Abstract
To examine the suppressive effect of suckling stimulus and ovarian progesterone and their interaction on the resumption of ovarian cyclicity during lactation, peripheral plasma progesterone levels and the length of diestrous periods were studied by using lactating rats of which post-partum ovulation was blocked by anti-LH serum. Plasma progesterone levels were less than 5.6ng/ml in the rats treated with antiserum despite of their size of litter that had been nursed. The diestrous period in antiserum treated rats nursing 12 pups (15.4±1.4 days) was shorter than that in the intact rats nursing the same number of pups (26.2±1.1 days). In the rats nursing 2 pups, antiserum treatment also shortened the di-estrous period (6.5±0.1 days) and this value was the same as those in the intact or anti-serum treated rat of which pups were removed immediately after parturition.
Plasma progesterone levels before and after removal of pups were studied in antiserum treated rats nursing 12 pups up to 10 days after parturition. They were received progester-one injection from 3 to 10 or to 16 days after parturition and were deprived of their litter 10 days after parturition. When progesterone was injected from 3 to 16 days after parturi-tion, the diestrous period following removal of pups was prolonged to 8.7 days. Cessation of progesterone treatment 10 days after parturition markedly shortened the diestrous period following removal of pups (3.4 days) and reduced the plasma progesterone to the basal level on the day following removal of pups.
