Carbon Dioxide and Fruit Odor Transduction in Drosophila Olfactory Neurons. What Controls their Dynamic Properties?

We measured frequency response functions between odorants and action potentials in two types of neurons in Drosophila antennal basiconic sensilla. CO₂ was used to stimulate ab1C neurons, and the fruit odor ethyl butyrate was used to stimulate ab3A neurons. We also measured frequency response functions for light-induced action potential responses from transgenic flies expressing H134R-channelrhodopsin-2 (ChR2) in the ab1C and ab3A neurons. Frequency response functions for all stimulation methods were well-fitted by a band-pass filter function with two time constants that determined the lower and upper frequency limits of the response. Low frequency time constants were the same in each type of neuron, independent of stimulus method, but varied between neuron types. High frequency time constants were significantly slower with ethyl butyrate stimulation than light or CO₂ stimulation. In spite of these quantitative differences, there were strong similarities in the form and frequency ranges of all responses. Since light-activated ChR2 depolarizes neurons directly, rather than through a chemoreceptor mechanism, these data suggest that low frequency dynamic properties of Drosophila olfactory sensilla are dominated by neuron-specific ionic processes during action potential production. In contrast, high frequency dynamics are limited by processes associated with earlier steps in odor transduction, and CO₂ is detected more rapidly than fruit odor.     

Flower and fruit volatiles assist host‐plant location in the Tomato fruit fly Neoceratitis cyanescens

Mature females of the tomato fruit fly Neoceratitis cyanescens can detect host fruit at a short distance using only visual stimuli, but little is known about the role of airborne volatile cues in the host searching strategy. A series of experiments is conducted in a laboratory wind tunnel, in which the behavioural responses of individual flies to volatiles from Solanaceae host plants (including tomato Lycopersicum esculentum Mill., bug weed Solanum mauritianum Scop. and Turkey berry Solanum torvum Sw.) are observed, according to some environmental (air speed) and physiological (age and mating status of females, time of day) factors. Mature females respond primarily to specific olfactory cues from blends of flowers or host fruit, preferentially unripe fruit for bug weed, as opposed to ripe fruit for Turkey berry or tomato. Males are also highly attracted by the odour of unripe fruit of bug weed. Wind plays a key role, as shown by the proportion of flies that reach the upwind section of the tunnel in the presence of both fruit odour and air flow (66.7%) and in the absence of either fruit odour (13.3%) or wind (36.7%). In response to fruit volatiles carried by wind, flies embark in a ‘plume tracking’ or ‘aim and shoot' flight, consistent with odour‐conditioned anemotaxis. Females respond to host fruit odour regardless of their age, egg load or mating status, and also more consistently in the afternoon, which is their preferential time of day for egg‐laying. Searching behaviour and response to host volatiles in N. cyanescens are discussed in the light of host‐finding and an adaptive strategy.     

The chemosensory basis for behavioral divergence involved in sympatric host shifts II: olfactory receptor neuron sensitivity and temporal firing pattern to individual key host volatiles

The Rhagoletis species complex has been a key player in the sympatric speciation debate for much of the last 50 years. Studies indicate that differences in olfactory preference for host fruit volatiles could be important in reproductively isolating flies infesting each type of fruit via premating barriers to gene flow. Single sensillum electrophysiology was used to compare the response characteristics of olfactory receptor neurons from apple, hawthorn, and flowering dogwood-origin populations of R. pomonella, as well as from the blueberry maggot, R. mendax (an outgroup). Eleven volatiles were selected as stimuli from behavioral/electroantennographic studies of the three R. pomonella host populations. Previously, we reported that differences in preference for host fruit volatile blends are not a function of alterations in the general class of receptor neurons tuned to key host volatiles. In the present study, population comparisons involving dose–response trials with the key volatiles revealed significant variability in olfactory receptor neuron sensitivity and temporal firing pattern both within and among Rhagoletis populations. It is concluded that such variability in peripheral sensitivity and temporal firing pattern could influence host preference and contribute to host fidelity and sympatric host shifts in the Rhagoletis complex.     

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.     

