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Does a Short Loop Feedback Mechanism for the Control of Luteinizing Hormone Secretion Exist in the Ewe?¹

Laboratory of Neuroendocrinology, The Babraham Institute (D.C.S., N.P.E.), Babraham, Cambridge, United Kingdom CB2 4AT; and Institut National de la Recherche Agronomique, Unité de Neuroendocrinologie Sexuelle, Station de Physiologie de la Reproduction (D.C.S., B.M.), Nouzilly 37380, France

Address all correspondence and requests for reprints to: Dr. Donal C. Skinner, Station de Physiologie de la Reproduction, Nouzilly 37380, France. E-mail: skinner{at}tours.inra.fr

It is not known whether a short loop feedback mechanism for the regulation of LH exists in sheep. This study on ovariectomized ewes investigated whether a bolus injection (10, 1, and 0.1 µg LH or 1 µg 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 determined by measurement of endogenous LH secretion. To assess the potential for involvement in a LH short loop feedback system and to estimate the level of LH in the hypothalamic milieu, the concentrations of LH in the peripheral circulation, portal circulation, and third ventricle were measured during an estradiol-induced preovulatory LH surge (n = 4).

Neither the bolus nor continuous administration of LH into the third ventricle had any effect on the mean interpulse interval, nadir, pulse amplitude, or circulating level of systemic LH. Furthermore, despite portal LH concentrations being more than 20-fold higher than jugular LH concentrations, LH levels in third ventricular cerebrospinal fluid remained barely detectable and did not reflect dynamic secretory events in the peripheral or hypothalamo-hypophyseal portal blood. These data demonstrate that in ewes, little pituitary LH reaches the third ventricle, and the small amount that does is unable to affect peripheral gonadotropin release. Our study suggests, therefore, that a short loop feedback system for LH does not exist in the ewe.