How smell develops

The neurobiological effects of senescence on dopaminergic system: A comprehensive review

Over time, the body undergoes a natural, multifactorial, and ongoing process named senescence, which induces changes at the molecular, cellular, and micro-anatomical levels in many body systems. The brain, being a highly complex organ, is particularly affected by this process, potentially impairing its numerous functions. The brain relies on chemical messengers known as neurotransmitters to function properly, with dopamine being one of the most crucial. This catecholamine is responsible for a broad range of critical roles in the central nervous system, including movement, learning, cognition, motivation, emotion, reward, hormonal release, memory consolidation, visual performance, sexual drive, modulation of circadian rhythms, and brain development. In the present review, we thoroughly examine the impact of senescence on the dopaminergic system, with a primary focus on the classic delimitations of the dopaminergic nuclei from A8 to A17. We provide in-depth information about their anatomy and function, particularly addressing how senescence affects each of these nuclei.

TMEM59 ablation leads to loss of olfactory sensory neurons and impairs olfactory functions via interaction with inflammation

The olfactory epithelium undergoes constant neurogenesis throughout life in mammals. Several factors including key signaling pathways and inflammatory microenvironment regulate the maintenance and regeneration of the olfactory epithelium. In this study, we identify TMEM59 (also known as DCF1) as a critical regulator to the epithelial maintenance and regeneration. Single-cell RNA-Seq data show downregulation of TMEM59 in multiple epithelial cell lineages with aging. Ablation of TMEM59 leads to apparent alteration at the transcriptional level, including genes associated with olfactory transduction and inflammatory/immune response. These differentially expressed genes are key components belonging to several signaling pathways, such as NF-κB, chemokine, etc. TMEM59 deletion impairs olfactory functions, attenuates proliferation, causes loss of both mature and immature olfactory sensory neurons, and promotes infiltration of inflammatory cells, macrophages, microglia cells and neutrophils into the olfactory epithelium and lamina propria. TMEM59 deletion deteriorates regeneration of the olfactory epithelium after injury, with significant reduction in the number of proliferative cells, immature and mature sensory neurons, accompanied by the increasing number of inflammatory cells and macrophages. Anti-inflammation by dexamethasone recovers neuronal generation and olfactory functions in the TMEM59-KO animals, suggesting the correlation between TMEM59 and inflammation in regulating the epithelial maintenance. Collectively, TMEM59 regulates olfactory functions, as well as neuronal generation in the olfactory epithelium via interaction with inflammation, suggesting a potential role in therapy against olfactory dysfunction associated with inflamm-aging.

OlfactionBase: a repository to explore odors, odorants, olfactory receptors and odorant-receptor interactions

Olfaction is a multi-stage process that initiates with the odorants entering the nose and terminates with the brain recognizing the odor associated with the odorant. In a very intricate way, the process incorporates various components functioning together and in synchronization. OlfactionBase is a free, open-access web server that aims to bring together knowledge about many aspects of the olfaction mechanism in one place. OlfactionBase contains detailed information of components like odors, odorants, and odorless compounds with physicochemical and ADMET properties, olfactory receptors (ORs), odorant- and pheromone binding proteins, OR-odorant interactions in Human and Mus musculus. The dynamic, user-friendly interface of the resource facilitates exploration of different entities: finding chemical compounds having desired odor, finding odorants associated with OR, associating chemical features with odor and OR, finding sequence information of ORs and related proteins. Finally, the data in OlfactionBase on odors, odorants, olfactory receptors, human and mouse OR-odorant pairs, and other associated proteins could aid in the inference and improved understanding of odor perception, which might provide new insights into the mechanism underlying olfaction. The OlfactionBase is available at https://bioserver.iiita.ac.in/olfactionbase/.

The role of the odorant receptors in the formation of the sensory map

In the olfactory system, odorant receptors (ORs) expressed at the cell membrane of olfactory sensory neurons detect odorants and direct sensory axons toward precise target locations in the brain, reflected in the presence of olfactory sensory maps. This dual role of ORs is corroborated by their subcellular expression both in cilia, where they bind odorants, and at axon terminals, a location suitable for axon guidance cues. Here, we provide an overview and discuss previous work on the role of ORs in establishing the topographic organization of the olfactory system and recent findings on the mechanisms of activation and function of axonal ORs.

Determination of quasi-primary odors by endpoint detection

It is known that there are no primary odors that can represent any other odors with their combination. Here, we propose an alternative approach: "quasi" primary odors. This approach comprises the following condition and method: (1) within a collected dataset and (2) by the machine learning-based endpoint detection. The quasi-primary odors are selected from the odors included in a collected odor dataset according to the endpoint score. While it is limited within the given dataset, the combination of such quasi-primary odors with certain ratios can reproduce any other odor in the dataset. To visually demonstrate this approach, the three quasi-primary odors having top three high endpoint scores are assigned to the vertices of a chromaticity triangle with red, green, and blue. Then, the other odors in the dataset are projected onto the chromaticity triangle to have their unique colors. The number of quasi-primary odors is not limited to three but can be set to an arbitrary number. With this approach, one can first find "extreme" odors (i.e., quasi-primary odors) in a given odor dataset, and then, reproduce any other odor in the dataset or even synthesize a new arbitrary odor by combining such quasi-primary odors with certain ratios.

Topographic organization in the olfactory bulb

The ability of the olfactory system to detect and discriminate a broad spectrum of odor molecules with extraordinary sensitivity relies on a wide range of odorant receptors and on the distinct architecture of neuronal circuits in olfactory brain areas. More than 1000 odorant receptors, distributed almost randomly in the olfactory epithelium, are plotted out in two mirror-symmetric maps of glomeruli in the olfactory bulb, the first relay station of the olfactory system. How does such a precise spatial arrangement of glomeruli emerge from a random distribution of receptor neurons? Remarkably, the identity of odorant receptors defines not only the molecular receptive range of sensory neurons but also their glomerular target. Despite their key role, odorant receptors are not the only determinant, since the specificity of neuronal connections emerges from a complex interplay between several molecular cues and electrical activity. This review provides an overview of the mechanisms underlying olfactory circuit formation. In particular, recent findings on the role of odorant receptors in regulating axon targeting and of spontaneous activity in the development and maintenance of synaptic connections are discussed.

