Selectivity and response characteristics of human olfactory neurons

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

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

Coronavirus infection in chemosensory cells

Clinical manifestations of human coronavirus (HCoV)-related diseases are mostly related to the respiratory system, although secondary complications such as headache, anosmia, ageusia, and myalgia have been reported. HCoV infection and replication in chemosensory cells associated with ageusia and anosmia is poorly understood. Here, we characterized HCoV-OC43 and SARS-CoV-2 infection in two types of chemosensory cells, olfactory and taste cells, with their unique molecular and histological characteristics. We first assessed HCoV-OC43 infection in in vitro cultured human olfactory epithelial cells (hOECs) and fungiform taste papilla (HBO) cells. Interestingly, while both cell types were susceptible to HCoV-OC43 infection, viral replication rates were significantly reduced in HBO cells compared to hOECs. More interestingly, while culture media from hOECs was able to produce secondary infection in Vero cells, there was very limited secondary infection from HBO cells, suggesting that HBO cells may not be able to release infectious virus. On the other hand, unlike HCoV-OC43, SARS-CoV-2 showed comparable levels of viral infection rates in both hOECs and HBO cells. Furthermore, our RT-qPCR-based gene array studies revealed that several key genes involved in taste and olfactory functions were significantly altered by SARS-CoV-2 infection. These results may suggest a possible mechanism associated with chemosensory symptoms, such as anosmia and ageusia in patients infected with SARS-CoV-2.

Preparation of Human Olfactory Epithelial Biopsies for Downstream Analysis

The olfactory mucosa, lining a portion of the nasal cavity, houses the primary olfactory sensory neurons responsible for odor transduction, along with supporting cell populations. Tremendous advances have come from studying the peripheral olfactory system in animal models, especially the mouse. However, acquired human olfactory disorders lack effective therapies, and many of these conditions involve pathology in the olfactory mucosa. Thus, the ability to obtain human olfactory biopsy samples from subjects with olfactory dysfunction, or controls, may be of value. Here, we describe established techniques for collecting olfactory tissue from human subjects and preparing samples for downstream assays such as immunohistochemistry, flow cytometry, single-cell RNA-sequencing, or chromatin studies.

Primary Culture of the Human Olfactory Neuroepithelium and Utilization for Henipavirus Infection In Vitro

The olfactory receptor neurons (ORNs) are a unique cell type involved in the initial perception of odors. These specialized epithelial cells are located in the neuroepithelium of the nasal cavities and directly connect the nasal cavity with the central nervous system (CNS) via axons, which traverse the cribriform plate to synapse within the olfactory bulb. ORNs are derived from precursor cells that lie adjacent to the basal lamina of the olfactory epithelium. These precursor cells divide several times and their progeny differentiate into mature sensory neurons throughout life. In addition to its major and critical role in sensory transduction, the olfactory neuroepithelium may be an important tissue for viral replication and represents a potential site for viral entry into the CNS. In general, to gain access to the CNS, neurotropic viruses such as henipaviruses can use peripheral neural pathways or the circulatory system. However, the olfactory system has been reported to provide a portal of entry to the CNS for henipaviruses. The ability to obtain biopsies from living human subjects and culture these cells in the laboratory provides the opportunity to examine viral replication and effects on a neuronal cell population. As the most exposed and unprotected segment of the nervous system, the olfactory neuroepithelium may have an important role in neuropathology and systemic dissemination of viruses with established CNS effects. This chapter presents methods for primary culture of human ORNs, which have been used successfully by multiple investigators. The protocol provides a consistent, heterogeneous olfactory epithelial cell population, which demonstrates functional responses to odorant mixtures and exhibits several key features of the olfactory receptor neuron phenotype, encompassing olfactory receptors and signaling pathways.

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

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

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

Widespread Inhibition, Antagonism, and Synergy in Mouse Olfactory Sensory Neurons In Vivo

Sensory information is selectively or non-selectively enhanced and inhibited in the brain, but it remains unclear whether and how this occurs at the most peripheral level. Using in vivo calcium imaging of mouse olfactory bulb and olfactory epithelium in wild-type and mutant animals, we show that odors produce not only excitatory but also inhibitory responses in olfactory sensory neurons (OSNs). Heterologous assays indicate that odorants can act as agonists to some but inverse agonists to other odorant receptors. We also demonstrate that responses to odor mixtures are extensively suppressed or enhanced in OSNs. When high concentrations of odors are mixed, widespread antagonism suppresses the overall response amplitudes and density. In contrast, a mixture of low concentrations of odors often produces synergistic effects and boosts the faint odor inputs. Thus, odor responses are extensively tuned by inhibition, antagonism, and synergy at the most peripheral level, contributing to robust sensory representations.

Spanish Validation for Olfactory Function Testing Using the Sniffin' Sticks Olfactory Test: Threshold, Discrimination, and Identification

The assessment of olfactory function is becoming increasingly relevant, especially in cases of cognitive decline (i.e., neurodegenerative diseases), where olfactory alterations may be relevant as potential early biomarkers. The Sniffin' Sticks Olfactory Test, developed in Germany and validated in several countries, is an objective measure of olfactory performance. This study aims to validate this test in a Spanish sample. This study included 209 healthy normosmic volunteers (154 females and 55 males) aged between 20 to 79 years (mean age = 50.11 ± 15.18 years) as the normative sample. From this group, 22 participants were retested in order to obtain test-retest reliability evidence. Odor familiarity for descriptors in the olfactory identification test was also studied on an independent healthy sample (n = 69), and required cultural modifications were applied. Results indicate that men and women, as well as smokers and non-smokers, performed equally in every test. However, significant differences were found between age groups in every score. The general trend is that olfactory function progressively decreases as a function of age, the elderly group (+60 years) being the one with the lowest scores. In conclusion, this normative data, in addition to the test's cultural modifications, allows the Sniffin' Sticks Olfactory Test to be administered on a Spanish population.

Superior turbinate eosinophilia correlates with olfactory deficit in chronic rhinosinusitis patients

OBJECTIVE

To evaluate if molecular markers of eosinophilia in olfactory-enriched mucosa are associated with olfactory dysfunction.

STUDY DESIGN

Cross-sectional study of tissue biopsies from 99 patients, and an additional 30 patients who underwent prospective olfactory testing prior to sinonasal procedures.

METHODS

Tissue biopsies were processed for analysis of inflammatory markers using quantitative real time polymerase chain reaction (qRT-PCR). Ipsilateral olfactory performance was assessed using the Sniffin' Sticks (Burghart, Wedel, Germany) threshold component and the University of Pennsylvania Smell Identification Test (Sensonics, Haddon Heights, NJ). Age-adjusted data was correlated with inflammatory marker expression and clinical measures of obstruction from computed tomography and endoscopy.

RESULTS

Gene expression of the eosinophil marker CLC (Charcot Leyden crystal protein) was elevated in superior turbinate (ST) tissue in chronic rhinosinusitis (CRS) with nasal polyps (CRSwNP) compared to ST and inferior turbinate tissue in CRS without nasal polyps (CRSsNP) and control patients (all P < 0.001, respectively). CLC in ST tissue was correlated with IL-5 and eotaxin-1 expression (all P < 0.001; P = 0.65, and 0.49, respectively). CLC expression was strongly correlated with eosinophilic cationic protein levels (P < 0.001; r = -0.76), and ST CLC expression was inversely related to olfactory threshold (P = 0.002, r = -0.57) and discrimination scores (P = 0.05, r = -0.42). In multiple linear regression of CLC gene expression, polyp status, and radiographic and endoscopic findings with olfactory threshold, CLC was the only significantly correlated variable (P < 0.05).

