Expression and changes of galanin in neurons and microglia in the hippocampus after transient forebrain ischemia in gerbils

In the present study, we investigated chronological changes of galanin (GAL), well known as the potassium channel opener, immunoreactivity and GAL protein level in the hippocampus of the gerbil at the various times after 5 min transient forebrain ischemia. In the sham-operated group, weak GAL immunoreactivity was found in non-pyramidal cells. At 12 h after ischemia-reperfusion, the number of GAL-immunoreactive neurons and GAL immunoreactivity were significantly increased in the hippocampus compared to 3 h after ischemic insult, especially in the hippocampal CA1 region. Thereafter the number of GAL-immunoreactive neurons and GAL immunoreactivity decrease time-dependently in the hippocampus. Four days after transient ischemia, GAL immunoreactivity was low as compared with the sham-operated group. At this time point after ischemic insult, GAL immunoreactivity was shown in microglia in the CA1 region because delayed neuronal death happened in the CA1 pyramidal cells. The result of Western blot showed the pattern of GAL expression similar to that of immunohistochemical data. These results suggest that the early increase of GAL in the CA1 pyramidal cells may be associated with the reduction of the excitotoxic damage, that long-lasting enhanced expression of endogenous GAL at 12 h-2 days after ischemia may be associated with efflux of potassium ion into the extracellular space, and that GAL expression in microglia 4 days after ischemia may be associated with reduction of ischemic damage.