Lamprey gonadotropin-releasing hormone-III selectively releases follicle stimulating hormone in the bovine

Recent studies have shown that lamprey gonadotropin-releasing hormone (l-GnRH) is localized in the mammalian brain, and that l-GnRH-III, can selectively induce FSH secretion in the rat both in vivo and in vitro. Consequently, the purpose of this study was to determine if l-GnRH-III could elicit selective FSH release in cattle and compare this response with that to mammalian luteinizing hormone releasing hormone (m-LHRH). Cattle were chosen as the animal model because previous studies have demonstrated that FSH and LH are secreted by separate gonadotropes in that species. For these studies, crossbred cycling heifers were implanted with jugular cannulae and l-GnRH-III was infused either between Days 9–14 or on Day 20 of the estrous cycle. Blood samples were collected both before and following peptide infusion. Our results demonstrate that during Days 9–14 of the estrous cycle (luteal phase), when progesterone levels averaged between 4 and 5 ng/ml, a dose of 0.25 mg of l-GnRH-III induced the release of FSH (P < 0.05), but not LH. A 0.5 mg dose of l-GnRH-III caused a greater release of FSH (P < 0.01), but still did not induce LH release. Higher doses of the peptide were capable of significantly releasing both gonadotropins. Importantly, during the luteal phase, doses of 0.5 and 2 mg of m-LHRH were ineffective in stimulating FSH, but did elicit marked increases (P < 0.001) in LH. Again, progesterone levels averaged 4–5 pg/ml. In order to assess gonadotropin releasing ability of l-GnRH-III at a different phase of the estrous cycle, some animals were administered the peptide on Day 20, when progesterone levels were below 1.0 pg/ml. At this time, the l-GnRH-III induced the release of LH (P < 0.01), but not FSH. Overall, our results demonstrate that l-GnRH-III can selectively induce FSH in cattle during the luteal phase, whereas m-LHRH was ineffective in that regard. Furthermore, the fact that l-GnRH-III can selectively stimulate FSH when serum progesterone is high, and LH when serum progesterone is low, suggests its actions are under strong control of this steroid. We suggest the FSH releasing capacity of l-GnRH-III in cattle could render this peptide useful for enhancement of reproductive efficiency in this species.