Olfactory receptor patterning in a higher primate

Comfort Foods in the Twenty-First Century: Friend or Foe?

The comfort food (CF) concept emerged during the latter half of the twentieth century. Although not well defined, CF can be described as familiar foods that elicit feelings of well-being and play a role in social interactions and psychological health. These foods are often calorically dense and nutrient-poor, and overconsumption of some CF may contribute to negative metabolic health outcomes. This is particularly relevant when considering the global increase in obesity, leading to the development of therapeutics for improved weight control and metabolic health. In this review, we aim to (a) provide a historical perspective of the CF concept, (b) detail some genetic, developmental, and cultural factors that determine food preference, (c) discuss the influence of diet on the gut-brain connection, hormones, nutrient absorption, and microbiome diversity, and (d) provide a perspective detailing possible future directions in which food technology may enable a new generation of CF with enhanced palatability and nutrient profiles while contributing to well-being and environmental sustainability.

Deorphanization of Olfactory Trace Amine-Associated Receptors

Olfaction is the primary sense used by most animals to perceive the external world. The mouse olfactory system is comprised of several sensory structures, the largest of which is the main olfactory epithelium (MOE). Olfactory sensory neurons (OSNs) located within the MOE detect odors and pheromones using dedicated seven transmembrane, G protein-coupled receptors (GPCRs). Two families of GPCRs are expressed in the MOE and are conserved in humans and other vertebrates: odorant receptors (ORs) and trace amine-associated receptors (TAARs). TAARs are distantly related to biogenic amine receptors, such as dopamine and serotonin receptors. Several TAARs detect volatile amines, including ethological odors that evoke innate animal behavioral responses. Mouse TAAR4 recognizes the aversive predator odor 2-phenylethylamine, while mouse TAAR5 detects the attractive male mouse odor trimethylamine. In zebra fish, TAAR13c detects the foul death-associated odor cadaverine that mediates innate avoidance behavior. TAARs thus provide an excellent model subsystem to study odor valence. Identification of additional high-affinity ligands for TAARs will provide extra tools for such study. Therefore, this chapter focuses on the so-called SEAP (secreted alkaline phosphatase) assay that has been successfully applied for TAAR deorphanization in different species.

Structures and functions of the normal and injured human olfactory epithelium

The olfactory epithelium (OE) is directly exposed to environmental agents entering the nasal cavity, leaving OSNs prone to injury and degeneration. The causes of olfactory dysfunction are diverse and include head trauma, neurodegenerative diseases, and aging, but the main causes are chronic rhinosinusitis (CRS) and viral infections. In CRS and viral infections, reduced airflow due to local inflammation, inflammatory cytokine production, release of degranulated proteins from eosinophils, and cell injury lead to decreased olfactory function. It is well known that injury-induced loss of mature OSNs in the adult OE causes massive regeneration of new OSNs within a few months through the proliferation and differentiation of progenitor basal cells that are subsequently incorporated into olfactory neural circuits. Although normal olfactory function returns after injury in most cases, prolonged olfactory impairment and lack of improvement in olfactory function in some cases poses a major clinical problem. Persistent inflammation or severe injury in the OE results in morphological changes in the OE and respiratory epithelium and decreases the number of mature OSNs, resulting in irreversible loss of olfactory function. In this review, we discuss the histological structure and distribution of the human OE, and the pathogenesis of olfactory dysfunction associated with CRS and viral infection.

Olfactory Function and Olfactory Disorders

The sense of smell is important. This became especially clear to patients with infection-related olfactory loss during the SARS-CoV-2 pandemic. We react, for example, to the body odors of other humans. The sense of smell warns us of danger, and it allows us to perceive flavors when eating and drinking. In essence, this means quality of life. Therefore, anosmia must be taken seriously. Although olfactory receptor neurons are characterized by regenerative capacity, anosmia is relatively common with about 5 % of anosmic people in the general population. Olfactory disorders are classified according to their causes (e. g., infections of the upper respiratory tract, traumatic brain injury, chronic rhinosinusitis, age) with the resulting different therapeutic options and prognoses. Thorough history taking is therefore important. A wide variety of tools are available for diagnosis, ranging from short screening tests and detailed multidimensional test procedures to electrophysiological and imaging methods. Thus, quantitative olfactory disorders are easily assessable and traceable. For qualitative olfactory disorders such as parosmia, however, no objectifying diagnostic procedures are currently available. Therapeutic options for olfactory disorders are limited. Nevertheless, there are effective options consisting of olfactory training as well as various additive drug therapies. The consultation and the competent discussion with the patients are of major importance.

Comparative Aspects of Ricin Toxicity by Inhalation

The pathogenesis of ricin toxicity following inhalation has been investigated in many animal models, including the non-human primate (predominantly the rhesus macaque), pig, rabbit and rodent. The toxicity and associated pathology described in animal models are broadly similar, but variation appears to exist. This paper reviews the published literature and some of our own unpublished data and describes some of the possible reasons for this variation. Methodological variation is evident, including method of exposure, breathing parameters during exposure, aerosol characteristics, sampling protocols, ricin cultivar, purity and challenge dose and study duration. The model species and strain used represent other significant sources of variation, including differences in macro- and microscopic anatomy, cell biology and function, and immunology. Chronic pathology of ricin toxicity by inhalation, associated with sublethal challenge or lethal challenge and treatment with medical countermeasures, has received less attention in the literature. Fibrosis may follow acute lung injury in survivors. There are advantages and disadvantages to the different models of pulmonary fibrosis. To understand their potential clinical significance, these factors need to be considered when choosing a model for chronic ricin toxicity by inhalation, including species and strain susceptibility to fibrosis, time it takes for fibrosis to develop, the nature of the fibrosis (e.g., self-limiting, progressive, persistent or resolving) and ensuring that the analysis truly represents fibrosis. Understanding the variables and comparative aspects of acute and chronic ricin toxicity by inhalation is important to enable meaningful comparison of results from different studies, and for the investigation of medical countermeasures.

Genome-Wide RNA Tomography in the Mouse Whole Olfactory Mucosa

Spatial transcriptomics allows for the genome-wide profiling of topographic gene expression patterns within a tissue of interest. Here, we describe our methodology to generate high-quality RNA-seq libraries from cryosections from fresh frozen mouse whole olfactory mucosae. This methodology can be extended to virtually any vertebrate organ or tissue sample.

MRI tractography reveals the human olfactory nerve map connecting the olfactory epithelium and olfactory bulb

The olfactory nerve map describes the topographical neural connections between the olfactory epithelium in the nasal cavity and the olfactory bulb. Previous studies have constructed the olfactory nerve maps of rodents using histological analyses or transgenic animal models to investigate olfactory nerve pathways. However, the human olfactory nerve map remains unknown. Here, we demonstrate that high-field magnetic resonance imaging and diffusion tensor tractography can be used to visualize olfactory sensory neurons while maintaining their three-dimensional structures. This technique allowed us to evaluate the olfactory sensory neuron projections from the nasal cavities to the olfactory bulbs and visualize the olfactory nerve maps of humans, marmosets and mice. The olfactory nerve maps revealed that the dorsal-ventral and medial-lateral axes were preserved between the olfactory epithelium and olfactory bulb in all three species. Further development of this technique might allow it to be used clinically to facilitate the diagnosis of olfactory dysfunction.

