Micropipette recording with juxtacellular Neurobiotin ejection, linked micropipette-microwire recording, and antidromic and orthodromic activation from the ventral tegmental area and locus coeruleus were used to identify hypocretin (Hcrt) cells in anesthetized rats and develop criteria for identification of these cells in unanesthetized, unrestrained animals. We found that Hcrt cells have broad action potentials with elongated later positive deflections that distinguish them from adjacent antidromically identified cells. They are relatively inactive in quiet waking but are transiently activated during sensory stimulation. Hcrt cells are silent in slow wave sleep and tonic periods of REM sleep, with occasional burst discharge in phasic REM. Hcrt cells discharge in active waking and have moderate and approximately equal levels of activity during grooming and eating and maximal activity during exploratory behavior. Our findings suggest that these cells are activated during emotional and sensorimotor conditions similar to those that trigger cataplexy in narcoleptic animals.

Hypocretin (Hcrt or orexin) somas are located in the hypothalamus and project widely to forebrain and brainstem regions, densely innervating monoaminergic and cholinergic cells. Loss of Hcrt function results in the sleep disorder narcolepsy. However, the normal pattern of Hcrt release across the sleep-wake cycle is unknown. We monitored Hcrt-1 release in the basal forebrain, perifornical hypothalamus, and locus ceruleus (LC) across the sleep-wake cycle using microdialysis in freely moving cats and a sensitive solid phase radioimmunoassay. We found that the peptide concentration in dialysates from the hypothalamus was significantly higher during active waking (AW) than during slow-wave sleep (SWS). Moreover, Hcrt-1 release was significantly higher during rapid eye movement (REM) sleep than during SWS in the hypothalamus and basal forebrain. We did not detect a significant difference in release across sleep-waking states in the LC, perhaps because recovered levels of the peptide were lower at this site. Because there was a trend toward higher levels of Hcrt-1 release during AW compared with quiet waking (QW) in our 10 min dialysis samples, we compared Hcrt-1 levels in CSF in 2 hr AW and QW periods. Hcrt-1 release into CSF was 67% higher during AW than during QW. Elevated levels of Hcrt during REM sleep and AW are consistent with a role for Hcrt in the central programming of motor activity.