Olfactory receptor antagonism between odorants

The detection of thousands of volatile odorants is mediated by several hundreds of different G protein-coupled olfactory receptors (ORs). The main strategy in encoding odorant identities is a combinatorial receptor code scheme in that different odorants are recognized by different sets of ORs. Despite increasing information on agonist-OR combinations, little is known about the antagonism of ORs in the mammalian olfactory system. Here we show that odorants inhibit odorant responses of OR(s), evidence of antagonism between odorants at the receptor level. The antagonism was demonstrated in a heterologous OR-expression system and in single olfactory neurons that expressed a given OR, and was also visualized at the level of the olfactory epithelium. Dual functions of odorants as an agonist and an antagonist to ORs indicate a new aspect in the receptor code determination for odorant mixtures that often give rise to novel perceptual qualities that are not present in each component. The current study also provides insight into strategies to modulate perceived odorant quality.     

Comparison of odorant specificity of two human olfactory receptors from different phylogenetic classes and evidence for antagonism

Humans are able to detect and discriminate myriads of odorants using only several hundred olfactory receptors (ORs) classified in two major phylogenetic classes representing ORs from aquatic (class I) and terrestrial animals (class II). Olfactory perception results in a combinatorial code, in which one OR recognizes multiple odorants and different odorants are recognized by different combinations of ORs. Moreover, recent data suggest that odorants could also behave as antagonists for other ORs, thus making the combinatorial coding more complex. Here we describe the odorant repertoires of two human ORs belonging to class I and class II, respectively. For this purpose, we set up an assay based on calcium imaging in which 100 odorants were screened using air-phase odorant stimulation at physiological doses. We showed that the human class I OR52D1 is functional, exhibiting a narrow repertoire related to that of its orthologous murine OR, demonstrating than this human class I OR is not an evolutionary relic. The class II OR1G1 was revealed to be broadly tuned towards odorants of 9-10 carbon chain length, with diverse functional groups. The existence of antagonist odorants for the class II OR was also demonstrated. They are structurally related to the agonists, with shorter carbon chain length.     

Perceptual blending in odor mixtures depends on the nature of odorants and human olfactory expertise

Our olfactory system is confronted with complex mixtures of odorants, often recognized as single entities due to odor blending (e.g., coffee). In contrast, we are also able to discriminate odors from complex mixtures (e.g., off-odors). Therefore, the olfactory system is able to engage either configural or elemental processes when confronted with mixtures. However, the rules that govern the involvement of these processes during odor perception remain poorly understood. In our first experiment, we examined whether simple odorant mixtures (binary/ternary) could elicit configural perception. Twenty untrained subjects were asked to evaluate the odor typicality of mixtures and their constituents. The results revealed a significant increase in odor typicality in some but not all mixtures as compared with the single components, which suggest that perceptual odor blending can occur only in specific mixtures (configural processing). In our second experiment, we tested the hypothesis that general olfactory expertise can improve elemental perception of mixtures. Thirty-two trained subjects evaluated the odor typicality of the stimuli presented during the first experiment, and their responses were compared with those obtained from the untrained panelists. The results support the idea that general training with odors increases the elemental perception of binary and ternary blending mixtures.     

Different thresholds for detection and discrimination of odors in the honey bee (Apis mellifera)

Naturally occurring odors used by animals for mate recognition, food identification and other purposes must be detected at concentrations that vary across several orders of magnitude. Olfactory systems must therefore have the capacity to represent odors over a large range of concentrations regardless of dramatic changes in the salience, or perceived intensity, of a stimulus. The stability of the representation of an odor relative to other odors across concentration has not been extensively evaluated. We tested the ability of honey bees to discriminate pure odorants across a range of concentrations at and above their detection threshold. Our study showed that pure odorant compounds became progressively easier for honey bees to discriminate with increasing concentration. Discrimination is, therefore, a function of odorant concentration. We hypothesize that the recruitment of sensory cell populations across a range of concentrations may be important for odor coding, perhaps by changing its perceptual qualities or by increasing its salience against background stimuli, and that this mechanism is a general property of olfactory systems.     

