Subsystem organization of the mammalian sense of smell

Olfactory receptors in neural regeneration in the central nervous system

Olfactory receptors are crucial for detecting odors and play a vital role in our sense of smell, influencing behaviors from food choices to emotional memories. These receptors also contribute to our perception of flavor and have potential applications in medical diagnostics and environmental monitoring. The ability of the olfactory system to regenerate its sensory neurons provides a unique model to study neural regeneration, a phenomenon largely absent in the central nervous system. Insights gained from how olfactory neurons continuously replace themselves and reestablish functional connections can provide strategies to promote similar regenerative processes in the central nervous system, where damage often results in permanent deficits. Understanding the molecular and cellular mechanisms underpinning olfactory neuron regeneration could pave the way for developing therapeutic approaches to treat spinal cord injuries and neurodegenerative diseases like Alzheimer's disease. Olfactory receptors are found in almost any cell of every organ/tissue of the mammalian body. This ectopic expression provides insights into the chemical structures that can activate olfactory receptors. In addition to odors, olfactory receptors in ectopic expression may respond to endogenous compounds and molecules produced by mucosal colonizing microbiota. The analysis of the function of olfactory receptors in ectopic expression provides valuable information on the signaling pathway engaged upon receptor activation and the receptor's role in proliferation and cell differentiation mechanisms. This review explores the ectopic expression of olfactory receptors and the role they may play in neural regeneration within the central nervous system, with particular attention to compounds that can activate these receptors to initiate regenerative processes. Evidence suggests that olfactory receptors could serve as potential therapeutic targets for enhancing neural repair and recovery following central nervous system injuries.

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.

Cystic fibrosis alters the structure of the olfactory epithelium and the expression of olfactory receptors affecting odor perception

A reduced sense of smell is a common condition in people with cystic fibrosis (CF) that negatively affects their quality of life. While often attributed to nasal mucosa inflammation, the underlying causes of the olfactory loss remain unknown. Here, we characterized gene expression in olfactory epithelium cells from patients with CF using single-nuclei RNA sequencing and found altered expression of olfactory receptors (ORs) and genes related to progenitor cell proliferation. We confirmed these findings in newborn, inflammation-free samples of a CF animal model and further identified ultrastructural alterations in the olfactory epithelium and bulbs of these animals. We established that CFTR, the anion channel whose dysfunction causes CF, is dispensable for odor-evoked signaling in sensory neurons, yet CF animals displayed defective odor-guided behaviors consistent with the morphological and molecular alterations. Our study highlights CF's major role in modulating epithelial structure and OR expression, shedding light on the mechanisms contributing to olfactory loss in CF.

Behavioral Assays in the Study of Olfaction

Olfaction is a fundamental sense in most animal species. In mammals, the olfactory system comprises several subpopulations of sensory neurons located throughout the nasal cavity, which detect a variety of chemical stimuli, including odorants, intraspecies, and interspecies chemical communication cues. Some of these compounds are important for regulating innate or learned behaviors and endocrine changes in response to other animals in the environment. With a particular focus on laboratory rodent species, this chapter provides a comprehensive description of the most important behavioral assays used for studying the olfactory system and is meant to be a practical guide for those who investigate olfaction-mediated behaviors or who have an interest in deciphering the molecular, cellular, or neural mechanisms through which the sense of smell controls the generation of adaptive behavioral outputs.

Calcium Imaging in Olfaction: Non-rodent Mammalian Species

The mammalian olfactory system plays a crucial role in detecting and transducing a diverse array of molecular cues present in the environment influencing in the display of behaviors such as feeding, reproductive function, and avoidance of predators. The recognition of these chemical signals primarily involves activation of olfactory and vomeronasal sensory neurons located in the main olfactory epithelium and the vomeronasal organ, respectively. Calcium imaging is a well-established method in rodents and insects to evaluate sensory cell activity, although its utilization in other species, particularly large mammals, is less prevalent. Here, we describe recent methodological advances using calcium imaging to scrutinize the response patterns of olfactory and vomeronasal sensory neurons isolated from goats and pigs, with potential applications in other species as well. Our approach reports cell responses to olfactory stimuli and provides a method for molecular characterization of the activated cells through immunolabeling. This methodology enables us to analyze the response patterns of single sensory cells aligning them with specific molecular profiles, providing a valuable tool for identifying mammalian chemosignals and their respective receptors.

Development of an rpS6-Based Ex Vivo Assay for the Analysis of Neuronal Activity in Mouse and Human Olfactory Systems

Olfactory sensitivity to odorant molecules is a complex biological function influenced by both endogenous factors, such as genetic background and physiological state, and exogenous factors, such as environmental conditions. In animals, this vital ability is mediated by olfactory sensory neurons (OSNs), which are distributed across several specialized olfactory subsystems depending on the species. Using the phosphorylation of the ribosomal protein S6 (rpS6) in OSNs following sensory stimulation, we developed an ex vivo assay allowing the simultaneous conditioning and odorant stimulation of different mouse olfactory subsystems, including the main olfactory epithelium, the vomeronasal organ, and the Grueneberg ganglion. This approach enabled us to observe odorant-induced neuronal activity within the different olfactory subsystems and to demonstrate the impact of environmental conditioning, such as temperature variations, on olfactory sensitivity, specifically in the Grueneberg ganglion. We further applied our rpS6-based assay to the human olfactory system and demonstrated its feasibility. Our findings show that analyzing rpS6 signal intensity is a robust and highly reproducible indicator of neuronal activity across various olfactory systems, while avoiding stress and some experimental limitations associated with in vivo exposure. The potential extension of this assay to other conditioning paradigms and olfactory systems, as well as its application to other animal species, including human olfactory diagnostics, is also discussed.

Object-oriented olfaction: challenges for chemosensation and for chemosensory research

Many animal species use olfaction to extract information about objects in their environment. Yet, the specific molecular signature that any given object emits varies due to various factors. Here, we detail why such variability makes chemosensory-mediated object recognition such a hard problem, and we propose that a major function of the elaborate chemosensory network is to overcome it. We describe previous work addressing different elements of the problem and outline future research directions that we consider essential for a full understanding of object-oriented olfaction. In particular, we call for extensive representation of olfactory object variability in chemical, behavioral, and electrophysiological analyses. While written with an emphasis on macrosmatic mammalian species, our arguments apply to all organisms that employ chemosensation to navigate complex environments.

The male's scent triggered a neural response in females despite ambiguous behavioral response in Asian house rats

Pheromones and olfactory communication play vital roles in sex recognition and mate choice in rodents. Asian house rats (Rattus tanezumi) (RT) often startle easily, making behavioral measurements difficult to carry out accurately in the laboratory. Here, the behavioral and olfactory preferences of the female RT between males and females were not observed using a conventional two-choice device; we then explored the neuro-immunohistochemical evidence in the brains of RT females. We found that male urine elicited significantly higher c-fos expression in the accessory olfactory system and sex-related brain regions in females than female urine did. On the other hand, the differences of volatile compounds and major urinary proteins (MUPs) in both voided urine and preputial glands (PGs) of the RT were detected using gas chromatography-mass spectrometer, sodium dodecyl sulfate-polyacrylamide gel electrophoresis, isoelectric focusing electrophoresis, and liquid chromatography-electrospray ionization mass spectrometry. We found that PG-derived 1-(4,5-dihydro-2-thiazolyl)-ethanone and total MUPs were more abundant in males versus females, suggesting these sexually divergent components might activate the female's accessory olfactory system. In conclusion, the neuro-immunohistochemical evidence indicated that potential sex pheromones might exist in RT; however, the strength of the chemical signal might be too weak to cause behavioral responses in females.

