Odor Interactions Research Articles

Interactions of Odorants with Olfactory Receptors and Other Preprocessing Mechanisms: How Complex and Difficult to Predict?

In this issue of Chemical Senses, Münch et al. present a thorough analysis of how mixtures of odorants interact with olfactory receptors (ORs) borne by olfactory receptor neurons (ORNs). Using fruit fly ORNs expressing the receptor OR22a, they provide a clear example of mixture interaction and confirm that the response of an ORN to a binary mixture can be sometimes predicted quantitatively knowing the ORN responses to its components as shown previously in rat ORNs. The prediction is based on a nonlinear model that assumes a classical 2-step activation of the OR and competition of the 2 odorants in the mixture for the same binding site. Can this success be generalized to all odorant-receptor pairs? This would be an encouraging perspective, especially for the fragrance and flavor industries, as it would permit the prediction of all mixtures. To address this question, I outline its conceptual framework and discuss the variety of mixture interactions found so far. In accordance with the effects described in the study of other receptors, several kinds of mixture interactions have been found that are not easily predictable. The relative importance of the predictable and less predictable effects thus appears as a major issue for future developments.

Effects of Stimulus Intensity on Odor Enhancement by Taste

Enhancement of retronasal odors by sucrose has been shown to be a reliable perceptual phenomenon of taste–odor interactions. It is unknown, however, whether stimulus intensity modulates the degree of odor enhancement. The present study was therefore designed to investigate how the intensity of odor and taste alone affects the degree of odor enhancement. In the first experiment, subjects rated the intensities of taste and odor for aqueous solutions of sucrose and citral at three concentrations, both alone and in binary mixtures. The results showed that sucrose significantly increased the “citrus” ratings to an extent that was inversely related to the intensity of citral alone. Interestingly, the increase in sucrose intensity had insignificant effects on odor enhancement. To test the reliability of the relationship between odor intensity and the degree of enhancement, a second experiment was conducted in which subjects inhaled three concentrations of citral in vapor phase via the mouth while tasting sucrose. This procedure also provided an opportunity to rule out the possibility that the odor enhancement is the result of, at least in part, physicochemical interactions between sucrose and volatiles. Consistent with the previous results, the presence of sucrose in the mouth significantly enhanced the “citrus” ratings compared to when citral was inhaled alone, and the odor intensity relationship was preserved. These findings demonstrate that odor enhancement by sucrose is a perceptual phenomenon that occurs independent of physicochemical interactions between flavor components and that the degree of enhancement is greater when retronasal odors are weak.

The ‘Mouth to Nose Merging System’: A novel approach to study the impact of odour on other sensory perceptions

The impact of odour on taste has been widely studied and it is less the case for aroma and texture perceptions. The relations between odour and texture notably are not very clear and results from previous works are not in accordance. Furthermore, there is little information on the impact of an odour, representative of an aroma released in human mouth, presented via the orthonasal route on other perceptions. Most of the time, investigations dealing with sensory interactions were carried out on food model matrices which lead to a problem for extrapolation of the results to real food products. The aim of the study was to measure the impact of an odorant flow on aroma, taste and texture perceptions when eating a complex real food. To fulfill the objective, apple was chosen as a complex and familiar real food matrix and an innovative device, called ‘Mouth to Nose Merging System’ was designed, validated and used in this work. It involved (1) an in vitro chewing simulator called ‘artificial mouth’ and delivering an odorant extract similar to the aroma released in human mouth; (2) in vivo olfactory receptors of a judge’s nose, from a trained panel. The study showed that apple texture perception can be modified by the apple odorant extract. The latter did not modify aroma and taste perception. The method gave promising results for investigating cognitive interactions resulting from food consumption as odour and texture interactions were underlined.

Interactions of odorants with olfactory receptors and receptor neurons match the perceptual dynamics observed for woody and fruity odorant mixtures

