Firing properties of identified interneuron populations in the mammalian hindlimb central pattern generator

Little is known about the network structure of the central pattern generator (CPG) controlling locomotor movements in mammals. The present experiments aim at providing such knowledge by focusing on commissural interneurons (CINs) involved in left-right coordination. During NMDA and 5-HT-initiated locomotor-like activity, we recorded intracellularly from caudally or descending projecting L2 and L3 CINs (dCINs) located in the ventromedial area of the lumbar spinal cord in newborn rats. This region is crucial for rhythmic motor output and left-right coordination. The overall sample of dCINs represented a heterogenous population with neurons that fired in all phases of the locomotor cycle and exhibited varying degrees of rhythmicity, from strongly rhythmic to nonrhythmic. Among the rhythmic, putative CPG dCINs were populations that fired in-phase with the ipsilateral or with the contralateral L2 locomotor-like activity. There was a high degree of organization in the dorsoventral location of rhythmic dCINs, with neurons in-phase with the ipsilateral L2 activity located more ventrally. Spikes of rhythmically active dCINs were superimposed on membrane oscillations that were generated predominantly by synaptic input, with little direct contribution from the intrinsic pacemaker hyperpolarization-activated inward current. For both ipsilaterally and contralaterally firing dCINs the dominant synaptic drive was in-phase with the ipsilateral L2 motor activity. This study provides the first characterization of putative CPG interneurons in the mammalian spinal cord. Our results suggest an anatomical and physiological separation of CPG commissural interneurons in the ventral horn and demonstrate that it is possible to target specific interneuron subpopulations in the mammalian locomotor network.