Mouse spinal cord in cell culture. III. Neuronal chemosensitivity and its relationship to synaptic activity.

1. Mouse spinal cord (SC) cells in dissociated cell cultures showed strong electrophysiologic responses to glutamate, gamma-aminobutyric acid (GABA), and glycine when these were iontophoretically applied to the neurons. 2. The extrapolated reversal potential for the glutamate response was 20-30 mV negative in contrast to the positive extrapolated reversal potential for the SC-SC excitatory postsynaptic potential. The data are interpreted as indicating different ionic mechanisms for the glutamate response and the EPSP. 3. The reversal potentials for the glycine and GABA responses were similar to one another and to the IPSP reversal potential. The time course of the glycine and GABA responses were quite different from each other, however. 4. While some SC cells showed a relatively uniform sensitivity over their surfaces to iontophoretically applied glutamate, discrete regions of higher sensitivity occurred on most cells. 5. Release of excitatory and inhibitory transmitter could be elicited by focal application of glutamate and, in favorable instances, this could be shown to be due to the sensitivity of presynaptic terminals to the applied glutamate. Considerable spatial resolution of regions from which transmitter release could be elicited was achieved by this technique. Some correspondence between glutamate "hot spots" and such release sites was found.