Hypocretin/Orexin Overexpression Induces An Insomnia-Like Phenotype in Zebrafish

Article (PDF Available)inThe Journal of Neuroscience : The Official Journal of the Society for Neuroscience 26(51):13400-10 · January 2007with123 Reads
DOI: 10.1523/JNEUROSCI.4332-06.2006 · Source: PubMed
Abstract
As many as 10% of humans suffer chronic sleep disturbances, yet the genetic mechanisms that regulate sleep remain essentially unknown. It is therefore crucial to develop simple and cost-effective vertebrate models to study the genetic regulation of sleep. The best characterized mammalian sleep/wake regulator is hypocretin/orexin (Hcrt), whose loss results in the sleep disorder narcolepsy and that has also been implicated in feeding behavior, energy homeostasis, thermoregulation, reward seeking, addiction, and maternal behavior. Here we report that the expression pattern and axonal projections of embryonic and larval zebrafish Hcrt neurons are strikingly similar to those in mammals. We show that zebrafish larvae exhibit robust locomotive sleep/wake behaviors as early as the fifth day of development and that Hcrt overexpression promotes and consolidates wakefulness and inhibits rest. Similar to humans with insomnia, Hcrt-overexpressing larvae are hyperaroused and have dramatically reduced abilities to initiate and maintain rest at night. Remarkably, Hcrt function is modulated by but does not require normal circadian oscillations in locomotor activity. Our zebrafish model of Hcrt overexpression indicates that the ancestral function of Hcrt is to promote locomotion and inhibit rest and will facilitate the discovery of neural circuits, genes, and drugs that regulate Hcrt function and sleep.
Behavioral/Systems/Cognitive
Hypocretin/Orexin Overexpression Induces An
Insomnia-Like Phenotype in Zebrafish
David A. Prober,
1
Jason Rihel,
1
Anthony A. Onah,
1
Rou-Jia Sung,
1
and Alexander F. Schier
1,2,3,4,5
1
Department of Molecular and Cellular Biology,
2
Division of Sleep Medicine,
3
Center for Brain Science,
4
Harvard Stem Cell Institute, and
5
Broad Institute,
Harvard University, Cambridge, Massachusetts 02138
As many as 10% of humans suffer chronic sleep disturbances, yet the genetic mechanisms that regulate sleep remain essentially unknown.
It is therefore crucial to develop simple and cost-effective vertebrate models to study the genetic regulation of sleep. The best character-
ized mammalian sleep/wake regulator is hypocretin/orexin (Hcrt), whose loss results in the sleep disorder narcolepsy and that has also
been implicated in feeding behavior, energy homeostasis, thermoregulation, reward seeking, addiction, and maternal behavior. Here we
report that the expression pattern and axonal projections of embryonic and larval zebrafish Hcrt neurons are strikingly similar to those
in mammals. We show that zebrafish larvae exhibit robust locomotive sleep/wake behaviors as early as the fifth day of development and
that Hcrt overexpression promotes and consolidates wakefulness and inhibits rest. Similar to humans with insomnia, Hcrt-
overexpressing larvae are hyperaroused and have dramatically reduced abilities to initiate and maintain rest at night. Remarkably, Hcrt
function is modulated by but does not require normal circadian oscillations in locomotor activity. Our zebrafish model of Hcrt overex-
pression indicates that the ancestral function of Hcrt is to promote locomotion and inhibit rest and will facilitate the discovery of neural
circuits, genes, and drugs that regulate Hcrt function and sleep.
Key words: hypocretin; orexin; sleep; insomnia; circadian rhythm; zebrafish
Introduction
Sleep remains a fundamental mystery of biology. It is unclear how
sleep and wakefulness are regulated, and the function of sleep
remains controversial (Hendricks et al., 2000a; Greenspan et al.,
2001; Hobson, 2005; Saper et al., 2005; Siegel, 2005). The genetic
mechanisms that regulate sleep are of great medical interest, be-
cause !10% of Americans suffer chronic sleep disorders (Colten
and Altevog, 2006). Insomnia, defined as an impaired ability to
initiate or maintain sleep (American Academy of Sleep Medicine,
2001), accounts for "50% of sleep-related complaints (Colten
and Altevog, 2006).
The identification of defective hypocretin/orexin (Hcrt) sig-
naling as a cause of mammalian sleep disorders highlighted the
power of genetic approaches to sleep research. Dogs and mice
that lack either the neuropeptide Hcrt or its G-protein-coupled
receptors have symptoms of narcolepsy, a syndrome character-
ized by excessive daytime sleepiness, fragmented sleep/wake
states, and sudden loss of muscle tone during waking (cataplexy)
(Chemelli et al., 1999; Lin et al., 1999; Hara et al., 2001; Willie et
al., 2003; Mochizuki et al., 2004; Siegel, 2004). Conversely, direct
injection of Hcrt protein into the brain increases locomotor ac-
tivity and decreases sleep for a few hours (Sakurai et al., 1998;
Hagan et al., 1999; Ida et al., 1999; Yamanaka et al., 1999, 2002;
Bourgin et al., 2000; John et al., 2000; Nakamura et al., 2000;
Piper et al., 2000; Huang et al., 2001; Jones et al., 2001; Espana et
al., 2002; Fujiki et al., 2003; Mieda et al., 2004; Nakamachi et al.,
2006). These studies led to the hypothesis that Hcrt signaling
maintains wakefulness. However, the long-term effects of in-
creased Hcrt signaling on behavior remain unknown but will
greatly impact the therapeutic potential of drugs that stimulate
Hcrt signaling (Zeitzer et al., 2006).
Here we introduce the zebrafish as a model system to analyze
the ancestral roles of Hcrt signaling and to determine the effects
of long-term Hcrt overexpression on behavior. Previous studies
demonstrated that late-larval zebrafish have sleep-like states sim-
ilar to mammals and Drosophila (Hendricks et al., 2000b; Shaw et
al., 2000; Zhdanova et al., 2001; Saper et al., 2005; Siegel, 2005).
Assays for locomotor activity revealed that zebrafish larvae are
less active and exhibit increased arousal thresholds to a mechan-
ical stimulus at night (Zhdanova et al., 2001). It has also been
shown that the single zebrafish Hcrt ortholog is expressed in
neurons in the posterior hypothalamus (Kaslin et al., 2004;
Faraco et al., 2006) that project to monoaminergic and cholin-
ergic nuclei in adult zebrafish (Kaslin et al., 2004), as in mammals
(Peyron et al., 1998; Chemelli et al., 1999; Date et al., 1999; Hagan
et al., 1999; Horvath et al., 1999; Nambu et al., 1999; Nakamura et
al., 2000). These studies raised the questions of whether Hcrt
regulates sleep/wake states in zebrafish and whether the Hcrt cir-
Received Oct. 4, 2006; revised Nov. 9, 2006; accepted Nov. 13, 2006.
This workwas supported by grants from the National Institutes of Health and the McKnight Endowment Fund for
Neuroscience (A.F.S.). D.A.P. was supported by a fellowship from the Helen Hay Whitney Foundation. J.R. is a
Bristol-MyersSquibb Fellow of the Life Sciences ResearchFoundation. We thank Wolfgang Driever, Su Guo,Shin-Ichi
Higashijima, and Steve Wilson for providing in situ probes, Steven Zimmerman for technical assistance, Amir Karger
for assistance with data analysis, Patrick Mabray and Irina Zhdanova for advice on behavioral assays, and Sebastian
Kraves for comments on this manuscript.
