Pesticides Principles of Toxicology Pesticides EPA definition substances

Pesticides Principles of Toxicology

Pesticides • EPA definition: “substances or mixtures of substances intended for preventing, destroying, repelling or mitigating any pest…”

A bit of history… • Sulfur – Chinese - 1000 BC – Europe - 1800 s – CA - today ! • Arsenic-containing • Strychnine • Nicotine (tobacco leaves extracts - 1690) • Pyrethrum (chrysanthemum extract) • Bordeaux mix: copper, lime (Ca(OH)2), water

Later… • 1930’s - modern era chemistry – Alkylthiocyanate – Dithiocarbamate – Bromide compounds • WWII – DDT – Dinitrocresol – 2, 4 D • Since then, synthesis with goal improved specificity, reduced toxicity… • No such a thing as “safe pesticide”

Integral part of crop and health protection Poisonings are anticipated… 3 mil acute cases annually (ww) 220, 000 deaths CA - 25, 000 pesticide related illnesses, annually USA - 80, 000

Efficacy of crop protection

Pesticide poisonings by occupational activity

Medical successes • DDT – Typhus in Naples, Italy – River blindness, West Africa – Malaria - Africa, Asia, Middle East There are still many parasitic and vectorborne diseases…

Regulations… • 1906 - First Federal Food and Drugs Act • 1938 - Federal Food, Drugs and Cosmetics Act • 1958 amend. - Delaney clause: “no additive shall be deemed safe if …found to induce cancer…” • 1947 - FIFRA: Federal Insecticide, Fungicide and Rodenticide Act: all pest control products under one law - by USDA • 1972 - FIFRA reorganized and passed to EPA • FIFRA Amendments - 1975, ‘ 78, ‘ 80, 84 • 1996 - Food Quality protection Act (children) • Developing countries adapt or lack regulations

Nervous System Central Peripheral Afferent Somatic Efferent Autonomic Para-Sympathetic ENS

Anatomic Classification CNS Skull and Spinal cord PNS 12 pairs of cranial nerves 31 pairs of spinal nerves afferent CNS PNS efferent

Sympathetic ANS Fight or flight Parasympathetic ANS Rest and digest

Parasympathetic: Cholinergic cranio-sacral Sympathetic: Adrenergic thoraco-lumbar

Parasympathetic nerves 4 Cranial III oculomotor VII facial IX glossopharyngial X vagus Sacral S 2 S 3 S 4

Sympathetic nerves T 1. . T 12 L 1 L 2 L 3

Nerves Afferent Efferent Neurons Mixed Spinal nerves are mixed for major length – then divide Afferent dorsal root Efferent ventral root

Structure of a typical neuron

Pre-ganglionic neuron Post-ganglionic neuron organ Ganglion Multi-neuron synapse Neuroeffector junction

Somatic Ach Parasympathetic -cholinergic Ach Sympathetic - adrenergic Ach Adr/NA

Feature Sympathetic Parasympathetic Start point thoraco-lumbar cranio-sacral Ganglion near spine on organ (terminal) Ganglion synapse Ach Pregangl. neuron short long Postgangl. neuron long short Effector synapse Adr Ach Effector organs throughout body limited

Typical Synapse

Cholinergic Transmission

Acetylcholine Synthesis O CH 3 C-O- + HO-CH 2 -N+(CH 3)3 Acetate Choline Coenzyme A Choline Acetylase O CH 3 C-O-CH 2 -N+(CH 3)3 Acetylcholine

Acetylcholine Catabolism O CH 3 C-O-CH 2 -N+(CH 3)3 Acetylcholine Cholinesterase O CH 3 C-O + HO-CH 2 -N+(CH 3)3 Acetate Choline

Cholinergic Receptors Muscarinic: M 1 CNS, sympathetic (exceptions), presynaptic M 2 Smooth muscle, heart, presynaptic M 3 Exocrine glands, blood vessels Nicotinic: NM skeletal muscle NN ganglia (post-), presynaptic

Acetylcholine Nicotinic Receptor

Nicotinic Receptor structure a b d g a Ion Channel

Na+ a g d a b Na+

Muscarinic receptor Agonist Outside Inside G protein

Muscarinic receptor G- proteins Excitatory action Phosholipase C Inhibitory action Adenylic cyclase K+ channels

Muscarinic receptor stimulation Gi Signal transduction cascade involving Adenylic cyclase - - Protein Kinase A -

Muscarinic receptor stimulation Gq Signal transduction cascade involving Phospholipase C

Cholinergic agonists

Adrenergic Transmission

Sympathetic Nerve Terminal Tyrosine 1 mitochondria contains MAO, oxidizes amines transport 4 2 3 exocytosis DA, Nepi stored with ATP in granules 5 re-uptake a, b Receptors

Adr (NA) synthesis

Adrenergic Receptors Alpha 1 most effector cells 2 presynaptic, lipocytes, platelets, some smooth muscle Beta b b 1 effector cells (*heart), brain, lipocytes, presynaptic b 2 smooth muscle and myocardium b 3 lipocytes

Adrenergic receptors: a 2 and b Adenylyl cyclase b receptor + - a 2 receptor

Adrenergic receptors: a 1 Phospholipase C a 1 receptor +

Pesticides • Organochlorines • Ch. E inhibitors • Organophosphates • Carbamates • Phenoxyherbicides • Pyrethroids • Bromine-based • Phenol- derivatives • Dipyridyl derivatives

