Glucocorticoids enhance regeneration of murine olfactory epithelium

Chronic intranasal corticosteroid treatment induces degeneration of olfactory sensory neurons in normal and allergic rhinitis mice

BACKGROUND

Nasal eosinophilic inflammation is the therapeutic target for olfactory dysfunction in allergic rhinitis (AR). Intranasal corticosteroids are commonly considered to offer targetable benefit given their immunosuppressive property. However, experimental evidence suggests that continuous corticosteroid exposure may directly cause olfactory damage by disrupting the turnover of olfactory sensory neurons (OSNs). This potentially deleterious effect of corticosteroids calls into question their long-term topical use for treating olfactory loss related to AR. The aim of this study was to assess the impacts of chronic intranasal corticosteroid treatment on olfactory function and OSN population in mice under normal and pathological conditions.

METHODS

BALB/c mice were intranasally treated with fluticasone propionate (FP, 0.3 mg/kg) for up to 8 weeks. Additional mice were used to establish an ovalbumin-induced mouse model of AR, followed by nasal challenge with ovalbumin for 8 weeks in the presence or absence of intranasal FP treatment. The authors examined olfactory function, OSN existence, neuronal turnover, and nasal inflammation using behavioral test, histological analyses, Western blotting, and enzyme-linked immunosorbent assay.

RESULTS

Intranasal treatment with FP for 8 weeks (FP-wk8) reduced odor sensitivity in normal mice. This reduction was concomitant with loss of OSNs and the axons projecting to the olfactory bulb, primarily resulting from increased neuronal apoptosis. In FP-wk8 AR mice, intranasal FP treatment attenuated olfactory impairment and eosinophilic inflammation but failed to reconstitute OSN population and axonal projections.

CONCLUSION

These results suggest that chronic intranasal corticosteroid treatment contributes to OSN degeneration that may reduce the therapeutic effectiveness for AR-related olfactory loss.

Morphogenic Effect of Exogenous Glucocorticoid Hormones in the Girardia tigrina Planarian (Turbellaria, Tricladida)

We have studied the effect of two glucocorticoid hormones: hydrocortisone and its synthetic analogue methylprednisolone on the regeneration activity of head and tail blastema of the Girardia tigrina planarian. The regeneration activity was studied in head and tail blastema formed after resection by means of lifetime computer morphometry and immunohistochemical labeling of neoblasts. The search for orthologous proteins-glucocorticoid receptors (hydrocortisone) was performed using the SmedGD database of the Schmidtea mediterranea planarian. The results indicate that both hormones influence the recovery rate of the regenerating head and tail blastema. The worms with regenerating tail blastema have less sensitivity to the hormones' treatment compared to the ones with regenerating head blastema. Hydrocortisone at a high concentration (10-3 M) suppressed the regeneration rate, while stimulating it at lower concentrations (10-4-10-6 M). The same concentrations of methylprednisolone inhibited the regeneration of head blastema, but did not affect the tail blastema regeneration. The two hormones acted differently: while hydrocortisone stimulated the proliferation of neoblasts in the periwound region, methylprednisolone reduced the mitotic activity, mainly on the tail zone furthest from the wound surface. We suggest that exogenous glucocorticoids can influence endogenous mechanisms of hormone-dependent regeneration.

A systematic review of the biological mediators of fat taste and smell

Taste and smell play a key role in our ability to perceive foods. Overconsumption of highly palatable energy-dense foods can lead to increased caloric intake and obesity. Thus there is growing interest in the study of the biological mediators of fat taste and associated olfaction as potential targets for pharmacologic and nutritional interventions in the context of obesity and health. The number of studies examining mechanisms underlying fat taste and smell has grown rapidly in the last 5 years. Therefore, the purpose of this systematic review is to summarize emerging evidence examining the biological mechanisms of fat taste and smell. A literature search was conducted of studies published in English between 2014 and 2021 in adult humans and animal models. Database searches were conducted using PubMed, EMBASE, Scopus, and Web of Science for key terms including fat/lipid, taste, and olfaction. Initially, 4,062 articles were identified through database searches, and a total of 84 relevant articles met inclusion and exclusion criteria and are included in this review. Existing literature suggests that there are several proteins integral to fat chemosensation, including cluster of differentiation 36 (CD36) and G protein-coupled receptor 120 (GPR120). This systematic review will discuss these proteins and the signal transduction pathways involved in fat detection. We also review neural circuits, key brain regions, ingestive cues, postingestive signals, and genetic polymorphism that play a role in fat perception and consumption. Finally, we discuss the role of fat taste and smell in the context of eating behavior and obesity.

