Early terminal and nodal sprouting of motor axons after botulinum toxin

Axonal sprouting in distal motor axons was studied in an attempt to answer two questions: (a) is the cell body required for early axonal sprouting?, and (b) do nodal, as well as terminal, axonal sprouts arise after muscle inactivity not caused by nerve injury? Botulinum toxin (BT) was used to induce axonal sprouting without nerve trauma. Mice were injected in the right calf with a sublethal dose of BT and the soleus muscle examined ultrastructurally at times varying from 3 h to 5 days post-injection. Terminal axonal sprouts were seen 2 days after injection, and based on the time taken for BT to act and the growth rate of sprouts, axons were calculated to sprout within 24 h of muscle inactivity. This short time suggests that early axonal regrowth is initiated and controlled at the distal axon. Sprouts were seen arising from the intramuscular nodes of Ranvier from 2 days after BT injection. Unlike the terminal sprouts which elongated over time, the nodal sprouts remained short and confined by the basal lamina overlying the node, probably because without structural denervation there were no empty perineural sheaths to act as pathways to the motor endplates. The finding of terminal and nodal sprouts after botulinum toxin supports the hypothesis that muscle inactivity gives rise to a single growth factor for both terminal and nodal sprouting.