Correspondence should be addressed to David A. Prober or Alexander F. Schier, Department of Molecular and
Cellular Biology, Harvard University, 16 Divinity Avenue, Cambridge, MA 02138. E-mail: schier@mcb.harvard.edu,
prober@fas.harvard.edu.
DOI:10.1523/JNEUROSCI.4332-06.2006
Copyright © 2006 Society for Neuroscience 0270-6474/06/2613400-11$15.00/0
13400 The Journal of Neuroscience, December 20, 2006 26(51):13400 –13410
cuit is functional during larval stages. To address these questions,
we generated an inducible, long-lasting genetic model of Hcrt
overexpression in zebrafish larvae and developed high-
throughput locomotor activity and arousal assays to study sleep/
wake behaviors.
Materials and Methods
Isolation of zebrafish hcrt receptor
Full-length hcrt receptor (HcrtR) cDNA was amplified by performing
reverse transcription-PCR with Superscript II (Invitrogen, Carlsbad,
CA) using primers based on sequence from 5# rapid amplification of
cDNA ends (RACE) (FirstChoice RLM-RACE; Ambion, Austin, TX) and
Ensembl exon prediction. The hcrt receptor cDNA sequence has been
deposited in GenBank under accession number EF122429.
In situ hybridization
Single in situ hybridizations were performed us-
ing standard protocols and developed using ni-
troblue-tetrazolium-chloride/5-bromo-4-chlor-
indolyl-phosphate (Roche, Indianapolis, IN).
Double-fluorescent in situ hybridizations were
performed using DIG- and 2,4-dinitrophenol
(DNP)-labeled riboprobes and the TSA Plus
DNP System (PerkinElmer, Wellesley, MA).
Immunofluorescence
Samples were fixed in 4% paraformaldehyde/
PBS overnight at 4°C and then washed with
0.25% Triton X-100/PBS (PBTx). Brains were
manually dissected and blocked for at least 1 h
in 2% sheep serum/2% DMSO/PBTx. Antibody
incubations were performed in block solution
overnight at 4°C. Primary antibodies were rab-
bit anti-orexin A (AB3704, 1:500; Chemicon,
Temecula, CA), rabbit anti-dopamine
!
hy-
droxylase (AB1585, 1:100; Chemicon), mouse
anti-tyrosine hydroxylase (MAB318, 1:100;
Chemicon), rabbit anti-histamine (AB5885,
1:1000; Chemicon), and rabbit anti-serotonin
(S5545, 1:1000; Sigma, St. Louis, MO). Kaslin et
al. used a blocking Hcrt peptide to demonstrate
that the rabbit anti-orexin A antibody specifi-
cally labels hypothalamic Hcrt neurons in adult
zebrafish (Kaslin et al., 2004). Alexa 594-
conjugated secondary antibodies (1:500; Invitro-
gen) were used. Samples were mounted in 100%
glycerolandimaged on a Zeiss(Oberkochen,Ger-
many) Pascal confocal microscope.
Generation of transgenic fish
hcrt enhanced green fluorescent protein. A 2.4
kb fragment of Fugu rubripes genomic DNA
containing 2 kb of upstream sequence, the pu-
tative hcrt first exon, intron, and the beginning
of the second exon (Ensembl Fugu Assembly
4.0; scaffold_15 nucleotides 1450910
1453276), was amplified using the Expand High
Fidelity kit (Roche). This sequence was sub-
cloned upstream of enhanced green fluorescent
protein (EGFP) and flanked by adeno-
associated viral inverted terminal repeat ele-
ments (Hsiao et al., 2001) and sites recognized
by the homing endonuclease I-SceI (Thermes et
al., 2002). For transient expression experi-
ments, "20 pg of plasmid DNA was injected
into embryos at the one-cell stage. Live hcrt
EGFP-expressing larvae were mounted in 0.8%
low-melting agarose and imaged on a Zeiss
LSM510 confocal microscope.
Heat-shock promoter driven Hcrt. Full-length
zebrafish hcrt cDNA was isolated using 5# and 3#
RACE (FirstChoice RLM-RACE; Ambion) and cloned downstream of
the zebrafish hsp70c promoter (Halloran et al., 2000) in a vector contain-
ing flanking ISceI sites. Stable transgenic fish were generated by injecting
plasmids with ISceI enzyme into the cytoplasm of embryos at the one-cell
stage.
Behavioral analysis
Locomotor activity analysis. Larvae were raised on a 14/10 h light/dark
(LD) cycle at 28.5°C. On the fourth day of development, single larva were
placed in each of 80 wells of a 96-well plate (7701-1651; Whatman,
Clifton, NJ), which allowed simultaneous tracking of each larva and
prevented the larvae from interfering with the activity of each other.
Locomotor activity was monitored for several days using an automated
video-tracking system (Videotrack; ViewPoint Life Sciences, Montreal,
Figure 1. Hcrt and Hcrt receptor expression during embryonic and larval stages. A, B, hcrt mRNA is expressed in two bilaterally
symmetricclusters of neuronsinthe posterior hypothalamus thateach contains two tofourneurons at 24hpf(A, enlarged in inset)
and7–10 neurons at 120 hpf (B,enlarged in inset). At 48 hpf, hcrt receptor mRNA is expressed in discreteclusters of neurons in the
forebrain, midbrain, and hindbrain (C) and in a row of neurons along the spinal cord (D). An Hcrt1 peptide-specific antibody (Ab)
(E,F )and hcrt–EGFP transgenic fish (F–I ) label Hcrt neurons and reveal extensive projections within the diencephalon (E, F, G,I ),
sparse projections to the forebrain (E, F ), dense projections to the locus ceruleus [labeled with a dopamine
!
hydroxylase (D
!
H)
antibody(G,H; arrowheads in I )], and projections down the spinal cord (arrowsin I ) at 120 hpf. EGFP-expressing Hcrt neurons are
boxed in I to distinguish them from autofluorescence in the eyes and skin. The hcrt receptor is expressed in diencephalic dopami-
nergic neurons that express the dopamine transporter (dat) (J, K ) and in noradrenergic neurons of the locus ceruleus that express
dopamine
!
hydroxylase (d
!
h) (L, M ). Boxed regions in G, J, and L are shown at higher magnification in H, K, and M. J is provided
fororientation but was imaged from a differentembryo than shown in K. Anterior is tothe left. A, B, I–K are dorsalviews, E–H are
ventral views, and C, D, L, M are side views. Scale bars: A–G, I–L, 50
"
m; H, M, 10
"
m.
