By Toby Lanser
With winter in full swing mid-February, the temperature is low and the days are short. This causes people to spend the majority of their time inside, sheltered under artificial light and warmth, which creates an environment that commonly causes people to get the ‘winter blues,’ resulting in what’s called Seasonal Affective Disorder (SAD). This change in mood from season to season is well known, and the reasons behind it are reasonably well-studied. For example, it is understood that people respond differently to the same stimuli when encountered under different shades of light.
But despite the fact that SAD is well known, a new study from groups of scientists at Brown University and Johns Hopkins are now determining the central mediators within the pathway between the retina and brain that plays a role in both mood and learning. Usually, the classical pathway of rod and cone photoreceptors (responsible for light/dark and color vision respectively) is how the brain receives and processes light. However, another type of cells in the retina, called intrinsically photosensitive retinal ganglion cells (ipGRGCs), also receive photons (light). As it turns out, ipGRGCs also assist in mood regulation, according to a previous study off which these researchers based their experiments. In the previous study, researchers had removed ipGRGCs in mice, which resulted in a decrease in overall mood.
To investigate the role of ipGRGCs in both mood and learning, the researchers first determined that the cells in question targeted specific regions of the brain, in this case the thalamus, which plays a role in relaying sensory signals. Cleverly, the scientists sent a signal from the ipGRGCs and traced, retroactively, the signal from its destination back to the group of cells. The result was that the signal passed through regions in the brain heavily associated with mood regulation and cognition. Additionally, they used calcium imaging, which is a common method used by neuroscientists to visualize the activity in the identified groups of neurons communicating with the ipGRGCs.
Through all of this rigorous mapping, the researchers were able to draw a map from the cells they hypothesized played a role in mood and cognition (ipGRGCs) to areas in the brain known to deal with those processes (fig 1). Given the increase in depression and suicides during the winter months, this research can act as a scaffold for new research in the coming years in terms of the effect of light on a person's mood and cognition throughout the seasons. This new pathway can also serve doctors providing diagnoses by changing a patient's prognosis to cater to this new mood affecting pathway.
References:
Fernandez, D. C., Fogerson, P. M., Ospri, L. L., Thomsen, M. B., Layne, R. M., Severin, D., . . . Hattar, S. (2018). Light Affects Mood and Learning through Distinct Retina-Brain Pathways. Cell, 175(1). doi:10.1016/j.cell.2018.08.004