Development of locomotor activity induced by NMDA receptor activation in the lumbar spinal cord of the rat fetus studied in vitro

The development of neuronal circuits generating locomotor activity was characterized in an isolated lumbar spinal cord preparation from of fetal and neonatal rats. Locomotor activity induced by bath application of the NMDA receptor agonists, NMA and NMDA, was monitored from both sides of the corresponding lumbar ventral roots. Activation of NMDA receptors first evoked rhythmic motor activity at E15.5. NMA-induced rhythmic motor activity was not observed under synaptic blockade by TTX or cadmium ions, suggesting that this activity was evoked by synaptic drive from the interneuronal circuits in the spinal cord. At E15.5-E16.5, the rhythmic motor activity on both sides was synchronized. Phase relationship of the rhythmic motor activity between both sides was variable at E17.5-E19,5. The rhythmic motor activity was alternating on both sides at E20.5. Mid-sagittal splitting of the spinal cord did not affect the rhythm generation at all stages examined, suggesting the existence of independent rhythm-generating circuits on each side. The rhythmic motor activity in the presence of strychnine was synchronized on both sides at all stages examined. These results indicate that the changes in rhythm pattern are mediated by development of glycinergic inhibitory pathways, while the basic rhythm can be generated without the glycinergic inhibitory pathways.