Spatial memory decline after masticatory deprivation and aging is associated with altered laminar distribution of CA1 astrocytes

Intriguing astrocyte responses in CA1 to reduced and rehabilitated masticatory function: Dorsal and ventral distinct perspectives in adult mice

OBJECTIVE

We sought to investigate the plasticity of diet-induced changes in astrocyte morphology of stratum lacunosum-moleculare (SLM) in CA1.

DESIGN

Three diet regimes were adopted in 15 mice, from the 21st postnatal day to 6 months. The first diet regimen was pellet feed, called Hard Diet (HD). The second, with reduced masticatory, received a pellet-diet followed by a powdered-diet, and it was identified as Hard Diet/Soft Diet (HD/SD). Finally, the group with rehabilitated masticatory was named Hard Diet/Soft Diet/Hard Diet (HD/SD/HD). In the end, euthanasia and brain histological processing were performed, in which astrocytic immunoreactivity to glial-fibrillary-acidic-protein (GFAP) was tested. In reconstructed astrocytes, morphometric analysis was performed.

RESULTS

Astrocyte morphometric revealed that changes in masticatory regimens impact astrocyte morphology. In the dorsal CA1, switching from a hard diet to a soft diet led to reductions in most variables, whereas in the ventral, fewer variables were affected, highlighting regional differences in astrocyte responses. Cluster analysis further showed that diet-induced changes in astrocyte morphology were reversible in the dorsal region, but not in the ventral region, indicating a persistent impact on astrocyte diversity and complexity in the ventral even after rehabilitation. Correlation tests between astrocyte morphology and behavioral performance demonstrated disrupted relationships under masticatory stress, with effects persisting after rehabilitation.

CONCLUSION

Changes in the diet result in significant alterations in astrocyte morphology, suggesting a direct link between dietary modulation and cellular structure. Morphometric analyses revealed distinct alterations in astrocyte morphology in response to changes in the masticatory regimen, with both dorsal/ventral regions displaying notable changes. Moreover, the regional differential effects on astrocytes underscore the complexity of mastication on neuroplasticity and cognitive function.

Pre-Adolescent Diet Normalization Restores Cognitive Function in Young Mice

Mastication is a fundamental function critical for human health. Controlled by the central nervous system (CNS), it influences CNS development and function. A poor masticatory performance causes cognitive dysfunction in both older adults and children. Improving mastication may prevent cognitive decline. However, no study has determined the period of masticatory dysfunction that impairs children's later acquisition of cognitive function. Herein, we developed an animal model wherein a soft diet was switched to a normal diet at early and late time points in young mice. We aimed to investigate the impact of restored mastication on learning and memory function. Behavioral studies were conducted to evaluate learning and memory. Micro-CT was used to evaluate orofacial structural differences, while histological and biochemical approaches were employed to assess differences in the hippocampal morphology and function. Correction to a hard-textured diet before adolescence restored mastication and cognitive function through the stimulation of neurogenesis, extracellular signal-regulated kinases, the cyclic adenosine monophosphate-response element-binding protein pathway, and the brain-derived neurotrophic factor, tyrosine receptor B. In contrast, post-adolescent diet normalization failed to rescue full mastication and led to impaired cognitive function, neuronal loss, and decreased hippocampal neurogenesis. These findings revealed a functional linkage between the masticatory and cognitive function in mice during the juvenile to adolescent period, highlighting the need for adequate food texture and early intervention for mastication-related cognitive impairment in children.

Long-Term Soft-Food Rearing in Young Mice Alters Brain Function and Mood-Related Behavior

The relationship between caloric and nutrient intake and overall health has been extensively studied. However, little research has focused on the impact of the hardness of staple foods on health. In this study, we investigated the effects of a soft diet on brain function and behavior in mice from an early age. Mice fed a soft diet for six months exhibited increased body weight and total cholesterol levels, along with impaired cognitive and motor function, heightened nocturnal activity, and increased aggression. Interestingly, when these mice were switched back to a solid diet for three months, their weight gain ceased, total cholesterol levels stabilized, cognitive function improved, and aggression decreased, while their nocturnal activity remained high. These findings suggest that long-term consumption of a soft diet during early development can influence various behaviors associated with anxiety and mood regulation, including weight gain, cognitive decline, impaired motor coordination, increased nocturnal activity, and heightened aggression. Therefore, the hardness of food can impact brain function, mental well-being, and motor skills during the developmental stage. Early consumption of hard foods may be crucial for promoting and maintaining healthy brain function.

