Step-by-step explanation:
The hypocretins (orexins) are recently described hypothalamic neuropeptides thought to have an important role in the regulation of sleep and arousal states1. Their discovery was reported independently by two groups using different techniques. de Lecea et al.2 identified the pro-hormone pre-prohypocretin, and its peptide products hypocretin-1 (Hcrt-1) and hypocretin-2 (Hcrt-2), by nucleotide sequencing. The discovery of the orexins, orexin-A (Orx-A) and orexin-B (Orx-B), was reported almost simultaneously by Sakurai et al.3 who used the technique of orphan receptor cloning. The terms orexin and hypocretin are synonymous and in this article we will use hypocretin (Hcrt). The finding that cerebrospinal fluid (CSF) levels of these peptides were abnormal in patients with narcolepsy has stimulated research on the potential role of these peptides in human disease. We present here an overview of the pertinent findings from animal studies and a review of the published data from human studies, with a particular emphasis on narcolepsy. Finally, we consider the possible roles of these peptides in neurological and psychiatric disorders.