Temperature dependence of membrane sealing following transection in mammalian spinal cord axons

Using an in vitro sucrose-gap recording chamber, sealing of cut axons in isolated strips of white matter from guinea pig spinal cord was measured by recording the "compound membrane potential". This functional sealing was found to correlate well with anatomical resealing, measured by a horseradish peroxidase uptake assay. Near-complete functional and anatomical recovery of the axonal membrane occurred routinely within 60 min following transection at 37 degrees C in regular Krebs' solution. The rate of membrane potential recovery is exponential, with a time-constant of 20+/-5 min. The sealing process at 31 degrees C was similar to that at 37 degrees C, and was effectively blocked at 25 degrees C, under which condition most axons continued to take up horseradish peroxidase for more than 1h, and failed to substantially recover their membrane potential. Seventy-five percent of the cords transected at 40 degrees C had similar sealing behavior to those at 37 degrees C and 31 degrees C. The balance failed to seal the cut end. Two-dimensional morphometric analysis has shown that raising the temperature from 25 degrees C to above 31 degrees C significantly decreases axonal permeabilization to horseradish peroxidase (increases the sealing of transected ends) across all areas of a transverse section of spinal cord. Moreover, this enhancement of sealing exists across all axon calibers. Since severe cooling compromises membrane resealing, caution needs to be taken when hypothermic treatment (below 25 degrees C) is applied within the first 60 min following mechanical injury. In summary, we have found that at normal temperature (37 degrees C), nerve fibers repair their damaged membrane following physical injury with an hour. This is similar at mildly lower (31 degrees C) and relatively higher (40 degrees C) temperature, although some fibers tend to collapse under this febrile temperature. Moreover, severely low temperature (25 degrees C) hindered the repair of damaged membranes. Based on our study, caution is needed in treating spinal cord injury with low temperatures.