The process of optogenetics

Strictly speaking, optogenetics involves “experimenting with a combination of genetic manipulation and optics” (Kasparov,2011). It can be used in several animal models, including theC. elegans, fly, zebrafish, mouse, rat, and primate (Fenno et al.,2011). Ultimately, optogenetics may be used to control the behavior of freely moving mammals by administering light. However, several steps need to be taken to achieve this, as illustrated in Figure1.

First, the opsins of interest have to be specified. Optogenetics has mostly focussed on microbial opsins, because these are capable of directly coupling light to rapid ion transport (Yizhar et al.,2011a; Han,2012). However, since microbial opsins serve strictly as ion flow modulators, animal opsins are being used for biochemical control (Fenno et al.,2011). Neuronal activation can be obtained by illuminating channelrhodopsin-2 (ChR2) with blue light, while neuronal inhibition results from illuminating halorhodopsin (NpHR) with yellow light. Receptor-mediated intracellular signaling can be achieved by shining green light on OptoXRs, see Figure1A.

The next step is to make sure that mammalian cells express microbial opsins, for example by using virus carrier systems (Figure1B). The virus carries the opsin gene and is injected into the cell of interest. There are several types of viral vectors that can be used for this purpose, the most common being the Lenti and adeno-associated virus (AAV).

Finally, light has to be delivered into the brain. This can be achieved by using a chronically implanted cannula (affixed to the skull) to which an optical fiber can be attached (Figure1C). Laser light can then be delivered via this optical fiber directly into the brain (Zhang et al.,2010).

The application of optogenetics

Many questions in cognitive neuroscience regarding the molecular, cellular and circuit-level underpinnings of behavior still remain unresolved due to the trade-off between temporal and spatial specificity. Since optogenetic tools have become available to the scientific community, numerous studies have applied this method successfully to answer such questions. The advantage of optogenetics over other neuromodulation techniques is its high-temporal specificity combined with cellular precision. For example, although electrical manipulation has a high temporal resolution, it is unable to achieve true inactivation or excitation of individual neurons. Pharmacological and genetic manipulations show the opposite pattern, they can target at least certain kinds or families of neurons but are lacking temporal precision (Fenno et al.,2011). Optogenetics can be applied to very diverse research topics, even outside neuroscience. Here, we will limit ourselves to a few studies that successfully applied optogenetic tools in areas that are relevant for cognitive neuroscience; for a broader picture see the special issue ofBiological Psychiatry on this topic (Deisseroth,2012).

Optogenetics has been successfully applied in neuromodulation research. For example, Witten et al. (2011) used optogenetic tools to clarify the relationship between dopamine (DA) neuron firing and positive reinforcement in genetically modified rats. They observed that optical stimulation of DA neurons in the ventral tegmental area of these rats led to vigorous intracranial self-stimulation. Likewise, Tsai et al. (2009) demonstrated that phasic dopaminergic activity is sufficient to mediate mammalian behavioral conditioning, by using an optogenetic approach. They emphasize that integrating optogenetics with other approaches (e.g., electrophysiological, behavioral and electrochemical methods) will reveal relevant interactions of DA neurons with other neuromodulatory circuits (e.g., monoaminergic and opioid circuits). The use of optogenetics further revealed opposite roles of D1+ and D2+ neurons (in the nucleus accumbens) in processing cocaine reward (Lobo et al.,2010). In this study the firing rate of D1+ and D2+ neurons was selectively controlled to investigate the resulting effects on cocaine reward. It was found that activation of D2+ neurons suppresses cocaine reward, while activation of D1+ neurons shows the opposite pattern. Another example of optogenetic neuromodulation shows how symptoms of Parkinson's disease can be either aggravated or improved (Kravitz et al.,2010). Kravitz et al. (2010) modulated the firing activity of single neurons, manipulating either direct or indirect pathways in the basal ganglia. Another study (Bass et al.,2010) showed how neuronal dopamine release patterns could be evoked in the dorsal part of the striatum in living rats. Results like these show that the use of optogenetics can lead to a better understanding of cause-effect relationships, for example in dopamine-based disorders.

