Regenerative glutamate release in the hippocampus of Rett syndrome model mice

Excess glutamate during intense neuronal activity is not instantly cleared and may accumulate in the extracellular space. This has various long-term consequences such as ectopic signaling, modulation of synaptic efficacy and excitotoxicity; the latter implicated in various neurodevelopmental and neurodegenerative diseases. In this study, the quantitative imaging of glutamate homeostasis of hippocampal slices from methyl-CpG binding protein 2 knock-out (Mecp2-/y) mice, a model of Rett syndrome (RTT), revealed unusual repetitive glutamate transients. They appeared in phase with bursts of action potentials in the CA1 neurons. Both glutamate transients and bursting activity were suppressed by the blockade of sodium, AMPA and voltage-gated calcium channels (T- and R-type), and enhanced after the inhibition of HCN channels. HCN and calcium channels in RTT and wild-type (WT) CA1 neurons displayed different voltage-dependencies and kinetics. Both channels modulated postsynaptic integration and modified the pattern of glutamate spikes in the RTT hippocampus. Spontaneous glutamate transients were much less abundant in the WT preparations, and, when observed, had smaller amplitude and frequency. The basal ambient glutamate levels in RTT were higher and transient glutamate increases (spontaneous and evoked by stimulation of Schaffer collaterals) decayed slower. Both features indicate less efficient glutamate uptake in RTT. To explain the generation of repetitive glutamate spikes, we designed a novel model of glutamate-induced glutamate release. The simulations correctly predicted the patterns of spontaneous glutamate spikes observed under different experimental conditions. We propose that pervasive spontaneous glutamate release is a hallmark of Mecp2-/y hippocampus, stemming from and modulating the hyperexcitability of neurons.

神经元强烈活动时产生的过量谷氨酸（glutamate）无法被立即清除，可能在细胞外间隙蓄积。这会引发多种长期效应，例如异位信号传导、突触效能调控异常以及兴奋性毒性（excitotoxicity）；其中兴奋性毒性与多种神经发育及神经退行性疾病密切相关。本研究通过对瑞特综合征（Rett syndrome, RTT）模型——甲基CpG结合蛋白2敲除（Mecp2-/y）小鼠的海马脑片（hippocampal slices）开展谷氨酸稳态定量成像，观察到了异常的重复性谷氨酸瞬态变化。这些瞬态变化与CA1神经元的动作电位（action potentials）爆发同步出现。谷氨酸瞬态变化与神经元爆发活动均可被钠通道（sodium channels）、AMPA受体及T、R型电压门控钙通道（voltage-gated calcium channels）的阻断剂抑制，而经超极化激活环核苷酸门控通道（HCN channels）抑制剂处理后则显著增强。RTT模型与野生型（wild-type, WT）CA1神经元中的HCN通道及钙通道，在电压依赖性与动力学特性上均存在差异。这两类通道均可调控突触后整合（postsynaptic integration）过程，并改变RTT模型小鼠海马内的谷氨酸峰电位模式。野生型标本中的自发性谷氨酸瞬态变化则显著更少，即便被观测到，其幅度与频率也更低。RTT模型小鼠的基础环境谷氨酸水平更高，且谷氨酸瞬态升高（包括自发性升高及谢弗侧支（Schaffer collaterals）刺激诱发的升高）的衰减速度更慢。上述两点均表明，RTT模型小鼠的谷氨酸摄取（glutamate uptake）效率更低。为阐释重复性谷氨酸峰电位的产生机制，我们构建了一种全新的谷氨酸诱导谷氨酸释放模型。该模型的仿真结果准确预测了不同实验条件下观测到的自发性谷氨酸峰电位模式。我们提出，广泛存在的自发性谷氨酸释放是Mecp2-/y小鼠海马的标志性特征，其源于神经元过度兴奋性（hyperexcitability）并可对其进行调控。