dI3 Neurons Mediate Spinal Motor Learning

Abstract

Motor learning, traditionally attributed to supraspinal structures, is increasingly recognized as a process that can occur within the spinal cord through intrinsic plasticity and circuit reorganization. Here, we investigate the role of a class of excitatory spinal interneurons - dorsal interneuron 3 (dI3) - in the acquisition of novel motor behaviors independent of supraspinal input. Using a real-time, closed-loop stimulation paradigm in spinalized mice, we promoted a persistent adaptation in the hindlimb position to be higher than its resting level by delivering saphenous nerve stimulation contingent on hindlimb position. The stimulation intensities were calibrated to selectively recruit low-threshold mechanoreceptors (LTMRs) that provide input to dI3s and mediate non-nociceptive sensory input relevant to motor control. To test the contribution of dI3s in this learning, inhibitory DREADD (hM4Di) receptors were expressed in Isl1⁺/VGLUT2⁺ cells, achieving reversible, cell-type-specific silencing of dI3s. Our results demonstrate that closed-loop stimulation reliably induces spinal motor learning, evidenced by a sustained behavioural change - elevation of hindlimb position above a preset vertical threshold. Chemogenetic silencing of dI3s abolished this motor learning at LTMR-selective stimulation intensities, and performance did not improve with repeated trials under dI3 inhibition. These findings indicate that dI3 activity is essential for the acquisition of new spinal motor skills with learning that is driven primarily by LTMR input. This work advances our understanding of the cellular and circuit mechanisms underlying spinal motor learning and highlights dI3s as a critical component of experience-dependent motor adaptation in the absence of supraspinal input.