Fast photoswitches for control of neuronal activity in the cochlea

Light stimulation of spiral ganglion neurons (SGNs) in the ear provides a future alternative to electrical stimulation used in cochlear implants. Optogenetic manipulation of neuronal activity is based on the expression of light-sensitive proteins, which requires gene therapy. An alternative to optogenetics is offered by photopharmacology, employing light sensitive drugs (photoswitches) to control native or engineered receptors. Among photoswitches, Targeted Covalent Photoswitches (TCPs) photocontrol endogenous ionotropic glutamate receptors. Upon ultraviolet light, neural activity can be triggered and reversible silenced with visible light (Izquierdo-Serra et al. 2016). However, their photophysical properties hamper their translational application in many fields. By chemical design and synthesis, we obtained a fast single wavelength-TCP (TCPfast). Using whole-cell patch clamp recordings of hippocampal neuron primary cultures, we show: i) the ability of TCPfast to photomodulate neural activity through AMPA receptors without genetic manipulation; ii) the chemicallydesigned fast-relaxation in dark. To further study its utility, we tested in-vivo the potential of TCPfast to photosensitize the SGNs of the cochlea. Following diffusion of the photoswitch into the gerbil cochlea, we recorded optically (λ = 473 nm) evoked compound action potentials (oCAP) in 50% of the cases. oCAPs were: i) evoked by light pulse as low as 1 µJ and increased in amplitude proportionally with the radiant flux; ii) the largest in response to 80 µs light pulse; iii) following repetition rate as high as 1.5 kHz. TCPfast pioneers the field of photopharmacological stimulation in the cochlea. Future studies should focus on improving stability and handling for application in the cochlea and potential hearing restoration.