Understanding the spinal circuits that organize rhythmic limb movements is critical for future treatment of patients with spinal cord injuries. The overall goal of this project is to further our understanding of a set of intemeurons that participate in the central pattern generator (CPG) for locomotion in the neonatal rat spinal cord. The commissural intemeurons (CINs) send their axons to the contralateral spinal cord and are thought to play critical roles in left-right coordination and rhythm generation during locomotion. I propose to characterize two of the ionic currents that control post-inhibitory rebound in three anatomically defined classes of CINs with ascending, descending or intrasegmental axons. Using whole cell voltage clamp recordings, I will measure the parameters of the hyperpolarization-activated inward current, Ih and the low-threshold calcium current, IT, in each cell type, and determine the effects of serotonin on these currents. I will perform single cell RT- PCR studies to determine which genes related to Ih and IT are expressed in each CIN class. These experiments should provide further criteria for rigorous identification of neuronal classes in the CPG for locomotion in the neonatal rat spinal cord.