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Simple Summary  The magnitude and duration of the pre-ovulatory release of LH is important in controlling the timing of ovulation in estrus synchronization and fixed-time artificial insemination (FTAI) protocols in cows. The neuropeptide kisspeptin (KISS1) stimulates GnRH neurons in the brain which in turn induce LH release from the pituitary gland. The present study used ovariectomized Nelore cows as a model to examine whether priming with the steroid estradiol benzoate (EB) for 12 h increased both the peak and duration of LH release after treatment with KISS1. It was found that cows pre-treated with EB for 12 h showed a greater LH peak and longer duration of LH release in response to KISS1 compared with cows that received EB at the same time as KISS1, or KISS1 alone. The incorporation of EB priming and KISS1 in estrus synchronization-FTAI protocols could improve the control of LH release and ovulation, and potentially increase fertilization and conception rates in cows. This would have important practical application in assisted breeding in beef and dairy cows.    Abstract  The present study examined whether priming with estradiol benzoate (EB) for 12 h increased both the peak and duration of LH release in response to kisspeptin (KISS1, KP) in cows. In a Latin square design, ovariectomized Nelore cows ( n  = 8) received: Control, i.m. 4 mL of 0.9% saline; KP, i.m. 4 mg murine KISS1-10; EBKP, i.m. 4 mg KISS1-10 + i.m. 2 mg EB simultaneously; EB12KP, i.m. 4 mg KISS1-10 + i.m. 2 mg EB 12 h before KISS1-10. Concentrations of LH were determined in blood samples obtained at time 0 (treatment), 20, 40, 60, 90, 120, 150, 180, 210 and 270 min. Concentrations of LH were analyzed by Proc GLIMMIX for repeated measures. In case of significance, the adjusted Tukey test was used to test for differences among treatments. GraphPad 8.0 PRISM ®  was used to determine the area under the LH-response curve (AUC) after injection of KISS1-10. Plasma LH remained relatively constant throughout sampling after treatment with saline. The peak in LH after injection of KISS1-10 occurred at 20 min in Groups KP and EBKP and at 40 min in Group EB12KP. The peak LH response (∆LH, ng/mL) was greater ( p  < 0.01) in Group EB12KP (5.6 ± 0.9) than in Groups KP (2.4 ± 0.9) and EBKP (3.5 ± 0.9), which did not differ. AUC (LH ng/mL*min) was greater ( p  = 0.02) in Group EB12KP (439 ± 73) than in Groups KP (176 ± 73) and EBKP (241 ± 73), with the latter two groups not differing. The findings indicated that 12 h priming with EB increased both the peak and duration of the LH response to treatment with KISS1. The incorporation of EB priming and KISS1 could improve the efficiency of estrus synchronization with fixed-time AI in cows. This would have an important practical application in assisted breeding in beef and dairy cattle.

Estradiol Priming Potentiates the Kisspeptin-Induced Release of LH in Ovariectomized Cows