tPA binds to injured neurons ex vivo. NMDA was injected into the mouse striatum. Twenty-four hours later 1-mm coronal slices were prepared and incubated in 100 to 400 nM tPA overnight. Sections were then washed, fixed, and subjected to anti-tPA immunofluorescence (B,D,F) with a DAPI counterstain (A,C,E,G). A 40-μm scale bar is shown for panels A through D and a 20-μm scale bar, for panels E through G. (A,C) DAPI staining indicates a reduction in density of DAPI-positive cells following NMDA injection (compare ipsilateral [ipsi] with contralateral [contra] regions). (B,D) The contralateral striatum is devoid of tPA-binding cells, whereas numerous tPA-binding cells are present within the injured ipsilateral striatum. Panels E through G are the same section stained for tPA (F) and DAPI (E,G). Arrowheads point to healthy cells that do not bind tPA within the ipsilateral striatum. High- and low-exposure DAPI counterstaining reveals that these non–tPA-binding cells possess distinct brightly stained, regular-shaped nuclei and are therefore deemed viable. In contrast, arrows point to several tPA-binding cells that possess shrunken or irregular-shaped nuclei and are therefore considered injured. The binding of tPA to injured cells within the lesion was observed in experiments conducted on 3 separate mice. No comparable tPA-binding cells were found in sections that were incubated in the absence of recombinant tPA (not shown).