Prober et al. Hypocretin Causes Insomnia Phenotype in Zebrafish J. Neurosci., December 20, 2006 26(51):13400 –13410 13401
Quebec, Canada) with a Dinion one-third inch Monochrome camera
(model LTC0385; Bosch, Fairport, NY) fitted with a fixed-angle
megapixel lens (M5018-MP; Computar) and infrared filter, and the
movement of each larva was recorded using Videotrack quantization
mode. The 96-well plate and camera were housed inside a custom-
modified Zebrabox (ViewPoint Life Sciences) that was continuously il-
luminated with infrared lights and was illuminated with white lights
from 9:00 A.M. to 11:00 P.M. The 96-well plate was housed in a chamber
filled with circulating water to maintain a constant temperature of
28.5°C. The Videotrack threshold parameters for detection were
matched to visual observation of the locomotion of single larva. The
Videotrack quantization parameters were set as follows: detection
threshold, 40; burst (threshold for very large movement), 25; freeze
(threshold for no movement), 4; bin size, 60 s. The data were further
analyzed using custom PERL software and Visual Basic Macros for Mi-
crosoft (Seattle, WA) Excel. Any 1 min bin with zero detectable move-
ment was considered 1 min of rest because this duration of inactivity was
correlated with an increased arousal threshold; a rest bout was defined as
a continuous string of rest minutes. Sleep latency was defined as the
length of time from lights out to the start of the first rest bout. An active
minute was defined as a 1 min bin with any detectable activity. An active
bout was considered any continuous stretch of 1 min bins with detectable
movement. Heat shocks were performed by placing the 96-well plate in a
37°C water bath for 1 h. Ten independent experiments totaling 302 heat-
shock promoter-driven Hcrt (HS-Hcrt) and 219 wild-type (WT) larvae
were analyzed. We occasionally observed that HS-Hcrt larvae were more
active than WT larvae even before heat shock, presumably attributable to
leakiness of the heat-shock promoter (Thummel et al., 2005). This was
more often observed in experiments performed under constant lighting
conditions, in which circadian oscillations do not dampen the effect of
overexpressed Hcrt (see Fig. 7A) (supplemental Fig. 9A, available at ww-
w.jneurosci.org as supplemental material).
Behavioral state transition analysis. To calculate transition frequencies
between inactive, low-active, and high-active states, the total number of
transitions from one state to another was divided by the total number of
minutes spent in that state. A low-active state was defined as any 1 min
period with activity that lasted 1 s or less, whereas a high-active state was
defined as any 1 min period with !1 s of activity. Using these parameters,
WT larvae in a normal LD cycle spend "30% of their time in each state.
Arousal threshold analysis. White lights were manipulated during the
first and second nights after heat shock with an automated timer to
precisely regulate each transition. Transitions were from full lights on to
full lights off in $1 s. The Videotrack parameters were the same as above,
except that the recorded bin size was shortened to 1 s to ensure that
changes in locomotor behavior were precisely synchronized with lighting
transitions. Response latency was defined as the interval between the light
transition and the first 1 s bin of activity. A total of 780 HS-Hcrt and 800
WT responses were recorded in three independent experiments. The
Institutional and Animal Care and Use Committee of Harvard Univer-
sity approved all animal experiments.
Results
Hcrt neurons project to Hcrt receptor-expressing neurons
associated with arousal
To study the Hcrt circuit in zebrafish, we cloned the zebrafish
Hcrt and HcrtR orthologs. We found that the zebrafish genome
encodes a single HcrtR ortholog that is more related to mamma-
lian HcrtR2 (70%) than HcrtR1 (60%) (supplemental Fig. 1C,D,
available at www.jneurosci.org as supplemental material). We
also confirmed that the single zebrafish hcrt gene encodes Hcrt1
and Hcrt2 peptides that are 45 and 54% identical, respectively, to
theirhumanorthologs (supplemental Fig.1A,B, available atwww.
jneurosci.org as supplemental material) (Faraco et al., 2006).
Using in situ hybridization, we found that the hcrt receptor is
expressed in several discrete clusters of neurons in the telenceph-
alon, diencephalon, and hindbrain (Fig. 1C), as well as in a row of
neurons along the spinal cord (Fig. 1D). Using in situ hybridiza-
tion (Fig. 1A, B), an Hcrt1-specific antibody (Fig. 1 E,F), and
transgenic fish in which EGFP expression is regulated by the hcrt
promoter (Fig. 1F), we confirmed that hcrt is expressed in 7–10
neurons in the posterior hypothalamus on the fifth day of devel-
opment (Faraco et al., 2006).
Mammalian Hcrt neurons project to widespread regions of
the brain, including aminergic and cholinergic cells that promote
arousal (Peyron et al., 1998; Chemelli et al., 1999; Date et al.,
1999; Hagan et al., 1999; Horvath et al., 1999; Nambu et al., 1999;
Nakamura et al., 2000). Using our hcrt–EGFP transgenic line, we
observed that, by the fifth day of development, the axons of Hcrt
neurons project to the four to five noradrenergic neurons of the
locus ceruleus (Fig. 1G,H ). We also observed extensive colocal-
ization of Hcrt neuronal processes in the diencephalon with those
of tyrosine hydroxylase-expressing dopaminergic neurons (sup-
plemental Fig. 2, available at www.jneurosci.org as supplemental
material). Double-fluorescent in situ hybridization revealed that
the hcrt receptor is expressed in all of the diencephalic dopami-
nergic neurons (Fig. 1J, K) and the noradrenergic neurons of the
locus ceruleus (Fig. 1L, M ), suggesting that Hcrt may directly
modulate the activity of these neurons. Thus, the projection pat-
tern of Hcrt neurons during early zebrafish larval development
bears striking similarities to those in adult zebrafish (Kaslin et al.,
2004) and mammals (Peyron et al., 1998; Chemelli et al., 1999;
Date et al., 1999; Hagan et al., 1999; Horvath et al., 1999; Nambu
et al., 1999; Nakamura et al., 2000). However, unlike mammals
Figure 2. Time-lapse images of a developing Hcrt neuron. Transient injection of the hcrt
EGFP transgene labels a single Hcrt neuron, which was repeatedly imaged up to day 11 of
development. As for all Hcrt neurons that we labeled in this manner, the axon initially projects
"50
"
m dorsally and then turns caudally and grows down the spinal cord (indicated with an
arrow in A, C, E, G, H ). By 30 hpf, the axon has already grown "400
"
m (A) and eventually
grows almost the entire length of the spinal cord (A–H ). A, C, E are shown at higher magnifi-
cationinB,D,F,revealingthedevelopmentof dendritic and axonal arbors within the dienceph-
alon. The eye is outlined in white for orientation. Anterior is to the left, and dorsal is up. Scale
bars: A, C, E, G, H, 200
"
m; B, D, F, 40
"
m.
13402 J. Neurosci., December 20, 2006 26(51):13400 –13410 Prober et al. Hypocretin Causes Insomnia Phenotype in Zebrafish
and adult zebrafish, we did not observe dense Hcrt projections to
the raphe serotonergic or tuberomammillary histaminergic neu-
rons on the fifth day of development (data not shown). Thus, at a
time in development when Hcrt can modulate locomotor activity
(see below), Hcrt neurons do not directly project to serotonergic
or histaminergic neurons.
