Lumbar spinal Shox2 interneurons receive monosynaptic excitatory input from the lateral paragigantocellular nucleus in the adult mouse.

Locomotor output in vertebrates is generated in the spinal cord but is initiated and controlled by descending projections from supraspinal structures. Spinal interneurons involved in locomotion have been revealed through manipulation of genetically identified interneurons in transgenic mouse lines. Lumbar spinal interneurons expressing the transcription factor Shox2 include putative locomotor rhythm generating neurons in mice. The direct connection between supraspinal and lumbar spinal locomotor-related interneurons is comprised of reticulospinal neurons which are thought to directly provide drive to spinal rhythm generating interneurons that receive descending input and convert it to a rhythmic output. Excitatory neurons in the lateral paragigantocellular nucleus (LPGi) within the medulla have been shown to provide this descending drive in the context of forward locomotor initiation. However, a direct connection between excitatory LPGi neurons and identified spinal rhythm generating neurons has yet to be demonstrated. Here, we performed viral tracing and electrophysiological recordings to test for direct connections between the LPGi and lumbar Shox2 interneurons in adult mice. Using both monosynaptic-restricted transsynaptic rabies and anterograde AAV tracing, we show that excitatory neurons from the LPGi make direct putative excitatory synaptic contacts onto lumbar spinal Shox2 interneurons. A monosynaptic connection was confirmed via recordings of excitatory postsynaptic potentials in Shox2 interneurons in lumbar spinal slices evoked by optogenetic activation of LPGi terminals. These results demonstrate that at least a subset of lumbar spinal Shox2 interneurons receive monosynaptic excitatory input from the LPGi in the medulla, a connection which may provide the substrate for the initiation of locomotion.