Major strides have been taken in the regulation of lead intoxication in the general population, but studies using genetic markers of susceptibility to environmental toxicants raise the question of whether genes can make certain individuals more vulnerable to environmental toxins such as lead. At least three polymorphic genes have been identified that potentially can influence the bioaccumulation and toxicokinetics of lead in humans. The first gene to be discussed in this review is the gene coding for delta-aminolevulinic acid dehydratase (ALAD), an enzyme of heme biosynthesis, that exists in two polymorphic forms. The resulting isozymes have been shown to affect the blood and bone lead levels in human populations. The effects of ALAD in lead intoxication have also been studied in laboratory mice that differ in the genetic dose for this enzyme. The second gene reviewed here is the vitamin D receptor (VDR) gene. The VDR is involved in calcium absorption through the gut and into calcium-rich tissues such as bone. Recent findings suggest that VDR polymorphism may influence the accumulation of lead in bone. Finally, the third gene to be discussed here that may influence the absorption of lead is the hemochromatosis gene coding for the HFE protein. The presence of mutations in the HFE gene leads to hemochromatosis in homozygotic individuals. Because of the associations between iron and lead transport, it is possible that polymorphisms in the HFE gene may also influence the absorption of lead, but this has not yet been studied. More studies will be needed to define the role of these genes in lead intoxication.ImagesFigure 2Figure 4

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Individual differences within strains of rats have been demonstrated for dopamine-mediated behaviours and responses to dopaminergic drugs. Differences in mesolimbic dopamine function may underlie individual differences in some of these behaviours, including sugar consumption and amphetamine hyperlocomotion. The present study addressed two potential mechanisms for these differences in dopamine-mediated behaviours. The possibility of functional differences in dopamine receptor subtypes was tested in LOW and HIGH sugar feeders. LOW and HIGH feeders did not differ in their response to the partial D1 agonist SKF-38393. The highest dose (2.5 mg/kg) of the D2 agonist quinpirole stimulated locomotor activity to a greater degree in a subset of HIGH sugar feeders as compared with LOW feeders. All doses of amphetamine induced a greater locomotor response in HIGH feeders as compared with LOW feeders, and HIGH feeders exhibited higher levels of extracellular dopamine in the nucleus accumbens than LOW feeders following exposure to sugar and treatment with amphetamine. These results support the interpretation that LOW and HIGH feeders exhibit differences in presynaptic nucleus accumbens dopamine function that account for the expression of individual differences in sugar consumption and response to amphetamine treatments. A subset of HIGH feeders may also exhibit greater D2 receptor function.