Combined high-field MRI and DTI analyses in post-mortem mouse, marmoset, and human samples provide insight into the neural connections between nasal cavities and olfactory bulbs.

Spatial organization of olfactory receptor gene choice in the complete V1R-related ORA family of zebrafish

The vertebrate sense of smell employs four main receptor families for detection of odors, among them the V1R/ORA family, which is unusually small and highly conserved in teleost fish. Zebrafish possess just seven ORA receptors, enabling a comprehensive analysis of the expression patterns of the entire family. The olfactory organ of zebrafish is representative for teleosts, cup-shaped, with lamella covered with sensory epithelium protruding into the cup from a median raphe. We have performed quantitative in situ hybridization on complete series of horizontal cryostat sections of adult zebrafish olfactory organ, and have analysed the location of ora-expressing cells in three dimensions, radial diameter, laminar height, and height-within-the-organ. We report broadly overlapping, but distinctly different distributions for all ora genes, even for ora3a and ora3b, the most recent gene duplication. Preferred positions in different dimensions are independent of each other. This spatial logic is very similar to previous reports for the much larger families of odorant receptor (or) and V2R-related olfC genes in zebrafish. Preferred positions for ora genes tend to be more central and more apical than those we observed for these other two families, consistent with expression in non-canonical sensory neuron types.

Evaluation of Approach to a Conspecific and Blood Biochemical Parameters in TAAR1 Knockout Mice

It is known that the trace amine-associated receptor 1 (TAAR1) receptor is involved in limbic brain functions by regulating dopamine transmission and putative reward circuitry. Moreover, other TAARs are expressed in the olfactory system of all studied vertebrate species, sensing innate socially-relevant odors, including pheromones. Therefore, one can assume that TAARs may play a role in rodent social and sexual behavior. A comparative behavioral and biochemical analysis of TAAR1 knockout (TAAR1-KO) and wild-type mice is also important for the preliminary evaluation of the potential side effects of future TAAR1-based therapies. In our studies, we adapted a sexual incentive motivation test for mice to evaluate the sexual behavior of TAAR1-KO and wild-type mice. Previously, similar methods were primarily applied to rats. Furthermore, we measured testosterone and other biochemical parameters in the blood. As a result, we found only minimal alterations in all of the studied parameters. Thus, the lack of TAAR1 does not significantly affect sexual motivation and routine lipid and metabolic blood biochemical parameters, suggesting that future TAAR1-based therapies should have a favorable safety profile.

The scaling of olfaction: Moths have relatively more olfactory surface area than mammals

Body size affects nearly every aspect of locomotion and sensing, but little is known how body size influences olfaction. One reason for this missing link is that olfaction differs fundamentally from vision and hearing in that molecules are advected by fluid before depositing on olfactory sensors. This critical role of fluid flow in olfaction leads to complexities and trade-offs. For example, a greater density of hairs and sensory neurons may lead to greater collection, but can also lead to reduced flow through hairs and additional weight and drag due to a larger olfactory organ. In this study, we report the surface area and sensory neuron density in olfactory organs of 95 species of moths and mammals. We find that approximately 12-14 percent of an olfactory system's surface area is devoted to chemosensors. Furthermore, total olfactory surface area and olfactory sensing surface area scale with body mass to the 0.49 and 0.38 powers respectively, indicating that moths have a higher proportion of olfactory surface area than mammals. The density of olfactory neurons appears to be near the limit, at 10,000 to 100,000 neurons per square mm across both insects and mammals. This study demonstrates the need for future work detailing how scaling of olfaction and other senses vary across taxa.

A 3D transcriptomics atlas of the mouse nose sheds light on the anatomical logic of smell

The sense of smell helps us navigate the environment, but its molecular architecture and underlying logic remain understudied. The spatial location of odorant receptor genes (Olfrs) in the nose is thought to be independent of the structural diversity of the odorants they detect. Using spatial transcriptomics, we create a genome-wide 3D atlas of the mouse olfactory mucosa (OM). Topographic maps of genes differentially expressed in space reveal that both Olfrs and non-Olfrs are distributed in a continuous and overlapping fashion over at least five broad zones in the OM. The spatial locations of Olfrs correlate with the mucus solubility of the odorants they recognize, providing direct evidence for the chromatographic theory of olfaction. This resource resolves the molecular architecture of the mouse OM and will inform future studies on mechanisms underlying Olfr gene choice, axonal pathfinding, patterning of the nervous system, and basic logic for the peripheral representation of smell.

Pattern of TAAR5 Expression in the Human Brain Based on Transcriptome Datasets Analysis

Trace amine-associated receptors (TAAR) recognize organic compounds, including primary, secondary, and tertiary amines. The TAAR5 receptor is known to be involved in the olfactory sensing of innate socially relevant odors encoded by volatile amines. However, emerging data point to the involvement of TAAR5 in brain functions, particularly in the emotional behaviors mediated by the limbic system which suggests its potential contribution to the pathogenesis of neuropsychiatric diseases. TAAR5 expression was explored in datasets available in the Gene Expression Omnibus, Allen Brain Atlas, and Human Protein Atlas databases. Transcriptomic data demonstrate ubiquitous low TAAR5 expression in the cortical and limbic brain areas, the amygdala and the hippocampus, the nucleus accumbens, the thalamus, the hypothalamus, the basal ganglia, the cerebellum, the substantia nigra, and the white matter. Altered TAAR5 expression is identified in Down syndrome, major depressive disorder, or HIV-associated encephalitis. Taken together, these data indicate that TAAR5 in humans is expressed not only in the olfactory system but also in certain brain structures, including the limbic regions receiving olfactory input and involved in critical brain functions. Thus, TAAR5 can potentially be involved in the pathogenesis of brain disorders and represents a valuable novel target for neuropsychopharmacology.

Principles of odor coding in vertebrates and artificial chemosensory systems

The biological olfactory system is the sensory system responsible for the detection of the chemical composition of the environment. Several attempts to mimic biological olfactory systems have led to various artificial olfactory systems using different technical approaches. Here we provide a parallel description of biological olfactory systems and their technical counterparts. We start with a presentation of the input to the systems, the stimuli, and treat the interface between the external world and the environment where receptor neurons or artificial chemosensors reside. We then delineate the functions of receptor neurons and chemosensors as well as their overall I-O relationships. Up to this point, our account of the systems goes along similar lines. The next processing steps differ considerably: while in biology the processing step following the receptor neurons is the "integration" and "processing" of receptor neuron outputs in the olfactory bulb, this step has various realizations in electronic noses. For a long period of time, the signal processing stages beyond the olfactory bulb, i.e., the higher olfactory centers were little studied. Only recently there has been a marked growth of studies tackling the information processing in these centers. In electronic noses, a third stage of processing has virtually never been considered. In this review, we provide an up-to-date overview of the current knowledge of both fields and, for the first time, attempt to tie them together. We hope it will be a breeding ground for better information, communication, and data exchange between very related but so far little connected fields.