Phosphodiesterase 5A regulates the vomeronasal pump in mice

The vomeronasal organ (VNO) is a specialized chemoreceptive structure in many vertebrates that detects chemical stimuli, mostly pheromones, which often elicit innate behaviors such as mating and aggression. Previous studies in rodents have demonstrated that chemical stimuli are actively transported to the VNO via a blood vessel-based pumping mechanism, and this pumping mechanism is necessary for vomeronasal stimulation in behaving animals. However, the molecular mechanisms that regulate the vomeronasal pump remain mostly unknown. In this study, we observed a high level of expression of phosphodiesterase 5A (PDE5A) in the vomeronasal blood vessel of mice. We provided evidence to support the potential role of PDE5A in vomeronasal pump regulation. Local application of PDE5A inhibitors-sildenafil or tadalafil-to the vomeronasal organ (VNO) reduced stimulus delivery into the VNO, decreased the pheromone-induced activity of vomeronasal sensory neurons, and attenuated male-male aggressive behaviors. PDE5A is well known to play a role in regulating blood vessel tone in several organs. Our study advances our understanding of the molecular regulation of the vomeronasal pump.

Parenting behaviors in mice: Olfactory mechanisms and features in models of autism spectrum disorders

Rodents, along with numerous other mammals, heavily depend on olfactory cues to navigate their social interactions. Processing of olfactory sensory inputs is mediated by conserved brain circuits that ultimately trigger social behaviors, such as social interactions and parental care. Although innate, parenting is influenced by internal states, social experience, genetics, and the environment, and any significant disruption of these factors can impact the social circuits. Here, we review the molecular mechanisms and social circuits from the olfactory epithelium to central processing that initiate parental behaviors and their dysregulations that may contribute to the social impairments in mouse models of autism spectrum disorders (ASD). We discuss recent advances of the crucial role of olfaction in parental care, its consequences for social interactions, and the reciprocal influence on social interaction impairments in mouse models of ASD.

Traumatic stress-enhanced alcohol drinking: Sex differences and animal model perspectives

Purpose of review

Stress is associated with alcohol drinking, and epidemiological studies document the comorbidity of alcohol use disorder (AUD) and post-traumatic stress disorder (PTSD), with higher comorbid prevalence in females than in males. The aim of this paper is to highlight information related to sex differences in stress-enhanced alcohol drinking from clinical studies and from preclinical studies utilizing an animal model of traumatic stress.

Recent findings

Stress is associated with alcohol drinking and relapse in males and females, but there are sex differences in the alcohol-related adaptation of stress pathways and in the association of different prefrontal regions with stress-induced anxiety. The predator stress model of traumatic stress produced enhanced alcohol drinking in a subgroup of stress-sensitive male and female animals, which could be associated with sex and subgroup differences in stress axis responsivity, behavioral responses to predator odors, and epigenetic mechanisms engaged by traumatic experiences.

Summary

While additional studies in females are necessary, existing clinical and preclinical evidence suggests that biological mechanisms underlying stress-enhanced drinking likely differ between males and females. Thus, effective treatment strategies may differ between the sexes.

Constitutive and evoked release of ATP in adult mouse olfactory epithelium

In adult olfactory epithelium (OE), ATP plays a role in constant cell turnover and post-injury neuroregeneration. We previously demonstrated that constitutive and ATP-evoked ATP release are present in neonatal mouse OE and underlie continuous cell turn-over and post-injury neuroregeneration, and that activation of purinergic P2X7 receptors is involved in the evoked release. We hypothesized that both releases are present in adult mouse OE. To study the putative contribution of olfactory sensory neurons to ATP release, we used olfactory sensory neuronal-like OP6 cells derived from the embryonic olfactory placode cells. Calcium imaging showed that OP6 cells and primary adult OE cell cultures express functional purinergic receptors. We monitored ATP release from OP6 cells and whole adult OE turbinates using HEK cells as biosensors and luciferin-luciferase assays. Constitutive ATP release occurs in OP6 cells and whole adult mouse OE turbinates, and P2X7 receptors mediated evoked ATP release occurs only in turbinates. The mechanisms of ATP release described in the present study might underlie the constant cell turn-over and post-injury neuroregeneration present in adult OE and thus, further studies of these mechanisms are warranted as it will improve our knowledge of OE tissue homeostasis and post-injury regeneration.

Natural and Pathological Aging Distinctively Impacts the Pheromone Detection System and Social Behavior

Normal aging and many age-related disorders such as Alzheimer's disease cause deficits in olfaction; however, it is currently unknown how natural and pathological aging impacts the detection of social odors which might contribute to the impoverishment of social behavior at old age further worsening overall health. Analysis of the vomeronasal organ, the main gateway to pheromone-encoded information, indicated that natural and pathological aging distinctively affects the neurogenic ability of the vomeronasal sensory epithelium. Whereas cell proliferation remained majorly preserved in 1-year-old APP/PS1 mice, naturally aged animals exhibited significant deficiencies in the number of mature, proliferative, and progenitor cells. These alterations may support age-related deficits in the recognition of social cues and the display of social behavior. Our findings indicate that aging disrupts the processing of social olfactory cues decreasing social odor exploration, discrimination, and habituation in both wild-type senescent (2-year-old) mice and in 1-year-old double mutant model of Alzheimer's disease (APP/PS1). Furthermore, social novelty was diminished in 1-year-old APP/PS1 mice, indicating that alterations in the processing of social cues are accelerated during pathological aging. This study reveals fundamental differences in the cellular processes by which natural and pathological aging disrupts the exploration of social information and social behavior.

Sensory detection of female olfactory cues as a central regulator of energy metabolism and body weight in male mice

Olfactory stimuli from food influence energy balance, preparing the body for digestion when food is consumed. Social chemosensory cues predict subsequent energetic changes required for social interactions and could be an additional sensory input influencing energy balance. We show that exposure to female chemostimuli increases metabolic rate in male mice and reduces body weight and adipose tissue expansion when mice are fed a high-fat diet. These responses are linked to detection of female chemostimuli via G-protein Gαo-expressing vomeronasal sensory neurons. Males with Gαo deleted in the olfactory system are fertile but do not show changes in body weight when paired with females and show severely blunted changes in energy expenditure when exposed to female bedding. These results establish that metabolic and reproductive responses to females can be partly uncoupled in male mice and that detection of female chemostimuli is a central regulator of energy metabolism and lipid storage.