A recent report used an hcrt–EGFP transgene to label Hcrt
neurons (Faraco et al., 2006). Using a similar reagent, we ex-
tended these studies by performing time-lapse imaging of indi-
vidual Hcrt neurons and made two novel observations. First, we
found that all Hcrt neurons extend their axons toward the spinal
cord at similar rates but terminate at different locations (Figs. 1I,
2) (supplemental Figs. 3– 6, available at www.jneurosci.org as
supplemental material) (data not shown). This contrasts with
mammals, in which only a subset of Hcrt neurons project axons
down the spinal cord (van den Pol, 1999), and indicates that the
small number of zebrafish Hcrt neurons efficiently innervate the
trunk. Second, we observed the development of extensive den-
dritic and axonal arbors as early as 30 h postfertilization (hpf)
(Fig. 2A, B) (supplemental Fig. 3 A, B, available at www.jneurosci.
org as supplemental material), many of which undergo dynamic
growth (Fig. 2D, F ) (supplemental Figs. 5, 6, available at www.
jneurosci.org as supplemental material) and retraction (supple-
mental Fig. 6, available at www.jneurosci.org as supplemental
material) over the first 2 weeks of development. Together, these
results reveal that, not only in adult mammals and adult zebrafish
but also in zebrafish larvae, Hcrt is expressed in neurons in the
posterior hypothalamus that project to aminergic neurons that
express the Hcrt receptor. In contrast to mammals, however,
zebrafish larvae have !100-fold fewer Hcrt neurons (de Lecea et
al., 1998; Lin et al., 1999; Peyron et al., 2000), whose growth can
be monitored during development. These results establish the
zebrafish as a simple yet powerful system to study the develop-
ment and circuitry of Hcrt neurons.
Hcrt overexpression promotes locomotor activity
The effect of ectopic Hcrt has been investigated in a variety of
animals by injecting Hcrt peptide into the brain (Sakurai et al.,
Figure3. Hcrtoverexpression consolidates active states and reduces rest. A, Ventral views of
7 d postfertilization brains labeled with an Hcrt1-specific antibody from HS-Hcrt transgenic
larva that either were not (top) or were (bottom) heat shocked 48 h earlier. The non-heat-
shocked brain shows endogenous Hcrt protein, whereas the heat-shocked brain shows high
Hcrt levels throughout much of the brain. Scale bars, 100
"
m. B, Hcrt overexpression increases
locomotor activity. Each data point represents the average seconds of locomotor activity every
10 min for 20 larvae of each genotype. Behavioral recording was initiated on day 4 of develop-
4
ment. HS-Hcrt and WT larvae were heat shocked for 1 h onday 5 (arrow). HS-Hcrt and WTlarvae
had similar activity levels before heat shock. Hcrt-overexpressing larvae became significantly
moreactive than WT larvae a few hours after heat shock and remainedmoreactivefor over 48 h.
Note that larvae of both genotypes became very active for several minutes after lights out. The
spike in activity during the afternoon on days 6 and 7 resulted from the addition of water to
offset evaporation. C, Activity plots of representative WT and Hcrt-overexpressing larvae during
1 h preceding and after lights out. Black squares represent 1 min periods during which any
locomotor activity is recorded and are referred to as active bouts. Rest latency refers to the time
between lights out and the first 1 min period with no activity. Rest bout refers to a period of at
least 1 min with no activity. D–H, Combined results from 10 independent experiments are
shown. D–G, Each bar represents the mean % SEM of 302 HS-Hcrt or 219 WT larvae. Hcrt
overexpression increases active bout length (D), decreases rest bout length at night (E), de-
creases total time at rest (F ), and decreases the number of rest bouts (G) (**p $ 0.01 by
two-tailed Student’s t test). H, Hcrt overexpression reduces rest in the entire larval population.
The graph represents the distribution of total rest times for HS-Hcrt and WT larvae during the
night after heat shock. I, Pie charts represent the percentage of time spent in each state, and
arrows represent the frequencies of transitions between states during the night after the heat
shock. HS-Hcrt larvae in the inactive and low-active states are more likely to transition to the
high-activestatethan WT larvae, as represented by thicker arrows. Frequency values inboldare
significantly different between HS-Hcrt and WT larvae ( p $ 0.05 by two-tailed Student’s t
test). For example, HS-Hcrt larvae spend 64% of their time in the high-active state compared
with 38% for WT, and the probability that WT larvae will transition from the inactive to the
high-active state is only 13 versus 26% for HS-Hcrt larvae. State transition frequencies do not
add up to 1 because larvae often remain in the same state for !1 min.
Prober et al. Hypocretin Causes Insomnia Phenotype in Zebrafish J. Neurosci., December 20, 2006 26(51):13400 –13410 13403
1998; Hagan et al., 1999; Ida et al., 1999;
Yamanaka et al., 1999; Bourgin et al., 2000;
John et al., 2000; Nakamura et al., 2000;
Piper et al., 2000; Huang et al., 2001; Jones
et al., 2001; Espana et al., 2002; Yamanaka
et al., 2002; Fujiki et al., 2003; Mieda et al.,
2004; Nakamachi et al., 2006). Because this
technique is invasive, labor intensive, and
only produces transient effects, we gener-
ated a genetic model of Hcrt overexpres-
sion that is non-invasive, easy to induce,
highly reproducible, and long lasting using
a heat-shock promoter to drive hcrt ex-
pression (HS-Hcrt) (Fig. 3A). After heat
shock, these larvae express hcrt mRNA in
all cells (supplemental Fig. 7B,F, available
at www.jneurosci.org as supplemental ma-
terial). Interestingly, overexpressed Hcrt1
peptide is only found in a subset of neu-
rons in the CNS (Fig. 3A) (supplemental
Fig. 7D,H, available at www.jneurosci.org
as supplemental material), likely attribut-
able to the restricted expression of a pro-
tease that cleaves the Hcrt propeptide. Ma-
ture Hcrt1 peptide levels begin to increase
3 h after heat shock (supplemental Fig. 7,
available at www.jneurosci.org as supple-
mental material) and return to normal 3– 4 d later (data not
shown).
To determine whether Hcrt regulates sleep/wake states in ze-
brafish, we characterized these behaviors in WT and HS-Hcrt
larvae. The existence of sleep-like states has been established in
organisms ranging from flies and zebrafish to mice and humans
(Campbell and Tobler, 1984; Hendricks et al., 2000a,b; Shaw et
al., 2000; Zhdanova et al., 2001; Hobson, 2005; Pack et al., 2006).
Sleep studies in mammals typically use electroencephalogram re-
cordings to define sleep and wake states based on patterns of
brain activity. This technique provides precise information re-
garding sleep/wake states but is labor intensive and thus not ideal
for high-throughput experiments (Pack et al., 2006). As an alter-
native means of monitoring sleep/wake states, behavioral studies
have shown that sleep-like states can be identified as periods of
inactivity associated with increased arousal thresholds (Campbell
and Tobler, 1984; Hendricks et al., 2000a; Shaw et al., 2000;
Greenspan et al., 2001; Shaw and Franken, 2003; Huber et al.,
2004). Studies in Drosophila assay locomotor activity by record-
ing interruption of an infrared beam by a moving fly, and arousal
levels are measured using locomotor responses to a variety of
mechanical, auditory, and thermal stimuli (Hendricks et al.,
2000b; Shaw et al., 2000; Huber et al., 2004; Cirelli et al., 2005).
