The effect of hypothermia on the expression of neurotrophin mRNA in the hippocampus following transient cerebral ischemia in the rat.
The expression of the mRNAs of nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), neurotrophin 3 (NT3) and the neurotrophin receptor, TrkB, was studied in the rat hippocampus by in situ hybridization following normothermic (37 degreesC) and protective hypothermic (33 degreesC) transient cerebral ischemia of 15 min duration. In the resistant dentate gyrus, normothermic ischemia transiently induced NGF mRNA at around 8 h of recovery, while the NT3 mRNA levels were depressed over at least a 24-h recovery period. The levels of BDNF and TrkB were transiently and markedly elevated with a maximal expression at 24 h of recovery. Intraischemic hypothermia reduced the induction of NGF mRNA, while the increase of BDNF mRNA expression occurred earlier during recovery, and the post-ischemic NT3 mRNA depression was not affected. Also, the expression of TrkB mRNA was enhanced, and occurred concomitantly with the elevation of BDNF mRNA. In contrast, there were no changes in neurotrophin and TrkB mRNA in the CA3 and CA1 regions. The expression of BDNF mRNA at 24 h after normothermic ischemia, was attenuated by intraischemic hypothermia. We conclude that, the expressions of NGF, BDNF, NT3 or TrkB mRNA in ischemia-sensitive hippocampal subregions are not increased by protective hypothermia. In contrast, hypothermia induces neurotrophin mRNA alterations in the ischemia-resistant dentate gyrus that may convey protection to sensitive regions.