After intramuscular injection, [125I] tetanus toxin is carried to spinal motor neurons by retrograde axonal transport. In the present study, we asked whether systemically administered toxin can be transported toward the spinal cord via this intraaxonal pathway. Mice were given [125I] tetanus toxin intraperitoneally, one sciatic nerve was ligated and, 24 h later, the animals were perfused. Autoradiograms showed labeled toxin accumulated within axons distal to the ligature. We interpret these findings as indicating that circulating toxin can leak out of intramuscular capillaries, be taken up at nerve terminals, and subsequently be carried toward the spinal cord by retrograde axonal transport. This pathway may be an important route by which systemically administered toxin can reach the CNS.
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