Figure 3. Preferential formation of electrical, but not chemical, synapses between sparsely labelled neocortical interneurons in clusters.

(a,b) Confocal images of a pair of sparsely labelled EGFP-expressing interneurons in cluster (green, 1 and 3) and two nearby non-EGFP-expressing non-clonally related interneurons (2 and 4) labelled at E12, examined by quadruple whole-cell patch-clamp recordings at P14, 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. The wavy line indicates electrical coupling between the two sparsely labelled interneurons. Scale bar, 50 μm. (c) Morphological reconstruction of the four recorded interneurons in (b). (d) Firing patterns of the four recorded interneurons in (b) responding to somatic current injections. Note that all four interneurons are non-FS. Scale bars, 50 mV and 200 ms. (e) Sample traces of the membrane potentials of the four interneurons in response to brief and long depolarizing and hyperpolarizing current injections (grey) under current-clamp mode. Scale bars, 1,200 pA (grey), 30 mV (red), 2 mV (blue and black) and 200 ms. (f, g) Summary of the frequencies of (f) electrical and (g) chemical synaptic connections between sparsely labelled interneurons in clusters and their nearby non-clonally related interneurons after P14. *P<0.05. NS, not significant (χ² test).