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

Olfactory Dysfunction following COVID-19 and the Potential Benefits of Olfactory Training

COVID-19 is associated with a common symptom of olfactory dysfunction, which may persist even after the infection is resolved. Olfactory training (OT) has emerged as the most effective intervention for post-viral olfactory dysfunction. OT involves daily exposure of the olfactory system to various odors. The current study aims to explore olfactory dysfunction following COVID-19 and the potential benefits of olfactory training.

METHODS

This is a cross-sectional study conducted among adults aged 18-60 living in Alahssa, Saudi Arabia. An online questionnaire containing an informed consent form and a survey to collect demographic data, vaccination status, level of loss of smell and taste, and the level of awareness about olfactory training (OT) was distributed among all participants who agreed to participate in this study.

RESULTS

The study included 524 participants and presented their baseline characteristics, including age, gender, COVID-19 infection status, and complaints. Most patients were female (66.0%), and 46.2% had previously been infected with COVID-19. About 54.8% of participants reported chemosensory dysfunction, while 286 had olfactory dysfunction. Of those, 29.8% had anosmia, 16.8% had hyposmia, and 8.0% had parosmia. Results suggest that being fully or partially vaccinated may offer some protection against olfactory dysfunction compared to being unvaccinated. Adherence to olfactory training was associated with improvement in the sense of smell.

CONCLUSIONS

The study highlights the importance of awareness and adherence to olfactory training, which may improve the sense of smell in individuals with chemosensory dysfunction. The findings of this study can inform public health policies and interventions aimed at reducing the burden of olfactory dysfunction associated with COVID-19 vaccination.

Enhancement of olfaction by femtomolar concentrations of zinc ions

Zinc is recognized as an important element for olfaction. Zinc nanoparticles enhance olfaction in response to odors; however, the mechanisms underlying this action remain unknown. Herein, the effect of zinc on olfactory receptors was deduced using electro-olfactogram (EOG) responses recorded from the isolated olfactory mucosae of bullfrogs (Rana catesbeiana) following the administration or chelation of zinc ions. Menthone and n-amyl acetate were used as odorants, whereas forskolin (an adenylate cyclase activator) and cholera toxin (a Gαolf activator) were used as intracellular signal transduction activators. The EOG responses provoked by the odorants and cholera toxin were suppressed by dithizone-mediated zinc ion chelation, and the EOG responses were recovered by administering non-chelated zinc. However, the EOG response to forskolin was not suppressed by dithizone. In contrast, the addition of femtomolar concentrations of zinc ions enhanced the EOG responses. The above-mentioned effects on EOG responses were examined by changing the concentration of zinc ions but not zinc nanoparticles. The results of this study suggest that Gαolf alone or both olfactory receptors and Gαolf likely require zinc ions for their activation.

COVID-19 Anosmia: High Prevalence, Plural Neuropathogenic Mechanisms, and Scarce Neurotropism of SARS-CoV-2?

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is the causative pathogen of coronavirus disease 2019 (COVID-19). It is known as a respiratory virus, but SARS-CoV-2 appears equally, or even more, infectious for the olfactory epithelium (OE) than for the respiratory epithelium in the nasal cavity. In light of the small area of the OE relative to the respiratory epithelium, the high prevalence of olfactory dysfunctions (ODs) in COVID-19 has been bewildering and has attracted much attention. This review aims to first examine the cytological and molecular biological characteristics of the OE, especially the microvillous apical surfaces of sustentacular cells and the abundant SARS-CoV-2 receptor molecules thereof, that may underlie the high susceptibility of this neuroepithelium to SARS-CoV-2 infection and damages. The possibility of SARS-CoV-2 neurotropism, or the lack of it, is then analyzed with regard to the expression of the receptor (angiotensin-converting enzyme 2) or priming protease (transmembrane serine protease 2), and cellular targets of infection. Neuropathology of COVID-19 in the OE, olfactory bulb, and other related neural structures are also reviewed. Toward the end, we present our perspectives regarding possible mechanisms of SARS-CoV-2 neuropathogenesis and ODs, in the absence of substantial viral infection of neurons. Plausible causes for persistent ODs in some COVID-19 convalescents are also examined.

Olfactory Ensheathing Cells Mediate Neuroplastic Mechanisms After Olfactory Training in Mouse Model

Background

Several studies have reported beneficial effects of olfactory training (OT) on the olfactory nervous system. However, the mechanisms underlying the regeneration of the olfactory system induced by OT are still under investigation.