Similarly, a study monitoring the locomotor activity of 7- to 14-
d-old zebrafish larvae demonstrated sleep/wake states and in-
creased arousal thresholds in response to a mechanical stimulus
at night (Zhdanova et al., 2001). To determine the earliest point
in zebrafish development in which a sleep-like state could be
studied, we developed an automated video-tracking system to
monitor the locomotor activity of large numbers of larvae in a
high-throughput manner (Fig. 4). Individual larvae were placed
in each of 80 wells of a 96-well plate on the fourth day of devel-
opment, and the locomotor activity of each larva was monitored
for several days using an infrared camera. We defined an active
bout as a period of at least 1 min with any detectable activity (Fig.
3C). A rest bout was defined as a 1 min period with zero activity,
because 1 min of inactivity is associated with significant changes
in arousal threshold, as is observed in sleep (see below). We ob-
served that WT larvae exhibit robust locomotor activity begin-
ning on the fifth day of development and are much more active
during the day than at night (Fig. 5A, B) (Hurd and Cahill, 2002).
Hcrt overexpression significantly increases locomotor activity
(Fig. 3B) and the length of active bouts (Fig. 3C,D) during both
the day and night (for movies, see http://www.mcb.harvard.edu/
schier). For example, the average active bout length during the
night after heat shock is 87 min in Hcrt-overexpressing larvae and
14 min in WT. These results suggest that Hcrt overexpression
stimulates locomotor activity and consolidates active states.
Hcrt-overexpressing larvae also spend less time in the resting
state (Fig. 3 F, H ) and have significantly fewer (Fig. 3C,G) and
shorter (Fig. 3C,E) rest bouts. These effects are particularly prom-
inent at night. For example, on the night after heat shock, Hcrt-
overexpressing larvae have an average of 26 rest bouts compared
with 64 for WT, and Hcrt-overexpressing larvae have a median
sleep latency of 77 min compared with 13 min for WT. Thus,
Hcrt-overexpressing larvae are severely impaired in their ability
to both initiate and maintain the resting state at night, displaying
the hallmark symptoms of insomnia (American Academy of
Sleep Medicine, 2001; Mahowald and Schenck, 2005).
Hcrt overexpression decreases arousal threshold
We next developed and applied an assay to test whether Hcrt
modulates arousal thresholds, a key criterion for sleep/wake reg-
ulators. We found that most larvae become active for several
minutes after exposure to sudden darkness, perhaps in response
to a perceived threat such as the shadow of a predator (Fig. 5C,D)
(for movie, see http://www.mcb.harvard.edu/schier). Almost all
larvae become active within 15 s of dark onset if they display any
activity during the previous minute (Fig. 6 A). In contrast, 1 min
or more of rest immediately before dark onset reduces the num-
ber of responding WT larvae (Fig. 6A) and increases the average
response latency (Fig. 6B). These results indicate that rest before
Figure 4. Zebrafish larval locomotor activity assay. WT or HS-Hcrt transgenic fish are mated, embryos are collected, and
individuallarvae are placed in each of 80 wells of a 96-well plate on the fourth day ofdevelopment.Theplateisplaced in a box that
is continuously illuminated by infrared lights and is illuminated by white lights from 9:00 A.M. to 11:00 P.M. The larvae are
monitoredby an infrared camera, and the locomotor activity of each larva is recorded byacomputer.Sample40 s activity traces for
a single larva during the day and at night are shown. Each movement of the larva is recorded as an upward deflection of the trace.
When that deflection reaches a threshold (green), it is recorded as movement by the software. Zebrafish larvae move in short
bursts and are mostly inactive at night. The small white deflections at night represent background noise. If necessary, larvae are
genotyped by PCR after the experiment to identify HS-Hcrt larvae.
13404 J. Neurosci., December 20, 2006 26(51):13400 –13410 Prober et al. Hypocretin Causes Insomnia Phenotype in Zebrafish
dark onset increases arousal thresholds in WT larvae. Responses
after 2 or more minutes of rest are not significantly different from
those after 1 min of rest (Fig. 6A,B), indicating that as little as 1
min of rest can be considered a sleep-like state. Strikingly, Hcrt-
overexpressing larvae have lower arousal thresholds than WT
larvae. A larger proportion of Hcrt-overexpressing larvae re-
spond to darkness after rest (Fig. 6 A), and response latency is
shorter for Hcrt-overexpressing larvae compared with WT (Fig.
6B). Thus, Hcrt-overexpressing larvae have reduced arousal
thresholds and, similar to humans with chronic insomnia, are
hypersensitive to arousing stimuli (Bonnet and Arand, 2000;
American Academy of Sleep Medicine, 2001; Nofzinger et al.,
2004; Mahowald and Schenck, 2005).
Hcrt overexpression affects transitions between active and
inactive states
As an alternative means of assaying arousal levels in Hcrt-
overexpressing larvae, we investigated the nature of transitions
between active and resting states. We subdivided the active state
into a high-active state (active for !1 s per minute) and a low-
active state (active for $1 s per minute) and determined the
frequency at which a larva switches among all states. For example,
when WT larvae are in the inactive state at night, they transition
into the low-active state with a frequency of 36% and into the
high-active state with a frequency of 13% (Fig. 3I). Thus, when
WT larvae transition from resting to waking, they are more likely
to first enter a low-active state before becoming more highly
active. Hcrt-overexpressing larvae at rest have similar transition
rates into the low-active state (frequency of 33% for HS-Hcrt vs
36% for WT) but are twice as likely as WT to become highly active
directly from rest (frequency of 26% for HS-Hcrt vs 13% for WT)
(Fig. 3I ). Furthermore, Hcrt-overexpressing larvae spend almost
twice as much time in the high-active state and a third as much
time in the inactive state as WT. Thus, Hcrt overexpression de-
stabilizes the inactive and low-active states in favor of the high-
active state. These results are consistent with our finding that
Hcrt-overexpressing larvae are hyperaroused (Fig. 6).
Interactions between hcrt and the circadian system
Hcrt-overexpressing larvae are less active at night than during the
day, revealing that Hcrt signaling modulates but does not extin-
guish circadian regulation of locomotor activity (Fig. 3B). Con-
versely, we asked whether normal circadian oscillations are re-
quired for Hcrt to increase locomotor activity. Such experiments
have not been performed in other systems because inducible,
long-term Hcrt expression systems have not been established.
WT larvae raised in either constant dark (DD) or constant light
(LL) conditions exhibit little or no oscillation in their locomotor
activity (supplemental Fig. 8, available at www.jneurosci.org as
supplemental material) (Hurd and Cahill, 2002). Remarkably,
Hcrt overexpression increases locomotor activity and decreases
rest in larvae raised in either DD (Fig. 7) or LL (supplemental Fig.
9, available at www.jneurosci.org as supplemental material). This
effect is particularly dramatic in DD conditions, in which WT
larvae exhibit very low activity levels. For example, in Hcrt-
overexpressing larvae, the average active bout length increases
23-fold and the number of sleep bouts decreases sevenfold com-
pared with WT from 12–24 h after heat shock. Furthermore, the
effects of Hcrt overexpression are much greater in DD or LL than
in larvae maintained in 14/10 h LD conditions. For example,
Figure 5. Characterization of wild-type larval zebrafish sleep/wake locomotor behavior. A, B, Each data point represents the seconds of locomotor activity every 10 min for a single larva (A) or
averagedfor 80 larvae (B). As shownin previous studies (Hurd and Cahill, 2002;Kaneko and Cahill, 2005), we foundthat zebrafish larvae exhibit high locomotor activitylevels during the day and low
levels at night beginning on the fifth day of development. Larvae move in short bursts of activity followed by pauses, such that the total amount of time a single larva moves each minute is typically
4 –10 s during the day and 0 –1 s at night. C, D, Pulses of sudden darkness provide a non-invasive assay of arousal levels. Each data point represents the seconds of locomotor activity every 30 s for
a single larva (C) or averaged for 40 larvae (D). Larvae were exposed to alternating 30 min periods of light and darkness during the night. Individual larvae respond to most dark stimuli (C). No
attenuation in the response was observed after multiple light/dark stimuli.
