Testing broad-spectrum and isoform-preferring HCN channel blockers for anticonvulsant properties in mice.

Hyperpolarization-activated cyclic nucleotide-gated (HCN) channels have been implicated in the pathogenesis of epilepsy and consequently as targets for anticonvulsant drugs. Consistent with this, broad-spectrum block of HCN-mediated current (Ih) reduces seizure susceptibility in a variety of epilepsy models. However, HCN channel isoforms have distinct biophysical characteristics and anatomical expression suggesting that they may play different roles in setting neuronal excitability. Here we confirm that the broad-spectrum blocker ivabradine is effective at reducing seizure susceptibility in the s.c.PTZ seizure assay and extend this, showing efficacy of this drug in a thermogenic assay that models febrile seizures. Ivabradine is also effective at reducing thermogenic seizures in the Scn1a mouse model of Dravet syndrome in which febrile seizures are a feature. HCN isoform-preferring drugs were tested in the s.c.PTZ seizure assay. We confirm that the HCN4-preferring drug, EC18, is efficacious in reducing seizure susceptibility. Conversely, the HCN2/1-preferring drug, MEL55A, increased seizure susceptibility in the s.c.PTZ seizure assay. MEL57A, an HCN1-preferring drug, had no effect on seizure susceptibility. Mouse pharmacokinetic studies (for MEL55A and MEL57A) and screening against additional ion channels have not been thoroughly investigated on the HCN isoform-preferring compounds. Our results need to be considered in this light. Nevertheless, these data suggest that HCN isoform-selective block can have a differential impact on seizure susceptibility. This motivates the need to develop more HCN isoform-selective compounds to better explore this idea.

The hyperpolarization-activated cyclic nucleotide-gated 4 channel as a potential anti-seizure drug target.

BACKGROUND AND PURPOSE: Hyperpolarization-activated cyclic nucleotide-gated (HCN) channels are encoded by four genes (HCN1-4) with distinct biophysical properties and functions within the brain. HCN4 channels activate slowly at robust hyperpolarizing potentials, making them more likely to be engaged during hyperexcitable neuronal network activity seen during seizures. HCN4 channels are also highly expressed in thalamic nuclei, a brain region implicated in seizure generalization. Here, we assessed the utility of targeting the HCN4 channel as an anti-seizure strategy using pharmacological and genetic approaches. EXPERIMENTAL APPROACH: The impact of reducing HCN4 channel function on seizure susceptibility and neuronal network excitability was studied using an HCN4 channel preferring blocker (EC18) and a conditional brain specific HCN4 knockout mouse model. KEY RESULTS: EC18 (10 mg·kg-1 ) and brain-specific HCN4 channel knockout reduced seizure susceptibility and proconvulsant-mediated cortical spiking recorded using electrocorticography, with minimal effects on other mouse behaviours. EC18 (10 µM) decreased neuronal network bursting in mouse cortical cultures. Importantly, EC18 was not protective against proconvulsant-mediated seizures in the conditional HCN4 channel knockout mouse and did not reduce bursting behaviour in AAV-HCN4 shRNA infected mouse cortical cultures. CONCLUSIONS AND IMPLICATIONS: These data suggest the HCN4 channel as a potential pharmacologically relevant target for anti-seizure drugs that is likely to have a low side-effect liability in the CNS.