Quantitative analysis of olfactory receptor neuron projections in the antennal lobe of the malaria mosquito, Anopheles gambiae

Molecular Basis of CO2 Sensing in Hyphantria cunea

Carbon dioxide (CO2) released by plants can serve as a cue for regulating insect behaviors. Hyphantria cunea is a widely distributed forestry pest that may use CO2 as a cue for foraging and oviposition. However, the molecular mechanism underlying its ability to sense CO2 has not been elucidated. Our initial study showed that CO2 is significantly attractive to H. cunea adults. Subsequently, 44 H. cunea gustatory receptors (GRs) were identified using transcriptome data, and 3 candidate CO2 receptors that are specifically expressed in the labial palps were identified. In vivo electrophysiological assays revealed that the labial palp is the primary organ for CO2 perception in H. cunea, which is similar to findings in other lepidopteran species. By using the Xenopus oocyte expression system, we showed that the HcunGR1 and HcunGR3 co-expressions produced a robust response to CO2, but HcunGR2 had an inhibitory effect on CO2 perception. Finally, immunohistochemical staining revealed sexual dimorphism in the CO2-sensitive labial pit organ glomerulus (LPOG). Taken together, our results clarified the mechanism by which H. cunea sense CO2, laying the foundation for further investigations into the role of CO2 in the rapid spread of H. cunea.

Olfaction in Anopheles mosquitoes

As vectors of disease, mosquitoes are a global threat to human health. The Anopheles mosquito is the deadliest mosquito species as the insect vector of the malaria-causing parasite, which kills hundreds of thousands every year. These mosquitoes are reliant on their sense of smell (olfaction) to guide most of their behaviors, and a better understanding of Anopheles olfaction identifies opportunities for reducing the spread of malaria. This review takes a detailed look at Anopheles olfaction. We explore a range of topics from chemosensory receptors, olfactory neurons, and sensory appendages to behaviors guided by olfaction (including host-seeking, foraging, oviposition, and mating), to vector management strategies that target mosquito olfaction. We identify many research areas that remain to be addressed.

Revisiting the Labial Pit Organ Pathway in the Noctuid Moth, Helicoverpa armigera

Lepidopteran species detect CO2 via a specialized organ located on the peripheral segment of the labial palps, the labial palp pit organ (LPO). Based on tracing of LPO sensory neurons targeting one distinct antennal-lobe glomerulus, Kent and her colleagues described the projections originating from the LPO in the sphinx moth as "an accessory olfactory pathway in Lepidoptera" already in the 1980 ties. In spite of similar reports from studies of other lepidopteran species, however, it has been an unresolved issue whether additional termination areas of the labial nerve, such as the gnathal ganglion (GNG) and the ventral nerve cord, are actually output sites of LPO neurons. Since the previous studies have interpreted slightly differently about the projection pattern occurring from the classical mass staining, we performed selective mass staining from the inside of the pit and from the outer surface of the peripheral palp. The results demonstrated that the LPO sensory neurons project exclusively to the LPO glomerulus (LPOG), whereas the non-LPO sensory neurons target the GNG and the ventral nerve cord. Additional iontophoretic staining of individual LPO sensory neurons, performed from the LPO and the LPOG, showed three morphological neuron types: one bilateral targeting the LPOG in both antennal lobes, one unilateral targeting the ipsilateral LPOG only, and one contralateral targeting the LPOG in the other antennal lobe. Finally, to explore putative differences in the projection pattern of neurons housed by two previously reported sensillum types in the pit, i.e., hair-shaped sensilla located distally and club-shaped sensilla located proximally, we performed mass staining from two different levels of the peripheral palp. We found a projection pattern implying stronger innervation of the ipsi- than the contralateral LPOG in both staining experiments.

Olfactory learning and chemical ecology of olfaction in disease vector mosquitoes: a life history perspective

Mosquitoes transmit many debilitating diseases including malaria, dengue and Zika. Odors mediate behaviors that directly impact disease transmission (blood-feeding) as well as life history events that contribute to mosquito survival and fitness (mating and oviposition, nectar foraging, larval foraging and predator avoidance). In addition to innate olfaction-mediated behaviors, mosquitoes rely on olfactory experience throughout their life to inform advantageous choices in many of these important behaviors. Previous reviews have addressed either the chemical ecology of mosquitoes, or olfactory-driven behaviors including host-feeding or oviposition. Adding to this literature, we use a holistic life history perspective to integrate and compare innate and learned olfactory behavior at various stages of mosquito development.

