The cells necessary for a light-induced arousal response are located in the hypothalamus, says a new study, thus reinforcing the perception that light and darkness governs the human cycle of sleep and waking up.
Jerome Siegel and his colleagues from the University of California, Los Angeles have identified the group of neurons that mediates whether light arouses us and said that these cells release a neurotransmitter called hypocretin.
For the study, the researchers examined the behavioural 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.
"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," Siegel said.
They tested the two groups while they performed a variety of tasks during both light and dark phases and found that the KO mice were only deficient at working for positive rewards during the light phase.
During the dark phase these mice learned at the same rate as their WT littermates and were completely unimpaired in working for the same rewards.
"The findings suggest that administering hypocretin and boosting the function of hypocretin cells will increase the light-induced arousal response.
"Conversely, blocking their function by administering hypocretin receptor blockers will reduce this response and thereby induce sleep," he added.
The study has been recently published in online edition of the Journal of Neuroscience.