norepinephrine
Norepinephrine is the sympathetic system's main on-the-ground messenger. Where adrenaline is the alarm hormone dumped into the blood, norepinephrine is released right at the nerve endings onto target tissues, like a local courier rather than a national broadcast.
It is a catecholamine made from the amino acid tyrosine through dopamine, stored in vesicles in sympathetic postganglionic terminals, and released when those nerves fire. It activates adrenergic receptors, with strong action at alpha receptors and beta-1 receptors and relatively weak action at beta-2 receptors. Its action is ended mainly by reuptake back into the nerve terminal, with smaller contributions from the enzymes monoamine oxidase and catechol-O-methyltransferase.
As a drug, norepinephrine is given by intravenous infusion to raise dangerously low blood pressure in shock, mainly by tightening blood vessels through alpha receptors. Many other medicines work by interfering with its life cycle: blocking its release, its reuptake, its receptors, or the enzymes that degrade it.
In septic shock, a norepinephrine infusion is the usual first-line drug to restore blood pressure by constricting widely dilated vessels.
First-line vasopressor in shock.
Norepinephrine and adrenaline differ by a single chemical group, but that small change shifts adrenaline toward stronger beta-2 activity, which is why adrenaline opens airways more effectively while norepinephrine is the stronger pure vasopressor.