Neural Representations of Airflow in Drosophila Mushroom Body

The Drosophila mushroom body (MB) is a higher olfactory center where olfactory and other sensory information are thought to be associated. However, how MB neurons of Drosophila respond to sensory stimuli other than odor is not known. Here, we characterized the responses of MB neurons to a change in airflow, a stimulus associated with odor perception. In vivo calcium imaging from MB neurons revealed surprisingly strong and dynamic responses to an airflow stimulus. This response was dependent on the movement of the 3^rd antennal segment, suggesting that Johnston''s organ may be detecting the airflow. The calyx, the input region of the MB, responded homogeneously to airflow on. However, in the output lobes of the MB, different types of MB neurons responded with different patterns of activity to airflow on and off. Furthermore, detailed spatial analysis of the responses revealed that even within a lobe that is composed of a single type of MB neuron, there are subdivisions that respond differently to airflow on and off. These subdivisions within a single lobe were organized in a stereotypic manner across flies. For the first time, we show that changes in airflow affect MB neurons significantly and these effects are spatially organized into divisions smaller than previously defined MB neuron types.     

Prospects for Developing Odour Baits To Control Glossina fuscipes spp., the Major Vector of Human African Trypanosomiasis

We are attempting to develop cost-effective control methods for the important vector of sleeping sickness, Glossina fuscipes spp. Responses of the tsetse flies Glossina fuscipes fuscipes (in Kenya) and G. f. quanzensis (in Democratic Republic of Congo) to natural host odours are reported. Arrangements of electric nets were used to assess the effect of cattle-, human- and pig-odour on (1) the numbers of tsetse attracted to the odour source and (2) the proportion of flies that landed on a black target (1×1 m). In addition responses to monitor lizard (Varanus niloticus) were assessed in Kenya. The effects of all four odours on the proportion of tsetse that entered a biconical trap were also determined. Sources of natural host odour were produced by placing live hosts in a tent or metal hut (volumes≈16 m³) from which the air was exhausted at ∼2000 L/min. Odours from cattle, pigs and humans had no significant effect on attraction of G. f. fuscipes but lizard odour doubled the catch (P<0.05). Similarly, mammalian odours had no significant effect on landing or trap entry whereas lizard odour increased these responses significantly: landing responses increased significantly by 22% for males and 10% for females; the increase in trap efficiency was relatively slight (5–10%) and not always significant. For G. f. quanzensis, only pig odour had a consistent effect, doubling the catch of females attracted to the source and increasing the landing response for females by ∼15%. Dispensing CO₂ at doses equivalent to natural hosts suggested that the response of G. f. fuscipes to lizard odour was not due to CO₂. For G. f. quanzensis, pig odour and CO₂ attracted similar numbers of tsetse, but CO₂ had no material effect on the landing response. The results suggest that identifying kairomones present in lizard odour for G. f. fuscipes and pig odour for G. f. quanzensis may improve the performance of targets for controlling these species.     

The chemosensory basis for behavioral divergence involved in sympatric host shifts. I. Characterizing olfactory receptor neuron classes responding to key host volatiles

The recent shift of Rhagoletis pomonella from its native host hawthorn to introduced, domestic apple has been implicated as an example of sympatric speciation. Recent studies suggest that host volatile preference might play a fundamental role in host shifts and subsequent speciation in this group. Single sensillum electrophysiology was used to test a proposed hypothesis that differences in R. pomonella olfactory preference are due to changes in the number or odor specificity of olfactory receptor neurons. Individuals were analyzed from apple, hawthorn, and flowering dogwood-origin populations, as well as from the blueberry maggot, Rhagoletis mendax Curran (an outgroup). Eleven compounds were selected as biologically relevant stimuli from previous electroantennographic/behavioral studies of the three R. pomonella populations to host fruit volatiles. Cluster analysis of 99 neuron responses showed that cells from all tested populations could be grouped into the same five classes, ranging from those responding to one or two volatiles to those responding to several host volatiles. Topographical mapping also indicated that antennal neuron locations did not differ by class or fly taxa. Our results do not support the hypothesis that differences in host preference among Rhagoletis populations are a result of alterations in the number or class of receptor neurons responding to host volatiles.     

