蚊虫嗅觉识别的神经机制

蚊虫主要依赖嗅觉系统与外界环境进行化学信息交流。蚊虫通过嗅觉感受系统寻找食物、 配偶和产卵场所, 进而做出相应的行为反应。本文综述了近年来蚊虫嗅觉系统对气味信号神经传导机制的研究进展。蚊虫的嗅觉感器主要位于触角和下颚须, 触角上的毛形感器和锥形感器感受氨水、 乳酸、 羧酸类化合物等人体和其他动物释放的微量气味物质, 下颚须上的锥形感器则感受呼出的二氧化碳以及一些其他的挥发性物质; 蚊虫嗅觉感器内部有受体神经细胞, 其上分布有嗅觉受体蛋白, 蚊虫对外界环境的化学感受就是通过气味物质与这些受体蛋白互作而得以实现; 根据对不同气味物质的反应谱差异, 嗅觉神经细胞被分为不同的功能类型; 来自嗅觉神经细胞的神经信号进一步从外周传导至中枢神经中脑触角叶内的神经小球, 在此对信息进行初步的处理, 通过评估嗅觉神经细胞的反应和触角叶内的神经小球相应被激活的区域, 不同小球被分别命名; 最后, 神经信号继续整合, 由投射神经传向前脑, 最终引发一系列昆虫行为反应。这些研究从理论上剖析了气味信号在蚊虫嗅觉系统中的神经转导通路, 对于我们深刻理解蚊虫的嗅觉系统具有重要意义, 同时也有助于进一步理解其他昆虫甚至人类的气味识别机制及进行更深层次神经科学的探索。     

Olfactory Coding in Antennal Neurons of the Malaria Mosquito, Anopheles gambiae

Olfactory receptor neurons (ORNs) in the antenna of insects serve to encode odors in action potential activity conducted to the olfactory lobe of the deuterocerebrum. We performed an analysis of the electrophysiological responses of olfactory neurons in the antennae of the female malaria mosquito Anopheles gambiae s.s. and investigated the effect of blood feeding on responsiveness. Forty-four chemicals that are known to be present in human volatile emanations were used as odor stimuli. We identified 6 functional types of trichoid sensilla and 5 functional types of grooved-peg sensilla (GP) based on a hierarchical cluster analysis. Generalist ORNs, tuned to a broad range of odors, moderate specialist ORNs and 2 ORNs tuned to only one odor were identified in different sensilla types. Neurons in GP were tuned to more polar compounds including the important behavioral attractant ammonia and its synergist L-lactic acid, responses to which were found only in GP. Combinatorial coding is the most plausible principle operating in the olfactory system of this mosquito species. We document for the first time both up- and downregulation of ORN responsiveness after blood feeding. Modulation of host-seeking and oviposition behavior is associated with both qualitative and quantitative changes in the peripheral sensory system.     

Antennal sensilla of whiteflies: Trialeurodes vaporariorum (Westwood), the glasshouse whitefly,and Aleyrodes proletella (Linnaeus), the cabbage whitefly, (Homoptera : Aleyrodidae). Part 2: Ultrastructure

A transmission electron microscope study of the antennal sensilla of the whitefly Trialeurodes vaporariorum and Aleyrodes proletella (Homoptera : Aleyrodidae) revealed that of the sensilla unique to the antennal flagellum (basiconic, coeloconic and small digitate-tipped sensory pegs), basiconic and coeloconic sensilla occur as subtypes. Four subtypes of basiconic cone sensilla occur on the flagella of T. vaporariorum and 3 on A. proletella. All the subtypes of basiconic sensilla have an ultrastructure typical of olfactory sensilla and probably have a primary olfactory function. Two subtypes of coeloconic sensilla occur on the flagella of both species. Their ultrastructure suggests primarily a chemosensory function. The digitate-tipped sensory peg of both species possesses a triad of neurones which have ultrastructural characteristics similar to the known thermo-/hygroreceptors of other insect species. The other sensilla, which occur on the antennae of the whiteflies, include cheatae, campaniform and subcuticular sensilla, all of which have an ultrastructure typical of mechanoreceptors.     

