Bender Roland A, Soleymani Sheila V, Brewster Amy L, Nguyen Snow T, Beck Heinz, Mathern Gary W, Baram Tallie Z
Department of Anatomy, University of California, Irvine, Irvine, California 92697, USA.
J Neurosci. 2003 Jul 30;23(17):6826-36. doi: 10.1523/JNEUROSCI.23-17-06826.2003.
Changes in the expression of ion channels, contributing to altered neuronal excitability, are emerging as possible mechanisms in the development of certain human epilepsies. In previous immature rodent studies of experimental prolonged febrile seizures, isoform-specific changes in the expression of hyperpolarization-activated cyclic nucleotide-gated cation channels (HCNs) correlated with long-lasting hippocampal hyperexcitability and enhanced seizure susceptibility. Prolonged early-life seizures commonly precede human temporal lobe epilepsy (TLE), suggesting that transcriptional dysregulation of HCNs might contribute to the epileptogenic process. Therefore, we determined whether HCN isoform expression was modified in hippocampi of individuals with TLE. HCN1 and HCN2 expression were measured using in situ hybridization and immunocytochemistry in hippocampi from three groups: TLE with hippocampal sclerosis (HS; n = 17), epileptic hippocampi without HS, or non-HS (NHS; n = 10), and autopsy material (n = 10). The results obtained in chronic human epilepsy were validated by examining hippocampi from the pilocarpine model of chronic TLE. In autopsy and most NHS hippocampi, HCN1 mRNA expression was substantial in pyramidal cell layers and lower in dentate gyrus granule cells (GCs). In contrast, HCN1 mRNA expression over the GC layer and in individual GCs from epileptic hippocampus was markedly increased once GC neuronal density was reduced by >50%. HCN1 mRNA changes were accompanied by enhanced immunoreactivity in the GC dendritic fields and more modest changes in HCN2 mRNA expression. Furthermore, similar robust and isoform-selective augmentation of HCN1 mRNA expression was evident also in the pilocarpine animal model of TLE. These findings indicate that the expression of HCN isoforms is dynamically regulated in human as well as in experimental hippocampal epilepsy. After experimental febrile seizures (i.e., early in the epileptogenic process), the preserved and augmented inhibition onto principal cells may lead to reduced HCN1 expression. In contrast, in chronic epileptic HS hippocampus studied here, the profound loss of interneuronal and principal cell populations and consequent reduced inhibition, coupled with increased dendritic excitation of surviving GCs, might provoke a "compensatory" enhancement of HCN1 mRNA and protein expression.
离子通道表达的变化会导致神经元兴奋性改变，这正逐渐成为某些人类癫痫发生发展的可能机制。在之前对未成熟啮齿动物进行的实验性长时间热性惊厥研究中，超极化激活的环核苷酸门控阳离子通道（HCNs）表达的亚型特异性变化与海马体长期的兴奋性过高以及癫痫易感性增强相关。长时间的早期惊厥通常先于人类颞叶癫痫（TLE）出现，这表明HCNs的转录失调可能促成癫痫发生过程。因此，我们确定了TLE患者海马体中HCN亚型的表达是否发生改变。我们使用原位杂交和免疫细胞化学方法，对三组海马体中的HCN1和HCN2表达进行了检测：伴有海马硬化（HS；n = 17）的TLE患者、无HS的癫痫海马体（非HS，NHS；n = 10）以及尸检材料（n = 10）。通过检查慢性TLE毛果芸香碱模型的海马体，验证了在慢性人类癫痫中获得的结果。在尸检和大多数NHS海马体中，HCN1 mRNA表达在锥体细胞层中较高，而在齿状回颗粒细胞（GCs）中较低。相比之下，一旦GC神经元密度降低超过5０%，癫痫海马体GC层和单个GC中的HCN1 mRNA表达就会显著增加。HCN1 mRNA的变化伴随着GC树突区域免疫反应性增强以及HCN2 mRNA表达的更适度变化。此外，在TLE的毛果芸香碱动物模型中，也明显出现了类似的HCN1 mRNA表达的强烈且亚型选择性增加。这些发现表明，HCN亚型的表达在人类以及实验性海马癫痫中受到动态调节。在实验性热性惊厥后（即在癫痫发生过程早期），对主要细胞的保留和增强抑制可能导致HCN1表达降低。相比之下，在本文研究的慢性癫痫HS海马体中，中间神经元和主要细胞群体的大量丧失以及随之而来的抑制作用减弱，再加上存活GC树突兴奋性增加，可能会引发HCN1 mRNA和蛋白质表达的“代偿性”增强。
