* Based on a “Imre Pacséri” Memorial Lecture held at the National Congress of the Hungarian Society of Occupational Health in Szombathely, September 3 - 5, 1997.
First of all I should like to thank for the “Pacséri Imre” award to the Hungarian Society of Occupational Health and speaking about my activity in occupational toxicology. I remember some essential ideas of Imre Pacseri on occupational hygiene. His main conception was that labour hygiene is a discipline of medical prevention and not only a technical question. He learned this idea in Boston, where he had a Rockefeller study trip in 1929. There he learned moreover that without basic knowledge of organ specific toxic properties of a given chemical, that kind of prevention does not work in the case of dangerous chemicals.
In the early 1960th “maximal allowable concentration” in the workplace (MAC) usually was established on the basis of the central nervous system functions, independently that are they transient, reversible or irreversible impairments.
At that time I was asked to organize a neurobehavioral laboratory inside the department of toxicology, however there was no available guideline for assessment of neurotoxicity. On the other hand, there were several data in the international literature about the neurotoxic properties of chemicals, these data usually based on human findings of severe epidemiological events from all over the world (e.g. Minamata disease) much rather than from animal experiments. Our aim was clear: safety evaluations based on animal experiments should prevent human neurotoxicity of chemicals. What is the main purpose for testing of neurotoxicity of chemicals in the workplace? According to the present guidelines they are as follows: mechanism of action, sensitive population, species sensitivity and differencies, time-dependent effects, dose-response relationship, reversibility and no effect level (NOEL) (Federal Register 1994).
On the basis of these findings one might – and need – to extrapolate the results from animal experiments to human, furthermore the human risk and risk assessment/ management/prevention should be established.
In our neurobehavioral toxicology laboratory we dealt with electrophysiology and behavioral tests as well depending on the type of chemical studied. Among our main interests were the effects of heavy metals, namely organomercurial fungicides.
Methoxyethylmercurychloride (MEMC) might cause not only kidney damage, but induce severe learning and memory disturbances, moreover altered shape and latency of the cortical evoked potencials in rats as well. As a major effect in rats, the impaired frequency spectrum of the spontaneous EEG activity has been revealed by EEG registrations (Lehotzky, 1968, 1974, 1976). Neurotoxicity of MEMC could be increased by using antidote Dicaptol, because penetration through the blood brain barrier is much intensive. Owing to these findings organomercurial fungicides were banned in Hungary.
Our interest turned towards the field of organophosphate cholinesterase inhibitor insecticides because of the intensive application in the Hungarian agricultural practice. Delayed neurotoxic action had been demonstrated after subacute Sumithion (fenitrothion) exposure in rats. Diminished maximal motor conduction velocity of the sciatic nerve in rabbits and in rats was demonstrated without any cholinergic signs, using conduction velocity measurements in the caudal nerve according to Miyoshi (1973). All of these findings are due to the so called “loose myelin” demyelinization process of the motor nerves (Lehotzky, Ungváry, 1976, Ecobichon et al. 1977). The so called “delayed neurotoxicity” has no correlation with the cholinesterase inhibition and atropin has no antidote effect. Very similarly two more organophophates could induce diminished motor conduction velocity after subacute administration in rats (e.g., Neviphos and Terra Sytam (dimefox)).
After having demonstrated by Otto (1981) the minimal brain dysfunction (MBD) i.e. hyperactivity, learning and memory disturbances of children exposed to low concentration of lead, our work has been started from the early 80ths in the field of the so called “behavioural teratology”.
There was an increased interest towards animal models in developmental neurotoxicity, after having revealed that prenatal administration of different drugs, neurotoxic agents, such as alcohol, amphetamine, morphine, heroin, methadon and lead, induce subtle neurobehavioral impairments and delayed development of nervous system functions without any morphological malformation (Vorhees et al. 1979).
Several expert groups have focused on the functions that should be included in behavioural test battery as follows: sensory systems, neuromotor development, locomotor activity, learning and memory, reactivity and habituation (Tilson 1980).
Using our own behavioural test battery it has been shown that after prenatal exposure (from the 7th-15th days of gestation) in addition of testing the physical development of pups as eye- and ear opening, body weight after delivery and weaning, gait and locomotion behavioural patterns in open field, motor coordination on rotorod, freezing reaction of stress situation in a swimming test in water cylinder, learning and memory ability in a conditioned avoidance reflex acquisition situation (latency and performance), as well as reconditioning (to check reversibility of impairments) could be tested in rat pups in different postnatal days, in order to assess the properties of neurotoxicity and dose-response relationships.
Testing of social interactions, the so called “observational learning” and “taste-aversion” proved to be sensitive methods as well. These tests were used to estimate transient, subtle associative functional impairments due to different prenatal exposures.
Using this behavioral test battery it has been shown, that organophosphate cholinesterase inhibitors (fenitrothion), organomercurial fungicide (MEMC), cadmiumchloride, carbondisulphide, trimetilbenzene, pyrethroids (deltamethrin and cypermethrin) induce dose-dependent delay in development of neuromotor functions and learning ability; after prenatal exposure, without any overt sign of toxicity of pups or dams (Lehotzky et al. 1982, 1985, 1989, 1990).
Conditioned avoidance learning, both of acquisition and extinction, moreover reconditioning after the 90th postnatal day, tested in rat pups proved to be the most sensitive method in our behavioural test battery.
Estimation of dose-response and no effect level (NOEL) might be the basis of the risk assessment in neurotoxicology as well, taking into consideration of the “worst case” situation.
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LEHOTZKY, K., and BORDÁS, S. (1968). “Study on the subacute Neurotoxic Effect of Methoxy ethyl mercury chloride in Rats.” Med. Del. Lavoro 30:241–249.
LEHOTZKY, K., and MÉSZÁROS, I. (1974). “Alteration of Electroencephalogram and Evoked Potential in Rats Induced by Organic Mercury.” Acta Pharmacol. Toxicol. 35:180–184
LEHOTZKY, K., and UNGVÁRY, Gy. (1976). “Experimental Data on the Neurotoxicity of Fenitrothion.” Acta Pharmacol. Toxicol. 39:374–382
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LEHOTZKY, K., SZEBERÉNYI, M. J., UNGVÁRY, Gy., and KISS, A. (1985). “Behavioural Effects of Prenatal Exposure to Carbondisulphide and to Aromatol in Rats.” Receptors and Other Targets for Toxic Substances. Arch. Toxicol. Suppl. (8):442–446.
LEHOTZKY, K., SZEBERÉNYI, J., and KISS, A. (1989). “Behavioral Consequences of Prenatal Exposure to the Organophosphate Insecticide Sumithion.” Neurotox. Teratol. 11:321–324.
LEHOTZKY, K., UNGVÁRY, Gy., POLINÁK, D., and KISS, A. (1990). “Behavioral Deficits Due to Prenatal Exposure to Cadmium Chloride in CFY Rat Pups.” Neurotox. Teratol. 12:169–172.
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TILSON, H. A., CABE, P. A., and BURNE, T. A. (1980). “Behavioral Procedures for the Assessment of Neurotoxicity.” in: Experimental and Clinical Neurotoxicology (Ed: P. S. Spencer) pp:758–767.
VORHEES, C. V., BUTCHER, R. E., BRUNNER, R. L., and SOBOTKA, T. J. (1979). “A Developmental Test Battery for Neurobehavioral Toxicity in Rats: a Preliminary Analysis Using Monosodium Glutamate, Calcium Carrageenan and Hydroxylurea.” Toxicol. Applied Pharmacol. 50:267–282.