What is Optogenetics? How Scientists are Using Light to Understand the Brain

Written by Anastasiia Caangay, PhD candidate at the University of Alabama-Birmingham

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The human central nervous system encompasses complex and sophisticated processes. The balance of neuronal excitation and inhibition, mediated by electrical activity, plays a critical role in facilitating or suppressing specific behaviors. However, sometimes signaling between cells is disrupted, resulting in disease manifestation. To better understand how neuronal processing contributes to disease development and/or progression, scientists can harness the power of light using a technique known as optogenetics.

Understanding Neuronal Communication

The information from our five basic senses is processed by neurons communicating through ion channels. The opening of ion channels causes the flow of ions into or out of the cell, generating an electrical signal called an action potential. This signal can then be passed to the next neuron and trigger responses such as muscle contractions.

Figure 1: Central Nervous System Neuron Anatomy and Signal Propagation

Several methods are used to study neuronal communication. For example, the use of electron microscopy allows scientists to better understand how neuronal structures like synaptic vesicles are involved in neurotransmitter release, as well as the anatomy and connectivity of neurons (Jurrus et al., 2010). Similarly, viral tracing allows for the mapping of neuronal pathways from the region of interest to other parts of the nervous system (Qiu et al., 2010). However, a major disadvantage of many of these techniques is that they do not allow for cell-specific targeting, making it difficult to determine which neuronal populations are associated with animal behavior.

To determine whether particular neurons and the communication between them are involved in disease manifestation, scientists can use optogenetics, a technique that allows for real-time monitoring of cell-specific modulation of neuronal activity by either exciting or inhibiting a population of neurons. This helps to identify causal relationships between neuronal activity and behavior. This is particularly valuable since some diseases are associated with disruptions in neuronal communication. Furthermore, optogenetics can target and manipulate neurons within large networks, such as the cortex, allowing for cell-specific manipulation of central nervous system activity.

What Happens When Neuronal Communication Goes Awry?

Neuronal health is essential for effective communication within the brain and between the brain and body. However, when neuronal activity is abnormal, it can disrupt these communication pathways, potentially leading to cognitive impairments, motor dysfunction, sensory disturbances, and other health issues.

Epilepsy is a classic example of impaired neuronal communication. Epileptic seizures are often associated with an imbalance between excitation and inhibition of neurons, making this disparity a central focus in epilepsy research (Bertram et al., 2013).

Another example is chronic pain, where altered neuronal activity plays an important role. One of the contributing factors to chronic pain is a phenomenon called central sensitization, where neurons become easier to activate and amplify pain signals to other neurons because of inflammation, injury, or hyperactivity of a neuron itself (Latremoliere & Woolf, 2009). In other words, in those with chronic pain, neurons in the central nervous system might persistently amplify pain signals, contributing to the state of ongoing pain.

What is Optogenetics and How Does it Work?

The use of optogenetics to study neuronal dysfunction involves manipulating neuronal activity with light. This technique has been clearly established in preclinical models, and particularly in rodents.

Optogenetics involves genetically engineering neurons that respond to photo stimulation. These neurons express opsins, light-gated channels or pumps for ions that respond to specific wavelengths of light. For example, halorhodopsin (NpHR) hyperpolarizes neurons, inhibiting their activity, while channelrhodopsin (ChR), depolarizes neurons, promoting excitation (Guru et al., 2015).

Figure 2: Example of opsin effects on neuronal activity. 

In 2005, Boyden et al. revolutionized the field of optogenetics by delivering a channelrhodopsin-2 (ChR2)-containing lentivirus into hippocampal neurons, demonstrating for the first time that light could precisely control neuronal activity in animals (Boyden et al., 2005). This landmark study served as a stepping stone for the development of modern optogenetic tools. 

Today, opsins can be introduced into animal models via viral transfection, typically using harmless viral vectors that carry the opsin gene. However, this approach alone does not distinguish between different neuron types, which can lead to off-target expression and complicate the interpretation of results (Guru et al., 2015). 

To achieve cell-type-specific opsin expression (those that express a particular biomarker, such as Nav1.8, for example), researchers use conditional expression systems based on the Cre-lox recombination system (Guru et al., 2015). In this setup, genetically engineered animals express an enzyme called Cre recombinase under the control of a cell-type-specific promoter. When a viral vector containing an opsin gene blocked by a stop sequence is introduced, for example by injection into a particular brain site, only cells expressing Cre recombinase will remove the stop sequence and activate opsin expression, ensuring specificity (Guru et al., 2015). 

