Identification of olfactory receptor neurons in Uraba lugens (Lepidoptera: Nolidae) and its implications for host range

Phytophagous insects detect volatile compounds produced by host and non-host plants, using species-specific sets of olfactory receptor neurons (ORNs). To investigate the relationship between the range of host plants and the profile of ORNs, single sensillum recordings were carried out to identify ORNs and corresponding active compounds in female Uraba lugens (Lepidoptera: Nolidae), an oligophagous eucalypt feeder. Based on the response profiles to 39 plant volatile compounds, 13 classes of sensilla containing 40 classes of ORNs were identified in female U. lugens. More than 95% (163 out of 171) of these sensilla contained 16 classes of ORNs with narrow response spectra, and 62.6% (107 out of 171) 18 classes of ORNs with broad response spectra. Among the specialized ORNs, seven classes of ORNs exhibited high specificity to 1,8-cineole, (±)-citronellal, myrcene, (±)-linalool and (E)-β-caryophyllene, major volatiles produced by eucalypts, while nine other classes of ORNs showed highly specialized responses to green leaf volatiles, germacrene D, (E)-β-farnesene and geranyl acetate that are not produced by most eucalypts. We hypothesize that female U. lugens can recognize their host plants by detecting key host volatile compounds, using a set of ORNs tuned to host volatiles, and discriminate them from non-host plants using another set of ORNs specialized for non-host volatiles. The ORNs with broad response spectra may enhance the discrimination between host and non-host plants by adding moderately selective sensitivity. Based on our finding, it is suggested that phytophagous insects use the combinational input from both host-specific and non-host specific ORNs for locating their host plants, and the electrophysiological characterization of ORN profiles would be useful in predicting the range of host plants in phytophagous insects.     

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.     

Interference of plant volatiles on pheromone receptor neurons of male Grapholita molesta (Lepidoptera: Tortricidae)

In moths, sex pheromone components are detected by pheromone-specific olfactory receptor neurons (ph-ORNs) housed in sensilla trichodea in the male antennae. In Grapholita molesta, ph-ORNs are highly sensitive and specific to the individual sex pheromone components, and thus help in the detection and discrimination of the unique conspecific pheromone blend. Plant odors interspersed with a sub-optimal pheromone dose are reported to increase male moth attraction. To determine if the behavioral synergism of pheromone and plant odors starts at the ph-ORN level, single sensillum recordings were performed on Z8-12:Ac and E8-12:Ac ph-ORNs (Z-ORNs and E-ORNs, respectively) stimulated with pheromone–plant volatile mixtures. First, biologically meaningful plant-volatile doses were determined by recording the response of plant-specific ORNs housed in sensilla auricillica and trichodea to several plant odorants. This exploration provided a first glance at plant ORNs in this species. Then, using these plant volatile doses, we found that the spontaneous activity of ph-ORNs was not affected by the stimulation with plant volatiles, but that a binary mixture of sex pheromone and plant odorants resulted in a small (about 15%), dose-independent, but statistically significant, reduction in the spike frequency of Z-ORNs with respect to stimulation with Z8-12:Ac alone. The response of E-ORNs to a combination of E8-12:Ac and plant volatiles was not different from E8-12:Ac alone. We argue that the small inhibition of Z-ORNs caused by physiologically realistic plant volatile doses is probably not fully responsible for the observed behavioral synergism of pheromone and plant odors.     

Structure and function of the antennal sensilla of the palm weevil Rhynchophorus palmarum (Coleoptera, Curculionidae)

The distribution, fine structure and function of the sensilla present on the antennal club of Rhynchophorus palmarum were studied. No sex dimorphism was observed. Scanning and transmission electron microscopy showed five types of hair-like structures, four of which were evenly distributed on the antennal club. Two types of hair (IV and V) showed wall pores, a characteristic of olfactory sensilla. The antenna numbers 11,190 +/- 3040 type IV and 7360 +/- 1500 type V hairs. Using single sensillum recording, we identified 17 types of olfactory receptor neurons (ORNs) on the basis of their responses to pheromone and host plant odors, triggering synergic behavioral responses. We characterized highly specific and sensitive ORNs tuned to the aggregation pheromone (18% ORNs; 0.01-1 ng response threshold) and to host plant odors such as ethyl acetate, ethanol, acetoin and guaiacol (10% ORNs; 1-10 ng response threshold). Eleven percent of the ORNs were more generalist, responding to several odors with low sensitivity. Nine percent of the ORNs showed a complex pattern of responses, being co-activated by the pheromone and plant odors. This suggests an interaction at the sensory neuron level between pheromone and plant odors, triggering synergic behavioral responses.     

