Molar loss induces hypothalamic and hippocampal astrogliosis in aged mice

Tooth loss in young mice is associated with cognitive decline and femur-bone mineral density

Osteoporosis is a prevalent disease that is associated with increased hip fractures which cause significant decline in quality of life. Tooth loss affects systemic condition such as cognitive function through various mechanism, but the link between tooth loss and femoral bone mineral density is still uncertain. This study aims to investigate whether tooth loss in young mice affects memory function and femoral bone mineral density. Eight-week-old male C57BL/6 J mice were allocated randomly into the control group with sham operation and the tooth-loss group extracted all maxillary molar. Step-through passive avoidance test as cognitive function test, micro-CT analysis and western blotting analysis were performed after 1- and 2-month observation period. Step-through passive avoidance test revealed that the tooth-loss group in 2-month observation period impaired cognitive function. Additionally, micro-CT analysis revealed a significant decrease in both the length of the mandible and bone mineral density in the femur of the tooth-loss group compared to the control group. Claudin-5 level in the hippocampus, which is one of the tight junction markers in blood-brain-barrier, was significantly decreased in the tooth-loss group. The findings of our present study suggested that tooth loss impair cognitive function accompanied by reduced tight-junction marker, mandibular growth and bone mineral density of femur.

Capsaicin modulates TRPV1, induces β-defensin expression, and regulates NF-κB in oral senescent cells and a murine model

Aging is associated with a decline in oral immune function, marked by reduced levels of antimicrobial peptides such as defensins. Capsaicin, a bioactive component found in chili peppers, has been theorized to modulate immune responses through specific receptor pathways. This study examined the effects of aging on oral defensin levels and the potential mitigating role of capsaicin, mediated by the immune response in oral tissues. We conducted a comparative analysis between young and aged mice, with or without capsaicin supplementation, for 3 months. The effect of capsaicin was also studied in vitro in senescence-induced human oral keratinocytes. We found that aging did not reduce defensin levels uniformly but did so in some instances. Capsaicin treatment increased defensin levels in these cases, potentially through transient receptor potential cation channel subfamily V member 1 (TRPV1)-mediated pathways in the oral cavity. Capsaicin supplementation may counteract age-related declines in oral defensin levels, enabling the maintenance of oral immune function during aging.

Search for unknown neural link between the masticatory and cognitive brain systems to clarify the involvement of its impairment in the pathogenesis of Alzheimer's disease

Brain degenerations in sporadic Alzheimer's disease (AD) are observed earliest in the locus coeruleus (LC), a population of noradrenergic neurons, in which hyperphosphorylated tau protein expression and β-amyloid (Aβ) accumulation begin. Along with this, similar changes occur in the basal forebrain cholinergic neurons, such as the nucleus basalis of Meynert. Neuronal degeneration of the two neuronal nuclei leads to a decrease in neurotrophic factors such as brain-derived neurotrophic factor (BDNF) in the hippocampus and cerebral cortex, which results in the accumulation of Aβ and hyperphosphorylated tau protein and ultimately causes neuronal cell death in those cortices. On the other hand, a large number of epidemiological studies have shown that tooth loss or masticatory dysfunction is a risk factor for dementia including AD, and numerous studies using experimental animals have also shown that masticatory dysfunction causes brain degeneration in the basal forebrain, hippocampus, and cerebral cortex similar to those observed in human AD, and that learning and memory functions are impaired accordingly. However, it remains unclear how masticatory dysfunction can induce such brain degeneration similar to AD, and the neural mechanism linking the trigeminal nervous system responsible for mastication and the cognitive and memory brain system remains unknown. In this review paper, we provide clues to the search for such "missing link" by discussing the embryological, anatomical, and physiological relationship between LC and its laterally adjoining mesencephalic trigeminal nucleus which plays a central role in the masticatory functions.

Effects of tooth loss on behavioral and psychological symptoms of dementia in app knock-in mice

OBJECTIVES

Many patients with Alzheimer's disease (AD) experience behavioral and psychological symptoms of dementia (BPSD), which significantly affect their quality of life. It is known that 5-Hydroxytryptamine (5-HT) plays a crucial role in the development of BPSD. While the relationship between tooth loss and AD symptoms has been acknowledged, the aspect of aggression has not been focused on until now. Despite the established importance of 5-HT in BPSD, how tooth loss is related to the exacerbation of AD symptoms, especially in terms of aggression, remains largely unexplored. Although nutritional status is known to influence the progression of dementia, the specific effect of tooth loss on peripheral symptoms, notably aggression, is not well understood.

