Higher transduction to the hippocampal neurons. (A) Concentrated Lv-Kir2.1 vector, prepared in the absence or presence of Ba2+, was injected under the same conditions (≈0.05 nl/site) into the extracellular space of the CA1 pyramidal layer in organotypic cultures of rat hippocampus. Seven days after injection, the cultured slices were fixed with 4% paraformaldehyde and immunostained with anti-NeuN antibody and a secondary antibody conjugated with Alexa Fluor 568. The single-plane images were taken with a confocal microscope (218 × 288 μm; FV300, Olympus). Bar, 50 μm. (B) Differences in the numbers of GFP-positive cells. Numbers of GFP-positive cells seen within the visual field described above were counted (n = 18 and 41, from three independent experiments; p < 0.00001, Student’s t test).