Presynaptic inhibition of norepinephrine release is a function of which receptor subtype?

• A. Beta 2 Receptors
• B. Beta 1 Receptors
• C. Alpha 1 Receptors
• D. Alpha 2 Receptors

Answer: (D)

Presynaptic inhibition of norepinephrine release is achieved by alpha-2 adrenergic receptors on the sympathetic nerve terminal. When these receptors are activated, they couple to Gi proteins, which inhibits adenylyl cyclase and lowers cAMP. This reduces the opening of voltage-gated calcium channels, so less Ca2+ enters the terminal to trigger vesicle fusion. With reduced calcium, fewer norepinephrine-containing vesicles are released, providing a negative feedback mechanism that dampens sympathetic outflow. Clinically, alpha-2 agonists like clonidine decrease NE release and sympathetic activity through this pathway. The other receptor subtypes don’t serve this presynaptic inhibitory role: alpha-1 receptors are postsynaptic and promote contraction via Gq signaling, while beta receptors modulate responses after release rather than acting as the primary brake on NE release.