Synaptic transmission between ventrolateral funiculus axons and lumbar motoneurons in the isolated spinal cord of the neonatal rat

1. We studied the projections of ventrolateral funiculus (VLF) axons to lumbar motoneurons in the in vitro spinal cord preparation of 1- to 6-day- old rats using extracellular and sharp-electrode intracellular recordings. 2. Ipsilateral and contralateral VLF projections to lumbar motoneurons (L₄- L₅) could be activated in the neonatal rat by stimulation of the surgically peeled VLF at the rostral (L₁-L₂) and caudal lumbar (L₆) cord. Motoneurons were activated ipsilaterally through short- and long-latency projections in all cases and contralaterally through long-latency projections in most cases. 3. Suppression of the excitatory components of VLF postsynaptic potentials (PSPs) by application of the specific antagonists of N-methyl D-aspartate (NMDA) and non-NMDA receptors, 2-amino-5-phosphonovaleric acid (APV) and 6- cyano-7-nitroquinoxaline-2,3-dione (CNQX), revealed depolarizing PSPs that could be reversed at -55 to -60 mV by injection of depolarizing current steps to the motoneurons. These depolarizing PSPs were blocked by addition of strychnine and bicuculline and are therefore suggested to be glycine and γ- aminobutyric acid-A (GABA(A)) receptor-mediated inhibitory PSPs. The identity of a small (≤0.2 mV) residual depolarizing component that persisted in the presence of APV, CNQX, strychnine, and bicuculline remains to be determined. 4. Short-latency excitatory PSPs (EPSPs) could be resolved from the ipsilaterally elicited VLF PSPs after the reduction of the polysynaptic activity in the preparation by administration of mephenesin, which was followed by suppression of the glycine and GABA(A) receptor-mediated components of the PSPs by bath application of strychnine and bicuculline. The latencies of these EPSPs were similar to those of the monosynaptic dorsal root afferent EPSPs recorded from the same motoneurons. These short-latency VLF EPSPs were shortened by the NMDA antagonist APV and revealed an NMDA receptor-mediated component after administration of the non-NMDA receptor antagonist CNQX. Addition of the GABA(B) receptor agonist L-(-) baclofen or the glutamate analogue L-2-amino-4-phosphonobutyric acid (L-AP4) attenuated the pharmacologically resolved short-latency EPSPs. 5. The amplitude of the short-latency EPSPs produced by low-frequency (0.016-0.5 Hz) repetitive stimulation of the ipsilateral VLF did not vary with the frequency of stimulation. Stimulation of dorsal root afferents at the same frequencies induced prolonged EPSP depression in the same motoneurons. 6. Double pulse (interpulse intervals 15 ms-1 s) and high-frequency (1-20 Hz) stimulation of the ipsilateral VLF revealed substantial facilitation and tetanic potentiation of the EPSPs. Stimulation of dorsal root afferents induced under the same conditions double pulse depression and severe tetanic depression of the EPSPs in the same motoneurons. Perfusion of the preparations with low- calcium, high-magnesium Krebs saline decreased the amplitude of the EPSPs produced by VLF and dorsal root stimulation, markedly increased the facilitation and frequency potentiation of VLF EPSPs, and decreased but did not abolished the tetanic depression of dorsal root afferent EPSPs. 7. The frequency potentiation developed during high-frequency stimulation of VLF was accompanied in many motoneurons by prolonged transmembrane depolarization (tetanic depolarization). The depolarization increased with the frequency and the intensity of VLF stimulation and decreased after shortening the duration of the EPSPs by application of the NMDA receptor blocker APV. Tetanic depolarization developed also on stimulation of the polysynaptic contralateral VLF. In this case, however, the depolarization decayed before the end of the stimulus train, with the appearance of intermittent polysynaptic transmission. These findings suggest that the tetanic depolarization reflects temporal summation of the VLF EPSPs. 8. We suggest that the combination of frequency potentiation and temporal summation of the PSPs elicited by VLF stimulation in the neonatal rat reflects early functional specialization of the pathways traveling in VLF and is used to assure safe synaptic transmission in these pathways. We discuss the relevance of these findings to the performance of the motor control functions in the developing mammalian spinal cord.