Melanopsin will help to adjust the biological clock
The retina of the mammalian eye contains rods and cones – photosensitive cells that are a kind of "sensors" necessary to obtain an image of the surrounding world. Only narrow specialists know about the third type of sensors – cells containing the photochromic pigment melanopsin. These cells do not participate in the formation and transmission of images to the brain, but assess the intensity of light falling on the retina and adjust the circadian (daily) biological clock of the body accordingly.
Visual information processing begins with photons hitting one or more of the 125 million photosensitive retinal cells. Sticks use the pigment rhodopsin to register dim light, and cones use a complex of pigments to distinguish colors.
This inner layer of cells converts the received signals into electrical impulses and transmits them to the cells of the middle layer, which, in turn, send them to the ganglion cells, the processes of which form the optic nerve.
Melanopsin differs from the enzymes of rods and cones that provide visual images. It acts as a photometer (a device for measuring illumination) and, in addition to adjusting the biological clock, regulates the pupil diameter, melatonin synthesis and physical activity of the body.
Melanopsin is synthesized by approximately 2000 specialized retinal ganglion cells (in the diagram, such a cell is highlighted in black), which send signals directly to the human biological clock – a cluster of cells located above the intersection of optical nerves. Through these signals, the body synchronizes its circadian rhythms in accordance with sunrise and sunset.
The absence of melanopsin does not lead to the loss of sensitivity to light by animals; conversely, blind mice without rods and cones retain the ability to adjust their biological clock, pupil diameter and light-dependent activity with the help of melanopsin-expressing cells.
Scientists at the Salk Institute for Biological Research (San Diego) have suggested that mice without melanopsin may trigger compensatory mechanisms during development that can affect the results of the study. To avoid this effect, they developed a system that specifically and effectively eliminates melanopsin-expressing cells in adult mice without damaging the retina. The researchers created genetically modified mice with melanopsin-expressing cells sensitive to diphtheria toxin. Treatment of the retina of 8-week-old mice with this toxin did not affect their visual abilities at all, but completely deprived the biological clock of the animals of sensitivity to light.
The authors believe that melanopsin-expressing cells are unique targets for therapeutic intervention in disorders associated with circadian rhythm disorders, including symptoms caused by flights across several time zones, shift work, seasonal changes in daylight hours, as well as insomnia and depression. They have already started screening small molecules in order to identify compounds that can change the sensitivity of melanopsin to light and thus control the tuning of the biological clock.
Portal "Eternal youth" www.vechnayamolodost.ru based on the materials of ScienceDaily
17.06.2008