Influence of occlusal support on learning memory and cholinergic neurons in the rat: a mini review

Abstract

In order to verify the influence of tooth loss and brain function in the rat, Wistar male aged rats were divided into the following groups: a control group (fed with solid diet), a soft diet group (fed with powder diet containing the same components as solid one), and a molar-crownless group (all molars were removed at 25 weeks and then fed with a powder diet. Experiment 1: To evaluate both learning ability and memory, rats were tested with a one-way step through type of passive avoidance apparatus divided into light and dark chambers at 40-weeks. There was no significant difference between the molar-crownless group and the control group in the response latency before the acquisition trails (non-stimulated period). At day 4 and 7 after the acquisition trials, the response latency of the molar-crownless group was significantly shorter than that in the control group (p < 0.05). After the passive avoidance test, determination of acetylcholine (ACh) concentration of the cerebral cortex and hippocampus was performed. The ACh levels of the molarcrownless group in the cerebral cortex and hippocampus were significantly lower than those of the control group (p < 0.05). Experiment 2: At 15 and 35 weeks, the number of choline acetyltransferase (ChAT)-positive neurons in the nucleus of the diagonal band/medial septal nucleus (NDB/MS) was significantly smaller in the molar-crownless group than in the control group (p < 0.01). The results of the study suggested that the soft diet and the decease of occlusal-masticatory function caused by occlusal support loss could accelerate learning ability and memory; moreover, this decrease of oral sensory information may have caused a reduction in the number of ChAT-positive neurons selectively in NDB/MS, which in turn caused a decline of ACh concentrations in the hippocampus.