Searching for the ligands of odorant receptors

Shedding light on human olfaction: Electrophysiological recordings from sensory neurons in acute slices of olfactory epithelium

The COVID-19 pandemic brought attention to our limited understanding of human olfactory physiology. While the cellular composition of the human olfactory epithelium is similar to that of other vertebrates, its functional properties are largely unknown. We prepared acute slices of human olfactory epithelium from nasal biopsies and used the whole-cell patch-clamp technique to record electrical properties of cells. We measured voltage-gated currents in human olfactory sensory neurons and supporting cells, and action potentials in neurons. Additionally, neuronal inward current and action potentials responses to a phosphodiesterase inhibitor suggested a transduction cascade involving cAMP as a second messenger. Furthermore, responses to odorant mixtures demonstrated that the transduction cascade was intact in this preparation. This study provides the first electrophysiological characterization of olfactory sensory neurons in acute slices of the human olfactory epithelium, paving the way for future research to expand our knowledge of human olfactory physiology.

Sensory Transduction in Photoreceptors and Olfactory Sensory Neurons: Common Features and Distinct Characteristics

The past decades have seen tremendous progress in our understanding of the function of photoreceptors and olfactory sensory neurons, uncovering the mechanisms that determine their properties and, ultimately, our ability to see and smell. This progress has been driven to a large degree by the powerful combination of physiological experimental tools and genetic manipulations, which has enabled us to identify the main molecular players in the transduction cascades of these sensory neurons, how their properties affect the detection and discrimination of stimuli, and how diseases affect our senses of vision and smell. This review summarizes some of the common and unique features of photoreceptors and olfactory sensory neurons that make these cells so exciting to study.

Is It Possible to Predict the Odor of a Molecule on the Basis of its Structure?

The olfactory sense is the dominant sensory perception for many animals. When Richard Axel and Linda B. Buck received the Nobel Prize in 2004 for discovering the G protein-coupled receptors’ role in olfactory cells, they highlighted the importance of olfaction to the scientific community. Several theories have tried to explain how cells are able to distinguish such a wide variety of odorant molecules in a complex context in which enantiomers can result in completely different perceptions and structurally different molecules. Moreover, sex, age, cultural origin, and individual differences contribute to odor perception variations that complicate the picture. In this article, recent advances in olfaction theory are presented, and future trends in human olfaction such as structure-based odor prediction and artificial sniffing are discussed at the frontiers of chemistry, physiology, neurobiology, and machine learning.

Olfactory receptor function

Olfaction plays a critical role in several aspects of life. Olfactory disorders are very common in the general population, and can lead to malnutrition, weight loss, food poisoning, depression, and other disturbances. Odorants are first detected in the upper region of the nose by the main olfactory epithelium (OE). In this region, millions of olfactory sensory neurons (OSNs) interact with odor molecules through the odorant receptors (ORs), which belong to the superfamily of G protein-coupled receptors. The binding of odors to the ORs initiates an electrical signal that travels along the axons to the main olfactory bulb of the brain. The information is then transmitted to other regions of the brain, leading to odorant perception and emotional and behavioral responses. In the OE, OSNs die and are continuously replaced from stem cells localized in the epithelium's basal region. Damage to this epithelium can be caused by multiple factors, leading to anosmia (smell loss). In this chapter, we introduce the basic organization of the OE and focus on the molecular mechanisms involved in odorant perception. We also describe recent experiments that address the mechanisms of OSNs regeneration in response to neuronal injury.

Sparsened neuronal activity in an optogenetically activated olfactory glomerulus

Glomeruli are the functional units of olfactory information processing but little remains known about their individual unit function. This is due to their widespread activation by odor stimuli. We expressed channelrhodopsin-2 in a single olfactory sensory neuron type, and used laser stimulation and simultaneous in vivo calcium imaging to study the responses of a single glomerulus to optogenetic stimulation. Calcium signals in the neuropil of this glomerulus were representative of the sensory input and nearly identical if evoked by intensity-matched odor and laser stimuli. However, significantly fewer glomerular layer interneurons and olfactory bulb output neurons (mitral cells) responded to optogenetic versus odor stimuli, resulting in a small and spatially compact optogenetic glomerular unit response. Temporal features of laser stimuli were represented with high fidelity in the neuropil of the glomerulus and the mitral cells, but not in interneurons. Increases in laser stimulus intensity were encoded by larger signal amplitudes in all compartments of the glomerulus, and by the recruitment of additional interneurons and mitral cells. No spatial expansion of the glomerular unit response was observed in response to stronger input stimuli. Our data are among the first descriptions of input-output transformations in a selectively activated olfactory glomerulus.

