Optogenetics provides a basis for linking behavioral functions with cellular and circuit-level activity patterns. Already, systematic optogenetic perturbation of distinct cell types and brain projections has begun to shed light on the cellular substrates of neurological and psychiatric disorders including Parkinson’s disease, anxiety, retinal degeneration, cocaine conditioning, social dysfunction, and depression [14
]. Tantalizing possibilities for the future of optogenetics lie in several directions, including: the extension of optogenetic tools with distinct wavelengths of sensitivity and novel modes of control beyond electrical signaling; the emergence of gene expression technologies capable of targeting optogenetic tools to functionally defined cell populations and circuit elements; and the continued intersection of optogenetics with a variety of readout modalities and especially with high-throughput system-wide electrical and genome-wide molecular profiling technologies.
The optogenetics toolbox of the future will enable multiplexed control of several populations of neurons within the same experimental setup and will enable the combinatorial testing of signal processing roles of distinct neural circuit elements in complex behavior. Genome prospecting and molecular engineering of opsin genes from diverse microbial species have already expanded the optogenetic toolbox with a diverse palette of neuronal activators and inhibitors with distinct wavelengths of excitation, ion selectivity, and kinetics [18
]. The search continues for proteins sensitive to far-red wavelengths which will not only enable probing of deeper brain regions, but will also facilitate the simultaneous investigation of multiple neural circuits when used in conjunction with other opsins. Recent successes in obtaining an atomic resolution structure of channelrhodopsin will catalyze opsin engineering efforts [60
]. In addition, proteins responsive to wavelengths from the ultrasound or magnetic frequency ranges will possibly enable the remote control of neural tissue without the necessity of any surgical implants. New opsins with selective conductance for potassium and calcium, ions important in synaptic events and intraneuronal signaling pathways, will result in precise dissection of the role of specific signaling processes in neural function.
Optogenetic control will also expand beyond electrical signaling by enabling precise modulation of endogenous transcription within specific neural populations. The development of light sensing protein domains from plant and microbial species [62
] will enable the design of photo-activatable enzymes and transcription factors. These light-sensing domains can be coupled with programmable DNA binding domains, including designer zinc finger proteins (ZFPs) [66
] and transcription activator-like effectors (TALEs) [67
], to achieve optically-controlled targeted modulation of endogenous transgene and RNA expression in mammalian genomes. The ability to regulate transcription within specific circuits in awake behaving animals will be important for dissecting the molecular and epigenetic basis of neuropsychiatric diseases.
In order to fully understand the circuit-level mechanisms underlying neuropsychiatric disease, it is important to pinpoint the specific circuits underlying behavioral dysfunctions. The targeting of optogenetic tools into specific cells based on their activity during behavioral testing allows the identification of functional circuits. This can be achieved using molecular markers of neural activity [71
] including immediate early genes (IEG) such as c-Fos
, and Npas4
. Leveraging the activity-dependent transcription regulatory elements of IEGs [72
], it is possible to target the expression of opsin genes to distinct groups of neurons that are active during behavioral testing. For example, in an animal model of posttraumatic stress disorder (PTSD), activity-dependent targeting of opsins will enable the identification of cells involved in the phenotype. Subsequent activation of the targeted cells in the absence of trauma-inducing stimuli can be used to determine the necessity or sufficiency of specific cell populations in the manifestation of the PTSD phenotype. The epigenetic profile of these cells can be further analyzed to identify molecular signatures of PTSD.
A third major avenue for the future will be to integrate the use of optogenetics with other system-level technologies to facilitate global analysis of brain functions. A combination of optogenetics and fMRI [74
] or large-scale multi-electrode arrays [75
] will allow us to identify target brain regions and activity patterns corresponding to specific behavioral functions. Subsequent genome-wide transcriptome [75
] and epigenome [76
] analysis of the identified circuits will bridge molecular signatures with circuit dynamics and behavior. The integration of optogenetics with a diverse range of readout modalities will further bridge our understanding of nervous system functions and disease processes from molecules and cells to circuits and behavior.
As the optogenetics technology matures, it will continue to augment our ability to dissect the mammalian brain with increasing precision and expanding modes of control. The ability to specifically modulate the electrical, biochemical, and transcriptional activity of specific neuronal circuits will bring us one step closer to understanding the mechanism of normal and pathological nervous system function. The intersection of optogenetics with other disciplines of biology and engineering will lead to novel therapeutic targets and innovative therapeutic interventions for neuropsychiatric diseases.