Figure 1. Method for recording and quantitating electrophysiologic responses. Diagram of the rat hippocampal slice and the evoked response. The stimulating electrode is shown in the Schaffer collateral pathway and the recording electrode in the CA1 pyramidal cell layer. Three CA1 pyramidal cells and one CA3 pyramidal cell are illustrated; there are hundreds of these cells in the hippocampal slice. The evoked population spike is due to the action potentials of many CA1 pyramidal cells. The Schaffer collateral pathway is composed of axons of the CA3 pyramidal cells, which form glutamatergic synapses onto the CA1 pyramidal cells. The trace at the bottom of the figure represents a response recorded from CA1 pyramidal cells after stimulation of the Schaffer collaterals. The first smaller spike on the trace (indicated by +) is caused by action potentials in the Schaffer collaterals, which are presynaptic to the pyramidal cells. The second larger spike (indicated by *), on which our measurements are based, is due to action potentials in CA1 pyramidal cells. The amplitude of this spike is measured as the average of the difference in millivolts between A and B and C and B.