|Optogenetics: what is it, why use it, and how to use it.|
The ability for the precise temporal and spatial control of neurons has long been seen as a holy grail in neuroscience. Commonplace techniques, such as lesion studies or electrical stimulation, can introduce numerous confounds into the data by inadvertently affecting nearby cell populations. The overall result is a diminished explanatory power for experiments and an inability to make strong, causal claims. Optogenetics couples genetic tools, which deliver histological specificity, with optical stimulation, which delivers physiologically-relevant millisecond-timescale control. The result is an ability to pick apart complex neural networks like never before, and reduce intricate, sophisticated behaviours and phenomena down to individual neurons in vivo.
The main toolkit of optogenetics are the opsins, light sensitive ion channels found primarily in bacteria and algae. These microbes use opsins to detect light so they can swim towards it, facilitating photosynthesis. Microbial opsins were first discovered in the 1970s, but it was not until 2005 that they were first reported as tools for neuronal control. Since that time, the available toolkit of opsins has expanded: opsins have been engineered that can hyperpolarize, depolarize, respond to different wavelengths of light, alter pH, and even act as G-proteins. Likewise, numerous novel techniques have been developed to fully utilise these newfound tools and apply them to diverse lines of scientific inquiry.
|A brief overview of optogenetics provided by Nature Methods|
Optogenetics is a relatively novel set of research techniques that allow fast and directed control of specific cellular functions. Essentially, optogenetics is the use of cells embedded with light sensitive opsins to control their function. The key to understanding what roles cells play and how they affect behaviour is being able to precisely modify and study different cell types without confounding factors, but previous studies undertaken used indefinite methods such as lesions or electrode stimulation that did not allow for confident determination of causality. Extreme, millisecond-level temporal specificity greatly improves our ability to perturb neuronal systems and devices like the optrode greatly improve our ability to observe them. With optogenetics, we can fundamentally understand and fluently speak the language of cells instead of having to go through translators, and it can all be done in freely moving mammals. There are two basic steps involved, first, a specimen has to be infused with the tools of interest, and second, a light delivery mechanism has to be installed to affect those tools. Both steps encompass a wide variety of choices, giving scientists a veritable grab bag of neuromodulation abilities to suit any experimental needs.
|Fig. 1: Steps involved in projection targeting, which allows anatomically-targeted optogenetic manipulation. For example, if one wishes to
target only the pathway from B to A, illumination at the same locus as virus injection (B) would be counterproductivesince C is affected.
Transfected axons can be illuminated directly at their terminals (A), allowing for simple, anatomy based targeting. Modified from .
Optogenetics has grown to be applied to nearly every subfield of neuroscience. As new lines of investigation are launched, optogenetic tools by themselves are no longer adequate, and need to be altered and used in new approaches. Opsins have been altered in different ways: they have been refined for better performance (ChETA); redshifted to allow different wavelengths of excitation (C1V1); slowed down to facilitate longer, subthreshold depolarizations; and new opsin classes continue to be investigated as tools for manipulating intracellular signaling (OptoXR). Optogenetics also continues to be applied in novel, creative experimental strategies: techniques such as projection targeting can achieve excellent anatomical specificity, and the integration of optogenetics with existing transgenic techniques has proven to be a powerful tool for targeting neurons based on their intrinsic biochemistries. These new tools and techniques have been applied in a number of ways, allowing for the unprecedented deconstruction of emergent and complex behavioural and pathological phenomena.