Figure 5.

Structured polysynaptic local circuit motifs in the dentate hilus. A, Simultaneous quadruple hilar cell recording with five monosynaptic connections, including two reciprocal mossy cell/interneuron connections (gray rectangles). Recordings also illustrate excitatory synaptic convergence (MC1 and MC2 both excite HI1; EPSP onset latencies = 2.1 and 1.5 ms, respectively) and inhibitory divergence (HI1 inhibits MC1, MC2, and HI2; latencies, 1.7, 1.8, and 2.0 ms). All known phenotypes except HI2 (distance metric, −4.23). Average postsynaptic response shown in bold trace superimposed on example unitary responses (gray traces). Vertical dashed lines indicate presynaptic AP timing (middle of rising phase). B, Simultaneous triple hilar cell recording demonstrating inhibitory convergence (HI1 and HI2 both inhibit MC; IPSP onset latencies, 2.4 and 1.8 ms) and a reciprocal mossy cell/interneuron connection (EPSP onset latency, 1.8 ms). All known phenotypes. C, Diagram representation of hilar circuitry expected on the basis of monosynaptic connection preferences alone (MC/HI reciprocal motif frequency expected = 0.9%; 1 in 114 pairs; top) and observed enrichment of reciprocal motifs in the dentate hilus (observed motif frequency = 3.5%; p < 0.05; binomial cumulative distribution; bottom).