Large Timescale Interrogation of Neuronal Function by Fiberless Optogenetics Using Lanthanide Micro-particles.

Optogenetics requires implantation of light-delivering optical fibers, as current light-sensitive opsins are activated by visible light, which cannot effectively penetrate biological tissues. Insertion of optical fibers and subsequent photostimulation inherently damages brain tissue, and fiber tethering can restrict animal behavior. To overcome these technical limitations, we developed minimally invasive "fiberless" optogenetics using lanthanide micro-particles (LMPs), which emit up-conversion luminescence in the visible spectrum in response to irradiation with tissue-penetrating near-infrared light. Depolarizing (C1V1) and hyperpolarizing (ACR1) opsins were strongly activated by up-conversion luminescence from green-emitting LMPs both in vitro and in vivo. Using this technique, we successfully manipulated locomotive behavior of mice by activating and inhibiting neurons in the dorsal striatum, at a depth of 2 mm from the brain surface. LMPs were retained and remained functional for >8 weeks at the injection site. Fiberless optogenetics offers opportunities to control neuronal function over longer time frames using freely behaving animals.