Abstracts

Rationale: Studies of patients with implanted electrodes reveal functional effects independent of electrical stimulation. These electrodes necessarily cause lesions in tissue simply due to their insertion. To understand better the nature of these lesions, we investigated their anatomy and extent in the case of unilateral electrode implantation into the ventral hippocampus of rats in the absence of electrical stimulation. Significant tissue degeneration is observed away from the electrode track as distant as the contralateral hippocampus. We have developed stereologically based methods to quantify degeneration and to compare with Glial Fibrillary Acidic Protein (GFAP) expression and microglial activation, which are standard damage markers.Methods: All work was performed with IACUC approval. Male Sprague-Dawley rats (300g) were anesthetized and either underwent sham surgery or were implanted with a 250 micron diameter stainless steel electrode into the ventral hippocampus. Some unilateral implants were transient (i.e. electrode was removed before end of surgery). Animals were allowed to recover 2, 5, 10 or 21 days before sacrifice and perfusion. All brains were histologically prepared and treated with Amino-Cupric Silver stain, which stains for degeneration, as well as stains for GFAP expression and microglial activation. Degeneration was quantified by calculating the area fraction of silver-stained tissue within very thin focal planes. The area fraction method was compared to a faster non-stereological image analysis method based on luminance. GFAP expression was quantified by the calculation of stained area fraction. Microglia were exhaustively counted. Results: Area fraction of stained tissue is stereologically the soundest estimate for degeneration in the tissue, though it is cumbersome. Our image analysis technique, however, proved to give a good approximation of the area fraction results while being much faster and more easily automated. We found that gliosis and activated microglia were contained to an area near the electrode track, and uncorrelated with areas showing silver-stained degeneration. Patterns of degeneration were consistent with known anterograde axonal projections within and from the hippocampus and stemmed from the electrode path.Conclusions: The extent of damage caused by electrode implantation is not obvious in tissue stained for GFAP or activated microglia. For investigators wishing to determine the full range of damage from implanted electrodes, a stain designed specifically for all parts of degenerating neurons is necessary. These results are relevant to clinicians using implanted electrodes for stimulation as well as for pre-operative evaluation. NIH grants R01EB001507, K022MH01493, and R01MH50006