The hypocretins (hcrts), also known as orexins, are two recently identified excitatory neuropeptides that in rat are produced by ϳ1200 neurons whose cell bodies are located in the lateral hypothalamus. The hypocretins/orexins have been implicated in the regulation of rapid eye movement (REM) sleep and the pathophysiology of narcolepsy. In the present study, we investigated whether the locus coeruleus (LC), a structure receiving dense hcrtergic innervation, which is quiescent during REM sleep, might be a target for hcrt to regulate REM sleep. Local administration of hcrt1 but not hcrt2 in the LC suppressed REM sleep in a dose-dependent manner and increased wakefulness at the expense of deep, slow-wave sleep. These effects were blocked with an antibody that neutralizes hcrt binding to hcrt receptor 1. In situ hybridization and immunocytochemistry showed the presence of hcrt receptor 1 but not the presence of hcrt receptor 2 in the LC. Iontophoretic application of hcrt1 enhanced the firing rate of LC neurons in vivo, and local injection of hcrt1 into the LC induced the expression of c-fos in the LC area. We propose that hcrt receptor 1 in the LC is a key target for REM sleep regulation and might be involved in the pathophysiological mechanisms of narcolepsy. Key words: norepinephrine; orexin; orexin receptors; c-fos; arousal; microinjection; immunocytochemistryThe hypocretins (hcrt1 and hcrt2), also called orexins, are two neuropeptides derived from the same precursor, which are expressed in a small set of neurons in the perifornical area of the hypothalamus Sakurai et al., 1998). The hypocretins are neuroexcitatory and bind to two different G-protein-coupled receptors, hcrt receptors 1 and 2 (hcrtr1 and hcrtr2, also known as OX1 and OX2 receptors) with different affinities (Sakurai et al., 1998). Recently, evidence has emerged that confirms a role for the hypocretins in arousal states. Lin et al. (1999) mapped the canine narcolepsy mutation (canarc-1) to hcrtr2. Knock-out experiments in mice demonstrated that the absence of hypocretin causes alterations in sleep architecture, particularly on the amount of rapid eye movement (REM) sleep during the dark period (Chemelli et al., 1999). In addition, hcrt-deficient mice display electroencephalographic patterns and behaviors that resemble those of narcoleptic attacks. Intracerebroventricular infusion of nanomolar amounts of hypocretin has recently been shown to increase arousal, reduce REM sleep, and affect neuroendocrine balance (Hagan et al., 1999). Nishino and colleagues (2000) found that seven of nine patients with narcolepsy had undetectable hcrt1 in CSF. These independent studies indicate that the hypocretins have a major role in the regulation of sleep, but the role of different brain structures and the contribution of each of the hcrt receptors remain unknown.The projections of hcrt-containing neurons extend widely throughout the brain Date et al., 1999). Four main hcrtergic afferent regions can be recognized from anatomical studies : an intrahypothalamic field...

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Analyses in mice deficient for the blue-light-sensitive photopigment melanopsin show that direct effects of light on behavior and EEG depend on the time of day. The data further suggest an unexpected role for melanopsin in sleep homeostasis.