Rewrite the textbook! Today Science discovers a new pathway to "from the eye to the brain."

For decades, biology textbooks have told us that our eyes send signals to the brain, and these signals are all "excitable.".and in today's latest issue of science, a research team from Northwestern University in the United States has provided practical evidence and found a "inhibitory" signaling pathway.this also gives us a new understanding of retinal ganglion cells.scientists know that retinal ganglion cells (RGCs) play an important role in vision.in fact, these cells can transmit light signals to more than 40 different brain regions, producing conscious visual perception, or performing some subconscious function (such as adjusting pupil size and biological rhythm).in general, these functions depend on the release of excitatory neurotransmitter glutamate by retinal ganglion cells at synapses.however, some previous studies have suggested that some retinal ganglion cells that release the "inhibitory" neurotransmitter gamma aminobutyric acid (GABA) may also exist in some mammals.to find clues to these cells, the researchers designed a clever experiment.they co expressed the enzyme responsible for GABA synthesis with Cre recombinase, and then expressed CRE dependent fluorescent protein in the single eye of mice.since only these ganglion cells can map to the brain in the retina, any axon that emits fluorescent signals represents the existence of retinal ganglion cells that produce GABA.} mouse retina: blue is the nucleus, purplish red is the inhibitory cell, and green is iprgc (photo source: reference [2]). Using this technology, scientists found the axons of these retinal ganglion cells, and found that they mostly exist in the "non imaging area" of the brain.as the name suggests, this region is not involved in visual imaging, but is related to the regulation of circadian rhythm.based on previous data, the researchers also found that "intrinsic photosensitive retinal ganglion cells" (iprgc) are the dominant retinal ganglion cells in these brain regions.so, is the fluorescence signal observed by the researchers really come from iprgc? A series of subsequent studies have shown that this is the case and confirmed that these cells release GABA.the next question is, what is the use of these iprgcs? Scientists suppressed the function of these cells and found that the circadian rhythm of the model mice became more sensitive to weak light."these inhibitory signals prevent our circadian clock from resetting in low light conditions and also prevent the pupil from contracting under low light conditions," co-author of this study, tiffany Professor Schmidt said: "I think our research provides a mechanistic explanation for why the eyes are so sensitive to light, but subconscious behavior is relatively less sensitive to light."this mechanism also has certain significance for organisms.when the environment becomes dark for a short time (such as entering a dark room), we certainly don't want to make big adjustments to the body's biological clock.only when there is really a lasting change of light (such as the alternation of day and night), do we expect the biological clock to respond to it.the researchers mentioned at the end of the paper that their study found a GABAergic circuit starting from the retina, which can reduce the sensitivity of non imaging areas of the brain to weak light, thus reducing the impact on biological rhythms.this also prevents the body's biological clock from making too many unnecessary adjustments.References: [1] Takuma Sonoda et al., (2020), a noncanical entrepreneurial circuit damp, behavioral sensitivity to light, science, DOI: 10.1126/ science.aay3152 [2] Eyes send an unexpected signal to the brain, Retrieved April 30, 2020, from