Dependence of c-fos Expression on Amplitude of High-Frequency Spinal Cord Stimulation in a Rodent Model

Objectives: Clinical high-frequency spinal cord stimulation (hfSCS) (>250 Hz) applied at subperception amplitudes reduces leg and low back pain. This study investigates, via labeling for c-fos—a marker of neural activation, whether 500 Hz hfSCS applied at amplitudes above and below the dorsal column (DC) compound action potential (CAP) threshold excites dorsal horn neurons. Materials and Methods: DC CAP thresholds in rats were determined by applying single biphasic pulses of SCS to T₁₂–T₁₃ segments using pulse widths of 40 or 200 μsec via a ball electrode placed over the left DC and increasing amplitude until a short latency CAP was observed on the L₅ DC and sciatic nerve. The result of this comparison allowed us to substitute sciatic nerve CAP for DC CAP. SCS at T₁₂–T₁₃ was applied continuously for two hours using: sham or hfSCS at 500 Hz SCS, 40 μsec pulse width, and 50, 70, 90, or 140% CAP threshold. Spinal cord slices from T₁₁–L₁ were immunolabeled for c-fos, and the number of c-fos-positive cells was quantified. Results: 500 Hz hfSCS applied at 90 and 140% CAP threshold produced substantial (≥6 c-fos + neurons on average per slice per segment) c-fos expression in more segments between T₁₁ and L₁ than did sham stimulation (p < 0.025, 90% CAP; p < 0.001, 140% CAP, Fisher's Exact Tests) and resulted in more c-fos-positive neurons on average per slice per segment ipsilateral to than contralateral to the SCS electrode at 70, 90, and 140% CAP threshold (p < 0.01, Wilcoxon Signed Rank Tests). Conclusions: The finding of enhanced c-fos expression in the ipsilateral superficial dorsal horn provides evidence for activation/modulation of neuronal circuitry associated with subperception hfSCS.