Evolutionary ecology of chemosensation and its role in sensory drive

All behaviors of an organism are rooted in sensory processing of signals from its environment, and natural selection shapes sensory adaptations to ensure successful detection of cues that maximize fitness. Sensory drive, or divergent selection for efficient signal transmission among heterogeneous environments, has been a useful hypothesis for describing sensory adaptations, but its current scope has primarily focused on visual and acoustic sensory modalities. Chemosensation, the most widespread sensory modality in animals that includes the senses of smell and taste, is characterized by rapid evolution and has been linked to sensory adaptations to new environments in numerous lineages. Yet, olfaction and gustation have been largely underappreciated in light of the sensory drive hypothesis. Here, we examine why chemosensory systems have been overlooked and discuss the potential of chemosensation to shed new insight on the sensory drive hypothesis and vice versa. We provide suggestions for developing a framework to better incorporate studies of chemosensory adaptation that have the potential to shape a more complete, coherent, and holistic interpretation of the sensory drive.

Combining In Vivo and In Vitro Approaches To Identify Human Odorant Receptors Responsive to Food Odorants

Olfactory perception plays an important role in food flavor. Humans have around 400 odorant receptors (ORs), which can be activated by an enormous number of odorants in a combinatorial fashion. To date, only a few odorant receptors have been linked to their respective odorants, due to the difficulties in expressing these receptor proteins in heterologous cell systems. In vivo approaches allow for the analysis of odorant-receptor interactions in their native environment and have the advantage that the complete OR repertoire is simultaneously tested. Once mouse odorant-receptor pairs are defined, one can search for the corresponding human orthologues, which can be validated against the odorants in heterologous cells. Thus, the combination of in vivo and in vitro methods should contribute to the identification of human ORs that recognize odorants of interest, such as key food odorants.

Olfactory Receptors in Non-Chemosensory Organs: The Nervous System in Health and Disease

Olfactory receptors (ORs) and down-stream functional signaling molecules adenylyl cyclase 3 (AC3), olfactory G protein α subunit (Gαolf), OR transporters receptor transporter proteins 1 and 2 (RTP1 and RTP2), receptor expression enhancing protein 1 (REEP1), and UDP-glucuronosyltransferases (UGTs) are expressed in neurons of the human and murine central nervous system (CNS). In vitro studies have shown that these receptors react to external stimuli and therefore are equipped to be functional. However, ORs are not directly related to the detection of odors. Several molecules delivered from the blood, cerebrospinal fluid, neighboring local neurons and glial cells, distant cells through the extracellular space, and the cells’ own self-regulating internal homeostasis can be postulated as possible ligands. Moreover, a single neuron outside the olfactory epithelium expresses more than one receptor, and the mechanism of transcriptional regulation may be different in olfactory epithelia and brain neurons. OR gene expression is altered in several neurodegenerative diseases including Parkinson’s disease (PD), Alzheimer’s disease (AD), progressive supranuclear palsy (PSP) and sporadic Creutzfeldt-Jakob disease (sCJD) subtypes MM1 and VV2 with disease-, region- and subtype-specific patterns. Altered gene expression is also observed in the prefrontal cortex in schizophrenia with a major but not total influence of chlorpromazine treatment. Preliminary parallel observations have also shown the presence of taste receptors (TASRs), mainly of the bitter taste family, in the mammalian brain, whose function is not related to taste. TASRs in brain are also abnormally regulated in neurodegenerative diseases. These seminal observations point to the need for further studies on ORs and TASRs chemoreceptors in the mammalian brain.

Olfr603, an orphan olfactory receptor, is expressed in multiple specific embryonic tissues

BACKGROUND

Olfactory receptors were initially believed to be expressed specifically within the olfactory neurons. However, accumulating genome-scale data has recently demonstrated more extensive expression. There are hundreds of olfactory receptor family members and the realisation of their widespread expression provides an opportunity to reveal new biology. However, existing data is predominantly based on RT-PCR, microarray and RNA-seq approaches and the details of tissue and cell-type specific expression are lacking.

