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。     

Advantage of the Highly Restricted Odorant Receptor Expression Pattern in Chemosensory Neurons of Drosophila

A fundamental molecular feature of olfactory systems is that individual neurons express only one receptor from a large odorant receptor gene family. While numerous theories have been proposed, the functional significance and evolutionary advantage of generating a sophisticated one-receptor-per neuron expression pattern is not well understood. Using the genetically tractable Drosophila melanogaster as a model, we demonstrate that the breakdown of this highly restricted expression pattern of an odorant receptor in neurons leads to a deficit in the ability to exploit new food sources. We show that animals with ectopic co-expression of odorant receptors also have a competitive disadvantage in a complex environment with limiting food sources. At the level of the olfactory system, we find changes in both the behavioral and electrophysiological responses to odorants that are detected by endogenous receptors when an olfactory receptor is broadly misexpressed in chemosensory neurons. Taken together these results indicate that restrictive expression patterns and segregation of odorant receptors to individual neuron classes are important for sensitive odor-detection and appropriate olfactory behaviors.     

Conserved, highly specialized olfactory receptor neurons for food compounds in 2 congeneric scarab beetles, Pachnoda interrupta and Pachnoda marginata

Few studies have systematically addressed evolutionary changes in olfactory neuron assemblies, either by genetic drift or as an adaptation to specific odor environments. We have studied the sense of olfaction in 2 congeneric scarab beetles, Pachnoda interrupta Olivier and Pachnoda marginata Drury (Coleoptera: Scarabaeidae: Cetoniinae), which are both opportunistic polyphages, feeding mainly on fruit and flowers. The 2 species occur in dissimilar habitats: P. interrupta is found in dry savannah, and P. marginata in tropical parts of equatorial Africa. To study how these species may have adapted their sense of olfaction to their odor environments, we utilized single-unit electrophysiology on olfactory sensilla with a wide selection of food-related compounds. Despite the differences in habitat, we found that the species shared most of the physiological types of olfactory receptor neurons (ORNs) encountered, although their proportions frequently varied between the species. The high degree of conservation in olfaction between the species implies that a similar sensory strategy is efficient for food search in both habitats. However, shifts in proportions of receptor neuron classes, and slight shifts in response profiles and/or presence of some ORN classes unique to either species, may reflect adaptation to a different set of hosts.     

Functional characteristics of a tiny but specialized olfactory system: olfactory receptor neurons of carrot psyllids (Homoptera: Triozidae)

With only approximately 50 olfactory receptor neurons (ORNs), the carrot psyllid Trioza apicalis (Homoptera: Psylloidea) may have the smallest olfactory system described in adult Neopteran insects. Using single sensillum recordings (SSR) and gas chromatograph-linked SSR, we characterized 4 olfactory sensilla forming a distinct morphological type, which together house approximately 25% of all ORNs. We recorded responses to extracts and single constituents from Daucus carota ssp. sativus, from the conifers Picea abies, Pinus sylvestris, and Juniperus communis, as well as from male and female T. apicalis. Receptor neurons were highly selective; only 9 compounds in total elicited repeatable responses, and each neuron responded to at most 3 individual compounds. Chemical profiles of carrot and conifers showed significant overlap, with 4 out of 9 electrophysiologically active compounds occurring in more than one type of extract, but a carrot-specific compound elicited the most repeated responses. We identified 4 tentative neuron classes and found a rather high degree of neuronal redundancy, with 1 neuron class present in 3 and another present in all 4 of the sensilla, respectively.     