Physiology and antennal lobe projections of olfactory receptor neurons from sexually isomorphic sensilla on male <Emphasis Type="Italic">Heliothis virescens</Emphasis>

The neurophysiology and antennal lobe projections of olfactory receptor neurons (ORNs) within sexually isomorphic short trichoid sensilla of male Heliothis virescens (Noctuidae: Lepidoptera) were investigated using cut-sensillum recording and cobalt-lysine staining. A total of 202 sensilla were sorted into 14 possible sensillar categories based on odor responses and physiology of ORNs within. Seventy-two percent of the sensilla identified contained ORNs stimulated by conspecific odors. In addition, a large number of ORNs were specifically sensitive to ß-caryophyllene, a plant-derived volatile (N = 41). Axons originating from ORNs associated with individual sensilla were stained with cobalt lysine (N = 67) and traced to individual glomeruli in the antennal lobe. ORNs with responses to female sex pheromone components exhibited similar axonal projections as those previously described from ORNs in long sensilla trichodea in male H. virescens. Antennal lobe axonal arborizations of ORNs sensitive to hairpencil components were also located in glomeruli near the base of the antennal nerve, whilst those sensitive to plant odorants projected to more medial glomeruli. Comparisons with ORNs described from female H. virescens supports the notion that glomeruli at the base of the antennal nerve are associated with conspecific and interspecific odorants, whereas those located medially are associated with plant volatiles.     

Analysis of immunocytochemical staining patterns in the antennal system of Drosophila melanogaster

By immunizing mice with homogenized brains, heads, or a mixture of heads and antennae of D. melanogaster, we obtained six monoclonal antibodies (mabs) that bind to the olfactory system of Drosophila with various degrees of specificity. They can be divided into three groups with respect to their staining pattern: (1) The antibodies ca51/2, na21/2, and nb230 label both in the third (olfactory) antennal segment and in the visual ganglia. All of them bind to antennal structures that can be correlated with basiconic sensilla. The antibody ca51/2 labels sensory neurons of these sensilla. In the antenna of the lozenge ³ mutant, which lacks basiconic sensilla, no labeling is present. In Western blots ca51/2 recognizes in the antenna an antigen of 43.5 kDa, which is expressed in the antenna only in the presence of basiconic sensilla. The antibody na21/2 binds to basiconic and coeloconic sensilla, most likely to the apical part of sheath cells. In immunoblots it recognizes in the antenna two antigens of 42.2 kDa and 46.7 kDa. The latter appears to be correlated in the antenna with the presence of basiconic sensilla. (2) The staining pattern of antibody nc10 is associated with the sheath cells of basiconic and coeloconic sensilla. Moreover, nc10 binds to a subset of glomeruli in the antennal lobe. (3) The staining pattern of the antibodies VG2 and I24B5 is restricted to the antenna. I24B5 recognizes coeloconic sensilla and VG2 recognizes both coeloconic and basiconic sensilla. Staining patterns in both cases include sheath cells.     

The Drosophila melanogaster Phospholipid Flippase dATP8B Is Required for Odorant Receptor Function

The olfactory systems of insects are fundamental to all aspects of their behaviour, and insect olfactory receptor neurons (ORNs) exhibit exquisite specificity and sensitivity to a wide range of environmental cues. In Drosophila melanogaster, ORN responses are determined by three different receptor families, the odorant (Or), ionotropic-like (IR) and gustatory (Gr) receptors. However, the precise mechanisms of signalling by these different receptor families are not fully understood. Here we report the unexpected finding that the type 4 P-type ATPase phospholipid transporter dATP8B, the homologue of a protein associated with intrahepatic cholestasis and hearing loss in humans, is crucial for Drosophila olfactory responses. Mutations in dATP8B severely attenuate sensitivity of odorant detection specifically in Or-expressing ORNs, but do not affect responses mediated by IR or Gr receptors. Accordingly, we find dATP8B to be expressed in ORNs and localised to the dendritic membrane of the olfactory neurons where signal transduction occurs. Localisation of Or proteins to the dendrites is unaffected in dATP8B mutants, as is dendrite morphology, suggesting instead that dATP8B is critical for Or signalling. As dATP8B is a member of the phospholipid flippase family of ATPases, which function to determine asymmetry in phospholipid composition between the outer and inner leaflets of plasma membranes, our findings suggest a requirement for phospholipid asymmetry in the signalling of a specific family of chemoreceptor proteins.     

