Sleep-wake Regulation and Hypocretin-Melatonin Interaction in Zebrafish

Proc Natl Acad Sci U S A. 2009 Dec 22;106(51):21942-7. doi: 10.1073/pnas.906637106. Epub 2009 Dec 4.

Abstract

In mammals, hypocretin/orexin (HCRT) neuropeptides are important sleep-wake regulators and HCRT deficiency causes narcolepsy. In addition to fragmented wakefulness, narcoleptic mammals also display sleep fragmentation, a less understood phenotype recapitulated in the zebrafish HCRT receptor mutant (hcrtr-/-). We therefore used zebrafish to study the potential mediators of HCRT-mediated sleep consolidation. Similar to mammals, zebrafish HCRT neurons express vesicular glutamate transporters indicating conservation of the excitatory phenotype. Visualization of the entire HCRT circuit in zebrafish stably expressing hcrt:EGFP revealed parallels with established mammalian HCRT neuroanatomy, including projections to the pineal gland, where hcrtr mRNA is expressed. As pineal-produced melatonin is a major sleep-inducing hormone in zebrafish, we further studied how the HCRT and melatonin systems interact functionally. mRNA level of arylalkylamine-N-acetyltransferase (AANAT2), a key enzyme of melatonin synthesis, is reduced in hcrtr-/- pineal gland during the night. Moreover, HCRT perfusion of cultured zebrafish pineal glands induces melatonin release. Together these data indicate that HCRT can modulate melatonin production at night. Furthermore, hcrtr-/- fish are hypersensitive to melatonin, but not other hypnotic compounds. Subthreshold doses of melatonin increased the amount of sleep and consolidated sleep in hcrtr-/- fish, but not in the wild-type siblings. These results demonstrate the existence of a functional HCRT neurons-pineal gland circuit able to modulate melatonin production and sleep consolidation.

Publication types

  • Research Support, N.I.H., Extramural
  • Research Support, Non-U.S. Gov't

MeSH terms

  • Animals
  • Intracellular Signaling Peptides and Proteins / physiology*
  • Melatonin / biosynthesis
  • Melatonin / physiology*
  • Neuropeptides / physiology*
  • Orexin Receptors
  • Orexins
  • Pineal Gland / metabolism
  • Receptors, G-Protein-Coupled / genetics
  • Receptors, G-Protein-Coupled / metabolism
  • Receptors, G-Protein-Coupled / physiology
  • Receptors, Neuropeptide / genetics
  • Receptors, Neuropeptide / metabolism
  • Receptors, Neuropeptide / physiology
  • Sleep*
  • Wakefulness*
  • Zebrafish / physiology*

Substances

  • Intracellular Signaling Peptides and Proteins
  • Neuropeptides
  • Orexin Receptors
  • Orexins
  • Receptors, G-Protein-Coupled
  • Receptors, Neuropeptide
  • Melatonin