Specificity-related suppression of responses to binary mixtures in olfactory receptors of the Colorado potato beetle

Odorant mixtures elicit less variable and faster responses than pure odorants

In natural environments, odors are typically mixtures of several different chemical compounds. However, the implications of mixtures for odor processing have not been fully investigated. We have extended a standard olfactory receptor model to mixtures and found through its mathematical analysis that odorant-evoked activity patterns are more stable across concentrations and first-spike latencies of receptor neurons are shorter for mixtures than for pure odorants. Shorter first-spike latencies arise from the nonlinear dependence of binding rate on odorant concentration, commonly described by the Hill coefficient, while the more stable activity patterns result from the competition between different ligands for receptor sites. These results are consistent with observations from numerical simulations and physiological recordings in the olfactory system of insects. Our results suggest that mixtures allow faster and more reliable olfactory coding, which could be one of the reasons why animals often use mixtures in chemical signaling.

Coevolution of Exploiter Specialization and Victim Mimicry can be Cyclic and Saltational

Darwin's Principle of Divergence explains sympatric speciation as gradual and directional. Contradicting evidence suggests that species' traits evolve saltationally. Here, we model coevolution in exploiter-victim systems. Victims (resource population) have heritable, mutable cue phenotypes with different levels of defense. Exploiters have heritable, mutable perceptual phenotypes. Our simulations reveal coevolution of victim mimicry and exploiter specialization in a saltational and reversible cycle. Evolution is gradual and directional only in the specialization phase of the cycle thereby implying that specialization itself is saltational in such systems. Once linked to assortative mating, exploiter specialization provides conditions for speciation.

A High-Bandwidth Dual-Channel Olfactory Stimulator for Studying Temporal Sensitivity of Olfactory Processing

Animals encounter fine-scale temporal patterns of odorant mixtures that contain information about the distance and number of odorant sources. To study the role of such temporal cues for odorant detection and source localization, one needs odorant delivery devices that are capable of mimicking the temporal stimulus statistics of natural odor plumes. However, current odorant delivery devices either lack temporal resolution or are limited to a single odorant channel. Here, we present an olfactory stimulator that features precise control of high-bandwidth stimulus dynamics, which allows generating arbitrary fluctuating binary odorant mixtures. We provide a comprehensive characterization of the stimulator's performance and use it to demonstrate that odor background affects the temporal resolution of insect olfactory receptor neurons, and we present a hitherto unknown odor pulse-tracking capability of up to 60 Hz in Kenyon cells, which are higher order olfactory neurons of the insect brain. This stimulator might help investigating whether and how animals use temporal stimulus cues for odor detection and source localization. Because the stimulator is easy to replicate it can facilitate generating the same odor stimulus dynamics at different experimental setups and across different labs.

Mixture processing and odor-object segregation in insects

When enjoying the scent of grinded coffee or cut grass, most of us are unaware that these scents consist of up to hundreds of volatile substances. We perceive these odorant mixtures as a unitary scent rather than a combination of multiple odorants. The olfactory system processes odor mixtures into meaningful odor objects to provide animals with information that is relevant in everyday tasks, such as habitat localization, foraging, social communication, reproduction, and orientation. For example, odor objects can be a particular flower species on which a bee feeds or the receptive female moth which attracts males by its specific pheromone blend. Using odor mixtures as cues for odor-driven behavior rather than single odorants allows unambiguous identification of a potentially infinite number of odor objects. When multiple odor objects are present at the same time, they form a temporally complex mixture. In order to segregate this mixture into its meaningful constituents, animals must have evolved odor-object segregation mechanisms which are robust against the interference by background odors. In this review, we describe how insects use information of the olfactory environment to either bind odorants into unitary percepts or to segregate them from each other.

Feeding-induced rearrangement of green leaf volatiles reduces moth oviposition

The ability to decrypt volatile plant signals is essential if herbivorous insects are to optimize their choice of host plants for their offspring. Green leaf volatiles (GLVs) constitute a widespread group of defensive plant volatiles that convey a herbivory-specific message via their isomeric composition: feeding of the tobacco hornworm Manduca sexta converts (Z)-3- to (E)-2-GLVs thereby attracting predatory insects. Here we show that this isomer-coded message is monitored by ovipositing M. sexta females. We detected the isomeric shift in the host plant Datura wrightii and performed functional imaging in the primary olfactory center of M. sexta females with GLV structural isomers. We identified two isomer-specific regions responding to either (Z)-3- or (E)-2-hexenyl acetate. Field experiments demonstrated that ovipositing Manduca moths preferred (Z)-3-perfumed D. wrightii over (E)-2-perfumed plants. These results show that (E)-2-GLVs and/or specific (Z)-3/(E)-2-ratios provide information regarding host plant attack by conspecifics that ovipositing hawkmoths use for host plant selection.

