A researcher studies the effect of a drug that selectively blocks presynaptic α2-adrenoreceptors at noradrenergic terminals. Which of the following best describes the expected physiological consequence?

• A Decreased norepinephrine release from the terminal due to loss of autoreceptor-mediated inhibition
• B Increased norepinephrine release because autoinhibitory feedback is removed ✓
• C Activation of postsynaptic Gi protein leading to decreased cAMP in target cells
• D Selective blockade of α2 postsynaptic heteroreceptors causing vasodilation

Explanation

Presynaptic α2-adrenoreceptors act as autoreceptors that sense ambient norepinephrine and, when activated, inhibit further neurotransmitter release via Gi-protein-coupled reduction in cAMP. Blocking these receptors removes autoinhibitory feedback, increasing norepinephrine release. Option A reverses the physiology. Option C describes what happens when α2 receptors are activated, not blocked. Option D confuses presynaptic autoreceptors with heteroreceptors on non-noradrenergic terminals.

Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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