A natural inbuilt timer, the circadian rhythm is a 24-hour cycle in all humans, plants, and animals determining sleep and feeding patterns. Circadian rhythms are endogenously generated and can be modulated by external cues such as sunlight or temperature. Nearly all mammalian tissues have functional, autonomous circadian clocks, which must be synchronized to the 24-hour day.
This synchronization, also known as entrainment, is hierarchal in mammals and located in the suprachiasmatic nucleus, a pair of distinct groups of cells located in the hypothalamus in the brain.
Photoreceptors in the skin
Now, a study from researchers at the University of Washington shows a type of opsin protein, a photoreceptor found in the eye, is expressed in the skin of mice, and synchronizes the skin’s circadian clock to the light-dark cycle, independent of the eyes or brain. The team states this is the first functional demonstration of opsin photoreceptors outside the eye directly controlling circadian rhythms in a mammal. The opensource study is published in the journal Current Biology.
Previous studies show squids, octopuses, cuttlefish, amphibians, and chameleon lizards can change the color of their skin. They have photoreceptors, proteins known as opsins, in their skin that operate independently of their brain.
Mammals have opsins in the retina which are responsible for color vision and vision in dim light. Researchers have suggested mammals might express opsin proteins outside the eye, however, there is limited data on what functions they might influence. The current study shows skin expresses its own photoreceptors using a member of the opsin gene family, known as neuroposin.
The current study utilizes mice with no retinal photoreceptors and a nonfunctional melanopsin gene. Normally, retinal photoreceptors combine with melanopsin to inform the brain’s circadian clock whether it is light or dark.
Thus experimental mice were unable to synchronize their behavior to the light-dark cycle using the brain. The mice were entrained to a 12-hour light/12-hour dark cycle for at least 3 weeks and then lights were turned off for 2 days of constant darkness. Results show the circadian rhythms of these animals’ skin remained synchronized to the local light-dark cycle. Some of the mice in the experiment woke up at dawn, and some mice woke up at dusk.
Separate skin circadian rhythm
The lab states as it has been shown the skin actually possesses its own autonomous circadian rhythm independent from the brain. Consequently, this means skin can sense whether it is day or night even when it’s cultured by itself in a dish. They go on to explain this has far-reaching implications for cancer and wound care.
The team surmises their data shows the skin use neuropsin to sense the light-dark cycle and keep time independently of the brain or eyes. For the future, the researchers hypothesize specific colors of light at certain times of day will influence how the skin heals.
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