HYPOTHALAMIC CONTROL OF DEVELOPMENT AND AGING OF THE THYMUS

Removal of pituitary gland results in atrophy of the thymus. As the pi tuitary gland is under the control of the hypothalamus, destruction of the anterior portion of the hypothalamus (AHTL) is expected to negati vely influence the thymic function. Contrary to our expectation, the t hymus became hypertrophic and the serum level of the growth hormone (G H) markedly increased, when the anterior portion of the hypothalamus w as widely destroyed in rats at 1 month and over. The results suggested that AHTL removed the cells secreting GHRIH (growth hormone release i nhibitory hormone), but not GHRH (growth hormone releasing hormone), l eading to increased secretion of GH from pituitary gland and thymic hy perplasia. In other words, the development and aging of thymus appears to be under the balance of the positive (GHRH) and negative (GHRIH) s ignals of the hypothalamus. It is most likely that the positive signal is high just after the birth and decreases thereafter with a concomit ant increase of the negative signal, leading to the onset of thymic at rophy at around puberty. (C) 1998 Elsevier Science Ireland Ltd.