CONCLUSION

Markers of eosinophils are elevated in the ST of patients with CRSwNP and correlate with olfactory loss. These findings support the hypothesis that olfactory dysfunction in CRS correlates local eosinophil influx into the olfactory cleft.

LEVEL OF EVIDENCE

NA. Laryngoscope, 127:2210-2218, 2017.

Helional induces Ca2+ decrease and serotonin secretion of QGP-1 cells via a PKG-mediated pathway

The secretion, motility and transport by intestinal tissues are regulated among others by specialized neuroendocrine cells, the so-called enterochromaffin (EC) cells. These cells detect different luminal stimuli, such as mechanical stimuli, fatty acids, glucose and distinct chemosensory substances. The EC cells react to the changes in their environment through the release of transmitter molecules, most importantly serotonin, to mediate the corresponding physiological response. However, little is known about the molecular targets of the chemical stimuli delivered from consumed food, spices and cosmetics within EC cells. In this study, we evaluated the expression of the olfactory receptor (OR) 2J3 in the human pancreatic EC cell line QGP-1 at the mRNA and protein levels. Using ratiofluorometric Ca(2+) imaging experiments, we demonstrated that the OR2J3-specific agonist helional induces a transient dose-dependent decrease in the intracellular Ca(2+) levels. This Ca(2+) decrease is mediated by protein kinase G (PKG) on the basis that the specific pharmacological inhibition of PKG with Rp-8-pCPT-cGMPS abolished the helional-induced Ca(2+) response. Furthermore, stimulation of QGP-1 cells with helional caused a dose-dependent release of serotonin that was comparable with the release induced by the application of a direct PKG activator (8-bromo-cGMP). Taken together, our results demonstrate that luminal odorants can be detected by specific ORs in QGP-1 cells and thus cause the directed release of serotonin and a PKG-dependent decrease in intracellular Ca(2.)

Sendai Virus Induces Persistent Olfactory Dysfunction in a Murine Model of PVOD via Effects on Apoptosis, Cell Proliferation, and Response to Odorants

Background

Viral infection is a common cause of olfactory dysfunction. The complexities of studying post-viral olfactory loss in humans have impaired further progress in understanding the underlying mechanism. Recently, evidence from clinical studies has implicated Parainfluenza virus 3 as a causal agent. An animal model of post viral olfactory disorders (PVOD) would allow better understanding of disease pathogenesis and represent a major advance in the field.

Objective

To develop a mouse model of PVOD by evaluating the effects of Sendai virus (SeV), the murine counterpart of Parainfluenza virus, on olfactory function and regenerative ability of the olfactory epithelium.

Methods

C57BL/6 mice (6–8 months old) were inoculated intranasally with SeV or ultraviolet (UV)-inactivated virus (UV-SeV). On days 3, 10, 15, 30 and 60 post-infection, olfactory epithelium was harvested and analyzed by histopathology and immunohistochemical detection of S-phase nuclei. We also measured apoptosis by TUNEL assay and viral load by real-time PCR. The buried food test (BFT) was used to measure olfactory function of mice at day 60. In parallel, cultured murine olfactory sensory neurons (OSNs) infected with SeV or UV-SeV were tested for odorant-mixture response by measuring changes in intracellular calcium concentrations indicated by fura-4 AM assay.

Results

Mice infected with SeV suffered from olfactory dysfunction, peaking on day 15, with no loss observed with UV-SeV. At 60 days, four out of 12 mice infected with SeV still had not recovered, with continued normal function in controls. Viral copies of SeV persisted in both the olfactory epithelium (OE) and the olfactory bulb (OB) for at least 60 days. At day 10 and after, both unit length labeling index (ULLI) of apoptosis and ULLI of proliferation in the SeV group was markedly less than the UV-SeV group. In primary cultured OSNs infected by SeV, the percentage of cells responding to mixed odors was markedly lower in the SeV group compared to UV-SeV (P = 0.007).

Conclusion

We demonstrate that SeV impairs olfaction, persists in OE and OB tissue, reduces their regenerative ability, and impairs the normal physiological function of OSNs without gross cytopathology. This mouse model shares key features of human post-viral olfactory loss, supporting its future use in studies of PVOD. Further testing and development of this model should allow us to clarify the pathophysiology of PVOD.

Office-based olfactory mucosa biopsies

BACKGROUND

Requests from researchers for olfactory mucosal biopsies are increasing as a result of advances in the fields of neuroscience and stem cell biology. Published studies report variable rates of success in obtaining true olfactory tissue, often below 50%. In cases where biopsies are not obtained carefully and confirmed through histological techniques, erroneous conclusions are made. Attention to the epithelium alone without submucosal analysis may add to the confusion. A consistent biopsy technique can help rhinologists obtain higher yields of olfactory mucosa. Confirmatory tissue staining analysis assures olfactory mucosa has been obtained, thereby strengthening clinical correlations and scientific conclusions.

METHODS

Biopsies of the septum within the anterior olfactory cleft were obtained under endoscopic guidance in an office procedure room using topical local anesthetic (lidocaine). After mucosal incision, a small, cupped, biopsy forceps was used to obtain specimens approximately 2 to 3 mm in size. Specimens were sectioned and analyzed with immunohistochemistry for presence of olfactory epithelium and/or olfactory fascicles.

RESULTS

A total of 14 subjects were biopsied in this analysis. Four subjects had biopsies in the operating room (OR). The remaining 10 underwent biopsies in the clinic. All biopsies obtained in the OR revealed evidence of olfactory mucosa. Of the 10 clinic biopsies, 8 (80%) revealed evidence of olfactory mucosa. No complications were encountered.

CONCLUSION

High yields of olfactory mucosa can be obtained safely in an office-based setting. Technique, including attention to the area of biopsy, and confirmatory analysis are important in assuring presence of olfactory tissue.

Damage to Olfactory Progenitor Cells Is Involved in Cigarette Smoke-Induced Olfactory Dysfunction in Mice

Exposure to cigarette smoke is a major cause of olfactory dysfunction. However, the underlying mechanisms by which cigarette smoke interferes with the highly regenerative olfactory nerve system remain unclear. To investigate whether cigarette smoke induces olfactory dysfunction by disrupting cell proliferation and cell survival in the olfactory epithelium (OE), we developed a mouse model of smoking that involved intranasal administration of a cigarette smoke solution (CSS). Immunohistological analyses and behavioral testing showed that CSS administration during a period of 24 days reduced the number of olfactory marker protein-positive mature olfactory receptor neurons (ORNs) in the OE and induced olfactory dysfunction. These changes coincided with a reduction in the number of SOX2(+) ORN progenitors and Ki-67(+) proliferating cells in the basal layer of the OE, an increase in the number of caspase-3(+) apoptotic cells, and an increase in the expression of mRNA for the inflammatory cytokines IL-1β and IL-6. Notably, the proliferating ORN progenitor population recovered after cessation of treatment with CSS, resulting in the subsequent restoration of mature ORN numbers and olfaction. These results suggest that SOX2(+) ORN progenitors are targets of CSS-induced impairment of the OE, and that by damaging the ORN progenitor population and increasing ORN death, CSS exposure eventually overwhelms the regenerative capacity of the epithelium, resulting in reduced numbers of mature ORNs and olfactory dysfunction.

