Changes of n-hexane neurotoxicity and its urinary metabolites by long-term co-exposure with MEK or toluene

It is well known that the neurotoxicity of n-hexane may be modified upon co-exposure with other organic solvents. In order to elucidate this mechanism further, rats were exposed to 500ppm n-hexane, 500ppm n-hexane plus 500ppm methyl ethyl ketone (MEK), 500ppm n-hexane plus 500ppm toluene, or air only for 8h per day for 33 weeks. The body weight, motor nerve conduction velocity (MCV) and distal latency (DL) were determined before exposure and after 4, 8, 12, 16, 20, 24, 29, and 33 weeks of exposure. From each group one rat was histologically examined after 33 weeks of exposure. To establish a relationship between the n-hexane neurotoxicity and changes in biotransformation, urinary metabolites (2-hexanol, methyl n-butyl ketone (MBK), 2,5-hexanedione, 2,5-dimethylfuran, and gamma-valerolactone) were measured by gas chromatography on the first exposure day, and after 1, 2, 4, 8, 12, 16, 20, 24, 29, and 33 weeks of exposure. The total amounts of metabolites of n-hexane in the urine significantly decreased upon co-exposure of n-hexane, with MEK as well as with toluene, in comparison with those of animals exposed to n-hexane alone. 2,5-Hexanedione, which is considered the ultimate neurotoxic metabolite of n-hexane, also decreased. Electrophysiological and histological studies did not reveal statistically significant differences between any two groups among the four groups. It is considered that the present results might explain the combined effects of n-hexane and toluene which decrease n-hexane neurotoxicity, but do not explain those of n-hexane and MEK. Therefore, other mechanisms of the combined effects of n-hexane and MEK should be studied.