Increase in masseter muscle activity by newly fabricated complete dentures improved brain function.

Purpose To reveal effects of improvement of masseter muscle activity on brain function in elderly people wearing complete dentures.Methods Subjects were 14 edentulous patients with a chief complaint of the inconvenience of their complete dentures. The surface electromyographic (EMG) activity of the masseter muscles was measured. Brain activities were analyzed with functional magnetic resonance imaging (fMRI), employing chewing gum as the task program. Cognitive functions were evaluated with Trail Making Test Part A (TMT-A), Rey Auditory Verbal Learning Test (RAVLT) and Rey-Osterrieth Complex Figure Test (R-OCFT). Those evaluations were performed in which subjects wore their old dentures (OD) or newly fabricated dentures (ND).Results We compared ND condition with OD condition. The masseter muscle activity significantly increased in ND condition (p < 0.05, Wilcoxon signed rank test). The brain activity increased significantly in the superior frontal gyrus, precentral gyrus, putamen, inferior parietal lobule, cerebellum, inferior frontal lobe, and middle frontal gyrus under the ND condition than under the OD condition (p < 0.01, uncorrected, cluster size > 10 voxels). Results of TMT-A, RAVLT, and R-OCFT were also significantly improved (p < 0.05, Wilcoxon signed rank test).Conclusions In the edentulous elderly, the brain activity was increased following the improvement of the masseter muscle activity. Consequently, it is possible that the improvement of the masseter muscle activity might influence on the attention, verbal skills, and visual memory.

Trans-trigeminal transport of masseter-derived neprilysin to hippocampus.

OBJECTIVE: To show the possible occurrence of exosomal transport of neprilysin from masseter muscle to hippocampus via trigeminal nerve in the living mouse. DESIGN: Mouse C2C12 myotube-derived exosomes were labeled with near-infrared (NIR) dye and injected into the masseter muscle to track their fluorescence from masseter muscle to hippocampus via trigeminal nerve. A plasmid vector encoding green fluorescent protein (GFP)-tagged neprilysin (GFP-neprilysin) was transfected into masseter muscle of C57BL/6 J mice. Expression of mRNA and encoded protein of the transgene was identified in masseter muscle, trigeminal nerve and hippocampus by RT-PCR and Western blot, respectively. RESULTS: Peak of exosomal NIR in masseter muscle at time 0 rapidly reduced at 3 h and 6 h along with the subsequent increases in trigeminal nerve and hippocampus. Expression of GFP-neprilysin mRNA was detected in masseter muscle, but not trigeminal nerve and hippocampus. On the other hand, the corresponding protein of GFP-neprilysin was identified in the three tissues on day 3 after transfection into masseter muscle as a single band on Western blots with anti-GFP and anti-neprilysin antibodies. CONCLUSION: The appearance of GFP-neprilysin protein in trigeminal nerve and hippocampus without a corresponding mRNA expression indicated the protein's origin from the masseter muscle. Concomitant migration of NIR-exosomes from masseter muscle to hippocampus via trigeminal nerve suggested the possible occurrence of exosomal transport of neprilysin.

Effects of tooth loss on brain structure: a voxel-based morphometry study.

PURPOSE: One of the most prominent issues in a super-aging society is the rapid increase in dementia patients. Cross-sectional studies in dentistry have indicated that patients with dementia have worse oral health compared to healthy people. The purpose of this study was to clarify the influence of tooth loss on brain structure by comparing the volumes of gray matter (GM) and white matter (WM) between edentulous and dentulous subjects. METHODS: Subjects were recruited from the Denture Clinic at Iwate Medical University Hospital Dental Center. Experiments were performed on edentulous (5 males, 8 females, 81.8±1.24years) and dentulous subjects (4 males, 7 females, 77.1±4.25years). Patients with dementia were excluded from this study. Brain volumes of GM and WM in edentulous and dentulous subjects were compared using intracranial volume, age, gender and history of hypertension as covariates. Analyzed brain areas were identified by transforming the Montreal Neurological Institute coordinate into the anatomical coordinate in edentulous subjects. RESULTS: The analysis of WM structural images found no morphological differences between dentulous and edentulous subjects. However, significant atrophy of GM was observed in the hippocampus, caudate nucleus and temporal pole of the right hemisphere in edentulous subjects. CONCLUSIONS: The results of this study suggest that tooth loss was a causal factor for volume reduction in brain areas related to memory, learning and cognition.