DOI:http://dx.doi.org/10.7554/eLife.00421.001

Plants have developed a variety of strategies to defend themselves against herbivorous animals, particularly insects. In addition to mechanical defences such as thorns and spines, plants also produce compounds known as secondary metabolites that keep insects and other herbivores at bay by acting as repellents or toxins. Some of these metabolites are produced on a continuous basis by plants, whereas others—notably compounds called green-leaf volatiles—are only produced once the plant has been attacked. Green-leaf volatiles—which are also responsible for the smell of freshly cut grass—have been observed to provide plants with both direct protection, by inhibiting or repelling herbivores, and indirect protection, by attracting predators of the herbivores themselves.

The hawkmoth Manduca sexta lays its eggs on various plants, including tobacco plants and sacred Datura plants. Once the eggs have hatched into caterpillars, they start eating the leaves of their host plant, and if present in large numbers, these caterpillars can quickly defoliate and destroy it. In an effort to defend itself, the host plant releases green-leaf volatiles to attract various species of Geocoris, and these bugs eat the eggs.

One of the green-leaf volatiles released by tobacco plants is known as (Z)-3-hexenal, but enzymes released by M. sexta caterpillars change some of these molecules into (E)-2-hexenal, which has the same chemical formula but a different structure. The resulting changes in the ‘volatile profile’ alerts Geocoris bugs to the presence of M. sexta eggs and caterpillars on the plant.

Now Allmann et al. show that adult female M. sexta moths can also detect similar changes in the volatile profile emitted by sacred Datura plants that have been damaged by M. sexta caterpillars. This alerts the moths to the fact that Geocoris bugs are likely to be attacking eggs and caterpillars on the plant, or on their way to the plant, so they lay their eggs on other plants. This reduces competition for resources and also reduces the risk of newly laid eggs being eaten by predators. Allmann et al. also identified the neural mechanism that allows moths to detect changes in the volatile profile of plants—the E- and Z- odours lead to different activation patterns in the moth brain.

DOI:http://dx.doi.org/10.7554/eLife.00421.002

Neuronal processing of complex mixtures establishes a unique odor representation in the moth antennal lobe

Animals typically perceive natural odor cues in their olfactory environment as a complex mixture of chemically diverse components. In insects, the initial representation of an odor mixture occurs in the first olfactory center of the brain, the antennal lobe (AL). The contribution of single neurons to the processing of complex mixtures in insects, and in particular moths, is still largely unknown. Using a novel multicomponent stimulus system to equilibrate component and mixture concentrations according to vapor pressure, we performed intracellular recordings of projection and interneurons in an attempt to quantitatively characterize mixture representation and integration properties of single AL neurons in the moth. We found that the fine spatiotemporal representation of 2–7 component mixtures among single neurons in the AL revealed a highly combinatorial, non-linear process for coding host mixtures presumably shaped by the AL network: 82% of mixture responding projection neurons and local interneurons showed non-linear spike frequencies in response to a defined host odor mixture, exhibiting an array of interactions including suppression, hypoadditivity, and synergism. Our results indicate that odor mixtures are represented by each cell as a unique combinatorial representation, and there is no general rule by which the network computes the mixture in comparison to single components. On the single neuron level, we show that those differences manifest in a variety of parameters, including the spatial location, frequency, latency, and temporal pattern of the response kinetics.