Just noticeable differences in component concentrations modify the odor quality of a blending mixture

The odors we perceive are mainly the result of mixtures of odorants that, however, are commonly perceived as single undivided entities; nevertheless, the processes involved remain poorly explored. It has been recently reported that perceptual blending based on configural olfactory processing can cause odorant mixtures to give rise to an emergent odor not present in the components. The present study examined whether specific component proportions are required to elicit an emergent odor. Starting from the composition of a ternary target mixture in which an emergent pineapple odor was perceived, 4 concentration levels of each component were chosen to elicit just noticeable differences (JNDs). Each combination of levels was used to design sample mixtures. Fifteen subjects evaluated the intensity, typicality, and pleasantness of each sample mixture against the target mixture in a paired-comparison protocol. Statistical modeling showed that a variation of less than 1 JND in one of the components was sufficient to induce a significant decrease in pineapple odor typicality in the ternary mixture. This finding confirms previous findings on perceptual blending in simple odorant mixtures and underscores the human ability to discriminate between odor percepts induced by mixtures including very similar odorant proportions.     

Molecular and cellular basis of human olfaction

The human olfactory systems recognize and discriminate a large number of different odorant molecules. The detection of chemically distinct odorants begins with the binding of an odorant ligand to a specific receptor protein in the ciliary membrane of olfactory neurons. To address the problem of olfactory perception at a molecular level, we have cloned, functionally expressed, and characterized some of the human olfactory receptors from chromosome 17. Our results show that a receptor protein is capable of recognizing the particular chemical substructure of an odor molecule and, therefore, is able to respond only to odorants that have a defined molecular structure. These findings represent the beginning of the molecular understanding of odorant recognition in humans. In the future, this knowledge could be used for the design of synthetic ideal receptors for specific odors (biosensors), or the perfect odor molecule for a given receptor.     

Odor representations in the rat olfactory bulb change smoothly with morphing stimuli

Many species of mammals are very good at categorizing odors. One model for how this is achieved involves the formation of "attractor" states in the olfactory processing pathway, which converge to stable representations for the odor. We analyzed the responses of rat olfactory bulb mitral/tufted (M/T) cells using stimuli "morphing" from one odor to another through intermediate mixtures. We then developed a phenomenological model for the representation of odors and mixtures by M/T cells and show that >80% of odorant responses to different concentrations and mixtures can be expressed in terms of smoothly summing responses to air and the two pure odorants. Furthermore, the model successfully predicts M/T cell responses to odor mixtures when respiration dependence is eliminated. Thus, odor mixtures are represented in the bulb through summation of components, rather than distinct attractor states. We suggest that our olfactory coding model captures many aspects of single and mixed odor representation in M/T cells.     

Olfactory receptor cells respond to odors in a tissue and semiconductor hybrid neuron chip

Olfactory systems of human beings and animals have the abilities to sense and distinguish varieties of odors. In this study, a bioelectronic nose was constructed by fixing biological tissues onto the surface of light-addressable potentiometric sensor (LAPS) to mimic human olfaction and realize odor differentiation. The odorant induced potentials on tissue-semiconductor interface was analyzed by sensory transduction theory and sheet conductor model. The extracellular potentials of the receptor cells in the olfactory epithelium were detected by LAPS. Being stimulated by different odorants, such as acetic acid and butanedione, olfactory epithelium activities were analyzed on basis of local field potentials and presented different firing modes. The signals fired in different odorants could be distinguished into different clusters by principal component analysis (PCA). Therefore, with cellular populations well preserved, the epithelium tissue and LAPS hybrid system will be a promising neuron chip of olfactory biosensors for odor detecting.     

