Elsevier

Neuroscience

Volume 91, Issue 1, June 1999, Pages 251-264
Neuroscience

Electrically mediated regeneration and guidance of adult mammalian spinal axons into polymeric channels

https://doi.org/10.1016/S0306-4522(98)00584-3Get rights and content

Abstract

An extracellular electric field has been shown to influence the regeneration of nerve fibers within the adult mammalian spinal cord. However, in these studies, few axons were labeled by local application of intracellular markers relative to the number of axons transected. This has limited an evaluation of the robustness of the response, and the direction of growth of regenerating axons that might be influenced by the orientation of the applied voltage gradient. In this study, a hollow silicone rubber tube (c. 6 mm×1 mm outside diameter) containing a cathodal (negative) electrode was inserted longitudinally into the dorsal half of the adult guinea-pig spinal cord. The electric field (∼100 μV/mm) was imposed within the damaged spinal cord with an implanted d.c. stimulator for about three weeks. Based on previous studies, this orientation of the electric field would be expected to both initiate axonal regeneration and guide growing axons to, and into, the silicone guidance channel. In experimental animals (n=20), a robust regeneration of axons into the tube was observed in more than half the cases. These axons were traced from surrounding white and gray matter by anterograde and retrograde labeling using a tetramethylrhodamine-conjugated dextran as an intracellular marker. Control animals (n=16) received tubes with inactive electrodes. It was rare to find any axons within control guidance channels, since adult mammalian central nervous system axons do not regenerate.

This report provides evidence for not only the facilitated regeneration of adult mammalian central axons, but also their guidance, by an imposed electric field.

Section snippets

The implant

The constant-current (d.c.) implant consisted of two components: the stimulator and a hollow silicone rubber guidance channel containing an active negative electrode (the cathode) or a sham electrode within its center.

The stimulator was similar to previously reported designs11., 15., 17. using one 3-V Lithium Cell (Ray o Vac 1225 BR) connected to a constant-current source (National Semiconductor LM 334 Z), which regulates the d.c. output, and a one-eighth-watt resistor to set the total current

General procedure

Animals tolerated the surgical procedures well, and the minor mortality was not significantly different between the experimental and control groups. All stimulator units were functional at the time of their removal (the battery would not be exhausted for another week or more of continuous operation). Although both ends of the tube were visually confirmed to be imbedded in the parenchyma of the spinal cord at surgery, in five cases (four control, one experimental), one end worked out of the cord

Discussion

This report demonstrates that a weak electric field imposed within a damaged adult guinea-pig spinal cord can both induce the regeneration of axons, and guide their growth, into the ends of a hollow silicone rubber tube inserted into the dorsal half of the cord. This is because a pair of electrodes produced a d.c. voltage gradient within the injured spinal cord, with the cathode located within the experimental tubes. This negative electrode drew current into the open ends of the tubes,

Acknowledgements

I dedicate this work to the memory of Professor Melvin Cohen of Yale University, mentor and friend. I gratefully acknowledge the expert technical assistance of Aaron Harbath for stimulator fabrication, Debra Bohnert for expert surgery and preparation of the tissues for analysis and viewing, and Andrea Street and Jenifer Sanson for manuscript preparation. David Williams provided the artwork used in the Experimental Procedures section. This work was supported by a grant from the Public Health

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