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In female rats bearing an average litter of 10 ± 0·28 fetuses, pituitary prolactin is secreted in a pattern of two daily surges during the first 10 days of pregnancy only. When the number of fetuses was experimentally reduced, prolactin surges continued for a longer period and pregnancy was correspondingly lengthened. The relationship found between a reduced number of fetuses, and delay in termination of prolactin surges further supports the view that, during pregnancy, rat placental lactogen exerts a major influence in inhibiting secretion of prolactin by the maternal pituitary gland. The constant relationship found in all groups between termination of nocturnal surges of prolactin and parturition indicates that it is the interval between implantation and termination of prolactin surges which is prolonged in female rats with small litters.
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Androgen-sterilized female rats were obtained by administering 10 μg testosterone propionate to pups on day 2 after birth. In contrast with ovariectomized adults, androgenized adult female rats are incapable of responding to cervical stimulation by secreting prolactin in the nocturnal surge pattern. In spite of the loss of this pattern the androgenized female rats still exhibited a daily circadian rhythm of prolactin secretion with afternoon levels three times higher than those after midnight.
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At mid-pregnancy in the rat, episodic secretion of pituitary'prolactin ceases when the placenta is sufficiently developed. At this time, sufficient placental lactogen is secreted by the placenta to inhibit prolactin secretion. The present study tested whether the fully developed placenta at mid-pregnancy can inhibit prolactin secreted by a donor pituitary gland implanted under the kidney capsule. Three pituitary glands were implanted in rats on day 7 of pregnancy; muscle fragments were implanted in controls. Blood was collected during the first and second halves of pregnancy. It was found that prolactin concentrations in the animals with the pituitary implants were high on days 9 and 10 of pregnancy and remained raised during the second half of pregnancy while in control animals nocturnal surges were absent in the second half of pregnancy, the last one occurring on day 10. This observation indicates that the placental hormone cannot act directly on the pituitary gland to inhibit prolactin secretion, but presumably exerts its suppressive effect on prolactin secretion through the hypothalamus.
