Neural mechanisms of context-dependent processing of CO2 avoidance behavior in fruit flies

The fruit fly, Drosophila melanogaster, innately avoids even low levels of CO₂. CO₂ is part of the so-called Drosophila stress odor produced by stressed flies, but also a byproduct of fermenting fruit, a main food source, making the strong avoidance behavior somewhat surprising. Therefore, we addressed whether feeding states might influence the fly’s behavior and processing of CO₂. In a recent report, we showed that this innate behavior is differentially processed and modified according to the feeding state of the fly. Interestingly, we found that hungry flies require the function of the mushroom body, a higher brain center required for olfactory learning and memory, but thought to be dispensable for innate olfactory behaviors. In addition, we anatomically and functionally characterized a novel bilateral projection neuron connecting the CO₂ sensory input to the mushroom body. This neuron was essential for processing of CO₂ in the starved fly but not in the fed fly. In this Extra View article, we provide evidence for the potential involvement of the neuromodulator dopamine in state-dependent CO₂ avoidance behavior. Taken together, our work demonstrates that CO₂ avoidance behavior is mediated by alternative neural pathways in a context-dependent manner. Furthermore, it shows that the mushroom body is not only involved in processing of learned olfactory behavior, as previously suggested, but also in context-dependent innate olfaction.     

Mushroom body input connections form independently of sensory activity in Drosophila melanogaster

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The olfactory circuit of the fruit fly Drosophila melanogaster

The olfactory circuit of the fruit fly Drosophila melanogaster has emerged in recent years as an excellent paradigm for studying the principles and mechanisms of information processing in neuronal circuits. We discuss here the organizational principles of the olfactory circuit that make it an attractive model for experimental manipulations, the lessons that have been learned, and future challenges.     

Structural plasticity of olfactory neuropils in relation to insect diapause

1. Many insects that live in temperate zones spend the cold season in a state of dormancy, referred to as diapause. As the insect must rely on resources that were gathered before entering diapause, keeping a low metabolic rate is of utmost importance. Organs that are metabolically expensive to maintain, such as the brain, can therefore become a liability to survival if they are too large.
2. Insects that go through diapause as adults generally do so before entering the season of reproduction. This order of events introduces a conflict between maintaining low metabolism during dormancy and emerging afterward with highly developed sensory systems that improve fitness during the mating season.
3. We investigated the timing of when investments into the olfactory system are made by measuring the volumes of primary and secondary olfactory neuropils in the brain as they fluctuate in size throughout the extended diapause life‐period of adult Polygonia c‐album butterflies.
4. Relative volumes of both olfactory neuropils increase significantly during early adult development, indicating the importance of olfaction to this species, but still remain considerably smaller than those of nondiapausing conspecifics. However, despite butterflies being kept under the same conditions as before the dormancy, their olfactory neuropil volumes decreased significantly during the postdormancy period.
5. The opposing directions of change in relative neuropil volumes before and after diapause dormancy indicate that the investment strategies governing structural plasticity during the two life stages could be functionally distinct. As butterflies were kept in stimulus‐poor conditions, we find it likely that investments into these brain regions rely on experience‐expectant processes before diapause and experience‐dependent processes after diapause conditions are broken.
6. As the shift in investment strategies coincides with a hard shift from premating season to mating season, we argue that these developmental characteristics could be adaptations that mitigate the trade‐off between dormancy survival and reproductive fitness.

     

Encoding of plant odour information in insects: peripheral and central mechanisms

Insects are suitable model organisms for studying mechanisms underlying olfactory coding and olfactory learning, by their unique adaptation to host plants in which the chemical senses are essential. Recent molecular biological studies have shown that a large number of genes in insects and other organisms are coding for olfactory receptor proteins. In general, one receptor type seems to be expressed in each neurone. The functional characterisations of olfactory receptor neurones have been extensive in certain insect species, demonstrating a fine-tuning of single neurones to biologically relevant odourants; both insect and plant produced volatiles. Stained neurones of the same functional type have been shown to project in one and the same glomerular unit in the primary olfactory centre, the antennal lobe. This corresponds to molecular biological studies, showing projections in one glomerulus by neurones expressing the same receptor type. Comparison of these findings with physiological and morphological characterisations of antennal lobe neurones has indicated correspondence between input and output of the glomerular units. Examples are presented from studies of heliothine moths. From the antennal lobe, the olfactory information is further conveyed to the mushroom bodies, particularly important for learning, and the lateral protocerebrum, a premotoric area. The three brain areas are regions of synaptic plasticity important in learning of odours, which is well studied in the honeybee but also in species of moths.     

