Neuroprotection of mild hypothermia: differential effects

To estimate whether mild hypothermia during repetitive hypoxia provides a neuroprotective effect on brain tissue, hippocampal slice preparations were subjected to repetitive hypoxic episodes under different temperature conditions. Slices of guinea pig hippocampus (n=40) were placed at the interface of artificial cerebrospinal fluid (aCSF) and gas (normoxia: 95% O₂, 5% CO₂; hypoxia: 95% N₂, 5% CO₂). Evoked potentials (EP) and direct current (DC) potentials were recorded from hippocampal CA1 region. Slices were subjected to two repetitive hypoxic episodes under the following temperature conditions: (A) 34°C/34°C, (B) 30°C/30°C and (C) 34°C/30°C. Hypoxic phases lasted until an anoxic terminal negativity (ATN) occurred. The recovery after first hypoxia lasted 30 min. Tissue function was assessed regarding the latency of ATN and the recovery of evoked potentials. The ATN latencies with protocol A (n=25) for the first and second hypoxia were 5.9±1.3 min (mean±S.E.M., 1st hypoxia) and 2.4±0.9 min (2nd hypoxia), with protocol B the latencies (n=7) were significantly longer: 25.2±7.1 min and 15.6±7.7 min. With protocol C (n=8), the latencies were 5.6±1.8 and 3.3±0.5 min. No differences were seen in the recovery of the EPs with protocols A–C. Our results suggest that a mild hypothermia is only neuroprotective if applied from an initial hypoxia onwards.