Shining Light on the New Wave of Neurobiology

Since its inception in 2005, optogenetics has revolutionised neurobiology—researchers are now closer than ever to determining the functions, behaviours, and pathologies of individual neurons (Boyden, 2011). As this technology continues to develop, it underscores the intersectionality of optics and genetics and shapes scientific perception of neural dysfunction and mood disorders (Guru, et al., 2015). An understanding of the uses and methodologies of optogenetics is crucial to predicting its impact on the next generation of circuit-based neural experimentation. 

The core purpose of optogenetic therapy is to manipulate the activity of individual neurons using light and to observe the results in real time (Adamczyk and Zawadzki, 2020). To accomplish this, researchers leverage the previously built knowledge surrounding electrical brain stimulation (Deng, et al., 2014). Once regarded as the leading neuromodulation technique, electrical stimulation involves the placement of an electrode into a target region of the brain and the transmission of a small electrical signal to the electrode to observe patient response. 

By recording patient movement in response to recurring stimulation of different areas of the brain, researchers began to form a visual understanding of the regions responsible for particular actions (Lim and LeDue, 2017). In the time following these findings, brain stimulation experiments evolved, and studies exposed the pitfalls of electrical stimulation. Its primary issue was the susceptibility of the brain to damage when an electrode was implanted. The second weakness was that electrical stimulation activates tissue in an inexact, non-selective way, as seen in Figure 1.

Figure 1: Stimulation of the brain under three different conditions. A) depicts electrical stimulation, which involves the insertion and delivery of an electrical signal to a metal electrode placed into a brain region of interest. Not only is this precarious to perform due to possible brain damage, but also it activates the brain (“On!”) in an imprecise and ineffective way, preventing researchers from accurately identifying the connections responsible for certain conditions. B) depicts stimulation of the brain using blue light. Due to the inhomogeneities present in brain tissue, light is significantly or entirely attenuated before it reaches the neuron. This produces no activation (no “On!”) during treatment. C) depicts optogenetic stimulation. Here, neurons have been genetically modified to code for opsins. The green neurons have been modified to produce channelrhodopsin-2 (ChR2), a highly popular opsin for optogenetics that responds exclusively to blue light. When blue light is shone, these neurons depolarize (“On!”), and launch ensuing action potentials in neurons within their circuit (Lim and LeDue, 2017). 

Contrarily, the technique of optogenetics uses the properties of light to stimulate light-sensitive ion channels via genetically modified neurons (Camporeze, et al., 2018; White, Mackay and Whittaker, 2020). To sensitise neurons, a highly specialised viral vector is injected to initiate transduction and long-term protein expression of opsins (White, Mackay and Whittaker, 2020). Opsins are a diverse family of algae-based photoreceptive molecules; besides existing as excitatory or inhibitory, different opsins are responsive to distinct wavelengths of light, making them applicable treatment alternatives to a variety of neural disorders. 

In clinical trials, light is shone onto the brain of the subject and physical reactions are observed (White, Mackay and Whittaker, 2020). For optogenetics to be successful, the target brain region must correspond with the wavelength of light applied. This implies that the deeper the target region or the larger the volume of target cells, the longer the wavelength of applied light, as seen in Figure 2.

Figure 2: The visible light spectrum and its associated distances of penetration through human tissues. Blue light possesses a shorter wavelength and higher energy, but travels very little through tissue. This is due to human skin consisting of wavelength-dependent scattering coefficients that deflect blue light before it passes 1 mm of tissue. Red light, however, passes some 4-5 mm beneath the skin’s surface. This is due to the inherent scattering properties of tissue that favour red light over shorter wavelength colours. Therefore, optogenetics treatments that require deep tissue stimulation require red light and opsins sensitive to red light wavelengths. Treatments that require surface-level stimulation warrant the use of blue light and opsins sensitive to blue light wavelengths (Ash, et al., 2017).

Once a suitable opsin has been inserted, researchers can apply the requisite wavelength of light to induce neural excitation (Wang, et al., 2018). Opsins are selective for specific types of light, meaning that neurons can only be stimulated as long as a particular wavelength is shining on them (Lim and LeDue, 2017). This enables researchers to have precise regulation of a neuron’s activity, and assess the circuit-based underpinnings of abnormal behaviour in a more straightforward manner (Guru, et al., 2015). 

The challenges involved in progressing optogenetics from animal models to human studies remains clear (White, Mackay and Whittaker, 2020). For instance: the viral vector must be injected precisely during procedure; opsin expression from the initial injection must be sufficient as to eliminate the need of additional injections; and an implantable device capable of administering sufficient optical stimulation without damaging brain tissue must be produced to ensure better long-term outcomes. 

Despite these difficulties, optogenetics continues to show considerable therapeutic potential for brain disorder management. Further research must be completed to overcome current hurdles and ensure safe, long-lasting brain stimulation. Although it is too early to carry out optogenetics on humans, we are inching ever closer to a comprehensive understanding of brain function.

References:

Adamczyk, A.K. and Zawadzki, P., 2020. The Memory-Modifying Potential of Optogenetics and the Need for Neuroethics. NanoEthics, 14(3), pp.207–225. https://doi.org/10.1007/s11569-020-00377-1.

Ash, C., Dubec, M., Donne, K. and Bashford, T., 2017. Effect of wavelength and beam width on penetration in light-tissue interaction using computational methods. Lasers in Medical Science, 32(8), pp.1909-1918. https://doi.org/10.1007/s10103-017-2317-4.

Boyden, E.S., 2011. A history of optogenetics: the development of tools for controlling brain circuits with light. F1000 Biology Reports, 3, pp.1-12. https://doi.org/10.3410/B3-11.

Camporeze, B., Manica, B.A., Bonafé, G.A., Ferreira, J.J.C., Diniz, A.L., Oliveira, C.T.P. de, Junior, L.R.M., Aguiar, P.H.P. de and Ortega, M.M., 2018. Optogenetics: the new molecular approach to control functions of neural cells in epilepsy, depression and tumors of the central nervous system. American Journal of Cancer Research, 8(10), pp.1900-1918.

Deng, W., Goldys, E.M., Farnham, M.M. and Pilowsky, P.M., 2014. Optogenetics, the intersection between physics and neuroscience: light stimulation of neurons in physiological conditions. American Journal of Physiology-Regulatory, Integrative and Comparative Physiology, 307(11), pp.R1292-R1302. https://doi.org/10.1152/ajpregu.00072.2014.

Guru, A., Post, R.J., Ho, Y.-Y. and Warden, M.R., 2015. Making Sense of Optogenetics. International Journal of Neuropsychopharmacology, 18(11), pp.1-8. https://doi.org/10.1093/ijnp/pyv079.

Lim, D.H. and LeDue, J., 2017. What Is Optogenetics and How Can We Use It to Discover More About the Brain? Frontiers for Young Minds, 5, p.51. https://doi.org/10.3389/frym.2017.00051.

Wang, Y., Hu, Z., Ju, P., Yin, S., Wang, F., Pan, O. and Chen, J., 2018. Viral vectors as a novel tool for clinical and neuropsychiatric research applications. General Psychiatry, 31(2), pp.1-9. https://doi.org/10.1136/gpsych-2018-000015.White, M., Mackay, M. and Whittaker, R.G., 2020. Taking Optogenetics into the Human Brain: Opportunities and Challenges in Clinical Trial Design. Open Access Journal of Clinical Trials, 12, pp.33–41. https://doi.org/10.2147/OAJCT.S259702.