Research Methods in Toxicology and Insecticide Resistance ...

elease of acetylcholine (a common neurotransmitter found in insects) from insect
vesicles into the synaptic space. The acetylcholine then quickly diffuses and reaches
channel receptors at the postsynaptic axonic membrane, where it binds to receptor
sites to open and close the sodium and potassium channels. As such, an action potential
is created at the postsynaptic axonic membrane with the simultaneous hydrolysis
of acetylcholine (deactivation of the neurotransmitter) into acetic acid and choline
catalyzed by acetylcholine esterase (reaction below). This enzyme is the common
target of most organophosphate and carbamate insecticides.
Acetylcholine esterase
Acetylcholine + water Acetic acid + choline
There are two types of postsynaptic acetylcholine receptors in animals: (1) nicotinic
acetylcholine receptors and (2) muscarinic acetylcholine receptors. The former
predominates in insects, the latter in mammals. Owing to this important factor, a new
group of neonicotinoid insecticide that targets only nicotinic acetylcholine receptors,
with much lower toxicity to mammals, has been developed.
Genetics: gene regulation in cells
Most insecticides act via inhibition of either target enzymes or receptors, all of
which are proteinaceous in nature. As such, it is pertinent to understand how genes
are regulated in the production of the necessary proteins to act as either enzymes or
receptors. Basically, there are two types of gene regulation, negative and positive. In
negative gene regulation, a repressor that binds and suppresses the promoter of a gene
requires the binding of an activator to form a complex. This allows a dissociation of
the repressor-activator complex from the promoter, which then combines with RNA
polymerase enzyme, allowing the expression of the gene by transcribing a messenger
RNA (mRNA). In positive gene regulation, an inactive activator binds with an activator
that then sits on the promoter, enabling the binding of RNA polymerase, resulting
in the transcription of mRNA.
The following fl ow chart shows the various activities or processing of DNA,
RNA, and protein that occur within the nucleus and cytosol:
a) Nucleus
DNA Packing, methylation, amplifi cation, rearrangements,
X-inactivation, heterochromatin, DNA organization
RNA transcript Promoters, enhancers, transcription factors, binding
proteins, repressors
Functional RNA Capping, polyA tail, splicing, variable splicing
16 K.L. Heong, K.H. Tan, C.P.F. Garcia, L.T. Fabellar, and Z. Lu