Larval chemosensory projections and invasion of adult afferents in the antennal lobe of Drosophila

We have studied the fate of olfactory afferents during metamorphic transformation of Drosophila melanogaster. Intracellular labeling of afferents from larval head chemosensilla suggests that the larval antennal lobe may be an olfactory target, whereas tritocerebral and suboesophageal centers are likely targets of gustatory sensilla. Application of monoclonal antibody 22C10 shows that the larval antennal nerve is the precursor of the adult antennal nerve and is used as a centripetal pathway for the adult afferents. Likely guidance cues are larval olfactory afferents that persist during early metamorphosis. P[GAL4] enhancer trap lines are introduced as efficient markers to follow the establishment of adult sensory projections. β-Galactosidase and the bovine TAU protein were used as reporter proteins, and their expression patterns are compared. P[GAL4] lines MT14 and KL116 demonstrate that adult antennal afferents have arrived in the antennal lobe 24 h after pupariation and extend to the contralateral lobe 6 h later. Line MT14 expresses GAL4 mostly in basiconic sensilla and in certain trichoid sensilla, whereas KL116 is specific for trichoid and a small subset of basiconic sensilla. In the antennal lobe, largely complementary subsets of glomeruli are labeled by the two lines, in agreement with the observation that particular types of sensilla project to particular target glomeruli. © 1997 John Wiley & Sons, Inc. J Neurobiol 32: 281–297, 1997.     

Sex pheromone blend of Manduca sexta: Responses of central olfactory interneurons to antennal stimulation in male moths

Recently, chemical analysis of solvent rinses of the external surfaces of pheromone glands from female Manduca sexta revealed a blend of 12 aldehydes, including the previously identified sex pheromone component, (E,Z)-10,12-hexadecadienal (bombykal). Previous electrophysiological studies showed that olfactory (deutocerebral) interneurons in the antennal lobes of males exhibited a wide range of responsiveness to pheromonal stimulation of the ipsilateral antenna. These experiments were performed with crude extracts of pheromone glands as well as two synthetic compounds: the major pheromone component, bombykal, and (E,Z)-11,13-pentadecadienal, a mimic of a second component of the female's pheromone blend. Using intracellular methods, we have now reexamined similar olfactory interneurons, using each of the 12 chemically identified components as well as synthetic blends of various combinations of them. Eight of the 12 components isolated from female glands elicited some form of response in olfactory interneurons in males. In accordance with biochemical and behavioral data, the most potent are bombykal and two trienals, (E,E,E)- and (E,E,Z)-10,12,14-hexadecatrienal. We also conclude that the C₁₅ dienal is selective for one of the trienal receptors on the antenna, but is much less potent than the natural trienal stimulant.     

Spatial Receptive Fields for Odor Localization

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Quantitative analysis of antennal mosaic generation in Drosophila melanogaster by the MARCM system

Mosaics have been used in Drosophila to study development and to generate mutant structures when a mutant allele is homozygous lethal. New approaches of directed somatic recombination based on FRT/FLP methods, have increased mosaicism rates but likewise multiple clones in the same individual appeared more frequently. Production of single clones could be essential for developmental studies; however, for cell-autonomous gene function studies only the presence of homozygous cells for the target recessive allele is relevant. Herein, we report the number and extension of antennal mosaics generated by the MARCM system at different ages. This information is directed to obtain the appropriated mosaic type for the intended application. By applying heat shock at 10 different developmental stages from 0-12 h to 6-7 days after egg laying, more than 50% of mosaics were obtained from 5,028 adults. Single recombinant clones appeared mainly at early stages while massive recombinant areas were observed with late treatments.     

