Upregulation of vascular endothelial growth factor receptors Flt-1 and Flk-1 in rat hippocampus after transient forebrain ischemia

This study characterizes the distribution of the two tyrosine kinase receptors for vascular endothelial growth factor (VEGF), Flt-1 and Flk-1, in the rat hippocampus following transient forebrain ischemia. The semi-quantitative reverse transcriptase-polymerase chain reaction (RT-PCR) analysis of Flt-1 and Flk-1 in hippocampal CA1 showed upregulation of these receptors following ischemic injury. Expression of Flt-1 and Flk-1 mRNA was restricted to neurons in the pyramidal cell and granule cell layers in control animals; however, upregulation was detected in activated glial cells and in the vascular endothelial cells rather than in neurons, in ischemic hippocampi. Most of the activated glial cells expressing Flt-1 and Flk-1 were reactive astrocytes, although some were microglial cells. The spatiotemporal expression of Flt-1 in the ischemic hippocampus mirrored that of Flk-1 expression. Expression of mRNA for both receptors was induced after 12 h, appeared to be increased progressively until 3 days when the highest expression was reached, and was sustained for more than 2 weeks. Flt-1 and Flk-1 immunoreactivity in the ischemic hippocampus matched the mRNA induction patterns except for a somewhat delayed onset. These data suggest that VEGF may be involved in the glial response via specific VEGF receptors in the rat hippocampus following transient forebrain ischemia.