Synaptic NMDA Receptor-Dependent Ca(2+) Entry Drives Membrane Potential and Ca(2+) Oscillations in Spinal Ventral Horn Neurons

During vertebrate locomotion, spinal neurons act as oscillators when initiated by glutamate release from descending systems. Activation of NMDA receptors initiates Ca(2+)-mediated intrinsic membrane potential oscillations in central pattern generator (CPG) neurons. NMDA receptor-dependent intrinsic oscillations require Ca(2+)-dependent K(+) (K(Ca)2) channels for burst termination. However, the location of Ca(2+) entry mediating K(Ca)2 channel activation, and type of Ca(2+) channel – which includes NMDA receptors and voltage-gated Ca(2+) channels (VGCCs) – remains elusive. NMDA receptor-dependent Ca(2+) entry necessitates presynaptic release of glutamate, implying a location at active synapses within dendrites, whereas VGCC-dependent Ca(2+) entry is not similarly constrained. Where Ca(2+) enters relative to K(Ca)2 channels is crucial to information processing of synaptic inputs necessary to coordinate locomotion. We demonstrate that Ca(2+) permeating NMDA receptors is the dominant source of Ca(2+) during NMDA-dependent oscillations in lamprey spinal neurons. This Ca(2+) entry is synaptically located, NMDA receptor-dependent, and sufficient to activate K(Ca)2 channels at excitatory interneuron synapses onto other CPG neurons. Selective blockade of VGCCs reduces whole-cell Ca(2+) entry but leaves membrane potential and Ca(2+) oscillations unaffected. Furthermore, repetitive oscillations are prevented by fast, but not slow, Ca(2+) chelation. Taken together, these results demonstrate that K(Ca)2 channels are closely located to NMDA receptor-dependent Ca(2+) entry. The close spatial relationship between NMDA receptors and K(Ca)2 channels provides an intrinsic mechanism whereby synaptic excitation both excites and subsequently inhibits ventral horn neurons of the spinal motor system. This places the components necessary for oscillation generation, and hence locomotion, at glutamatergic synapses.