NMDA receptor activation triggers voltage oscillations, plateau potentials and bursting in neonatal rat lumbar motoneurons in vitro.

Whole-cell recordings of lumbar motoneurons in the intact neonatal rat spinal cord in vitro were undertaken to examine the effects of N-methyl-D-aspartate (NMDA) receptor activation on membrane behaviour. Bath application of NMDA induced rhythmic voltage oscillations of 5.9+/-2.1 mV (SD) at a frequency of 4.4+/-1.5 Hz. Amplitude, but not frequency, of the voltage oscillations was membrane potential-dependent. Voltage oscillations could recruit action potentials and/or plateau potentials with or without superimposed bursting. Blockade of synaptic transmission with tetrodotoxin (TTX) sometimes resulted in a loss of oscillatory activity which could then be restored by increasing the NMDA concentration. After application of TTX, the trajectory of NMDA-induced oscillations was similar to the trajectory induced in the presence of intact synaptic networks, although the mean oscillation duration was longer and the oscillation frequency was slower (1.8+/-1.1 Hz). Current ramps delivered after bath application of NMDA demonstrated bistable membrane properties which may underlie the plateau potentials. Injection of intracellular current pulses could initiate, entrain and terminate individual plateau potentials. The results suggest that membrane depolarization produced by oscillations may activate other intrinsic conductances which generate plateau potentials, thereby providing the neuron with enhanced voltage sensitivity, compared to that produced by NMDA receptor activation alone. These oscillatory events may have a role in the regulation of motor output in a variety of rhythmic behaviours including locomotion.