Ultrastructural study of mouse olfactory epithelium following destruction by ZnSO4 and its subsequent regeneration

Single or Repeated Ablation of Mouse Olfactory Epithelium by Methimazole

Odor-detecting olfactory sensory neurons residing in the nasal olfactory epithelium (OE) are the only neurons in direct contact with the external environment. As a result, these neurons are subjected to chemical, physical, and infectious insults, which may be the underlying reason why neurogenesis occurs in the OE of adult mammals. This feature makes the OE a useful model for studying neurogenesis and neuronal differentiation, with the possibility for systemic as well as local administration of various compounds and infectious agents that may interfere with these cellular processes. Several different chemical compounds have been shown to cause toxic injury to the OE, which can be used for OE ablation. We, and others, have found that the systemic administration of the hyperthyroid drug, methimazole, reliably causes olfactotoxicity as a side effect. Here, we outline an OE lesioning protocol for single or repeated ablation by methimazole. A single methimazole administration can be used to study neuroepithelial regeneration and stem cell activation, while repeated ablation and regeneration of OE enable the study of tissue stem cell exhaustion and generation of tissue metaplasia.

Diving into the streams and waves of constitutive and regenerative olfactory neurogenesis: insights from zebrafish

The olfactory system is renowned for its functional and structural plasticity, with both peripheral and central structures displaying persistent neurogenesis throughout life and exhibiting remarkable capacity for regenerative neurogenesis after damage. In general, fish are known for their extensive neurogenic ability, and the zebrafish in particular presents an attractive model to study plasticity and adult neurogenesis in the olfactory system because of its conserved structure, relative simplicity, rapid cell turnover, and preponderance of neurogenic niches. In this review, we present an overview of the anatomy of zebrafish olfactory structures, with a focus on the neurogenic niches in the olfactory epithelium, olfactory bulb, and ventral telencephalon. Constitutive and regenerative neurogenesis in both the peripheral olfactory organ and central olfactory bulb of zebrafish is reviewed in detail, and a summary of current knowledge about the cellular origin and molecular signals involved in regulating these processes is presented. While some features of physiologic and injury-induced neurogenic responses are similar, there are differences that indicate that regeneration is not simply a reiteration of the constitutive proliferation process. We provide comparisons to mammalian neurogenesis that reveal similarities and differences between species. Finally, we present a number of open questions that remain to be answered.

Age-Related Olfactory Dysfunction: Epidemiology, Pathophysiology, and Clinical Management

Like other sensory systems, olfactory function deteriorates with age. Epidemiological studies have revealed that the incidence of olfactory dysfunction increases at the age of 60 and older and males are more affected than females. Moreover, smoking, heavy alcohol use, sinonasal diseases, and Down’s syndrome are associated with an increased incidence of olfactory dysfunction. Although the pathophysiology of olfactory dysfunction in humans remains largely unknown, studies in laboratory animals have demonstrated that both the peripheral and central olfactory nervous systems are affected by aging. Aged olfactory neuroepithelium in the nasal cavity shows the loss of mature olfactory neurons, replacement of olfactory neuroepithelium by respiratory epithelium, and a decrease in basal cell proliferation both in the normal state and after injury. In the central olfactory pathway, a decrease in the turnover of interneurons in the olfactory bulb (OB) and reduced activity in the olfactory cortex under olfactory stimulation is observed. Recently, the association between olfactory impairment and neurodegenerative diseases, such as Alzheimer’s disease (AD) and Parkinson’s disease (PD), has gained attention. Evidence-based pharmacotherapy to suppress or improve age-related olfactory dysfunction has not yet been established, but preliminary results suggest that olfactory training using odorants may be useful to improve some aspects of age-related olfactory impairment.

Stem and progenitor cells of the mammalian olfactory epithelium: Taking poietic license

The capacity of the olfactory epithelium (OE) for lifelong neurogenesis and regeneration depends on the persistence of neurocompetent stem cells, which self-renew as well as generating all of the cell types found within the nasal epithelium. This Review focuses on the types of stem and progenitor cells in the epithelium and their regulation. Both horizontal basal cells (HBCs) and some among the population of globose basal cells (GBCs) are stem cells, but the two types plays vastly different roles. The GBC population includes the basal cells that proliferate in the uninjured OE and is heterogeneous with respect to transcription factor expression. From upstream in the hierarchy to downstream, GBCs encompass 1) Sox2⁺ /Pax6⁺ stem-like cells that are totipotent and self-renew over the long term, 2) Ascl1⁺ transit-amplifying progenitors with a limited capacity for expansive proliferation, and 3) Neurog1⁺ /NeuroD1⁺ immediate precursor cells that make neurons directly. In contrast, the normally quiescent HBCs are activated to multipotency and proliferate when sustentacular cells are killed, but not when only OSNs die, indicating that HBCs are reserve stem cells that respond to severe epithelial injury. The master regulator of HBC activation is the ΔN isoform of the transcription factor p63; eliminating ΔNp63 unleashes HBC multipotency. Notch signaling, via Jagged1 ligand on Sus cells and Notch1 and Notch2 receptors on HBCs, is likely to play a major role in setting the level of p63 expression. Thus, ΔNp63 becomes a potential therapeutic target for reversing the neurogenic exhaustion characteristic of the aged OE. J. Comp. Neurol. 525:1034-1054, 2017. © 2016 Wiley Periodicals, Inc.

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.

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.

Injury in aged animals robustly activates quiescent olfactory neural stem cells

While the capacity of the olfactory epithelium (OE) to generate sensory neurons continues into middle age in mice, it is presumed that this regenerative potential is present throughout all developmental stages. However, little experimental evidence exists to support the idea that this regenerative capacity remains in late adulthood, and questions about the functionality of neurons born at these late stages remain unanswered. Here, we extend our previous work in the VNO to investigate basal rates of proliferation in the OE, as well as after olfactory bulbectomy (OBX), a commonly used surgical lesion. In addition, we show that the neural stem cell retains its capacity to generate mature olfactory sensory neurons in aged animals. Finally, we demonstrate that regardless of age, a stem cell in the OE, the horizontal basal cell (HBC), exhibits a morphological switch from a flattened, quiescent phenotype to a pyramidal, proliferative phenotype following chemical lesion in aged animals. These findings provide new insights into determining whether an HBC is active or quiescent based on a structural feature as opposed to a biochemical one. More importantly, it suggests that neural stem cells in aged mice are responsive to the same signals triggering proliferation as those observed in young mice.

