Odorant concentration differentiator for intermittent olfactory signals

Spike frequency adaptation facilitates the encoding of input gradient in insect olfactory projection neurons

The olfactory system in insects has evolved to process the dynamic changes in the concentration of food odors or sex pheromones to localize the nutrients or conspecific mating partners. Experimental studies have suggested that projection neurons (PNs) in insects encode not only the stimulus intensity but also its rate-of-change (input gradient). In this study, we aim to develop a simple computational model for a PN to understand the mechanism underlying the coding of the rate-of-change information. We show that the spike frequency adaptation is a potential key mechanism for reproducing the phasic response pattern of the PN in Drosophila. We also demonstrate that this adaptation mechanism enables the PN to encode the rate-of-change of the input firing rate. Finally, our model predicts that the PN exhibits the intensity-invariant response for the pulse and ramp odor stimulus. These results suggest that the developed model is useful for investigating the coding principle underlying olfactory information processing in insects.

Restoration of mitochondria axonal transport by adaptor Disc1 supplementation prevents neurodegeneration and rescues visual function

Deficits in mitochondrial transport are a common feature of neurodegenerative diseases. We investigated whether loss of components of the mitochondrial transport machinery impinge directly on metabolic stress, neuronal death, and circuit dysfunction. Using multiphoton microscope live imaging, we showed that ocular hypertension, a major risk factor in glaucoma, disrupts mitochondria anterograde axonal transport leading to energy decline in vulnerable neurons. Gene- and protein-expression analysis revealed loss of the adaptor disrupted in schizophrenia 1 (Disc1) in retinal neurons subjected to high intraocular pressure. Disc1 gene delivery was sufficient to rescue anterograde transport and replenish axonal mitochondria. A genetically encoded ATP sensor combined with longitudinal live imaging showed that Disc1 supplementation increased ATP production in stressed neurons. Disc1 gene therapy promotes neuronal survival, reverses abnormal single-cell calcium dynamics, and restores visual responses. Our study demonstrates that enhancing anterograde mitochondrial transport is an effective strategy to alleviate metabolic stress and neurodegeneration.

Multisensory-motor integration in olfactory navigation of silkmoth, Bombyx mori, using virtual reality system

Most animals survive and thrive due to navigational behavior to reach their destinations. In order to navigate, it is important for animals to integrate information obtained from multisensory inputs and use that information to modulate their behavior. In this study, by using a virtual reality (VR) system for an insect, we investigated how the adult silkmoth integrates visual and wind direction information during female search behavior (olfactory behavior). According to the behavioral experiments using a VR system, the silkmoth had the highest navigational success rate when odor, vision, and wind information were correctly provided. However, the success rate of the search was reduced if the wind direction information provided was different from the direction actually detected. This indicates that it is important to acquire not only odor information but also wind direction information correctly. When the wind is received from the same direction as the odor, the silkmoth takes positive behavior; if the odor is detected but the wind direction is not in the same direction as the odor, the silkmoth behaves more carefully. This corresponds to a modulation of behavior according to the degree of complexity (turbulence) of the environment. We mathematically modeled the modulation of behavior using multisensory information and evaluated it using simulations. The mathematical model not only succeeded in reproducing the actual silkmoth search behavior but also improved the search success relative to the conventional odor-source search algorithm.

Pheromone binding protein is involved in temporal olfactory resolution in the silkmoth

Male moths utilize spatio-temporal female sex pheromone information to orient toward conspecific females. Pheromones are distributed as discontinuous plumes owing to air turbulence; thus, efficient tracking of intermittent stimuli is expected to require a high temporal resolution. Here, using pheromone binding protein (BmPBP1)-knockout silkmoths, we showed that a loss of functional PBP lowered the temporal sensory resolution of male antennae. This altered temporal resolution resulted in significantly reduced straight walking and longer turning behavior, which respectively occurred when males detected and lost contact with pheromones, indicating that temporal resolution was also lowered at the behavioral level. BmPBP1-knockout males required significantly longer time than wild-type males in locating pheromone sources and female moths. Our results suggest that BmPBP1 plays a critical role in determining olfactory response kinetics. Accordingly, high temporal olfactory and behavioral resolutions, as shaped by PBP, are essential for tracking pheromone plumes and locating females efficiently.