The present study aimed to create a direct bridge between observations on peripheral and central responses to odorant mixtures and their components. Three experiments were performed using mixtures of fruity (isoamyl acetate; ISO) and woody (whiskey lactone; WL) odorants known to contribute to some of the major notes in Burgundy red wine. These experiments consisted of (i) calcium imaging of human embryonic kidney cells (HEK293T) transfected with olfactory receptors (ORs); (ii) single-unit electrophysiological recordings from olfactory receptor neurons (ORNs) and analyses of electro-olfactogram (EOG) responses in the rat nose in vivo; and (iii) psychophysical measurements of the perceived intensity of the mixtures as rated by human subjects. The calcium imaging and electrophysiological results revealed that ISO and WL can act simultaneously on single ORs or ORNs and confirm that receptor responses to mixtures are not the result of a simple sum of the effects of the individual mixture compounds. The addition of WL to ISO principally suppressed the ORN activation induced by ISO alone and was found to enhance this activation in a subset of cases. In the human studies, the addition of high concentrations of WL to ISO decreased the perceived intensity of the ISO. In contrast, the addition of low concentrations of WL enhanced the perceived intensity of the fruity note (ISO) in this mixture, as it enhanced EOG responses in ORNs. Thus, both OR and ORN responses to ISO + WL mixtures faithfully reflected perceptual response changes, so the odour mixture information is set up after the peripheral stage of the olfactory system.

A first glance on the molecular mechanisms of pheromone-plant odor interactions in moth antennae

A first glance on the molecular mechanisms of pheromone-plant odor interactions in moth antennae

Bio-inspired solutions to the challenges of chemical sensing

Bio-inspired solutions to the challenges of chemical sensing

The Mouse Eugenol Odorant Receptor: Structural and Functional Plasticity of a Broadly Tuned Odorant Binding Pocket

Molecular interactions of odorants with their olfactory receptors (ORs) are of central importance for the ability of the mammalian olfactory system to detect and discriminate a vast variety of odors with a limited set of receptors. How a particular OR binds and distinguishes different odorant molecules remains largely unknown on a structural basis. Here we investigated this question for the mouse eugenol receptor (mOR-EG). By screening a large odorant library, we discovered a wide range of chemical structures activating the receptor in heterologous mammalian cells. Potent agonists comprise (i) benzene, (ii) cyclohexane, or (iii) polycyclic structures substituted with alcohol, aldehyde, keto, ether, or esterified carboxylic groups. To detect those amino acids within the receptor that are in contact with a particular bound odorant molecule, we investigated how distinct mOR-EG point mutants were activated by the different odorant agonists found for the wild-type receptor. We identified 11 amino acids as a part of the receptor's ligand binding pocket. Molecular modeling predicted 10 of these residues in transmembrane helices TM3-TM6 and one in the extracellular loop between TM2 and TM3. These amino acids participate in odorant binding with variable importance depending on the type of odorant, revealing functional "fingerprints" of ligand-receptor interactions.

AN EXPECTATIONS-BASED APPROACH TO EXPLAINING THE CROSSMODAL INFLUENCE OF COLOR ON ORTHONASAL OLFACTORY IDENTIFICATION: ASSESSING THE INFLUENCE OF TEMPORAL AND SPATIAL FACTORS

In a previous series of experiments, the present authors demonstrated that when people are asked to identify the flavors of various drinks on the basis of orthonasal olfactory cues, a number of experimental variables can modulate whether the color of the drinks influence their judgments. Here we examined two additional variables that we hypothesized might also mediate color's effect on flavor identification behavior: the timing and spatial positioning of color cues with respect to the positioning of olfactory cues. In Experiment 1, participants were exposed to color cues before, during/after, or after exposure to the olfactory cues. Color had a significant influence on flavor identification behavior in the latter two conditions, but not in the former condition. In Experiment 2, color cues were spatially separated from the olfactory cues, and no significant effect of color on flavor identification behavior was found. These results therefore reveal that manipulating spatial and temporal factors can have differential effects on crossmodal color–odor interactions. PRACTICAL APPLICATIONS This research has the potential to inform the decisions made by manufacturers regarding the placement of color and odor cues in products and packaging. While color cues (which, critically, often precede our consumption of foodstuffs) have been shown to influence our olfactory judgments of food and beverage identity, it is important for manufacturers to consider the specific temporal and spatial relationships that either encourage (or discourage) such crossmodal interactions. By taking these factors into account, our experience and enjoyment of food and drink can be optimized – especially if color–odor interactions are used to facilitate the most favorable interpretation of a stimulus.

