In many nonprimate species, regression of the corpus luteum (CL) is initiated by the uterine release of prostaglandin F2alpha (PGF_(2alpha). In fact, this knowledge has been exploited for decades in development of estrous synchronization protocols. When a fully functional CL is present, a single, large-dose injection of PGF_(2alpha) will result in luteolysis followed by the onset of estrus and ovulation, allowing a renewed opportunity for breeding. This procedure has been extensively utilized to study the events involved in the regression of the CL. At the time of natural luteolysis, PGF_(2alpha) is released from the uterus in a pulsatile fashion, and evidence suggests that the pattern of decline in progesterone differs between animals that receive a single dose of PGF_(2alpha) compared to repeated doses [1, 2]. The problem has been that it is impossible to predict the precise timing of the onset of uterine release of PGF_(2alpha) and luteal regression in a natural cycle, making it difficult to study this stage of the estrous cycle experimentally.
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