It is estimated that 30% of those affected with epilepsy do not have control over their seizures, which are considered the hallmark of the neurological disorder . A better understanding of the inner neurobiological mechanisms of these seizures may lead us to new treatments for epilepsy altogether. However, the rise of optogenetics, a technique using light to control genetically modified neurons, has transformed epilepsy research. This technique has given insights into the roles that cell types play in mediating seizures within the development of epilepsy . The targeting of neuron populations within optogenetics has introduced prospects of therapies that can stop seizures on command . This article aims to illustrate the neurobiological basis behind these breakthroughs in optogenetics, which are bound to direct epilepsy research in the coming years.
Neurobiology of Epilepsy
Epilepsy is one of the most common neurological disorders; however, we still do not have a complete understanding of the detailed pathophysiology of this condition as well as its most effective treatments. The defining characteristic of epilepsy is the recurring, uncontrollable seizures it induces in its affected patients.
A seizure is an alteration of the mind’s function caused by the excessive discharge of neurons in the brain. An epileptic seizure is specified as a seizure caused by abnormal neuronal firing from a nonepileptic event, such as a psychogenic seizure (having a psychological basis) .
Seizures typically occur when there is a disruption of inner mechanisms that normally form a balance between excitation and inhibition (being the firing and prevention of an electrical synapse between neurons, respectively) . Typically, there are controls within the brain that hinder neurons from excessive action potential discharge (excitation); however, there exist mechanisms that induce excitation so that the nervous system can function . Disrupting these mechanisms that inhibit or excite firing leads to seizures.
The nervous system is a function of its environment made up of ions, which are the chemical and electrical gradients that form the setting for electrical activity in the brain and spine . The way in which the nervous system maintains this ionic environment is by setting a resting membrane potential. A resting potential is set normally to ensure that neurons are not firing in excess but are close enough to the excitability threshold so that they are able to do it . The control of resting potential becomes vital in the prevention of excessive discharge that is associated with epileptic seizures.
Within a neuron, there is usually a high potassium concentration inside the cell, and a high sodium concentration outside the cell. This gives a net transmembrane potential of approximately -60mV . However, if this balance is disrupted (e.g. too much potassium outside of the neuron), the endings of the neurons may depolarize (become positive), leading to action potential discharge and firing. There are pumps present in the plasma membrane to maintain the chemical and electrical gradients (e.g. sodium-potassium ATPase). If an abnormality occurs within these pumps, seizures may be facilitated . Thus, the blockade of the sodium-potassium ATPase increases the likelihood that a seizure is induced, indicating that it plays a role in the inner mechanisms of epilepsy . Along with pumps, glial cells (bring physical and chemical support for neurons) also provide controls on extracellular ion concentration, which suggests that glia may be very important in the regulation of seizure activity . Therefore, the control of the ionic environment of the nervous system provides a target for treatments.
However, seizures themselves can lead to changes in the transmembrane potentials, as seizures may be followed by a rise in extracellular potassium, which fires neurons. This means that the transmembrane potential (controlled by pumps and glia) is a control that, if altered negatively, could begin a cycle of seizures .
Overview of Optogenetics
Optogenetics is a research tool that gives neuroscientists the ability to control neurons. In order to understand the mechanisms of optogenetics, the green algae Chlamydomonas Reinhardtii must be mentioned . This simple organism utilizes photosynthesis in order to create energy that is vital for its survival. In order to make this process efficient, it has an eyespot, which is a light-sensitive part of the cell that informs a single-celled alga which direction light is coming from, so it can move into a better position . To activate this eyespot, C. Reinhardtii moves ions across a membrane through ion channels (which regulate what enters the cell) . When light of the correct wavelength hits the ion channels, it causes a change in their morphology, opening the channels so that ions can flow across the membrane. The most common ion channel for stimulation in optogenetics is known as Channelrhodopsin-2 .
Neurons are triggered in the same way, hence why researchers have used genetics to express the light-activated ion channels on neurons within the brain. When light hits the ion channels, they open, leading to ions entering the cell, causing them to fire a synapse . To get these ion channels expressed in the brain, a genetically modified virus was made, which is able to recombine its DNA with the DNA of the host cells when it is injected into an area of the brain . Once the recombination occurs, the cell’s dynamics and machinery necessary to express the ion channel gene is available, leading to the cell expressing the channels on its membrane .
Various types of neurons express a different group of genes, which differentiates them from one another. Most of these genes have a promoter region which causes the gene to be expressed if it is active. Thus, it is possible to express the light sensitive ion channels in those cells by using the promoter specific to a certain cell type using the transgenic virus.
Applications for Epilepsy
It should first be noted that there is difficulty in genetically modifying cells in a patient's brain . That being said, the initial steps to achieve this are already being taken, as genes that inhibit neuronal activity are being inserted into regions of the brain that induce seizures in patients who have severe forms of epilepsy. Genes that decrease the activity of neurons are being inserted into seizure-generating areas in patients with advanced stages or forms epilepsy within numerous clinical trials .
Since epilepsy is defined by the overactivity of excitatory neurons, many of the current treatments are aiming to find ways in which inhibiting these neurons is possible. A first step towards this was the ability to selectively target hippocampal neurons of mammals, achieved through using inhibitory opsin (protein that is released by the action of light) of mice hippocampi . Another mouse model injected genes (the lentivirus gene in particular, which is a family of viruses that are responsible for diseases such as AIDS) inside neurons resulted in hyperpolarization in response to orange light . Action potentials were decreased in force, and depolarization was curbed altogether . In addition, the presence of the virally transfected opsins into neurons did not affect the physiological properties of the cells in any way whatsoever .
Temporal lobe seizure interventions similarly activate regions of the cerebellum with optogenetic lasers, and the duration and timing between seizures was altered significantly as a result. Exposing the lateral cortex to light reduced the length of seizures, but did not seem to affect the time between seizures. When another region within the cerebellum known as the vermis was exposed to light, the seizures were shorter, and the duration between seizures increased significantly .
More recently, expression of yellow light exposure in hippocampal neurons have been used to successfully reduce the frequency of chemically induced epileptiform bursts in chronically epileptic animals. Other than inhibiting excitatory cells using optogenetics, precise modulation of inhibitory interneurons can also be used as a strategy to help impede seizures. Within cortical regions of the brain, there are at least 16 types of inhibitory interneurons. Certain interneuron subtypes provide inhibition to farther dendrites of neurons (dendritic inhibition). Dendritic inhibition is regarded as holding importance in regulating plasticity of excitatory dendritic inputs from other neurons and remote excitatory endings .
As much as optogenetics is a promising treatment, much remains to be learned from the use of opsins and light delivery into cells, and many logistical aspects must be understood before complete application onto affected individuals with epilepsy. Among these aspects are the investigation of the human immune response to foreign proteins that may be injected and the optimization of light delivery with chronically implanted devices . Applications of optogenetics have been successful in inducing both normal and pathological mechanisms and, additionally, therapeutic applications have been considered as well. However, there is still much to be investigated, understood and solidified before optogenetics are widely used as a treatment for epilepsy.
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