Nonlinear Firing Dynamics in Spinal Interneurons May Delineate the Presence or Absence of Spinal Cord Injury-related Neuropathic Pain.

Spinal cord injury (SCI) often results in hyperexcitability of sensory-dominant regions of the spinal cord, leading to muscle spasms, spasticity, and SCI-related neuropathic pain (SCI-NP). In part, these symptoms are due to injury-induced alterations in neuromodulatory drive. Altered neuromodulatory drive changes a neuron's input-output function, increasing or decreasing its sensitivity to a given excitatory or inhibitory input through actions on metabotropic receptors on the dendrites. Many of these actions result in downstream effects on the voltage-gated ion channels that mediate persistent inward currents (PICs), which can markedly exaggerate the gain of a given spinal neuron to a synaptic input(s). Here, we hypothesize that SCI-NP is associated with altered neuromodulatory drive, revealed through the firing dynamics of sensory interneurons in the spinal cord. We find evidence of altered PICs and associated self-sustained firing as well as firing state changes in rats with and rats without SCI-NP, however we find a greater prevalence of features associated with altered neuromodulation in animals with SCI-NP than those without.