insecticides
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Insecticides632 Lecture 6 - Application of cellular neuroscience to a practical problem
Cellular Neuroscience - Revision
Resting potential Action potential Channels:
voltage gated, ligand gated, ionotropic &
metabotropic Chemical synaptic transmission
Aims of lecture to know problems of effective
application of insecticides to know the main types of insecticides to know their site(s) of action possible mechanisms of resistance
Reading Matters Web see http://biolpc22.york.ac.uk/632 Book:
Tomlin, CD S (1997) The pesticide manual Papers
Narahashi T (1996) Neuronal ion channels as the target sites of insecticides Pharmacology & Toxicology 79: 1-14
ffrench-Constant, RH et al (1998) Why are there so few resistance-associated mutations in insecticide target genes? Phil.Trans. R. Soc. B 353 1685-1693
Matsuda et al (2001) Neonicotinoids: insecticides acting on insect nicotinic acetylcholine receptors Trends pharm. 11:573-580
Delivering insecticide effectively?
rapidity specificity
to target species side effects
stability light & air (oxygen) not too persistent
solubility cheap
Main targets development
ecdysis [moulting] specific to insects cuticle specific to insects
respiration CNS
Why Knockdown resting insects have low metabolic
demand unlike mammals general respiratory or muscular
poisons not so good? knockdown insecticides
disable insect quickly OK to kill slowly target CNS
Main classes organochlorine (1940s) cyclodiene organophosphorus pyrethroids (1975-) Imidacloprid (1990s) phenyl pyrazoles
Organophosphorus example: malathion carbamates have similar action more toxic to insects phosphorylate acetylcholinesterase raises [ACh], so use atropine as
antidote if humans are poisoned
Organophosphorus phosphate group, with two CH3 / C2H5
and one longer side chain often S replaces O
malathion
Carbamates also related
originally derived from calabar beans in W Africa
aldicarb LD50 5mg/kg
Insects OP oxidase much
more toxic cytochrome P450
oxidase in mitochondria, etc
Vertebrates OP carboxyesterase
non-toxic
More toxic to insects
Phosphorylate acetylcholinesterase
active site of enzyme has serine - OH
active site binds P from phosphate half like very long (80 min)
acetylcholine maloxon
Cyclodiene e.g. Dieldrin, Lindane
once widely used like other
organochlorines, very lipid soluble
Cyclodiene mode of action
affects GABAA which carry Cl- currents binds to picrotoxin
site not GABA site enhances current faster
desensitisation
dieldrin
GABA induced Cl- current
Cyclodiene sensitivity insects are more
sensitive to GABAA insecticides because receptor is a
pentamer the -subunit binds
the insecticide insect homooligomer
3 receptors mammals have
heterooligomer
Phenyl pyrazoles fipronil
also targets GABAA receptors
same site as Lindane
Organochlorine DDT low solubility in water, high in lipids at main peak of use, Americans ate
0.18mg/day human mass 80kg
Na Channel effect more toxic to insects
DDT symptoms of
poisoning are bursty discharges
Na current effect Na current is slower to end in DDT
orange bar marks stimulus
Pyrethroids very quick knockdown need an oxidase inhibitor photostable and effective
30g/hectare (1% of previous insecticides\)
Pyrethroids major current
insecticide derived from
chrysanthemum Na channel effect more toxic because
of differences in Na sequence
may also have other effects ?
typically esters of chrysanthemic acid
typical pyrethroids ...aromatic rings & Cl or Br contribute to toxicity
Deltamethrin most toxic
No CN hyperexcitatio
n convulsions
CN next to ester bond
hypersensitive paralysis
Na channel effect Sodium current
lasts longer Voltage clamp
Note tail current
control tetramethrin
single voltage
voltage series
Na channel effect - ii
Unitary sodium current lasts longer patch clamp type II open even
less often but for even longer
more toxic because of differences in Na channel sequence rat mutant isoleucine methionine in
intracellular loop of domain 2 (I874M)
other effects ? Pyrethroids have been reported to
affect calcium channels GABA, ACh, glutamate receptors
Imidacloprid newer nicotinic binds to ACh
receptor
Imidacloprid iistimulate nerve and record EPSP apply carbamylcholine
Summary so far Na+ channels targets of DDT,
pyrethroids AChEsterase targets of OPs ACh receptor target of Imidacloprid GABAA receptor target of cyclodienes
& fipronil
Problem of Resistance resistance means that the insects survive! some species never develop,
e.g. tsetse flies - few offspring most very quick
e.g. mosquitoes - rapid life, many offspring cross resistance, e.g. to DDT and
pyrethroids because same target is used. [behavioural resistance]
Resistance mechanisms
organophosphates organochlorine cyclodiene pyrethroids
see Ann Rev Entomology 2000
Organophosphates carboxylesterase genes
amplified e.g. in mosquito, Culex, up
to 250 x copies of gene/cell carboxylesterase gene
mutated higher kinetics and affinity
(Tribolium) detoxified by
glutathione-S-transferases (i.e. addition of glutathione)
Organochlorine DDT detoxified by
glutathione-S-transferases (i.e. addition of glutathione)
Na channel resistance
Cyclodiene target site change known as Rdl
resistance to dieldrin GABAA receptor
alanine 302 serine [or glycine] change affects cyclodiene, picrotoxin
binding and reduces
desensitisation
Pyrethroids non-target resistance P450 oxidase
more transcription giving more expression
leads to cross-resistance to organophosphates & carbamates
target resistance Na+ channel
Na+ channel kdr : leucine alanine (L1014F)
9 Musca strains super-kdr : methionine threonine
(M918T)
Effect on currents
M918T blocks current completely
Comparative mutations
Conclusions Cellular neuroscience helps understand
many insecticide actions binding to channel proteins
ligand-gated voltage gated
mutation leads to resistance target site enzymatic degradation
Web page http://biolpc22.york.ac.uk/632/
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