Prolactin

Prolactin is a peptide hormone primarily produced and secreted by lactotroph cells in the anterior pituitary gland. It plays a crucial role in various physiological processes, most notably in lactation and reproductive functions. Prolactin is involved in the regulation of breast development and milk production postpartum, but it also exerts multiple effects on metabolism, immune regulation, and behavior.

Structure and Synthesis

Prolactin is a single-chain polypeptide composed of approximately 198 amino acids in its mature form, with a molecular weight of about 23 kDa. It belongs to the family of class I cytokines, sharing structural similarities with growth hormone and placental lactogen. The gene coding for prolactin is located on chromosome 6 in humans.

The synthesis of prolactin occurs in specialized lactotroph cells within the anterior pituitary. Its release is primarily controlled by hypothalamic factors, with dopamine acting as the main inhibitory regulator through D2 receptors. Other hypothalamic peptides such as thyrotropin-releasing hormone (TRH) and vasoactive intestinal peptide (VIP) stimulate prolactin secretion. Prolactin is released into the bloodstream in a pulsatile manner, with levels varying according to physiological states.

Physiological Functions

Lactation and Mammary Gland Development

The most well-known function of prolactin is its role in the initiation and maintenance of lactation. During pregnancy, prolactin promotes the growth and differentiation of mammary alveolar epithelium, preparing the breast tissue for milk production. After childbirth, prolactin stimulates the synthesis of milk proteins and lipids essential for nourishing the newborn. Prolactin acts synergistically with other hormones such as estrogen, progesterone, and oxytocin to ensure effective milk secretion.

Reproductive Functions

Prolactin influences reproductive physiology beyond lactation. Elevated prolactin levels can suppress the hypothalamic-pituitary-gonadal axis by inhibiting gonadotropin-releasing hormone (GnRH) secretion, leading to reduced secretion of luteinizing hormone (LH) and follicle-stimulating hormone (FSH). This suppressive effect can result in hypogonadism, menstrual irregularities, and infertility, particularly when prolactin is excessively elevated (hyperprolactinemia).

Metabolic and Immune Effects

Prolactin receptors are widely expressed in various tissues, including the liver, adipose tissue, and immune cells, indicating broader systemic roles. Prolactin modulates lipid and carbohydrate metabolism and has been implicated in immune regulation, potentially enhancing immune cell proliferation and cytokine production. It may also influence osmoregulation and behavior, including parental and sexual behaviors.

Regulation of Prolactin Secretion

Prolactin secretion is unique among pituitary hormones because it is predominantly under inhibitory control by dopamine, which is released from the hypothalamus and acts on D2 receptors on lactotrophs to suppress prolactin release. When dopamine secretion decreases, prolactin secretion increases.

Stimulatory factors include:

• Thyrotropin-releasing hormone (TRH)
• Vasoactive intestinal peptide (VIP)
• Estrogens, which sensitize lactotrophs and increase prolactin gene expression
• Suckling stimulus during breastfeeding, which inhibits dopamine release, thereby enhancing prolactin secretion to maintain milk production

The circadian rhythm also influences prolactin levels, with peak concentrations typically occurring during sleep.

Clinical Significance

Hyperprolactinemia

An abnormally high serum prolactin level can result from various causes including pituitary adenomas (prolactinomas), hypothyroidism, renal failure, certain medications (e.g., antipsychotics, dopamine antagonists), and chest wall injuries. Hyperprolactinemia commonly leads to galactorrhea (inappropriate milk secretion), amenorrhea, infertility in women, and hypogonadism in men.

Hypoprolactinemia

Although less common, low prolactin levels may be observed in cases of pituitary insufficiency or damage to the lactotroph cells, potentially impairing lactation postpartum.

Diagnostic Measurement

Serum prolactin measurement is used to evaluate suspected pituitary disorders and reproductive dysfunction. Normal levels vary by sex and physiological state, with higher baseline levels in women, especially during pregnancy and lactation.

Prolactin Receptors and Signal Transduction

Prolactin exerts its effects by binding to the prolactin receptor (PRLR), a member of the cytokine receptor superfamily. PRLR exists in multiple isoforms generated by alternative splicing and is expressed in many tissues. Upon prolactin binding, the receptor undergoes dimerization, activating intracellular signaling cascades primarily via the Janus kinase 2 (JAK2) and signal transducer and activator of transcription 5 (STAT5) pathway. Other pathways such as MAPK and PI3K/Akt may also be involved, mediating gene transcription changes responsible for prolactin’s diverse biological effects.

Summary of Key Points

Prolactin remains a hormone of considerable clinical and physiological importance due to its diverse roles, particularly in reproduction and lactation, as well as its emerging functions in metabolism and immune regulation. Understanding its regulation and actions is critical in diagnosing and managing disorders related to pituitary function and reproductive health.