L'insomnie du poisson zèbre peut-elle expliquer notre sommeil ?
Par Jean-Luc Goudet - Futura-Sciences
Quand on les empêche de dormir la nuit, les poissons zèbres ne récupèrent pas pendant le jour, comme le feraient la plupart des mammifères. Cette étrange observation s'explique peut-être par une différence dans l'action d'un récepteur du système nerveux, également impliqué dans la narcolepsie. Mais au fait, pourquoi dort-on ?
http://www.futura-sciences.com/fr/sinformer/actualites/news/t/zoologie/d/linsomnie-du-poisson-zebre-peut-elle-expliquer-notre-sommeil_13271/
http://www.nature.com/news/2007/071016/full/news.2007.167.html
Fish insomnia sheds light on sleep
Studies across species could reveal how sleep evolved.
Emma Marris
Zebrafish won't be caught napping after a sleepless night.
Zebrafish
don't nap more during daylight hours when sleep deprived, a new study
shows. The work suggests that fish are better able to use light cues to
stay awake during the day than mammals, hinting that evolution has
produced different systems for regulating sleep in different groups of
animals.
Every animal sleeps, but many do so in ways that
humans would hardly recognize. Cows stand stock still on their four
legs; dolphins take a separate nap in each hemisphere of their brains
so they can keep swimming. Even fruitflies catch forty winks now and
again in their short lives.
The way you can tell a zebrafish
is asleep, says Emmanuel Mignot at Stanford University in Palo Alto,
California, is that its tail droops, it hangs immobile at the bottom of
the tank, and it requires more of a prod - a mild electric current will
do - to get it swimming than when it is awake.
Mignot and
his colleagues are keen to keep zebrafish awake to study how sleep - or
the lack of it - affects this often-studied fish. No one really
understands why people sleep; how sleep evolved is equally mysterious,
says Mignot. "Sleep is one of the basic mysteries remaining, in terms
of why it has been selected for." To understand that, he is studying
sleep in animals from dogs to zebrafish. "It is better to understand
how we sleep across evolution, and then we will understand the reason
for sleep," he says.
Dodging the sleep debt
The team found that if you keep zebrafish awake all night and then
leave the lights off, they will make up for lost snoozing time. But if
you keep them awake and then flip on the lights, they won't nap more
during the next day, as mammals would1.
To
find out what was causing this behaviour, Mignot and his colleagues
looked at what happens when a mutation knocks out the only receptor for
the sleep neuropeptide hypocretin (also known as orexin) in the
zebrafish.
In mammals, such a gap in the hypocretin system
causes narcolepsy, a syndrome featuring crushing daytime sleepiness and
night-time insomnia as well as bouts of muscle collapse called
cataplexy. But in the fish, this seems to cause only the night-time
insomnia.
Mignot says he thinks that light and the hormone it
triggers - melatonin - suppress sleep in the fish so strongly, they
overrule any 'sleep debt' from the night before. Unlike mammals, they
only need their hypocretin system to regulate sleep at night.
Physiological studies support this theory. Some birds seem to behave
the same way, Mignot adds, hinting that light and melatonin-dominated
day regulation might span the non-mammalian animal family tree.
"I
do believe the role of light and melatonin came to a kind of
crossroads. At some point it became less effective, and animals had to
develop a different way to promote wakefulness," says Mignot.
The
finding contradicts previous work that has found that hypocretin
increases wakefulness during the day as well as at night in zebrafish -
though this was in larvae rather than adults.
"Further analysis of hypocretin-receptor mutants is needed to resolve
these discrepancies," says Alexander Schier of Harvard University in
Cambridge, Massachusetts, one of the authors of the previous study.
Drowsy dogs
Mignot has been mapping the neurochemistry of sleep in humans, rodents, and a pack of narcoleptic dogs for years.
He
says he would like to see much more work done across the animal
kingdom, to create an evolutionary tree of sleep. He's going to stick
with his zebrafish for now, he adds, but he would love to study the
same system in an animal such as a platypus - from the most ancient and
reptile-like family of mammals. "I would dream to do this in a
monotreme, to find out if we could have a narcoleptic platypus."
The
narcoleptic dogs, incidentally, are no longer an active area of
research - the dogs have all been retired into private life. "We have
one last dog, and he was just adopted," says Mignot. "He's now mine."
References
- Yokogawa, T. et al. PLoS Biol. 5, e277 (2007).
- Prober, D. A., Rihel, J., Onah, A. A., Sung, R.-J. & Schier, A. F. J. Neuroscience. 26, 13400-13410 (2006
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