Hindbrain V2a neurons impose rhythmic activity on motor neurons in an in vitro reticulospinal circuit

Abstract

The reticulospinal system is an evolutionarily conserved pathway among vertebrates that relays locomotor control signals from the hindbrain to the spinal cord. Recent studies have identified specific hindbrain cell types that participate in this circuit, including Chx10+ neurons of the medullary reticular formation, which project to the spinal cord and are active during periods of locomotion. To create a system in which reticulospinal neurons communicate with spinal motor effectors, we have constructed an in vitro model using two purified excitatory neuronal subtypes: HB9+ spinal motor neurons and Chx10+ hindbrain neurons. Cultured separately, these neurons exhibit cell type-specific patterns of activity; the Chx10+ cultures developed regular, synchronized bursts of activity that recruited neurons across the entire culture, whereas motor neuron activity consisted of an irregular pattern. A combination of the two subtypes produced cultures in which Chx10+ neurons recruited the motor neurons into synchronized network bursts, which were dependent on AMPA receptors. In addition to demonstrating that the activity of in vitro networks can depend on the developmental identity of their constituent neurons, we provide a new model with genetically specified nerve cell types to study the activity of a reticulospinal circuit.\n\nSignificance statementModels of the brain that use cultured neurons are usually comprised of a complex mixture of different kinds of cells, making it hard to determine how each cell type contributes to the overall pattern of activity. We made a simplified culture containing two cell types known to form a reticulospinal circuit in vivo. While in isolated cultures, each cell type had a distinct pattern of activity, in coculture the activity of one cell type came to dominate, indicating that the patterns observed in complex neuronal cultures arise in part from the distinctive properties of the constituent neurons.