A pharmacology researcher studies the effects of a drug that selectively binds to presynaptic alpha-2 adrenoceptors on sympathetic nerve terminals. When given to a patient with hypertension, which hemodynamic profile is expected?

• A Increased blood pressure by stimulating postsynaptic alpha-2 receptors in vascular smooth muscle
• B Increased blood pressure by stimulating alpha-2 receptors in the brainstem causing increased sympathetic outflow
• C Decreased blood pressure by reducing norepinephrine release from peripheral sympathetic terminals ✓
• D No effect on blood pressure because alpha-2 receptors are only in the CNS

Explanation

Presynaptic alpha-2 autoreceptors serve as negative feedback regulators of norepinephrine release; their activation inhibits further NE exocytosis from sympathetic nerve terminals. A drug selectively targeting peripheral presynaptic alpha-2 receptors would reduce norepinephrine overflow into the synapse, decreasing vascular tone and blood pressure. This is distinct from the central alpha-2 agonist action of clonidine, which reduces sympathetic outflow from the brainstem. Postsynaptic vascular alpha-2 receptors do mediate vasoconstriction, but the presynaptic autoreceptor action predominates in NE regulation.

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

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