Objectives

To determine the key mechanisms involved in the olfactory system recovery and to assess the neuroplastic effects of OT.

Methods

Thirty healthy female C57BL/6 mice were randomly allocated to 4 groups: control, n = 6; anosmia (no treatment), n = 8; OT, n = 8; and steroid treatment; n = 8. Except for the control group, mice were administered 3-methylindole. Anosmia was assessed using a food-finding test (FFT). The olfactory neuroepithelium was for histological examinations, gene ontology with pathway analyses, RNA, and protein studies.

Results

FFT was significantly reduced at 3 weeks in the OT mice versus steroids (78.27 s vs 156.83 s, P < .008) and controls (78.27 s vs 13.14 s, P < .003), although final outcome in the FFT was similar in these groups. Expression of olfactory and neurogenesis marker was higher in the olfactory neuroepithelium of the OT group than in the anosmia group without treatment. The mechanisms underlying olfactory regeneration might be related to early olfactory receptor stimulation, followed by neurotrophic factor stimulation of neuronal plasticity.

Conclusion

OT can improve olfactory function and accelerate olfactory recovery. The mechanisms underlying olfactory regeneration might be related to an initial stimulation of olfactory receptors followed by neurogenesis. Olfactory ensheathing cells might play an important role in olfactory regeneration following OT, based on the observed changes in messenger ribonucleic acid (mRNA) and protein expression, as well as the findings of the gene analysis.

Reversible deafferentation of the zebrafish olfactory bulb with wax plug insertion

BACKGROUND

Deafferentation of the zebrafish olfactory bulb allows investigation of neuroplasticity in a particularly dynamic brain region of a popular model animal known for its regenerative abilities. Current methods to remove sensory input to the zebrafish olfactory bulb differ in the extent of deafferentation and potential for recovery.

NEW METHOD

We present a novel method of olfactory bulb deafferentation using continuous wax plug insertions into the nasal cavity of zebrafish. Wax plugs were placed in the nasal cavity and replaced if needed over 1wk or 3wk survival periods. Wax plugs were removed from fish after 1wk of occlusion to analyze the potential recovery of the olfactory organ and bulb.

RESULTS

Wax plug insertions caused a dramatic reduction in olfactory organ size and structure and significantly reduced afferent input to the olfactory bulb after 1wk and 3wk. Removal of the wax plugs after 1wk allowed for recovery of the olfactory organ and subsequent reinnervation of the olfactory bulb.

COMPARISONS WITH EXISTING METHODS

Chemical ablation with detergent causes partial, temporary deafferentation of the olfactory bulb. Cautery ablation causes complete, permanent deafferentation of the olfactory bulb. Wax plug insertions cause nearly complete, temporary deafferentation, allowing both significant deafferentation and the potential for reinnervation of the olfactory bulb.

CONCLUSIONS

The wax plug insertion method of deafferentation described here is unique in that it destroys almost completely the structure of the olfactory organ and removes almost completely sensory input to the olfactory bulb, yet the organ returns to its typical morphology and afferent innervation returns.

Mild Fluid Percussion Injury Induces Diffuse Axonal Damage and Reactive Synaptic Plasticity in the Mouse Olfactory Bulb

Despite the regenerative capacity of the olfactory bulb (OB), head trauma causes olfactory disturbances in up to 30% of patients. While models of olfactory nerve transection, olfactory receptor neuron (ORN) ablation, or direct OB impact have been used to examine OB recovery, these models are severe and not ideal for study of OB synaptic repair. We posited that a mild fluid percussion brain injury (mFPI), delivered over mid-dorsal cortex, would produce diffuse OB deafferentation without confounding pathology. Wild type FVB/NJ mice were subjected to mFPI and OB probed for ORN axon degeneration and onset of reactive synaptogenesis. OB extracts revealed 3 d postinjury elevation of calpain-cleaved 150-kDa αII-spectrin, an indicator of axon damage, in tandem with reduced olfactory marker protein (OMP), a protein specific to intact ORN axons. Moreover, mFPI also produced a 3-d peak in GFAP+ astrocyte and IBA1+ microglial reactivity, consistent with postinjury inflammation. OB glomeruli showed disorganized ORN axons, presynaptic degeneration, and glial phagocytosis at 3 and 7 d postinjury, all indicative of deafferentation. At 21 d after mFPI, normal synaptic structure re-emerged along with OMP recovery, supporting ORN afferent reinnervation. Robust 21 d postinjury upregulation of GAP-43 was consistent with the time course of ORN axon sprouting and synapse regeneration reported after more severe olfactory insult. Together, these findings define a cycle of synaptic degeneration and recovery at a site remote to non-contusive brain injury. We show that mFPI models diffuse ORN axon damage, useful for the study of time-dependent reactive synaptogenesis in the deafferented OB.