Figure 3. Example of Cre-lox recombination system

Once the neurons of interest have been “tagged” with opsins, researchers can use various techniques to study the effects of neuronal excitation or inhibition in response to light stimulation. In some studies, light is delivered through implanted LED devices, allowing scientists to monitor behavioral responses to light activation or silencing of specific neurons in awake animals (Michoud et al., 2018). To assess neural activity more directly, some studies perform in vivo single-unit electrophysiological recordings, which measure the firing patterns of individual neurons in live, anesthetized animals (Moore et al., 2014). For in vitro experiments, researchers may use cultured living cells or brain tissue slices, enabling real-time imaging for electrophysiological recordings under controlled conditions (Morton et al., 2019).  

Applications of Optogenetics  

Optogenetics has demonstrated wide-ranging potential in neuroscience research and continues to be a powerful tool for understanding neuronal communication and elucidating potential therapeutic interventions in various disease models.  

Stroke 

In animal models of stroke, stimulation of ChR2-expressing cells in a region adjacent to the lesion site resulted in increased neuroplasticity and cerebral blood flow, which correlated with improved animal behavior (Cheng et al., 2014). Others have found that optogenetic stimulation of thalamocortical axons improves sensorimotor function of the forepaw after stroke, which could be associated with increased formation of axonal boutons (Tennant et al., 2017).   

Epilepsy 

 Promising results have been shown in studies of epilepsy, with optogenetic inhibition of different populations of neurons resulting in reduced seizure activity (Schuele & Lüders, 2008; Tønnesen et al., 2009). In a mouse model of temporal lobe epilepsy, for example, optogenetic inhibition of excitatory principal cells or activation of certain GABAergic cells in the hippocampus aborts seizures immediately (Krook-Magnuson et al., 2013). Optogenetic manipulation of GABAergic cells can also contribute to increased performance on memory tasks in addition to shortening seizure duration (Kim et al., 2020).  

Neurodegeneration  

Research from neurodegenerative conditions, like Alzheimer’s and Parkinson’s Disease, also suggests that optogenetics can help elucidate disease mechanisms and identify potential therapeutic strategies (Degos et al., 2019; Lee et al., 2023). For instance, in a mouse model of Alzheimer’s disease, optogenetic activation of glutaminergic neurons in the dentate gyrus improves working and short-term memory (Wang et al., 2019). Conversely, in studies of Parkinson’s disease, optogenetic activation of glutamatergic neurons in the cuneiform nucleus results in adverse Parkinson-like symptoms in mice, potentially offering a novel treatment target (Fougère et al., 2021).  

Anxiety and PTSD 

Fear related memory recall can be reversed using hippocampal neuron inhibition, which, in the future, could serve as a therapeutic for those with anxiety and post-traumatic stress disorder (Goshen et al., 2011). On the other hand, optogenetic facilitation of serotonergic neurons in median, and not dorsal, raphe nucleus enhances anxiety behavior in mice (Ohmura et al, 2014). Interestingly, optogenetic activation of hypothalamic cells associated with melanin also improves fear-associated memory processing in addition to improved sleep, which is often impaired in those with PTSD (Davis et al., 2020). 

Chronic Pain 

In studies of chronic and acute pain, optogenetics is also being used as a potential pain treatment via different routes of action (Daou et al., 2016; Kc et al., 2023). For example, optogenetic stimulation of inhibitory neurons in anterior cingulate cortex significantly reduced pain-like behavior in mice and the neuronal activity of this region (Gu et al., 2020). Ontogenetically targeting Nav1.8+ sensory neurons also can result in robust pain behavior by just shining light on the surface of the skin. Activation of the Nav1.8+ afferents in vivo are also shown to induce central sensitization, which provides new ideas about acute to chronic pain transition mechanisms (Daou et al., 2013).  

Whole Brain Activity  

A novel approach to understand whole brain activity combines optogenetics and fMRI, where the activity of the whole brain can be monitored during the optogenetics manipulation (Desai et al., 2011). Activation of several brain regions like prefrontal, striatal, and limbic was found to be enhanced when medial prefrontal cortex was ontogenetically activated in mouse during fMRI. Since neuronal dysfunction in the medial prefrontal cortex is associated with many diseases (Chai et al., 2011: Goldstein et al., 2011; Koenigs & Grafman, 2009), understanding the effect of this region on the rest of the brain can provide valuable insights on complex circuitry involving medial prefrontal region (Liang et al., 2015).  