Expansion of murine and human olfactory epithelium/mucosa colonies and generation of mature olfactory sensory neurons under chemically defined conditions

Olfactory dysfunctions, including hyposmia and anosmia, affect ~100 million people around the world and the underlying causes are not fully understood. Degeneration of olfactory sensory neurons and incapacity of globose basal cells to generate olfactory sensory neurons are found in elder people and patients with smell disorders. Thus, olfactory stem cell may function as a promising tool to replace inactivated globose basal cells and to generate sensory neurons. Methods: We established clonal expansion of cells from the murine olfactory epithelium as well as colony growth from human olfactory mucosa using Matrigel-based three-dimensional system. These colonies were characterized by immunostaining against olfactory epithelium cellular markers and by calcium imaging of responses to odors. Chemical addition was optimized to promote Lgr5 expression, colony growth and sensory neuron generation, tested by quantitative PCR and immunostaining against progenitor and neuronal markers. The differential transcriptomes in multiple signaling pathways between colonies under different base media and chemical cocktails were determined by RNA-Seq. Results: In defined culture media, we found that VPA and CHIR99021 induced the highest Lgr5 expression level, while LY411575 resulted in the most abundant yield of OMP⁺ mature sensory neurons in murine colonies. Different base culture media with drug cocktails led to apparent morphological alteration from filled to cystic appearance, accompanied with massive transcriptional changes in multiple signaling pathways. Generation of sensory neurons in human colonies was affected through TGF-β signaling, while Lgr5 expression and cell proliferation was regulated by VPA. Conclusion: Our findings suggest that targeting expansion of olfactory epithelium/mucosa colonies in vitro potentially results in discovery of new source to cell replacement-based therapy against smell loss.

Calcium Imaging in the Zebrafish

The zebrafish (Danio rerio) has emerged as a widely used model system during the last four decades. The fact that the zebrafish larva is transparent enables sophisticated in vivo imaging, including calcium imaging of intracellular transients in many different tissues. While being a vertebrate, the reduced complexity of its nervous system and small size make it possible to follow large-scale activity in the whole brain. Its genome is sequenced and many genetic and molecular tools have been developed that simplify the study of gene function in health and disease. Since the mid 90's, the development and neuronal function of the embryonic, larval, and later, adult zebrafish have been studied using calcium imaging methods. This updated chapter is reviewing the advances in methods and research findings of zebrafish calcium imaging during the last decade. The choice of calcium indicator depends on the desired number of cells to study and cell accessibility. Synthetic calcium indicators, conjugated to dextrans and acetoxymethyl (AM) esters, are still used to label specific neuronal cell types in the hindbrain and the olfactory system. However, genetically encoded calcium indicators, such as aequorin and the GCaMP family of indicators, expressed in various tissues by the use of cell-specific promoters, are now the choice for most applications, including brain-wide imaging. Calcium imaging in the zebrafish has contributed greatly to our understanding of basic biological principles during development and adulthood, and the function of disease-related genes in a vertebrate system.

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.

Data-driven nanomechanical sensing: specific information extraction from a complex system

Smells are known to be composed of thousands of chemicals with various concentrations, and thus, the extraction of specific information from such a complex system is still challenging. Herein, we report for the first time that the nanomechanical sensing combined with machine learning realizes the specific information extraction, e.g. alcohol content quantification as a proof-of-concept, from the smells of liquors. A newly developed nanomechanical sensor platform, a Membrane-type Surface stress Sensor (MSS), was utilized. Each MSS channel was coated with functional nanoparticles, covering diverse analytes. The smells of 35 liquid samples including water, teas, liquors, and water/EtOH mixtures were measured using the functionalized MSS array. We selected characteristic features from the measured responses and kernel ridge regression was used to predict the alcohol content of the samples, resulting in successful alcohol content quantification. Moreover, the present approach provided a guideline to improve the quantification accuracy; hydrophobic coating materials worked more effectively than hydrophilic ones. On the basis of the guideline, we experimentally demonstrated that additional materials, such as hydrophobic polymers, led to much better prediction accuracy. The applicability of this data-driven nanomechanical sensing is not limited to the alcohol content quantification but to various fields including food, security, environment, and medicine.

Odor identity coding by distributed ensembles of neurons in the mouse olfactory cortex

Olfactory perception and behaviors critically depend on the ability to identify an odor across a wide range of concentrations. Here, we use calcium imaging to determine how odor identity is encoded in olfactory cortex. We find that, despite considerable trial-to-trial variability, odor identity can accurately be decoded from ensembles of co-active neurons that are distributed across piriform cortex without any apparent spatial organization. However, piriform response patterns change substantially over a 100-fold change in odor concentration, apparently degrading the population representation of odor identity. We show that this problem can be resolved by decoding odor identity from a subpopulation of concentration-invariant piriform neurons. These concentration-invariant neurons are overrepresented in piriform cortex but not in olfactory bulb mitral and tufted cells. We therefore propose that distinct perceptual features of odors are encoded in independent subnetworks of neurons in the olfactory cortex.

Plasticity in the Olfactory System: Lessons for the Neurobiology of Memory

We are rapidly advancing toward an understanding of the molecular events underlying odor transduction, mechanisms of spatiotemporal central odor processing, and neural correlates of olfactory perception and cognition. A thread running through each of these broad components that define olfaction appears to be their dynamic nature. How odors are processed, at both the behavioral and neural level, is heavily dependent on past experience, current environmental context, and internal state. The neural plasticity that allows this dynamic processing is expressed nearly ubiquitously in the olfactory pathway, from olfactory receptor neurons to the higher-order cortex, and includes mechanisms ranging from changes in membrane excitability to changes in synaptic efficacy to neurogenesis and apoptosis. This review will describe recent findings regarding plasticity in the mammalian olfactory system that are believed to have general relevance for understanding the neurobiology of memory.