Encoding of plant odours by receptor neurons in the pine weevil (Hylobius abietis) studied by linked gas chromatography-electrophysiology

Receptor neuron responses to plant volatiles, trapped by head-space procedures, were examined in the pine weevil Hylobius abietis, using gas chromatography linked with electrophysiological recordings from single neurons. Seventy-two receptor neurons were tested 173 times for various plant volatile mixtures, either via a polar or a non-polar column.

1. All responses appeared as increased firing rates which followed the concentration profiles of the GC-eluted compounds.
2. The neurons were classified separately for the two column types in 17 and 19 groups respectively, according to the compounds they responded to. It suggests that the plant odour information is encoded by a large, but limited number of receptor neuron types.
3. Most neurons responded to a limited number of compounds (1–5) and showed a marked best response to one of them, whereas additional responses to several other components which seems to be structurally similar, was recorded for some neurons. It suggests that the plant odour receptor neurons are rather narrowly than broadly tuned, and that each neuron is specialized for receiving information about one or a few related compounds.
4. Most neurons responded to monoterpenes, whereas the other neurons responded to compounds of other categories.
5. Both major and minor plant volatile components activated specifically receptor neurons.

     

Concurrent modulation of neuronal and behavioural olfactory responses to sex and host plant cues in a male moth

Mating has profound effects on animal physiology and behaviour, not only in females but also in males, which we show here for olfactory responses. In cotton leafworm moths, Spodoptera littoralis, odour-mediated attraction to sex pheromone and plant volatiles are modulated after mating, producing a behavioural response that matches the physiological condition of the male insect. Unmated males are attracted by upwind flight to sex pheromone released by calling females, as well as to volatiles of lilac flowers and green leaves of the host plant cotton, signalling adult food and mating sites, respectively. Mating temporarily abolishes male attraction to females and host plant odour, but does not diminish attraction to flowers. This behavioural modulation is correlated with a response modulation in the olfactory system, as shown by electro-physiological recordings from antennae and by functional imaging of the antennal lobe, using natural odours and synthetic compounds. An effect of mating on the olfactory responses to pheromone and cotton plant volatiles but not to lilac flowers indicates the presence of functionally independent neural circuits within the olfactory system. Our results indicate that these circuits interconnect and weigh perception of social and habitat odour signals to generate appropriate behavioural responses according to mating state.     

Oriented responses of the triatomine bugs Rhodnius prolixus and Triatoma infestans to vertebrate odours on a servosphere

Oriented responses of both R. prolixus and T. infestans adults were recorded on a servosphere to mouse-odour, one of its components (CO₂), and to rabbit urine-odour. The volatiles were delivered in an air-stream under controlled conditions which excluded other sensory modalities. In stimulus-free air the triatomines walked preferentially downwind in straight bouts interrupted by stops or periods at relatively low speeds, all of variable duration. In odour-laden air, bugs maintained their typical walking habit but switched from negative to positive anemotaxis. The characteristic response to odour onset was to stop, sample the air with the antennae, turn upwind in situ, and then walk off in the direction of the source for at least a few seconds, i.e., odour mediated anemotaxis. Mouse-odour caused T. infestans to increase its speed to 5.3 cms^-1. Both species continued with the upwind response for some time after odour delivery ceased, but the crosswind component of the tracks was more prominent during this period — an effort, we presume, by the bugs to re-contact an odour plume. This investigation provides unequivocal evidence for host finding in triatomines by olfactory cues alone.     

Interactions of mechanical stimuli and sex pheromone information in antennal lobe neurons of a male moth,<Emphasis Type="Italic"> Spodoptera littoralis</Emphasis>

Male moths respond to sex pheromone sources with up-wind flight behaviour. Localization of the odour source requires not only detection of the olfactory stimulus, but also other sensory input regarding, e.g. visual and mechanical stimuli. Thus, integration of different types of sensory input is necessary. It is, however, not known where in the central nervous system the integration of information regarding different sensory modalities takes place. Using intracellular recording and staining techniques, we investigated neurons in the antennal lobe of Spodoptera littoralis, during stimulation with a mechanical stimulus and a sex pheromone. Fifteen percent of all the neurons investigated responded to the mechanical stimulus and the majority of these neurons showed altered responses if the olfactory stimulus was added. A receptor neuron responding only to the wind stimulus was found to arborise in the antennal lobe. Most projection neurons responded with an enhanced action potential frequency to the combined stimulus. In local interneurons, enhancement, depression, or no change of the responses to the wind stimulus was found when the olfactory stimulus was added. The results suggest that neurons present in the antennal lobe integrate mechanosensory and olfactory input, possibly assisting the moths to orient during up-wind flight towards an odour source.     

