Date: 22th June,2020
Reporter: Guo-Cheng Li
😸😃😏
大家好，今天分享一本关于昆虫嗅觉感受的一本书，2019年出版在Springer Nature上。分为上下两卷。我将讲下卷的第Ⅱ，Ⅳ章。主要是关于昆虫嗅觉系统的介绍和嗅觉的编码模式。
Many insects rely on their sense of smell to ==identify food, prey or mates==. Odors are detected by the sensory organs and the resulting signals processed by the brains before they can lead to behaviors.
Here we provide a detailed comparison of the molecular, anatomical, and physiological parameters of olfactory systems across species including flies, moths, bees and mosquitoes
Insects usually have a pair of major olfactory sensory organs: ==the antennae== and ==the maxillary palps==, which are covered with hair-like, porous, cuticular structures called sensilla. Each sensillum contains multiple olfactory receptor neurons (ORNs) surrounded by sensory lymph.
It is currently assumed that odors entering through the pores of sensilla are transported across the lymph by odorant binding proteins (OBPs) to the olfactory receptors (ORs) present on the membrane of ORNs.
Various insect species can have different mechanisms for the use of OBPs and ORs in recognition of general odor and/or sex pheromone compounds.
The number of olfactory receptor neurons also varies considerably among insect species
Olfactory receptor neurons are ==spontaneously active== and their ==firing frequency== changes when an odor binds to the olfactory receptors.
Each ORN expresses a single or very few types of olfactory receptors. Some odorant receptors are broadly tuned and respond to multiple odors while some are narrowly tuned.
Olfactory systems employ a ==combinatorial== code for encoding odors: ==a single ORN may be activated by multiple== ==odors, and a single odor can activate many ORNs.==
The olfactory receptor (OR) is a G-protein-coupled seven transmembrane protein with the N-terminus facing the cytoplasmic side and the C-terminus facing the extracellular side, a topology opposite to that observed in vertebrate ORs .
Each insect OR forms a heteromeric complex with a species-specific odorant co-receptor.
This complex is a ligand-gated ion channel that opens in response to odor binding and determines the sensitivity and specificity of the ORN in which it is expressed.
A new class of olfactory receptors called ionotropic receptors (IRs) was recently identified. They belong to a subfamily of ==ionotropic glutamate receptors (iGluRs)== and are known to respond to environmental as well as cellular signals. ==While ORs have seven transmembrane domains, IRs are expected to have only three.==
==IR的ORN投射；CO
2的投射==
The antennal lobe is the first major information processing center in the insect olfactory system. It is a ==spheroidal== structure located in the ==deutocerebrum== of insect brain and receives direct input from ORNs. Different types of neurons innervating the lobe help in ==reformatting olfactory information== and propagating it to higher brain centers.
The antennal lobe is composed of several ==glomeruli== (small spheroidal neuropils) where olfactory receptor ==neurons(ORN), local neurons (LNs), projection neurons (PNs) and other modulatory neurons== interconnect. Their ==numbers, sizes, and organization== vary across species.
The antennal lobe is the first processing center for olfactory information, and in ways that are still being elucidated, functions to transform and organize olfactory information. Such transformation is influenced by local interneurons (LNs) that innervate much of the antennal lobe. Olfactory information is then relayed by the projection neurons (PNs), which send their axons to synapse with the mushroom bodies (the center of learning/memory) and also the lateral horn.
LNs, as their name suggests, are neurons contained within the antennal lobe that synapse with ORN terminals, PNs and other LNs. LNs play an important role in formatting the input (from ORNs) into the output (PNs) of the antennal lobe.
LNs are mostly inhibitory and release ==neurotransmitter== GABA (γ-aminobutyric acid); Less common are the inhibitory ==histaminergic(组胺能的)== LNs observed in Bombyx and Apis; In Drosophila, LNs expressing ==acetylcholine and glutamate==.
Projection neurons, the only ==efferent== neurons of the insect antennal lobe, carry the reformatted output to higher brain areas. Inside the lobe they exhibit both ==uniglomerular and multiglomerular dendritic innervation==.
Their ==cell bodies== are found at the periphery of the antennal lobe. The ==axon bundle== of PNs projects to the mushroom body and the lateral horn through multiple fiber tracts.
==PNs can be both excitatory and inhibitory==. Excitatory PNs expressing acetylcholine as their neurotransmitter, Inhibitory PNs releasing GABA.
