Prolactin

Introduction

• Source
  • lactotrophs in the anterior pituitary synthesize prolactin
    • during pregnancy there is an increase in lactotrophs
    • it is chemically similar to growth hormone
• Regulation
  • prolactin secretion is regulated by chemicals that alter prolactin gene transcription
    • stimulatory
      • thyrotropin-related hormone (TRH)
      • pregnancy
      • breastfeeding
        • suckling stimulates afferent fibers to inhibit hypothalamic dopamine secretion
      • dopamine antagonists such as anti-psychotics
      • sleep
      • stress
    • inhibitory
      • dopamine
        • this is a catecholamine that tonically inhibits prolactin secretion in non-pregnant and non-lactating patients 
        • dopamine mainly arises from hypothalamic dopaminergic neurons
          • there are also dopaminergic neurons in the posterior lobe of the pituitary
      • dopamine agonists
        • cabergoline 
        • bromocriptine
      • prolactin
        • it inhibits its own secretion via upregulating hypothalamic dopamine synthesis and secretion (negative feedback)
      • somatostatin
• Function
  • milk production (lactogenesis)
    • prolactin stimulates the production of milk components (e.g., lactose and casein)
      • during pregnancy elevated levels of estrogen and progesterone decrease prolactin receptor numbers in the breast
        • this is why lactation does not occur during pregnancy but at parturition
          • recall that at parturition estrogen and progesterone levels decrease
  • breast development
    • stimulates the mammary ducts to proliferate and branch
      • this is accomplished with the help of estrogen and progesterone
  • inhibits ovulation
    • prolactin inhibits gonadotropin-releasing hormone (GnRH) synthesis and secretion
      • this explains why fertility is decreased during breastfeeding
• Clinical correlate
  • hyperprolactinemia
    • etiology
      • prolactinoma
      • antipsychotics
    • symptoms
      • galactorrhea
      • infertility
    • treatment
      • dopamine agonists such as
        • cabergoline
        • bromocriptine