Fig. 9.

Somatic and AIS muscimol differentially affect temporal summation of excitatory postsynaptic potential (EPSP)-like events (aPSPs) and input–output relationships. A1: muscimol application to the AIS failed to evoke an action potential during the 4th aPSP, which routinely reached threshold in interleaved control sweeps. Notice that aPSP size and shape are similar in the absence (red traces) and presence (black traces) of muscimol. Bottom: current waveform used to stimulate the cell. The peak amplitude was 175 pA. All insets are magnification of the boxed area, with calibration bars 20 mV and 2 ms. A2: somatic muscimol failed to evoke an action potential at the 4th aPSP. Notice the difference in aPSP peak and time course in the absence (black) and presence (red) of muscimol. B1: with larger current amplitude (250 pA peak amplitude), muscimol application to AIS just before the 2nd aPSP depolarized the voltage threshold and delayed firing. B2: somatic muscimol under the same current-injection conditions as B1 caused an action potential failure at the 2nd aPSP. All traces are from the same cell, which is representative of the 4 cells tested. C: input/output curves for a representative granule cell subjected to 200 ms current pulses of varied amplitudes, with muscimol applied for 50 ms to either the AIS or to the soma immediately before current onset. The y-axis gives the number of action potentials (APs) elicited by the given current amplitude. Muscimol concentration was 5 μM, applied either to the AIS (red circles) or to the soma (red squares). Baseline input/output curve is given as black circles. D: raw traces (top) and voltage thresholds (bottom) in the absence and presence of AIS muscimol application for the cell represented in C. Traces were matched for number of action potentials (8), which corresponded to a current amplitude of 200 pA for control (black trace) and 250 pA for muscimol (red trace). Bottom: voltage thresholds calculated from phase-plot analysis for each action potential in the train. The effect of muscimol on voltage threshold may be reduced compared with the effect in other figures as a result of the larger amplitude of current injection used. Increased current amplitudes tended to hyperpolarize action potential threshold. E: summary of effect represented by the example in C (n = 5). The number of action potentials at a fixed current amplitude is plotted. The current amplitude was that needed to elicit 50% of the maximum firing rate under baseline conditions (175 pA for the example in C). F: summary of effect represented by the example in D (n = 5). Average action potential threshold for the final 4 action potentials in each condition is plotted.