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McGregor R, Wu MF, Thannickal TC, Siegel JM. Opiate anticipation, opiate induced anatomical changes in hypocretin (Hcrt, orexin) neurons and opiate induced microglial activation are blocked by the dual Hcrt receptor antagonist suvorexant, while opiate analgesia is maintained. BIORXIV : THE PREPRINT SERVER FOR BIOLOGY 2024:2023.09.22.559044. [PMID: 37790444 PMCID: PMC10542511 DOI: 10.1101/2023.09.22.559044] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 10/05/2023]
Abstract
We previously found that heroin addiction in humans is accompanied by an increase in the number of detected Hcrt neurons and a decrease in their soma size. We now show that the increased number of Hcrt cells visible after morphine treatment is likely the result of increased Hcrt production in neurons having sub-detection levels of the peptides. We find that morphine increases Hcrt projections to the ventral tegmental area (VTA), the level of tyrosine hydroxylase enzyme (TH) and the number of TH positive cells in VTA, with no changes in the adjacent substantia nigra. We find that the dual Hcrt receptor antagonist suvorexant prevents morphine-induced changes in the number and size of Hcrt neurons, microglial activation and morphine anticipatory behavior, but does not diminish morphine analgesia. These findings suggest that combined administration of opiates and suvorexant may be a less addictive way of administering opiates for pain relief in humans.
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McGregor R, Wu MF, Holmes B, Lam HA, Maidment NT, Gera J, Yamanaka A, Siegel JM. Hypocretin/Orexin Interactions with Norepinephrine Contribute to the Opiate Withdrawal Syndrome. J Neurosci 2022; 42:255-263. [PMID: 34853083 PMCID: PMC8802943 DOI: 10.1523/jneurosci.1557-21.2021] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/30/2021] [Revised: 09/21/2021] [Accepted: 10/09/2021] [Indexed: 11/21/2022] Open
Abstract
We previously found that human heroin addicts and mice chronically exposed to morphine exhibit a significant increase in the number of detected hypocretin/orexin (Hcrt)-producing neurons. However, it remains unknown how this increase affects target areas of the hypocretin system involved in opioid withdrawal, including norepinephrine containing structures locus coeruleus (LC) and A1/A2 medullary regions. Using a combination of immunohistochemical, biochemical, imaging, and behavioral techniques, we now show that the increase in detected hypocretin cell number translates into a significant increase in hypocretin innervation and tyrosine hydroxylase (TH) levels in the LC without affecting norepinephrine-containing neuronal cell number. We show that the increase in TH is completely dependent on Hcrt innervation. The A1/A2 regions were unaffected by morphine treatment. Manipulation of the Hcrt system may affect opioid addiction and withdrawal.SIGNIFICANCE STATEMENT Previously, we have shown that the hypothalamic hypocretin system undergoes profound anatomic changes in human heroin addicts and in mice exposed to morphine, suggesting a role of this system in the development of addictive behaviors. The locus coeruleus plays a key role in opioid addiction. Here we report that the hypothalamic hypocretin innervation of the locus coeruleus increases dramatically with morphine administration to mice. This increase is correlated with a massive increase in tyrosine hydroxylase expression in locus coeruleus. Elimination of hypocretin neurons prevents the tyrosine hydroxylase increase in locus coeruleus and dampens the somatic and affective components of opioid withdrawal.