Input-timing-dependent plasticity at incoming synapses of the mushroom body facilitates olfactory learning in Drosophila

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昆虫嗅觉系统结构与功能研究进展

昆虫的脑由前脑、中脑和后脑组成,其中前脑含有高级感觉中枢,如蘑菇体和中央复合体,控制昆虫的学习、记忆和运动等高级神经活动;中脑包含触角叶,是嗅觉神经中心;而后脑则通常不发达,主要包括内分泌神经元和控制进食与消化的运动神经元。不同于其他物种,昆虫由于其特殊的生活习性,听觉和视觉系统相对退化,主要依赖嗅觉来捕食、交流和求偶,因此嗅觉系统尤其发达。本文综述了目前对昆虫的脑部主要神经结构和功能(中央复合体、蕈形体和触角叶结构)以及昆虫脑部结构遗传变异(性别异构,不同发育时期、不同昆虫以及昆虫与其他动物的脑部结构差异)的研究进展,并总结了目前昆虫脑对信号的加工处理和识别机制的研究结果。     

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     

Misexpression screen for genes altering the olfactory map in Drosophila

Despite the identification of a number of guidance molecules, a comprehensive picture has yet to emerge to explain the precise anatomy of the olfactory map. From a misexpression screen of 1,515 P{GS} lines, we identified 23 genes that, when forcibly expressed in the olfactory receptor neurons, disrupted the stereotyped anatomy of the Drosophila antennal lobes. These genes, which have not been shown previously to control olfactory map development, encode novel proteins as well as proteins with known roles in axonal outgrowth and cytoskeletal remodeling. We analyzed Akap200, which encodes a Protein Kinase A-binding protein. Overexpression of Akap200 resulted in fusion of the glomeruli, while its loss resulted in misshapen and ectopic glomeruli. The requirement of Akap200 validates our screen as an effective approach for recovering genes controlling glomerular map patterning. Our finding of diverse classes of genes reveals the complexity of the mechanisms that underlie olfactory map development.     

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.     

Olfactory responses and sensilla morphology of the blackcurrant leaf midge Dasineura tetensi

Olfactory responses of Dasineura tetensi (Rubs) (Diptera; Cecidomyiidae) to leaf volatiles of blackcurrant (Ribes nigrum) were tested in a 4-way olfactometer. Newly emerged virgin females showed no response to the leaf volatiles emitted from a blackcurrant shoot. Newly emerged males (which are known to respond to a pheromone released by the female) also showed no response to the leaf volatiles. Two hours after mating females responded positively, indicating that leaf volatiles may play a role in host plant finding. Scanning (SEM) and transmission (TEM) electron microscopy of the antennae of D. tetensi showed that males and females share five sensillum types; sensilla chaetica (mechanoreceptors), sensilla trichodea, sensilla basiconica, uniporous peg sensilla and circumfila (chemoreceptors). The sensilla chaetica and sensilla trichodea resemble those found on other insects. Sensilla basiconica were found on all antennal subsegments except the tip. These are multiporous receptors with five unbranched dendrites filling the lumen. Small peg sensilla located on the tips of both male and female antennae may function as contact-chemoreceptors. The circumfila, which are a unique type of sensilla found only on cecidomyiid antennae, form loops around each of the antennal subsegments, being attached to the surface by a series of stalks. TEM revealed that each stalk consisted of one sensillum containing a single highly branched dendrite. The distal regions of the walls of each sensillum are fused together to form the circumfila. Circumfila have multiporous walls and a lumen filled with multiple branches of dendrites. Their structure suggests that they are important olfactory receptors in both the male and female.     

A Drosophila nonvisual arrestin is required for the maintenance of olfactory sensitivity

