Interactions of neurotoxicants with neurotransmitter systems.

Many neurotoxic compounds have been shown to interfere with neurotransmission both in vitro and following acute and chronic administration. Various parameters of neurotransmission can be directly affected by neurotoxicants; these include the enzyme(s) synthesizing a neurotransmitter, the release and uptake processes, the enzyme(s) which metabolize the neurotransmitter, the receptors, and post-synaptic events associated with receptor activation. Some neurotoxicants can interfere with neurotransmission indirectly, by interacting for example with energy metabolism, sodium channels or ATPases. Furthermore, measured alterations of any parameter of neurotransmission can be the result of neuronal death, due to a cytotoxic effect of the neurotoxicants. Chemicals which have been shown to alter neurotransmission include solvents (e.g. carbon disulfide), metals and organometals (e.g. lead, mercury, trimethyltin) and pesticides (e.g. organophosphates, pyrethroids, organochlorines, formamidines). An example of the various alterations in neurotransmitter parameters, which can occur following acute or chronic administration, is represented by the organophosphates. Organophosphorus insecticides owe their acute toxicity to inhibition of acetylcholinesterase and accumulation of acetylcholine at cholinergic receptors. Chronic exposure to these compounds results in the development of tolerance to their toxicity which is associated with a decrease in the density of muscarinic and nicotinic receptors in both the central and peripheral nervous system. Other examples of the interactions of neurotoxicants with neurotransmitters are also described.