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.

Histopathology of the tegument of rabbits infested by Rhipicephalus sanguineus (ACARI: IXODIDAE) ticks and exposed to selamectin (active principle of acaricide Revolution, Pfizer)

Ticks are hematophagous ectoparasites which can transmit several diseases to the host during their feeding process. When ticks mechanically damage the tissue, they eventually induce inflammatory responses on the skin spot where they are fixed. One of the alternatives to control these ectoparasites is the use of chemical substances like selamectin-the active principle of Pfizer's antiparasitic Revolution-a macrocyclic lactone capable of doing neurotoxic damage to the tick and eventually eliminating infestation in dogs and cats. The purpose of this study was to analyze, using histological and histochemical techniques, the occurrence of morphophysiological alterations in the skin of the host rabbits exposed to selamectin and infested with Rhipicephalus sanguineus (Acari: Ixodidae). Histologically, the exposed and infested rabbits showed a partial and/or total decrease in the stratum corneum and the epithelium decreased in the number of cell layers, consequently reducing the stratification (thinning) and quite pronounced formations of sub-epidermal edemas with consequent disorganization of collagen fibers in the dermal layer's connective tissue. Histochemical tests showed strong periodic acid-Schiff-positive reaction in the hair follicle and some regions of the dermis, besides resynthesis of collagen fibers detected by Mallory's trichrome technique. The obtained results showed that selamectin acts like a toxicant agent when in contact with the skin of the rabbit infested with ticks, inducing morphophysiological alterations in the acute inflammatory process in the animal's tegument. Selamectin is a chemical substance which has a dose-dependent action since higher concentrations cause greater morphophysiological damage in the skin of rabbits.

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.

Nxnl2 splicing results in dual functions in neuronal cell survival and maintenance of cell integrity

The rod-derived cone viability factors, RdCVF and RdCVF2, have potential therapeutical interests for the treatment of inherited photoreceptor degenerations. In the mouse lacking Nxnl2, the gene encoding RdCVF2, the progressive decline of the visual performance of the cones in parallel with their degeneration, arises due to the loss of trophic support from RdCVF2. In contrary, the progressive loss of rod visual function of the Nxnl2-/- mouse results from a decrease in outer segment length, mediated by a cell autonomous mechanism involving the putative thioredoxin protein RdCVF2L, the second spliced product of the Nxnl2 gene. This novel signaling mechanism extends to olfaction as shown by the progressive impairment of olfaction in aged Nxnl2-/- mice and the protection of olfactory neurons by RdCVF2. This study shows that Nxnl2 is a bi-functional gene involved in the maintenance of both the function and the viability of sensory neurons.

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.

Telomere shortening impairs regeneration of the olfactory epithelium in response to injury but not under homeostatic conditions

Atrophy of the olfactory epithelium (OE) associated with impaired olfaction and dry nose represents one of the most common phenotypes of human aging. Impairment in regeneration of a functional olfactory epithelium can also occur in response to injury due to infection or nasal surgery. These complications occur more frequently in aged patients. Although age is the most unifying risk factor for atrophic changes and functional decline of the olfactory epithelium, little is known about molecular mechanisms that could influence maintenance and repair of the olfactory epithelium. Here, we analyzed the influence of telomere shortening (a basic mechanism of cellular aging) on homeostasis and regenerative reserve in response to chemical induced injury of the OE in late generation telomere knockout mice (G3 mTerc(-/-)) with short telomeres compared to wild type mice (mTerc(+/+)) with long telomeres. The study revealed no significant influence of telomere shortening on homeostatic maintenance of the OE during mouse aging. In contrast, the regenerative response to chemical induced injury of the OE was significantly impaired in G3 mTerc(-/-) mice compared to mTerc(+/+) mice. Seven days after chemical induced damage, G3 mTerc(-/-) mice exhibited significantly enlarged areas of persisting atrophy compared to mTerc(+/+) mice (p = 0.031). Telomere dysfunction was associated with impairments in cell proliferation in the regenerating epithelium. Deletion of the cell cycle inhibitor, Cdkn1a (p21) rescued defects in OE regeneration in telomere dysfunctional mice. Together, these data indicate that telomere shortening impairs the regenerative capacity of the OE by impairing cell cycle progression in a p21-dependent manner. These findings could be relevant for the impairment in OE function in elderly people.

Age-related changes of the regeneration mode in the mouse peripheral olfactory system following olfactotoxic drug methimazole-induced damage

We investigated age-related changes in the mode of regeneration in the mouse peripheral olfactory system after olfactotoxic drug-induced damage. Mice at postnatal ages of 10 days, 3 months, and 16 months were given an intraperitoneal injection of methimazole to produce damage in the olfactory neuroepithelium. The olfactory neuroepithelia were harvested and analyzed immunohistochemically at various postlesion timepoints, from 1 day through to 94 days, to investigate neuroepithelial cell proliferation, the time course of neuronal differentiation, the reconstitution of neuroepithelium, and the innervation of the olfactory bulb. Functional recovery was assessed using the vanillin avoidance behavioral test. The chronological pattern in the expression of Ki67, beta III tubulin, and olfactory marker protein, molecular markers for neuronal cell proliferation and differentiation, changed similarly among the different age groups. In contrast, the extent of neuroepithelial cell proliferation after injury decreased with age, and the final histological recovery of the olfactory neuroepithelium and the innervation of the olfactory bulb were significantly smaller in the 16-month-old group compared to the younger age groups. These results suggest that the age-related decline in the capacity of olfactory neuroepithelium to reconstitute neuroepithelium is associated with its age-related decrease in proliferative activity after the neuroepithelial injury rather than changes in the process of neuronal differentiation. In spite of these incomplete anatomical recoveries, 16-month-old mice regained the ability to avoid vanillin solution by 1 month postlesion, suggesting that the extent of anatomical epithelial damage is not necessarily proportional to the threshold of olfactory perception.