Organization of the antennal lobes in the praying mantis (Tenodera aridifolia)

Olfaction in insects plays pivotal roles in searching for food and/or for sexual partners. Although many studies have focused on the olfactory processes of nonpredatory insect species, little is known about those in predatory insects. Here, we investigated the anatomical features of the primary olfactory center (antennal lobes) in an insect predator whose visual system is well developed, the praying mantis Tenodera aridifolia. Both sexes of T. aridifolia were found to possess 54 glomeruli, and each glomerulus was identified based on its location and size. Moreover, we found a sexual dimorphism in three glomeruli (macroglomeruli) located at the entrance of the antennal nerves, which are 15 times bigger in males than their homologs in females. We additionally deduced the target glomeruli of olfactory sensory neurons housed in cognate types of sensilla by degenerating the sensory afferents. The macroglomeruli received sensory inputs from grooved peg sensilla, which are present in a large number at the proximal part of the males' antennae. Furthermore, our findings suggest that glomeruli at the posteriodorsal part of the antennal lobes receive sensory information from putative hygro- and thermosensitive sensilla. The origins of projections connected to the protocerebrum are also discussed. J. Comp. Neurol. 525:1685-1706, 2017. © 2016 Wiley Periodicals, Inc.

Learning and Memory in Disease Vector Insects

Learning and memory plays an important role in host preference and parasite transmission by disease vector insects. Historically there has been a dearth of standardized protocols that permit testing their learning abilities, thus limiting discussion on the potential epidemiological consequences of learning and memory to a largely speculative extent. However, with increasing evidence that individual experience and associative learning can affect processes such as oviposition site selection and host preference, it is timely to review the recently acquired knowledge, identify research gaps and discuss the implication of learning in disease vector insects in perspective with control strategies.

Neuronal projections from the Haller's organ and palp sensilla to the synganglion of Amblyomma americanum§

Abstract The present study was conducted to elucidate the neuronal pathways between peripheral olfactory and taste sensilla and the synganglion in an Ixodidae tick species. The tarsus of the front legs (olfactory nerves) and the fourth palpal segment (gustatory nerves) of unfed Amblyomma americanum males and females were excised. A neuronal tracer, dextran tetramethylrhodamine, was used for filling of the sensory neurons. The synganglion preparations were examined using a confocal microscope. Neuronal arborizations from the Haller’s organ were confined to the olfactory lobes and the first pedal ganglion. The estimated number of olfactory glomeruli ranged from 16 to 22 per olfactory lobe in the females. The number of glomeruli was not counted in males because they were densely packed. Sensory neurons associated with sensilla at the distal end of the palpal organ projected into the palpal ganglion in the synganglion through the palpal nerve. Gustatory sensory neurons associated with palpal sensilla projected into a commissure with several bulges, which are confined in the palpal ganglion. The findings of distinct projection patterns of sensory neurons associated with the Haller’s organ and palpal organ in the lone star tick from this study advanced our knowledge on mechanisms of sensory information processing in ticks.

Morphological characterization of the antennal lobes in the Mediterranean fruit fly Ceratitis capitata

The medfly Ceratitis capitata is one of the most important pests for horticulture worldwide. The knowledge about anatomy and function of the medfly olfactory system is still limited. The first brain structure to process olfactory information in insects is the antennal lobe (AL), which is composed of its functional and morphological units, the olfactory glomeruli. Here, we present a morphological three-dimensional reconstruction of AL glomeruli in adult brains. We used unilateral antennal backfills of olfactory receptor neurons (ORNs) with neural tracers, revealing the AL structure. We recorded confocal stacks acquired from whole-mount specimens, and analyzed them with the software AMIRA. The ALs in C. capitata are organized in glomeruli which are more tightly packed in the anterior part than the posterior one. Axons of ORNs bilaterally connect the ALs through a commissure between the two ALs. This commissure is formed by several distinct fascicles. Contralateral dye transfer suggests the presence of gap junctions connecting ORNs from both antennae. There was no statistical difference between the average volumes of female ALs (204,166 ± 12,554 μm(3)) and of male ALs (190,287 ± 11,823 μm(3)). In most specimens, we counted 53 glomeruli in each AL, seven of which were sexually dimorphic in size.

