The discovery of hypocretin (Hcrt)/orexin in 1998 and its link to human narcolepsy in 2001 has led to the publication of more than 1,100 studies of its anatomical distribution, physiology and role in pathology. Conspicuously absent in these publications were any studies of Hcrt cell (unit) firing, or Hcrt release, in relation to behavior. Such studies are an essential step in gaining an understanding of the functioning of this system. During the last cycle of this grant we have solved the problem of how to identify Hcrt units in the freely moving animal. We have also developed highly sensitive RIA techniques that allowed, for the first time, measurement of Hcrt release over individual sleep and waking states. We propose to use these findings and techniques to address key issues concerning the function of Hcrt neurons. We will determine the effects of food deprivation, eating, and glucose and insulin administration on Hcrt unit activity in freely behaving rats to test the hypothesis that these neurons are """"""""orexigenic."""""""" We will investigate our hypothesis that Hcrt cell discharge in rodents is primarily related to motivated motor activity in a series of studies. We will use operant reinforcement in rats to produce high or low firing rates in Hcrt units to provide a novel and objective assessment of the behavioral correlates of their activity. We will use classical and operant conditioning to test our hypothesis that their activity is increased during reward anticipation and decreased during anxiety. We will test our hypothesis that animals without Hcrt are impaired in their ability to work for reward, but are not impaired in avoidance tasks. We will identify the neurotransmitters responsible for the changes in Hcrt unit activity across the sleep-waking cycle with reverse microdialysis. Unit recording studies and the well established species-specific nature of cataplexy indicate that the behavioral correlates of Hcrt cell activity may vary substantially across species. Accordingly, we will take advantage of a unique opportunity to directly measure Hcrt release in human subjects. We will measure Hcrt release in these subjects during a range of emotions and behaviors by microdialysis studies of temporal and frontal lobe regions. The proposed studies should clarify the nature of neurotransmitter control of Hcrt neurons, their involvement in narcolepsy and depression, and their normal physiological and behavioral roles in rodents and humans.
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