Influence of high-fluoride on thyroid function and brain damage in rats

Objective To study the influence of high-fluoride on thyroid function and brain damage. Methods Thirty-six Wistar rats were randondy divided, according to weight and gender into 3 groups(12 rats each), i.e. control group, high fluoride group, and high fluoride plus thyroid tablet treatment group. The rats were fed with normal tap water containing no more than 5 mg/L NaF and the tap water added 100,100 mg/L NaF, respectively. After 7 months of experiment, the rats in high fluoride plus thyroid tablet treatment group were given with 0.04% thyroid tablet( 1.8 ml·kg~(-1)·d~(-1)) by gastric perfusion for three weeks. The contents of TT_3 and TT_4 in serum were detected by radio-immunological assay; the histomorphology in thyroids and brains were observed under microscopy; and the protein level of NMDAR2B subunit of glutamate receptor in the hippocampal CA1 and CA3 was measured by immunohistochemistry. Results As compared to the values of TT_3 and TT_4 in serum of rats in control group[ (0.97 ± 0.15), (84.03 ± 12.45)nmol/L], TT_3 and TT_4 in high fluoride group were obviously lower [(0.24 ± 0.07), (15.16 ± 2.08)nmol/L, all P < 0.01]; while no changes in TT_3 and TT_4 were detected in high fluoride plus thyroid tablet treatment group[ (1.02 ± 0.19), (85.63 ± 9.55)nmol/L, all P < 0.05] as compared to controls, but higher than those in high fluoride group(all P < 0.01 ). The pathological changes including partial hyperplasy, arrangement disorder, atrophy, and decreased colloid of the thyroid follicular epithelial cells in high fluoride group were observed under microscopy. In high fluoride plus thyroid tablet treatment group, the degree of the thyroid cellular hyperplasy was relatively slight as compared to high fluoride group. The swelling and disarrangement of neurons in the hippocampus were observed in high fluoride group, whereas the changes of the neurons were not so obvious in high fluoride plus thyroid tablet treatment group. The grey values of NMDAR2B positive cells in the hippocampal CA1 and CA3 in high fluoride group(167.05 ± 7.31 ) were significantly increased as compared to controls (92.53 ± 9.67 ) or high fluoride plus thyroid tablet treatment group( 101.66 ± 12.21, all P < 0.01 ). Conclusions High fluoride can induce the decreased function and changed histomorphology in thyroid and result in pathological damages in the brains of rats. However, treated with thyroid tablet to those having damages induced by high fluoride, the thyroid function and morphology can be normal, and the brain damages can be alleviated. The results indicate that hypothyroidism caused by high fluoride might be an important participating factor in brain damages caused by fluorosis. Key words: Fluoride; Thyroid gland;  Brain injuries;  Hippocampus