Organochlorine insecticides

Organochlorine insecticides • DDT – first commercially produced insecticide (1940’s) – banned in the US in the 1970’s but is still manufactured and exported (1 ton/day) • Cyclodienes – Most toxic (CNS) and persistent pesticides known • HCH and Cl-benzene – Mixtures of isomers – Medicinal use (lice shampoo) (lindane) • t 1/2 = 7 -30 y • Bioaccumulates • Persistent • Lipophilic • Non-selective • Endocrine disrupter • Reproductive toxins • Neurotoxins (Lindane)

Observed effects • DDT – Enzyme induction – Competes with estradiol for receptor • Cyclodienes – Reproductive toxicity (reduced fertility, loss of pups, teratogenic) – CNS toxicity • HCH and Cl-benzene – CNS toxicity – Increased hepatocellular tumors (mice)

Mechanisms of action • DDT – Peripheral sensory neurons – prolonged negative afterpotential in neurons – K+ transport, inactivate Na+ channel closure, inhibit Na+ /K+ and Ca 2+ /Mg 2+ ATPases, inhibit calmodulin-transport of Ca 2+ (fig. 22 -4) • Cyclodienes – CNS localized – GABAA receptor/channel antagonists, inhibit Cl-uptake and Na+ /K+ and Ca 2+ /Mg 2+ ATPases • HCH and Cl-benzene – Suggested similar to cyclodienes but unknown

Cholinesterase inhibitors • Organophosphates (OP) and Carbamates – Strong Acute neurotoxicity - ACh. E inhibition (cholinergic effects) – Nervous system toxins - nerve gas (sarin)

1 st OP: TEPP (tetraethylpyrophosphate), followed by parathion 1 st carbamic: 1930 WWII chemical warfare 1988 Iraq- against Kurds 1994 Japan 1995 Tokyo subway

Cholinesterase O CH 3 C - O - CH 2 - N(CH 3)3 + Esteratic Anionic O + CH 3 C-O- + HO-CH 2 -N(CH 3)3

Organophosphates R 1 R 2 O P R 1 -2 = aliphatic X Parathion Malathion Soman Ecothiophate X = e- withdrawing

Organophosphates R 1 R 2 O P Esteratic X Strong Covalent Bond, Inactivates Enzyme (stable >100 h) Aging of complex Anionic

Organophosphates are slower to release from Ach. E - “also aging effect” Carbamates are faster: reversible Phase I metabolic activation Multiple metabolic reactions

Neurobehavioral, muscular and cognitive effects Delayed Neuropathy (OPIDN) - ginger jake WHO has a battery of neuropsychological tests

OPs causing OPIDN

Phase I and II biotransformation

Antidote pralidoxime, removing OP from enzyme site

Pyrethroids Newer (1980) but were 30% of all use by 1982 Extensive agricultural use Indoor use Pet flee control Household plants Modify Na+ channel kinetics Abnormal repetitive discharges Type A shorter action than type B

Avermectins

New generation pesticides

Herbicides • 1. 9% increase/year between 1980 -1985 (x 2 of insecticides) due to: – Monoculture – Mechanization of agricultural processes • Categories by application: – Pre-planting – Pre-emergent – Post-emergent • Low mammal toxicity • Suspected mutagens, carcinogens, teratogens • Skin irritants

Phenoxyherbicides • Introduced in 1946 • 2, 4 Dichloro- and 2, 4, 5 Trichloro phenoxy acetic acids • Defoliants (Vietnam war) - Forestry • Nerve toxicity, peripheral neuropathy • Controversy about NHL and HL • Contaminants may be responsible for toxicity

Dipyridyl derivatives “startling human toxicity” Banned in many countries but still in use in 130 others Lung is the most susceptible target organ Highly polar- poor GI absorption (5 -10%) LD 50=22 -262 mg/kg LD 50=100 -400 mg/kg

Chloroacetanilides Only slight acute toxicity but Carcinogens of category 2 B Metabolic activation to mutagenic metabolite (DEBQ 1) 1985 Canada incident (well water contamination)

Phosphomonomethyl aminoacids • Non-selective systemic herbicides • Free acids or salts ocular and mucus membrane irritants • Class E carcinogens (EPA) • Solvent may be the toxic compound (POEA)

Fungicides • Lipophilic, accumulate • 90% are carcinogenic in animals --> 75 mil pounds produced annually • 10% acreage but 60% of total dietary carcinogenic risk • Contaminants are dioxins and furans • • Hexachlorobenzene (banned) Pentachlorophenol (banned) Phthalimides Dithiocarbamates

Fungicides • Dithiocarbamates – Ferbam, ziram, maneb, zineb, nabam (metal-based names) – Some reported as teratogenic – Degradation to ethylene thiourea (ETU): a known mutagen, carcinogen, teratogen and antithyroid compound. – Some neurotoxicity at high doses – May cross into CNS if bound to divalent metals

Fumigants • Very volatile - inhalation exposure • Non-selective, highly reactive and cytotoxic – – – acrylonitrile carbon disulfide carbon tetrachloride ethylene dibromide (gastric carcinomas, sterility) ethylene oxide (carcinogen, developmental tox. ) phosphine (PH 3) released from aluminum phosphide (Al. P) in moist conditions (grain storage)

Rodenticides • Rodents: vectors of disease – Zinc phosphide - PH 3 (cell toxicity, necrosis, GI, liver, kidneys) – Fluoroacetic acid and derivatives (Fluoroacetyl-Co. A --> fluorocitrate: Krebs cycle collapse) – -naphthyl thiourea (ANTU) must be metabolically activated --> resistance – Anticoagulants (coumadin, warfarin) - antagonist of vit. K in synthesis of clotting factors; requires multiple doses; resistance