Electrophysiological demonstration of independent olfactory receptor types and associated neuronal responses in the trout olfactory bulb

The present study attempts to highlight the principles by which peripheral olfactory information of across- and within-class odorant signals is transformed into bulbar neuron responses. For this purpose, we performed electro-olfactogram cross-adaptation and mixture experiments as well as single unit recording of olfactory bulb neurons using amino acid, bile acid and F-prostaglandin stimulants in brown and rainbow trout. The results show that amino acids, a bile acid and a F-prostaglandin activate independent receptor types. However, within the class of amino acids, different receptor types are only partially independent. Neurons responsive to bile acid and amino acids were segregated to the mid-dorsal and latero-posterior olfactory bulb, respectively. Of the 43 responsive olfactory bulb neurons studied in brown trout, 41 showed specificity for one odorant class. Olfactory bulb neurons gained responsiveness to new amino acids with increasing stimulant concentration. We conclude that different odorant classes activate specific neurons located in different regions of the trout olfactory bulb, and that information distinguishing related amino acids can be represented in a limited number of bulbar neurons with distinct response profiles under the conditions investigated.     

Onset of odorant receptor gene expression during olfactory sensory neuron regeneration

Individual olfactory sensory neurons are thought to express only one odorant receptor gene from a repertoire of hundreds to thousands of genes. How do these sensory neurons choose just one specific odorant receptor to express during their differentiation? As an initial attempt toward understanding the process of odorant receptor gene regulation, we studied when odorant receptor expression is activated during sensory neuron regeneration. We find that receptor gene expression is activated in postmitotic neurons and can occur in the absence of the olfactory bulb. These results suggest that receptor expression is restricted to the terminal stages of olfactory neuron differentiation, and sensory neurons do not simply inherit the odorant receptor that is already expressed in mitotic precursor cells. Our results also support a model in which odorant receptor gene expression occurs independent of the olfactory bulb.     

A comparative study of pheromone perception in two species of Noctuid moths

The electrical activity of single olfactory receptor neurons in male soybean looper (SBL) Pseudoplusia includens(Walker) and cabbage looper (CL) Trihoplusia ni(Hübner) moths was evaluated in response to stimulation with fixed amounts of the individual components of their respective pheromone blends. In common with earlier observations in the CL, there are at least two classes of morphologically distinct pheromone sensitive sensilla on the antenna of male SBL, each of which contains two olfactory receptor neurons. In both species, one class of sensilla contains an olfactory receptor neuron sensitive to (Z)-7-dodecen-1-ol acetate (Z-7, 12:AC), the major component in each insect's blend, and a companion receptor neuron which is sensitive to (Z)-7-dodecen-1-ol (Z7,12: OH). In both species the second class of sensilla contains an olfactory receptor neuron which is sensitive to one of the minor components of the pheromone blend. (Z)-5-dodecen-1-ol acetate (Z-5,12:AC) is an effective stimulus in SBL, whereas (Z)-7-tetradecen-1-ol acetate (Z-7,14:AC) is an effective stimulus in CL. However, these two stimulatory compounds have been identified only in the female CL gland; neither has been found in the SBL gland. Thus, in contrast to the CL, which has receptor neurons which are responsive exclusively to conspecific pheromone components, the SBL has a class of receptor neurons which is responsive to a minor component of another species' pheromone blend. Field-trapping assays in which Z-5,12:AC is added to the SBL blend suggest that this single CL component is a powerful inhibitor of male SBL behavioral responses to conspecific pheromone blends. The difference observed in the specificity of the receptor neurons in this second class of sensilla are thus believed to play an integral role in the isolation processes that are maintained between these two species and may well account for the observed behavioral differences in their responses to heterospecific pheromone blends.     

Olfactory receptor database: a sensory chemoreceptor resource

The Olfactory Receptor Database (ORDB) is a WWW-accessible database that has been expanded from an olfactory receptor resource to a chemoreceptor resource. It stores data on six classes of G-protein-coupled sensory chemoreceptors: (i) olfactory receptor-like proteins, (ii) vomeronasal receptors, (iii) insect olfactory receptors, (iv) worm chemo-receptors, (v) taste papilla receptors and (vi) fungal pheromone receptors. A complementary database of the ligands of these receptors (OdorDB) has been constructed and is publicly available in a pilot mode. The database schema of ORDB has been changed from traditional relational to EAV/CR (Entity-Attribute-Value with Classes and Relationships), which allows the interoperability of ORDB with other related databases as well as the creation of intra-database associations among objects. This inter-operability facilitates users to follow information from odor molecule binding to its putative receptor, to the properties of the neuron expressing the receptor, to a computational model of activity of olfactory bulb neurons. In addition, tools and resources have been added allowing users to access interactive phylogenetic trees and alignments of sensory chemoreceptors. ORDB is available via the WWW at http://ycmi.med. yale.edu/senselab/ordb/     

