Excitatory and inhibitory transmission from dorsal root afferents to neonate rat motoneurons in vitro

Intracellular recordings were made from antidromically identified motoneurons in neonate (12-22 days) rat transverse spinal cord slices and the transmitters and receptors probably involved in initiating the excitatory (EPSP) and inhibitory (IPSP) postsynaptic potentials were investigated. Stimulation of dorsal roots elicited in motoneurons an EPSP, an IPSP, or an EPSP followed by an IPSP. EPSPs in 70% of motoneurons had a short latency (≤1 ms) and in the remaining cells a latency longer than 1 ms. The IPSPs had a long latency (≥1 ms). Short- and long-latency EPSPs were enhanced by the acidic amino acid uptake inhibitor l-aspartic acid-ß-hydroxamate (AAH) and depressed by the non-selective glutamate receptor antagonists γ-d-glutamylglycine (DGG) and kynurenic acid. Short-latency EPSPs were suppressed by the quisqualate/kainate (QA/KA) receptor antagonist 6,7-dinitroquinoxaline-2,3-dione (DNQX) but not by the N-mthyl-d-aspartate (NMDA) receptor antagonists d-(-)-2-amino-5-phosphonovaleric acid (APV) and ketamine. Long-latency EPSPs were reduced by DNQX as well as by APV and ketamine. Superfusion of the slices with a Mg-free solution increased the EPSPs and unmasked a late, APV-sensitive component. The IPSP was reduced by the glycine antagonist strychnine as well as by APV and ketamine but resistant to DNQX. The results indicate that stimulation of dorsal roots elicited in motoneurons a monosynaptic EPSP mediated by glutamate/aspartate acting predominantly on the QA/KA subtype of glutamate receptors; an NMDA component can be unveiled in Mg-free solution. The IPSP appears to be mediated by glycine released from inhibitory interneurons subsequent to their activation by glutamate/aspartate. Furthermore, NMDA receptors are probably present on excitatory and inhibitory interneurons and involved in interneuronal transmission.