[How we smell and what it means to us: basic principles of the sense of smell]

The origins of the sense of smell lie in the perception of environmental molecules and go back to unicellular organisms such as bacteria. Odors transmit a multitude of information about the chemical composition of our environment. The sense of smell helps people and animals with orientation in space, warns of potential threats, influences the choice of sexual partners, regulates food intake and influences feelings and social behavior in general. The perception of odors begins in sensory neurons residing in the olfactory epithelium that express G protein-coupled receptors, the so-called olfactory receptors. The binding of odor molecules to olfactory receptors initiates a signal transduction cascade that converts olfactory stimuli into electrical signals. These signals are then transmitted to the olfactory bulb, the first relay center in the olfactory pathway, via the axons of the sensory neurons. The olfactory information is processed in the bulb and then transferred to higher olfactory centers via axons of mitral cells, the bulbar projection neurons. This review describes the mechanisms involved in peripheral detection of odorants, outlines the further processing of olfactory information in higher olfactory centers and finally gives an overview of the overall significance of the ability to smell.

Mechanisms underlying odorant-induced and spontaneous calcium signals in olfactory receptor neurons of spiny lobsters, Panulirus argus

We determined if a newly developed antennule slice preparation allows studying chemosensory properties of spiny lobster olfactory receptor neurons under in situ conditions with Ca(2+) imaging. We show that chemical stimuli reach the dendrites of olfactory receptor neurons but not their somata, and that odorant-induced Ca(2+) signals in the somata are sufficiently stable over time to allow stimulation with a substantial number of odorants. Pharmacological manipulations served to elucidate the source of odorant-induced Ca(2+) transients and spontaneous Ca(2+) oscillations in the somata of olfactory receptor neurons. Both Ca(2+) signals are primarily mediated by an influx of extracellular Ca(2+) through voltage-activated Ca(2+) channels that can be blocked by CoCl2 and the L-type Ca(2+) channel blocker verapamil. Intracellular Ca(2+) stores contribute little to odorant-induced Ca(2+) transients and spontaneous Ca(2+) oscillations. The odorant-induced Ca(2+) transients as well as the spontaneous Ca(2+) oscillations depend on action potentials mediated by Na(+) channels that are largely TTX-insensitive but blocked by the local anesthetics tetracaine and lidocaine. Collectively, these results corroborate the conclusion that odorant-induced Ca(2+) transients and spontaneous Ca(2+) oscillations in the somata of olfactory receptor neurons closely reflect action potential activity associated with odorant-induced phasic-tonic responses and spontaneous bursting, respectively. Therefore, both types of Ca(2+) signals represent experimentally accessible proxies of spiking.

Time course of structural and functional maturation of human olfactory epithelial cells in vitro

The unique ability of olfactory neurons to regenerate in vitro has allowed their use for the study of olfactory function, regeneration, and neurodegenerative disorders; thus, characterization of their properties is important. This present study attempts to establish the timeline of structural (protein expression) and functional (odorant sensitivity) maturation of human olfactory epithelial cells (hOE) in vitro using biopsy-derived cultured tissue. Cells were grown for 7 days; on each day, cells were tested for odorant sensitivity using calcium imaging techniques and then protein expression of each cell was tested using immunocytochemistry for proteins typically used for characterizing olfactory cells. Previous studies have shown that mature olfactory neurons in vitro attain a unique "phase-bright" morphology and express the olfactory marker protein (OMP). By day 3 in vitro, a variety of cells were odorant-sensitive, including both "phase-bright" and "phase-dark" cells that have previously been considered glial-like cells. The functional maturation of these hOEs appears to take place within 4 days. Interestingly, the emergence of an odorant sensitivity profile of both phase-bright and phase-dark cells preceded the expression of marker protein expression for OMP (which is expressed only by mature neurons in vivo). This structural maturation took 5 days, suggesting that the development of odorant sensitivity is not coincident with the expression of marker molecules that are hallmarks of structural maturation. These results have important implications for the use of hOEs as in vitro models of olfactory and neuronal function.

Ca2+-activated Cl- channels of the ClCa family express in the cilia of a subset of rat olfactory sensory neurons

The Ca²⁺-activated Cl⁻ channel is considered a key constituent of odor transduction. Odorant binding to a specific receptor in the cilia of olfactory sensory neurons (OSNs) triggers a cAMP cascade that mediates the opening of a cationic cyclic nucleotide-gated channel (CNG), allowing Ca²⁺ influx. Ca²⁺ ions activate Cl⁻ channels, generating a significant Cl⁻ efflux, with a large contribution to the receptor potential. The Anoctamin 2 channel (ANO2) is a major constituent of the Cl⁻ conductance, but its knock-out has no impairment of behavior and only slightly reduces field potential odorant responses of the olfactory epithelium. Likely, an additional Ca²⁺-activated Cl⁻ channel of unknown molecular identity is also involved. In addition to ANO2, we detected two members of the ClCa family of Ca²⁺-activated Cl⁻ channels in the rat olfactory epithelium, ClCa4l and ClCa2. These channels, also expressed in the central nervous system, may correspond to odorant transduction channels. Whole Sprague Dawley olfactory epithelium nested RT-PCR and single OSNs established that the mRNAs of both channels are expressed in OSNs. Real time RT-PCR and full length sequencing of amplified ClCa expressed in rat olfactory epithelium indicated that ClCa4l is the most abundant. Immunoblotting with an antibody recognizing both channels revealed immunoreactivity in the ciliary membrane. Immunochemistry of olfactory epithelium and OSNs confirmed their ciliary presence in a subset of olfactory sensory neurons. The evidence suggests that ClCa4l and ClCa2 might play a role in odorant transduction in rat olfactory cilia.

Primary culture of the human olfactory neuroepithelium

The central cell type involved in the initial perception of odors and transduction of the sensory signal are the olfactory receptor neurons (ORNs) located in the olfactory neuroepithelium of the nasal cavities. The olfactory epithelium is a unique system similar to the neuroepithelium of the embryonic neural tube, in which new neurons are continually generated throughout adult life. Olfactory neurons are derived from precursor cells that lie adjacent to the basal lamina of the olfactory epithelium; these precursor cells divide several times and their progeny differentiate into mature sensory neurons throughout life. Thus, the human olfactory epithelium has the potential to be used as a tool to examine certain human disorders resulting from abnormal development of the nervous system. This chapter presents methods for primary culture of human ORNs, which have been used successfully by multiple investigators. The protocol provides a consistent, heterogeneous cell population, which demonstrates functional responses to odorant mixtures and exhibits a complex neuronal phenotype, encompassing receptors and signaling pathways pertinent to both olfaction and other aspects of CNS function. These cultured neural cells exhibit neurotransmitter pathways important in a number of neuropsychiatric disorders, and the ability to culture cells from living human subjects provides a tool for assessing cellular neuropathology at the individual patient level.

