Parasympathomimetics

Parasympathomimetics (the majority of parasympathomimetics) are substances whose effect is caused by activation of the muscarinic receptors (see below acetylcholine, acetylcholine receptors). A distinction is made between 2 substance classes:

• direct parasympathomimetics
• indirect parasympathomimetics.

Direct parasympathomimetics bind directly to the acetylcholine receptor (see below acetylcholine) of the muscarinic type and activate it.

Direct parasympathomimetics are substances like:

• carbachol
• Bethanechol
• Methacholine
• Pilocarpine
• Arecolin

Indirect parasympathomimetics are substances like:

• physostigmine
• Rivastigmine
• Pyridostigmine
• Neostigmine
• Distigmine

Indirect parasympathomimetics act by increasing the synaptic concentration of acetylcholine. They reduce acetylcholine degradation by inhibiting the enzyme acetylcholinesterase. However, the term "indirect parasympathomimetics" is misleading because acetylcholine is a transmitter in the parasympathetic and sympathetic nervous system as well as at the motor end plate of the skeletal muscles. Acetylcholinesterase is also inhibited there. The substances that are so effective should therefore be better called cholinesterase inhibitors (Graefe KH 2016)