The Zonal Organization of Odorant Receptor Gene Choice in the Main Olfactory Epithelium of the Mouse

A mature olfactory sensory neuron (OSN) of the main olfactory epithelium (MOE) typically expresses one allele of one odorant receptor (OR) gene. It is widely thought that the great majority of the 1,141 intact mouse OR genes are expressed in one of four MOE zones (or bands or stripes), which are largely non-overlapping. Here, we develop a multiplex method to map, in 3D and MOE-wide, the expression areas of multiple OR genes in individual, non-genetically modified mice by three-color fluorescence in situ hybridization, semi-automated image segmentation, and 3D reconstruction. We classify the expression areas of 68 OR genes into 9 zones. These zones are highly overlapping and strikingly complex when viewed in 3D reconstructions. There could well be more zones. We propose that zones reflect distinct OSN types that are each restricted in their choice to a subset of the OR gene repertoire.

Odor coding in the mammalian olfactory epithelium

Noses are extremely sophisticated chemical detectors allowing animals to use scents to interpret and navigate their environments. Odor detection starts with the activation of odorant receptors (ORs), expressed in mature olfactory sensory neurons (OSNs) populating the olfactory mucosa. Different odorants, or different concentrations of the same odorant, activate unique ensembles of ORs. This mechanism of combinatorial receptor coding provided a possible explanation as to why different odorants are perceived as having distinct odors. Aided by new technologies, several recent studies have found that antagonist interactions also play an important role in the formation of the combinatorial receptor code. These findings mark the start of a new era in the study of odorant-receptor interactions and add a new level of complexity to odor coding in mammals.

Olfactory signaling via trace amine-associated receptors

Trace amine-associated receptors (TAARs) are a family of G protein-coupled receptors that function as odorant receptors in the main olfactory system of vertebrates. TAARs are monoallelically expressed in primary sensory neurons where they couple to the same transduction cascade as canonical olfactory receptors and are mapped onto glomeruli within a specific region of the olfactory bulb. TAARs have a high affinity for volatile amines, a class of chemicals that are generated during the decomposition of proteins and are ubiquitous physiological metabolites that are found in body fluids. Thus, amines are proposed to play an important role in intra- and interspecific communication such as signaling the sex of the conspecific, the quality of the food source, or even the proximity of a predator. TAARs have a crucial role in the perception of these behaviorally relevant compounds as the genetic deletion of all or even individual olfactory TAARs can alter the behavioral response and reduce the sensitivity to amines. The small size of this receptor family combined with the ethological relevance of their ligands makes the TAARs an attractive model system for probing olfactory perception. This review will summarize the current knowledge on the olfactory TAARs and discuss whether they represent a unique subsystem within the main olfactory system.

Sequence Variants in TAAR5 and Other Loci Affect Human Odor Perception and Naming

Olfactory receptor (OR) genes in humans form a special class characterized by unusually high DNA sequence diversity, which should give rise to differences in perception and behavior. In the largest genome-wide association study to date based on olfactory testing, we investigated odor perception and naming with smell tasks performed by 9,122 Icelanders, with replication in a separate sample of 2,204 individuals. We discovered an association between a low-frequency missense variant in TAAR5 and reduced intensity rating of fish odor containing trimethylamine (p.Ser95Pro, p_combined = 5.6 × 10^-15). We demonstrate that TAAR5 genotype affects aversion to fish odor, reflected by linguistic descriptions of the odor and pleasantness ratings. We also discovered common sequence variants in two canonical olfactory receptor loci that associate with increased intensity and naming of licorice odor (trans-anethole: lead variant p.Lys233Asn in OR6C70, p_combined = 8.8 × 10^-16 and p_combined = 1.4 × 10^-9) and enhanced naming of cinnamon (trans-cinnamaldehyde; intergenic variant rs317787-T, p_combined = 5.0 × 10^-17). Together, our results show that TAAR5 genotype variation influences human odor responses and highlight that sequence diversity in canonical OR genes can lead to enhanced olfactory ability, in contrast to the view that greater tolerance for mutations in the human OR repertoire leads to diminished function.

Cadaverine and Spermine Elicit Ca2+ Uptake in Human CP Cells via a Trace Amine-Associated Receptor 1 Dependent Pathway

The choroid plexus (CP) constitutes a barrier between the blood and the cerebrospinal fluid (CSF) which regulates the exchange of substances between these two fluids through mechanisms that are not completely understood. Polyamines as spermine, spermidine and putrescine are produced by all cells and are present in the CSF. Interestingly, their levels are altered in some neuronal disorders as Alzheimer's and Parkinson's diseases, thus increasing the interest in their signalling in the central nervous system (CNS). Cadaverine, on the other hand, is synthetized by the intestinal microbiome, suggesting that the presence of this bacterial metabolite in the CSF requires that it is up taken to the CNS across brain barriers. We knew that polyamines are detected by the olfactory signalling cascade operating at the CP, but the receptor involved had not been identified. The zebrafish TAAR13c was the only receptor known to bind a polyamine-cadaverine. Thus, we searched for a human receptor with homology to TAAR13c and found that some human TAARs including TAAR1 showed great homology. Then, we confirmed the expression of TAAR1 mRNA and protein in a human cell line of the CP, and in human CP samples. Calcium imaging assays after TAAR1 knockdown in these cells with a specific siRNA against TAAR1 showed a consistent reduction in the responses of these cells to cadaverine and spermidine, but not to spermine, suggesting that TAAR1 is activated by cadaverine and spermidine, but not spermine.

Functional evolution of vertebrate sensory receptors

Sensory receptors enable animals to perceive their external world, and functional properties of receptors evolve to detect the specific cues relevant for an organism's survival. Changes in sensory receptor function or tuning can directly impact an organism's behavior. Functional tests of receptors from multiple species and the generation of chimeric receptors between orthologs with different properties allow for the dissection of the molecular basis of receptor function and identification of the key residues that impart functional changes in different species. Knowledge of these functionally important sites facilitates investigation into questions regarding the role of epistasis and the extent of convergence, as well as the timing of sensory shifts relative to other phenotypic changes. However, as receptors can also play roles in non-sensory tissues, and receptor responses can be modulated by numerous other factors including varying expression levels, alternative splicing, and morphological features of the sensory cell, behavioral validation can be instrumental in confirming that responses observed in heterologous systems play a sensory role. Expression profiling of sensory cells and comparative genomics approaches can shed light on cell-type specific modifications and identify other proteins that may affect receptor function and can provide insight into the correlated evolution of complex suites of traits. Here we review the evolutionary history and diversity of functional responses of the major classes of sensory receptors in vertebrates, including opsins, chemosensory receptors, and ion channels involved in temperature-sensing, mechanosensation and electroreception.

