Role of HCN Currents (Ih) in Epileptiform Activity in Subicular Neurons

Abstract

Subicular burst firing neurons are established as primary instigators of seizure onset and propagation in Temporal Lobe Epilepsy (TLE), a form of focal epilepsy that accounts for approximately 60% of all epilepsy cases in adults. Despite their pivotal role, the mechanisms governing epileptogenic activity within these bursting neurons remain poorly understood. Investigating the cellular and ionic mechanisms underlying seizure activity in these neuronal subtypes is critical for developing targeted therapies for TLE. In this study, we sought to elucidate the neuronal mechanisms shaping epileptiform activity in burst firing neurons and interneurons. We observed that epileptogenic 4AP-0Mg induced different patterns of epileptiform discharges in burst firing neurons and interneurons. Hyperpolarization-activated cyclic nucleotide-gated (HCN) channels regulate the intrinsic excitability of the neurons by governing the neuronal firing properties and membrane potential. To study the role of Ih (HCN currents) in epileptiform activity in subicular neurons, we modeled subicular HCN currents in the dynamic clamp that mimicked downregulation and overexpression observed in epilepsy-associated pathophysiology. Our results indicated that the burst firing neurons contribute to the HCN-mediated epileptic firing characteristics in the subiculum. We subsequently investigated the homeostatic modulation of HCN during the epileptiform activity in subicular burster cells. Our study is the first report showing Ih in rat subicular neurons during 4AP 0Mg-induced epileptogenic activity undergoes modulation on a time scale of a few minutes. Additionally, we observed that the changes in sag and chirp responses were persistent after the wash-out of 4AP-0Mg; thus, the changes appear irreversible. Our studies further showed that the neuronal excitability changes paralleled the changes in the HCN conductance during epileptogenesis. We conclude that a very rapid decline in somatic HCN function during epileptiform activity represents a previously unidentified mechanism of homeostatic dysfunction over a very short period, impeding the neuron’s ability to reestablish its regulatory processes in the subicular burster cells. These findings highlight the critical role of HCN channels in mediating seizure dynamics and underscore their potential as therapeutic drug targets and neuromodulation strategies aimed at preventing or mitigating seizures to counteract aberrant neuronal excitability in epilepsy.