What is the primary mechanism of toxicity for organophosphate and carbamate insecticides?

• A. Inhibition of acetylcholinesterase
• B. Blockade of GABA receptors
• C. Uncoupling of oxidative phosphorylation
• D. Inhibition of DNA synthesis

Answer: (A)

Inhibition of acetylcholinesterase is the primary mechanism. Organophosphates and carbamates block the enzyme that normally breaks down acetylcholine at cholinergic synapses and the neuromuscular junction, causing acetylcholine to accumulate and overstimulate both muscarinic and nicotinic receptors. This leads to the characteristic cholinergic syndrome with increased secretions, bronchoconstriction, miosis, muscle fasciculations, weakness, and central nervous system effects. The two classes differ in how they inhibit the enzyme: organophosphates form a covalent, phosphorylated enzyme that can age and become irreversibly disabled, while carbamates carbamoylate the enzyme but inhibition is reversible as the enzyme eventually reactivates. This mechanism explains why antidotes like atropine (to block muscarinic effects) and oximes (to reactivate AChE in organophosphate poisoning if given early) are effective. Other mechanisms—such as blocking GABA receptors, uncoupling oxidative phosphorylation, or inhibiting DNA synthesis—do not account for the acute cholinergic toxidrome produced by these insecticides.