The 2005 announcement in Science that the inhalation of hydrogen sulfide gas (H2S) could induce a reversible hypometabolic state in mice, similar to suspended animation, set off a flurry of speculation and investigation into the mechanisms and potential therapies suggested by the discovery, from the possibility of extended space travel to improving the outcome after a stroke. Breathing the gas decreased body temperature,respiratory and heart rates, and activity levels, all without incurring brain damage. The initial report was followed by the determination that mice could tolerate hours of otherwise lethal hypoxia if exposed to the gas first,suggesting H2S as a potential therapy for hypoxia-related diseases. Cessation of blood flow (ischemia) and reperfusion, as occurs in stroke or during a heart attack, causes both cell death (apoptosis) and inflammation,and the gas has proved to improve survival by combating both of these pathologies. Hydrogen gas alone is also protective against ischemia–reperfusion injuries, primarily by neutralizing free radicals,but utilizing highly flammable hydrogen gas in a clinical setting poses a safety risk. This led Jianmei Cai and his colleagues at the Medical Universities in Shanghai and Shandong to wonder if injecting saline saturated with hydrogen gas could similarly protect newborn rats against ischemia, as reductions in blood flow or oxygen during birth can lead to significant brain damage.
The investigators first tested three concentrations of hydrogen saline in rat pups subjected to low oxygen (hypoxia) and brain ischemia (reduced blood flow) to determine the best concentration. Hydrogen-saturated saline was injected into the peritoneal cavity and the levels of brain damage determined by staining brain slices to look for dead vs live cells; by examining activity levels of an apoptotic marker (caspase-3) that indicates that cells have died; and by measuring levels of oxidatively damaged lipids, as lipid and protein damage are symptoms of oxidative stress, which continues to damage cells for hours to days after ischemia–reperfusion. The rat pups also underwent behavioral testing to look for overt signs of brain damage.
Cai's group found that 5 ml kg–1 of H2 saline almost completely suppressed the damage that occurred in hypoxic–ischemic rats. The number of live cells in the cortex and hippocampus was significantly higher in hypoxic–ischemic animals that had been treated with H2 saline than in animals that were hypoxic–ischemic but not treated with H2 saline, while the volume of dead cells in H2-treated rats was reduced to non-hypoxic levels. H2 saline also dramatically decreased levels of oxidatively damaged lipids, apoptotic activity and behavioral deficits. The H2-treated rats that had been exposed to ischemia also sustained less brain damage than the untreated rats. They were able to escape a water maze in approximately half the time that it took for ischemic rats to escape,had better postural responses, and wandered less in their cages.
The key neuroprotective effect of hydrogen is apparently its ability to neutralize free radicals, and while suspended animation and space travel may yet be a long way off, research such as this offers hope that we can begin to significantly decrease the mortality and morbidity associated with strokes,heart attacks and neonatal brain disorders.