Accumulation of waterborne mercury(II) in specific areas of fish brain

We used whole-body autoradiography to study the distribution of Hg-203(II) in the central nervous system of brown (Salmo trutta) and rainbow (Oncorhyn chus mykiss) trout. Fish were either exposed to waterborne Hg(ll) for 7 and 21 d or they received an intravenous injection of the metal and were sacri ficed 1 and 21 d later. Mercury did not accumulate in the brain after intra venous injection, indicating that the blood-brain barrier is impervious to Hg in plasma. In contrast, Hg was accumulated in specific areas of the brai n (olfactory system, eminentia granulares and medulla of cerebellum, optic nerve and tectum, and rhombencephalon) and spinal cord (ventral horn gangli a) following water exposure. The specificity of the ac cumulation sites str ongly suggests that waterborne Hg was taken up by water-exposed receptor ce lls of sensory nerves and subsequently transferred toward the brain by axon al transport, a normal physiological process for the transport of organelle s and dissolved neuronal constituents along nerve axons. Accumulation of Hg in ventral horn ganglia is probably the result of leaching of metal from b lood into muscle followed by uptake in motor plates. Axonal transport allow s waterborne inorganic Hg, and possibly other xenobiotics, to circumvent th e blood-brain barrier. Considering the importance of complex behavior in th e life of fish, a nd the well-known deleterious effects of mercury on the n ervous system, the toxicological significance of this uptake route needs to be assessed.