Concha bullosa surgery and the distribution of human olfactory neuroepithelium

In bullous middle turbinate surgery, controversy exists over which side of the bullous middle turbinate should be removed, as the distribution of human olfactory neuroepithelium is unclear. This study evaluated whether the middle turbinate tissue of patients undergoing endoscopic concha bullosa surgery contains functional olfactory epithelium. This prospective clinical study was conducted in tertiary referable center. It detected 70 conchae bullosa in 48 patients with sinonasal symptoms, who underwent paranasal computed tomography (CT) that showed pneumatization of the middle concha. All samples were obtained under general anesthesia. Three samples were obtained from each bullous middle turbinate: one each from the anterior, medial, and lateral portions. The mucosa from each sample was stained with olfactory marker protein (OMP). In total, 210 middle turbinate samples were taken from 48 patients during endoscopic surgery for conchae bullosa. The patients were 22 females and 26 males. Of the 70 conchae bullosa, OMP-stained nerve tissue was found in the lateral, anterior and medial aspects of 57 (81.4 %), 42 (60.0 %) and 23 (32.8 %) of the bullous middle turbinates, respectively. OMP-stained nerve tissue was found in 122 (58.1 %) of the 210 bullous middle turbinate tissue samples. OMP-stained nerve tissue was found on the lateral surface of the bullous middle turbinate more often than the medial surface. Therefore, during the concha bullosa surgery, OMP-stained nerve tissue found at least in the medial part of concha, suggested that the opening of the medial part of middle concha.

Age-associated loss of selectivity in human olfactory sensory neurons

We report a cross-sectional study of olfactory impairment with age based on both odorant-stimulated responses of human olfactory sensory neurons (OSNs) and tests of olfactory threshold sensitivity. A total of 621 OSNs from 440 subjects in 2 age groups of younger (≤ 45 years) and older (≥ 60 years) subjects were investigated using fluorescence intensity ratio fura-2 imaging. OSNs were tested for responses to 2 odorant mixtures, as well as to subsets of and individual odors in those mixtures. Whereas cells from younger donors were highly selective in the odorants to which they responded, cells from older donors were more likely to respond to multiple odor stimuli, despite a loss in these subjects' absolute olfactory sensitivity, suggesting a loss of specificity. This degradation in peripheral cellular specificity may impact odor discrimination and olfactory adaptation in the elderly. It is also possible that chronic adaptation as a result of reduced specificity contributes to observed declines in absolute sensitivity.

Human nasal olfactory epithelium as a dynamic marker for CNS therapy development

Discovery of new central nervous system (CNS) acting therapeutics has been slowed down by the lack of useful applicable biomarkers of disease or drug action often due to inaccessibility of relevant human CNS tissue and cell types. In recent years, non-neuronal cells, such as astrocytes, have been reported to play a highly significant role in neurodegenerative diseases, CNS trauma, as well as psychiatric disease and have become a target for small molecule and biologic therapies. We report the development of a method for measuring pharmacodynamic changes induced by potential CNS therapeutics using nasal olfactory neural tissue biopsy. We validated this approach using a potential astrocyte-targeted therapeutic, thiamphenicol, in a pre-clinical rodent study as well as a phase 1 human trial. In both settings, analysis of the olfactory epithelial tissue revealed biological activity of thiamphenicol at the drug target, the excitatory amino acid transporter 2 (EAAT2). Therefore, this biomarker approach may provide a reliable evaluation of CNS glial-directed therapies and hopefully improve throughput for nervous system drug discovery.

Imaging ensemble activity in arthropod olfactory receptor neurons in situ

We show that lobster olfactory receptor neurons (ORNs), much like their vertebrate counterparts, generate a transient elevation of intracellular calcium (Ca(i)) in response to odorant activation that can be used to monitor ensemble ORN activity. This is done in antennal slice preparation in situ maintaining the polarity of the cells and the normal micro-environment of the olfactory cilia. The Ca(i) signal is ligand-specific and increases in a dose-dependent manner in response to odorant stimulation. Saturating stimulation elicits a robust increase of up to 1 μM free Ca(i) within 1-2s of stimulation. The odor-induced Ca(i) response closely follows the discharge pattern of extracellular spikes elicited by odorant application, with the maximal rise in Ca(i) matching the peak of the spike generation. The Ca(i) signal can be used to track neuronal activity in a functional subpopulation of rhythmically active ORNs and discriminate it from that of neighboring tonically active ORNs. Being able to record from many ORNs simultaneously over an extended period of time not only allows more accurate estimates of neuronal population activity but also dramatically improves the ability to identify potential new functional subpopulations of ORNs, especially those with more subtle differences in responsiveness, ligand specificity, and/or transduction mechanisms.

Neuropathology of the olfactory mucosa in chronic rhinosinusitis

BACKGROUND

Chronic rhinosinusitis (CRS) is a complex heterogeneous inflammatory disease that affects the nasal cavity, but the pathological examination of the olfactory mucosa (OM) in this disease has been limited.

METHODS

Nasal biopsy specimens were obtained from 20 control subjects and 50 CRS patients in conjunction with clinical assessments. Histopathology of these nasal biopsy specimens was performed and immunohistochemistry was used to characterize nonneuronal, neuronal, and inflammatory cells in the OM. These OM characteristics were then evaluated to determine the degree to which pathological features may be related to smell loss in CRS.

RESULTS

Histopathological examination of control and CRS OM revealed changes in the normal pseudostratified olfactory epithelium (OE): intermixing of goblet cells, metaplasia to squamous-like cells, and erosion of the OE. Lower percentages of normal epithelium and olfactory sensory neurons were found in CRS OE compared with controls. Relative to other CRS patients, those with anosmia had the greatest amount of OE erosion, the highest density of eosinophils infiltrating the OE, and exhibited the most extensive abnormalities on CT and endoscopic examination, including being significantly more likely to exhibit nasal polyposis.

CONCLUSION

Our results suggest that OM pathology observed in nasal biopsy specimens can assist in understanding the degree of epithelial change and sensorineural damage in CRS and the potential for olfactory loss.

Cellular basis for the olfactory response to nicotine

Smokers regulate their smoking behavior on the basis of sensory stimuli independently of the pharmacological effects of nicotine (Rose J. E., et al. (1993) Pharmacol., Biochem. Behav.44 (4), 891-900). A better understanding of sensory mechanisms underlying smoking behavior may help to develop more effective smoking alternatives. Olfactory stimulation by nicotine makes up a considerable part of the flavor of tobacco smoke, yet our understanding of the cellular mechanisms responsible for olfactory detection of nicotine remains incomplete. We used biophysical methods to characterize the nicotine sensitivity and response mechanisms of neurons from olfactory epithelium. In view of substantial differences in the olfactory receptor repertoire between rodent and human (Mombaerts P. (1999) Annu. Rev. Neurosci.22, 487-509), we studied biopsied human olfactory sensory neurons (OSNs), cultured human olfactory cells (Gomez G., et al. (2000) J. Neurosci. Res.62 (5), 737-749), and rat olfactory neurons. Rat and human OSNs responded to S(-)-nicotine with a concentration dependent influx of calcium and activation of adenylate cyclase. Some rat OSNs displayed some stereoselectivity, with neurons responding to either enantiomer alone or to both. Freshly biopsied and primary cultured human olfactory neurons were less stereoselective. Nicotinic cholinergic antagonists had no effect on the responses of rat or human OSNs to nicotine. Patch clamp recording of rat OSNs revealed a nicotine-activated, calcium-sensitive nonspecific cation channel. These results indicate that nicotine activates a canonical olfactory receptor pathway rather than nicotinic cholinergic receptors on OSNs. Further, because the nicotine-sensitive mechanisms of rodents appear generally similar to those of humans, this animal model is an appropriate one for studies of nicotine sensation.

