PROJECT SUMMARY Spinal cord stimulators (SCS) and related devices are commonly used to treat refractory pain conditions, although mechanisms underlying pain reduction remain unclear. Improved understanding of SCS and the development of biomarkers are critical for improving device design and stimulation patterns and optimizing patient selection for device treatment. The main hypothesis of this proposal is that SCS devices reduce pain by modulating the excitability of peripheral sensory nerve fibers that project within the spinal dorsal columns, and this effect can be leveraged for biomarker development. The rationale for this hypothesis is that first order low-threshold mechanoreceptors project from the periphery through the dorsal column/medial lemniscus system to the brainstem. These are the neurons that (1) have closest proximity to the epidural space, the location of stimulation, and (2) have processes in both the periphery and spinal cord tracts. We will apply specialized tests of peripheral nerve excitability, threshold tracking nerve conduction studies, to detect changes in the excitability exerted on these neurons by SCS. We will also perform secondary measurements to determine other potential mechanisms of SCS in the peripheral and central nervous systems: microneurography, to detect modulation of primary nociceptor neuron excitability; PET-MRI, to measure changes in brain neuroinflammation; plasma cytokines and chemokines, to detect systemic neuroinflammatory effects due to SCS. We will perform sensory phenotyping to identify mechanisms in specific subgroups and pain and psychometric instruments to quantify responses. Aim 1 will use a multiple crossover design in subjects who have stably implanted SCS devices, and SCS settings will be toggled between therapeutic and minimal settings. Crossover intervals will include both two-week periods and rapid two-hour periods between setting changes. Aim 2 will consist of a prospective assessment of subjects prior and after implantation of new SCS devices, with a goal of identifying predictors of pain reduction response. The successful completion of this study will yield new mechanisms by which SCS reduces pain, relevant biomarkers, and further development of promising outcomes for broad pain research.