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Affiliation(s)
- Ronald McGregor
- Department of Psychiatry and Biobehavioral Sciences, University of California, Los Angeles, Los Angeles, California 90095
- Veterans Administration Greater Los Angeles Healthcare System, North Hills, Los Angeles, California 91343
| | - Ming-Fung Wu
- Department of Psychiatry and Biobehavioral Sciences, University of California, Los Angeles, Los Angeles, California 90095
- Veterans Administration Greater Los Angeles Healthcare System, North Hills, Los Angeles, California 91343
| | - Brent Holmes
- Veterans Administration Greater Los Angeles Healthcare System, North Hills, Los Angeles, California 91343
- Department of Medicine, University of California, Los Angeles, Los Angeles, 90095
| | - Hoa Anh Lam
- Department of Psychiatry and Biobehavioral Sciences, University of California, Los Angeles, Los Angeles, California 90095
- Hatos Center for Neuropharmacology, University of California, Los Angeles, Los Angeles, California 90095
| | - Nigel T Maidment
- Department of Psychiatry and Biobehavioral Sciences, University of California, Los Angeles, Los Angeles, California 90095
- Brain Research Institute, University of California, Los Angeles, Los Angeles, California 90095
- Hatos Center for Neuropharmacology, University of California, Los Angeles, Los Angeles, California 90095
| | - Joseph Gera
- Veterans Administration Greater Los Angeles Healthcare System, North Hills, Los Angeles, California 91343
- Department of Medicine, University of California, Los Angeles, Los Angeles, 90095
- Jonnson Comprehensive Cancer Center, University of California, Los Angeles, Los Angeles, 90095
- Molecular Biology Institute, University of California, Los Angeles, Los Angeles, 90095
| | - Akihiro Yamanaka
- Department of Neuroscience II, Research Institute of Environmental Medicine, Nagoya University, Nagoya 464-8601, Japan
| | - Jerome M Siegel
- Department of Psychiatry and Biobehavioral Sciences, University of California, Los Angeles, Los Angeles, California 90095
- Veterans Administration Greater Los Angeles Healthcare System, North Hills, Los Angeles, California 91343
- Brain Research Institute, University of California, Los Angeles, Los Angeles, California 90095
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McGregor R, Thannickal TC, Siegel JM. Pleasure, addiction, and hypocretin (orexin). HANDBOOK OF CLINICAL NEUROLOGY 2021; 180:359-374. [PMID: 34225941 DOI: 10.1016/b978-0-12-820107-7.00022-7] [Citation(s) in RCA: 8] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/24/2022]
Abstract
The hypocretins/orexins were discovered in 1998. Within 2 years, this led to the discovery of the cause of human narcolepsy, a 90% loss of hypothalamic neurons containing these peptides. Further work demonstrated that these neurons were not simply linked to waking. Rather these neurons were active during pleasurable behaviors in waking and were silenced by aversive stimulation. This was seen in wild-type mice, rats, cats, and dogs. It was also evident in humans, with increased Hcrt release during pleasurable activities and decreased release, to the levels seen in sleep, during pain. We found that human heroin addicts have, on average, an increase of 54% in the number of detectable Hcrt neurons compared to "control" human brains and that these Hcrt neurons are substantially smaller than those in control brains. We found that in mice, chronic morphine administration induced the same changes in Hcrt neuron number and size. Our studies in the mouse allowed us to determine the specificity, dose response relations, time course of the change in the number of Hcrt neurons, and that the increased number of Hcrt neurons after opiates was not due to neurogenesis. Furthermore, we found that it took a month or longer for these anatomical changes in the mouse brain to return to baseline. Human narcoleptics, despite their prescribed use of several commonly addictive drugs, do not show significant evidence of dose escalation or substance use disorder. Similarly, mice in which the peptide has been eliminated are resistant to addiction. These findings are consistent with the concept that an increased number of Hcrt neurons may underlie and maintain opioid or cocaine use disorders.
