A researcher is studying the effects of clonidine at presynaptic alpha-2 receptors on a sympathetic nerve terminal. Activation of these receptors will:
- A Increase cAMP via Gs coupling and augment norepinephrine release
- B Decrease cAMP via Gi coupling and inhibit norepinephrine release ✓
- C Activate phospholipase C, increasing IP3 and intracellular calcium
- D Open potassium channels causing membrane depolarization
Correct answer: B. Decrease cAMP via Gi coupling and inhibit norepinephrine release
Explanation
Presynaptic alpha-2 receptors are coupled to Gi proteins; their activation inhibits adenylyl cyclase, lowers cAMP, and reduces norepinephrine exocytosis (negative feedback autoreceptor). Clonidine acts at both central (locus coeruleus) and peripheral presynaptic alpha-2 sites to diminish sympathetic outflow. Alpha-2 does not signal through Gq/phospholipase C, and its channel effects (opening K+ channels) cause hyperpolarization, not depolarization.
Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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