A history of optogenetics: the development of tools for controlling brain circuits with light

Abstract

Understanding how different kinds of neuron in the brain work together to implement sensations, feelings, thoughts, and movements, and how deficits in specific kinds of neuron result in brain diseases, has long been a priority in basic and clinical neuroscience. "Optogenetic" tools are genetically encoded molecules that, when targeted to specific neurons in the brain, enable their activity to be driven or silenced by light. These molecules are microbial opsins, seven-transmembrane proteins adapted from organisms found throughout the world, which react to light by transporting ions across the lipid membranes of cells in which they are genetically expressed. These tools are enabling the causal assessment of the roles that different sets of neurons play within neural circuits, and are accordingly being used to reveal how different sets of neurons contribute to the emergent computational and behavioral functions of the brain. These tools are also being explored as components of prototype neural control prosthetics capable of correcting neural circuit computations that have gone awry in brain disorders. This review gives an account of the birth of optogenetics and discusses the technology and its applications.

Figure 1.. Adaptation of microbial opsins from nature for the optical control of neural activity

(A-C) Diagrams depicting the physiological responses of (A) archaerhodopsins and bacteriorhodopsins (light-driven outward proton pumps), (B) halorhodopsins (light-driven inward chloride pumps), and (C) channelrhodopsins (light-gated inward nonspecific cation channels), when expressed in the plasma membranes of neurons and exposed to light. (D) Demonstration of use of Halobacterium sodomense archaerhodopsin-3 (Arch) to mediate light-driven neural silencing in cortical pyramidal neurons of awake mice. Top: Neural activity in a representative neuron before, during, and after 5 seconds of yellow light illumination, shown as a spike raster plot (upper panel), and as a histogram of instantaneous firing rate averaged across trials (lower panel; bin size, 20 ms). Bottom: Population average of instantaneous firing rate before, during, and after yellow light illumination (black line, mean; gray lines, mean ± SE; n = 13 units). Adapted from [36]. (E) Demonstration of use of Natronomonas pharaonis halorhodopsin (Halo/NpHR) to mediate light-driven spike quieting, demonstrated for a representative hippocampal neuron in vitro. Top: (“Current injection”), neuronal firing of 20 spikes at 5 Hz, induced by pulsed somatic current injection (~300 pA, 4 ms). Middle: (“Light”), membrane hyperpolarization induced by two periods of yellow light, timed so as to be capable of blocking spikes 7–11 and spike 17 out of the train of 20 spikes. Bottom: (“Current injection + Light”), yellow light drives Halo to block neuron spiking (note absence of spikes 7–11 and of spike 17), while leaving intact the spikes elicited during periods of darkness. Adapted from [34]. (F) Demonstration of use of Chlamydomonas reinhardtii channelrhodopsin-2 (ChR2) to mediate light-driven spiking in two different hippocampal neurons, in response to the same train of blue light pulses (with timings selected from a Poisson distribution with mean interval λ = 100 ms). Adapted from [26]. BR, bacteriorhodopsin.

Figure 2.. Early data showing that channelrhodopsin-2 could mediate light-driven spiking in neurons

Raw voltage trace recorded from a current-clamped neuron in vitro, expressing channelrhodopsin-2 and exhibiting light-activated spikes, here driven by four 15 ms-duration blue light pulses. The data here shown were acquired from the first channelrhodopsin-2-expressing neuron recorded in the study that culminated in [26].

Figure 3.. Genomic diversity of opsins enables multicolor silencing of different neural populations

Neurons expressing the Natronomonas pharaonis halorhodopsin (Halo) (top) were selectively quieted by red light and not by blue light, whereas neurons expressing the Leptosphaeria maculans opsin (Mac) were selectively quieted by blue light and not by red light (bottom). Data were acquired from opsin-expressing neurons current-clamped in vitro, and spikes were induced by pulsed somatic current injection, as in Figure 1E. Adapted from [36].