Elimination of superfluous neuromuscular junctions in rat calf muscles recovering from botulinum toxin-induced paralysis.

In order to determine the fate of the superfluous neuromuscular junctions (NMJs) formed during the course of botulinum toxin (BoTx)-induced paralysis, we have quantified the change in the total length of the nerve muscle contact area(s) following BoTx injection into rat calf muscles. The results indicate that: (1) at least some of the superfluous NMJs are eliminated following muscle recovery; (2) synapse elimination is a slow process, as 4 months after recovery it was not yet complete; (3) muscles with different content of type I and II fibers follow a different time course during synapse formation and elimination. We further investigated the possibility that the neural cell adhesion molecule (NCAM) would be the element whose loss from the NMJ might play a role in synapse elimination. Using immunofluorescence and immunoelectron microscopy we show that NCAM is exclusively localized between nerve terminals and Schwann cells and not between nerve terminals and muscle. This localization was maintained throughout paralysis and following recovery, suggesting that NCAM does not play a role in synapse elimination.