Single Sensillum Recordings in the Insects <em>Drosophila melanogaster</em> and <em>Anopheles gambiae</em>

The sense of smell is essential for insects to find foods, mates, predators, and oviposition sites3. Insect olfactory sensory neurons (OSNs) are enclosed in sensory hairs called sensilla, which cover the surface of olfactory organs. The surface of each sensillum is covered with tiny pores, through which odorants pass and dissolve in a fluid called sensillum lymph, which bathes the sensory dendrites of the OSNs housed in a given sensillum. The OSN dendrites express odorant receptor (OR) proteins, which in insects function as odor-gated ion channels4, 5. The interaction of odorants with ORs either increases or decreases the basal firing rate of the OSN. This neuronal activity in the form of action potentials embodies the first representation of the quality, intensity, and temporal characteristics of the odorant6, 7. Given the easy access to these sensory hairs, it is possible to perform extracellular recordings from single OSNs by introducing a recording electrode into the sensillum lymph, while the reference electrode is placed in the lymph of the eye or body of the insect. In Drosophila, sensilla house between one and four OSNs, but each OSN typically displays a characteristic spike amplitude. Spike sorting techniques make it possible to assign spiking responses to individual OSNs. This single sensillum recording (SSR) technique monitors the difference in potential between the sensillum lymph and the reference electrode as electrical spikes that are generated by the receptor activity on OSNs1, 2, 8. Changes in the number of spikes in response to the odorant represent the cellular basis of odor coding in insects. Here, we describe the preparation method currently used in our lab to perform SSR on Drosophila melanogaster and Anopheles gambiae, and show representative traces induced by the odorants in a sensillum-specific manner.

Single Sensillum Recordings in the Insects <em>Drosophila melanogaster</em> and <em>Anopheles gambiae</em>

The sense of smell is essential for insects to find foods, mates, predators, and oviposition sites3. Insect olfactory sensory neurons (OSNs) are enclosed in sensory hairs called sensilla, which cover the surface of olfactory organs. The surface of each sensillum is covered with tiny pores, through which odorants pass and dissolve in a fluid called sensillum lymph, which bathes the sensory dendrites of the OSNs housed in a given sensillum. The OSN dendrites express odorant receptor (OR) proteins, which in insects function as odor-gated ion channels4, 5. The interaction of odorants with ORs either increases or decreases the basal firing rate of the OSN. This neuronal activity in the form of action potentials embodies the first representation of the quality, intensity, and temporal characteristics of the odorant6, 7.Given the easy access to these sensory hairs, it is possible to perform extracellular recordings from single OSNs by introducing a recording electrode into the sensillum lymph, while the reference electrode is placed in the lymph of the eye or body of the insect. In Drosophila, sensilla house between one and four OSNs, but each OSN typically displays a characteristic spike amplitude. Spike sorting techniques make it possible to assign spiking responses to individual OSNs. This single sensillum recording (SSR) technique monitors the difference in potential between the sensillum lymph and the reference electrode as electrical spikes that are generated by the receptor activity on OSNs1, 2, 8. Changes in the number of spikes in response to the odorant represent the cellular basis of odor coding in insects. Here, we describe the preparation method currently used in our lab to perform SSR on Drosophila melanogaster and Anopheles gambiae, and show representative traces induced by the odorants in a sensillum-specific manner.

Odor Interaction between Bourgeonal and Its Antagonist Undecanal

The perceived quality of a binary mixture will, as a rule of thumb, be dominated by the quality of the stronger unmixed component. On the other hand, there are mechanisms that, in theory, suggest that this will not always be true; one example being receptor antagonism. Undecanal has been indicated as an antagonist for bourgeonal-sensitive receptors in the human olfactory epithelium. Therefore, we investigated mixtures of isointense concentrations of bourgeonal and undecanal and, as a control, mixtures of isointense concentrations of bourgeonal and n-butanol. Both mixture types were investigated at 2 levels of concentration. The particular aim was to see if the bourgeonal-undecanal mixtures would exhibit asymmetric odor quality favoring the perception of the antagonist and the control mixture would not. For the control mixture, indeed odor quality tended to be dominated by the strongest component before mixing as would be suggested from previous studies. In line with the hypothesis, the bourgeonal-undecanal mixture was dominated by the antagonist's quality, but only when mixed at higher concentrations, altogether suggesting the effects of a low-affinity receptor antagonism. This is, to our knowledge, the first demonstration of how antagonistic interaction at the level of the receptor can affect the perception of odor mixtures in humans.

