Synaptic interactions between mammalian central neurons in cell culture. I. Reversal potential for excitatory postsynaptic potentials.

Intracellular recording and stimulation techniques were used to study the electrical properties of neurons in cell cultures from fetal mouse spinal cord (SC). The morphology of SC neurons and the distribution on SC neurons of boutons formed by synaptically connected SC or dorsal root ganglion (DRG) neurons were demonstrated with horseradish peroxidase (HRP) injection. Postsynaptic polarization in conjunction with synaptic activation of SC neurons was used to determine the reversal potential for excitatory postsynaptic potentials (EPSPs). Tetraethylammonium ions were injected postsynaptically in order to obtain reversal of the EPSPs. Both SC-SC and DRG-SC excitatory connections could be reversed by postsynaptic depolarization. The average reversal potential for the SC-SC EPSP was -4 +/- 12.2 (SD) mV and that for the DRG-SC EPSP was +8 +/- 7.9 (SD) mV, a statistically significant difference (Wilcoxon two-sample rank; P less than 0.05). Scatter was quite large, particularly for the SC-SC connection. While some neurons gave clear electrophysiological evidence of significant dendritic effects, the average total electrotonic length was small (0.58 +/- 0.65 (SD) of a length constant). The morphological extent of the dendrites of SC neurons was substantially less than that of mature motoneurons in vivo. We concluded that both SC-SC and DRG-SC EPSPs were mediated by a conventional conductance increase and that most synaptic input was not far removed electrically from the recording site in the neuron cell body.