Pan-Inhibitory Hippocampal Neuron Ablation Reveals Insights into the Role of Interneurons in Epileptogenesis

Impaired inhibition-and the resulting disruption of the brain's excitatory-inhibitory (E-I) balance-has historically been theorized to play a critical role in epileptogenesis and seizures (Treiman, 2001).This concept is supported by numerous studies showing impaired function of gamma-aminobutyric acid (GABA)-ergic function in both genetic and acquired animal models of epilepsy and in human epileptic brain tissue.Furthermore, medications that enhance GABAergic signaling have an anticonvulsant effect whereas GABA antagonists are proconvulsant.Yet, despite a large body of evidence implicating GABA in epileptogenesis and epilepsy, the direct causal role of inhibitory interneuron loss (or impairment) has remained incompletely understood.In the case of acquired models of epilepsy, such as temporal lobe epilepsy, interneuron loss is central to signature pathophysiology but occurs alongside multiple other pathological and neuroplastic changes that are also likely to influence disease progression.In the case of genetic epilepsies, such as Dravet syndrome, interneurons display prominent pathology (Ogiwara et al., 2007), but developmental abnormalities likewise complicate interpretation.Several studies have attempted to selectively inhibit interneurons in the adult hippocampus to isolate their effects on epileptogenesis, but these manipulations were limited to specific inhibitory cell subpopulations (Drexel et al., 2017) or hippocampal subregions (Spampanato and Dudek, 2017) and the results were modest.Thus, definitively implicating inhibitory interneuron loss has remained an elusive goal.A recent study by Dusing et al. ( 2024) aimed to answer this longstanding question of how interneuron loss-in isolation of other factors-contributes to epileptogenesis and epilepsy.Their innovative experimental approach employed inducible, pan-inhibitory neuron ablation or silencing, targeted widely across the hippocampus.Specifically, the authors first used diphtheria toxin-mediated ablation targeted to vesicular GABA transporter (VGAT)-expressing interneurons in the dorsal and ventral bilateral hippocampus.This led to the development of frequent and severe seizure clusters and persistent epileptiform activity as measured by EEG.Surprisingly, 1 week later, there was a dramatic decline in seizure frequency, with only occasional breakthrough seizures thereafter.Postexperimental immunostaining showed significant reductions in the density of parvalbumin (PV) and somatostatin (SST) immunoreactive interneurons across the dorsal and ventral hippocampus, supporting the assumption that most inhibitory interneurons were indeed ablated during the experiment.Interestingly, upon closer examination of perisomatic inhibitory inputs onto dentate gyrus granule cells (GCs), while the number of PV puncta was markedly reduced (consistent with the decrease in PV interneuron number), the density of postsynaptic inhibitory synapses (assayed by immunostaining for gephyrin, a postsynaptic scaffolding protein at inhibitory synapses) was unchanged.Next, the authors conducted transient silencing of the same VGAT neuronal population by activating the chemogenetic inhibitory receptor, hM4DGi, expressed within those cells.Chemogenetic inhibition of VGAT neurons led to interictal spikes and seizures, though the observed phenotype was significantly milder than had been observed with interneuron ablation.