Apoptosis in the Aging Olfactory Epithelium

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Abstract
Olfactory receptor neurons undergo apoptosis at a baseline rate, probably secondary to environmental damage even in the absence of gross disease. The study demonstrates age-related changes in expression of genes known to regulate apoptosis in the rat olfactory mucosa. These results are compared with gene expression in young rats and rats that have undergone surgical deafferentation of the olfactory receptor neurons. The olfactory mucosae from three groups of rats were studied: young, normal rats (age, 12 wk); old, normal rats (age, 24 mo); and young rats 9 days after bilateral removal of the olfactory bulb. Bulbectomy is known to produce an initial wave of apoptotic cell death in the population of olfactory neurons. At 9 days after the injury, the olfactory mucosae consist of an enhanced population of regenerating neurons destined to also undergo apoptosis, since their synaptic target (bulb) has been removed. Ribonuclease protection assays and histological analysis of the three groups were performed. Ribonuclease protection assay analysis indicates that age induces increases in the expression of pro-apoptotic genes in the olfactory mucosae similar to the increase seen after deafferentation (bulbectomy). Specifically, the expression of procaspase-3 and bax was increased in aged animals and bulbectomized animals when compared with young, normal animals. Aging induces changes in gene expression in the olfactory mucosae that appear to favor apoptosis, probably associated with increased fragility of olfactory receptor neurons in older animals. These changes may, at least in part, explain the age-related decline in olfactory sensation and loss of olfactory receptor neurons seen in elderly patients.