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Telomerase is a reverse transcriptase that adds repeats of a six-base DNA sequence to chromosome ends and thereby prevents their shortening during successive cell divisions. Telomerase activity and expression of the catalytic subunit of telomerase (TERT) are high in brain cells during embryonic development, but are undetectable in the adult. We now report that telomerase activity and expression of TERT are induced in the hippocampus of adult mice after administration of the seizure-inducing excitotoxin kainate. Telomerase activity is present at 24 h and 7 days after kainate administration, but is no longer detectable at 4 weeks. Because the time course of telomerase induction was similar to the time course of microglial activation, we performed studies to determine whether microglia were the source of the telomerase activity. Examination of brain sections immunostained with a TERT antibody and an antibody against a microglia-specific antigen revealed that TERT was not detectable in the uninjured brain, and was present in microglia 24 h and 7 days after kainate administration. This is the first evidence that telomerase can be induced by brain injury and the first evidence that TERT can be expressed in microglia, suggesting roles for telomerase in microglial responses to brain injury. [Copyright &y& Elsevier]