Central alpha-2 agonists
Stimulating alpha-2 receptors in the brainstem to cut sympathetic outflow.
Also called centrally acting alpha-2 agonists and central α2-agonists.
Class card
- Drugs
- Clonidine and Methyldopa
- Used for
- High blood pressure, not usually first-line
- High blood pressure in pregnancy (methyldopa)
- ADHD (clonidine)
- Opioid withdrawal symptoms (clonidine)
- Known for
- Drowsiness and sedation
- Dry mouth
- Rebound hypertension if clonidine is stopped suddenly
- Name stems
- -onidine
- Part of
- sympathomimetics
3 cards: the class, and each drug in it.
Mechanism of action
How do central alpha-2 agonists work?
They stimulate alpha-2 adrenergic receptors in the brainstem, which act as a brake on sympathetic nerve traffic. Less norepinephrine is released, so peripheral resistance, heart rate and blood pressure fall. Clonidine acts directly and also binds imidazoline I1 receptors; methyldopa is a prodrug whose metabolite, alpha-methylnorepinephrine, probably acts in the same way.
The drugs
Central alpha-2 agonists with a card
- ClonidineUnlike methyldopa it needs no conversion to work. It comes as tablets, a skin patch and an epidural infusion for cancer pain, and even small amounts swallowed by a child can be toxic.
- MethyldopaIts long record in pregnancy, with no clear harm found in follow-up of children whose mothers took it, keeps it a standard choice for high blood pressure in pregnancy.
Uses
What are central alpha-2 agonists used for?
- High blood pressure, not usually first-line
- High blood pressure in pregnancy (methyldopa)
- ADHD (clonidine)
- Opioid withdrawal symptoms (clonidine)
- Menopausal hot flashes (clonidine)
Side effects
Side effects of central alpha-2 agonists
- Drowsiness and sedationKnown for
- Dry mouthKnown for
- Rebound hypertension if clonidine is stopped suddenlyKnown forSerious
- Slow heart rate
- Heart block (AV block)Serious
- Dizziness on standing (orthostatic hypotension)
- Depression and nightmares
- Erectile dysfunction and low libido
- Hemolytic anemia (methyldopa)Serious
- Liver injury, including hepatitis (methyldopa)Serious
What the class has in common. Each drug's card has its own, and Known for marks what an exam expects you to know.
Watch for
Cautions
- Never stop clonidine suddenly: taper it over days, or blood pressure can surge with agitation, headache and tremor.
- Sedation adds up with alcohol, benzodiazepines and other CNS depressants.
- Clonidine with a beta blocker, verapamil, diltiazem or digoxin can cause severe bradycardia and heart block.
- Methyldopa: check blood count, Coombs test and liver tests; avoid it in active liver disease and with MAOIs.
Remember them
A way to remember them
Alpha-2 is the brake: press it to slow down, but never let go suddenly.
Brainstem alpha-2 receptors work as a brake on sympathetic outflow. Clonidine and methyldopa's metabolite press it, so heart rate and blood pressure fall and people feel drowsy. Stop clonidine abruptly and the brake comes off: catecholamines surge and blood pressure rebounds, which is why it is always tapered.
Written for biofideX
Name stems
-onidine means central alpha-2 agonist (clonidine type)
Clonidine stimulates central alpha-2 receptors and cuts sympathetic outflow; moxonidine and brimonidine share the stem. WHO writes it -(o)nidine, part of -nidine.
- Every drug name stem
Don't mix them up
Classes easily confused with these
- Alpha-1 blockersBlocking alpha-1 receptors, relaxing blood vessels and prostate muscle
- Noradrenergic and specific serotonergic antidepressantsBlocking α2 autoreceptors, so more norepinephrine and serotonin are released
- Beta blockersBlocking beta receptors, so the heart beats slower and less forcefully
Sources
Checked against
Written for biofideX in plain words, after checking these references:
- Clonidine hydrochloride tablets: label (DailyMed)
- Clonidine hydrochloride extended-release tablets: label (DailyMed)
- Methyldopa tablets: label (DailyMed)
- Clonidine (NCI Thesaurus)
- Methyldopa (NCI Thesaurus)
- Common questions about clonidine (NHS)
- WHO recommendations on drug treatment for non-severe hypertension in pregnancy (WHO)
- Guidelines for the psychosocially assisted pharmacological treatment of opioid dependence (WHO)
For learning pharmacology, not for treatment decisions: no doses are given, and a prescriber, pharmacist or the product's own label is the word on any one patient. Spotted a mistake? Tell us and we'll correct it.
Blood pressure
Practice central alpha-2 agonists with the other classes in this group: what each drug is, how it works, what it treats and what it's known for.
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