EDITOR'S CHOICE IN NEUROSCIENCE
D.C. Fernandez et al., “Light affects mood and learning through distinct retina-brain pathways,” Cell, 175:71–84.e18, 2018.
Depressive feelings associated with fewer hours of daylight in winter were once considered an indirect consequence of circadian rhythm disruption. But in 2012, chronobiologist Samer Hattar, then of Johns Hopkins University, and colleagues showed that light can boost mood scores—along with learning ability—in mice, even when sleep and circadian rhythms are unperturbed.
THE THIRD CELL
To understand these effects, the researchers looked at recently discovered photoreceptors known as intrinsically photosensitive retinal ganglion cells (ipRGCs), which unlike rods and cones play no role in image formation. “Anatomical data suggested that [the] cells can directly influence several brain areas involved in mood and learning functions,” study coauthor Diego Fernandez of the National Institute of Mental Health, where Hattar now works, writes in an email.
FORK IN THE ROAD
Unexpectedly, transgenic mice with different populations of ipRGCs ablated revealed two independent pathways mediating mood and learning. One set of ipRGCs projected to the suprachiasmatic nucleus, a brain region associated with circadian function—although rhythms were unaffected in the animals. That pathway mediated light’s effects on learning, while cells projecting to the perihabenular nucleus in the thalmus regulated mood. “We were stunned that they are completely dissociable,” Hattar says.
OUT OF THE DARK
The results further support the circadian-clock independence of some of light’s effects, and could illuminate the mechanisms behind neuropsychiatric disorders associated with certain light conditions, says Lily Yan, a neuropsychologist at Michigan State University. Hattar’s team is now keen to understand more about light’s effects, he says. “Why should light enhance your mood?”