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Affiliation(s)
- Ronald McGregor
- Neuropsychiatric Institute and Brain Research Institute, University of California, Los Angeles, CA, United States; Neurobiology Research, Veterans Administration Greater Los Angeles Healthcare System, Los Angeles, CA, United States
| | - Thomas C Thannickal
- Neuropsychiatric Institute and Brain Research Institute, University of California, Los Angeles, CA, United States; Neurobiology Research, Veterans Administration Greater Los Angeles Healthcare System, Los Angeles, CA, United States
| | - Jerome M Siegel
- Neuropsychiatric Institute and Brain Research Institute, University of California, Los Angeles, CA, United States; Neurobiology Research, Veterans Administration Greater Los Angeles Healthcare System, Los Angeles, CA, United States
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Li SB, Nevárez N, Giardino WJ, de Lecea L. Optical probing of orexin/hypocretin receptor antagonists. Sleep 2018; 41:5060288. [PMID: 30060151 PMCID: PMC6454482 DOI: 10.1093/sleep/zsy141] [Citation(s) in RCA: 21] [Impact Index Per Article: 3.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/16/2018] [Revised: 06/10/2018] [Indexed: 01/17/2023] Open
Abstract
Study Objectives The present study investigated the function of Hypocretin (Hcrt or Orexin/OX) receptor antagonists in sleep modulation and memory function with optical methods in transgenic mice. Methods We used Hcrt-IRES-Cre knock-in mice and AAV vectors expressing channelrhodopsin-2 (ChR2) to render Hcrt neurons sensitive to blue light stimulation. We optogenetically stimulated Hcrt neurons and measured latencies to wakefulness in the presence or absence of OX1/2R antagonists and Zolpidem. We also examined endogenous Hcrt neuronal activity with fiber photometry. Changes in memory after optogenetic sleep disruption were evaluated by the novel object recognition test (NOR) and compared for groups treated with vehicle, OX1/2R antagonists, or Zolpidem. We also analyzed electroencephalogram (EEG) power spectra of wakefulness, rapid eye movement (REM) sleep, and non-REM (NREM) sleep following the injections of vehicle, OX1/2R antagonists, and Zolpidem in young adult mice. Results Acute optogenetic stimulation of Hcrt neurons at different frequencies resulted in wakefulness. Treatment with dual OX1/2R antagonists (DORAs) DORA12 and MK6096, as well as selective OX2R antagonist MK1064 and Zolpidem, but not selective OX1R antagonist 1SORA1, significantly reduced the bout length of optogenetic stimulation-evoked wakefulness episode. Fiber photometry recordings of GCaMP6f signals showed that Hcrt neurons are active during wakefulness, even in the presence of OXR antagonists. Treatment with dual OX1/2R antagonists improved memory function despite optogenetic sleep fragmentation caused impaired memory function in a NOR test. Conclusions Our results show DORAs and selective OX2R antagonists stabilize sleep and improve sleep-dependent cognitive processes even when challenged by optogenetic stimulation mimicking highly arousing stimuli.
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Affiliation(s)
- Shi-Bin Li
- Department of Psychiatry and Behavioral Sciences, Stanford University School of Medicine, Stanford, CA
| | - Natalie Nevárez
- Department of Psychiatry and Behavioral Sciences, Stanford University School of Medicine, Stanford, CA
| | - William J Giardino
- Department of Psychiatry and Behavioral Sciences, Stanford University School of Medicine, Stanford, CA
| | - Luis de Lecea
- Department of Psychiatry and Behavioral Sciences, Stanford University School of Medicine, Stanford, CA
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Ahmed SS, Steinman L. Narcolepsy and influenza vaccination-induced autoimmunity. ANNALS OF TRANSLATIONAL MEDICINE 2017; 5:25. [PMID: 28164110 PMCID: PMC5253292 DOI: 10.21037/atm.2016.12.63] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Received: 11/26/2016] [Accepted: 12/01/2016] [Indexed: 11/06/2022]
Affiliation(s)
- S. Sohail Ahmed
- Immunology, Inflammation and Infectious Diseases, Roche Pharma Research & Early Development, Basel 4070, Switzerland
| | - Lawrence Steinman
- Neurology and Neuroscience, Stanford University School of Medicine, Stanford, CA 94305, USA
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6
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Abstract
We previously reported an increased frequency of antibodies to hypocretin (HCRT) receptor 2 in sera obtained from narcoleptic patients who received the European AS03-adjuvanted vaccine Pandemrix (GlaxoSmithKline Biologicals, s.a.) for the global influenza A H1N1 pandemic in 2009 [A(H1N1)pdm09]. These antibodies cross-reacted with a particular fragment of influenza nucleoprotein (NP) - one of the proteins naturally contained in the virus used to make seasonal influenza vaccine and pandemic influenza vaccines. The purpose of this commentary is to provide additional insights and interpretations of the findings and share additional data not presented in the original paper to help the reader appreciate the key messages of that publication. First, a brief background to narcolepsy and vaccine-induced narcolepsy will be provided. Then, additional insights and clarification will be provided on the following topics: 1) the critical difference identified in the adjuvanted A(H1N1)pdm09 vaccines, 2) the contributing factor likely for the discordant association of narcolepsy between the AS03-adjuvanted pandemic vaccines Pandemrix and Arepanrix (GlaxoSmithKline Biologicals, s.a.), 3) the significance of detecting HCRT receptor 2 (HCRTr2) antibodies in some Finnish control subjects, 4) the approach used for the detection of HCRTr2 antibodies in vaccine-associated narcolepsy, and 5) the plausibility of the proposed mechanism involving HCRTr2 modulation in vaccine-associated narcolepsy.
