Traumatic axonal injury induces calcium influx modulated by tetrodotoxin-sensitive sodium channels

Diffuse axonal injury (DAI) is one of the most common and important patholo gies resulting from the mechanical deformation of the brain during trauma. It has been hypothesized that calcium influx into axons plays a major role in the pathophysiology of DAI. However, there is little direct evidence to support this hypothesis, and mechanisms of potential calcium entry have not been explored. In the present study, we used an in vitro model of axonal s tretch injury to evaluate the extent and modulation of calcium entry after trauma. Using a calcium-sensitive dye, we observed a dramatic increase in i ntra-axonal calcium levels immediately after injury. Axonal injury in a cal cium-free extracellular solution resulted in no change in calcium concentra tion, suggesting an extracellular source for the increased post-traumatic c alcium levels. We also found that the post-traumatic change in intra-axonal calcium was completely abolished by the application of the sodium channel blocker tetrodotoxin or by replacement of sodium with N-methyl-D-glucamine. In addition, application of the voltage-gated calcium channel (VGCC) block er omega -conotoxin MVIIC attenuated the post-traumatic increase in calcium . Furthermore, blockade of the Na+-Ca2+ exchanger with bepridil modestly re duced the calcium influx after injury. In contrast to previously proposed m echanisms of calcium entry after axonal trauma, we found no evidence of cal cium entry through mechanically produced pores (mechanoporation). Rather, o ur results suggest that traumatic deformation of axons induces abnormal sod ium influx through mechanically sensitive Na+ channels, which subsequently triggers an increase in intraaxonal calcium via the opening of VGCCs and re versal of the Na+-Ca2+ exchanger.