Opioid regulation of luteinizing hormone in amenorrheic patients after therapy for induction of ovulation

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.

Opioid control of luteinizing hormone secretion in humans

The observation that heroin-addicted subjects are amenorrheic and/or hypogonadic suggested a possible role of endogenous opioid peptides (EOP) in the regulation of the hypothalamus-pituitary-gonodal axis. Because EOP are localized all through the axis, they may influence the reproductive function acting at various levels. The injection of morphine decreases plasma LH levels, abolishing the pulsatile pattern of secretion. The evidence that naloxone, an opiate receptor blockade, increases LH levels suggests a tonic inhibitory action of EOP. The naloxone-induced LH increase is not observed before pubertal maturation, in both sexes, and is absent in children with delayed or precocious puberty and in those with Klinefelter's or Turner's syndrome, suggesting a role of gonadal function on the opioid control of LH secretion. In adult women this LH response to naloxone is present during the periovulatory and the luteal phase of the menstrual cycle, suggesting a permissive role of estradiol and mainly of progesterone on the action of EOP on the LH secretion. Indeed, in amenorrheic subjects naloxone lacks to stimulate plasma LH levels and the treatment for the induction of ovulation restores this activity. An increased inhibitory action of EOP on GnRH release may explain the inefficacy of naloxone to stimulate LH secretion in hypogonadotropinic patients, while a decreased action has been hypothesized in postmenopausal women. The clinical implications of EOP in reproductive medicine appear to be promising.