TAAR Agonists

Trace amine-associated receptors (TAARs) are a family of G protein-coupled receptors (GPCRs) that are evolutionarily conserved in vertebrates. The first discovered TAAR1 is mainly expressed in the brain, and is able to detect low abundant trace amines. TAAR1 is also activated by several synthetic compounds and psychostimulant drugs like amphetamine. Activation of TAAR1 by specific agonists can regulate the classical monoaminergic systems in the brain. Further studies have revealed that other TAAR family members are highly expressed in the olfactory system which are termed olfactory TAARs. In vertebrates, olfactory TAARs can specifically recognize volatile or water-soluble amines. Some of these TAAR agonists are produced by decarboxylation of amino acids. In addition, some TAAR agonists are ethological odors that mediate animal innate behaviors. In this study, we provide a comprehensive review of TAAR agonists, including their structures, biosynthesis pathways, and functions.

Sex differences in main olfactory system pathways involved in psychosexual function

We summarize literature from animal and human studies assessing sex differences in the ability of the main olfactory system to detect and process sex-specific olfactory signals ("pheromones") that control the expression of psychosexual functions in males and females. A case is made in non primate mammals for an obligatory role of pheromonal signaling via the main olfactory system (in addition to the vomeronasal-accessory olfactory system) in mate recognition and sexual arousal, with male-specific as well as female-specific pheromones subserving these functions in the opposite sex. Although the case for an obligatory role of pheromones in mate recognition and mating among old world primates, including humans, is weaker, we review the current literature assessing the role of putative human pheromones (eg, AND, EST, "copulin"), detected by the main olfactory system, in promoting mate choice and mating in men and women. Based on animal studies, we hypothesize that sexually dimorphic effects of putative human pheromones are mediated via main olfactory inputs to the medial amygdala which, in turn, transmits olfactory information to sites in the hypothalamus that regulate reproduction.

A transcriptomic atlas of mammalian olfactory mucosae reveals an evolutionary influence on food odor detection in humans

The mammalian olfactory system displays species-specific adaptations to different ecological niches. To investigate the evolutionary dynamics of olfactory sensory neuron (OSN) subtypes across mammalian evolution, we applied RNA sequencing of whole olfactory mucosa samples from mouse, rat, dog, marmoset, macaque, and human. We find that OSN subtypes, representative of all known mouse chemosensory receptor gene families, are present in all analyzed species. Further, we show that OSN subtypes expressing canonical olfactory receptors are distributed across a large dynamic range and that homologous subtypes can be either highly abundant across all species or species/order specific. Highly abundant mouse and human OSN subtypes detect odorants with similar sensory profiles and sense ecologically relevant odorants, such as mouse semiochemicals or human key food odorants. Together, our results allow for a better understanding of the evolution of mammalian olfaction in mammals and provide insights into the possible functions of highly abundant OSN subtypes.

In Silico Peptide Repertoire of Human Olfactory Receptor Proteomes on High-Stringency Mass Spectrometry

Human olfactory receptors (ORs) are seven-pass transmembrane G-protein coupled receptors (GPCR) involved in smell perception and many other signaling pathways. They are primarily expressed in the olfactory epithelium and ectopically expressed in several other organs and tissues. neXtProt contains 4 PE1 (protein existence 1, evidenced at the protein level) ORs, determined on the basis of either protein interaction data (i.e., OR1D4 and OR2AG1) or convincing genetic, haplotype, or biochemical data (i.e., OR1D2 and OR2J3). Not a single OR currently qualifies for neXtProt PE1 status based on mass spectrometry (MS) evidence. Many reasons for this absence of MS-based identification have been proposed, including (i) confined or spatiotemporal or developmental expression, (ii) low copy number, (iii) OR repertoire gene silencing, and (iv) limited tissue availability. OR transmembrane domains (TMDs) inherently limit MS identification because the hydrophobic nature restricts the access of trypsin to potential cleavage sites. Equally, the extremely low frequency or lack of accessible arginine and lysine residues in TMDs renders trypsin cleavage ineffective. Here, we demonstrate an analytical approach specifically focused on the hydrophilic (trypsin-accessible) domains of ORs [i.e., with all transmembrane segments and anchored peptides excluded). We predicted the ability of OR soluble (hydrophilic) domains to yield 2 or more >9 amino acids (aa) length unique mapping (unique to a protein only), non-nested (peptides with varying length at the N or C terminal but containing the same core sequence), leucine/isoleucine (I/L) switch examined (I and L have same mass and cannot be distinguished by MS) tryptic peptides. Our analysis showed that ∼58% of the human OR proteome could potentially generate tryptic peptides that satisfy current the Human Proteome Project data interpretation guidelines (version 2.1) when no missed cleavages are allowed and increases to ∼78% when one missed cleavage is allowed. The utilization of current biological data (adjuvant genomics, expression profile, transcriptomics, epigenome silencing data, etc.) and the adoption of a non-conventional proteomics approach (e.g., Confetti multiprotease digestion, CNBr cleavage of TMDs, and more-extreme chromatographic and MS methods) could aid in the detection of the remaining ORs.

Deorphanization of Olfactory Trace Amine-Associated Receptors

Olfaction is the primary sense used by most animals to perceive the external world. The mouse olfactory system is composed of several sensory structures, the largest of which is the main olfactory epithelium (MOE). Olfactory sensory neurons (OSNs) located within the MOE detect odors and pheromones using dedicated seven-transmembrane G protein-coupled receptors (GPCRs). Two families of GPCRs are expressed in the MOE and are conserved in humans and other vertebrates: odorant receptors (ORs) and trace amine-associated receptors (TAARs). TAARs are distantly related to biogenic amine receptors, such as dopamine and serotonin receptors. Several TAARs detect volatile amines including ethological odors that evoke innate animal behavioral responses. Mouse TAAR4 recognizes the aversive predator odor 2-phenylethylamine, while mouse TAAR5 detects the attractive male mouse odor trimethylamine. In zebrafish, TAAR13c detects the foul death-associated odor cadaverine that mediates innate avoidance behavior. TAARs thus provide an excellent model subsystem to study odor valence. And identification of additional high-affinity ligands for TAARs will provide extra tools for such study. Therefore, this chapter focuses on the so-called SEAP assay that has been successfully applied for TAAR deorphanization in different species.

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.

Sense of Smell: Structural, Functional, Mechanistic Advancements and Challenges in Human Olfactory Research

Olfaction, the sense of smell detects and discriminate odors as well as social cues which influence our innate responses. The olfactory system in human beings is found to be weak as compared to other animals; however, it seems to be very precise. It can detect and discriminate millions of chemical moieties (odorants) even in minuscule quantities. The process initiates with the binding of odorants to specialized olfactory receptors, encoded by a large family of Olfactory Receptor (OR) genes belonging to the G-protein-coupled receptor superfamily. Stimulation of ORs converts the chemical information encoded in the odorants, into respective neuronal action-potentials which causes depolarization of olfactory sensory neurons. The olfactory bulb relays this signal to different parts of the brain for processing. Odors are encrypted using a combinatorial approach to detect a variety of chemicals and encode their unique identity. The discovery of functional OR genes and proteins provided an important information to decipher the genomic, structural and functional basis of olfaction. ORs constitute 17 gene families, out of which 4 families were reported to contain more than hundred members each. The olfactory machinery is not limited to GPCRs; a number of non- GPCRs is also employed to detect chemosensory stimuli. The article provides detailed information about such olfaction machinery, structures, transduction mechanism, theories of odor perception, and challenges in the olfaction research. It covers the structural, functional and computational studies carried out in the olfaction research in the recent past.

