Therapeutic Hypothermia Influences Cell Genesis and Survival in the Rat Hippocampus following Global Ischemia

Delayed hypothermia salvages CA1 neurons from global ischemic injury. However, the effects of this potent neuroprotectant on endogenous repair mechanisms, such as neurogenesis, have not been clearly examined. In this study, we quantified and phenotyped newly generated cells within the hippocampus following untreated and hypothermia-treated ischemia. We first show that CA1 pyramidal neurons did not spontaneously regenerate after ischemia. We then compared the level of neuroprotection when hypothermia was initiated either during or after ischemia. Treatment efficacy decreased with longer delays, but hypothermia delayed for up to 12 hours was neuroprotective. Although bromodeoxyuridine (BrdU) incorporation was elevated in ischemic groups, CA1 neurogenesis did not occur as the BrdU label did not colocalize with neuronal nuclei (NeuN) in any of the groups. Instead, the majority of BrdU-labeled cells were Iba-positive microglia, and neuroprotective hypothermia decreased the delayed generation of microglia during the third postischemic week. Conversely, hypothermia delayed for 12 hours significantly increased the survival of newly generated dentate granule cells at 4 weeks after ischemia. Thus, our findings show that CA1 neurogenesis does not contribute to hypothermic neuroprotection. Importantly, we also show that prolonged hypothermia positively interacts with postischemic repair processes, such as neurogenesis, resulting in improved functional outcome.