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Affiliation(s)
| | - Lawrence Steinman
- b Neurology and Neuroscience , Stanford University School of Medicine , Stanford , CA , USA
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Steinman L, Ahmed SS. Response to comment on “Antibodies to influenza nucleoprotein cross-react with human hypocretin receptor 2”. Sci Transl Med 2015; 7:314lr2. [DOI: 10.1126/scitranslmed.aad6789] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/06/2023]
Affiliation(s)
- Lawrence Steinman
- Neurology and Neuroscience, Stanford University School of Medicine, Stanford, CA 94305, USA
| | - S. Sohail Ahmed
- Global Clinical Sciences, Novartis Vaccines Srl, Siena 53100, Italy
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Brown JA, Woodworth HL, Leinninger GM. To ingest or rest? Specialized roles of lateral hypothalamic area neurons in coordinating energy balance. Front Syst Neurosci 2015; 9:9. [PMID: 25741247 PMCID: PMC4332303 DOI: 10.3389/fnsys.2015.00009] [Citation(s) in RCA: 65] [Impact Index Per Article: 7.2] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/28/2014] [Accepted: 01/15/2015] [Indexed: 12/26/2022] Open
Abstract
Survival depends on an organism’s ability to sense nutrient status and accordingly regulate intake and energy expenditure behaviors. Uncoupling of energy sensing and behavior, however, underlies energy balance disorders such as anorexia or obesity. The hypothalamus regulates energy balance, and in particular the lateral hypothalamic area (LHA) is poised to coordinate peripheral cues of energy status and behaviors that impact weight, such as drinking, locomotor behavior, arousal/sleep and autonomic output. There are several populations of LHA neurons that are defined by their neuropeptide content and contribute to energy balance. LHA neurons that express the neuropeptides melanin-concentrating hormone (MCH) or orexins/hypocretins (OX) are best characterized and these neurons play important roles in regulating ingestion, arousal, locomotor behavior and autonomic function via distinct neuronal circuits. Recently, another population of LHA neurons containing the neuropeptide Neurotensin (Nts) has been implicated in coordinating anorectic stimuli and behavior to regulate hydration and energy balance. Understanding the specific roles of MCH, OX and Nts neurons in harmonizing energy sensing and behavior thus has the potential to inform pharmacological strategies to modify behaviors and treat energy balance disorders.
