Impact of the opioid system on the reproductive axis

Endogenous opioids, first described more than 40 years ago, have long been recognized for their main role as important neuromodulators within the central nervous system. More recently endogenous opioids and their receptor have been identified in a variety of reproductive and nonreproductive tissues outside the central nervous system. Their role within these tissues and organs, however, is only incompletely understood. In the central nervous system, endogenous opioids inhibit pulsatile GnRH release, in part mediating the stress response within the central nervous-pituitary gonadal axis, resulting in hypothalamic amenorrhea. In the ovary, the presence of endogenous opioids primarily produced by granulosa cells has been demonstrated within the follicular fluid, likely influencing oocyte maturation. In hypothalamic amenorrhea, normal cycles can be restored by the administration of opioid antagonists, such as naltrexone. In polycystic ovarian syndrome, endogenous opioids have found to be elevated and may stimulate insulin secretion from the endocrine pancreas. This effect can be inhibited by opioid antagonists, resulting in a decrease of circulating insulin levels in response to glucose challenge. Endogenous opioids may also play a role in the pathogenesis of ovarian hyperstimulation syndrome. In summary, endogenous opioids exert a wide variety of actions within the reproductive system and are worthy of further scientific study.