Plant terpenes affect intensity and temporal parameters of pheromone detection in a moth

In moths, the components of the female pheromone blend are detected in the male antennae by pheromone olfactory receptor neurons (Ph-ORNs) expressing narrowly tuned olfactory receptors. Responses to sex pheromones have generally been thought to be independent from the odorant background. However, interactions between pheromone components and plant volatiles have been reported at behavioral and detection levels. To document the mechanisms of such interactions, we analyzed Ph-ORN responses of Spodoptera littoralis to the main pheromone component, Z9E11-14:Ac, in the presence of 4 monoterpenes. To mimic natural contexts in which plant odors and pheromone emanate from different sources, the 2 stimuli were presented with different temporal patterns and from independent sources. Linalool reversibly reduced the firing response to Z9E11-14:Ac and produced an off effect. Geraniol and geranyl and linalyl acetates reduced the responses to Z9E11-14:Ac with a longer time course. Pulses of linalool over prolonged pheromone stimulation resulted in a discontinuous firing activity. Pulses of pheromone were better separated over a background of linalool, compared with odorless air. The data confirm that plant compounds may modulate the intensity and the temporal coding by Ph-ORNs of pheromone information. This modulation might positively affect mate location at high pheromone density especially nearby a pheromone source.

Interactions between acetoin, a plant volatile, and pheromone in Rhynchophorus palmarum: behavioral and olfactory neuron responses

Aggregation of Rhynchophorus palmarum weevils on host plants is mediated by a male pheromone (rhynchophorol: R) and host-plant volatiles (PVs) acting in synergy. Synthetic PV blends synergizing pheromone contain acetoin (A) and ethyl acetate (EtAc). R, A, and EtAc are detected by specialized olfactory receptor neurons (ORNs). In addition, particular types of ORNs are tuned to both A and R. To specify the role played by acetoin in pheromone perception, we recorded the responses of ORNs to 100 ng of A or R presented either separately or mixed. Behavioral responses to R, A, and EtAc were studied in a four-armed olfactometer and by field trapping. We screened 59 R-, A-, and AR-tuned ORNs by recording specific responses to odors presented either separately or mixed. Stimulations by blends elicited complex response profiles from the three ORN types: some gave synergistic responses, others were inhibited, and the remainder responded as though both odors were detected independently. Several gave either a weak or no response to a first stimulation by R, but responded clearly to a second stimulation after an intercalary stimulation by A. In the olfactometer, both sexes were more attracted to a blend of A + R (1 + 0.01 ng/sec) than to pure compounds, whereas EtAc did not enhance response to R. Pheromone-baited traps (1 mg/day) containing PV blends (650 mg/day) based on an ethanol/EtAc blend (1:1), plus either 5 or 10% A, or a more complex reference blend, or sugarcane (natural pheromone synergist), caught similar numbers of weevils and about twice as many insects as a control ethanol/EtAc blend. Traps with only pheromone caught about 10 times fewer insects. Behavioral results support the role of acetoin as a pheromone synergist for R. palmarum, and electrophysiological data provide evidence of modulation of peripheral sensory responses to pheromone by acetoin. Sexual dimorphism was observed neither at the ORN nor at the behavioral levels.

Responses in highly selective sensory neurons to blends of pheromone components in the moth Agrotis segetum

Pheromone detecting sensory neurons in moths are known to be highly sensitive and selective. Female-emitted sexual pheromones are normally mixtures of a few to several components. However, not much is known about how receptor neurons respond to blends of compounds. In the present study we investigated how four physiological types of pheromone component-selective neurons responded to binary mixtures or to the complete blend in the turnip moth Agrotis segetum. We found that responses to mixtures only rarely differed from that to the excitatory component alone. The mixture interactions were exclusively suppressive and occurred only at high concentrations. Therefore we conclude that the, in A. segetum, commonly observed mixture interactions observed in higher brain centra are mainly the result of central nervous processing and that information about the pheromone components reaches the antennal lobes virtually unaltered. In addition, we found a physiological type of receptor neuron, responding selectively to one of the female-emitted pheromone components, that has previously not been observed in the Swedish population.

Olfactory receptor neurons detecting plant odours and male volatiles in Anomala cuprea beetles (Coleoptera: Scarabaeidae)

We have identified several types of olfactory receptor neurons in male and female Anomala cuprea beetles. The receptor neurons were sensitive to female sex pheromone components, flower volatiles, green leaf volatiles and unknown volatiles from males. Olfactory sensilla were located on three lamellae forming the antennal club. There was a clear spatial separation between some types of sensilla on each lamella. Receptor neurons for the two sex pheromone components were situated in sensilla placodea covering a specific area on each lamella in both males and females. All sex pheromone receptor neurons were found in these sensilla. Most other receptor neurons were located in a longitudinal, heterogeneous streak formed by various types of sensilla. Receptor neurons for plant-derived compounds appeared to be specialists with a high sensitivity to their respective key compound. The most remarkable among these are the green leaf volatile-specific receptor neurons, which were both sensitive and selective, with the key compound being at least 1000 times as effective as any other compound. These green leaf volatile detectors are apparently homologous to detectors recently found in the scarab Phyllopertha diversa. Our results emphasize the role of single-sensillum recordings as a tool in the identification of biologically active odours.

