Kleis, P. | bioRxiv Science

bioRxiv2023

On-demand low-frequency stimulation for seizure control: efficacy and behavioral implications

Mesial temporal lobe epilepsy (MTLE), the most common form of focal epilepsy in adults, is often refractory to medication and associated with hippocampal sclerosis. Deep brain stimulation represents an alternative treatment option for drug-resistant patients who are ineligible for resective brain surgery. In clinical practice, closed-loop stimulation at high frequencies is applied to interrupt ongoing seizures, yet with a high incidence of false detections, the drawback of delayed seizure-suppressive intervention and limited success in sclerotic tissue. More recently, hippocampal low-frequency stimulation (LFS) has been shown to reduce excitability in clinical settings and prevent seizures in experimental MTLE when applied continuously. However, as the hippocampus is important for navigation and memory, it would be beneficial to stimulate it only on-demand to reduce its exposure to LFS pulses, and to investigate LFS-related effects on cognition. Using the intrahippocampal kainate mouse model, which recapitulates the key features of MTLE, we developed an on-demand LFS setup and investigated its effects on spontaneous seizure activity and hippocampal function. Specifically, our online detection algorithm monitored epileptiform activity in hippocampal local field potential recordings and identified short epileptiform bursts preceding focal seizure clusters, triggering hippocampal LFS to stabilize the network state. In addition, we investigated the acute influence of LFS on behavioral performance, including anxiety-like behavior in the light-dark box test, spatial and non-spatial memory in the object location memory and novel object recognition test, as well as spatial navigation and long-term memory in the Barnes maze. Compared to open-loop stimulation protocols, on-demand LFS was more efficient in preventing focal seizure clusters, as the strong anti-epileptic effect was achieved with a reduced stimulation load. In behavioral tests, chronically epileptic mice were as mobile as healthy controls but showed increased anxiety, an altered spatial learning strategy and impaired memory performance. Most importantly, our experiments ruled out deleterious effects of hippocampal LFS on cognition and even showed alleviation of deficits in long-term memory recall. Taken together, our findings may provide a promising alternative to current therapies, overcoming some of their major limitations, and inspire further investigation of LFS for seizure control in MTLE.

neuroscience↗

bioRxiv2021

In vivo characterization and application of a novel potassium channel-based optogenetic silencer in the healthy and epileptic mouse hippocampus

The performance of available optogenetic inhibitors remains insufficient due to low light sensitivity, short-lasting photocurrents, and unintended changes in ion distributions. To overcome these limitations, a novel potassium channel-based optogenetic silencer was developed and successfully applied in various in vitro and acute in vivo settings (Bernal Sierra et al., 2018). This tool, a two-component construct called PACK, comprises a photoactivated adenylyl cyclase (bPAC) and a cAMP-dependent potassium channel (SthK). Here, we examined the long-term inhibitory action and side effects of the PACK construct in healthy and epileptic adult male mice. We targeted hippocampal CA1 pyramidal cells using a viral vector and enabled illumination of these neurons via an implanted optic fiber. Local field potential (LFP) recordings from the CA1 of freely moving mice revealed significantly reduced neuronal activity during 50-minute intermittent illumination, especially in the beta and gamma frequency ranges. Adversely, PACK expression in healthy mice induced chronic astrogliosis, dispersion of pyramidal cells, and generalized seizures. These side effects were independent of the light application and were also present in mice expressing bPAC without the potassium channel. Additionally, light-activation of bPAC alone increased neuronal activity, presumably via enhanced cAMP signaling. In chronically epileptic mice, the dark activity of bPAC/PACK in CA1 prevented the spread of spontaneous epileptiform activity from the seizure focus to the contralateral bPAC/PACK-expressing hippocampus. Taken together, the PACK tool is a potent optogenetic inhibitor but requires refinement of its light-sensitive domain to avoid unexpected physiological changes. Significance statementOptogenetics allows precise manipulation of neuronal activity via genetically encoded light-sensitive proteins. Unfortunately, available optogenetic inhibitors are not suitable for prolonged use. The newly developed two-component potassium channel-based optogenetic inhibitor, PACK, has been identified as a potent silencer of neurons in various acute experiments. Here, we characterized the PACK construct in freely behaving healthy and epileptic mice. We targeted the PACK silencer specifically to CA1 pyramidal neurons, where illumination with short light pulses at low frequencies reliably reduced neuronal activity. In chronically epileptic mice, PACK prevented the spread of epileptiform activity from the seizure focus to the contralateral PACK-expressing hippocampus. The major disadvantage of the PACK silencer is its light-sensitive domain, the bPAC adenylyl cyclase, which may induce side effects.

neuroscience↗