How Female Mice Attract Males: A Urinary Volatile Amine Activates a Trace Amine-Associated Receptor That Induces Male Sexual Interest

Individuals of many species rely on odors to communicate, find breeding partners, locate resources and sense dangers. In vertebrates, odorants are detected by chemosensory receptors of the olfactory system. One class of these receptors, the trace amine-associated receptors (TAARs), was recently suggested to mediate male sexual interest and mate choice. Here we tested this hypothesis in mice by generating a cluster deletion mouse (Taar2-9^−/−) lacking all TAARs expressed in the olfactory epithelium, and evaluating transduction pathways from odorants to TAARs, neural activity and behaviors reflecting sexual interest. We found that a urinary volatile amine, isobutylamine (IBA), was a potent ligand for TAAR3 (but not TAAR1, 4, 5, and 6). When males were exposed to IBA, brain regions associated with sexual behaviors were less active in Taar2-9^−/− than in wild type males. Accordingly, Taar2-9^−/− males spent less time sniffing both the urine of females and pure IBA than wild type males. This is the first demonstration of a comprehensive transduction pathway linking odorants to TAARs and male sexual interest. Interestingly, the concentration of IBA in female urine varied across the estrus cycle with a peak during estrus. This variation in IBA concentration may represent a simple olfactory cue for males to recognize receptive females. Our results are consistent with the hypothesis that IBA and TAARs play an important role in the recognition of breeding partners and mate choice.

A Near-Complete Spatial Map of Olfactory Receptors in the Mouse Main Olfactory Epithelium

Different regions of the mammalian nose smell different odors. In the mouse olfactory system, spatially regulated expression of >1000 olfactory receptors (ORs) along the dorsomedial-ventrolateral (DV) axis forms a topological map in the main olfactory epithelium (MOE). However, the locations of most ORs along the DV axis are currently unknown. By sequencing mRNA of 12 isolated MOE pieces, we mapped out the DV locations-as quantified by "zone indices" on a scale of 1-5-of 1033 OR genes with an estimated error of 0.3 zone indices. Our map covered 81% of all intact OR genes and 99.4% of the total OR mRNA abundance. Spatial regulation tended to vary gradually along chromosomes. We further identified putative non-OR genes that may exhibit spatial expression along the DV axis.

Trace Amine-Associated Receptors as Novel Therapeutic Targets for Immunomodulatory Disorders

Trace amines and their receptors (trace amine-associated receptors; TAARs) are an emerging pharmacological target for the treatment of human disorders. While most studies have focused on their therapeutic potential for neurologic and psychiatric disorders, TAARs are also expressed throughout the periphery, including prominent expression in human leukocytes. Furthermore, recent independent, unbiased metabolomic studies have consistently identified one or more TAAR ligands as potential etiologic factors in inflammatory bowel disease (IBD). The putative role of TAARs in diseases such as IBD that are associated with hyperactive immune responses has not, however, previously been systematically addressed. Here, we review the current state of the knowledge of the effects of TAARs on leukocyte function, in particular in the context of mucosal epithelial cells that interface with the environment; developing a model whereby TAARs may be considered as a novel therapeutic target for disorders associated with dysregulated immune responses to environmental factors. In this model, we hypothesize that altered trace amine homeostasis results in hyperactivity of the immune system. Such loss of homeostasis can occur through many different mechanisms including TAAR polymorphisms and altered trace amine load due to changes in host synthesis and/or degradative enzymes, diet, or microbial dysbiosis. The resulting alterations in TAAR functioning can then lead to a loss of homeostasis of leukocyte chemotaxis, differentiation, and activation, as well as an altered ability of members of the microbiota to adhere to and penetrate the epithelial cell layers. Such changes would generate a pro-inflammatory state at mucosal epithelial barrier layers that can manifest as clinical symptomatology such as that seen in IBD. These alterations may also have the potential to induce systemic effects, which could possibly contribute to immunomodulatory disorders in other systems, including neurological diseases.

Trace Amines and Their Receptors

Trace amines are endogenous compounds classically regarded as comprising β-phenylethyalmine, p-tyramine, tryptamine, p-octopamine, and some of their metabolites. They are also abundant in common foodstuffs and can be produced and degraded by the constitutive microbiota. The ability to use trace amines has arisen at least twice during evolution, with distinct receptor families present in invertebrates and vertebrates. The term "trace amine" was coined to reflect the low tissue levels in mammals; however, invertebrates have relatively high levels where they function like mammalian adrenergic systems, involved in "fight-or-flight" responses. Vertebrates express a family of receptors termed trace amine-associated receptors (TAARs). Humans possess six functional isoforms (TAAR1, TAAR2, TAAR5, TAAR6, TAAR8, and TAAR9), whereas some fish species express over 100. With the exception of TAAR1, TAARs are expressed in olfactory epithelium neurons, where they detect diverse ethological signals including predators, spoiled food, migratory cues, and pheromones. Outside the olfactory system, TAAR1 is the most thoroughly studied and has both central and peripheral roles. In the brain, TAAR1 acts as a rheostat of dopaminergic, glutamatergic, and serotonergic neurotransmission and has been identified as a novel therapeutic target for schizophrenia, depression, and addiction. In the periphery, TAAR1 regulates nutrient-induced hormone secretion, suggesting its potential as a novel therapeutic target for diabetes and obesity. TAAR1 may also regulate immune responses by regulating leukocyte differentiation and activation. This article provides a comprehensive review of the current state of knowledge of the evolution, physiologic functions, pharmacology, molecular mechanisms, and therapeutic potential of trace amines and their receptors in vertebrates and invertebrates.

Pleasantness and trigeminal sensations as salient dimensions in organizing the semantic and physiological spaces of odors

A major issue in human olfaction research is to characterize the main dimensions that organize the space of odors. The present study examines this question and shows that, beside pleasantness, trigeminal sensations, and particularly irritation, play an important role. These results were consistent along two different spaces constructed using semantic description and physiological responses to 105 odorants, smelled and described by human participants. Taken together, these findings suggest that salient trigeminal features, in conjunction with pleasantness, are involved in detecting relevant emotional stimuli, and modify the way organisms categorize smells. These results shed light on the importance of trigeminal sensitivity in the well-established defensive function of olfaction.

Overlapping but distinct topology for zebrafish V2R-like olfactory receptors reminiscent of odorant receptor spatial expression zones

Background

The sense of smell is unrivaled in terms of molecular complexity of its input channels. Even zebrafish, a model vertebrate system in many research fields including olfaction, possesses several hundred different olfactory receptor genes, organized in four different gene families. For one of these families, the initially discovered odorant receptors proper, segregation of expression into distinct spatial subdomains within a common sensory surface has been observed both in teleost fish and in mammals. However, for the remaining three families, little to nothing was known about their spatial coding logic. Here we wished to investigate, whether the principle of spatial segregation observed for odorant receptors extends to another olfactory receptor family, the V2R-related OlfC genes. Furthermore we thought to examine, how expression of OlfC genes is integrated into expression zones of odorant receptor genes, which in fish share a single sensory surface with OlfC genes.