RESULTS

As a proof of principle, we selected Olfr603 for expression analysis. We generated an antibody against a non-conserved epitope of Olfr603 and characterised its expression in E8.5-E12.5 mouse embryos using immunohistochemistry. This analysis demonstrated a dynamic pattern of expression in diverse cell types within the developing embryo unrelated to the olfactory system. Expression was detected in migrating neural crest, endothelial precursors and vascular endothelium, endocardial cells, smooth muscle, neuroepithelium and within the ocular tissues. This complex distribution does not conform to any apparent germ layer or tissue origin.

CONCLUSIONS

This initial characterisation of Olfr603 expression highlights the potential for a broad role for this receptor in the development of many tissues.

Identification of agonists for a group of human odorant receptors

Olfaction plays a critical role in several aspects of the human life. Odorants are detected by hundreds of odorant receptors (ORs) which belong to the superfamily of G protein-coupled receptors. These receptors are expressed in the olfactory sensory neurons of the nose. The information provided by the activation of different combinations of ORs in the nose is transmitted to the brain, leading to odorant perception and emotional and behavioral responses. There are ~400 intact human ORs, and to date only a small percentage of these receptors (~10%) have known agonists. The determination of the specificity of the human ORs will contribute to a better understanding of how odorants are discriminated by the olfactory system. In this work, we aimed to identify human specific ORs, that is, ORs that are present in humans but absent from other species, and their corresponding agonists. To do this, we first selected 22 OR gene sequences from the human genome with no counterparts in the mouse, rat or dog genomes. Then we used a heterologous expression system to screen a subset of these human ORs against a panel of odorants of biological relevance, including foodborne aroma volatiles. We found that different types of odorants are able to activate some of these previously uncharacterized human ORs.

In vivo identification of eugenol-responsive and muscone-responsive mouse odorant receptors

Our understanding of mammalian olfactory coding has been impeded by the paucity of information about the odorant receptors (ORs) that respond to a given odorant ligand in awake, freely behaving animals. Identifying the ORs that respond in vivo to a given odorant ligand from among the ∼1100 ORs in mice is intrinsically challenging but critical for our understanding of olfactory coding at the periphery. Here, we report an in vivo assay that is based on a novel gene-targeted mouse strain, S100a5-tauGFP, in which a fluorescent reporter selectively marks olfactory sensory neurons that have been activated recently in vivo. Because each olfactory sensory neuron expresses a single OR gene, multiple ORs responding to a given odorant ligand can be identified simultaneously by capturing the population of activated olfactory sensory neurons and using expression profiling methods to screen the repertoire of mouse OR genes. We used this in vivo assay to re-identify known eugenol- and muscone-responsive mouse ORs. We identified additional ORs responsive to eugenol or muscone. Heterologous expression assays confirmed nine eugenol-responsive ORs (Olfr73, Olfr178, Olfr432, Olfr610, Olfr958, Olfr960, Olfr961, Olfr913, and Olfr1234) and four muscone-responsive ORs (Olfr74, Olfr235, Olfr816, and Olfr1440). We found that the human ortholog of Olfr235 and Olfr1440 responds to macrocyclic ketone and lactone musk odorants but not to polycyclic musk odorants or a macrocyclic diester musk odorant. This novel assay, called the Kentucky in vivo odorant ligand-receptor assay, should facilitate the in vivo identification of mouse ORs for a given odorant ligand of interest.

