STUDIES CONCERNING MECHANISM CONTROLLING INITIATION OF LACTATION AT PARTURITION IV.INFLUENCE OF SUCKLING ON LACTOGEN CONTENT OF PITUITARY OF POSTPARTUM RABBITS1,2

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Abstract
The effects of the presence and absence of the nursing stimulus after parturition on the lactogen content of the pituitary were detd. in 72 New Zealand White rabbits. At 2 days postpartum the lactogen content of the pituitary of both suckled and non-suckled rabbits was the same; at 5 days postpartum there was 66% more hormone in the pituitaries of the suckled rabbits; at 10 days postpartum the pituitaries of the suckled rabbits contained 67% more lactogen; and at 20 days after parturition there was 100% more lactogenic hormone in the pituitaries of the suckled rabbits. The peak in lactogen content of the pituitary was reached in both nursed and non-nursed rabbits on the 5th day after parturition, after which it assumed a parallel decline. The pituitaries of the nursed rabbits were somewhat heavier in the nursed than those of the non-nursed does. The amt. of milk secretion seemed to parallel the lactogen content of the pituitary in the non-nursed does, and to a lesser extent, also in the nursed does. These studies indicate that in the rabbit the stimulus of nursing plays a role in the initiation of lactation to the extent that maximum, lactation and maximum pituitary lactogen cannot be obtained without it. Furthermore, in the absence of the suckling stimulus after parturition, the lactogen content of the pituitary declines rapidly and lactation soon ceases.