Morphological study of the effects of intranasal zinc sulfate irrigation on the mouse olfactory epithelium and olfactory bulb

Toxicological inhalation studies in rats to substantiate grouping of zinc oxide nanoforms

BACKGROUND

Significant variations exist in the forms of ZnO, making it impossible to test all forms in in vivo inhalation studies. Hence, grouping and read-across is a common approach under REACH to evaluate the toxicological profile of familiar substances. The objective of this paper is to investigate the potential role of dissolution, size, or coating in grouping ZnO (nano)forms for the purpose of hazard assessment. We performed a 90-day inhalation study (OECD test guideline no. (TG) 413) in rats combined with a reproduction/developmental (neuro)toxicity screening test (TG 421/424/426) with coated and uncoated ZnO nanoforms in comparison with microscale ZnO particles and soluble zinc sulfate. In addition, genotoxicity in the nasal cavity, lungs, liver, and bone marrow was examined via comet assay (TG 489) after 14-day inhalation exposure.

RESULTS

ZnO nanoparticles caused local toxicity in the respiratory tract. Systemic effects that were not related to the local irritation were not observed. There was no indication of impaired fertility, developmental toxicity, or developmental neurotoxicity. No indication for genotoxicity of any of the test substances was observed. Local effects were similar across the different ZnO test substances and were reversible after the end of the exposure.

CONCLUSION

With exception of local toxicity, this study could not confirm the occasional findings in some of the previous studies regarding the above-mentioned toxicological endpoints. The two representative ZnO nanoforms and the microscale particles showed similar local effects. The ZnO nanoforms most likely exhibit their effects by zinc ions as no particles could be detected after the end of the exposure, and exposure to rapidly soluble zinc sulfate had similar effects. Obviously, material differences between the ZnO particles do not substantially alter their toxicokinetics and toxicodynamics. The grouping of ZnO nanoforms into a set of similar nanoforms is justified by these observations.

Olfactory Loss in Rhinosinusitis: Mechanisms of Loss and Recovery

Chronic rhinosinusitis (CRS) is a highly prevalent disease and up to 83% of CRS patients suffer from olfactory dysfunction (OD). Because OD is specifically seen in those CRS patients that present with a type 2 eosinophilic inflammation, it is believed that type 2 inflammatory mediators at the level of the olfactory epithelium are involved in the development of this olfactory loss. However, due to the difficulties in obtaining tissue from the olfactory epithelium, little is known about the true mechanisms of inflammatory OD. Thanks to the COVID-19 pandemic, interest in olfaction has been growing rapidly and several studies have been focusing on disease mechanisms of OD in inflammatory conditions. In this paper, we summarize the most recent data exploring the pathophysiological mechanisms underlying OD in CRS. We also review what is known about the potential capacity of olfactory recovery of the currently available treatments in those patients.

326K at E Protein Is Critical for Mammalian Adaption of TMUV

Outbreaks of Tembusu virus (TMUV) infection have caused huge economic losses to the poultry industry in China since 2010. However, the potential threat of TMUV to mammals has not been well studied. In this study, a TMUV HB strain isolated from diseased ducks showed high virulence in BALB/c mice inoculated intranasally compared with the reference duck TMUV strain. Further studies revealed that the olfactory epithelium is one pathway for the TMUV HB strain to invade the central nervous system of mice. Genetic analysis revealed that the TMUV HB virus contains two unique residues in E and NS3 proteins (326K and 519T) compared with duck TMUV reference strains. K326E substitution weakens the neuroinvasiveness and neurovirulence of TMUV HB in mice. Remarkably, the TMUV HB strain induced significantly higher levels of IL-1β, IL-6, IL-8, and interferon (IFN)-α/β than mutant virus with K326E substitution in the brain tissue of the infected mice, which suggested that TMUV HB caused more severe inflammation in the mouse brains. Moreover, application of IFN-β to infected mouse brain exacerbated the disease, indicating that overstimulated IFN response in the brain is harmful to mice upon TMUV infection. Further studies showed that TMUV HB upregulated RIG-I and IRF7 more significantly than mutant virus containing the K326E mutation in mouse brain, which suggested that HB stimulated the IFN response through the RIG-I-IRF7 pathway. Our findings provide insights into the pathogenesis and potential risk of TMUV to mammals.

Effects of nasal inflammation on the olfactory bulb

Sinonasal diseases, such as rhinosinusitis, affect up to 12% of individuals each year which constitutes these diseases as some of the most common medical conditions in the world. Exposure to environmental pathogens and toxicants via the nasal cavity can result in a severe inflammatory state commonly observed in these conditions. It is well understood that the epithelial and neuronal cells lining the olfactory mucosa, including olfactory sensory neurons (OSNs), are significantly damaged in these diseases. Prolonged inflammation of the nasal cavity may also lead to hyposmia or anosmia. Although various environmental agents induce inflammation in different ways via distinct cellular and molecular interactions, nasal inflammation has similar consequences on the structure and homeostatic function of the olfactory bulb (OB) which is the first relay center for olfactory information in the brain. Atrophy of the OB occurs via thinning of the superficial OB layers including the olfactory nerve layer, glomerular layer, and superficial external plexiform layer. Intrabulbar circuits of the OB which include connectivity between OB projection neurons, OSNs, and interneurons become significantly dysregulated in which synaptic pruning and dendritic retraction take place. Furthermore, glial cells and other immune cells become hyperactivated and induce a state of inflammation in the OB which results in upregulated cytokine production. Moreover, many of these features of nasal inflammation are present in the case of SARS-CoV-2 infection. This review summarizes the impact of nasal inflammation on the morphological and physiological features of the rodent OB.

Songbirds use scent cues to relocate to feeding sites after displacement: An experiment in great tits (Parus major)

Air-borne chemicals are highly abundant sensory cues and their use in navigation might be one of the major evolutionary mechanisms explaining the development of olfaction in animals. Despite solid evidence for the importance of olfaction in avian life (e.g., foraging or mating), the importance of chemical cues in avian orientation remains controversial. In particular, songbirds are sorely neglected models, despite their remarkable orientation skills. Here we show that great tits (Parus major) require olfactory cues to orientate toward winter-feeding sites within their home range after displacement. Birds that received an olfaction-depriving treatment were impaired in homing. However, the return rates between olfaction-deprived and control individuals did not differ. Birds with decreased perception of olfactory cues required more time to return to the winter feeding sites. This effect became apparent when the distance between the releasing and capture sites was greater. Our results indicate that even in a familiar environment with possible visual landmarks, scent cues might serve as an important source of information for orientation.

