A new fast electrophysiological response measured extracellularly in a mouse hippocampal brain slice

In a hippocampal mouse brain slice the CA1 stratum radiatum field potential evoked by electrical stimulation of the Schaffer collaterals was used to analyze the very early responses. Parallel bipolar electrodes were utilized to lower the voltage induced by the stimulating current in the recording electrode, and to keep the induced voltage within the linear range of the headstage preamplifier and the recording amplifiers. When the stimulating and recording electrodes are in saline in the same plane and same depth, the shapes of the recorded voltage and stimulating current are similar. The shape of the recorded voltage significantly changes when the recording electrode is in the stratum radiatum of CA1. The signal recorded in the first 2 ms, before the presynaptic volley, indicates the existence of a fast response shown as a positive deflection. For times slightly longer than the stimulating pulse width, the fast response overlaps with the voltage induced by the discharging of the polarization charge accumulated in the double layer capacitance at the interface between metal and saline. For times longer than about 1.4 ms the fast response and presynaptic volley are superimposed. The fast response is about 0.5 mV when the stimulating charge is 15-25 nC. The demonstration that the fast response disappears upon bath perfusion of tetrodotoxin is convincing support for its axonal origin. We postulate that this fast response represents the current source corresponding to the Schaffer collaterals excitatory sink. An undistorted recording of the first 2 ms of the sweep gives useful information about the fast neuronal activity measured extracellularly, and cannot be regarded as a simple stimulus artifact.