Isoflurane Conditioning‐Induced Delayed Cerebral Ischemia Protection in Subarachnoid Hemorrhage—Role of Inducible Nitric Oxide Synthase

BACKGROUND: Recent evidence implicates inflammation as a key driver in delayed cerebral ischemia after aneurysmal subarachnoid hemorrhage (SAH). Inducible nitric oxide synthase (iNOS) is one of the known major mediators of inflammation. We previously showed that an inhalational anesthetic, isoflurane, provides strong protection against delayed cerebral ischemia after SAH. Our current study aims to define the role of iNOS in isoflurane conditioning‐induced protection against delayed cerebral ischemia in a mouse model of SAH. METHODS AND RESULTS: The experiments used 10‐ to 14‐week‐old male wild‐type (C57BL/6) and iNOS global knockout mice. Anesthetic conditioning was initiated 1 hour after SAH with isoflurane 2% for 1 hour. Isoflurane‐induced changes in iNOS expression were measured. N‐(3‐(aminomethyl) benzyl) acetamidine, a highly selective iNOS inhibitor, was injected intraperitoneally immediately after SAH and then daily. Vasospasm, microvessel thrombosis, and neurological assessment was performed. Data were analyzed by 1‐way ANOVA and 2‐way repeated measures ANOVA followed by Student Newman Keuls comparison test. Statistical significance was set at P<0.05. Isoflurane conditioning downregulated iNOS expression in naïve and SAH mice. N‐(3‐(aminomethyl) benzyl) acetamidine attenuated large artery vasospasm and microvessel thrombosis and improved neurological deficits in wild‐type animals. iNOS knockout mice were significantly resistant to vasospasm, microvessel thrombosis, and neurological deficits induced by SAH. Combining isoflurane with N‐(3‐(aminomethyl) benzyl) acetamidine did not offer extra protection, nor did treating iNOS knockout mice with isoflurane. CONCLUSIONS: Isoflurane conditioning‐induced delayed cerebral ischemia protection appears to be mediated by downregulating iNOS. iNOS is a potential therapeutic target to improve outcomes after SAH.