Yes-associated protein regulates glutamate homeostasis through promoting the expression of excitatory amino acid transporter-2 in astrocytes via β-catenin signaling

The homeostasis of glutamate is mainly regulated by the excitatory amino acid transporters (EAATs), especially by EAAT2 in astrocytes. Excessive glutamate in the synaptic cleft caused by dysfunction or dysregulation of EAAT2 can lead to excitotoxicity, neuronal death and cognitive dysfunction. However, it remains unclear about the detailed regulation mechanism of expression and function of astrocytic EAAT2. In this study, first, we found increased neuronal death and impairment of cognitive function in YAP^GFAP -CKO mice (conditionally knock out Yes-associated protein [YAP] in astrocytes), and identified EAAT2 as a downstream target of YAP through RNA sequencing. Second, the expression of EAAT2 was decreased in cultured YAP^-/- astrocytes and the hippocampus of YAP^GFAP -CKO mice, and glutamate uptake was reduced in YAP^-/- astrocytes, but increased in YAP-upregulated astrocytes. Third, further investigation of the mechanism showed that the mRNA and protein levels of β-catenin were decreased in YAP^-/- astrocytes and increased in YAP-upregulated astrocytes. Wnt3a activated YAP signaling and up-regulated EAAT2 through β-catenin. Furthermore, over-expression or activation of β-catenin partially restored the downregulation of EAAT2, the impairment of glutamate uptake, neuronal death and cognitive decline that caused by YAP deletion. Finally, activation of EAAT2 also rescued neuronal death and cognitive decline in YAP^GFAP -CKO mice. Taken together, our study identifies an unrecognized role of YAP signaling in the regulation of glutamate homeostasis through the β-catenin/EAAT2 pathway in astrocytes, which may provide novel insights into the pathogenesis of brain diseases that closely related to the dysfunction or dysregulation of EAAT2, and promote the development of clinical strategy.

The Masticatory Activity Interference in Quantitative Estimation of CA1, CA3 and Dentate Gyrus Hippocampal Astrocytes of Aged Murine Models and under Environmental Stimulation

Studies indicating the influence of masticatory dysfunction, due to a soft diet or lack of molars, on impairing spatial memory and learning have led to research about neuronal connections between areas and cell populations possibly affected. In this sense, with scarce detailed data on the subfields of hippocampus in dementia neurodegeneration, there is no information about astrocytic responses in its different layers. Thus, considering this context, the present study evaluated the effects of deprivation and rehabilitation of masticatory activity, aging, and environmental enrichment on the stereological quantification of hippocampal astrocytes from layers CA1, CA3, and DG. For this purpose, we examined mature (6-month-old; 6M), and aged (18-month-old; 18M) mice, subjected to distinct masticatory regimens and environments. Three different regimens of masticatory activity were applied: continuous normal mastication with hard pellets (HD); normal mastication followed by deprived mastication with equal periods of pellets followed by soft powder (HD/SD); or rehabilitated masticatory activity with equal periods of HD, followed by powder, followed by pellets (HD/SD/HD). Under each specific regimen, half of the animals were raised in standard cages (impoverished environment (IE)) and the other half in enriched cages (enriched environment (EE)), mimicking sedentary or active lifestyles. Microscopic stereological, systematic, and random sampling approaches with an optical dissector of GFAP-immunolabeled astrocytes were done, allowing for an astrocyte numerical estimate. Stratum moleculare and hilus, from the dentate gyrus (DG) and Strata Lacunosum-Moleculare, Oriens, and Radiatum, similarly to the dentate gyrus, showed no significant change in any of the investigated variables (age, diet, or environment) in these layers. However, in Stratum radiatum, it was possible to observe significant differences associated with diet regimens and age. Therefore, diet-related differences were found when the HD 18M IE group was compared to the HD/SD/HD 18-month-old group in the same environment (IE) (p = 0.007). In the present study, we present modulatory factors (masticatory function, environmental enrichment, and aging) for the differentiated quantitative laminar response in the hippocampal regions, suggesting other studies to read the plasticity and responsiveness of astrocytes, including the molecular background.

