Intrinsic brainstem circuits comprised of Chx10-expressing neurons contribute to reticulospinal output in mice

Glutamatergic reticulospinal neurons in the gigantocellular reticular nucleus (GRN) of the medullary reticular formation can function as command neurons, transmitting motor commands to spinal cord circuits to instruct movement. Recent advances in our understanding of this neuron-dense region have been facilitated by the discovery of expression of the transcriptional regulator, Chx10, in excitatory reticulospinal neurons. Here, we address the capacity of local circuitry in the GRN to contribute to reticulospinal output. We define two subpopulations of Chx10-expressing neurons in this region, based on distinct electrophysiological properties and soma size (small and large), and show that these populations correspond to local interneurons and reticulospinal neurons, respectively. Using focal release of caged glutamate combined with patch clamp recordings, we demonstrated that Chx10 neurons form microcircuits in which the Chx10 local interneurons project to and facilitate the firing of Chx10 reticulospinal neurons. We discuss the implications of these microcircuits in terms of movement selection. NEW & NOTEWORTHY Reticulospinal neurons in the medullary reticular formation integrate inputs from higher regions to effectively instruct spinal motor circuits. Using photoactivation of neurons in brainstem slices, we studied connectivity of reticular formation neurons that express the transcriptional regulator, Chx10. We show that a subpopulation of these neurons functions as local interneurons that affect descending commands. The results shed light on the internal organization and microcircuit formation of reticular formation neurons.