Humans and Mice Express Similar Olfactory Preferences

In humans, the pleasantness of odors is a major contributor to social relationships and food intake. Smells evoke attraction and repulsion responses, reflecting the hedonic value of the odorant. While olfactory preferences are known to be strongly modulated by experience and learning, it has been recently suggested that, in humans, the pleasantness of odors may be partly explained by the physicochemical properties of the odorant molecules themselves. If odor hedonic value is indeed predetermined by odorant structure, then it could be hypothesized that other species will show similar odor preferences to humans. Combining behavioral and psychophysical approaches, we here show that odorants rated as pleasant by humans were also those which, behaviorally, mice investigated longer and human subjects sniffed longer, thereby revealing for the first time a component of olfactory hedonic perception conserved across species. Consistent with this, we further show that odor pleasantness rating in humans and investigation time in mice were both correlated with the physicochemical properties of the molecules, suggesting that olfactory preferences are indeed partly engraved in the physicochemical structure of the odorant. That odor preferences are shared between mammal species and are guided by physicochemical features of odorant stimuli strengthens the view that odor preference is partially predetermined. These findings open up new perspectives for the study of the neural mechanisms of hedonic perception.     

果蝇嗅觉分子机理研究进展

黑腹果蝇Drosophila melanogaster是生物学研究的重要模式生物,也是探索研究生物体嗅觉奥秘的理想材料。近年来,由于分子生物学技术在神经科学领域的广泛应用,黑腹果蝇嗅觉机理研究取得了许多重大突破, 对气味分子受体及其识别机理、 嗅觉神经电信号的产生和传递、嗅觉信息的加工、编码以及记忆等方面都有了深入的了解。研究表明, 果蝇约1 300个嗅神经元(olfactory receptor neurons, ORNs)共表达62种不同的气味受体蛋白（olfactory receptor proteins, ORs）, 用以检测和识别其所感受的所有化学气味分子。许多OR所识别的气味分子配体已鉴定出来,普通的气味（如水果的气味）由数种不同的OR组合来识别,而信息素（pheromone）分子则由单种特定的OR来检测。气味信息在嗅神经元内转换成神经电信号,嗅觉电信号沿嗅神经元的轴突传递到触角叶, 再经投射神经元（projection neurons, PNs）将信息送至高级中枢如蘑菇体（mushroom body, MB）和侧角（lateral horn, LH）,最终引发行为反应。在黑腹果蝇嗅觉信息传递通路中,某些蛋白如Dock,N-cadherin,Fruitless等起着重要作用,缺失这些蛋白会导致嗅觉异常。本文对这些研究进展作一综述。     

Insect Odorant Response Sensitivity Is Tuned by Metabotropically Autoregulated Olfactory Receptors

Insects possess one of the most exquisitely sensitive olfactory systems in the animal kingdom, consisting of three different types of chemosensory receptors: ionotropic glutamate-like receptors (IRs), gustatory receptors (GRs) and odorant receptors (ORs). Both insect ORs and IRs are ligand-gated ion channels, but ORs possess a unique configuration composed of an odorant-specific protein OrX and a ubiquitous coreceptor (Orco). In addition, these two ionotropic receptors confer different tuning properties for the neurons in which they are expressed. Unlike IRs, neurons expressing ORs are more sensitive and can also be sensitized by sub-threshold concentrations of stimuli. What is the mechanistic basis for these differences in tuning? We show that intrinsic regulation of Orco enhances neuronal response to odorants and sensitizes the ORs. We also demonstrate that inhibition of metabotropic regulation prevents receptor sensitization. Our results indicate that Orco-mediated regulation of OR sensitivity provides tunable ionotropic receptors capable of detecting odors over a wider range of concentrations, providing broadened sensitivity over IRs themselves.     

