Reduced mastication stimulates impairment of spatial memory and degeneration of hippocampal neurons in aged SAMP8 mice

Molecular mechanisms underlying the effect of tooth shortening on memory dysfunction in Wistar male rat

OBJECTIVE

We investigated the effects of molar tooth shortening on the mRNA expression of the AβPP/BACE1, BDNF/TrkB, and Bax/Bcl-2 signaling pathways in the Wistar male rat hippocampal regions.

DESIGN

Four groups (n = 5 per group) of male Wistar rats (control, SRM (shortened right molar), SLM (shortened left molar), and SBM (shortened bilateral molar)) were used. RNA was isolated from the hippocampus and transformed into cDNA. Real-time quantitative PCR was used to evaluate the mRNA expression levels of AβPP, BACE1, Bax, Bcl-2, BDNF, and TrkB.

RESULTS

Differential mRNA expression was observed in rat groups. SBM significantly upregulated the AβPP, BACE1, and Bax mRNA expressions, whereas the expression levels of Bcl-2, BDNF, and TrkB were decreased. SRM and SLM approximately had the same effect on the expression enhancement of AβPP, BACE1, and Bax; however, SRM was more effective than SLM in increasing the expression of these genes.

CONCLUSIONS

Symmetrical molar teeth shortening affected the mRNA expression of AβPP and BACE1, which is related to learning and memory dysfunction.

Effects of early tooth loss on chronic stress and progression of neuropathogenesis of Alzheimer's disease in adult Alzheimer's model AppNL-G-F mice

Introduction

Alzheimer's disease (AD), the most common neurodegenerative disease, is characterized by accumulated amyloid-β (Aβ) plaques, aggregated phosphorylated tau protein, gliosis-associated neuroinflammation, synaptic dysfunction, and cognitive impairment. Many cohort studies indicate that tooth loss is a risk factor for AD. The detailed mechanisms underlying the association between AD and tooth loss, however, are not yet fully understood.

Methods

We explored the involvement of early tooth loss in the neuropathogenesis of the adult AppNL-G-F mouse AD model. The maxillary molars were extracted bilaterally in 1-month-old male mice soon after tooth eruption.

Results

Plasma corticosterone levels were increased and spatial learning memory was impaired in these mice at 6 months of age. The cerebral cortex and hippocampus of AD mice with extracted teeth showed an increased accumulation of Aβ plaques and phosphorylated tau proteins, and increased secretion of the proinflammatory cytokines, including interleukin 1β (IL-1β) and tumor necrosis factor α (TNF-α), accompanied by an increased number of microglia and astrocytes, and decreased synaptophysin expression. AD mice with extracted teeth also had a shorter lifespan than the control mice.

Discussion

These findings revealed that long-term tooth loss is a chronic stressor, activating the recruitment of microglia and astrocytes; exacerbating neuroinflammation, Aβ deposition, phosphorylated tau accumulation, and synaptic dysfunction; and leading to spatial learning and memory impairments in AD model mice.

The effect of single-implant overdentures on cognitive function in older adults: A 3-year follow-up report

OBJECTIVE

To investigate the occurrence of mild cognitive impairment in edentulous patients up to 3 years after they received single-implant overdentures (1-IODs).

METHODS

This was a follow-up study for a crossover randomized controlled trial comparing 1-IODs and conventional complete dentures, with patient satisfaction as the primary outcome. In the present study, the cognitive function of the patients, measured with the Japanese version of the Montreal cognitive assessment (MoCA-J), was evaluated as the secondary outcome at baseline and 2 months, 1 year, 2 years, and 3 years after 1-IOD placement. The total and domain (memory, executive function, visuospatial skills, language, attention, and orientation) MoCA-J scores of the patients at each timepoint were analyzed and compared.

RESULTS

Twenty-two patients with edentulous mandibles received 1-IODs. Within-group comparisons revealed that the total MoCA-J scores at 1 year and 3 years after 1-IOD placement were significantly increased compared with baseline scores. Additionally, the memory domain scores at all timepoint were significantly increased compared with the baseline scores, and the executive function domain scores at the 2-month, 2-year and 3-year timepoint were significantly increased compared with the baseline scores.

CONCLUSION

Patients with edentulous mandibles who underwent 1-IOD placement showed significantly improved total scores of MoCA-J after 1 year and 3 years of wearing 1-IODs. In addition, they showed significantly improved memory domain scores at 2 months, 1 year, 2 years, and 3 years after 1-IOD placement and executive function domain scores at 2 months, 2 years, and 3 years after 1-IOD placement.

CLINICAL SIGNIFICANCE

The results of this study suggest that 1-IOD treatment for older adults, especially those with edentulous mandibles, may prevent cognitive decline regardless of the condition of the maxilla.

Increased vertical dimension of occlusion for varying periods differentially impairs learning and memory in guinea pigs

There is an increasing number of studies showing that occlusal dysfunction impairs learning and memory. We previously demonstrated that the brain has a mechanism to calibrate between the activities of spindle afferents and periodontal-mechanoreceptor afferents for controlling the chewing movement, and the accurate calibration can be done only at the proper vertical dimension of occlusion (VDO). Then, the chewing at an inappropriate VDO may induce a severe mental stress due to a mal-calibration. However, it is not clear how the impairment of learning/memory progresses over the period of stress induced by occlusal dysfunction. We investigated by passive avoidance test how the behavior and learning/memory are altered in guinea pigs in which the VDO was raised by 2-3 mm over the period up to 8 weeks. We found that the guinea pigs reared under the raised occlusal-condition (ROC) for 1 week showed a very high sensitivity to electrical stimulation whereas this did not cause the memory consolidation in the 1st-day retention trial, suggesting that such hypersensitivity rather hampered the fear learning. In the guinea pigs reared under the ROC for 2 and 8 weeks, the learning ability was not largely affected and memory consolidation occurred similarly whereas the memory retention deteriorated more severely in the latter guinea pigs than in the former ones. In the guinea pigs reared under the ROC for 3 and 4 weeks, learning was severely impaired, and memory consolidation did not occur. These results suggest that the occlusal dysfunction for varying periods differentially impairs learning and memory.

Molar tooth shortening induces learning and memory impairment in Wistar rat

OBJECTIVE

This study aimed to investigate the relationship between different patterns of molar crown loss and the association between symmetrical and asymmetrical shortening molar teeth with memory impairment.

MATERIALS AND METHODS

Male Wistar rats were divided into four groups (n = 10) including control, SLM (shortened left molar), SRM (shortened right molar), and SBM (shortened bilateral molar) groups. Morris water maze (MWM) and passive avoidance test (PAT) were performed to assess spatial and fear memory, respectively. Besides, histological assessment of hippocampus and gingival tissues was done.