Nasal neuron PET imaging quantifies neuron generation and degeneration

Olfactory dysfunction is broadly associated with neurodevelopmental and neurodegenerative diseases and predicts increased mortality rates in healthy individuals. Conventional measurements of olfactory health assess odor processing pathways within the brain and provide a limited understanding of primary odor detection. Quantification of the olfactory sensory neurons (OSNs), which detect odors within the nasal cavity, would provide insight into the etiology of olfactory dysfunction associated with disease and mortality. Notably, OSNs are continually replenished by adult neurogenesis in mammals, including humans, so OSN measurements are primed to provide specialized insights into neurological disease. Here, we have evaluated a PET radiotracer, [¹¹C]GV1-57, that specifically binds mature OSNs and quantifies the mature OSN population in vivo. [¹¹C]GV1-57 monitored native OSN population dynamics in rodents, detecting OSN generation during postnatal development and aging-associated neurodegeneration. [¹¹C]GV1-57 additionally measured rates of neuron regeneration after acute injury and early-stage OSN deficits in a rodent tauopathy model of neurodegenerative disease. Preliminary assessment in nonhuman primates suggested maintained uptake and saturable binding of [¹⁸F]GV1-57 in primate nasal epithelium, supporting its translational potential. Future applications for GV1-57 include monitoring additional diseases or conditions associated with olfactory dysregulation, including cognitive decline, as well as monitoring effects of neuroregenerative or neuroprotective therapeutics.

Patterns of olfactory bulb neurogenesis in the adult zebrafish are altered following reversible deafferentation

Adult brain plasticity can be investigated using reversible methods that remove afferent innervation but allow return of sensory input. Repeated intranasal irrigation with Triton X-100 in adult zebrafish diminishes innervation to the olfactory bulb, resulting in a number of alterations in bulb structure and function, and cessation of the treatment allows for reinnervation and recovery. Using bromodeoxyuridine, Hu, and caspase-3 immunoreactivity we examined cell proliferation, differentiation, migration, and survival under conditions of acute and chronic deafferentation and reafferentation. Cell proliferation within the olfactory bulb was not influenced by acute or chronic deafferentation or reafferentation, but cell fate (including differentiation, migration, and/or survival of newly formed cells) was affected. We found that chronic deafferentation caused a bilateral increase in the number of newly formed cells that migrated into the bulb, although the amount of cell death of these new cells was significantly increased compared to untreated fish. Reafferentation also increased the number of newly formed cells migrating into both bulbs, suggesting that the deafferentation effect on cell fate was maintained. Reafferentation resulted in a decrease in newly formed cells that became neurons and, although death of newly formed cells was not altered from control levels, survival was reduced in relation to that seen in chronically deafferented fish. The potential effect of age on cell genesis was also examined. While the amount of cell migration into the olfactory bulbs was not affected by fish age, more of the newly formed cells became neurons in older fish. Younger fish displayed more cell death under conditions of chronic deafferentation. In sum, our results show that reversible deafferentation affects several aspects of cell fate, including cell differentiation, migration, and survival, and age of the fish influences the response to deafferentation.

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.

Effect of IP3R3 and NPY on age-related declines in olfactory stem cell proliferation

Losing the sense of smell because of aging compromises health and quality of life. In the mouse olfactory epithelium, aging reduces the capacity for tissue homeostasis and regeneration. The microvillous cell subtype that expresses both inositol trisphosphate receptor type 3 (IP3R3) and the neuroproliferative factor neuropeptide Y (NPY) is critical for regulation of homeostasis, yet its role in aging is undefined. We hypothesized that an age-related decline in IP3R3 expression and NPY signaling underlie age-related homeostatic changes and olfactory dysfunction. We found a decrease in IP3R3(+) and NPY(+) microvillous cell numbers and NPY protein and a reduced sensitivity to NPY-mediated proliferation over 24 months. However, in IP3R3-deficient mice, there was no further age-related reduction in cell numbers, proliferation, or olfactory function compared with wild type. The proliferative response was impaired in aged IP3R3-deficient mice when injury was caused by satratoxin G, which induces IP3R3-mediated NPY release, but not by bulbectomy, which does not evoke NPY release. These data identify IP3R3 and NPY signaling as targets for improving recovery following olfactotoxicant exposure.

