A soft diet influences the dopaminergic system and learning ability

Objectives: The aim of this study is to understand the effects of dysfunctional mastication caused by soft-diet feeding on the dopaminergic neural system and learning ability in both the normal and Alzheimer's disease (AD) brain.

Methods: 72 male Wistar rats were 3 weeks old at the beginning of the experiment. We divided the 72 rats into four experimental groups: 1) normal rats fed with hard food; 2) normal rats fed with soft food; 3) Amyloid-â protein infused rats (AD rat) fed with hard food; and 4) AD rats fed with soft food. We assessed dopamine (DA) release in the hippocampus using a micro-dialysis approach and learning ability and memory using step-through passive avoidance tests. Furthermore, we assessed the levels of tyrosine hydroxylase (TH), the rate-limiting enzyme for the biosynthesis of dopamine in the ventral tegmental area (VTA), by immunohistochemical methods. Differences among groups were statically evaluated using one-way analysis of variance (ANOVA) and post hoc test.

Results: Our results showed that when both normal and AD rats were fed a soft diet, a significant decrease in DA release in the hippocampus occurred and impaired learning ability and memory were detected when comparing to rats fed hard food (P<0.001). However, a change in the number of TH-immunopositive neurons in rats fed a soft diet was not observed in the VTA.

Conclusions:These observations suggest that soft-diet feeding may enhance a decline in hippocampal dopaminergic neuronal systems and result in impaired learning ability and memory in both normal and AD brain.