THE BRAIN AS A TARGET ORGAN OF GONADAL-STEROIDS

Gonadal steroids have many effects in the central nervous system. Through a feedback mechanism, they influence the synthesis and release of hypothalamic gonadotropin-releasing hormone (GnRH) and/or pituitary gonadotropic hormones (luteinizing hormone, LH, and follicle stimulating hormone, FSH). Endogenous opioid peptides (EOPs) represent one of the key factors modulating the activity of sex steroids on the hypothalamus-pituitary-gonadal (HPG) axis. In particular, these peptides control the secretion of LH by inhibiting the activity of the hypothalamic neurons which produce GnRH. The EOP effect is dependent on the steroid hormone milieu, as shown by different responses to naloxone administration, both in animals and in humans. For the naloxone-induced increase in LH secretion to occur, relatively high levels of sex steroids are required. In humans, LH release is absent before sexual maturation. In fertile women, naloxone administration increases LH levels in the luteal phase but not in the follicular phase. In the postmenopausal period, naloxone has no effect on LH release; estrogen/progestin therapy does restore the LH response.