Fig. 1.

Subthreshold impedance and resonance in GAD67-GFP+ (GFP+) interneurons. A1: membrane voltage traces recorded in a representative example of a nonresonant GFP+ cell. The cell was stimulated with a sequence of current steps followed by a chirp function (0–40 Hz, 2 Hz/s chirp rate). The stimulus is shown schematically at the bottom of the panel. Stimulus amplitude (I_stim) was 40 pA. Voltage traces were recorded at a membrane potential of −70 mV (top) or −75 mV (bottom) in the same neuron by injection of DC current. The rectification ratio (Rr), calculated as the ratio of the of the steady-state membrane voltage response to depolarizing (ΔV₁) and hyperpolarizing (ΔV₂) current steps (indicated by dashed horizontal lines), was 1.13 at −70 mV and 1.05 at −75 mV. Note the depolarizing membrane voltage “sag” in response to hyperpolarizing current pulses. A2: corresponding impedance curves of this cell. Impedance magnitude (see methods) is plotted against chirp frequency (1 to 40 Hz) for the 2 chirp stimuli shown in A1. Membrane impedance decays monotonically in both cases from a maximum at the minimum chirp frequency of 1 Hz, with increasing chirp frequency up to 40 Hz. B1: membrane voltage traces recorded in a representative example of a resonant GFP+ cell. This cell was stimulated with a sequence of current steps (250 ms long in this cell) followed by a chirp function (0–40 Hz; current stimulus is shown schematically at the bottom). Stimulus amplitude was 60 pA (top trace), 80 pA (middle trace), or 100 pA (bottom trace), all at the same resting membrane potential (−78 mV). Action potentials (APs, clipped at the top) were evoked during the depolarizing half of the third and fifth sine wave cycle of the chirp at stimulus strength 100 pA, but note the lack of spiking during the preceding depolarizing step current pulse of the same amplitude as the chirp. In this cell, and all other resonant GFP+ cells, there was a “hump” in the depolarizing voltage envelope when the stimulus amplitude was just below the amplitude necessary to evoke spiking with a maximum amplitude at the upper resonance frequency (middle trace, arrow labeled f_upp). When amplitude was increased to evoke spiking, APs preferentially occurred in cycles at f_upp (compare middle and bottom voltage traces). Note the asymmetry in the membrane voltage response to positive and negative current steps and the asymmetry in the depolarizing and hyperpolarizing voltage envelope during chirp stimulation (Rr was 0.90 in this cell). B2: corresponding impedance curves for the voltage traces shown on the left. Arrow points to a peak in impedance magnitude at the resonant chirp frequency f_res. In this cell f_res was found at 3.7 Hz. Note the “noise” in the impedance curve when spikes were evoked (bottom panel).