Photocontrol of neural activity: biophysical mechanisms and performance in vivo.

Nervous systems run on electricity. Information is represented as differences in electric potential and processed by active devices—nerve cells—that generate stereotyped output signals (action potentials or spikes) in response to variation in input potential. Like most bioelectric potentials, the electrical signals of neurons are diffusion potentials; they are established by ionic concentration gradients across the external neuronal membrane and controlled by different types of ion-selective conductances (see section 3.1). Some of these conductances (voltage-gated ion channels) are regulated locally, by virtue of their ability to sense the electric potential across the membrane in which they sit, whereas others respond to chemicals (neurotransmitters) that report potential changes in distant, synaptically connected cells. Loosely speaking, the interplay of local membrane potential differences, electrotonic currents driven by these differences, and voltage-gated conductances defines the information-processing capabilities of a single neuron. Neurons, however, do not operate in isolation. They are embedded into circuits of often staggering complexity in which each cell is influenced by, and influences, many others via thousands of afferent and efferent synaptic connections. This multitude of interactions supports a vast array of biophysical phenomena that allow circuits to perform functions that would be impossible to implement with single cells. Unfortunately, however, the distributed nature of neural circuits also makes them exceedingly difficult objects for experimental study: the standard approach of recording the electrical activity of a single neuron is often inadequate for understanding the function of a circuit. Optical methods are potentially better matched than electrodes to the distributed information-processing architecture of the nervous system. Optical recording of voltage-controlled ionic fluxes or cellto-cell communication, for example, can reveal the dynamics of signaling in entire populations of neurons with cellular or synaptic resolution. Yet a full arsenal of experimental tools for studying the nervous system must include techniques to manipulate neural activity as well as record it. Aside from patterned sensory stimulation, the most common methods for controlling neuronal signaling have been lesioning, pharmacology, and electrical stimulation. Only in recent years have optical methods also begun to have a significant impact in this domain. This trend is due, in large part, to three advantages. The first is the excellent spatial and temporal resolution of many optical approaches. The second is the ability to control neural activity in a “hands-off” manner, which allows probing of many tissue sites simultaneously or in rapid succession. The third, and most recent, advantage arises when a pinch of genetics is added to the optical mix; neuronal activity can then be recorded and manipulated with “genetic resolution”. By encoding an optically responsive protein in DNA and using cell-type specific promoters or localized DNA delivery to control expression, photosensitivity can be restricted to a subset of cells in a given anatomical region, based on the cells’ functional identity or connectivity, rather than just their location. The power of genetically targeted photomanipulation was foreseen by Francis Crick, who in his 1999 Kuffler Lectures raised the “far-fetched” possibility that molecular biologists could “engineer a particular cell type to be sensitive to light”. But such foresight was by no means universal. When the first experiments realizing Crick’s vision were submitted * To whom correspondence should be addressed. E-mail: gero.miesenboeck@ dpag.ox.ac.uk. † Yale University School of Medicine. ‡ University of Oxford. Chem. Rev. 2008, 108, 1588–1602 1588