Spinal cord uptake of lead injected into muscle

It has been hypothesised that some motor neuron diseases may result from lead (Pb) being retrogradely transported in axons from muscle to spinal cord. To test this hypothesis, we injected mice in the right hindlimb with 0.1 ml of 0.25% or 2.5% Pb. In some animals the sciatic nerve was cut before injection. Pb levels were measured in the lumbar and thoracic spinal cord and the kidney with atomic absorption spectrophotometry (AAS). Four weeks after injection, Pb was found in the spinal cord on AAS, with higher levels in the lumbar than the thoracic cord only in the high-Pb group. Sciatic nerve cut did not inhibit the passage of Pb to the cord. The results suggest that most Pb injected into muscle reaches the spinal cord via the blood or damaged axons, rather than via intact axons. These data do not support the hypothesis of Pb-induced motor neuron disease via retrograde axonal transport from muscle.