Plasma Thyrotropin Responses to Thyrotropin-Releasing Hormone after Injection into the Third Ventricle, Systemic Circulation, Median Eminence and Anterior Pituitary

To determine whether the cerebrospinal fluid provides a potential route by which hypothalamic hypophysiotropic hormones might be transported to the anterior pituitary, thyrotropin releasing hormone (TRH) was injected into the third ventricle and plasma radioimmunoassayable thyroid stimulating hormone (TSH) measured. Comparisons were made with injections into the median eminence, anterior pituitary and a peripheral vein. In one series of experiments, intraventricular TRH did not increase plasma TSH significantly above baseline, but did prevent the expected post—anesthetic fall. In a second series, intraventricular TRH caused a small but significant increase in plasma TSH. By contrast injections of similar amounts into the systemic circulation, median eminence and anterior pituitary led to a much greater rise in plasma TSH at five min post injection: third ventricular 27.9 ± 12.7, iv 70.8 ± 21, median eminence 107 ± 35, and pituitary 188 ± 22.6 mU TSH/100 ml. These studies indicate that TRH can reach the anterior pituitary from the third ventricle, but that this route of administration is the least effective of those used. (Endocrinology91: 696, 1972)