NBB 4910 Principles of Neurobiology Jeff Scott Guest Lecture - - PDF document

Jeff Scott Guest Lecture Spring 2013

NBB 4910 Principles of Neurobiology

INSECTICIDE NEUROTOXICOLOGY

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-Verlag Berlin, Heidelberg.E.

• J. R. Bloomquist, 1996. Ion channels as

targets for insecticides, Ann. Rev.

• Entomol. 41: 163-90.
• T. Narahashi, 1992. Nerve membrane Na

+ channels as targets of insecticides,

Trends Pharmacol. Sci. 13: 236-241.

References

Pesticides

Long, long ago Inorganics Long ago botanicals

Historical Perspective

1880 Commercial production (inorganics) 1941 DDT 1946 Cyclodienes 1947 Organophosphates 1950 Carbamates 1970 Insect Growth regulators 1980 Pyrethroids 1995 Genetic engineering 2000 Neonicotinoids

Effective insect control

Historical Perspective

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

• f AChE)

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

Neurophysiology of insecticides acting 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. Subsequently won the Nobel prize in 1948.

• DDT opened the era of synthetic
• rganic 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 (LD50 1 ‑ 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

• f 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 In vi vivo vo pois poisoning

• ning

sym symptom toms

American Cockroach CNS

Cockroach cercal nerve-giant axon preparation

In In si situ tu pois poisoning

• ning

sym symptom toms

Voltage Clamp

Voltage Clamp Normal

Voltage Clamp TTX

Voltage Clamp Normal vs. TTX

Voltage Clamp TEA

Voltage Clamp Normal vs. TEA

Voltage Clamp DDT

Voltage Clamp Normal vs. DDT

Voltage Clamp DDT + TEA

Voltage Clamp DDT alone vs.. DDT + TEA

Voltage Clamp DDT + TTX

Voltage Clamp TTX alone vs.. DDT + TTX

Do the voltage clamp experiments explain the repetitive discharges? Do repetitive discharges explain the symptoms?

Pyrethroids

• Based upon a natural

product ‑ pyrethrins

• First pyrethroids

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 "early pyrethroids"

But then something unusual happened

Thes hese e ins insect ecticides icides appear ppear to

• be

be “s “super uper DDT” ” Sympt mptoms

• ms ar

are e similar imilar Bot

• th

h caus cause e repet epetit itiv ive e dis dischar harges ges Bot

• th

h pr prolong

• long the

he opening

• pening of
• f

the he sodium

• dium channel

hannel Bot

• th

h ar are e mor more e toxic xic at lo lower er temper emperatur ures es

Cl C CCl3 H

DDT

Cl O O C O H H

H

Pyrethrin I

= = VIDEO

Photostable pyrethroids

Symptoms of poisoning are radically different from DDT or Type I pyrethroids

VIDEO

There are no repetitive discharges

VIDEO

Structures of Type I and Type II pyrethroids

C O N O O O Cl C C CN O O C O O O CN Cl Cl C O O O Cl Cl C O O O CN C O O O O

Type I Type II

tetramethrin fenvalerate cypermethrin permethrin cyphenothrin phenothrin

Identify the site of Action Type II Pyrethroids

What would you do?

Patch clamp

How does this explain the lack of repetitive discharges?

DDT and pyrethroids (both Type I and Type II) both hold the voltage gated sodium channel in the open state Differences in symptoms (and potencies) can be explained by differences in kinetics