RESULTS

In the MWM test, SBM and SLM groups had higher escape latency over training trials and spent less time in the target quadrant in the probe trial (p < 0.01). In the PAT, step-through latency was significantly reduced in three groups, and time spent in the dark compartment increased in SBM (p < 0.01) and SLM (p < 0.05) groups. In addition, each teeth shortening group indicated a reduction in density (p < 0.01) and thickness layer (p < 0.05) of pyramidal cells. Gingival was normal after shortening of the molar crown.

CONCLUSIONS

Different patterns of molar teeth shortening induced learning and memory impairment; however, symmetrical molar teeth shortening has more effects on memory impairment.

Does the Loss of Teeth Have an Impact on Geriatric Patients’ Cognitive Status?

Significant changes in the microstructure of the brain cause dementia and other mental declines associated with aging and disease. Although research has established a connection between oral health and dementia, the underlying pathologic mechanisms are still unknown. Aim: Our aim was to evaluate dentures’ impact on the cognitive state of geriatric patients. Material and methods: A total of 108 individuals seeking treatment at the Faculty of Dental Medicine in Iasi, Romania, participated in the study, which ran from May 2022 to October 2022. Cognitive dysfunction was assessed using the Mini-Mental State Examination. The acquired data were analyzed with IBM SPSS 26.0, and the p-value was set at 0.05. Results: The average value of the MMSE score was 21.81 ± 3.872. Differences between groups of wearer/non-wearer subjects were statistically significant for most of the questions in the questionnaire. Linear regression analysis showed that individuals with a high MMSE score have prosthodontic treatment. A decrease in the MMSE score is associated with a decrease in masticatory efficiency (B = 1.513, p = 0.268). Conclusions: This study provides further evidence that tooth loss is associated with worse cognitive performance. It is thus conceivable that the necessary effects can be achieved by increasing the efforts dedicated to preventing tooth loss in the adult population.

Comparison of the effect of conventional and implant-retained overdentures on brain activity and cognition in a geriatric population - A Functional MRI study

PURPOSE

This study aims to evaluate the effect of rehabilitation with complete dentures versus implant-retained overdenture on activity in various parts of the brain cognition in a geriatric edentulous population via Functional MRI (fMRI) studies and the Mini-Mental State Examination (MMSE).

METHODS

Ten completely edentulous patients were rehabilitated with both complete dentures and two-implant retained overdentures for three months each. fMRI studies were performed for each modality during chewing and recall tasks at three time periods: T0: Completely Edentulous(CE) T1: after three months of wearing Conventional Complete Dentures(CD) and T2: after three months of wearing Implant-retained Overdentures (IOD). The Z scores obtained from the fMRI at these phases of examination were tabulated and correlated with MMSE scores obtained at the corresponding time periods.

RESULTS

Z scores obtained during the memory recall tasks at T2 were the greatest (Prefrontal Cortex (p=0.059) and Hippocampus (p=0.036). The MMSE scores obtained were significantly higher for the IODs when compared to the CDs and Baseline values (p < 0.05)Conclusion: IODs may potentially result in superior sensory feedback in edentulous patients and lead to improved cognitive performance when compared to conventional complete dentures.

The Effect of Alteration of Vertical Dimension of Occlusion on Brain Activity in Complete Denture Wearers

BACKGROUND: The establishment of appropriate vertical dimension of occlusion (VDO) is paramount for successful and functioning complete dentures (CDs). AIM: The aim of this study was to assess the effect of alteration of VDO on the brain activity in CD wearers. MATERIALS AND METHODS: Ten completely edentulous patients participated in this study. Each patient received three sets of CDs: one control set and two duplicates’ CDs. The control CDs was designed with appropriate VDO (control - VDO), while the duplicate CDs were designated as followed: one with VDO lowered by –3 mm (low - VDO) the other one VDO was increased by +5 mm (high - VDO). The effect of alteration of VDO on brain activity was assessed by measuring alpha waves changes using electroencephalogram (EEG) before and after chewing gum. Statistical analysis was carried out using one-way ANOVA and dependent t-test. The significance level was set at p < 0.05. RESULTS: There was a significant increase in the alpha waves mean values after chewing (p < 0.05) gum compared to before except for low - VDO CDs (p > 0.05). The control-VDO revealed a statistically significant higher mean values compared to the duplicate dentures. CDs with high - VDO showed higher mean value than low - VDO dentures, however, it was statistically insignificant (p > 0.05). CONCLUSION: Appropriate determination of VDO is not only important for the success of CDs but also for the functional augmentation of brain activity.

Lack of occlusal support did not impact amyloid β deposition in APP knock-in mice

PURPOSE

The lack of occlusal support is an epidemiological risk factor linked to Alzheimer's disease. This study sought to assess the relationship between amyloid β (Aβ) deposition and the lack of occlusal support in amyloid precursor protein (APP) knock-in mice.

METHODS

Sixteen experimental animals were divided into two groups. The upper molars were extracted in the extraction group (group E), and a sham operation was performed in the control group (group C). The Morris water maze test was performed 4 months after the tooth extraction. Aβ immunohistochemical staining and Nissl staining of the hippocampus were performed. Hippocampal plasma corticosterone and Aβ protein levels were measured.

RESULTS

In the maze task, the escape latency was significantly longer in group E than in group C. In the probe trials, the time elapsed in the target quadrant was significantly shorter in group E than in group C. The number of hippocampal neurons decreased in group E. There was no significant difference in the plasma corticosterone levels between the two groups, indicating that there was no effect of chronic stress on the behavioral results. Hippocampal Aβ40 and Aβ42 protein levels and Aβ deposition areas by immunohistochemical staining were not significantly different between the two groups.

CONCLUSIONS

Aβ deposition was not increased in the hippocampus of molarless APP knock-in mice. As such, it appears that cognitive impairment due to a lack of occlusal support was not related to Aβ deposition.

Cognitive changes and neural correlates after oral rehabilitation procedures in older adults: a protocol for an interventional study

BACKGROUND

Epidemiological studies show an association between masticatory function and cognitive impairment. This has further strengthened the notion that tooth loss and impaired masticatory function may be risk factors for dementia and cognitive decline. Animal experiments have indicated a causal relationship and several possible mechanisms have been discussed. This evidence is, however, lacking in humans. Therefore, in the current interventional study, we aim to investigate the effect of rehabilitation of masticatory function on cognition in older adults.

METHODS

Eighty patients indicated for prosthodontic rehabilitation will be randomly assigned to an experimental or a control group. Participants will conduct neuropsychological assessments, masticatory performance tests, saliva tests, optional magnetic resonance imaging, and answer questionnaires on oral health impact profiles and hospital anxiety and depression scale before, 3 months, and 1 year after oral rehabilitation. The difference between the two groups is that the control group will be tested an additional time, (at an interval of about 3 months) before the onset of the oral rehabilitation procedure. The primary outcome is a change in measures of episodic memory performance.

