Astrocyte Inositol Triphosphate Receptor Type 2 and Cytosolic Phospholipase A2 Alpha Regulate Arteriole Responses in Mouse Neocortical Brain Slices

Functional hyperemia of the cerebral vascular system matches regional blood flow to the metabolic demands of the brain. One current model of neurovascular control holds that glutamate released by neurons activates group I metabotropic glutamate receptors (mGluRs) on astrocytes, resulting in the production of diffusible messengers that act to regulate smooth muscle cells surrounding cerebral arterioles. The acute mouse brain slice is an experimental system in which changes in arteriole diameter can precisely measured with light microscopy. Stimulation of the brain slice triggers specific cellular responses that can be correlated to changes in arteriole diameter. Here we used inositol trisphosphate receptor type 2 (IP3R2) and cytosolic phospholipase A2 alpha (cPLA2α) deficient mice to determine if astrocyte mGluR activation coupled to IP3R2-mediated Ca2+ release and subsequent cPLA2α activation is required for arteriole regulation. We measured changes in astrocyte cytosolic free Ca2+ and arteriole diameters in response to mGluR agonist or electrical field stimulation in acute neocortical mouse brain slices maintained in 95% or 20% O2. Astrocyte Ca2+ and arteriole responses to mGluR activation were absent in IP3R2−/− slices. Astrocyte Ca2+ responses to mGluR activation were unchanged by deletion of cPLA2α but arteriole responses to either mGluR agonist or electrical stimulation were ablated. The valence of changes in arteriole diameter (dilation/constriction) was dependent upon both stimulus and O2 concentration. Neuron-derived NO and activation of the group I mGluRs are required for responses to electrical stimulation. These findings indicate that an mGluR/IP3R2/cPLA2α signaling cascade in astrocytes is required to transduce neuronal glutamate release into arteriole responses.

脑血管系统的功能性充血（functional hyperemia）可使局部脑血流量与大脑代谢需求相匹配。当前主流的神经血管调控模型认为，神经元释放的谷氨酸会激活星形胶质细胞表面的I组代谢型谷氨酸受体（group I metabotropic glutamate receptors, mGluRs），进而诱导可扩散信使的产生，这些信使可作用于脑小动脉周围的平滑肌细胞。急性小鼠脑切片是一种实验体系，可通过光学显微镜精准测量小动脉直径的变化；对脑切片施加刺激可触发特异性细胞应答，该应答可与小动脉直径的变化相关联。本研究使用2型三磷酸肌醇受体（inositol trisphosphate receptor type 2, IP3R2）及胞质磷脂酶A2α（cytosolic phospholipase A2 alpha, cPLA2α）缺陷型小鼠，旨在探究星形胶质细胞的mGluR激活偶联IP3R2介导的钙释放以及后续cPLA2α激活是否为小动脉调控所必需。我们在95%或20%氧浓度下培养的急性小鼠新皮层脑切片中，检测了星形胶质细胞胞质游离钙离子浓度以及小动脉直径在mGluR激动剂或电场刺激下的变化。IP3R2基因缺陷（IP3R2−/−）的脑切片中，星形胶质细胞对mGluR激活的钙应答完全缺失。敲除cPLA2α不会改变星形胶质细胞对mGluR激活的钙应答，但会消除其对mGluR激动剂或电刺激诱导的小动脉应答。小动脉直径变化的极性（扩张/收缩）同时依赖于刺激类型与氧浓度。神经元源性一氧化氮（nitric oxide, NO）及I组mGluRs的激活是电刺激应答所必需的。本研究结果表明，星形胶质细胞内的mGluR/IP3R2/cPLA2α信号级联，是将神经元释放的谷氨酸信号转化为小动脉血管应答所必需的。