Plasticity of glomeruli and olfactory-mediated behavior in zebrafish following detergent lesioning of the olfactory epithelium

The zebrafish olfactory system is a valuable model for examining neural regeneration after damage due to the remarkable plasticity of this sensory system and of fish species. We applied detergent to the olfactory organ and examined the effects on both morphology and function of the olfactory system in adult zebrafish. Olfactory organs were treated once with Triton X-100 unilaterally to study glomerular innervation patterns or bilaterally to study odor detection. Fish were allowed to recover for 4-10 days and were compared to untreated control fish. Axonal projections were analyzed using whole mount immunocytochemistry with anti-keyhole limpet hemocyanin, a marker of olfactory axons in teleosts. Chemical lesioning of the olfactory organ with a single dose of Triton X-100 had profound effects on glomerular distribution in the olfactory bulb at 4 days after treatment, with the most significant effects in the medial region of the bulb. Glomeruli had returned by 7 days post-treatment. Analysis of the ability of the fish to detect cocktails of amino acids or bile salts consisted of counting the number of turns the fish made before and after odorant delivery. Control fish turned more after exposure to both odorants. Fish tested 4 and 7 days after chemical lesioning made more turns in response to amino acids but did not respond to bile salts. At 10 days post-lesion, these fish had regained the ability to detect bile salts. Thus, the changes seen in bulbar innervation patterns correlated to odorant-mediated behavior. We show that the adult zebrafish brain has the capacity to recover rapidly from detergent damage of the olfactory epithelium, with both glomerular distribution and odorant-mediated behavior returning in 10 days.

From chemical neuroanatomy to an understanding of the olfactory system

The olfactory system is the appropriate model for studying several aspects of neuronal physiology spanning from the developmental stage to neural network remodelling in the adult brain. Both the morphological and physiological understanding of this system were strongly supported by classical histochemistry. It is emblematic the case of the Olfactory Marker Protein (OMP) staining, the first, powerful marker for fully differentiated olfactory receptor neurons and a key tool to investigate the dynamic relations between peripheral sensory epithelia and central relay regions given its presence within olfactory fibers reaching the olfactory bulb (OB). Similarly, the use of thymidine analogues was able to show neurogenesis in an adult mammalian brain far before modern virus labelling and lipophilic tracers based methods. Nowadays, a wealth of new histochemical techniques combining cell and molecular biology approaches is available, giving stance to move from the analysis of the chemically identified circuitries to functional research. The study of adult neurogenesis is indeed one of the best explanatory examples of this statement. After defining the cell types involved and the basic physiology of this phenomenon in the OB plasticity, we can now analyze the role of neurogenesis in well testable behaviours related to socio-chemical communication in rodents.

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.

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.

Blocking P2X receptors can inhibit the injury-induced proliferation of olfactory epithelium progenitor cells in adult mouse

OBJECTIVE

The olfactory epithelium (OE) is unusual for its remarkable regenerative capacity and sustained neurogenesis of olfactory receptor neurons (ORNs) throughout adult life. Regeneration of ORNs is accomplished by basal cells in the OE, including stem cells and progenitor cells. Although there is considerable knowledge about the roles of OE basal cells in ORN turnover, the molecular mechanism that regulates the proliferation and differentiation of adult OE basal cells is not fully understood. As intercellular signaling molecules, purines have been reported to meditate proliferation, differentiation and migration of many kinds of neural stem cells. However, it is still unclear whether ATP, which could be released by injured ORNs, plays a role in regulating neurogenesis in ORN turnover.

METHODS

RT-PCR and immunohistochemistry were used to detect the expression of ionotropic purinergic receptors-P2X receptors in adult mouse OE. By using the olfactory bulbectomy model and in vivo administration of P2X receptors antagonists, the function of P2X receptors in regulating the proliferation of OE progenitor cell was evaluated.

RESULTS

We found that basal cells in the adult mouse OE express functional P2X receptors, and blocking the activities of P2X receptors can significantly inhibit the injury-induced proliferation of OE basal cells.

