We describe a hypothalamus-specific mRNA that encodes preprohypocretin, the putative precursor of a pair of peptides that share substantial amino acid identities with the gut hormone secretin. The hypocretin (Hcrt) protein products are restricted to neuronal cell bodies of the dorsal and lateral hypothalamic areas. The fibers of these neurons are widespread throughout the posterior hypothalamus and project to multiple targets in other areas, including brainstem and thalamus. Hcrt immunoreactivity is associated with large granular vesicles at synapses. One of the Hcrt peptides was excitatory when applied to cultured, synaptically coupled hypothalamic neurons, but not hippocampal neurons. These observations suggest that the hypocretins function within the CNS as neurotransmitters.

The novel neuropeptides called hypocretins (orexins) have recently been identified as being localized exclusively in cell bodies in a subregion of the tuberal part of the hypothalamus. The structure of the hypocretins, their accumulation in vesicles of axon terminals, and their excitatory effect on cultured hypothalamic neurons suggest that the hypocretins function in intercellular communication. To characterize these peptides further and to help understand what physiological functions they may serve, we undertook an immunohistochemical study to examine the distribution of preprohypocretin-immunoreactive neurons and fibers in the rat brain. Preprohypocretin-positive neurons were found in the perifornical nucleus and in the dorsal and lateral hypothalamic areas. These cells were distinct from those that express melanin-concentrating hormone. Although they represent a restricted group of cells, their projections were widely distributed in the brain. We observed labeled fibers throughout the hypothalamus. The densest extrahypothalamic projection was found in the locus coeruleus. Fibers were also seen in the septal nuclei, the bed nucleus of the stria terminalis, the paraventricular and reuniens nuclei of the thalamus, the zona incerta, the subthalamic nucleus, the central gray, the substantia nigra, the raphe nuclei, the parabrachial area, the medullary reticular formation, and the nucleus of the solitary tract. Less prominent projections were found in cortical regions, central and anterior amygdaloid nuclei, and the olfactory bulb. These results suggest that hypocretins are likely to have a role in physiological functions in addition to food intake such as regulation of blood pressure, the neuroendocrine system, body temperature, and the sleep–waking cycle.

A new orexigenic peptide called hypocretin (orexin) has recently been described in neurons of the lateral hypothalamus and perifornical area. The medial and lateral hypothalamus have been loosely called satiety and feeding centers of the brain, respectively. Approximately one-third of all medial and lateral hypothalamic neurons tested, but not hippocampal neurons, show a striking nanomolar sensitivity to hypocretin. As studied with calcium digital imaging with fura-2, hypocretin raises cytoplasmic calcium via a mechanism based on G-protein enhancement of calcium influx through plasma membrane channels. The peptide has a potent effect at both presynaptic and postsynaptic receptors. Most synaptic activity in hypothalamic circuits is attributable to axonal release of GABA or glutamate. With whole-cell patch-clamp recording, we show that hypocretin, acting directly at axon terminals, can increase the release of each of these amino acid transmitters. Two hypocretin peptides, hypocretin-1 and hypocretin-2, are coded by a single gene; neurons that respond to one peptide also respond to the other. In addition to its effect on feeding, we find that this peptide also regulates the synaptic activity of physiologically identified neuroendocrine neurons studied in hypothalamic slices containing the arcuate nucleus, suggesting a second function of hypocretin in hormone regulation. The widespread distribution of hypocretin axons, coupled with the strong response to the peptide at both presynaptic and postsynaptic sites, suggests that the peptide probably modulates a variety of hypothalamic regulatory systems and could regulate the axonal input to these regions presynaptically.

The hypothalamic neuropeptides hypocretins (orexins) play a crucial role in the stability of arousal and alertness. We tested whether the hypocretinergic system is a critical component of the stress response activated by the corticotropin-releasing factor (CRF). Our results show that CRF-immunoreactive terminals make direct contact with hypocretin-expressing neurons in the lateral hypothalamus and that numerous hypocretinergic neurons express the CRF-R1/2 receptors. We also demonstrate that application of CRF to hypothalamic slices containing identified hypocretin neurons depolarizes membrane potential and increases firing rate in a subpopulation of hypocretinergic cells. CRF-induced depolarization was tetrodotoxin insensitive and was blocked by the peptidergic CRF-R1 antagonist astressin. Moreover, activation of hypocretinergic neurons in response to acute stress was severely impaired in CRF-R1 knock-out mice. Together, our data provide evidence of a direct neuroanatomical and physiological input from CRF peptidergic system onto hypocretin neurons. We propose that, after stressor stimuli, CRF stimulates the release of hypocretins and that this circuit contributes to activation and maintenance of arousal associated with the stress response.

The molecules originally described as the hypocretins and subsequently as the orexins were initially implicated in the control of food intake. Recent observations implicate this newly-described neurotransmitter system in the sleep disorder narcolepsy and, potentially, in the regulation of normal sleep processes. This article reviews the research that led to the isolation of the hypocretin/orexin peptides, their receptors and the activity of these molecules as we currently understand them. A model is proposed in which the cells that make these peptides might be involved in arousal state control.