A multicenter real-life study to determine the efficacy of corticosteroids and olfactory training in improving persistent COVID-19-related olfactory dysfunction

Background

No definitive treatment exists to effectively restore function in patients with persistent post-infectious olfactory dysfunction (OD). Corticosteroids have been considered as a therapeutic option in post-infectious OD but their benefit in COVID-19-related OD remains unexplored. We aim to determine the role of the combination of corticosteroids plus olfactory training (OT) in improving persistent COVID-19-related OD.

Methods

A multicenter real-life cohort study was conducted between December 2020 and April 2022 on patients with reported COVID-19-related OD. Only patients with confirmed OD at Sniffin' Sticks (S'S) and those who attended their 6-month follow-up were included. Patients were started on a combined treatment of corticosteroids and OT. Patients refusing corticosteroids or not doing any treatment formed the control groups. Visual analogue scale (VAS) for sense of smell and SNOT-22 were used to assess patients reported symptoms.

Results

Sixty-seven subjects with reported COVID-19-related OD were initially seen. Normosmic patients at S'S (n = 14) and those not attending their follow-up (n = 9) were excluded. Of the 44 patients included in the analysis, 19 patients had the combined treatment (group A), 16 patients refused to take corticosteroids and did the OT alone (group B) whereas 9 patients did not do any treatment (group C). An improvement of threshold + discrimination + identification (TDI) score (p = .01) and VAS for smell (p = .01) was found in group A whereas only the TDI score improved in group B (p = .04). Presence of comorbidities, age, sex (male), and length of OD negatively influenced olfactory recovery.

Conclusions

Our study confirms the importance of OT in long-term OD suggesting that the addition of corticosteroids may give a benefit in terms of patient's perceived olfaction.

Level of Evidence

2b.

Postinfectious Olfactory Dysfunction: Oral Steroids and Olfactory Training versus Olfactory Training Alone: Is There any Benefit from Steroids?

INTRODUCTION

There are limited treatment options for postinfectious olfactory dysfunction (PIOD). Olfactory training has recently been used in clinical practice, but no medical treatment is widely accepted. Although there is weak evidence for their value, some physicians use oral corticosteroids as first-line treatment. The aim of this study was to compare combined oral methylprednisolone and olfactory training with olfactory training alone in the management of PIOD.

METHODS

This prospective cohort study included 131 patients with PIOD over a 2-year period before the COVID-19 pandemic. Seventy-eight patients who were treated with oral methylprednisolone and olfactory training (group A) were compared with 53 patients who were treated with olfactory training only (group B). Olfactory function was evaluated with "Sniffin' Sticks" at baseline and 2, 8, and 16 weeks after initial assessment. Patients who improved after steroid treatment underwent magnetic resonance imaging of the paranasal sinuses, skin prick tests, lung spirometry, and sputum eosinophil assessment.

RESULTS

Oral steroids improved 19.23% of patients (n = 15) of group A. History, clinical evaluation, imaging, and laboratory tests identified an inflammatory background in half of them (n = 8). The remaining 7 had no findings of nasal inflammation, and all had a short history of olfactory dysfunction. Both groups significantly improved in olfactory testing results at the end of the olfactory training scheme without significant difference between them.

CONCLUSIONS

The percentage of improved patients after oral methylprednisolone was relatively low to suggest it as first-line treatment. Half of the improved patients had an underlying upper airway inflammatory condition not related to the infection that caused the acute loss of olfactory function.

Construction of an irreversible allergic rhinitis-induced olfactory loss mouse model

Clinical data show that part of patients with sinonasal diseases suffered from olfactory dysfunction, especially with allergic rhinitis (AR) and chronic rhinosinusitis (CRS). However, the mechanisms responsible for AR-induced olfactory loss are still largely unknown. Because of the difficulty to obtain human olfactory mucosa, an AR-induced olfactory loss animal model needs to be constructed to clarify the mechanism. The AR mouse model was induced by intraperitoneal sensitizing with ovalbumin (OVA) followed by intranasal challenge lasted from 1 to 12 weeks. For groups with recovery, mice were housed for another 4-week long without any treatment after the last intranasal challenge. Olfactory function, olfactory receptor neurons (ORNs) density and leukocytes infiltration were examined at different time points. Olfactory loss occurs immediately after 1-week intranasal challenge and deteriorates almost to anosmia after 8th week, and after that olfactory loss become irreversible. Nasal inflammation induces significant infiltration of leukocytes into olfactory epithelium (OE), which negatively correlated with the density of ORNs and mouse olfaction in a time dependent manner. The neutrophilic subtype dominates in number amongst the total infiltrated leukocytes, indicating its pivotal role in nasal inflammation-induced olfactory dysfunction. In this study, we constructed a persistent AR-induced olfactory loss mouse model, losing the ability to recover from dysfunction if the disease duration more than eight weeks, which implies that timely treatments are necessary. Meanwhile, this mouse model could provide an easy and reliable system to clarify the mechanisms of AR-induced irreversible olfactory dysfunction.