CONCLUSION

Our research provides evidence in support of the hypothesis that purinergic signaling can serve as a paracrine signal in regulating the neurogenesis of OE in adult mouse.

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.

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.

Dissociating space and trace in dorsal and ventral hippocampus

Emerging evidence suggests that the hippocampus can be anatomically and functionally dissociated along its septotemporal axis into dorsal and ventral subregions. With respect to function, we have recently demonstrated that pre-training excitotoxic lesions of ventral, but not dorsal, hippocampus impair the acquisition of trace fear conditioning, whereas post-training lesions of either dorsal or ventral hippocampus impair the subsequent expression of trace fear conditioning (Yoon and Otto (2007) Neurobiol Learn Mem 87:464-475). In addition to trace fear conditioning, dorsal and ventral hippocampus appear to be differentially involved in a number of spatial memory tasks. The present study examined the effects of temporary inactivation of dorsal or ventral hippocampus on the acquisition and expression of trace fear conditioning and on performance of a spatial delayed reinforced alternation task. The findings demonstrate a double dissociation of dorsal and ventral hippocampal function: inactivation of ventral, but not dorsal, hippocampus attenuated the acquisition and expression of trace fear conditioning, whereas inactivation of dorsal, but not ventral, hippocampus dramatically impaired performance in the delayed reinforced alternation task. These data further support the notion that dorsal and ventral hippocampus contribute differentially to performance in a variety of paradigms.

Amphibian larvae and zinc sulphate: a suitable model to study the role of brain-derived neurotrophic factor (BDNF) in the neuronal turnover of the olfactory epithelium

The vertebrate olfactory system has fascinated neurobiologists over the last six decades because of its ability to replace its neurons and synaptic connections continuously throughout adult life, under both physiological and pathological conditions. Among the factors that are proposed to be involved in this regenerative potential, brain-derived neurotrophic factor (BDNF) is a candidate for having an important role in the neuronal turnover in the olfactory epithelium (OE) because of its well-documented neurogenic and trophic effects throughout the nervous system. The aim of the present study was to generate a suitable model to study the participation of BDNF in the recovery of the OE after injury in vivo. We developed an experimental design in which the OE of Rhinella arenarum tadpoles could be easily and selectively damaged by immersing the animals in ZnSO(4) solutions of various concentrations for differing time periods. Image analysis of histological sections showed that different combinations of each of these conditions produced statistically different degrees of injury to the olfactory tissue. We also observed that the morphology of the OE was restored within a few days of recovery after ZnSO(4) treatment. Immunohistochemical analysis of BDNF was performed with an antiserum whose specificity was confirmed by Western blotting, and which showed drastic changes in the abundance and distribution pattern of this neurotrophin in the damaged olfactory system. Our results thus suggest that BDNF is involved in the regeneration of the OE of amphibian larvae, and that our approach is suitable for further investigations of this topic.

Adult neurogenesis and cellular brain repair with neural progenitors, precursors and stem cells

Recent work in neuroscience has shown that the adult central nervous system (CNS) contains neural progenitors, precursors and stem cells that are capable of generating new neurons, astrocytes and oligodendrocytes. While challenging the previous dogma that no new neurons are born in the adult mammalian CNS, these findings bring with them the future possibilities for development of novel neural repair strategies. The purpose of this review is to present the current knowledge about constitutively occurring adult mammalian neurogenesis, highlight the critical differences between 'neurogenic' and 'non-neurogenic' regions in the adult brain, and describe the cardinal features of two well-described neurogenic regions-the subventricular zone/olfactory bulb system and the dentate gyrus of the hippocampus. We also provide an overview of presently used models for studying neural precursors in vitro, mention some precursor transplantation models and emphasize that, in this rapidly growing field of neuroscience, one must be cautious with respect to a variety of methodological considerations for studying neural precursor cells both in vitro and in vivo. The possibility of repairing neural circuitry by manipulating neurogenesis is an intriguing one, and, therefore, we also review recent efforts to understand the conditions under which neurogenesis can be induced in non-neurogenic regions of the adult CNS. This work aims towards molecular and cellular manipulation of endogenous neural precursors in situ, without transplantation. We conclude this review with a discussion of what might be the function of newly generated neurons in the adult brain, and provide a summary of present thinking about the consequences of disturbed adult neurogenesis and the reaction of neurogenic regions to disease.

