Abstract
Spontaneous persistent estrus (SPE), although a common feature of senility, may occur in middle-aged rats. Thus, in Charles River CD rats it often appears in virgin females 6-12 months old. Whereas in aged SPE rats a single injection of LH is commonly followed either by pseudopregnancies (PSPs) in repetition or by PSP succeeded by a number of short estrous cycles, the usual result in middle-aged CD SPE rats is the immediate occurrence of I or more short cycles, after which SPE returns. Among 46 such animals, only 10 (22%) presented PSP, while 30 (65%) merely presented short cycles. The other 6 rats returned promptly to SPE after a brief diestrus. From these results it is postulated that events after a luteinizing dose of LH or the introduction of a set of corpora lutea by any other means are determined by the amount of basal PRL secretion in the days after treatment. If PRL is minimal, SPE returns promptly; if moderately elevated, short cycles appear because of low grade progesterone secretion from the co...
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