PHEV infection: A promising model of betacoronavirus-associated neurological and olfactory dysfunction

Porcine hemagglutinating encephalomyelitis virus (PHEV) is a highly neurotropic coronavirus belonging to the genus Betacoronavirus. Similar to pathogenic coronaviruses to which humans are susceptible, such as SARS-CoV-2, PHEV is transmitted primarily through respiratory droplets and close contact, entering the central nervous system (CNS) from the peripheral nerves at the site of initial infection. However, the neuroinvasion route of PHEV are poorly understood. Here, we found that BALB/c mice are susceptible to intranasal PHEV infection and showed distinct neurological manifestations. The behavioral study and histopathological examination revealed that PHEV attacks neurons in the CNS and causes significant smell and taste dysfunction in mice. By tracking neuroinvasion, we identified that PHEV invades the CNS via the olfactory nerve and trigeminal nerve located in the nasal cavity, and olfactory sensory neurons (OSNs) were susceptible to viral infection. Immunofluorescence staining and ultrastructural observations revealed that viral materials traveling along axons, suggesting axonal transport may engage in rapid viral transmission in the CNS. Moreover, viral replication in the olfactory system and CNS is associated with inflammatory and immune responses, tissue disorganization and dysfunction. Overall, we proposed that PHEV may serve as a potential prototype for elucidating the pathogenesis of coronavirus-associated neurological complications and olfactory and taste disorders.

Chronic thinner inhalation alters olfactory behaviors in adult mice

Volatile solvents exposure can result in various behavioral impairments that have been partly associated to altered adult hippocampal neurogenesis. Despite recent evidence supporting this association, few studies have been devoted to examine the impact on olfactory functioning and olfactory bulb (OB) neurogenesis, although olfactory system is directly in contact with volatile molecules. Thus, this study was designed to evaluate in adult mice the potential modifications of the olfactory functioning after acute (1 day), subchronic (6 weeks) and chronic (12 weeks) exposure to thinner vapor at both behavioral and cellular levels. Firstly, behavioral evaluations showed that chronic thinner exposure impacts on odor detection ability of treated mice but does not affect mice ability to efficiently discriminate between two different odors. Moreover, chronic thinner exposure produces impairment in the olfactory-mediated associative memory. Secondly, analysis of the effects of thinner exposure in the subventricular zone (SVZ) of the lateral ventricle and in the OB revealed that thinner treatments do not induce apoptosis nor glial activation. Thirdly, immunohistochemical quantification of different markers of adult olfactory neurogenesis showed that inhalant treatments do not change the number of proliferating progenitors in the SVZ and the rostral migratory stream (RMS), as well as the number of newborn cells reaching and integrating in the OB circuitry. Altogether, our data highlight that the impaired olfactory performances in chronically-exposed mice are not associated to an alteration of adult neurogenesis in the SVZ-OB system.

Immune responses in the injured olfactory and gustatory systems: a role in olfactory receptor neuron and taste bud regeneration?

Sensory cells that specialize in transducing olfactory and gustatory stimuli are renewed throughout life and can regenerate after injury unlike their counterparts in the mammalian retina and auditory epithelium. This uncommon capacity for regeneration offers an opportunity to understand mechanisms that promote the recovery of sensory function after taste and smell loss. Immune responses appear to influence degeneration and later regeneration of olfactory sensory neurons and taste receptor cells. Here we review surgical, chemical, and inflammatory injury models and evidence that immune responses promote or deter chemosensory cell regeneration. Macrophage and neutrophil responses to chemosensory receptor injury have been the most widely studied without consensus on their net effects on regeneration. We discuss possible technical and biological reasons for the discrepancy, such as the difference between peripheral and central structures, and suggest directions for progress in understanding immune regulation of chemosensory regeneration. Our mechanistic understanding of immune-chemosensory cell interactions must be expanded before therapies can be developed for recovering the sensation of taste and smell after head injury from traumatic nerve damage and infection. Chemosensory loss leads to decreased quality of life, depression, nutritional challenges, and exposure to environmental dangers highlighting the need for further studies in this area.

The role of olfactory transport in the penetration of manganese oxide nanoparticles from blood into the brain

There is no doubt that various nanoparticles (NPs) can enter the brain from the nasal cavity. It is assumed that NPs can penetrate from blood into the central nervous system (CNS) only by breaking the blood–brain barrier (BBB). The accumulation of NPs in CNS can provoke many neurological diseases; therefore, the understanding of its mechanisms is of both academic and practical interest. Although hitting from the surface of the lungs into the bloodstream, NPs can accumulate in various mucous membranes, including the nasal mucosa. Thus, we cannot rule out the ability of NPs to be transported from the bloodstream to the brain through the olfactory uptake. To test this hypothesis, we used paramagnetic NPs of manganese oxide (Mn3O4-NPs), whose accumulation patterns in the mouse brain were recorded using T1-weighted magnetic resonance imaging. The effect of intranasal application of endocytosis and axonal transport inhibitors on the brain accumulation patterns of intranasally or intravenously injected Mn3O4-NPs was evaluated. A comparative analysis of the results showed that the transport of Mn3O4-NPs from the nasal cavity to the brain is more efficient than their local permeation through BBB into CNS from the bloodstream, for example with the accumulation of Mn3O4NPs in the dentate gyrus of the hippocampus, and through the capture and transport of NPs from the blood by olfactory epithelium cells. Also, experiments with the administration of chlorpromazine, a specific inhibitor of clathrin-dependent endocytosis, and methyl-β-cyclodextrin, inhibitor of the lipid rafts involved in the capture of substances by endothelium cells, showed differences in the mechanisms of NP uptake from the nasal cavity and from the bloodstream. In this study, we show a significant contribution of axonal transport to NP accumulation patterns in the brain, both from the nasal cavity and from the vascular bed. This explains the accumulation of different sorts of submicron particles (neurotropic viruses, insoluble xenobiotics, etc.), unable to pass BBB, in the brain. The results will add to the understanding of the pathogenesis of various neurodegenerative diseases and help studying the side effects of therapeutics administered intravenously.

Anatomical basis and physiological role of cerebrospinal fluid transport through the murine cribriform plate

Cerebrospinal fluid (CSF) flows through the brain, transporting chemical signals and removing waste. CSF production in the brain is balanced by a constant outflow of CSF, the anatomical basis of which is poorly understood. Here, we characterized the anatomy and physiological function of the CSF outflow pathway along the olfactory sensory nerves through the cribriform plate, and into the nasal epithelia. Chemical ablation of olfactory sensory nerves greatly reduced outflow of CSF through the cribriform plate. The reduction in CSF outflow did not cause an increase in intracranial pressure (ICP), consistent with an alteration in the pattern of CSF drainage or production. Our results suggest that damage to olfactory sensory neurons (such as from air pollution) could contribute to altered CSF turnover and flow, providing a potential mechanism for neurological diseases.

Olfactory dysfunction revisited: a reappraisal of work-related olfactory dysfunction caused by chemicals

Occupational exposure to numerous individual chemicals has been associated with olfactory dysfunction, mainly in individual case descriptions. Comprehensive epidemiological investigations into the olfactotoxic effect of working substances show that the human sense of smell may be impaired by exposure to metal compounds involving cadmium, chromium and nickel, and to formaldehyde. This conclusion is supported by the results of animal experiments. The level of evidence for a relationship between olfactory dysfunction and workplace exposure to other substances is relatively weak.

Olfactory dysfunction revisited: a reappraisal of work-related olfactory dysfunction caused by chemicals

Occupational exposure to numerous individual chemicals has been associated with olfactory dysfunction, mainly in individual case descriptions. Comprehensive epidemiological investigations into the olfactotoxic effect of working substances show that the human sense of smell may be impaired by exposure to metal compounds involving cadmium, chromium and nickel, and to formaldehyde. This conclusion is supported by the results of animal experiments. The level of evidence for a relationship between olfactory dysfunction and workplace exposure to other substances is relatively weak.

