Sodium-dependent regenerative responses in dendrites of axotomized motoneurons in the cat

Abstract

Ten days after extradural axotomy, partial spikes are found in greater than 20% of cat L7 motoneurons, while 15-21 days after axotomy the incidence increases to 60%. These responses are produced in excitable (hot) spots in the dendrites by synaptic excitation. Intracellular injection of QX-314, a lidocaine derivative and effective blocker of Na+ channels from within neurons, results in elimination of partial spikes before blocking somadendritic spikes. The action of QX-314 does not depend on changes in passive membrane properties or on changes in synaptic properties. Injections of Cs+ and Cl- ions rule out any major role for calcium, potassium, or chloride currents in the production of partial spikes. The partial spikes represent an unusual Na+-dependent dendritic phenomenon induced by axotomy when carried out relatively near the soma. It is reasonable to postulate that the partial spikes result from a higher concentration of Na+ channels in the dendrites. This may be the consequence of a high rate of production of Na+-channel proteins that are intended for the cut end of the axon; alternatively, they may result from the reflection from the cut end of such proteins produced at either a normal or an increased rate. These aberrant channels are inserted into somatic and dendritic membranes in higher concentrations than normal and, as well as producing local dendritic regions of low safety factor responsible for the partial spikes, also produce somadendritic spikes of unusually fast rise time and lower than usual threshold, which are relatively resistant to QX-314.