Increased axonal transport in the rat optic system after systemic exposure to methylmercury: differential effects in local vs systemic exposure conditions

Axonal transport was studied by several techniques in the optic system of adult female Long-Evans rats following systemic exposure to methylmercury in 5 mM Na₂CO₃. Control rats were treated with the buffer alone. Four mg Hg/kg body weight for 4-6 days, or for 12 days, induced significant changes in the rate of protein synthesis in the retinal cells and in the rate of propagation of protein-bound radioactivity along the ganglion cell axons. Axonal transport of particulate material in both groups treated with methylmercury increased to a rate of 147 mm/day compared to 93 mm/day in controls. Methylmercury was distributed evenly throughout the retinogeniculate system. No clinical neuropathy was evident in either mercury-treated group. It is proposed that the increased rates of transport may represent an adaptive compensatory response to distal axonopathy caused by methylmercury. To investigate why systemic dosing produced effects opposite to those observed with local application of MeHg, various doses of MeHg were tested in the local and systemic paradigms, including doses which yielded equal concentrations of Hg in the retina. The results indicate that the differential response between the two treatment conditions is not a function of local dose, per se. Local and systematic application produce different dose-effect curves, which do not coincide at any dose.