Influence of a long-term powdered diet on the social interaction test and dopaminergic systems in mice

Impact of habitual chewing on gut motility via microbiota transition

The gut environment, including the microbiota and its metabolites and short-chain fatty acids (SCFA), is essential for health maintenance. It is considered that functional recovery treatment for masticatory dysphagia affects the composition of the gut microbiota, indicating that habitual mastication, depending on the hardness of the food, may affect the gut microbiota and environment. However, the impact of chronic powdered diet feeding on the colonic condition and motility remains unclear. Here, we evaluated various colonic features in mice fed with powdered diets for a long-term and a mouse model with masticatory behavior. We observed a decreased abundance of the SCFA-producing bacterial genera in the ceca of the powdered diet-fed mice. Based on the importance of SCFAs in gut immune homeostasis and motility, interestingly, powdered diet feeding also resulted in constipation-like symptoms due to mild colitis, which were ameliorated by the administration of a neutrophil-depleting agent and neutrophil elastase inhibitors. Lastly, the suppressed colonic motility in the powdered diet-fed mice was significantly improved by loading masticatory activity for 2 h. Thus, feeding habits with appropriate masticatory activity and stimulation may play a key role in providing a favorable gut environment based on interactions between the gut microbiota and host immune system.

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.

Involvement of catecholaminergic and GABAAergic mediations in the anxiety-related behavior in long-term powdered diet-fed mice

Dietary habits are important factors which affect metabolic homeostasis and the development of emotion. We have previously shown that long-term powdered diet feeding in mice increases spontaneous locomotor activity and social interaction (SI) time. Moreover, that diet causes changes in the dopaminergic system, especially increased dopamine turnover and decreased dopamine D4 receptor signals in the frontal cortex. Although the increased SI time indicates low anxiety, the elevated plus maze (EPM) test shows anxiety-related behavior and impulsive behavior. In this study, we investigated whether the powdered diet feeding causes changes in anxiety-related behavior. Mice fed a powdered diet for 17 weeks from weaning were compared with mice fed a standard diet (control). The percentage (%) of open arm time and total number of arm entries were increased in powdered diet-fed mice in the EPM test. We also examined the effects of diazepam, benzodiazepine anti-anxiety drug, bicuculline, GABA-A receptor antagonist, methylphenidate, dopamine transporter (DAT) and noradrenaline transporter (NAT) inhibitor, atomoxetine, selective NAT inhibitor, GBR12909, selective DAT inhibitor, and PD168077, selective dopamine D4 receptor agonist, on the changes of the EPM in powdered diet-fed mice. Methylphenidate and atomoxetine are clinically used to treat attention deficit/hyperactivity disorder (ADHD) symptoms. The % of open arm time in powdered diet-fed mice was decreased by treatments of atomoxetine, methylphenidate and PD168077. Diazepam increased the % of open arm time in control diet-fed mice, but not in powdered diet-fed mice. The powdered diet feeding induced a decrease in GABA transaminase, GABA metabolic enzymes, in the frontal cortex. Moreover, the powdered diet feeding induced an increase in NAT expression, but not DAT expression, in the frontal cortex. These results suggest that the long-term powdered diet feeding may cause low anxiety or impulsivity, possibly via noradrenergic and/or dopaminergic, and GABAAergic mediations and increase the risk for onset of ADHD-like behaviors.

Chewing Stimulation Reduces Appetite Ratings and Attentional Bias toward Visual Food Stimuli in Healthy-Weight Individuals

Based on the theory of incentive sensitization, the exposure to food stimuli sensitizes the brain’s reward circuits and enhances attentional bias toward food. Therefore, reducing attentional bias to food could possibly be beneficial in preventing impulsive eating. The importance of chewing has been increasingly implicated as one of the methods for reducing appetite, however, no studies to investigate the effect of chewing on attentional bias to food. In this study, we investigated whether chewing stimulation (i.e., chewing tasteless gum) reduces attentional bias to food as well as an actual feeding (i.e., ingesting a standardized meal) does. We measured reaction time, gaze direction and gaze duration to assess attentional bias toward food images in pairs of food and non-food images that were presented in a visual probe task (Experiment 1, n = 21) and/or eye-tracking task (Experiment 2, n = 20). We also measured appetite ratings using visual analog scale. In addition, we conducted a control study in which the same number of participants performed the identical tasks to Experiments 1 and 2, but the participants did not perform sham feeding with gum-chewing/actual feeding between tasks and they took a rest. Two-way ANOVA revealed that after actual feeding, subjective ratings of hunger, preoccupation with food, and desire to eat significantly decreased, whereas fullness significantly increased. Sham feeding showed the same trends, but to a lesser degree. Results of the visual probe task in Experiment 1 showed that both sham feeding and actual feeding reduced reaction time bias significantly. Eye-tracking data showed that both sham and actual feeding resulted in significant reduction in gaze direction bias, indexing initial attentional orientation. Gaze duration bias was unaffected. In both control experiments, one-way ANOVAs showed no significant differences between immediately before and after the resting state for any of the appetite ratings, reaction time bias, gaze direction bias, or gaze duration bias. In conclusion, chewing stimulation reduced subjective appetite and attentional bias to food, particularly initial attentional orientation to food. These findings suggest that chewing stimulation, even without taste, odor, or ingestion, may affect reward circuits and help prevent impulsive eating.