Home-cage odors spatial cues elicit theta phase/gamma amplitude coupling between olfactory bulb and dorsal hippocampus

The brain oscillations may play a critical role in synchronizing neuronal assemblies in order to establish appropriate sensory-motor integration. In fact, studies have demonstrated phase-amplitude coupling of distinct oscillatory rhythms during cognitive processes. Here we investigated whether olfacto-hippocampal coupling occurs when mice are detecting familiar odors located in a spatially restricted area of a new context. The spatial olfactory task (SOT) was designed to expose mice to a new environment in which only one quadrant (target) contains odors provided by its own home-cage bedding. As predicted, mice showed a significant higher exploration preference to the target quadrant; which was impaired by olfactory epithelium lesion (ZnSO₄). Furthermore, mice were able to discriminate odors from a different cage and avoided the quadrant with predator odor 2,4,5-trimethylthiazoline (TMT), reinforcing the specificity of the SOT. The local field potential (LFP) analysis of non-lesioned mice revealed higher gamma activity (35-100Hz) in the main olfactory bulb (MOB) and a significant theta phase/gamma amplitude coupling between MOB and dorsal hippocampus, only during exploration of home-cage odors (i.e. in the target quadrant). Our results suggest that exploration of familiar odors in a new context involves dynamic coupling between the olfactory bulb and dorsal hippocampus.

After oxidation, zinc nanoparticles lose their ability to enhance responses to odorants

Electrical responses of olfactory sensory neurons to odorants were examined in the presence of zinc nanoparticles of various sizes and degrees of oxidation. The zinc nanoparticles were prepared by the underwater electrical discharge method and analyzed by atomic force microscopy and X-ray photoelectron spectroscopy. Small (1.2 ± 0.3 nm) zinc nanoparticles significantly enhanced electrical responses of olfactory neurons to odorants. After oxidation, however, these small zinc nanoparticles were no longer capable of enhancing olfactory responses. Larger zinc oxide nanoparticles (15 nm and 70 nm) also did not modulate responses to odorants. Neither zinc nor zinc oxide nanoparticles produced olfactory responses when added without odorants. The enhancement of odorant responses by small zinc nanoparticles was explained by the creation of olfactory receptor dimers initiated by small zinc nanoparticles. The results of this work will clarify the mechanisms for the initial events in olfaction, as well as to provide new ways to alleviate anosmia related to the loss of olfactory receptors.

Exposure to Zinc Sulfate Results in Differential Effects on Olfactory Sensory Neuron Subtypes in Adult Zebrafish

Zinc sulfate is a known olfactory toxicant, although its specific effects on the olfactory epithelium of zebrafish are unknown. Olfactory organs of adult zebrafish were exposed to zinc sulfate and, after 2, 3, 5, 7, 10 or 14 days, fish were processed for histological, immunohistochemical, ultrastructural, and behavioral analyses. Severe morphological disruption of the olfactory organ was observed two days following zinc sulfate exposure, including fusion of lamellae, epithelial inflammation, and significant loss of anti-calretinin labeling. Scanning electron microscopy revealed the apical surface of the sensory region was absent of ciliated structures, but microvilli were still present. Behavioral analysis showed significant loss of the ability to perceive bile salts and some fish also had no response to amino acids. Over the next several days, olfactory organ morphology, epithelial structure, and anti-calretinin labeling returned to control-like conditions, although the ability to perceive bile salts remained lost until day 14. Thus, exposure to zinc sulfate results in rapid degeneration of the olfactory organ, followed by restoration of morphology and function within two weeks. Zinc sulfate appears to have a greater effect on ciliated olfactory sensory neurons than on microvillous olfactory sensory neurons, suggesting differential effects on sensory neuron subtypes.

Neural regeneration dynamics of Xenopus laevis olfactory epithelium after zinc sulfate-induced damage

Neural stem cells (NSCs) of the olfactory epithelium (OE) are responsible for tissue maintenance and the neural regeneration after severe damage of the tissue. In the normal OE, NSCs are located in the basal layer, olfactory receptor neurons (ORNs) mainly in the middle layer, and sustentacular (SUS) cells in the most apical olfactory layer. In this work, we induced severe damage of the OE through treatment with a zinc sulfate (ZnSO₄) solution directly in the medium, which resulted in the loss of ORNs and SUS cells, but retention of the basal layer. During recovery following injury, the OE exhibited increased proliferation of NSCs and rapid neural regeneration. After 24h of recovery, new ORNs and SUS cells were observed. Normal morphology and olfactory function were reached after 168h (7 days) of recovery after ZnSO₄ treatment. Taken together, these data support the hypothesis that NSCs in the basal layer activate after OE injury and that these are sufficient for complete neural regeneration and olfactory function restoration. Our analysis provides histological and functional insights into the dynamics between olfactory neurogenesis and the neuronal integration into the neuronal circuitry of the olfactory bulb that restores the function of the olfactory system.

Unravelling the Olfactory Sense: From the Gene to Odor Perception

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

Sensory deprivation disrupts homeostatic regeneration of newly generated olfactory sensory neurons after injury in adult mice

Although it is well known that injury induces the generation of a substantial number of new olfactory sensory neurons (OSNs) in the adult olfactory epithelium (OE), it is not well understood whether olfactory sensory input influences the survival and maturation of these injury-induced OSNs in adults. Here, we investigated whether olfactory sensory deprivation affected the dynamic incorporation of newly generated OSNs 3, 7, 14, and 28 d after injury in adult mice. Mice were unilaterally deprived of olfactory sensory input by inserting a silicone tube into their nostrils. Methimazole, an olfactotoxic drug, was also injected intraperitoneally to bilaterally ablate OSNs. The OE was restored to its preinjury condition with new OSNs by day 28. No significant differences in the numbers of olfactory marker protein-positive mature OSNs or apoptotic OSNs were observed between the deprived and nondeprived sides 0-7 d after injury. However, between days 7 and 28, the sensory-deprived side showed markedly fewer OSNs and mature OSNs, but more apoptotic OSNs, than the nondeprived side. Intrinsic functional imaging of the dorsal surface of the olfactory bulb at day 28 revealed that responses to odor stimulation were weaker in the deprived side compared with those in the nondeprived side. Furthermore, prevention of cell death in new neurons 7-14 d after injury promoted the recovery of the OE. These results indicate that, in the adult OE, sensory deprivation disrupts compensatory OSN regeneration after injury and that newly generated OSNs have a critical time window for sensory-input-dependent survival 7-14 d after injury.

The regeneration of P2 olfactory sensory neurons is selectively impaired following methyl bromide lesion

The capacity of the peripheral olfactory system to recover after injury has not been thoroughly explored. P2-IRES-tauLacZ mice were exposed to methyl bromide, which causes epithelial damage and kills 90% of the P2 neurons. With subsequent neuronal regeneration, P2 neurons recover within their usual territory to equal control numbers by 1 month but then decline sharply to roughly 40% of control by 3 months. At this time, the P2 projection onto the olfactory bulb is erroneous in several respects. Instead of converging onto 1 or 2 glomeruli per surface, small collections of P2 axons innervate multiple glomeruli at roughly the same position in the bulb as in controls. Within these glomeruli, the P2 axons are aggregated near the edge, whereas the remainder of the glomerulus contains olfactory marker protein (+), non-P2 axons, violating the one receptor-one glomerulus rule normally observed. The aggregates are denser than found in control P2-innervated glomeruli, suggesting that the P2 axons may not be synaptically connected. Based on published literature and other data, we hypothesize that P2 neurons lose out in an activity-based competition for synaptic territory within the glomeruli and are not maintained at control numbers due to a lack of trophic support from the bulb.

