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

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.

Effects of low occlusal loading on the neuromuscular behavioral development of cortically-elicited jaw movements in growing rats

The effect of altered occlusal force on masticatory-related neuromuscular control, which projects from the anterior part of the cortical masticatory area (A-CMA), during growth remains unclear. This study sought to evaluate the effect of occlusal hypofunction on neuromuscular development of jaw muscle activities and cortically-induced rhythmic jaw movements (RJMs) in growing rats. Sixty-four 2-week-old male albino Wistar rats were divided into the control (fed normal diet) and experimental (fed soft diet) groups soon after weaning. Electromyographic activity was recorded at 5, 7, 9, and 11 weeks from the right masseter and anterior digastric along with RJMs. We found a significantly longer onset latency and smaller peak-to-peak amplitude in the experimental group than that in the control group. The RJMs showed an increase in gape size and lateral excursion until up to 9 weeks in both groups. However, both the average gape size and lateral excursion were significantly smaller in the experimental group than that in the control group after 9 weeks. The jaw movement pattern also showed a significant decrease at the maximum opening period in the experimental group. Our findings indicate that inadequate occlusal function during growth alters neuromuscular control of masticatory behaviors and impairs the pattern of RJMs.

Food hardness influences the progression of age-related hearing loss in mice

C57BL/6J and DBA/2J mice are often used for hearing research because of their early onset and progression of age-related hearing loss (AHL). Here, we report that the hardness of the diet affects the progression of AHL in these mice. When C57BL/6J mice and DBA/2J mice were fed a pellet-type or powder-type standard AIN93M diet, the pellet diet significantly promoted AHL. AHL promotion was eliminated by crushing the pellet diet to a powder. Subsequently, when C57BL/6J mice were fed the pellet-type AIN93M diet obtained from three different manufacturers, two of them significantly promoted AHL. The hardness of the diets was measured, and it was found that the two diets that promoted AHL were significantly harder than the other diet. Next, we attempted to reduce diet hardness by replacing some nutritional ingredients with dried eggs or phosphatidylcholine (PC), and we succeeded in obtaining brittle diets with lower hardness values. Then, C57BL/6J mice were bred with brittle diets for 6 months and the promotion of AHL was suppressed to the equivalent level as the powder diet. Furthermore, when senescence-accelerated mice, SAMP8, were fed a brittle diet for one year, the progression of AHL was also suppressed; however, it did not affect other aging indexes, such as mental and physical performance. We also confirmed that a high-fat pellet diet, which is soft even in pellet form, did not promote AHL. Time-restricted feeding (tRF), which is a chrono-nutritional method to delay aging, ameliorated the promotion of AHL by the hard AIN93M pellets in C57BL/6J mice. These results indicate that the physical form and hardness of diets affect the progression of AHL in mouse models.

Effects of Selective Serotonin Reuptake Inhibitors on Depression-Like Behavior in a Laser-Induced Shock Wave Model

Primary blast injury can result in depression-like behavior in the long-term. However, the effects of the selective serotonin reuptake inhibitor (SSRI) on the depression induced by mild blast traumatic brain injury (bTBI) in the long-term remain unclear. We generated a mouse model of mild bTBI using laser-induced shock wave (LISW) and administered an SSRI to mice by oral gavage for 14 days after LISW exposure. This study aimed to investigate the mechanisms of SSRI-mediated alleviation of depression-like behavior induced by mild bTBI. Animals were divided into three groups: sham, LISW-Vehicle, and LISW-SSRI. LISW was applied to the head of anesthetized mice at 0.5 J/cm². Twenty-eight days after the LISW, mice in the LISW-SSRI group exhibited reduced depression-like behavior, a significant increase in the number of cells co-stained for 5-bromo-2'-deoxyuridine (Brd-U) and doublecortin (DCX) in the dentate gyrus (DG) as well as increased brain-derived neurotrophic factor (BDNF) and serotonin levels in the hippocampus compared to the sham and LISW-Vehicle groups. Additionally, levels of phosphorylated cAMP response element binding protein (pCREB) in the DG were significantly decreased in the LISW-Vehicle group compared to that in the sham group. Importantly, pCREB levels were not significantly different between LISW-SSRI and sham groups suggesting that SSRI treatment may limit the downregulation of pCREB induced by mild bTBI. In conclusion, recovery from depression-like behavior after mild bTBI may be mediated by hippocampal neurogenesis induced by increased BDNF and serotonin levels as well as the inhibition of pCREB downregulation in the hippocampus.

