Epoxy Fatty Acids and Inhibition of the Soluble Epoxide Hydrolase Selectively Modulate GABA Mediated Neurotransmission to Delay Onset of Seizures

In the brain, seizures lead to release of large amounts of polyunsaturated fatty acids including arachidonic acid (ARA). ARA is a substrate for three major enzymatic routes of metabolism by cyclooxygenase, lipoxygenase and cytochrome P450 enzymes. These enzymes convert ARA to potent lipid mediators including prostanoids, leukotrienes and epoxyeicosatrienoic acids (EETs). The prostanoids and leukotrienes are largely pro-inflammatory molecules that sensitize neurons whereas EETs are anti-inflammatory and reduce the excitability of neurons. Recent evidence suggests a GABA-related mode of action potentially mediated by neurosteroids. Here we tested this hypothesis using models of chemically induced seizures. The level of EETs in the brain was modulated by inhibiting the soluble epoxide hydrolase (sEH), the major enzyme that metabolizes EETs to inactive molecules, by genetic deletion of sEH and by direct administration of EETs into the brain. All three approaches delayed onset of seizures instigated by GABA antagonists but not seizures through other mechanisms. Inhibition of neurosteroid synthesis by finasteride partially blocked the anticonvulsant effects of sEH inhibitors while the efficacy of an inactive dose of neurosteroid allopregnanolone was enhanced by sEH inhibition. Consistent with earlier findings, levels of prostanoids in the brain were elevated. In contrast, levels of bioactive EpFAs were decreased following seizures. Overall these results demonstrate that EETs are natural molecules which suppress the tonic component of seizure related excitability through modulating the GABA activity and that exploration of the EET mediated signaling in the brain could yield alternative approaches to treat convulsive disorders.在大脑中，癫痫发作会释放大量多不饱和脂肪酸，其中包括花生四烯酸（arachidonic acid, ARA）。ARA是环氧合酶（cyclooxygenase）、脂氧合酶（lipoxygenase）以及细胞色素P450（cytochrome P450）这三类主要代谢酶促通路的底物。这些酶可将ARA转化为多种强效脂质介质，包括前列腺素类（prostanoids）、白三烯（leukotrienes）以及环氧二十碳三烯酸（epoxyeicosatrienoic acids, EETs）。前列腺素类与白三烯多为促炎分子，可使神经元致敏；而EETs则具有抗炎作用，能够降低神经元的兴奋性。近期研究证据表明，该过程存在一种与γ-氨基丁酸（gamma-aminobutyric acid, GABA）相关的作用模式，且可能由神经甾体（neurosteroids）介导。本研究通过化学诱导癫痫发作模型对该假说进行了验证。我们通过三种方式调节大脑内EETs水平：一是抑制可溶性环氧化物水解酶（soluble epoxide hydrolase, sEH）——这是将EETs代谢为无活性分子的核心酶类；二是通过基因敲除sEH；三是直接向脑内注射EETs。这三种干预手段均能延缓GABA受体拮抗剂诱导的癫痫发作，但对其他机制引发的痫性发作无显著效果。使用非那甾胺（finasteride）抑制神经甾体合成后，可部分阻断sEH抑制剂的抗惊厥作用；而sEH抑制则能够增强低剂量无活性的神经甾体别孕烷醇酮（allopregnanolone）的药效。与此前的研究结果一致，大脑内前列腺素类物质的水平出现升高。与之相反，癫痫发作后具有生物活性的环氧脂肪酸（epoxy fatty acids, EpFAs）水平会下降。综合以上结果，本研究证实EETs是一类天然分子，可通过调节GABA的活性来抑制癫痫相关兴奋性的紧张性成分；同时，针对大脑内EET介导的信号通路开展研究，有望为惊厥性疾病（convulsive disorders）的治疗提供全新的干预策略。