A lifetime of neurogenesis in the olfactory system

Neurogenesis continues well beyond embryonic and early postnatal ages in three areas of the nervous system. The subgranular zone supplies new neurons to the dentate gyrus of the hippocampus. The subventricular zone supplies new interneurons to the olfactory bulb, and the olfactory neuroepithelia generate new excitatory sensory neurons that send their axons to the olfactory bulb. The latter two areas are of particular interest as they contribute new neurons to both ends of a first-level circuit governing olfactory perception. The vomeronasal organ and the main olfactory epithelium comprise the primary peripheral olfactory epithelia. These anatomically distinct areas share common features, as each exhibits extensive neurogenesis well beyond the juvenile phase of development. Here we will discuss the effect of age on the structural and functional significance of neurogenesis in the vomeronasal and olfactory epithelia, from juvenile to advanced adult ages, in several common model systems. We will next discuss how age affects the regenerative capacity of these neural stem cells in response to injury. Finally, we will consider the integration of newborn neurons into an existing circuit as it is modified by the age of the animal.

Transitory and activity-dependent expression of neurogranin in olfactory bulb tufted cells during mouse postnatal development

Neurogranin (Ng) is a brain-specific postsynaptic calmodulin-binding protein involved in synaptic activity-dependent plasticity. In the adult olfactory bulb (OB), Ng is expressed by a large population of GABAergic interneurons in the granule cell layer. We show here that, during postnatal development, Ng is also expressed by OB neurons in the superficial external plexiform layer (sEPL) and glomerular layer (GL). These Ng-positive neurons display morphological and neurochemical features of superficial and external tufted cells. Ng expression in these cells is transient during OB development: few elements express Ng at postnatal day (P) 5, increasing in number and reaching a peak at P10, then progressively decreasing. At P30, Ng is rarely detectable in these neurons. Ng expression in developing tufted cells is also modulated at the cellular level: at earlier stages, Ng labeling is distributed throughout the cell body and dendritic arborization in the GL, but, at P20, when the glomerular circuits are fully matured, Ng becomes restricted to the soma and proximal portion of tufted cell apical dendrites. We show that olfactory deprivation at early postnatal stages induces a strong increase in Ng-positive tufted cells from P10 to P20, whereas no changes have been observed following olfactory deprivation in adult mice. These findings demonstrate that Ng expression in sEPL-GL is restricted to developmental stages and indicate its activity-dependent regulation in a time window critical for glomerular circuit development, suggesting a role for Ng in maturation and dendritic remodeling of tufted cells.

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.

Reversible deafferentation of the adult zebrafish olfactory bulb affects glomerular distribution and olfactory-mediated behavior

The olfactory system is a useful model for studying central nervous system recovery from damage due to its neuroplasticity. We recently developed a novel method of deafferentation by repeated exposure of Triton X-100 to the olfactory organ of adult zebrafish. This long-term, reversible method of deafferentation allows both degeneration and regeneration to be observed in the olfactory bulb. The aim of the present study is to examine olfactory bulb innervation, glomerular patterns, and olfactory-mediated behavior with repeated Triton X-100 treatment and the potential for recovery following cessation of treatment. Olfactory bulbs of control, chronic-treated, and recovery animals were examined for the presence or absence of glomeruli that have been identified in the zebrafish glomerular map. Following chronic treatment, the number of glomeruli was dramatically reduced; however, partial innervation remained in the lateral region of the bulb. When animals were given time to recover, complete glomerular distribution returned. A behavioral assay was developed to determine if innervation remaining correlated with behavior of the fish. Chronic-treated fish did not respond to odorants involved with social behavior but continued to react to odorants that mediate feeding behavior. Following recovery, responses to odorants involved with social behavior returned. The morphological and behavioral effects of chronic Triton X-100 treatment in the olfactory system suggest there may be differential susceptibility or resistance to external damage in a subset of sensory neurons. The results of this study demonstrate the remarkable regenerative ability of the olfactory system following extensive and long-term injury.

p63 regulates olfactory stem cell self-renewal and differentiation

The olfactory epithelium is a sensory neuroepithelium that supports adult neurogenesis and tissue regeneration following injury, making it an excellent model for investigating neural stem cell regulation in vivo. Previous studies have identified the horizontal basal cell (HBC) as the neural stem cell of the postnatal olfactory epithelium. However, the molecules and pathways regulating HBC self-renewal and differentiation are unknown. In the present study, we demonstrate that the transcription factor p63, a member of the p53 tumor suppressor gene family known to regulate stem cell dynamics in other epithelia, is highly enriched in HBCs. We show that p63 is required cell autonomously for olfactory stem cell renewal and further demonstrate that p63 functions to repress HBC differentiation. These results provide critical insight into the genetic regulation of the olfactory stem cell in vivo and more generally provide an entrée toward understanding the coordination of stem cell self-renewal and differentiation.

Effects of pyridine inhalation exposure on olfactory epithelium in mice

Olfactory neurons in the nasal mucosa have the capacity to regenerate continuously along the lifespan by neurogenesis processes starting with progenitor cells close to the basal lamina. The cellular turnover into olfactory neuroepithelium may be modified by environmental stimuli insofar as nasal mucosa is directly in contact with airborne chemicals. However, few studies have been focused on selective changes, especially those concerning mature olfactory neurons and basal cells during specific inhalation exposure. Among chemicals, solvents are known to induce changes in smell abilities and concomitant histological and cellular modifications related to the type of molecule, concentration and time of exposure. This study was designed to characterize smell sensitivity (using behavioral tests) and immunohistochemical effects on olfactory neuroepithelium induced by pyridine exposure in mice. Olfactory marker protein (OMP) and proliferating cell nuclear antigen (PCNA) were used to characterize respectively mature olfactory neurons and basal cells. Results showed that inhalation exposure to pyridine had no impact on smell sensitivity whatever the concentration used and the time of exposure. These findings were in agreement with immunohistochemical measurements showing the same cellular kinetic whatever the condition of exposition to pyridine. Indeed, OMP-positive cells increased and PCNA-positive cells decreased as early as the beginning of exposure and cell amounts remained stable at this level until the end of exposure. These findings suggest that pyridine could have the property to rapidly activate a cellular turnover from basal cell progenitors. Rather than toxic effects, the present findings suggest that the metabolites of pyridine might have cell cycle activation properties.