DISCUSSION

Although tooth loss and masticatory function are widespread in older people, it is still an underexplored modifiable risk factor potentially contributing to the development of cognitive impairment. If rehabilitation of masticatory function shows positive effects on the neurocognitive function, this will have great implications on future health care for patients with impaired masticatory status. The present project may provide a new avenue for the prevention of cognitive decline in older individuals.

TRIAL REGISTRATION

The protocol for the study was retrospectively registered in ClinicalTrials.gov Identifier: NCT04458207, dated 02-07-2020.

Sedentary Life and Reduced Mastication Impair Spatial Learning and Memory and Differentially Affect Dentate Gyrus Astrocyte Subtypes in the Aged Mice

To explore the impact of reduced mastication and a sedentary lifestyle on spatial learning and memory in the aged mice, as well as on the morphology of astrocytes in the molecular layer of dentate gyrus (MolDG), different masticatory regimens were imposed. Control mice received a pellet-type hard diet, while the reduced masticatory activity group received a pellet diet followed by a powdered diet, and the masticatory rehabilitation group received a pellet diet, followed by powder diet and then a pellet again. To mimic sedentary or active lifestyles, mice were housed in an impoverished environment of standard cages or in an enriched environment. The Morris Water Maze (MWM) test showed that masticatory-deprived group, regardless of environment, was not able to learn and remember the hidden platform location, but masticatory rehabilitation combined with enriched environment recovered such disabilities. Microscopic three-dimensional reconstructions of 1,800 glial fibrillary acidic protein (GFAP)-immunolabeled astrocytes from the external third of the MolDG were generated using a stereological systematic and random sampling approach. Hierarchical cluster analysis allowed the characterization into two main groups of astrocytes with greater and lower morphological complexities, respectively, AST1 and AST2. When compared to compared to the hard diet group subjected to impoverished environment, deprived animals maintained in the same environment for 6 months showed remarkable shrinkage of astrocyte branches. However, the long-term environmental enrichment (18-month-old) applied to the deprived group reversed the shrinkage effect, with significant increase in the morphological complexity of AST1 and AST2, when in an impoverished or enriched environment. During housing under enriched environment, complexity of branches of AST1 and AST2 was reduced by the powder diet (pellet followed by powder regimes) in young but not in old mice, where it was reversed by pellet diet (pellet followed by powder and pellet regime again). The same was not true for mice housed under impoverished environment. Interestingly, we were unable to find any correlation between MWM data and astrocyte morphological changes. Our findings indicate that both young and aged mice subjected to environmental enrichment, and under normal or rehabilitated masticatory activity, preserve spatial learning and memory. Nonetheless, data suggest that an impoverished environment and reduced mastication synergize to aggravate age-related cognitive decline; however, the association with morphological diversity of AST1 and AST2 at the MolDG requires further investigation.

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 influ ence on the attention, verbal skills, and visual memory.

Association between cognitive health and masticatory conditions: a descriptive study of the national database of the universal healthcare system in Japan

Cognitive health is subject to decline with increasing numbers of lost teeth which impacts mastication. This study is a descriptive data analysis of the association between masticatory and cognitive conditions using a large database. We obtained the dental and medical records from Japan's universal healthcare system (UHCS) from the national database in 2017. The data from 94% of the Japanese population aged 65 and over is included. It is inclusive of diagnostic codes for various types of cognitive impairment, as well as dental treatment records from 2012 to 2017. The cognitive impairment group was compared to those without a diagnosis of cognitive impairment. Crude odds ratio between loss of mastication with natural teeth (exposure) and cognitive impairments (outcome) were compared. Patients who have lost masticatory function are likely to have cognitive impairment with an odds ratio of 1.89 (p<0.0001) for early elderly (aged 65-75) and 1.33 (p<0.0001) for advanced elderly (over 75). Patients who are edentulous and function with complete dentures are likely to have cognitive impairment with an odds ratio of 2.38 (p<0.0001) and 1.38 (p<0.0001), respectively. The data shows a convincing and significant result of an association between cognitive health and oral health, related to masticatory conditions.

Tooth Loss Induces Memory Impairment and Gliosis in App Knock-In Mouse Models of Alzheimer's Disease

BACKGROUND

Epidemiological studies have shown that tooth loss is associated with Alzheimer's disease (AD) and dementia. However, the molecular and cellular mechanisms by which tooth loss causes AD remain unclear.

OBJECTIVE

We investigated the effects of tooth loss on memory impairment and AD pathogenesis in AppNL-G-F mice.

METHODS

Maxillary molar teeth on both sides were extracted from 2-month-old AppNL-G-F mice, and the mice were reared for 2 months. The short- and long-term memory functions were evaluated using a novel object recognition test and a passive avoidance test. Amyloid plaques, amyloid-β (Aβ) levels, glial activity, and neuronal activity were evaluated by immunohistochemistry, Aβ ELISA, immunofluorescence staining, and western blotting. The mRNA expression levels of neuroinflammatory cytokines were determined by qRT-PCR analysis.

RESULTS

Tooth loss induced memory impairment via an amyloid-cascade-independent pathway, and decreased the neuronal activity, presynaptic and postsynaptic protein levels in both the cortex and hippocampus. Interestingly, we found that tooth loss induced glial activation, which in turn leads to the upregulation of the mRNA expression levels of the neuroinflammation cytokines tumor necrosis factor-α (TNF-α), interleukin-6 (IL-6), and IL-1β in the hippocampus. We also found that tooth loss activated a stress-activated protein kinase, c-Jun N-terminal kinase (JNK), and increased heat shock protein 90 (HSP90) levels in the hippocampus, which may lead to a glial activation.

CONCLUSION

Our findings suggest that taking care of teeth is very important to preserve a healthy oral environment, which may reduce the risk of cognitive dysfunction.

The path from trigeminal asymmetry to cognitive impairment: a behavioral and molecular study

Trigeminal input exerts acute and chronic effects on the brain, modulating cognitive functions. Here, new data from humans and animals suggest that these effects are caused by trigeminal influences on the Locus Coeruleus (LC). In humans subjects clenching with masseter asymmetric activity, occlusal correction improved cognition, alongside with reductions in pupil size and anisocoria, proxies of LC activity and asymmetry, respectively. Notably, reductions in pupil size at rest on the hypertonic side predicted cognitive improvements. In adult rats, a distal unilateral section of the trigeminal mandibular branch reduced, on the contralateral side, the expression of c-Fos (brainstem) and BDNF (brainstem, hippocampus, frontal cortex). This counterintuitive finding can be explained by the following model: teeth contact perception loss on the lesioned side results in an increased occlusal effort, which enhances afferent inputs from muscle spindles and posterior periodontal receptors, spared by the distal lesion. Such effort leads to a reduced engagement of the intact side, with a corresponding reduction in the afferent inputs to the LC and in c-Fos and BDNF gene expression. In conclusion, acute effects of malocclusion on performance seem mediated by the LC, which could also contribute to the chronic trophic dysfunction induced by loss of trigeminal input.