Prober et al. Hypocretin Causes Insomnia Phenotype in Zebrafish J. Neurosci., December 20, 2006 26(51):13400 –13410 13405
during the night after heat shock, the average active bout length
of Hcrt-overexpressing larvae is !2.5-fold longer in DD or LL
than in LD (compare Figs. 3D, 7B) (supplemental Fig. 9B, avail-
able at www.jneurosci.org as supplemental material), and Hcrt-
overexpressing larvae in DD or LL spend less than half as much
time at rest as larvae in LD (compare Figs. 3F, 7D) (supplemental
Fig. 9D, available at www.jneurosci.org as supplemental
material).
Dramatic differences in state transitions were also observed
under constant lighting conditions. For example, in DD condi-
tions, Hcrt-overexpressing larvae are far more likely than WT to
transition from the inactive to the high-active state (frequency of
53% for HS-Hcrt vs 15% for WT) (Fig. 7G). HS-Hcrt larvae are
also far more likely than WT larvae to transition from the low-
active to the high-active state (frequency of 71% for HS-Hcrt vs
28% for WT) and far less likely to transition into the inactive state
from either active state (HS-Hcrt low-active to inactive fre-
quency, 8 vs 21% for WT; HS-Hcrt high-active to inactive fre-
quency, 2 vs 10% for WT) (Fig. 7G). The effect of overexpressed
Hcrt in DD or LL is also greater than in LD. For example, Hcrt-
overexpressing larvae spend 64% of the time in the high-active
state in LD (Fig. 3I ) but 86 or 90% of the time in DD or LL
conditions, respectively (Fig. 7G) (supplemental Fig. 9G, avail-
able at www.jneurosci.org as supplemental material). Further-
more, Hcrt-overexpressing larvae transition directly from the in-
active to the high-active state 26% of the time in LD but 53 or 46%
of the time in DD or LL conditions, respectively. Because the
effect of overexpressed Hcrt in DD or LL conditions is signifi-
cantly greater than in LD, we conclude that normal circadian
oscillations dampen the effects of Hcrt signaling. These results
also demonstrate that Hcrt can function in the absence of normal
circadian oscillations in locomotor activity.
Discussion
Our results reveal that the neural circuitry and function of the
Hcrt system is conserved from mammals to zebrafish. We find
that 5-d-old wild-type larvae exhibit robust locomotive sleep/
wake behaviors and have higher arousal thresholds during peri-
ods of rest, indicative of a sleep-like state (Campbell and Tobler,
1984; Hendricks et al., 2000a; Shaw et al., 2000; Greenspan et al.,
2001; Zhdanova et al., 2001; Shaw and Franken, 2003). Our in-
ducible Hcrt transgenic expression system reveals that Hcrt-
overexpressing larvae are hyperaroused and have dramatically
reduced abilities to initiate and maintain a sleep-like state, similar
to humans with insomnia (Bonnet and Arand, 2000; American
Academy of Sleep Medicine, 2001; Nofzinger et al., 2004; Ma-
howald and Schenck, 2005). Strikingly, Hcrt can increase loco-
motor activity independent of normal circadian oscillations in
locomotor activity, and these oscillations dampen the effects of
Hcrt overexpression. Because our experiments were performed
on larvae, the results are not confounded by previous experience,
feeding, thermoregulation, social interactions, or other complex
homeostatic processes or behaviors in which Hcrt has been im-
plicated in mammals (Sakurai et al., 1998; Szekely et al., 2002;
Yamanaka et al., 2003; Harris et al., 2005; Borgland et al., 2006;
D’Anna and Gammie, 2006). Our experiments therefore clarify
the primary function of Hcrt and suggest that the most basal and
ancestral role of Hcrt signaling is the promotion of wakefulness.
Our experiments also establish zebrafish as a system to study the
genetics and neural circuitry of sleep.
Our results indicate that the Hcrt neural circuit in zebrafish
larvae has strong similarity to its counterparts in adult zebrafish
and mammals. Hcrt is exclusively expressed in the zebrafish pos-
terior hypothalamus (Fig. 1) (Kaslin et al., 2004; Faraco et al.,
2006) as in mammals (de Lecea et al., 1998; Sakurai et al., 1998),
but zebrafish have !100-fold fewer Hcrt neurons (de Lecea et al.,
1998; Sakurai et al., 1998; Lin et al., 1999; Peyron et al., 2000).
Although this highly specific expression is regulated at the tran-
scriptional level, our misexpression studies suggest that posttran-
scriptional mechanisms can also limit production of mature Hcrt
peptide (supplemental Fig. 7, available at www.jneurosci.org as
supplemental material). Larval zebrafish Hcrt neurons project to
diencephalic dopaminergic neurons and the noradrenergic neu-
rons of the locus ceruleus, as they do in mammals (Peyron et al.,
1998; Chemelli et al., 1999; Date et al., 1999; Hagan et al., 1999;
Horvath et al., 1999; Nambu et al., 1999; Nakamura et al., 2000)
and adult zebrafish (Kaslin et al., 2004). These neurons express
the single zebrafish Hcrt receptor ortholog, indicating that they
are likely directly activated by Hcrt. In contrast to mammals
(Peyron et al., 1998; Date et al., 1999; Nambu et al., 1999) and
Figure 6. Hcrt overexpression decreases arousal threshold. A, Hcrt overexpression increases
the response rate to sudden darkness. Each data point represents the percentage of larvae that
become active within 15 s of sudden darkness, after continuous rest for at least the indicated
periodsof time. Nearly 100% of larvae of bothgenotypesrespondto a dark stimulus if they were
active during the previous minute. One minute of prestimulus rest reduces the response rate by
30% for WT larvae but only by 10% for Hcrt-overexpressing larvae. For each time point ana-
lyzed, significantly more Hcrt-overexpressing larvae respond than WT larvae (*p $ 0.05;
**p $ 0.01 by
#
2
test). B, Hcrt overexpression reduces the response latency to sudden dark
-
ness. Each data point represents the average elapsed time between initiation of the dark stim-
ulus and the onset of locomotor activity, after continuous rest for at least the indicated periods
of time. Hcrt-overexpressing larvae respond significantly faster than WT larvae (*p $ 0.05;
**p $ 0.01 by two-tailed Student’s t test). One minute of prestimulus rest increases the re-
sponse latency by fivefold for WT larvae but only by threefold for Hcrt-overexpressing larvae.
Even after at least 5 min of continuous rest, the response latency of WT larvae is twice that of
Hcrt-overexpressinglarvae.Forbothgenotypesin A and B, responses after 1 or more minutes of
prestimulus rest are all significantly different from those after 0 min of rest ( p $ 0.001).
