Acute physiological response of mammalian central neurons to axotomy: ionic regulation and electrical activity

The transection of the axon of central neurons has dramatic consequences on the damaged cells and nerves. Injury activates molecular programs leading to a complex repertoire of responses that, depending on the cellular context, include activation of sprouting, axonal degeneration, and cell death. Although the cellular mechanisms started at the time of lesion are likely to shape the changes affecting injured cells, the acute physiological reaction to trauma of mammalian central neurons is not completely understood yet. To characterize the physiology of the acute response to axonal transection, we have developed a model of in vitro axotomy of neurons cultured from the rodent cortex. Imaging showed that axotomy caused an increase of calcium in the soma and axon. Propagation of the response to the soma required the activation of voltage-dependent sodium channels, since it was blocked by tetrodotoxin. The electrophysiological response to axotomy was recorded in patched neurons kept in the current clamp configuration: injury was followed by vigorous spiking activity that caused a sodium load and the activation of transient calcium currents that were opened by each action potential. The decrease of the electrochemical gradient of sodium caused inversion of the Na-Ca exchanger that provided an additional mean of entry for calcium. Finally, we determined that inhibition of the physiological response to axotomy hindered the regeneration of a new neurite. These data provide elements of the framework required to link the axotomy itself to the downstream molecular machinery that contributes to the determination of the long-term fate of injured neurons and axons.