Physiological Properties of Supragranular Cortical Inhibitory Interneurons Expressing Retrograde Persistent Firing

Figure 1

Induction mechanisms and properties of persistent firing in layer 2/3 interneurons. (a) Representative traces showing a series of depolarizing current steps of increasing amplitude (duration of each step: 1 sec) in a current-clamped layer 2/3 interneuron. This protocol eventually led to the generation of persistent firing outlasting the termination of the current step. (b) Graph showing the mean number of current injection-induced somatic spikes needed to evoke persistent firing (somatic) and the mean number of spikes participating in persistent firing (persistent). (c) Spike waveform of one representative somatic and persistent spike in the same neuron (top). The derivative of the rectangular dashed area is represented stretched in time in the bottom traces. Note that the peak of the derivative represents the steepest point of the slope during the spike upstroke; meanwhile at the peak of the spike waveform the value of the derivative is equal to zero. (d) Phase plot showing the rate of change in membrane potential (dVm/dt) as a function of the membrane potential (Vm) for a train of somatic (black traces) and persistent (grey traces) spikes. Note in (c) and (d) the biphasic course of the derivative of the persistent spike. The first component (1) represents the back-propagation of the action potential in the axonal compartment while the second phase (2) represents the spike invasion in the somatodendritic compartment of the neuron. These two phases are better separated and therefore clearly visible only in ectopic spikes due to the long latency for the spike to back-propagate from a distal site in the axon to the somatodentritic compartment of the neuron.

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