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Affiliation(s)
- Juliette A Brown
- Department of Pharmacology and Toxicology, Michigan State University East Lansing, MI, USA ; Center for Integrative Toxicology East Lansing, MI, USA
| | | | - Gina M Leinninger
- Center for Integrative Toxicology East Lansing, MI, USA ; Department of Physiology, Michigan State University East Lansing, MI, USA
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Dauvilliers Y, Siegel JM, Lopez R, Torontali ZA, Peever JH. Cataplexy—clinical aspects, pathophysiology and management strategy. Nat Rev Neurol 2014; 10:386-95. [PMID: 24890646 DOI: 10.1038/nrneurol.2014.97] [Citation(s) in RCA: 116] [Impact Index Per Article: 11.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/12/2022]
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10
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Merlo Pich E, Melotto S. Orexin 1 receptor antagonists in compulsive behavior and anxiety: possible therapeutic use. Front Neurosci 2014; 8:26. [PMID: 24592206 PMCID: PMC3923148 DOI: 10.3389/fnins.2014.00026] [Citation(s) in RCA: 34] [Impact Index Per Article: 3.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/30/2013] [Accepted: 01/27/2014] [Indexed: 11/16/2022] Open
Abstract
Fifteen years after the discovery of hypocretin/orexin a large body of evidence has been collected supporting its critical role in the modulation of several regulatory physiological functions. While reduced levels of hypocretin/orexin were initially associated with narcolepsy, increased levels have been linked in recent years to pathological states of hypervigilance and, in particular, to insomnia. The filing to FDA of the dual-activity orexin receptor antagonist (DORA) suvorexant for the indication of insomnia further corroborates the robustness of such evidences. However, as excessive vigilance is also typical of anxiety and panic episodes, as well as of abstinence and craving in substance misuse disorders. In this review we briefly discuss the evidence supporting the development of hypocretin/orexin receptor 1 (OX1) antagonists for these indications. Experiments using the OX1 antagonist SB-334867 and mutant mice have involved the OX1 receptor in mediating the compulsive reinstatement of drug seeking for ethanol, nicotine, cocaine, cannabinoids and morphine. More recently, data have been generated with the novel selective OX1 antagonists GSK1059865 and ACT-335827 on behavioral and cardiovascular response to stressors and panic-inducing agents in animals. Concluding, while waiting for pharmacologic data to become available in humans, risks and benefits for the development of an OX1 receptor antagonist for Binge Eating and Anxiety Disorders are discussed.
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Blouin AM, Siegel JM. Relation of melanin concentrating hormone levels to sleep, emotion and hypocretin levels. Sleep 2013; 36:1777. [PMID: 24293749 DOI: 10.5665/sleep.3194] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/11/2023] Open
Affiliation(s)
- Ashley M Blouin
- Department of Metabolism and Aging and Department of Neuroscience, The Scripps Research Institute, Jupiter, FL
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Orexin/hypocretin system modulates amygdala-dependent threat learning through the locus coeruleus. Proc Natl Acad Sci U S A 2013; 110:20260-5. [PMID: 24277819 DOI: 10.1073/pnas.1320325110] [Citation(s) in RCA: 135] [Impact Index Per Article: 12.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
Survival in a dangerous environment requires learning about stimuli that predict harm. Although recent work has focused on the amygdala as the locus of aversive memory formation, the hypothalamus has long been implicated in emotional regulation, and the hypothalamic neuropeptide orexin (hypocretin) is involved in anxiety states and arousal. Nevertheless, little is known about the role of orexin in aversive memory formation. Using a combination of behavioral pharmacology, slice physiology, and optogenetic techniques, we show that orexin acts upstream of the amygdala via the noradrenergic locus coeruleus to enable threat (fear) learning, specifically during the aversive event. Our results are consistent with clinical studies linking orexin levels to aversive learning and anxiety in humans and dysregulation of the orexin system may contribute to the etiology of fear and anxiety disorders.
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Blouin AM, Fried I, Wilson CL, Staba RJ, Behnke EJ, Lam HA, Maidment NT, Karlsson KÆ, Lapierre JL, Siegel JM. Human hypocretin and melanin-concentrating hormone levels are linked to emotion and social interaction. Nat Commun 2013; 4:1547. [PMID: 23462990 PMCID: PMC3595130 DOI: 10.1038/ncomms2461] [Citation(s) in RCA: 169] [Impact Index Per Article: 15.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/28/2012] [Accepted: 01/09/2013] [Indexed: 12/12/2022] Open
Abstract
The neurochemical changes underlying human emotions and social behaviour are largely unknown. Here we report on the changes in the levels of two hypothalamic neuropeptides, hypocretin-1 and melanin-concentrating hormone, measured in the human amygdala. We show that hypocretin-1 levels are maximal during positive emotion, social interaction and anger, behaviours that induce cataplexy in human narcoleptics. In contrast, melanin-concentrating hormone levels are minimal during social interaction, but are increased after eating. Both peptides are at minimal levels during periods of postoperative pain despite high levels of arousal. Melanin-concentrating hormone levels increase at sleep onset, consistent with a role in sleep induction, whereas hypocretin-1 levels increase at wake onset, consistent with a role in wake induction. Levels of these two peptides in humans are not simply linked to arousal, but rather to specific emotions and state transitions. Other arousal systems may be similarly emotionally specialized.
