Abstracts

This abstract has been invited to present during the Basic Mechanisms platform session
This abstract has been invited to present during the Basic Science Poster Highlights poster session

Rationale: Traditional focal epilepsy therapies often rely on systemic antiseizure drugs that alter brain-wide activity, rather than addressing underlying aberrant firing in specific neuronal populations. People with epilepsy therefore endure diverse side effects due to off-target neuromodulation impacting normal cognition and behavior. Chemogenetic approaches, such as Designer Receptors Exclusively Activated by Designer Drugs (DREADDs), allow more targeted neuronal regulation and have demonstrated efficacy in animal epilepsy models. However, inhibitory DREADD metabotropic signaling relies on downstream effectors and activates non-specific second messenger pathways. Therefore, we have instead evaluated controlling focal epilepsy with engineered chimeric ligand-gated chloride channels. These channels are activated by select α7 nicotinic acetylcholine (ACh) receptor agonists, while also retaining sensitivity to endogenous ACh fluxes.

Methods: Adult male C57BL/6 mice were injected in both hippocampi with AAV vectors containing a pan-neuronal CODA71 chimeric receptor expression cassette or a control scrambled cassette. Evoked hippocampal seizure threshold shifts were measured in response to perforant pathway electrical stimulation. For spontaneous seizure experiments, mice received unilateral intrahippocampal kainic acid (IHKA) injections to induce epilepsy, 3 weeks before AAV injection and EEG electrode placement. Hippocampal ACh was monitored in vivo with 2‑P iAChSnFr sensor imaging.

Results: CODA71 chimeric chloride channels suppress hippocampal neuronal firing in acute slices after selective agonist TC-5619 application (TC-5619 rheobase: 368±39.7 pA; vehicle rheobase: 80±15.4 pA; p=0.002). The chemogenetic approach also altered in vivo susceptibility to evoked, focal, hippocampal electrographic seizures. Minimal threshold potentials to evoke electrographic seizures rose more after i.p. TC-5619 (100 mg/kg) in CODA71 animals (139±48.0%) than in scramble controls (-10±13.2%; p=0.0166). Furthermore, we evaluated how well this chemogenetic approach controlled spontaneous seizures in the IHKA focal epilepsy model, which replicates many human temporal lobe epilepsy features. Electrographic seizure frequency was significantly reduced by i.p. TC-5619 injections in epileptic mice expressing CODA71, compared to in scramble controls (p=0.0011). We are continuing to investigate how the dynamics of seizure-dependent changes in hippocampal ACh may interact with ACh-sensitive receptor activation.

Conclusions: Our data show that chimeric chloride channels, like the CODA71 receptor, allow targeted focal seizure suppression, while avoiding potential pitfalls of systemic and DREADD-based therapies. We continue to explore how further restricting receptor expression to more select neuronal subsets or altering sensitivity to endogenous ACh signaling can further improve the potential of this approach to treat focal epilepsies.

Funding: CODA Biotherapeutics, Inc. provided funding and reagents, as well as guidance on AAV and small molecule dosing.