Olfactory responses to attractants and repellents in tsetse

The aims of this study were to investigate how antennal olfactory cells of tsetse (Diptera: Glossinidae) code odour quality and how they are able to discriminate between attractive and repellent odours. For Glossina pallidipes Austen, a survey is presented of the cells' responses to attractive (1-octen-3-ol, acetone, 3-methylphenol, carbon dioxide) and repellent stimuli (2-methoxyphenol, acetophenone, lactic acid, naphthalene). In addition, the responses of these cells to binary mixtures and the dose-response curves of 1-octen-3-ol, 3-methylphenol, 2-methoxyphenol and acetophenone are presented. A minority of the cells responded to one attractant or repellent only, whereas the vast majority were excited by more than one of the attractive and/or repellent stimuli. It is proposed that the peripheral olfactory cells of tsetse discriminate between different compounds via an across-fibre pattern coding, in which the cells that specifically code for attractants or repellents may play a substantial role in composing a unique excitation pattern that informs the central nervous system about the specificity of odours.

Electrophysiological responses of antennal receptor neurons in female Australian sheep blowflies, Lucilia cuprina, to host odours

Studies were conducted to investigate the electrophysiological responses of receptor neurons on the antennae of female Lucilia cuprina to putative stimulants. Three distinct types of olfactory neurons were identified. Tests with 25 compounds showed 27 out of the 75 neurons examined responded best to racemic 1-octen-3-ol (Oct-best neuron), nine to dimethyldisulphide (DMDS-best neuron) and seven to 2-phenylethanol (PE-best neuron). The remaining cells were not stimulated by any chemical stimulus presented in this study. The olfactory neurons showed low spontaneous activities, 2.2 ~ 2.7 impulses/s, which increased significantly on stimulation with test compounds. Dose-dependent responses were observed with Oct-best neurons. Another type of neuron that had a much higher spontaneous activity (26 impulses/s) was observed. This neuron did not respond to any of the candidate compounds tested but showed a decrease in activity with increased airflow over the antennae and was termed the Airflow-sensitive neuron. The spontaneous activities of Oct-, DMDS-, PE-best and Airflow-sensitive neurons were inhibited by butanoic, pentanoic, hexanoic acids, and 3-methylindole.

The influence of odor type on the discrimination and identification of odorants in multicomponent odor mixtures

Using a limited set of odorants, previous studies have indicated that the ability of humans to discriminate and identify the components of olfactory mixtures is limited to approximately four. However, the ability to generalize these results may have been limited by specific neural or cognitive interactions among the particular odorants used. In the present experiment, 41 subjects examined the influence of odor type (different individual odorants), from two very different odor sets, on the perception of the components of complex mixtures. One set contained odors that were selected by an expert panel to blend well in mixtures (good blenders), whereas the other contained odors that blended poorly in mixtures (poor blenders). The stimuli were common, dissimilar odorants of equivalent, moderate intensity, each of which was a single chemical. A computer-controlled air dilution olfactometer delivered a single odorant or a mixture containing up to eight odorants. Although the poor blenders were more easily discriminated, this superiority was displayed within a narrow range, and the ability of subjects to identify mixture components with either odor set was limited to approximately four. The results indicate that, whereas odor type can alter which odorants will be perceived in a mixture, the limited capacity to discriminate mixture components is independent of the type of odorants. These findings are discussed in terms of their implications for olfactory coding.