Dorsal hippocampal contributions to unimodal contextual conditioning

Although there is general consensus that the hippocampus is not critically involved in the acquisition of fear conditioned to an explicit conditioned stimulus (CS), the extent to which the hippocampus participates in contextual fear conditioning remains unclear. To further characterize the potential role of the hippocampus in contextual fear conditioning, the present experiments examined the effect of excitotoxic lesions of dorsal hippocampus on the acquisition of a novel contextual fear conditioning paradigm in which a unimodal (olfactory) cue served to disambiguate discrete "contexts" within a single behavioral training chamber. Selective lesions of dorsal hippocampus severely attenuated olfactory contextual conditioning without affecting conditioning to an explicit auditory or olfactory CS. Additional experiments indicate that these contextual conditioning deficits cannot be attributed to a lesion-induced decrement in olfactory perception, a preferential impairment of "weak" forms of conditioning, or hyperactivity. Thus, the hippocampus appears to contribute importantly to the acquisition of fear conditioned to explicitly nonspatial, unimodal, temporally, and spatially diffuse contextual stimuli.

Apoptosis induced by prolonged exposure to odorants in cultured cells from rat olfactory epithelium

Multicellular organisms undergo programmed cell death (PCD) as a mechanism for tissue remodeling during development and tissue renewal throughout adult life. Overdose of some neuronal receptor agonists like glutamate can trigger a PCD process termed excitotoxicity in neurons of the central nervous system. Calcium has an important role in PCD processes, especially in excitotoxicity. Since the normal turnover of olfactory receptor neurons (ORNs) relies, at least in part, on an apoptotic mechanism and odor transduction in ORNs involves an increase in intracellular Ca2+ concentration ([Ca2+]i), we investigated the possibility that long-term exposures to odorants could trigger an excitotoxic process in olfactory epithelial cells (EC). We used single-cell [Ca2+]i determinations and fluorescence microscopy techniques to study the effects of sustained odorant exposures in olfactory EC in primary culture. Induction of PCD was evaluated successively by three independent criteria: (1) measurements of DNA fragmentation, (2) translocation of phosphatidylserine to the external leaflet of the plasma membrane, and (3) caspase-3 activation. Our results support the notion of an odorant-induced PCD in olfactory EC. This odorant-induced PCD was prevented by LY83583, an odorant response inhibitor, suggesting that ORNs are the main epithelial cell population undergoing odorant-induced PCD.

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.

Adult neurogenesis and repair of the adult CNS with neural progenitors, precursors, and stem cells

Recent work in neuroscience has shown that the adult central nervous system contains neural progenitors, precursors, and stem cells that are capable of generating new neurons, astrocytes, and oligodendrocytes. While challenging previous dogma that no new neurons are born in the adult mammalian CNS, these findings bring with them future possibilities for the development of novel neural repair strategies. The purpose of this review is to present current knowledge about constitutively occurring adult mammalian neurogenesis, to highlight the critical differences between "neurogenic" and "non-neurogenic" regions in the adult brain, and to describe the cardinal features of two well-described neurogenic regions-the subventricular zone/olfactory bulb system, and the dentate gyrus of the hippocampus. We also provide an overview of currently used models for studying neural precursors in vitro, mention some precursor transplantation models, and emphasize that, in this rapidly growing field of neuroscience, one must take caution with respect to a variety of methodological considerations for studying neural precursor cells both in vitro and in vivo. The possibility of repairing neural circuitry by manipulating neurogenesis is an intriguing one, and, therefore, we also review recent efforts to understand the conditions under which neurogenesis can be induced in non-neurogenic regions of the adult CNS. This work aims toward molecular and cellular manipulation of endogenous neural precursors in situ, without transplantation. We conclude this review with a discussion of what the function might be of newly generated neurons in the adult brain and provide a summary of current thinking about the consequences of disturbed adult neurogenesis and the reaction of neurogenic regions to disease.