Reconstructing the ecology of a Cretaceous cockroach: destructive and high-resolution imaging of its micro sensory organs

Animals highly depend on their sensory organs to detect information about their surrounding environment. Among animal sensory organs, those of insects have a notable ability to detect information despite their small size, which might be, therefore, one of the reasons for the evolutionary success of insects. However, insect sensory organs are seldom fossilized in sediments due to their small size and fragility. A potential solution for this problem is the study of exceptionally well-preserved fossil material from amber. Unfortunately, the resolution of existing non-destructive analysis is insufficient to observe details of these micro sensory organs even with amber preservation. Here, we focus on the analysis of the micro sensory organs of an extinct male cockroach (Huablattula hui Qiu et al., 2019) in Cretaceous amber by combining destructive and non-destructive methods. Compared to extant species inhabiting dark environments, H. hui has relatively large compound eyes, and all the antennal sensilla for detecting multimodal information observed here are fewer or smaller. The characteristics of these sensory organs support the diurnality of the bright habitats of H. hui in contrast to many extant cockroaches. Like extant male mantises, grooved basiconic type sensilla exist abundantly on the antenna of the fossilized specimen. The abundance of grooved basiconic sensilla in mantid males results from using sex pheromones, and therefore, H. hui may have likewise used mantis-like intersexual communication. These lines of evidence suggest that the ecology and behavior of Cretaceous cockroaches were more diverse than those of related extant species.

A novel framework based on a data-driven approach for modelling the behaviour of organisms in chemical plume tracing

We propose a data-driven approach for modelling an organism's behaviour instead of conventional model-based strategies in chemical plume tracing (CPT). CPT models based on this approach show promise in faithfully reproducing organisms' CPT behaviour. To construct the data-driven CPT model, a training dataset of the odour stimuli input toward the organism is needed, along with an output of the organism's CPT behaviour. To this end, we constructed a measurement system comprising an array of alcohol sensors for the measurement of the input and a camera for tracking the output in a real scenario. Then, we determined a transfer function describing the input-output relationship as a stochastic process by applying Gaussian process regression, and established the data-driven CPT model based on measurements of the organism's CPT behaviour. Through CPT experiments in simulations and a real environment, we evaluated the performance of the data-driven CPT model and compared its success rate with those obtained from conventional model-based strategies. As a result, the proposed data-driven CPT model demonstrated a better success rate than those obtained from conventional model-based strategies. Moreover, we considered that the data-driven CPT model could reflect the aspect of an organism's adaptability that modulated its behaviour with respect to the surrounding environment. However, these useful results came from the CPT experiments conducted in simple settings of simulations and a real environment. If making the condition of the CPT experiments more complex, we confirmed that the data-driven CPT model would be less effective for locating an odour source. In this way, this paper not only poses major contributions toward the development of a novel framework based on a data-driven approach for modelling an organism's CPT behaviour, but also displays a research limitation of a data-driven approach at this stage.

Refinement of ectopic protein expression through the GAL4/UAS system in Bombyx mori: application to behavioral and developmental studies

Silkmoth, Bombyx mori, is one of the important model insects in which transgenic techniques and the GAL4/UAS system are applicable. However, due to cytotoxicity and low transactivation activity of GAL4, effectiveness of the GAL4/UAS system and its application in B. mori are still limited. In the present study, we refined the previously reported UAS vector by exploiting transcriptional and translational enhancers, and achieved 200-fold enhancement of reporter GFP fluorescence in the GAL4/UAS system. Enhanced protein expression of membrane-targeted GFP and calcium indicator protein (GCaMP5G) drastically improved visualization of fine neurite structures and neural activity, respectively. Also, with the refined system, we generated a transgenic strain that expresses tetanus toxin light chain (TeTxLC), which blocks synaptic transmission, under the control of GAL4. Ectopic TeTxLC expression in the sex pheromone receptor neurons inhibited male courtship behavior, proving effectiveness of TeTxLC on loss-of-function analyses of neural circuits. In addition, suppression of prothoracicotropic hormone (PTTH) or insulin-like peptide (bombyxin) secretion impaired developmental timing and growth rate, respectively. Furthermore, we revealed that larval growth is sex-differentially regulated by these peptide hormones. The present study provides important technical underpinnings of transgenic approaches in silkmoths and insights into mechanisms of postembryonic development in insects.

