Mode of Action
Neonicotinoids, like nicotine, are nicotinic acetylcholine receptor agonists. This receptor is normally activated by the neurotransmitter acetylcholine. In mammals, these receptors are located in both the central and peripheral nervous systems. In insects the receptors are limited to the CNS. While low to moderate activation of these receptors causes nervous stimulation, high levels overstimulate and block the receptors. This receptor blockage causes paralysis and death. Normally, acetylcholine is broken down by acetylcholinesterase to terminate signals from these receptors. However, acetylcholinesterase cannot break down neonicotinoids, and the binding is irreversible. Because most neonicotinoids bind much more strongly to insect neuron receptors than to mammal neuron receptors, these insecticides are selectively more toxic to insects than mammals.
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