Figure 4.

Spike timing of somatostatin-positive interneurons determines the time course of recurrent inhibition. (a) A train of spikes at 100 Hz in a layer 2/3 pyramidal cell (black trace) elicits action potentials in an SOM interneuron (threshold for action potential generation was achieved in some (blue races; 12 superimposed sweeps) but not all (gray traces, 14 superimposed sweeps) trials (V_m interneuron, −63 mV)). Inset, the interneuron was reciprocally connected with the pyramidal cell: spiking of the interneuron (blue trace) triggered outward currents in the pyramidal cell (black trace). (b) Summary graph of the distribution of spike times in SOM interneurons in response to trains of action potentials at 100Hz in the presynaptic pyramidal cells (n = 12). (c) The blue trace illustrates the result of the convolution of the spike time distribution (in b) with a fit to an average unitary IPSC (sum of two exponential functions; τ_rise, 1.7 ms; τ_decay, 11 ms). The convolution is superimposed onto the time course, averaged over all experiments, of the outward current elicited by the spiking of a single pyramidal cell onto a neighboring pyramidal cell (gray trace, from Fig. 1b). Note the similarity of the rising and decaying phase of the two currents. Inset: gray trace, standard IPSC; black trace, unitary IPSC from a. (d) Summary graph of the distribution of spike times in interneurons receiving depressing inputs in response to trains of action potentials at 100 Hz in the presynaptic pyramidal cells (n = 2). Inset, a train of spikes at 100 Hz in a layer 2/3 pyramidal cell (black trace) elicits action potentials in an interneuron receiving depressing inputs (blue traces; five superimposed sweeps where threshold for action potential generation was achieved (V_m interneuron, −63 mV)). (e) Convolution (blue trace) of the spike distribution (in d) with the fit of an average IPSC (sum of two exponential functions; τ_rise, 0.8 ms; τ_decay, 9.4 ms). The convolution is superimposed onto the time course, averaged over all experiments, of the outward current elicited by the spiking of a single pyramidal cell onto a neighboring pyramidal cell (gray trace, from Fig. 1b). Note the very different rising and decaying phases of the two currents. (f) Simultaneous recording from three layer 2/3 pyramidal cells (blue, PC1; black, PC2; green, PC3). A train of spikes in PC2 alone leads to inhibition with late onset in PC1 (V_H, −40 mV; black traces), similar to that illustrated in Figures 1 and 2. A train of spikes in PC3 alone leads to no inhibition in PC1 (green traces). A simultaneous train of spikes in PC2 and PC3 leads to the appearance of an early component of inhibition (open arrow) followed by the late component (black arrow) in PC1 (blue trace).