Mosquito Attraction: Crucial Role of Carbon Dioxide in Formulation of a Five-Component Blend of Human-Derived Volatiles

Behavioral responses of the malaria mosquito Anopheles coluzzii (An. gambiae sensu stricto molecular 'M form') to an expanded blend of human-derived volatiles were assessed in a dual-port olfactometer. A previously documented attractive three-component blend consisting of NH3, (S)-lactic acid, and tetradecanoic acid served as the basis for expansion. Adding 4.5% CO2 to the basic blend significantly enhanced its attractiveness. Expansion of the blend with four human-derived C4-volatiles was then assessed, both with and without CO2. Only when CO2 was offered simultaneously, did addition of a specific concentration of 3-methyl-1-butanol or 3-methyl-butanoic acid significantly enhance attraction. The functional group at the terminal C of the 3-methyl-substituted C4 compounds influenced behavioral effectiveness. In the absence of CO2, addition of three concentrations of butan-1-amine caused inhibition when added to the basic blend. In contrast, when CO2 was added, butan-1-amine added to the basic blend strongly enhanced attraction at all five concentrations tested, the lowest being 100,000 times diluted. The reversal of inhibition to attraction by adding CO2 is unique in the class Insecta. We subsequently augmented the three-component basic blend by adding both butan-1-amine and 3-methyl-1-butanol and optimizing their concentrations in the presence of CO2 in order to significantly enhance the attractiveness to An. coluzzii compared to the three- and four-component blends. This novel blend holds potential to enhance malaria vector control based on behavioral disruption.

Sensillum-specific, topographic projection patterns of olfactory receptor neurons in the antennal lobe of the cockroach Periplaneta americana

In vertebrates and many invertebrates, olfactory signals detected by peripheral olfactory receptor neurons (ORNs) are conveyed to a primary olfactory center with glomerular organization in which odor-specific activity patterns are generated. In the cockroach, Periplaneta americana, ORNs in antennal olfactory sensilla project to 205 unambiguously identifiable antennal lobe (AL) glomeruli that are classified into 10 glomerular clusters (T1-T10 glomeruli) innervated by distinct sensory tracts. In this study we employed single sensillum staining techniques and investigated the topographic projection patterns of individual ORNs to elucidate the relationship between sensillum types and glomerular organization in the AL. Axons of almost all ORNs projected to individual glomeruli. Axons of ORNs in perforated basiconic sensilla selectively innervated the anterodorsal T1-T4 glomeruli, whereas those in trichoid and grooved basiconic sensilla innervated the posteroventral T5-T9 glomeruli. About 90% of stained ORNs in trichoid sensilla sent axons to the T5 glomeruli and more than 90% of ORNs in grooved basiconic sensilla innervated the T6, T8, and T9 glomeruli. The T5 and T9 glomeruli exclusively receive sensory inputs from the trichoid and grooved basiconic sensilla, respectively. All investigated glomeruli received convergent input from a single type of sensillum except F11 glomerulus in the T6 glomeruli, which was innervated from both trichoid and grooved basiconic sensilla. These results suggest that ORNs in distinct sensillum types project to glomeruli in distinct glomerular clusters. Since ORNs in distinct sensillum types are each tuned to distinct subsets of odorant molecules, the AL is functionally compartmentalized into groups of glomeruli.

Constancy and variability of identified glomeruli in antennal lobes: computational approach in Spodoptera littoralis

The primary olfactory centres share striking similarities across the animal kingdom. The most conspicuous is their subdivision into glomeruli, which are spherical neuropil masses in which synaptic contacts between sensory and central neurons occur. Glomeruli have both an anatomical identity (being invariant in location, size and shape) and a functional identity (each glomerulus receiving afferents from olfactory receptor neurons that express the same olfactory receptor). Identified glomeruli offer a favourable system for analysing quantitatively the constancy and variability of the neuronal circuits, an important issue for understanding their function, development and evolution. The noctuid moth Spodoptera littoralis with its well-studied pheromone communication system has become a model species for olfaction research. We analyse here its glomerular organisation based on ethyl-gallate-stained and synapsin-stained preparations. Although we have confirmed that the majority of glomeruli can be individually identified in various antennal lobes, we have recognised several types of biological variability. Some glomeruli are absent, possibly indicating the lack of the corresponding receptor neuron type or its misrouting during development. The antennal lobes vary in global shape and, consequently, the spatial location of the glomerular changes. Although they do not prevent glomerulus identification when quantitative analysis methods are used, these variations place limits on the straightforward identification of glomeruli in functional studies, e.g. calcium-imaging or single-cell staining, when using conventional three-dimensional maps of individual antennal lobes.

