Abstract
Based on events occurring during the genesis of a preovulatory surge of luteinizing hormone (LH) and an ovulatory failure of central origin, the hypothalamic median eminence (ME) is probably a major neuroendocrine control site for reproductive activity in the hen. The ratio of facilitatory to inhibitory (F:I) inputs on LH-releasing hormone (LHRH) neuronal terminals is an important determinant of the ME control site. The word "facilitatory" is defined as "stimulatory or increasing responsiveness to inputs". A relative increase in the F:I ratio of inputs on LHRH neuronal terminals at the ME is apparently involved in the genesis of the preovulatory surge of LH. Both an increase in neuropeptide Y (NPY) facilitatory inputs and a decrease in beta-endorphin (beta END) inhibitory inputs to ME-LHRH neuronal terminals are involved in the increase in the F:I ratio preceding the preovulatory surge of LH. Although the NPY component of this ratio (F) is apparently driven by the preovulatory surge of progesterone (P4) itself, its beta END component (I) might be related to the preovulatory surge of estradiol accompanying or preceding that of P4. As the egg-producing life of the hen progresses, a relative decrease in the F:I ratio on ME-LHRH neuronal terminals occurs. As a consequence of this decrease, stress-related inputs (e.g., feed restriction) can induce an hypothalamic failure of central origin in laying hens ending their 1st yr of production, but not in birds at the start of their egg-laying life. An increase in ME dopaminergic inhibitory inputs (I) on LHRH neuronal terminals is an apparent cause of the ovulatory failure induced by feed restriction.