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Leptin Acts at the Bovine Adenohypophysis to Enhance Basal and Gonadotropin-Releasing Hormone-Mediated Release of Luteinizing Hormone: Differential Effects Are Dependent upon Nutritional History¹

Animal Reproduction Laboratory,³ Texas A&M University Agricultural Research Station, Beeville, Texas 78102 Department of Animal Science⁴ Center for Animal Biotechnology and Genomics,⁵ Texas A&M University, College Station, Texas 77843 Department of Pathobiology,⁶ College of Veterinary Medicine, Texas A&M University, College Station, Texas 77843 Department of Animal and Wildlife Sciences,⁷ Texas A&M University-Kingsville, Kingsville, Texas 78363

Recombinant ovine leptin (oleptin) stimulates an acute increase in the secretion of LH in fasted, but not in normal-fed, cows through an augmentation of the magnitude of individual pulses of LH. Herein, we tested the hypothesis that this effect could be accounted for by functional changes at the adenohypophyseal (AP) level. Eleven ovariectomized, estradiol-implanted cows were assigned to one of two dietary groups: normal-fed (n = 6) and fasted (fasted for 72 h; n = 5). After the animals were killed, the adenohypophyses were collected and AP explants were perifused with Krebs-Ringer bicarbonate buffer (KRB) for a total of 6.5 h, including a 2-h treatment at 2.5 h with KRB or increasing doses of oleptin and a challenge at 4.5 h with 50 ng of GnRH. To test for effects of leptin at the hypothalamic level, explants encompassing the medial basal hypothalamus-infundibular complex (HYP) were incubated in KRB alone (control) or in KRB containing 1000 ng of oleptin. Basal release of LH from AP explants treated with leptin was greater (P < 0.02) than that from control-treated explants in fasted, but not in normal-fed, cows. To the contrary, leptin-treated explants from normal-fed, but not from fasted, cows released more (P < 0.001) LH in response to GnRH than control-treated tissues. Neither fasting nor leptin affected (P > 0.1) the secretion of GnRH from HYP explants. These observations support the hypothesis that leptin modulates the secretion of LH in mature cows, to a large extent, by its direct actions at the AP. Differential manifestations of these effects are dependent upon nutritional history.

¹ Supported by USDA-NRI 00-35203-9132. A preliminary report of this work was presented at the 35th Annual Meeting of the Society for the Study of Reproduction.

² Correspondence: G.L. Williams, Animal Reproduction Laboratory, Texas A&M University Agricultural Research Station, 3507 Hwy 59 E, Beeville, TX 78102-9410. FAX: 361 358 4930; glw{at}fnbnet.net

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