It is suggested that masticatory dysfunction affects the central nervous system; however, the underlying mechanism remains unknown. Brain-derived neurotrophic factor (BDNF) and its receptor, TrkB, are known to play important roles in memory and learning. In this study, we examined the effects of mastication on memory, the expression levels of BDNF and TrkB, and the number of neurons in the hippocampus of mice. Male C57 BL/6J mice (3 weeks old) were randomly divided into the control group (N = 7) fed chow pellets and the experimental group (N = 7) fed a liquid diet, which reduces mastication during eating. At 14 weeks of age, we performed a passive avoidance test and found that memory and learning ability were impaired in the experimental group compared with the control group. After the behavioral experiment, brains were harvested and analyzed morphologically and biochemically. In the hippocampus of the experimental group, the expression levels of BDNF were significantly higher, whereas those of TrkB were lower than those of the control group. In the cerebral cortex, these levels remained unchanged between the two groups. The ratio of phospho-p44/42 ERK/pan ERK, a downstream molecule of BDNF/TrkB signaling, in the experimental group was significantly lower than that of the control group in the cortex and hippocampus. The number of pyramidal neurons in the hippocampus was lower in the experimental group than in the control group. These findings suggest that reduced mastication induced by a liquid diet in early childhood may impair memory and learning ability, accompanied by neuronal loss in the hippocampus.

Mouth breathing caused by nasal obstruction affects the normal growth and development of craniofacial structures, including changes in the orofacial muscles. Tongue muscles play an important role in patency of the pharyngeal airway, and changes in the breathing pattern may influence tongue function. The purpose of this study was to evaluate the effect of unilateral nasal obstruction during growth on contractile properties of the tongue-protruding muscles. Sixty 6-day-old male Wistar albino rats were divided randomly into control (n = 30) and experimental (n = 30) groups. Rats in the experimental group underwent a unilateral nasal obstruction after cauterization of the external nostril at the age of 8 days, and muscle contractile characteristics were measured at 5, 7, and 9 wk of age. The specific parameters measured were twitch force, contraction time, half-decay time, tetanic force, and fatigue index. Repeated-measures multivariate analysis of variance was used for intergroup and intragroup statistical comparisons. Twitch contraction force and half-decay time were significantly increased in the experimental group at all ages. Tetanic forces at 60 and 80 Hz were significantly higher in the experimental group at all ages. The fatigue index was decreased significantly in the experimental group at the age of 5 wk. These results suggest that early unilateral nasal obstruction may increase the contraction force of the tongue-protruding muscles and prolong the duration of muscle contraction, which may influence the shape and development of the craniofacial complex.

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Nasal obstruction can occur at different life stages. In early stages of life the respiratory system is still under development, maturing during the growth period. Previous studies have shown that nasal obstruction in neonatal rats alters craniofacial function. However, little is known about the effects of nasal obstruction that develops during later growth periods. The aim of this study was to investigate the effects of nasal obstruction during later periods of growth on the functional characteristics of the jaw-opening reflex (JOR) and tongue-protruding muscles. In total, 102 6-day-old male Wistar rats were randomized into either a control or experimental group (both n = 51). In order to determine the appropriate timing of nasal obstruction, the saturation of arterial oxygen (SpO2) was monitored at 8 days, and at 3, 5, 7, 9, and 11 weeks in the control group. Rats in the experimental group underwent unilateral nasal obstruction at the age of 5 weeks. The SpO2 was monitored at 7, 9, and 11 weeks in the experimental group. The electromyographic responses of JOR and the contractile properties of the tongue-protruding muscles were recorded at 7, 9, and 11 weeks. In the control group, SpO2 decreased until 5 weeks of age, and remained relatively stable until 11 weeks of age. The SpO2 was significantly lower in the experimental group than in the control. In the experimental group, JOR changes included a longer latency and smaller peak-to-peak amplitude, while changes in the contractile properties of the tongue-protruding muscles included larger twitch and tetanic forces, and a longer half-decay time. These results suggest that nasal obstruction during later growth periods may affect craniofacial function.

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