Olfactory bulb recovery following reversible deafferentation with repeated detergent application in the adult zebrafish

The neuroplasticity and regenerative properties of the olfactory system make it a useful model for studying the ability of the nervous system to recover from damage. We have developed a novel method for examining the effects of long-term deafferentation and regeneration of the olfactory organ and resulting influence on the olfactory bulb in adult zebrafish. To test the hypothesis that repeated damage to the olfactory epithelium causes reduced olfactory bulb afferent input and cessation of treatment allows recovery, we chronically ablated the olfactory organ every 2-3 days for 3 weeks with the detergent Triton X-100 while another group was allowed 3 weeks of recovery following treatment. Animals receiving chronic treatment showed severe morphological disruption of the olfactory organ, although small pockets of epithelium remained. These pockets were labeled by anti-calretinin, indicating the presence of mature olfactory sensory neurons (OSNs). Following a recovery period, the epithelium was more extensive and neuronal labeling increased, with three different morphologies of sensory neurons observed. Repeated peripheral exposure to Triton X-100 also affected the olfactory bulb. Bulb volumes and anti-tyrosine hydroxylase-like immunoreactivity, which is an indicator of afferent activity, were diminished in the olfactory bulb of the chronically treated group compared to the control side. In the recovery group, there was little difference in bulb volume or antibody staining. These results suggest that repeated, long-term nasal irrigation with Triton X-100 eliminates a substantial number of mature OSNs and reduces afferent input to the olfactory bulb. It also appears that these effects are reversible and regeneration will occur in both the peripheral olfactory organ and the olfactory bulb when given time to recover following cessation of treatment. We report here a new method that allows observation not only of the effects of deafferentation on the olfactory bulb but also the effects of reinnervation.

Sensory experience selectively regulates transmitter synthesis enzymes in interglomerular circuits

Sensory experience influences brain organization and function. A particularly striking example is in the olfactory bulb where reduction of odorant sensory signals profoundly down-regulates dopamine in glomerular neurons. There are two large populations of glomerular inhibitory interneurons: (1) GABAergic periglomerular (PG) cells, whose processes are limited to a single glomerulus, regulate intraglomerular processing and (2) DAergic-GABAergic short axon (SA) cells, whose processes contact multiple glomeruli, regulate interglomerular processing. The inhibitory neurotransmitter GABA is synthesized from L-glutamic acid by the enzyme glutamic acid decarboxylase (GAD) of which there are two major isoforms: GAD65 and GAD67. GAD65 is expressed in uniglomerular PG cells. GAD67 is expressed by SA cells, which also co-express the rate-limiting enzyme for dopamine synthesis, tyrosine hydroxylase (TH). Deafferentation or sensory deprivation decreases TH expression but it is not known if sensory input alters GAD isoforms. Here we report that either deafferentation or reduction of sensory input by nares occlusion significantly reduced GAD67 protein and the number of SA cells expressing GAD67. However, neither manipulation altered GAD65 protein or the number of GAD65 PG cells. These findings show that sensory experience strongly impacts transmitter regulation in the circuit that controls neural processing across glomeruli but not in the circuit that regulates intraglomerular processing.

Comparative Fear-Related Behaviors to Predator Odors (TMT and Natural Fox Feces) before and after Intranasal ZnSO(4) Treatment in Mice

The possibility that synthetic 2,4,5-trimethylthiazoline (TMT), frequently used to induce unconditioned fear in rodents, could be more a pungent odor activating intranasal trigeminal nerve fibers rather than a predator odor index is currently discussed. In order to explore this question, the present study compared fear-related behaviors to predator odors (synthetic 10% TMT and natural fox feces) and toluene (as an irritant compound without ecological significance) before and after intranasal ZnSO₄ perfusion which is known to provoke transient anosmia. Results show that natural fox feces could be consider as a pure olfactory (CN I) nerve stimulant while 10% TMT appeared to be a mixed olfactory (CN I) and trigeminal (CN V) nerves stimulant with a great olfactory power and a low trigeminal power. These findings suggest that behavioral neuroscience studies should use concentrations lower than 10% TMT to obtain fear-related behaviors similar to those obtained with natural fox feces odor.

Novel strategies in brachial plexus repair after traumatic avulsion

Clinical trials in spinal cord injury (SCI) can be affected by many confounding variables including spontaneous recovery, variation in the lesion type and extend. However, the clinical need and the paucity of effective therapies has spawned a large number of animal studies and clinical trials for SCI. In this review, we suggest that brachial plexus avulsion injury, a longitudinal spinal cord lesion, is a simpler model to test methods of spinal cord repair. We explore reconstructive techniques currently explored for the repair of brachial plexus avulsion and focus on the use of olfactory ensheathing cell transplantation as an adjunct treatment in brachial plexus repair.

Neurotoxic, inflammatory, and mucosecretory responses in the nasal airways of mice repeatedly exposed to the macrocyclic trichothecene mycotoxin roridin A: dose-response and persistence of injury

Macrocyclic trichothecene mycotoxins encountered in water-damaged buildings have been suggested to contribute to illnesses of the upper respiratory tract. Here, the authors characterized the adverse effects of repeated exposures to roridin A (RA), a representative macrocyclic trichothecene, on the nasal airways of mice and assessed the persistence of these effects. Young, adult, female C57BL/6 mice were exposed to single daily, intranasal, instillations of RA (0.4, 2, 10, or 50 microg/kg body weight [bw]) in saline (50 microl) or saline alone (controls) over 3 weeks or 250 microg/kg RA over 2 weeks. Histopathologic, immunohistochemical, and morphometric analyses of nasal airways conducted 24 hr after the last instillation revealed that the lowest-effect level was 10 microg/kg bw. RA exposure induced a dose-dependent, neutrophilic rhinitis with mucus hypersecretion, atrophy and exfoliation of nasal transitional and respiratory epithelium, olfactory epithelial atrophy and loss of olfactory sensory neurons (OSNs). In a second study, the persistence of lesions in mice instilled with 250 microg/kg bw RA was assessed. Nasal inflammation and excess luminal mucus were resolved after 3 weeks, but OSN loss was still evident in olfactory epithelium (OE). These results suggest that nasal inflammation, mucus hypersecretion, and olfactory neurotoxicity could be important adverse health effects associated with short-term, repeated, airborne exposures to macrocyclic trichothecenes.

Rapid degeneration and regeneration of the zebrafish olfactory epithelium after triton X-100 application

The effects of Triton X-100 on the olfactory epithelium (OE) of adult zebrafish were examined to study neuronal turnover in this model system. Fish were killed at various time points after detergent application and stained with hematoxylin and eosin to examine olfactory structures, immunocytochemistry to examine cell types, or DiI to examine connections to the olfactory bulb. A significant decrease in epithelial thickness of treated sides was observed 1-day posttreatment. Epithelium thickness recovered by 5 days. The most significant reduction in the OE following Triton X-100 treatment corresponded to the region of supporting cells and mature olfactory sensory neurons. Labeling for all neurons with anti-Hu and for the 3 sensory neuron subtypes of the zebrafish OE (ciliated, microvillous, and crypt neurons) diminished 1 day after lesion and returned by 5 days posttreatment. Retrograde labeling from the olfactory bulb showed that the majority of mature olfactory sensory neurons disappeared in 1 day and reappeared by 5 days after treatment. Anti-proliferating cell nuclear antigen was used to show mitotic activity, and after chemical lesion, there was an increase in proliferation in specific regions of the OE. Thus, chemical ablation causes temporary reduction with swift regeneration of the OE occurring within a week.

