Effects of Thyrotropin-Releasing Hormone on Plasma Thyroxine in the Calf: Comparison of Intraventricular and Intravenous Injection Routes

Abstract

SummaryVarious doses of TRH (12.5, 25, 50, 100, and 200 μg) were injected into the lateral ventricle and jugular vein of unanesthetized calves to examine the possibility of CSF transport of thyrotropin-re-leasing hormone (TRH). At higher doses (100 and 200 μg), there was no major difference in effectiveness of jugular and ventricular TRH to elevate thyroxine levels. Thyroxine levels were significantly elevated as early as 1.5 hr and peaks occurred 6 to 12 hr after 100 μg of TRH. At the lower doses (<100 μg), the effectiveness of TRH to elevate thyroxine levels was clearly route dependent. All TRH doses (<100 μg) by jugular injection were effective, while only the 50-μg intraventricular dose was clearly effective. These observations indirectly indicate that TRH is transferred from CSF to the vascular system. This transfer process, possibly involving cells of the median eminence, may impede the movement of TRH to blood and thus account for the reduced effectiveness of intraventricular TRH.