Prolactin (Lactogenic hormone)
Prolactin (Lactogenic hormone) or PRL is a single‑chain peptide consisting of 199 amino acids and three disulfide bridges.
Prolactin (Lactogenic hormone) is a necessary hormone for maintenance of Corpus luteum in rat, dog.
PRL is structurally similar to GH and has many overlapping biological functions with GH. The most important influence of PRL secretion is the combination of pregnancy, estrogen and nursing of newborn. PRL secretion increases steadily during pregnancy.
PRL secretion rises at night similar to Growth Hormone. Suckling inhibits dopamine (PIF) secretion, thus stimulates PRL release.
TRH and VIP stimulate PRL synthesis and release and Dopamine, catecholamines, LH ‑ RH inhibits PRL release. PRL is the principle hormone responsible for lactogenesis(milk production).
PRL together with estrogen, progestrone, cortisol and GH cause proliferation of duct system of the mammary gland. During pregnancy prolactin, estrogen and progesterone cause development of secretory alveoli in the mammary gland thus causes initiation and maintenance of lactation in domestic animals.
PRL level increases just prior to parturition and after parturition, milk synthesis and secretion require PRL along with cortisol and insulin. PRL induces enzymes necessary for lactose synthesis. It provokes synthesis of milk constituents including lactalbumin, casein, and lipids. Prolactin increases in the peripheral blood of the cow during milking.
In pigeon it causes the crop gland to hypertrophy and stimulates the secretion of crop milk (this is used in bioassay of prolactin).
In ewes, prolactin stimulates corpus luteum referred to as the luteotropic effect; hence it is also referred as luteotropin .
An excess of PRL blocks the synthesis and release of LH‑RH which inhibits gonadotropin (FSH/LH) secretions from the pituitary and prevents ovulation and spermatogenesis.
High PRL concentration also inhibits the synthesis of gonadal steroids both in male and female.
In animals PRL provides productive maternal behavior toward the newborn. Broodiness in birds is controlled by PRL.
Prolactin may have a growth mediating role similar to GH and reduces lipid synthesis and Helps in partitioning of nutrients.