ACUTE INCREASE IN RESPONSIVENESS OF LUTEINIZING-HORMONE (LH)-RELEASING HORMONE NERVE-TERMINALS TO NEUROPEPTIDE-Y STIMULATION BEFORE THE PREOVULATORY LH SURGE

Neuropeptide-Y (NPY) neurons regulate LH secretion in part through fac ilitation of LHRH release. We tested the hypothesis that responsivenes s of LHRH neurons to NPY's facilitatory actions is physiologically reg ulated during the estrous cycle, and specifically, that it may be incr eased as a component of the gonadotropin surge-generating process. A d ynamic superfusion paradigm was used to examine the role of cycle stag e and time of day on LHRH responsiveness to NPY stimulation, using med ian eminence tissue from animals killed at 0900, 1400, and 1800 h on m etestrus and proestrus. Tissue obtained at 0900 and 1800 h on metestru s did not exhibit significant LHRH responses to 10(-7) M NPY, and only moderate responses were seen at 1400 h on metestrus and 0900 h on pro estrus. At 1400 h on proestrus, however, median eminence responsivenes s to the same concentration of NPY was significantly increased, with L HRH responses to NPY being 2- to 5-fold greater than those at 0900 (P < 0.01), 1400 (P < 0.05), and 1800 h on metestrus (P < 0.01) and at 09 00 h on proestrus (P < 0.05). Neither cycle-related changes in basal L HRH release nor changes in the releasability of LHRH in response to de polarization could account for the accentuated responses in the 1400 h proestrous group. These data clearly demonstrate that the responsiven ess of LHRH terminals and/or their afferents to the actions of NPY is acutely enhanced during a brief window of time on proestrus, viz. imme diately before generation of gonadotropin surges. Our findings are con sistent with the hypothesis that the preovulatory endocrine milieu per mits an acute increase in the responsiveness of LHRH nerve terminals t o the actions of NPY, perhaps by prompting increases in the number and /or affinity of NPY receptors.