Immunohistochemical characterization of human olfactory tissue

OBJECTIVES/HYPOTHESIS

The pathophysiology underlying human olfactory disorders is poorly understood because biopsying the olfactory epithelium (OE) can be unrepresentative and extensive immunohistochemical analysis is lacking. Autopsy tissue enriches our grasp of normal and abnormal olfactory immunohistology and guides the sampling of the OE by biopsy. Furthermore, a comparison of the molecular phenotype of olfactory epithelial cells between rodents and humans will improve our ability to correlate human histopathology with olfactory dysfunction.

STUDY DESIGN

An immunohistochemical analysis of human olfactory tissue using a comprehensive battery of proven antibodies.

METHODS

Human olfactory mucosa obtained from 21 autopsy specimens was analyzed with immunohistochemistry. The position and extent of olfactory mucosa was assayed by staining whole mounts (WMs) with neuronal markers. Sections of the OE were analyzed with an extensive group of antibodies directed against cytoskeletal proteins and transcription factors, as were surgical specimens from an esthesioneuroblastoma.

RESULTS

Neuron-rich epithelium is always found inferior to the cribriform plate, even at advanced age, despite the interruptions in the neuroepithelial sheet caused by patchy respiratory metaplasia. The pattern of immunostaining with our antibody panel identifies two distinct types of basal cell progenitors in human OE similar to rodents. The panel also clarifies the complex composition of esthesioneuroblastoma.

CONCLUSIONS

The extent of human olfactory mucosa at autopsy can easily be delineated as a function of age and neurologic disease. The similarities in human versus rodent OE will enable us to translate knowledge from experimental animals to humans and will extend our understanding of human olfactory pathophysiology.

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.

Expression of pax6 and sox2 in adult olfactory epithelium

The olfactory epithelium maintains stem and progenitor cells that support the neuroepithelium's life-long capacity to reconstitute after injury. However, the identity of the stem cells--and their regulation--remain poorly defined. The transcription factors Pax6 and Sox2 are characteristic of stem cells in many tissues, including the brain. Therefore, we assessed the expression of Pax6 and Sox2 in normal olfactory epithelium and during epithelial regeneration after methyl bromide lesion or olfactory bulbectomy. Sox2 is found in multiple kinds of cells in normal epithelium, including sustentacular cells, horizontal basal cells, and some globose basal cells. Pax6 is co-expressed with Sox2 in all these, but is also found in duct/gland cells as well as olfactory neurons that innervate necklace glomeruli. Most of the Sox2/Pax6-positive globose basal cells are actively cycling, but some express the cyclin-dependent kinase inhibitor p27(Kip1), and are presumably mitotically quiescent. Among globose basal cells, Sox2 and Pax6 are co-expressed by putatively multipotent progenitors (labeled by neither anti-Mash1 nor anti-Neurog1) and neuron-committed transit amplifying cells (which express Mash1). However, Sox2 and Pax6 are expressed by only a minority of immediate neuronal precursors (Neurog1- and NeuroD1-expressing). The assignment of Sox2 and Pax6 to these categories of globose basal cells is confirmed by a temporal analysis of transcription factor expression during the recovery of the epithelium from methyl bromide-induced injury. Each of the Sox2/Pax6-colabeled cell types is at a remove from the birth of neurons; thus, suppressing their differentiation may be among the roles of Sox2/Pax6 in the olfactory epithelium.

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.

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 stimulation with scent of lavender oil affects autonomic neurotransmission and blood pressure in rats

Previously, we observed that olfactory stimulation with scent of lavender oil (SLVO) suppressed sympathetic nerve activities and elevated gastric vagal (parasympathetic) nerve activity (GVNA), decreased plasma glycerol concentration and body temperature, and enhanced appetite in rats. Here, we further showed that olfactory stimulation with SLVO lowered renal sympathetic nerve activity (RSNA) and blood pressure (BP) and elevated GVNA in urethane-anesthetized rats. Olfactory stimulation with linalool, a component of lavender oil, also elicited decreases in RSNA and BP and an increase in GVNA in urethane-anesthetized rats. Anosmia induced by pretreatment of the nasal cavity by application of ZnSO4 eliminated the effects of both SLVO and scent of linalool on RSNA, GVNA and BP. Furthermore, intracerebroventricular administration of thioperamide, a histaminergic H3-antagonist, abolished the suppression of RSNA and BP as well as the elevation of GVNA mediated by both SLVO and scent of linalool. Finally, bilateral lesions of the hypothalamic suprachiasmatic nucleus (SCN) eliminated RSNA and BP suppression and the elevation of GVNA due to SLVO or linalool. Thus, it was concluded that scent of lavender oil and its active component, linalool, affects autonomic neurotransmission and reduces blood pressure through the central histaminergic nervous system and the SCN.

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.

Olfactory stimulation with scent of essential oil of grapefruit affects autonomic neurotransmission and blood pressure

Previously, we observed that olfactory stimulation with scent of grapefruit oil (SGFO) enhances sympathetic nerve activities and suppresses gastric vagal (parasympathetic) nerve activity (GVNA), increases plasma glycerol concentration and body temperature, and decreases appetite in rats. Here, we show that olfactory stimulation with SGFO for 10 min elevates renal sympathetic nerve activity (RSNA) and blood pressure (BP) and lowers GVNA in urethane-anesthetized rats. Olfactory stimulation with limonene, a major component of grapefruit oil, also elicited increases in RSNA and BP in urethane-anesthetized rats. Anosmic treatment with ZnSO(4) eliminated both the effects of SGFO and scent of limonene on RSNA and BP. Intracerebral administration of diphenhydramine, a histaminergic H1-antagonist, abolished SGFO- or scent of limonene-mediated increases in RSNA and BP as well as the decrease in GVNA. Moreover, bilateral lesions of the hypothalamic suprachiasmatic nucleus (SCN) eliminated the SGFO- and limonene-mediated increases in RSNA and BP and decrease in GVNA, but bilateral lesions of the cerebral cortex did not have any affect on these parameters. These findings suggest that scent of grapefruit oil and its active component, limonene, affect autonomic neurotransmission and blood pressure through central histaminergic nerves and the suprachiasmatic nucleus.