Food Hardness Modulates Behavior, Cognition, and Brain Activation: A Systematic Review of Animal and Human Studies

Food hardness is one of the dietary features that may impact brain functions. We performed a systematic review to evaluate the effect of food hardness (hard food versus soft food diet) on behavior, cognition, and brain activation in animals and humans (PROSPERO ID: CRD42021254204). The search was conducted on 29 June 2022 using Medline (Ovid), Embase, and Web of Science databases. Data were extracted, tabulated by food hardness as an intervention, and summarized by qualitative synthesis. The SYRCLE and JBI tools were used to assess the risk of bias (RoB) of individual studies. Of the 5427 studies identified, 18 animal studies and 6 human studies met the inclusion criteria and were included. The RoB assessment indicated that 61% of animal studies had unclear risks, 11% had moderate risks, and 28% had low risks. All human studies were deemed to have a low risk of bias. The majority (48%) of the animal studies showed that a hard food diet improved behavioral task performance compared to soft food diets (8%). However, 44% of studies also showed no differential effects of food hardness on behavioral tests. It was also evident that certain regions of the brain were activated in response to changes in food hardness in humans, with a positive association between chewing hard food, cognition performance, and brain function. However, variations in the methodologies of the included studies hindered the meta-analysis execution. In conclusion, our findings highlight the beneficial effects of dietary food hardness on behavior, cognition, and brain function in both animals and humans, however, this effect may depend on several factors that require further understanding of the causality.

Common eating habit patterns are associated with a high maximum occlusal force and pre-eating cardiac vagal tone

Background

Masticatory function is associated with nervous function, including autonomic nervous function, and both functions are influenced by human habits. In a previous preliminary study of 53 young women, we found that eating habit patterns were associated with occlusal force as an indicator of masticatory function. Therefore, we hypothesized that relationships exist between occlusal force, the autonomic nervous system, and eating habit patterns.

Methods

To test our hypothesis, we analyzed the relationship between heart rate variability measured before and after lunch in 53 young women, and measured and surveyed maximum occlusal force and eating habit patterns, respectively, in these participants.

Results

High occlusal force was associated with an increased high-frequency (HF) component (vagal tone index) of the heart rate variability index immediately before lunch (standardized regression coefficient (β) = 0.417, P = 0.002). Each of the eating habit items surveyed in a questionnaire showed a similar tendency for the HF component immediately before lunch and maximum occlusal force; in particular, "Habit of eating breakfast" and "Number of meals per day" were significantly associated with both variables. Additionally, total eating habit scores related to higher maximum occlusal force were associated with an increase in the HF component immediately before lunch (β = 0.514, P < 0.001). The maximum occlusal force and the pre-eating HF component values were stratified by total eating habit scores (into low, medium, high categories), and the high scores were significantly higher than the medium or low scores.

Conclusions

Occlusal force and the pre-eating cardiac vagal response of individuals were characterized by their common eating habit patterns, indicating that eating habits may be simultaneously associated with the development of masticatory function, nervous system development, and cardiovascular rhythm. Although further research is needed to investigate these relationships in detail, our findings provide insights that will inform the study of physical functions, neurodevelopment, habitual behaviors, and health in humans.