In vivo identification of eugenol-responsive and muscone-responsive mouse odorant receptors

Our understanding of mammalian olfactory coding has been impeded by the paucity of information about the odorant receptors (ORs) that respond to a given odorant ligand in awake, freely behaving animals. Identifying the ORs that respond in vivo to a given odorant ligand from among the ∼1100 ORs in mice is intrinsically challenging but critical for our understanding of olfactory coding at the periphery. Here, we report an in vivo assay that is based on a novel gene-targeted mouse strain, S100a5-tauGFP, in which a fluorescent reporter selectively marks olfactory sensory neurons that have been activated recently in vivo. Because each olfactory sensory neuron expresses a single OR gene, multiple ORs responding to a given odorant ligand can be identified simultaneously by capturing the population of activated olfactory sensory neurons and using expression profiling methods to screen the repertoire of mouse OR genes. We used this in vivo assay to re-identify known eugenol- and muscone-responsive mouse ORs. We identified additional ORs responsive to eugenol or muscone. Heterologous expression assays confirmed nine eugenol-responsive ORs (Olfr73, Olfr178, Olfr432, Olfr610, Olfr958, Olfr960, Olfr961, Olfr913, and Olfr1234) and four muscone-responsive ORs (Olfr74, Olfr235, Olfr816, and Olfr1440). We found that the human ortholog of Olfr235 and Olfr1440 responds to macrocyclic ketone and lactone musk odorants but not to polycyclic musk odorants or a macrocyclic diester musk odorant. This novel assay, called the Kentucky in vivo odorant ligand-receptor assay, should facilitate the in vivo identification of mouse ORs for a given odorant ligand of interest.

The state of the art of odorant receptor deorphanization: a report from the orphanage

The odorant receptors (ORs) provide our main gateway to sensing the world of volatile chemicals. This involves a complex encoding process in which multiple ORs, each of which detects its own set of odorants, work as an ensemble to produce a distributed activation code that is presumably unique to each odorant. One marked challenge to decoding the olfactory code is OR deorphanization, the identification of a set of activating odorants for a particular receptor. Here, we survey various methods used to try to express defined ORs of interest. We also suggest strategies for selecting odorants for test panels to evaluate the functional expression of an OR. Integrating these tools, while retaining awareness of their idiosyncratic limitations, can provide a multi-tiered approach to OR deorphanization, spanning the initial discovery of a ligand to vetting that ligand in a physiologically relevant setting.

Single cell RT-PCR identification of odorant receptors expressed by olfactory neurons

Mammals have between 400 and 1,300 functional odorant receptor (OR) genes in their genomes. Each olfactory sensory neuron in the nose expresses only one single type of OR out of this vast repertoire. The OR expressed by an olfactory sensory neuron determines its functional activity and wiring to the olfactory bulb. Therefore, the identification of the OR type expressed by individual neurons contributes to the understanding of the mechanisms of odorant perception. Here we describe the protocol that combines single cell RT-PCR and degenerate PCR to identify the OR expressed by single olfactory neurons. In a primary PCR reaction, cDNAs corresponding to all mRNAs present in a single neuron are synthesized. In a secondary PCR reaction, the cDNAs corresponding to the OR expressed by this neuron are amplified using degenerate primers that match OR sequences.

Functional genomics reveals dysregulation of cortical olfactory receptors in Parkinson disease: novel putative chemoreceptors in the human brain

Parkinson disease (PD) is no longer considered a complex motor disorder but rather a systemic disease with variable nonmotor deficits that may include impaired olfaction, depression, mood and sleep disorders, and altered cortical function. Increasing evidence indicates that multiple metabolic defects occur in regions outside the substantia nigra, including the cerebral cortex, even at premotor stages of the disease. We investigated changes in gene expression in the frontal cortex in PD patient brains using a transcriptomics approach. Functional genomics analysis indicated that cortical olfactory receptors (ORs) and taste receptors (TASRs) are altered in PD patients. Olfactory receptors OR2L13, OR1E1, OR2J3, OR52L1, and OR11H1 and taste receptors TAS2R5 and TAS2R50 were downregulated, but TAS2R10 and TAS2R13 were upregulated at premotor and parkinsonian stages in the frontal cortex area 8 in PD patient brains. Furthermore, we present novel evidence that, in addition to the ORs, obligate downstream components of OR function adenylyl cyclase 3 and olfactory G protein (Gαolf), OR transporters, receptor transporter proteins 1 and 2 and receptor expression enhancing protein 1, and OR xenobiotic removing UDP-glucuronosyltransferase 1 family polypeptide A6 are widely expressed in neurons of the cerebral cortex and other regions of the adult human brain. Together, these findings support the concept that ORs and TASRs in the cerebral cortex may have novel physiologic functions that are affected in PD patients.

