Sloviter Robert S, Zappone Colin A, Harvey Brian D, Frotscher Michael
Department of Pharmacology and the Graduate Program in Neuroscience, University of Arizona College of Medicine, Tucson, Arizona 85724, USA.
J Comp Neurol. 2006 Feb 20;494(6):944-60. doi: 10.1002/cne.20850.
Kainic acid-induced neuron loss in the hippocampal dentate gyrus may cause epileptogenic hyperexcitability by triggering the formation of recurrent excitatory connections among normally unconnected granule cells. We tested this hypothesis by assessing granule cell excitability repeatedly within the same awake rats at different stages of the synaptic reorganization process initiated by kainate-induced status epilepticus (SE). Granule cells were maximally hyperexcitable to afferent stimulation immediately after SE and became gradually less excitable during the first month post-SE. The chronic epileptic state was characterized by granule cell hyper-inhibition, i.e., abnormally increased paired-pulse suppression and an abnormally high resistance to generating epileptiform discharges in response to afferent stimulation. Focal application of the gamma-aminobutyric acid type A (GABA(A)) receptor antagonist bicuculline methiodide within the dentate gyrus abolished the abnormally increased paired-pulse suppression recorded in chronically hyper-inhibited rats. Combined Timm staining and parvalbumin immunocytochemistry revealed dense innervation of dentate inhibitory interneurons by newly formed, Timm-positive, mossy fiber terminals. Ultrastructural analysis by conventional and postembedding GABA immunocytochemical electron microscopy confirmed that abnormal mossy fiber terminals of the dentate inner molecular layer formed frequent asymmetrical synapses with inhibitory interneurons and with GABA-immunopositive dendrites as well as with GABA-immunonegative dendrites of presumed granule cells. These results in chronically epileptic rats demonstrate that dentate granule cells are maximally hyperexcitable immediately after SE, prior to mossy fiber sprouting, and that synaptic reorganization following kainate-induced injury is temporally associated with GABA(A) receptor-dependent granule cell hyper-inhibition rather than a hypothesized progressive hyperexcitability. The anatomical data provide evidence of a possible anatomical substrate for the chronically hyper-inhibited state.
海人酸诱导的海马齿状回神经元丢失可能通过触发正常情况下不相连的颗粒细胞之间反复兴奋性连接的形成，导致致痫性过度兴奋。我们通过在由海人酸诱导的癫痫持续状态（SE）引发的突触重组过程的不同阶段，在同一清醒大鼠体内反复评估颗粒细胞兴奋性，来验证这一假设。SE后颗粒细胞对传入刺激立即表现出最大程度的过度兴奋，并在SE后的第一个月逐渐变得兴奋性降低。慢性癫痫状态的特征是颗粒细胞过度抑制，即配对脉冲抑制异常增加以及对传入刺激产生癫痫样放电的抗性异常高。在齿状回局部应用A型γ-氨基丁酸（GABA(A)）受体拮抗剂甲硫双环脲可消除慢性过度抑制大鼠中记录到的异常增加的配对脉冲抑制。联合Timm染色和小白蛋白免疫细胞化学显示，新形成的、Timm阳性的苔藓纤维终末对齿状抑制性中间神经元有密集的神经支配。通过常规和包埋后GABA免疫细胞化学电子显微镜进行的超微结构分析证实，齿状内分子层的异常苔藓纤维终末与抑制性中间神经元、GABA免疫阳性树突以及推测的颗粒细胞的GABA免疫阴性树突形成频繁的不对称突触。这些在慢性癫痫大鼠中的结果表明，齿状颗粒细胞在SE后苔藓纤维发芽之前立即表现出最大程度的过度兴奋，并且海人酸诱导损伤后的突触重组在时间上与GABA(A)受体依赖性颗粒细胞过度抑制相关，而不是与假设的进行性过度兴奋相关。解剖学数据为慢性过度抑制状态提供了可能的解剖学基础的证据。
