EXCITATORY POSTSYNAPTIC CURRENTS AND GLUTAMATE RECEPTORS IN NEONATAL RAT SYMPATHETIC PREGANGLIONIC NEURONS IN-VITRO

1 We obtained whole cell patch-clamp recordings from visually identified sympathetic preganglionic neurons (SPNs) in thin (200-300 mu m) transverse spinal cord slices of neonatal rats (1-14 days postnatal). Exogenous application of glutamate (100 mu M), N-methyl-D-aspartate (NMDA; 100 mu M), kainate (100 mu M), quisqualate (1 mu M), and alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA; 50 mu M) induced inward currents at a holding potential of -30 mV. 2. Excitatory postsynaptic currents (EPSCs) were evoked by electrical stimulation either in the dorsal horn or the lateral funiculus. They reversed at 1.2 +/- 4.6 (SD) mV and could in most cases (49 of 51) be separated into two components. 3. In the presence of DL-2-amino-5-phosphonovalerate (10-40 mu M) the current-voltage (I-V) relationship of the remaining EPSC was linear. When stimulated in the lateral funiculus, its rise time (10-90%) and the time constant of the monoexponential decay were 1.6 +/- 1.0 and 5.5 +/- 2.7 ms, respectively. By contrast, when stimulated in the dorsal horn, this component had a rise time (10-90%) of 3.0 +/- 0.8 ms and a decay time constant of 13.7 +/- 7.6 ms. 4. We studied the NMDA receptor-mediated component of the EPSCs after superfusion of 6-cyano-7-nitroquinoxaline-2,3-dione (5 mu M) The I-V relationship of this component had a region of negative slope conductance between -30 and -80 mV, which was abolished in Mg2+-free saline. The rise time (10-90%) ranged from 3.3 to 9.5 ms acid the decay was biexponential. Both decay time constants increased with depolarization. Mg2+-free saline reduced this voltage sensitivity. 5. At a membrane potential of -80 mV and in 1 mM extracellular Mg2+, the NMDA receptor-mediated component represented 74.8 +/- 11.2% of the total charge carried by the EPSCs evoked by stimulation in the dorsal horn. In contrast, when stimulated from the lateral funiculus, 28.9 +/- 18.9% of the total charge carried during the EPSC was mediated by the NMDA receptor-mediated component. The contribution of the NMDA receptor-mediated component increased in both cases with depolarization. In addition, in 2 of 18 SPNs the EPSC evoked in the dorsal horn was exclusively carried by NMDA receptors. 6. We conclude that L-glutamate or a related substance mediates the fast excitatory input onto SPNs. Viscerosomatic and supraspinal inputs form synapses with different topographical locations on the SPN. Furthermore, our results may also indicate that the synapses of these two inputs have different relative amounts of the NMDA and AMPA/kainate types of receptors.