Structure of atropine (hyoscyamine)

Atropine is an alkaloid extracted from the deadly nightshade (Atropa belladonna) and other plants of the family Solanaceae. It is a secondary metabolite of these plants and serves as a drug with a wide variety of effects.

Physiological effects and uses

Generally, atropine lowers the "rest and digest" activity of all muscles and glands regulated by the parasympathetic nervous system. This occurs because atropine is a competitive inhibitor of the muscarinic acetylcholine receptors. (Acetylcholine is the neurotransmitter used by the parasympathetic nervous system.)

Mydriatic

The main use of atropine is in dilating the pupil (mydriasis) before undergoing eye exams, although tropicamide is now preferred due to atropine's slow degradation. Atropine-containing drops are directly applied to the eye. The mechanism of action involves blocking the contraction of the pupillary constrictor muscle (which is normally stimulated by acetylcholine release), thereby allowing the radial dilator muscle to contract and dilate the pupil.

Resuscitation

Injections of atropine are used in the treatment of bradycardia (an extremely low heart rate) and pulseless electrical activity (PEA) in cardiac arrest. This works because the main action of the vagus nerve of the parasympathetic system on the heart is to slow it down. Atropine blocks that action and therefore may speed up the heart rate.

Secretions and brochoconstriction

Atropine reduces bronchial and salivary secretion (which are also directed by the parasympathetic system); a dry mouth and increased heart rate are usually among the first effects of the drug.

Antidote for organophosphate poisoning

By blocking the action of acetylcholine, atropine also serves as an antidote for poisoning by organophosphate insecticides and nerve gases. Troops who are likely to be attacked with chemical weapons often carry autoinjectors with atropine and obidoxime which can be quickly injected into the thigh.

Side effects and overdoses

Adverse reactions to atropine include ventricular fibrillation, supraventricular or ventricular tachycardia, giddiness, nausea, blurred vision, loss of balance, dilated pupils, photophobia, and possibly, notably in the elderly, confusion, hallucinations and excitation. These latter effects are due to the fact that atropine is able to cross the blood-brain barrier. Because of the hallucinogenic properties, some have used the drug recreationally, though this is very dangerous.

In overdoses, atropine is poisonous. Atropine is sometimes added to other potentially addictive drugs; abuse of those drugs is then prevented by the unpleasant effects of atropine overdose. The antidote to atropine itself is physostigmine or pilocarpine.

Chemistry and pharmacology

Atropine is a racemic mixture of D-hyoscyamine and L-hyoscyamine, with most of its physiological effects due to L-hyoscyamine.

The most common atropine compound used in medicine is atropine sulfate (C₁₇H₂₃NO₃)₂. H₂SO₄ . H₂O, the full chemical name is 1α H, 5α H-Tropan-3-α ol (±)-tropate(ester), sulfate monohydrate.

History

In the Renaissance, women used the juice of the berries of Atropa belladonna to enlarge the pupils of their eyes, for cosmetic reasons; "belladonna" is Italian for "beautiful woman".

Atropine and its mydriatic effects were discovered in 1833 by the German chemist Friedrich Ferdinand Runge (1795-1867).