Relationship between cerebral atrophy and number of present teeth in elderly individuals with cognitive decline

BACKGROUND AND AIM

Recent shifts in the distribution of Japan's population towards older ages, have meant that the chance of encountering patients with dementia in dental clinics is increasing. Many studies have shown that the brain volume decreases along with the progression of dementia. Although previous studies have reported a relationship between tooth loss or periodontitis and the onset of dementia, the pathological mechanisms underlying this association have not been elucidated. In this study, we aimed to examine the relationship between the oral condition and brain atrophy and to discuss how to adequately deal with patients with dementia.

PARTICIPANTS AND METHODS

This cross-sectional study included 15 participants who underwent brain magnetic resonance imaging (MRI). The participants were 60 years or older and presented with cognitive decline, including Alzheimer's disease (AD) and mild cognitive impairment (MCI), diagnosed by a neurologist. We obtained information on the oral condition, lifestyle, cognitive function, and brain atrophy. Cognitive function was assessed using the Mini-Mental State Examination (MMSE). MR images of each patient were analyzed using the voxel-based specific regional analysis system for Alzheimer's disease (VSRAD) to provide a quantitative measure of the degree of brain atrophy.

RESULTS

The study population included 4 male and 11 female patients. The mean age and mean number of present teeth were 75.9 years (SD 6.7) and 15.0 (SD 11.1), respectively. The median MMSE score was 25.6 (SD 3.7). The degree of atrophy of the whole brain was significantly correlated with the number of present teeth (ρ = -0.72, p < 0.05) and the presence of a daily exercise habit (ρ = -0.66, p < 0.05).

CONCLUSION

This study demonstrated that the number of present teeth could be an indicator of the progress of dementia. Preserving the teeth as well as the acquisition of a regular exercise habit might be important for preventing progression of dementia. Further research examining a larger study population and analyzing a greater number of factors is warranted.

Oral dysfunctions and cognitive impairment/dementia

With global increases in the aging population, the number of patients with dementia is greatly increasing, which has become a big social problem. Many studies have shown strong associations between oral disorders and systemic disorders, such as diabetes, arthritis, sepsis, aspiration pneumonia, arteriosclerosis, bacterial endocarditis, and other cardiovascular diseases. Similarly, numerous cross-sectional studies showed that patients with dementia usually have poor oral conditions and tooth loss. These have long been considered as a result of difficulty with oral care due to impaired cognitive function, memory, and physical ability in patients with dementia. Indeed, even in patients with mild cognitive impairment, oral care becomes insufficient owing to decreases in spontaneity of grooming and finger dexterity. However, recent studies have shown that tooth loss and occlusal dysfunction may affect brain function and trigger the onset of dementia found in neurodegenerative diseases including Alzheimer's disease. In this review, we highlight the relationships among aging, oral dysfunction, and the development of dementia. Increasing evidence suggests that oral dysfunction is not only a result of dementia in the elderly people, but could also be a causative factor for the onset of dementia.

Memory impairment of chewing-side preference mice is associated with 5-HT-BDNF signal pathway

Although tooth loss is a known risk factor of cognitive function, whether and how the chewing-side preference (CSP) affects memory impairment still remains unclear. This study evaluates the behavior changes in mice after the loss of teeth on one side and explores the role of serotonin (5-HT) and brain-derived neurotrophic factor (BDNF) signal pathway within these changes. To this end, CSP mouse models with either the removal of left unilateral molars (CSP-L) or right unilateral molars (CSP-R) were established. Morris water maze test and passive avoidance test were performed to evaluate the mice's learning and memory capacity in the 4th and 8th weeks. The correlation between CSP and brain function changes was validated with changes in 5-HT and BDNF levels. CSP mice's cognitive function was found to be decreased, along with a significant decline in 5-HT1A level, especially in CSP-R mice. BDNF and TrkB levels in CSP-R mice were also significantly lowered. These findings suggest that CSP results in memory impairment, which is associated with the 5-HT-BDNF signaling pathway.

The association between mastication and mild cognitive impairment in Korean adults

Currently, a few of studies revealed that there is an association between mastication and cognitive impairment. There is no study of Korean adult representative samples in relation to mastication and cognitive decline. This study was to investigate the relationship between mastication and mild cognitive impairment in Korean adults.A population-based cross-sectional study was conducted in which a total of 7029 subjects (2987 men and 4042 women) over 45 years old were surveyed from the Korea Longitudinal Study on Aging (KLoSA), Round 5th survey. Logistic regression analysis was performed for the study data controlling for confounding factors such as age, gender, education, income, smoking, drinking, exercise, wearing denture, and the number of chronic diseases.Decreased chewing function is associated with mild cognitive impairment (odds ratio [OR] = 3.24, 95% confidence interval [CI] = 2.67-3.93) after controlling for confounding variables. In the participants who did not wear dentures, the reduction of chewing function was strongly correlated with mild cognitive impairment (OR = 3.97, 95% CI = 3.11-5.08).Mastication was associated with mild cognitive impairment. To prevent cognitive decline, health specialists should pay more attention to the decline of the mastication in people without dentures.

Loss of Functional Dentition is Associated with Cognitive Impairment

BACKGROUND

Although tooth loss is known to increase the risk of cognitive impairment and dementia, few studies have investigated the association between functional teeth including rehabilitated lost teeth and cognitive functionObjective:We investigated the associations of the numbers of functional teeth and functional occlusal units with cognitive impairment and cognitive function in late life.

METHODS

The current study was conducted as a part of the Korean Longitudinal Study on Cognitive Aging and Dementia (KLOSCAD), a community-based elderly cohort study. We analyzed 411 participants who have agreed with the additional dental exam. Geriatric psychiatrists and neuropsychologists administered the Consortium to Establish a Registry for Alzheimer's disease Assessment Packet Clinical and Neuropsychological Assessment Battery to all participants, and dentists examined their dental status.

RESULTS

Higher number of functional teeth (OR = 0.955, 95% CI = 0.914-0.997, p = 0.037) and higher number of functional occlusal units (OR = 0.900, 95% CI = 0.813-0.996, p = 0.042) were associated with lower odds of cognitive impairment. When we analyzed these relationships separated by the location of teeth, only the numbers of functional teeth (OR = 0.566, 95% CI = 0.373-0.857, p = 0.007) and functional occlusal units (OR = 0.399, 95% CI = 0.213-0.748, p = 0.004) in the premolar area were associated with lower odds of cognitive impairment.

