Olfactory physiology in the Drosophila antenna and maxillary palp: acj6 distinguishes two classes of odorant pathways.

This article provides characterization of the electrical response to odorants in the Drosophila antenna and provides physiological evidence that a second organ, the maxillary palp, also has olfactory function in Drosophila. The acj6 mutation, previously isolated by virtue of defective olfactory behavior, affects olfactory physiology in the maxillary palp as well as in the antenna. Interestingly, abnormal chemosensory jump 6 (acj6) reduces response in the maxillary palp to all odorants tested except benzaldehyde (odor of almond), as if response to benzaldehyde is mediated through a different type of odorant pathway from the other odorants. In other experiments, different parts of the antenna are shown to differ with respect to odorant sensitivity. Evidence is also provided that antennal response to odorants varies with age, and that odorants differ in their age dependence.     

Maxillary palps can mediate taste rejection of plant allelochemicals by caterpillars

All caterpillars possess a pair of maxillary palps that “drum” the surface of foods during feeding. These chemosensory organs contain over 65% of a caterpillar's taste receptor cells, but their functional significance remains largely unknown. We examined their role in rejection of plant allelochemicals, using the tobacco hornworm (Manduca sexta) as a model insect and an extract from a plant species (Grindeliaglutinosa) as a model stimulus. We selected this system because hornworms reject foods containing Grindelia extract, and because preliminary studies indicated that their maxillary palps respond to this extract. We hypothesized that Grindelia extract elicits rejection through stimulating: (1) olfactory receptor cells, (2) taste receptor cells, (3) oral mechanoreceptors, and/or (4) a postingestive response mechanism. Our results were consistent only with hypothesis 2: caterpillars approached Grindelia-treated diets without apparent hesitation, but rejected it within 6 s of initiating biting; Grindelia-treated solutions stimulated taste receptor cells in the maxillary palp, but not the other gustatory chemosensilla; and ablating the maxillary palps eliminated rejection of Grindelia-treated diets. Our results demonstrate that taste receptor cells in the maxillary palps mediate rejection of Grindelia extract, and provide the first direct evidence for a role of maxillary palps in rejection of plant allelochemicals. Accepted: 25 January 1998     

Olfactory physiology in the Drosophila antenna and maxillary palp: acj6 Distinguishes two classes of odorant pathways

This article provides characterization of the electrical response to odorants in the Drosophila antenna and provides physiological evidence that a second organ, the maxillary palp, also has olfactory function in Drosophila. The acj6 mutation, previously isolated by virtue of defective olfactory behavior, affects olfactory physiology in the maxillary palp as well as in the antenna. Interestingly, abnormal chemosensory jump 6 (acj6) reduces response in the maxillary palp to all odorants tested except benzaldehyde (odor of almond), as if response to benzaldehyde is mediated through a different type of odorant pathway from the other odorants. In other experiments, different parts of the antenna are shown to differ with respect to odorant sensitivity. Evidence is also provided that antennal response to odorants varies with age, and that odorants differ in their age dependence. © 1992 John Wiley & Sons, Inc.     

A spatial map of olfactory receptor expression in the Drosophila antenna

Insects provide an attractive system for the study of olfactory sensory perception. We have identified a novel family of seven transmembrane domain proteins, encoded by 100 to 200 genes, that is likely to represent the family of Drosophila odorant receptors. Members of this gene family are expressed in topographically defined subpopulations of olfactory sensory neurons in either the antenna or the maxillary palp. Sensory neurons express different complements of receptor genes, such that individual neurons are functionally distinct. The isolation of candidate odorant receptor genes along with a genetic analysis of olfactory-driven behavior in insects may ultimately afford a system to understand the mechanistic link between odor recognition and behavior.     

Odor coding in the maxillary palp of the malaria vector mosquito Anopheles gambiae

BACKGROUND: Many species of mosquitoes, including the major malaria vector Anopheles gambiae, utilize carbon dioxide (CO(2)) and 1-octen-3-ol as olfactory cues in host-seeking behaviors that underlie their vectorial capacity. However, the molecular and cellular basis of such olfactory responses remains largely unknown. RESULTS: Here, we use molecular and physiological approaches coupled with systematic functional analyses to define the complete olfactory sensory map of the An. gambiae maxillary palp, an olfactory appendage that mediates the detection of these compounds. In doing so, we identify three olfactory receptor neurons (ORNs) that are organized in stereotyped triads within the maxillary-palp capitate-peg-sensillum population. One ORN is CO(2)-responsive and characterized by the coexpression of three receptors that confer CO(2) responses, whereas the other ORNs express characteristic odorant receptors (AgORs) that are responsible for their in vivo olfactory responses. CONCLUSIONS: Our results describe a complete and highly concordant map of both the molecular and cellular olfactory components on the maxillary palp of the adult female An. gambiae mosquito. These results also facilitate the understanding of how An. gambiae mosquitoes sense olfactory cues that might be exploited to compromise their ability to transmit malaria.     