Role of ortho-retronasal olfaction in mammalian cortical evolution

Fossils of mammals and their extinct relatives among cynodonts give evidence of correlated transformations affecting olfaction as well as mastication, head movement, and ventilation, and suggest evolutionary coupling of these seemingly separate anatomical regions into a larger integrated system of ortho-retronasal olfaction. Evidence from paleontology and physiology suggests that ortho-retronasal olfaction played a critical role at three stages of mammalian cortical evolution: early mammalian brain development was driven in part by ortho-retronasal olfaction; the bauplan for neocortex had higher-level association functions derived from olfactory cortex; and human cortical evolution was enhanced by ortho-retronasal smell.

Tag1 deficiency results in olfactory dysfunction through impaired migration of mitral cells

The olfactory system provides mammals with the abilities to investigate, communicate and interact with their environment. These functions are achieved through a finely organized circuit starting from the nasal cavity, passing through the olfactory bulb and ending in various cortical areas. We show that the absence of transient axonal glycoprotein-1 (Tag1)/contactin-2 (Cntn2) in mice results in a significant and selective defect in the number of the main projection neurons in the olfactory bulb, namely the mitral cells. A subpopulation of these projection neurons is reduced in Tag1-deficient mice as a result of impaired migration. We demonstrate that the detected alterations in the number of mitral cells are well correlated with diminished odor discrimination ability and social long-term memory formation. Reduced neuronal activation in the olfactory bulb and the corresponding olfactory cortex suggest that Tag1 is crucial for the olfactory circuit formation in mice. Our results underpin the significance of a numerical defect in the mitral cell layer in the processing and integration of odorant information and subsequently in animal behavior.

Shh-proteoglycan interactions regulate maturation of olfactory glomerular circuitry

The olfactory system relies on precise circuitry connecting olfactory sensory neurons (OSNs) and appropriate relay and processing neurons of the olfactory bulb (OB). In mammals, the exact correspondence between specific olfactory receptor types and individual glomeruli enables a spatially precise map of glomerular activation that corresponds to distinct odors. However, the mechanisms that govern the establishment and maintenance of the glomerular circuitry are largely unknown. Here we show that high levels of Sonic Hedgehog (Shh) signaling at multiple sites enable refinement and maintenance of olfactory glomerular circuitry. Mice expressing a mutant version of Shh (Shh(Ala/Ala)), with impaired binding to proteoglycan co-receptors, exhibit disproportionately small olfactory bulbs containing fewer glomeruli. Notably, in mutant animals the correspondence between individual glomeruli and specific olfactory receptors is lost, as olfactory sensory neurons expressing different olfactory receptors converge on the same glomeruli. These deficits arise at late stages in post-natal development and continue into adulthood, indicating impaired pruning of erroneous connections within the olfactory bulb. In addition, mature Shh(Ala/Ala) mice exhibit decreased proliferation in the subventricular zone (SVZ), with particular reduction in neurogenesis of calbindin-expressing periglomerular cells. Thus, Shh interactions with proteoglycan co-receptors function at multiple locations to regulate neurogenesis and precise olfactory connectivity, thereby promoting functional neuronal circuitry.

Nuclear architecture as an epigenetic regulator of neural development and function

The nervous system of higher organisms is characterized by an enormous diversity of cell types that function in concert to carry out a myriad of neuronal functions. Differences in connectivity, and subsequent physiology of the connected neurons, are a result of differences in transcriptional programs. The extraordinary complexity of the nervous system requires an equally complex regulatory system. It is well established that transcription factor combinations and the organization of cis-regulatory sequences control commitment to differentiation programs and preserve a nuclear plasticity required for neuronal functions. However, an additional level of regulation is provided by epigenetic controls. Among various epigenetic processes, nuclear organization and the control of genome architecture emerge as an efficient and powerful form of gene regulation that meets the unique needs of the post-mitotic neuron. Here, we present an outline of how nuclear architecture affects transcription and provide examples from the recent literature where these principles are used by the nervous system.

Taste and smell function in chronic disease: a review of clinical and biochemical evaluations of taste and smell dysfunction in over 5000 patients at The Taste and Smell Clinic in Washington, DC

PURPOSE

To describe systematic methods developed over 40 years among over 5000 patients at The Taste and Smell Clinic in Washington, DC to evaluate taste and smell dysfunction.

MATERIALS AND METHODS

A tripartite methodology was developed. First, methods to determine clinical pathology underlying the multiple disease processes responsible for taste and smell dysfunction were developed. Second, methods to determine biochemical parameters responsible for these pathologies were developed. Third, methods to implement these techniques were developed to form a unified basis upon which treatment strategies can be developed to treat these patients.

RESULTS

Studies were performed in 5183 patients. Taste loss was present in 62% of patients, smell loss in 87%. Most patients with taste loss (52%) exhibited Type II hypogeusia; most patients with smell loss (56%) exhibited Type II hyposmia. Sensory distortions were present in 60%. Four common diagnostic entities were found: post influenza-type hyposmia and hypogeusia (27% of patients), idiopathic causes (16%), allergic rhinitis (15%) and post head injury (14%). Regardless of clinical diagnosis the major biochemical abnormality found in most patients (~70%) was diminished parotid salivary and nasal mucus secretion of cAMP and cGMP.

CONCLUSIONS

Taste and smell dysfunctions are common clinical problems associated with chronic disease processes. These symptoms require a systematic, integrated approach to understand their multiple and complex components. The approach presented here can and has led to effective treatment.