The Sedentary Lifestyle and Masticatory Dysfunction: Time to Review the Contribution to Age-Associated Cognitive Decline and Astrocyte Morphotypes in the Dentate Gyrus

As aging and cognitive decline progresses, the impact of a sedentary lifestyle on the appearance of environment-dependent cellular morphologies in the brain becomes more apparent. Sedentary living is also associated with poor oral health, which is known to correlate with the rate of cognitive decline. Here, we will review the evidence for the interplay between mastication and environmental enrichment and assess the impact of each on the structure of the brain. In previous studies, we explored the relationship between behavior and the morphological features of dentate gyrus glial fibrillary acidic protein (GFAP)-positive astrocytes during aging in contrasting environments and in the context of induced masticatory dysfunction. Hierarchical cluster and discriminant analysis of GFAP-positive astrocytes from the dentate gyrus molecular layer revealed that the proportion of AST1 (astrocyte arbors with greater complexity phenotype) and AST2 (lower complexity) are differentially affected by environment, aging and masticatory dysfunction, but the relationship is not straightforward. Here we re-evaluated our previous reconstructions by comparing dorsal and ventral astrocyte morphologies in the dentate gyrus, and we found that morphological complexity was the variable that contributed most to cluster formation across the experimental groups. In general, reducing masticatory activity increases astrocyte morphological complexity, and the effect is most marked in the ventral dentate gyrus, whereas the effect of environment was more marked in the dorsal dentate gyrus. All morphotypes retained their basic structural organization in intact tissue, suggesting that they are subtypes with a non-proliferative astrocyte profile. In summary, the increased complexity of astrocytes in situations where neuronal loss and behavioral deficits are present is counterintuitive, but highlights the need to better understand the role of the astrocyte in these conditions.

Tooth Loss, Denture Use, and Cognitive Impairment in Chinese Older Adults: A Community Cohort Study

BACKGROUND

Evidence regarding the associations of tooth loss and denture use with incident cognitive impairment is inconclusive in older adults, and few prospective studies have examined the potential interaction between tooth loss and denture use in these specific populations.

METHODS

Data were assessed from 17 079 cognitively normal older adults aged ≥65 years, participating in the Chinese Longitudinal Healthy Longevity Survey. The outcome of interest was cognitive impairment (assessed by the Chinese version of Mini-Mental State Examination). The number of natural teeth and status of denture use were collected by a structural questionnaire.

RESULTS

A total of 6456 cases of cognitive impairment were recorded during 88 627 person-years of follow-up. We found that compared with participants with 20+ teeth, those with 10-19, 1-9, and 0 teeth had increased risks of incident cognitive impairment (p-trend < .001). Participants without dentures also had a higher risk of incident cognitive impairment, compared with those who wore dentures. Effect modification by denture use was observed (p-interaction = .010). Specifically, among those without dentures, the adjusted hazard ratio (95% confidence interval) for participants with 10-19, 1-9, and 0 teeth were 1.19 (1.08, 1.30), 1.28 (1.17, 1.39), and 1.28 (1.16, 1.41), respectively, as compared to those with 20+ teeth. In contrary, among denture users, detrimental effect was only observed among those with 0 teeth (hazard ratio 1.14, 95% confidence interval: 1.16, 1.41).

CONCLUSIONS

In Chinese older adults, maintaining 20+ teeth is important for cognitive health; denture use would attenuate the detrimental effects of tooth loss, especially for partial tooth loss, on cognitive impairment.

Brain-Derived Neurotrophic Factor Systemic Response in the Periodontium, Trigeminal Nucleus Caudalis, and Hippocampus Induced by Occlusal Trauma

Occlusal trauma (OT), by causing periodontal tissue damage, can activate and enhance the activity of the peripheral and central nervous system (CNS) neuropeptides. The brain-derived neurotrophic factor (BDNF) gene is activity-dependent and exhibits marked alterations, characterized by protection against injury and repair. Our results show the possible molecular mechanism through which noxious environmental stimuli induce alterations in BDNF activity in the local periodontal tissue, the primary sensory neurons-Vc, and the hippocampus, suggesting systemic impairment. BDNF serves a more positive and enduring trauma protection and repair function in Vc compared to that in local dental tissue.

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.

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.

