University of Basel researchers have discovered that clown loaches, a species of fish in South Asia, do not need orexin to stay awake. This molecule is thought to be necessary for the normal rhythm of wakefulness and sleep in vertebrates. People without orexin experience narcolepsy.
Until recently, it was assumed that vertebrates shared similar mechanisms that regulate sleep behavior. That’s why researchers have been using fish for the past 20 years as a model organism to study sleep and its regulation.
Now the team has made a surprising discovery in a South Asian species of fish: Clown loaches, also found in zoos and aquariums, show normal sleep rhythms, but their sleep is regulated differently. way. They lack the so-called orexin signaling pathway (also known as the Hypocretin signaling pathway), which was once thought to be important for regulating sleep and wakefulness in all vertebrates.
The results of the study were published in Current Biology.
Sleep-Wake Rhythm Without Orexin
“We were very surprised that clown loaches show normal sleep rhythms and, more importantly, they wake up easily despite a defective orexin pathway,” said first author Vassilis Bitsikas, PhD , in a release.
Thus, this type of carp does not fall into a comatose state, as is common with narcolepsy and does not require orexin to regulate its sleep-wake cycle.
Initially, the idea was to study the orexin signaling pathway in clown loaches in more detail. Since these fish only stop swimming when they sleep and rest on their sides, it is easy to observe them when they are actually sleeping. “It seemed like the perfect model organism for our sleep studies. However, further investigations revealed that clown loaches do not have a functional orexin signaling pathway,” Bitsikas said in a release.
Narcolepsy in Mammals
In humans, a functional orexin signaling pathway is essential to maintain sleep-wake rhythms. Deficiencies in this pathway lead to narcolepsy. Patients experiencing this condition experience excessive daytime sleepiness, cataplexy, and uncontrollable sleep attacks, from which they hardly ever wake up.
This neurological disease is caused by the loss of nerve cells in the brain that produce orexin, a neurotransmitter that keeps us awake.
“So far, it has been assumed that orexin deficiency disrupts normal sleep-wake behavior in all vertebrates. Now it turns out that this assumption is clearly wrong,” says professor Alex Schier, PhD, at the Biozentrum, University of Basel, who led the team of researchers, in a release.
Fish Regulate Their Sleep Differently
The researchers also discovered that not only clown loaches but also zebrafish can regulate their sleep-wake rhythm without relying on orexin.
“They still maintain normal sleep and wake behavior despite a defective orexin signaling pathway. So, they don’t rely on it to stay awake,” says Bitsikas in a release.
Accordingly, this fish species may have developed a separate or compensatory sleep control mechanism by comparison with mammals. “It will be interesting to know when and why different control systems evolved in vertebrates,” Schier said in a release.
Fish are often used as model organisms for studying the evolution of sleep. “The new findings have changed our understanding of the regulation of sleep and wakefulness. Fish may have some secrets that may help us understand why some animals are more vulnerable to narcolepsy than others, ” Bitsikas said in a release.
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