Endogenous opioid peptides have a profound inhibitory influence on the secretion of luteinizing hormone. Recently, it has been proposed that a reduction in the activity of these peptides in the hypothalamus may be the initial neural stimulus for the generation of the preovulatory luteinizing hormone surge which induces ovulation. Furthermore, this action may involve alterations in the activity of monoaminergic neurones which synapse on the luteinizing hormone-releasing hormone neurones in the hypothalamus. This investigation was undertaken to determine the effects of exogenous agonists of various opioid receptor subtypes on, (i) the magnitude of the preovulatory luteinizing hormone surge, and (ii) the activity of monoaminergic neurotransmitters in the hypothalamic areas which contain luteinizing hormone-releasing hormone neurones at the time when the surge is initiated
