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Neuroendocrinology |
Animal Reproduction Laboratory,3 Texas A&M University Agricultural Research Station, Beeville, Texas 78102
Federal University of Santa Maria–CAPES,4 Santa Maria, RS, Brazil
Department of Animal Science,5 Center for Animal Biotechnology and Genomics, Texas A&M University, College Station, Texas 77843
Department of Animal Science,6 University of Missouri, Columbia, Missouri 65211
We tested the hypothesis that leptin could prevent fasting-mediated reductions in pulsatile secretion and modify GnRH-mediated release of LH in heifers approaching puberty. Thirteen crossbred, prepubertal heifers (13.5–16 mo; 280–350 kg) exhibiting frequencies of pulses of LH between 0.67 and 1 pulse/h, were assigned randomly to two groups: 1) control (n = 6), fasted for 72 h with s.c. injections of saline at 12-h intervals, and 2) leptin (n = 7), fasted for 72 h with s.c. injections of oleptin (19.2 µg/kg) at 12-h intervals. Blood samples were collected intensively for 6 h on Days 0 and 3. This was followed on Day 3 with sequential administration of physiological (0.0011 µg/kg, i.v.) and pharmacological (0.22 µg/kg, i.v.) doses of GnRH and additional blood sampling. Leptin treatment increased (P = 0.0003) plasma concentrations of leptin 5–6-fold compared to controls. Fasting caused a marked decline (P = 0.01) between Days 0 and 3 in the frequency of LH pulses in controls; however, this effect was prevented in the leptin group, with pulse frequency increasing (P < 0.008) from Day 0 to 3. Leptin treatment increased GnRH-induced release of LH at both low (P = 0.04) and high (P = 0.02) doses. Plasma insulin and insulin-like growth factor-1 were reduced by fasting and unaffected by leptin. Leptin increased mean concentrations of growth hormone. Results indicate, for the first time, that exogenous leptin can prevent fasting-mediated reductions in the frequency of LH pulses and modify GnRH-mediated release of LH in intact, prepubertal heifers.
2 Correspondence: G.L. Williams, Animal Reproduction Laboratory, Texas A&M University Agricultural Research Station, 3507 Highway 59E, Beeville, TX 78102-9410. FAX: 361 358 4930; glw{at}fnbnet.net
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