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Neurotoxic Insecticides — Modes of Action

 

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Zip File Size -275 MB

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Zip File Size -178 MB

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This animation was developed as an instructional tutorial to educate the public and those that work regularly with insecticides on how some of the most common insecticides cause their toxic effects. It is also appropriate for instruction in toxicology and entomology courses, especially those taught by distance education. The tutorial was intended as an independent self-teaching unit to allow an individual with some biological knowledge to follow a logical sequence of events that describes how nervous systems function (using an insect model), and to illustrate how neurotoxic insecticides disrupt the actions of nervous systems to produce toxic effects.  Students that study this tutorial will gain a basic knowledge of the structural and functional importance of nerve cells and animal nervous systems and of cell communication, in addition to learning about four different neurotoxic actions by insecticidal chemicals. 

The tutorial has a running time of approximately 45 min.  and consists of 13 scenes

Scene 1: Title and Objectives- presents structural comparisons of insect and human nervous systems. Compares the advantages and disadvantages of using chemicals that disrupt the nervous system. Finally, it presents the objectives of the tutorial and summarizes why it is important to understand how the nervous system functions and how insecticides disrupt these functions.

Scene 2: Insecticide Entry - illustrates how insecticides are applied to surfaces, then taken up by the insect wax layer, absorbed into the body and distributed by the circulation to the nervous system.

Scene 3: Insect Nervous System - demonstrates the organization of the insect nervous system. The structure of a neuron (nerve cell) is described including the role of each part of the neuron in conveying nervous information. Finally the scene ends with a view of the organization of a ganglion (region containing neuron cell bodies). This information is critical to understanding how insecticides are toxic to insects and other animals by disrupting normal nervous functions.

Scene 4: Membrane Electrical Charge and the Nerve Impulse - Since insecticides exert their actions on nerve cells, it is important to understand how nerve cells produce and use electrical charges to convey information. This animation summarizes these critical processes.

Scene 5: The Synapse - The synapse is the junction between two nerve cells where information is transferred. The synapse is a major target for insecticide toxicity. The scene illustrates synapse organization and how information is conveyed across the synapse by chemicals released from the presynaptic nerve cell and detected by special receptors on the postsynaptic nerve cell. The neurotransmitter chemicals cause new electrical responses in the postsynaptic nerve cell to transmit the information on through the nervous system.

Scene 6: Nerve-Muscle Interactions - Nerves stimulate muscle contraction. This scene explains why uncontrolled muscle twitching is a characteristic symptom of insecticide poisoning.

Scene 7: Acetylcholinesterase Action and Inhibition - Acetylcholinesterase is a synaptic enzyme that degrades acetylcholine, a neurotransmitter chemical that carries information between nerve cells at the synapse. Acetylcholinesterase is also an important target for carbamate and organophosphate insecticides. The animation illustrates the normal role of acetylcholinesterase in the synapse and explains how organophosphate inhibition disrupts the synapse function and its effect on the insect.

Scene 8: Organophosphate Inhibition of Acetylcholinesterase - The animation shows how acetylcholinesterase interacts normally with acetylcholine. The animation explains why organophosphates also interact with the enzyme and demonstrate that "tight" binding by organophosphates permanently destroys acetylcholinesterase activity.

Scene 9: Carbamate Inhibition of Acetylcholinesterase - Carbamates block acetylcholinesterase by competing with natural acetylcholine for the enzyme's active site. Although this competition inhibits normal enzyme activity like organophosphates, the enzyme is, nonetheless, still functional and, unlike organophosphate effects, the enzyme can eventually recover its normal activity after carbamate exposure.

Scene 10: Neonicotinoids - Imidacloprid - an Acetylcholine Mimic - How neonicotinoids mimic acetylcholine and hyperstimulate the acetylcholine receptor to hyperactivate the nervous system leading to nervous and muscular exhaustion.

Scene 11: Pyrethroids - Axon Sodium Channel Inhibitors - Pyrethroids are derivatives of pyrethrum, a natural, plant-derived insecticide. The scene depicts how pyrethroids inhibit closure of sodium channels in nerve cell membranes. Inhibition of sodium channel closure produces a constant electrical discharge in the nerve cells resulting in a fatal hyper-stimulation of the nervous and muscular systems.

Scene 12.1 and 12.2: Insecticide Resistance and Resistance Management - Using organophosphates as the example, the animation illustrates how insecticide resistance occurs at both the molecular and population levels in insects. It shows how minor mutations in a target molecule can diminish insecticide sensitivity and toxicity, and how this leads to populations of insects that are resistant to insecticides. Resistance management is illustrated by using knowledge of insecticide mode of action to treat a resistant population with an alternative insecticide that has a different mode of action. 

This last scene (12.1 and 12.2) can be obtained as a separate item by educators interested in focusing only on the topic of insecticide resistance.

 

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