ReaChR: a red-shifted variant of channelrhodopsin enables deep transcranial optogenetic excitation

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Abstract
In this technical report, the authors describe a new, red-shifted variant of channelrhodopsin (called red-activatable channelrhodopsin or ReaChR) that shows faster kinetics and greater photocurrents than currently available red-shifted probes. In addition, they show that ReaChR can be activated in awake mice through the intact skull. Channelrhodopsins (ChRs) are used to optogenetically depolarize neurons. We engineered a variant of ChR, denoted red-activatable ChR (ReaChR), that is optimally excited with orange to red light (λ ∼590–630 nm) and offers improved membrane trafficking, higher photocurrents and faster kinetics compared to existing red-shifted ChRs. Red light is less scattered by tissue and is absorbed less by blood than the blue to green wavelengths that are required by other ChR variants. We used ReaChR expressed in the vibrissa motor cortex to drive spiking and vibrissa motion in awake mice when excited with red light through intact skull. Precise vibrissa movements were evoked by expressing ReaChR in the facial motor nucleus in the brainstem and illumination with red light through the external auditory canal. Thus, ReaChR enables transcranial optical activation of neurons in deep brain structures without the need to surgically thin the skull, form a transcranial window or implant optical fibers.

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