In this article we will discuss about:- 1. Definition of Prolactin 2. Actions of Prolactin 3. Control of Prolactin Secretion 4. Applied Aspects 5. Deficiency.
Definition of Prolactin:
Prolactin (PRL) is a 199-amino acid single-chain protein and has considerable structural similarity to human growth hormone and hCS.
The Prolactin receptor resembles the GH receptor and belongs to the cytokine receptor superfamily. Therefore, PRL acts through a JAK/STAT signaling pathway.
Prolactin circulates unbound to serum proteins and thus has a relatively short half-life of about 20 minutes.
Actions of Prolactin:
i. During pregnancy, Prolactin levels increase steadily until term and under the influence of this hormone plus the high levels of estrogens and progesterone, full lobuloalveolar development of the breasts takes place.
ii. Prolactin causes milk secretion from the breast after estrogen and progesterone priming. After expulsion of the placenta at parturition, there is an abrupt decline in circulating estrogens and progesterone. The drop in circulating estrogen initiates lactation. PRL and estrogen synergize in producing breast growth but estrogen antagonizes the milk- producing effect of PRL on the breast. In addition, PRL facilitates the maternal behavior.
iii. Prolactin inhibits GnRH secretion and antagonizes the action of gonadotropins on the ovaries. Ovulation is inhibited, and the ovaries are inactive, so estrogen and progesterone output falls to low levels. Nursing stimulates PRL secretion; consequently, nursing is associated with lactational amenorrhea, which refers to cessation of menstrual periods in women who frequently and regularly nurse a baby. This effect of prolactin has been called “nature’s contraceptive” and hence, nursing is a natural but unreliable method of birth control.
iv. The function of PRL in normal males is unsettled but hyperprolactinemia in men is associated with impotence and hypogonadism that disappear when PRL secretion is reduced.
Control of Prolactin Secretion:
Prolactin is unique among the anterior pituitary hormones in that its release is normally under tonic inhibition by the hypothalamus. This is exerted by dopaminergic tracts that secrete dopamine (prolactin-inhibiting hormone, PIH) at the median eminence. The cell bodies of these dopaminergic neurons are located in the arcuate nuclei. Dopamine binds to the D2 receptor, which is linked to a Gi signaling pathway.
Lactotropes increase in size and number during pregnancy. The human pituitary enlarges two to three-folds in volume during pregnancy. The increase in lactotropes is due to stimulation by placental estrogen in response to estrogens.
Prolactin is secreted continuously at low basal rates throughout life, regardless of sex. It is one of the many hormones released in response to stress. Surgery, fear, stimuli causing arousal and exercise are all effective stimuli. As with GH, sleep increases Prolactin secretion and Prolactin has a pronounced sleep-associated diurnal rhythm.
Applied Aspects of Prolactin:
The prolactinoma (Prolactin -secreting tumor) is the most common form of hormone-secreting pituitary tumor. Hyperprolactinemia in women is associated with oligomenorrhea or amenorrhea and infertility. GnRH release, the gonadotrope response to GnRH and the ovarian response to LH all decrease. The hypogonadism produced by prolactinomas is associated with osteoporosis due to estrogen deficiency.
Hyperprolactinemia can produce infertility in men. While breast enlargement can occur, true gynecomastia (inappropriate growth of mammary glandular tissue) and galactorrhea (inappropriate flow of milk) are rare.
As Prolactin synthesis and secretion is predominantly under inhibitory control by the hypothalamus, disruption of the pituitary stalk and the hypothalamo-hypophyseal portal vessels (e.g., due to surgery or physical trauma) results in an increase in PRL levels.
ii. Chiari-Frommel Syndrome:
This is a rare condition in which there is persistence of lactation (galactorrhea) and amenorrhea in women who do not nurse after delivery. It is due to persistent prolactin secretion without the secretion of the FSH and LH necessary to produce maturation of new follicles and ovulation.
The only pathologic problem known to be associated with a deficiency in PRL secretion is the inability to initiate postpartum lactation.