Calcium and unit response decrement to locally applied glutamate on rat hippocampal CA1 neurons.

In rat hippocampal slices, sensitivity of CA1 neuronal somata to iontophoretically applied glutamate pulses was reduced subsequent to: (1) a 20 Hz tetanic stimulation of stratum radiatum; (2) an application of N-methyl-DL-aspartate or glutamate or Ca2+. This decreased responsiveness was sometimes followed by an increase. Ca2+ antagonists, verapamil and Mn2+, interfered with the induction of the above changes in sensitivity to glutamate pulses. No correlation was observed between alterations in sensitivity of these neurons to glutamate and changes in synaptically-induced responses. These results suggest that a build-up of CA1 cell Ca2+ could be involved in decreasing the responsiveness (desensitization of receptors?) of the neuron to applied glutamate leading to a compensatory increase in the sensitivity.