Stimulation of alpha-2 adrenoreceptors in the central nervous system produces which physiological effect that makes clonidine useful as an antihypertensive?

• A Peripheral vasoconstriction via alpha-2 receptors on vascular smooth muscle
• B Presynaptic inhibition of norepinephrine release from peripheral sympathetic terminals
• C Activation of central inhibitory alpha-2 receptors in the rostral ventrolateral medulla reducing sympathetic outflow ✓
• D Stimulation of cardiac vagal centers increasing heart rate and reducing cardiac output

Explanation

Clonidine acts as an alpha-2 agonist at presynaptic/autoreceptor sites in the rostral ventrolateral medulla (RVLM) — the primary sympathetic cardiovascular control center. Activating these inhibitory receptors reduces central sympathetic outflow to the heart and blood vessels, lowering heart rate, cardiac output, and total peripheral resistance. Option B describes peripheral presynaptic alpha-2 effects, which contribute minimally at antihypertensive doses. Option A is actually a vasoconstrictor effect of peripheral alpha-2 stimulation. Option D is incorrect as clonidine reduces (not increases) heart rate.

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

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