Neurotoxicity and Behavioral Effects of Environmental Chemicals - Organic Pollutants: An Ecotoxicological Perspective - page 303

BOX 16.2 In vItRO aSSayS fOR neuROtOXICIty
There has long been an interest in the development of in vitro assays for detect-
ing neurotoxic effects of chemicals from the point of view of both human
risk assessment and environmental risk assessment. The effects of neurotoxic
chemicals on laboratory animals is a major concern of animal welfare organi-
zations. An outstanding problem is that, because of the complexity of the ner-
vous system, some neurotoxic effects can only be detected in vivo—in whole
animal systems (Dewar 1983, Atterwill et al. 1991). Thus, it is difficult to fore-
see the total banning of in vivo tests. However, in vitro assays can still make an
important contribution to testing protocols for chemicals. These protocols can
include a combination of in vivo and in vitro tests, with a consequent reduction
in the use of animals for testing procedures (Atterwill et al. 1991).
Atterwill et al. (1991) list six categories of nervous system culture that have
been used in in vivo testing procedures. These are dispersed cell cultures,
explant cultures, whole organ cultures, reaggregate cultures, whole embryo
models, and cell lines. It is possible in cultures such as these to measure the
cellular response to neurotoxic chemicals. Electrophysiological measurements
can be made even on single cells, revealing effects of chemicals upon ion cur-
rents and transmembrane potential. Also, there is the possibility of following
effects on the release of chemical messengers such as cyclic AMP from post-
synaptic membranes, when neurotransmitters interact with their receptors.
In one example (Lawrence and Casida 1984, Abalis et al. 1985) rat brain
microsacs were used to test the action of cyclodiene insecticides such as
dieldrin and endrin on the GABA receptors contained therein. The influx of
radiolabeled Cl − into the microsacs via the pore channel of the receptor was
inhibited by these chemicals. A similar assay was developed using microsacs
from cockroach nerve. Assays with this preparation showed again the inhibi-
tory effect of a cyclodiene (this time heptachlor epoxide) on Cl − influx. Also,
that microsacs from cyclodiene resistant cockroaches were insensitive to the
inhibitory effect of picrotoxinin, which binds to the same site on the GABA
receptor (Kadous et al. 1983).
of the responses of different species and strains of insects to insecticides. Returning
to the examples given in Table 16.1, both DDT and pyrethroid insecticides interact
with the Na + channel of the axonal membrane of insects. With repeated use of DDT,
insects such as houseflies came to develop kdr and super kdr resistance against the
insecticide. Both types of resistance are due to the appearance of forms of the Na +
channel that are insensitive to the insecticide (see Chapter 4, Section 4.5, and Chapter
12, Section 12.6). The fact that these strains also show marked cross-resistance to
pyrethroids is compelling evidence that this pore channel represents the principal
site of action for both types of insecticide in insects.
The effects of DDT on nerve action potential are illustrated in Figure 16.1. In
nerves poisoned by the insecticide, there is a prolongation of the sodium current
and a consequent delay in returning to the resting potential. This can result in the