Question

Prolactin secretion will be inhibited by

• A. Haloperidol
• B. GABA
• C. Neurophysin
• D. Dopamine

Hint:

Dopamine is the hypothalamic prolactin inhibiting factor, acting on D2 receptors of lactotrophs.

Answer 1

The Correct Option is D

Step 1: Understanding the Question:
The question asks which substance normally inhibits the release of prolactin from the anterior pituitary gland.

Step 2: Key Formula or Approach:
Prolactin is unusual among pituitary hormones because its main control from the hypothalamus is inhibitory rather than stimulatory. The hypothalamus releases a substance called prolactin inhibiting factor, or PIF, into the portal blood supplying the pituitary, and this factor has been identified as dopamine.

Step 3: Detailed Explanation:
Dopamine travels down the tuberoinfundibular pathway from the hypothalamus to the anterior pituitary and binds to D2 receptors on lactotroph cells, the cells that make prolactin. This binding switches off prolactin gene expression and secretion. Anything that reduces dopamine, such as dopamine receptor blocking drugs, removes this brake and raises prolactin levels instead of lowering them.
Haloperidol is a dopamine receptor antagonist used in psychiatry, it blocks the very receptor dopamine uses, so it actually raises prolactin levels, one of its well known side effects, rather than lowering them.
GABA has some inhibitory effects on hormone release in general, but it is not the physiological prolactin inhibiting factor, dopamine holds that role.
Neurophysin is a carrier protein bound to oxytocin and vasopressin as they travel down the axon to the posterior pituitary, it has no role in prolactin control.

Step 4: Final Answer:
Dopamine is the physiological inhibitor of prolactin secretion, so the correct option is (D).