Effects of neonatal mercuric chloride administration on growth and biochemical development of neuronal and non-neuronal tissues in the rat: comparison with methylmercury.

Mercuric chloride (HgCl2) was administered at 3 dosage levels (0.5, 1, and 2.5 mg/kg s.c.) daily to newborn rats beginning at 1 day of age and continuing through weaning. HgCl2 produced a dose-dependent inhibition of body growth which was apparent only after weaning and which worsened as the animals approached adulthood; brain growth was also adversely affected, but less so than was body weight. Growth of other tissues (heart, kidney, liver) was influenced in a different manner, with initial increases over control organ weights and a subsequent decline toward normal (kidney) or to subnormal levels (heart, liver). Examination of ornithine decarboxylase (ODC) activity, an index of cellular maturation, confirmed that HgCl2 produced separable types of effects in different organ systems. Although the patterns of growth alterations caused by HgCl2 resembled those seen with methylmercury, the effects on ODC were dissimilar, suggesting that there were some differences in the underlying biochemical mechanisms. In addition to causing generalized alterations of cellular development, HgCl2 produced specific effects on catecholamine neurotransmitter systems, with increases in brain norepinephrine levels and turnover as well as elevations in synaptosomal uptake capabilities for norepinephrine. Dopamine levels and turnover were slightly reduced or unchanged, but synaptosomal uptake was still elevated. Target-specific effects were also apparent in the peripheral sympathetic nervous system, where renal but not cardiac norepinephrine was elevated in the HgCl2 group. Again, some but not all of these changes can be produced by comparable exposure to methylmercury; many of the neuronal effects in animals exposed to HgCl2 were in the opposite direction from those seen with the organomercurial.