Morphological identification of dentate gyrus BCs in young (P6–P10) and mature (P18–P25) GAD67–GFP-expressing mice. A, B, Confocal image stacks from a horizontal section (50 μm) of the dentate gyrus in a young (P6) and a mature (P21) transgenic mouse. C, D, GFP-expressing cells were filled with biocytin during the recording and subsequently visualized using DAB as chromogen. Light-microscopic images show a pair of young (left) and of mature (right) synaptically coupled BCs in the mouse dentate gyrus. Note that the axonal arborizations are located in the GC layer, identifying the cells as BCs. Dashed lines indicate borders of the granule cell layer. Arrowheads point to axon collaterals. E–G, Confocal images confirm colocalization of GAD67–GFP and PV in an identified BC at P12. E, A GABAergic interneuron was identified in the slice preparation of the dentate gyrus on the basis of the GAD67–GFP signal. The arrow points to the cell body of the selected cell. F, The same cell was biocytin filled and stained with Alexa Fluor 647-conjugated streptavidin. Arrowheads point to axonal collaterals in the granule cell layer. G, The BC was identified as PV positive by immunohistochemistry. Note the weaker GFP and PV labeling of the recorded cell compared with neighboring cells because of intracellular dialysis with the pipette solution (recording time, ∼10 min). Arrow points to the same cell as in E. gcl, Granule cell layer; ml, molecular layer.