Long-term hippocampal low-frequency stimulation alleviates focal seizures, memory deficits and synaptic pathology in epileptic mice

Background Mesial temporal lobe epilepsy (MTLE) is a prevalent form of focal epilepsy characterized by seizures originating from the hippocampus and adjacent regions. Neurostimulation presents an alternative for surgery-ineligible patients with intractable seizures. However, conventional approaches have limited efficacy and require refinement for better seizure control. While hippocampal low-frequency stimulation (LFS) has shown promising seizure reduction in animal studies and small clinical cohorts, its mechanisms, sex-specific outcomes, and long-term effects remain unknown.

We aimed to identify the long-term antiepileptic and cognitive outcomes and potential underlying mechanisms of hippocampal LFS in chronically epileptic male and female mice.

We used the intrahippocampal kainate mouse model replicating the features of MTLE: spontaneous seizures, hippocampal sclerosis, and memory deficits. We applied 1 Hz electrical LFS in the sclerotic hippocampus 6 hours a day, four times a week for 5 weeks and examined its effects on epileptiform activity, spatial memory, and kainate-induced pathological features at cellular and synaptic levels.

Long-term hippocampal LFS consistently diminished focal seizures in epileptic male and female mice, with seizure reduction extending beyond the stimulation period. Additionally, LFS relieved spatial memory deficits and reversed pathological long-term potentiation-like changes at perforant path-dentate granule cell synapses. LFS had no significant effect on generalized convulsive seizures, anxiety-like behaviour, neurogenesis, hippocampal sclerosis, excitatory synapse marker expression, or presynaptic vesicles in perforant path fibers.

These findings provide clinically relevant insights into the seizure type-specific effects of hippocampal LFS, which, alongside synaptic and behavioural improvements, could contribute to enhanced seizure control and quality of life in MTLE patients. Competing Interest Statement The authors have declared no competing interest. Abbreviations - AAV - adeno-associated virus - CA - Cornu ammonis - CaMKIIα - Ca2+/calmodulin-dependent protein kinase II α - CEMT - Center for Experimental Models and Transgenic Service - cHC - contralateral hippocampus - DBS - deep brain stimulation - DGC - dentate granule cell - Doublecortin - DCX - GFAP - glial fibrillary acidic protein - HFS - high-frequency stimulation - HL - high-load - iHC - ipsilateral hippocampus - KA - kainate - LFP - local field potential - LFS - low-frequency stimulation - LL - low-load - LTD - long-term depression - LTP - long-term potentiation - ML - medium load - MML - middle molecular layer - MTLE - mesial temporal lobe epilepsy - NeuN - neuronal nuclei - PB - phosphate buffer - PFA - paraformaldehyde - PP - perforant path - PSD - postsynaptic density - RM - repeated measures - RNS - responsive neurostimulation - ROI - region of interest - Sal - Saline - vGluT1 - vesicular glutamate transporter 1