THE MEDIAN-EMINENCE AS A SITE FOR NEUROENDOCRINE CONTROL OF REPRODUCTION IN HENS

Based on events occurring during the genesis of a preovulatory surge o f luteinizing hormone (LH) and an ovulatory failure of central origin, the hypothalamic median eminence (ME) is probably a major neuroendocr ine control site for reproductive activity in the hen. The ratio of fa cilitatory to inhibitory (F:I) inputs on LH-releasing hormone (LHRH) n euronal terminals is an important determinant of the ME control site. The word ''facilitatory'' is defined as ''stimulatory or increasing re sponsiveness to inputs''. A relative increase in the F:I ratio of inpu ts on LHRH neuronal terminals at the ME is apparently involved in the genesis of the preovulatory surge of LH. Both an increase in neuropept ide Y (NPY) facilitatory inputs and a decrease in beta-endorphin (beta END) inhibitory inputs to ME-LHRH neuronal terminals are involved in t he increase in the F:I ratio preceding the preovulatory surge of LH. A lthough the NPY component of this ratio (F) is apparently driven by th e preovulatory surge of progesterone (P4) itself, its betaEND componen t (I) might be related to the preovulatory surge of estradiol accompan ying or preceding that of P4. As the egg-producing life of the hen pro gresses, a relative decrease in the F:I ratio on ME-LHRH neuronal term inals occurs. As a consequence of this decrease, stress-related inputs (e.g., feed restriction) can induce an hypothalamic failure of centra l 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 dopamin ergic inhibitory inputs (I) on LHRH neuronal terminals is an apparent cause of the ovulatory failure induced by feed restriction. An additio nal cause might be the decrease in the ready releasable pool of LHRH a t the ME. Thus, the ME is probably a major neuroendocrine control site for reproductive activity in the hen.