Odorant moiety and odor mixture perception in free-flying honey bees (Apis mellifera)

Two experiments are described that employ a Y-tube odor-trainingparadigm to address questions relating to olfactory perceptionin free-flying worker honey bees. The first is designed to evaluatehow easily bees can be conditioned to discriminate between twoodors and how willing they are to generalize between closelyrelated odors. In particular, we demonstrate that individualworker bees have no trouble learning to discriminate betweenalkyl ketones or alcohols that differ by only one carbon atom(e.g. heptanone versus octanone) or between a ketone and alcoholfunctional group attached to the same alkyl radical; but theygeneralize between compounds with the same functional groupmuch more readily than those with the same alkyl radical. Thesecond experiment is designed to explore the relationship betweenthe perception of a mixture of odorants and the perception ofthe individual odorants themselves. Our results suggest thatthere appears to be a stronger relationship between a two-odorantmixture and its constituents than would be suggested by themixture being an odor intermediate between the two constituentodorants. We also include a comprehensive discussion on theproblem of extracting quality and concentration informationfrom an odor stimulus and we explore ideas relating to the perceptionof the constituent odorant components of complex odors.     

Olfactory-based discrimination learning in the moth, Manduca sexta

Moths possess highly tuned olfactory capabilities, which can detect very low concentrations of pheromonal odorants. Much is known about the structure and function of the moth olfactory system with respect to detection of pheromones. However, we lack an understanding of the broader olfactory system, in particular, to what degree are moths capable of detecting and discriminating odorants that are not components of pheromone blends. Here we describe a methodology used to investigate the discriminability of nonpheromonal odors in moths. In a series of experiments we show that the moth Manduca sexta can (1) discriminate a number of different odors but (2) that methyl jasmonate, neither readily conditions to a food reward nor is it readily discriminated from another odor. The lack of a response to methyl jasmonate may be related to its role in host plant defense. This work provides a basis for future mapping of physiological and pharmacological studies of nonpheromonal coding in insects onto learned behavioral responses to those odorants.     

Emotional processing of odors: evidence for a nonlinear relation between pleasantness and familiarity evaluations

Pleasantness, familiarity, and intensity are 3 interdependent dimensions commonly used to describe the perceived qualities of an odor. In particular, many empirical studies have demonstrated a positive correlation between familiarity and pleasantness. However, on the basis of both theoretical and methodological perspectives, we questioned the validity of such a relation for malodors. We report 2 studies based on subjective judgments of a large sample of odorants (Experiment 1) associated with autonomic recordings (Experiment 2). Multivariate exploratory analysis performed on the data splits the whole odorant set into 2 subsets composed, respectively, of unpleasant and pleasant odorants. Subsequent correlation analyses have shown that the relation between pleasantness and familiarity is specific for the pleasant odors in the 2 experiments. Moreover, autonomic activity was more important in response to malodors than to pleasant odors and was significantly correlated with unpleasantness ratings in the subset of unpleasant odors. These 2 studies argue in favor of a functional dissociation in the relations between both subjective and autonomic responses to odors as a function of pleasantness and indicate that researchers in the olfactory domain should consider the relations between pleasantness and familiarity as more complex than linear.     

Olfaction and odor discrimination are mediated by the C. elegans guanylyl cyclase ODR-1

Animals in complex environments must discriminate between salient and uninformative sensory cues. Caenorhabditis elegans uses one pair of olfactory neurons called AWC to sense many different odorants, yet the animal can distinguish each odorant from the others in discrimination assays. We demonstrate that the transmembrane guanylyl cyclase ODR-1 is essential for responses to all AWC-sensed odorants. ODR-1 appears to be a shared signaling component downstream of odorant receptors. Overexpression of ODR-1 protein indicates that ODR-1 can influence odor discrimination and adaptation as well as olfaction. Adaptation to one odorant, butanone, is disrupted by ODR-1 overexpression. Olfactory discrimination is also disrupted by ODR-1 overexpression, probably by overproduction of the shared second messenger cGMP. We propose that AWC odorant signaling pathways are insulated to permit odor discrimination.     

