Is the First or Second Periovulatory Surge of FSH Responsible for Follicular Recruitment in the Hamster?

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Abstract
Proestrous hamsters were injected at 1300 h with phenobarbital to block ovulation. The animals were bled via cardiac puncture every 6 h beginning at 1600 h of proestrus until 1000 h of estrus. The ovaries were then removed and a quantitative evaluation of follicular development carried out. In saline-injected proestrous hamsters, there was a peak of serum FSH and LH [lutropin] at 1600 h followed by a 2nd prolonged peak of FSH at 0400 and 1000 h of estrus accompanied by low tonic levels of LH. This hormone profile was associated at 1000 h of estrus with an average of 9.3 large preantral follicles (stages IV follicles) per ovary. In contrast, the phenobarbital-treated hamsters showed no increases in either gonadotropin at any time and the ovaries contained large antral follicles and an average of only 1.7 stage IV follicles per ovary (3 of 4 animals). Injection of 1 mg progesterone concurrent with phenobarbital restored the 1st surge of LH but not FSH and partially restored the 2nd peak of serum FSH; the ovaries contained newly formed corpora lutea and 10.5 stage IV follicles per ovary. Injection of 1 .mu.g estradiol along with phenobarbital did not restore the 1st peak of LH or FSH and partially restored the 2nd FSH surge; the ovaries contained large Graafian follicles but also an average of 10.8 stage IV follicles. Concurrent injection of progesterone and estradiol at the same time as phenobarbital restored the 2nd FSH peak to the magnitude of control values and again the ovaries contained 10.8 stage IV follicles. By using the same animals for periovulatory measurements of gonadotropins and for ovarian histology, these experiments clearly demonstrate that the 2nd prolonged release of FSH is responsible for the recruitment at estrus at the large preantral (stage IV) follicles which represent the pool from which the follicles ovulating 4 days later are selected.

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