The human olfactory mucosa

Studies of the tissues of the human olfactory mucosa have been performed to investigate olfactory dysfunction and, more recently, olfactory mucosa has attracted a novel interest of investigators because it can be used as an early marker of neurodegenerative conditions of the brain and as a source of multipotent neural stem cells, with applications in regenerative medicine. The olfactory mucosa is readily available to the otolaryngologist, but the harvesting of this tissue must be safe, effective, and reliable, obtaining as little tissue as necessary, while avoiding unnecessary harm to the remaining olfactory tissue and function. The purpose of this review is to summarize the results of the most important studies and knowledge with regard to the human olfactory mucosa and its applications, emphasizing the issue of the distribution of the olfactory mucosa in the nasal cavities.

An odor-specific threshold deficit implicates abnormal intracellular cyclic AMP signaling in schizophrenia

OBJECTIVE

Although olfactory deficits are common in schizophrenia, their underlying pathophysiology remains unknown. Recent evidence has suggested that cAMP signaling may be disrupted in schizophrenia. Since cAMP mediates signal transduction in olfactory receptor neurons, this could contribute to the etiology of observed olfactory deficits. This study was designed to test this hypothesis by determining odor detection threshold sensitivities to two odorants that differ in their relative activations of this intracellular cAMP signaling cascade.

METHOD

Thirty schizophrenia patients, 25 healthy comparison subjects, and 19 unaffected first-degree relatives of schizophrenia patients were studied. Odor detection threshold sensitivities were measured for the two odorants citralva and lyral. Although both have fruity/floral scents, citralva strongly activates adenylyl cyclase to increase cAMP levels, while lyral is a very weak activator of adenylyl cyclase.

RESULTS

There was a significant group-by-odor interaction. Both schizophrenia patients and unaffected first-degree relatives were impaired in their ability to detect lyral versus citralva. Comparison subjects were equally sensitive to both odorants. This selective deficit could not be explained by differences in age, sex, smoking, clinical symptom profile, or medication use.

CONCLUSIONS

This study establishes the presence of an odor-specific hyposmia that may denote a disruption of cAMP-mediated signal transduction in schizophrenia. The presence of a parallel deficit in the patients' unaffected first-degree relatives suggests that this deficit is genetically mediated. Although additional physiological studies are needed to confirm the underlying mechanism, these results offer strong inferential support for the hypothesis that cAMP signaling is dysregulated in schizophrenia.

Human olfactory epithelial cells generated in vitro express diverse neuronal characteristics

The olfactory epithelium constitutes the sole source of regenerating neural cells that can be obtained from a living human. As such, primary cultures derived from human olfactory epithelial biopsies can be utilized to study neurobiological characteristics of individuals under different conditions and disease states. Here, using such human cultures, we report in vitro generation of cells that exhibit a complex neuronal phenotype, encompassing receptors and signaling pathways pertinent to both olfaction and other aspects of CNS function. Using in situ hybridization, we demonstrate for the first time the native expression of olfactory receptors in cultured cells derived from human olfactory epithelial tissue. We further establish the presence and function of olfactory transduction molecules in these cells using immunocytochemistry, calcium imaging and molecular methods. Western blot analysis revealed the expression of neurotransmitter receptors for dopamine (D2R), 5-HT (5HT2C) and NMDA subtypes 1 and 2A/2B. Stimulation with dopamine or 5-HT enhanced receptor G protein activation in a subtype specific manner, based on 35S-guanosine triphosphate incorporation assay. Functional characteristics of the cultured cells are demonstrated through enhanced tyrosine phosphorylation of NMDAR 2A/2B and recruitment of signaling partners in response to NMDA stimulation. The array of neuronal characteristics observed here establishes that proliferating cells derived from the human olfactory epithelium differentiate in vitro to express functional and molecular attributes of mature olfactory neurons. These cultured neural cells exhibit neurotransmitter pathways important in a number of neuropsychiatric disorders. Their ready availability from living humans thus provides a new tool to link functional and molecular features of neural cells with clinical characteristics of individual living patients.

The peripheral olfactory system of the domestic chicken: physiology and development

Olfaction is a ubiquitous sensory system found in all terrestrial vertebrates. Birds use olfaction for several important activities such as feeding and mating; thus, understanding bird biology would also require the systematic study olfaction. In addition, the olfactory system has several unique features that are useful for the study of nervous system function and development, including a large multigene family for olfactory receptor expression, peripheral neurons that regenerate, and a complex system for sensory innervation of the olfactory bulb. We focused on physiological, anatomical and behavioral approaches to study the chick olfactory neurons and the olfactory bulb. Chick olfactory neurons displayed some properties similar to those found in mature neurons of other vertebrate species, and other properties that were unique. Since information from these neurons is initially processed in the olfactory bulb, we also conducted preliminary studies on the developmental timeline of this structure and showed that glomerular structures are organized in ovo during a critical time period, during which embryonic chicks can form behavioral associations with odorants introduced in ovo. Lastly, we have shown that chick olfactory neurons can grow and mature in vitro, allowing their use in cell culture studies. These results collectively demonstrate some of the features of the olfactory system that are common to all vertebrates, and some that are unique to birds. These highlight the potential for the use of the physiology and development of the olfactory system as a model system for avian brain neurobiology.

TRPM5-Expressing Solitary Chemosensory Cells Respond to Odorous Irritants

Inhaled airborne irritants elicit sensory responses in trigeminal nerves innervating the nasal epithelium, leading to protective reflexes. The sensory mechanisms involved in the detection of odorous irritants are poorly understood. We identified a large population of solitary chemosensory cells expressing the transient receptor potential channel M5 (TRPM5) using transgenic mice where the promoter of TRPM5 drives the expression of green fluorescent protein (GFP). Most of these solitary chemosensory cells lie in the anterior nasal cavity. These GFP-labeled solitary chemosensory cells exhibited immunoreactivity for synaptobrevin-2, a vesicle-associated membrane protein important for synaptic transmission. Concomitantly, we found trigeminal nerve fibers apposed closely to the solitary chemosensory cells, indicating potential transmission of sensory information to trigeminal fibers. In addition, stimulation of the nasal cavity with high concentrations (0.5–5 mM) of a variety of odorants elicited event-related potentials (ERPs) in areas rich in TRPM5-expressing solitary chemosensory cells. Furthermore, odorous chemicals and trigeminal stimuli induced changes in intracellular Ca2+ levels in isolated TRPM5-expressing solitary chemosensory cells in a concentration-dependent manner. Together, our data show that the TRPM5-expressing cells respond to a variety of chemicals at high exposure levels typical of irritants and are positioned in the nasal cavity appropriately to monitor inhaled air quality.