Human olfactory receptors: recombinant expression in the baculovirus/Sf9 insect cell system, functional characterization, and odorant identification

Cell surface expression of recombinant olfactory receptors (ORs) is a major limitation in characterizing their functional nature. We have shown that the recombinant expression of a human OR, OR 17-210, in the baculovirus/Sf9 insect cell system allows this protein to be expressed at the cell surface. We used Ca(2+) imaging to demonstrate that recombinant OR 17-210 produces cellular activities upon odorant stimulation with ketones. Furthermore, this expression and functional system has been used to show that the preincubation of Human Odorant Binding Protein 2A decrease the calcium response of OR 17-210 following stimulation by acetophenone and beta ionone.

Immunohistochemical detection of olfactory-specific sensory transduction proteins in olfactory neuroblastoma

Olfactory neuroblastoma (ONB) is a malignant tumor found in the superior nasal cavity. Although there is not adequate molecular evidence, ONB is considered to develop from olfactory sensory neurons (OSNs). Here, we immunohistochemically detected intense signals for the olfactory-specific sensory transduction proteins, G(αolf), adenylyl cyclase III, and cyclic-nucleotide-gated channel A2 in ONB, together with the conventional ONB marker, neuron-specific enolase, while the established OSN markers, olfactory marker protein and growth-associated protein 43, were not detected. These results indicate that ONB and OSNs share the same lineage and that the detected transduction proteins could serve as specific tumor markers for ONB.

Origin of basal activity in mammalian olfactory receptor neurons

Mammalian odorant receptors form a large, diverse group of G protein-coupled receptors that determine the sensitivity and response profile of olfactory receptor neurons. But little is known if odorant receptors control basal and also stimulus-induced cellular properties of olfactory receptor neurons other than ligand specificity. This study demonstrates that different odorant receptors have varying degrees of basal activity, which drives concomitant receptor current fluctuations and basal action potential firing. This basal activity can be suppressed by odorants functioning as inverse agonists. Furthermore, odorant-stimulated olfactory receptor neurons expressing different odorant receptors can have strikingly different response patterns in the later phases of prolonged stimulation. Thus, the influence of odorant receptor choice on response characteristics is much more complex than previously thought, which has important consequences on odor coding and odor information transfer to the brain.

Odor detection by humans of lineal aliphatic aldehydes and helional as gauged by dose-response functions

We have measured concentration detection (i.e., psychometric) functions to determine the odor detectability of homologous aliphatic aldehydes (propanal, butanal, hexanal, octanal, and nonanal) and helional. Subjects (16 < or = n < or = 18) used a 3-alternative forced-choice procedure against carbon-filtered air (blanks), under an ascending concentration approach. Generation, delivery, and control of each vapor were achieved via an 8-station vapor delivery device. Gas chromatography served to quantify the concentrations presented. Group and individual functions were modeled by a sigmoid (logistic) equation. Odor detection thresholds (ODTs) were defined as the concentration producing a detectability (P) halfway (P = 0.5) between chance (P = 0.0) and perfect detection (P = 1.0). ODTs decreased with carbon chain length: 2.0, 0.46, 0.33, and 0.17 ppb, respectively, from propanal to octanal, but the threshold increased for nonanal (0.53 ppb), revealing maximum sensitivity for the 8-carbon member. The strong olfactory receptor (OR) ligands octanal and helional (0.14 ppb) showed the lowest thresholds. ODTs fell at the lower end of previously reported values. Interindividual variability (ODT ratios) amounted to a factor ranging from 10 to 50, lower than typically reported, and was highest for octanal and hexanal. The behavioral dose-response functions emerge at concentrations 2-5 orders of magnitude lower than those required for functions tracing the activation of specific human ORs by the same aldehydes in cell/molecular studies, after all functions were expressed as vapor concentrations.

The biological and toxicological activity of gases and vapors

A large amount of data on the biological and toxicological activity of gases and vapors has been collected from the literature. Processes include sensory irritation thresholds, the Alarie mouse test, inhalation anesthesia, etc. It is shown that a single equation using only five descriptors (properties of the gases and vapors) plus a set of indicator variables for the given processes can correlate 643 biological and non-lethal toxicological activities of 'non-reactive' compounds with a standard deviation of 0.36 log unit. The equation is scaled to sensory irritation thresholds obtained by the procedure of Cometto-Muñiz, and Cain provides a general equation for the prediction of sensory irritation thresholds in man. It is suggested that differences in biological/toxicological activity arise primarily from transport from the gas phase to a receptor phase or area, except for odor detection thresholds where interaction with a receptor(s) is important.

