REVIEWS NEURAL CIRCUITS Optogenetic investigation of neural circuits underlying brain disease in animal models Kay M. Tye1,2 and Karl Deisseroth1,3,4,5 Abstract | Optogenetic tools have provided a new way to establish causal relationships between brain activity and behaviour in health and disease. Although no animal model captures human disease precisely, behaviours that recapitulate disease symptoms may be elicited and modulated by optogenetic methods, including behaviours that are relevant to anxiety, fear, depression, addiction, autism and parkinsonism. The rapid proliferation of optogenetic reagents together with the swift advancement of strategies for implementation has created new opportunities for causal and precise dissection of the circuits underlying brain diseases in animal models. Opsins To improve understanding of psychiatric and neurologi- electrophysiological, pharmacological and behavioural Membrane-bound proteins cal disorders, it will be important to identify the under- assessments. We also highlight the advantages and that can incorporate small lying neural circuits, to pinpoint the precise nature of practical limitations of these approaches for the study organic ‘retinal’ molecules to the causally important aberrations in these circuits of psychiatric and neurological disease. become a light receptor. and to modulate circuit and behavioural dysfunction with precise and specific experimental interventions. Technological advances in optogenetics However, such a deep, circuit-level understanding of The optogenetic toolbox includes a rapidly expand- neuropsychiatric disorders, or indeed even of normal ing array of available opsin variants that offer both CNS circuit function, has been challenging to achieve distinct advantages and individual limitations in with traditional methods. The complexity of neural cir- controlling cellular activity or signalling3,12–21. Other 1Department of Bioengineering, Stanford cuitry has historically precluded the use of genetically important components of the toolbox are light-delivery 6,9,22–28 16,29–31 University, 318 Campus Drive, and temporally precise manipulations to probe detailed methods , targeting strategies and trans- Clark Center, Stanford, mechanisms of function and dysfunction. genic rodent lines that increase the range of available California 94305-5444, USA. Optogenetics1,2 describes the now widespread use specific cellular targets32–34. For example, the recent 2Picower Institute of Learning opsins3 4 35,36 37 and Memory, Department of of microbial , or related tools , that can be acti- development of devices and transgenic rat lines Brain and Cognitive Sciences, vated by illumination to manipulate cells with high that facilitate integration of optogenetic techniques Massachusetts Institute of specificity and temporal precision5–7 even within intact with measures of neural activity have advanced the Technology, Cambridge, tissue or behaving animals8–11. Here, we briefly review application of optogenetic tools to investigate the neu- Massachusetts how optogenetic approaches have been used to dissect ral bases of complex behaviours that are relevant to 02139-4307, USA. 3Department of Psychiatry, neural circuits in animal models of symptoms that are neuropsychiatric disease. Stanford University, 401 relevant to fear, anxiety, depression, schizophrenia, Quarry Road, Stanford, addiction, social dysfunction, Parkinson’s disease and Integration of optogenetics with mapping techniques. California 94305-5717, USA. epilepsy. Successful probing of complex diseases in The recent integration of fMRI with optogenetic manip- 4CNC Program, Stanford University, Stanford, this way will depend on the validity of animal mod- ulation, now referred to as ofMRI, has not only vali- California 94305, USA. els used to identify the crucial circuit elements and dated a previously assumed interpretation of the fMRI 5Howard Hughes Medical activity patterns that are involved in each cluster of BOLD signal38 (that increased neuronal activity in local Institute, Stanford University symptoms, and the precision and efficiency of inter- excitatory neurons can causally trigger, rather than Medical School, Stanford, ventions designed to selectively target these elements simply correlate with, an increase in the local BOLD California 94305-5323, USA. e‑mails: [email protected]; or patterns. Therefore, we also discuss new strategies signal) but has also shown that it is possible to assay [email protected] for targeting opsins to specific cells or circuit ele- the effects of precise optogenetic manipulations on doi:10.1038/nrn3171 ments and principles for integrating optogenetics with global brain activity. Given that many neuropsychiatric NATURE REVIEWS | NEUROSCIENCE VOLUME 13 | APRIL 