The effect of direct current field polarity on recovery after acute experimental spinal cord injury

Recent evidence indicates that direct current (DC) fields promote recovery of acutely injured central and peripheral nervous system axons. The polarity of the applied DC field may play an important role in modulating these effects. In the present study, the effect of DC field polarity on recovery of injured spinal cord axons was examined anatomically, electrophysiologically and behaviourly in a rat model. After a 53 g clip compression injury of the cord at T1, 30 adult rats were randomly and blindly allocated to one of three groups (n = 10 each): one group received implantation of a DC stimulator (14 microA) with the cathode caudal to the injury site; the second group received implantation of a similar stimulator with the cathode rostral to the injury site; and the third group received a sham (O microA) stimulator. Clinical neurological function was assessed by the inclined plane technique and axonal function was assessed by motor- and somatosensory-evoked potentials (MEP and SSEP). A quantitative assessment of axonal integrity was performed by counting neurons in the brain retrogradely labelled by the axonal tracer horseradish peroxidase (HRP) and by counting axons at the injury site. The inclined plane scores (P less than 0.0001), MEP amplitude (P less than 0.02), counts of neurons retrogradely labelled by HRP (P less than 0.0001), and axon counts at the injury site (P less than 0.01) were significantly greater in the group treated with a DC field with the cathode caudal to the lesion than in the other two groups. Conversely, the cathode rostral DC field caused a decrease in the number of neurons retrogradely labelled by HRP (P less than 0.05) compared to the sham and cathode caudal groups. These data confirm our previous finding that DC fields promote recovery of acutely injured spinal cord axons. Furthermore, the polarity of the applied field is of critical importance to this effect.