Responses after 2 or more minutes of prestimulus rest are not significantly different from those
after 1 min of rest ( p ! 0.05). A total of 780 HS-Hcrt and 800 WT responses from three
experiments were analyzed.
13406 J. Neurosci., December 20, 2006 26(51):13400 –13410 Prober et al. Hypocretin Causes Insomnia Phenotype in Zebrafish
adult zebrafish (Kaslin et al., 2004), we do not observe Hcrt pro-
jections to the raphe serotonergic or tuberomammillary hista-
minergic neurons on the fifth day of development. Thus, at a time
in development when Hcrt can regulate locomotor activity, direct
modulation of serotonergic and histaminergic neurons may not
be necessary to mediate Hcrt function and may therefore reveal
the basic neural circuitry required for Hcrt function. Moreover,
zebrafish brains lack a cortex, indicating that their relatively basal
brain structures are sufficient to mediate Hcrt function. The ac-
cessibility of zebrafish for in vivo imaging (Higashijima et al.,
2003; Gahtan and Baier, 2004) and the small number of Hcrt
neurons should make zebrafish a powerful system to study the
development and function of Hcrt neural circuits.
To generate a non-invasive, inducible, and robust model of
Hcrt overexpression, we created transgenic zebrafish that ex-
press Hcrt under the control of a heat-shock promoter. Most
previous Hcrt overexpression studies required the invasive and
labor-intensive injection of Hcrt peptide into the brain, which
increased wakefulness and inhibited rest for only a few hours, and
thus required that behavior be assayed immediately after han-
dling the animal (Sakurai et al., 1998; Hagan et al., 1999; Ida et al.,
1999; Bourgin et al., 2000; John et al., 2000; Nakamura et al.,
2000; Piper et al., 2000; Huang et al., 2001; Jones et al., 2001;
Espana et al., 2002; Yamanaka et al., 2002; Fujiki et al., 2003;
Nakamachi et al., 2006). One study used transgenic mice that
constitutively overexpressed Hcrt in the brain to rescue Hcrt de-
ficiency, but the effects of Hcrt-overexpression on wild-type be-
havior were not described (Mieda et al., 2004). In contrast to
these studies, our transgenic fish express high levels of Hcrt for at
least 3 d, allowing us to assay behavior long after the heat shock.
We find that Hcrt overexpression promotes and consolidates ac-
tive states, decreases the frequency and length of rest bouts, in-
creases sleep latency after lights out, and decreases arousal thresh-
old. Hcrt-overexpressing larvae also spend more time in highly
active states compared with wild-type and are more likely to tran-
sition from states of no or low activity into a high-activity state.
Our findings indicate that long-term overexpression of Hcrt in-
creases wakefulness and arousal, whereas long-term absence of
Hcrt in mice and humans reduces wakefulness and arousal. How-
ever, both loss- and gain-of-Hcrt function result in altered sleep/
wake states and abnormal transitions between activity states
(Chemelli et al., 1999; Hara et al., 2001; Willie et al., 2003; Mo-
chizuki et al., 2004). Thus, Hcrt levels must be tightly regulated to
allow optimal control of sleep/wake states, providing a potential
challenge to the development of therapeutics that target the Hcrt
system.
We found that Hcrt-overexpressing larvae are hyperaroused
and have impaired abilities to initiate and maintain rest at night,
thus displaying similar behavioral characteristics as humans with
insomnia (American Academy of Sleep Medicine, 2001). Al-
though there is no direct evidence implicating Hcrt in insomnia,
it has been proposed that chronic insomnia is a general disorder
Figure 7. Hcrt overexpression dramatically consolidates active states and reduces rest in
constantdarkconditions. A, Hcrt overexpression dramatically increases locomotor activity in DD
conditions. Each data point represents the average seconds of locomotor activity every 10 min
for 40 larvae of each genotype. Embryos were exposed to light for 2–3 h after fertilization but
were then raised in the dark. Behavioral recording was initiated on day 4 of development, and
larvae were heat shocked for 1 h on day 5 (arrow). Larvae exhibit activity levels in DD similar to
those observed during the dark phase of a 14/10 h light/dark cycle. Hcrt-overexpressing larvae
are slightly more active than WT larvae before heat shock, presumably attributable to leaky
expression driven by the heat-shock promoter, but become significantly more active after heat
shock. The slight oscillation in locomotor activity after heat shock likely results from the expo-
sure to low levels of red light immediately before and after heat shock, to handling of the
96-well plate as it is transferred to the 37°C water bath, or to the temperature stimulus of the
heat shock itself. B–E, Each bar represents the mean % SEM of 40 larvae. Hcrt overexpression
4
increases active bout length (B), decreases rest bout length (C), decreases total time at rest (D),
and decreases the number of rest bouts (E) in DD conditions (**p $ 0.01 by two-tailed Stu-
dent’s t test). F, Hcrt overexpression decreases rest in the entire larval population in DD condi-
tions. The graph represents the distribution of total rest times for HS-Hcrt and WT larvae 12–24
hafter heat shock. G, Pie chartsrepresentthepercentage of time spent in eachstate,andarrows
represent the frequencies of transitions between states during the 12 h after the heat shock.
HS-Hcrt larvae in the inactive and low-active states are more likely to transition to the high-
active state than WT larvae, as represented by thicker arrows. All frequency values are signifi-
cantly different between HS-Hcrt and WT larvae ( p $ 0.05 by two-tailed Student’s t test).
Prober et al. Hypocretin Causes Insomnia Phenotype in Zebrafish J. Neurosci., December 20, 2006 26(51):13400 –13410 13407
of hyperarousal that may involve dysfunction of the Hcrt system
(Horvath and Gao, 2005; Saper et al., 2005). In fact, Hcrt neurons
are regulated by minimal inhibitory input, making them suscep-
tible to abnormal excitation (Horvath and Gao, 2005). Although
one study found normal Hcrt levels in human subjects with in-
somnia (Mignot et al., 2002), only 12 subjects were studied. Fur-
thermore, this study would not detect other Hcrt-dependent ef-
fects, such as hyperactivity of Hcrt-expressing neurons or
activation of Hcrt signaling downstream of the Hcrt ligand. Al-
though models with reduced sleep have been generated via brain
lesions in mammals (Lu et al., 2000) or through genetic loss-of-
function in Drosophila (Cirelli et al., 2005), our novel gain-of-
function model is uniquely suited for the large-scale testing of
environmental signals and drugs that suppress insomnia in ver-
tebrates. Such models are urgently needed, because insomnia ac-
counts for "50% of sleep-related complaints (Colten and Al-
tevog, 2006).
Our experiments reveal novel interactions between Hcrt sig-
naling and the circadian system that were not identified previ-
ously because a long-term Hcrt overexpression system was not
available. First, Hcrt-overexpressing larvae are less active at night
than during the day. This indicates that Hcrt-overexpression
modulates but does not abolish the effects of the circadian system
on locomotor activity. Second, Hcrt overexpression more dra-
matically increases locomotor activity under constant lighting
conditions, in which larvae exhibit little or no oscillation in loco-
motor activity. Previous studies showed that the mammalian su-
prachiasmatic nucleus (SCN), which acts as the master pace-
maker for circadian rhythms in vertebrates, indirectly projects to
Hcrt neurons (Abrahamson et al., 2001; Aston-Jones et al., 2001;
Chou et al., 2003; Deurveilher and Semba, 2005; Yoshida et al.,
2006) and that Hcrt levels fluctuate with a circadian rhythm that
is abolished after SCN ablation (Deboer et al., 2004; Zhang et al.,
2004). These findings suggest that the circadian system regulates
Hcrt levels, perhaps by regulating the activity of Hcrt neurons.
