Accumulation of Waterborne Mercury(II) in Specific Areas of Fish Brain

Abstract

We used whole-body autoradiography to study the distribution of 203Hg(II) in the central nervous system of brown (Salmo trutta) and rainbow (Oncorhynchus mykiss) trout. Fish were either exposed to waterborne Hg(II) for 7 and 21 d or they received an intravenous injection of the metal and were sacrificed 1 and 21 d later. Mercury did not accumulate in the brain after intravenous injection, indicating that the blood−brain barrier is impervious to Hg in plasma. In contrast, Hg was accumulated in specific areas of the brain (olfactory system, eminentia granulares and medulla of cerebellum, optic nerve and tectum, and rhombencephalon) and spinal cord (ventral horn ganglia) following water exposure. The specificity of the accumulation sites strongly suggests that waterborne Hg was taken up by water-exposed receptor cells of sensory nerves and subsequently transferred toward the brain by axonal transport, a normal physiological process for the transport of organelles and dissolved neuronal constituents along nerve axons. Accumulation of Hg in ventral horn ganglia is probably the result of leaching of metal from blood into muscle followed by uptake in motor plates. Axonal transport allows waterborne inorganic Hg, and possibly other xenobiotics, to circumvent the blood−brain barrier. Considering the importance of complex behavior in the life of fish, and the well-known deleterious effects of mercury on the nervous system, the toxicological significance of this uptake route needs to be assessed.