Results

To select representative genes, we performed a comprehensive phylogenetic study of the zebrafish OlfC family, which identified a novel OlfC gene, reduced the number of pseudogenes to 1, and brought the total family size to 60 intact OlfC receptors. We analyzed the spatial pattern of OlfC-expressing cells for seven representative receptors in three dimensions (height within the epithelial layer, horizontal distance from the center of the olfactory organ, and height within the olfactory organ). We report non-random distributions of labeled neurons for all OlfC genes analysed. Distributions for sparsely expressed OlfC genes are significantly different from each other in nearly all cases, broad overlap notwithstanding. For two of the three coordinates analyzed, OlfC expression zones are intercalated with those of odorant receptor zones, whereas in the third dimension some segregation is observed.

Conclusion

Our results show that V2R-related OlfC genes follow the same spatial logic of expression as odorant receptors and their expression zones intermingle with those of odorant receptor genes. Thus, distinctly different expression zones for individual receptor genes constitute a general feature shared by teleost and tetrapod V2R/OlfC and odorant receptor families alike.

Electronic supplementary material

The online version of this article (10.1186/s12864-018-4740-8) contains supplementary material, which is available to authorized users.

Structure and zonal expression of olfactory receptors in the olfactory epithelium of the goat, Capra hircus

The mammalian olfactory system employs sophisticated mechanisms to detect and recognize an extensive range of smells. In rodents, the olfactory epithelium (OE), situated within the nasal cavity, mainly comprises four defined endoturbinates and several ectoturbinates. Olfactory receptors (ORs) belong to a large family, comprising over 1,000 genes in rodents, which are expressed in olfactory sensory neurons in the OE that detect odor molecules. The rodent OE is divided into four topographically distinct zones, defined by individual OR distribution. However, although the structural complexity and the zonal organization of mammalian OE may contribute to successfully interpreting olfactory information, it remains poorly understood. In this study, we investigated the nasal cavity structure and zonal organization of the OE in goats. Morphological observations revealed that the goat nasal cavity possessed well-developed endoturbinates and ectoturbinates and had a structure similar to that of rodents and sheep, previously reported in other studies. In situ hybridization was used to analyze the expression pattern of ORs, NADPH:quinone oxidoreductase 1, and olfactory cell adhesion molecules as markers of zonal organization in the goat OE. Based on the expression patterns of these genes, we concluded that the goat OE was divided into four zones. The well-developed structure of the nasal cavity and distribution of each OR in the OE were similar to those found in rodents, suggesting that these features were highly conserved between mammals and may have fundamental roles in discriminating among numerous odor molecules in the environment.

Olfactory epithelium: Cells, clinical disorders, and insights from an adult stem cell niche

Disorders causing a loss of the sense of smell remain a therapeutic challenge. Basic research has, however, greatly expanded our knowledge of the organization and function of the olfactory system. This review describes advances in our understanding of the cellular components of the peripheral olfactory system, specifically the olfactory epithelium in the nose. The article discusses recent findings regarding the mechanisms involved in regeneration and cellular renewal from basal stem cells in the adult olfactory epithelium, considering the strategies involved in embryonic olfactory development and insights from research on other stem cell niches. In the context of clinical conditions causing anosmia, the current view of adult olfactory neurogenesis, tissue homeostasis, and failures in these processes is considered, along with current and future treatment strategies.

Level of Evidence

NA.

The Case for TAAR1 as a Modulator of Central Nervous System Function

TAAR1 is widely expressed across the mammalian brain, particularly in limbic and monoaminergic areas, allegedly involved in mood, attention, memory, fear, and addiction. However, the subcellular distribution of TAAR1 is still unclear, since TAAR1 signal is largely intracellular. In vitro, TAAR1 is activated with nanomolar to micromolar affinity by some endogenous amines, particularly p-tyramine, beta-phenylethylamine, and 3-iodothyronamine (T1AM), the latter representing a novel branch of thyroid hormone signaling. In addition, TAAR1 responds to a number of psychoactive drugs, i.e., amphetamines, ergoline derivatives, bromocriptine and lisuride. Trace amines have been identified as neurotransmitters in invertebrates, and they are considered as potential neuromodulators. In particular, beta-phenylethylamine and p-tyramine have been reported to modify the release and/or the response to dopamine, norepinephrine, acetylcholine and GABA, while evidence of cross-talk between TAAR1 and other aminergic receptors has been provided. Systemic or intracerebroventricular injection of exogenous T1AM produced prolearning and antiamnestic effects, reduced pain threshold, decreased non-REM sleep, and modulated the firing rate of adrenergic neurons in locus coeruleus. However each of these substances may have additional molecular targets, and it is unclear whether their endogenous levels are sufficient to produce significant TAAR1 activation in vivo. TAAR1 knock out mice show a worse performance in anxiety and working memory tests, and they are more prone to develop ethanol addiction. They also show increased locomotor response to amphetamine, and decreased stereotypical responses induced by apomorphine. Notably, human genes for TAARs cluster on chromosome 6 at q23, within a region whose mutations have been reported to confer susceptibility to schizophrenia and bipolar disorder. For human TAAR1, around 200 non-synonymous and 400 synonymous single nucleotide polymorphisms have been identified, but their functional consequences have not been extensively investigated yet. In conclusion, the bulk of evidence points to a significant physiological role of TAAR1 in the modulation of central nervous system function and a potential pharmacological role of TAAR1 agonists in neurology and/or psychiatry. However, the specific effects of TAAR1 stimulation are still controversial, and many crucial issues require further investigation.

Hydrogen Sulfide Specifically Alters NAD(P)H Quinone Dehydrogenase 1 (NQO1) Olfactory Neurons in the Rat

The regions of the olfactory epithelium affected by hydrogen sulfide (H₂S) toxicity in the rat present a striking similarity with the developmental olfactory zone 1 described in the mouse. This zone which is the only region containing neurons expressing NAD(P)H quinone dehydrogenase 1 (NQO1) is involved in complex behavioral responses in rodents, and other mammals, triggered by specific olfactory stimuli. We therefore sought to determine whether (1) olfactory neurons expressing NQO1 are located in the same regions in the rats and in the mice and (2) there is an overlap between olfactory neurons expressing this protein and those affected by the toxicity of H₂S. Rats were exposed to H₂S - 200 ppm during 3 h, three consecutive days- and displayed symmetric acute segmental necrosis of the neurons and sustentacular cells of the olfactory epithelium in the dorsomedial nasal cavity. We found that expression of NQO1 in Sprague-Dawley rats spatially recapitulated that of the mouse. The degree of agreement or overlap between these two populations of neurons (necrosis vs. NQO1 expression) reached 80.2%. Although the underlying mechanisms accounted for the high sensitivity for NQO1 neurons -or the relative protection of non NQO1 neurons- to sulfide toxicity remain to be established, this observation is offering an intriguing approach that could be used to acutely suppress the pool of neural cells in olfactory zone I and to understand the mechanisms of toxicity and protection of other populations of neurons exposed to sulfide.