Notch Is Required in Adult Drosophila Sensory Neurons for Morphological and Functional Plasticity of the Olfactory Circuit

Olfactory receptor neurons (ORNs) convey odor information to the central brain, but like other sensory neurons were thought to play a passive role in memory formation and storage. Here we show that Notch, part of an evolutionarily conserved intercellular signaling pathway, is required in adult Drosophila ORNs for the structural and functional plasticity of olfactory glomeruli that is induced by chronic odor exposure. Specifically, we show that Notch activity in ORNs is necessary for the odor specific increase in the volume of glomeruli that occurs as a consequence of prolonged odor exposure. Calcium imaging experiments indicate that Notch in ORNs is also required for the chronic odor induced changes in the physiology of ORNs and the ensuing changes in the physiological response of their second order projection neurons (PNs). We further show that Notch in ORNs acts by both canonical cleavage-dependent and non-canonical cleavage-independent pathways. The Notch ligand Delta (Dl) in PNs switches the balance between the pathways. These data define a circuit whereby, in conjunction with odor, N activity in the periphery regulates the activity of neurons in the central brain and Dl in the central brain regulates N activity in the periphery. Our work highlights the importance of experience dependent plasticity at the first olfactory synapse.     

Central projections of olfactory receptor neurons from single antennal and palpal sensilla in mosquitoes

In insects, olfactory receptor neurons (ORNs) are located in cuticular sensilla, that are present on the antennae and on the maxillary palps. Their axons project into spherical neuropil, the glomeruli, which are characteristic structures in the primary olfactory center throughout the animal kingdom. ORNs in insects often respond specifically to single odor compounds. The projection patterns of these neurons within the primary olfactory center, the antennal lobe, are, however, largely unknown.We developed a method to stain central projections of intact receptor neurons known to respond to host odor compounds in the malaria mosquito, Anopheles gambiae. Terminal arborizations from ORNs from antennal sensilla had only a few branches apparently restricted to a single glomerulus. Axonal arborizations of the different neurons originating from the same sensillum did not overlap.ORNs originating from maxillary palp sensilla all projected into a dorso-medial area in both the ipsi- and contralateral antennal lobe, which received in no case axon terminals from antennal receptor neurons. Staining of maxillary palp receptor neurons in a second mosquito species (Aedes aegypti) revealed unilateral arborizations in an area at a similar position as in An. gambiae.     

Mixture interactions in moth olfactory physiology: examining the effects of odorant mixture, concentration, distal stimulation, and antennal nerve transection on sensillar responses

The insect olfactory system is challenged to decipher valid signals from among an assortment of chemical cues present in the airborne environment. In the moth, Heliothis virescens, males rely upon detection and discrimination of a unique blend of components in the female sex pheromone to locate mates. The effect of variable odor mixtures was used to examine physiological responses from neurons within sensilla on the moth antenna sensitive to female sex pheromone components. Increasing concentrations of heliothine sex pheromone components applied in concert with the cognate stimulus for each neuronal type resulted in mixture suppression of activity, except for one odorant combination where mixture enhancement was apparent. Olfactory receptor neuron (ORN) responses were compared between moths with intact and transected antennal nerves to determine whether specific instances of suppression might be influenced by central mechanisms. Type A sensilla showed little variation in response between transected and intact preparations; however, recordings from type B sensilla with transected antennal nerves exhibited reduced mixture suppression. Testing by parallel stimulation of distal antennal segments while recording and stimulating proximal segments dismissed the possibility of interneuronal or ephaptic effects upon sensillar responses. The results indicate that increasing concentrations of "noncognate" odorants in an odor mixture or antennal nerve transection can produce variation in the intensity and temporal dynamics of physiological recordings from H. virescens ORNs.     

Atypical membrane topology and heteromeric function of Drosophila odorant receptors in vivo

Drosophila olfactory sensory neurons (OSNs) each express two odorant receptors (ORs): a divergent member of the OR family and the highly conserved, broadly expressed receptor OR83b. OR83b is essential for olfaction in vivo and enhances OR function in vitro, but the molecular mechanism by which it acts is unknown. Here we demonstrate that OR83b heterodimerizes with conventional ORs early in the endomembrane system in OSNs, couples these complexes to the conserved ciliary trafficking pathway, and is essential to maintain the OR/OR83b complex within the sensory cilia, where odor signal transduction occurs. The OR/OR83b complex is necessary and sufficient to promote functional reconstitution of odor-evoked signaling in sensory neurons that normally respond only to carbon dioxide. Unexpectedly, unlike all known vertebrate and nematode chemosensory receptors, we find that Drosophila ORs and OR83b adopt a novel membrane topology with their N-termini and the most conserved loops in the cytoplasm. These loops mediate direct association of ORs with OR83b. Our results reveal that OR83b is a universal and integral part of the functional OR in Drosophila. This atypical heteromeric and topological design appears to be an insect-specific solution for odor recognition, making the OR/OR83b complex an attractive target for the development of highly selective insect repellents to disrupt olfactory-mediated host-seeking behaviors of insect disease vectors.     

