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-meth yl-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 freque ncy, of the voltage oscillations was membrane potential-dependent. Vol tage oscillations could recruit action potentials and/or plateau poten tials wi ih or without superimposed bursting. Blockade of synaptic tra nsmission with tetrodotoxin (TTX) sometimes resulted in a loss of osci llatory activity which could then be restored by increasing the NMDA c oncentration. 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). Curr ent ramps delivered after bath application of NMDA demonstrated bistab le membrane properties which may underlie the plateau potentials. inje ction of intracellular current pulses could initiate, entrain and term inate 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 r ole in the regulation of motor output in a variety of rhythmic behavio urs including locomotion.