In the past we have used PEI to knockdown gene in both DRG and SC.
Briefly, siRNA was dissolved in RNase-free water at the concentration of 1 μg/μl as stock solution, and mixed with polyethyleneimine (PEI, Fermentas Inc., Glen Burnie, MD), 10 min before injection, to increase cell membrane penetration. PEI was dissolved in 5% glucose, and 1 μg of siRNA was mixed with 0.18 μl of PEI. siRNA (2 μg) was intrathecally injected twice 2 and 1 day before the experiment.
To note, we were mostly targeting DRG in my experiments, but same protocol has been used in the lab to target SC.
For the siRNA, I suggest the Trilencer-27 siRNA kit from Origene. It contains three different siRNAs targeting different regions of the gene of interest and minimal interferon response (http://www.origene.com/siRNA/).