Competitive and Noncompetitive Odorant Interactions in the Early Neural Coding of Odorant Mixtures

Most olfactory receptor neurons (ORNs) express a single type of olfactory receptor that is differentially sensitive to a wide variety of odorant molecules. The diversity of possible odorant-receptor interactions raises challenging problems for the coding of complex mixtures of many odorants, which make up the vast majority of real world odors. Pure competition, the simplest kind of interaction, arises when two or more agonists can bind to the main receptor site, which triggers receptor activation, although only one can be bound at a time. Noncompetitive effects may result from various mechanisms, including agonist binding to another site, which modifies the receptor properties at the main binding site. Here, we investigated the electrophysiological responses of rat ORNs in vivo to odorant agonists and their binary mixtures and interpreted them in the framework of a quantitative model of competitive interaction between odorants. We found that this model accounts for all concentration-response curves obtained with single odorants and for about half of those obtained with binary mixtures. In the other half, the shifts of curves along the concentration axis and the changes of maximal responses with respect to model predictions, indicate that noncompetitive interactions occur and can modulate olfactory receptors. We conclude that, because of their high frequency, the noncompetitive interactions play a major role in the neural coding of natural odorant mixtures. This finding implies that the CNS activity caused by mixtures will not be easily analyzed into components, and that mixture responses will be difficult to generalize across concentration.

Wine flavor: chemistry in a glass

Although hundreds of chemical compounds have been identified in grapes and wines, only a few compounds actually contribute to sensory perception of wine flavor. This critical review focuses on volatile compounds that contribute to wine aroma and provides an overview of recent developments in analytical techniques for volatiles analysis, including methods used to identify the compounds that make the greatest contributions to the overall aroma. Knowledge of volatile composition alone is not enough to completely understand the overall wine aroma, however, due to complex interactions of odorants with each other and with other nonvolatile matrix components. These interactions and their impact on aroma volatility are the focus of much current research and are also reviewed here. Finally, the sequencing of the grapevine and yeast genomes in the past approximately 10 years provides the opportunity for exciting multidisciplinary studies aimed at understanding the influences of multiple genetic and environmental factors on grape and wine flavor biochemistry and metabolism (147 references).

Second messenger signalling in olfaction.

Odorous molecules are recognized by specific receptor proteins located in the ciliary membrane of olfactory receptor neurons. These receptors have been identified using molecular cloning--they are members of the seven-transmembrane-domain G protein-coupled receptor superfamily. Specific receptor subtypes are expressed in subsets of olfactory neurons spatially segregated within certain areas of the olfactory epithelium. Interaction of odorants with receptors initiates the primary reaction of olfactory signalling. Intracellular reaction cascades are activated via specific G proteins, leading to a rapid and transient rise in second messenger levels; odorous compounds elicit mutually exclusive cAMP or inositol 1,4,5-trisphosphate responses. Odorant-induced second messenger signalling is terminated via kinase-mediated negative feedback loops uncoupling the reaction cascades by phosphorylation of receptor proteins. Strong odour stimuli elicit a delayed response of another messenger system, the nitric oxide/cGMP cascade. cGMP may control some adaptive reactions in olfactory receptor neurons.

Searching for the Ligands of Odorant Receptors

Through the sense of smell mammals can detect and discriminate between a large variety of odorants present in the surrounding environment. Odorants bind to a large repertoire of odorant receptors located in the cilia of olfactory sensory neurons of the nose. Each olfactory neuron expresses one single type of odorant receptor, and neurons expressing the same type of receptor project their axons to one or a few glomeruli in the olfactory bulb, creating a map of odorant receptor inputs. The information is then passed on to other regions of the brain, leading to odorant perception. To understand how the olfactory system discriminates between odorants, it is necessary to determine the odorant specificities of individual odorant receptors. These studies are complicated by the extremely large size of the odorant receptor family and by the poor functional expression of these receptors in heterologous cells. This article provides an overview of the methods that are currently being used to investigate odorant receptor-ligand interactions.

Structural determinants of odorant recognition by the human olfactory receptors OR1A1 and OR1A2

An interaction of odorants with olfactory receptors is thought to be the initial step in odorant detection. However, ligands have been reported for only 6 out of 380 human olfactory receptors, with their structural determinants of odorant recognition just beginning to emerge. Guided by the notion that amino acid positions that interact with specific odorants would be conserved in orthologs, but variable in paralogs, and based on the prediction of a set of 22 of such amino acid positions, we have combined site-directed mutagenesis, rhodopsin-based homology modelling, and functional expression in HeLa/Olf cells of receptors OR1A1 and OR1A2. We found that (i) their odorant profiles are centred around citronellic terpenoid structures, (ii) two evolutionary conserved amino acid residues in transmembrane domain 3 are necessary for the responsiveness of OR1A1 and the mouse ortholog Olfr43 to ( S)-(−)-citronellol, (iii) changes at these two positions are sufficient to account for the differential ( S)-(−)-citronellol responsiveness of the paralogs OR1A1 and OR1A2, and (iv) the interaction sites for ( S)-(−)-citronellal and ( S)-(−)-citronellol differ in both human receptors. Our results show that the orientation of odorants within a homology modelling-derived binding pocket of olfactory receptor orthologs is defined by evolutionary conserved amino acid positions.