Drosophila Pheromone-Sensing Neurons Expressing the ppk25 Ion Channel Subunit Stimulate Male Courtship and Female Receptivity

As in many species, gustatory pheromones regulate the mating behavior of Drosophila. Recently, several ppk genes, encoding ion channel subunits of the DEG/ENaC family, have been implicated in this process, leading to the identification of gustatory neurons that detect specific pheromones. In a subset of taste hairs on the legs of Drosophila, there are two ppk23-expressing, pheromone-sensing neurons with complementary response profiles; one neuron detects female pheromones that stimulate male courtship, the other detects male pheromones that inhibit male-male courtship. In contrast to ppk23, ppk25, is only expressed in a single gustatory neuron per taste hair, and males with impaired ppk25 function court females at reduced rates but do not display abnormal courtship of other males. These findings raised the possibility that ppk25 expression defines a subset of pheromone-sensing neurons. Here we show that ppk25 is expressed and functions in neurons that detect female-specific pheromones and mediates their stimulatory effect on male courtship. Furthermore, the role of ppk25 and ppk25-expressing neurons is not restricted to responses to female-specific pheromones. ppk25 is also required in the same subset of neurons for stimulation of male courtship by young males, males of the Tai2 strain, and by synthetic 7-pentacosene (7-P), a hydrocarbon normally found at low levels in both males and females. Finally, we unexpectedly find that, in females, ppk25 and ppk25-expressing cells regulate receptivity to mating. In the absence of the third antennal segment, which has both olfactory and auditory functions, mutations in ppk25 or silencing of ppk25-expressing neurons block female receptivity to males. Together these results indicate that ppk25 identifies a functionally specialized subset of pheromone-sensing neurons. While ppk25 neurons are required for the responses to multiple pheromones, in both males and females these neurons are specifically involved in stimulating courtship and mating.     

Mutants in Drosophila TRPC Channels Reduce Olfactory Sensitivity to Carbon Dioxide

Background

Members of the canonical Transient Receptor Potential (TRPC) class of cationic channels function downstream of Gαq and PLCβ in Drosophila photoreceptors for transducing visual stimuli. Gαq has recently been implicated in olfactory sensing of carbon dioxide (CO₂) and other odorants. Here we investigated the role of PLCβ and TRPC channels for sensing CO₂ in Drosophila.

Methodology/Principal Findings

Through behavioral assays it was demonstrated that Drosophila mutants for plc21c, trp and trpl have a reduced sensitivity for CO₂. Immuno-histochemical staining for TRP, TRPL and TRPγ indicates that all three channels are expressed in Drosophila antennae including the sensory neurons that express CO₂ receptors. Electrophysiological recordings obtained from the antennae of protein null alleles of TRP (trp³⁴³) and TRPL (trpl³⁰²), showed that the sensory response to multiple concentrations of CO₂ was reduced. However, trpl³⁰²; trp³⁴³ double mutants still have a residual response to CO₂. Down-regulation of TRPC channels specifically in CO₂ sensing olfactory neurons reduced the response to CO₂ and this reduction was obtained even upon down-regulation of the TRPCs in adult olfactory sensory neurons. Thus the reduced response to CO₂ obtained from the antennae of TRPC RNAi strains is not due to a developmental defect.

Conclusion

These observations show that reduction in TRPC channel function significantly reduces the sensitivity of the olfactory response to CO₂ concentrations of 5% or less in adult Drosophila. It is possible that the CO₂ receptors Gr63a and Gr21a activate the TRPC channels through Gαq and PLC21C.     

