Endogenous opioid peptides and the control of gonadotrophin secretion

The endogenous opioid peptides are a group of recently discovered compounds which occur in the brain of a wide variety of species. Originally named because of their opiate-like activity, they have since been demonstrated to have multifaceted actions, one of which appears to be the modulation of luteinising hormone (LH) secretion. Because of the prime position of LH in the ovulatory process, this role for the opioids has attracted considerable interest. Their mode of action is essentially one of suppression and they work by inhibiting the release of hypothalamic gonadotrophin releasing hormone. Through this mechanism they have been implicated in the suppression of LH secretion during the prepubertal period and the modulation of LH during the oestrous cycle. It is well established that gonadal steroids suppress LH secretion by negative feedback upon the hypothalamic-pituitary axis, and this action may be brought about, in part, through intermediary opioidergic neurones. Much of the research to date has been carried out upon laboratory rodents and primates, but there is evidence now accruing that the opioids have similar actions in domestic animals. Knowledge of the role of these compounds may therefore aid in the understanding of an area of commercial importance, namely the control of ovulation in farm livestock.