Application of low-frequency photostimulation of parvalbumin interneurons to control epileptiform activity in the hippocampus
Authors: Trofimova A. M. , Postnikova T. Yu. , Proskurina E. Yu. , Zaitsev A. V. 
Published: May 2024

Low-frequency electrical stimulation of the brain is used to suppress seizure activity in people with resistant forms of epilepsy [1]. Low-frequency stimulation of certain cell types, such as optogenetic activation of inhibitory parvalbumin (PV) interneurons, can be considered as a promising method for treatment of resistant forms of epilepsy [2]. In this work, we investigated the effect of PV interneuron photostimulation on epileptiform activity in the mouse hippocampus and entorhinal cortex.
This work was performed on 4-month-old B6.129P2-Pvalbtm1(cre)Arbr/J (JacksonLab) mice expressing Cre recombinase in PV interneurons. Adenoassociated viral construct (AAV9-EF1a-DIO-hChR2(H134R)-mCherry) carrying the canalorhodopsin type 2 gene (ChR2) was injected into the CA1 field of the hippocampus at the border with the entorhinal cortex using stereotactic coordinates (AP: -4 mm, ML: 3.5 mm, DV: -3.5 mm). Experiments were performed after 4-5 weeks on surviving brain slices. ChR2-expressing interneurons were activated by 470-nm wavelength light using a laser diode-fiber light source. Epileptiform activity was induced in the slice by application of pro-epileptic solution with 4-aminopyridine (100 μM). Biophysical properties of neurons were recorded by the patch-clamp method in a whole-cell configuration. Epileptiform activity in the slice was recorded by the field potential withdrawal method.
We determined the optimal frequency and duration of photostimulation affecting epileptiform activity in the hippocampus of mice. We tested how PV interneuron photostimulation affects pyramidal neurons in the hippocampus using the patch-clamp method. The following parameters were assessed: intensity, duration, and frequency of PV interneuron photostimulation under normal conditions. Thus, at low photostimulation frequency we observed synchronized responses of the pyramidal cells. And the optimal duration of photostimulation should not have exceeded 25 ms. Then we decided to check how the selected photostimulation parameters affect the seizure activity in the slice. For this purpose, we used the method of recording field potentials. In the CA1 field of the hippocampus, photostimulation with a frequency of 1 Hz and a light flash duration of 10 ms induced regular interictal activity. This induced interictal activity completely suppressed the occurrence of ictal discharges in the brain slice. After cessation of photostimulation, the frequency of intrinsic epileptic-like events in the CA1 field of the hippocampus decreased compared with the pre-stimulatory level.
We found that photostimulation of PV interneurons results in discharges in response to light turn off, indicating synchronous activation of pyramidal neurons. Low-frequency photostimulation of PV interneurons is more effective in modulating epileptiform activity in the CA1 field of the hippocampus. The use of low-frequency optogenetic stimulation of PV interneurons seems to be a promising approach in the control and suppression of seizure activity.