Jeff ScottGuest LectureSpring 2015

NBB 4910Principles of Neurobiology

INSECTICIDE NEUROTOXICOLOGY

Soderlund, D., 2012 Molecular mechanisms of pyrethroid insecticide neurotoxicity: recent advances. Archives of Toxicology 86: 165–181.

Raymond-Delpuch, V., K. Matsuda, B. M. Sattelle, J. J. Rauh & D. B. Sattelle. 2005. Ion channels: molecular targets of neuroactive insecticides. Invert Neurosci 5: 119-133.

Zlotkin, E. 2001. Insecticides Affecting Voltage-Gated Ion Channels In: Biochemical Sites of Insecticide Action and Resistance (I. Ishaaya ed) pp. 43-76. Springer-VerlagBerlin, Heidelberg.E.

J. R. Bloomquist, 1996. Ion channels as targets for insecticides, Ann. Rev. Entomol. 41: 163-90.

References

Pesticides

The USA uses 1.0-1.5 billion pounds of pesticides (herbicides, insecticides, etc.) per year

Long, long ago Inorganics

Long ago botanicals

Historical Perspective

1880 Commercial production(inorganics)

1941 DDT1946 Cyclodienes1947 Organophosphates1950 Carbamates1970 Insect Growth regulators1980 Pyrethroids1995 Genetic engineering2000 Neonicotinoids

Eff

ectiv

e in

sect

con

trol

#1

#2

Long, long ago Inorganics

Long ago botanicals

Historical Perspective

1880 Commercial production(inorganics)

1941 DDT1946 Cyclodienes1947 Organophosphates1950 Carbamates1970 Insect Growth regulators1980 Pyrethroids1995 Genetic engineering2000 Neonicotinoids

Eff

ectiv

e in

sect

con

trol

#1

#2

Perspective and Predictions

In addition to pest control, neurotoxic insecticides have played key roles in characterization of the nervous system (e.g. DFP characterized serine in active site of AChE)

Synthetic insecticides will continue to be a major factor for pest control into the foreseeable future.

Neurophysiology of insecticidesacting on sodium channels

DDT & pyrethroids

Usually both provide very useful information

To investigate the mechanism of action of neurotoxins there are two main types of data one can gather:

behavior and neurophysiology

HISTORY OF DDT

(DichloroDiphenylTrichloroethane)

• Synthesized in 1874 by Ziedler• Insecticidal activity discovered by Paul Muller of Geigy in 1939. • Won the Nobel prize in 1948.• DDT opened the era of synthetic organic insecticides and

"efficient" insect control. Called "miracle insecticide" or "the insecticide that won the war" in the mid 1940s.

• Extremely effective against a broad range of insects (LD501 - 1000 ug/g to insects).

Cl C

CCl3

H

Cl

HISTORY OF DDT

• Relatively safe to humans (oral LD50 to humans 250 mg/kg, 115 mg/kg to rats, practically non-toxic by dermal exposure).

• Cheap !!!!!!! ~$0.50 per pound. • DDT has saved millions of lives. Controls vectors of

diseases such as typhus, yellow fever or malaria. Use of DDT nearly eradicated malaria from the planet. Some areas still are malaria free while other have resurgence in the number of cases.

• >2,000,000 tons of DDT have been manufactured (Mellanby 1992)

• 1962 Silent Spring labels DDT the “Elixir of Death”

Military use of DDT

Civilian use of DDT

In vivopoisoning symptoms

American Cockroach CNS

Cockroach cercal nerve-giant axon preparation

In situpoisoning symptoms

Voltage Clamp

Voltage ClampNormal

Voltage ClampTTX

Voltage ClampNormal vs. TTX

Voltage ClampTEA

Voltage ClampNormal vs. TEA

Voltage ClampDDT

Voltage ClampNormal vs. DDT

Voltage ClampDDT + TEA

Voltage ClampDDT alone vs..

DDT + TEA

Voltage ClampDDT + TTX

Voltage ClampTTX alone vs..

DDT + TTX

Do the voltage clamp experiments explain the repetitive discharges?

Do repetitive dischargesexplain the symptoms?

Pyrethroids

• Based upon a natural product - pyrethrins• First pyrethroids were synthesized in the

1940s, but they were not stable under field conditions.

• Field stable pyrethroids discovered in the 1960s.

• The 2nd largest class of insecticides in use today

Pyrethrins and the first pyrethroids

But then something unusual happened

These insecticides appear to be “super DDT”Symptoms are similarBoth cause repetitive dischargesBoth prolong the opening of the sodium channelBoth are more toxic at lower temperatures

Cl C

CCl3

H

DDT

Cl

O

OC

O

H

H

H

Pyrethrin I

?=

VIDEO

α-CN pyrethroids (Type II)Symptoms of poisoning are radically different

from DDT or Type I pyrethroids

VIDEO

There are no repetitive discharges

VIDEO

Structures

CO

N

O

O

O

Cl

C CCN

OO

CO

OO

CNClClC

OO

OClCl

CO

OO

CN

CO

OO

O

Type I Type II

tetramethrinfenvalerate

cypermethrinpermethrin

cyphenothrinphenothrin

Identify the site of action type II pyrethroids

What would you do?

Patch clamp data

Type I

Yamamoto et al (1983)

Patch clamp

dataType II

Control 10 μM deltamethrin

Chinn and Narahashi 1986

Patch clamp summary

How does this explain the lack of repetitive discharges?

Insecticide Open Timenone 1-2 msecDDT 10-20 msecType I pyrethroid 20-80 msecType II pyrethroid >80 msec

Both DDT and pyrethroids (both Type I and Type II) hold the voltage gated sodium channel in the open state

Differences in symptoms (and potencies)

can be explained by differences in kinetics

Any questions?