Zicam-induced damage to mouse and human nasal tissue

Intranasal medications are used to treat various nasal disorders. However, their effects on olfaction remain unknown. Zicam (zinc gluconate; Matrixx Initiatives, Inc), a homeopathic substance marketed to alleviate cold symptoms, has been implicated in olfactory dysfunction. Here, we investigated Zicam and several common intranasal agents for their effects on olfactory function. Zicam was the only substance that showed significant cytotoxicity in both mouse and human nasal tissue. Specifically, Zicam-treated mice had disrupted sensitivity of olfactory sensory neurons to odorant stimulation and were unable to detect novel odorants in behavioral testing. These findings were long-term as no recovery of function was observed after two months. Finally, human nasal explants treated with Zicam displayed significantly elevated extracellular lactate dehydrogenase levels compared to saline-treated controls, suggesting severe necrosis that was confirmed on histology. Our results demonstrate that Zicam use could irreversibly damage mouse and human nasal tissue and may lead to significant smell dysfunction.

Prolactin may stimulate proliferation in the olfactory epithelium of the female mouse

BACKGROUND

Although it is well known that olfactory receptor neurons differentiated from stem cells have an unusual lifelong ability to continue neurogenesis, effective treatment of patients with sensorineural olfactory loss is still lacking. The purpose of this study was to determine whether intranasal prolactin (PRL) is an effective inducer of proliferation in the olfactory epithelium (OE).

METHODS

The OE of mice treated with PRL for 6 consecutive days and of pregnant mice on gestation day (GD) 7 was investigated by immunohistochemical study using antibodies to bromodeoxyuridine.

RESULTS

The number of bromodeoxyuridine -labeled cells in the OE increased significantly in PRL-treated female mice and GD 7 mice (but not PRL-treated male mice) compared with controls.

CONCLUSION

Pregnancy may induce proliferation in the OE, and administration of intranasal PRL may have almost the same effect as pregnancy on the OE of female mice.

Role of the olfactory receptor neurons in the direct transport of inhaled uranium to the rat brain

Uranium presents numerous industrial and military uses and one of the most important risks of contamination is dust inhalation. In contrast to the other modes of contamination, the inhaled uranium has been proposed to enter the brain not only by the common route of all modes of exposure, the blood pathway, but also by a specific inhalation exposure route, the olfactory pathway. To test whether the inhaled uranium enter the brain directly from the nasal cavity, male Sprague-Dawley rats were exposed to both inhaled and intraperitoneally injected uranium using the (236)U and (233)U, respectively, as tracers. The results showed a specific frontal brain accumulation of the inhaled uranium which is not observed with the injected uranium. Furthermore, the inhaled uranium is higher than the injected uranium in the olfactory bulbs (OB) and tubercles, in the frontal cortex and in the hypothalamus. In contrast, the other cerebral areas (cortex, hippocampus, cerebellum and brain residue) did not show any preferential accumulation of inhaled or injected uranium. These results mean that inhaled uranium enters the brain via a direct transfer from the nasal turbinates to the OB in addition to the systemic pathway. The uranium transfer from the nasal turbinates to the OB is lower in animals showing a reduced level of olfactory receptor neurons (ORN) induced by an olfactory epithelium lesion prior to the uranium inhalation exposure. These results give prominence to a role of the ORN in the direct transfer of the uranium from the nasal cavity to the brain.

Harmful effects of cadmium on olfactory system in mice

The inhalation of certain metals can result in olfactory epithelial injury, an altered sense of smell, and direct delivery of the metal from the olfactory epithelium to the olfactory bulbs and other parts of the central nervous system. The purpose of this study was to examine whether mice given an intranasal instillation of cadmium would develop altered olfactory function and to assess whether cadmium may be transported directly from the olfactory epithelium to the central nervous system. To evaluate cadmium's ability to induce anosmia and on the basis of olfactory epithelium sensitivity to metals, the aim of this study was first to study cadmium effects on the olfactory function and secondly to check whether cadmium may be transported from the nasal area to the central nervous system. After an intranasal instillation of a solution containing CdCl2 at 136 mM, we observed in treated mice: (1) a partial destruction of the olfactory epithelium, which is reduced to three or four basal cell layers followed by a progressive regeneration; (2) a loss of odor discrimination with a subsequent recovery; and (3) a cadmium uptake by olfactory bulbs demonstrated using atomic absorption spectrophotometry, but not by other parts of the central nervous system. Cadmium was delivered to the olfactory bulbs, most likely along the olfactory nerve, thereby bypassing the intact blood-brain barrier. We consider that cadmium can penetrate olfactory epithelium and hence be transported to olfactory bulbs. The olfactory route could therefore be a likely way to reach the brain and should be taken into account for occupational risk assessments for this metal.

Carbon dioxide effects on olfactory functioning: behavioral, histological and immunohistochemical measurements

Most studies on toxic inhalation focus on solvent effects and few have dealt with gases on olfactory functioning. Among gases, the effects of carbon dioxide on general physiology have been well investigated contrary to the impact on olfactory neuroepithelium. Thus, this work was designed to evaluate in mice the possible effects of 3% CO(2) in two exposure periods: a 5h/day and a 12h/day conditions. Behavioral, histological and immunohistochemical observations were conducted every 2 weeks, i.e. before (W0), during (W2, W4) and after exposure (W6, W8). Firstly, behavioral evaluations of odor sensitivity showed differences in relation to the odor tested, i.e. no effect with congener urine odor and a reinforcement of 2,4,5-trimethythiazoline (TMT) (predator odor) repulsion. Secondly, histological evaluations showed a similar evolution of the epithelium thickness, i.e. a decrease along the exposure as well as during the post-exposure period and an increase of cell number (whatever the phenotype) although the kinetic appeared different in both experimental conditions. Thirdly, immunohistochemical quantification of olfactory marker protein (OMP)- and proliferating cell nuclear antigen (PCNA)-positive cells revealed that the number of mature olfactory neurons increased at the early beginning of exposure period in both conditions. While a decrease was observed in the following weeks (W4-W8) for the 12h/day condition, a stable amount of OMP-positive cells was maintained in the 5h/day condition. In contrast, the number of PCNA-positive cells followed a similar evolution, i.e. a constant decrease along the experiment. These findings indicate that the effects of CO(2) inhalation exposure are selectively dose-dependent.

The effect of Ginkgo biloba on the expression of intermediate-early antigen (c-fos) in the experimentally induced anosmic mouse

OBJECTIVE

Treatment of olfactory dysfunction is very difficult and has limited modality. Treatment with steroids has been used in patients with olfactory dysfunction but the side effects of steroid need to be weighed against its potential benefits. In the present study, the effect of systemic administration of dexamethasone and EGb 761 on damage to olfactory mucosa produced by zinc sulfate was examined. Expression of the immediate-early antigen (IEG), c-fos, in the olfactory bulb and piriform cortex was used to determine the effects of treatment.

