Effects of thyroid hormone, TRH and TSH on pro-TRH concentrations in various organs of rats

The effects of large doses of thyroid hormone, thyrotropin (TSH) or thyrotropin-releasing hormone (TRH) on pro-TRH concentrations in the rat hypothalamus, cerebrum, cerebellum, brain stem, stomach and eye were studied. The rats were administered T4 (2.5 mg/kg), T3 (375 micrograms/kg), TRH (1.0 mg/kg) or bovine TSH (1.25 IU/kg) i.p. and seven rats in each subgroup were decapitated at 4 or 24 h after the injection. Pro-TRH, TRH, TSH and thyroid hormone were measured by each radioimmunoassay. Immunoreactive pro-TRH (ir-pro-TRH) and immunoreactive TRH (ir-TRH) were found in the hypothalamus, cerebrum, brain stem, stomach and eye. Ir-pro-TRH concentrations in the hypothalamus decreased significantly after T4 and T3 injection and tended to decrease after TRH and TSH injection, but not significantly. In contrast, ir-pro-TRH concentrations in other tissues showed no changes after T4, T3, TRH or TSH injection. Ir-TRH concentrations in tissues did not change significantly after these hormone injection. The plasma TSH levels decreased significantly, while these of thyroid hormone increased significantly after thyroid hormone injection. Elution profile of acetic acid extracts of hypothalamus, stomach or eye was identical to that of synthetic pro-TRH. The findings suggest that the large dose of thyroid hormone inhibits pro-TRH synthesis in the hypothalamus, and that TRH synthesis in the tissues except the hypothalamus may not be regulated by thyroid hormone.