Immunohistochemical studies of the cellular changes in the peripheral olfactory system after zinc sulfate nasal irrigation

The peripheral olfactory system has a remarkable capacity for repair. We have performed an immunohistochemical study of the cellular changes that occur after zinc sulfate irrigation of the nasal cavity. The rapid loss of epithelial cells was followed by the proliferation of basal cells and the restoration of the epithelium with olfactory tissue. Horizontal basal cell markers, anti-cytokeratin 5/6 (CK5/6), and the Bandeiraea simplicifolia (BS-1) lectin initially co-localized on day 1 after treatment but rapidly displayed a disparity in their staining profile, with CK5/6 immunoreactive cells having a profile more akin to cells expressing the sustentacular marker cytokeratin 18 (CK18). This suggests CK5/6 and BS-1 label a different subset of horizontal basal cells. Axonal degeneration and regeneration was studied with a panel of markers to olfactory receptor neurons, their terminals, and olfactory bulb dendrites. The glial cells of the peripheral olfactory system, olfactory ensheathing cells, remained in position, with little change in immunoreactivity to laminin, although an increase in the expression of glial fibrillary acidic protein was observed. The events and the extent of reconstitution of the olfactory system after degeneration serves as a foundation for future studies designed to understand the unique regenerative capacity of the olfactory system.

Response of olfactory ensheathing cells to the degeneration and regeneration of the peripheral olfactory system and the involvement of the neuregulins

In this study we examined the proliferative response of olfactory ensheathing cells (OECs) to olfactory receptor neuron injury induced by zinc sulfate (ZnSO4) irrigation and related the response of OECs within the peripheral system to the inflammatory response induced by injury and the expression profile of neuregulins. After ZnSO4 treatment, degeneration in the epithelium is reproducible and rapid, with regeneration following after 4 days, and is morphologically complete by 5 weeks. Changes in the olfactory bulb are less dramatic, although degeneration of both the outer and the glomerular layers occurred. Treatment also induced a marked inflammatory response in both the epithelium and the bulb. Unlike Schwann cell changes associated with Wallerian degeneration, OECs did not proliferate or obviously migrate within the olfactory system in response to axonal loss, suggesting that the new nerves generated from the epithelium regrow back through conduits already formed by the glia. Expression of neuregulin 1alpha was maintained in the nerve by OECs, and changes in neuregulin 1 mRNA and erbB2 mRNA expression were detected, indicating that these growth factors may play a role in the regeneration of the peripheral olfactory system but not in OEC proliferation.

Globose basal cells are required for reconstitution of olfactory epithelium after methyl bromide lesion

Despite a remarkable regenerative capacity, recovery of the mammalian olfactory epithelium can fail in severely injured areas, which subsequently reconstitute as aneuronal respiratory epithelium (metaplasia). We contrasted the cellular response of areas of the rat epithelium that recover as olfactory after methyl bromide lesion with those undergoing respiratory metaplasia in order to identify stem cells that restore lesioned epithelium as olfactory. Ventral olfactory epithelium is at particular risk for metaplasia after lesion and patches of it are rendered acellular by methyl bromide exposure. In contrast, globose basal cells (GBCs, marked by staining with GBC-2) are preserved in surrounding ventral areas and uniformly throughout dorsal epithelium, which consistently and completely recovers as olfactory after lesion. Over the next few days, neurons reappear, but only in those areas in which GBCs are preserved and multiply. In contrast, parts of the epithelium in which GBCs are destroyed are repopulated in part by Bowman's gland cells, which pile up above the basal lamina. Electron microscopy confirms the reciprocity between gland cells and globose basal cells. By 14 days after lesion, the areas that are undergoing metaplasia are repopulated by typical respiratory epithelial cells. As horizontal basal cells are eliminated from all parts of the ventral epithelium, the data suggest that GBC-2(+) cells are ultimately responsible for regenerating olfactory neuroepithelium. In contrast, GLA-13(+) cells may give rise to respiratory metaplastic epithelium where GBCs are eliminated. Thus, we support the idea that a subpopulation of GBCs is the neural stem cell of the olfactory epithelium.

Administration of transforming growth factor-alpha enhances anatomical and behavioral recovery following olfactory nerve transection

Although replacement of olfactory receptor neurons (ORNs) and subsequent reinnervation of the olfactory bulb occur following ORN injury, the intrinsic and extrinsic factors that contribute to the regulation of this dynamic process have not yet been fully identified. Recent research indicates that several growth factors have neurogenic effects on ORNs in vitro, and that chronic in vivo administration of either basic fibroblast growth factor, epidermal growth factor, or transforming growth factor-alpha (TGF-alpha) following chemical lesion can enhance the normal rate of ORN reinnervation of the olfactory bulb. The primary goal of the present experiments was to further assess the extent to which growth factor-related enhancements in the rate of anatomical recovery during ORN reconstitution and subsequent reinnervation of olfactory bulb are accompanied by enhancements in the rate of recovery of odor-guided behavior.A series of experiments in rats was conducted to initially characterize the time course of the anatomical and behavioral recovery normally observed following ORN reconstitution as a consequence of olfactory nerve transection, and to subsequently characterize the anatomical and behavioral effects of TGF-alpha administration on this normal rate of recovery. Consistent with a host of prior studies, olfactory nerve transection produced consistent and substantial deafferentation of olfactory bulb followed by a time-dependent anatomical recovery which was significantly enhanced by administration of TGF-alpha. The effect of TGF-alpha on functional recovery following olfactory nerve transection was also assessed using an odor-guided fear conditioning task. ORN lesioned animals receiving injections of TGF-alpha during recovery were found to display enhanced conditioned responding to an olfactory stimulus compared to untreated subjects. Further behavioral analyses suggested that this enhanced functional recovery was likely not due to non-specific effects of TGF-alpha on cognition or motor activity, but rather to enhanced olfactory input to the CNS. Future studies will likely reveal the exact mechanism of action mediating the anatomical and concomitant behavioral effects of this growth factor. Since ORNs are one of only a few populations of neurons capable of regeneration or replacement, the continued study of the cellular and molecular factors that coordinate this regenerative process may ultimately lead to the development of therapeutic strategies to promote an enhanced functional recovery following injury to other neuronal populations.

