Liquid diet induces memory impairment accompanied by a decreased number of hippocampal neurons in mice

Abstract

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.

The impact of mastication on cognition: Evidence for intervention and the role of adult hippocampal neurogenesis

“…The soft diet fed mice showed signs of decreased hippocampal function, which manifested as decreased memory and learning ability, such as measured by the passive avoidance test [33]. Cell proliferation assessed by immunohistochemically labelling progenitor cells was also found to be reduced in the DG in soft diet groups at a range of different ages, demonstrating that masticatory impairment causing potential problems with memory and cognition may not be a problem just for the elderly [10].…”

“…Liquid diets were also found to increase BDNF levels and decrease TrkB levels [33]. The BDNF levels are thought to be higher due to reduced uptake by TrkB, as expression of this receptor is reduced.…”

“…Adult offspring of these dams showed attenuated prenatal stress-induced learning deficits and decreased dentate gyrus cell proliferation [33] C57BL/6J mice Liquid diet Liquid diet group showed impaired memory and learning ability. In the hippocampus, the number of pyramidal neurons and expression of Tropomyosin receptor kinase B was reduced, and expression of BDNF was increased [34] Sprague-Dawley rats Liquid diet No differences in neuronal differentiation and survival of newborn neurons were seen.…”

“…Experimental studies discussed, mainly done on mice, have consistently shown that impairing masticatory function results in both cellular and behavioural changes. A recurring theme identified is that of a decrease in neural stem cells proliferation in the hippocampus in many of the animal studies, when masticatory impairment was simulated by feeding mice a soft/powdered/liquid diet [10,19,33,34].…”

“…Association found between cognitive function in the normal MMSE score range and the number of sound/healthy and decayed teeth [39] >61 back on TrkB expression level [33]. The disruption in BDNF/TrkB signalling leads to reduced induction of the MAPK cascade, which is necessary for memory and learning, and the findings suggest that the reduction of BDNF/TrkB signalling decreases the number of neuronal cells in the hippocampus, resulting in impaired memory and learning ability [33].…”