Long considered essential to the regulation of the biological clock, cones were neglected with the discovery of melanopsin, and are now considered again for their role in regulating our biological clock.
Why is it that we are tired when it gets dark and active when the sun shines? The answer lies in our circadian clock. We all have an internal clock that regulates the activity of our cells and our metabolism. From an evolutionary point of view, this clock probably seemed to maintain the biological rhythms of different species despite different light conditions, depending on geographical region or seasons.
Yet this internal clock adapts to light conditions and we are all able to synchronize our physiological processes with different types of light. This phenomenon is possible thanks to our retinas and the light-absorbing pigments they contain.
A few pointers
Your retina contains neurons that respond to light and its various wavelengths. This allows light information to be transmitted to the areas of the brain responsible for processing it. There are five known neurons: Photoreceptors (cones and rods), bipolar cells, ganglion cells, horizontal cells, and amacrine cells. In this article, we will focus on photoreceptors and ganglion cells.
Rods and cones contain light-sensitive pigments that, through molecular processes and the action of other cell types, convert perceived light information into an electrical message (an action potential) and then into a chemical message (neurotransmitter). This phenomenon is called phototransduction. The rods specialize in processing the nuances of light, while the cones allow us to see colors. However, the melanopsin ganglion cells play a role in the transmission of luminance because they contain a specific photopigment, melanopsin, which, unlike the photopigments in the cones and rods, causes depolarization of the light-sensitive ganglion cells.
Moving away from simplistic assumptions
The phenomenon of light adaptation in the circadian cycle is not a simple one. But in the past, many scientists clung to one-dimensional hypotheses: In the early days of research on these topics, it was thought that cones and rods were responsible for photoemission. When melanopsin was discovered in 1998, there was much excitement and some people assumed that this was the only photopigment involved in the process. Gradually, the hegemony of melanopsin was reconsidered, thanks to experimental evidence showing that in mouse models lacking melanopsin, the adaptive response to light had not been completely abolished.
We now know that rods and cones are involved in this process. However, there are still important questions that need to be answered by researchers. A recent study, published in the journal Proceedings National Academy of Science, has clarified the different roles of certain cone subtypes in the biological clock.
What the recent work shows
This study examined the specific role of cones by analyzing electrophysiological and behavioral responses in two types of mice. In this study, the researchers compared a wild-type mouse model and a genetically modified mouse model in which only the photopigments present in the cones were expressed. The researchers discovered that the photopigments present in the cones were not expressed in the mouse model. They made interesting observations, such as the difference in response of the suprachiasmatic nucleus between the two models and the importance of the presence of cones sensitive to short wavelengths in the photoreceptor, which does not saturate as quickly as previously thought in response to light.