Norepinephrine

Beta-blockers antagonize beta-1 and beta-2 receptors which are the usual targets of the sympathetic nervous system, including epinephrine and norepinephrine.

Alpha receptor agonists are substances that activate the sympathetic nervous system by stimulation of alpha receptors, similar to the effects of epinephrine and norepinephrine.

Norepinephrine is a hormone that is mediated predominantly through the sympathetic nervous system and acts by stimulating alpha and beta receptors.

Clonidine is considered a centrally acting alpha-2 receptor agonist.