatory local neurons. Merely silencing receptor neurons was not sufficient to elicit these changes, implying that severing receptor neuron axons is the relevant signal. When we expressed the neuroprotective gene Wallerian degeneration slow (Wld(S)) in receptor neurons before severing their axons, this blocked the induction of plasticity. Because expressing Wld(S) prevents severed axons from recruiting glia, this result suggests a role for glia. Consistent with this, we found that blocking endocytosis in ensheathing glia blocked the induction of plasticity. In sum, these results reveal a novel injury response whereby severed sensory axons recruit glia, which in turn signal to central neurons to upregulate their activity. By strengthening excitatory interactions between neurons in a deafferented brain region, this mechanism might help boost activity to compensate for lost sensory input.

Mechanisms of maximum information preservation in the Drosophila antennal lobe.

We examined the presence of maximum information preservation, which may be a fundamental principle of information transmission in all sensory modalities, in the Drosophila antennal lobe using an experimentally grounded network model and physiological data. Recent studies have shown a nonlinear firing rate transformation between olfactory receptor neurons (ORNs) and second-order projection neurons (PNs). As a result, PNs can use their dynamic range more uniformly than ORNs in response to a diverse set of odors. Although this firing rate transformation is thought to assist the decoder in discriminating between odors, there are no comprehensive, quantitatively supported studies examining this notion. Therefore, we quantitatively investigated the efficiency of this firing rate transformation from the viewpoint of information preservation by computing the mutual information between odor stimuli and PN responses in our network model. In the Drosophila olfactory system, all ORNs and PNs are divided into unique functional processing units called glomeruli. The nonlinear transformation between ORNs and PNs is formed by intraglomerular transformation and interglomerular interaction through local neurons (LNs). By exploring possible nonlinear transformations produced by these two factors in our network model, we found that mutual information is maximized when a weak ORN input is preferentially amplified within a glomerulus and the net LN input to each glomerulus is inhibitory. It is noteworthy that this is the very combination observed experimentally. Furthermore, the shape of the resultant nonlinear transformation is similar to that observed experimentally. These results imply that information related to odor stimuli is almost maximally preserved in the Drosophila olfactory circuit. We also discuss how intraglomerular transformation and interglomerular inhibition combine to maximize mutual information.

Origins of correlated activity in an olfactory circuit.

Multineuronal recordings often reveal synchronized spikes in different neurons. The manner in which correlated spike timing affects neural codes depends on the statistics of correlations, which in turn reflects the connectivity that gives rise to correlations. However, determining the connectivity of neurons recorded in vivo can be difficult. We investigated the origins of correlated activity in genetically labeled neurons of the Drosophila antennal lobe. Dual recordings showed synchronized spontaneous spikes in projection neurons (PNs) postsynaptic to the same type of olfactory receptor neuron (ORN). Odors increased these correlations. The primary origin of correlations lies in the divergence of each ORN onto every PN in its glomerulus. Reciprocal PN-PN connections make a smaller contribution to correlations and PN spike trains in different glomeruli were only weakly correlated. PN axons from the same glomerulus reconverge in the lateral horn, where pooling redundant signals may allow lateral horn neurons to average out noise that arises independently in these PNs.

Homeostatic matching and nonlinear amplification at identified central synapses.

Here we describe the properties of a synapse in the Drosophila antennal lobe and show how they can explain certain sensory computations in this brain region. The synapse between olfactory receptor neurons (ORNs) and projection neurons (PNs) is very strong, reflecting a large number of release sites and high release probability. This is likely one reason why weak ORN odor responses are amplified in PNs. Furthermore, the amplitude of unitary synaptic currents in a PN is matched to the size of its dendritic arbor. This matching may compensate for a lower input resistance of larger dendrites to produce uniform depolarization across PN types. Consistent with this idea, a genetic manipulation that lowers input resistance increases unitary synaptic currents. Finally, strong stimuli produce short-term depression at this synapse. This helps explain why PN odor responses are transient, and why strong ORN odor responses are not amplified as powerfully as weak responses.

Activity-dependent regulation of synaptic size in Drosophila neuromuscular junctions.

One of the fundamental questions in neural development is how neurons form synapses of the appropriate size for the efficient transfer of information across neural circuits. Here we investigated the mechanisms that bring about the size correlation between synapses and postsynaptic cells during development of Drosophila neuromuscular junctions (NMJs). To do this, we made use of a unique system in which two neighboring muscles (M6 and M7) are innervated by the same neurons. In mature NMJs, synaptic size on M6 is normally larger than that on M7, in accordance with the difference in muscle volume; this ensures the same extent of contraction of both muscles, and we refer to this correspondence as "matching". We found that matching was apparent in larvae 8 h after hatching, but not in newly hatched larvae despite the difference in muscle volume. When sensory inputs were suppressed by the expression of tetanus toxin in sensory neurons, matching did not occur, although synapses were able to grow. Matching was also suppressed by the inhibition of motoneuronal activity. These results suggest that matching is induced by regulating the rate of synaptic growth on M6 and M7 in an experience- and activity-dependent manner. It seems most likely that retrograde signals from the postsynaptic to the presynaptic cell convey the information about muscle cell size. We thus examined whether a candidate of retrograde signaling in NMJs, BMP signaling, is involved in matching. However, there was no effect on matching in BMP type II receptor gene mutants, suggesting that other experience-driven mechanisms besides BMP signaling are involved in the proper development of synapses.