PARENT SESSION
PLATFORM SESSION 21: REGULATION OF GONADOTROPINS AND THEIR RECEPTORS: II
Chair: LaBarberra, Andrew¹, ¹
Co-chair: Meehan, Thomas¹, ¹
Harborside D
2:00 PM-4:00 PM

(451) LEPTIN MODULATES BASAL SECRETION OF LH AND ENHANCES GONADOTROPH RESPONSIVENESS TO GnRH IN ADENOHYPOPHYSEAL EXPLANTS FROM FASTED COWS.

Amstalden, Marcel^1,2,3, Zieba, Dorota^1,2,3, Gallino, Joanna^1,2,3, Morton, Stephanie^1,2,3, Edwards, John⁴, Harms, Paul², Welsh, Thomas², Stanko, Randy^1,5, Keisler, Duane⁶, Williams, Gary^1,2,3, ¹ Animal Reproduction Laboratory, Beeville, TX² Department of Animal Science, College Station, TX³ Center for Animal Biotechnology and Genomics, College Station, TX⁴ Department of Pathobiology, College Station, TX⁵ Department of Animal and Wildlife Sciences, Kingsville, TX⁶ Department of Animal Science, Columbia, MO

ABSTRACT- We have reported that intracerebroventricular infusions of recombinant ovine leptin (oleptin) stimulate hypersecretion of LH in fasted, but not in control-fed cows. The response was characterized by an increase in the magnitude of LH pulses and mean plasma concentrations of LH. However, in recent experiments, leptin failed to affect the secretion of GnRH from hypothalamic explants or to modify release of LH in cultured adenohypophyseal cells from normal-fed bulls or steers. Objectives of the current experiment were to determine whether the stimulatory effect of leptin on secretion of LH in fasted cows is mediated at the hypothalamic or pituitary level. Ten mature, ovariectomized cows in moderately thin body condition, each bearing an estradiol implant, were used. Cows were assigned randomly to one of two dietary groups: 1) Normal-Fed (n=5) and 2) Fasted (72-h fasting; n=5). At the end of the 72-h period, cows were euthanized and hypothalami and pituitaries collected. Hypothalami were transected sagittally and each half of the medial basal hypothalamus connected to the infundibulum was incubated in Krebs-Ringer bicarbonate buffer (KRB). After a period of equilibration, hypothalamic (HYP) explants from each cow were incubated in the presence of either KRB alone or KRB containing 1000 ng of oleptin. Adenohypophyseal (AP) explants were cut into 0.5 x 2 mm strips and perifused with KRB for 6.5 h. At 2.5 h, AP explants were perifused for 2 h with KRB containing 0, 5, 10, 50, or 100 ng/ml of oleptin. At 4.5 h, AP explants were challenged with 50 ng of GnRH. Neither fasting nor leptin affected (P > 0.1) the secretion of GnRH from HYP explants. However, AP explants from fasted cows released more (P < 0.001) LH during the equilibration period than those from normal-fed cows. After equilibration and before GnRH treatment, leptin-treated AP explants from fasted cows secreted more (P < 0.04) LH than control-treated explants. Moreover, AP explants from fasted, but not normal-fed, cows treated with 10, 50, and 100 ng/ml of leptin released 27% more (P < 0.001) LH in response to GnRH than control-treated explants. These data provide evidence that leptin modulates basal secretion of LH in fasted cows and enhances responsiveness of anterior pituitary gonadotrophs to GnRH.

KEY WORDS: Leptin, LH, GnRH, Bovine