Lifespan extension in female mice by early, transient exposure to adult female olfactory cues

Several previous lines of research have suggested, indirectly, that mouse lifespan is particularly susceptible to endocrine or nutritional signals in the first few weeks of life, as tested by manipulations of litter size, growth hormone levels, or mutations with effects specifically on early-life growth rate. The pace of early development in mice can also be influenced by exposure of nursing and weanling mice to olfactory cues. In particular, odors of same-sex adult mice can in some circumstances delay maturation. We hypothesized that olfactory information might also have a sex-specific effect on lifespan, and we show here that the lifespan of female mice can be increased significantly by odors from adult females administered transiently, that is from 3 days until 60 days of age. Female lifespan was not modified by male odors, nor was male lifespan susceptible to odors from adults of either sex. Conditional deletion of the G protein Gαo in the olfactory system, which leads to impaired accessory olfactory system function and blunted reproductive priming responses to male odors in females, did not modify the effect of female odors on female lifespan. Our data provide support for the idea that very young mice are susceptible to influences that can have long-lasting effects on health maintenance in later life, and provide a potential example of lifespan extension by olfactory cues in mice.

Food preference acquired by social transmission is altered by the absence of the olfactory marker protein in mice

Food preference is conserved from the most primitive organisms to social animals including humans. A continuous integration of olfactory cues present both in food and in the different environmental and physiological contexts favors the intake of a given source of food or its avoidance. Remarkably, in mice, food preference can also be acquired by olfactory communication in-between conspecifics, a behavior known as the social transmission of food preference (STFP). STFP occurs when a mouse sniffs the breath of a conspecific who has previously eaten a novel food emitting specific odorants and will then develop a preference for this never encountered food. The efficient discrimination of odorants is performed by olfactory sensory neurons (OSNs). It is essential and supports many of the decision-making processes. Here, we found that the olfactory marker protein (OMP), an enigmatic protein ubiquitously expressed in all mature olfactory neurons, is involved in the fine regulation of OSNs basal activity that directly impacts the odorant discrimination ability. Using a previously described Omp null mouse model, we noticed that although odorants and their hedonic-associated values were still perceived by these mice, compensatory behaviors such as a higher number of sniffing events were displayed both in the discrimination of complex odorant signatures and in social-related contexts. As a consequence, we found that the ability to differentiate the olfactory messages carried by individuals such as those implicated in the social transmission of food preference were significantly compromised in Omp null mice. Thus, our results not only give new insights into the role of OMP in the fine discrimination of odorants but also reinforce the fundamental implication of a functional olfactory system for food decision-making.

Pheromone sensing in Drosophila requires support cell-expressed Osiris 8

Background

The nose of most animals comprises multiple sensory subsystems, which are defined by the expression of different olfactory receptor families. Drosophila melanogaster antennae contain two morphologically and functionally distinct subsystems that express odorant receptors (Ors) or ionotropic receptors (Irs). Although these receptors have been thoroughly characterized in this species, the subsystem-specific expression and roles of other genes are much less well-understood.

Results

Here we generate subsystem-specific transcriptomic datasets to identify hundreds of genes, encoding diverse protein classes, that are selectively enriched in either Or or Ir subsystems. Using single-cell antennal transcriptomic data and RNA in situ hybridization, we find that most neuronal genes—other than sensory receptor genes—are broadly expressed within the subsystems. By contrast, we identify many non-neuronal genes that exhibit highly selective expression, revealing substantial molecular heterogeneity in the non-neuronal cellular components of the olfactory subsystems. We characterize one Or subsystem-specific non-neuronal molecule, Osiris 8 (Osi8), a conserved member of a large, insect-specific family of transmembrane proteins. Osi8 is expressed in the membranes of tormogen support cells of pheromone-sensing trichoid sensilla. Loss of Osi8 does not have obvious impact on trichoid sensillar development or basal neuronal activity, but abolishes high sensitivity responses to pheromone ligands.

Conclusions

This work identifies a new protein required for insect pheromone detection, emphasizes the importance of support cells in neuronal sensory functions, and provides a resource for future characterization of other olfactory subsystem-specific genes.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12915-022-01425-w.

Olfactory modulation of the medial prefrontal cortex circuitry: Implications for social cognition

Olfactory dysfunction is manifested in a wide range of neurological and psychiatric diseases, and often emerges prior to the onset of more classical symptoms and signs. From a behavioral perspective, olfactory deficits typically arise in conjunction with impairments of cognition, motivation, memory, and emotion. However, a conceptual framework for explaining the impact of olfactory processing on higher brain functions in health and disease remains lacking. Here we aim to provide circuit-level insights into this question by synthesizing recent advances in olfactory network connectivity with other cortical brain regions such as the prefrontal cortex. We will focus on social cognition as a representative model for exploring and critically evaluating the relationship between olfactory cortices and higher-order cortical regions in rodent models. Although rodents do not recapitulate all dimensions of human social cognition, they have experimentally accessible neural circuits and well-established behavioral tests for social motivation, memory/recognition, and hierarchy, which can be extrapolated to other species including humans. In particular, the medial prefrontal cortex (mPFC) has been recognized as a key brain region in mediating social cognition in both rodents and humans. This review will highlight the underappreciated connectivity, both anatomical and functional, between the olfactory system and mPFC circuitry, which together provide a neural substrate for olfactory modulation of social cognition and social behaviors. We will provide future perspectives on the functional investigation of the olfactory-mPFC circuit in rodent models and discuss how to translate such animal research to human studies.

Low signaling efficiency from receptor to effector in olfactory transduction: A quantified ligand-triggered GPCR pathway

G protein-coupled receptor (GPCR) signaling is ubiquitous. As an archetype of this signaling motif, rod phototransduction has provided many fundamental, quantitative details, including a dogma that one active GPCR molecule activates a substantial number of downstream G protein/enzyme effector complexes. However, rod phototransduction is light-activated, whereas GPCR pathways are predominantly ligand-activated. Here, we report a detailed study of the ligand-triggered GPCR pathway in mammalian olfactory transduction, finding that an odorant-receptor molecule when (one-time) complexed with its most effective odorants produces on average much less than one downstream effector. Further experiments gave a nominal success probability of tentatively ∼10-4 (more conservatively, ∼10-2 to ∼10-5). This picture is potentially more generally representative of GPCR signaling than is rod phototransduction, constituting a paradigm shift.

Gene duplication, conservation, and divergence of activating transcription factor 5 gene in zebrafish

Activating transcription factor 5 (Atf5) is a member of the ATF/CREB family of transcription factors and involved in diverse cellular functions and diseases in mammals. However, the function of atf5 remains largely unknown in fish. Here, we report the expression pattern and function of duplicated atf5 genes in zebrafish. The results showed that the gene structures of zebrafish atf5a and atf5b were similar to their mammalian orthologs. Zebrafish Atf5a and Atf5b shared an amino acid sequence identity of 40.7%. Zebrafish atf5a and atf5b had maternal origin with dynamic expression during embryonic development. Zebrafish atf5a mRNA is mainly enriched in olfactory epithelium, midbrain, and hindbrain, while zebrafish atf5b mRNA is mainly detected in midbrain, hindbrain, and liver during embryogenesis. The results of acute hypoxia experiment showed that atf5a mRNA was significantly upregulated in the brain, liver, and muscle, while atf5b mRNA was just increased significantly in the brain. Functional analysis showed that knockdown of atf5a affects the development of the ciliated neurons in zebrafish embryos. The effect was enhanced when atf5a MO was co-injected with atf5b MO. The development of ciliated neurons in zebrafish embryos was not affected by injection of atf5b MO alone. atf5a knockdown also affects the development of early-born olfactory neurons. The effects caused by atf5a knockdown could be rescued by atf5b mRNA. These results suggest that the duplicated atf5 genes may have evolved divergently and play redundant biological roles in the development of olfactory sensory neurons in zebrafish.

