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-nitroquin-oxaline-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 gamma-aminobutyric acid-A (GABAA) receptor-mediated inhibitory PSPs. The identity of a small (< or = 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 GABAA 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 GABAB receptor agonist L-(-) baclofen or the glutamate analogue L-2-amino-4-phosphonobutyric acid (L-AP4) attenuated the pharmacologically resolved short-latency EPSPs.(ABSTRACT TRUNCATED AT 400 WORDS)