Human olfactory detection of homologous n-alcohols measured via concentration-response functions

We explored in humans concentration-detection functions for the odor of the homologous n-alcohols ethanol, 1-butanol, 1-hexanol, and 1-octanol. These functions serve to establish structure-activity relationships, and reflect the pharmacology of the olfactory sense at the behavioral level. We tested groups of 14 to 17 subjects (half of them females), averaging 31 to 35 years old. An 8-station vapor delivery device (VDD8) presented the stimulus under a three-alternative forced-choice procedure against carbon-filtered air. The VDD8 was built to meet the demands of typical human sniffs in a short-term (<5 s) olfactory detection task, and to accurately control odorant generation, delivery, and stability. Actual stimulus concentration was quantified by gas chromatography before and during testing. The functions obtained were log normally distributed and were accurately modeled by a sigmoid (logistic) function, both at the group and at the individual level. Sensitivity to ethanol was the lowest and to 1-octanol the highest. Functions became steeper with increasing carbon chain length. For all alcohols the concentration detected halfway between chance and perfect detection (threshold) was at the ppb (or nM) level. Females were slightly more sensitive than males. Intersubject variability across participants was between one and two orders of magnitude. The present odor thresholds were lower than many reported in the past but their relative pattern across alcohols paralleled that in our earlier data and in compilation studies. A previously described quantitative structure-activity relationship for odor potency holds promise to model thresholds that, like those obtained here, best reflect the intrinsic sensitivity of human olfaction.

Novel stimulus-induced calcium efflux in Drosophila mushroom bodies

The mushroom body (MB) is an important part of the Drosophila brain, and is involved in many behaviors, including olfactory learning and memory and some visual cognition. However, the physiological properties of MB neurons remain elusive. Here we used a calcium-imaging technique to study calcium signals in Drosophila MB. We found that, rather than increasing calcium spread, electrical stimuli dramatically decreased calcium signals in the terminals of MB fibers. This novel calcium decrease occurred at all developmental stages from larvae to adults, but was specific for certain regions of the MB neurons. GABA receptor blockade promoted calcium propagation through the MB fibers, but did not disrupt the stimulus-induced decrease in calcium. Furthermore, this decrease in calcium was independent of extracellular calcium concentration and was not due to altered uptake by intracellular calcium stores and mitochondria. Rather, we found that inhibition of sodium-calcium exchangers significantly attenuated the stimulus-induced decrease in calcium, whereas the decrease persisted when membrane calcium pumps were blocked. Our findings indicate that MB neurons exhibit a novel stimulus-induced calcium efflux, which may be importantly regulated by sodium-calcium exchangers in the Drosophila MB.

Normative data for the “Sniffin’ Sticks” including tests of odor identification, odor discrimination, and olfactory thresholds: an upgrade based on a group of more than 3,000 subjects

"Sniffin' Sticks" is a test of nasal chemosensory function that is based on pen-like odor dispensing devices, introduced some 10 years ago by Kobal and co-workers. It consists of tests for odor threshold, discrimination, and identification. Previous work established its test-retest reliability and validity. Results of the test are presented as "TDI score", the sum of results obtained for threshold, discrimination, and identification measures. While normative data have been established they are based on a relatively small number of subjects, especially with regard to subjects older than 55 years where data from only 30 healthy subjects have been used. The present study aimed to remedy this situation. Now data are available from 3,282 subjects as compared to data from 738 subjects published previously. Disregarding sex-related differences, the TDI score at the tenth percentile was 24.9 in subjects younger than 15 years, 30.3 for ages from 16 to 35 years, 27.3 for ages from 36 to 55 years, and 19.6 for subjects older than 55 years. Because the tenth percentile has been defined to separate hyposmia from normosmia, these data can be used as a guide to estimate individual olfactory ability in relation to subject's age. Absolute hyposmia was defined as the tenth percentile score of 16-35 year old subjects. Other than previous reports the present norms are also sex-differentiated with women outperforming men in the three olfactory tests. Further, the present data suggest specific changes of individual olfactory functions in relation to age, with odor thresholds declining most dramatically compared to odor discrimination and odor identification.

Morituri te salutant? Olfactory signal transduction and the role of phosphoinositides

During the past 150 years, researchers have investigated the cellular, physiological, and molecular mechanisms underlying the sense of smell. Based on these efforts, a conclusive model of olfactory signal transduction in the vertebrate's nose is now available, spanning from G-protein-mediated odorant receptors to ion channels, which are linked by a cyclic adenosine 3',5'-monophosphate-mediated signal transduction cascade. Here we review some historical milestones in the chronology of olfactory research, particularly emphasising the role of cyclic nucleotides and inositol trisphosphate as alternative second messengers in olfactory cells. We will describe the functional anatomy of the nose, outline the cellular composition of the olfactory epithelium, and describe the discovery of the molecular backbone of the olfactory signal transduction cascade. We then summarize our current model, in which cyclic adenosine monophosphate is the sole excitatory second messenger in olfactory sensory neurons. Finally, a possible significance of microvillous olfactory epithelial cells and inositol trisphosphate in olfaction will be discussed.

Humans as an animal model for systems-level organization of olfaction

The past 15 years have seen significant advances in the study of olfaction, with particular emphasis on elucidating the molecular building blocks of the sensory process. However, much of the systems-level organization of olfaction remains unexplored. Here, we provide an overview at this level, highlighting results obtained from studying humans, whom we think provide an underutilized, yet critical, animal model for olfaction.

Once and again: Retinoic acid signaling in the developing and regenerating olfactory pathway

Retinoic acid (RA), a member of the steroid/thyroid superfamily of signaling molecules, is an essential regulator of morphogenesis, differentiation, and regeneration in the mammalian olfactory pathway. RA-mediated teratogenesis dramatically alters olfactory pathway development, presumably by disrupting retinoid-mediated inductive signaling that influences initial olfactory epithelium (OE) and bulb (OB) morphogenesis. Subsequently, RA modulates the genesis, growth, or stability of subsets of OE cells and OB interneurons. RA receptors, cofactors, and synthetic enzymes are expressed in the OE, OB, and anterior subventricular zone (SVZ), the site of neural precursors that generate new OB interneurons throughout adulthood. Their expression apparently accommodates RA signaling in OE cells, OB interneurons, and slowly dividing SVZ neural precursors. Deficiency of vitamin A, the dietary metabolic RA precursor, leads to cytological changes in the OE, as well as olfactory sensory deficits. Vitamin A therapy in animals with olfactory system damage can accelerate functional recovery. RA-related pathology as well as its potential therapeutic activity may reflect endogenous retinoid regulation of neuronal differentiation, stability, or regeneration in the olfactory pathway from embryogenesis through adulthood. These influences may be in register with retinoid effects on immune responses, metabolism, and modulation of food intake.

Phosphatidyl-inositide signalling proteins in a novel class of sensory cells in the mammalian olfactory epithelium

Ciliated sensory neurons, supporting cells and basal stem cells represent major cellular components of the main olfactory epithelium in mammals. Here we describe a novel class of sensory cells in the olfactory neuroepithelium. The cells express phospholipase C beta-2 (PLC beta2), transient receptor potential channels 6 (TRPC6) and inositol 3, 4, 5-trisphosphate receptors type III (InsP3R-III). Unlike ciliated olfactory neurons, they express neither olfactory marker protein nor centrin, adenylyl cyclase or cyclic nucleotide-gated cation channels. Typical components of the cytoskeleton of microvilli, ezrin and actin are found co-localized with PLC beta2 and TRPC6 in apical protrusions of the cells. In Ca2+-imaging experiments, the cells responded to odours. They express neuronal marker proteins and possess an axon-like process, but following bulbectomy the cells do not degenerate. Our results suggest a novel class of microvillous secondary chemosensory cells in the mammalian olfactory system. These cells, which utilize phosphatidyl-inositides in signal transduction, represent about 5% of all olfactory cells. Their abundance indicates that they play an important role in stimulus-dependent functions and/or the regeneration of the olfactory system.