PNs are spontaneously active, and respond to odors with volleys of spikes that are often organized in ==temporal patterns==. The temporal patterning of responses could enhance the capacity of PNs to encode a large number of odors appears to be a shared feature among the commonly studies insect species.
The antennal lobe receives ==modulatory input from the rest of the brain via feedback neurons==. With their dendrites in different regions of the protocerebrum and axonal terminals in the antennal lobe, the feedback neurons modulate antennal lobe activity by releasing ==serotonin, dopamine, octopamine or histamine==.
==The mushroom body (MB)== is a paired structure that follows the antennal lobe in the olfactory cascade, and is thought to be important for ==olfactory learning and memory==.(嗅觉系统的可塑性)
MB receives its primary inputs from the PNs and provides output to several efferents that project to other higher centers in the brain.
Each MB is composed of a cup-shaped structure called the ==calyx== (or head) at which it receives its major inputs. The cup is attached to a stalk of parallel fibers known as the ==pedunculus== which further differentiates into different lobes: the vertical or α-lobe; the medial or β-lobe; and the γ-lobe.
The mushroom body itself is composed of small cells, called ==Kenyon Cells (KCs)==, that have their ==dendrites in the calyx region and axon terminals in the lobes.==
In most insects, the MB receives input from PNs originating in the antennal lobe. These neurons provide olfactory input through randomized connections with the KCs in the calyx. ==One PN can project to multiple KCs and conversely, a single KC can receive inputs from multiple PNs==; KCs could thus function as ==coincidence detectors== for synchronous PN spikes.
A GABAergic feedback neuron, connecting the MB lobes to the calyces. The feedback neuron receives input from KCs, and then normalizes their responses by providing inhibitory feedback to KCs proportional to their overall activity; this normalization helps KCs in maintaining their sparse odor representations, which may be important for KCs in their role in forming associative memories.
KCs are the intrinsic neurons of the MB, and are generally described as having ==very little extra-nuclear protoplasm==, which distinguishes them from other neurons in the brain.
Their cell bodies lie in and around the calyx region and are generally very small.
KCs ==arborize== extensively in the calyces. ==KCs are among the most populous cell type in the brain== – a feature that allows KCs to encode a large number of potential odors with minimal overlap in the representations.
The physiological properties of KCs typically include large input resistance and small capacitance.
KC axons pass through the pedunculus and divide into the α-lobe and β-lobe or γ-lobe, and provide output to the different efferent neurons.
==The large population of KCs converges onto relatively few neurons that take the output of the MB to other brain areas==. These neurons, called the output neurons or the extrinsic neurons of the MB, can receive inputs in either one or a combination of the lobes and can have contact with KCs at several points.
These neurons project to different MB regions and to the protocerebral lobe.
These neurons have extended arborization outside the MB and respond to multimodal inputs.
Olfactory receptor neurons in the antennae and maxillary palps send axons to specific glomeruli in the antennal lobe. All olfactory receptor neurons expressing the same odorant receptor complement (same colour) converge at the same glomerulus. There they ==form synaptic contacts with projection neurons and local neurons==. Projection neurons send axons either directly to the lateral horn neuropile (green projection neuron) or ==indirectly via the calyx of the mushroom bodies (red and blue projection neurons), where they form synapses with Kenyon cells==.
