Discussion

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follicular and luteal phase condition for a relatively short period. Previous study demonstrated that the same dosage of senktide was able to stimulate rapid response of LH secretion during follicular phase in several goats, however the changes in mean LH concentration were considered insignificant [25]. This previous finding is considered distinguishable compared to the present study, where a rapid and significant increase of LH secretions could be clearly detected for a relatively short period.

In the FP group, increasing LH secretion after the B21-750 injection is unlikely to be the cause of estradiol elevation. The increase of estradiol in this group is speculated to occur due to the developing follicles during the follicular phase. It is a general consensus that during the follicular phase, the developing follicles produces estradiol, which in turn will send a positive feedback action necessary for preovulatory LH surge [45, 116]. After the removal of CIDR, the progesterone milieu started to decline and this gave a negative feedback to the release of LH [65]. The increase release of LH during this period stimulated the maturation of follicles, which in turn produces estradiol and gives a rise in the circulating estradiol concentration [112, 118].

Therefore, the high plasma estradiol in the FP group of this study is considered within the natural manner of the follicular phase.

Since the estradiol level of the FP group is considered to be unaffected by the B21-750 treatment, so does the ovulation time, which is occurred around 3 to 4 days after the injection. Contrary to this result, continuous intravenous infusion of the other NK3R agonist, senktide, was able to stimulate ovulation as early as within 2 days after treatment [25]. The ability of senktide to induce early ovulation was due to the capability of maintaining the sustained increase of LH secretion, which induced acceleration of follicular maturation. This effect could not be obtained by a single

59 injection of B21-750 in the present study.

In the LP group of this present study, a rapid increase of LH secretion was stimulated for a short period, i.e. one hour after injection of B21-750. Interestingly, a significant increase of LH, with lower percentage level compared to the first hour, was detected during the 5 to 6 h after injection. However, it is difficult to determine whether this “second” increase was due to the B21-750 treatment or coincidently caused by the normal pattern of LH pulse release during the luteal phase. It has been reported that during the luteal phase in a goat, LH pulse may occurred once every 6 h [40].

In line with the stimulation of LH increase for a short period in the LP group, the ovarian steroid hormone, i.e. progesterone and estradiol were not stimulated. Although a gradual increase pattern of estradiol could be detected in one goat (#12-LP) of the LP group, it was likely due to development of follicles. It has been reported that several follicles may grow coincidently during each follicular wave without affecting each other’s growth [68], suggesting that overlapping growth of follicle might occurred in the

#12-LP, indicated by the increase of estradiol.

Based on the experiment II-1 and II-2 of this chapter, acute injection of NK3R agonist was able to induce a rapid but short period increase of LH, and the stimulatory effect was considered to be similar in the FP and LP groups. In contrast with the result in this study, Billings et al. [7] demonstrated that local administration of NK3R agonist into the retrochiasmatic region of hypothalamus stimulated release of LH in the follicular, but not luteal phase in ewe. This study suggested that the response of LH secretion to the NK3R depends on the stage of estrous cycle and implies that B21-750 has high potency to stimulate LH secretion.

Despite of the dependence of the estrous phase, presence of functional gonads in

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the goats of this present study may play a role in the mechanism of NK3R stimulatory effect on GnRH/LH secretion. Study in arcuate nucleus of mice showed that high levels of NK3R mRNA expression were detected in the ovariectomized mice. Surprisingly, replacement of estradiol to those ovariectomized mice declined the number NK3R mRNA expression [76]. On the other hand, Amstalden et al. [3] found that NK3R immunoreactive cells were distributed in the preoptic area and arcuate nucleus of ewe during the follicular and luteal phase. It is suggested that NK3R may actively involved in the reproductive regulation of gonadal intact small ruminant species.

To the best of my knowledge, NK3R agonist needs to be administered either intermittently for several times or continuously for a certain minutes/hours in order to adequately stimulate LH secretion [7, 24, 25, 50]. Recent studies have proven that during the continuous administration period, sustain increase of LH secretion could be maintained [7, 25, 50].

4.4.3. Conclusion

Based on the result from experiment II-1 and II-2, it is postulated that hypothalamus and anterior pituitary can continuously release GnRH and LH in response to senktide action when senktide is present in the circulation. Moreover, it is suggested that sufficient blood level of NK3R agonist was not maintained in the circulation with a single injection of B21-750.

In summary, under the current experimental protocol in this chapter, conclusion can be drawn as follows. (1) Stimulation of pulsatile LH by senktide is considerably effective when administered low-dose continuous-infusion. Further studies might be needed to examine the effective dose for stimulating LH surge via intravaginal routes.

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(2) B21-750 was able to induce a rapid response of LH secretion for a short period after a single injection in both follicular and luteal phase, which is insufficient to stimulate development of ovarian follicles as well as the steroid hormone. Thus the overall estrous cycle stayed in the normal range. Further studies are needed to determine whether it is more necessary to increase the dose of B21-750 or increase the frequency of B21-750 administration in order to obtain adequate amplitude LH secretion for inducing follicular or luteal development.