Prolactin (PRL)

1. Overview

Prolactin is a peptide hormone primarily involved in the initiation and maintenance of lactation in females. It also plays roles in metabolism, reproductive health, immune function, and behavior. Although it is best known for its function in the female reproductive system, prolactin is present in both sexes and has a wide variety of physiological effects. It is part of the somatotropin/prolactin hormone family, which also includes growth hormone.

2. Location

Prolactin is synthesized and secreted by lactotroph cells in the anterior pituitary gland. Its release is under tonic inhibition by the hypothalamus, primarily via the neurotransmitter dopamine (also called prolactin-inhibiting hormone). When dopamine inhibition is reduced—such as during pregnancy or after childbirth—prolactin secretion increases. Small amounts of prolactin are also produced in other tissues like the uterus, immune cells, and adipose tissue, but their physiological relevance is limited.

3. Structure

Prolactin is a single-chain polypeptide composed of 199 amino acids in humans, with a molecular weight of approximately 23 kDa. It shares structural homology with growth hormone (GH) and human placental lactogen (hPL). It is encoded by the PRL gene on chromosome 6. Prolactin exerts its effects by binding to the prolactin receptor (PRLR), which belongs to the cytokine receptor superfamily and activates the JAK-STAT signaling pathway.

4. Function

Prolactin has multiple endocrine, metabolic, and behavioral functions:

• Lactation: Stimulates the mammary glands to produce milk after childbirth.

• Mammary gland development: Promotes growth and differentiation of breast tissue during pregnancy.

• Inhibits ovulation: Suppresses the hypothalamic-pituitary-gonadal axis by inhibiting GnRH, reducing LH and FSH secretion.

• Immunomodulation: Affects immune cell activity and may contribute to immune tolerance during pregnancy.

• Behavioral effects: Involved in maternal bonding, anxiety modulation, and potentially in sexual gratification.

5. Physiological role(s)

Prolactin contributes to several processes in both females and males:

• During pregnancy: High levels of prolactin prepare the breasts for lactation, although milk production is inhibited by high estrogen and progesterone levels until after delivery.

• After childbirth: Prolactin levels surge in response to suckling, stimulating milk synthesis and maintaining lactation.

• Fertility suppression: Elevated prolactin inhibits ovulation and menstruation during lactation (lactational amenorrhea), serving as a natural form of contraception.

• In males: Prolactin modulates Leydig cell function, testosterone production, and sexual behavior, although its exact role remains less defined than in females.

• Stress and immune function: Prolactin levels can rise in response to physical or emotional stress and may enhance immune responses under certain conditions.

6. Clinical Significance

Abnormal prolactin levels can lead to several medical conditions:

• Hyperprolactinemia:

  • Most common pituitary hormone disorder, often caused by prolactin-secreting tumors (prolactinomas).

  • Symptoms in women: amenorrhea, galactorrhea, infertility.

  • Symptoms in men: decreased libido, erectile dysfunction, gynecomastia, infertility.

  • Other causes include hypothyroidism, dopamine antagonist medications (e.g., antipsychotics), chest trauma, and pregnancy.

  • Treatment involves dopamine agonists like cabergoline or bromocriptine, and occasionally surgery or radiotherapy.

• Prolactinomas:

  • Benign pituitary tumors that overproduce prolactin.

  • Can cause mass effect symptoms like headaches or visual field defects if large.

  • Typically managed with medical therapy and monitoring.

• Hypoprolactinemia:

  • Rare and usually due to pituitary insufficiency or Sheehan syndrome.

  • May impair lactation postpartum but usually has minimal systemic effects.

• Fertility and contraception:

  • Prolactin’s natural suppression of ovulation during breastfeeding forms the basis of the lactational amenorrhea method (LAM) for contraception.