The influence of chemical complexity on the perception of multicomponent odor mixtures

The present study investigates the hypothesis that complex object odors (odors that emanate from flowers, foods, sewage, etc.) that consist of dozens of odorants are processed and encoded as discrete entities, as if each was a single chemical odor. To test this hypothesis, the capacity of trained subjects to discriminate and identify the components of stimuli consisting of one to eight object odors was determined. The results indicated that subjects could only identify up to four object odors in a mixture, which is similar to earlier findings with mixtures that contained only single chemical odors. The limited capacity was also reflected in the number of odors selected, regardless of whether the choices were correct or incorrect, in confidence ratings, and in decision times. The identification of a limited number of object odors in every mixture that was presented suggests that both associative (synthetic) and dissociative (analytic) processes are involved in the perceptual analysis of odor mixtures.

Generalization among related complex odorant mixtures and their components: analysis of olfactory perception in the spiny lobster

We investigated the processing of odorant mixtures containing two to seven components by the spiny lobster Panulirus argus. The chemicals tested were food-related compounds that are attractive to spiny lobsters, and include adenosine-5'-monophosphate, betaine, L-cysteine, L-glutamate, DL-succinate, taurine, and ammonium. Components were tested at concentrations that produced search behavioral responses of equal magnitude in unconditioned animals. Responses of unconditioned animals to mixtures and their components reveal hypoadditivity, in which the response to a mixture is less than the sum of the responses to that mixture's components. Aversive conditioning coupled with generalization testing was used to evaluate generalization and hence perceptual similarity between related mixtures. Animals were conditioned to either an individual odorant, a four-compound mixture, or a seven-compound mixture, followed by generalization testing with submixtures or larger mixtures containing the conditioned stimulus. Animals tended not to generalize, but significant generalization between a more simple conditioned stimulus and more complex mixtures containing that conditioned stimulus occurred in 2 of 11 cases, and significant generalization between a conditioned mixture and its submixtures was observed in 4 of 9 cases. Both the number and chemical identity of components of mixtures may contribute to the degree of generalization between mixtures. Overshadowing, in which the ability to learn about a chemical is affected by simultaneous presentation of other chemicals, occurred in two of three cases. We discuss implications of these findings with respect to elemental and configural processing of odorant mixtures in the spiny lobster, possible neural mechanisms responsible for these results, and the potential utility of generalization and overshadowing to the spiny lobster's natural behavior.

Mixture suppression without inhibition for binary mixtures from whole cell patch clamp studies of in situ olfactory receptor neurons of the spiny lobster

Whole cell patch clamping was used to investigate mechanisms of mixture suppression for in situ olfactory receptor neurons (ORNs) of the spiny lobster Panulirus argus. We used a set of single compounds and binary mixtures that have been used in previous biochemical studies of receptor-odorant binding, electrophysiological studies of spiking output from ORNs, and behavioral studies. These odorants were adenosine 5'-monophosphate (AMP), betaine (Bet), L-cysteine (Cys), L-glutamate (Glu), taurine (Tau), ammonium chloride, D,L-succinate, binary mixtures of these compounds, as well as a 33-component artificial oyster mixture (AOM). For the 40 ORNs studied, these stimuli more frequently elicited inward than outward currents. AMP, Glu, Tau and Bet evoked the largest and most numerous inward currents; Cys most commonly evoked outward currents. Na+ was an important charge-carrying ion for the Glu-evoked response in one ORN and the Bet-evoked response in another ORN. Mixture suppression, defined conservatively in this study as cases where the response to a binary mixture was less than the response to the more excitatory component of that mixture, was observed in 6 ORNs. In all 6 cases, neither component of the mixture evoked an outward conductance (i.e. neither was inhibitory). Five of these cases of mixture suppression involved a mixture containing two excitatory compounds (i.e. producing inward conductances): four ORNs were excited by both Glu and AMP, and one ORN was excited by both Tau and Glu. One case of mixture suppression occurred for a compound (Tau) tha did not produce a current when presented alone but which when added to Bet suppressed the inward current generated by Bet. Mechanisms for these suppressions are discussed, including inhibition of receptor binding by the components of a binary mixture and effects on second messengers or ion channels.