Topical Dexamethasone Administration Impairs Protein Synthesis and Neuronal Regeneration in the Olfactory Epithelium

Chronic inflammatory process in the nasal mucosa is correlated with poor smell perception. Over-activation of immune cells in the olfactory epithelium (OE) is generally associated with loss of olfactory function, and topical steroidal anti-inflammatory drugs have been largely used for treating such condition. Whether this therapeutic strategy could directly affect the regenerative process in the OE remains unclear. In this study, we show that nasal topical application of dexamethasone (DEX; 200 or 800 ng/nostril), a potent synthetic anti-inflammatory steroid, attenuates OE lesion caused by Gram-negative bacteria lipopolysaccharide (LPS) intranasal infusion. In contrast, repeated DEX (400 ng/nostril) local application after lesion establishment limited the regeneration of olfactory sensory neurons after injury promoted by LPS or methimazole. Remarkably, DEX effects were observed when the drug was infused as 3 consecutive days regimen. The anti-inflammatory drug does not induce OE progenitor cell death, however, disturbance in mammalian target of rapamycin downstream signaling pathway and impairment of protein synthesis were observed during the course of DEX treatment. In addition, in vitro studies conducted with OE neurospheres in the absence of an inflammatory environment showed that glucocorticoid receptor engagement directly reduces OE progenitor cells proliferation. Our results suggest that DEX can interfere with the intrinsic regenerative cellular mechanisms of the OE, raising concerns on the use of topical anti-inflammatory steroids as a risk factor for progressive olfactory function impairment.

Intranasal application of glucocorticoid alleviates olfactory dysfunction in mice with allergic rhinitis

The aim of the present study was to investigate the effect of glucocorticoid intervention on olfactory dysfunction in mice with allergic rhinitis (AR). An AR animal model was established by intraperitoneal injection and intranasal application of ovalbumin to mice. The olfactory function of the mice was evaluated using a buried food test, and morphological changes in the nasal mucosa were determined using hematoxylin and eosin staining. The expression of olfactory marker protein (OMP) in the olfactory mucosa was tested by immunohistochemistry, and was observed on days 7 and 14 after the application of glucocorticoid. The incidence rate of olfactory dysfunction in AR mice was 75.34%, and the olfactory epithelium became thinner in mice with AR compared to the control group. In addition, the expression of OMP in the olfactory epithelium was downregulated in mice with AR compared with the control group. Expression of OMP in the olfactory epithelium was upregulated in the budesonide group A and betamethasone group A compared with the medicine-free group, whereas the expression of OMP in the olfactory epithelium of budesonide group A or betamethasone group A was not significantly different from the control group. Moreover, the expression of OMP in the budesonide group B was similar to budesonide group A, and expression of OMP in betamethasone group B was similar to betamethasone group A. The expression of OMP in olfactory mucosa is downregulated in AR mice with olfactory dysfunction. Following the application of glucocorticoid, the expression of OMP in the olfactory mucosa in mice is upregulated. Moreover, intranasal local glucocorticoid has a low incidence of systemic adverse reactions, and is recommended for the treatment of olfactory dysfunction in AR.

Long-Lasting Metabolic Imbalance Related to Obesity Alters Olfactory Tissue Homeostasis and Impairs Olfactory-Driven Behaviors

Obesity is associated with chronic food intake disorders and binge eating. Food intake relies on the interaction between homeostatic regulation and hedonic signals among which, olfaction is a major sensory determinant. However, its potential modulation at the peripheral level by a chronic energy imbalance associated to obese status remains a matter of debate. We further investigated the olfactory function in a rodent model relevant to the situation encountered in obese humans, where genetic susceptibility is juxtaposed on chronic eating disorders. Using several olfactory-driven tests, we compared the behaviors of obesity-prone Sprague-Dawley rats (OP) fed with a high-fat/high-sugar diet with those of obese-resistant ones fed with normal chow. In OP rats, we reported 1) decreased odor threshold, but 2) poor olfactory performances, associated with learning/memory deficits, 3) decreased influence of fasting, and 4) impaired insulin control on food seeking behavior. Associated with these behavioral modifications, we found a modulation of metabolism-related factors implicated in 1) electrical olfactory signal regulation (insulin receptor), 2) cellular dynamics (glucorticoids receptors, pro- and antiapoptotic factors), and 3) homeostasis of the olfactory mucosa and bulb (monocarboxylate and glucose transporters). Such impairments might participate to the perturbed daily food intake pattern that we observed in obese animals.