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.     

Smoke,pheromone and kairomone olfactory receptor neurons in males and females of the pine sawyer Monochamus galloprovincialis (Olivier) (Coleoptera: Cerambycidae)

The response of antennal olfactory receptor neurons (ORNs) of Monochamus galloprovincialis to several odourants was tested using single sensillum electrophysiology. Behaviourally active pheromone, and kairomone (host and sympatric bark beetle pheromone) odours were tested alongside smoke compounds released by burnt wood that are potentially attractive to the insect. The antennae bore several types of sensilla. Two plate areas in the proximal and distal ends of each antennal segment were covered with basiconic sensilla that responded to the odour stimuli. Sensilla basiconica contained one or two cells of different spike amplitude. The 32 male and 38 female ORNs tested responded with excitations or inhibitions to the different plant odours. In general the response of male and female receptors was very similar so they were pooled to perform a cluster analysis on ORN responses. Six ORNs were clearly specialised for pheromone reception. Responses to kairomone and smoke odours were less specific than those of pheromone, but a group of 9 cells was clearly excited by smoke compounds (mainly eugenol and 4-methyl 2-methoxyphenol), a group of 8 cells was very responsive to α-pinene, β-pinene and cis-verbenol, and a group of 14 cells responded to a wider range of compounds. The rest of the cells (47%) were either non-responsive or slightly inhibited by smoke compounds. Dose–response curves were obtained for several compounds. Different compounds induced significantly different latencies and these appeared to be unrelated to their boiling point.     

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.     

Electrophysiological response patterns of 16 olfactory neurons from the trichoid sensilla to odorant from fecal volatiles in the locust, locusta migratoria manilensis

Locusts are the most serious pests of crops in greater part of the world. They locate their host plants primarily through olfactory cues, using antennal chemosensilla, which house olfactory receptor neurons (ORNs). Despite the great economical interest of these species, their olfactory neurons have been poorly investigated at the functional level. In this study, we have used single sensillum recordings (SSRs) to obtain response patterns of ORNs from the antennal trichoid sensilla to various chemicals in the oriental locust Locusta migratoria. On the basis of their spontaneous spike amplitudes, trichoid sensilla could be distinguished into two types, housing two or three ORNs, respectively. These two structural types could be further classified into seven functional subtypes. Nine different odorants that are present in the locust feces were used as stimulants during SSRs. In particular, benzaldehyde elicited inhibitory responses in most of the ORNs tested. Moreover, in a majority of these ORNs, the excitatory responses obtained with trans-2-hexenal or 2-heptanone was inhibited when benzaldehyde was mixed with these stimulants. At least 16 response patterns of these ORNs to nine chemicals were identified by SSRs, suggesting a high complexity of the cellular mechanisms underlying chemoreception in locusts.     

东北大黑鳃金龟嗅感器超微结构

利用扫描电镜和透射电镜对东北大黑鳃金龟Holotrichia diomphalia成虫触角嗅感器进行超微结构研究。结果表明： 其嗅感器集中于触角鳃片上,着生在表皮内陷形成的凹腔里。嗅感器包括锥形感器和板形感器两种,锥形感器根据锥体形状的差异可分为4种类型,板形感器根据盘体形状的不同可分为5种类型。嗅感器表皮为单壁,壁上具有微孔和孔道微管。嗅感器内神经元的数目并不一致,1～3个不等。雄性触角鳃片的长度长于雌性触角鳃片,并且雄性触角嗅感器的总数远远多于雌性,其中雄性板形感器的数目与雌性差异不大,但雄性锥形感器的数目却远远的多于雌性,几乎是雌性的9倍。由此推测锥形感器是感受性信息素的感器,而板形感器用于感受植物气味。     

Influence of blood meal on the responsiveness of olfactory receptor neurons in antennal sensilla trichodea of the yellow fever mosquito, Aedes aegypti

