SYNAPTIC EXCITATION OF ALPHA-MOTONEURONS BY DORSAL-ROOT AFFERENTS IN THE NEONATAL RAT SPINAL-CORD

1. Excitatory synaptic transmission in mono- and polysynaptic pathways between dorsal root afferents and alpha-motoneurons studied in the sp inal cord preparation of the neonatal rat isolated in vitro, using sha rp-electrode intracellular recordings. 2. The duration of monosynaptic excitatory postsynaptic potentials (EPSPs) elicited in lumbar motoneu rons were shortened after addition of the specific N-methyl-D-aspartat e (NMDA) receptor blocker 2-amino-5-phosphonovaleric acid (APV) to the perfusate. The EPSPs were then completely blocked by the non-NMDA rec eptor blocker 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX). 3. A robust NMDA-receptor-mediated component of monosynaptic EPSPs was revealed b y addition of CNQX to the bathing medium. This component reached as mu ch as 30% of the EPSP amplitude, was evident at resting potential leve l in both low and normal Mg2+ Krebs saline, and could be completely ab olished by addition of APV. These findings suggest that the NMDA-recep tor-mediated component may contribute to monosynaptic excitation under normal conditions. 4. Polysynaptic EPSPs evoked in motoneurons in the fifth lumbar segment by stimulation of the fourth lumbar dorsal root in the presence of the glycine and gamma-aminobutyric acid A (GABA(A)) receptor blockers strychnine and bicuculline could be completely or p artially blocked by application of either APV or CNQX. Suprathreshold activity could be then elicited in these motoneurons by increasing the stimulation intensity by a factor of 2 to 3. A complete blockade of p olysynaptic excitation at these stimulation intensities was obtained o nly in the presence of both APV and CNQX. These results suggest that b oth receptor subtypes make a significant contribution to polysynaptic excitation of alpha-motoneurons by dorsal root afferents. 5. Analysis of variation in the amplitudes of the non-NMDA-receptor-mediated compo nent of the monosynaptic EPSP and of the estimated (occurring 25 ms af ter the EPSP initiation) and the pharmacologically resolved NMDA compo nent was done during low-frequency repetitive stimulation of the dorsa l root. The kinetics of the initial decrease in EPSP amplitude during repetitive stimulation and the dependence of the EPSP amplitude on the stimulation frequency was similar for the NMDA- and non-NMDA-receptor -mediated components of the EPSPs. Addition of the GABA(B) receptor ag onist L-(-) baclofen to the perfusate decreased the EPSP amplitude and reduced the frequency-dependent synaptic depression of both the NMDA- and non-NMDA-receptor-mediated components of monosynaptic EPSPs to th e same level. These results are consistent with the hypothesis that th e excitatory amino acid (EAA) transmitter activating the NMDA and non- NMDA receptor subtypes is released from the same set of presynaptic fi bers. 6. Bath application of the glutamate analogue 2-amino-4-phosphon obutyric acid (AP4), reduced the amplitude of the non-NMDA- and NMDA-r eceptor-mediated components of monosynaptic EPSPs by a factor of 2 to 5. This reduction was not accompanied by changes in the time course of the EPSPs or in the passive properties of the motoneuron membrane. We suggest that AP4 reduces the EPSPs by a presynaptic mechanism. 7. Des pite the two- to fivefold decrease in their amplitude, monosynaptic EP SPs recorded in the presence of AP4 were substantially depressed durin g low-frequency stimulation, exhibiting a prolonged synaptic depressio n that was similar to that of the untreated control preparations. Cont rary to this, reduction of the EPSPs by lowering the Ca2+/Mg2+ ratio o f the perfusate or by bath application Of L-(-) baclofen has been show n to alleviate the prolonged synaptic depression. We therefore suggest that the action of AP4 may not be accomplished by a direct reduction of the presynaptic calcium influx and the EAA release probability. Alt ernative mechanisms involving branch point blockade of presynaptic aff erents, partial depletion of releasable transmitter stores, or inactiv ation of presynaptic release sites are suggested to underlie the AP4 a ction.