Recovery following peripheral destruction of olfactory neurons in young and adult mice

Olfactory neurons (ON) which are located in the olfactory epithelium are responsible of odorous molecule detection. A unique feature of these cells is their continuous replacement throughout life due to the proliferation and differentiation of local neural precursors, the basal cells. Thus, experimental destruction of all ON induces a stimulation of basal cell division followed by tissue regeneration. The fact that ON precursors display such proliferative and neurogenic activity in adults makes these cells particularly attractive as a potential tool for nervous system repair. However, basal cell proliferation and, thus, ON production, decrease in relation to age; mostly during the first months of life. Therefore, we aimed to seek whether the ability of ON precursors to yield new functional ON in regenerative conditions was consequently impaired in adult. ZnSO4 intranasal perfusion administered to young (1 month) and adult (6 months) mice leads in a few days to total ON destruction and to hyposmia. Tissue and function restoration occurred in the following weeks in both mice groups and was preceded by a transient peak of cell division. In adults, although neurogenesis in the impaired olfactory epithelium was less efficient than in young mice, neural precursors retain their ability to provide new functional ON as indicated by the butanol detection recovery. This was achieved more rapidly than total ON regeneration, suggesting that a reduced number of reconnected ON may be sufficient for odor discrimination.

Neural regeneration and the peripheral olfactory system

The peripheral olfactory system is able to recover after injury, i.e., the olfactory epithelium reconstitutes, the olfactory nerve regenerates, and the olfactory bulb is reinnervated, with a facility that is unique within the mammalian nervous system. Cell renewal in the epithelium is directed to replace neurons when they die in normal animals and does so at an accelerated pace after damage to the olfactory nerve. Neurogenesis persists because neuron-competent progenitor cells, including transit amplifying and immediate neuronal precursors, are maintained within the population of globose basal cells. Notwithstanding events in the neuron-depleted epithelium, the death of both non-neuronal cells and neurons directs multipotent globose basal cell progenitors, to give rise individually to sustentacular cells and horizontal basal cells as well as neurons. Multiple growth factors, including TGF-alpha, FGF2, BMPs, and TGF-betas, are likely to be central in regulating choice points in epitheliopoiesis. Reinnervation of the bulb is rapid and robust. When the nerve is left undisturbed, i.e., by lesioning the epithelium directly, the projection of the reconstituted epithelium onto the bulb is restored to near-normal with respect to rhinotopy and in the targeting of odorant receptor-defined neuronal classes to small clusters of glomeruli in the bulb. However, at its ultimate level, i.e., the convergence of axons expressing the same odorant receptor onto one or a few glomeruli, specificity is not restored unless a substantial number of fibers of the same type are spared. Rather, odorant receptor-defined subclasses of neurons innervate an excessive number of glomeruli in the rough vicinity of their original glomerular targets.

Functional consequences following infection of the olfactory system by intranasal infusion of the olfactory bulb line variant (OBLV) of mouse hepatitis strain JHM

The present study assessed the functional consequences of viral infection with a neurotropic coronavirus, designated MHV OBLV, that specifically targets central olfactory structures. Using standard operant techniques and a 'go, no-go' successive discrimination paradigm, six BALB/c mice were trained to discriminate between the presentation of an air or odor stimulus (three mice for each of the odorants propanol and propyl acetate). Two additional BALB/c mice were trained to discriminate between the presentation of air and the presentation of either vanillin or propionic acid. Following criterion performance, each mouse received an additional 2000 trials of overtraining. At completion of overtraining one mouse from the propanol and propyl acetate groups were allocated as untreated. The remaining six mice were inoculated with 300 microl of the OBLV stock per nostril for a total of 1.5 x 10(6) p.f.u. in 600 microl. Following a 1 month rest, untreated and inoculated animals were again tested on their respective air versus odor discrimination task. Untreated animals immediately performed at criterion levels. In contrast, inoculated animals varied in their capacity to discriminate between air and odorant. Five of the six inoculated mice showed massive disruption of the olfactory bulb, including death of mitral cells; the other was more modestly affected. In addition, the density of innervation of the olfactory mucosa by substance P-containing trigeminal fibers is also affected by inoculation. Those mice that remained anosmic to the training odorants had the most severe reduction in mitral cell number and substance P fiber density among the inoculated animals.

Comparison of neuregulin-1 expression in olfactory ensheathing cells, Schwann cells and astrocytes

Recently we demonstrated that a member of the neuregulin-1 (NRG-1) family of growth factors is a mitogen and survival factor for olfactory ensheathing cells (OECs). OECs are specialized glial cells within the olfactory system that are believed to play a role in the continual nerve re-growth of this tissue. OECs share properties with both astrocytes and Schwann cells but are likely to be a distinct glial cell type. NRG-1s have been found to be important regulators of Schwann cells in vivo, but the role of NRG-1 for OECs is less clear. The nrg-1 gene produces at least 12 different isoforms, that are likely to have different functions, due to alternative splicing of its mRNA. In this study, the expression of NRG-1 mRNAs in OECs was compared with other glial cells and their corresponding tissue sources. Cultured glial cells, unlike their tissue sources, expressed NRG-1 mRNAs containing the alpha EGF-like domain and expressed only the type 1beta isoform that lacks the glycosylated spacer domain. This correlated with expression of these isoforms during olfactory nerve degeneration in vivo. Although OECs expressed mRNA for all NRG-1 isoforms, the protein could not be detected in concentrated supernatant, or on the cell surface by immunofluorescence, but was detected in the nucleus or cytoplasm (depending on the isoform). These data support the hypothesis that NRG-1s play a functional role in OEC biology.

Regeneration of olfactory receptor neurons following chemical lesion: time course and enhancement with growth factor administration

Although it has been known for over 50 years that olfactory receptor neuron (ORN) neurogenesis and subsequent reinnervation of the olfactory bulb (OB) occurs following ORN injury, the precise intrinsic and extrinsic factors that regulate this dynamic process have not yet been fully identified. In the first of two experiments, we characterized the time course of anatomical recovery following zinc sulfate (ZnSO(4)) lesion of ORNs in adult male Sprague-Dawley rats. ZnSO(4) produced a near complete deafferentation of OB within 3 days following intranasal administration. A time-dependent increase in ORN reinnervation of OB was observed following 10, 20, and 30 day recovery intervals. Given the evidence that bFGF, EGF, and TGF-alpha have mitogenic effects on ORNs in vitro, a second experiment examined the extent to which these growth factors (GFs) might enhance ORN regeneration and subsequent reinnervation of OB in vivo. Rats received intranasal infusions of ZnSO(4) on day 0, followed by subcutaneous injections of either bFGF (5, 10, or 50 microgram/kg), EGF (5, 10, or 50 microgram/kg), or TGF-alpha (5 or 10 microgram/kg) on days 3-6. Horseradish peroxidase (HRP) histochemistry of OB following a 10-day recovery period revealed a dose-related enhancement in reinnervation of OB for each of the three growth factors examined, with the greatest enhancement produced by TGF-alpha. These data suggest that GFs may regulate ORN mitogenesis in vivo in a way similar to that which has been characterized in vitro.