In female Aedes aegypti L. mosquitoes, a blood meal induces physiological and behavioral changes. Previous studies have shown that olfactory receptor neurons (ORNs) housed in grooved peg sensilla on the antennae of Ae. aegypti down-regulate their sensitivity to lactic acid, a key component driving host-seeking behavior, which correlates with observed changes in the host-seeking behavior of this species. In the present study, we performed electrophysiological recordings from the most abundant antennal sensillum type, sensilla trichodea. Our results indicate that the response spectra of ORNs contained within most trichoid sensilla do not change in response to blood feeding. However, we observe an increase in sensitivity to primarily indole and phenolic compounds in neurons housed within four of the five functional types of short blunt tipped II trichoid sensilla, both at 24 and 72 h post-blood feeding, which was more pronounced at 24 h than 72 h. Furthermore, sensitivity to undecanone, acetic acid and propionic acid was observed to increase 72 h post-blood meal. Considering the timing of these changes, we believe that these neurons may be involved in driving the orientation behavior of female mosquitoes to oviposition sites, which are known to release these compounds.     

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.     

Antennal olfactory sensory neurones responsive to host and nonhost plant volatiles in gorse pod moth Cydia succedana

We investigate the response profiles of the antennal olfactory sensory neurones (OSNs) in male and female gorse pod moth Cydia succedana to host and nonhost volatiles, using the single sensillum recording technique. Eight different classes of olfactory sensilla are identified in female C. succedana and five different classes of olfactory sensilla in males. Nineteen different classes of OSNs are identified from the sensilla in females, and nine different classes of OSNs in the male sensilla. All classes of sensilla, except class F7 and class M1 sensilla, co‐compartmentalize two or three OSNs in each sensillum, and the OSNs present in the same sensillum are specialized for different volatiles. Most plant‐volatile OSNs exhibit phasic‐tonic type of temporal responses, whereas the pheromone OSNs in male C. succedana show rather phasic responses. The majority of OSNs identified in C. succedana display highly specialized responses to a narrow range of volatiles, whereas only a small proportion of OSNs show broad response spectra. Two most abundant classes of OSNs exhibit highly specialized responses to β‐myrcene and (E)‐β‐ocimene, two major volatiles released by gorse (Ulex europaeus), the main host of C. succedana. By contrast, several other classes of OSNs exhibit highly specialized responses to geraniol, (Z)‐3‐hexen‐1‐ol, (±)‐α‐terpineol, citral and benzyl acetate, which are produced by various nonhost plants. Taking the results of the present study together, we suggest that C. succedana use the combinational input from a set of highly specialized OSNs for host plant volatiles and another set of highly specialized OSNs for nonhost volatiles to discriminate between hosts and nonhosts.     

Measuring activity in olfactory receptor neurons in Drosophila: Focus on spike amplitude

Olfactory responses at the receptor level have been thoroughly described in Drosophila melanogaster by electrophysiological methods. Single sensilla recordings (SSRs) measure neuronal activity in intact individuals in response to odors. For sensilla that contain more than one olfactory receptor neuron (ORN), their different spontaneous spike amplitudes can distinguish each signal under resting conditions. However, activity is mainly described by spike frequency.Some reports on ORN response dynamics studied two components in the olfactory responses of ORNs: a fast component that is reflected by the spike frequency and a slow component that is observed in the LFP (local field potential, the single sensillum counterpart of the electroantennogram, EAG). However, no apparent correlation was found between the two elements.In this report, we show that odorant stimulation produces two different effects in the fast component, affecting spike frequency and spike amplitude. Spike amplitude clearly diminishes at the beginning of a response, but it recovers more slowly than spike frequency after stimulus cessation, suggesting that ORNs return to resting conditions long after they recover a normal spontaneous spike frequency. Moreover, spike amplitude recovery follows the same kinetics as the slow voltage component measured by the LFP, suggesting that both measures are connected.These results were obtained in ab2 and ab3 sensilla in response to two odors at different concentrations. Both spike amplitude and LFP kinetics depend on odorant, concentration and neuron, suggesting that like the EAG they may reflect olfactory information.     