Using insects to drive mobile robots - hybrid robots bridge the gap between biological and artificial systems

The use of mobile robots is an effective method of validating sensory-motor models of animals in a real environment. The well-identified insect sensory-motor systems have been the major targets for modeling. Furthermore, mobile robots implemented with such insect models attract engineers who aim to avail advantages from organisms. However, directly comparing the robots with real insects is still difficult, even if we successfully model the biological systems, because of the physical differences between them. We developed a hybrid robot to bridge the gap. This hybrid robot is an insect-controlled robot, in which a tethered male silkmoth (Bombyx mori) drives the robot in order to localize an odor source. This robot has the following three advantages: 1) from a biomimetic perspective, the robot enables us to evaluate the potential performance of future insect-mimetic robots; 2) from a biological perspective, the robot enables us to manipulate the closed-loop of an onboard insect for further understanding of its sensory-motor system; and 3) the robot enables comparison with insect models as a reference biological system. In this paper, we review the recent works regarding insect-controlled robots and discuss the significance for both engineering and biology.

Presynaptic GABA Receptors Mediate Temporal Contrast Enhancement in Drosophila Olfactory Sensory Neurons and Modulate Odor-Driven Behavioral Kinetics

Contrast enhancement mediated by lateral inhibition within the nervous system enhances the detection of salient features of visual and auditory stimuli, such as spatial and temporal edges. However, it remains unclear how mechanisms for temporal contrast enhancement in the olfactory system can enhance the detection of odor plume edges during navigation. To address this question, we delivered to Drosophila melanogaster flies pulses of high odor intensity that induce sustained peripheral responses in olfactory sensory neurons (OSNs). We use optical electrophysiology to directly measure electrical responses in presynaptic terminals and demonstrate that sustained peripheral responses are temporally sharpened by the combined activity of two types of inhibitory GABA receptors to generate contrast-enhanced voltage responses in central OSN axon terminals. Furthermore, we show how these GABA receptors modulate the time course of innate behavioral responses after odor pulse termination, demonstrating an important role for temporal contrast enhancement in odor-guided navigation.

The neurobiological basis of orientation in insects: insights from the silkmoth mating dance

Counterturning is a common movement pattern during orientation behavior in insects. Once male moths sense sex pheromones and then lose the input, they demonstrate zigzag movements, alternating between left and right turns, to increase the probability to contact with the pheromone plume. We summarize the anatomy and function of the neural circuit involved in pheromone orientation in the silkmoth. A neural circuit, the lateral accessory lobe (LAL), serves a role as the circuit module for zigzag movements and controls this operation using a flip-flop neural switch. Circuit design of the LAL is well conserved across species. We hypothesize that this zigzag module is utilized in a wide range of insect behavior. We introduce two examples of the potential use: orientation flight and the waggle dance in bees.

Intrinsic and Network Mechanisms Constrain Neural Synchrony in the Moth Antennal Lobe

Projection-neurons (PNs) within the antennal lobe (AL) of the hawkmoth respond vigorously to odor stimulation, with each vigorous response followed by a ~1 s period of suppression—dubbed the “afterhyperpolarization-phase,” or AHP-phase. Prior evidence indicates that this AHP-phase is important for the processing of odors, but the mechanisms underlying this phase and its function remain unknown. We investigate this issue. Beginning with several physiological experiments, we find that pharmacological manipulation of the AL yields surprising results. Specifically, (a) the application of picrotoxin (PTX) lengthens the AHP-phase and reduces PN activity, whereas (b) the application of Bicuculline-methiodide (BIC) reduces the AHP-phase and increases PN activity. These results are curious, as both PTX and BIC are inhibitory-receptor antagonists. To resolve this conundrum, we speculate that perhaps (a) PTX reduces PN activity through a disinhibitory circuit involving a heterogeneous population of local-neurons, and (b) BIC acts to hamper certain intrinsic currents within the PNs that contribute to the AHP-phase. To probe these hypotheses further we build a computational model of the AL and benchmark our model against our experimental observations. We find that, for parameters which satisfy these benchmarks, our model exhibits a particular kind of synchronous activity: namely, “multiple-firing-events” (MFEs). These MFEs are causally-linked sequences of spikes which emerge stochastically, and turn out to have important dynamical consequences for all the experimentally observed phenomena we used as benchmarks. Taking a step back, we extract a few predictions from our computational model pertaining to the real AL: Some predictions deal with the MFEs we expect to see in the real AL, whereas other predictions involve the runaway synchronization that we expect when BIC-application hampers the AHP-phase. By examining the literature we see support for the former, and we perform some additional experiments to confirm the latter. The confirmation of these predictions validates, at least partially, our initial speculation above. We conclude that the AL is poised in a state of high-gain; ready to respond vigorously to even faint stimuli. After each response the AHP-phase functions to prevent runaway synchronization and to “reset” the AL for another odor-specific response.

