Adrenergic Transmission: Noradrenaline from Synthesis to Reuptake
The cholinergic system destroys its transmitter the instant it's used. The adrenergic system does something cleverer — it sucks the transmitter back up to use again. That one difference, reuptake, is why cocaine causes a pounding heart and blown pupils, why some antidepressants work, and why a plate of aged cheese can be deadly for the wrong patient. Follow noradrenaline through its life cycle and these puzzles solve themselves.
A young man is brought to the emergency room after using cocaine: his heart is racing dangerously, his blood pressure is sky-high, his pupils are wide, and he's sweating and agitated. It looks exactly like a massive sympathetic 'fight-or-flight' surge — because that's precisely what it is. Cocaine didn't add any adrenaline to his body. Instead, it blocked the single most important step in adrenergic transmission: the reuptake that normally clears noradrenaline away. With the cleanup jammed, his own noradrenaline keeps hammering its receptors over and over. To understand him — and half of adrenergic pharmacology — we trace noradrenaline from birth to reuptake.
The noradrenaline life cycle
This is the single biggest difference between the two transmitter systems. Acetylcholine is destroyed in the synapse by an enzyme (AChE); noradrenaline is mostly recycled by reuptake. That's why the star drug targets differ: for cholinergic drugs, it's the enzyme (anticholinesterases); for adrenergic drugs, it's the reuptake transporter (cocaine, tricyclic antidepressants) and the storage vesicle (reserpine, amphetamine).
Cocaine blocks the noradrenaline reuptake transporter (NET), so every pulse of noradrenaline the sympathetic nerves release lingers far longer and keeps re-stimulating its receptors. The α1 receptors on vessels drive up blood pressure, the β1 receptors race the heart, and the α1 in the eye dilate the pupils — a full-body sympathetic storm produced not by extra transmitter, but by a blocked exit. That's why treatment leans on calming the sympathetic drive rather than adding to it.
- NE synthesis: tyrosine → DOPA (tyrosine hydroxylase = rate-limiting) → dopamine → noradrenaline.
- Termination is mainly REUPTAKE (NET), not enzymatic destruction in the cleft.
- MAO & COMT then break down surplus noradrenaline.
- Cocaine/tricyclics block reuptake; reserpine depletes stores; amphetamine forces release.
- The α2 autoreceptor is a negative-feedback brake on further release.
- Thinking an enzyme in the synapse ends the noradrenaline signal. Reuptake is the main mechanism.
- Forgetting tyrosine hydroxylase (not DOPA decarboxylase) is the rate-limiting synthesis step.
- Assuming cocaine adds catecholamines. It blocks their reuptake, amplifying the body's own.
- Overlooking the α2 autoreceptor — it's why α2 agonists REDUCE sympathetic output.
How does noradrenaline's signal mainly get terminated at the synapse?
- Noradrenaline is made from tyrosine (tyrosine hydroxylase = rate-limiting), stored, released, binds α/β receptors.
- Its signal ends mainly by REUPTAKE (NET), then MAO/COMT break down the surplus.
- Cocaine/tricyclics block reuptake; reserpine depletes stores; amphetamine/tyramine force release.
- Reuptake (not an enzyme) is the key difference from cholinergic transmission.
- Katzung BG. Basic & Clinical Pharmacology — Adrenergic transmission & noradrenaline metabolism.
- Brunton LL, et al. Goodman & Gilman's The Pharmacological Basis of Therapeutics — Adrenergic (noradrenergic) neurotransmission.
- Rang HP, Dale MM, et al. Rang & Dale's Pharmacology — Noradrenergic transmission.
- Guyton & Hall Textbook of Medical Physiology — Norepinephrine synthesis, uptake & metabolism.
- Whalen K. Lippincott Illustrated Reviews: Pharmacology — Adrenergic neurons & neurotransmission.
