Figure 8. Lineage-related preferential electrical coupling correlates with common inhibitory synaptic outputs to nearby pyramidal neurons.

(a,b) Confocal images of a pair of EGFP-expressing sparsely labelled interneurons in clusters (green, 1 and 4) labelled at E12 and two nearby non-EGFP-expressing pyramidal neurons (2 and 3) examined by quadruple whole-cell patch-clamp recordings at P15 and stained with 4′,6-diamidino-2-phenylindole (DAPI) (blue). Alexa 568 hydrazide (red) was included in the recording pipettes to confirm the identity of the recorded neurons. Scale bar, 50 μm. (c) Morphological reconstruction of the four recorded neurons in (a). (d) Firing patterns of the four recorded neurons in (a) responding to somatic current injections. Note that cells 1 and 4 are non-FS and cells 2 and 3 are regular spiking pyramidal neurons (PN). Scale bars, 50 mV and 200 ms. (e,f) Sample traces of the (e) membrane potentials or (f) currents of the four neurons in response to (e) extended or (f) brief depolarizing and/or hyperpolarizing current injections (grey) under (e) current- or (f) voltage- clamp mode. Arrowheads indicate electrical synapses and arrows indicate chemical synapses. Open arrowheads indicate the lack of responses to hyperpolarizations, confirming electrical but not chemical synapses. Scale bars: (e) 1,000 pA (grey), 60 mV (red), 4 mV (blue and black), 1 mV (green) and 200 ms; (f) 1,000 pA (grey), 60 mV (red), 40 pA (blue, green and black) and 25 ms. (g) Synaptic connectivity pattern of the four recorded neurons in (a). The wavy lines indicate electrical synapses and the bar-headed lines indicate inhibitory chemical synapses. (h) Summary of the frequency of sparsely labelled interneuron pairs in clusters (green) or non-clonally (orange) labelled interneuron pairs that both provide presynaptic output to the same nearby pyramidal neurons with regard to their electrical coupling. *P<0.05; **P<0.01; NS, not significant (χ² test).