Physiological Properties of Supragranular Cortical Inhibitory Interneurons Expressing Retrograde Persistent Firing

Figure 5

Layer 1 interneurons displayed spontaneous ectopic action potentials (eAPs). (a) One representative biocytin-filled layer 1 interneuron. (b) Representative traces showing a series of depolarizing current steps of increasing amplitude (duration of each step: 1 sec) in the one and only layer 1 interneuron (out of 27 cells) which displayed persistent firing. Note that, in this neuron, persistent firing consisted entirely of spikelets. (c) Voltage trace from one representative layer 1 neuron displaying one spontaneous spike with physiological properties reminescent of ectopically originated action potentials (eAPs). This eAP can be distinguished from the two subsequent “standard” spikes (originated following a spontaneous membrane potential depolarization) due to the different spike waveform and the abrupt, more negative spike threshold. In the inset on top the eAP (left) and the couple of “standard spikes” (right) are streched in time to better visualize their waveforms. (d) Spike waveform of one representative somatic and eAP recorded in a layer 1 interneuron (top) to better distinguish between the two firing “modes.” The derivative of the rectangular dashed area is represented stretched in time in the bottom traces. (e) Phase plot showing the rate of change in membrane potential (dVm/dt) as a function of the membrane potential (Vm) for a few somatic (black traces) and a few ectopic (grey traces) spikes. Similarly to persistent firing, it is possible to observe a biphasic course in the derivative of the spike waveform of the eAP, in (d) (bottom) and (e) (1, 2). (f) Bar graphs showing the mean input resistance (left-axis) and spike half-width (right-axis) for ectopic firing (ectopic firing rate > 0.01 Hz) and nonectopic firing (or ectopic firing at a frequency below 0.01 Hz) layer 1 interneurons.

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