CONCLUSION

Loss of functional teeth and functional occlusal units (especially in the premolar region) were associated with increased cognitive impairment.

Tooth loss early in life induces hippocampal morphology remodeling in senescence-accelerated mouse prone 8 (SAMP8) mice

Long-term tooth loss is associated with the suppression of hippocampal neurogenesis and impairment of hippocampus-dependent cognition with aging. The morphologic basis of the hippocampal alterations, however, remains unclear. In the present study, we investigated whether tooth loss early in life affects the hippocampal ultrastructure in senescence-accelerated mouse prone 8 (SAMP8) mice, using transmission electron microscopy. Male SAMP8 mice were randomized into control or tooth-loss groups. All maxillary molar teeth were removed at 1 month of age. Hippocampal morphologic alterations were evaluated at 9 months of age. Tooth loss early in life induced mitochondrial damage and lipofuscin accumulation in the hippocampal neurons. A thinner myelin sheath and decreased postsynaptic density length were also observed. Our results revealed that tooth loss early in life may lead to hippocampal ultrastructure remodeling and subsequent hippocampus-dependent cognitive impairment in SAMP8 mice with aging.

Occlusal force predicted cognitive decline among 70- and 80-year-old Japanese: A 3-year prospective cohort study

PURPOSE

Dementia is a growing health problem for countries with aging populations, but few effective dementia treatments are available. However, there is increasing interest in oral health as a modifiable risk factor in interventions to prevent cognitive decline. This study aimed to investigate the impact of oral health on the decline of cognitive function over 3 years among Japanese people aged 70 and 80 years.

METHODS

Participants (n = 860) were community-dwelling older adults who participated in baseline and follow-up surveys (at baseline: 69-71 years n = 423; 79-81 years, n = 437). Registered dentists examined the number of teeth, number of functional teeth, number of periodontal teeth, and occlusal force. The Japanese version of the Montreal Cognitive Assessment was used to evaluate cognitive function. We also evaluated socioeconomic factors, medical history, drinking and smoking habits, physical performance, genetic factors, and C-reactive protein concentration in blood. A generalized estimating equation (GEE) was used to examine how oral health at baseline influenced cognitive decline over 3 years.

RESULTS

The GEE showed that the number of teeth (non-standardized coefficient: B = 0.031, p = 0.022) and occlusal force (B = 0.103, p = 0.004) at baseline were associated with cognitive function at follow-up, even after adjusting for other risk factors. Furthermore, maintaining more teeth (B = 0.009, p = 0.004) and a stronger occlusal force (B = 0.020, p = 0.040) buffered cognitive decline.

CONCLUSIONS

Number of teeth and occlusal force predict cognitive decline over 3 subsequent years in Japanese older adults aged 70 and 80 years.

Impact of Tooth Loss and Other Risk Factors on Cognitive Impairment in Saudi Female Population

Introduction

It is known that cognitive impairment is linked to aging and neurobiological, psychological, and social factors. Recently, however, mastication and the number of teeth has also attracted attention, with a previous case control study reporting a correlation between the loss of teeth and Alzheimer's disease.

Objective

To investigate possible relationships between cognitive function and various demographic variables, stress, medical history, and number of natural teeth in a specified female population.

Materials & Methods

A sample of the Saudi female population, 40-65 years of age, who visited the King Khalid University College of Dentistry (Abha, Saudi Arabia), was studied. Education, occupation, perceived stress, and medical history, along with the number of remaining teeth, were assessed. The Mini-Mental State Examination (MMSE) was used to assess cognitive performance and the results were statistically analyzed.

Results

Subjects were divided into those with mild, moderate, and severe impairment based on MMSE score; the association between age, education, occupation, medical history, and cognitive function demonstrated statistically significant results. Fifty percent of subjects with 0-16 teeth exhibited severe cognitive impairment. Of the cognitive abilities, attention, recall, and language skills were linked to the number of remaining teeth. When subjects were categorized into only high and low cognitive impairment based on MMSE score, regression analysis did not reveal a significant correlation between any of the studied variables and cognitive impairment.

Conclusion

Results of the present study add to the recent data and head towards the theory of likely connection between the number of teeth and hippocampus-dependent cognitive functioning. Results of regression analysis revealed an absence of conclusive relation in the latter part of study. Longitudinal analyses including comprehensive clinical dental data with brain-imaging will shed further light on probable causal relationship(s).

Influences of mouth breathing on memory and learning ability in growing rats

This study was to investigate the influences of habitual mouth breathing on memory and learning ability during the growth period. At age 5 weeks, the experimental rats were subjected to surgery to close completely one side of the nasal cavity. An 8-arm radial maze was used to evaluate memory and learning ability at age 7, 11, and 15 weeks. Moreover, the brain was extracted at age 7, 11, and 15 weeks, and subjected to histomorphometric examination for the distribution and number of pyramidal cells in the hippocampal CA1 and CA3 regions after Nissl staining. The trial time to accomplish each task was significantly longer in the experimental group than in the control group throughout the experimental period. The number of pyramidal cells was significantly less in the experimental rats than in controls in both the CA1 and CA3 regions for the entire experimental period. Thus, the functional deterioration of the respiratory system during the growth phase exerts a substantial effect on the growth and development of the central nervous system.

Effects of the loss and reconstruction of molar occlusal support on memory retrieval and hippocampal neuron density in rats

PURPOSE

Animal experiments have shown that the loss of occlusal support causes impairments in spatial cognition. Many reports have focused on the memory encoding process, and only few studies have investigated the effect on memory retrieval. This study aimed to examine the effects of both the loss and reconstruction of occlusal support on the memory retrieval process and on the number of hippocampal pyramidal cells.

METHODS

The experimental animals were divided into a molarless group, in which molars were extracted, a denture-wearing molarless group with experimental dentures inserted after molar extraction, and a control group. Radial maze trials were performed for 20 consecutive days (acquisition phase). The rats were tested on long-term memory retention following the acquisition phase in eight trials every five days, and in an additional trial 20 days later (probe phase).

RESULTS

The denture-wearing molarless group showed higher levels of spatial learning and memory than the molarless group. There were no significant differences in memory retrieval during the probe phase between the denture-wearing molarless and the control group. The molarless group showed significantly worse spatial learning and memory and had fewer neurons in the hippocampus than the control group.

CONCLUSIONS

Our results suggest that the loss of occlusal support decreases the number of pyramidal cells in the hippocampus and impairs memory decoding and retrieval. However, this effect is suppressed by the reconstruction of occlusal support.