Or83b encodes a broadly expressed odorant receptor essential for Drosophila olfaction

Fruit flies are attracted by a diversity of odors that signal the presence of food, potential mates, or attractive egg-laying sites. Most Drosophila olfactory neurons express two types of odorant receptor genes: Or83b, a broadly expressed receptor of unknown function, and one or more members of a family of 61 selectively expressed receptors. While the conventional odorant receptors are highly divergent, Or83b is remarkably conserved between insect species. Two models could account for Or83b function: it could interact with specific odor stimuli independent of conventional odorant receptors, or it could act in concert with these receptors to mediate responses to all odors. Our results support the second model. Dendritic localization of conventional odorant receptors is abolished in Or83b mutants. Consistent with this cellular defect, the Or83b mutation disrupts behavioral and electrophysiological responses to many odorants. Or83b therefore encodes an atypical odorant receptor that plays an essential general role in olfaction.     

Developmental regulation of olfactory circuit formation in mice

In mammals, odorants induce various behavioral responses that are critical to the survival of the individual and species. Binding signals of odorants to odorant receptors (ORs) expressed in the olfactory epithelia are converted to an odor map, a pattern of activated glomeruli, in the olfactory bulb (OB). This topographic map is used to identify odorants for memory-based learned decisions. In the embryo, a coarse olfactory map is generated in the OB by a combination of dorsal-ventral and anterior-posterior targeting of olfactory sensory neurons (OSNs), using specific sets of axon-guidance molecules. During the process of OSN projection, odor signals are sorted into distinct odor qualities in separate functional domains in the OB. Odor information is then conveyed by the projection neurons, mitral/tufted cells, to various regions in the olfactory cortex, particularly to the amygdala for innate olfactory decisions. Although the basic architecture of hard-wired circuits is generated by a genetic program, innate olfactory responses are modified by neonatal odor experience in an activity-dependent manner. Stimulus-driven OR activity promotes post-synaptic events and dendrite selection in the responding glomeruli making them larger. As a result, enhanced odor inputs in neonates establish imprinted olfactory memory that induces attractive responses in adults, even when the odor quality is innately aversive. In this paper, I will provide an overview of the recent progress made in the olfactory circuit formation in mice.     

The role of the coreceptor Orco in insect olfactory transduction

Insects sense odorants with specialized odorant receptors (ORs). Each antennal olfactory receptor neuron expresses one OR with an odorant binding site together with a conserved coreceptor called Orco which does not bind odorants. Orco is necessary for localization of ORs to dendritic membranes and, thus, is essential for odorant detection. It forms a spontaneously opening cation channel, activated via phosphorylation by protein kinase C. Thereafter, Orco is also activated via cyclic adenosine monophosphate (cAMP). Orco forms homo—as well as heteromers with ORs with unknown stoichiometry. Contradictory publications suggest different mechanisms of olfactory transduction. On the one hand, evidence accumulates for the employment of more than one G protein-coupled olfactory transduction cascade in different insects. On the other hand, results from other studies suggest that the OR–Orco complex functions as an odorant-gated cation channel mediating ionotropic signal transduction. This review analyzes conflicting hypotheses concerning the role of Orco in insect olfactory transduction. In conclusion, in situ studies in hawkmoths falsify the hypothesis that Orco underlies odorant-induced ionotropic signal transduction in all insect species. Instead, Orco forms a metabotropically gated, slow cation channel which controls odorant response threshold and kinetics of the sensory neuron.     

An odorant derivative as an antagonist for an olfactory receptor

Different odorants are recognized by different combinations of G protein-coupled olfactory receptors, and thereby, odor identity is determined by a combinatorial receptor code for each odorant. We recently demonstrated that odorants appeared to compete for receptor sites to act as an agonist or an antagonist. Therefore, in natural circumstances where we always perceive a mixture of various odorants, olfactory receptor antagonism between odorants may result in a receptor code for the mixture that cannot be predicted from the codes for its individual components. Here we show that stored isoeugenol has an antagonistic effect on a mouse olfactory receptor, mOR-EG. However, freshly purified isoeugenol did not have an inhibitory effect. Instead, an isoeugenol derivative produced during storage turned out to be a potent competitive antagonist of mOR-EG. Structural analysis revealed that this derivative is an oxidatively dimerized isoeugenol that naturally occurs by oxidative reaction. The current study indicates that as odorants age, they decompose or react with other odorants, which in turn affects responsiveness of an olfactory receptor(s).