What is a belladonna alkaloid?

Where does the term belladonna come from?

A belladonna alkaloid is a group of alkaloids, including atropine and scopolamine, found in plants such as belladonna and jimsonweed. They are used in medicine to dilate the pupils of the eyes, dry respiratory passages, prevent motion sickness, and relieve cramping of the intestines and bladder.

The term belladonna comes from an Italian/Spanish origin meaning “beautiful woman”. The reason it was named this was because its original discoverers used the plant it is derived from to dilate the eyes of people so what they were looking at was “more beautiful”.

These drugs are antimuscarinic agents and may be used to dry secretions, increase HR and dilate bronchioles + pupils. These drugs have affinity for the receptor but no intrinsic activity. In other words they occupy the receptor without producing an effect - plus - they keep ACh from binding to the receptor and producing an effect.

All of these drugs are from the classification of antimuscarinics. They all are used to “dry” patients secreations. The way they do this is by blocking the muscarinic receptors, rendering that receptor unusable. They also block the few sympathetic receptors that are cholinergic, such as the salivary and sweat glands. This is one of the reasons that they are used in anesthesia. These drugs have little action on skeletal muscle.

Atropine – works on the muscarinic receptors, where it binds competitively with ACh. It works both centrally and peripherally. When given in small doses, it can actually cause the heart rate to drop due to the blockade of the M1 receptors in the presynaptic ganglion. With higher doses, the M2 receptors on the SA node are blocked and the HR increases.

Scopalamine – Works much like atropine, but with greater action on the CNS and a longer duration causing more sedation and anitsalagogue effect. Used as an anti-motion sickness drug. May cause temporary short-term memory loss.Note - this is active ingredient in "Scope patch". Much less water soluble than atropine so crosses BBB more readily and more likely to get CNS effects.

Ipatropium – derivitave of atropine used to treat asthma/COPD who are unable to take adrenergic agonists. It is available in IH form, and does not enter the systemic circulation. Ipatropiums antimuscarinic effect by inhibiting IP3 release which causes bronchoconstriction.This is "atrovent". Very water soluble, little is absorbed systemically when given by inhalation.

Glycopyralate – Used in anesthesia for its anticholinergic effects and used to increase HR. Works like atropine. 

Glycopyrrolate is less lipid soluble than atropine so less likely to cause CNS effects like sedation and confusion. Make sure you know where NDMRs, atropine and neostigmine work (see for example fig. 5.2).

Low doses of nicotine causes ganglionic stimulation by depolarization. At higher doses, nicotine causes ganglionic blockade.