Brain toxicity mercury exposure

Mercury (Hg) is a toxic metal—it is one of the so-called heavy metals. Hg is a neurotoxin that causes damage to the central nervous system (CNS), which consists of the brain and associated parts. Hg is active at about 50pg/100mL of blood (500 ppb). Central nervous system damage manifests itself as quarrelsome behavior, headaches, depression, and muscle tremors. The classic example of mercury poisoning is the mad hatter, caused by mercury exposure during the felt-making process. 

Studies in mice indicate that toxicity from exposure to dimethylmercury is the result of metabolic conversion of dimethylmercury to methylmercury, and that dimethylmercury does not enter the brain until it has been metabolized to methylmercury, which occurs over the first several days following absorption (Ostland 1969). Nierenberg et al. (1998) report the results of an analyses of mercury content in the hair of a 48-year-old female who died subsequent to an acute exposure to dimethylmercury. The results are consistent with the kinetic profdes for methylmercury, and support the hypothesis of a rapid conversion of dimethylmercury to a methylmercury metabolite. 

Rice DC (1989) Brain and tissue levels of mercury after chronic methyl mercury exposure in the monkey. ] Toxicol Environ Health 27 189-198. Roberts MC, Seawright AA and Ng JC (1979) Chronic phenylmercuric acetate toxicity in a horse. Vet Hum Toxicol 21 321-32. 

Van-Hoek AN, de-Jong MD, van-Os CH (1990) Effects of dimethylsulfoxide and mercurial sulfhydryl reagents on water and solute permeability of rat kidney brush border membranes. Biochim Biophys Acta 1030 203-210 Verity MA, Sarafian T (1991) Role of oxidative injury in the pathogenesis of methylmercury neurotoxicity. In Suzuki T, Imura N, Clarkson TW (eds) Advances in mercury toxicology. Plenum, New York, pp 209-222 Waku K, Nakazawa Y (1979) Toxic effects of several mercury compounds on SH-and non-SH enzymes. Toxicol Lett 4 49-55 Warfvinge K, Hua J, Berlin M (1992) Mercury distribution in the rat brain after mercury vapor exposure. Toxicol Appl Pharmacol 117 46-52 Wasteneys GO, Cardin M, Reuhl KR, Brown DL (1988) The effects of methylmercury on the cytoskeleton of murine embryonal carcinoma cells. Cell Biol Toxicol 4 41-60... 

A knowledge of physiology and pharmacokinetics is needed (Fanis et al. 1993 Monteiro and Furness 2001). Levels of mercuiy normally vary among internal tissues, and the time to equilibrate within each tissue varies. For example, blood mercury levels normally reflect veiy recent exposure, while brain and liver levels reflect longer-term exposure. Tissue-specific mechanisms of detoxification and seqnestration, among other processes, must be understood to define the bioactive moiety in observed tissue bmdens before a clear expression of toxicity can be derived (Woodetal. 1997). 

Mercury dimethyl is a highly toxic substance by all routes of exposure. Several cases of human poisoning are well documented. (Patnaik, P. 1999. A Comprehensive Guide to the Hazardous Properties of Chemical Substances, 2nJohn Wiley Sons.) The compound can accumulate in the brain and blood of humans. Intake of small quantities can cause death. 

During long-term constant exposure (several months) to methyl mercury in food, there is a linear relationship between daily intake of methyl mercury and the concentration of mercury in blood. The mercury concentration in blood (pg/L) corresponds to the daily intake of methyl mercury (pg/ day) multiplied by 0.5-1. When exposure is continuous, the blood mercury concentration is proportional to the concentration in the brain, the critical organ for methyl mercury toxicity. Because of mercury s short half-life in the blood (2-4 days), evaluation of blood mercury is of limited clinical value if a substantial amount of time has passed since time of exposure [43]. 

In a case report of four patients who were exposed to ethyl mercury, toxicity was seen in the brain, spinal motor neurons, peripheral nerves, skeletal muscles, and myocardium. Several case studies of accidental occupational exposure have also been documented. The most common signs of ethylmercury toxicity are paraesthesia, dysarthria, and constriction of the visual field. However, none of the symptoms of ethylmercury toxicity are specific and death is a common outcome if exposure levels are high. 

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