Which receptor types mediate sympathetic responses to norepinephrine?

• A. Muscarinic receptors
• B. Dopaminergic receptors
• C. Nicotinic receptors
• D. Adrenergic receptors (α and β)

Answer: (D)

Adrenergic receptors mediate sympathetic responses to norepinephrine. When norepinephrine is released from sympathetic nerve endings, it binds to receptors on target tissues that are part of the adrenergic receptor family, which includes alpha and beta subtypes. Activation of these receptors produces typical sympathetic effects: alpha-1 on vascular smooth muscle causes vasoconstriction, beta-1 in the heart increases heart rate and contractility, and beta-2 can promote bronchodilation and select vasodilations in some beds. Some alpha-2 receptors modulate neurotransmitter release and help regulate sympathetic tone.

Other receptor types don’t primarily mediate norepinephrine’s actions in the target organs. Muscarinic receptors respond to acetylcholine in the parasympathetic system, nicotinic receptors are involved at autonomic ganglia and the neuromuscular junction, and dopaminergic receptors respond to dopamine.