Roles and effects of environmental carbon dioxide in insect life

Carbon dioxide (CO(2)) is a ubiquitous sensory cue that plays multiple roles in insect behavior. In recent years understanding of the well-known role of CO(2) in foraging by hematophagous insects (e.g., mosquitoes) has grown, and research on the roles of CO(2) cues in the foraging and oviposition behavior of phytophagous insects and in behavior of social insects has stimulated interest in this area of insect sensory biology. This review considers those advances, as well as some of the mechanistic bases of the modulation of behavior by CO(2) and important progress in our understanding of the detection and CNS processing of CO(2) information in insects. Finally, this review briefly addresses how the ongoing increase in atmospheric CO(2) levels may affect insect life.

The antennal lobe of the African malaria mosquito, Anopheles gambiae - innervation and three-dimensional reconstruction

Antibody labelling and subsequent three-dimensional reconstructions of the primary olfactory centres, the antennal lobes, of male and female African malaria mosquitoes, Anopheles gambiae, revealed 61 and 60 glomerular neuropils respectively. In addition to the small difference in number of glomeruli, sexual dimorphism was observed in both the size of the antennal lobe and of individual glomeruli. Furthermore, sexual specificity was observed within the array. Anterograde staining of afferents from peripheral olfactory organs support the reconstruction of the glomerular array. Although anterograde stainings support an organotopic organization of the antennal lobe, convergence of afferents originating from different organs into single glomeruli is observed. This finding, in both A. gambiae and A. aegypti, may shed new light upon the development and function of the olfactory system.

Multiple olfactory receptor neurons and their axonal projections in the antennal lobe of the honeybee Apis mellifera

The poreplate sensilla of honeybees are equipped with multiple olfactory receptor neurons (ORNs), which innervate glomeruli of the antennal lobe (AL). We investigated the axonal projection pattern in glomeruli of the AL (glomerular pattern), formed by the multiple ORNs of individual poreplate sensilla. We used the different glomerular patterns to draw conclusions about the equipment of poreplate sensilla with different ORN types. ORNs of single poreplate sensilla were stained and analyzed by laser-scanning confocal microscopy and 3D software (AMIRA). In 13 specimens we found between 7 and 23 ORNs. This is in accordance with data found in the literature (5-35 ORNs) suggesting that all ORNs of the single poreplate sensilla were stained. The ORNs innervate the AL via all four sensory tracts (T1-T4), and glomeruli of the anterior part of the AL are more often innervated. Each ORN innervates a single glomerulus (uniglomerular), and all ORNs of one poreplate sensillum project to different glomeruli. Visual inspection and individual identification of glomeruli, based on the honeybee digital AL atlas, were used to evaluate mapping of glomeruli by a rigid transformation of the experimental ALs onto a reference AL. ORNs belonging to individual poreplate sensilla form variable glomerular patterns, and we did not find a common organization of glomerular patterns. We conclude that poreplate sensilla are equipped with different ORN types but that the same ORN types can be found in different poreplate sensilla. The equipment of poreplate sensilla with ORNs is overlapping. The mapping of glomeruli by rigid transformation is revealed to be a powerful tool for comparative neuroanatomy.

Neuronal architecture of the mosquito deutocerebrum

Mosquito behavior is heavily dependent on olfactory and mechanosensory cues, which are detected by receptor neurons on the antenna and on the palps. Recent progress in mosquito sensory genomics highlights the need for an up-to-date understanding of the neural architecture of the mosquito brain. Here we present a detailed description of the neural structure of the primary target of the majority of these neurons, the deutocerebrum, in the African malaria (Anopheles gambiae) and yellow fever (Aedes aegypti) mosquitoes. Special focus is made on the olfactory system, the antennal lobe (AL), where we present high-resolution three-dimensional models of the ALs of male and female Ae. aegypti. These models reveal a sexual dimorphism in the number of glomeruli, 49 and 50 glomeruli in male and female mosquitoes, respectively, and in the size of several of the identified glomeruli. The fine structure of receptor neuron terminations in the AL and the rest of the deutocerebrum is described, as are the arborizations of intrinsic deutocerebral neurons and neurons providing output to higher brain areas. In the AL a specific and very large center receiving input from the mechanosensory Johnston's organ is revealed as a multilobed structure receiving peripheral input according to a somatotopic pattern. Within the antennal nerve a specific neuropil containing early, bouton-like ramifications of receptor neurons is described. Within the glomerular array of the AL, neurons providing a possible feedback circuit to antennal receptor neurons are shown. With these results we provide a new resolution in mosquito deutocerebral architecture.