A TAP1 null mutation leads to an enlarged olfactory bulb and supernumerary, ectopic olfactory glomeruli

Major histocompatibility class I (MHCI) molecules are well known for their immunological role in mediating tissue graft rejection. Recently, these molecules were discovered to be expressed in distinct neuronal subclasses, dispelling the long-held tenet that the uninjured brain is immune-privileged. Here, we show that MHCI molecules are expressed in the main olfactory bulb (MOB) of adult animals. Furthermore, we find that mice with diminished levels of MHCI expression have enlarged MOBs containing an increased number of small, morphologically abnormal and ectopically located P2 glomeruli. These findings suggest that MHCI molecules may play an important role in the proper formation of glomeruli in the bulb.

Odorant receptors in the formation of the olfactory bulb circuitry

In mammals, smell is mediated by odorant receptors expressed by sensory neurons in the nose. These specialized receptors are found both on olfactory sensory neurons' cilia and axon terminals. Although the primary function of ciliary odorant receptors is to detect odorants, their axonal role remains unclear but is thought to involve axon guidance. This review discusses findings that show axonal odorant receptors are indeed functional and capable of modulating neural connectivity.

Odor memory stability after reinnervation of the olfactory bulb

The olfactory system, particularly the olfactory epithelium, presents a unique opportunity to study the regenerative capabilities of the brain, because of its ability to recover after damage. In this study, we ablated olfactory sensory neurons with methimazole and followed the anatomical and functional recovery of circuits expressing genetic markers for I7 and M72 receptors (M72-IRES-tau-LacZ and I7-IRES-tau-GFP). Our results show that 45 days after methimazole-induced lesion, axonal projections to the bulb of M72 and I7 populations are largely reestablished. Furthermore, regenerated glomeruli are re-formed within the same areas as those of control, unexposed mice. This anatomical regeneration correlates with functional recovery of a previously learned odorant-discrimination task, dependent on the cognate ligands for M72 and I7. Following regeneration, mice also recover innate responsiveness to TMT and urine. Our findings show that regeneration of neuronal circuits in the olfactory system can be achieved with remarkable precision and underscore the importance of glomerular organization to evoke memory traces stored in the brain.

Wired for behaviors: from development to function of innate limbic system circuitry

The limbic system of the brain regulates a number of behaviors that are essential for the survival of all vertebrate species including humans. The limbic system predominantly controls appropriate responses to stimuli with social, emotional, or motivational salience, which includes innate behaviors such as mating, aggression, and defense. Activation of circuits regulating these innate behaviors begins in the periphery with sensory stimulation (primarily via the olfactory system in rodents), and is then processed in the brain by a set of delineated structures that primarily includes the amygdala and hypothalamus. While the basic neuroanatomy of these connections is well-established, much remains unknown about how information is processed within innate circuits and how genetic hierarchies regulate development and function of these circuits. Utilizing innovative technologies including channel rhodopsin-based circuit manipulation and genetic manipulation in rodents, recent studies have begun to answer these central questions. In this article we review the current understanding of how limbic circuits regulate sexually dimorphic behaviors and how these circuits are established and shaped during pre- and post-natal development. We also discuss how understanding developmental processes of innate circuit formation may inform behavioral alterations observed in neurodevelopmental disorders, such as autism spectrum disorders, which are characterized by limbic system dysfunction.

Glomerular latency coding in artificial olfaction

Sensory perception results from the way sensory information is subsequently transformed in the brain. Olfaction is a typical example in which odor representations undergo considerable changes as they pass from olfactory receptor neurons (ORNs) to second-order neurons. First, many ORNs expressing the same receptor protein yet presenting heterogeneous dose–response properties converge onto individually identifiable glomeruli. Second, onset latency of glomerular activation is believed to play a role in encoding odor quality and quantity in the context of fast information processing. Taking inspiration from the olfactory pathway, we designed a simple yet robust glomerular latency coding scheme for processing gas sensor data. The proposed bio-inspired approach was evaluated using an in-house SnO₂ sensor array. Glomerular convergence was achieved by noting the possible analogy between receptor protein expressed in ORNs and metal catalyst used across the fabricated gas sensor array. Ion implantation was another technique used to account both for sensor heterogeneity and enhanced sensitivity. The response of the gas sensor array was mapped into glomerular latency patterns, whose rank order is concentration-invariant. Gas recognition was achieved by simply looking for a “match” within a library of spatio-temporal spike fingerprints. Because of its simplicity, this approach enables the integration of sensing and processing onto a single-chip.

Calcium imaging in the zebrafish

The zebrafish (Danio rerio) has emerged as a new model system during the last three decades. The fact that the zebrafish larva is transparent enables sophisticated in vivo imaging. While being the vertebrate, the reduced complexity of its nervous system and small size make it possible to follow large-scale activity in the whole brain. Its genome is sequenced and many genetic and molecular tools have been developed that simplify the study of gene function. Since the mid 1990s, the embryonic development and neuronal function of the larval, and later, adult zebrafish have been studied using calcium imaging methods. The choice of calcium indicator depends on the desired number of cells to study and cell accessibility. Dextran indicators have been used to label cells in the developing embryo from dye injection into the one-cell stage. Dextrans have also been useful for retrograde labeling of spinal cord neurons and cells in the olfactory system. Acetoxymethyl (AM) esters permit labeling of larger areas of tissue such as the tectum, a region responsible for visual processing. Genetically encoded calcium indicators have been expressed in various tissues by the use of cell-specific promoters. These studies have contributed greatly to our understanding of basic biological principles during development and adulthood, and of the function of disease-related genes in a vertebrate system.

Genetics of canine olfaction and receptor diversity

Olfaction is a particularly important sense in the dog. Humans selected for this capacity during the domestication process, and selection has continued to be employed to enhance this ability. In this review we first describe the different olfactory systems that exist and the different odorant receptors that are expressed in those systems. We then focus on the dog olfactory receptors by describing the olfactory receptor gene repertoire and its polymorphisms. Finally, we discuss the different uses of dog olfaction and the questions that still need to be studied.

