Spinal dI3 neurons are involved in sustained motor adaptation elicited by low-threshold cutaneous afferents

Adaptation of muscle activity to meet a certain target or intention is traditionally attributed to supraspinal structures. However, evidence is mounting that this process can occur within the spinal cord through intrinsic plasticity and circuit reorganization. Here, we investigate the role of a class of excitatory spinal interneurons, called 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 toe elevation. The stimulation intensities were calibrated to selectively recruit low-threshold mechanoreceptors (LTMRs). To test the contribution of dI3s in this motor adaptation, inhibitory DREADD (hM4Di) receptors were expressed in Isl1⁺/Vglut2⁺ cells, achieving reversible, cell-type-specific silencing of dI3s. Our results demonstrate that stimulation of cutaneous inputs to the spinal cord contingent on a certain positional goal can generate sustained changes in motor activity, in this case, in the form of elevation of toe position above a preset vertical threshold. Chemogenetic silencing of dI3s abolished this motor adaptation induced by activation of LTMRs. These findings indicate that dI3 activity is essential for a particular type of motor adaptation driven primarily by LTMR input.