Intracellular injection of astrocytes in lightly fixed tissue slices was performed as previously described, with some modifications (Buhl et al., [1990], PMID: 2262598). The brain was placed in ice-cold PBS and cut into coronal slices with a vibratome at a thickness of 100 µm, and stored in PBS at 4°C until used. Individual cells within the slices were identified using infrared-DIC optics, impaled and iontophoretically injected with dye using 1-second pulses of negative current (0.5 Hz) for 1-2 minutes. After several cells were filled, the slices were placed in ice-cold 4% PFA for at least 1 hour. The slices were then ready to be immunolabeled. Slices were coverslipped using Gelvatol (Harlow and Lane, [1988]) and allowed to set overnight at room temperature before they were examined. Image acquisition and analysis: Specimens were examined using a Radiance2000 laser scanning confocal system (Bio-Rad, Hercules, CA) attached to a Nikon E600FN microscope (Kanagawa, Japan). A 60x oil immersion (NA 1.4) objective was used to image filled astrocytes.