Effects of fluoride exposure on thyroid hormone level and intelligence in rats

Objective To investigate the effects of fluoride exposure on the thyroid hormone level and intelligence in rats and to investigate the biomarkers of intellectual impairment induced by high fluoride exposure. Methods A total of 24 clean healthy Sprague-Dawley rats were randomly divided into control group (tap water containing 0.344 mg/L fluoride) and low-, medium-, and high-fluoride exposure groups (tap waters containing 10, 50, and 100 mg/L sodium fluoride, respectively). One male rat was cohabited with two female rats in the same group. After the offspring rats were weaned, 12 offspring rats (male/female ratio=1∶1) with a similar body weight in each group were subjected to the same treatment for the parental offspring. The offspring rats were sacrificed on the 60th day after birth. The weight of offspring rats was measured. Serum thyrotropin (TSH) , free triiodothyronine (FT3) , and free thyroxine (FT4) levels were measured by enzyme-linked immunosorbent assay. The learning and memory abilities of the rats were evaluated by Morris water maze test. The expression of mitochondrial fission 1 (Fis1) and mitofusin 1 (Mfn1) in blood was measured by Western blot. Results The offspring rats in the medium-and high-fluoride exposure groups had significantly lower serum TSH and FT4 levels than those in the control group (P<0.05). The place navigation test showed that the offspring rats in the medium-and high-fluoride exposure groups had significantly longer escape latency than those in the control group (P<0.05) , and the high-fluoride exposure group had a significantly longer escape distance than those in the control group (P<0.05). The spatial probe test showed that the offspring rats in the low-, medium-, and high-fluoride exposure groups had significantly shorter swimming time and distance in the target quadrant and total swimming time and distance than those in the control group (P<0.05). Compared with the offspring rats in the control group, those in the low-, medium-, and high-fluoride exposure groups had significantly higher expression of Fis1 (P<0.05) , and those in the low- and medium-fluoride exposure groups had significantly higher expression of Mfn1 (P<0.05) . Conclusion High fluoride exposure can reduce the secretion of thyroid hormone and the abnormality of mitochondrial dynamics in peripheral lymphocytes may provide a clue to identifying the biomarkers of intellectual impairment induced by fluoride exposure. Key words: Rats; Fluorine; Thyroid hormone; Intelligence; Mitochondrial dynamics