Oral health and healthy chewing for healthy cognitive ageing: A comprehensive narrative review

INTRODUCTION

Ageing leads to physiological cognitive decline that it is worsened in people with neurodegenerative diseases such as Alzheimer's disease. Despite the ongoing search for a solution to this cognitive decline, no effective remedies have been established. It has been determined that modifiable external factors, such as oral health and occlusal function, prevent cognitive decline.

OBJECTIVE

To analyse the primary interactions between occlusal function and cognitive functions.

MAIN FINDINGS

Masticatory function is related to cognitive functions. In particular, current evidence, from both animal and human studies, suggests that the activation of masticatory muscles and proper mastication, with natural teeth or dental prosthesis, induces the release of several mediators and the activation of specific brain areas. Together, they result in higher neuronal activity, neurotrophic support, blood flow and the prevention of amyloid-beta plaque formation. Thus, all the components of the masticatory system must work together in order to preserve cognitive function.

CONCLUSIONS

Available evidence suggests that oral and cognitive health are more interconnected than previously thought. Therefore, maintenance and adequate restoration of the whole masticatory system are important for the prevention of cognitive decline. In summary, oral and chewing health lead to healthy cognitive ageing.

Altered mastication adversely impacts morpho-functional features of the hippocampus: A systematic review on animal studies in three different experimental conditions involving the masticatory function

Recent results have established that masticatory function plays a role not only in the balance of the stomatognathic system and in the central motor control, but also in the trophism of the hippocampus and in the cognitive activity. These implications have been shown in clinical studies and in animal researches as well, by means of histological, biochemical and behavioural techniques. This systematic review describes the effects of three forms of experimentally altered mastication, namely soft-diet feeding, molar extraction and bite-raising, on the trophism and function of the hippocampus in animal models. Through a systematic search of PubMed, Embase, Web of Science, Scopus, OpenGray and GrayMatters, 645 articles were identified, 33 full text articles were assessed for eligibility and 28 articles were included in the review process. The comprehensiveness of reporting was evaluated with the ARRIVE guidelines and the risk of bias with the SYRCLE RoB tool. The literature reviewed agrees that a disturbed mastication is significantly associated with a reduced number of hippocampal pyramidal neurons in Cornu Ammonis (CA)1 and CA3, downregulation of Brain Derived Neurotrophic Factor (BDNF), reduced synaptic activity, reduced neurogenesis in the Dentate Gyrus (DG), glial proliferation, and reduced performances in behavioural tests, indicating memory impairment and reduced spatial orientation. Moreover, while the bite-raised condition, characterized by occlusal instability, is known to be a source of stress, soft-diet feeding and molar extractions were not consistently associated with a stress response. More research is needed to clarify this topic. The emerging role of chewing in the preservation of hippocampal trophism, neurogenesis and synaptic activity is worthy of interest and may contribute to the study of neurodegenerative diseases in new and potentially relevant ways.

Environmental Impoverishment, Aging, and Reduction in Mastication Affect Mouse Innate Repertoire to Explore Novel Environments and to Assess Risk

Studies indicate that inhibition of adequate masticatory function, due to soft diet, occlusal disharmony, or molar losses affects the cognitive behavior of rodents. However, no study has tested the effects on new environments exploration and risk assessment coupled with a combination of masticatory function rehabilitation and environmental enrichment. In the present report, we tested the hypothesis that age, environment, and masticatory changes may interact and alter exploratory patterns of locomotor activity and mice preferences in an open field (OF) arena. As OF arenas are widely used to measure anxiety-like behavior in rats and mice. We examined in an open arena, the exploratory and locomotor activities of mature (6-month-old; 6M), late mature (12-month-old; 12M), and aged (18-month-old; 18M) mice, subjected to distinct masticatory regimens and environments. Three different regimens of masticatory activity were used: continuous normal mastication with hard pellets (HD); normal mastication followed by reduced mastication with equal periods of pellets followed by soft powder – HD/SD; or rehabilitated masticatory activity with equal periods of HD, followed by powder, followed by pellets – HD/SD/HD). Under each diet regimen, half of the individuals were raised in standard cages [impoverished environment (IE)] and the other half in enriched cages [enriched environment (EE)]. Animals behavior on the open field (OF) task were recorded by webcam and analyzed with Any Maze software (Stöelting). The locomotor and exploratory activities in OF task declined with age, and this was particularly evident in 18M HD EE mice. Although all groups kept their preference by the peripheral zone, the outcomes were significantly influenced by interactions between environment, age, and diet. Independent of diet regime, 6M young mice maintained in an EE where voluntary exercise apparatus is available, revealed significant less body weight than all other groups. Although body weight differences were minimized as age progressed, 18M EE group revealed intragroup significant influence of diet regimens. We suggest that long life environmental enrichment reduces the tendency to avoid open/lit spaces (OF) and this is particularly influenced by masticatory activity. These measurements may be useful in discussions of anxiety-related tasks.

