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(131) LEPTIN ACTS AT THE HYPOTHALAMUS TO STIMULATE GnRH SECRETION IN THE FASTED COW. Zieba, Dorota1, 2, Amstalden, Marcel1, 2, Morton, Stephanie1, 2, Keisler, Duane 3, Williams, Gary1, 2, 1 Texas A&M University Agricultural Research Station, Beeville, TX2 Texas A&M University, College Station, TX3 University of Missouri, Columbia, MO ABSTRACT- The leptin receptor has been shown to be present at both hypothalamic and adenohyphyseal loci in several mammalian species, but few studies have confirmed the ability of leptin to stimulate directly hypothalamic GnRH and adenohypophyseal LH secretion, respectively. In the sexually-immature, fasted heifer, we have reported evidence of direct hypothalamic effects of leptin in its ability to modulate the frequency of LH pulses. However, in the sexually-mature female, our studies to date have confirmed only that leptin can modulate LH secretory dynamics through direct effects at the adenohypophyseal level. The current experiment tested the hypothesis that the ability of leptin to increase circulating concentrations of LH in fasted cows may also be attributed to effects on hypothalamic GnRH secretion. Five mature, ovariectomized, estradiol-implanted beef cows were used. Cows were in moderate to good body condition (BCS= 4-5; 1-9 scale) and were fitted surgically with third ventricle (IIIV) cannulas for the collection of cerebrospinal fluid (csf). Treatments consisted of: 1) physiological saline (Control), and 2) oLeptin (0.2 KEY WORDS: cows, leptin, LH, GnRH |
g/kg body weight) in a switchback design, such that each cow received each of the treatments in random order approximately 4 weeks apart. During each experimental period, cows were fasted for 72 h with free access to water. Jugular blood was collected simultaneously with IIIV csf at 10-min intervals for 5 h on Days 0 and 3. On Day 3, treatments (saline or oleptin) were injected i.v. 3 times at 0, 1 and 2 h, with the first sample collected immediately before the first injection at time 0. Leptin treatment increased mean plasma concentrations of LH (P< 0.001) and csf GnRH (P < 0.05) compared to saline. Changes were accompanied by simultaneous increases in the amplitude of LH pulses (P <0.04) and size (area under the curve) of GnRH pulses (P < 0.03). As expected, the frequencies of GnRH and LH pulses were not affected. Results indicate that leptin modulates LH secretion in fasted cows, in part, through its stimulatory effects on hypothalamic GnRH secretion (USDA-NRI 00-35203-9132).