Olfactory receptor trafficking involves conserved regulatory steps

Olfactory receptors are difficult to functionally express in heterologous cells. They are typically retained in the endoplasmic reticulum of cells commonly used for functional expression studies and are only released to the plasma membrane in mature cells of the olfactory receptor neuron lineage. A recently developed olfactory cell line, odora, traffics olfactory receptors to the plasma membrane when differentiated. We found that undifferentiated odora cells do not traffic olfactory receptors to their surface, even though they release the receptors to the Golgi apparatus and endosomes. This behavior differs from other cell lines tested thus far. Differentiated odora cells also properly traffic vomeronasal receptors of the VN1 type, which lack sequence similarity to olfactory receptors. ODR-4, a protein that is necessary for plasma membrane trafficking of a chemosensory receptor in nematodes, facilitates trafficking of rat olfactory receptor U131 in odora and Chinese hamster ovary cells. Olfactory receptor trafficking from the endoplasmic reticulum to the plasma membrane involves at least two steps whose regulation depends on the maturation state of cells in the olfactory receptor neuron lineage. These results also indicate that some components of the regulatory mechanism are conserved.     

Ion channels from chemosensory olfactory neurons

The olfactory epithelium has the ability to respond to a large number of volatile compounds of small molecular weight. Ultimately, such a property lies on a specialized type of neuron, the olfactory receptor cell. In the presence of odorants, the olfactory receptor neuron responds with action potentials whose frequency depends on odorant concentration. The primary events in the process of olfactory transduction are thought to occur at the cilia of olfactory receptor neurons and involve the binding of odorants to receptor molecules followed by the opening of ion channels. A crucial step in understanding olfactory transduction requires identifying the mechanisms that regulate the electrical activity of olfactory cells. In the last couple of years, patch-clamp recording from isolated olfactory cells and reconstitution of olfactory membranes in planar lipid bilayers have begun to shed light on some of these mechanisms. Although the information emerging from such studies is still preliminary, there are already well-defined hypotheses on the molecular events that might underlie the primary events in olfactory transduction. Currently, attention is being focused on the notions that second messengers might be involved in the activation of ion channels in olfactory cilia, and that odorant binding to a receptor molecule might lead directly to the gating of ion channels in chemosensory olfactory membranes. The coming years promise to be exciting ones in the field of olfactory transduction. We have now the necessary tools to be able to confront hypotheses and experimental facts.     

Crypt neurons express a single V1R-related ora gene

Both ciliated and microvillous olfactory sensory neuron populations express large families of olfactory receptor genes. However, individual neurons generally express only a single receptor gene according to the "one neuron-one receptor" rule. We report here that crypt neurons, the third type of olfactory neurons in fish species, use an even more restricted mode of expression. We recently identified a novel olfactory receptor family of 6 highly conserved G protein-coupled receptors, the v1r-like ora genes. We show now that a single member of this family, ora4 is expressed in nearly all crypt neurons, whereas the other 5 ora genes are not found in this cell type. Consistent with these findings, ora4 is never coexpressed with any of the remaining 5 ora genes. Furthermore, several lines of evidence indicate the absence of any other olfactory receptor families in crypt neurons. These results suggest that the vast majority of the crypt neuron population may select one and the same olfactory receptor gene, a "one cell type-one receptor" mode of expression. Such an expression pattern is familiar in the visual system, with rhodopsin as the sole light receptor of rod photoreceptor cells, but unexpected in the sense of smell.     

