Activity of Retinal Neurons Can Be Modulated by Tunable Near-Infrared Nanoparticle Sensors

The vision of patients rendered blind by photoreceptor degeneration can be partially restored by exogenous stimulation of surviving retinal ganglion cells (RGCs). Whereas conventional electrical stimulation techniques have failed to produce naturalistic visual percepts, nanoparticle-based optical sensors have recently received increasing attention as a means to artificially stimulate the RGCs. In particular, nanoparticle-enhanced infrared neural modulation (NINM) is a plasmonically mediated photothermal neuromodulation technique that has a demonstrated capacity for both stimulation and inhibition, which is essential for the differential modulation of ON-type and OFF-type RGCs. Gold nanorods provide tunable absorption through the near-infrared wavelength window, which reduces interference with any residual vision. Therefore, NINM may be uniquely well-suited to retinal prosthesis applications but, to our knowledge, has not previously been demonstrated in RGCs. In the present study, NINM laser pulses of 100 μs, 500 μs and 200 ms were applied to RGCs in explanted rat retinae, with single-cell responses recorded via patch-clamping. The shorter laser pulses evoked robust RGC stimulation by capacitive current generation, while the long laser pulses are capable of inhibiting spontaneous action potentials by thermal block. Importantly, an implicit bias toward OFF-type inhibition is observed, which may have important implications for the feasibility of future high-acuity retinal prosthesis design based on nanoparticle sensors.

Use of an invertebrate animal model (Aplysia californica) to develop novel neural interfaces for neuromodulation

New tools for monitoring and manipulating neural activity have been developed with steadily improving functionality, specificity, and reliability, which are critical both for mapping neural circuits and treating neurological diseases. This review focuses on the use of an invertebrate animal, the marine mollusk Aplysia californica, in the development of novel neurotechniques. We review the basic physiological properties of Aplysia neurons and discuss the specific aspects that make it advantageous for developing novel neural interfaces: First, Aplysia nerves consist only of unmyelinated axons with various diameters, providing a particularly useful model of the unmyelinated C fibers in vertebrates that are known to carry important sensory information, including those that signal pain. Second, Aplysia's neural tissues can last for a long period in an ex vivo experimental setup. This allows comprehensive tests such as the exploration of parameter space on the same nerve to avoid variability between animals and minimize animal use. Third, nerves in large Aplysia can be many centimeters in length, making it possible to easily discriminate axons with different diameters based on their conduction velocities. Aplysia nerves are a particularly good approximation of the unmyelinated C fibers, which are hard to stimulate, record, and differentiate from other nerve fibers in vertebrate animal models using epineural electrodes. Fourth, neurons in Aplysia are large, uniquely identifiable, and electrically compact. For decades, researchers have used Aplysia for the development of many novel neurotechnologies. Examples include high-frequency alternating current (HFAC), focused ultrasound (FUS), optical neural stimulation, recording, and inhibition, microelectrode arrays, diamond electrodes, carbon fiber microelectrodes, microscopic magnetic stimulation and magnetic resonance electrical impedance tomography (MREIT). We also review a specific example that illustrates the power of Aplysia for accelerating technology development: selective infrared neural inhibition of small-diameter unmyelinated axons, which may lead to a translationally useful treatment in the future. Generally, Aplysia is suitable for testing modalities whose mechanism involves basic biophysics that is likely to be similar across species. As a tractable experimental system, Aplysia californica can help the rapid development of novel neuromodulation technologies.