Effects of chronic exposure to cadmium, lead and mercury of brain biogenic amines in the rat.

Effects of chronic (45 days) treatment with different doses of cadmium chloride (0.25 and 1.0 mg/kg/day), methylmercury chloride (0.4 and 4.0 mg/kg/day) and lead acetate (0.2 and 1.0 mg/kg/day) and of 28-day withdrawal of treatment on the levels of acetylcholine (ACh) and activity of acetylcholinesterase (AChE) in cerebral cortex, and concentration of norepinephrine (NE) and 5-hydroxytryptamine (5-HT) in brain-stem were examined in rats. Exposure to both cadmium and methylmercury produced significant decreases in cortical ACh and brain-stem 5-HT levels. In addition, brain-stem NE concentration was increased in methylmercury-treated rats. In contrast, chronic treatment with lead resulted in enhanced cerebrocortical ACh levels but a decreased brain-stem NE concentration. Treatment with cadmium also produced a transient enhancement of striatal dopamine levels. Cadmium-induced decrease in brain-stem 5-HT and lead-induced accumulation of cortical ACh persisted even after 28 day withdrawal of treatment. The data indicate that chronic exposure to low doses of heavy metals produces differential changes in regional levels of various brain biogenic amines. These changes may represent the early signs of adverse effects on CNS function since they occur before any overt symptoms of neurotoxic effects of heavy metals become apparent.