脑由前脑( protocerebrum ) 、中脑( deutocerebrum ) 和后脑 ( tritocerebrum) 组成。前脑的左右两侧有突出的视叶( optical lobe) ，直接与复眼相连接，背面突出3根细长的单眼柄分别和背单眼和侧单眼相连。中脑包括两个膨大的中脑叶，由此发出触角神经分布到触角肌上，主要包括==触角叶( antennal lobe，AL)和触角神经( antennal nerve)== 。后脑是由第1 体节的一对神经节特化而成，连接在中脑的下面，左右各成一叶; 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。
前脑在脑的最前端，脑体由密集的神经纤维群和神经纤维球( 由神经末梢聚集而成的小团) 组成，共有4 种，即==脑桥体、中央复合体( central complex) 、蕈形体( mushroom body) 和腹体==，其中以中央复合体和蕈形体的作用最为重要。
蕈形体是一种与嗅觉有关位于前脑背侧的结构，又称蘑菇体，与嗅觉学习记忆有关。包含数量众多的密集排列的内源性肯扬细胞( Kenyon cells，KCs) 。它可以分为3 大部分: ==蕈体冠( calyx) 、蕈体柄( pedunculus) 和蕈体叶( lobes)==。
昆虫触角叶是位于中脑的球形结构，是昆虫嗅觉系统的第一级神经中枢，==神经纤维球是早期嗅觉信息处理的基本功能单位==，神经纤维球在不同物种及性别间形状、大小和相对位置有所不同。
触角叶内含有3 类神经元: ==投射神经元( projection neuron，PN) 、局部神经元( local neuron，LN) 和离心神经元( centrifugal neuron，CN )== 。PN 的树突可以收集信息并传到下一级中枢，它的轴突从触角叶投射到特定的高级脑中 枢。
LN 的显著特征是没有轴突，广泛分布于触角叶中，调节嗅觉受体神经元输入的嗅觉信号。多数LN释放γ-氨基丁酸( GABA) 作为神经递质，参与抑制性的神经调节，因此被称为抑制性局部神经元( inhibitory local neuron，iLN) 。触角叶中也存在兴奋性局部神经元( excitatory local neuron，eLN)。
==昆虫脑部结构存在一定的性别差异==，在许多雄性个体中，存在扩大型神经纤维球复合体( macro glomerular complex，MGC) ，它由3 个亚结构组成，分别是==云状体( cumulus) 、圈形体1( toroid 1)和圈形体2( toroid 2)== ，负责对性信息素的加工处理。
Three-dimensional reconstruction and diagrammatic representation of sexually isomorphicglomeruli (G) and sexually dimorphic glomeruli in a female and b male Manduca. ==latLFGlateral large female glomerulus, medLFG medial large female glomerulus==, C cumulus, T1 toroid-1, T2 toroid-2, G glomerulus.
==雌性个体特异的glomerulus.==
幼虫的嗅觉系统比成虫的简单，尤其表现在感觉水平和解剖结构上，在外周化学感受器上，幼虫和成虫最大的区别在于==嗅觉受体神经元( olfactory receptor neuron，ORN) 的急剧增多==。而味觉受体神经元( gustatory receptor neuron，GRN) 的差别则相对小些。
另一个幼虫特有的特征是==感觉水平上气味和味道功能的混合==，即使它们位于两个不同的感受器，这可能由于是气味和味道的区别对幼体取食不是特别重要。
相对于成虫蕈体冠含有数百个神经纤维球，==幼虫的蕈体冠仅含有少数几个==，幼虫嗅觉回路没有会聚和分散，而==成虫的嗅觉回路则有会聚和分散==。
果蝇的1300 个ORN聚集到43个神经纤维球，再分散至150 个PN 和成百个蕈体冠神经纤维球。
气味分子经过嗅觉感受器上的小孔进入感受器内的淋巴液内环境，与淋巴液中的气味结合蛋白结合在一起。气味结合蛋白最早被发现存在于蛾类昆虫的感觉器淋巴液中。昆虫体内主要存在两种气味结合蛋白( odour binding protein，OBP) ，一种只与普通气味结合被称为普通气味结合蛋白( general odour binding protein，GOBP) ，这种蛋白在雌雄个体中均存在。另一种则特异性结合雌性信息素，被称为性信息素结合蛋白( pheromone binding protein，PBP) ，这类蛋白一般存在于雄性个体中。
GOBP 多数分布锥形感器中，而PBP 则分布于感受性信息素的毛形感器中。OBPs 将气味分子携带到嗅觉受体神 经末梢与树突膜上的气味受体结合后，气味分子从OBPs 上解离下来并结合到气味受体上，气味分子和它的受体之间的相互作用会导致神经细胞膜上的去极化，这个极性的变化刺激昆虫的ORN,ORN随即将化学信号转变为电信号，电信号则以动作电位的形式沿轴突传播到更高级的感觉中心，并在这里对各种电信号进行整合，释放神经脉冲，指导昆虫产生特定的生理和行为反应。
在此期间，气味分子必须在很短的时间内失活(odorant degrading enzymes (ODEs))，以保障受体神经对随后气味刺激的反应，以便能够对周围环境中气味质与量的变化做出即时和准确的分辨。
哺乳动物的嗅觉受体属于G 蛋白偶联受体( G-proteincoupled receptor) ，其具有7 个跨膜α-螺旋结构，分别由3 个胞外和3 个胞内的环状结构相连，受体整段肽链的氨基端在胞外，羧基端在胞内。而昆虫的嗅觉受体与哺乳动物的嗅觉受体的同源性很低，只有30%。昆虫嗅觉受体肽链的方向和哺乳动物的相反，氨基端在胞内，而羧基端在胞外。而且昆虫嗅觉发挥作用需要ORco存在。
❓ 自然界的气味分子有成千上万种，而昆虫触角上的ORN数目相对要少很多，昆虫触角叶内的神经纤维球数量又相对比ORN少了许多，昆虫是如何利用这些数量有限的神经纤维球来编码这些成千上万的气味分子的?