Presynapses in Kenyon cell dendrites in the mushroom body calyx of Drosophila

Plastic changes at the presynaptic sites of the mushroom body (MB) principal neurons called Kenyon cells (KCs) are considered to represent a neuronal substrate underlying olfactory learning and memory. It is generally believed that presynaptic and postsynaptic sites of KCs are spatially segregated. In the MB calyx, KCs receive olfactory input from projection neurons (PNs) on their dendrites. Their presynaptic sites, however, are thought to be restricted to the axonal projections within the MB lobes. Here, we show that KCs also form presynapses along their calycal dendrites, by using novel transgenic tools for visualizing presynaptic active zones and postsynaptic densities. At these presynapses, vesicle release following stimulation could be observed. They reside at a distance from the PN input into the KC dendrites, suggesting that regions of presynaptic and postsynaptic differentiation are segregated along individual KC dendrites. KC presynapses are present in γ-type KCs that support short- and long-term memory in adult flies and larvae. They can also be observed in α/β-type KCs, which are involved in memory retrieval, but not in α'/β'-type KCs, which are implicated in memory acquisition and consolidation. We hypothesize that, as in mammals, recurrent activity loops might operate for memory retrieval in the fly olfactory system. The newly identified KC-derived presynapses in the calyx are, inter alia, candidate sites for the formation of memory traces during olfactory learning.

In vivo layer visualization of rat olfactory bulb by a swept source optical coherence tomography and its confirmation through electrocoagulation and anatomy

Here, we report in vivo 3-D visualization of the layered organization of a rat olfactory bulb (OB) by a swept source optical coherence tomography (SS-OCT). The SS-OCT operates at a wavelength of 1334 nm with respective theoretical depth and lateral resolutions of 6.7 μm and 15.4 μm in air and hence it is possible to get a 3D structural map of OB in vivo at the micron level resolution with millimeter-scale imaging depth. Up until now, with methods such as MRI, confocal microscopy, OB depth structure in vivo had not been clearly visualized as these do not satisfy the criterion of simultaneously providing micron-scale spatial resolution and imaging up to a few millimeter in depth. In order to confirm the OB's layered organization revealed by SS-OCT, we introduced the technique of electrocoagulation to make landmarks across the layered structure. To our knowledge this is such a first study that combines electrocoagulation and OCT in vivo of rat OB. Our results confirmed the layered organization of OB, and moreover the layers were clearly identified by electrocoagulation landmarks both in the OCT structural and anatomical slice images. We expect such a combined study is beneficial for both OCT and neuroscience fields.

Role of sensory activity on chemospecific populations of interneurons in the adult olfactory bulb

The olfactory bulb (OB) retains a remarkable capacity to renew its interneuronal populations throughout the lifespan of animals. Neuronal precursors giving rise to the bulbar interneurons are generated in the subventricular zone and have to migrate long distances before reaching the OB. In the adult OB these neuronal precursors differentiate into distinct neuronal types, including GABAergic cells located in the granule cell layer and a diverse set of neurons in the glomerular layer comprising GABAergic and dopaminergic interneurons, as well as other neuronal subtypes expressing calretinin and calbindin. While the role of sensory activity in the integration and/or survival of newly generated cells in the olfactory system is well established, very little is known about how odorant-induced activity affects fate specification of newborn cells as well as survival and fate maintenance of preexisting neuronal populations generated in adulthood. The present study demonstrates that sensory deprivation diminishes not only the number of newborn cells in the OB, but also reduces the density of granule and periglomerular cells generated before nostril occlusion. It also shows that sensory activity has an important influence on the development and expression of dopaminergic, but not GABAergic, calretinin or calbindin phenotypes. Our data reveal that odorant-induced activity is important for the survival of both newborn and preexisting OB interneurons generated at adulthood and suggests that these chemospecific populations are differentially affected by sensory deprivation.

The vomeronasal organ mediates interspecies defensive behaviors through detection of protein pheromone homologs

Potential predators emit uncharacterized chemosignals that warn receiving species of danger. Neurons that sense these stimuli remain unknown. Here we show that detection and processing of fear-evoking odors emitted from cat, rat, and snake require the function of sensory neurons in the vomeronasal organ. To investigate the molecular nature of the sensory cues emitted by predators, we isolated the salient ligands from two species using a combination of innate behavioral assays in naive receiving animals, calcium imaging, and c-Fos induction. Surprisingly, the defensive behavior-promoting activity released by other animals is encoded by species-specific ligands belonging to the major urinary protein (Mup) family, homologs of aggression-promoting mouse pheromones. We show that recombinant Mup proteins are sufficient to activate sensory neurons and initiate defensive behavior similarly to native odors. This co-option of existing sensory mechanisms provides a molecular solution to the difficult problem of evolving a variety of species-specific molecular detectors.

Integration of the antennal lobe glomeruli and three projection neurons in the standard brain atlas of the moth heliothis virescens

Digital three dimensional standard brain atlases (SBAs) are valuable tools for integrating neuroimaging data of different preparations. In insects, SBAs of five species are available, including the atlas of the female Heliothis virescens moth brain. Like for the other species, the antennal lobes (ALs) of the moth brain atlas were integrated as one material identity without internal structures. Different from the others, the H. virescens SBA exclusively included the glomerular layer of the AL. This was an advantage in the present study for performing a direct registration of the glomerular layer of individual preparations into the standard brain. We here present the H. virescens female SBA with a new model of the AL glomeruli integrated into the atlas, i.e. with each of the 66 glomeruli identified and labelled with a specific number. The new model differs from the previous H. virescens AL model both in respect to the number of glomeruli and the numbering system; the latter according to the system used for the AL atlases of two other heliothine species. For identifying female specific glomeruli comparison with the male AL was necessary. This required a new male AL atlas, included in this paper. As demonstrated by the integration of three AL projection neurons of different preparations, the new SBA with the integrated glomruli is a helpful tool for determining the glomeruli innervated as well as the relative position of the axonal projections in the protocerebrum.

