Hourly Administration of Luteinizing Hormone Induces Ovulation in Prepubertal Female Sheep*

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Abstract
The effects of repeated injections of lutenizing hormone [LH] on ovarian function in immature sheep .apprx. 12 weeks before the time of the first expected spontaneous ovulation were examined. The frequency of endogenous LH pulses during the pretreatment period was .apprx. 1 pulse each 3 h. The 1st experiment determined that rapid injection (i.v.) of 15.5 .mu.g LH replicated the amplitude of endogenous pulses. Hourly injection of this dose for 48 h to simulate the rapid LH pulse frequency of the follicular phase of the postpubertal female induced an LH surge and ovulation in 2 of 3 lambs. Administration of 33% of the dose over the 48 h period did not [5.2 .mu.g/h (3 lambs) or 15.5 .mu.g each 3 h (3 lambs)]. The 2nd experiment (7 lambs) determined the time course of the preovulatory estradiol rise produced in response to hourly LH pulses (15.5 .mu.g/injection), as well as the length of luteal phase after the induced LH surge. Four lambs produced a sustain edestradiol rise, a LH surge, and ovulation. The luteal phase was normal (13 days) in 1 and short in 3 lambs (6-11 days). In the remaining 3 prepubertal females that did not ovulate in response to 48-h injections of LH the estradiol rise was not sustained. Circulating estradiol in 5 untreated control lambs exhibited only transient increases during the course of the study. Hourly adiministration of physiological quantities of LH over a relatively brief period (48 h) can produce a follicular phase culminating in 1st ovulation in the immature lamb. In the context of the mechanism proposed for puberty in the female sheep, the hypothalamus, through its modulation of LH pulse frequency, governs the initiation of ovulation.