“标记线条编码”(labeled-line code)认为触角叶对气味分子的编码很直接: 一种气味分子激活相对应的神经纤维球，这些神经纤维球的输出神经元则把信息传到位于前脑相对应的高级中枢，这种编码方式不仅专一强而且速度很快。
“交叉纤维编码”(cross-fiber code)则认为一种气味分子同时与多种嗅觉受体结合，因此激活不同类型的嗅神经元，再而激活多个神经纤维球。不同气味分子激活的神经纤维球有重复，但又不完全一样。这种编码方式的优越性在于它成指数增长地扩大了编码空间，几十个神经纤维球的排列组合远远超过几十种，但昆虫的触角叶系统是同时具备这两种编码方式。
ORN对气味分子通常起兴奋反应，而神经纤维球的输出神经元既能产生兴奋反应，也能产生抑制反应，而且在兴奋反应里包含着抑制成分 。抑制反应是触角叶水平上中枢神经元的重要特点之一。这是由于在触角叶里的神经纤维球之间由局部神经元连接，而局部神经元的一种重要传递递质就是抑制性的γ-氨基丁酸( γ-aminobutyric acid，GABA) ，因此，一个神经纤维球的兴奋可以造成周围神经纤维球的抑制。如编码不同信息素的神经纤维球之间就存在侧向抑制作用，而且编码信息素和编码其他植物气味的神经纤维球之间也存在侧向抑制的作用，==神经纤维球之间的相互抑制作用可能是为了提高气味之间的对比而加强对某一种气味的识别==。
==信噪比( signal-to-noise ratio) 和增益控制( gain control)== 是保证嗅觉信号准确快速传递到下一嗅觉中枢的重要神经活动。前者重在放大微弱的信号，后者重在抑制较强的信号。
在脊椎动物和昆虫中，一个典型的提高信噪比的内在嗅觉结构基础便是ORN的汇聚作用。
- 多条表达相同OR的ORN汇聚成一个膨大的神经纤维球，而一个神经纤维球通常只发出1 ～ 2 条PN 至嗅觉更高级中枢。ORN和PN 之间的信号传递呈非线性关系，即使当ORN受到微弱的刺激其信号峰靠近基线几乎检测不到时，在PN 中却能检测到较为强烈的[信号](####4.4 Output Neurons of MB)。
神经系统往往还面临频繁的、相同的信号刺激，那么这些“多余的”信号是怎么被屏蔽掉的呢? 这就有赖于神经系统的增益控制。
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触角叶内的神经纤维球之间存在侧向抑制作用，LN 上的神经递质GABAA 和GABAB 受体共同参与调节了信号的抑制作用。
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嗅觉神经系统还可以通过PN 和ORN编码气味的时间差达到增益控制的目的，在信号传递的时PN 比ORN更先到达最高值。这样就避免了其他噪音信号对PN 的干扰。
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增益控制的另一机制是ORN和PN 之间的短期抑制( short-term depression) 。ORN放电频率越大，突触后电流却越小，原因可能是过快的脉冲导致突触前递质囊泡释放减少。
提高信噪比以及增益控制即是嗅觉神经系统将微弱的信号和过强的信号控制在一个同等的数量级的机制，这可能有利于下一级嗅觉中枢对嗅觉信号的进一步处理，同时这也可能保证了信号传导的真实性和客观性。
当然，昆虫嗅觉系统也具有一定程度的可塑性。
这部分由莫宝童师兄讲过，我在此不做赘述。
Moth’s survival directly depends on olfactory cues for feeding and reproduction(mating and egg-laying),reflected by the large part of the brain devoted to olfaction. Their well-developed olfactory system makes them able to discriminate among thousands of air-born organic molecules appearing in discontinuous odor plumes interrupted by a tremendous level of background noise.
Plants scent compositions are dynamical, varying periodically (diurnal and seasonal), and according to different conditions; like pollination, nutrition access, microbial infestation, herbivory, ambient temperature, humidity and auto-oxidation by the surrounding air.
Based on biosynthetic origin these volatile compounds have been classified into four main groups; ==the terpenes, aliphatic fatty acid derivatives , aromatic compounds and other amino acid derivatives.==
The ==C6 aldehydes, alcohols and their esters, known as green leaf volatiles==. Terpenes (mono-, homo-, and sesquiterpenes) are the largest class of secondary metabolites present in both leaves and reproductive parts. Aromatic compounds are more typical in floral scents.
