Prolactin

Prolactin (PRL) is a 199 amino acid hormone that is synthesized and secreted by the anterior pituitary and other tissues. It was discovered in the early 1900s and was named based on its role in lactation. PRL is the only pituitary hormone regulated by the hypothalamus, mainly by tonic inhibition. Inhibition of PRL action is mainly mediated by dopamine, while thyrotropin-releasing hormone, vasoactive intestinal peptide, serotonin, and oxytocin act as PRL-releasing factors. Pituitary PRL secretion lacks the classical target hormone feedback system, as its primary target, the mammary gland, is an exocrine gland. PRL also controls its own secretion through a short negative-feedback loop that involves the tuberoinfundibular system. PRL actions are mediated by its receptors, derived by differential splicing from a single gene and whose expression is tissue specific. While its principal role is to stimulate proliferation and differentiation of mammary cells required for lactation, studies in animal models have attributed multiple actions in different species, and novel findings in this field have accumulated in recent years. PRL expression in a variety of extra-pituitary tissues explains its involvement in regulation of reproduction, immune function, metabolism, osmoregulation, and tumorigenesis, even though mechanisms of action have yet to be better elucidated, especially in humans. In clinical practice, the most relevant condition involving PRLR is hyperprolactinemia; PRL deficiency is very rare and mostly associated with other pituitary defects. Clinical and laboratory investigations are important to determine the etiology of hyperprolactinemia.