Functional Analysis of Rhythmic Jaw Movements Evoked by Electrical Stimulation of the Cortical Masticatory Area During Low Occlusal Loading in Growing Rats

The maturation of rhythmic jaw movements (RJMs) and related neuromuscular control has rarely been studied in animals, though this process is essential for regulating the development of stomatognathic functions. Previous studies have shown that occlusal hypofunction during growth alters masticatory performance. However, little is known about patterns of cortically-induced RJMs under conditions of soft-diet feeding during development. The aim of this study is to clarify the effect of low occlusal loading on the pattern of cortically induced RJMs and related neuromuscular responses in growing rats. Sixty-four 2-week-old male albino Wistar rats were randomly divided into two groups and fed on either a normal diet (control) or soft diet (experimental) soon after weaning. At 5, 7, 9, and 11 weeks of age, electromyographic (EMG) activity was recorded from the right masseter and anterior digastric muscles along with corresponding kinematic images in RJMs during repetitive intracortical microstimulation of the left cortical masticatory area (CMA). Rats in both groups showed an increase in gape size and lateral excursion until 9 weeks of age. The vertical jaw movement speed in both groups showed no significant difference between 5 and 7 weeks of age but increased with age from 9 to 11 weeks. Compared to the control group, the average gape size and vertical speed were significantly lower in the experimental group, and the pattern and rhythm of the jaw movement cycle were similar between both groups at each recording age. EMG recordings showed no age-related significant differences in onset latency, duration, and peak-to-peak amplitude. Moreover, we found significantly longer onset latency, smaller peak-to-peak amplitude, and greater drop-off mean and median frequencies in the experimental group than in the control group, while there was no significant difference in the duration between groups. These findings indicate that a lack of enough occlusal function in infancy impedes the development of patterns of RJMs and delays the neuromuscular response from specific stimulation of the CMA.

A novel mouse model of mild traumatic brain injury using laser-induced shock waves

Blast-induced mild traumatic brain injury (mild bTBI) has been a frequent battlefield injury in soldiers during the conflicts in Iraq and Afghanistan. Understanding the pathophysiology and determining effective treatments for mild bTBI has become an international problem in the field of neurotrauma research. Contributing to this problem is a lack of an experimental model that accurately mimics the characteristics of mild bTBI. To date, the "mild'' versions of common experimental models of TBI have simply been less severe degrees of traumatic injury; these animals do not necessarily exhibit the clinical characteristics of mild bTBI seen in humans. Therefore, our first objective was to develop a highly controlled mouse model of bTBI using laser-induced shockwaves (LISWs). We established the parameters necessary to cause a reproducible injury of very mild severity, the most important feature seen in clinical practice. We defined very mild bTBI as having no traumatic change on the head visible to the naked eye after the insult was applied using very mild shockwaves to the heads of mice. Our very mild bTBI mouse model exhibited neurobehavioral changes in the chronic phase, such as cognitive impairment and depression-like behavior. We also observed an increase in 5-bromo-2'-deoxyuridine-positive, proliferating cells in the dentate gyrus during the acute phase and a subsequent decrease during the chronic phase. This model appears to be an accurate representation of the damage occurring in actual mild bTBI patients. We also found that an increase in cell proliferation in the dentate gyrus during the acute phase is the most prominent feature after a TBI.