Time-course of apoptosis in the olfactory epithelium of rainbow trout exposed to a low copper level

Copper at low doses is known to specifically induce olfactory neuron death in fish olfactory epithelium. Using light and electron transmission microscopy, we have investigated the features and the time-course of receptor cell death in rainbow trout exposed for 15 days to 20 mug Cu(2+)/l. Ultrastructural observations demonstrate that degenerating cells, which included both mature and immature neurons, exhibited morphological changes characteristic of a cell death by apoptosis. Quantitative analysis shows that the number of apoptotic cells increased significantly already after 1 day of exposure, reaching a peak at day 5. From this timepoint of exposure, no more mature neuron was noted in the olfactory epithelium. Following a significant decrease in the number of apoptotic cells at day 10, a second wave of neuron death was noted at day 15. These findings argue for the occurrence of a neurogenesis process to balance the receptor cell death, despite continued copper exposure, and for a higher vulnerability to the metal of olfactory neurons presenting more advanced stages of cell differentiation. The molecular mechanisms by which copper may induce olfactory neuron apoptosis are discussed.     

Single unit recording from olfactory cilia.

Sensory cilia from olfactory receptor cells can be pulled into a patch pipette located above the mucus layer of an olfactory mucosa. While the pipette does not form a tight electrical seal with the ciliary membrane, it nevertheless allows to record current transients driven by action potentials arising in the olfactory neuron. This method is an alternative to single-unit-recording with electrodes pushed into the mucosa and, in some respects, to patch clamp recordings from isolated olfactory cells. Its advantage is technical simplicity and minimal disturbance of the neuron from which signals are derived. Less than 5% of the chemosensitive apical surface of the neuron is covered by the pipette. The neuron remains in situ and its cilia remain covered with some mucus. (However, mucus is in part dissolved by the bathing solution). Odorant thresholds in the picomolar range were thus obtained.     

Odorant receptor gene choice in olfactory sensory neurons: the one receptor-one neuron hypothesis revisited

Designed for general chemical recognition, the mammalian olfactory system shares many similarities with the immune system. Among these is the popular notion that a single olfactory sensory neuron expresses a single odorant receptor gene, while all other approximately 1000 genes of this type remain silent. Here, I examine the evidence supporting the one receptor-one neuron hypothesis. I conclude that, contrary to widespread belief, it is far from being proven. I propose an hypothesis of a developmental phase of oligogenic expression that is followed by positive and negative selection resulting usually in cells with one expressed receptor. Curiously, selective processes are well established and widely accepted for lymphocytes, but these concepts are essentially ignored for olfactory sensory neurons, despite the analogies that are frequently made between these two systems. More attention must be paid to odorant receptor gene choice and expression during development and neuronal differentiation.     

The sox gene Dichaete is expressed in local interneurons and functions in development of the Drosophila adult olfactory circuit

In insects, the primary sites of integration for olfactory sensory input are the glomeruli in the antennal lobes. Here, axons of olfactory receptor neurons synapse with dendrites of the projection neurons that relay olfactory input to higher brain centers, such as the mushroom bodies and lateral horn. Interactions between olfactory receptor neurons and projection neurons are modulated by excitatory and inhibitory input from a group of local interneurons. While significant insight has been gleaned into the differentiation of olfactory receptor and projection neurons, much less is known about the development and function of the local interneurons. We have found that Dichaete, a conserved Sox HMG box gene, is strongly expressed in a cluster of LAAL cells located adjacent to each antennal lobe in the adult brain. Within these clusters, Dichaete protein expression is detected in both cholinergic and GABAergic local interneurons. In contrast, Dichaete expression is not detected in mature or developing projection neurons, or developing olfactory receptor neurons. Analysis of novel viable Dichaete mutant alleles revealed misrouting of specific projection neuron dendrites and axons, and alterations in glomeruli organization. These results suggest noncell autonomous functions of Dichaete in projection neuron differentiation as well as a potential role for Dichaete‐expressing local interneurons in development of the adult olfactory circuitry. © 2012 Wiley Periodicals, Inc. Develop Neurobiol, 2013