Uptake of waterborne tributyltin in the brain of fish: axonal transport as a proposed mechanism.

In previous studies, it was shown that waterborne Hg(II), Cd(II), and Mn(II) enter nerves innervating water-exposed sensory organs of fish and are transported to the brain by axonal transport. However, it is not known if organometals, such as tributyltin (TBT), can reach the brain of fish via the same route. In this work, we exposed rainbow trout (Oncorhynchus mykiss) to waterborne [113Sn]-TBT (4.2 kBq/L). Three fish were sampled after a 2-week exposure, and three others were sampled after a 2-week depuration period. Another group of four fish received an intravenous injection of [113Sn]-TBT and were sampled after 2 and 14 d. Distribution of the radiolabel was visualized and quantified by quantitative whole-body autoradiography. The brain accumulated a significant amount of 113Sn, with hot spots being found in parts receiving sensory nerves from water-exposed sensory organs, such as eminentia granulares (lateral lines organs). Labeling of the brain was also seen for i.v.-injected fish, indicating that the blood-brain barrier is not impervious to TBT or its metabolites. Nevertheless, the distribution of radioactivity in the brain was much more uniform, with no evident hot spot. Though the transfer [water --> gills --> blood stream --> blood-brain barrier --> brain] may account for a certain proportion of the radiolabel accumulation in fish brain, exposure to [113Sn]-TBT via water resulted in higher accumulation in some areas of the brain, of which the specific location strongly suggests that it was taken up in different water-exposed sensory nerve terminals and transported directly to the brain by axonal transport, as the parent compound or as a metabolite. The resulting local enhancement of the accumulation of butyltins might jeopardize the integrity of nervous system. Further work is needed to assess the toxicological significance of this process.