Two types of local interneurons are distinguished by morphology, intrinsic membrane properties, and functional connectivity in the moth antennal lobe

Neurons in the silkmoth antennal lobe (AL) are well characterized in terms of their morphology and odor-evoked firing activity. However, their intrinsic electrical properties including voltage-gated ionic currents and synaptic connectivity remain unclear. To address this, whole cell current- and voltage-clamp recordings were made from second-order projection neurons (PNs) and two morphological types of local interneurons (LNs) in the silkmoth AL. The two morphological types of LNs exhibited distinct physiological properties. One morphological type of LN showed a spiking response with a voltage-gated sodium channel gene expression, whereas the other type of LN was nonspiking without a voltage-gated sodium channel gene expression. Voltage-clamp experiments also revealed that both of two types of LNs as well as PNs possessed two types of voltage-gated potassium channels and calcium channels. In dual whole cell recordings of spiking LNs and PNs, activation of the PN elicited depolarization responses in the paired spiking LN, whereas activation of the spiking LN induced no substantial responses in the paired PN. However, simultaneous recording of a nonspiking LN and a PN showed that activation of the nonspiking LN induced hyperpolarization responses in the PN. We also observed bidirectional synaptic transmission via both chemical and electrical coupling in the pairs of spiking LNs. Thus our results indicate that there were two distinct types of LNs in the silkmoth AL, and their functional connectivity to PNs was substantially different. We propose distinct functional roles for these two different types of LNs in shaping odor-evoked firing activity in PNs.

A Circuit for Gradient Climbing in C. elegans Chemotaxis

Animals have a remarkable ability to track dynamic sensory information. For example, the nematode Caenorhabditis elegans can locate a diacetyl odor source across a 100,000-fold concentration range. Here, we relate neuronal properties, circuit implementation, and behavioral strategies underlying this robust navigation. Diacetyl responses in AWA olfactory neurons are concentration and history dependent; AWA integrates over time at low odor concentrations, but as concentrations rise, it desensitizes rapidly through a process requiring cilia transport. After desensitization, AWA retains sensitivity to small odor increases. The downstream AIA interneuron amplifies weak odor inputs and desensitizes further, resulting in a stereotyped response to odor increases over three orders of magnitude. The AWA-AIA circuit drives asymmetric behavioral responses to odor increases that facilitate gradient climbing. The adaptation-based circuit motif embodied by AWA and AIA shares computational properties with bacterial chemotaxis and the vertebrate retina, each providing a solution for maintaining sensitivity across a dynamic range.

Dynamical feature extraction at the sensory periphery guides chemotaxis

Behavioral strategies employed for chemotaxis have been described across phyla, but the sensorimotor basis of this phenomenon has seldom been studied in naturalistic contexts. Here, we examine how signals experienced during free olfactory behaviors are processed by first-order olfactory sensory neurons (OSNs) of the Drosophila larva. We find that OSNs can act as differentiators that transiently normalize stimulus intensity—a property potentially derived from a combination of integral feedback and feed-forward regulation of olfactory transduction. In olfactory virtual reality experiments, we report that high activity levels of the OSN suppress turning, whereas low activity levels facilitate turning. Using a generalized linear model, we explain how peripheral encoding of olfactory stimuli modulates the probability of switching from a run to a turn. Our work clarifies the link between computations carried out at the sensory periphery and action selection underlying navigation in odor gradients.

DOI:http://dx.doi.org/10.7554/eLife.06694.001

Fruit flies are attracted to the smell of rotting fruit, and use it to guide them to nearby food sources. However, this task is made more challenging by the fact that the distribution of scent or odor molecules in the air is constantly changing. Fruit flies therefore need to cope with, and exploit, this variation if they are to use odors as cues.

Odor molecules bind to receptors on the surface of nerve cells called olfactory sensory neurons, and trigger nerve impulses that travel along these cells. While many studies have investigated how fruit flies can distinguish between different odors, less is known about how animals can use variation in the strength of an odor to guide them towards its source.

