DOES A SHORT LOOP FEEDBACK MECHANISM FOR THE CONTROL OF LUTEINIZING-HORMONE SECRETION EXIST IN THE EWE

It is not known whether a short loop feedback mechanism for the regula tion of LH exists in sheep. This study on ovariectomized ewes investig ated whether a bolus injection (10, 1, and 0.1 mu g LH or 1 mu g BSA; n = 4) or a 3-h continuous infusion of exogenous LH (100 or 1 ng/min; n = 7) into the third ventricle through a permanent indwelling cannula could influence the activity of the GnRH pulse generator as determine d by measurement of endogenous LH secretion. To assess the potential f or involvement in a LH short loop feedback system and to estimate the level of LH in the hypothalamic milieu, the concentrations of LH in th e peripheral circulation, portal circulation, and third ventricle were measured during an estradiol-induced preovulatory LH surge (n = 4). N either the bolus nor continuous administration of LH into the third ve ntricle had any effect on the mean interpulse interval, nadir, pulse a mplitude, or circulating level of systemic LH. Furthermore, despite po rtal LH concentrations being more than 20-fold higher than jugular LH concentrations, LH levels in third ventricular cerebrospinal fluid rem ained barely detectable and did not reflect dynamic secretory events i n the peripheral or hypothalamo-hypophyseal por tal blood. These data demonstrate that in ewes, little pituitary LH reaches the third ventri cle, and the small amount that does is unable to affect peripheral gon adotropin release. Our study suggests, therefore, that a short loop fe edback system for LH does not exist in the ewe.