Functional expression of chemoreceptors with the help of a Guanine nucleotide exchange factor

Odorant receptors and other chemoreceptors are usually poorly expressed in the plasma membrane of heterologous cells. A key point of regulation in G protein-mediated signaling is the interconversion between the active GTP-bound and inactive GDP-bound states of the Galpha subunit, which regulatory proteins, such as guanine nucleotide exchange factors (GEFs), can control. GEFs stimulate formation of the GTP-bound state of Galpha and therefore are considered to work as positive regulators of G protein-coupled receptor signaling. Ric-8B, a GEF that is specifically expressed in olfactory sensory neurons, promotes functional expression of odorant receptors in HEK293T cells because it amplifies the initially low receptor signaling through Galphaolf. This same strategy could be used to functionally express other types of chemoreceptors.

Mammalian olfactory receptors

Perception of chemical stimuli from the environment is essential to most animals; accordingly, they are equipped with a complex olfactory system capable of receiving a nearly unlimited number of odorous substances and pheromones. This enormous task is accomplished by olfactory sensory neurons (OSNs) arranged in several chemosensory compartments in the nose. The sensitive and selective responsiveness of OSNs to odorous molecules and pheromones is based on distinct receptors in their chemosensory membrane; consequently, olfactory receptors play a key role for a reliable recognition and an accurate processing of chemosensory information. They are therefore considered as key elements for an understanding of the principles and mechanisms underlying the sense of smell. The repertoire of olfactory receptors in mammals encompasses hundreds of different receptor types which are highly diverse and expressed in distinct subcompartments of the nose. Accordingly, they are categorized into several receptor families, including odorant receptors (ORs), vomeronasal receptors (V1Rs and V2Rs), trace amine-associated receptors (TAARs), formyl peptide receptors (FPRs), and the membrane guanylyl cyclase GC-D. This large and complex receptor repertoire is the basis for the enormous chemosensory capacity of the olfactory system.

Calcium-activated chloride currents in olfactory sensory neurons from mice lacking bestrophin-2

Olfactory sensory neurons use a chloride-based signal amplification mechanism to detect odorants. The binding of odorants to receptors in the cilia of olfactory sensory neurons activates a transduction cascade that involves the opening of cyclic nucleotide-gated channels and the entry of Ca(2+) into the cilia. Ca(2+) activates a Cl(-) current that produces an efflux of Cl(-) ions and amplifies the depolarization. The molecular identity of Ca(2+)-activated Cl(-) channels is still elusive, although some bestrophins have been shown to function as Ca(2+)-activated Cl(-) channels when expressed in heterologous systems. In the olfactory epithelium, bestrophin-2 (Best2) has been indicated as a candidate for being a molecular component of the olfactory Ca(2+)-activated Cl(-) channel. In this study, we have analysed mice lacking Best2. We compared the electrophysiological responses of the olfactory epithelium to odorant stimulation, as well as the properties of Ca(2+)-activated Cl(-) currents in wild-type (WT) and knockout (KO) mice for Best2. Our results confirm that Best2 is expressed in the cilia of olfactory sensory neurons, while odorant responses and Ca(2+)-activated Cl(-) currents were not significantly different between WT and KO mice. Thus, Best2 does not appear to be the main molecular component of the olfactory channel. Further studies are required to determine the function of Best2 in the cilia of olfactory sensory neurons.

From Pheromones to Behavior

In recent years, considerable progress has been achieved in the comprehension of the profound effects of pheromones on reproductive physiology and behavior. Pheromones have been classified as molecules released by individuals and responsible for the elicitation of specific behavioral expressions in members of the same species. These signaling molecules, often chemically unrelated, are contained in body fluids like urine, sweat, specialized exocrine glands, and mucous secretions of genitals. The standard view of pheromone sensing was based on the assumption that most mammals have two separated olfactory systems with different functional roles: the main olfactory system for recognizing conventional odorant molecules and the vomeronasal system specifically dedicated to the detection of pheromones. However, recent studies have reexamined this traditional interpretation showing that both the main olfactory and the vomeronasal systems are actively involved in pheromonal communication. The current knowledge on the behavioral, physiological, and molecular aspects of pheromone detection in mammals is discussed in this review.

