A group of brain neurons have been identified in mice that are influenced by light or darkness, according to new research.
Jerome Siegel, a professor of psychiatry at UCLA, and colleagues discovered the cells necessary for a light-induced arousal response are located in the hypothalamus, an area at the base of the brain responsible for, among other things, control of the autonomic nervous system, body temperature, hunger, thirst, fatigue — and sleep.
Their findings are published in the online edition of the Journal of Neuroscience.
Siegel’s team discovered the cells release a neurotransmitter called hypocretin. In the lab, researchers compared mice with and without hypocretin and found that those who didn’t have it were unable to stay awake in the light, while those who had it showed intense activation of these cells in the light but not while they were awake in the dark.
Prior research has found that a loss of hypocretin is responsible for narcolepsy and the sleepiness associated with Parkinson’s disease. But the neurotransmitter’s role in normal behavior was, until now, unclear.
“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.
In the study, researchers examined the behavioral capabilities of mice that had their hypocretin genetically “knocked-out” (KO mice) and compared them with the activities of normal, wild-type mice (WT) that still had their hypocretin neurons.
The researchers tested the two groups while they performed a variety of tasks during both light and dark phases.
“The findings suggest that administering hypocretin and boosting the function of hypocretin cells will increase the light-induced arousal response,” Siegel said. “Conversely, blocking their function by administering hypocretin receptor blockers will reduce this response and thereby induce sleep.”
Thus, the research suggests a potential menchanism to increase arousal or induce sleep.
Siegel continues, “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.”