Parasympathomimetic agents, also known as cholinergic agonists or cholinomimetics, are drugs that mimic the effects of the neurotransmitter acetylcholine, the primary neurotransmitter of the parasympathetic nervous system.
They work by either directly stimulating the cholinergic receptors or by inhibiting the enzyme that breaks down acetylcholine in the synaptic cleft, thereby increasing the concentration of acetylcholine available to stimulate the receptors.
SAR of Parasympathomimetic agent:
The Structure-Activity Relationship (SAR) of parasympathomimetic agents, or cholinomimetics, revolves around the following key elements:
1.Quaternary Nitrogen Atom:
A positively charged nitrogen atom is crucial for interaction with the acetylcholine receptor.
2.Ester Linkage:
An ester bond is essential for binding to the receptor's esteratic site.
3.Two Carbon Spacer:
A two-carbon atom spacer between the nitrogen and the carbonyl carbon is needed for proper alignment with the receptor.
4.Hydrophobic Substituent:
A hydrophobic (non-polar) substituent is often attached to the nitrogen atom to increase the potency of the molecule.
Parasympathomimetic agents can be divided into two main types based on the type of cholinergic receptors they act upon: Direct acting and Indirect acting
Direct-Acting Parasympathomimetics:
These drugs directly stimulate cholinergic receptors. They can be further subdivided based on the receptor subtype they act upon:
Muscarinic Agonists: These drugs directly stimulate muscarinic acetylcholine receptors. Examples include pilocarpine (used in glaucoma) and bethanechol (used in urinary retention).
Nicotinic Agonists: These drugs directly stimulate nicotinic acetylcholine receptors. An example is nicotine.
