Insect Nervous System Disruption

The target sites within the nervous system of insects known at present are very restricted. They consist of the sodium channel, the components of the nicotinic cholinergic synapse and the y-aminobutyric acid (GABA) and octopamine receptors. Benson4 (1991) lists potential target sites within the insect neuronal and muscular system. These are shown in Table 3.1. 

At the end of an axon, where it meets another nerve cell or an effector cell (a cell such as a muscle or a gland cell), there is a gap or junction that is usually about 10 to 20 nm wide and this is known as a synapse. The passage of the nerve impulse across this synapse is chemical rather than electrical. When the nerve impulse reaches a synapse it causes the release of a chemical transmitter that is usually acetylcholine. Other transmitters have been identified and these include L-glutamate and y-aminobutyric acid (GABA). The released acetylcholine interacts with a receptor on the 

Neurotransmitter receptor ligand recognition sites Cholinergic 

A wide range of insecticides (and acaricides and nematicides) show this mode of action. In the 11th Edn of Pesticide Manual there are 71 organophosphorus and 18 carbamate insecticides listed.5 

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Chemicals with the potential to disrupt the mammalian nervous system may occur naturally (neurotoxins) or arise by synthesis (neurotoxicants). While chemicals with neurotoxic potential are conveniently termed neurotoxins or neurotoxicants , this is not an intrinsic property but rather the description of an effect that may occur if the tissue concentration exceeds a certain threshold. Biological chemicals with neurotoxic properties often have high target specificity and toxic potency, discrete biological actions, and are among the best understood mechanistically. Examples of chemicals with direct or indirect neurotoxic potential are found in bacteria, algae, fungi, plants, coelenterates, insects, arachnids, moll-usks, amphibians, reptiles, fish, and certain mammals (Table 1). 

A large number of commercially important insecticides act on the nervous system of the insect. By investigating certain areas of this system in detail, it may be possible to find new classes of chemicals which can disrupt the normal nervous activity, and this may lead to the development of new insecticides. It is important to look not only at chemicals which kill the insect, but also at those groups of chemicals which can disrupt the insect s behavior sufficiently to prevent its survival in the field. 

Carbary I (1-naphthyl methylcarbamate) is a chemical in the carbamate family used chiefly as an insecticide. It is a colorless white crystalline solid. Carbaryl disrupts the nervous system by adding a carbamyl moiety to the active site of the acetylcholinesterase enzyme, which prevents it from interacting with acetylcholine.1 It is classified as a likely human carcinogen by the EPA. The pesticide is used indiscriminately, so the toxicity has raised public concern about the ecosystem and human health. Carbaryl is lethal to many non-target insects such as the honeybee. Accumulation of the pesticide occurs in many aquatic organisms such as catfish and algae.2 Due to public health and ecosystem concerns a number of analytical procedures have been used to determine carbaryl concentrations. 

I. Mechanism of toxicity. In Insects, pyrethrins and pyrethroids rapidly cause death by paralyzing the nervous system through disruption of the membrane ion transport system in nerve axons, and pyrethroids prolong sodium influx and also... 

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