Glomerular interactions in olfactory processing channels of the antennal lobes

An open question in olfactory coding is the extent of interglomerular connectivity: do olfactory glomeruli and their neurons regulate the odorant responses of neurons innervating other glomeruli? In the olfactory system of the moth Manduca sexta, the response properties of different types of antennal olfactory receptor cells are known. Likewise, a subset of antennal lobe glomeruli has been functionally characterized and the olfactory tuning of their innervating neurons identified. This provides a unique opportunity to determine functional interactions between glomeruli of known input, specifically, (1) glomeruli processing plant odors and (2) glomeruli activated by antennal stimulation with pheromone components of conspecific females. Several studies describe reciprocal inhibitory effects between different types of pheromone-responsive projection neurons suggesting lateral inhibitory interactions between pheromone component-selective glomerular neural circuits. Furthermore, antennal lobe projection neurons that respond to host plant volatiles and innervate single, ordinary glomeruli are inhibited during antennal stimulation with the female’s sex pheromone. The studies demonstrate the existence of lateral inhibitory effects in response to behaviorally significant odorant stimuli and irrespective of glomerular location in the antennal lobe. Inhibitory interactions are present within and between olfactory subsystems (pheromonal and non-pheromonal subsystems), potentially to enhance contrast and strengthen odorant discrimination.     

Discrimination training with multimodal stimuli changes activity in the mushroom body of the hawkmoth Manduca sexta

Background

The mushroom bodies of the insect brain play an important role in olfactory processing, associative learning and memory. The mushroom bodies show odor-specific spatial patterns of activity and are also influenced by visual stimuli.

Methodology/Principal Findings

Functional imaging was used to investigate changes in the in vivo responses of the mushroom body of the hawkmoth Manduca sexta during multimodal discrimination training. A visual and an odour stimulus were presented either together or individually. Initially, mushroom body activation patterns were identical to the odour stimulus and the multimodal stimulus. After training, however, the mushroom body response to the rewarded multimodal stimulus was significantly lower than the response to the unrewarded unimodal odour stimulus, indicating that the coding of the stimuli had changed as a result of training. The opposite pattern was seen when only the unimodal odour stimulus was rewarded. In this case, the mushroom body was more strongly activated by the multimodal stimuli after training. When no stimuli were rewarded, the mushroom body activity decreased for both the multimodal and unimodal odour stimuli. There was no measurable response to the unimodal visual stimulus in any of the experiments. These results can be explained using a connectionist model where the mushroom body is assumed to be excited by olfactory stimulus components, and suppressed by multimodal configurations.

Conclusions

Discrimination training with multimodal stimuli consisting of visual and odour cues leads to stimulus specific changes in the in vivo responses of the mushroom body of the hawkmoth.     

Functional and molecular evolution of olfactory neurons and receptors for aliphatic esters across the Drosophila genus

Insect olfactory receptor (Or) genes are large, rapidly evolving gene families of considerable interest for evolutionary studies. They determine the responses of sensory neurons which mediate critical behaviours and ecological adaptations. We investigated the evolution across the genus Drosophila of a subfamily of Or genes largely responsible for the perception of ecologically relevant aliphatic esters; products of yeast fermentation and fruits. Odour responses were recorded from eight classes of olfactory receptor neurons known to express this Or subfamily in D. melanogaster and from homologous sensilla in seven other species. Despite the fact that these species have diverged over an estimated 40 million years, we find that odour specificity is largely maintained in seven of the eight species. In contrast, we observe extensive changes in most neurons of the outgroup species D. virilis, and in two neurons across the entire genus. Some neurons show small shifts in specificity, whilst some dramatic changes correlate with gene duplication or loss. An olfactory receptor neuron response similarity tree did not match an Or sequence similarity tree, but by aligning Or proteins of likely functional equivalence we identify residues that may be relevant for odour specificity. This will inform future structure–function studies of Drosophila Ors.     