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Affiliation(s)
- Ashley M. Blouin
- Department of Psychiatry and Biobehavioral Sciences, University of California at Los Angeles, Los Angeles, CA 90095
- Department of Psychiatry and Behavioral Sciences, Johns Hopkins University, Baltimore, Maryland 21205
| | - Itzhak Fried
- Department of Neurosurgery, University of California at Los Angeles, Los Angeles, CA 90095
- Brain Research Institute, University of California at Los Angeles, Los Angeles, CA 90095
| | - Charles L. Wilson
- Brain Research Institute, University of California at Los Angeles, Los Angeles, CA 90095
- Department of Neurology, University of California at Los Angeles, Los Angeles, CA 90095
| | - Richard J. Staba
- Department of Neurology, University of California at Los Angeles, Los Angeles, CA 90095
| | - Eric J. Behnke
- Department of Neurosurgery, University of California at Los Angeles, Los Angeles, CA 90095
| | - Hoa A. Lam
- Department of Psychiatry and Biobehavioral Sciences, University of California at Los Angeles, Los Angeles, CA 90095
| | - Nigel T. Maidment
- Department of Psychiatry and Biobehavioral Sciences, University of California at Los Angeles, Los Angeles, CA 90095
- Brain Research Institute, University of California at Los Angeles, Los Angeles, CA 90095
| | - Karl Æ. Karlsson
- Department of Psychiatry and Biobehavioral Sciences, University of California at Los Angeles, Los Angeles, CA 90095
- Dept of Biomedical Engineering, School of Science and Engineering, Reykjavik University, Reykjavik, Iceland
| | - Jennifer L. Lapierre
- Department of Psychiatry and Biobehavioral Sciences, University of California at Los Angeles, Los Angeles, CA 90095
| | - Jerome M. Siegel
- Department of Psychiatry and Biobehavioral Sciences, University of California at Los Angeles, Los Angeles, CA 90095
- Brain Research Institute, University of California at Los Angeles, Los Angeles, CA 90095
- Neurobiology Research, Veterans Administration Greater Los Angeles Healthcare System, 16111 Plummer St., North Hills, CA 91343
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Highly specific role of hypocretin (orexin) neurons: differential activation as a function of diurnal phase, operant reinforcement versus operant avoidance and light level. J Neurosci 2011; 31:15455-67. [PMID: 22031892 DOI: 10.1523/jneurosci.4017-11.2011] [Citation(s) in RCA: 73] [Impact Index Per Article: 5.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022] Open
Abstract
Hypocretin (Hcrt) cell loss is responsible for narcolepsy, but Hcrt's role in normal behavior is unclear. We found that Hcrt knock-out mice were unable to work for food or water reward during the light phase. However, they were unimpaired relative to wild-type (WT) mice when working for reward during the dark phase or when working to avoid shock in the light or dark phase. In WT mice, expression of Fos in Hcrt neurons occurs only in the light phase when working for positive reinforcement. Expression was seen throughout the mediolateral extent of the Hcrt field. Fos was not expressed when expected or unexpected unearned rewards were presented, when working to avoid negative reinforcement, or when given or expecting shock, even though these conditions elicit maximal electroencephalogram (EEG) arousal. Fos was not expressed in the light phase when light was removed. This may explain the lack of light-induced arousal in narcoleptics and its presence in normal individuals. This is the first demonstration of such specificity of arousal system function and has implications for understanding the motivational and circadian consequences of arousal system dysfunction. The current results also indicate that comparable and complementary specificities must exist in other arousal systems.
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