(A) Paired recording between a presynaptic fast-spiking interneuron (FSI) and a NG2 cell. An action current evoked in the presynaptic interneuron (upper trace) elicits PSCs recorded in the NG2 cell (bottom trace; average of 200 traces). (B, D) Current-clamp recordings of the FSI recorded in A (B), and a non-fast-spiking interneurons (NFSI) (D) connected to a NG2 cell during injections of −150 pA and 200 pA. Note differences on spike properties between the two cells (insets, right). (C) The connected FSI was loaded with biocytin and was immunoreactive for PV (stacks of 17 Z-sections; each 2 µm). (E) 3D plot of the three major electrophysiological parameters distinguishing connected FSIs (gray) from NFSIs (black). (F) Histograms showing the fraction of Venus+ interneurons that are PV+ (n = 6 mice). (G) Percentages of tested and connected FSI (gray) and NFSI (black) with respect to all tested interneurons. (H) Percentages of connected and unconnected FSIs (gray) and NFSIs (black) with respect to each group of interneurons separately. (I) Connection probability of FSI and NFSI as a function of three postnatal stages (45, 44, and 44 tested pairs at PN8-9, PN10-11, and P12-13, respectively).