Lifespan of mature olfactory sensory neurons varies with location in the mouse olfactory epithelium and age of the animal

Histological changes in the olfactory bulb and rostral migratory stream due to interruption of olfactory input

OBJECTIVE

Periglomerular and granule cells in the adult mammalian olfactory bulb modulate olfactory signal transmission. These cells originate from the subventricular zone, migrate to the olfactory bulb via the Rostral Migratory Stream (RMS), and differentiate into mature cells within the olfactory bulb throughout postnatal life. While the regulation of neuroblast development is known to be affected by external stimuli, there is a lack of information concerning changes that occur during the recovery process after injury caused by external stimuli. To address this gap in research, the present study conducted histological observations to investigate changes in the olfactory bulb and RMS occurring after the degeneration and regeneration of olfactory neurons.

METHODS

To create a model of olfactory neurodegeneration, adult mice were administered methimazole intraperitoneally. Nasal tissue and whole brains were removed 3, 7, 14 and 28 days after methimazole administration, and EdU was administered 2 and 4 h before removal of these tissues to monitor dividing cells in the RMS. Methimazole-untreated mice were used as controls. Olfactory nerve fibers entering the olfactory glomerulus were observed immunohistochemically using anti-olfactory marker protein. In the brain tissue, the entire RMS was observed and the volume and total number of cells in the RMS were measured. In addition, the number of neuroblasts and dividing neuroblasts passing through the RMS were measured using anti-doublecortin and anti-EdU antibodies, respectively. Statistical analysis was performed using the Tukey test.

RESULTS

Olfactory epithelium degenerated was observed after methimazole administration, and recovered after 28 days. In the olfactory glomeruli, degeneration of OMP fibers began after methimazole administration, and after day 14, OMP fibers were reduced or absent by day 28, and overall OMP positive fibers were less than 20%. Glomerular volume tended to decrease after methimazole administration and did not appear to recover, even 28 days after recovery of the olfactory epithelium. In the RMS, EdU-positive cells decreased on day 3 and began to increase on day 7. However, they did not recover to the same levels as the control methimazole-untreated mice even after 28 days.

CONCLUSION

These results suggest that the division and maturation of neuroblasts migrating from the RMS was suppressed by olfactory nerve degeneration or the disruption of olfactory input.

The Role of the Stimulus in Olfactory Plasticity

Plasticity, the term we use to describe the ability of a nervous system to change with experience, is the evolutionary adaptation that freed animal behavior from the confines of genetic determinism. This capacity, which increases with brain complexity, is nowhere more evident than in vertebrates, especially mammals. Though the scientific study of brain plasticity dates back at least to the mid-19th century, the last several decades have seen unprecedented advances in the field afforded by new technologies. Olfaction is one system that has garnered particular attention in this realm because it is the only sensory modality with a lifelong supply of new neurons, from two niches no less! Here, we review some of the classical and contemporary literature dealing with the role of the stimulus or lack thereof in olfactory plasticity. We have restricted our comments to studies in mammals that have used dual tools of the field: stimulus deprivation and stimulus enrichment. The former manipulation has been implemented most frequently by unilateral naris occlusion and, thus, we have limited our comments to research using this technique. The work reviewed on deprivation provides substantial evidence of activity-dependent processes in both developing and adult mammals at multiple levels of the system from olfactory sensory neurons through to olfactory cortical areas. However, more recent evidence on the effects of deprivation also establishes several compensatory processes with mechanisms at every level of the system, whose function seems to be the restoration of information flow in the face of an impoverished signal. The results of sensory enrichment are more tentative, not least because of the actual manipulation: What odor or odors? At what concentrations? On what schedule? All of these have frequently not been sufficiently rationalized or characterized. Perhaps it is not surprising, then, that discrepant results are common in sensory enrichment studies. Despite this problem, evidence has accumulated that even passively encountered odors can "teach" olfactory cortical areas to better detect, discriminate, and more efficiently encode them for future encounters. We discuss these and other less-established roles for the stimulus in olfactory plasticity, culminating in our recommended "aspirations" for the field going forward.

Characterization of human oxidoreductases involved in aldehyde odorant metabolism

Oxidoreductases are major enzymes of xenobiotic metabolism. Consequently, they are essential in the chemoprotection of the human body. Many xenobiotic metabolism enzymes have been shown to be involved in chemosensory tissue protection. Among them, some were additionally shown to be involved in chemosensory perception, acting in signal termination as well as in the generation of metabolites that change the activation pattern of chemosensory receptors. Oxidoreductases, especially aldehyde dehydrogenases and aldo-keto reductases, are the first barrier against aldehyde compounds, which include numerous odorants. Using a mass spectrometry approach, we characterized the most highly expressed members of these families in the human nasal mucus sampled in the olfactory vicinity. Their expression was also demonstrated using immunohistochemistry in human epitheliums sampled in the olfactory vicinity. Recombinant enzymes corresponding to three highly expressed human oxidoreductases (ALDH1A1, ALDH3A1, AKR1B10) were used to demonstrate the high enzymatic activity of these enzymes toward aldehyde odorants. The structure‒function relationship set based on the enzymatic parameters characterization of a series of aldehyde odorant compounds was supported by the X-ray structure resolution of human ALDH3A1 in complex with octanal.