• Clinical science

Parasympathomimetic drugs (Cholinomimetic drugs)

Summary

Parasympathomimetic drugs activate the parasympathetic nervous system (PSNS). As the neurotransmitter of the PSNS is acetylcholine (ACh), parasympathomimetics are also called cholinomimetic agents. These are classified according to whether they act as direct agonists of acetylcholine receptors (AChR) or indirect agonists of AChR (also called anticholinesterase). While direct agonists act by binding directly to muscarinic or nicotinic ACh receptors, indirect agonists prolong the action of endogenous acetylcholine by inhibiting acetylcholinesterase (AChE). Direct agonists of AChR are used topically in ophthalmology to induce miosis, while indirect agonists are used to treat conditions such as postoperative ileus, urinary retention, and myasthenia gravis.

Overview

Overview of direct and indirect parasympathomimetics

Classification Drugs Mechanism of action Pharmacological characteristics Indications
Direct parasympathomimetics

Bethanechol

  • Bind to muscarinic/nicotinic AChR direct AChR agonism
  • No nicotinic agonism [1]
  • Resistant to AChE

Carbachol

  • Nicotinic and muscarinic agonism [2]
  • Resistant to AChE
Cevimeline
  • Predominantly muscarinic agonism [3]

Pilocarpine

Methacholine

Indirect parasympathomimetics

Neostigmine

  • Inhibit AchE → ↓ breakdown of ACh ACh levels

Pyridostigmine

Edrophonium

  • Very short duration of action (∼ 10 minutes) [6]

Physostigmine

Distigmine

Echothiophate

  • Irreversible AChE inhibitor
  • Long-acting

Donepezil

Rivastigmine

Galantamine

  • Centrallly acting AChE inhibitors

Pharmacodynamics

Effects of parasympathomimetics on the different receptor types [7]
ACh receptors Organ/tissue Effects of parasympathomimetics
M1, M4, M5

Central nervous system

  • Influences neurologic functions (e.g., memory)
M2

Heart

M3

Smooth muscle

Exocrine glands

  • ↑ Secretions
    • Sweat
    • Gastric acid
Nicotinic Skeletal muscle
  • ↑ Muscle contraction and tone

Adverse effects

Direct parasympathomimetics

Indirect parasympathomimetics

Cholinergic crisis (cholinergic syndrome)

DUMBBELLSS: Diarrhea, Urination, Miosis, Bradycardia, Bronchospasm, Emesis, Lacrimation, Lethargy, Sweating, and Salivation are the main symptoms of cholinergic crisis.

We list the most important adverse effects. The selection is not exhaustive.