The role of biochemical markers in peripheral body fluids in assessment of human neurotoxicity

To date the evaluation of chemically-induced neurotoxic effects on humans has been dependent mostly on electrophysiological measurements, neurobehavioral tests and biological exposure assessment. However, recently attempts have been made to develop biochemical parameters in peripheral body fluids which can be easily obtained from humans and which can represent markers for the same parameters in nervous tissue. The approach of this kind is logically based on the following facts: 1) Blood cells (e.g., platelets and lymphocytes) possess some characteristics of monoaminergic neurons such as the existence of storage vesicles of monoamines, membrane neurotransmitter receptors, high affinity uptake sites and neurotransmitter-related metabolizing enzymes. 2) Leakage of nerve-specific markers from nervous tissue to peripheral body fluids may occur following damages of target neuronal cells or macromolecules. 3) Quantitative and/or qualitative alterations of peripheral biochemical markers (e.g. neurotransmitter receptors) can be induced by the regulation mechanisms of neuronal, endocrinal and immunologic interactions when the nervous functions are perturbed by various exogenous or endogenous factors. Erythrocyte acetyl cholinesterase (AChE), free erythrocyte protoporphyrin (FEP), lymphocyte neurotoxicity target enzyme (NTE), blood aminolevulinic acid dehydratase (ALA-D), and carboxyhemoglobin (CO-Hb) are well-known peripheral markers of the effects induced by organophosphates (AChE, NTE), lead (FEP, ALA-D) and carbon monoxide (CO-Hb). Many studies have been made on the effects of organic solvents, heavy metals and pesticides on neurotransmission parameters in blood cells such as neurotransmitter uptake, receptor binding and enzyme activity. This paper summarizes the present knowledge on the development and clinical applications of some peripheral biochemical markers such as neurotransmission parameters in blood cells and neuronal or glial cell marker proteins in CSF, blood and urine. The role of these peripheral biochemical markers in the assessment of environmental chemically-induced human neurotoxicity is also discussed.