[Paracrine regulation of anterior pituitary hormones by neuropeptides]

The hypothalamus is the source of neuropeptides which, being secreted into the portal system, control the synthesis and the secretion of the anterior pituitary hormones. Besides the well characterized hypothalamic central control and the hormonal peripheral control, recent studies have shown, in the anterior pituitary, the expression, among many other regulatory factors, of neuropeptides that are identical to those produced by the hypothalamus and that seem involved in the local control of anterior pituitary functions through autocrine or paracrine mechanisms. The presence of the neuropeptide mRNAs, precursors and mature forms of the peptides in anterior pituitary tissues as well as the secretion of the mature peptides argue in favor of the intrinsic ability of the normal and tumoral anterior pituitary to express neuropeptides. This expression of neuropeptides occurring in tissues bearing functional receptors for these ligands, anterior pituitary control could rely, at least in part, on endogenous neuropeptides acting locally. Correlations between neuropeptide contents in the anterior pituitary and the plasma levels of anterior pituitary hormones suggest that neuropeptides of anterior pituitary origin can play a local regulatory role, complementary of the classical hypothalamic central control. In the normal anterior pituitary which remains under hypothalamic control, it is presently difficult to evaluate the relative importance of the local and central control. However, anterior pituitary hyperplasia and pituitary tumors represent two models in which the specific contribution of the local control is easier to define.