Environmental Enrichment and Successful Aging

The human brain sustains a slow but progressive decline in function as it ages and these changes are particularly profound in cognitive processing. A potential contributor to this deterioration is the gradual decline in the functioning of multiple sensory systems and the effects they have on areas of the brain that mediate cognitive function. In older adults, diminished capacity is typically observed in the visual, auditory, masticatory, olfactory, and motor systems, and these age-related declines are associated with both a decline in cognitive proficiency, and a loss of neurons in regions of the brain. We will review how the loss of hearing, vision, mastication skills, olfactory impairment, and motoric decline accompany cognitive loss, and how improved functioning of these systems may aid in the restoration of the cognitive abilities in older adults. The human brain appears to require a great deal of stimulation to maintain its cognitive efficacy as people age and environmental enrichment may aid in its maintenance and recovery.

Dental deafferentation and brain damage: A review and a hypothesis

In the last few decades, neurobiological and human brain imaging research have greatly advanced our understanding of brain mechanisms that support perception and memory, as well as their function in daily activities. Knowledge of the neurobiological mechanisms behind the deafferentation of stomatognathic systems has also expanded greatly in recent decades. In particular, current studies reveal that the peripheral deafferentations of stomatognathic systems may be projected globally into the central nervous system (CNS) and become an associated critical factor in triggering and aggravating neurodegenerative diseases. This review explores basic neurobiological mechanisms associated with the deafferentation of stomatognathic systems. Further included is a discussion on tooth loss and other dental deafferentation (DD) mechanisms, with a focus on dental and masticatory apparatuses associated with brain functions and which may underlie the changes observed in the aging brain. A new hypothesis is presented where DD and changes in the functionality of teeth and the masticatory apparatus may cause brain damage as a result of altered cerebral circulation and dysfunctional homeostasis. Furthermore, multiple recurrent reorganizations of the brain may be a triggering or contributing risk factor in the onset and progression of neurodegenerative conditions such as Alzheimer's disease (AD). A growing understanding of the association between DD and brain aging may lead to solutions in treating and preventing cognitive decline and neurodegenerative diseases.

Trigeminal, Visceral and Vestibular Inputs May Improve Cognitive Functions by Acting through the Locus Coeruleus and the Ascending Reticular Activating System: A New Hypothesis

It is known that sensory signals sustain the background discharge of the ascending reticular activating system (ARAS) which includes the noradrenergic locus coeruleus (LC) neurons and controls the level of attention and alertness. Moreover, LC neurons influence brain metabolic activity, gene expression and brain inflammatory processes. As a consequence of the sensory control of ARAS/LC, stimulation of a sensory channel may potential influence neuronal activity and trophic state all over the brain, supporting cognitive functions and exerting a neuroprotective action. On the other hand, an imbalance of the same input on the two sides may lead to an asymmetric hemispheric excitability, leading to an impairment in cognitive functions. Among the inputs that may drive LC neurons and ARAS, those arising from the trigeminal region, from visceral organs and, possibly, from the vestibular system seem to be particularly relevant in regulating their activity. The trigeminal, visceral and vestibular control of ARAS/LC activity may explain why these input signals: (1) affect sensorimotor and cognitive functions which are not directly related to their specific informational content; and (2) are effective in relieving the symptoms of some brain pathologies, thus prompting peripheral activation of these input systems as a complementary approach for the treatment of cognitive impairments and neurodegenerative disorders.

Preceding functional tooth loss delays recovery from acute cerebral hypoxia and locomotor hypoactivity after murine subarachnoid haemorrhage

Tooth loss and related changes in the functionality may lead to worse outcome of stroke patients, but the effect on hemorrhagic stroke remains unclear. This study aimed to determine the impact of impaired masticatory function on acute cerebral oxygenation and locomotor activity after experimental subarachnoid haemorrhage (SAH). Twenty C57BL/6 mice with (MC-treated group) or without (control group) prior treatment of cutting off the upper molars were subjected to SAH by endovascular perforation. Grading of SAH and acute cerebral infarction were assessed by MR images. Brain tissue oxygen saturation (SbtO₂ ) by photoacoustic imaging and parameters related to locomotor activity by open-field test were analyzed serially after SAH. In all mice, global SbtO2 depression was notable immediately after SAH induction (P <.001), which recovered close to the baseline levels until day 3. However, MC-treated mice demonstrated a prolonged relative cerebral hypoxia (<40% of the baseline SbtO2) as compared to the control (3 ± 1 vs 1 ± 1 days; P <.05). The average distance travelled on day 7 and the ratio of central-area distance/total travelled distance by open-field test between days 7 and 14 were significantly lower in MC-treated mice than in the control mice (P <.05), although the occurrences of new infarction were not statistically different (P >.05). These data suggest a possible link between preceding masticatory impairment and early brain injury to deteriorate neurobehavioural function in patients after SAH.

Mastication and oral health-related quality of life in removable denture wearers with Alzheimer disease

STATEMENT OF PROBLEM

Alzheimer disease (AD) can affect masticatory function, affecting oral health-related quality of life (OHRQoL). Whether oral rehabilitation with conventional removable prostheses can restore masticatory function and improve OHRQoL in these individuals is unknown.

PURPOSE

The purpose of this clinical study was to evaluate the influence of oral rehabilitation with removable prostheses on masticatory efficiency and OHRQoL in elders with and without AD.

MATERIAL AND METHODS

Thirty-two elders with mild AD (n=16, mean age=76.7 ±6.3 years) or without AD (n=16, mean age=75.2 ±4.4 years) were recruited. All participants first underwent masticatory efficiency and OHRQoL evaluations, and 2 months after insertion of new removable prostheses, the variables were reassessed. Masticatory efficiency was determined using the sieving method, and OHRQoL was measured by applying the Geriatric Oral Health Assessment Index (GOHAI). The data from the baseline and after insertion of the new removable prostheses were compared by paired t test. Group differences at each time point were assessed by t test (α=.05).

RESULTS

After insertion of the new removable prostheses, masticatory efficiency and OHRQoL improved in both the elders with AD and the control. At baseline, elders with AD had lower masticatory efficiency and higher OHRQoL than controls (P<.05). After removable prosthesis insertion, elders with AD continued to show lower masticatory efficiency values than controls, but their OHRQoL was similar.

CONCLUSIONS

Oral rehabilitation with new removable prostheses improved the masticatory efficiency and OHRQoL of elders with and without AD, although masticatory efficiency did not reach control levels in elders with AD.