METHODS

Young adult CD1 mice (6 to 8 weeks old, male) were used. After anosmic mice were made by bilateral intranasal irrigation with 0.2 ml of 5% (0.17 M) zinc sulfate, anosmia was confirmed by a food finding test. Four groups of anosmic mice were studied: a steroid group (steroid injection group, n=12), an EGb group (EGb injection group, n=12), a steroid-EGb group (steroid and EGb injection group, n=12), and a control group (anosmic mice and no Tx. n=12). The olfactory bulb and piriform cortex of four mice in each group were obtained at 1, 2, and 3 weeks after instillation of zinc sulfate by cardiac perfusion, and immunohistochemical staining for c-fos was also performed to evaluate brain activity. In approximately 10 well-defined glomeruli of the olfactory bulb and in one side of the piriform cortex, c-fos (+) cells were counted. Statistical analyses were performed by Kruskal-Wallis one-way analysis of variance (ANOVA) by rank.

RESULTS

In all experimental groups, c-fos (+) cells increased in a time-dependent manner. The combination treatment of steroid and EGb was the most effective and the no-treatment group the least effective 1 week later after zinc sulfate irrigation. However, 3 weeks later after zinc sulfate irrigation, there was no statistically significant differences in the number of c-fos positive cells among all 4 groups (3 treatment groups and the control group).

CONCLUSION

The combination treatment of EGb and steroid enhanced the regeneration of the olfactory pathway after olfactory mucosal injury by zinc sulfate. Our study suggests that EGb could be an effective treatment option for olfactory dysfunction.

Sensory deafferentation transsynaptically alters neuronal GluR1 expression in the external plexiform layer of the adult mouse main olfactory bulb

Altered distribution of the alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptor subunit GluR1 has been linked to stimulation-dependent changes in synaptic efficacy, including long-term potentiation and depression. The main olfactory bulb (OB) remains plastic throughout life; how GluR1 may be involved in this plasticity is unknown. We have previously shown that neonatal naris occlusion reduces numbers of interneuron cell bodies that are immunoreactive for GluR1 in the external plexiform layer (EPL) of the adult mouse OB. Here, we show that immunoreactivity of mouse EPL interneurons for GluR1 is also dramatically reduced following olfactory deafferentation in adulthood. We further show that expression of glutamic acid decarboxylase (GAD) 65, 1 of 2 GAD isoforms expressed by adult gamma-aminobutyric acidergic interneurons, is reduced, but to a much smaller extent, and that in double-labeled cells, immunoreactivity for the Ca(2+)-binding protein parvalbumin (PV) is also reduced. In addition, GluR1 expression is reduced in presumptive tufted cells and interneurons that are negative for GAD65 and PV. Consistent with previous reports, sensory deafferentation resulted in little neuronal degeneration in the adult EPL, indicating that these differences were not likely due to death of EPL neurons. Together, these results suggest that olfactory input regulates expression of the GluR1 AMPA receptor subunit by tufted cells that may in turn regulate GluR1 expression by interneurons within the OB EPL.

Olfactory ensheathing glia: Their contribution to primary olfactory nervous system regeneration and their regenerative potential following transplantation into the injured spinal cord

Olfactory ensheathing glia (OEG) are a specialized type of glia that guide primary olfactory axons from the neuroepithelium in the nasal cavity to the brain. The primary olfactory system is able to regenerate after a lesion and OEG contribute to this process by providing a growth-supportive environment for newly formed axons. In the spinal cord, axons are not able to restore connections after an injury. The effects of OEG transplants on the regeneration of the injured spinal cord have been studied for over a decade. To date, of all the studies using only OEG as a transplant, 41 showed positive effects, while 13 studies showed limited or no effects. There are several contradictory reports on the migratory and axon growth-supporting properties of transplanted OEG. Hence, the regenerative potential of OEG has become the subject of intense discussion. In this review, we first provide an overview of the molecular and cellular characteristics of OEG in their natural environment, the primary olfactory nervous system. Second, their potential to stimulate regeneration in the injured spinal cord is discussed. OEG influence scar formation by their ability to interact with astrocytes, they are able to remyelinate axons and promote angiogenesis. The ability of OEG to interact with scar tissue cells is an important difference with Schwann cells and may be a unique characteristic of OEG. Because of these effects after transplantation and because of their role in primary olfactory system regeneration, the OEG can be considered as a source of neuroregeneration-promoting molecules. To identify these molecules, more insight into the molecular biology of OEG is required. We believe that genome-wide gene expression studies of OEG in their native environment, in culture and after transplantation will ultimately reveal unique combinations of molecules involved in the regeneration-promoting potential of OEG.

Transport of intranasally instilled fine Fe2O3 particles into the brain: micro-distribution, chemical states, and histopathological observation

It has been demonstrated that inhaled fine (d < 2.5 microm) and ultrafine (d < 100 nm) particles produce more severe toxicity than coarse particles. Some recent data support the concept that the central nervous system (CNS) may be a target for the inhaled fine particulates. This work describes initial observation of the transport of intranasally instilled fine ferric oxide (Fe2O3) particles in animal brain. The iron micro-distribution and chemical state in the mice olfactory bulb and brain stem on day 14 after intranasal instillation of fine Fe2O3 particle (280 +/- 80 nm) suspension at a single dose of 40 mg/kg body weight were analyzed by synchrotron radiation x-ray fluorescence and x-ray absorption near-edge structure (XANES). The micro-distribution map of iron in the olfactory bulb and brain stem shows an obvious increase of Fe contents in the olfactory nerve and the trigeminus of brain stem, suggesting that Fe2O3 particles were possibly transported via uptake by sensory nerve endings of the olfactory nerve and trigeminus. The XANES results indicate that the ratios of Fe (III)/Fe (II) were increased in the olfactory bulb and brain stem. The further histopathological observation showed that the neuron fatty degeneration occurred in the CA3 area of hippocampus. Such results imply an adverse impact of inhalation of fine Fe2O3 particles on CNS.

Transition of mouse de novo methyltransferases expression from Dnmt3b to Dnmt3a during neural progenitor cell development

Dnmt3a and Dnmt3b, which are known as functional de novo methyltransferases, are responsible for creating genomic methylation patterns during mammalian development. Recently, we have shown that specific expression of Dnmt3b in epiblast, embryonic ectoderm, hematopoietic progenitor cells and spermatogonia cells is followed by Dnmt3a expression (Watanabe D, Suetake I, Tada T, Tajima S (2002) Stage- and cell-specific expression of Dnmt3a and Dnmt3b during embryogenesis. Mech Dev 118:187-190; Watanabe D, Suetake I, Tajima S, Hanaoka K (2004) Expression of Dnmt3b in mouse hematopoietic progenitor cells and spermatogonia at specific stages. Gene Expr Patterns 5:43-49). In this study, we analyzed the expression of mouse de novo methyltransferases during development of the nervous systems. In the embryonic olfactory epithelium (OE), Dnmt3b was specifically expressed in Mash1 positive globose basal cells (i.e. transiently amplifying neural progenitor cells), while Dnmt3a was expressed in immature olfactory receptor neurons. Dnmt3b-positive cells were rarely observed in the adult OE, but were increased in regenerating OE with intranasal ZnSO(4) administration. Dnmt3b was also detected in the E8.5 neural plate, E10.5 spinal cord and retina cells, while Dnmt3a was expressed in postmitotic young neurons. Furthermore, Dnmt3b was specifically expressed in ES cells, while Dnmt3a was transiently expressed during neural cell differentiation of ES cells. Dnmt3b is specifically expressed in progenitor cells during hematopoiesis, spermatogenesis and neurogenesis, suggesting an important role in the initial steps of progenitor cell differentiation. Dnmt3a is expressed in postmitotic young neurons following the Dnmt3b expression. Dnmt3a may be required for the establishment of tissue-specific methylation patterns of the genome. The coordinated expression of de novo methyltransferases from Dnmt3b to Dnmt3a suggests conserved mechanisms of de novo methylation of the genome and different functions for Dnmt3b and Dnmt3a during progenitor cell development.

