Suppression of Hyperpolarization-Activated Cyclic Nucleotide-Gated Channel Function in Thalamocortical Neurons Prevents Genetically Determined and Pharmacologically Induced Absence Seizures

Abstract

Hyperpolarization-activated cyclic nucleotide-gated (HCN) channels and the Ih current they generate contribute to the pathophysiological mechanisms of absence seizures (ASs), but their precise role in neocortical and thalamic neuronal populations, the main components of the network underlying AS generation, remains controversial. In diverse genetic AS models, Ih amplitude is smaller in neocortical neurons and either larger or unchanged in thalamocortical (TC) neurons compared with nonepileptic strains. A lower expression of neocortical HCN subtype 1 channels is present in genetic AS-prone rats, and HCN subtype 2 knock-out mice exhibit ASs. Furthermore, whereas many studies have characterized Ih contribution to “absence-like” paroxysmal activity in vitro, no data are available on the specific role of cortical and thalamic HCN channels in behavioral seizures. Here, we show that the pharmacological block of HCN channels with the antagonist ZD7288 applied via reverse microdialysis in the ventrobasal thalamus (VB) of freely moving male Genetic Absence Epilepsy Rats from Strasbourg decreases TC neuron firing and abolishes spontaneous ASs. A similar effect is observed on γ-hydroxybutyric acid-elicited ASs in normal male Wistar rats. Moreover, thalamic knockdown of HCN channels via virally delivered shRNA into the VB of male Stargazer mice, another genetic AS model, decreases spontaneous ASs and Ih-dependent electrophysiological properties of VB TC neurons. These findings provide the first evidence that block of TC neuron HCN channels prevents ASs and suggest that any potential anti-absence therapy that targets HCN channels should carefully consider the opposite role for cortical and thalamic Ih in the modulation of absence seizures.SIGNIFICANCE STATEMENT Hyperpolarization-activated cyclic nucleotide-gated (HCN) channels play critical roles in the fine-tuning of cellular and network excitability and have been suggested to be a key element of the pathophysiological mechanism underlying absence seizures. However, the precise contribution of HCN channels in neocortical and thalamic neuronal populations to these nonconvulsive seizures is still controversial. In the present study, pharmacological block and genetic suppression of HCN channels in thalamocortical neurons in the ventrobasal thalamic nucleus leads to a marked reduction in absence seizures in one pharmacological and two genetic rodent models of absence seizures. These results provide the first evidence that block of TC neuron HCN channels prevents absence seizures.

Abolishing cAMP sensitivity in HCN2 pacemaker channels induces generalized seizures

“…be part of the pathomechanism leading to seizures (29,42). Previous rodent models for absence epilepsy have focused on abnormal cortical activity and on a perturbation of cortically controlled feedforward inhibition of TC cells through the nRT (43,44). Absence epilepsy was also found in animals deficient in corticoreticular excitation, which leads to a hyperexcitability of TC cells due to the lack of feedforward inhibition (45).…”

Cav3.2 T‐type calcium channel mutation influences kindling‐induced thalamic neuronal firing patterns in Genetic Absence Epilepsy Rats From Strasbourg

“…Channelopathies in several ion channels have been identified as possible pathophysiological mechanisms of absence seizures in both humans and animal models . Among these different voltage‐gated ion channels, hyperpolarization‐activated cyclic nucleotide‐gated (HCN) channels and the mediated I h currents are proposed to contribute to network excitability and seizure expression in genetic models of absence epilepsy . Moreover, a reduction in HCN2 mRNA levels was also observed in fully kindled animals in the CA3 region .…”

Ectopic HCN4 expression drives mTOR-dependent epilepsy

“…Indeed, several studies have reported alterations in the expression of HCN, in particular, HCN1 and HCN2, in different types of epilepsy in both mice and humans (for review (41,54)). One recent study reported that blocking HCN1 channel activity prevented absence seizures (55), suggesting that an increase in HCN1 channel expression contributes to the generation of absence seizures. Another study reported no change in HCN4 expression in the hippocampus following febrile seizures despite changes in HCN1 (56).…”