A patient on long-term dopamine agonist therapy for Parkinson's disease develops amenorrhoea and galactorrhoea. The mechanism involves dopamine's role as:

• A A direct stimulator of lactotroph cells when its agonist binds pituitary D2 receptors
• B A stimulator of GnRH from the hypothalamus reducing LH/FSH
• C An inhibitor of somatostatin allowing excess GH to suppress FSH
• D The physiological inhibitor (PIF) of prolactin secretion from lactotrophs ✓

Explanation

Dopamine is the principal prolactin-inhibiting factor (PIF) secreted by tuberoinfundibular neurons into the hypophyseal portal blood. It tonically suppresses prolactin release by binding D2 receptors on lactotroph cells. Paradoxically, dopamine agonists (like bromocriptine, cabergoline) used for Parkinson's disease mimic this inhibitory action and lower prolactin. The amenorrhoea-galactorrhoea in the question must be from a co-existing prolactinoma, not the drug itself. If the agonist is the cause stated, the scenario would be from dopamine antagonists (metoclopramide, antipsychotics) not agonists — the key teaching point is that dopamine is PIF.

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.