Electrically Evoked Compound Action Potentials in Spinal Cord Stimulation: Implications for Preclinical Research Models

Objectives The study aimed to assess the feasibility of recording electrically evoked compound action potentials (ECAPs) from the rat spinal cord. To achieve this, we characterized electrophysiological responses of dorsal column (DC) axons from electrical stimulation and quantified the relationship between ECAP and motor thresholds (ECAPTs and MTs). Material and Methods Naive, anesthetized and freely behaving rats were implanted with a custom-made epidural spinal cord stimulation (SCS) lead. Epidural stimulation and recordings were performed on the same lead using specifically designed equipment. Results The ECAPs recorded from the rat spinal cord demonstrated the expected triphasic morphology. Using 20 mu sec pulse duration and 2 Hz frequency rate, the current required in anesthetized rats to generate ECAPs was 0.13 +/- 0.02 mA, while the average current required to observe MT was 1.49 +/- 0.14 mA. In unanesthetized rats, the average current required to generate ECAPs was 0.09 +/- 0.02 mA, while the average current required to observe MT was 0.27 +/- 0.04 mA. Thus, there was a significant difference between the ECAPT and MT in both anesthetized and unanesthetized rats (MT was 13.39 +/- 2.40 and 2.84 +/- 0.33 times higher than ECAPT, respectively). Signal analysis revealed average conduction velocities (CVs) suggesting that predominantly large, myelinated fibers were activated. In addition, a morphometric evaluation of spinal cord slices indicated that the custom-made lead may preferentially activate DC axons. Conclusions This is the first evidence demonstrating the feasibility of recording ECAPs from the rat spinal cord, which may be more useful in determining parameters of SCS in preclinical SCS models than MTs. Thus, this approach may allow for the development of a novel model of SCS in rats with chronic pain that will translate better between animals and humans.