Every neuron contains a variety of pump and channel proteins that control the flow of ions across its membrane, maintaining a negative membrane potential in the resting neuron. Activation signals, for example from neurotransmitters, cause positively-charged ions to flow into the cell from the external environment via these channel proteins, resulting in membrane depolarization. At a certain threshold, this triggers an action potential — a rapid influx of sodium ions that effectively reverses the voltage inside the cell, initiating a chain reaction of sodium-ion influx that propagates down the length of the axon, eventually causing the release of neurotransmitters that stimulate or inhibit the production of electrical impulses in neighbouring neurons.
Schematic representation of the action of channelrhodopsin and halorhodopsin on neural cells. Microelectrodes have historically proven useful for the direct stimulation less so for the inhibition of neurons in neurophysiological studies, although the poor resolution limit imposed by this experimental regime has left neuroscientists hungry for alternatives.
Both of these proteins can readily be introduced into target cells by various techniques, allowing scientists to rapidly and accurately turn individual neurons on and off without the need for additional drugs or chemicals. Although this field of optogenetics is relatively new, scientists have already made remarkable progress in mapping functional brain circuitry over long distances — for example, charting neuronal processes that link the two hemispheres of the cerebral cortex in mice.
A lot of effort is currently being devoted to refining optogenetic techniques. Although viruses offer an effective and clinically applicable means for delivering the genes that encode these rhodopsins, it is still a laborious process to develop constructs that maximize the efficiency of gene delivery and expression. In addition, although naturally-occurring channel rhodopsin and halorhodopsin work well, there is a suggestion that modified versions of these proteins might offer improved light sensitivity and therefore more rapid switching.
Better modes of light delivery will be required to improve the accuracy and efficiency of optogenetic strategies 3 ; alternatives under investigation include arrays of separately addressable light-emitting diodes LEDs that effectively cover multiple brain sectors of interest, and infrared illumination systems that can penetrate deep within the dense tissues of the brain.
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The core optogenetic methodologies are well established; having shown that these light-activated rhodopsins are tolerated and functional in the mammalian brain, scientists are now focused on using these tools for basic and clinical research. Breakthroughs in materials science now allow the cultivation of neurons in complex predetermined patterns. The capacity to stimulate or silence individual cells selectively within these engineered cultures, in conjunction with reagents that allow the direct visualization of neuronal activity, promises to yield insights that could inform the design of artificial neural networks based on the natural principles underlying brain structure and function.
Automated light-based mapping of the mouse motor cortex. Anaesthetized transgenic mice are stimulated by laser beam. Motor response is detected by electromyography EMG electrodes. Even the complex environment of the living brain is within reach of these techniques 4. These successes in animal models are merely a prelude to a longer-term goal: Happy holidays from all of the team at Scientifica!
Here, we have collated a variety of recently published neuroscience research that used optogenetics. These include a less-invasive way to deliver light deeper into the mouse brain, using light as a potential therapy to treat chronic pain and using optogenetics to map cells in the brain.
Optogenetics is a research technology that enables neuroscientists to use light to control neurons with high spatial and temporal resolution. Specific neurons in the brain are genetically modified to express light-sensitive proteins.
Controllling neural circuits with light
When light hits these neurons, the proteins are activated, causing the neuron to fire. Scientists are therefore able to specifically switch signalling cascades on and off using light. Using optogenetics, neuroscientists can precisely measure the effects of turning a set of neurons on or off in real time in freely moving animals.
They can see the behaviours they contribute to, allowing them to infer what they are needed for and identify cells that contribute to or remedy a disease state. Light delivery in optogenetics often uses invasive optical fibres, which can cause tissue damage and alter behaviour. Developing ways to less-invasively, and even non-invasively, deliver light to specific neurons in the brain, will improve reliability of research as well as animal welfare.
A team of researchers have developed a way to less-invasively deliver light deeper into the mouse brain.
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The near-infrared light is delivered from outside the skull, negating the need for optical fibres to be implanted into the brain, and can penetrate deeper into brain tissue. By emitting visible wavelengths of light, the upconversion nanoparticles remotely deliver light to cells that express light-sensitive proteins, enabling near-infrared light to be used to activate or inhibit cells deeper in the brain. In studies, the nanoparticles appeared to be suitable for long-term use, due to their stability and biocompatibility. Tapered Optical Fibers enable light to be delivered to the living brain with reduced invasiveness and under tight spatiotemporal control.
