KISSPEPTIN FUNCTION IN FEMALE BOVINE REPRODUCTION

Abstract

Kisspeptin is a modulator of GnRH and LH secretion in mammalian species. The regulator effect of kisspeptin on the hypothalamic-hypophysial-gonadal axis is unknown in the bovine species. The overall objective of the thesis was to determine the mechanism of action of kisspeptin on the GnRH-LH system and its effect on ovarian follicular dynamics in pubertal cattle. In the first study, the distribution of kisspeptin immunoreactive cells and their neuronal association with GnRH positive neurons was characterized during the proestrous, metestrous and diestrous stages of the estrous cycle in cows (n=7). Kisspeptin perikarya were distributed in two main populations in cows: a cranial cluster located in the preoptic area of the brainstem, and a caudal aggregation in the arcuate nucleus of the hypothalamus. The percentage of kisspeptin immunoreactive fibers in the preoptic area (P=0.01) and the close association with GnRH perikarya (P=0.09) increased during the low progesterone period and proestrous phase of the estrous cycle in cows respectively. In the second study, the effect of peripheral administration of the shortest form of kisspeptin (kisspeptin-10) was evaluated in sexually mature female cattle (n=52) during the luteal period of the estrous cycle. Intravenous treatment with the human kisspeptin-10 sequence enhanced the plasma LH concentration to a greater extent than intramuscular treatment (P<0.01). Kisspeptin administration induced increased LH in a dose dependent fashion (P<0.01) and increased the dominant follicular size when given at high dose (15mg) during the first ovarian follicular wave. The objective of the third study was to compare the effect of a single intravenous bolus versus multiple injections of the human versus murine kisspeptin on Luteinizing hormone secretion; furthermore, the fate of the dominant follicle was compared during the low-progesterone environment in pubertal heifers (n=72). A single bolus injection or multiple intravenous injections of human kisspeptin-10 given over 2 hours period increased LH plasma concentrations more than the respective treatments with murine kisspeptin-10 (P<0.01). Also, 45 mg of human kisspeptin-10 given over a period of two hours (3 to 9 intravenous injections) induced a similar ovulation rate as GnRH treatment (P=0.44). The mechanism of peripheral administration of kisspeptin on LH release and ovulation was elucidated in the last study (n=21). After two hours of administration, kisspeptin-10 did not enhance GnRH neuron activation in the preoptic area or the hypothalamus of cows. However, after cows were given a GnRH antagonist, kisspeptin treatment was not able to increase plasma LH concentrations and induce ovulation in pubertal heifers. This observation suggests that LH release is mediated by GnRH secretion, but may not induce de-novo GnRH synthesis after exogenous peripheral administration of kisspeptin. In summary, we have determined the mechanism of action of peripheral administration of kisspeptin on reproductive control in mature cattle. For the first time, we have demonstrated that the peripheral use of kisspeptin induces ovulation and follicular wave emergence in cattle, and that this effect is dependent on the GnRH system.