Molecular tuning of odorant receptors and its implication for odor signal processing

The discovery of the odorant receptor (OR) family by Buck and Axel in 1991 provided a quantum jump in our understanding of olfactory function. However, the study of the responsiveness of ORs to odor ligands was challenging due to the difficulties in deorphanizing the receptors. In this manuscript, we review recent findings of OR responsiveness that have come about through improved OR deorphanization methods, site-directed mutagenesis, structural modeling studies, and studies of OR responses in situ in olfactory sensory neurons. Although there has been a major leap in our understanding of receptor-ligand interactions and how these contribute to the input to the olfactory system, an improvement of our understanding of receptor structure and dynamics and interactions with intracellular and extracellular proteins is necessary.

Human olfaction: from genomic variation to phenotypic diversity

The sense of smell is a complex molecular device, encompassing several hundred olfactory receptor proteins (ORs). These receptors, encoded by the largest human gene superfamily, integrate odorant signals into an accurate 'odor image' in the brain. Widespread phenotypic diversity in human olfaction is, in part, attributable to prevalent genetic variation in OR genes, owing to copy number variation, deletion alleles and deleterious single nucleotide polymorphisms. The development of new genomic tools, including next generation sequencing and CNV assays, provides opportunities to characterize the genetic variations of this system. The advent of large-scale functional screens of expressed ORs, combined with genetic association studies, has the potential to link variations in ORs to human chemosensory phenotypes. This promises to provide a genome-wide view of human olfaction, resulting in a deeper understanding of personalized odor coding, with the potential to decipher flavor and fragrance preferences.

Concentration-detection functions for the odor of homologous n-acetate esters

Using air-dilution olfactometry, we measured concentration-response functions for the odor detection of the homologous esters ethyl, butyl, hexyl, and octyl acetate. Stimuli were delivered by means of an 8-station vapor delivery device (VDD-8) specifically designed to capture odor detection performance by humans under environmentally realistic conditions. Groups of 16-17 (half female) normosmic (i.e., having a normal olfaction) non-smokers (ages 18-38) were tested intensively. The method involved a three-alternative forced-choice procedure against carbon-filtered air, with an ascending concentration approach. Delivered concentrations were confirmed by gas chromatography before and during actual testing. A sigmoid (logistic) model provided an excellent fit to the odor detection functions both at the group and individual levels. Odor detection thresholds (ODTs) (defined as the half-way point between chance and perfect detection) decreased from ethyl (245 ppb by volume), to butyl (4.3 ppb), to hexyl acetate (2.9 ppb), and increased for octyl acetate (20 ppb). Interindividual threshold variability was near one and always below two orders of magnitude. The steepness of the functions increased slightly but significantly with carbon chain length. The outcome showed that the present thresholds lie at the very low end of those previously reported, but share with them a similar relative trend across n-acetates. On this basis, we suggest that a recent quantitative structure-activity relationship (QSAR) for ODTs can be applied to these and additional optimized data, and used to describe and predict not just ODTs but the complete underlying psychometric odor functions.

The electrochemical basis of odor transduction in vertebrate olfactory cilia

Most vertebrate olfactory receptor neurons share a common G-protein-coupled pathway for transducing the binding of odorant into depolarization. The depolarization involves 2 currents: an influx of cations (including Ca2+) through cyclic nucleotide-gated channels and a secondary efflux of Cl- through Ca2+-gated Cl- channels. The relation between stimulus strength and receptor current shows positive cooperativity that is attributed to the channel properties. This cooperativity amplifies the responses to sufficiently strong stimuli but reduces sensitivity and dynamic range. The odor response is transient, and prolonged or repeated stimulation causes adaptation and desensitization. At least 10 mechanisms may contribute to termination of the response; several of these result from an increase in intraciliary Ca2+. It is not known to what extent regulation of ionic concentrations in the cilium depends on the dendrite and soma. Although many of the major mechanisms have been identified, odor transduction is not well understood at a quantitative level.