Interestingly, mice with homozygous deletions of the prolactin gene fail to show significant abnormalities in immune responses. A considerable amount of research is in progress to delineate the role of prolactin in normal and pathologic immune responses. At the current time, however, the significance of these potential functions remains poorly understood. In contrast to what is seen with all the other pituitary hormones, the hypothalamus tonically suppresses prolactin secretion from the pituitary. If the pituitary stalk is cut, prolactin secretion increases, while secretion of all the other pituitary hormones fall dramatically due to loss of hypothalamic releasing hormones.
Dopamine serves as the major prolactin-inhibiting factor or brake on prolactin secretion.
Dopamine is secreted into portal blood by hypothalamic neurons, binds to receptors on lactotrophs, and inhibits both the synthesis and secretion of prolactin. Agents and drugs that interfere with dopamine secretion or receptor binding lead to enhanced secretion of prolactin. Stimulation of the nipples and mammary gland, as occurs during nursing, leads to prolactin release. This effect appears to be due to a spinal reflex arc that causes release of prolactin-stimulating hormones from the hypothalamus. Estrogens provide a well-studied positive control over prolactin synthesis and secretion , and increasing blood concentrations of estrogen during late pregnancy appear responsible for the elevated levels of prolactin that are necessary to prepare the mammary gland for lactation at the end of gestation.