The neural underpinnings of sleep have also been investigated with optogenetic methods (Adamantidis et al.,2007; de Lecea et al.,2012). In a recent review de Lecea et al. (2012) discuss the use of optogenetics in sleep research as well as in studying the interactions between neuromodulatory systems [e.g., hypocretin (Hcrt) and locus coeruleus/norepinephrine systems]. The authors stress the importance of optogenetics for controlling neural circuits to examine boundaries between sleep and wakefulness (de Lecea et al.,2012). In line with this, researchers stimulated Hcrt producing neurons in freely moving mice (Adamantidis et al.,2007), which led to an increased probability of sleeping mice becoming awake (either from slow wave sleep or rapid eye movement sleep). Interestingly, Hcrt deficiency is associated with the neurological disorder narcolepsy in which sleeping patterns are altered (Adamantidis et al.,2007), and an optogenetic approach may provide further insights into such disorders.

Another area that has successfully been studied with optogenetics is depression (Lobo et al.,2012). For instance, optogenetic stimulation of the medial prefrontal cortex (mPFC) was found to initiate rapid antidepressant-like responses in mice (Covington et al.,2010). Lobo et al. (2012) stress the importance of using optogenetics to study depression, since it allows for answering important questions regarding depression, which have been left unanswered thus far.

A recent review (Yizhar,2012) on neural circuitries in social functioning revealed how optogenetics might improve our understanding of social behavior and psychiatric impairments, and possibly lead to the development of novel treatment methods. Yizhar et al. (2011b) propose the excitation and inhibition (E/I) balance hypothesis, stating that imbalance in the inhibition and excitation pattern within neural circuitries is involved in several psychiatric diseases and behavioral deficits (e.g., autism, schizophrenia). Using an optogenetic approach, it was indeed found that elevations of cellular E/I balance in the mPFC led to increased high-frequency power (30–80 Hz range) and behavioral impairment (Yizhar et al.,2011b). Optogenetics has further been applied to cortical oscillations (synchronized neural activity), that are associated with various cognitive processes as well as psychiatric conditions such as anxiety, autism, and schizophrenia (Sohal,2012). Sohal (2012) demonstrated how a particular class of inhibitory interneurons play a causal role in the occurrence of gamma oscillations, which is important for the way neurons communicate.

In addition, several studies used optogenetics to investigate neural circuits that underlie fear conditioning and memory formation (see Johansen et al.,2012, for a review). Other work on contextual fear memories revealed that optogenetic inhibition of CA1 hippocampal neurons can reverse contextual fear memory recall, even weeks after training (Goshen et al.,2011). Studies like these play an important role in the understanding of fundamental cognitive processes like memory formation, but also in anxiety disorders and posttraumatic stress disorder, which are characterized by disturbing, recurring contextual memories.

Finally, a promising methodological approach of optogenetics is the combination with functional Magnetic Resonance Imaging (fMRI). This combination has been termed “opto-fMRI” or “ofMRI” (Desai et al.,2011; Deisseroth,2012), and has been applied to several domains (Desai et al.,2011; Abe et al.,2012). Although still facing some challenges (Christie et al.,2012), the combination of fMRI with optogenetics provides a unique possibility to observe how functional changes in the brain are brought about as a result of optogenetic manipulation.

Conclusion

In sum, optogenetics is a promising tool for cognitive neuroscience and we believe that it might be applied to human subjects in the long run. Although the area still faces many obstacles, the field of optogenetics is growing rapidly and new advances are continuously being made to improve the technique (for a review see Dugue et al.,2012). For instance, optogenetic modulation in primate neurons has been investigated (Diester et al.,2011) and it has already been demonstrated that ChR2 can function within human neurons (Weick et al.,2010). Furthermore, results from optogenetic studies could be used to determine more effectively the specific region for applying Transcranial Magnetic Stimulation or Deep Brain Stimulation (DBS). Since in DBS all cells in a certain region are stimulated, many side effects have been reported (Frank et al.,2007; Serranová et al.,2013). Optogenetic stimulation would not result in such side effects because of its spatial specificity and could therefore potentially even replace DBS treatment in the future (Lalumiere,2011). In addition, approaches like opto-fMRI might be translatable to humans in the medium term (Bullmore,2012). Taken together, we believe that optogenetics complements other neuroscientific methods and should be used on a wider scale within cognitive neuroscience.

References