Oscillatory properties of the central pattern generator for locomotion in neonatal rats.

1. The oscillatory properties of the lumbar spinal networks that generate locomotor activity in mammals were studied. These experiments were performed on an in vitro isolated spinal cord preparation from newborn rats. Adding to the saline serotonin (5-HT) and N-methyl-D-aspartate (NMDA) excitatory amino acid receptor agonist (N-methyl-D,L-aspartate, NMA) induced rhythmic locomotor-like activity that was recorded in the ventral roots. 2. The period of the rhythmic locomotor-like activity could be set by combining the neurotransmitters at various concentrations. The combined transmitters also acted on the instantaneous variations of the period value. The stability of the period from one cycle to another increased when 5-HT was mixed with NMA compared with the activity induced by bath application of the compound alone. 3. K(+)-induced depolarizations were used to set the neuron membrane potential. These systematic changes in the K+ concentration resulted in a change in the period value. 4. Stimulation of the dorsal roots reset the ongoing rhythm. Phase-response curves were drawn up that showed that the spinal networks exhibit a differential sensitivity to the same stimulus depending on the phase at which the stimulus is applied. The free-running activity could be entrained by the cyclic volleys of sensory discharges. Entrainment occurred, on average, between -28 and 36% of the rest period. Because the lumbar segments were completely isolated by means of a sucrose blockade of all descending neuronal activity, the entrainment of the locomotor activity must take place directly at the lumbar level. 5. In conclusion, our data show that, at birth, the rhythmic activity produced by the spinal networks is based on a neuronal architecture that displays oscillatory properties. At this early age, the peripheral sensory afferents have already established connections with the central pattern generator. These sensory inputs are able to control the cycling activity of the spinal networks step by step.