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The effect of suckling and breast stimulation on plasma prolactin measured by homologous radioimmunoassay has been studied in nursing women, in normal, nonlactating women and men, in chlorpromazine treated women, in pregnant women, and in women with galactorrhea. Nursing women had increased plasma prolactin before nursing during the first 6 weeks postpartum; mean prolactin rose rapidly throughout the period of suckling and reached a peak 8.5 times the baseline value just after the termination of nursing. Women studied between 7 and 28 weeks postpartum had normal prolactin levels before nursing; with suckling they had smaller but significant elevations, averaging 6.0 times the baseline value at the end of nursing. When nursing women were allowed to play with but not nurse their infants, prolactin did not rise in spite of the occurrence of milk let-down. Substitution of a breast pump for the infant at a regular period of nursing caused prolactin elevations similar in timing and in magnitude to those occurring during suckling. Stimulation of the breast with a breast pump for 30 min produced significant prolactin increases in 7 out of 18 normal women and failed to increase prolactin in normal men. Self-administered stimulation of the nipple and areola also produced prolactin increases in a minority of normal women. Stimulation of nursing in normal women by frequent daily periods of breast stimulation for 2 to 14 days produced no increase in basal prolactin or in prolactin response to breast manipulation in 2 subjects. Chronic chlorpromazine treatment raised basal prolactin but did not enhance the sensitivity to breast stimulation in 4 subjects. Breast pump stimulation caused prolactin rises in all of 3 pregnant women and induced lactation in 1 woman in the seventh month of pregnancy. Of 9 women with galactorrhea, only 1 had increased prolactin after breast pump or manual stimulation. We conclude that sensory stimuli arising from mechanical stimulation of the breast can release prolactin in a minority of normal women; after pregnancy and parturition the effect is seen in all women, and persists with diminishing force as nursing is continued. While estrogenic stimulation during pregnancy may sensitize the breast and pituitary to facilitate prolactin release around the time of delivery, estrogen differences cannot explain the differences in sensitivity seen among normal women, or the persistence of sensitivity in nursing mothers many weeks postpartum.
NOEL et al. (Fri,) studied this question.