Masticatory hypofunction effects induced by BTXA injection of hippocampal neurons in developing rats

BACKGROUND AND OBJECTIVE

In clinical practice, malocclusion is often encountered during the period of growth and development of individuals. In addition to nutritional imbalance, some studies have found that mastication affects learning and memory ability. Tooth loss and masticatory hypofunction have been suggested as risk factors of Alzheimer disease. However, relatively little research has been done in developing animals. The present study evaluated the relationship between masticatory hypofunction and neuropathological changes of the hippocampus in developing rats.

DESIGN

Four-week-old Wistar rats were randomly divided into saline-injected and botulinum toxin type A (BTXA)-injected groups. After an experiment period of 4 weeks, the rats were sacrificed for evaluation of neuropathological changes in the hippocampus through Nissl staining and phosphorylated cyclic AMP (cAMP) response element binding protein (CREB) immunohistochemistry.

RESULTS

Nissl staining revealed a significant reduction in the density of neurons in the BTXA-injected rats. The BTXA-injected rats exhibited a decreased level of CREB phosphorylation. The degree of p-CREB immunoreactivity differed significantly between the two groups.

CONCLUSION

The BTXA-injected rats exhibited a reduction in neuron density and phosphorylated CREB, indicating that mastication might influence the learning and memory ability during the growth period. Therefore, it is strongly suggested that malocclusion be corrected as soon as possible during growth and development.

Occlusal force predicts global motion coherence threshold in adolescent boys

Background

Beneficial effects of mastication on cognitive abilities in the elderly have been shown in human studies. However, little is currently known about the effect of masticatory stimulation on cognitive and perceptual ability in younger populations. The purpose of the present study is to investigate the influences of masticatory stimulation on perceptual ability in adolescent boys.

Methods

The present study examined the relationship between occlusal force (i.e., masticatory stimulation) and visual perception ability in adolescent boys. Visual perception ability was quantified by measuring global motion coherence threshold using psychophysical method. As an index of masticatory stimulation, occlusal force was measured by pressure sensitive film. We also measured participants’ athletic ability, e.g. aerobic capacity and grip strength, as potential confounding factor.

Results

The multiple regression analysis revealed a significant negative correlation between global motion coherence threshold and occlusal force, which persisted after controlling for confounding factors such as age and aerobic capacity.

Conclusions

This finding indicates that masticatory stimulation enhances visual perception in adolescent boys, indicating the possibility that beneficial effects of masticatory stimulation are observed not only in the elderly but in developing population consistently with the findings of the previous animal studies.

Tooth loss is associated with accelerated cognitive decline and volumetric brain differences: a population-based study

Tooth loss has been related to cognitive impairment; however, its relation to structural brain differences in humans is unknown. Dementia-free participants (n = 2715) of age ≥60 years were followed up for up to 9 years. A subsample (n = 394) underwent magnetic resonance imaging at baseline. Information on tooth loss was collected at baseline, and cognitive function was assessed using the Mini-Mental State Examination at baseline and at follow-ups. Data were analyzed using linear mixed effects models and linear regression models. At baseline, 404 (14.9%) participants had partial tooth loss, and 206 (7.6%) had complete tooth loss. Tooth loss was significantly associated with a steeper cognitive decline (β: -0.18, 95% confidence interval [CI]: -0.24 to -0.11) and remained significant after adjusting for or stratifying by potential confounders. In cross-sectional analyses, persons with complete or partial tooth loss had significantly lower total brain volume (β: -28.89, 95% CI: -49.33 to -8.45) and gray matter volume (β: -22.60, 95% CI: -38.26 to -6.94). Thus, tooth loss may be a risk factor for accelerated cognitive aging.