Volatile components from the mandibular glands of the turtle ants, Cephalotes alfaroi and Cephalotes cristatus

GC–MS analysis of whole head extracts from the turtle ants, Cephalotes alfaroi and Cephalotes cristatus, showed that 4-heptanone and 4-heptanol were the major volatile components in the mandibular glands. 4-Heptanone and 4-heptanol have rarely been identified in mandibular gland secretions from other ant genera. Thus, these compounds may be chemotaxonomic markers for the genus Cephalotes, since they have been identified in the mandibular glands from all members of this genus that have been investigated to date. Minor components identified in the whole head extracts of these ants were 3-methyl-1-butanol, 3-heptanone, 3-hexanol, 2- and 3-methylbutanoic acids, 2-methyl-4-heptanone, 2-phenylethanol and phenol. To our knowledge, this is the first time that 2-methyl-4-heptanone and phenol have been reported in the mandibular gland secretion from any Formicid.     

Conus venom peptides: correlating chemistry and behavior

Chemical communication in scarab beetles involves female-released long-distance sex pheromones. Electrophysiological recordings using tungsten microelectrodes demonstrated two types of olfactory receptor neurons in the scarab beetle Anomala cuprea, each specific for one of the two pheromone components (R)-buibuilactone and (R)-japonilure, respectively. No neurons were found that responded specifically to enantiomers of the pheromone compounds, i.e. (S)-buibuilactone and (S)-japonilure. Pheromone receptor neurons are present in high numbers on both the male and the female antenna, with a lower sensitivity in the females. As in bark beetles and moths, the pheromone receptor neurons in A. cuprea are very sensitive and selective. The difference in response thresholds between (R)- and (S)-enantiomers is almost three orders of magnitude. Pheromone receptor neurons are found in sensilla placodea located in a defined area on each lamella in the antennal club. (R)-buibuilactone and (R)-japonilure neurons are always found in different sensilla. Both types of sensilla contain two neurons, with the pheromone-sensitive neuron displaying a high spike amplitude and the second neuron, not responding to any of the tested compounds, always with a lower spike amplitude. Accepted: 19 December 1998     

苹毛丽金龟触角嗅感器超微结构

利用扫描电镜和透射电镜对苹毛丽金龟Proagopertha lucidula (Faldermann)雌雄成虫触角嗅感器超微结构特征进行研究。结果表明:嗅感器集中于触角鳃片上,分布在触角鳃片表皮内陷形成的凹腔里,有5种感器,以板状感器为主,以外还有锥状感器、腔状感器、腔锥状感器和毛状感器。板状感器根据盘体形状的不同可分为4种类型,锥状感器根据锥体形状的差异可分为2种类型,腔锥状感器根据形状分为2种类型。嗅感器表皮有微孔和孔道微管。嗅感器内神经元的数目并不一致,1～2个不等。雄虫鳃片嗅感器总数显著多于雌性,是雌虫嗅感器总数1.8倍。其中雄性锥状感器的数目是雌性2倍,雌虫腔锥状感器数量是雄虫的4倍,雄虫板状感器数量是雌虫的2倍,雌雄腔状感器的数目无显著性差异。     

Antennal lobe projection destinations of Helicoverpa zea male olfactory receptor neurons responsive to heliothine sex pheromone components

We used single sensillum recordings to define male Helicoverpa zea olfactory receptor neuron physiology followed by cobalt staining to trace the axons to destination glomeruli of the antennal lobe. Receptor neurons in type A sensilla that respond to the major pheromone component, (Z)-11-hexadecenal, projected axons to the cumulus of the macroglomerular complex (MGC). In approximately 40% of these sensilla a second receptor neuron was stained that projected consistently to a specific glomerulus residing in a previously unrecognized glomerular complex with six other glomeruli stationed immediately posterior to the MGC. Cobalt staining corroborated by calcium imaging showed that receptor neurons in type C sensilla sensitive to (Z)-9-hexadecenal projected to the dorsomedial posterior glomerulus of the MGC, whereas the co-compartmentalized antagonist-sensitive neurons projected to the dorsomedial anterior glomerulus. We also discovered that the olfactory receptor neurons in type B sensilla exhibit the same axonal projections as those in type C sensilla. Thus, it seems that type B sensilla are anatomically type C with regard to the projection destinations of the two receptor neurons, but physiologically one of the receptor neurons is now unresponsive to everything except (Z)-9-tetradecenal, and the other responds to none of the pheromone-related odorants tested.