MECHANISMS OF PROTEIN FEEDBACK ON GONADOTROPIN-SECRETION

The classical concept that the control of follicle-stimulating hormone (FSH) and luteinising hormone (LH) secretion by the pituitary gland i s achieved by the stimulation of gonadotropin-releasing hormone (GnRH) and negative feedback by steroid secretion from the gonads has recent ly been modified by the identification of several proteins with the ca pacity to modulate FSH secretion. These proteins, inhibin, activin and follistatin, are produced in the ovary and testis and have the capaci ty to act as long loop protein feedback signals which modulate FSH sec retion. Further studies indicate that inhibin probably acts as a circu lating hormone to inhibit FSH secretion. More recent data suggest that activin and follistatin may act as local mediators to control FSH sec retion at the pituitary by paracrine or autocrine phenomena. The produ ction of activin, which stimulates FSH, and its binding protein follis tatin, by pituitary cells raises the possibility that the local produc tion modulates FSH secretion in addition to the long loop feedback sig nals emanating from the gonads through the steroid hormones estradiol and testosterone as well as circulating inhibin levels. Further studie s are necessary to confirm the nature of these regulatory processes. ( C) 1998 Elsevier Science Ireland Ltd. All rights reserved.