Alex Benison, PhD, University of Colorado, Department of Psychology and Neuroscience
Channelrhodopsins (ChRs) are light-gated ion channels originating from microalgae of the genus Chlamydomonas. In algae they are light sensitive proteins that are used by the microorganism to navigate towards or away from light sources. These light activated proteins are similar to rhodopsin proteins found in the vertebrate eye. ChRs are transmembrane proteins that change conformation when exposed to light, allowing positive cations to enter the cell. In algae, as in the mammalian eye, this influx of ions can be used to transfer information via neurons. Under resting conditions neurons are hyperpolarized, or they have a negative membrane potential in relation to the extracellular environment. If there is an influx of positively charged ions the neuron begins to depolarize, and if depolarized above a certain threshold voltage gated ion channels open and the neuron fires an action potential. This is the primary way neurons communicate with one another in the brain.
Neurons have many inherent ion channels that can cause neuronal firing, but in the early 2000's researchers Karl Deisseroth and Edward Boyden began to experiment with the idea of using ChRs inserted into mammalian neurons to allow light driven control of these cells and in 2005 optogenetics was officially born (Boyden et al., 2005). Since then, many exciting new tools have been added to the optogenetics toolbox helping facilitate the exponential expansion of this field of research. Using light to both turn on, and turn off neurons and glial cells, in a biologically relevant manner, with anatomical and cell type specificity, has realized a neuroscientific goal that had been impossible previously. One of the most exciting aspects of this emerging technology is the potential for its use in behaving animals.
Optogenetic light stimulation in rodents is primarily done using two techniques, either using LED or laser light emission. This light is then delivered to the animal using large, multimode, fiber optic cables that attach to optrodes (thin, unclad sections of fiber) embedded in the brain region of interest. These large cable diameters are necessary to both deliver the visible light wavelengths needed to activate opsins (450-650nm depending on the opsin) as well as have a sufficient light intensity and light spread in the tissue. The amount of light transmitted through brain tissue is dependent on 1) light scattering inside, 2) light absorption by the tissue, and on 3) the conical spreading of light after it exits the optical fiber ("Geometric decrease"). This can be formalized using the Kubelka-Munk theory of light propagation in scattering and absorptive media (Aravanis et al., 2007). Because of this, fiber diameters are often 100-400uM with the widest numerical aperture available to facilitate light spread (0.39 NA or greater). An online calculator has been developed to calculate light spread taking into account all these variables. Delivering optical stimulation in awake, unrestrained animals is vital to understanding how different brain regions affect behavior, but it also provides a few challenges as well.
Fiber optic light stimulation in freely moving animals requires fiber optic rotary joints. As an unrestrained animal moves in a circular direction, elastic force in the opposite direction of the rotation is built up in the fiber optic cable tethered to the head of the animal. Fiber optic cables are very susceptible to fracturing under these stresses. To counteract this rotational shearing force it is essential that the fiber optic patch cables can rotate freely, with very low friction to allow freedom of movement to the animal with minimal stress. We use a Princetel R series rotary joint that comes with FC receptacles (Model RFC). This way we get to decide on the type of fiber cable and the ideal length. It is also unique in that it allows us to use one light source for multiple animals that are chronically teatherd without having to disconnect their head mount. This is very helpful in situations where the stress of handling can affect the data. The R series features extremely low insertion loss and impressive return loss performance for singlemode and multimode fibers typically used in optogenetic research.
The ever expanding toolbox and unique advantages of temporal and neuron specific cellular manipulation afforded by optogenetics have made this the fastest growing, most exciting new field of research in neuroscience. As this field continues to grow exponentially, companies that see the benefit of staying at the forefront of solving the engineering challenges inherent in optogenetics, such as Princetel, will help nurture the innovation that will lead to future scientific breakthroughs, both in how we understand brain function, and treat medical conditions. The future of optogenetic research truly is bright.
Aravanis AM, Wang L-P, Zhang F, Meltzer LA, Mogri MZ, Schneider MB, Deisseroth K (2007) An optical neural interface: in vivo control of rodent motor cortex with integrated fiberoptic and optogenetic technology. J Neural Eng 4:S143-S156.
Boyden ES, Zhang F, Bamberg E, Nagel G, Deisseroth K (2005) Millisecond-timescale, genetically targeted optical control of neural activity. Nat Neurosci 8:1263-1268.