Autophagy activation is involved in neuroprotection induced by hyperbaric oxygen preconditioning against focal cerebral ischemia in rats

Our previous studies have demonstrated that hyperbaric oxygen (HBO) preconditioning induces tolerance to focal cerebral ischemia. The present study aimed to investigate whether autophagy is involved in the neuroprotection elicited by HBO preconditioning in a rat model of transient focal cerebral ischemia. Twenty-four hours after the completion of HBO preconditioning (2.5 atm absolute in 100% oxygen for 60 min per day for 5 consecutive days), male Sprague–Dawley rats were subjected to focal cerebral ischemia by middle cerebral artery occlusion (MCAO) for 120 min. The neurobehavioral score and infarct volume were used to evaluate cerebral ischemic injury. An intracerebroventricular injection of the autophagy inhibitor 3-methyladenine (3-MA) or the autophagy inducer rapamycin was administered before HBO preconditioning or MCAO. We found that after reperfusion the protein expression of LC3-II and Beclin 1 and the formation of autophagosomes were increased by HBO preconditioning or ischemia, but the increase following HBO preconditioning was higher than the increase following ischemia. 3-MA suppressed the increases in LC3-II and Beclin 1 induced by HBO preconditioning and attenuated the neuroprotection of HBO preconditioning against cerebral ischemia. Furthermore, 3-MA treatment before MCAO aggravated subsequent cerebral ischemic injury. In contrast, pretreatment with rapamycin up-regulated LC3-II and Beclin 1 after reperfusion and mimicked the neuroprotective effect of HBO preconditioning. These results indicate that HBO preconditioning elevates autophagic activity, which elicits a neuroprotective effect against ischemic injury in the brain, and suggest a novel mechanism of HBO preconditioning-induced tolerance against transient focal cerebral ischemia.