Odor blocking of stress hormone responses

Scents have been employed for millennia to allay stress, but whether or how they might do so is largely unknown. Fear and stress induce increases in blood stress hormones controlled by hypothalamic corticotropin releasing hormone neurons (CRHNs). Here, we report that two common odorants block mouse stress hormone responses to three potent stressors: physical restraint, predator odor, and male-male social confrontation. One odorant inhibits restraint and predator odor activation of excitatory neurons upstream of CRHNs in the bed nucleus of the stria terminalis (BNSTa). In addition, both activate inhibitory neurons upstream of CRHNs in the hypothalamic ventromedial nucleus (VMH) and silencing of VMH inhibitory neurons hinders odor blocking of stress. Together, these findings indicate that odor blocking can occur via two mechanisms: (1) Inhibition of excitatory neurons that transmit stress signals to CRHNs and (2) activation of inhibitory neurons that act directly or indirectly to inhibit stressor activation of CRHNs.

Mapping transgene insertion sites reveals the α-Cre transgene expression in both developing retina and olfactory neurons

The Tg(Pax6-cre,GFP)^2Pgr (α-Cre) mouse is a commonly used Cre line thought to be retinal-specific. Using targeted locus amplification (TLA), we mapped the insertion site of the transgene, and defined primers useful to deduce zygosity. Further analyses revealed four tandem copies of the transgene. The insertion site mapped to clusters of vomeronasal and olfactory receptor genes. Using R26R and Ai14 Cre reporter mice, we confirmed retinal Cre activity, but also detected expression in Gα₀⁺ olfactory neurons. Most α-Cre⁺ olfactory neurons do not express Pax6, implicating the influence of neighboring regulatory elements. RT-PCR and buried food pellet test did not detect any effects of the transgene on flanking genes in the nasal mucosa and retina. Together, these data precisely map α-Cre, show that it does not affect surrounding loci, but reveal previously unanticipated transgene expression in olfactory neurons. The α-Cre mouse can be a valuable tool in both retinal and olfactory research.

The Pax6-α-Cre mouse line used in retinal studies actually contains four transgene insertion within gene clusters of olfactory and vomeronasal receptors, leading to expression in not just retinal, but also olfactory and vomeronasal sensory neurons.

Olfactory-induced locomotion in lampreys

The olfactory system allows animals to navigate in their environment to feed, mate, and escape predators. It is well established that odorant exposure or electrical stimulation of the olfactory system induces stereotyped motor responses in fishes. However, the neural circuitry responsible for the olfactomotor transformations is only beginning to be unraveled. A neural substrate eliciting motor responses to olfactory inputs was identified in the lamprey, a basal vertebrate used extensively to examine the neural mechanisms underlying sensorimotor transformations. Two pathways were discovered from the olfactory organ in the periphery to the brainstem motor nuclei responsible for controlling swimming. The first pathway originates from sensory neurons located in the accessory olfactory organ and reaches a single population of projection neurons in the medial olfactory bulb, which, in turn, transmit the olfactory signals to the posterior tuberculum and then to downstream brainstem locomotor centers. A second pathway originates from the main olfactory epithelium and reaches the main olfactory bulb, the neurons of which project to the pallium/cortex. The olfactory signals are then conveyed to the posterior tuberculum and then to brainstem locomotor centers. Olfactomotor behavior can adapt, and studies were aimed at defining the underlying neural mechanisms. Modulation of bulbar neural activity by GABAergic, dopaminergic, and serotoninergic inputs is likely to provide strong control over the hardwired circuits to produce appropriate motor behavior in response to olfactory cues. This review summarizes current knowledge relative to the neural circuitry producing olfactomotor behavior in lampreys and their modulatory mechanisms.

Modification of the Peripheral Olfactory System by Electronic Cigarettes

Electronic cigarettes (e-cigs) are used by millions of adolescents and adults worldwide. Commercial e-liquids typically contain flavorants, propylene glycol, and vegetable glycerin with or without nicotine. These chemical constituents are detected and evaluated by chemosensory systems to guide and modulate vaping behavior and product choices of e-cig users. The flavorants in e-liquids are marketing tools. They evoke sensory percepts of appealing flavors through activation of chemical sensory systems to promote the initiation and sustained use of e-cigs. The vast majority of flavorants in e-liquids are volatile odorants, and as such, the olfactory system plays a dominant role in perceiving these molecules that enter the nasal cavity either orthonasally or retronasally during vaping. In addition to flavorants, e-cig aerosol contains a variety of by-products generated through heating the e-liquids, including odorous irritants, toxicants, and heavy metals. These harmful substances can directly and adversely impact the main olfactory epithelium (MOE). In this article, we first discuss the olfactory contribution to e-cig flavor perception. We then provide information on MOE cell types and their major functions in olfaction and epithelial maintenance. Olfactory detection of flavorants, nicotine, and odorous irritants and toxicants are also discussed. Finally, we discuss the cumulated data on modification of the MOE by flavorant exposure and toxicological impacts of formaldehyde, acrolein, and heavy metals. Together, the information presented in this overview may provide insight into how e-cig exposure may modify the olfactory system and adversely impact human health through the alteration of the chemosensory factor driving e-cig use behavior and product selections. © 2021 American Physiological Society. Compr Physiol 11:2621-2644, 2021.

Lipidomic profile of human nasal mucosa and associations with circulating fatty acids and olfactory deficiency

The nasal mucosa (NM) contains olfactory mucosa which contributes to the detection of odorant molecules and the transmission of olfactory information to the brain. To date, the lipid composition of the human NM has not been adequately characterized. Using gas chromatography, liquid chromatography coupled to mass spectrometry and thin layer chromatography, we analyzed the fatty acids and the phospholipid and ceramide molecular species in adult human nasal and blood biopsies. Saturated and polyunsaturated fatty acids (PUFAs) accounted for 45% and 29% of the nasal total fatty acids, respectively. Fatty acids of the n-6 family were predominant in the PUFA subgroup. Linoleic acid and arachidonic acid (AA) were incorporated in the main nasal phospholipid classes. Correlation analysis revealed that the nasal AA level might be positively associated with olfactory deficiency. In addition, a strong positive association between the AA levels in the NM and in plasma cholesteryl esters suggested that this blood fraction might be used as an indicator of the nasal AA level. The most abundant species of ceramides and their glycosylated derivatives detected in NM contained palmitic acid and long-chain fatty acids. Overall, this study provides new insight into lipid species that potentially contribute to the maintenance of NM homeostasis and demonstrates that circulating biomarkers might be used to predict nasal fatty acid content.