Optogenetics is a technique that allows neuroscientists to control the activities of individual nerve cells, simply by shining light on to them. Because fruit fly larvae are almost transparent, optogenetics can be used on freely moving animals. Now, Schulze, Gomez-Marin et al. have used optogenetics in these larvae to trigger patterns of activity in individual olfactory sensory neurons that mimic the activity patterns elicited by real odors. These virtual realities were then used to study, in detail, some of the principles that control the sensory navigation of a larva—as it moves using a series of forward ‘runs’ and direction-changing ‘turns’.

Olfactory sensory neurons responded most strongly whenever light levels changed rapidly in strength (which simulated a rapid change in odor concentration). On the other hand, these neurons showed relatively little response to constant light levels (i.e., constant odors). This indicates that the activity of olfactory sensory neurons typically represents the rate of change in the concentration of an odor. An independent study by Kim et al. found that olfactory sensory neurons in adult fruit flies also respond in a similar way.

Schulze, Gomez-Marin et al. went on to show that the signals processed by a single type of olfactory sensory neuron could be used to predict a larva's behavior. Larvae tended to turn less when their olfactory sensory neurons were highly active. Low levels and inhibition of activity in the olfactory sensory neurons had the opposite effect; this promoted turning. It remains to be determined how this relatively simple control principle is implemented by the neural circuits that connect sensory neurons to the parts of a larva's nervous system that are involved with movement.

DOI:http://dx.doi.org/10.7554/eLife.06694.002

Targeted disruption of a single sex pheromone receptor gene completely abolishes in vivo pheromone response in the silkmoth

Male moths use species-specific sex pheromones to identify and orientate toward conspecific females. Odorant receptors (ORs) for sex pheromone substances have been identified as sex pheromone receptors in various moth species. However, direct in vivo evidence linking the functional role of these ORs with behavioural responses is lacking. In the silkmoth, Bombyx mori, female moths emit two sex pheromone components, bombykol and bombykal, but only bombykol elicits sexual behaviour in male moths. A sex pheromone receptor BmOR1 is specifically tuned to bombykol and is expressed in specialized olfactory receptor neurons (ORNs) in the pheromone sensitive long sensilla trichodea of male silkmoth antennae. Here, we show that disruption of the BmOR1 gene, mediated by transcription activator-like effector nucleases (TALENs), completely removes ORN sensitivity to bombykol and corresponding pheromone-source searching behaviour in male moths. Furthermore, transgenic rescue of BmOR1 restored normal behavioural responses to bombykol. Our results demonstrate that BmOR1 is required for the physiological and behavioural response to bombykol, demonstrating that it is the receptor that mediates sex pheromone responses in male silkmoths. This study provides the first direct evidence that a member of the sex pheromone receptor family in moth species mediates conspecific sex pheromone information for sexual behaviour.

A simple behaviour provides accuracy and flexibility in odour plume tracking—the robotic control of sensory-motor coupling in silkmoths

Odour plume tracking is an essential behaviour for animal survival. A fundamental strategy for this is moving upstream and then across-stream. Male silkmoths, Bombyx mori, display this strategy as a pre-programmed sequential behaviour. They walk forward (surge) in response to the female sex pheromone and perform a zigzagging “mating dance”. Though pre-programmed, the surge direction is modulated by the bilateral olfactory input and optic flow. However, the nature of interaction between these two sensory modalities and contribution of the resultant motor command to localizing an odour source are still unknown. We evaluated the ability of the silkmoth to localize the odour source under conditions of disturbed sensory-motor coupling, using a silkmoth-driven mobile robot. The significance of the bilateral olfaction of the moth was confirmed by inverting the robot's olfactory input that is acquired using two flexible air-suction tubes and delivered to the antennae of the moth, or its motor output. Inversion of the motor output induced consecutive circling, which was inhibited by covering the visual field of the moth. This suggests that the corollary discharge from the motor command and the reafference of self-generated optic flow generates compensatory signals to guide the surge accurately. Additionally, after inverting the olfactory input, the robot successfully tracked the odour plume by using a combination of behaviours. These results indicate that accurate guidance of the reflexive surge by integrating bilateral olfactory and visual information with innate pre-programmed behaviours increases the flexibility to track an odour plume even under disturbed circumstances.