What is Next? 

Optogenetics holds immense potential as a therapeutic tool for a variety of neurological conditions. However, several challenges remain in translating this technique to clinical use. First, the delivery of opsins is currently invasive, and the field must develop alternative methods that minimize side effects while maintaining high spatial precision. Additionally, delivering light to deep brain regions is also invasive and presents challenges such as limited access and potential heat generation. Despite these hurdles, the therapeutic promise of optogenetics is substantial. With continued innovation, it may ultimately revolutionize the treatment of brain disorders and perhaps receive further global recognition.  

References 

Bertram, E. H. (2013). Neuronal circuits in epilepsy: do they matter?. Experimental neurology, 244, 67-74. 

Boyden, E. S., Zhang, F., Bamberg, E., Nagel, G., & Deisseroth, K. (2005). Millisecond-timescale, genetically targeted optical control of neural activity. Nature neuroscience, 8(9), 1263-1268. 

Chai, X. J., Whitfield-Gabrieli, S., Shinn, A. K., Gabrieli, J. D., Nieto Castañón, A., McCarthy, J. M., ... & Öngür, D. (2011). Abnormal medial prefrontal cortex resting-state connectivity in bipolar disorder and schizophrenia. Neuropsychopharmacology, 36(10), 2009-2017. 

Cheng, M. Y., Wang, E. H., Woodson, W. J., Wang, S., Sun, G., Lee, A. G., ... & Steinberg, G. K. (2014). Optogenetic neuronal stimulation promotes functional recovery after stroke. Proceedings of the National Academy of Sciences, 111(35), 12913-12918. 

Daou, I., Beaudry, H., Ase, A. R., Wieskopf, J. S., Ribeiro-da-Silva, A., Mogil, J. S., & Séguéla, P. (2016). Optogenetic silencing of Nav1. 8-positive afferents alleviates inflammatory and neuropathic pain. Eneuro, 3(1). 

Daou, I., Tuttle, A. H., Longo, G., Wieskopf, J. S., Bonin, R. P., Ase, A. R., ... & Séguéla, P. (2013). Remote optogenetic activation and sensitization of pain pathways in freely moving mice. Journal of Neuroscience, 33(47), 18631-18640. 

Davis, C. J., & Vanderheyden, W. M. (2020). Optogenetic sleep enhancement improves fear-associated memory processing following trauma exposure in rats. Scientific Reports, 10(1), 18025. 

Degos, B., Vandecasteele, M., Valverde, S., Piette, C., Gangarossa, G., Derousseaux, W., ... & Venance, L. (2019). Deep brain stimulation-guided optogenetic rescue of parkinsonian symptoms. Movement Disorders, 34, S887-S888. 

Desai, M., Kahn, I., Knoblich, U., Bernstein, J., Atallah, H., Yang, A., ... & Boyden, E. S. (2011). Mapping brain networks in awake mice using combined optical neural control and fMRI. Journal of neurophysiology, 105(3), 1393-1405. 

Fougère, M., van der Zouwen, C. I., Boutin, J., Neszvecsko, K., Sarret, P., & Ryczko, D. (2021). Optogenetic stimulation of glutamatergic neurons in the cuneiform nucleus controls locomotion in a mouse model of Parkinson’s disease. Proceedings of the National Academy of Sciences, 118(43), e2110934118. 

Goldstein, R. Z., & Volkow, N. D. (2011). Dysfunction of the prefrontal cortex in addiction: neuroimaging findings and clinical implications. Nature reviews neuroscience, 12(11), 652-669. 

Goshen, I., Brodsky, M., Prakash, R., Wallace, J., Gradinaru, V., Ramakrishnan, C., & Deisseroth, K. (2011). Dynamics of retrieval strategies for remote memories. Cell, 147(3), 678-689. 

Gu, L., Uhelski, M. L., Anand, S., Romero-Ortega, M., Kim, Y. T., Fuchs, P. N., & Mohanty, S. K. (2015). Pain inhibition by optogenetic activation of specific anterior cingulate cortical neurons. PloS one, 10(2), e0117746. 

