WESTWOOD—Scientists at UCLA have recently discovered a group of neurons that dictates if light stimulates humans or not. In the current online edition of the Journal of Neuroscience, Jerome Siegel, professor of psychiatry with the Semel Institute for Neuroscience at UCLA, along with colleagues, report that the cells required for a light-induced response are located in the hypothalamus, an area at the base of the human brain that is in charge of the autonomic nervous system, body temperature, hunger, thirst, fatigue as well as sleep.

Semel Institute for Neuroscience. Photo courtesy of UCLA.

According to UCLA’s website, Siegel says that these cells release a neurotransmitter called hypocretin. The team observed mice both with and without hypocretin and discovered that the mice that did not have hypocretin were not able to stay awake in the light, compared with those with the neurotransmitter, which showed elevated activity in these cells in the light, but not while they were awake in the dark.

The group doing this research previously found that a loss in hypocretin resulted in narcolepsy and the fatigue associated with Parkinson’s disease. The role of hypocretin in normal everyday behavior was previously  unknown.

"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," said Siegel, who is also a member of the UCLA Brain Research Institute and chief of neurobiology research at the Sepulveda Veterans Affairs Medical Center in Mission Hills, Calif.

Previous studies of hypocretin in mice had only analyzed its use in light phases, or normal sleep time for mice, or dark phases, or normal wake time, but never both. Also previous studies only researched the rodents while performing a single task. In this study, however, the team examined the behavior of mice that had the neurotransmitter genetically removed (KO mice), and contrasted those behaviors with that of normal mice or wild-type (WT) that still had hypocretin.

It was discovered that the KO mice only showed deficiencies in working for rewards during the light phase. In the dark phase, the KO mice showed no difference in learning when compared to WT mice.

Researchers also found that keeping in line with the KO mice, the hypocretin in the WT mice was maximized while working for rewards in the light phase, but cells were not active performing the same tasks in the dark phase.

According to Siegel, "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."

The study was supported by the National Institutes of Health and the Medical Research Service of the Department of Veteran Affairs.

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