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Kisspeptins are the product of the Kiss1 gene and potently stimulate GnRH secretion. In sheep, Kiss1 mRNA-expressing cells are found in the arcuate nucleus (ARC) and dorsal-lateral preoptic area and both appear to mediate the positive feedback effect of estradiol to generate the preovulatory GnRH/LH surge. To determine the role of kisspeptin in transmitting estrogen-positive feedback in the hypothalamus, we administered the kisspeptin antagonist p-271 to ewes subjected to an estradiol benzoate-induced LH surge. Kisspeptin antagonist treatment significantly attenuated these LH surges. We further examined the response to kisspeptin treatment prior to the LH surge. Kisspeptin significantly stimulated GnRH secretion into the hypophysial portal system, but the response to kisspeptin was similar in luteal and late-follicular phase ewes. Kiss1r mRNA expression in GnRH neurons was also similar across the estrous cycle. To examine alternative pathways for kisspeptin stimulation of GnRH neurons, we examined the origin of kisspeptin neuronal fibers in the external zone of the median eminence (ME) using neuronal tracing and immunohistochemical techniques. ARC populations of kisspeptin neurons project fibers to the ME. Finally, we showed kisspeptin stimulates GnRH release from ovine ME-cultured explants. This suggests direct kisspeptin to GnRH terminal-to-terminal communication within the ME. Overall, these data indicate an essential role for kisspeptin in receiving stimulatory estrogen signals and generating the full positive feedback GnRH/LH surge. Kisspeptin neurons of the ARC project to the external zone of the ME and kisspeptin acts upon the GnRH fibers at this level.

Kisspeptin Is Essential for the Full Preovulatory LH Surge and Stimulates GnRH Release from the Isolated Ovine Median Eminence