Extraneuronal saxitoxin binding sites in rabbit myelinated nerve

The changes in binding of 3H-labeled saxitoxin (STX) to rabbit sciatic nerve during axonal regeneration (after nerve crush) and during axonal degeneration (after nerve section) were measured and compared with the corresponding changes in the sciatic nerves of other mammals (rat, guinea pig, and cat). In the rabbit and rat, regeneration after nerve crush is associated with a 2- to 4-fold increase in STX binding capacity, consistent with the known corresponding increase in the number of nodes of Ranvier in regenerating nerve. Furthermore, consistent with the disappearance of nodes that occurs with Wallerian degeneration, nerve section leads to a disappearance of all, or most, of the STX binding in rat and guinea pig nerve, similar to that previously found for cat nerve. However, in the rabbit, nerve section leads to a large maintained increase in STX binding. Intraneural injection of diphtheria toxin, which is known to damage Schwann cells and which causes an increase in STX binding in intact nerves, abolishes the binding in cut nerves. It is suggested that the increased binding in cut nerves is to nonneuronal sites situated on the surface membrane of the Schwann cells, which have greatly proliferated in number as axonal degeneration has progressed. The reason for the difference between rabbits and other species and the possibility that the binding sites of rabbit Schwann cells represent functional sodium channels remain to be investigated.