We used the original rat cDNA clone 35 to isolate full-length cDNAs for both rat and mouse. The 569-nucleotide rat sequence (de Lecea et al. 1998) suggested that the corresponding mRNA encoded a 130-residue putative secretory protein with four pairs of tandem basic residues for potential proteolytic processing. We then isolated a cDNA clone from mouse libraries and its sequence revealed only that two of the four possible rat maturation products were conserved. Both of these terminated with glycine residues, which in proteolytically processed secretory peptides typically are substrates for peptidylglycine alpha-amidating monooxygenase, leaving a C-terminal amide in the mature peptide. These features suggested that the product of the clone 35 hypothalamic mRNA served as a preprohormone for two C-terminally amidated, secreted peptides. One of these, which was later to be named hypocretin 2 (hcrt2), was, on the basis of the putative preprohormone amino-acid sequence, predicted to contain precisely 28 residues.

The C-terminal 19 residues of these two putative peptides shared 13 amino acid identities. This region of one of the peptides contained a 7/7 match with secretin, suggesting that the preprohormone gave rise to two peptide products that were structurally related closely to each other and distantly to secretin. We initially commented on the secretin similarity. Sequence similarities with various members of the incretin family, especially secretin, suggested that the gene was formed from the secretin gene by three genetic rearrangements: first, a duplication of the secretin gene; second, deletions of the N-terminal portion of the 5′ duplicate and the C-terminal portion of the 3′ duplicate to yield a secretin with its N- and C- termini leap-frogged (circularly permuted); and third, a further duplication of the permuted gene, followed by modifications, to form a secretin derivative that encoded two related hypocretin peptides.

As we began to write the paper describing our discovery of the peptides via cDNA cloning, their immunohistochemical detection, their presence in dense core vesicles at synapses, and their neuroexcitatory properties on hypothalamic neurons, we realized that we needed a name other than clone 35. There were several possible functions for the peptides, but direct evidence for none. Based on our previous discovery of Cortistatin (which we named after its predominantly cortical expression and its similarity with somatostatin), (de Lecea et al. 1996) we came up with several non-pejorative possibilities, most of which were variations on syllables abstracted from hypothalamic member of the incretin family. The most straightforward of these possibilities was “hypocretin”. At the 1997 Society for Neuroscience Meeting, Greg Sutcliffe presented a poster describing the sequence of the new protein, the expression of its mRNA exclusively in a small number of neurons in the dorsolateral hypothalamus, the electron microscopic detection of immunoreactive vesicles in presynaptic boutons, and the neuroexcitatory properties of the amidated hcrt2 peptide. At the adjacent poster, Cristelle Peyron and Tom Kilduff presented the data detecting hypocretin immunoreactivity in the dorsolateral hypothalamic neurons and immunoreactive fibers through the CNS (Peyron et al. 1998).

At the poster, a ballot listing several of the names under consideration was listed and asked poster attendees to express their preference. The plurality of the votes were cast for hypocretin. Thus, this became the first democratically named neurotransmitter.

We sent a reformatted text using the revised name hypocretin to the PNAS, where it was accepted and published on January 6, 1998 (de Lecea et al. 1998). In the paper we noted that the existence of two hcrt peptides that differ in their amino acid sequences might indicate two Hcrt receptor subtypes (de Lecea et al. 1998).

The February 20, 1998 issue of Cell carried an article describing the identification of two endogenous peptides that stimulated calcium flux in cells transfected with an orphan G protein-coupled receptor (Sakurai et al. 1998). Sakurai and colleagues demonstrated that intracerebroventricular administration of either hcrt1 or hcrt2 increased food consumption in rats. Furthermore, rats fasted for 48 h increased the concentration of hypocretin mRNA and peptides. Based on these observations, they proposed the alternative name, orexins, for the hypocretin peptides (Sakurai et al. 1998).

This study demonstrated the actual presence of the two proteolytically processed hypocretin peptides within the brain, and elucidated by mass spectroscopy the exact structures of these endogenous peptides, which could not be deduced from nucleic acid sequences alone. The structure of orexin B was the same as that predicted from the hcrt2 cDNA sequence. The N-terminus of orexin A (hcrt1) was defined and found to correspond to a genetically encoded glutamine derivatized as pyroglutamate. The two intrachain disulfide bonds within hcrt1 were also defined.

A commentary about new hypothalamic factors accompanied the Cell report. It mentioned both the hypocretins and the orexins, without realizing that they were the same peptides. The omission was inadvertent but, unfortunately, this was the beginning of a dual nomenclature. The Yanagisawa group published an addendum in the March 6 Cell indicating that the orexins are the same as the hypocretins (Sakurai et al. 1998).

The papers describing the independent discovery and characterization of the hypocretins/orexins have catalyzed a great body of work. There are now already over 12,000 papers concerning aspects of these neurotransmitters, including their prominent role in arousal state transitions, brain reward, stress, panic and other neuropsychiatric conditions. The perifornical region of the hypothalamus has been associated with nutritional homeostasis, blood pressure and thermal regulation, neural control of endocrine secretion, brain reward and arousal. Thus, these activities ranked among those affected by the peptides (Sakurai 2014; Li et al. 2014).

The association of Hcrt deficiency with narcolepsy conducted in parallel in two different laboratories using, again, two independent approaches, clearly demonstrated that Hcrt system is a non-redundant peptidergic system whose critical function is to stabilize arousal states (de Lecea and Huerta 2014).

How do Hcrt neurons maintain wakefulness? In 2005, the groups of Siegel and Jones independently reported the recording of identified Hcrt neurons from awake animals (Mileykovskiy et al. 2005; Lee et al. 2005). These recordings revealed that Hcrt neurons are phasically active during active waking, and practically silent during quiet waking and sleep. Interestingly, the highest activity was observed during transitions between rapid eye movement sleep and wakefulness. This discovery also showed that the pharmacological approach to studying the role of Hcrt in sleep and wakefulness was not appropriate, as drugs take several minutes to diffuse into brain structures and therefore span several sleep/wake cycles in rodents. In order to determine precisely whether Hcrt release was instructive or permissive to wakefulness, other methodologies needed to be applied.

To further explore the mechanism by which phasic Hcrt activity maintains wakefulness, we used a newly developed optogenetic approach to manipulate the activity of Hcrt neurons in vivo (Adamantidis et al. 2007). Optogenetics uses actuator opsin molecules (e.g., channelrhodopsin-2 (ChR2) or halorhodopsin—NpHR) to selectively activate or silence genetically-targeted cells, respectively, with flashes of light at specific wavelength (Boyden et al. 2005). Further information about optogenetic technology can be found in many other excellent reviews (Fenno et al. 2011).

In an effort to better understand the temporal dynamics of neural circuits of wakefulness, we applied optogenetics to reversibly and selectively manipulate the activity of hcrt neurons in freely-moving animals. To deliver light to the hcrt or LC field, we designed optical-neural interfaces in which optical fibers were chronically implanted on the mouse skull (Fig. 2). Using this strategy, we were able to control hcrt neural activity both in vitro and in vivo with millisecond-precise optical stimulation (Adamantidis et al. 2007). The high temporal and spatial precision of stimulation allowed us to mimic the physiological range of hypocretin neu ron discharge rates (1–30 Hz) (Lee et al. 2005; Hassani et al. 2009).