Gucy2d selectively marks inhibitory dynorphin neurons in the spinal dorsal horn but is dispensable for pain and itch sensitivity

Introduction

Inhibitory neurons in the spinal dorsal horn can be classified based on expression of neurochemical marker genes. However, these marker genes are often expressed throughout the central nervous system, which poses challenges for manipulating genetically identified spinal neurons without undesired off-target effects.

Objectives

We investigated whether Gucy2d, previously identified as a highly selective marker of dynorphin-lineage neurons in the dorsal horn, is expressed in other locations within the adult mouse spinal cord, dorsal root ganglia (DRG), or brain. In addition, we sought to molecularly characterize Gucy2d-expressing dorsal horn neurons and investigate whether the disruption of Gucy2d gene expression affects sensitivity to itch or pain.

Methods

In situ hybridization experiments assessed Gucy2d mRNA expression in the adult mouse spinal cord, DRG, and brain, and its colocalization with Pax2, Bhlhb5, and Pde2a in dorsal horn neurons. Knockout mice lacking Gucy2d expression were compared with littermate controls to assess sensitivity to chloroquine-induced itch and dry skin-mediated chronic itch, as well as heat, cold, or mechanical stimuli.

Results

Gucy2d is selectively expressed in dynorphin-lineage neurons in lamina I-III of the adult mouse spinal cord but not in the brain or DRG. Spinal Gucy2d-expressing neurons are inhibitory neurons that also express the transcription factor Bhlhb5 and the cGMP-dependent phosphodiesterase Pde2a. Gucy2d knockout mice did not exhibit altered responses to itch or pain.

Conclusions

The selective expression of Gucy2d within a subpopulation of inhibitory dorsal horn neurons may yield a means to selectively manipulate inhibitory signaling at the level of the spinal cord without effects on the brain.

Age-dependent appearance of SARS-CoV-2 entry sites in mouse chemosensory systems reflects COVID-19 anosmia-ageusia symptoms

COVID-19 pandemic has given rise to a collective scientific effort to study its viral causing agent SARS-CoV-2. Research is focusing in particular on its infection mechanisms and on the associated-disease symptoms. Interestingly, this environmental pathogen directly affects the human chemosensory systems leading to anosmia and ageusia. Evidence for the presence of the cellular entry sites of the virus, the ACE2/TMPRSS2 proteins, has been reported in non-chemosensory cells in the rodent’s nose and mouth, missing a direct correlation between the symptoms reported in patients and the observed direct viral infection in human sensory cells. Here, mapping the gene and protein expression of ACE2/TMPRSS2 in the mouse olfactory and gustatory cells, we precisely identify the virus target cells to be of basal and sensory origin and reveal the age-dependent appearance of viral entry-sites. Our results propose an alternative interpretation of the human viral-induced sensory symptoms and give investigative perspectives on animal models.

Brechbühl et al characterise the gene and protein expression of ACE2/TMPRSS2 in the mouse olfactory and gustatory cells, which reveals that SARS-CoV-2 target cells are of basal and sensory origin. They also demonstrate an age-dependent appearance of viral entry-sites, which could inform the use of mouse models in the investigation of SARS-CoV-2 effects on olfaction.

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.

INPP5E controls ciliary localization of phospholipids and the odor response in olfactory sensory neurons

The lipid composition of the primary cilia membrane is emerging as a critical regulator of cilia formation, maintenance and function. Here, we show that conditional deletion of the phosphoinositide 5′-phosphatase gene Inpp5e, mutation of which is causative of Joubert syndrome, in terminally developed mouse olfactory sensory neurons (OSNs), leads to a dramatic remodeling of ciliary phospholipids that is accompanied by marked elongation of cilia. Phosphatidylinositol (4,5)-bisphosphate [PI(4,5)P₂], which is normally restricted to the proximal segment redistributed to the entire length of cilia in Inpp5e knockout mice with a reduction in phosphatidylinositol (3,4)-bisphosphate [PI(3,4)P₂] and elevation of phosphatidylinositol (3,4,5)-trisphosphate [PI(3,4,5)P₃] in the dendritic knob. The redistribution of phosphoinositides impaired odor adaptation, resulting in less efficient recovery and altered inactivation kinetics of the odor-evoked electrical response and the odor-induced elevation of cytoplasmic Ca²⁺. Gene replacement of Inpp5e through adenoviral expression restored the ciliary localization of PI(4,5)P₂ and odor response kinetics in OSNs. Our findings support the role of phosphoinositides as a modulator of the odor response and in ciliary biology of native multi-ciliated OSNs.

Summary: Cilia of olfactory sensory neurons have a unique lipid composition. Localization of phospholipids is controlled by the INPP5E phosphatase and is involved in modulation of the odor response.

Fast odour dynamics are encoded in the olfactory system and guide behaviour

Odours are transported in turbulent plumes, which result in rapid concentration fluctuations^1,2 that contain rich information about the olfactory scenery, such as the composition and location of an odour source^2-4. However, it is unclear whether the mammalian olfactory system can use the underlying temporal structure to extract information about the environment. Here we show that ten-millisecond odour pulse patterns produce distinct responses in olfactory receptor neurons. In operant conditioning experiments, mice discriminated temporal correlations of rapidly fluctuating odours at frequencies of up to 40 Hz. In imaging and electrophysiological recordings, such correlation information could be readily extracted from the activity of mitral and tufted cells-the output neurons of the olfactory bulb. Furthermore, temporal correlation of odour concentrations⁵ reliably predicted whether odorants emerged from the same or different sources in naturalistic environments with complex airflow. Experiments in which mice were trained on such tasks and probed using synthetic correlated stimuli at different frequencies suggest that mice can use the temporal structure of odours to extract information about space. Thus, the mammalian olfactory system has access to unexpectedly fast temporal features in odour stimuli. This endows animals with the capacity to overcome key behavioural challenges such as odour source separation⁵, figure-ground segregation⁶ and odour localization⁷ by extracting information about space from temporal odour dynamics.

Encoding innately recognized odors via a generalized population code

Odors carrying intrinsic values often trigger instinctive aversive or attractive responses. It is not known how innate valence is encoded. An intuitive model suggests that the information is conveyed through specific channels in hardwired circuits along the olfactory pathway, insulated from influences of other odors, to trigger innate responses. Here, we show that in mice, mixing innately aversive or attractive odors with a neutral odor and, surprisingly, mixing two odors with the same valence, abolish the innate behavioral responses. Recordings from the olfactory bulb indicate that odors are not masked at the level of peripheral activation and glomeruli independently encode components in the mixture. In contrast, crosstalk among the mitral and tufted (M/T) cells changes their patterns of activity such that those elicited by the mixtures can no longer be linearly decoded as separate components. The changes in behavioral and M/T cell responses are associated with reduced activation of brain areas linked to odor preferences. Thus, crosstalk among odor channels at the earliest processing stage in the olfactory pathway leads to re-coding of odor identity to abolish valence associated with the odors. These results are inconsistent with insulated labeled lines and support a model of a common mechanism of odor recognition for both innate and learned valence associations.

