Effects of hypophysectomy on pro-thyrotropin-releasing hormone concentrations in rats

The effects of hypophysectomy on pro-thyrotropin-releasing hormone (pro-TRH) and TRH concentrations in the rat hypothalamus, cerebrum, cerebellum and brain stem, stomach and eye were studied. The hypophysectomy was performed via ear route and the rats were used for experiments at seven days after the operation. The hypophysectomized rats were administered T4 (500 micrograms/kg), T3 (100 micrograms/kg), TRH (1.0 mg/kg), or bovine TSH (1.25 IU/kg) was injected ip, and five rats in each subgroup were decapitated at four hours after the injection. Pro-TRH, TRH, TSH and thyroid hormone were measured by their radioimmunoassays. Immunoreactive pro-TRH (ir-pro-TRH) concentrations in the hypothalamus increased significantly after hypophysectomy, while its concentrations in the other tissues showed no changes. The immunoreactive TRH (ir-TRH) concentrations in the hypothalamus decreased significantly after hypophysectomy, but its concentrations did not change in the other tissues. In the hypophysectomized rats, ir-pro-TRH concentrations in the hypothalamus decreased significantly after T4 or T3 injection and tended to decrease after TRH or TSH injection. The ir-TRH concentrations in the hypothalamus increased significantly after T4, T3, TSH or TRH injection in the hypophysectomized rats. However, ir-pro-TRH and ir-TRH concentrations in the other tissues did not change after these hormone injections. The findings suggest that hypophysectomy stimulates TRH synthesis and release in the hypothalamus, and that pro-TRH synthesis in the tissues except the hypothalamus may not be regulated with thyroid hormone.