Effects Of Thyroid Hormone On Auditory Evoked Potentials And Learning And Memory In Developmental Period Of Brain In Rat

Objective:The changes of fast component of brainstem auditory evoked potentials (FC-BAEP), slow component of brainstem auditory evoked potentials (SC-BAEP), middle latency responses (MLR) and functions of learning and memory had been observed in the rat with congenital hypothyroidism and the rat with congenital hypothyroidism treated with replacement of thyroid hormone at different age stages during the first postnatal month to further probe into the actions of thyroid hormone on brain development and the related critical action times, and provide new experimental evidence for the application of FC-BAEP, SC-BAEP and MLR in early diagnosis and curative effect evaluation in the patients suffering from congenital hypothyroidism.Methods:The rat pups with congenital hypothyroidism were induced by methimazole solution of 0.3g-L-1 mass concentration added in the drinking water for their mothers from the pregnant day 1 to the postnatal day 30, and the injection of L-thyroxine (L-T4) of 0.02mg·kg-1 mass fraction into abdominal cavity in the related each day for replacing treatment. The rat pups were randomly divided into 6 groups, group H (hypothyroidism), group RP1 (replacing treatment from postnatal day 1 to 30), group RP7 (replacing treatment from postnatal day 7 to 30), group RP14 (replacing treatment from postnatal day 14 to 30), group RP21 (replacing treatment from postnatal day 21 to 30), and group N (control). Serum free triiodothyronine (FT3) and thyroxine or tetraiodothyronine (FT4) of the rat were determined by radioimmunoassay. FC-BAEP, SC-BAEP and MLR were recorded with the summation-average technique of computer from the skull surface of the rat from postnatal day 14 to 60. The functions of learning and memory of the rat were determined with Morris water maze at postnatal day 60. Results:1. FC-BAEP,SC-BAEP and MLR of the rat in group RP1 emerged normally. But FC-BAEP,SC-BAEP and MLR of the rat in group RP7 and RP14 emerged later, and FC-BAEP,SC-BAEP and MLR of the rat in group RP21 and H emerged further later.2. The prolonged peak latency (PL) and interpeak latency (IPL) of FC-BAEP,SC-BAEP and MLR in rat of group RP1, RP7, RP14 and RP21 could be corrected significantly during 60 days after birth, and became normal in an ordered delayed times respectively. But the PL and IPL of FC-BAEP,SC-BAEP and MLR in the rat of group H at postnatal day 60 were still prolonged significantly.3. The escaped latency, swimming distance and swimming velocity in the Morris water maze test in the rat of group RP1 and RP7 were normal at postnatal day 60. However, the escaped latency and swimming distance were prolonged significantly and the swimming velocity was slowed significantly in the rat of group RP14 and RP21 at postnatal day 60, and these abnormal changes were more severe in the rat of group H.4. The platform search strategies in the Morris water maze test at postnatal day 60 were straight model and trending one in rat of group N, RP1 and RP7, random model in the rat of group RP14, random model and margin one in the rat of group RP21 and margin model in the rat of group H.Conclusions:1. The developments of auditory function and learning and memory are retarded significantly in the rat with congenital hypothyroidism, which can be corrected obviously with the replacing treatment of thyroid hormone, and the earlier the start of treatment and the better the outcome.2. The critical action times of thyroid hormone on the development of auditory function and learning and memory are different.3. The application of FC-BAEP, SC-BAEP and MLR is full of promise in early diagnosis and curative effect evaluation in the patients suffering from congenital hypothyroidism.