Olfactory subsystems associated with the necklace glomeruli in rodents

The necklace glomeruli are a loosely defined group of glomeruli encircling the caudal main olfactory bulb in rodents. Initially defined by the expression of various immunohistochemical markers, they are now better understood in the context of the specialized chemosensory neurons of the main olfactory epithelium and Grueneberg ganglion that innervate them. It has become clear that the necklace region of the rodent main olfactory bulb is composed of multiple distinct groups of glomeruli, defined at least in part by their afferent inputs. In this review, we will explore the necklace glomeruli and the chemosensory neurons that innervate them.

Olfaction across the water-air interface in anuran amphibians

Extant anuran amphibians originate from an evolutionary intersection eventually leading to fully terrestrial tetrapods. In many ways, they have to deal with exposure to both terrestrial and aquatic environments: (i) phylogenetically, as derivatives of the first tetrapod group that conquered the terrestrial environment in evolution; (ii) ontogenetically, with a development that includes aquatic and terrestrial stages connected via metamorphic remodeling; and (iii) individually, with common changes in habitat during the life cycle. Our knowledge about the structural organization and function of the amphibian olfactory system and its relevance still lags behind findings on mammals. It is a formidable challenge to reveal underlying general principles of circuity-related, cellular, and molecular properties that are beneficial for an optimized sense of smell in water and air. Recent findings in structural organization coupled with behavioral observations could help to understand the importance of the sense of smell in this evolutionarily important animal group. We describe the structure of the peripheral olfactory organ, the olfactory bulb, and higher olfactory centers on a tissue, cellular, and molecular levels. Differences and similarities between the olfactory systems of anurans and other vertebrates are reviewed. Special emphasis lies on adaptations that are connected to the distinct demands of olfaction in water and air environment. These particular adaptations are discussed in light of evolutionary trends, ontogenetic development, and ecological demands.

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.

Peripheral oxytocin injection modulates vomeronasal sensory activity and reduces pup-directed aggression in male mice

Behaviors are shaped by hormones, which may act either by changing brain circuits or by modifying sensory detection of relevant cues. Pup-directed behaviors have been previously shown to change via action of hormones at the brain level. Here, we investigated hormonal control of pup-induced activity in the vomeronasal organ, an olfactory sensory structure involved in the detection of non-volatile chemosignals. Vomeronasal activity decreases as males switch from a pup-aggressive state to a non-aggressive parenting state, after they socially contact a female. RNA sequencing, qPCR, and in situ hybridization were used to identify expression, in the vomeronasal sensory epithelium, of candidate GPCR hormone receptors chosen by in silico analyses and educated guesses. After identifying that oxytocin and vasopressin receptors are expressed in the vomeronasal organ, we injected the corresponding hormones in mice and showed that oxytocin administration reduced both pup-induced vomeronasal activity and aggressive behavior. Conversely, injection of an oxytocin receptor antagonist in female-primed male animals, which normally exhibit reduced vomeronasal activity, significantly increased the number of active vomeronasal neurons. These data link oxytocin to the modulation of olfactory sensory activity, providing a possible mechanism for changes in male behavior after social experience with females.

Amyotrophic Lateral Sclerosis Is Accompanied by Protein Derangements in the Olfactory Bulb-Tract Axis

Amyotrophic lateral sclerosis (ALS) is a fatal disease characterized by progressive muscle paralysis due to the degeneration of upper and lower motor neurons. Recent studies point out an involvement of the non-motor axis during disease progression. Despite smell impairment being considered a potential non-motor finding in ALS, the pathobiochemistry at the olfactory level remains unknown. Here, we applied an olfactory quantitative proteotyping approach to analyze the magnitude of the olfactory bulb (OB) proteostatic imbalance in ALS subjects (n = 12) with respect to controls (n = 8). Around 3% of the quantified OB proteome was differentially expressed, pinpointing aberrant protein expression involved in vesicle-mediated transport, macroautophagy, axon development and gliogenesis in ALS subjects. The overproduction of olfactory marker protein (OMP) points out an imbalance in the olfactory signal transduction in ALS. Accompanying the specific overexpression of glial fibrillary acidic protein (GFAP) and Bcl-xL in the olfactory tract (OT), a tangled disruption of signaling routes was evidenced across the OB–OT axis in ALS. In particular, the OB survival signaling dynamics clearly differ between ALS and frontotemporal lobar degeneration (FTLD), two faces of TDP-43 proteinopathy. To the best of our knowledge, this is the first report on high-throughput molecular characterization of the olfactory proteostasis in ALS.

Sensory Glia Detect Repulsive Odorants and Drive Olfactory Adaptation

Glia are typically considered as supporting cells for neural development and synaptic transmission. Here, we report an active role of a glia in olfactory transduction. As a polymodal sensory neuron in C. elegans, the ASH neuron is previously known to detect multiple aversive odorants. We reveal that the AMsh glia, a sheath for multiple sensory neurons including ASH, cell-autonomously respond to aversive odorants via G-protein-coupled receptors (GPCRs) distinct from those in ASH. Upon activation, the AMsh glia suppress aversive odorant-triggered avoidance and promote olfactory adaptation by inhibiting the ASH neuron via GABA signaling. Thus, we propose a novel two-receptor model where the glia and sensory neuron jointly mediate adaptive olfaction. Our study reveals a non-canonical function of glial cells in olfactory transduction, which may provide new insights into the glia-like supporting cells in mammalian sensory procession.

Representation of Olfactory Information in Organized Active Neural Ensembles in the Hypothalamus

The internal representation of sensory information via coherent activation of specific pathways in the nervous system is key to appropriate behavioral responses. Little is known about how chemical stimuli that elicit instinctive behaviors lead to organized patterns of activity in the hypothalamus. Here, we study how a wide range of chemosignals form a discernible map of olfactory information in the ventromedial nucleus of the hypothalamus (VMH) and show that different stimuli entail distinct active neural ensembles. Importantly, we demonstrate that this map depends on functional inputs from the vomeronasal organ. We present evidence that the spatial locations of active VMH ensembles are correlated with activation of distinct vomeronasal receptors and that disjunct VMH ensembles exhibit differential projection patterns. Moreover, active ensembles with distinct spatial locations are not necessarily associated with different behavior categories, such as defensive or social, calling for a revision of the currently accepted model of VMH organization.

Sensory neurons expressing the atypical olfactory receptor guanylyl cyclase D are required for the acquisition of odor preferences by mice in diverse social contexts

Animals use social communication to learn important information from conspecifics that can guide appropriate behavioral choices. For example, during the social transmission of food preference (STFP), conspecific semiochemicals detected by mouse olfactory sensory neurons (OSNs) expressing the atypical olfactory receptor guanylyl cyclase D (GC-D+ OSNs) promote the acquisition of food preferences in the recipient animal, mitigating the risk of ingesting food contaminated with toxins or pathogens. However, it is unclear if GC-D+ OSNs mediate preference learning outside this specific context. Here, we report that GC-D+ OSNs are required for the acquisition of odor preferences by both adult and juvenile mice, and that GC-DD-dependent preference could be formed for conditionally aversive odors. We used a two-choice olfactory behavioral test to assess odor preferences in adult Gucy2d +/+, +/- and -/- mice that encountered novel odors together with GC-D+ OSN stimuli (guanylin family peptides), during social investigation of a live conspecific, or during suckling as pups. Gucy2d +/+ and +/- mice (which express functional GC-D), but not Gucy2d -/- littermates, successfully acquire a preference for the demonstrated odor in any of these behavioral paradigms. Mice could even acquire a GC-D-dependent preference for odors to which they had recently formed a conditioned aversion. Together, these results demonstrate that GC-D+ OSNs mediate the acquisition of socially-transmitted odor preferences in different social and experiential contexts and at different life stages.

