Figure 4. Preferential electrical synapse formation between sparsely labelled neocortical interneuron clusters originated from MGE/PoA.

(a) Overview of the experimental procedure. (b) Confocal image of three EGFP/mCherry-expressing sparsely labelled interneurons (green/red, 1, 2 and 4) and a nearby mCherry-expressing non-clonally labelled interneuron (red, 3) labelled at E12, examined by quadruple whole-cell patch clamp recordings at P18. Alexa 405 hydrazide (blue) was included in the recording pipettes to confirm the identity of the recorded neurons. Scale bar, 50 μm. (c) 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. (d) Confocal images of the four recorded interneurons in (b) stained for PV (white). Note that none is PV-positive. Scale bar, 50 μm. (e) Morphological reconstruction of the four recorded interneurons in (b). (f,g) Sample traces of the (f) membrane potentials or (g) currents of the four interneurons in response to extended or brief depolarizing and/or hyperpolarizing current injections (grey) under (f) current-clamp or (g) voltage-clamp mode, respectively. 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, (f) 600 pA (grey), 50 mV (red), 0.25 mV (blue), 0.5 mV (black) and 200 ms; (g) 1,200 pA (grey), 50 mV (red), 2.5 pA (black), 1.25 pA (green) and 20 ms. (h) Synaptic connectivity pattern of the four recorded interneurons in (b). The wavy lines indicate electrical synapses and the bar-headed lines indicate inhibitory chemical synapses. (i and j) Summary of the frequencies of (i) electrical and (j) chemical synaptic connections between sparsely labelled and nearby non-clonally labelled neocortical interneurons that are generated in the MGE/PoA at the same time. ***P<0.001; NS, not significant (χ² test).