Prolactine, a protein that in humans is best known for its role in enabling females, to produce milk. It is sometimes classified as a gonadotropin although in humans it has only a weak luteotropic effect while the effect of suppressing classical gonadotropic hormones is more important. In humans, prolactin (Pic. 1) is produced at least in the anterior pituitary, decidua, myometrium, breast, lymphocytes, leukocytes and prostate. Prolactin within the normal reference ranges (Pic. 2) can act as a weak gonadotropin but at the same time suppresses GnRH secretion. Dopamine (DA) holds a predominant role in the regulation of prolactin (PRL) secretion. DA provides inhibitory control over the secretion of prolactin (Pic. 3). Physiologic levels of prolactin in males enhance luteinizing hormone-receptors in Leydig cells, resulting in testosterone secretion, which leads to spermatogenesis.
Prolactin has a wide range of effects:
- Stimulates the mammary glands to produce milk (lactation): increased serum concentrations of prolactin during pregnancy cause enlargement of the mammary glands of the breasts and prepare for the production of milk. Milk production normally starts when the levels of progesterone fall by the end of pregnancy and a suckling stimulus is present.
- Provides the body with sexual gratification after sexual acts: The hormone counteracts the effect of dopamine, which is responsible for sexual arousal. This is thought to cause the sexual refractory period. The amount of prolactin can be an indicator for the amount of sexual satisfaction and relaxation. Unusually high amounts are suspected to be responsible for impotence and loss of libido.
Efect of hormonal action during pregnancy and after childbirth:
- During pregnancy, high circulating concentrations of estrogen and progesterone increase prolactin levels by 10- to 20-fold. However, at the same time, estrogen, as well as progesterone, inhibit the stimulatory effects of prolactin on milk production. It is the abrupt drop of estrogen and progesterone levels following delivery that allows prolactin — which temporarily remains high — to induce lactation.
- After childbirth, prolactin levels fall as the internal stimulus for them is removed. Sucking by the baby on the nipple then promotes further prolactin release, maintaining the ability to lactate. The sucking activates mechanoreceptors in and around the nipple. These signals are carried bynerve fibers through the spinal cord to the hypothalamus, where changes in the electrical activity of neurons that regulate the pituitary gland cause increased prolactin secretion. The suckling stimulus also triggers the release of oxytocin from the posterior pituitary gland, which triggers milk let-down: Prolactin controls milk production (lactogenesis) but not the milk-ejection reflex; the rise in prolactin fills the breast with milk in preparation for the next feed.
High prolactin levels can also contribute to mental health issues
Prolactin levels peak during REM sleep (the stage of sleep during which most brain activity and dreams occur), and in the early morning. Levels can rise after:
- minor surgical procedures
- following epileptic seizures
- physical or emotional stress
Prolactin levels have also been found to rise with use of the drug MDMA (Ecstasy), leading to speculation that prolactin may have a role in the post-orgasmic state as well as decreased sexual desire.
Prolactin receptors are present in the mammillary glands, ovaries, pituitary glands, heart, lung, thymus, spleen, liver, pancreas, kidney, adrenal gland, uterus, skeletal muscle, skin, and areas of the central nervous system. When prolactin binds to the receptor, it causes it to dimerize with another prolactin receptor.
Hypersecretion of prolactin is more common than hyposecretion. Hyperprolactinemia is the most frequent abnormality of the anterior pituitary tumors, termed prolactinomas. Prolactinomas may disrupt the hypothalamic-pituitary- gonadal axis as prolactin tends to suppress the secretion of gonadotropin-releasing hormone (GnRH) from the hypothalamus and in turn decreases the secretion of follicle-stimulating hormone (FSH) and luteinizing hormone (LH) from the anterior pituitary, therefore disrupting the ovulatory cycle in females. Such hormonal changes may manifest as amenorrhea and infertility in females as well as impotence in males. Inappropriate lactation (galactorrhoea) is another important clinical sign of prolactinomas, which may be treated with the administration of bromocriptine, which is an agonist of the dopamine D2 receptor.
Hyperprolactinaemia, or excess serum prolactin, is associated with hypoestrogenism, anovulatory infertility, oligomenorrhoea, unexpected lactation, and loss of libido in women, and erectile dysfunction and loss of libido in men.