Changes of Mitochondrial DNA and Heat Shock Protein Gene Expressions in Gerbil Hippocampus after Transient Forebrain Ischemia

Hippocampal CA₁ neurons are the most vulnerable to transient cerebral ischemia. However, the mechanism has not been fully understood. The level of mRNA for cytochrome C oxidase (COX) subunit I (COX-I), which is encoded by mitochondrial (mt) DNA, progressively decreased in the hippocampal CA₁ neurons of gerbils from 3 h of reperfusion after 3.5 min of transient forebrain ischemia and completely disappeared at 7 days. The activity of COX protein also showed an early decrease in CA₁ cells and was followed by reduction of the level of COX-I DNA after 2 days. However, succinic dehydrogenase, an mt enzyme encoded by nuclear DNA, maintained normal activity until 1 day in the CA₁ cells and significantly decreased at 7 days. The mRNA for mt heat shock protein (HSP) 60 began to increase at 3 h in the CA₁ cells and was sustained until 1 day. The mRNAs for 72-kDa heat shock protein and 73-kDa heat shock cognate protein, which are located mainly in the cytoplasm, were induced together in the CA₁ cells with a peak at 1–2 days. These results suggest that a disturbance of mt DNA expression occurred in the CA₁ neurons at the early stage of reperfusion and was aggravated over the course of time. The disturbance could cause progressive failure of energy production of the cells that eventually results in neuronal cell death.