Expression of Notch1 and Hes5 in the developing olfactory epithelium

CONCLUSION

Notch signaling pathway may play an important role in the development of the olfactory epithelium (OE). Objectives. To elucidate whether the Notch signaling pathway mediates the developmental processes in the developing OE.

MATERIALS AND METHODS

The expression of Notch1 and Hes5 in the developing OE of mice with ages ranging from embryonic day (E) 11 to postnatal day (PN) 14 was examined.

RESULTS

As detected by in situ hybridization, Notch1 was expressed in scattered cells located in the basal portion of the embryonic OE and later in the cell layer adjacent to the basal lamina from E11 to PN14. Hes5 was expressed in scattered cells located in the basal portion of the embryonic OE from E11. However, at the late embryonic stage, the number of Hes5-positive cells decreased and after birth distinct Hes5-positive cells were not observed in the OE.

Effects of toluene inhalation exposure on olfactory functioning: behavioral and histological assessment

Exposure to pollutant or toxic substances is known to induce adverse health effects but few studies have been devoted to study the impact on olfactory functioning although neuroreceptors in the nasal cavity are directly in contact with volatile molecules. Thus, this work was designed to evaluate in mice the potential modifications of the olfactory functioning during (1 month) and after (1 month) a prolonged toluene exposure at both sensitive/perceptive and cellular levels. Mice were exposed to 1000ppm of toluene for 5h/day, 5days/week for 4 weeks. Firstly, behavioral evaluation (T-maze test) to toluene sensitivity showed a constant decrease during all the 4 weeks of exposure (W1-W4) which continued during 2 weeks after the exposure (W5, W6). In contrast, during the last 2 weeks of the experiment (W7, W8), the sensitivity of mice to toluene went back to normal. Secondly, structural modifications, i.e. density of cells and thickness of olfactory epithelium were observed soon after the outset of exposure. The number of cells did not change at the beginning of exposure (W1, W2), decreased markedly later (W3, W4), increased significantly the first week of the recovery period (W5) and stayed stable during the following weeks (W6-W8). Concerning the thickness of neuroepithelium, the results at W1 showed a decrease followed by an increase suggesting an inflammatory process (W2, W3). In contrast, the results of W4 revealed an abrupt decrease of the thickness whereas the return to normal arose immediately at the outset of recovery period.

Conditional ablation of mature olfactory sensory neurons mediated by diphtheria toxin receptor

The vertebrate olfactory epithelium provides an excellent model system to study the regulatory mechanisms of neurogenesis and neuronal differentiation due to its unique ability to generate new sensory neurons throughout life. The replacement of olfactory sensory neurons is stimulated when damage occurs in the olfactory epithelium. In this study, transgenic mice, with a transgene containing human diphtheria toxin receptor under the control of the olfactory marker protein promoter (OMP-DTR), were generated in which the mature olfactory sensory neurons could be specifically ablated when exposed to diphtheria toxin. Following diphtheria toxin induced neuronal ablation, we observed increased numbers of newly generated growth associated protein 43 (GAP43)-positive immature olfactory sensory neurons. OMP-positive neurons were continuously produced from the newly generated GAP43-positive cells. The expression of the signal transduction components adenylyl cyclase type III and the G-protein alpha subunit G(alpha olf) was sensitive to diphtheria toxin exposure and their levels decreased dramatically preceding the disappearance of the OMP-positive sensory neurons. These data validate the hypothesis that OMP-DTR mice can be used as a tool to ablate the mature olfactory sensory neurons in a controlled fashion and to study the regulatory mechanisms of the neuronal replacement.

Changes in reelin expression in the mouse olfactory bulb after chemical lesion to the olfactory epithelium

To explore the functional roles of Reelin in the adult olfactory system, we examined changes in the expression of reelin mRNA and Reelin protein in the olfactory bulb (OB) of adult mice after a chemical lesion to the olfactory epithelium. Following intranasal irrigation with 2% zinc sulphate solution, animals were perfused at various times between 5 and 40 days post-lesion. The expression of reelin mRNA in mitral cells in the OB was slightly reduced at 5 days post-lesion, completely abolished by 20 days, but restored almost to the normal level at 40 days post-lesion. Similarly, the expression of Reelin protein in mitral cells of the deafferented OB also recovered, although not to the normal level. No recovery of either reelin mRNA or Reelin immunoreactivity was seen in the periglomerular cells and external tufted cells. The expression profile of reelin mRNA and Reelin protein in the OB coincided with the time course of degeneration and regeneration of olfactory nerves, as indicated by anterograde labeling of olfactory nerves with WGA-HRP. These results suggest that expression of reelin mRNA in the adult OB is regulated by olfactory inputs.

Dehydroepiandrosterone regulates astroglia reaction to denervation of olfactory glomeruli

Effects of dehydroepiandrosterone (DHEA) on glial reactions of the peripherally denervated olfactory bulb were studied in adult male rats. Denervation was achieved by destroying the olfactory mucosa with ZnSO(4) (0.17 M) irrigation of the nasal cavities. In one series of experiments, chronic DHEA treatment was applied (daily injections for 7 days, i.p., 10 mg/kg b.w. and 25 mg/kg b.w.); in the other series of experiments, animals received a single injection of DHEA (i.p., 10 mg/kg b.w., 25 mg/kg b.w. and 50 mg/kg b.w.) 2 h following ZnSO(4) treatment. To determine whether DHEA conversion to estradiol was involved in the mechanism of DHEA action on glia, a third series of experiments was carried out in which the aromatase inhibitor fadrozole (4.16 mg/ml) was administered using subcutaneously implanted osmotic minipumps. Rats were killed on day 7 after chemical denervation, and the reaction of glial cells was monitored within the olfactory bulb, using GFAP and vimentin immunohistochemistry. Qualitative changes in GFAP expression were analyzed by Western blot. Chronic DHEA treatment with both doses (10 mg/kg b.w. and 25 mg/kg b.w.) and acute DHEA treatment with the highest dose applied (50 mg/kg b.w.), inhibited the increase in GFAP expression induced by the denervation of the olfactory bulb. Furthermore, GFAP and vimentin immunostaining in the glomerular layer of the olfactory bulb were diminished in the denervated and DHEA treated groups. However, when DHEA treatment was combined with fadrozole administration, such a decrease in GFAP expression could not be detected in the chemically denervated olfactory bulb. These findings indicate that DHEA, depending on the dose applied and the mode of administration, attenuates glial reaction to denervation and may regulate glial plasticity in the olfactory glomeruli. These effects are likely to be mediated at least in part by the conversion of DHEA to estradiol.