In vivo and in vitro neurogenesis in human olfactory epithelium

The birth and differentiation of neurons have been extensively studied in the olfactory epithelium (OE) of rodents but not in humans. The goal of this study was to characterize cellular composition and molecular expression of human OE in vivo and in vitro. In rodent OE, there are horizontal basal cells and globose basal cells that are morphologically and functionally distinct. In human OE, however, there appears to be no morphological distinction among basal cells, with almost all cells having round cell bodies similar to rodent globose basal cells. Unlike the case in rodents, human basal cells, including putative neuronal precursors, express p75NGFR, suggesting a distinctive role for p75NGFR in human OE neurogenesis. Molecular expression of neuronal cells during differentiation in human OE grossly follows that in rodents. However, the topographical organization of immature and mature ORNs in human OE differs from that of rodents, in that immature and mature ORNs in humans are dispersed throughout the OE, whereas rodent counterparts have a highly laminar organization. These observations together suggest that the birth and differentiation of neuronal cells in human OE differ from those in rodents. In OE explant culture, neuronal cells derived from human OE biopsy express markers for immature and mature neurons, grossly recapitulating neuronal differentiation of olfactory neurons in vivo. Furthermore, small numbers of cells are doubly label for bromodeoxyuridine and olfactory marker protein, indicating that neuronal cells born in vitro reach maturity. These data highlight species-related differences in OE development and demonstrate the utility of explant culture for experimental studies of human neuronal development.

Animal models of depression: olfactory lesions affect amygdala, subventricular zone, and aggression

Psychiatric or depressed patients show alterations in both olfactory projection areas and mucosa. In rodents, ablation of olfactory bulbs causes a depression-like syndrome, useful to test antidepressant agents. We studied in mice the behavioral symptoms and neuroanatomical correlates after mucosal damage or ablation of the olfactory bulb. Our results are based on a battery of tests exploiting anxious, aggressive, and depressive behavior, on morphological and immunohistochemical analysis. We found similar results in both sensory-damaged and bulbectomized animals, with a behavioral dissociation concerning different forms of aggression. These findings do not support a simple downregulation of social interactions in damaged mice. The most prominent modifications in the brains of sensory damaged and bulbectomized mice are detected in the subventricular zone (SVZ), the source area of neural stem cells, and in the content of cAMP-dependent protein kinase within the amygdala, suggesting a central role of this structure in the functional modulation of behavior.

Binding sites of gamma-secretase inhibitors in rodent brain: distribution, postnatal development, and effect of deafferentation

gamma-Secretase is a multimeric complex consisted of presenilins (PSs) and three other proteins. PSs appear to be key contributors for the enzymatic center, the potential target of a number of recently developed gamma-secretase inhibitors. Using radiolabeled and unlabeled inhibitors as ligands, this study was aimed to determine the in situ distribution of gamma-secretase in the brain. Characterization using PS-1 knock-out mouse embryos revealed 50 and 80% reductions of gamma-secretase inhibitor binding density in the heterozygous (PS-1(+/-)) and homozygous (PS-1-/-) embryos, respectively, relative to the wild type (PS-1(+/+)). The pharmacological profile from competition binding assays suggests that the ligands may target at the N- and C-terminal fragments of PS essential for gamma-secretase activity. In the adult rat brain, the binding sites existed mostly in the forebrain, the cerebellum, and discrete brainstem areas and were particularly abundant in areas rich in neuronal terminals, e.g., olfactory glomeruli, CA3-hilus area, cerebellar molecular layer, and pars reticulata of the substantia nigra. In the developing rat brain, diffuse and elevated expression of binding sites occurred at the early postnatal stage relative to the adult. The possible association of binding sites with neuronal terminals in the adult brain was further investigated after olfactory deafferentation. A significant decrease with subsequent recovery of binding sites was noted in the olfactory glomeruli after chemical damage of the olfactory epithelium. The findings in this study support a physiological role of PS or gamma-secretase complex in neuronal and synaptic development and plasticity.

Regenerative Medicine for Diseases of the Head and Neck: Principles ofIn vivoRegeneration

The application of endogenous regeneration in regenerative medicine is based on the concept of inducing regeneration of damaged or lost tissues from residual tissues in situ. Therefore, endogenous regeneration is also termed in vivo regeneration as opposed to mechanisms of ex vivo regeneration which are applied, for example, in the field of tissue engineering. The basic science foundation for mechanisms of endogenous regeneration is provided by the field of regenerative biology. The ambitious vision for the application of endogenous regeneration in regenerative medicine is stimulated by investigations in the model organisms of regenerative biology, most notably hydra, planarians and urodeles. These model organisms demonstrate remarkable regenerative capabilities, which appear to be conserved over large phylogenetical stretches with convincing evidence for a homologue origin of an endogenous regenerative capability. Although the elucidation of the molecular and cellular mechanisms of these endogenous regenerative phenomena is still in its beginning, there are indications that these processes have potential to become useful for human benefit. Such indications also exist for particular applications in diseases of the head and neck region. As such epimorphic regeneration without blastema formation may be relevant to regeneration of sensorineural epithelia of the inner ear or the olphactory epithelium. Complex tissue lesions of the head and neck as they occur after trauma or tumor resections may be approached on the basis of relevant mechanisms in epimorphic regeneration with blastema formation.

