Abstracts

Rationale: Focal seizures originate from aberrant firing in a subset of neurons, yet most antiepileptic drugs rely on systemic compound administration and alter brain-wide activity. Thus, patients with epilepsy endure substantial side effects due to unintentional modulation of neurons involved in normal cognition and undesirable off-target changes in other biological systems. Ideal focal epilepsy treatments would only alter activity of the specific neurons responsible for seizure generation. Recently, using chemogenetic Designer Receptors Exclusively Activated by Designer Drugs (DREADDs) to alter seizure activity has produced promising results in animal epilepsy models. However, inhibitory DREADDs rely on G protein-coupled receptors indirectly activating ion channels through second messengers and can inconsistently change neuron electrical potentials. We therefore investigated controlling focal epileptiform activity using engineered chimeric ligand-gated chloride channels activated by modified α7 nicotinic acetylcholine receptor agonists.

Methods: Adult male C57BL/6 mice were bilaterally injected with AAV constructs into the hippocampus to express either Coda71 chimeric receptors or scrambled control vectors. Changes to evoked hippocampal seizure thresholds were measured in response to perforant pathway electrical stimulation. For spontaneous seizure experiments, mice were unilaterally injected intrahippocampally with kainic acid (KA) to induce epilepsy, 3 weeks prior to AAV injection and EEG electrode implantation.

Results: Coda71 chimeric chloride channels express well in hippocampal neurons and effectively suppress neuronal firing in acute slices, following selective agonist TC-5619 application (TC-5619 rheobase: 368±39.7 pA; vehicle rheobase: 80±15.4 pA; p=0.002). We also evaluated how our chemogenetic approach altered in vivo susceptibility to evoked, focal, hippocampal electrographic seizures. The minimal threshold potential to evoke electrographic seizures increased much more following i.p. TC-5619 (100 mg/kg) in Coda71 animals (139±48.0%) than in scramble controls (-10±13.2%; p=0.0166). Finally, we tested the efficacy of our chemogenetic approach to control spontaneous seizures in the intrahippocampal KA focal epilepsy model, which replicates many human temporal lobe epilepsy features. Both electrographic seizure duration and frequency were significantly reduced by i.p. TC-5619 injections in epileptic mice (p=0.0011; p=0.0325).

Conclusions: Our data demonstrate the potential of the Coda71 receptor, and other similar chimeric chloride channel variants, to allow targeted focal seizure suppression, while avoiding potential pitfalls of systemic and DREADD-based therapies. We aim to determine whether further restricting receptor expression to more select neuronal subsets or altering sensitivity to endogenous cholinergic signaling can further improve the potential of our approach to treat focal epilepsies.

Funding: Please list any funding that was received in support of this abstract.: Coda Biotherapeutics, Inc. provided reagents and funding for these experiments, but was not involved in specifics of experimental design, data collection or analysis.