Use of bipolar parallel electrodes for well-controlled microstimulation in a mouse hippocampal brain slice

In a hippocampal brain slice two types of stimulating electrodes [single (SE) or monopolar and parallel bipolar (PE)] were used to determine the optimal protocol for single pulse microstimulation. We show that even for a constant-current power source the amplitude of stimulating current (SC) is not constant, especially for short pulse widths (PW) (<200 micros). Recording the stimulating current and computing the amount of electric charge that is passed through the microelectrode gives the best estimate of the strength of electrical stimulation. For SE the evoked response is obstructed for a time interval larger than three times the PW. The stimulus artifact (SA) substantially decreases when a PE is used. The orientation of the stimulating current relative to the position of the targeted fibers (Schaffer collaterals) was controlled when using a PE. The use of PEs allowed the accurate recording of the physiological response that contains three clearly defined peaks. Stimulation can be elicited at PW as short as 30 micros when the main current is capacitive. The charge needed to elicit physiological responses was in the range of 1-40 nC (the lower values for the PE) suggesting that use of PEs is most advantageous for well-controlled microstimulation studies in brain slices.