Figure 12. Properties of recurrent interneuron excitation.
Properties of unitary EPSPs elicited in a bistratified cell (see Fig. 11). A, distinct EPSPs in the interneuron (I) were elicited by spontaneous APs in the concomitantly recorded pyramidal cell (P). The baseline membrane potential of the interneuron was tonically depolarized (from a Vrest value of -65 mV to -61 mV). Interneuron AHPs prevented some EPSPs to become suprathreshold, effectively filtering the roughly 15 Hz AP frequency in the pyramical cell to a 7·5 Hz rhythm of synaptic APs in the interneuron. Distinct superimposed EPSPs were observed in the interneuron (marked by arrows). B, typical examples of early (a, open bars in C) and late APs (b, filled bars in C) on top of unitary EPSPs. Numbers denote latencies between peaks of pyramidal cell AP and unitary EPSP-triggered APs in the coupled interneuron (Δt(P,AP, peak-I,AP,peak)), 4·7 ms (a) and 52·6 ms (b). Latency to onset of unitary EPSP (inset on right) was 1·1 ms. C, distribution of Δt(P,AP,peak-I,AP,peak) (n= 235 measurements). The open bars depict latencies to early APs, filled bars latencies to delayed APs. D, orthodromic responses in the same pair of pyramidal cell and interneuron shown in A-C. Orthodromic responses in interneuron (top) and pyramidal cell (bottom) were obtained at 100 μA (a) and 200 μA (b). In a, the delayed pyramidal cell AP triggered a unitary EPSP with the typical delay (1·1 ms). In b, orthodromic stimulation elicited a double spike response in the interneuron. The peak of pyramidal cell AP preceded peak of the second interneuron spike by 2·5 ms.