2012 | 251 © 2012 Macmillan Publishers Limited. All rights reserved REVIEWS diseases are likely to involve distributed perturbations, As SSFO also has an enhanced sensitivity to light, it global approaches such as ofMRI may be crucial for enables the non-invasive light-induced activation of identifying and mapping the downstream effects of SSFO-expressing neurons up to 3 mm below the sur- cell-type or projection-specific manipulations (in an face of the brain when an optical fibre is placed just unbiased fashion). above the brain surface48. Thus, the development of the Local, detailed circuit-mapping has also benefited SSFO may facilitate research in large brain regions or greatly from optogenetics. Continuing a long-standing in large-brained animals. tradition of mapping neural circuitry in mammals Another group of noteworthy opsins is the red- with optical approaches39, and in certain cases using shifted activation wavelength ChR1/VChR1 chimaera new classes of light delivery40, several elegant optoge- (C1V1) family, which includes variants that are sig- netic studies have already made substantial advances nificantly more potent48 than ChR2 and approximately in detailed circuit mapping41–43. These studies have fourfold more potent than the Volvox channelrho- helped to clarify the role of specific cortical layers in dopsin 1 (VChR1), a red-shifted opsin developed the regulation of activity flow, as well as to delineate previously14. Members of the C1V1 family and their the detailed pattern of synaptic inputs arising from associated variants have a peak activation wavelength distinct cortical layers onto distinct subcellular loca- of ~560 nm and can readily be activated by 590 nm tions in neocortical principal cells. By providing a rich light, thus increasing the feasibility of combinatorial fMRI (Functional magnetic source of information that would have been difficult excitation or depolarization of different populations of 21,48 resonance imaging). This or impossible to obtain by other means, these studies neurons at distinct experimental epochs or patterns . method can use detection of may lay the groundwork for identifying circuit or con- These opsins have been used to investigate behaviours blood oxygen levels as a proxy nectivity phenotypes that can go awry in disease states. that are relevant to autism and schizophrenia48, and are for neural activity, and offers a promising candidates for the study of other diseases of non-invasive method to globally assay brain activity in New opsin variants. Earlier optogenetic tools, such the brain. 5,13 humans. as channelrhodopsin 2 (ChR2) — which enables action potential elicitation to be time-locked to light Use of transgenic rodents in optogenetics. Tools for BOLD pulses — or halorhodopsin (NpHR)16,18,44–46 and pro- conferring genetic specificity to opsin delivery are also (Blood oxygen level 16,19,21 Cre recombinase dependent). The BOLD signal ton pumps — which enable hyperpolarization improving. A large number of -driver is one kind of signal that fMRI of membranes to inhibit the production of action mouse lines have already been used in optogenetic can use to assess neural potentials — are still useful. However, the expan- research (reviewed in REF. 6). Although transgenic activity. sion of the optogenetic toolbox (FIG. 1) now provides mouse lines have proven to be very useful in the study greater flexibility in experimental design and more of cells, circuits and behaviours that are relevant to Channelrhodopsin 31,48–55 A light-driven cation channel, powerful and refined manipulations. For example, disease , the more complex behavioural and elec- found in algae, that can be engineered channelrhodopsin variants (including the trophysiological assays available in rats could provide used to depolarize cell ChETA family20,21 and ChIEF47) can be used to evoke important additional insight. However, optogenetic membranes. ultra-fast firing frequencies (up to 200 Hz or more) in research in rats has been hampered by a lack of genetic Halorhodopsin fast-spiking neurons. tools for targeting opsins to specific cell types that are A light-driven chloride ion Although the ability to elicit action potentials that implicated in disease. The recent development of two pump found in phylogenetically are time-locked to light pulses is powerful, the syn- Cre recombinase-driver rat lines targeting tyrosine ancient archaea, known as chrony and patterning of an experimentally delivered hydroxylase and choline acetyltransferase neurons, halobacteria, that can be used illumination pattern may not represent the physiologi- with cell-type-specific promoter or enhancer regions to hyperpolarize cell 4 vectors37 membranes. cal neural code. OptoXRs (opsin–receptor chimae- that were too large to