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.

Reduced Mastication Impairs Memory Function

Mastication is an indispensable oral function related to physical, mental, and social health throughout life. The elderly tend to have a masticatory dysfunction due to tooth loss and fragility in the masticatory muscles with aging, potentially resulting in impaired cognitive function. Masticatory stimulation has influence on the development of the central nervous system (CNS) as well as the growth of maxillofacial tissue in children. Although the relationship between mastication and cognitive function is potentially important in the growth period, the cellular and molecular mechanisms have not been sufficiently elucidated. Here, we show that the reduced mastication resulted in impaired spatial memory and learning function owing to the morphological change and decreased activity in the hippocampus. We used an in vivo model for reduced masticatory stimuli, in which juvenile mice were fed with powder diet and found that masticatory stimulation during the growth period positively regulated long-term spatial memory to promote cognitive function. The functional linkage between mastication and brain was validated by the decrease in neurons, neurogenesis, neuronal activity, and brain-derived neurotrophic factor (BDNF) expression in the hippocampus. These findings taken together provide in vivo evidence for a functional linkage between mastication and cognitive function in the growth period, suggesting a need for novel therapeutic strategies in masticatory function-related cognitive dysfunction.

Power spectrum density analysis for the influence of complete denture on the brain function of edentulous patients - pilot study

PURPOSE

This pilot study was to find the influence of complete denture on the brain activity and cognitive function of edentulous patients measured through Electroencephalogram (EEG) signals.

MATERIALS AND METHODS

The study recruited 20 patients aged from 50 to 60 years requiring complete dentures with inclusion and exclusion criteria. The brain function and cognitive function were analyzed with a mental state questionnaire and a 15-minute analysis of power spectral density of EEG alpha waves. The analysis included edentulous phase and post denture insertion adaptive phase, each done before and after chewing. The results obtained were statistically evaluated.

RESULTS

Power Spectral Density (PSD) values increased from edentulous phase to post denture insertion adaption phase. The data were grouped as edentulous phase before chewing (EEG p1-0.0064), edentulous phase after chewing (EEG p2-0.0073), post denture insertion adaptive phase before chewing (EEG p3-0.0077), and post denture insertion adaptive phase after chewing (EEG p4-0.0096). The acquired values were statistically analyzed using paired t-test, which showed statistically significant results (P<.05).

CONCLUSION

This pilot study showed functional improvement in brain function of edentulous patients with complete dentures rehabilitation.

Influence of chewing behaviour on memory and spatial learning in albino BALB/c mice

INTRODUCTION

Since the relationship between chewing and cognitive functions has not been fully elucidated, this study aimed to determine the impact of chewing behaviour on spatial learning and memory in albino male BALB/c mice.

METHODS

Twenty mice aged 8 weeks were divided into 2 equal groups. The regular chewing group was fed with uncrushed grains (the same diet given to all 20 mice since they were weaned) and the limited chewing group was fed with crushed grains. At 16 weeks of age, the mice were evaluated over 5 days, including a 4-day acquisition phase prior to a probe test of spatial learning and memory in the Morris water maze on the fifth day.

RESULTS

A comparison of the regular chewing group and the limited chewing group found no significant differences in either the acquisition phase or the probe test. However, there were significant differences in the acquisition phase for just the regular chewing group when comparing results from the first day to those from the other 3 days.

CONCLUSIONS

The results suggest that regular chewing affects spatial learning and memory since mice in the regular chewing group decreased their times to find the hidden platform during the acquisition phase.