Nonvisual arrestins are a family of multifunctional adaptor molecules that regulate the activities of diverse families of receptors including G protein-coupled receptors, frizzled, and transforming growth factor-beta receptors. These activities indicate broad roles in both physiology and development for nonvisual arrestins. Drosophila melanogaster has a single nonvisual arrestin, kurtz, which is found at high levels within the adult olfactory receptor neurons (ORNs), suggesting a role for this gene in modulating olfactory sensitivity. Using heat-induced expression of a krz cDNA through development, we rescued krz(1) lethality. The resulting adults lacked detectable levels of krz in the olfactory system. The rescued krz(1) homozygotes have an incompletely penetrant antennal structural defect that was completely rescued by the neural expression of a krz cDNA. The krz(1) loss-of-function adults without visible antennal defects displayed diminished behavioral responsiveness to both aversive and attractive odors and also demonstrated reduced olfactory receptor potentials. Both the behavioral and electrophysiological phenotypes were rescued by the targeted expression of the krz cDNA within postdevelopmental ORNs. Thus, krz is required within the nervous system for antennal development and is required later in the ORNs for the maintenance of olfactory sensitivity in Drosophila. The reduced receptor potentials in krz(1) antenna indicate that nonvisual arrestins are required for the early odor-induced signaling events within the ORNs.     

Factors that influence the development of cultured neurons from the brain of the mothManduca sexta

Summary During metamorphic adult development, neurons and glial cells in the developing olfactory (antennal) lobes of the moth undergo characteristic and extensive changes in shape. These changes depend on an interplay among these two cell types and ingrowing sensory axons. All of the direct cellular interactions occur against a background of changing steroid hormone titers. Antennal-lobe (AL) neurons dissociated from stage-5 (of 18 stages) metamorphosing animals survive at least 3 wk in primary cell culture. We describe here the morphological influences on AL neurons of (1) exposure to the steroid hormone 20-hydroxyecdysone, (2) exposure to sensory axons, and (3) interactions among the AL neurons. Cultured AL neurons respond only weakly, if at all, to 20-hydroxyecdysone. They do, however, show greater total outgrowth and branching when they had been exposed in vivo to sensory axons. Because there is no direct contact between some of the neuronal types and the sensory axons at the time of dissociation, the increase in outgrowth must have been mediated via a diffusible factor(s). When AL cells (neurons and glia) are plated at high density in low volumes of medium, or when the cells are plated at low density but in the presence of medium conditioned by high-density cultures, neurite outgrowth and cell survival are increased. Nerve growth factor (NGF), epidermal growth factor (EGF), fibroblast growth factor-basic (bFGF), transforming growth factor-β (TGF _β ) and insulin-like growth factor (ILGF) had no obvious effect on neuronal morphology and thus are unlikely to underlie these effects. Our results suggest that the mature shape of AL neurons depends on developmental interactions among a number of diffusible factors.     

昆虫气味认知的嗅觉神经结构及分子机制

大多数昆虫主要通过气味认知感知外界环境的变化,维持生命活动。探究昆虫气味认知的嗅觉系统神经结构及分子机制,对于完善气味认知神经生物学理论及利用其原理进行仿生学研究等有重要的科学意义。近年,关于昆虫气味认知科学研究有了很大的进展。本文从昆虫神经生物学的视角详细综述了近年关于昆虫气味认知的嗅觉神经结构、分子机制及气味信号的神经传导途径等方面的基本理论及最新研究成果。综述结果显示:昆虫对气味的认知是通过嗅觉神经系统的触角感器、触角叶(AL)、蕈形体(MB)等脑内多层信号处理神经结构来实现的。当外界气味分子进入触角感器内后,由感器内特定的气味识别蛋白(OBP)将气味分子运载到达嗅觉感受神经元(ORN)树突膜上的受体位点,气味分子与表达特定气味的受体(OR)结合产生电信号,并以动作电位的形式通过ORN的轴突传到脑内的触角叶。在触角叶经过嗅觉纤维球对气味信息选择性加工处理,再由投射神经元(PNs)将初步的识别和分类的气味信息传到蕈形体和外侧角(LH)等神经中枢,实现对气味的识别和认知。虽然,近年昆虫气味认知神经生物学的研究有了很大的进步,但是,我们认为目前的研究成果还不能完全阐明昆虫气味认知的神经机制,还有很多问题,例如,触角叶上众多的嗅觉纤维球是如何对嗅觉感受神经元传入的气味信息进行编码处理的?等有待进一步深入研究。为了搞清这些疑难问题,我们认为需要提高现有的实验技术水平,加强电生理学和分子神经生物学相结合的实验研究,从分子水平探究气味认知的神经机制可能是未来研究的热点。