Wiring Olfaction: The Cellular and Molecular Mechanisms that Guide the Development of Synaptic Connections from the Nose to the Cortex

Within the central nervous system, the olfactory system fascinates by its developmental and physiological particularities, and is one of the most studied models to understand the mechanisms underlying the guidance of growing axons to their appropriate targets. A constellation of contact-mediated (laminins, CAMs, ephrins, etc.) and secreted mechanisms (semaphorins, slits, growth factors, etc.) are known to play different roles in the establishment of synaptic interactions between the olfactory epithelium, olfactory bulb (OB) and olfactory cortex. Specific mechanisms of this system (including the amazing family of about 1000 different olfactory receptors) have been also proposed. In the last years, different reviews have focused in partial sights, specially in the mechanisms involved in the formation of the olfactory nerve, but a detailed review of the mechanisms implicated in the development of the connections among the different olfactory structures (olfactory epithelium, OB, olfactory cortex) remains to be written. In the present work, we afford this systematic review: the different cellular and molecular mechanisms which rule the formation of the olfactory nerve, the lateral olfactory tract and the intracortical connections, as well as the few data available regarding the accessory olfactory system. These mechanisms are compared, and the implications of the differences and similarities discussed in this fundamental scenario of ontogeny.

Olfactory sensitivity for alkylpyrazines-a comparative study in CD-1 mice and spider monkeys

Using a conditioning paradigm, the olfactory sensitivity of four CD-1 mice for six alkylpyrazines was investigated. With all six stimuli, the animals discriminated concentrations <or=0.1 ppm (parts per million) from the odorless solvent, and with three of the six stimuli the animals were even able to detect concentrations <or=0.1 ppb (parts per billion). Four spider monkeys tested in parallel were found to detect five of the same six stimuli at concentrations <1 ppm and with one stimulus they were able to discriminate concentrations <1 ppb from the solvent. The results showed CD-1 mice to be more sensitive than spider monkeys with five of the six alkylpyrazines tested. There was a significant positive correlation between sensitivity and the number of alkyl groups attached to the pyrazine (Pyr) ring in both species. A comparison of the detection thresholds obtained here to those obtained in human subjects suggests that neither the number of functional olfactory receptor genes nor the absolute or the relative size of the olfactory bulbs reliably predict a species' olfactory sensitivity. These threshold data may provide useful information for the choice of adequate stimulus concentrations in electrophysiological or imaging studies of the olfactory system or investigations of the discriminative abilities of mice and spider monkeys.

Olfactory discrimination of aliphatic odorants at 1 ppm: too easy for CD-1 mice to show odor structure-activity relationships?

Using an operant conditioning paradigm we tested the ability of CD-1 mice to discriminate between 25 odorants comprising members of five homologous series of aliphatic odorants (C4-C8) presented at a gas phase concentration of 1 ppm. We found (a) that all mice significantly discriminated between all 50 stimulus pairs that involved odorants sharing the same functional group, but differing in carbon chain length, as well as between all 50 stimulus pairs that involved odorants sharing the same carbon chain length but differing in functional group, (b) a significant negative correlation between discrimination performance and structural similarity of odorants in terms of differences in carbon chain length with the acetic esters and the 2-ketones, but not with the 1-alcohols, n-aldehydes, and n-carboxylic acids tested, and (c) that odorant pairs differing in functional group were significantly better discriminated than odorant pairs differing in carbon chain length. These findings demonstrate that CD-1 mice have excellent discrimination ability for structurally related aliphatic odorants, that correlations between discrimination performance and structural similarity of odorants are odorant class-specific rather than a general phenomenon, and that both carbon chain length and type of functional group play an important role for odor quality coding in mice.

Loss of short dystrophin isoform Dp71 in olfactory ensheathing cells causes vomeronasal nerve defasciculation in mouse olfactory system

The Duchenne muscular dystrophy (DMD) gene encodes dystrophin, which is a protein defective in DMD patients, as well as a number of shorter isoforms, which have been shown to be expressed in various non-muscle, primarily neural, tissues. As of yet, the physiological function of the various dystrophin isoforms is not fully understood. In the present study, we investigated the neurological phenotype that arises in the DMD-null mice, where expression of all dystrophin isoforms had been disrupted. We demonstrate that vomeronasal axons in the DMD-null mice are defasciculated, and some of the defasciculated vomeronasal axons aberrantly entered into the main olfactory bulb, which indicates that the product(s) of the DMD gene plays an important role in vomeronasal nerve organization. Through western blot and immunofluorescence analyses, we determined that the dystrophin isoform Dp71 was exclusively expressed in the mouse olfactory system: mainly in the olfactory ensheathing cells (OECs), an olfactory system-specific glia cell that ensheaths fascicles of the olfactory nerve. In the OECs, Dp71 was co-localized with beta-dystroglycan, utrophin, laminin, and perlecan. Since beta-dystroglycan and perlecan expression was decreased in the OECs of DMD-null mice, we hypothesize that Dp71 expressed in the OECs participates in fasciculation of the vomeronasal nerve, most likely through interactions with extracellular matrix.

Atypical expression of Drosophila gustatory receptor genes in sensory and central neurons

Members of the Drosophila gustatory receptor (Gr) gene family are generally expressed in chemosensory neurons and are known to mediate the perception of sugars, bitter substrates, CO(2), and pheromones. The Gr gene family consists of 68 members, many of which are organized in gene clusters of up to six genes, yet only expression of about 15 Gr genes has been characterized in detail prior to this study. Here we describe the first comprehensive expression analysis of six highly conserved Gr genes, Gr28a and Gr28b.a to Gr28b.e. Four of these Gr genes are not only expressed in the characteristic pattern associated with previously analyzed Gr genes-chemosensory neurons of the gustatory and olfactory system-but several other types of sensory neurons and neurons in the brain. Specifically, we show that several of the Gr28 genes are expressed in abdominal multidendritic neurons, putative hygroreceptive neurons of the arista, neurons associated with the Johnston's organ, peripheral proprioceptive neurons in the legs, neurons in the larval and adult brain, and oenocytes. Thus, our findings suggest that some Gr genes are utilized in nongustatory roles in the nervous system and tissues involved in proprioception, hygroreception, and other sensory modalities. It is also possible that the Gr28 genes have chemosensory roles in the detection of internal ligands.