Fitness benefits in recognizing the presence or absence of particular volatiles may directly reflect nutritional demands, immune-protection , toxicity, and indirectly the presence of competitors, parasitoids and predators.
In this odor rich environment the moth olfactory system adapts and evolves in a co-evolutionary arms race of plant defense and herbivore response.
- The class of odorant receptors (ORs) are seven-transmembrane domain proteins with a unique membrane topology; they are considered to be involved in the detection of a vast array of chemically diverse compounds, including pheromones.
- The second class of olfactory receptors comprises so-called ionotropic receptors (IRs); they seem to be specialized in the detection of short-chain organic acids, amines and aldehydes.
- The third class of olfactory receptors are assigned to the insect gustatory receptor (GR) family. These receptor types are involved in the detection of carbon dioxide in various insects and of pheromones in Drosophila melanogaster.
Utilizing genome sequences of D. melanogaster, a combination of bioinformatics and cloning strategies has led to the identification of candidate OR genes that were found to be expressed in distinct subsets of OSNs in the antennae or the maxillary palps of the fly.
Functional analyses demonstrated that the candidate OR proteins conferred odor-specific responses and that the OR repertoire provides the molecular basis for the detection and discrimination of a large number of different odorous compounds.
Orco is considered as a marker for OR-expressing OSNs in insects. Functionally, Orco seems to be necessary for membrane targeting of canonical ORs and for forming ==heteromeric complexes composed of at least two subunits, the co-receptor Orco and an odorant-binding OR, which operate as nonselective cation channels.==
Insect ORs are predicted as heterotetramers consisting of two subunits of narrowly tuned ORs and two subunits of highly conserved odorant receptor co receptor (ORco).
However, it is an open question ==whether the size of the OR gene family in an insect is linked to the complexity of its chemical environment.==
The finding is noteworthy since receptors of the IR and the GR families appeared early in evolution and are found in a diversity of organisms across the Protostomia.
Receptors for the detection of pheromones appear to represent a subset of the insect OR repertoire. The first ==pheromone receptors (PRs)== were identified in two lepidopteran species, the tobacco budworm H. virescens and the silkmoth Bombyx mori.
For candidate PRs of several other lepidopteran species, similar sequences were found, indicating that in the order Lepidoptera, ==PRs form a separate group within the otherwise widely diverse OR family.==
The similarity of the receptor protein structure is in line with the similar chemical structure of the lepidopteran pheromones that are mostly ==long-chain unsaturated acetates, alcohols and aldehydes or polyenic hydrocarbons.==
Firstly, it is important to know, how many and which OR genes are actually expressed ==in different olfactory tissues== ==(antennae and maxillary palps) or in distinct developmental stages==.
Secondly, it may have great functional implications that ==distinct OR subtypes are expressed in different numbers== ==of cells==. In the antenna of adult fruit flies, individual OR subtypes are expressed in subsets of 2–50 cells of the total number of 1200 OSNs.
Whether ==the number of cells expressing a certain OR subtype reflects the relevance of the corresponding odorants== is still elusive.
Thirdly, it was originally envisioned that each OSN usually expresses one ligand-specific OR type; however, several exceptions from this general “one-OR/one-OSN” rule have been found recently. ==Co-expression of several OR types in a single cell may be a widespread phenomenon and a mechanism to broaden the response spectrum of an OSN.==
Genes encoding the co-expressed ORs are arranged as a cluster within the genome. Moreover, analyses of transcripts from ==the clustered OR genes led to the discovery of polycistronic RNA== (encodes two or more proteins) suggesting that the ==co-expressed OR proteins are translated from the same primary transcript==.
==Whereas some ORs appear to be quite narrowly tuned, most ORs are broadly tuned==; i.e. they respond to multiple chemical compounds. Moreover, most of the compounds activated several receptor types. These features are reminiscent of the vertebrate olfactory system and suggest ==a combinatorial coding of odor quality==.
One critical parameter is ==the number and selection of chemical compounds== employed in the functional assays.
In addition, ==the stimulus concentration== is of great importance when determining the response spectrum of ORs; it is mostly unclear to what extent the stimulus concentrations ==reflect the natural odorant concentration an insect encounters==.
