Systematic characterization of spinal cord stimulation effects on dorsal horn populations

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There is a substantial need to understand the fundamental biological mechanisms of neuromodulation therapies in order to improve clinical delivery and outcomes (RFA-NS-20-006). Intractable chronic pain of the back and limbs continues to be challenging to treat clinically, and spinal cord stimulation (SCS) devices have experienced tremendous market growth despite a lack of an accepted mechanistic basis. There is minimal knowledge of how SCS engages dorsal horn circuits, primarily due to technical limitations associated with traditional electrophysiological recordings, such as stimulation-induced electrical noise artifacts and low sampling power. Multiphoton microscopy provides a novel and powerful approach to characterize the dorsal horn circuits modulated by SCS in transgenic mice genetically engineered to express calcium indicators in molecularly defined populations. An implantable miniature bipolar SCS electrode will be utilized to study the entire spectrum of clinically-relevant stimulation parameters on superficial dorsal horn populations. A wide range of frequencies, duty cycles, and waveforms will be investigated. Preliminary experiments demonstrated that SCS at 50 Hz drives sustained firing preferentially in GABAergic populations. We plan to systematically characterize the effects of stimulation parameters on GABAergic (Aim 1) and glutamatergic (Aim 2) populations. Neuronal networks distal and proximal to the SCS electrode will be examined, allowing the quantification of dorsal column and electrical field mediated effects, respectively. Nociceptive-range stimulation of afferent pathways will be combined with SCS to characterize responses in labeled output projection neurons, both in vitro and in intact mice implanted with a chronic imaging window (Aim 3). The Gate Control Theory suggested that activation of dorsal columns activates inhibitory neurons residing in laming II; this notion will be directly tested in Aim 1 using imaging techniques with superior sampling power and impervious to stimulation artifacts. This approach will provide an unprecedented understanding of the impact of SCS parameters, including energy delivery, on the activity of dorsal horn neurons over prolonged periods. This project's central goals are closely aligned with the RFA by proposing experiments to characterize cellular responses to neurostimulation in a relevant mouse model, with clear translational implications. Findings from these studies are expected to lay the foundation for the design and refinement of next-generation neuromodulation devices and substantially impact patient care.