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

Locomotion in vertebrates is generated in the spinal cord but initiated by supraspinal centers. Spinal interneurons expressing Shox2 include putative locomotor rhythm generating neurons in mice. Reticulospinal neurons directly provide drive to spinal rhythm generating interneurons, which then convey rhythmic output. Excitatory neurons in the lateral paragigantocellular nucleus (LPGi) have been shown to provide this descending drive during locomotor initiation. Here, we performed viral tracing and electrophysiology to test for direct connections between the LPGi and lumbar Shox2 interneurons in adult mice. Using monosynaptic-restricted rabies tracing, we show that excitatory neurons from the LPGi make direct synaptic connections onto lumbar Shox2 interneurons. This connection from the ventral caudal medulla to Shox2 interneurons was confirmed via anterograde tracing and recordings of excitatory postsynaptic potentials in Shox2 interneurons. Thus, a subset of Shox2 interneurons receives monosynaptic excitatory input from the LPGi, which may provide the substrate for locomotor initiation.