Optogenetic stimulation suppresses ictal activity in a 4-aminopyridine model of epileptic activity <i>in vitro</i>

Abstract

The WHO estimates that nearly 1% of the population suffers from epilepsy. Despite advances in the development of new antiepileptic drugs, seizures cannot be completely eliminated in nearly one-third of patients. One promising approach to the treatment of epilepsy may be gene therapy. Because epileptic activity is caused by an imbalance between excitation and inhibition, researchers have focused primarily on regulating neuronal excitability. Initially, the main approaches were based on the hyperexpression of inhibitory peptides such as galanin or NPY, or the suppression of neuronal excitability by the hyperexpression of potassium channels in neurons. However, these effects should be well calculated and strictly dosed, as it is difficult to correct the expression later. If the expression is too low, the anticonvulsant effect will not be achieved, and if the expression is too high, the neuronal networks will be impaired due to strong inhibition. For this reason, methodological approaches to treatment in which the effect on the neuronal excitability in the epileptic focus can be controlled are of great interest. Optogenetic methods offer such an advantage. Optogenetics uses light to alter the excitability of specific neuronal populations and can also be used in a biofeedback paradigm in which the light source is activated only at the risk of generating seizure activity. A number of optogenetic tools have now been developed, including light-activated cationic (e.g., ChR2) and anion channels (ACR), metabotropic receptors, pumps (NpHR, Arch), and enzymes. However, the optogenetic approach has a number of technical difficulties in delivering the light source and the risk of developing an immune response to the expression of rhodopsins. This report reviews the experience of practical application of optogenetic tools in the use of 4-aminopyridine in vitro model of epileptiform activity in experiencing slices of the entorhinal cortex. We tested the efficacy of suppressing ictal activity using several variants of optogenetic stimulation: (1) activation of excitatory and inhibitory neurons (in Thy1-ChR2-YFP mice), (2) activation of inhibitory interneurons only (in PV-Cre mice after virus injection with the channelodopsin-2 gene), hyperpolarization of excitatory neurons after expression of (3) archaeodopsin or (4) a light-dependent sodium pump. We found that ictal activity was successfully suppressed when low-frequency optogenetic stimulation induced regular interictal activity. Usually, interictal activity was induced by rhythmic synchronous activation of the principal neurons of the entorhinal cortex. In other cases, the ictal activity was preserved, although its characteristics may have changed. We determined the parameters of optogenetic stimulation that were most effective in suppressing ictal activity. The availability of a wide range of gene therapy approaches for epilepsy that have demonstrated efficacy in preclinical studies suggests that clinical trials of some of these approaches will begin in the next few years.