Conservation of Glomerular Organization in the Main Olfactory Bulb of Anuran Larvae

The glomerular array in the olfactory bulb of many vertebrates is segregated into molecularly and anatomically distinct clusters linked to different olfactory functions. In anurans, glomerular clustering is so far only described in Xenopus laevis. We traced olfactory projections to the bulb in tadpoles belonging to six distantly related anuran species in four families (Pipidae, Hylidae, Bufonidae, Dendrobatidae) and found that glomerular clustering is remarkably conserved. The general bauplan consists of four unequally sized glomerular clusters with minor inter-species variation. During metamorphosis, the olfactory system undergoes extensive remodeling. Tracings in metamorphotic and juvenile Dendrobates tinctorius and Xenopus tropicalis suggest a higher degree of variation in the glomerular organization after metamorphosis is complete. Our study highlights, that the anatomical organization of glomeruli in the main olfactory bulb (MOB) is highly conserved, despite an extensive ecomorphological diversification among anuran tadpoles, which suggests underlying developmental constraints.

The Vomeronasal System Can Learn Novel Stimulus Response Pairings

Behavioral responses can be classified as innate or learned and are often mediated by distinct neuronal pathways. In many animals, chemical cues are crucial for directing behaviors, and multiple chemosensory subsystems serve this purpose. The major subsystems in vertebrates are the main olfactory system (MOS) and the vomeronasal system (VNS). While the MOS has well-documented associative capabilities, the VNS is known for its role in mediating innate responses to sensory cues with clear ethological significance. However, it remains unknown whether the VNS can map arbitrary sensory activation to novel behavioral outputs. To address this question, we used several optogenetic strategies for selective vomeronasal activation and tested whether mice could associate stimulation patterns with particular reward locations. Our experiments indicate that mice can, indeed, exploit VNS activity to direct novel behavioral responses, implying that the VNS holds a substantial capacity for redirecting and adapting behavioral responses to given stimulation patterns.

Depletion of Ric-8B leads to reduced mTORC2 activity

mTOR, a serine/threonine protein kinase that is involved in a series of critical cellular processes, can be found in two functionally distinct complexes, mTORC1 and mTORC2. In contrast to mTORC1, little is known about the mechanisms that regulate mTORC2. Here we show that mTORC2 activity is reduced in mice with a hypomorphic mutation of the Ric-8B gene. Ric-8B is a highly conserved protein that acts as a non-canonical guanine nucleotide exchange factor (GEF) for heterotrimeric Gαs/olf type subunits. We found that Ric-8B hypomorph embryos are smaller than their wild type littermates, fail to close the neural tube in the cephalic region and die during mid-embryogenesis. Comparative transcriptome analysis revealed that signaling pathways involving GPCRs and G proteins are dysregulated in the Ric-8B mutant embryos. Interestingly, this analysis also revealed an unexpected impairment of the mTOR signaling pathway. Phosphorylation of Akt at Ser473 is downregulated in the Ric-8B mutant embryos, indicating a decreased activity of mTORC2. Knockdown of the endogenous Ric-8B gene in cultured cell lines leads to reduced phosphorylation levels of Akt (Ser473), further supporting the involvement of Ric-8B in mTORC2 activity. Our results reveal a crucial role for Ric-8B in development and provide novel insights into the signals that regulate mTORC2.

Gene inactivation in mice can be used to identify genes that are involved in important biological processes and that may contribute to disease. We used this approach to study the Ric-8B gene, which is highly conserved in mammals, including humans. We found that Ric-8B is essential for embryogenesis and for the proper development of the nervous system. Ric-8B mutant mouse embryos are smaller than their wild type littermates and show neural tube defects at the cranial region. This approach also allowed us to identify the biological pathways that potentially contribute to the observed phenotypes, and uncover a novel role for Ric-8B in the mTORC2 signaling pathway. mTORC2 plays particular important roles in the adult brain, and has been implicated in neurological disorders. Our mutant mice provide a model to study the complex molecular and cellular processes underlying the interplay between Ric-8B and mTORC2 in neuronal function.

Multi-glomerular projection of single olfactory receptor neurons is conserved among amphibians

Individual receptor neurons in the peripheral olfactory organ extend long axons into the olfactory bulb forming synapses with projection neurons in spherical neuropil regions, called glomeruli. Generally, odor map formation and odor processing in all vertebrates is based on the assumption that receptor neuron axons exclusively connect to a single glomerulus without any axonal branching. We comparatively tested this hypothesis in multiple fish and amphibian species (both sexes) by applying sparse cell electroporation to trace single olfactory receptor neuron axons. Sea lamprey (jawless fish) and zebrafish (bony fish) support the unbranched axon concept, with 94% of axons terminating in single glomeruli. Contrastingly, axonal projections of the axolotl (salamander) branch extensively before entering up to six distinct glomeruli. Receptor neuron axons labeled in frog species (Pipidae, Bufonidae, Hylidae, and Dendrobatidae) predominantly bifurcate before entering a glomerulus and 59 and 50% connect to multiple glomeruli in larval and postmetamorphotic animals, respectively. Independent of developmental stage, lifestyle, and adaptations to specific habitats, it seems to be a common feature of amphibian olfactory receptor neuron axons to frequently bifurcate and connect to multiple glomeruli. Our study challenges the unbranched axon concept as a universal vertebrate feature and it is conceivable that also later diverging vertebrates deviate from it. We propose that this unusual wiring logic evolved around the divergence of the terrestrial tetrapod lineage from its aquatic ancestors and could be the basis of an alternative way of odor processing.

Uncovering the mouse olfactory long non-coding transcriptome with a novel machine-learning model

Very little is known about long non-coding RNAs (lncRNAs) in the mammalian olfactory sensory epithelia. Deciphering the non-coding transcriptome in olfaction is relevant because these RNAs have been shown to play a role in chromatin modification and nuclear architecture reorganization, processes that accompany olfactory differentiation and olfactory receptor gene choice, one of the most poorly understood gene regulatory processes in mammals. In this study, we used a combination of in silico and ex vivo approaches to uncover a comprehensive catalogue of olfactory lncRNAs and to investigate their expression in the mouse olfactory organs. Initially, we used a novel machine-learning lncRNA classifier to discover hundreds of annotated and unannotated lncRNAs, some of which were predicted to be preferentially expressed in the main olfactory epithelium and the vomeronasal organ, the most important olfactory structures in the mouse. Moreover, we used whole-tissue and single-cell RNA sequencing data to discover lncRNAs expressed in mature sensory neurons of the main epithelium. Candidate lncRNAs were further validated by in situ hybridization and RT-PCR, leading to the identification of lncRNAs found throughout the olfactory epithelia, as well as others exquisitely expressed in subsets of mature olfactory neurons or progenitor cells.

