Introduction to Epilepsy

Epilepsy is a chronic brain disorder characterized by an unpredictable recurrence of seizures. Seizures are categorized as either focal or generalized [1]. Several different cognitive and emotional dysfunctions can manifest depending on the area of the brain that is affected by epilepsy [2]. The names of the anatomical areas implicated in epilepsy are used to categorize the disorder, such as parietal, temporal, frontal, and occipital-lobe epilepsy [3]. The prevalence of epilepsy is remarkable as it includes 1% of the world population, which highlights the significance of research in this area [2]. Epileptogenesis refers to the causative cascades that resulted in the onset of spontaneous seizures or epilepsy [3]. Epileptogenesis can be influenced by both acquired and genetic factors [Full size image

Patch Clamp Recording

Patch clamp recording provides direct and accurate measurements of membrane potential and current flow through cell membranes [56, 57]. In the patch clam** technique, voltage clamp (V-clamp) and current clamp (I-clamp) are used to address certain research questions [57]. In a nutshell, the V-clamp approach allows to precisely quantify the ionic currents that flow through the cell by clam** the membrane potential at a predefined level [57]. In contrast, the I-clamp approach allows to monitor changes in membrane potential by clam** the current rather than the voltage [57]. I-clamp is used to record resting membrane potential and synaptic potentials while V-clamp is employed to record firing activity [56]. Consequently, patch clamp recording techniques enable to get substantial knowledge of the cellular and network dynamics involved in seizures [56,57,58]. Seizures are commonly identified by atypical and synchronous firing of neurons; therefore, during a seizure, patch clamp recording can capture the action potentials of specific neurons, providing data on their firing rates, amplitudes, and altered patterns [57]. This knowledge helps the comprehension of the processes involved in the genesis and spread of seizures [58]. Patch clamp recording has however some limitations that require attention. One such concern relates to the invasive nature of inserting a glass pipette into the cell membrane for the purpose of tight seals formed during recording [57]. This invasive approach to monitor cellular activity raises legitimate concerns, as studies have indicated that even the composition of the pipette itself may result in variations in recorded data [57]. Moreover, patch clamp recordings are often carried out in vitro, so they might not accurately reflect the complicated in vivo environment [56, 58]. However, employing acute slices, patch clamp recording has been shown to be a successful method for investigating epilepsy in vitro [27]. By directly monitoring the electrical activity of individual neurons, patch clamp recording enables the characterization of abnormal firing patterns and alterations in membrane potentials related to seizures [57]. Patch clamp recordings also allow studying how pharmacological agents alter neuronal excitability, advancing the search for new anti-seizure treatments [56, 59]. Additionally, one can evaluate and set up the appropriate dosage of novel drugs using the concentration–response curve [Multi Electrode Array Recording

A MEA is designed to record extracellular electrical activity of all excitable or electrogenic cells and tissues [54, 62]. MEAs typically have insulated microelectrodes covered in conductive materials like indium-tin oxide, palladium, or gold on a photoetched glass chip [62]. Depending on the purpose of the study, the number, arrangement, and size of the MEA chips can vary. For example, arrays have a number of electrodes that vary from few tenths up to many thousands (high density MEA or HD-MEA). Each electrode has a size from 10 to 30 µm [62]. The ability to record local field potentials (LFPs) or high-frequency single spikes through MEAs depends on both the cell culture density and the proximity of cells to the electrodes. In cases of high cell density or recording from acute slices, the resulting electrical activity could lead to either constructive or destructive interference, influencing the expected outcome of LFP recordings [62, 63].

EA recordings provides valuable data regarding action potentials (spikes) and local field potentials (LFPs), which reflect the collective activity of small groups of neurons. These data give insights into the spike rate of action potentials or into the burst rate. The burst represents grou**s of action potentials, which are rapid and synchronized sequences of spikes. In addition, the overall spike and burst rates within the network can be recorded [62]. Furthermore, by conducting more advanced analysis, valuable information can be obtained, including autocorrelograms, cross-correlograms, and advanced statistical models of network connectivity [62, 75, 84]. Additionally, ChR2 can successfully prevent seizures when they are ectopically expressed in inhibitory interneurons [75]. NpHR is a light-gated chloride pump that is generated from halobacteria [85]. Yellow (590 nm) light activates NpHR, prompting chloride ions to enter neurons [85]. This influx of chloride ions hyperpolarizes the neuron, effectively decreasing its excitability and raising the threshold for action potential firing [85]. Thus, NpHR permits to selectively inhibit neuronal activity of target neurons within different brain circuits [75, 85]. This controlled inhibition of action potentials offers valuable opportunities for studying neural processes and their contributions to various functions. Similar to NpHR, Arch functions as an inhibitory opsin. In fact, when subjected to green light at 565 nm, this archaea-derived opsin operates as a proton pump, facilitating the transport of protons into the neuron [85]. The neural action potential is inhibited as a result of this proton pum** by raising the threshold for depolarization. Arch is distinguished from other optogenetic tools by its singular capacity to produce consistent and long-lasting inhibition, which makes it essential for implementing long-term changes in neuronal activity [13]. These optogenetic approaches can help better understanding the complex processes behind epileptic activity. By precisely activating or inhibiting specific neurons, optogenetics can in fact reveal details on how these neurons promote the development and progression of seizures, enabling investigations into the dynamics of neural networks and the relationships between neuronal activity and epileptogenesis. Additionally, optogenetics has a potential for studying and modifying several biochemical processes within neuronal cells, resulting in novel therapeutic treatments different from those offered by traditional pharmacological methods [13, 75, 86]. An in vivo study highlights the effectiveness of this advanced technique. The study showed that light-stimulation of the deep and intermediate layers of the superior colliculus via Chr2 could suppress spontaneous limbic seizures across various experimental epilepsy models [87]. In this way, optogenetic tools emerge as a promising avenue to target specific pathways implicated in epilepsy onset in vivo. Translating this understanding into novel applications holds the potential for develo** more effective drugs to suppress drug-resistant epilepsy [84].