Revisiting the link between cognitive decline and masticatory dysfunction

Age-related decline in cognitive functions and dementia are major challenges in geriatric healthcare. Accumulating evidence from clinical, epidemiological and animal research suggests that tooth loss may be a risk factor for the decline of cognitive functions. This issue highlights the role of the brain-stomatognathic axis in geriatric medicine. Whether input from the stomatognathic apparatus can affect the brain remains an open debate. By revisiting the evidence published in the past five years, we argue that the hypothesis regarding the association between cognitive decline and masticatory dysfunction should be carefully interpreted. Most of the available clinical and epidemiological studies present only cross-sectional data. With respect to the prospective studies, important confounding factors, such as nutritional and physical conditions, were not fully controlled for. Animal research has revealed that hippocampal deficits may play key roles in the observed cognitive decline. However, experimental intervention and outcome assessments may not capture the condition of human subjects. Brain neuroimaging research may be suitable for bridging the gap between clinical and animal research, potentially contributing to (a) the clarification of the brain network associated with mastication, (b) the identification of brain imaging markers for exploring the mechanisms underlying long-term changes in masticatory functions, and (c) the elucidation of interactions between mastication and other cognitive-affective processing systems. Three potential models of the brain-stomatognathic axis and relevant hypotheses are summarized, focusing on the sensory feedback mechanisms, the compensation of motor control, and cerebellar deficits. Finally, we highlight four critical aspects of study and experimental design that should be considered in future research: (a) the refinement of the considered behavioral assessments, (b) the inclusion of baseline changes in mental and physical conditions,

Animal Models for Elucidation of the Mechanisms of Neuropsychiatric Disorders Induced by Sleep and Dietary Habits

Numerous changes in human lifestyle in modern life increase the risk of disease. Especially, modern sleep and dietary habits are crucial factors affecting lifestyle disease. In terms of sleep, decreases in total sleep time and in rapid eye movement sleep time have been observed in attention-deficit/hyperactivity disorder (ADHD) patients. From a dietary perspective, mastication during eating has several good effects on systemic, mental, and physical functions of the body. However, few animal experiments have addressed the influence of this decline in sleep duration or of long-term powdered diet feeding on parameters reflecting systemic health. In our studies, we examined both the influence of intermittent sleep deprivation (SD) treatment and long-term powdered diet feeding on emotional behavior in mice, and focused on the mechanisms underlying these impaired behaviors. Our findings were as follows: SD treatment induced hypernoradrenergic and hypodopaminergic states within the frontal cortex. Furthermore, hyperactivity and an explosive number of jumps were observed. Both the hypernoradrenergic state and the jumps were improved by treatment with ADHD therapeutic drugs. On the other hand, long-term powdered diet feeding increased social interaction behaviors. The feeding affected the dopaminergic function of the frontal cortex. In addition, the long-term powdered diet fed mice presented systemic illness signs, such as elevations of blood glucose, and hypertension. This review, describing the SD mice and long-term powdered diet fed mice can be a useful model for elucidation of the mechanism of neuropsychiatric disorders or the discovery of new therapeutic targets in combatting effects of the modern lifestyle.

Long-term feeding on powdered food causes hyperglycemia and signs of systemic illness in mice

AIMS

Dietary habits are crucial factors affecting metabolic homeostasis. However, few animal experiments have addressed the effects of long-term feeding with soft food on parameters reflecting systemic health.

MAIN METHODS

Using mice, we compared the effects of short (3 days) and long (17 weeks from weaning) feeding periods between powdered food and normal pellet food on the levels of blood glucose, serum levels of insulin, catecholamines, and corticosterone, blood pressure, and/or social interaction behaviors. In addition, the effects of a human glucagon-like peptide-1 analog, liraglutide (a new drug with protective effects against neuronal and cardiovascular diseases), were compared between the powder and pellet groups.

KEY FINDING

(i) Powdered food, even for such a short period, resulted in a greater glycemic response than pellet food, consistent with powdered food being more easily digested and absorbed. (ii) Long-term feeding on powdered food induced hyperglycemia and related systemic signs of illness, including increases in serum adrenaline, noradrenaline, and corticosterone, higher blood pressures (especially diastolic), and increased social interaction behaviors. (iii) Liraglutide, when administered subcutaneously for the last 2 weeks of the 17-week period of feeding, improved these changes (including those in social interaction behaviors).

SIGNIFICANCE

The hyperglycemia associated with long-term powdered-food feeding may lead to certain systemic illness signs, such as elevations of blood glucose, hypertension, and abnormal behaviors in mice. Mastication of food of adequate hardness may be very important for the maintenance of systemic (physical and mental) health, possibly via reduction in the levels of blood glucose and/or adrenal stress hormones (catecholamines and glucocorticoids).