Morphological and ultrastructural characterization of olfactory sensilla in Drosophila suzukii: Scanning and transmission electron microscopy

Drosophila suzukii is a serious horticultural and quarantine pest, damaging various berry crops. Although the active use of olfactory communication in D. suzukii is well-known, their olfactory sensory system has not been comprehensively reported. Therefore, the present study was carried out to understand the morphology, distribution and ultrastructure of olfactory sensilla present in the antennae and maxillary palps of D. suzukii, through scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The olfactory sensilla on the antennae of D. suzukii in both sexes could be classified into three major morphological types, basiconic, trichoid and coeloconic sensilla, according to their shapes. The antennal basiconic sensilla were further divided into three subtypes and the antennal trichoid sensilla into two subtypes, respectively, according to the size of individual sensillum. In contrast to the antennal olfactory sensilla showing diverse morphology, basiconic sensilla was the only type of olfactory sensilla in the maxillary palps of D. suzukii. The basiconic sensilla in the maxillary palps could be further classified into three subtypes, based on their size. Our SEM and TEM observations indicated that multiple nanoscale pores are present on the surface of all types of olfactory sensilla in the antennae and maxillary palps, except coeloconic sensilla. The difference in the morphological types and the distribution of olfactory sensilla suggests that their olfactory functions are different between antennae and maxillary palps in D. suzukii. The results of this study provide useful information for further studies to determine the function of olfactory sensilla in D. suzukii and to understand their chemical communication system.     

Flagellar sensilla of the eusocial gall-inducing thrips Kladothrips intermedius and its kleptoparasite, Koptothrips dyskritus (Thysanoptera: Phlaeothripinae)

Insect antennal flagella host a multitude of sensory organs fulfilling different functions. Chemoreception, for example, is essential for insects in many contexts. Both olfaction and contact chemoreception are involved in host-plant selection, as well as in the integrity of insect societies, especially for nestmate recognition. Kladothrips intermedius is a eusocial gall-inducing thrips with two castes: dispersers and soldiers. Koptothrips dyskritus is a specialist in invading Kl. intermedius galls, killing the occupants, and thereby gaining the food and shelter offered by galls. In this study, we compared the morphology and ultrastructure of the flagellar sensilla of Kl. intermedius and Ko. dyskritus via scanning and transmission electron microscopy in order to facilitate future investigations of their sensory ecology, with an emphasis on chemical ecology. The two species show a very similar sensillar array. There are a few mechanosensory trichoid and a second type of mechanosensory sensilla, thermo-hygroreceptive sensilla, olfactory single-walled basiconic and double-walled coeloconic sensilla as well as contact chemoreceptive chaetic sensilla. The latter are sexually dimorphic in Kl. intermedius. Dispersers and soldiers of Kl. intermedius do not present noteworthy morphological differences, but the ultrastructural investigations revealed that soldiers have fewer ORNs, possibly an adaptation to their gall-cloistered lifestyle.     

Analysis of immunocytochemical staining patterns in the antennal system of Drosophila melanogaster

By immunizing mice with homogenized brains, heads, or a mixture of heads and antennae of D. melanogaster, we obtained six monoclonal antibodies (mabs) that bind to the olfactory system of Drosophila with various degrees of specificity. They can be divided into three groups with respect to their staining pattern: (1) The antibodies ca51/2, na21/2, and nb230 label both in the third (olfactory) antennal segment and in the visual ganglia. All of them bind to antennal structures that can be correlated with basiconic sensilla. The antibody ca51/2 labels sensory neurons of these sensilla. In the antenna of the lozenge ³ mutant, which lacks basiconic sensilla, no labeling is present. In Western blots ca51/2 recognizes in the antenna an antigen of 43.5 kDa, which is expressed in the antenna only in the presence of basiconic sensilla. The antibody na21/2 binds to basiconic and coeloconic sensilla, most likely to the apical part of sheath cells. In immunoblots it recognizes in the antenna two antigens of 42.2 kDa and 46.7 kDa. The latter appears to be correlated in the antenna with the presence of basiconic sensilla. (2) The staining pattern of antibody nc10 is associated with the sheath cells of basiconic and coeloconic sensilla. Moreover, nc10 binds to a subset of glomeruli in the antennal lobe. (3) The staining pattern of the antibodies VG2 and I24B5 is restricted to the antenna. I24B5 recognizes coeloconic sensilla and VG2 recognizes both coeloconic and basiconic sensilla. Staining patterns in both cases include sheath cells.     