Leukemia inhibitory factor is a key signal for injury-induced neurogenesis in the adult mouse olfactory epithelium

The mammalian olfactory epithelium (OE) is composed of primary olfactory sensory neurons (OSNs) that are renewed throughout adulthood by local, restricted neuronal progenitor cells. The molecular signals that control this neurogenesis in vivo are unknown. Using olfactory bulb ablation (OBX) in adult mice to trigger synchronous mitotic stimulation of neuronal progenitors in the OE, we show the in vivo involvement of a cytokine in the cellular events leading to the regeneration of the OE. We find that, of many potential mitogenic signals, only leukemia inhibitory factor (LIF) is induced before the onset of neuronal progenitor proliferation. The rise in LIF mRNA expression peaks at 8 hr after OBX, and in situ RT-PCR and immunocytochemistry indicate that LIF is upregulated, in part, in the injured neurons themselves. This rise in LIF is necessary for injury-induced neurogenesis, as OBX in the LIF knock-out mouse fails to stimulate cell proliferation in the OE. Moreover, delivery of exogenous LIF to the intact adult OE using an adenoviral vector stimulates BrdU labeling in the apical OE. Taken together, these results suggest that injured OSNs release LIF as a stimulus to initiate their own replacement.

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.

Zinc sulphate-induced anosmia decreases the nerve fibre density in the anterior cerebral artery of the rat

Detailed quantitative studies have demonstrated a topographical heterogeneity of nerve fibre densities in the cerebral arteries at the base of the brain as well as local changes in ageing and Alzheimer's patients. In this study, we test the hypothesis that local patterns of innervation are influenced by changes in flow fluctuations. This was investigated by inducing chronic anosmia and monitoring the nerve fibre density in the basal cerebral arteries in the adult rat. The olfactory epithelium was examined after staining with hematoxylin and eosin and showed a marked reduction of thickness in the anosmic group compared to the control group. The olfactory bulb was histochemically stained for succinate dehydrogenase (SDH) activity and showed a reduced staining in the anosmic group compared to the controls. Whole mount preparations of the basal cerebral arteries were immunostained for the general neural marker protein gene product (PGP) 9.5. The nerve fibre densities of the vessel walls were quantified by image analysis and expressed as area percentage and intercept density. This analysis showed a significant reduction in area percentage for the first part of the anterior cerebral artery, as well as for the second part of the anterior cerebral artery, and a significant reduction in intercept density for the second part of the anterior cerebral artery in the anosmic group. We conclude that peripherally induced anosmia decreases nerve fibre density in the anterior cerebral artery that may be due to a decreased metabolic activity in the rhinencephalon and, as a consequence, a reduction of flow fluctuations in the blood vessels supplying this area occurs.

In vivo induction of massive proliferation, directed migration, and differentiation of neural cells in the adult mammalian brain

The development of an in vivo procedure for the induction of massive proliferation, directed migration, and neurodifferentiation (PMD) in the damaged adult central nervous system would hold promise for the treatment of human neurodegenerative disorders such as Parkinson's disease. We investigated the in vivo induction of PMD in the forebrain of the adult rat by using a combination of 6-hydroxydopamine lesion of the substantia nigra dopaminergic neurons and infusions of transforming growth factor alpha (TGFalpha) into forebrain structures. Only in animals with both lesion and infusion of TGFalpha was there a rapid proliferation of forebrain stem cells followed by a timed migration of a ridge of neuronal and glial progenitors directed toward the region of the TGFalpha infusion site. Subsequently, increasing numbers of differentiated neurons were observed in the striatum. In behavioral experiments, there was a significant reduction of apomorphine-induced rotations in animals receiving the TGFalpha infusions. These results show that the brain contains stem cells capable of PMD in response to an exogenously administered growth factor. This finding has significant implications with respect to the development of treatments for both acute neural trauma and neurodegenerative diseases.