Red Nucleus Excitatory Neurons Initiate Directional Motor Movement in Mice

Here, we systematically investigate the role of RN neurons in lateralized motor behavior. Using a combination of fiber photometry and optrode recordings in mice performing standardized T-maze tasks, we reveal RN neuronal ensembles with preferential activation during ipsilateral turning movements. Through cell-type-specific optogenetic manipulations, we establish a causal relationship between RN neuronal activity and directional motor control: selective activation or inhibition of RN glutamatergic neurons facilitates or block ipsilateral turning behavior. Anterograde trans-synaptic tracing with H129 reveals selective RN projections to spinal interneuron populations that govern ipsilateral flexion. Together, these findings redefine the RN as a specialized hub in midbrain networks mediating lateralized movements and provide a mechanistic framework for understanding asymmetric motor control, with potential implications for therapeutic approaches to movement disorders. The dataset primarily includes the original data for 5 key figures and the corresponding analysis code.