Masticatory dysfunction-induced neural deficits in aged mice

So far, little attention has been paid to the interaction between masticatory dysfunction and the brain function including learning and memory. However, the fact that senile dementia often developes when the hospitalized elderly receives parenteral nutrition, and that their dementia is improved by an appropriate house dental care to allow taking food by themselves, if not all, strongly suggests the link between oral function and senile dementia. Thus, in senescence-accelerated mice (SAMP8), we examined the effects of masticatory dysfunction (molarless condition) on spatial cognition that is hippocampus-dependent and impaired at the earlier stage of dementia. In aged mice, molarless condition caused a marked spatial learning impairment as compared with age-matched molar-intact control mice; however, no effect was seen in young molarless mice. In the aged molarless mice, both the density of pyramidal neurons and the number of Fos-positive neurons in the hippocampal CA1 region were significantly decreased. In these mice, the molarless condition also enhanced the age-dependent increase in the density and hypertrophy of GFAP-labeled astrocytes in the CA1 region. Furthermore, decreased activities in the septohippocampal cholinergic system was detected in the molarless condition. These effects increased the longer the molarless condition persisted. In contrast, the impaired spatial memory in aged molarless mice was improved by repair of molars with artificial crown when repair was done before the memory deficit was getting severe. These experimental data strongly suggest that, in the elderly, keeping healthy condition of mastication may be a key factor to reduce a risk for senile dementia.