Did you know that travelling light can reach our brains? This reality alongside the increasing severity of global light pollution has led to a growing interest amongst researchers to investigate the impact of artificial light on the brain in recent years. In this post, we will be looking at a newly released study by Fasciani et al. (2020), on the effect of artificial light on dopamine neurons and possibly as an environmental risk factor for Parkinson’s Disease. To start off…

How Does Light Reach the Brain?

The shortest known path (indicated by the dotted black lines in Figure 1) that light takes to reach the substantia nigra (SN) would be through our eyes or our sinus cavities (Romeo et al., 2014). 

Figure 1. Shortest Light Path To The Brain

From there, it travels through the retro-orbital tissue, and in some cases continues to reach the superior orbital fissure (Figure 2) thereby successfully entering the internal cavity of the skull and reaching the mesencephalon.

Figure 2. Superior Orbital Fissure

It is also important to note that the depth of light penetration in biological tissues varies according to two factors — the wavelength of light and composition of the irradiated tissue. For instance, light with longer wavelengths has the potential to penetrate more deeply into biological tissues. Specifically, the study reveals that the penetration depth of light is greatest near the infrared range between 600 and 1300nm (Taroni et al., 2003). 

Figure 3. Electromagnetic spectrum Diagram

Attention has also been drawn to the fact that 660nm is within the range of visible light. This means that the lights that light up our cities, streets, homes and phones are highly invasive in nature. But how does light exactly inflict damage to our brains?

How Light Damages Dopamine Neurons

Light has the potential to increase tissue sensitivity to damage through decreased melatonin secretion in the brain. The significance of light in affecting this process can be attributed to the location of the pineal gland which is responsible for melatonin secretion. The pineal gland being located at a depth reachable by light hence adversely affects melatonin secretion under excessive light exposure (Figure 4). 

Figure 4. Pineal Gland Diagram

This is significant because melatonin plays a neuroprotective role in defending the brain against neurodegenerative diseases. Case in point, the study shows that melatonin reduces nigral dopaminergic neurons damage caused by neurotoxins in the substantia nigra (refer to SN in Figure 1) of mice (Naskar et al., 2015). These results have huge implications on the effect of light pollution on Parkinson’s disease. Since Parkinson’s disease occurs when approximately 60–80% of the dopaminergic neurons in substantia nigra are damaged, a light-induced reduction in melatonin secretion increases damage to dopamine neurons and hence increasing the risk of Parkinson’s disease. 

Why It Matters

The prevalence of Parkinson’s disease has been found to have a strong correlation with average satellite-observed sky light pollution (Romeo et al., 2013). While the association between light pollution and Parkinson’s disease is still highly speculative, this does not mean that we should not take it seriously. Much indirect evidence has strengthened the association of this relationship. To illustrate, people in the education and healthcare industries (Firestone et al., 2010; Park et al., 2005; Tsui et al., 1999), and those who have pursued higher education (Frigerio et al., 2005), have been found to be at a greater risk of Parkinson’s disease, likely the outcome of long exposure of artificial light through computers. Hence living in a world where light intensity and exposure is constantly growing, we ought to manage our light sources to protect not just our eyes or brains, but the overall health of our society.

Till next time!

Trudie

References:

• BD Editors. (2019, May 17). Pineal Gland Diagram [Diagram]. Biology Dictionary. https://biologydictionary.net/pineal-gland/
• Centers for Disease Control and Prevention. (2015, December 7). Electromagnetic Spectrum [Diagram]. Centers for Disease Control and Prevention. https://www.cdc.gov/nceh/radiation/spectrum.html
• Fasciani, I., Petragnano, F., Aloisi, G., Marampon, F., Rossi, M., Coppolino, M. F., Rossi, R., Longoni, B., Scarselli, M., & Maggio, R. (2020). A new threat to dopamine neurons: The downside of artificial light. Neuroscience, 432, 216-228. https://doi.org/10.1016/j.neuroscience.2020.02.047
• Firestone, J. A., Lundin, J. I., Powers, K. M., Smith‐Weller, T., Franklin, G. M., Swanson, P. D., … & Checkoway, H. (2010). Occupational factors and risk of Parkinson’s disease: A population‐based case–control study. American journal of industrial medicine, 53(3), 217-223.
• Frigerio, R., Elbaz, A., Sanft, K. R., Peterson, B. J., Bower, J. H., Ahlskog, J. E., … & Rocca, W. A. (2005). Education and occupations preceding Parkinson disease: a population-based case-control study. Neurology, 65(10), 1575-1583.
• Mjc.edu. (n.d.). [Diagram]. Columbia Asia. https://www.columbiaasia.com/malaysia/health-articles/ocular-anatomy
• Naskar, A., Prabhakar, V., Singh, R., Dutta, D., & Mohanakumar, K. P. (2015). Melatonin enhances L‐DOPA therapeutic effects, helps to reduce its dose, and protects dopaminergic neurons in 1‐methyl‐4‐phenyl‐1, 2, 3, 6‐tetrahydropyridine‐induced parkinsonism in mice. Journal of pineal research, 58(3), 262-274.
• Park, J., Yoo, C. I., Sim, C. S., Kim, H. K., Kim, J. W., Jeon, B. S., … & Jung, K. Y. (2005). Occupations and Parkinson’s disease: a multi-center case-control study in South Korea. Neurotoxicology, 26(1), 99-105.
• Romeo, S., Viaggi, C., Di Camillo, D., Willis, A. W., Lozzi, L., Rocchi, C., … & Caleo, M. (2013). Bright light exposure reduces TH-positive dopamine neurons: implications of light pollution in Parkinson’s disease epidemiology. Scientific reports, 3, 1395.
• Romeo, S., Di Camillo, D., Splendiani, A., Capannolo, M., Rocchi, C., Aloisi, G., … & Maggio, R. (2014). Eyes as gateways for environmental light to the substantia nigra: relevance in Parkinson’s disease. The Scientific World Journal, 2014.
• Taroni, P., Pifferi, A., Torricelli, A., Comelli, D., & Cubeddu, R. (2003). In vivo absorption and scattering spectroscopy of biological tissues. Photochemical & Photobiological Sciences, 2(2), 124-129.
• Tsui, J. K., Calne, D. B., Wang, Y., Schulzer, M., & Marion, S. A. (1999). Occupational risk factors in Parkinson’s disease. Canadian journal of public health, 90(5), 334-337.