Lip closure training improves eating behaviors and prefrontal cortical hemodynamic activity and decreases daytime sleep in elderly persons

Previous research suggests that aging-related deterioration of oral functions causes not only eating/swallowing disorders but also various conditions such as sleep disorders and higher-order brain dysfunction. The aim of the present study was to examine the effects of lip closure training on eating behavior, sleep, and brain function in elderly persons residing in an elder care facility. The 20 elderly subjects (mean age, 86.3 ± 1.0 years) were assigned to a control group or a lip closure training (LCT) group, in which an oral rehabilitation device was used for daily LCT sessions over a 4-week period. Before and after the 4-week intervention period, maximal lip closure force was measured, and prefrontal cortical hemodynamic activity (changes in oxygenated hemoglobin concentration) during lip closure movements was measured with (LCT group) or without (control group) use of the oral rehabilitation device. We also analyzed eating behavior and daytime sleep before and after the intervention period. Compared with the control group, the LCT group showed improved maximal lip closure force, shortened eating time, decreased food spill rates, and decreased daytime sleeping. Furthermore, compared with the control group, the LCT group showed a significant increase in prefrontal cortical activity during lip closure. In addition, the increase rate in the right dorsolateral prefrontal cortical activity after the intervention period was significantly correlated with the increase rate in the maximal lip closure force after the intervention period. These findings suggest that LCT is useful in elderly individuals with decreased eating/oral and cognitive functions without the risk of pulmonary aspiration during training.

Association between Mastication, the Hippocampus, and the HPA Axis: A Comprehensive Review

Mastication is mainly involved in food intake and nutrient digestion with the aid of teeth. Mastication is also important for preserving and promoting general health, including hippocampus-dependent cognition. Both animal and human studies indicate that mastication influences hippocampal functions through the end product of the hypothalamic-pituitary-adrenal (HPA) axis, glucocorticoid (GC). Epidemiologic studies suggest that masticatory dysfunction in aged individuals, such as that resulting from tooth loss and periodontitis, acting as a source of chronic stress, activates the HPA axis, leading to increases in circulating GCs and eventually inducing various physical and psychological diseases, such as cognitive impairment, cardiovascular disorders, and osteoporosis. Recent studies demonstrated that masticatory stimulation or chewing during stressful conditions suppresses the hyperactivity of the HPA axis via GCs and GC receptors within the hippocampus, and ameliorates chronic stress-induced hippocampus-dependent cognitive deficits. Here, we provide a comprehensive overview of current research regarding the association between mastication, the hippocampus, and HPA axis activity. We also discuss several potential molecular mechanisms involved in the interactions between mastication, hippocampal function, and HPA axis activity.

Decreased hippocampal brain-derived neurotrophic factor and impaired cognitive function by hypoglossal nerve transection in rats

The hypoglossal nerve controls tongue movements, and damages of it result in difficulty in mastication and food intake. Mastication has been reported to maintain hippocampus‐dependent cognitive function. This study was conducted to examine the effect of tongue motor loss on the hippocampus‐dependent cognitive function and its underlying mechanism. Male Sprague Dawley rats were subjected to the initial training of Morris water maze task before or after the bilateral transection of hypoglossal nerves (Hx). When the initial training was given before the surgery, the target quadrant dwelling time during the probe test performed at a week after the surgery was significantly reduced in Hx rats relative to sham‐operated controls. When the initial training was given after the surgery, Hx affected the initial and reversal trainings and probe tests. Brain‐derived neurotrophic factor (BDNF) expression, cell numbers and long‐term potentiation (LTP) were examined in the hippocampus on the 10^th day, and BrdU and doublecortin staining on the 14^th day, after the surgery. Hx decreased the hippocampal BDNF and cells in the CA1/CA3 regions and impaired LTP. BrdU and doublecortin staining was decreased in the dentate gyrus of Hx rats. Results suggest that tongue motor loss impairs hippocampus‐dependent cognitive function, and decreased BDNF expression in the hippocampus may be implicated in its underlying molecular mechanism in relation with decreased neurogenesis/proliferation and impaired LTP.

Molar loss and powder diet leads to memory deficit and modifies the mRNA expression of brain-derived neurotrophic factor in the hippocampus of adult mice

BACKGROUND

It is known that tooth loss is known to be a risk factor for Alzheimer's disease and soft diet feeding induces memory impairment. Recent studies have shown that brain-derived neurotrophic factor (BDNF) is associated with tooth loss or soft diet in young animal model, and that BDNF expression is decreased in patients with Alzheimer's disease. However, single or combined effect of tooth loss and/or soft diet on brain function has not fully understood. Here we examined the effect of molar loss and powder diet on memory ability and the expression of BDNF mRNA in the hippocampus of adult C57BL/6J mice. Twenty eight-weeks-old C57BL/6J mice were divided into intact molar group and extracted molar group. They were randomly divided into the I/S group (Intact upper molar teeth/Solid diet feeding), the E/S group (Extracted upper molar teeth/Solid diet feeding), the I/P group (Intact upper molar teeth/Powder diet feeding), and the E/P group (Extracted upper molar teeth/Powder diet feeding). The observation periods were 4 and 16-week. To analyze the memory ability, the step-through passive avoidance test was conducted. BDNF-related mRNA in the hippocampus was analyzed by real-time polymerase chain reaction (RT-PCR).

RESULTS

At 4 weeks later, we performed memory test and isolated brains to analyze. There were no differences in memory function and BDNF mRNA level between these four groups. However, at 16 weeks later, E/S and E/P group showed memory impairment, and decreased level of BDNF mRNA. Whereas, the powder diet had no effect on memory function and BDNF mRNA level even at 16 weeks later.

CONCLUSIONS

These results suggest that the effect of molar loss and powder diet on memory function and BDNF mRNA levels were different, molar loss may have a greater long-term effect on memory ability than powder diet does.

Chronic Powder Diet After Weaning Induces Sleep, Behavioral, Neuroanatomical, and Neurophysiological Changes in Mice

The purpose of this study is to clarify the effects of chronic powder diet feeding on sleep patterns and other physiological/anatomical changes in mice. C57BL/6 male mice were divided into two groups from weaning: a group fed with solid food (SD) and a group fed with powder food (PD), and sleep and physiological and anatomical changes were compared between the groups. PD exhibited less cranial bone structure development and a significant weight gain. Furthermore, these PD mice showed reduced number of neurogenesis in the hippocampus. Sleep analysis showed that PD induced attenuated diurnal sleep/wake rhythm, characterized by increased sleep during active period and decreased sleep during rest period. With food deprivation (FD), PD showed less enhancement of wake/locomotor activity compared to SD, indicating reduced food-seeking behavior during FD. These results suggest that powder feeding in mice results in a cluster of detrimental symptoms caused by abnormal energy metabolism and anatomical/neurological changes.