Reinnervation of the rat olfactory bulb after methyl bromide-induced lesion: timing and extent of reinnervation

We used the inhalation of methyl bromide gas to produce a near-complete destruction of the rat olfactory epithelium and analyzed the reinnervation of the bulb during reconstitution of the epithelium. The degeneration of olfactory axons elicits a transient up-regulation of glial cell proliferation and glial fibrillary acidic protein expression in the olfactory nerve and olfactory nerve layer of the bulb. Anterograde transport after intranasal infusion of wheat germ agglutinin conjugated horseradish peroxidase demonstrates that the first nascent axons reach the bulb within the first week after lesion. Subsequently, a massive wave of fibers arrives at the bulb between 1 and 2 weeks postlesion, and enters the glomeruli between 2 and 3 weeks postlesion. However, the olfactory projection does not stabilize until 8 weeks after lesion judging from the return in growth associated protein-43 expression to control levels. The extent of reinnervation after lesion is correlated with the completeness with which the epithelium reconstitutes itself. In rats that are lesioned while fed ad libitum, there is near-complete reconstitution of the neuronal population, and the projection onto the bulb fills the glomerular layer in its entirety. However, in rats that are lesioned while food restricted, a significant fraction of olfactory epithelium becomes respiratory during its reconstitution, and the population of reinnervating fibers is less. As a consequence, the posterior half of the bulb remains hypoinnervated overall and denervated at its caudal margin. The preferential reinnervation of the anterior bulb in the food-restricted, methyl bromide gas-lesioned animals indicates that the mechanisms that guide the growth of the olfactory axons and restore receptotopy do not operate with the same precision in this setting as they do during development or during the lower level of turnover associated with the "normal" laboratory existence. Accordingly, we hypothesize that the persistence of a significant population of pre-existing neurons is needed to preserve receptotopy during reinnervation. In addition, the results suggest that in the face of massive turnover and a reduced afferent population, there is a tendency for reinnervating axons to fill available synaptic space.

Methimazole toxicity in rodents: covalent binding in the olfactory mucosa and detection of glial fibrillary acidic protein in the olfactory bulb

Methimazole is an antithyroid drug reported to affect the sense of smell and taste in humans. The aim of the present study was to examine the distribution and effects of methimazole on the olfactory system in rodents. Autoradiography showed a selective covalent binding of 3H-labeled methimazole in the Bowman's glands in the olfactory mucosa, bronchial epithelium in the lungs, and centrilobular parts of the liver following an iv injection in mice. Histopathology showed an extensive lesion in the olfactory mucosa that was efficiently repaired 3 months after two consecutive ip doses of methimazole. The effect of methimazole on various brain regions was studied by determining levels and location of glial fibrillary acidic protein (GFAP). The results showed a threefold increase of GFAP in the olfactory bulb 2 weeks after treatment with methimazole whereas no change was observed 4 days after treatment. Pretreatment of mice with thyroxine did not protect against the methimazole-induced toxicity in the olfactory mucosa and bulb. In contrast, pretreatment with the cytochrome P450 inhibitor metyrapone completely prevented the covalent binding and toxicity of methimazole in the olfactory mucosa and bulb. The present results suggest that the methimazole-induced toxicity in the olfactory mucosa is mediated by a cytochrome P450-dependent metabolic activation of the compound into reactive metabolites that are bound to various tissues including the olfactory mucosa. The increase of GFAP in the olfactory bulb of methimazole-treated mice is suggested to be a secondary phenomenon due to the primary damage in the olfactory mucosa.

Stress induces neurogenesis in non-neuronal cell cultures of adult olfactory epithelium

Among the basal cells of the olfactory epithelium is a stem cell which divides and whose progeny differentiate into new sensory neurons throughout adult life. Olfactory neurogenesis is highly regulated, for example it is stimulated by epithelial damage. Previous reports implicate several growth factors in progenitor cell proliferation and neuronal differentiation in vitro but these studies differ in growth conditions and age of donors making it difficult to determine precisely the roles of neurogenic stimuli and their sites of action. The aims of the present study were to develop purified basal cell cultures from adult olfactory epithelium and to stimulate neurogenesis in defined growth conditions in order to elucidate the cellular mechanisms by which neurogenesis is stimulated after epithelial damage. We show here that differentiated olfactory sensory neurons arise after biochemical or mechanical stress of rat and mouse olfactory epithelial cell cultures in the absence of growth factors, complex media (e.g., serum, conditioned media, pituitary and hypothalamic extracts), or other cells (e.g., explants, feeder layers of glia, or other non-epithelial cells). Prior to the stress, these cultures contained basal cells and supporting cells but not neurons. After the stress, some cells differentiated into bipolar neurons expressing a number of neuronal proteins including olfactory marker protein. Bromodeoxyuridine experiments show that the differentiated neurons arose from recently divided cells which did not divide again before differentiating. We conclude that stress disrupts cell surface contacts to induce the immediate neuronal precursors to undergo final differentiation into olfactory sensory neurons. This may be a mechanism for enhanced neurogenesis after epithelial damage.

