Comparison of Luteinizing Hormone Surge Responses to Ovarian Steroids in Cyclic and Spontaneously Persistent Estrous Rats of Middle Age1

Female rats of the Charles River CD strain, when 6 to 12 months old, present spontaneous persistent estrus (SPE) in increasing numbers, while others remain cyclic. Apparently as a prelude to SPE, some rats still cycling will fail to ovulate early in response to estrogen or progesterone administration during diestrus. When estradiol benzoate (EB) was given on Day 2 of the 5-day cycle or when progesterone was given on Day 3, the luteinizing hormone (LH) surges, if they occurred at all, tended to be much smaller than the normal surge of proestrus. The normal surges could be equalled only when EB was given on Day 2 and followed by progesterone on Day 3. In SPE rats, progesterone treatment usually induces ovulation only after SPE has been interrupted for a few days and the rat has returned to proestrus-estrus. LH surges induced in that way by progesterone were usually substantial. Thus, the ability of the LH-release apparatus to function had returned within 1 week after interruption of the persistent estrogenic status. Comparison were made of LH surge responses to EB administration in cyclic and SPE rats during pseudopregnancies produced by cervical stimulation (after LH injection of SPE rats) or by daily injection of progesterone. In previously cyclic rats, proestrus-like surges of LH were registered consistently, while in previously SPE rats the levels attained were generally lower. Unexpectedly, if rats were castrated when daily progesterone treatment started, few produced large amounts of LH. This was especially true in the SPE group, 14 of 20 rats failing to show any LH surge. Hence, although capacity to produce the LH surge in response to estrogen + progesterone can return within a few days after SPE cases, some unknown ovarian activity plays an important role. Such activity may also take part in the normal cycle.