Dorsal-zone-specific reduction of sensory neuron density in the olfactory epithelium following long-term exercise or caloric restriction

Abstract

Exercise (Ex) and caloric restriction (CR) reduce oxidative stress and improve organ function. For instance, voluntary Ex or CR is known to reduce age-related cochlear damage in male C57BL/6J mice. However, the effect of Ex and CR on the olfactory system is unknown. In this study, we confirmed the positive effect of Ex and CR on age-related cochlear damage, but found that Ex and CR affected negatively cell dynamics in the olfactory epithelium (OE) by reducing the number of mature olfactory sensory neurons (OSNs) and increasing the number of proliferative basal cells and apoptotic OSNs in the dorsal zone of the olfactory epithelium (OE), which contains neurons expressing NADPH quinone oxido-reductase 1 (NQO1). In addition, these interventions resulted in lower odor-induced c-fos expression in areas of the olfactory bulb receiving projections from dorsal-zone OSNs than in areas receiving ventral-zone projections. Further, we observed substantial oxidative stress in NQO1-positive cells and apoptotic OSNs in the dorsal zone in Ex and CR animals. These results suggest that, in contrast to their positive effects in other organs, Ex and CR facilitate oxidative stress and negatively impact structure and function in dorsal-zone OSNs, probably in association with NQO1 bioactivation.

Zone-specific damage of the olfactory epithelium under protein restriction

“…It is reported that such age-related degenerative changes in the cochlea are reduced by some non-genetic interventions other than PR. For example, long-term voluntary exercise significantly delayed the progression of AHL and reduced OHC loss in the apical turn of the cochlear in aged mice 23,41 . Long-term CR of 10 month duration suppressed apoptotic cell death in the apical turn of the cochlea compared with non-CR mice 23,42 .…”

“…For example, long-term voluntary exercise significantly delayed the progression of AHL and reduced OHC loss in the apical turn of the cochlear in aged mice 23,41 . Long-term CR of 10 month duration suppressed apoptotic cell death in the apical turn of the cochlea compared with non-CR mice 23,42 . In an analogy with these interventions, PR also had preventive effects on age-related degenerative changes in the OHCs in the apical turn.…”

“…2A,B). In the analysis of IHCs and OHCs, we calculated the survival rates of HCs, i.e., the percentage of surviving HCs in the organ of Corti (see "Materials and methods") 23 . In both control and PR mice, age-related degeneration of HCs was observed specifically in the basal turn compared with the apical turn (Fig.…”

“…Control mice were fed with a diet of 353.6 kcal/100 g. Protein-restricted (PR) mice were fed with a diet of 353.8 kcal/100 g containing a 30% reduction in casein from AIN-93M, which was replaced with a cornstarch supplement. areas, we determined an NQO1-negative square (a 30 μm square) as one in which the staining was below two SDs of the mean NQO1 intensity in the NQO1 positive area, and an NQO1-positive square as just a square adjacent to the NQO1 negative square, as described in a previous report 23 . We then compared the signal intensity of the OMP-staining between the NQO1-positive and negative squares (Fig.…”

“…The numbers of remaining HCs at the basal and apical turns were counted for at least 10 sections per animal 65 . The inner hair cell (IHC) and outer hair cell (OHC) survival rates were calculated using the following formulae: IHC survival rate % = 100 × (the number of present IHCs in examined specimens/the number of examined specimens); OHC survival rate % = 100 × (the number of present OHCs in examined specimens/the number of examined specimens) 23,65 . The spiral ganglion cell (SGC) densities (number/mm 2 ) at each turn of each animal were calculated by dividing the number of SGCs by the area of Rosenthal's canal measured using ImageJ software (NIH) 65 .…”