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.     

Maxillary palp glomeruli and ipsilateral projections in the antennal lobe of<Emphasis Type="Italic">Drosophila melanogaster</Emphasis>

The antennal lobe was examined by Golgi-silver impregnation to differentiate the glomeruli depending on the source and types of inputs. Thirty-five of the 43 ‘identified’ olfactory glomeruli were Golgi-silver impregnated in the present study. Seven glomeruli compared to three, reported previously, were found to be targets of maxillary palp chemosensory neurons. These include glomeruli VA3, VC2, VM5, VA7m/VA7l of the ventral antennal lobe and DC2, DC3, DM5 of the dorsal antennal lobe. The number of glomeruli receiving the maxillary palp sensory projections tallies with the number ofDrosophila olfactory receptors (seven) reported to be expressed exclusively in the maxillary palp. Twenty-eight Golgi-impregnated glomeruli were found to receive input from the antennal nerve. The ratio of glomeruli serving the maxillary palp to those serving the antenna (∼1:5) matches with the ratio ofDrosophila olfactory receptors expressed in these two olfactory organs respectively. In addition to glomerulus V, glomeruli VP1-3, VL1, VL2a/2p and VC3m/3l were found to receive ipsilateral projections. Thus, additional ipsilateral glomeruli have been identified.     

Olfactory physiology in the Drosophila maxillary palp requires the visual system gene rdgB

We describe the kinetics of odorant response in the maxillary palp of Drosophila, and show that the rate of recovery from odorant stimulation is affected by mutation of the rdgB (retinal degeneration B) gene. We use immunocytochemistry to confirm that the rdgB gene product is expressed in the maxillary palp. rdgB has recently been shown to encode a protein with Ca²⁺-binding sites and sequence similarity to rat brain phosphatidylinositol transfer protein; it is located near the rhabdomeric membranes in photoreceptor cells, where it has been suggested to play a role in membrane transport. The delay in recovery kinetics that we observe in olfactory tissue may reflect a defect in membrane restoration at the conclusion of the olfactory transduction cascade. The use of common molecules in the physiology of two olfactory organs, and in both visual and olfactory physiology, is discussed.Abbreviations EAG electroantennogram - EPG electropalpogram - ERG electroretinogram - norpA no receptor potential A - PBS phosphate buffered saline - rdgB retinal degeneration B - PI phosphatidylinositol     

Adaptive variability of the gustatory system receptor part of the carp Cyprinus carpio (Cyprinidae, Teleostei) after chronic anosmia

To confirm the existence in fish of the olfactogustatory interactions of adaptive significance, the study was performed of reactions of the receptor part of the carp gustatory system to the complete olfactory deafferentation with use of the method of scanning electron microscopy. Peculiarities of morphological changes of the taste receptor apparatus of the external and oral localization were revealed, and the dynamic of development of the taste receptor adaptive changes was traced for 1–9 months after production of anosmia. Receptors of the intraoral gustatory system had reactions of low statistical significance which were delayed in time by 5–6 months. The receptor apparatus of the external gustatory system receptors of the intraoral taste system showed pronounced hypertrophic changes that appeared since 2–3 months after the beginning of anosmia. The changes consisted in an increase of the area of the taste bud (TB) sensory field and of their number in the sensory zones of the maxillary and mandibular parts. The taste receptors located in the maxillary barbels and in the mandible gular zone had the most expressed reactions to the olfactory deafferentation. In the fish with anosmia, formation of the additional structures of the “external taste” was detected in the form of epidermal processes under the lower lip, which were covered with taste papillae. Thus, it has been shown that the fish olfactory and gustatory systems are functionally interrelated; under conditions of olfactory deprivation the external gustatory system is able to undergo compensatory morphofunctional changes aimed at vicariation of the lost distant chemical reception.     

The maxillary palp of Drosophila : ultrastructure and physiology depends on the lozenge gene

The ultrastructure and physiology of the maxillary palp of Drosophila melanogaster have been studied in wild-type and lozenge mutants. Olfactory physiology in the maxillary palp is shown to depend upon the lozenge(lz) gene. Reduced response amplitudes were recorded for all odorants tested, and the physiological defect was shown to map to the lz locus. The structure of the maxillary palp sensilla is described by scanning electron microscopy (SEM) at high magnification, initially in the wild-type. A linear arrangement of pores, connected by furrows, was found in one class of sensilla, the basiconic sensilla. In the lz ³ mutant, morphological alterations in the basiconic sensilla and duplications of sensilla are documented by SEM. The correlation of structural abnormalities in the lz sensilla and physiological abnormalities in odorant response are consistent with an olfactory role for the basiconic sensilla of the maxillary palp. Accepted: 10 September 1996     

