Gonadal steroids and neuropeptide Y-opioid-LHRH axis: interactions and diversities

We report that the two classes of regulatory neuropeptides, neuropeptide Y (NPY) and endogenous opioid peptides (EOP), modulate luteinizing hormone (LH) release in diverse fashion in gonad-intact rats. Each neuropeptide acts at two loci, the hypothalamus and pituitary, to excite (NPY) or inhibit (EOP) LH release. At the hypothalamic level, NPY stimulates luteinizing hormone releasing hormone (LHRH) release, a response mediated by alpha 2-adrenoreceptors and amplified in the presence of adrenergic agonists. At the pituitary level, NPY acts in concert with LHRH to amplify the LH response. In contrast, EOP inhibit LHRH release by decreasing the supply of excitatory adrenergic signals in the vicinity of LHRH neurons in the preoptic-tuberal pathway, and at the pituitary level, they decrease LH release in response to LHRH. Further, the gonadal steroidal milieu facilitates NPY neurosecretion and postsynaptic expression of NPY in concert with adrenergic system; a similar clear-cut facilitatory effect of gonadal steroids on EOP secretion is not yet obvious. Our additional studies imply that the EOP system has the potential to increase sensitivity towards gonadal steroids and that to induce the preovulatory LH surge the neural clock may decrease the inhibitory EOP tone prior to the critical period on proestrus. This antecedent neural event allows the excitatory adrenergic and NPY signals to evoke LHRH secretion at a higher frequency approximating that seen in ovariectomized rats. Further studies are under way to delineate the steroid-induced subcellular events that integrate the action of these regulatory peptides in the control of the episodic LHRH secretion pattern which sustains basal and cyclic gonadotropin release in the rat.