Partitioning non-linearities in the response of honey bee olfactory receptor neurons to binary odors

In many organisms, of which honey bees are one example, a general (i.e., non-pheromonal) olfactory receptor neuron may respond to some odorants by increasing its firing rate and to others by decreasing its firing rate. In the latter case, this decrease will be with respect to a background firing rate determined by intrinsic (internal noise) and extrinsic (background odors) factors. To analyse receptor neurons of this complexity, we extend Beidler's model of receptor protein activation dynamics to account for the competition between depolarizing and hyperpolarizing pathways and couple the model to a phenomenological description of the non-linear relationship between the proportion of activate membrane receptors and the receptor cell spike generation rates. We then examine the implications of this theory for predicting the response of receptor neurons to odor mixtures based on their response to pure odorants at concentrations matched to the mixture. We derive inequalities that must be satisfied under our normative model, and propose that deviations from the model be designated as synergisms and inhibitions, depending on the direction in which various equalities and inequalities are violated. We then apply our inequalities to identifying synergisms and inhibitions in data analysed in a different way elsewhere (Akers, R.P. and Getz, W.M. Response of olfactory receptor neurons in honey bees to odorants and their binary mixtures. J. Comp. Physiol. (in press)). In these data regarding the response of honey bee placode sensilla to a number of odorants and their binary combinations, we demonstrate the presence of synergisms and inhibitions--that is, elevated or repressed responses that are not due to competitive interactions of mixture component odorants for receptor sites or Beidler (Beidler, L.M., 1962. Taste receptor stimulation. Prog. Biophys. Biophys. Chem. 12, 107-151) saturation mechanisms.

Central processing of sex pheromone, host odour, and oviposition deterrent information by interneurons in the antennal lobe of female Spodoptera littoralis (Lepidoptera: Noctuidae)

Physiological and anatomical characteristics of antennal lobe interneurons in female Spodoptera littoralis (Boisd.) were investigated using intracellular recording and staining techniques. Responses of local interneurons and projection neurons to female sex pheromone components, host plant odours, and behaviourally active oviposition deterrents were recorded. We found local interneurons and projection neurons that responded specifically to only one or two of the tested odours, but we also found less specific cells, and neurons that responded to most of the tested odourants. These findings show that there are not only specific olfactory pathways in female moths up to the protocerebral level, but also that integration can begin in the antennal lobe. No correlation was found between the degree of specificity of either local interneurons or projection neurons and their respective morphological characteristics. Specialized and unspecialized local interneurons arborized throughout the antennal lobe. Specialized and unspecialized projection neurons had uniglomerular arborizations in the antennal lobe and sent their axons to the calyces of the mushroom body, and to the lateral horn of the protocerebrum. One specific projection neuron had multiglomerular arborizations and projected only to the lateral horn of the protocerebrum. Projection neurons arborizing in the glomeruli closest to the entrance of the antennal nerve always responded to pheromone components. No other correlations were found between the arborization pattern of projection neurons in the antennal lobe or in the protocerebrum and their response characteristics. The sensitivity of local interneurons and projection neurons was in the same range as that of receptor neurons in olfactory sensilla on the antennae, suggesting a much lower convergence in the central nervous system in females than in the pheromone-processing pathway in males.

Honeybee olfactory sensilla behave as integrated processing units

Honeybee placode sensilla contain 18-35 olfactory receptor neurons. In insects, such neurons are thought to not interact with one another before reaching the central nervous system. Extracellular, multiunit recordings were made from the placodes and separated into spike shape classes, termed subplacode units. An analysis of the response spectra of subplacode units demonstrated that subplacode units with similar response spectra were more likely to be found in different placodes than in the same placode. An analysis was made of the mean interspike intervals and its variation for whole placodes and subplacode units. The coefficient of variation for whole placodes was less than that for subplacode units. Whole placode spike trains are therefore more uniform than subplacode spike trains, indicating that neurons might not be firing independently of each other.

Chemosensory responses to mixtures: a model based on composition of receptor cell types

Previous mixture models have assumed that members of a population of chemoreceptor cells are homogeneous in type, i.e., with either single shared or multiple independent receptor sites. In reality, many chemosensory systems actually consist of a heterogeneous population of receptor cells, consisting of both highly specific cells as well as more broadly and variably tuned cells. A mixed receptor composition model for binary mixtures is described which can be applied to chemosensory systems with heterogeneous receptor cell compositions. The model incorporates information on a) the number of receptor sites/transduction processes per cell, b) the specificity of receptor cells, and c) the contribution of the magnitude of response of each receptor cell to the overall response magnitude of the population of all receptor cells. The predictions of this model can be compared to behavioral responses of animals towards binary mixtures, or at any level of sensory processing which involves the input of the receptor cell population, in order to detect possible mixture interactions.