Okazaki M M, Molnár P, Nadler J V
Department of Pharmacology and Cancer Biology and Department of Neurobiology, Duke University Medical Center, Durham, North Carolina 27710, USA.
J Neurophysiol. 1999 Apr;81(4):1645-60. doi: 10.1152/jn.1999.81.4.1645.
A common feature of temporal lobe epilepsy and of animal models of epilepsy is the growth of hippocampal mossy fibers into the dentate molecular layer, where at least some of them innervate granule cells. Because the mossy fibers are axons of granule cells, the recurrent mossy fiber pathway provides monosynaptic excitatory feedback to these neurons that could facilitate seizure discharge. We used the pilocarpine model of temporal lobe epilepsy to study the synaptic responses evoked by activating this pathway. Whole cell patch-clamp recording demonstrated that antidromic stimulation of the mossy fibers evoked an excitatory postsynaptic current (EPSC) in approximately 74% of granule cells from rats that had survived >10 wk after pilocarpine-induced status epilepticus. Recurrent mossy fiber growth was demonstrated with the Timm stain in all instances. In contrast, antidromic stimulation of the mossy fibers evoked an EPSC in only 5% of granule cells studied 4-6 days after status epilepticus, before recurrent mossy fiber growth became detectable. Notably, antidromic mossy fiber stimulation also evoked an EPSC in many granule cells from control rats. Clusters of mossy fiber-like Timm staining normally were present in the inner third of the dentate molecular layer at the level of the hippocampal formation from which slices were prepared, and several considerations suggested that the recorded EPSCs depended mainly on activation of recurrent mossy fibers rather than associational fibers. In both status epilepticus and control groups, the antidromically evoked EPSC was glutamatergic and involved the activation of both AMPA/kainate and N-methyl-D-aspartate (NMDA) receptors. EPSCs recorded in granule cells from rats with recurrent mossy fiber growth differed in three respects from those recorded in control granule cells: they were much more frequently evoked, a number of them were unusually large, and the NMDA component of the response was generally much more prominent. In contrast to the antidromically evoked EPSC, the EPSC evoked by stimulation of the perforant path appeared to be unaffected by a prior episode of status epilepticus. These results support the hypothesis that recurrent mossy fiber growth and synapse formation increases the excitatory drive to dentate granule cells and thus facilitates repetitive synchronous discharge. Activation of NMDA receptors in the recurrent pathway may contribute to seizure propagation under depolarizing conditions. Mossy fiber-granule cell synapses also are present in normal rats, where they may contribute to repetitive granule cell discharge in regions of the dentate gyrus where their numbers are significant.
颞叶癫痫以及癫痫动物模型的一个共同特征是海马苔藓纤维向齿状分子层生长，其中至少有一些纤维支配颗粒细胞。由于苔藓纤维是颗粒细胞的轴突，反复的苔藓纤维通路为这些神经元提供单突触兴奋性反馈，这可能会促进癫痫放电。我们使用毛果芸香碱诱导的颞叶癫痫模型来研究激活该通路所诱发的突触反应。全细胞膜片钳记录表明，对苔藓纤维进行逆向刺激在毛果芸香碱诱导的癫痫持续状态后存活超过10周的大鼠的约74%的颗粒细胞中诱发了兴奋性突触后电流（EPSC）。在所有情况下，均通过Timm染色证实了反复的苔藓纤维生长。相比之下，在癫痫持续状态后4 - 6天，即在反复的苔藓纤维生长可检测到之前，对苔藓纤维进行逆向刺激仅在5%的所研究颗粒细胞中诱发了EPSC。值得注意的是，对苔藓纤维进行逆向刺激在许多对照大鼠的颗粒细胞中也诱发了EPSC。在制备切片的海马结构水平，苔藓纤维样Timm染色簇通常存在于齿状分子层的内三分之一处，并且有几个因素表明记录到的EPSC主要依赖于反复的苔藓纤维而非联合纤维的激活。在癫痫持续状态组和对照组中，逆向诱发的EPSC都是谷氨酸能的，并且涉及AMPA/海人藻酸受体和N - 甲基 - D - 天冬氨酸（NMDA）受体的激活。在有反复苔藓纤维生长的大鼠的颗粒细胞中记录到的EPSC在三个方面与在对照颗粒细胞中记录到的EPSC不同：它们更频繁地被诱发，其中一些异常大，并且反应的NMDA成分通常更突出。与逆向诱发的EPSC相反，刺激穿通路径诱发的EPSC似乎不受先前癫痫持续状态发作的影响。这些结果支持这样的假设，即反复的苔藓纤维生长和突触形成增加了对齿状颗粒细胞的兴奋性驱动，从而促进了重复的同步放电。在反复通路中NMDA受体的激活可能在去极化条件下促进癫痫传播。苔藓纤维 - 颗粒细胞突触在正常大鼠中也存在，在齿状回中它们数量较多的区域，可能有助于颗粒细胞的重复放电。
