Department of Neurobiology, David Geffen School of Medicine at the University of California, Los Angeles, Los Angeles, California 90095, USA.
J Neurosci. 2013 Sep 4;33(36):14392-405. doi: 10.1523/JNEUROSCI.2045-13.2013.
Axonal sprouting of excitatory neurons is frequently observed in temporal lobe epilepsy, but the extent to which inhibitory interneurons undergo similar axonal reorganization remains unclear. The goal of this study was to determine whether somatostatin (SOM)-expressing neurons in stratum (s.) oriens of the hippocampus exhibit axonal sprouting beyond their normal territory and innervate granule cells of the dentate gyrus in a pilocarpine model of epilepsy. To obtain selective labeling of SOM-expressing neurons in s. oriens, a Cre recombinase-dependent construct for channelrhodopsin2 fused to enhanced yellow fluorescent protein (ChR2-eYFP) was virally delivered to this region in SOM-Cre mice. In control mice, labeled axons were restricted primarily to s. lacunosum-moleculare. However, in pilocarpine-treated animals, a rich plexus of ChR2-eYFP-labeled fibers and boutons extended into the dentate molecular layer. Electron microscopy with immunogold labeling demonstrated labeled axon terminals that formed symmetric synapses on dendritic profiles in this region, consistent with innervation of granule cells. Patterned illumination of ChR2-labeled fibers in s. lacunosum-moleculare of CA1 and the dentate molecular layer elicited GABAergic inhibitory responses in dentate granule cells in pilocarpine-treated mice but not in controls. Similar optical stimulation in the dentate hilus evoked no significant responses in granule cells of either group of mice. These findings indicate that under pathological conditions, SOM/GABAergic neurons can undergo substantial axonal reorganization beyond their normal territory and establish aberrant synaptic connections. Such reorganized circuitry could contribute to functional deficits in inhibition in epilepsy, despite the presence of numerous GABAergic terminals in the region.
兴奋神经元的轴突发芽在颞叶癫痫中经常观察到，但抑制性中间神经元是否经历类似的轴突重组尚不清楚。本研究的目的是确定海马 CA1 区的 SOM 表达神经元是否在匹罗卡品癫痫模型中超越其正常区域进行轴突发芽，并支配齿状回颗粒细胞。为了获得 SOM 表达神经元在 s. oriens 中的选择性标记，使用 Cre 重组酶依赖性构建体将通道视紫红质 2 融合到增强型黄色荧光蛋白 (ChR2-eYFP) 中，将其递送到 SOM-Cre 小鼠的该区域。在对照小鼠中，标记的轴突主要局限于 s. lacunosum-moleculare。然而，在匹罗卡品处理的动物中，丰富的 ChR2-eYFP 标记纤维和末梢丛延伸到齿状分子层。用免疫金标记的电子显微镜显示，标记的轴突末端在该区域形成与颗粒细胞支配相一致的树突轮廓的对称突触。在 CA1 的 s. lacunosum-moleculare 和齿状分子层中对 ChR2 标记的纤维进行模式化照明，可在匹罗卡品处理的小鼠而非对照组中诱发齿状颗粒细胞的 GABA 能抑制反应。在两组小鼠的齿状回门区进行类似的光刺激，均未引起颗粒细胞产生明显反应。这些发现表明，在病理条件下，SOM/GABA 能神经元可以超越其正常区域进行大量轴突重组，并建立异常的突触连接。这种重组的电路可能导致癫痫抑制的功能缺陷，尽管该区域存在大量 GABA 能末梢。