Taste perception and coding in Drosophila

BACKGROUND: Discrimination between edible and contaminated foods is crucial for the survival of animals. In Drosophila, a family of gustatory receptors (GRs) expressed in taste neurons is thought to mediate the recognition of sugars and bitter compounds, thereby controlling feeding behavior. RESULTS: We have characterized in detail the expression of eight Gr genes in the labial palps, the fly's main taste organ. These genes fall into two distinct groups: seven of them, including Gr66a, are expressed in 22 or fewer taste neurons in each labial palp. Additional experiments show that many of these genes are coexpressed in partially overlapping sets of neurons. In contrast, Gr5a, which encodes a receptor for trehalose, is expressed in a distinct and larger set of taste neurons associated with most chemosensory sensilla, including taste pegs. Mapping the axonal targets of cells expressing Gr66a and Gr5a reveals distinct projection patterns for these two groups of neurons in the brain. Moreover, tetanus toxin-mediated inactivation of Gr66a- or Gr5a-expressing cells shows that these two sets of neurons mediate distinct taste modalities-the perception of bitter (caffeine) and sweet (trehalose) taste, respectively. CONCLUSION: Discrimination between two taste modalities-sweet and bitter-requires specific sets of gustatory receptor neurons that express different Gr genes. Unlike the Drosophila olfactory system, where each neuron expresses a single olfactory receptor gene, taste neurons can express multiple receptors and do so in a complex Gr gene code that is unique for small sets of neurons.     

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。     

Temporal target restriction of olfactory receptor neurons by Semaphorin-1a/PlexinA-mediated axon-axon interactions

Axon-axon interactions have been implicated in neural circuit assembly, but the underlying mechanisms are poorly understood. Here, we show that in the Drosophila antennal lobe, early-arriving axons of olfactory receptor neurons (ORNs) from the antenna are required for the proper targeting of late-arriving ORN axons from the maxillary palp (MP). Semaphorin-1a is required for targeting of all MP but only half of the antennal ORN classes examined. Sema-1a acts nonautonomously to control ORN axon-axon interactions, in contrast to its cell-autonomous function in olfactory projection neurons. Phenotypic and genetic interaction analyses implicate PlexinA as the Sema-1a receptor in ORN targeting. Sema-1a on antennal ORN axons is required for correct targeting of MP axons within the antennal lobe, while interactions amongst MP axons facilitate their entry into the antennal lobe. We propose that Sema-1a/PlexinA-mediated repulsion provides a mechanism by which early-arriving ORN axons constrain the target choices of late-arriving axons.     

Sensilla on maxillary and labial palps in a helicophagous ground beetle larva (Coleoptera,Carabidae)

Sensilla on the labial and maxillary palp of Carabus lefebvrei Dejean, 1826 larvae were investigated using scanning and transmission electron microscopy. Two identical sets of sensilla were present on the tips of both palp pairs, and four morphological types of sensilla were identified: sensilla basiconica types 1 and 2, sensilla coeloconica and sensilla digitiformia. Ultrastructure indicates that the sensilla basiconica type 1 and coeloconica have a chemical role as gustatory and olfactory receptors, respectively, while sensilla basiconica type 2 are mechanoreceptors and the sensilla digitiformia are probably thermo‐ and hygroreceptors. Their function is discussed in relation to specialized prey detection and habitat adaptations.     

The olfactory responses of the antenna and maxillary palp of the fleshfly,Neobellieria bullata (Diptera: Sarcophagidae), and their sensitivity to blockage of nitric oxide synthase

The relative sensitivities of the olfactory receptors in the antenna and maxillary palp of the fleshfly, Neobellieria bullata, were assessed using simultaneous electroantennograms (EAGs) and electropalpograms (EPGs). In general, the antennae and maxillary palps were more sensitive to odors related to animals (blood extract and saturated carboxylic acid) than to odors that were plant-derived (citral, hexenol, hexenal). In addition, the maxillary palps were relatively less sensitive to plant-derived odorants than the antennae, perhaps related to their anatomical position. Scanning electron microscopy was also used to assess the types of sensilla found on the two organs. In addition, NADPH-diaphorase histochemistry was used in an attempt to localize the enzyme nitric oxide synthase (NOS) in the antenna and the maxillary palps. We found evidence of NADPH-diaphorase staining in both organs, with localized staining in the antennal cells and more general staining in the maxillary palps. When NOS was selectively blocked using the antagonist L-NAME, the amplitude of the EAGs and EPGs to odorants fell by 30-50%. In contrast, application of the inactive enantiomer, D-NAME, did not change the amplitude of the EAGs or the EPGs. Our results indicate that NOS is involved in the function of olfactory receptor cells in the fleshfly.