Enhanced Expression of a Specific Hyperpolarization-Activated Cyclic Nucleotide-Gated Cation Channel (HCN) in Surviving Dentate Gyrus Granule Cells of Human and Experimental Epileptic Hippocampus

Abstract

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.

Formation of heteromeric hyperpolarization-activated cyclic nucleotide-gated (HCN) channels in the hippocampus is regulated by developmental seizures

“…Data acquisition and analysis of in situ hybridizations were carried out as described elsewhere (Bender et al, 2003;Brewster et al, 2002) on sections run concurrently, and always without knowledge of treatment. Semi-quantitative mRNA analysis was done by measuring optical density of the incorporated radioactivity using the image analysis program ImageTool.…”

“…Differential regulation of HCN1 and HCN2 mRNA expression has been found in pathological states (Bender et al, 2003;Bräuer et al, 2001;Brewster et al, 2002), including experimental febrile and kainate-induced seizures, and has been associated with altered properties of the I h (Chen et al, 2001a). Interestingly, the properties of I h after experimental febrile seizures resembled neither those of heterologously expressed homomeric HCN1 or HCN2 channels, nor their arithmetic intermediates (Chen et al, 2001a,b;Ulens and Tytgat, 2001).…”

Effects of the Ih Blockers CsCl and ZD7288 on Inherited Epilepsy in Mongolian Gerbils

“…Seizure grade 2 Generalized seizure: rearing, tonic-clonic convulsion of forelimbs and (Score: 2) hindlimbs, mastication, falling, opisthotonus posture and wild running followed by posture in arrest state, jerking straub tail, and bloody salivation.…”

“…In a recent study using genetically modified mice in which HCN2 had been deleted, the mice exhibited spontaneous absence seizures, suggesting that the absence of HCN2 is strongly proconvulsive [24]. Profound loss of interneuronal and principal cell populations and consequent reduced inhibition may induce a compensatory increase in HCN1 mRNA and protein expression [2]. The Yang (=excitatory) aspects include blockade of Ih channels by CsCl or ZD7288, which has been found to raise the afterdischarge threshold in an in vivo model of stimulus-evoked seizures [19].…”

Neuropeptide Y: Potential role in recurrent developmental seizures

“…limbic (temporal lobe) epilepsy, as adults [32,33]. This may be due in part to profound and long-lasting changes in the expression of the hyperpolarization-activated cyclic nucleotide-gated cation (HCN) channel genes after FS [12,18,19,66]. These data together suggest that prolonged experimental FS induce plastic processes in the brain that render the neuronal network hyperexcitable [18,19,23,24,32,33].…”