Two Preputial Gland-Secreted Pheromones Evoke Sexually Dimorphic Neural Pathways in the Mouse Vomeronasal System

Hexadecanol (16OH) and hexadecyl acetate (16Ac) are two pheromones secreted in a large quantity by mouse preputial glands and act on male and female mice differentially. Yet the underlying molecular and cellular mechanisms remain to be elucidated. In this study, we examined the activation of vomeronasal sensory neurons (VSNs) by these two pheromones and mapped the downstream neural circuits that process and relay their chemosignals. Using the calcium imaging method and immunohistochemistry, we found that a small number of VSNs were activated by 16OH, 16AC, or both in the male and female mice, most of which were located apically in the vomeronasal epithelium, and their numbers did not increase when the concentrations of 16OH and 16Ac were raised by 10,000-fold except that of female VSNs in response to 16OH. In the accessory olfactory bulb (AOB), the two pheromones evoked more c-Fos+ neurons in the anterior AOB (aAOB) than in the posterior AOB (pAOB); and the increases in the number of c-Fos+ neurons in both aAOB and pAOB were dose-dependent; and between sexes, the female AOB responded more strongly to 16OH than to 16Ac whereas the male AOB had the opposite response pattern. This sexual dimorphism was largely retained in the downstream brain regions, including the bed nucleus of the stria terminalis (BNST), the medial amygdaloid nucleus (MeA), the posteromedial cortical amygdaloid nucleus (PMCo), the medial preoptic area (MPA), and the ventromedial hypothalamic nucleus (VmH). Taken together, out data indicate that there is one V1r receptor each for 16OH, 16Ac, or both, and that activation of these receptors evokes sexually dimorphic neural circuits, directing different behavioral outputs and possibly modulating other pheromone-induced responses.

Signal Detection and Coding in the Accessory Olfactory System

In many mammalian species, the accessory olfactory system plays a central role in guiding behavioral and physiological responses to social and reproductive interactions. Because of its relatively compact structure and its direct access to amygdalar and hypothalamic nuclei, the accessory olfactory pathway provides an ideal system to study sensory control of complex mammalian behavior. During the last several years, many studies employing molecular, behavioral, and physiological approaches have significantly expanded and enhanced our understanding of this system. The purpose of the current review is to integrate older and newer studies to present an updated and comprehensive picture of vomeronasal signaling and coding with an emphasis on early accessory olfactory system processing stages. These include vomeronasal sensory neurons in the vomeronasal organ, and the circuitry of the accessory olfactory bulb. Because the overwhelming majority of studies on accessory olfactory system function employ rodents, this review is largely focused on this phylogenetic order, and on mice in particular. Taken together, the emerging view from both older literature and more recent studies is that the molecular, cellular, and circuit properties of chemosensory signaling along the accessory olfactory pathway are in many ways unique. Yet, it has also become evident that, like the main olfactory system, the accessory olfactory system also has the capacity for adaptive learning, experience, and state-dependent plasticity. In addition to describing what is currently known about accessory olfactory system function and physiology, we highlight what we believe are important gaps in our knowledge, which thus define exciting directions for future investigation.

Critical review of electronic nose and tongue instruments prospects in pharmaceutical analysis

Electronic nose (enose, EN) and electronic tongue (etongue, ET) have been designed to simulate human senses of smell and taste in the best possible way. The signals acquired from a sensor array, combined with suitable data analysis system, are the basis for holistic analysis of samples. The efficiency of these instruments, regarding classification, discrimination, detection, monitoring and analytics of samples in different types of matrices, is utilized in many fields of science and industry, offering numerous practical applications. Popularity of both types of devices significantly increased during the last decade, mainly due to improvement of their sensitivity and selectivity. The electronic senses have been employed in pharmaceutical sciences for, among others, formulation development and quality assurance. This paper contains a review of some particular applications of EN and ET based instruments in pharmaceutical industry. In addition, development prospects and a critical summary of the state of art in the field were also surveyed.

G protein γ subunit Gγ13 is essential for olfactory function and aggressive behavior in mice

Most olfactory receptors in vertebrates are G protein-coupled receptors, whose activation by odorants initiates intracellular signaling cascades through heterotrimeric G proteins consisting of α, β, and γ subunits. Abolishment of the α subunits such as Gαolf in the main olfactory epithelium and Gαi2 and Gαo in the vomeronasal organ resulted in anosmia and/or impaired behavioral responses. In this study, we report that a G protein γ subunit, Gγ13, is expressed in a spatiotemporal manner similar to those of Gαolf and Gαi2 in the olfactory system and vomeronasal organ, respectively. In addition, Gγ13 was found in the glomeruli of the main olfactory bulb but was largely absent in the glomeruli of the accessory olfactory bulb. Using the Cre-loxP system, the Gγ13's gene Gng13 was nullified in the mature olfactory sensory neurons and apical vomeronasal sensory neurons where the Cre recombinase was expressed under the promoter of the Omp gene for the olfactory marker protein. Immunohistochemistry indicated much reduced expression of Gγ13 in the apical vomeronasal epithelium of the mutant mice. Behavioral experiments showed that the frequency and duration of aggressive encounters in the male mutant mice were significantly lower than in WT male mice. Taken together, these data suggest that the Gγ13 subunit is a critical signaling component in both the main olfactory epithelium and apical vomeronasal epithelium, and it plays an essential role in odor-triggered social behaviors including male-male aggression.

Anatomy of the olfactory mucosa

The classic notion that humans are microsmatic animals was born from comparative anatomy studies showing the reduction in the size of both the olfactory bulbs and the limbic brain relative to the whole brain. However, the human olfactory system contains a number of neurons comparable to that of most other mammals, and humans have exquisite olfactory abilities. Major advances in molecular and genetic research have resulted in the identification of extremely large gene families that express receptors for sensing odors. Such advances have led to a renaissance of studies focused on both human and nonhuman aspects of olfactory physiology and function. Evidence that olfactory dysfunction is among the earliest signs of a number of neurodegenerative and neuropsychiatric disorders has led to considerable interest in the use of olfactory epithelial biopsies for potentially identifying such disorders. Moreover, the unique features of the olfactory ensheathing cells have made the olfactory mucosa a promising and unexpected source of cells for treating spinal cord injuries and other neural injuries in which cell guidance is critical. The olfactory system of humans and other primates differs in many ways from that of other species. In this chapter we provide an overview of the anatomy of not only the human olfactory mucosa but of mucosae from a range of mammals from which more detailed information is available. Basic information regarding the general organization of the olfactory mucosa, including its receptor cells and the large number of other cell types critical for their maintenance and function, is provided. Cross-species comparisons are made when appropriate. The polemic issue of the human vomeronasal organ in both the adult and fetus is discussed, along with recent findings regarding olfactory subsystems within the nose of a number of mammals (e.g., the septal organ and Grüneberg ganglion).