Question

Metoclopramide produces its pharmacological effects through ________.

• A. \(\text{5-HT agonist}\)
• B. \(\text{dopamine D}_2 \text{ antagonist}\)
• C. \(\text{H}_1 \text{ antagonist}\)
• D. \(\text{muscarinic antagonist}\)

Hint:

Metoclopramide Summary: - Primary Class: $D_2$ receptor antagonist. - Core Uses: Antiemetic, diabetic gastroparesis (prokinetic). - Caution: Can cause extrapyramidal side effects (dystonia, parkinsonian symptoms) due to central dopamine blockade.

Answer 1

The Correct Option is B

Concept: Metoclopramide is a widely used prokinetic and antiemetic agent. It exerts its therapeutic effects through a dual mechanism of action targeting receptors in both the central nervous system and the gastrointestinal tract:
• Central Mechanism: It blocks Dopamine $D_2$ receptors in the Chemoreceptor Trigger Zone (CTZ) of the medulla oblongata, preventing dopamine-mediated activation of the emetic reflex.
• Peripheral Mechanism: It blocks peripheral enteric $D_2$ receptors and exhibits partial $5\text{-HT}_4$ receptor agonist properties, which enhances acetylcholine release from myenteric neurons. This stimulates upper GI tract motility and accelerates gastric emptying.

Step 1: Select the primary mechanism from the choices.
The primary mechanism responsible for metoclopramide's antiemetic effect is its action as a dopamine $D_2$ antagonist, making option B the correct answer.