Reproducibility of odor maps by fMRI in rodents

The interactions of volatile odorants with the ∼1000 types of olfactory receptor neurons in the olfactory mucosa are represented in the olfactory bulb by glomerular spatial activity maps. If these spatial maps underlie the perceptual identification of odorants then, for a given organism, they must be both specific and reproducible. However, this intra-organism reproducibility need not be present between organisms because genetic and developmental studies of olfactory bulb wiring suggest that there is substantial variation between the glomerular arrangements of closely related organisms and even between the two bulbs in a given animal. The ability of functional MRI (fMRI) to record responses of the entire rodent olfactory bulb repeatedly within the same subject has made it possible to assess the reproducibility of odor-induced spatial activity maps both within and between subjects exposed to equivalent stimuli. For a range of odorants, representing multiple chemical classes, a level of fMRI reproducibility (at 7.0 T and 9.4 T) comparable or superior to other cortical regions was demonstrated. While the responses of different bulbs to the same odorant could be localized within the same broad regions of the glomerular sheet, the precise magnitude and topology of the response within those regions were both often highly variable. These results demonstrate the robustness of high-field fMRI as a tool for assaying olfactory bulb function and provide evidence that equivalent perceptual outcomes may arise from divergent neural substrates.

Odor and Taste Interaction on Brain Responses in Humans

Central neural integration of sensory input from different modalities is a prerequisite for flavor perception. Chemosensory information as part of flavor perception is mediated by (i) specific and (ii) general sensory systems. The mouth and the nose are areas housing specific sensory systems, i.e. the gustatory and the olfactory systems. In the gustatory and olfactory systems, as well as in the vomeronasal organ, information is mediated by chemoreceptors. In the somatosensory system, which is classified here as the general sensory system, chemoreceptors are mainly nociceptors. But there are other modalities present as well, such as mechanoand thermoreceptors. It is commonly accepted that there are plenty of opportunities for the above-mentioned systems to interact. This can happen at several levels of the information processing before reaching the cortical level and even peripheral interactions seem possible. In the psychophysical literature there are numerous demonstrations of taste/smell interactions. However, our understanding of the CNS mechanisms and detailed characteristics thereof are rather limited. In humans, some experiments have recently been published addressing this phenomenon by using imaging technologies (Small et al., 1997, 2004; Cerf-Ducastel et al., 2001; de Araujo et al., 2003; Cerf-Ducastel and Murphy, 2004). We wanted to know if the method of event-related potential (ERP) recording could be a useful tool for the investigation of taste and smell interactions. The reason for this approach is that ERPs have the highest possible time resolution compared to most imaging techniques except magnetic source imaging (MSI). Although we do not report any results here, it has now become possible to obtain information about areas of activation by applying electrical source localization methods based on ERPs. In addition, we have demonstrated that ERP components, namely the N1/P2 portion of the ERP, are generated in the insular cortex (Kettenmann et al., 1997) and also postulated that this area might play a crucial role in integrating taste and smell information. This assumption has been recently supported by Fu et al. (2004).

Taste + odor interactions in compound aversion conditioning

In three experiments with rats, taste + odor interactions in compound aversion conditioning were investigated. In Experiment 1, two odors (0.02% almond and 0.02% orange) were compared on single-element odor aversions, taste (denatonium) potentiated odor aversions, and potentiated odor aversions following taste extinction. Although no odor differences were seen following single-element conditioning, both types of potentiated orange odor aversions were stronger than their almond odor counterparts. These data show that odors of similar conditionability are differentially potentiated by the same taste. To determine whether these differences were due to unique perceptual representations, the effects of elemental extinction or compound extinction on aversions to the compound were investigated in Experiments 2 and 3. In Experiment 2, orange odor extinction weakened responding to the compound significantly more than taste extinction did. In contrast, almond odor extinction and taste extinction produced similar decrements in responding to the compound in Experiment 3. These results suggest that the perceptual representation of these specific taste + odor compounds are different, and they are discussed in regard to configural and within-compound association accounts of potentiation.

Odor interactions and learning in a model of the insect antennal lobe

Odor interactions and learning in a model of the insect antennal lobe