METHODS

In our study, we conducted maxillary molar extractions in aged C57BL/6J and AppNL-G-F mice and observed their condition over a 3-month period. During this time, we documented significant behavioral and genetic differences between mice in the control groups and mice that underwent tooth extraction. Notably, mice that underwent tooth extraction exhibited a considerable decline in cognitive function and increased in aggression 3 months after tooth extraction compared with the control groups (C57BL/6J and AppNL-G-Fmice).

RESULTS

Our findings suggest that molar loss may lead to reduced 5-HT levels in the hippocampus, possibly mediated by the trigeminal nerve, contributing to the development of aggression and BPSD in AD.

CONCLUSION

This study sheds light on the intricate relationships between oral health, 5-HT, and AD symptoms, offering valuable insights into potential therapeutic avenues for managing BPSD in patients with dementia.

Premature gray hair development in the interbrow region owing to the loss of maxillary first molars in young mice

The exact sites of premature hair graying and whether tooth loss causes this condition remain unknown. In this study, we aimed to explore the effect of reduced mastication on premature hair graying. Maxillary first molars were extracted from young mice, and the mice were observed for 3 months, along with non-extraction control group mice. After 3 months, gray hair emerged in the interbrow region of mice in the tooth extraction group but not in the control group. The expression of tyrosinase-related protein-2 (TRP-2) mRNA was lower in the interbrow tissues of young mice without maxillary molars than in those with maxillary molars. Tooth loss leads to interbrow gray hair growth, possibly because of weakened trigeminal nerve input, suggesting that reduced mastication causes premature graying. Thus, prompt prosthetic treatment after molar loss is highly recommended.

Long-term sorbitol consumption affects the hippocampus and alters cognitive function in aged mice

The systemic effects of the artificial sweetener sorbitol on older adult individuals have not been elucidated. We assessed the effects of sorbitol consumption on cognitive and gingival health in a mouse model. Aged mice were fed 5% sorbitol for 3 months before their behavior was assessed, and brain and gingival tissues were collected. Long-term sorbitol consumption inhibited gingival tissue aging in aged mice. However, it caused cognitive decline and decreased brain-derived neurotrophic factor (BDNF) in the hippocampus. Sorbitol consumption did not affect homeostatic function; however, it may exert effects within the brain, particularly in the hippocampus.

Risk factors of cognitive impairment: Impact of decline in oral function

Cognitive impairment and subsequent dementia are the major causes of disability and need for nursing care among older people in worldwide. The purpose is to review well-known risk factors for cognitive impairment and dementia, focusing on the relationship between decline in oral function and current prevention strategies. Various non-modifiable and modifiable risk factors are related to cognitive impairment. Effects of oral function to cognitive impairment is not yet well recognized in the medical community, although masticatory function, occlusal force, and number of teeth have been reported to be related to cognitive function. Furthermore, occlusal force rather than number of teeth was significantly related to the early stages of cognitive impairment, and that a decline in occlusal force seemed to lead to cognitive impairment directly and indirectly through dietary intake. This relationship was significant only for occlusal force, which may be associated with the early stages of cognitive decline. Nutritional change caused by reduced masticatory function is suggested as a possible explanation. Therefore, rehabilitation or maintenance of oral function should be sought to prevent cognitive impairment.

Long-Term Capsaicin Administration Ameliorates the Dysfunction and Astrogliosis of the Brain in Aged Mice with Missing Maxillary Molars

Tooth loss and decreased masticatory function reportedly affect cognitive function; tooth loss allegedly induces astrogliosis and aging of astrocytes in the hippocampus and hypothalamus, which is a response specific to the central nervous system owing to homeostasis in different brain regions. Capsaicin, a component of red peppers, has positive effects on brain disorders in mice. Decreased expression of transient receptor potential vanilloid 1, a receptor of capsaicin, is associated with the development of dementia. In this study, we investigated the effect of capsaicin administration in aged mice (C57BL/6N mice) with reduced masticatory function owing to the extraction of maxillary molars to investigate preventive/therapeutic methods for cognitive decline attributed to age-related masticatory function loss. The results demonstrated that mice with impaired masticatory function showed decreased motor and cognitive function at the behavioral level. At the genetic level, neuroinflammation, microglial activity, and astrogliosis, such as increased glial fibrillary acidic protein levels, were observed in the mouse brain. The mice with extracted molars fed on a diet containing capsaicin for 3 months demonstrated improved behavioral levels and astrogliosis, which suggest that capsaicin is useful in maintaining brain function in cases of poor oral function and prosthetic difficulties.

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.

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.