Local Blockade of Sodium Channels by Tetrodotoxin Ameliorates Tissue Loss and Long-Term Functional Deficits Resulting from Experimental Spinal Cord Injury

Abstract

Although relatively little is known of the mechanisms involved in secondary axonal loss after spinal cord injury (SCI), recent data from in vitro models of white matter (WM) injury have implicated abnormal sodium influx as a key event. We hypothesized that blockade of sodium channels after SCI would reduce WM loss and long-term functional deficits. To test this hypothesis, a sufficient and safe dose (0.15 nmol) of the potent Na ϩ channel blocker tetrodotoxin (TTX) was determined through a doseresponse study. We microinjected TTX or vehicle (VEH) into the injury site at 15 min after a standardized contusive SCI in the rat. Behavioral tests were performed 1 d after injury and weekly thereafter. Quantitative histopathology at 8 weeks postinjury showed that TTX treatment significantly reduced tissue loss at the injury site, with greater effect on sparing of WM than gray matter. TTX did not change the pattern of chronic histopathology typical of this SCI model, but restricted its extent, tripled the area of residual WM at the epicenter, and reduced the average length of the lesions. Serotonin immunoreactivity caudal to the epicenter, a marker for descending motor control axons, was nearly threefold that of VEH controls. The increase in WM at the epicenter was significantly correlated with the decrease in functional deficits. The TTX group exhibited a significantly enhanced recovery of coordinated hindlimb functions, more normal hindlimb reflexes, and earlier establishment of a reflex bladder. The results demonstrate that Na ϩ channels play a critical role in WM loss in vivo after SCI.

Minocycline inhibits contusion-triggered mitochondrial cytochrome c release and mitigates functional deficits after spinal cord injury

“…As described before in similar SCI models (19,21,24), all experimental rats demonstrated complete hind limb paralysis at 1 day p.i., including areflexia and lack of coordinated motor functions, such as locomotion. Thereafter, partial recovery of function was observed over the next few weeks.…”

“…was conducted as previously described (23). At each time point a battery of tests of hind limb reflexes, as well as coordinated use of hind limbs, was performed by using the BBB scale for overall hind limb function, as previously described (23)(24)(25).…”

“…Solvent blue-and hematoxylin͞eosin-stained sections at the lesion epicenter and 1, 2, 3, and 4 mm rostral and caudal to the epicenter were digitized by using the SPOT VERSION 3.5.9.1 for MAC OS program (Diagnostic Instruments, Sterling Heights, MI) and analyzed with the MCID ELITE program (Imaging Research, St. Catherine's, ON, Canada). Lesion areas and areas of white matter sparing were defined as previously described (21,24,26).…”

Effects of MK801 on evoked potentials, spinal cord blood flow and cord edema in acute spinal cord injury in rats

“…Thus, we believe that treatment with NMDA receptor antagonist would improve the longterm conduction function of cord after the drug is washed out and would provide neuroprotection in a manner similar to Na channel blocker tetrodotoxin in the traumatic SCI. 103 …”

Clinical canine spinal cord injury provides an opportunity to examine the issues in translating laboratory techniques into practical therapy

“…Such models have allowed exploration of the underlying biochemical and cellular events that give rise to tissue destruction and have therefore suggested new targets for therapeutic intervention. [30][31][32][33] Modelling of contusion has also proved useful because variations in the severity of injury can be correlated with functional outcome, 34 allowing deductions to be made regarding the tissuesparing efficacy of putative therapeutic interventions.…”