Characteristics of odorant elicited calcium fluxes in acutely-isolated chick olfactory neurons

To understand avian olfaction, it is important to characterize the peripheral olfactory system of a representative bird species. This study determined the functional properties of olfactory receptor neurons of the chicken olfactory epithelium. Individual neurons were acutely isolated from embryonic day-18 to newborn chicks by dissection and enzymatic dissociation. We tested single olfactory neurons with behaviorally relevant odorant mixtures and measured their responses using ratiometric calcium imaging; techniques used in this study were identical to those used in other studies of olfaction in other vertebrate species. Chick olfactory neurons displayed properties similar to those found in other vertebrates: they responded to odorant stimuli with either decreases or increases in intracellular calcium, calcium increases were mediated by a calcium influx, and responses were reversibly inhibited by 100 microM L: -cis-diltiazem, 1 mM Neomycin, and 20 microM U73122, which are biochemical inhibitors of second messenger signaling. In addition, some cells showed a complex pattern of responses, with different odorant mixtures eliciting increases or decreases in calcium in the same cell. It appears that there are common features of odorant signaling shared by a variety of vertebrate species, as well as features that may be peculiar to chickens.

Evidence for multiple calcium response mechanisms in mammalian olfactory receptor neurons

Olfactory receptor neurons employ a diversity of signaling mechanisms for transducing and encoding odorant information. The simultaneous activation of subsets of receptor neurons provides a complex pattern of activation in the olfactory bulb that allows for the rapid discrimination of odorant mixtures. While some transduction elements are conserved among many species, some species-specificity occurs in certain features that may relate to their particular physiology and ecological niche. However, studies of olfactory transduction have been limited to a relatively small number of vertebrate and invertebrate species. To better understand the diversity and evolution of olfactory transduction mechanisms, we studied stimulus-elicited calcium fluxes in olfactory neurons from a previously unstudied mammalian species, the domestic cat. Isolated cells from cat olfactory epithelium were stimulated with odorant mixtures and biochemical agents, and cell responses were measured with calcium imaging techniques. Odorants elicited either increases or decreases in intracellular calcium; odorant-induced calcium increases were mediated either by calcium fluxes through the cell membrane or by mobilization of intracellular stores. Individual cells could employ multiple signaling mechanisms to mediate responses to different odorants. The physiological features of these olfactory neurons suggest greater complexity than previously recognized in the role of peripheral neurons in encoding complex odor stimuli. The investigation of novel and unstudied species is important for understanding the mechanisms of odorant signaling that apply to the olfactory system in general and suggests both broadly conserved and species-specific evolutionary adaptations.

Cascades of response vectors of olfactory receptor neurons in Xenopus laevis tadpoles

Olfactory receptor neurons (ORNs) of Xenopus laevis tadpoles respond to water-born stimuli such as amino acids. Their sensitivity spectra with respect to amino acids have recently been shown to become more selective over ontogenetic stages [Manzini & Schild (2004) J. Gen. Physiol., 123, 99-107]. In this paper, we undertake a theoretical analysis of this data set and determine the correlational relationships among odorant responses represented as binary response vectors. We first show that, on the one hand, the number of 204 ORN classes (out of 283 recorded ORNs) cannot be explained by a random expression pattern of olfactory receptors (ORs). On the other hand, this number does not appear to be reconcilable with the idea that individual ORNs express one type of OR each. The covariance matrix of stimulus responses shows that the responses to some stimuli are correlated to those of others. Furthermore, the response vectors show positive as well as negative correlations among each other. While the positive correlations can partly be explained by the differing response frequencies to the odorants used, the negative ones cannot. Finally, we analyse the similarity among responses using the Hamming distance as a distance measure, the result being that most response vectors differ from others by small Hamming distances. Such vectors are shown to form pattern cascades, possibly reflecting a decreasing number of ORs being expressed over ontogenetic stages.

Odorant responses of dual polarity are mediated by cAMP in mouse olfactory sensory neurons

Some olfactory sensory neurons (OSNs) respond to odors with hyperpolarization. Although transduction for excitatory responses is mediated by opening of a cyclic nucleotide-gated (CNG) channel, there is controversy on the mechanism underlying inhibitory responses. We find that mouse OSNs respond to odorants by either depolarizing or hyperpolarizing responses in loose-patch measurements. In the perforated-patch configuration, OSNs not only responded with a current consistent with CNG channel-mediated excitation but also displayed enhancement of outward currents, consistent with inhibitory responses. Increasing cAMP levels pharmacologically elicited excitatory or inhibitory responses in different OSNs. In addition, OSNs from mice defective for the CNGA2 subunit of the CNG channel displayed neither excitatory nor inhibitory responses. Thus CNG channels mediate inhibitory olfactory responses.

Odors detected by mice deficient in cyclic nucleotide-gated channel subunit A2 stimulate the main olfactory system

It is believed that odor transduction in the mammalian main olfactory system only involves the cAMP-signaling pathway. Here, we report on odor responsiveness in mice with a disrupted cyclic nucleotide-gated (CNG) channel subunit A2. Several odorants, including putative pheromones, can be detected and discriminated by these mice behaviorally. These odors elicit responses in the olfactory epithelium, main olfactory bulb, and olfactory (piriform) cortex of CNGA2 knock-out mice. In addition, responses to odors detected by CNGA2 knock-out mice are relatively insensitive to inhibitors of the cAMP pathway. These results provide strong evidence that cAMP-independent pathways in the main olfactory system of mammals participate in detecting a subset of odors.

Olfaction and anosmia in rhinosinusitis

Olfactory disorders can cause serious consequences from the inability to detect many olfactory warning signals (eg, smoke, spoiled food, and gas leaks) and can significantly impact nutritional status, eating satisfaction, and many other issues related to quality of life. More than 200,000 people a year seek treatment for impaired olfactory ability, and available evidence suggests this figure is a significant underestimate of those affected. Rhinitis and rhinosinusitis are the primary etiologies for olfactory loss or distortion among patients presenting to chemosensory clinics, and they are among the most common chronic medical conditions in the United States, affecting an estimated 33 million people. Both allergic and nonallergic rhinitis/rhinosinusitis can result in diminished, distorted, or absent olfactory ability. Current therapies are only partially or transiently effective in reversing olfactory loss. The underlying mechanisms by which rhinitis/rhinosinusitis impact olfactory ability are likely to be multifactorial and might include altered air flow and odor deposition, changes in mucus composition, and effects of inflammatory mediators on receptor cell differentiation, maturation, or function.