Molecular, anatomical, and functional organization of the Drosophila olfactory system

BACKGROUND: Olfactory receptor neurons (ORNs) convey chemical information into the brain, producing internal representations of odors detected in the periphery. A comprehensive understanding of the molecular and neural mechanisms of odor detection and processing requires complete maps of odorant receptor (Or) expression and ORN connectivity, preferably at single-cell resolution. RESULTS: We have constructed near-complete maps of Or expression and ORN targeting in the Drosophila olfactory system. These maps confirm the general validity of the "one neuron--one receptor" and "one glomerulus--one receptor" principles and reveal several additional features of olfactory organization. ORNs in distinct sensilla types project to distinct regions of the antennal lobe, but neighbor relations are not preserved. ORNs grouped in the same sensilla do not express similar receptors, but similar receptors tend to map to closely appositioned glomeruli in the antennal lobe. This organization may serve to ensure that odor representations are dispersed in the periphery but clustered centrally. Integrated with electrophysiological data, these maps also predict glomerular representations of specific odorants. Representations of aliphatic and aromatic compounds are spatially segregated, with those of aliphatic compounds arranged topographically according to carbon chain length. CONCLUSIONS: These Or expression and ORN connectivity maps provide further insight into the molecular, anatomical, and functional organization of the Drosophila olfactory system. Our maps also provide an essential resource for investigating how internal odor representations are generated and how they are further processed and transmitted to higher brain centers.     

Characterization of olfactory receptor neurons for pheromone candidate and plant volatile compounds in the clover root weevil,Sitona lepidus

Antennal olfactory receptor neurons (ORNs) for pheromone and plant volatile compounds were identified and characterized in male and female clover root weevil, Sitona lepidus (Gyllenhal), using the single sensillum recording technique with five pheromone-related compounds, and 40 host and non-host plant volatile compounds. Overall, seven different types of olfactory sensilla containing specialized ORNs were identified in each sex of S. lepidus. Among them, three different types of sensilla in the males and two types in the females housed ORNs specialized for pheromone-related compounds. The ORNs in males were specialized for 4-methyl-3,5-heptanedione or one or more of four stereoisomers of 5-hydroxy-4-methyl-3-heptanone. In contrast, female sensilla did not contain ORNs sensitive to 4-methyl-3,5-heptanedione while they contained ORNs sensitive to and specialized for the stereoisomers of (4S,5S)-5-hydroxy-4-methyl-3-heptanone. In addition to the pheromone-related ORNs, four types of olfactory sensilla contained ORNs responsive to plant volatile compounds in male S. lepidus, and five types in females. Most of the ORNs identified in S. lepidus showed a high degree of specificity to specific volatile compounds although some of the active compounds showed overlapping response spectra in the ORNs across different types of sensilla. The most active plant volatile compounds were the four green leaf volatile compounds, (E)-2-hexenol, (Z)-2-hexenol, (Z)-3-hexenol and (E)-2-hexenal, and isomers of two monoterpenols, (±)-linalool and (±)-α-terpineol, all eliciting strong responses from relatively large numbers of ORNs in male and female S. lepidus. Our study indicates that S. lepidus has a set of highly sensitive and selective ORNs for pheromone and plant volatile compounds. Further work is needed to elucidate the behavioral implications of these findings.     

A broadly tuned odorant receptor in neurons of trichoid sensilla in locust,Locusta migratoria

Insects have evolved sophisticated olfactory reception systems to sense exogenous chemical signals. Odorant receptors (ORs) on the membrane of chemosensory neurons are believed to be key molecules in sensing exogenous chemical cues. ORs in different species of insects are diverse and should tune a species to its own specific semiochemicals relevant to their survival. The orthopteran insect, locust (Locusta migratoria), is a model hemimetabolous insect. There is very limited knowledge on the functions of locust ORs although many locust OR genes have been identified in genomic sequencing experiments. In this paper, a locust OR, LmigOR3 was localized to neurons housed in trichoid sensilla by in situ hybridization. LmigOR3 was expressed as a transgene in Drosophila trichoid olfactory neurons (aT1) lacking the endogenous receptor Or67d and the olfactory tuning curve and dose-response curves were established for this locust receptor. The results show that LmigOR3 sensitizes neurons to ketones, esters and heterocyclic compounds, indicating that LmigOR3 is a broadly tuned receptor. LmigOR3 is the first odorant receptor from Orthoptera that has been functionally analyzed in the Drosophila aT1 system. This work demonstrates the utility of the Drosophila aT1 system for functional analysis of locust odorant receptors and suggests that LmigOR3 may be involved in detecting food odorants, or perhaps locust body volatiles that may help us to develop new control methods for locusts.