PARACRINE REGULATION OF ANTERIOR-PITUITAR Y 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 hypothala mic central control and the hormonal peripheral control, recent studie s have shown, in the anterior pituitary, the expression, among many ot her regulatory factors, of neuropeptides that are identical to those p roduced by the hypothalamus and that seem involved in the local contro l of anterior pituitary functions through autocrine or paracrine mecha nisms. The presence of the neuropeptide mRNAs, precursors and mature f orms of the peptides in anterior pituitary tissues as well as the secr etion of the mature peptides argue in favor of the intrinsic ability o f the normal and tumoral anterior pituitary to express neuropeptides. This expression of neuropeptides occuring in tissues bearing functiona l receptors for these ligands, anterior pituitary control could rely, at least in part, on endogenous neuropeptides acting locally. Correlat ions between neuropeptide contents in the anterior pituitary and the p lasma levels of anterior pituitary hormones suggest that neuropeptides of anterior pituitary origin can play a local regulatory role, comple mentary of the classical hypothalamic central control. In the normal a nterior pituitary which remains under hypothalamic control, it is pres ently difficult to evaluate the relative importance of the local and c entral control. However, anterior pituitary hyperplasia and pituitary tumors represent two models in which the specific contribution of the local control is easier to define.