A Hopkins research team led by Samer Hattar, of the Department of Biology, recently conducted a study on the specific contributions of rods, light-sensitive cells found in the retina, to the signaling pathway for circadian photoentrainment, the process by which our bodies synchronize to the solar day.
Photoreceptors, which include both rods and cones, are not only responsible for the transduction of visual stimuli, thus allowing us to perceive the shapes and forms of the world around us, but also play a critical role in photoentrainment. Photoentrainment controls our internal clock to regulate our biological rhythms such as sleep and hormone regulation.
When light enters the eye and hits the retina, which is found at the back of the eyeball, rod cells are hyperpolarized, and ions flow out of the cells. This activates the phototransduction pathway and eventually leads to the activation of ganglion cells, which have axons that lead directly to the central nervous system.
While the retinal ganglion cells send information to the lateral geniculate nucleus, the intrinsically photosensitive retinal ganglion cells (ipRGC), which are also located in the retina but further from the surface, project to the suprachiasmatic nucleus (SCN), which is responsible for setting the circadian rhythm.
Unlike other retinal ganglion cells, ipRGCs can be activated directly by light, since ipRGCs contain melanopsin, a photopigment capable of capturing photons and activating the signal transduction pathway. However, melanopsin itself is relatively insensitive, therefore requiring rod and cone contributions.
Although it has been known that ipRGCs can fully account for non-image forming visual functions, such as pupil constriction and circadian rhythm maintenance, the relative contributions of rods and cones have been unclear.
This study used several lines of transgenic mice to isolate a specific group of photoreceptors in order to observe their precise effect on photoentrainment: The researchers engineered transgenic mice who lacked solely the rods pathway or solely the cones pathway, without the degradation of other retinal functions.
Results suggest that rods play a role in driving photoentrainment across a wide range of light intensities, including high intensities at which rods would not contribute to image-forming function.
It was observed that rods accomplish this via two distinct circuits. At low intensities, rods send their input through the rod bipolar pathway. At high intensities, however, rod-signaling via cone photoreceptors is necessary for photoentrainment.
Comparison across rod-defective and cone-defective mice models also indicates that rods are the major contributors to circadian responses. Even mice with highly attenuated rod function demonstrated better photoentrainment than mice with fully functional cones who lacked rods altogether.
Extending results of this finding to humans, Hattar points out the significance of daily light exposure to maintain healthy sleep-wake cycles and other biological rhythms.
Though it is a well-established fact that staying awake into the wee hours of the morning and sleeping until 3 o’clock in the afternoon is not the most beneficial habit, this study gives one additional reason to turn out the lights earlier.
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