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Control of Prolactin Secretion
In contrast to what is seen with all the other pituitary hormones, the hypothalamus tonically suppresses prolactin secretion from the pituitary. In other words, there is usually a hypothalamic "brake" set on the lactotroph, and prolactin is secreted only when the brake is released. 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.
In addition to tonic inhibition by dopamine, prolactin secretion is positively regulated by several hormones, including thyroid-releasing hormone, gonadotropin-releasing hormone and vasoactive intestinal polypeptide. 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. The 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.
Disease States
Excessive secretion of prolactin - hyperprolactinemia - is a relative common disorder in humans. This condition has numerous causes, including prolactin-secreting tumors and therapy with certain drugs.
Common manifestations of hyperprolactinemia in women include amenorrhea (lack of menstrural cycles) and galactorrhea (excessive or spontaneous secretion of milk). Men with hyperprolactinemia typically show hypogonadism, with decreased sex drive, decreased sperm production and impotence. Such men also often show breast enlargement (gynecomastia), but very rarely produce milk.
In contrast to what is seen with all the other pituitary hormones, the hypothalamus tonically suppresses prolactin secretion from the pituitary. In other words, there is usually a hypothalamic "brake" set on the lactotroph, and prolactin is secreted only when the brake is released. 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.
In addition to tonic inhibition by dopamine, prolactin secretion is positively regulated by several hormones, including thyroid-releasing hormone, gonadotropin-releasing hormone and vasoactive intestinal polypeptide. 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. The 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.
Disease States
Excessive secretion of prolactin - hyperprolactinemia - is a relative common disorder in humans. This condition has numerous causes, including prolactin-secreting tumors and therapy with certain drugs.
Common manifestations of hyperprolactinemia in women include amenorrhea (lack of menstrural cycles) and galactorrhea (excessive or spontaneous secretion of milk). Men with hyperprolactinemia typically show hypogonadism, with decreased sex drive, decreased sperm production and impotence. Such men also often show breast enlargement (gynecomastia), but very rarely produce milk.