A ventral glomerular deficit in Parkinson's disease revealed by whole olfactory bulb reconstruction

Olfactory dysfunction is common in Parkinson's disease and is an early symptom, but its pathogenesis remains poorly understood. Hindering progress in our mechanistic understanding of olfactory dysfunction in Parkinson's disease is the paucity of literature about the human olfactory bulb, both from normal and Parkinson's disease cases. Qualitatively it is well established that the neat arrangement of the glomerular array seen in the mouse olfactory bulb is missing in humans. But rigorous quantitative approaches to describe and compare the thousands of glomeruli in the human olfactory bulb are not available. Here we report a quantitative approach to describe the glomerular component of the human olfactory bulb, and its application to draw statistical comparisons between olfactory bulbs from normal and Parkinson's disease cases. We subjected horizontal 10 µm sections of olfactory bulbs from six normal and five Parkinson's disease cases to fluorescence immunohistochemistry with antibodies against vesicular glutamate transporter-2 and neural cell adhesion molecule. We scanned the immunostained sections with a fluorescence slide scanner, segmented the glomeruli, and generated 3D reconstructions of whole olfactory bulbs. We document the occurrence of atypical glomerular morphologies and glomerular-like structures deep in the olfactory bulb, both in normal and Parkinson's disease cases. We define a novel and objective parameter: the global glomerular voxel volume, which is the total volume of all voxels that are classified immunohistochemically as glomerular. We find that the global glomerular voxel volume in Parkinson's disease cases is half that of normal cases. The distribution of glomerular voxels along the dorsal-ventral dimension of the olfactory bulb in these series of horizontal sections is significantly altered in Parkinson's disease cases: whereas most glomerular voxels reside within the ventral half of olfactory bulbs from normal cases, glomerular voxels are more evenly spread among the ventral and dorsal halves of olfactory bulbs from Parkinson's disease cases. These quantitative whole-olfactory bulb analyses indicate a predominantly ventral deficit in the glomerular component in Parkinson's disease, consistent with the olfactory vector hypothesis for the pathogenesis of this neurodegenerative disease. The distribution of serine 129-phosphorylated α-synuclein immunoreactive voxels correlates with that of glomerular voxels. The higher the serine 129-phosphorylated α-synuclein load of an olfactory bulb from a Parkinson's disease case, the lower the global glomerular voxel volume. Our rigorous quantitative approach to the whole olfactory bulb will help understand the anatomy and histology of the normal human olfactory bulb and its pathological alterations in Parkinson's disease.

Torpor: The Rise and Fall of 3-Monoiodothyronamine from Brain to Gut-From Gut to Brain?

3-Monoiodothyronamine (T1AM), first isolated from rat brain, is reported to be an endogenous, rapidly acting metabolite of thyroxine. One of its numerous effects is the induction of a "torpor-like" state in experimental animals. A critical analysis of T1AM, to serve as an endogenous cryogen, is given. The proposed biosynthetic pathway for formation of T1AM, which includes deiodinases and ornithine decarboxylase in the upper intestinum, is an unusual one. To reach the brain via systemic circulation, enterohepatic recycling and passage through the liver may occur. The possible role of gut microbiota is discussed. T1AM concentrations in human serum, measured by a specific monoclonal assay are up to three orders of magnitude higher compared to values obtained by MS/MS technology. The difference is explained by the presence of a high-affinity binder for T1AM (Apolipoprotein B-100) in serum, which permits the immunoassay to measure the total concentration of the analyte but limits MS/MS technology to detect only the unbound (free) analyte, a view, which is contested here.

Effects of Propofol General Anesthesia on Olfactory Relearning

How general anesthesia interferes with sensory processing to cause amnesia remains unclear. Here, we show that activation of a learning-associated immediate early gene in rat olfactory cortices is uninterrupted by propofol, an intravenous general anesthetic with putative actions on the inhibitory GABA_A receptors. Once learned under anesthesia, a novel odor can no longer re-activate the same high-level transcription programming during subsequent conscious relearning. Behavioral tests indicate that the animals’ ability to consciously relearn a pure odorant, first experienced under general anesthesia, is indeed compromised. In contrast, when a mixture of two novel odorants is first experienced under anesthesia and then relearned consciously in pairs with one of the components, the animals show a deficit in relearning only the component but not the mixture. Our results reveal a previously unknown mechanism of unconscious memory due to irreplaceable neuronal commitment under general anesthesia and support the notion that general anesthesia acts at stages beyond cellular coding to disrupt sensory integration for higher-order association.

Sensory Biology: Novel Peripheral Organization for Better Smell

Sensory systems have adopted various ways to enhance detection and discrimination. A recent study shows a novel spatial organization of sensory cells in the peripheral olfactory system in mice for better odor detection.

Combinatorial effects of odorants on mouse behavior

The mechanisms by which odors induce instinctive behaviors are largely unknown. Odor detection in the mouse nose is mediated by >1, 000 different odorant receptors (ORs) and trace amine-associated receptors (TAARs). Odor perceptions are encoded combinatorially by ORs and can be altered by slight changes in the combination of activated receptors. However, the stereotyped nature of instinctive odor responses suggests the involvement of specific receptors and genetically programmed neural circuits relatively immune to extraneous odor stimuli and receptor inputs. Here, we report that, contrary to expectation, innate odor-induced behaviors can be context-dependent. First, different ligands for a given TAAR can vary in behavioral effect. Second, when combined, some attractive and aversive odorants neutralize one another's behavioral effects. Both a TAAR ligand and a common odorant block aversion to a predator odor, indicating that this ability is not unique to TAARs and can extend to an aversive response of potential importance to survival. In vitro testing of single receptors with binary odorant mixtures indicates that behavioral blocking can occur without receptor antagonism in the nose. Moreover, genetic ablation of a single receptor prevents its cognate ligand from blocking predator odor aversion, indicating that the blocking requires sensory input from the receptor. Together, these findings indicate that innate odor-induced behaviors can depend on context, that signals from a single receptor can block innate odor aversion, and that instinctive behavioral responses to odors can be modulated by interactions in the brain among signals derived from different receptors.

Spatial Mapping in the Rat Olfactory Bulb by Odor and Direct Electrical Stimulation

OBJECTIVES

To directly measure the spatial mapping in the olfactory bulb by odor presentation and by direct electrical stimulation.

STUDY DESIGN

Experimental (animal).

SETTING

University research laboratory.

SUBJECTS AND METHODS

Odor (n = 8) and electrical stimulation (n = 4) of the olfactory bulb in rats were used to demonstrate the spatial mapping of neural responses in the olfactory bulb. Both multiunit responses to odor stimulation and evoked potential responses to localized electrical stimulation were measured in different regions of the olfactory bulb.

RESULTS

Responses that were recorded simultaneously from an array of 32 electrodes positioned at different locations within the olfactory bulb were mapped. Results show different spatial patterns of neural activity for different odors (odor maps). Direct stimulation of the olfactory bulb with electrical current pulses from electrodes positioned at different locations was also effective in generating spatial patterns of neural activity.

