Further study of oestrous cycles that follow interruption of spontaneous persistent oestrus in middle-aged rats

Spontaneous persistent oestrus in certain middle-aged rats, when interrupted by an injection of LH, is usually followed by one or more short ovulatory cycles. A previous study disclosed that brief treatment with bromocriptine early in dioestrus blocks completion of the initial cycle whereupon persistent oestrus promptly returns, indirect evidence that the cycles are sustained through low-grade activation of the corpora lutea by prolactin. Further evidence of this is now presented by demonstration that administration of extrinsic prolactin at the time of the bromocriptine injection overcomes the blockage, apparently rescuing the corpora lutea and allowing completion of the cycle with renewed luteinization. Whereas bromocriptine treatment shortens the current dioestrus, the added prolactin restores the duration to at least that shown by 'LH-only' controls. In a related experiment, when persistent oestrus was interrupted by an injection of phenobarbitone (without corpus luteum formation), most of the rats remained dioestrous for only 2 days. In a few rats, this was followed by spontaneous completion of an ovulatory 4-day cycle. Hence, in the 'LH-only' group the small number of 4-day cycles may actually have gone to completion without dependence on the original set of corpora lutea. In some of the 'LH-only' group, a possible relationship was examined between the prolactin level at the time of LH injection and the subsequent ability of the rat to sustain multiple cycles.(ABSTRACT TRUNCATED AT 250 WORDS)