US scientists have found a group of brain cells that controls whether light arouses us (or not). They suggest the cells rely on a neurotransmitter to tell them whether they should be active or not in response to light. You can read about their study in the 26 October issue of The Journal of Neuroscience.

Bright light wakes us up, and makes it easier to stay awake. In fact as well as arousing us, very bright light is also known to act as an antidepressant. On the other hand, darkness makes us feel sleepy: it’s one reason people wear masks at bedtime, to help them get to sleep and not be woken up if a light goes on.

Now researchers at the University of California Los Angeles (UCLA) have identified a group of neurons that arouse us when there is light.

The light-sensitive cells in question are found in the hypothalamus, an area at the base of the brain responsible for sleep and also other things like controlling the autonomic nervous system, body temperature, hunger, thirst, and fatigue.

Senior author Jerome Siegel, a professor of psychiatry at the Semel Institute for Neuroscience and Human Behavior at UCLA, and colleagues, found that the cells release hypocretin, a neurotransmitter, that appears to play a key role in the process of keeping us awake.

Mice that didn’t have hypocretin couldn’t stay awake in the light, while the light-sensitive brain cells of those that did were active in the light, but inactive in the dark when the mice were awake.

The team had earlier found that narcolepsy, a sleep disorder that causes excessive sleepiness and frequent sleep attacks during the day, and the sleepiness associated with Parkinson’s disease, were both caused by loss of hypocretin, but it wasn’t clear if the neurotransmitter also played a role in normal everyday function.

Siegel, who is also chief of neurobiology research at the Sepulveda Veterans Affairs Medical Center in Mission Hills, California, and a member of UCLA’s Brain Research Institute, told the press:

“This current finding explains prior work in humans that found that narcoleptics lack the arousing response to light, unlike other equally sleepy individuals, and that both narcoleptics and Parkinson’s patients have an increased tendency to be depressed compared to others with chronic illnesses.”

While previous studies had explored how hypocretin behaves in rodents, none had looked at both light and dark phases together (mice normally sleep when it’s light and are active when it’s dark). Also, those studies only had the rodents doing one task.

For this study, Siegel and colleagues compared two groups of mice: one had their hypocretin genetically “knocked-out” (KO mice), and the other did not, these were the normal, or wild-type (WT) mice.

They then observed the two groups during both light and dark phases, while they carried out a range of tasks.

To their suprise, they found that the knockout (KO) mice, only had problems working for positive rewards during the light phase: their learning was impaired.

The researchers said that the hypocretin in the light-sensitive neurons of the WT mice ensured they were able to work maximally for positive rewards during the light phase, unlike their KO littermates.

But when it was dark, the KO mice learned at the same rate as their WT counterparts. This was because hypocretin only works in response to light, so the KO mice were not at a disadvantage.

Siegel said their findings suggest boosting the light-sensitive neurons with hypocretin, for instance by administering it, increases the light-induced arousal response. And conversely, blocking the action of hypocretin on the cells, for instance by administering receptor blockers, induces sleep.

“The administration of hypocretin may also have antidepressant properties, and blocking it may increase tendencies toward depression. So we feel this work has implications for treating sleep disorders as well as depression,” he added.

Written by Catharine Paddock PhD