Odors detected by mice deficient in cyclic nucleotide-gated channel subunit A2 stimulate the main olfactory system

It is believed that odor transduction in the mammalian main olfactory system only involves the cAMP-signaling pathway. Here, we report on odor responsiveness in mice with a disrupted cyclic nucleotide-gated (CNG) channel subunit A2. Several odorants, including putative pheromones, can be detected and discriminated by these mice behaviorally. These odors elicit responses in the olfactory epithelium, main olfactory bulb, and olfactory (piriform) cortex of CNGA2 knock-out mice. In addition, responses to odors detected by CNGA2 knock-out mice are relatively insensitive to inhibitors of the cAMP pathway. These results provide strong evidence that cAMP-independent pathways in the main olfactory system of mammals participate in detecting a subset of odors.

The superior turbinate as a source of functional human olfactory receptor neurons

OBJECTIVES

The function of human olfactory receptor neurons (ORNs) remains incompletely understood, in part because of the difficulty of obtaining viable olfactory tissue for study. During endoscopic sphenoidotomy, a portion of the superior turbinate is often removed to achieve wide and safe access to the sphenoid sinus. The purpose of this study was to determine whether functional olfactory mucosa could be obtained from such superior turbinate tissue.

STUDY DESIGN/METHODS

Superior turbinate tissue was resected from 4 patients undergoing transnasal endoscopic approaches to the sphenoid sinus. The gross appearance of the turbinate mucosa was normal at the time of surgery. The specimens were placed directly into cold cell culture media and transferred to the laboratory. A portion of the mucosa was fixed and embedded for histology and immunohistochemistry. The remaining tissue was enzymatically dissociated, and the resulting cell suspension was either prepared for immediate calcium imaging or placed into cell culture. Cultured ORNs underwent calcium imaging after several weeks to assess their ability to respond to odorants.

RESULTS

Histologic analysis of superior turbinate tissue revealed the presence of patchy olfactory neuroepithelium staining positive for olfactory marker protein. Acutely dissociated ORNs were capable of generating calcium responses to odorant mixtures. ORNs could be maintained in mixed culture and retained their ability to respond to odorants.

CONCLUSIONS

Superior turbinate tissue removed during endoscopic sphenoidotomy can provide a valuable source of human olfactory neuroepithelium for functional or histologic study. Superior turbinate tissue yields stem cells and immature neurons capable of differentiating into ORNs that retain many of their functional characteristics even after growth in culture.

A psychophysical study of the relationship between temporal processing in odor mixtures and transduction pathways

Depending on the odorant, transduction during the olfactory reception process is reported to be mediated by the second messengers cyclic adenosine 3',5'-monophosphate (cAMP) or inositol 1,4,5-triphosphate (IP(3)). The present study with humans investigates the relationship between temporal processing in mixtures and the type of transduction process used. The most common outcomes were reciprocal temporal interactions which were primarily dependent on odorant concentration and independent of the type of transduction process. The results are consistent with the bulk of evidence that each receptor neuron commonly has only one type of receptor.

Characteristics of odorant elicited calcium changes in cultured human olfactory neurons

An important step in establishing and utilizing a cell culture system for the in vitro study of olfaction is assessing whether the cultured cells possess physiological properties similar to those of mature olfactory neurons. Various investigators have successfully established proliferating cell lines from olfactory tissue, but few have demonstrated the characteristics of odor sensitivity of these cells. We successfully established cultured cell lines from adult human olfactory tissue obtained using an olfactory biopsy procedure and measured their ability to respond to odor stimulation using calcium imaging techniques. A subset of the human olfactory cells in culture displayed a distinct morphology and specifically expressed immunocytochemical markers characteristic of mature human olfactory neurons such as OMP, G(olf), NCAM and NST. Under defined growth conditions, these cultured cells responded to odorant mixes that have been previously shown to elicit intracellular calcium changes in acutely-isolated human olfactory neurons. These odorant-elicited calcium responses displayed characteristics similar to those found in mature human olfactory neurons. First, cultured cells responded with either increases or decreases in intracellular calcium. Second, increases in calcium were abolished by removal of extracellular calcium. Third, inhibitors of the olfactory signal transduction cascades reversibly blocked these odorant-elicited intracellular calcium changes. Our results demonstrate that cultures of adult human olfactory cells established from olfactory biopsies retain some of the in vivo odorant response characteristics of acutely isolated cells from the adult olfactory epithelium. This work has important ramifications for investigation of olfactory function and dysfunction using biopsy procedures and in vitro assays of odor sensitivity.

Modulation of odor-induced increases in [Ca(2+)](i) by inhibitors of protein kinases A and C in rat and human olfactory receptor neurons

Protein kinases A and C have been postulated to exert multiple effects on different elements of signal transduction pathways in olfactory receptor neurons. However, little is known about the modulation of olfactory responses by protein kinases in intact olfactory receptor neurons. To further elucidate the details of the modulation of odorant responsiveness by these protein kinases, we investigated the action of two protein kinase inhibitors: H89, an inhibitor of protein kinase A, and N-myristoylated EGF receptor, an inhibitor of protein kinase C, on odorant responsiveness in intact olfactory neurons. We isolated individual olfactory neurons from the adult human and rat olfactory epithelium and measured responses of the isolated cells to odorants or biochemical activators that have been shown to initiate cyclic AMP or inositol 1,4,5-trisphospate production in biochemical preparations. We employed calcium imaging techniques to measure odor-elicited changes in intracellular calcium that occur over several seconds. In human olfactory receptor neurons, the protein kinase A and C inhibitors affected the responses to different sets of odorants. In rats, however, the protein kinase C inhibitor affected responses to all odorants, while the protein kinase A inhibitor had no effect. In both species, the effect of inhibition of protein kinases was to enhance the elevation and block termination of intracellular calcium levels elicited by odorants. Our results show that protein kinases A and C may modulate odorant responses of olfactory neurons by regulating calcium fluxes that occur several seconds after odorant stimulation. The effects of protein kinase C inhibition are different in rat and human olfactory neurons, indicating that species differences are an important consideration when applying data from animal studies to apply to humans.

Expression of mRNAs encoding for two different olfactory receptors in a subset of olfactory receptor neurons

Evidence has accumulated to support a model for odorant detection in which individual olfactory receptor neurons (ORNs) express one of a large family of G protein-coupled receptor proteins that are activated by a small number of closely related volatile chemicals. However, the issue of whether an individual ORN expresses one or multiple types of receptor proteins has yet to be definitively addressed. Physiological data indicate that some individual ORNs can be activated by odorants differing substantially in structure and/or perceived quality, suggesting multiple receptors or one nonspecific receptor per cell. In contrast, molecular biological studies favor a scheme with a single, fairly selective receptor per cell. The present studies directly assessed whether individual rat ORNs can express multiple receptors using single-cell PCR techniques with degenerate primers designed to amplify a wide variety of receptor sequences. We found that whereas only a single OR sequence was obtained from most ORNs examined, one ORN produced two distinct receptor sequences that represented different receptor gene families. Double-label in situ hybridization studies indicated that a subset of ORNs co-express two distinct receptor mRNAs. A laminar segregation analysis of the cell nuclei of ORNs labeled with the two OR mRNA probes showed that for one probe, the histogram of the distribution of the cell nuclei along the depth of the epithelium was bimodal, with one peak overlapping the (unimodal) histogram for the other probe. These results are consistent with co-expression of two OR mRNAs in a population of single ORNs.