Neurotoxicity and Behavioral Effects of Environmental Chemicals - Organic Pollutants: An Ecotoxicological Perspective - page 293

16 Neurotoxicity and

Behavioral Effects of
Environmental Chemicals

16.1 IntroductIon
In the previous chapters there have been many examples of environmental chemicals,
both natural and human-made, that have harmful effects on the nervous system of
animals. Many of these compounds are toxic both to vertebrates and invertebrates.
Interestingly, five major groups of insecticides, organochlorine insecticides (OCs),
organophosphorous insecticides (OPs), carbamate insecticides, pyrethroids, and neo-
nicotinoids, all owe their insecticidal toxicity largely or entirely to their action on
sites in the nervous system. A few of these compounds have also been used to con-
trol vertebrate pests (e.g., the cyclodiene endrin has been used for vole control, and
the OP insecticides fenthion and parathion for controlling birds). Separate chapters
have been devoted to the OCs (Chapter 5), OPs and carbamates (Chapter 10), and the
pyrethroids (Chapter 12). Other human-made pollutants also have harmful effects on
the nervous system of animals, although they are not used with the intention of doing
so. Examples include the organomercury fungicides and tetraethyl lead, which has
been used as an antiknock in petrol (both in Chapter 8). It would appear, therefore,
that the nervous system represents an “Achilles heel” within both vertebrates and
invertebrates when it comes to the toxic action of chemicals. When pesticide manu-
facturers have screened for insecticidal activity across a wide diversity of organic
chemicals, many of the substances that have proved successful in subsequent com-
mercial development have been neurotoxic.
This line of argument can be extended to natural toxins as well (Chapter 1). Thus,
many plant toxins such as the pyrethrins, physostigmine, strychnine, veratridine,
aconitine, etc., all act upon the nervous system. As discussed earlier, the presence
of such compounds in plants is taken as evidence for a coevolutionary arms race
between higher plants and the animals that graze upon them. The production of these
compounds may protect the plants against grazing by vertebrates and invertebrates.
Apart from plants, animals and microorganisms also produce neurotoxins that have
deadly effects upon vertebrates or invertebrates or both in the living environment.
For example, snakes, spiders, and scorpions all produce neurotoxins, which they
inject into their prey to immobilize them (see Chapter 1, Section 1.3.1). Also, tetrado-
toxin is stored within the puffer fish, and ergot alkaloids are produced by the fungus

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