Xiang Wu1,Guosong Hong1
Stanford University1
Understanding complex neural circuitry and its correlation with specific behaviors requires spatially and temporally precise modulation of neuron subtypes in certain brain regions. For decades, neural stimulation has been predominantly achieved with traditional electrical stimulation. More recently, optogenetics has gained great popularity due to its rapid control of neural activities with visible light and its dissection of neural circuitry by selectively modulating specific neuron subtypes. However, deep-brain neural modulation usually involves invasive implantation of stimulation electrodes and optical fibers due to the limited penetration of electric fields and visible light in the brain tissue. These brain implants lead to acute damage and chronic gliosis in the brain tissue, while tethering the animal to an electrical wire or an optical fiber interferes with naturalistic behaviors of freely moving subjects.<br/>To address these challenges, we developed ultrasound and second near-infrared (NIR-II, 1000-1700 nm) responsive nanomaterials for non-invasive neuromodulation. First, we developed ultrasound-mediated nanoscopic light sources—mechanoluminescent nanomaterials for non-invasive optogenetics. These nanomaterials can be delivered via the endogenous circulatory system and provide millisecond timescale switching of local light emission upon deep-penetrating focused ultrasound stimulation. We demonstrated that the ultrasound-mediated light emission from the nanomaterials circulating in the blood was sufficient to activate channelrhodopsin-2 (ChR2)-expressing neurons in the brain and trigger behavioral response in live mice in a non-invasive manner, a technique which we termed “sono-optogenetics”. We envisage that sono-optogenetics provides a unique tool of rapid screening of different target regions in the brain for optogenetic neuromodulation, owing to the ease of changing the location of ultrasound focus in the brain by eliminating fiber optic implantation. <b>This work was published in PNAS (Wu, X. et al. Sono-optogenetics facilitated by a circulation-delivered rechargeable light source for minimally invasive optogenetics. <i>Proc. Natl. Acad. Sci. USA</i> 116, 26332–26342 (2019)) and was awarded the Science and PINS Prize for Neuromodulation in 2020.</b><br/>Second, to afford non-invasive, remote neuromodulation in freely behaving animals, we developed polymeric nanomaterials termed MINDS (macromolecular infrared nanotransducers for deep-brain stimulation). MINDS can efficiently absorb light in one of the biological transparency windows, the NIR-II window, modulating neural activity via temperature-sensitive transient receptor potential (TRP) channels. MINDS have a photothermal conversion efficiency of 71% at 1064 nm, the wavelength at which light attenuation by brain tissue is minimized. As a result, through-scalp wild-field NIR-II illumination can activate TRP channels sensitized by MINDS up to a depth of 5 mm in the mouse brain, leveraging a bioinspired mechanism underlying infrared sensitivity in rattlesnakes (as demonstrated by the Nobel Laureate David Julius). Specifically, MINDS act as an NIR-II sensitizer to activate TRP-expressing neurons in the hippocampus, motor cortex and ventral tegmental area of mice with minimal tissue damage under wide-field NIR-II illumination, resulting in substantial behavioral and electrophysiological changes. Due to its non-invasive and tether-free interface, our NIR-II neuromodulation technique enables the study of animal behavior in its most natural state and is particularly suitable for modulating neural activities in socially interacting experiments involving multiple subjects, such as in the IntelliCage, in the future. T<b>his work was recently published in <i>Nat. Biomed. Eng.</i> (Wu, X. <i>et al.</i> Tether-free photothermal deep-brain stimulation in freely behaving mice via wide-field illumination in the near-infrared-II window. <i>Nat. Biomed. Eng.</i> https://doi.org/10.1038/s41551-022-00862-w (2022)).</b>