Expression and nuclear translocation of glucocorticoid receptors in type 2 taste receptor cells

Stress increases the secretion of glucocorticoids (GCs), potent steroid hormones that exert their effects on numerous target tissues by acting through glucocorticoid receptors (GRs). GC signaling significantly affects ingestive behavior and taste preferences in humans and rodent models, but far less is known about the hormonal modulation of the peripheral sensory system that detects and assesses nutrient content of foods. A previous study linked restraint stress in rats to diminished expression of mRNA for one subunit of the sweet taste receptor (Tas1r3) in taste tissue and reduced gustatory nerve excitation by sweet compounds. Using RT-PCR, we detected mRNAs for GRα in circumvallate taste papillae and in oral epithelium devoid of taste buds ("non-taste" tissue). Further, circumvallate tissue was significantly enriched in GR mRNA compared to non-taste tissue based on quantitative PCR. Histologically, GR protein was expressed in all taste bud populations examined (circumvallate, foliate and fungiform papillae). Using transgenic mice expressing green fluorescent protein, almost all (97%) Tas1r3-positive taste cells (sweet-/umami-sensitive) expressed GR compared to a significantly smaller percentage (89%) of TrpM5-positive taste cells (sweet-, umami- and bitter-sensitive). When mice (n=4) were restrain stressed, GR protein mobilized to the nucleus in Tas1r3-GFP taste cells (1.7-fold over controls). Our results suggest that GR can be activated in taste receptor cells and may play a role in specific taste qualities (e.g., sweet, umami, and bitter) to shape how the taste system responds to stress.

Sex hormone binding globulin in the rat olfactory system

Ovarian steroids are known to act on the olfactory system. Their mode of action, however, is mostly unclear to date since nuclear receptors are lacking in sensory neurons. Here we used immunocytochemistry and RT-PCR to study expression and distribution of sex hormone binding globulin (SHBG) in the rat olfactory system. Single sensory cells in the olfactory mucosa and their projections in the olfactory bulb showed specific SHBG immunostaining as determined by double immunofluorescence with olfactory marker protein OMP. Larger groups of SHBG stained sensory cells occurred in the vomeronasal organ (VNO). A portion of the olfactory glomeruli in the accessory olfactory bulb showed large networks of SHBG positive nerve fibres. Some of the mitral cells showed SHBG immune fluorescence. RT-PCR revealed SHBG encoding mRNA in the olfactory mucosa, in the VNO and in the olfactory bulbs indicating intrinsic expression of the binding globulin. The VNO and its related projections within the limbic system are known to be sensitive to gonadal steroid hormones. We conclude that SHBG may be of functional importance for rapid effects of olfactory steroids on limbic functions including the control of reproductive behaviours through pheromones.

Expression of corticosteroid binding globulin in the rat olfactory system

Glucocorticoids are known to act on the olfactory system although their mode of action is still unclear since nuclear glucocorticoid receptors are mostly absent in the olfactory mucosa. In this study we used immunocytochemistry, in situ hybridization, and RT-PCR to study the expression and distribution of corticosteroid binding globulin (CBG) in the rat olfactory system. Mucosal goblet cells could be immunostained for CBG. Nasal secretion contained measurable amounts of CBG suggesting that CBG is liberated. CBG immunoreactivity was localized in many of the basal cells of the olfactory mucosa, while mature sensory cells contained CBG only in processes as determined by double immunostaining with the olfactory marker protein OMP. This staining was most pronounced in the vomeronasal organ (VNO). The appearance of CBG in the non-sensory and sensory parts of the VNO and in nerve terminals in the accessory bulb indicated axonal transport. Portions of the periglomerular cells, the mitral cells and the tufted cells were also CBG positive. CBG encoding transcripts were confirmed by RT-PCR in homogenates of the olfactory mucosa and VNO. Olfactory CBG may be significant for uptake, accumulation and transport of glucocorticoids, including aerosolic cortisol.