V1R promoters are well conserved and exhibit common putative regulatory motifs

Background

The mouse vomeronasal organ (VNO) processes chemosensory information, including pheromone signals that influence reproductive behaviors. The sensory neurons of the VNO express two types of chemosensory receptors, V1R and V2R. There are ~165 V1R genes in the mouse genome that have been classified into ~12 divergent subfamilies. Each sensory neuron of the apical compartment of the VNO transcribes only one of the repertoire of V1R genes. A model for mutually exclusive V1R transcription in these cells has been proposed in which each V1R gene might compete stochastically for a single transcriptional complex. This model predicts that the large repertoire of divergent V1R genes in the mouse genome contains common regulatory elements. In this study, we have characterized V1R promoter regions by comparative genomics and by mapping transcription start sites.

Results

We find that transcription is initiated from ~1 kb promoter regions that are well conserved within V1R subfamilies. While cross-subfamily homology is not evident by traditional methods, we developed a heuristic motif-searching tool, LogoAlign, and applied this tool to identify motifs shared within the promoters of all V1R genes. Our motif-searching tool exhibits rapid convergence to a relatively small number of non-redundant solutions (97% convergence). We also find that the best motifs contain significantly more information than those identified in controls, and that these motifs are more likely to be found in the immediate vicinity of transcription start sites than elsewhere in gene blocks. The best motifs occur near transcription start sites of ~90% of all V1R genes and across all of the divergent subfamilies. Therefore, these motifs are candidate binding sites for transcription factors involved in V1R co-regulation.

Conclusion

Our analyses show that V1R subfamilies have broad and well conserved promoter regions from which transcription is initiated. Results from a new motif-finding algorithm, LogoAlign, designed for this context and more generally for searching large, hierarchical datasets, suggest the existence of common information-rich regulatory motifs that are shared across otherwise divergent V1R subfamilies.

An early critical period for long-term plasticity and structural modification of sensory synapses in olfactory cortex

Critical periods for plasticity of thalamic sensory inputs play an important role in developing neocortical circuits. During an early postnatal time window, pyramidal cells of visual, auditory, and somatosensory cortex undergo structural refinement and possess an enhanced ability for activity-dependent synaptic plasticity. In olfactory cortex, however, pyramidal cells receive direct sensory input from the olfactory bulb, and it is unclear whether the development of olfactory sensory circuits is governed by a critical period. Here, we show that NMDA receptor-dependent long-term potentiation and dendritic spine maturation occur only during a brief postnatal time window at sensory synapses of olfactory cortex pyramidal cells. In contrast, associational synapses onto the same cells retain the capacity for plasticity into adulthood.

Topological reorganization of odor representations in the olfactory bulb

Odors are initially represented in the olfactory bulb (OB) by patterns of sensory input across the array of glomeruli. Although activated glomeruli are often widely distributed, glomeruli responding to stimuli sharing molecular features tend to be loosely clustered and thus establish a fractured chemotopic map. Neuronal circuits in the OB transform glomerular patterns of sensory input into spatiotemporal patterns of output activity and thereby extract information about a stimulus. It is, however, unknown whether the chemotopic spatial organization of glomerular inputs is maintained during these computations. To explore this issue, we measured spatiotemporal patterns of odor-evoked activity across thousands of individual neurons in the zebrafish OB by temporally deconvolved two-photon Ca²⁺ imaging. Mitral cells and interneurons were distinguished by transgenic markers and exhibited different response selectivities. Shortly after response onset, activity patterns exhibited foci of activity associated with certain chemical features throughout all layers. During the subsequent few hundred milliseconds, however, MC activity was locally sparsened within the initial foci in an odor-specific manner. As a consequence, chemotopic maps disappeared and activity patterns became more informative about precise odor identity. Hence, chemotopic maps of glomerular input activity are initially transmitted to OB outputs, but not maintained during pattern processing. Nevertheless, transient chemotopic maps may support neuronal computations by establishing important synaptic interactions within the circuit. These results provide insights into the functional topology of neural activity patterns and its potential role in circuit function.

Many sensory brain areas contain topographic maps where the physical location of neuronal activity contains information about a stimulus feature. In the first central processing center of the olfactory pathway, the olfactory bulb, chemically distinct odors often elicit spatially segregated input activity so that general chemical features are initially represented in a topographic fashion. It is, however, unclear whether this “chemotopic” organization of odor representations is maintained at subsequent stages of odor processing. To address this question, we visualized activity patterns across thousands of individual neurons in the intact olfactory bulb of zebrafish over time using two-photon calcium imaging. Our results demonstrate that odor-evoked activity across the output neurons of the olfactory bulb is chemotopically organized shortly after stimulus onset but becomes more widely distributed during the subsequent few hundred milliseconds of the response. This reorganization of olfactory bulb output activity is most likely mediated by inhibitory feedback and reduces the redundancy in activity patterns evoked by related stimuli. These results indicate that topographically organized activity maps in the olfactory bulb are not maintained during information processing, but contribute to the function of local circuits.

Two-photon calcium imaging in the zebrafish olfactory bulb reveals that mitral cells show more selective responses to odors than interneurons, and odor-evoked firing patterns of populations of mitral cells evolve over hundreds of milliseconds to become more distinct for different odors, thus providing more information about odor identity.

Internal representations of smell in the Drosophila brain

Recent advances in sensory neuroscience using Drosophila olfaction as a model system have revealed brain maps representing the external world. Once we understand how the brain's built-in capability generates the internal olfactory maps, we can then elaborate how the brain computes and makes decision to elicit complex behaviors. Here, we review current progress in mapping Drosophila olfactory circuits and discuss their relationships with innate olfactory behaviors.