Physiology and glomerular projections of olfactory receptor neurons on the antenna of female Heliothis virescens (Lepidoptera: Noctuidae) responsive to behaviorally relevant odors

The neurophysiology and antennal lobe projections of olfactory receptor neurons housed within short trichoid sensilla of female Heliothis virescens F. (Noctuidae: Lepidoptera) were investigated using a combination of cut-sensillum recording and cobalt-lysine staining techniques. Behaviorally relevant odorants, including intra- and inter-sexual pheromonal compounds, plant and floral volatiles were selected for testing sensillar responses. A total of 184 sensilla were categorized into 25 possible sensillar types based on odor responses and sensitivity. Sensilla exhibited both narrow (responding to few odors) and broad (responding to many odors) response spectra. Sixty-six percent of the sensilla identified were stimulated by conspecific odors; in particular, major components of the male H. virescens hairpencil pheromone (hexadecanyl acetate and octadecanyl acetate) and a minor component of the female sex pheromone, (Z)-9-tetradecenal. Following characterization of the responses, olfactory receptor neurons within individual sensilla were stained with cobalt lysine (N=39) and traced to individual glomeruli in the antennal lobe. Olfactory receptor neurons with specific responses to (Z)-9-tetradecenal, a female H. virescens sex pheromone component, projected to the female-specific central large female glomerulus (cLFG) and other glomeruli. Terminal arborizations from sensillar types containing olfactory receptor neurons sensitive to male hairpencil components and plant volatiles were also localized to distinct glomerular locations. This information provides insight into the representation of behaviorally relevant odorants in the female moth olfactory system. Electronic Supplementary Material Supplementary material is available for this article at and is accessible for authorized users.     

Elevated CO2 impairs olfactory‐mediated neural and behavioral responses and gene expression in ocean‐phase coho salmon (Oncorhynchus kisutch)

Elevated concentrations of CO₂ in seawater can disrupt numerous sensory systems in marine fish. This is of particular concern for Pacific salmon because they rely on olfaction during all aspects of their life including during their homing migrations from the ocean back to their natal streams. We investigated the effects of elevated seawater CO₂ on coho salmon (Oncorhynchus kisutch) olfactory‐mediated behavior, neural signaling, and gene expression within the peripheral and central olfactory system. Ocean‐phase coho salmon were exposed to three levels of CO₂, ranging from those currently found in ambient marine water to projected future levels. Juvenile coho salmon exposed to elevated CO₂ levels for 2 weeks no longer avoided a skin extract odor that elicited avoidance responses in coho salmon maintained in ambient CO₂ seawater. Exposure to these elevated CO₂ levels did not alter odor signaling in the olfactory epithelium, but did induce significant changes in signaling within the olfactory bulb. RNA‐Seq analysis of olfactory tissues revealed extensive disruption in expression of genes involved in neuronal signaling within the olfactory bulb of salmon exposed to elevated CO₂, with lesser impacts on gene expression in the olfactory rosettes. The disruption in olfactory bulb gene pathways included genes associated with GABA signaling and maintenance of ion balance within bulbar neurons. Our results indicate that ocean‐phase coho salmon exposed to elevated CO₂ can experience significant behavioral impairments likely driven by alteration in higher‐order neural signal processing within the olfactory bulb. Our study demonstrates that anadromous fish such as salmon may share a sensitivity to rising CO₂ levels with obligate marine species suggesting a more wide‐scale ecological impact of ocean acidification.     

Discrimination between enantiomers of linalool by olfactory receptor neurons in the cabbage moth Mamestra brassicae (L.)

Plants emit complex blends of volatiles, including chiral compounds that might be detected by vertebrates and invertebrates. Insects are ideal model organisms for studying the underlying receptor neuron mechanisms involved in olfactory discrimination of enantiomers. In the present study, we have employed two-column gas chromatography linked to recordings from single olfactory receptor neurons of Mamestra brassicae, in which separation of volatiles in a polar and a chiral column was performed. We here present the response properties of olfactory receptor neurons tuned to linalool. The narrow tuning of these receptor neurons was demonstrated by their strong responses to (R)-(-)-linalool, the weaker responses to the (+)-enantiomer as well as a few structurally related compounds, and no responses to the other numerous plant released volatiles. The enantioselectivity was verified by parallel dose-response curves, that of (R)-(-)-linalool shifted 1 log unit to the left of the (S)-(+)-linalool curve. A complete overlap of the temporal response pattern was found when comparing the responses of the same strength. Analysis of the spike amplitude and waveform indicated that the responses to the two enantiomers originated from the same neuron.