Effects of dexamethasone on TRH, TRH-glycine and pre-pro-TRH (178-199) levels in various rat organs.

The effect of dexamethasone administration on the concentration of thyrotropin releasing hormone (TRH) and pre-pro-TRH connecting peptides TRH-glycine (TRH-Gly), and pre-pro-TRH (178-199) in various rat organs was studied. Three groups of 35 rats each were injected dexamethasone (Group A: 25 micrograms/100 g; Group B: 500 micrograms/100 g) or saline (Group C: control). The subgroups of 7 rats each were decapitated at 1, 2, 3, 4 and 24 h after the injection and the levels of TRH-Gly, pre-pro-TRH (178-199) and TRH in the hypothalamus, cerebrum, cerebellum and brain stem, stomach, retina were estimated by specific radioimmunoassays. The level of TRH-Gly and pre-pro-TRH (178-199) in the hypothalamus decreased significantly in groups A and B at 1-4 hours after the injection, and then returned to pretreatment levels at 24 h after the injection. In contrast, TRH levels in the hypothalamus increased significantly in groups A and B at 1-4 h after the injection. The TRH-Gly, pre-pro-TRH (178-199) and TRH levels in other organs showed no changes after dexamethasone injection. From these findings it is concluded that dexamethasone inhibits the synthesis and secretion of TRH and the maturation of pro-TRH in the hypothalamus, while its effects on other organs is different from the hypothalamus.