The role of hippocampal interneurons in epileptogenesis

Temporal lobe epilepsy (TLE) is the most common and difficult to treat epilepsy syndrome. It is characterized by spontaneous recurrent seizures arising from limbic brain structures in particular from the hippocampus. Circuitries of the hippocampus are crucial for the initiation of single seizures as well as for the manifestation of recurrent spontaneous seizures (epilepsy). Multiple classes of interneurons exhibiting specific functions are modulating the information transmitted by excitatory pathways of the hippocampus. Physiological functions of the individual interneurons and their role in seizure precipitation are, however, poorly understood. We recently demonstrated that selective silencing of parvalbumin (PV)-containing basket cells induces recurrent seizures. In the present project, we will investigate the role of three other types of interneurons for the development of epilepsy. These are: Cholecystokinin (CCK)-containing basket cells expressing in addition to GABA and CCK either (1) the vesicular glutamate transporter 3 (VGLUT3) and the cannabinoid receptor CB1, or (2) vasoactive intestinal polypeptide (VIP). These interneurons (like parvalbumin neurons) exert feed-forward inhibition upon pyramidal cells. (3) The third group are somatostatin-containing O-LM cells mediating potent feed-back inhibition upon pyramidal cell dendrites. We will use local injection of a viral vector expressing tetanus toxin light chain into the hippocampus of respective transgenic mice. This treatment results in selective functional silencing of these neurons. We will then investigate development of spontaneous seizures (epilepsy) by continuous telemetric EEG recordings. Functional impairment of the affected interneurons will be demonstrated by electrophysiology in brain slices obtained from the mice. We will also express an artificial ion channel in these interneurons that will allow us to selectively activate the neurons and to investigate possible modulation of their GABA release by presynaptic receptors. In a third experimental approach we will investigate by immunohistochemistry and in situ hybridization a possible activation of endogenous anticonvulsant mechanisms leading to the termination of spontaneous seizures. The central aim of the project is to identify circuitry-based mechanisms that are crucial in the development of spontaneous seizures, and of mechanisms that terminate or protect from these seizures. A major advantage of our experimental approach is that it allows investigation of single components of the hippocampal circuitries and their role in development of recurrent seizures without neurodegeneration. It therefore resembles models of non-lesional TLE.

How Specialized Brain Cells Influence Epilepsy: A New Perspective on Inhibition and Seizure Control This research project provided important insights into how specific inhibitory brain cells affect seizure activity in temporal lobe epilepsy (TLE), one of the most common and drug-resistant forms of epilepsy. Contrary to earlier assumptions, the study revealed that not all inhibitory neurons are protective-some can even promote seizures depending on the brain's condition. The project focused on two subtypes of inhibitory interneurons: those expressing the neuropeptides VIP (vasoactive intestinal peptide) and somatostatin (SOM). These cells regulate brain activity through different mechanisms and contribute in distinct ways to the balance of excitation and inhibition in neural circuits. Using genetically modified mouse models, we selectively silenced or activated these neurons in the subiculum, a brain region closely involved in seizure generation. The key findings were striking: - In epileptic mice, silencing VIP-expressing interneurons led to a clear reduction in seizure frequency and severity. In contrast, activating these cells increased epileptiform activity. - In healthy mice, neither manipulation caused seizures, demonstrating a context-dependent role. VIP-interneurons may promote network instability in the diseased brain by disinhibiting excitatory neurons. - Silencing SOM-expressing interneurons had the opposite effect: it triggered spontaneous, recurrent seizures in previously healthy mice. This highlights SOM-interneurons as essential for maintaining inhibitory control and preventing seizures. These findings reshape the traditional view that all inhibitory cells are uniformly beneficial. Instead, they suggest that the type, location, and functional connections of interneurons must be considered when developing targeted epilepsy therapies. In addition to these functional discoveries, the study also investigated the activation of brain-derived protective mechanisms after seizures. The neuropeptide Y (NPY) was found to be overexpressed in both inhibitory and excitatory neurons after seizures, helping to suppress further activity. Blocking NPY's function pharmacologically led to intensified seizures, reinforcing its role as a natural anticonvulsant and a potential therapeutic target. The project also introduced a machine learning-based method for identifying sleep states by analyzing the high-frequency components of a single-channel EEG (without another electrode recording muscle activity)-a technical advance that simplifies brain monitoring and could improve preclinical epilepsy and sleep research. Additionally, an in vitro epilepsy model using hippocampal neurons cultured on high-density electrode arrays was successfully established. This setup allows long-term monitoring of neural activity and targeted manipulation of specific cell types as well as testing of new anti-seizure medications. The project has yielded several peer-reviewed publications, contributed to the training of several young researchers, and fostered international collaborations. These findings provide a deeper understanding of circuit dysfunction in epilepsy and lay the groundwork for new treatment strategies targeting specific interneuron types or enhancing the brain's own regulatory systems.