Combined Treatment of Xenon and Hypothermia in Newborn Rats - Additive or Synergistic Effect?

Background: Breathing the inert gas Xenon (Xe) enhances hypothermic (HT) neuroprotection after hypoxia-ischemia (HI) in small and large newborn animal models. The underlying mechanism of the enhancement is not yet fully understood, but the combined effect of Xe and HT could either be synergistic (larger than the two effects added) or simply additive. A previously published study, using unilateral carotid ligation followed by hypoxia in seven day old (P7) rats, showed that the combination of mild HT (35 degrees C) and low Xe concentration (20%), both not being neuroprotective alone, had a synergistic effect and was neuroprotective when both were started with a 4 h delay after a moderate HI insult. To examine whether another laboratory could confirm this finding, we repeated key aspects of the study. Design/ Methods: After the HI-insult 120 pups were exposed to different post-insult treatments: three temperatures (normothermia (NT) NT37 degrees C, HT35 degrees C, HT32 degrees C) or Xe concentrations (0%, 20% or 50%) starting either immediately or with a 4 h delay. To assess the synergistic potency of Xe-HT, a second set (n = 101) of P7 pups were exposed to either HT35 degrees C+Xe-0%, NT+ Xe-20% or a combination of HT35 degrees C+Xe-20% starting with a 4 h delay after the insult. Brain damage was analyzed using relative hemispheric (ligated side/ unligated side) brain tissue area loss after seven day survival. Results: Immediate HT32 degrees C (p = 0.042), but not HT35 degrees C significantly reduced brain injury compared to NT37 degrees C. As previously shown, adding immediate Xe-50% to HT32 degrees C increased protection. Neither 4 h-delayed Xe-20%, nor Xe-50% at (37 degrees C) significantly reduced brain injury (p>0.050). In addition, neither 4 h-delayed HT35 degrees C alone, nor HT35 degrees C+Xe-20% reduced brain injury. Wefound no synergistic effect of the combined treatments in this experimental model. Conclusions: Combining two treatments that individually were ineffective (delayed HT35 degrees C and delayed Xe-20%) did not exert neuroprotection when combined, and therefore did not show a synergistic treatment effect.