Acrylamide, 2,5-hexanedione, Tri-o-tolyl phosphate and leptophos belong to three fundamentally different chemical classes but all four chemicals cause central-peripheral distal axonopathy. Some of these compounds have been shown to alter brain steady state levels of neurotransmitters or to inhibit the activities of adenosine triphosphatases which are involved in the uptake and storage of biogenic amines. Tests were performed to determine alterations in steady state levels of rat brain norepinephrine and dopamine in response to doses of the above chemicals and of the central nervous system toxin, methyl mercuric chloride sufficient to cause ataxia. Catecholamine synthesis rate constant estimations were performed. Specific activities of tyrosine in brains of control and treatment groups following intravenous injection of labelled tyrosine were compared to determine if passage of tyrosine across the blood-brain barrier were affected by treatments. Levels of the dopamine metabolite, dihydroxyphenylacetic acid were assayed in all cases. Levels of the norepinephrine metabolite, 3-methoxy-4-hydroxymethylethyleneglycol sulfate, were assayed in response to acrylamide administration. Animal weights were recorded at the beginning and end of the treatment period. Rats treated with a cumulative dose of 250 mg/kg acrylamide had significantly lower norepinephrine levels than controls. 2,5-hexanedione administration significantly increased the dopamine synthesis rate constant at a cumulative dose of 210 mg/kg. Cumulative doses of 700 and 2100 mg/kg also appeared to elevate norepinephrine and dopamine synthesis rate constants, but values were not statistically significant. Leptophos caused a slight but significant increase in dopamine levels in rats administered a cumulative dose of 75 mg/kg. Methyl mercuric chloride caused variable effects to norephinephrine synthesis rate and lowered dopamine synthesis rate constant at cumulative doses of 5 to 50 mg/kg. No other alterations were seen in levels of catecholamines or of their metabolites, nor in synthesis rate constants of the catecholamines in response to administration of the five neurotoxic compounds. No evidence of altered blood-brain transport of tyrosine was observed at any level of neurotoxins administered. Rats given the highest cumulative doses of all neurotoxins except tri-o-tolyl phosphate gained significantly less weight than control animals. It was concluded that the four compounds which cause delayed distal neurotoxicity do not alter levels of turnover rates of brain catecholamines in a consistent manner.

Several industrial and environmental chemicals cause distal and/or central neuropathy among other diverse toxic effects. Spague-Dawley derived rats were fed doses of 2,5-hexanedione, acrylamide, tri-o-tolyl phosphate, leptophos and methylmercury via gavage. The dose levels and administration periods were established in previous experiments designed to assess clinical neuropathy using rats trained to walk on a rotorod apparatus fitted with an electrode floor. After intravenous injections of 3H-Tryptophan, whole rat brain homogenates were analyzed using liquid scintillation and spectrofluorometric techniques for levels of tryptophan, serotonin and 5-hydroxyindoleacetic acid. Serotonin turnover rates were calculated using the specific activities of tryptophan and serotonin at two different time periods. The levels of serotonin as well as the serotonin turnover rates were unaffected by dosages of 5 to 50 mg acrylamide/kg given daily doses, while whole brain concentrations of 5-hydroxyindoleacetic acid increased significantly in a dose-dependent manner. the rise in 5-hydroxyindoleacetic acid levels coupled with no effects on the other levels in acrylamide and 2,5-hexanedione-fed animals suggests a possible inhibition of the energy-dependent 5-hydroxyindoleacetic acid efflux system in the brain. Animals given five doses of Leptophos (4.5 to 45 mg/kg) or six doses from 30 to 300 mg/kg tri-o-tolyl phosphate, administered every third day, showed slightly eleveated, non-significant, serotonin turnover rates while levels of serotonin and tryptophan remained unchanged with a slight decrease in 5-hydroxyindoleacetic acid levels at the highest dosages. Levels of endogenous indole compounds in methylmercury treated rats showed no significant differences from control values; however, the turnover rates and levels of serotonin were slightly lower in the two lower treatment levels, while the highest dose level had no apparent effect on turnover rates or concentrations. Further studies involving longer treatment periods, alternate species or examination of discrete brain areas, may further clarify the effects of these chemicals on brain biochemistry.