Department of Neurobiology, Civitan International Research Center, The University of Alabama at Birmingham, Birmingham, AL 35294-2182, USA.
J Neurophysiol. 2011 Apr;105(4):1768-84. doi: 10.1152/jn.00800.2010. Epub 2011 Feb 9.
Dysfunctions of neuronal and network excitability have emerged as common features in disorders associated with intellectual disabilities, autism, and seizure activity, all common clinical manifestations of Rett syndrome (RTT), a neurodevelopmental disorder caused by loss-of-function mutations in the transcriptional regulator methyl-CpG-binding protein 2 (MeCP2). Here, we evaluated the consequences of Mecp2 mutation on hippocampal network excitability, as well as synapse structure and function using a combination of imaging and electrophysiological approaches in acute slices. Imaging the amplitude and spatiotemporal spread of neuronal depolarizations with voltage-sensitive dyes (VSD) revealed that the CA1 and CA3 regions of hippocampal slices from symptomatic male Mecp2 mutant mice are highly hyperexcitable. However, only the density of docked synaptic vesicles and the rate of release from the readily releasable pool are impaired in Mecp2 mutant mice, while synapse density and morphology are unaffected. The differences in network excitability were not observed in surgically isolated CA1 minislices, and blockade of GABAergic inhibition enhanced VSD signals to the same extent in Mecp2 mutant and wild-type mice, suggesting that network excitability originates in area CA3. Indeed, extracellular multiunit recordings revealed a higher level of spontaneous firing of CA3 pyramidal neurons in slices from symptomatic Mecp2 mutant mice. The neuromodulator adenosine reduced the amplitude and spatiotemporal spread of VSD signals evoked in CA1 of Mecp2 mutant slices to wild-type levels, suggesting its potential use as an anticonvulsant in RTT individuals. The present results suggest that hyperactive CA3 pyramidal neurons contribute to hippocampal dysfunction and possibly to limbic seizures observed in Mecp2 mutant mice and RTT individuals.
神经元和网络兴奋性的功能障碍已成为与智力障碍、自闭症和癫痫活动相关的疾病的共同特征，这些都是 Rett 综合征（RTT）的常见临床表现，RTT 是一种神经发育障碍，由转录调节因子甲基化 CpG 结合蛋白 2（MeCP2）的功能丧失突变引起。在这里，我们使用急性切片中的成像和电生理方法相结合，评估了 Mecp2 突变对海马网络兴奋性、突触结构和功能的影响。用电压敏感染料（VSD）成像神经元去极化的幅度和时空扩展，结果显示症状性雄性 Mecp2 突变小鼠的海马切片 CA1 和 CA3 区高度超兴奋性。然而，只有 Mecp2 突变小鼠的停泊突触囊泡密度和从易释放池中释放的速度受损，而突触密度和形态不受影响。在手术分离的 CA1 小切片中没有观察到网络兴奋性的差异，并且 GABA 能抑制的阻断以相同的程度增强了 Mecp2 突变型和野生型小鼠的 VSD 信号，这表明网络兴奋性起源于 CA3 区。事实上，细胞外多单位记录显示，来自症状性 Mecp2 突变小鼠的切片中 CA3 锥体神经元的自发放电水平更高。神经调质腺苷降低了 Mecp2 突变型切片 CA1 中诱发的 VSD 信号的幅度和时空扩展，使其有望作为 RTT 个体的抗惊厥药。本研究结果表明，过度活跃的 CA3 锥体神经元可能导致 Mecp2 突变小鼠和 RTT 个体中观察到的海马功能障碍和可能的边缘性癫痫发作。
