Optogenetics and thermogenetics: technologies for controlling the activity of targeted cells within intact neural circuits

Abstract

In recent years, interest has grown in the ability to manipulate, in a temporally precise fashion, the electrical activity of specific neurons embedded within densely wired brain circuits, in order to reveal how specific neurons subserve behaviors and neural computations, and to open up new horizons on the clinical treatment of brain disorders. Technologies that enable temporally precise control of electrical activity of specific neurons, and not these neurons' neighbors-whose cell bodies or processes might be just tens to hundreds of nanometers away-must involve two components. First, they require as a trigger a transient pulse of energy that supports the temporal precision of the control. Second, they require a molecular sensitizer that can be expressed in specific neurons and which renders those neurons specifically responsive to the triggering energy delivered. Optogenetic tools, such as microbial opsins, can be used to activate or silence neural activity with brief pulses of light. Thermogenetic tools, such as thermosensitive TRP channels, can be used to drive neural activity downstream of increases or decreases in temperature. We here discuss the principles underlying the operation of these two recently developed, but widely used, toolboxes, as well as the directions being taken in the use and improvement of these toolboxes.

Figure 1

Molecular sensitizers and energy-delivery devices, for optical control of neurons. A, Diagrams depicting the responses to light of (i) halorhodopsins (light-driven inward chloride pumps, which hyperpolarize neurons in which they are expressed when illuminated), (ii) channelrhodopsins (light-gated inward nonspecific cation channels, which depolarize neurons in which they are expressed when illuminated), and (iii) archaerhodopsins (light-driven outward proton pumps, which hyperpolarize neurons in which they are expressed when illuminated). B, Electrophysiological data demonstrating the use of (i) the N. pharaonis halorhodopsin (adapted from [17]), (ii) the C. reinhardtii channelrhodopsin ChR2 (adapted from [21]), and (iii) the H. sodomense archaerhodopsin Arch (adapted from [24]), to mediate the control of neural voltage in response to light. C, Methods for the delivery of light into the brain. (i) Chronically implanted optical fiber, to be inserted into the brain, with ferrule connector that emerges from the brain for easy connection to a corresponding ferrule on an optical fiber coupled to a laser. (ii) Arrays of small raw-die LEDs (top left, showing two LEDs from Cree), which can be wirelessly powered and controlled via a small (~1-2 gram) radio-powered receiver (bottom left). Fibers can also be coupled directly to the LEDs for deep light delivery; fiber tip irradiances can easily exceed 200 mW/mm² (top right, bottom right, showing a fourteen-LED array designed for targeting bilateral hippocampus). Adapted from [64]. (iii) Microfabricated waveguide arrays for delivery of light to multiple points along the axis of a single, miniature inserted probe (adapted from [65]).

Figure 2

Molecular sensitizers and energy-delivery devices, for thermal control of neurons. A, Diagrams depicting the responses to changes in temperature of (i) the warmth-activated dTRPA1 channel and (ii) the cold-activated rTRPM8 channel. Both dTRPA1 and rTRPM8 are non-selective cation channels of high conductance that depolarize cells upon appropriate thermal stimulation. B, Electrophysiological data demonstrating the use of dTRPA1 to control a fly motor neuron (adapted from [83]). EJP: excitatory junction potential of post-synaptic target muscle. C, Methods for delivering thermal stimulation into the brain. (i) Changes in ambient temperature are commonly used to control thermogenetic tools in Drosophila. (ii) Magnetic field-mediated heating of MnFe₂O₄ nanoparticles has been proposed as a possible strategy for delivering local thermal stimulation within the mammalian brain [106]. (iii) Focused ultrasound is a powerful strategy for localized tissue warming deep in the mammalian brain, and it could potentially be adapted to control thermogenetic tools.