Unlike common light delivery methods, Tapered Optical Fibers can deliver light to regions deeper than 1mm without being highly invasive. Previously these researchers, in a collaborative framework between the Center for Biomolecular Nanotechnologies of the Italian Institute of Technology Lecce, Italy and Harvard Medical School Boston , developed a thin Tapered Optical Fiber that could administer light to brain regions more than 1.
Here, the researchers analysed the use of high-numerical aperture NA fibers to obtain wide volume or spatially-selective light delivery over 3mm, which can cover entire brain regions of several animal models. The results give quantitative data on Tapered Optical Fibers as light delivery tools which can be used to design future experiments using a range of opsins in multiple brain regions.
Recent advances in optogenetics research
A Scientifica SliceScope was used to detect fluorescence emission and capture fluorescence images in this research. As optogenetics can be used to switch neurons on and off, it has great promise as a potential method of treating chronic pain. By selectively switching off pain-sensing neurons, specific pain signals can be deactivated, eliminating sensations of pain. Researchers at Washington University School of Medicine in St Louis and the University of Illinois at Urbana-Champaign used optogenetics for the first time to reduce bladder pain in mice. These results offer hope for treating interstitial cystitis also known as bladder pain syndrome which affects millions.
The mice were bred to express light-sensitive proteins in their pain-sensing neurons and were implanted with wirelessly controlled LED devices that delivered light to the bladder. This enabled the researchers to activate the light-sensitive proteins using LED light; switching on the light activated the proteins, which silenced the neuronal signals, resulting in pain relief. This method could be new way to treat pain in humans without using drugs, which can be addictive.
After discovering that the nerve cells in the skin that respond to gentle touch are the same cells that cause neuropathic pain, Scientists at EMBL Rome have developed a light-sensitive chemical that selectively binds to these nerve cells in the skin. The chemical is injected into the affected area of skin and then illuminated by near-infrared light. Other cell types, responsible for other sensations such as cold, vibration and normal pain, are not affected by the light treatment.
The skin is only desensitised to gentle touch. The pain relief lasts for a few weeks, after which the nerve endings grow back. This research was carried out in mice, but, as human skin contains the same cells, it is hoped the technique can one day be used to treat humans who suffer from neuropathic pain.
The team are now actively looking for new collaborations to develop this method further.
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The ability to activate and inactivate neurons using optogenetics has enabled researchers to see the role they have in normal functions, for example, learning and memory, vision and anxiety, as well as their implications in diseases. Scientists at the University of Maryland School of Medicine used optogenetics to find a direct link between dopamine and avoidance-behaviour related to pain and fear.
The researchers optogenetically stimulated dopamine neurons in the nucleus accumbens of rats to release either more or less dopamine. They found that animals with high levels of dopamine in this brain region learnt to avoid small electric shocks more quickly than those with lower levels of dopamine, indicating that dopamine drives animals to avoid unpleasant or painful situations and stimuli.
In a study by Okinawa Institute of Science and Technology Graduate University, scientists used optogenetics to investigate the role of cholinergic interneurons in the striatum in regulating the response to unexpected stimuli. Cholinergic interneurons in the striatum are in a near-constant state of activity, but if the brain receives an unexpected stimulus from the environment, these interneurons briefly stop firing. Optogenetics enabled the researchers to switch the cholinergic interneurons on and off as mice moved around their cage.
They discovered that when they paused cholinergic interneuron firing, the spiny projection neurons that they are connected to were stimulated less. These spiny projection neurons send impulses from the striatum to the rest of the brain, causing it to respond to the stimuli, therefore pauses in cholinergic interneurons may mediate how animals respond to unexpected stimuli.
At the University of Tennessee Health Science Center, researchers used optogenetics to study the role of acetylcholine in dishabituation of glomeruli in the olfactory bulb to prolonged odours.
Optogenetics - Wikipedia
The team used optogenetics to control the release of acetylcholine in the olfactory bulb during prolonged exposure to odour. They discovered that acetylcholine can rapidly enhance the post-synaptic glomerular responses that are habituated to an odour and increase the salience of the odour. They also discovered that this response in the olfactory bulb can be blocked by a cholinergic antagonist.
Optogenetics were utilised in this research to investigate how neuronal protrusions called filopodia 'know' where to make connections and how they make stable connections with neighbouring neurons. The researchers, from Jefferson Philadelphia University and Thomas Jefferson University , found that the same molecule can both repel and connect synapses. The receptor, EphB2 kinase, on the tip of the filopodium receives signals from outside the cell and relays these inside the cell.
Using a light-activated version of EphB2 kinase, they found that when EphB2 kinase received a fast activation signal, the filopodium would retract, rejecting any possible connections. However, when it received a slow activation signal, it enabled filopodia to make stable connections.