Traumatic brain injury and hemorrhagic hypotension suppress neuroprotective gene expression in injured hippocampal neurons.

Background: After traumatic brain injury, memory dysfunction is due in part to damage to the hippocampus. To study the molecular mechanisms of this selective vulnerability, the authors used laser capture microdissection of neurons stained with Fluoro-Jade to directly compare gene expression in injured (Fluoro-Jade-positive) and adjacent uninjured (FluoroJade-negative) rat hippocampal neurons after traumatic brain injury and traumatic brain injury plus hemorrhagic hypotension. Methods: Twelve isoflurane-anesthetized Sprague-Dawley rats underwent moderate (2.0 atm) fluid percussion traumatic brain injury followed by either normotension or hemorrhagic hypotension. Animals were killed 24 h after injury. Frozen brain sections were double stained with 1% cresyl violet and 0.001% Fluoro-Jade. RNA from 10 Fluoro-Jade-positive neurons and 10 Fluoro-Jade-negative neurons, obtained from the hippocampal CA1, CA3, and dentate gyrus subfields using laser capture microdissection, was linearly amplified and analyzed by quantitative ribonuclease protection assay for nine neuroprotective and apoptosis-related genes. Results: In injured CA3 neurons, expression of the neuroprotective genes glutathione peroxidase 1, heme oxygenase 1, and brain-derived neurotrophic factor was significantly decreased compared with that of adjacent uninjured neurons. Superimposition of hemorrhagic hypotension was associated with down-regulation of neuroprotective genes in both injured and uninjured neurons of all subregions. Expression of apoptosis-related genes did not vary between injured and uninjured neurons, with or without superimposed hemorrhage. Conclusions: The authors show, in the first direct comparison of messenger RNA levels in injured and uninjured hippocampal neurons, that injured neurons express lower levels of neuroprotective genes than adjacent uninjured neurons.