The availability of synthetic CRH has made it possible to answer several classical questions about hypothalamic-pituitary-adrenal control. The effects of CRH on the pituitary are inhibited by cortisol, thus confirming a direct pituitary feedback effect (Fig. 1.23). ADH, which previously had been shown to have CRH-like activity, has now been shown to potentiate CRH action, and in some settings, may be even more important than CRF as a stimulator of ACTH release. An important element in the negative feedback control of ACTH secretion by cortisol is the striking inhibitory effect of this steroid on the synthesis and secretion of CRH and ADH.