Guru, A., Post, R. J., Ho, Y. Y., & Warden, M. R. (2015). Making sense of optogenetics. International Journal of Neuropsychopharmacology, 18(11), pyv079. 

Jurrus, E., Paiva, A. R., Watanabe, S., Anderson, J. R., Jones, B. W., Whitaker, R. T., ... & Tasdizen, T. (2010). Detection of neuron membranes in electron microscopy images using a serial neural network architecture. Medical image analysis, 14(6), 770-783. 

Kc, E., Islam, J., Kim, H. K., & Park, Y. S. (2023). GFAP-NpHR mediated optogenetic inhibition of trigeminal nucleus caudalis attenuates hypersensitive behaviors and thalamic discharge attributed to infraorbital nerve constriction injury. The Journal of Headache and Pain, 24(1), 137. 

Kim, H. K., Gschwind, T., Nguyen, T. M., Bui, A. D., Felong, S., Ampig, K., ... & Soltesz, I. (2020). Optogenetic intervention of seizures improves spatial memory in a mouse model of chronic temporal lobe epilepsy. Epilepsia, 61(3), 561-571. 

Koenigs, M., & Grafman, J. (2009). The functional neuroanatomy of depression: distinct roles for ventromedial and dorsolateral prefrontal cortex. Behavioural brain research, 201(2), 239-243. 

Krook-Magnuson, E., Armstrong, C., Oijala, M., & Soltesz, I. (2013). On-demand optogenetic control of spontaneous seizures in temporal lobe epilepsy. Nature communications, 4(1), 1376. 

Latremoliere, A., & Woolf, C. J. (2009). Central sensitization: a generator of pain hypersensitivity by central neural plasticity. The journal of pain, 10(9), 895-926. 

Lee, Y. F., Russ, A. N., Zhao, Q., Perle, S. J., Maci, M., Miller, M. R., ... & Kastanenka, K. V. (2023). Optogenetic targeting of astrocytes restores slow brain rhythm function and slows Alzheimer’s disease pathology. Scientific reports, 13(1), 13075. 

Liang, Z., Watson, G. D., Alloway, K. D., Lee, G., Neuberger, T., & Zhang, N. (2015). Mapping the functional network of medial prefrontal cortex by combining optogenetics and fMRI in awake rats. Neuroimage, 117, 114-123. 

Michoud, F., Sottas, L., Browne, L. E., Asboth, L., Latremoliere, A., Sakuma, M., ... & Lacour, S. P. (2018). Optical cuff for optogenetic control of the peripheral nervous system. Journal of neural engineering, 15(1), 015002. 

Moore, A. K., & Wehr, M. (2014). A guide to in vivo single-unit recording from optogenetically identified cortical inhibitory interneurons. Journal of visualized experiments: JoVE, (93), 51757. 

Morton, A., Murawski, C., Deng, Y., Keum, C., Miles, G. B., Tello, J. A., & Gather, M. C. (2019). Photostimulation for in vitro optogenetics with high‐power blue organic light‐emitting diodes. Advanced Biosystems, 3(3), 1800290. 

Ohmura, Y., Tanaka, K. F., Tsunematsu, T., Yamanaka, A., & Yoshioka, M. (2014). Optogenetic activation of serotonergic neurons enhances anxiety-like behaviour in mice. International Journal of Neuropsychopharmacology, 17(11), 1777-1783. 

Qiu, L., Zhang, B., & Gao, Z. (2022). Lighting up neural circuits by viral tracing. Neuroscience Bulletin, 38(11), 1383-1396. 

Schuele, S. U., & Lüders, H. O. (2008). Intractable epilepsy: management and therapeutic alternatives. The Lancet Neurology, 7(6), 514-524. 

Tennant, K. A., Taylor, S. L., White, E. R., & Brown, C. E. (2017). Optogenetic rewiring of thalamocortical circuits to restore function in the stroke injured brain. Nature communications, 8(1), 15879. 

Tønnesen, J., Sørensen, A. T., Deisseroth, K., Lundberg, C., & Kokaia, M. (2009). Optogenetic control of epileptiform activity. Proceedings of the National Academy of Sciences, 106(29), 12162-12167. 

Wang, K. W., Ye, X. L., Huang, T., Yang, X. F., & Zou, L. Y. (2019). Optogenetics-induced activation of glutamate receptors improves memory function in mice with Alzheimer’s disease. Neural regeneration research, 14(12), 2147-2155.