CONCLUSION

These data suggest that by programming an array of stimulating electrodes, it should be possible to selectively activate different regions of the olfactory bulb, generating unique patterns of neural activity as seen in normal smell.

Unravelling the Olfactory Sense: From the Gene to Odor Perception

Although neglected by science for a long time, the olfactory sense is now the focus of a panoply of studies that bring new insights and raises interesting questions regarding its functioning. The importance in the clarification of this process is of interest for science, but also motivated by the food and perfume industries boosted by a consumer society with increasingly demands for higher quality standards. In this review, a general overview of the state of art of science regarding the olfactory sense is presented with the main focus on the peripheral olfactory system. Special emphasis will be given to the deorphanization of the olfactory receptors (ORs), a critical issue because the specificity and functional properties of about 90% of human ORs remain unknown mainly due to the difficulties associated with the functional expression of ORs in high yields.

Timberol® Inhibits TAAR5-Mediated Responses to Trimethylamine and Influences the Olfactory Threshold in Humans

In mice, trace amine-associated receptors (TAARs) are interspersed in the olfactory epithelium and constitute a chemosensory subsystem that is highly specific for detecting volatile amines. Humans possess six putative functional TAAR genes. Human TAAR5 (hTAAR5) is highly expressed in the olfactory mucosa and was shown to be specifically activated by trimethylamine. In this study, we were challenged to uncover an effective blocker substance for trimethylamine-induced hTAAR5 activation. To monitor blocking effects, we recombinantly expressed hTAAR5 and employed a commonly used Cre-luciferase reporter gene assay. Among all tested potential blocker substances, Timberol®, an amber-woody fragrance, is able to inhibit the trimethylamine-induced hTAAR5 activation up to 96%. Moreover, human psychophysical data showed that the presence of Timberol® increases the olfactory detection threshold for the characteristic fishy odor of trimethylamine by almost one order of magnitude. In conclusion, our results show that among tested receptors Timberol® is a specific and potent antagonist for the hTAAR5-mediated response to trimethylamine in a heterologous system. Furthermore, our data concerning the observed shift of the olfactory detection threshold in vivo implicate that hTAAR5 or other receptors that may be inhibited by Timberol® could be involved in the high affinity olfactory perception of trimethylamine in humans.

Non-classical amine recognition evolved in a large clade of olfactory receptors

Biogenic amines are important signaling molecules, and the structural basis for their recognition by G Protein-Coupled Receptors (GPCRs) is well understood. Amines are also potent odors, with some activating olfactory trace amine-associated receptors (TAARs). Here, we report that teleost TAARs evolved a new way to recognize amines in a non-classical orientation. Chemical screens de-orphaned eleven zebrafish TAARs, with agonists including serotonin, histamine, tryptamine, 2-phenylethylamine, putrescine, and agmatine. Receptors from different clades contact ligands through aspartates on transmembrane α-helices III (canonical Asp^3.32) or V (non-canonical Asp^5.42), and diamine receptors contain both aspartates. Non-classical monoamine recognition evolved in two steps: an ancestral TAAR acquired Asp^5.42, gaining diamine sensitivity, and subsequently lost Asp^3.32. Through this transformation, the fish olfactory system dramatically expanded its capacity to detect amines, ecologically significant aquatic odors. The evolution of a second, alternative solution for amine detection by olfactory receptors highlights the tremendous structural versatility intrinsic to GPCRs.

DOI:http://dx.doi.org/10.7554/eLife.10441.001

Many organisms make molecules called biogenic amines. These molecules, which include the human hormones adrenaline and histamine, have important roles in regulating the biology and behaviour of many animals. Some biogenic amines bind to receptor proteins called GPCRs on the surface of cells. Many drugs can affect the activity of GPCRs, so understanding how different GPCRs work is an important goal of the pharmaceutical industry. Like all proteins, GPCRs are made of chains of molecules called amino acids. The GPCRs that can detect biogenic amines use a particular amino acid named Asp^3.32, and when this amino acid is mutated, these GPCRs become unable to bind to their target amine.

Trace amine-associated receptors (TAARs) are a type of GPCR that are found in many animals to detect odors. Most TAARs in mammals contain the Asp^3.32 residue, and recognize amine odors. However, many fish TAARs do not contain Asp^3.32, and it was not clear what molecules these fish receptors detect.

Here Li et al. find that these fish TAARs also recognize amines, and use a different amino acid called Asp^5.42. Also, some TAARs contain both Asp^3.32 and Asp^5.42, and recognize chemicals with two amines named diamines. Some diamines that bind to TAARs are foul smelling odors; for example, cadaverine and putrescine are repulsive smells emitted by decomposing flesh. In total, the experiments identified amines that can bind to eleven zebrafish TAARs that previously had no odor partner.

Li et al. propose that some fish TAARs lost the Asp^3.32 during the course of evolution to leave the Asp^5.42 as the main interaction site for amines. This change dramatically altered how these TAARs interact with amines, which probably expanded the number of different amines that fish can detect. These findings open up new ways to study how the fish brain processes information about its surroundings.

DOI:http://dx.doi.org/10.7554/eLife.10441.002

Human amygdala activations during nasal chemoreception

This review serves as a comprehensive discussion of chemosensory stimulation of the amygdala in healthy humans. Following an introduction of the neuroanatomy of chemosensory processing in primary and secondary olfactory structures, functional resonance magnetic imaging and positron imaging tomography studies are systematically categorized based on valence of stimuli, stimulus concentration, and paradigm-dependent amygdala activation. The amygdala shows patterns of lateralization due to stimulus valence. Main findings include pleasant odors being associated with bilateral or left amygdala activation, and unpleasant odors being associated with activation of the right amygdala, suggesting a crucial role of the right amygdala in evolutionary preservation. Potentially threatening social stimuli, however, might be processed apart from the olfactory system and tend to activate the left amygdala. Amygdala response to chemosensory stimuli correlated with simultaneous activation in the orbitofrontal cortex (OFC), piriform cortex (PC), and insula, suggesting a close-knit network of these areas during stimulus processing.

The smell of death: evidence that putrescine elicits threat management mechanisms

The ability to detect and respond to chemosensory threat cues in the environment plays a vital role in survival across species. However, little is known about which chemical compounds can act as olfactory threat signals in humans. We hypothesized that brief exposure to putrescine, a chemical compound produced by the breakdown of fatty acids in the decaying tissue of dead bodies, can function as a chemosensory warning signal, activating threat management responses (e.g., heightened alertness, fight-or-flight responses). This hypothesis was tested by gaging people's responses to conscious and non-conscious exposure to putrescine. In Experiment 1, putrescine increased vigilance, as measured by a reaction time task. In Experiments 2 and 3, brief exposure to putrescine (vs. ammonia and a scentless control condition) prompted participants to walk away faster from the exposure site. Experiment 3 also showed that putrescine elicited implicit cognitions related to escape and threat. Experiment 4 found that exposure to putrescine, presented here below the threshold of conscious awareness, increased hostility toward an out-group member. Together, the results are the first to indicate that humans can process putrescine as a warning signal that mobilizes protective responses to deal with relevant threats. The implications of these results are briefly discussed.