Supporting cell proliferation in the olfactory epithelium decreases postnatally

It is well known that progenitor cells in the basal layer of olfactory epithelium proliferate continuously throughout life; the offspring of these dividing cells produce replacements for receptor neurons. In the rat the number of proliferating basal cells/mm length of epithelium (proliferation density) decreases with postnatal age while the area of the olfactory sheet increases. The supporting cells, which act as the glia of the olfactory epithelium, also divide. We examined in detail some aspects of the dynamics of olfactory supporting cell proliferation to determine whether their rate of proliferation changes with age, and how it compares with the rate in basal progenitor cells. Using BrdU to label dividing cells, we determined the proliferation density of supporting cells and basal cells in 10 microm coronal sections from six different anterior-posterior regions in rats ranging in age from birth (P1) until P333. We observed a dramatic decrease in supporting cell proliferation density from P1 (80 cells/mm) to P11 (32 cells/mm) to P21 (12 cells/mm); the density decreases continuously to P333 (0.4 cells/mm). This reduction was even more dramatic than that in the basal cell population (Weiler and Farbman, 1997). Analysis of the data for correlation between basal and supporting cell proliferation revealed a weak correlation in neonates but no correlation in older animals. This suggests that the mechanisms that regulate proliferation of the two cell types are different. Our data also indicate that the proliferation of supporting cells is related only to growth in surface area of the epithelium. No turnover seems to occur in the supporting cells as it does in the olfactory neurons, where proliferation of basal cells is necessary for both growth and replacement.

Effects of dichlobenil on ultrastructural morphology and cell replication in the mouse olfactory mucosa

The herbicide dichlobenil (2,6-dichlorobenzonitrile) is a tissue-specific inducing necrosis in the olfactory mucosa. Transmission electron microscopy showed vacuolations (1 hr) and necrosis (4 hr) in the Bowman's glands following intraperitoneal injection of dichlobenil (25 mg/kg) in C57B1 mice, whereas no lesions were noted in the olfactory neuroepithelium at these timepoints. Twenty-four hr following injection (25 mg/kg), most of the propria and the olfactory epithelium were severely damaged; the basal lamina, nerves, and blood vessels in the lamina propria, however, remained intact. Following a lower dose (12 mg/kg), the lesions were not as pronounced. The effects of dichlobenil (6, 12, and 25 mg/kg) on cell replication in the olfactory mucosa, as determined by incorporation of 3H-thymidine 3 days later, were more pronounced in the lamina propria than in the neuroepithelium and occurred at a lower dose in the lamina propria than in the neuroepithelium. Together these studies support the previous proposal that dichlobenil induces a primary damage in the Bowman's glands.

Reconstitution of the rat olfactory epithelium after methyl bromide-induced lesion

The olfactory epithelium and its neuronal population are known to have a substantial capacity to recover after either direct injury or damage to the olfactory nerve. However, the mechanisms underlying that capacity for recovery, and indeed the limits on the recovery process, are not well understood. The aim of this study is to describe in detail the way in which the olfactory epithelium reconstitutes after direct injury. Adult male rats were exposed to 330 ppm methyl bromide (MeBr) gas for a single 6-hour period. The exposure destroys all of the neurons and sustentacular cells in over 95% of the olfactory epithelium of food-restricted rats and in over 90% of the epithelium in ad-libitum-fed rats of the same weight, yet substantial recovery of the olfactory epithelium occurs. In response to the lesion, cellular proliferation increases markedly beginning between 24 and 48 hours, peaks at 1 week, and persists at levels higher than the control level for more than 4 weeks after MeBr exposure. Even though proliferation accelerates promptly, the beginning of neuronal reconstitution is delayed; only a few immature neurons are observed 3 days after the lesion, yet they reappear in large numbers by the end of the first week. The first mature neurons emerge between 7 and 14 days after lesion and increase to near normal numbers by 4-6 weeks. In association with the restoration of the neuronal population, basal cell proliferation returns to control levels between 4 and 6 weeks after damage. Likewise, sustentacular cells, identifiable by anticytokeratin 18 labeling, reappear rapidly and reform a distinct lamina in the superficial aspect of the epithelium. They closely resemble their counterparts in control epithelium with regard to disposition and shape by 3 weeks after lesion and with regard to expression of olfactory-specific cytochrome P450s by 8 weeks. Thus, most areas of the epithelium are restored to a near normal appearance and cellular composition by the end of 8 weeks, suggesting that the MeBr paradigm for lesioning the epithelium offers significant advantages over techniques such as Triton X-100 or ZnSO4 irrigation. However, not all measures of epithelial status are normal even at 8 weeks. Immature neurons remain slightly more numerous than normal at this time. Furthermore, some areas of the olfactory epithelium do not recover after MeBr lesion and are replaced by respiratory epithelium.(ABSTRACT TRUNCATED AT 400 WORDS)

Histopathologic changes in the olfactory epithelium in mice after exposure to sulfur dioxide

To investigate the effects of sulfur dioxide (SO2) on olfactory epithelium, an experiment was performed with 56 mice from the same colony. Experimental animals were divided into three groups consisting of a 30-min exposure group (group 1), a 60-min exposure group (group 2), and a 120-min exposure group (group 3). The olfactory mucosa in these mice were studied by light microscopy immediately, and after 24 h, 48 h, or 72 h exposure to 20 ppm of SO2. Edema, loss of cilia, epithelial thinning, and epithelial desquamation in the olfactory epithelium were observed in groups 2 and 3. The basal lamina and the connective tissue were well preserved throughout the entire mucosa. Injuries to olfactory epithelium became severer with exposure time. These changes were further pronounced 24 h after exposure. Regenerated epithelia were not observed in any group. Scanning electron microscopic findings were consistent with light microscopic findings. Olfactory epithelial surface were consistent with light microscopic findings. Olfactory epithelial surface was sloughed off and revealed, underlining intact basal lamina. The results of this study suggest that early lesions of olfactory epithelium after exposure to SO2 may be primarily degenerative.

Effects of intranasal ZnSO4 irrigation on olfactory and trigeminal cues

Intranasal irrigation with ZnSO4 solutions is used for experimental induction of anosmia. It is, however, unknown whether the trigeminal nerve is affected by the treatment. One day after irrigation (concentrations investigated were between 0.05-1%) the ability of food finding, an olfactory cue, was decreased in a concentration-dependent manner. The trigeminal effect was investigated from a reflexively induced decrease in respiratory rate due to n-propanol exposure. No impairment occurred at 1% ZnSO4. Anosmia was also seen 2-3 h after an irrigation with solutions of 0.05-1% ZnSO4. At the same time, 0.2 and 1% solutions in themselves decreased the respiratory rate due to reflexes from the upper and lower respiratory tract. A conspicuous systemic effect can be ruled out as the Zn++ antidote, CaNa2EDTA, had no effect on the decrease. A direct activation of the trigeminal nerve due to a reaction with a thiol group may explain the effect from the upper airways.