Mechanisms of hippocampal astrocytes mediation of spatial memory and theta rhythm by gliotransmitters and growth factors

Our knowledge about encoding and maintenance of spatial memory emphasizes the integrated functional role of the grid cells and the place cells of the hippocampus in the generation of theta rhythm in spatial memory formation. However, the role of astrocytes in these processes is often underestimated in their contribution to the required structural and functional characteristics of hippocampal neural network operative in spatial memory. We show that hippocampal astrocytes, by the secretion of gliotransmitters, such as glutamate, d-serine, and ATP and growth factors such as BDNF and by the expression of receptors and channels such as those of TNFα and aquaporin, have several diverse fuctions in spatial memory. We specifically focus on the role of astrocytes on five phases of spatial memory: (1) theta rhythm generation, (2) theta phase precession, (3) formation of spatial memory by mapping data of entorhinal grid cells into the place cells, (4) storage of spatial information, and (5) maintenance of spatial memory. Finally, by reviewing the literature, we propose specific mechanisms mentioned in the form of a hypothesis suggesting that astrocytes are important in spatial memory formation.

Enriched environment and masticatory activity rehabilitation recover spatial memory decline in aged mice

Background

To measure the impact of masticatory reduction on learning and memory, previous studies have produced experimental masticatory reduction by modified diet or molar removal. Here we induced spatial learning impairment in mice by reducing masticatory activity and then tested the effect of a combination of environmental enrichment and masticatory rehabilitation in recovering spatial learning at adulthood and in later life. For 6 months (6M) or 18 months (18M), we fed three groups of mice from postnatal day 21 respectively with a hard diet (HD) of pellets; pellets followed by a powdered, soft diet (HD/SD, divided into equal periods); or pellets followed by powder, followed by pellets again (HD/SD/HD, divided into equal periods). To mimic sedentary or active lifestyles, half of the animals from each group were raised from weaning in standard cages (impoverished environment; IE) and the other half in enriched cages (enriched environment; EE). To evaluate spatial learning, we used the Morris water maze.

Results

IE6M-HD/SD mice showed lower learning rates compared with control (IE6M-HD) or masticatory rehabilitated (IE6MHD/SD/HD) animals. Similarly, EE-HD/SD mice independent of age showed lower performance than controls (EE-HD) or rehabilitated mice (EE-HD/SD/HD). However, combined rehabilitation and EE in aged mice improved learning rate up to control levels. Learning rates did not correlate with swim speed.

Conclusions

Reduction in masticatory activity imposed on mice previously fed a hard diet (HD/SD) impaired spatial learning in the Morris water maze. In adults, masticatory rehabilitation recovered spatial abilities in both sedentary and active mice, and rehabilitation of masticatory activity combined with EE recovered these losses in aged mice.

Nutrition, adult hippocampal neurogenesis and mental health

INTRODUCTION

Over the last 8 years, emerging studies bridging the gap between nutrition and mental health have resolutely established that learning and memory abilities as well as mood can be influenced by diet. However, the mechanisms by which diet modulates mental health are still not well understood. Sources of data In this article, a review of the literature was conducted using PubMed to identify studies that provide functional implications of adult hippocampal neurogenesis (AHN) and its modulation by diet.

AREAS OF AGREEMENT

One of the brain structures associated with learning and memory as well as mood is the hippocampus. Importantly, the hippocampus is one of the two structures in the adult brain where the formation of newborn neurons, or neurogenesis, persists.

AREAS OF CONTROVERSY

The exact roles of these newborn neurons in learning, memory formation and mood regulation remain elusive.

GROWING POINTS

Nevertheless, there has been accumulating evidence linking cognition and mood to neurogenesis occurring in the adult hippocampus. Therefore, modulation of AHN by diet emerges as a possible mechanism by which nutrition impacts on mental health.

AREAS TIMELY FOR DEVELOPING RESEARCH

This area of investigation is new and needs attention because a better understanding of the neurological mechanisms by which nutrition affect mental health may lead to novel dietary approaches for disease prevention, healthier ageing and discovery of new therapeutic targets for mental illnesses.