Sleep-wake regulation and hypocretin-melatonin interaction in zebrafish.
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.
Appelbaum
Lior
L
Center for Narcolepsy, Department of Psychiatry and Behavioral Sciences, Howard Hughes Medical Institute, Stanford University, Palo Alto, CA 94305, USA.
Wang
Gordon X
GX
Maro
Geraldine S
GS
Mori
Rotem
R
Tovin
Adi
A
Marin
Wilfredo
W
Yokogawa
Tohei
T
Kawakami
Koichi
K
Smith
Stephen J
SJ
Gothilf
Yoav
Y
Mignot
Emmanuel
E
Mourrain
Philippe
P
D000818:Animals; D047908:Intracellular Signaling Peptides and Proteins / Q000502:physiology*; D008550:Melatonin / Q000096:biosynthesis / Q000502:physiology*; D009479:Neuropeptides / Q000502:physiology*; D064446:Orexin Receptors; D000068797:Orexins; D010870:Pineal Gland / Q000378:metabolism; D043562:Receptors, G-Protein-Coupled / Q000235:genetics / Q000378:metabolism / Q000502:physiology; D018013:Receptors, Neuropeptide / Q000235:genetics / Q000378:metabolism / Q000502:physiology; D012890:Sleep; D014851:Wakefulness; D015027:Zebrafish / Q000502:physiology*
D047908:Intracellular Signaling Peptides and Proteins; D009479:Neuropeptides; D064446:Orexin Receptors; D000068797:Orexins; D043562:Receptors, G-Protein-Coupled; D018013:Receptors, Neuropeptide; D008550:Melatonin