❓A third aspect one has to keep in mind when assigning ==the ligand specificity of an OR type== based on the results obtained from heterologous expression systems is ==a possible role of odorant binding proteins (OBPs)==.
sensory neuron membrane protein 1(SNMP1) seems to be expressed in ==all pheromone-responsive OSNs== and is supposed to be positioned in close proximity to PR proteins in the membrane. In cells expressing PRs, SNMP1 contributes to ==highly sensitive responses as well as to a rapid activation and termination of pheromone-induced reactions.==
In heterologous expression systems, it was found that ==Orco can form a non-specific, spontaneously opening cation channel permeable for Ca2+ and also for Na+ and K+==.
The functional role of Orco in pheromone-responsive OSNs may primarily be involved in controlling the spontaneous activity of the neurons and thus contributing to ==the thresholds and kinetics of pheromone-induced responses.==
The spontaneous gating activity of the Orco channel was considered a reason for the diminished spontaneous electrical activity of OSNs in Orco-deficient flies and an increased activity upon application of the Orco-specific agonist VUAA1.
(a) Membrane topology of insect odorant receptor proteins.
(b) Olfactory signal transduction: response to general odorants.
Binding of an odorant to the ORx subunit activates the channel and elicits an ionotropic current. In addition, the ORx subunit may activate a ==G protein (Gαs)== leading to an enhanced activity of ==adenylyl cyclase (AC)== and a rising level of the second messenger ==cAMP==. The second messenger may contribute to an increased conductance of the heteromeric ORx/Orco channel or may activate homomeric Orco channels (not shown). The activity of Orco can also be modulated by ==calmodulin (CaM)== or phosphorylation through ==protein kinase C (PKC)==
(c) Olfactory signal transduction: response to pheromones.
The extracellular domain of SNMP seems to funnel the pheromone molecule directly to the binding site of the PRx. Activation of the PRx/Orco channel complex leads to an influx of cations and a depolarization of the cell. Alternatively or in parallel, activation of the PRx may elicit a G protein-mediated (Gαq) pathway leading to an enhanced activity of ==phospholipase Cβ (PLCβ)== that generates the second messengers ==inositol trisphosphate (IP3)== and ==diacylglycerol(DAG)==. Increased levels of IP3 may open ==calcium-selective channels (CaC)== in the plasma membrane. A rise of the Ca2+ concentration in turn could induce the opening of Ca2+-activated cation channels (CC). Both Ca2+ and DAG could influence channel activities via DAG-activated PKC or via Ca2/CaM.
In Drosophila flies, two GR subtypes, GR21a and GR63a, are co-expressed in the same antennal neuron.
In the Dengue vector Aedes aegypti and other mosquitoes, they are named Gr1, Gr2, and Gr3 but in the Malaria vector A.gambiae Gr22, Gr23, and Gr24.
For flies, CO2 is an important signal in the search for ==fermenting food== , but for female mosquitoes, the perception of CO2 is of crucial importance ==to locate the blood host (cattle or human) by plumes of exhaled carbon dioxide==.
Besides the detection of carbon dioxide, some insect GRs seem to be involved in the reception of specific pheromone components controlling sexual behavior of flies.
The observation that neurons in sensilla ==coeloconica== do not express ORs and Orco but nevertheless do respond to chemical stimuli, primarily to ==organic acids and amines.==
From the 66 IR genes of the fly, 16 are expressed in the primary olfactory organs, i.e. antennae and maxillary palps; the remaining IRs were found in other tissues, including chemosensory tissues such as mouthparts and legs.
Two of the IR subtypes, ==IR8a and IR25a, were found to be expressed in many neurons of the coeloconic sensilla and co-expressed with other IR subtypes==. This observation has led to the concept that IR8a and IR25a may function as ==co-receptor (IRco)== of distinct IR receptor types.
Schematic structure of the odor-specific receptor protein IRx and the IR co-receptor IRco. Both IRX and IRco proteins have ==three membrane-spanning domains==, ==a pore region (P)== and an extracellular region representing a ==bipartite ligand-binding domain (LBD)== composed of ==two lobes (S1 and S2)==. In addition, the IRco protein is characterized by an extended ==amino-terminal domain (ATD).==
[1]:Picimbon, J.-F. (Ed.), 2019. Olfactory Concepts of Insect Control - Alternative to insecticides: Volume 2. Springer International Publishing, Cham. https://doi.org/10.1007/978-3-030-05165-5
[2]:万新龙, 杜永均, 2015. 昆虫嗅觉系统结构与功能研究进展. 昆虫学报 58, 688–698.
