Prolactin

 

 

Function

Prolactin's principal function is promoting milk production in lactating women. Levels are tonically elevated for the first three weeks after birth, after which levels fall to a lower but still elevated level. Increased pulsatile prolactin secretion is maintained as long as the mother continues to nurse.

Its effects include:

It is expressed in men as well, but its effects are not known.

 

Regulation of Expression

Prolactin is produced in the anterior pituitary by lactotrophs. It is constitutively released unless inhibited by dopamine, released by the arcuate nucleus in the hypothalamus.

Suckling is the most powerful physiologic cause of prolactin release, acting through spinal pathways to inhibit dopaminergic neurons in the hypothalamus. Treating women with dopamine-receptor agonists rapidly inhibits prolactin secretion and milk production. Prolactin releasing hormones from the hypothalamus include VIP and PHI-27.

 

Prolactin releasing factors include TRH, angiotensin II, substance P, beta-endorphin, and ADH. Estradiol increases prolactin secretion by increasing lactotroph sensitivity to TRH and decreasing sensitivity to dopamine. However, estrogen also blocks prolactin activity at the breast.

 

Consequences of Overexpression

 

Hyperprolactinemia is most commonly caused by prolactinomas of the anterior pituitary.

Increased levels lead to amenorrhea, galactorrhea, loss of libido, and infertility.

 

Pathological hyperprolactinemia can result from lactotroph hyperplasia in the pituitary, as a result to inhibitory hypothalamic dopamine neurons.

 

Several classes of drugs can increase prolactin levels, including:

Renal failure, estrogens, and hypothyroidism, Cushing's disease, and acromegaly, can also cause elevated prolactin levels.

 

Hyperprolactinemia reduces estrogen, leading to long term osteoporosis and a loss of cardioprotection.

 

Normal LH/FSH but without normal pulsatility and the LH surge = no estrogen and anovulation, though the menstrual cycle can still be occurring.

 

can inhibit granulosa cells, preventing aromatase conversion to estradiol

shortens luteal phase.

 

 

Consequences of Underexpression

I haven't read anything about this, but I'd imagine it would cause decreased milk production...

 

 

Pharmacologic Interventions

 

 

Signalling Pathways

Both PRL and GH have similiar affinities for the tyrosine kinase-associated PRL receptor. The JAK family of kinases is involved in mediating its effects, which include transcription of milk proteins such as lactalbumin and casein.

 

Structure

Prolactin is a 199 amino acid hormone with 16% homology to growth hormone.