Summary: A new study sheds insight into how mice see in color. The findings could have implications for treating human visual disorders.
Source: University of Colorado Anschutz Medical Campus.
Researchers at the University of Colorado Anschutz Medical Campus have discovered that color vision in mice is far more complex than originally thought, opening the door to experiments that could potentially lead to new treatments for humans.
The study was published this week in the journal Neuron.
The scientists, led by Maureen Stabio, PhD, assistant professor of anatomy and neurobiology at the University of Colorado School of Medicine, discovered a new property of a little understood cell called the M5.
They knew that mice possessed light-sensitive proteins called opsins that allowed them to detect a limited range of colors. But as they investigated the role M5 cells played in this, Stabio discovered that the mice also had neurons that could compare signals from the different opsins and then send those color signals to the brain for interpretation.
“We are the first to discover this particular color vision circuit in mice,” Stabio said. “We knew they had opsins but we didn’t know they possessed the other two requirements for color vision.”
Stabio’s work focuses primarily on the cells and circuits of the retina, including a group called intrinsically photosensitive retinal ganglion cells or ipRGCs which includes the M5. These cells are primarily involved in a kind of vision known as non-image forming vision.
IpRGCs typically don’t process contrast, color, faces or art. Instead, they react to ambient light levels and send that information to the brains internal clock to put the body in synch with the rising and setting of the sun (aka the circadian rhythms).
Stabio and her colleagues found that the M5 cell, the least understood of the group, might play a role in both image and non-image forming vision.
“This adds to growing evidence that image forming and non-image forming pigments, cell types, and circuits are not as distinct as once imagined,” she said. “The two appear to be intersecting.”
But the biggest surprise was discovering that the M5 cells also process color information in mice. Mice are nocturnal and generally have poor vision. They navigate chiefly by using their nose and whiskers.
“What exactly they are doing with this color information remains to be discovered, but we know now there is a circuit for it and it’s getting to the mouse’s brain,” Stabio said.
Source: David Kelly – University of Colorado Anschutz Medical Campus
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Original Research: Abstract for “The M5 Cell: A Color-Opponent Intrinsically Photosensitive Retinal Ganglion Cell” by Maureen E. Stabio, Shai Sabbah5, Lauren E. Quattrochi, Marissa C. Ilardi, P. Michelle Fogerson, Megan L. Leyrer, Jordan M. Renna, Min Tae Kim, Inkyu Kim, Matthew Schiel, Kevin L. Briggman, and David M. Bersonin Neuron. Published online December 14 2017 doi:10.1016/j.neuron.2017.11.030
The M5 Cell: A Color-Opponent Intrinsically Photosensitive Retinal Ganglion Cell
•M5 cells are a morphologically and functionally distinct unique ipRGC type
•They have both melanopsin responses and chromatically opponent cone-based signals
•They receive color-opponent signal (UV-ON, green-OFF) via Types 6–9 bipolar cells
•M5 cells innervate the dorsal lateral geniculate nucleus (dLGN)
Intrinsically photosensitive retinal ganglion cells (ipRGCs) combine direct photosensitivity through melanopsin with synaptically mediated drive from classical photoreceptors through bipolar-cell input. Here, we sought to provide a fuller description of the least understood ipRGC type, the M5 cell, and discovered a distinctive functional characteristic—chromatic opponency (ultraviolet excitatory, green inhibitory). Serial electron microscopic reconstructions revealed that M5 cells receive selective UV-opsin drive from Type 9 cone bipolar cells but also mixed cone signals from bipolar Types 6, 7, and 8. Recordings suggest that both excitation and inhibition are driven by the ON channel and that chromatic opponency results from M-cone-driven surround inhibition mediated by wide-field spiking GABAergic amacrine cells. We show that M5 cells send axons to the dLGN and are thus positioned to provide chromatic signals to visual cortex. These findings underscore that melanopsin’s influence extends beyond unconscious reflex functions to encompass cortical vision, perhaps including the perception of color.
“The M5 Cell: A Color-Opponent Intrinsically Photosensitive Retinal Ganglion Cell” by Maureen E. Stabio, Shai Sabbah5, Lauren E. Quattrochi, Marissa C. Ilardi, P. Michelle Fogerson, Megan L. Leyrer, Jordan M. Renna, Min Tae Kim, Inkyu Kim, Matthew Schiel, Kevin L. Briggman, and David M. Bersonin Neuron. Published online December 14 2017 doi:10.1016/j.neuron.2017.11.030