| Objective: We aimed to study the effects of thyroid hormone (TH )on the expression of neurotrophin-3 (NT-3 )and its receptor TrkC in rat cerebellum and hippocampus during the key development period and further to investigate the relationship between TH and NT-3 on the regulation of rat brain development .Methods: In the study, experimental animals were divided into three groups: (1) Hypothyroidism group: Pregnant rats had been given 0.02% methimazole (MM)water since E5, then the infant rats became innate hypothyroidism. (2)T4 hyperthyroidism group: Infant rats were intraperitoneal injected with T4 (0.4ug/ g·d ) at the beginning of birth. (3) Control group: We feed standard food and water to the pregnant rats and the infant rats were used as the matched control. NT-3mRNA and TrkCmRNA were measured by in site hybridization. Comparisons between groups were tested by One-Way ANOVA analysis and LSD test.Result: (1)The expressions of NT-3mRNA and TrkCmRNA in rat cerebellum and hippocampus have different characteristics on distribution. NT-3mRNA was expressed only on granule cells on inner layer while TrkCmRNA was expressed on both granule cells and Purkinj cells. NT-3 mRNA was expressed on CA1 and CA2 pyramidal cells and DG granule cells in hippocampus. TrkCmRNA was expressed on CA1 ,CA2 and CA3 pyramidal cells and DG granule cells in hippocampus. (2) NT-3mRNA expressed on cerebellum in hypothyroidism group was significantly lower than those in control group (P<0.05 ).(3) NT-3mRNA expressed on cerebellum in hyperthyroidism group on day P10,P15,P20,P25 and P30 was significantly higher than those in control group ( P<0.05) (.4) TrkCmRNA expressed in hyperthyroidism and hypothyroidism group was not significantly different from control group. (5)NT-3mRNA expressed in hypothyroidism group was significantly lower than those in control group.(6) NT-3mRNA expressed in hyperthyroidism group was significantly lower than those in control group on day P5,P10,P15and P20 ( P<0.05) .(7) In hippocampus of hypothyroid rats, as compared with control group, we found no significant difference between the expression of TrkCmRNA during the whole investigation period.(8)In hippocampus of hyperthyroid rats, as compared with control group, we found higher TrkCmRNA level only on day P10 .Conclusion: In the study we found that hypothyroidism decrease the NT-3mRNA level in hippocampus and cerebellum, however, hyperthyroidism increase the NT-3mRNA level in hippocampus and cerebellum. TH did not affect the TrkCmRNA expression. As a result, we suggested that the modulation by thyroid hormone of the expression of neurotrophin-3 expression rather than its receptor contributes to rat brain development.
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