However, it has also been shown that Hcrt signaling is not neces-
sary for the normal circadian amplitude and timing of sleep/wake
rhythms (Mochizuki et al., 2004), indicating that the circadian
system can function independently of Hcrt signaling. Interest-
ingly, SCN neurons indirectly project to and regulate the activity
of locus ceruleus neurons (Aston-Jones et al., 2001), suggesting
that both the circadian and Hcrt systems likely regulate sleep/
wake states, at least in part, by modulating the activity of locus
ceruleus neurons. Together, the mammalian loss-of-function
and our gain-of-function studies suggest that the Hcrt and circa-
dian systems can function independently of each other but also
appear to cross regulate. Additional study is required to clearly
determine whether the circadian and Hcrt systems use separate or
common mechanisms to regulate sleep/wake states.
More generally, our study establishes zebrafish as a model
system for the genetic analysis of sleep. Zebrafish will comple-
ment mammals and Drosophila because it combines the experi-
mental amenability of Drosophila (Driever et al., 1996; Granato et
al., 1996; Hendricks et al., 2000a; Greenspan et al., 2001; Orger et
al., 2004; Gross et al., 2005; Zon and Peterson, 2005) with neural
substrates similar to mammals (Reichert et al., 1996; Mueller and
Wullimann, 2005). Our high-throughput, quantitative system to
assay locomotor activity and arousal in zebrafish allows us to
simultaneously analyze hundreds of larvae. A similar technology
has provided the basis for large-scale genetic screens in Drosoph-
ila (Cirelli et al., 2005). Our system should therefore allow high-
throughput zebrafish screens for novel genes and drugs that reg-
ulate sleep.
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    • "In goldfish central injection of both OXA and OXB stimulates appetite and food consumption, with a more potent action for OXA (Volkoff et al., 1999). In zebrafish, the neuronal population expressing orexins regulates sleep and wake (Prober et al., 2006; Yokogawa et al., 2007; Elbaz et al., 2012). Orexins and their receptors are extensively expressed in the gastroenteric apparatus of mammals as described by immunohistochemical, PCR and in situ hybridization studies (Kirchgessner, 2002; Nakabayashi et al., 2003; Korczynsky et al., 2006; Heinonen et al., 2008; Roma nsky and Go Harłajczuk, 2009). "
    [Show abstract] [Hide abstract] ABSTRACT: Orexin A and B peptides and the receptor OX2R were studied in sea bass and goldfish gastroenteric tract by immunoblotting combined with densitometric analysis using NIH Image J software and immunohistochemical techniques. These teleost species present a different gut organization and diverse feeding habits. Immunoblotting experiments showed one band of 16 kDa corresponding to prepro-orexin, and one band of 38 kDa corresponding to the OX2R receptor. Immunohistochemical localization of OXA and OXB was observed in the enteric nervous system throughout the gastroenteric tract of both species. OXA and OXB immunoreactive cells were found in the gastric and intestinal regions of sea bass, and were mainly found in the basal region of folds in intestinal bulb, and in the midgut and hindgut of goldfish. The distribution of OX2R was mainly detected in the mucosa of the gastroenteric tract of sea bass and goldfish. This distribution suggests an endocrine action of OXA and OXB in the gastrointestinal tract as well as involvement in the peripheral control of food intake and digestive processes in both species. This study might also serve to determine the productive factors in breeding and as a baseline for future experimental studies on the regulation of the gastroenteric functions in non-mammalian vertebrates. This article is protected by copyright. All rights reserved.
    Full-text · Article · May 2016
    • "This species is a very prolific small vertebrate that, due to its size and the social nature, can be held in large numbers in small rack systems (up to 4000 fish in 250L water volume), demanding little cost and technical maintenance. In addition, the zebrafish shares many molecular pathways, genes and protein products with mammals (Faraco et al., 2006; Holzschuh et al., 2001; Kaslin, 2004; Kaslin and Panula, 2001; McLean and Fetcho, 2004; Mueller et al., 2004; Prober et al., 2006). Moreover, the majority of the genes already identified in this species is conserved and have homologs in mammals (Cerdà et al., 1998; Crollius and Weissenbach, 2005), which makes it an ideal model organism for embryology, development and disease studies (Sison and Gerlai, 2010). "
    [Show abstract] [Hide abstract] ABSTRACT: Many studies regarding the effects of drugs investigate the acute and chronic use of alcohol, but only a few address the effects of caffeine and alcohol combined to the performance of the zebrafish in cognitive tasks. The zebrafish is an important model for studying the effects of drugs on learning, because it has large genetic similarities to humans and the non-invasive administration of the substances favors translational bias of research. In this study, we observed the effects of alcohol and caffeine on zebrafish cognition through an object discrimination test. We noticed that animals subjected to acute alcohol dose and those under alcohol or caffeine withdrawal did not show discrimination. When fish were treated with associated alcohol and caffeine, those chronically treated with alcohol and subjected to moderate acute dose of caffeine showed learning of the task. Our results reinforce the harmful effects of the alcohol use on cognitive tasks, and suggest that continued use of high doses of caffeine cause cognitive impairment during withdrawal of the substance. However, the acute use of caffeine appears to reverse the harmful effects of alcohol withdrawal, allowing discriminative performance equivalent to control fish. Finally, we reiterate the use of zebrafish as a model for drug effects screening and search for active compounds that modulate the alcohol and caffeine effects.
    Full-text · Article · Feb 2016
    • "The Hcrt system appears to be an evolutionarily conserved circuit for arousal, as the genetic ablation of Hcrt neurons also causes a narcoleptic phenotype in zebrafish [80]. Conversely, the overexpression of Hcrt in zebrafish promotes wakefulness and inhibits rest, leading to a state of hyperarousal similar to insomnia [81] . In human patients , abnormal levels of Hcrt strongly correlate with narcolepsy ; for instance, studies have established low levels of Hcrt-1 in the CSF as a diagnostic criteria for narcolepsy [82]. "
    [Show abstract] [Hide abstract] ABSTRACT: The hypocretins (Hcrts), also known as orexins, have been among the most intensely studied neuropeptide systems since their discovery about two decades ago. Anatomical evidence shows that the hypothalamic neurons that produce hypocretins/orexins project widely throughout the entire brain, innervating the noradrenergic locus coeruleus, the cholinergic basal forebrain, the dopaminergic ventral tegmental area, the serotonergic raphe nuclei, the histaminergic tuberomammillary nucleus, and many other brain regions. By interacting with other neural systems, the Hcrt system profoundly modulates versatile physiological processes including arousal, food intake, emotion, attention, and reward. Importantly, interruption of the interactions between these systems has the potential to cause neurological and psychiatric diseases. Here, we review the modulation of diverse neural systems by Hcrts and summarize potential therapeutic strategies based on our understanding of the Hcrt system’s role in physiology and pathophysiological processes.
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