Semaphorin-1a controls receptor neuron-specific axonal convergence in the primary olfactory center of Drosophila

In the olfactory system of Drosophila, 50 functional classes of sensory receptor neurons (ORNs) project in a highly organized fashion into the CNS, where they sort out from one another and converge into distinct synaptic glomeruli. We identified the transmembrane molecule Semaphorin-1a (Sema-1a) as an essential component to ensure glomerulus-specific axon segregation. Removal of sema-1a in ORNs does not affect the pathfinding toward their target area but disrupts local axonal convergence into a single glomerulus, resulting in two distinct targeting phenotypes: axons either intermingle with adjacent ORN classes or segregate according to their odorant receptor identity into ectopic sites. Differential Sema-1a expression can be detected among neighboring glomeruli, and mosaic analyses show that sema-1a functions nonautonomously in ORN axon sorting. These findings provide insights into the mechanism by which afferent interactions lead to synaptic specificity in the olfactory system.

Leptin and its receptors are present in the rat olfactory mucosa and modulated by the nutritional status

Leptin is an adipocyte-derived cytokine that regulates body weight mainly via the long form of the leptin receptor (Ob-Rb). Leptin and its receptors are expressed in several tissues, suggesting that leptin might also be effective peripherally. We hypothesized that, as shown in taste cells, leptin and its receptors isoforms (Ob-Rs) could be present in the rat olfactory mucosa (OM). Using RT-PCR, light and electron microscopy immunohistochemistry (ICC), we found that different isoforms of the receptor were expressed in OM and localized in sustentacular cells and in a subpopulation of maturating neurons; in addition, immunoreactivity was also present in differentiated neurons and enriched at the cilia membranes, where the odorants bind to their receptors. Moreover, using RT-PCR, ICC and RIA measurements, we showed that leptin is synthesized locally in the olfactory mucosa. In addition, we demonstrate that fasting causes a significant enhanced transcription of both leptin and Ob-Rs in rat OM by quantitative RT-PCR data. Altogether, these results strongly suggested that leptin, acting as an endocrine or a paracrine factor, could be an important regulator of olfactory function, as a neuromodulator of the olfactory message in cilia of mature olfactory receptors neurons (ORN), but also for the homeostasis of this complex tissue, acting on differentiating neurons and on sustentacular cells.

Expression of pheromone receptor gene families during olfactory development in the mouse: expression of a V1 receptor in the main olfactory epithelium

In the mouse, two large gene families, V1R and V2R, encoding putative pheromone receptors have been described. Studies have suggested a homotypic recognition role for V1Rs and V2Rs during development in the targeting of vomeronasal axons to specific sets of glomeruli in the accessory olfactory bulb (AOB). Analysis of the onset of expression of the V1R and V2R gene families in developing vomeronasal neurons using polymerase chain reaction and in situ hybridization now suggests that a role for these receptors in the organization of axon projections is only likely at the final stages of targeting within the AOB. Surprisingly, our studies reveal expression of a V1Rd receptor in scattered cells within the main olfactory epithelium, suggesting that limited pheromone detection may also take place in this structure. The pheromone sensory neurons of the vomeronasal system and the neuroendocrine gonadotrophin-releasing hormone (GnRH) neurons that regulate fertility both arise from progenitor cells of the nasal placode. The development of these two cell types is intimately linked, and the GnRH neuron population migrates into the forebrain during embryogenesis in close association with a subset of vomeronasal sensory axons; how GnRH neurons recognize this axon subset is unknown. We report selective expression of a V1Ra gene in the clonal NLT GnRH cell line, raising the possibility of a similar role for V1Rs or V2Rs in the directed migration of GnRH neurons. However, no expression of this gene or of other V1Rs and V2Rs is detectable at the cellular level in migrating GnRH neurons in the mouse.

Morphological and molecular features of the mammalian olfactory sensory neuron axons: What makes these axons so special?

The main organization and gross morphology of the mammalian olfactory primary pathway, from the olfactory epithelium to the olfactory bulb, has been initially characterized using classical anatomical and ultrastructural approaches. During the last fifteen years, essentially thanks to the cloning of the odorant receptor genes, and to the characterization of a number of molecules expressed by the olfactory sensory neuron axons and their environment, significant new insights have been gained into the understanding of the development and adult functioning of this system. In the course of these genetic, biochemical and neuroanatomical studies, however, several molecular and structural features were uncovered that appear somehow to be unique to these axons. For example, these axons express odorant receptors in their terminal segment, and transport several mRNA species and at least two transcription factors. In the present paper, we review these unusual structural and molecular features and speculate about their possible functions in the development and maintenance of the olfactory system.

The alpha6 integrin subunit in the developing mouse olfactory bulb

Integrins are heterodimeric cell surface receptors that mediate developmental events by binding extracellular matrix ligands. Several lines of evidence suggest a role for integrins, specifically the alpha 6 subunit, in neuronal migration, neurite outgrowth, and axon guidance during olfactory development. Therefore, we undertook an analysis of the expression of the alpha 6 subunit in the olfactory system of the embryonic and early postnatal mouse to understand the role it may play during neural development. In addition, as a functional assay we examined the developmental effects of the loss of this subunit on olfactory development by analyzing an alpha 6 knockout (alpha 6-/-). Immunohistochemical analyses and confocal microscopy were used to examine alpha 6 expression in the CD-1 embryonic and early postnatal olfactory system and also to examine the organization of the olfactory system in the alpha 6-/- mouse. In CD-1 mice from E13 to E17, alpha 6 localizes in radial patterns extending from the core of the olfactory bulb to the nerve layer and colocalizes with RC2, an antibody specific for radial glia. By the day of birth (P0; approximately E19), expression is limited to the external plexiform layer and the olfactory nerve layer, where it colocalizes with laminin and p75. In the alpha 6-/- mouse, areas of ectopic granule cells were observed in the mitral cell layer of the olfactory bulb. These ectopias coincided with areas of disorganization of the radial glial processes and breaks in the mitral cell layer. These observations suggest a role for alpha 6 integrin in neural migration during olfactory development, likely secondary to organization of the radial glial scaffold.