Parasympathomimetic Agents Classification

• Parasympathomimetic agents, also known as cholinergic agonists, are drugs that mimic the action of the parasympathetic nervous system (PNS) by stimulating cholinergic receptors (muscarinic and/or nicotinic).

A. Direct-Acting Parasympathomimetic Agents

• These agents bind directly to cholinergic receptors, mimicking the effect of acetylcholine.

1. Mechanism

   • Act as agonists at muscarinic (and in some cases nicotinic)
   • Not dependent on endogenous acetylcholine levels.

2. Examples:

   • Acetylcholine
   • Carbachol
   • Bethanechol
   • Methacholine
   • Pilocarpine

B. Indirect-Acting Parasympathomimetic Agents (Cholinesterase Inhibitors)

• These agents inhibit acetylcholinesterase (AChE), the enzyme that breaks down acetylcholine.
• This leads to an increase in ACh levels at synapses, enhancing parasympathetic activity.
• Subtypes:

1. Reversible Cholinesterase Inhibitors

   • Bind non-covalently or reversibly to the active site of AChE.
   • Duration of action is short to moderate.
   • Examples:
     • Physostigmine
     • Neostigmine
     • Pyridostigmine
     • Edrophonium chloride
     • Tacrine hydrochloride
     • Ambenonium chloride

2. Irreversible Cholinesterase Inhibitors

   • Typically, organophosphates.
   • Bind covalently to AChE, causing long-lasting inhibition.
   • Require a reactivator to reverse the inhibition.
   • Examples:
     • Isofluorphate
     • Echothiophate iodide
     • Parathion
     • Malathion

C. Cholinesterase Reactivator

• Used in the treatment of poisoning by irreversible cholinesterase inhibitors.

  • Mechanism

    • Reactivates AChE by removing the phosphate group from the active site.

  • Example:

    • Pralidoxime chloride

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