Hypothalamic control of thymic function

Removal of the pituitary gland results in atrophy of the thymus. As the for mer is under the control of hypothalamus, destruction of anterior portion o f the hypothalamus (AHTL) would be expected to negatively influence the thy mic function. Contrary to our expectation, however, the thymus became hyper trophic and serum level of growth hormone (GH) markedly increased, when the anterior portion of the hypothalamus was destroyed in rats at 1 month of a ge and older. The results suggested that AHTL removed the cells secreting G HRH (growth hormone release inhibitory hormone), but not GHRH (growth hormo ne releasing hormone), leading to increased pituitary secretion of GH. This high serum level of GH appeared to be responsible for the thymic hyperplas ia occurring after AHTL. In other words, the development and aging of the t hymus appear to be dependent on the serum level of GH which is under the ba lance of positive (GHRH) and negative (GHRIH) signals ti-om the hypothalamu s. In rats and mice, the serum level of GH is very high just after birth, q uickly declines in young adults and does not change greatly thereafter. Thu s, it is likely that the initial positive signal is high just after birth a nd decreasing thereafter with a concomitant increase of negative signal, le ading to the onset of thymic atrophy at around puberty, in association with sex steroid release.