A tapered optical fiber delivers spatially precise and efficient optogenetic illumination to the rodent brain.
Optogenetic manipulation of neuronal activity has become an invaluable tool for researchers trying to gain insight into the workings of the mouse brain. However, it can still be a challenge to achieve desired illumination patterns in the mouse brain without causing excessive tissue damage, especially when illuminating very large areas, spatially confined areas, or multiple areas at once.
Bernardo Sabatini from Harvard Medical School in Boston remembers that his first steps into this problem began when a mutual friend suggested he speak with Ferruccio Pisanello and Massimo De Vittorio from the Italian Institute of Technology in Lecce. When Sabatini discussed with them how to steer light in the brain without having to move the light delivery device, they immediately said, “Oh, we know how to do that.” They suggested tapered fibers, which they could use to emit light at different places, depending on the mode of light propagation.
This discussion culminated in a 2014 Neuron paper in which the three researchers and their teams described optical fibers that could emit light through different windows in their coating. However, generating these fibers with windows was technically difficult. “It was much later that we realized that we really didn't need those windows, that we could simply take a bare fiber and achieve much the same result in a much simpler and cheaper and much easier-to-manufacture manner,” says Sabatini.
In the current design, the tapered fibers are pulled from very hard glass to a length of a few millimeters and a tip cross-section of a few hundred nanometers. Light emission through these fibers is then dependent on the numerical aperture and the angle of the incident light. When light enters the tapered fiber at a particular angle, the rays travel along the fiber by bouncing off the walls via total internal reflection until a critical angle is reached and the light is no longer reflected. By varying the input angle, the site of emission along the tapered fiber can be selected. On the other hand, if the fiber is illuminated using its full aperture, light is emitted along a broad swath of the fiber.
In addition to the precise control over light emission, another advantage of the tapered fibers is the low amount of damage they cause upon insertion into brain areas of interest. “We are using [the tapered fibers] in the brainstem to target very small nuclei that we haven't been able to target with flat-cleaved fibers very efficiently,” says Sabatini. “For these small nuclei in the brainstem, the flat-cleaved fiber tends to produce a lesion, whereas with the tapered fiber we can illuminate them nicely.” Compared to standard flat-cleaved fibers, Sabatini says that his lab uses lower light power with the tapered fibers because the illumination is very local. Because of these advantages, his lab is using the tapered fibers more and more. “We don't really see any reason to use the flat ones,” he says.
Many applications benefit from the use of tapered fibers. In addition to their use in optogenetic manipulation of small nuclei that are easily damaged by flat-faced fibers, the tapered fibers can be used to scan for brain areas that evoke a strong behavioral effect when optogenetically activated. “For example, if we stimulate within the ventral tegmental area we can have the animal report for us where we are getting the best excitation of the ventral tegmental area,” explains Sabatini. Moreover, long tapered fibers can be exploited to excite multiple parts of the brain, which can be useful for conducting epistasis experiments.
Sabatini and his collaborators envision further applications for their tapered fibers. Rather than emitting light through the tapered fibers, they are exploring the fibers' use in collecting light in fiber photometric applications. This will allow them to determine where in the brain light is emitted from, for example from calcium sensors in active brain areas. In addition, they are repurposing the coated fibers they previously described as electrodes, an application which is made possible by their metal coating. They can record local field potentials and even single units with their prototypes. “We are excited to continue developing this technology,” says Sabatini.
Pisanello, F. et al. Dynamic illumination of spatially restricted or large brain volumes via a single tapered optical fiber. Nat. Neurosci. doi:10.1038/nn.4591 (2017).
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Vogt, N. Tailoring optogenetic illumination through tapered fibers. Nat Methods 14, 763 (2017). https://doi.org/10.1038/nmeth.4381
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