Synaptic mechanisms underlying modulation of locomotor-related motoneuron output by premotor cholinergic interneurons

Spinal motor networks are formed by diverse populations of interneurons that set the strength and rhythmicity of behaviors such as locomotion. A small cluster of cholinergic interneurons, expressing the transcription factor Pitx2, modulates the intensity of muscle activation via 'C-bouton' inputs to motoneurons. However, the synaptic mechanisms underlying this neuromodulation remain unclear. Here, we confirm in mice that Pitx2 interneurons are active during fictive locomotion and that their chemogenetic inhibition reduces the amplitude of motor output. Furthermore, after genetic ablation of cholinergic Pitx2 interneurons, M2 receptor-dependent regulation of the intensity of locomotor output is lost. Conversely, chemogenetic stimulation of Pitx2 interneurons leads to activation of M2 receptors on motoneurons, regulation of Kv2.1 channels and greater motoneuron output due to an increase in the inter-spike afterhyperpolarization and a reduction in spike half-width. Our findings elucidate synaptic mechanisms by which cholinergic spinal interneurons modulate the final common pathway for motor output.