Hippocampus-dependent spatial memory impairment due to molar tooth loss is ameliorated by an enriched environment

BACKGROUND AND OBJECTIVE

Teeth are crucial, not only for mastication, but for overall nutrition and general health, including cognitive function. Aged mice with chronic stress due to tooth loss exhibit impaired hippocampus-dependent learning and memory. Exposure to an enriched environment restores the reduced hippocampal function. Here, we explored the effects of an enriched environment on learning deficits and hippocampal morphologic changes in aged senescence-accelerated mouse strain P8 (SAMP8) mice with tooth loss.

DESIGN

Eight-month-old male aged SAMP8 mice with molar intact or with molars removed were housed in either a standard environment or enriched environment for 3 weeks. The Morris water maze was performed for spatial memory test. The newborn cell proliferation, survival, and differentiation in the hippocampus were analyzed using 5-Bromodeoxyuridine (BrdU) immunohistochemical method. The hippocampal brain-derived neurotrophic factor (BDNF) levels were also measured.

RESULTS

Mice with upper molars removed (molarless) exhibited a significant decline in the proliferation and survival of newborn cells in the dentate gyrus (DG) as well as in hippocampal BDNF levels. In addition, neuronal differentiation of newly generated cells was suppressed and hippocampus-dependent spatial memory was impaired. Exposure of molarless mice to an enriched environment attenuated the reductions in the hippocampal BDNF levels and neuronal differentiation, and partially improved the proliferation and survival of newborn cells, as well as the spatial memory ability.

CONCLUSION

These findings indicated that an enriched environment could ameliorate the hippocampus-dependent spatial memory impairment induced by molar tooth loss.

Multiple tooth loss is associated with vascular cognitive impairment in subjects with acute ischemic stroke

BACKGROUND AND OBJECTIVE

Emerging evidence shows that tooth loss is associated with cognitive impairment and dementia. Vascular cognitive impairment (VCI) is a common consequence of ischemic stroke. This study investigated the association of tooth loss with VCI in patients with acute stroke.

MATERIAL AND METHODS

A total of 161 subjects with acute ischemic stroke were recruited. Within 1 wk after admission, fasting blood tests were undertaken and the number of teeth present was recorded. VCI was evaluated with the Montreal Cognitive Assessment (MoCA).

RESULTS

The patients with loss of ≥ 8 teeth exhibited significantly lower MoCA values as compared to those with loss of ≤ 7 teeth (13.2 ± 6.6 vs. 17.3 ± 6.0, p < 0.001). Multivariate logistic regression analysis showed that loss of ≥ 8 teeth (OR = 3.1, 95% CI: 1.2-7.9, p = 0.02) and stroke history (OR = 3.8, 95% CI: 1.1-14.1, p = 0.04) were significantly associated with VCI (MoCA score ≤ 20.0).

CONCLUSION

Within the limitations of this study, the current findings provide the first evidence that multiple tooth loss is independently associated with VCI in patients with acute ischemic stroke.

Decision-making deficits associated with disrupted synchronization between basolateral amygdala and anterior cingulate cortex in rats after tooth loss

Human studies have shown that multiple teeth loss was significantly associated with cognitive impairment, dementia and Alzheimer's disease. However, the causal relationship between tooth loss and cognitive deficits has not been clarified. Rodents demonstrate human-like cognitive faculties. In this study by performing rat gambling task (RGT), we reported that prolonged tooth loss condition by extracting all left molars in the rats led to an increase in the proportion of poor decision-makers, and decrease in the proportion of good decision-makers compared with controls. No influence was detected on the general activity and motivation after tooth loss. Recent experiments have shown that decision-making performances in the RGT rely on the functional integrity of the amygdala and anterior cingulate cortex (ACC). The theta band brain oscillation has been acknowledged for extensive cognitive functions. Here, we performed multiple-electrode array recordings of local field potential (LFP) in anesthetized rats. The results exhibited an increase in accumulative power of the theta frequency of LFP in the basolateral amygdala (BLA) and decrease of theta power in the ACC in tooth loss rats. Furthermore, cross-correlation analysis displayed that tooth loss suppressed the synchronization of theta frequency of LFP between the BLA and ACC, indicating reduced neuronal communications between these two regions. In conclusion, we demonstrate for the first time that tooth loss leads to higher-order cognitive deficits accompanied by the alteration of theta frequency of LFP in brain circuitries and disruption of neural network integrity.

Chew the Pain Away: Oral Habits to Cope with Pain and Stress and to Stimulate Cognition

The acute effects of chewing gum on cognitive performance, stress, and pain have been intensively studied in the last decade. The results have been contradicting, and replication studies proved challenging. Here, we review some of the recent findings of this topic and explore possible explanations for these discrepancies by incorporating knowledge derived from studies into oral habits and bruxism. Both stress and cerebral functional specialization (i.e., the involvement of specific brain structures in distinctive cognitive processes) are hypothesized to play a major role in the underlying physiological mechanisms of the diverse effects of chewing gum on cognition, stress, and pain.

Age-dependent loss of cholinergic neurons in learning and memory-related brain regions and impaired learning in SAMP8 mice with trigeminal nerve damage

The tooth belongs to the trigeminal sensory pathway. Dental damage has been associated with impairments in the central nervous system that may be mediated by injury to the trigeminal nerve. In the present study, we investigated the effects of damage to the inferior alveolar nerve, an important peripheral nerve in the trigeminal sensory pathway, on learning and memory behaviors and structural changes in related brain regions, in a mouse model of Alzheimer's disease. Inferior alveolar nerve transection or sham surgery was performed in middle-aged (4-month-old) or elderly (7-month-old) senescence-accelerated mouse prone 8 (SAMP8) mice. When the middle-aged mice reached 8 months (middle-aged group 1) or 11 months (middle-aged group 2), and the elderly group reached 11 months, step-down passive avoidance and Y-maze tests of learning and memory were performed, and the cholinergic system was examined in the hippocampus (Nissl staining and acetylcholinesterase histochemistry) and basal forebrain (choline acetyltransferase immunohistochemistry). In the elderly group, animals that underwent nerve transection had fewer pyramidal neurons in the hippocampal CA1 and CA3 regions, fewer cholinergic fibers in the CA1 and dentate gyrus, and fewer cholinergic neurons in the medial septal nucleus and vertical limb of the diagonal band, compared with sham-operated animals, as well as showing impairments in learning and memory. Conversely, no significant differences in histology or behavior were observed between middle-aged group 1 or group 2 transected mice and age-matched sham-operated mice. The present findings suggest that trigeminal nerve damage in old age, but not middle age, can induce degeneration of the septal-hippocampal cholinergic system and loss of hippocampal pyramidal neurons, and ultimately impair learning ability. Our results highlight the importance of active treatment of trigeminal nerve damage in elderly patients and those with Alzheimer's disease, and indicate that tooth extraction should be avoided in these populations.