Estrogen-receptor occurrence in the female mouse brain: effects of maternal experience, ovariectomy, estrogen and anosmia

Maternal behavior (ultrasound-induced pup-searching and retrieving) was studied in eight groups of female house mice with various hormonal backgrounds, experience with pups and function of the olfactory system. In their brains, estrogen receptor immunoreactive (ER-IR) cells were localized and quantified. All animals of all groups had ER-IR cells in a 'reliable subset' of brain areas, the medial preoptic area (MPOA) and ventromedial (VMH) and arcuate nucleus (ARH) of the hypothalamus. In another subset of brain areas, the anterior hypothalamic area (AHA) and cortical (CA) and medial (MA) amygdaloid nucleus, ER-IR cells can be expected in at least some animals of all experimental groups ('expected subset'). In a variable subset of additional brain areas (bed nucleus of the stria terminalis, BNST; suprachiasmatic nucleus, SC; lateral septal nuclei, LS; paraventricular nucleus of the hypothalamus, PVH; entorhinal and piriform cortex, ENT, PIR; subiculum, SUB; hippocampus, HPC; periventricular gray of the midbrain, PVG), ER-IR cells occurred only in some animals of some groups. Numbers of ER-IR cells in a given brain area, volumes occupied by these cells, and cell densities varied considerably among the groups. A covariation of cell counts and volumes was significant for most brain areas indicating that increases of numbers of ER-IR cells relate mainly to volume increases within a given brain area. Experience with pups correlated with an increase of ER presence in the AHA, VMH, ENT, PIR, SUB, HPC and PVG, however, only in the presence of estrogen. Estrogen and pup-experience together led to an increased ER presence in mainly the VMH, ENT and PIR, however, only in females with intact olfaction. Full maternal behavior (retrieving, ultrasound recognition) occurred after the high pregnancy- or experience-induced ER content was reduced to lower levels. The ER occurrence in lactating and experienced virgin females differed, however, in the AHA, BNST, SC, PVH, ENT, PIR, SUB, HPC and PVG showing that the maintenance of maternal behavior can run under different profiles of ER content in the brain. Ovariectomy and/or prolonged high blood-estrogen levels correlated significantly with decreased levels of ER-IR cells in most brain areas which could not be increased by pup-experience.

Morphological studies on the rodent main and accessory olfactory systems: the regio olfactoria and vomeronasal organ

The present study on the main olfactory system (MOS) and the accessory olfactory system (AOS) documents the functional morphology of the rodent olfactory region and that of the vomeronasal organ (VNO) using light and electron microscopical techniques. Special attention is given to the cytoarchitecture of the sensory epithelia, i. e. the olfactory epithelium (OE) of the regio olfactoria and the neuroepithelium of the VNO (VNO-NE). Both sensory epithelia consist of a pseudostratified columnar epithelium composed of three types of cells, i. e. receptor cells, supporting cells and progenitor cells. Even at the light microscopical level, however, distinctive morphological features can be distinguished which illustrate important differences between the two sensory epithelia. For example, the height of the respective epithelia differs considerably, the VNO-NE is approximately 170 microns tall and the OE is only about 90 microns. The receptors of the VNO-NE lack olfactory knobs which are typically found in the sensory cells of the OE. The perikarya of the receptor cells of the VNO-NE are very large when compared to those of the sensory cells of the OE. In contrast to the OE, blood vessels are found within the neuroepithelial layer of the VNO. The progenitor cells of the OE are located in a clearly distinguishable cell layer which is lacking in the rodent VNO-NE. The differences between the two epithelial layers become more obvious at the electron microscopical level. The olfactory knobs of the sensory cell dendrites of the OE reach the nasal cavity with numerous cilia. These olfactory hairs, on average 11 per knob, consist of a short proximal segment and a long and thin distal segment. This distal segment runs parallel to the epithelial surface and is embedded in the neuroepithelial mucosal layer. The dendrites of the receptor cells of the VNO-NE reach the lumen of the VNO with numerous branched microvilli which are also embedded in the mucous layer. Horizontal ultrathin sections through the apical portion of the OE reveal that each supporting cell completely envelopes several dendrites. This glia-like relationship is not found in the corresponding layer of the VNO-NE. The sensory cell perikarya of the OE contain only a few endoplasmatic reticulum (ER) profiles while the receptor cells of the VNO are characterized by an extensive smooth endoplasmatic reticulum (SER). In contrast to the fila olfactoria, numerous axons within the vomeronasal nerve show ellipsoidal varicosities without synaptic vesicles which may indicate the existence of at least two vomeronasal nerve fibers.(ABSTRACT TRUNCATED AT 400 WORDS)

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

The effects of intranasal zinc sulfate (ZnSO4) irrigation on the morphology of the olfactory epithelium and olfactory bulb were studied in mice with short survival times (as early as 1 day) and with long survival times (up to 593 days) after the irrigation procedure. As in several previous studies, the olfactory epithelium was completely destroyed within a few days after the ZnSO4 treatment. Within 2-4 days, the septum and turbinates were covered by a new, cuboidal epithelium, the cells of which differed significantly from any cells normally seen in the olfactory epithelium. Slowly, over several months, small areas of the olfactory epithelium regenerated in many of the animals. The ultrastructural changes occurring in the olfactory bulb from 1 to 25 days (the reactive stage) were characterized by degenerating olfactory axons and axon terminals, hypertrophy of astroglial cell processes, and proliferation of or extravasation by phagocytic cells. By 25 days after intranasal ZnSO4 irrigation, the number of reactive glial processes and phagocytic cells returned to normal. In some mice with survival times of 150 days or longer, there was reinnervation of small areas of the olfactory bulb by regenerated olfactory axons. These new olfactory axons innervated only superficial glomeruli or the outer portions of deeper glomeruli, but they formed synaptic contacts with mitral/tufted cells and periglomerular cells that did not differ from control animals. These findings were supported by tract-tracing experiments with 3H-amino acids and by behavioral analysis. In summary, the ultrastructural changes observed in the olfactory bulb in this study were not significantly different from those observed after surgical lesions of the olfactory epithelium or nerve.(ABSTRACT TRUNCATED AT 250 WORDS)