CIBA FOUND.SYMPOSIUM NO.45: BREAST FEEDING AND THE MOTHER

Lactation is usually differentiated into three stages - mammary growth, initiation of milk secretion and maintenance of established milk secretion. Each stage is regulated by a complex of metabolic and reproductive hormones: prolactin, growth hormone, corticoids, thyroxin, insulin and, especially for mammary growth, oestrogens and progesterone. For lactation to be maintained the mammary gland must also be stimulated by suckling (or milking) and ejection of milk. The effects of hormones on established lactation differ from species to species. Prolactin seems to play a central part in the hormonal control of lactation. Suckling, milking or other stimulation of the teat seems to be an adequate physiological stimulus for the release of prolactin and of oxytocin (which is essential for milk ejection). Two compounds, both of which are potent in human and animals, are helpful in investigations of the physiological role of prolactin in lactation in vivo: a specific inhibitor of the release of prolactin, the synthetic ergot alkaloid bromocriptine (2-bromo-α-ergocriptine, CB154), and a stimulator, the synthetic tripeptide of hypothalamic origin - thyroliberin (TRH). Experiments with these compounds indicate that large concentrations of prolactin in the circulation (especially after milking) are not needed for lactation to be maintained in ruminants but that prolactin is essential for the onset of lactation in these species. In contrast, prolactin is an essential hormone for lactogenesis and galactopoiesis in monogastric species like the rabbit, rat, pig, dog and human. The suckling stimulus is necessary for the maintenance of lactation in these species.