Nobody should have to sleep with fish, but new research on zebrafish suggest that we sleep as their.
The sleeping zebrafish has a brain activity similar to that of deep slow-wave sleep and rapid eye movement, or REM sleep, found in mammals, researchers said on July 10. Nature. And the team may have found the cells that trigger REM sleep.
The results suggest that the basics of sleep evolved at least 450 million years ago in zebrafish ancestors, before the evolution of animals that give birth to live young instead of laying eggs . 150 million years earlier than scientists thought when they discovered that lizards slept like mammals and birds (SN: 28/05/16, p. 9).
In addition, sleep may have evolved underwater, says Louis C. Leung, neuroscientist at the Stanford University School of Medicine. "These signatures [of sleep] really have important functions – even if we do not know what they are – that have survived hundreds of millions of years of evolution. "
In mammals, birds and lizards, sleep involves several stages characterized by specific electrical signals. During slow-wave sleep, the brain is largely silent, except for synchronized waves of electrical activity. The heart rate decreases and the muscles relax. During paradoxical or paradoxical sleep, the brain lights up with activity almost as if it were awake. But the muscles are paralyzed (with the exception of rapid eye contractions) and the heart beats irregularly.
Scientists have known for many years that fruit flies, nematodes, fish, octopuses and other creatures have rest periods that are reminiscent of sleep. But until now, no one was able to measure the electrical activity of the brain of these animals to see if this rest was identical to that of mammal sleep.
Leung and his colleagues have developed a system to do exactly that in zebrafish by genetically engineering it to form a fluorescent molecule that lights up when it encounters calcium, which is released when nerve cells and muscles are active. By following flashes of light with the help of an optic optical microscope, researchers tracked the brain and muscle activity of naturally transparent fish larvae.
The next task was to lull the fish into the microscope. In some experiments, the team added fish to drugs that trigger slow sleep or REM sleep in mammals. In other cases, the researchers deprived the fish of sleep for one night or hid them with a lot of activity during the day. The results of all nap-inducing methods were the same.
The team discovered that sleeping fish had two types of brain activity while they slept. One, similar to slow wave sleep, was characterized by short bursts of activity in some nerve cells of the brain. Researchers call this state slow sleep. REM sleep, which the researchers termed "propagative wave sleep," was characterized by frenetic brain activity that propagated like a wave in the brain. Researchers do not call slow sleep or slow sleep phases because there are some minor differences between fish and mammal sleep.
WAVE OF SLEEP By using genetically modified zebrafish, researchers have observed fish under a magnifying glass. A molecule that lights up (in red) when nerves and muscles become active has been used to monitor brain and body functions. As the zebrafish prepares for REM sleep, a wave of activity moves to the tail, causing the fish muscles to relax (about seven seconds). Then another wave of activity (about 11 seconds) sweeps the fish. This wave is similar to the one that triggers rapid sleep in mammals.
A group of cells that carp hollow spaces called ventricles in the brain seems to trigger this wave of brain activity similar to that of EMR. These ependymal cells plunge finger-like eyelashes into the cerebrospinal fluid that bathes the ventricles and the central nervous system. The researchers found that the cells appeared to beat their eyelashes faster when the amount of a well-known hormone that promotes sleep, called the hormone-concentrating melanin, increases in the fluid.
It is not known how ependymal cells communicate with the rest of the brain to trigger an activity similar to that of EMR. Such cells are also present in mammals, but no one has yet been able to see this as deeply in the brains of sleepy mammals to determine whether cells play a role in sleep. However, knowing these cells could help researchers develop better sleep aids, says Leung.
As in mammals, whole bodies of zebrafish are affected during sleep. Their muscles relax during sleep and their heart slows down to about 200 beats per minute when they are awake at around 110 to 120 beats per minute when they sleep during slow wave-shaped sleep. During REM sleep, the heart slows even further, at around 90 beats per minute, and loses its regular rhythm. And the muscles of the fish relax completely. The only characteristic missing from the fish is the rapid movement of the eyes. The co-author of the study, Philippe Mourrain, a biologist at the Stanford University School of Medicine, said his eyes were returning to their orbits.
Lack of eye movement could indicate that parts of the brain that treat emotions, such as the amygdala, are not as active in zebrafish as in mammals, said Allan Pack, sleep researcher, of the Perelman School of Medicine, University of Pennsylvania. With their monitoring of brain activity, researchers have pushed sleep research "to a higher level," says Pack, and "they present fairly compelling evidence" of a slow sleep pattern similar to that paradoxical sleep in fish.
Researchers' involvement of the entire body of evidence reinforces the argument that fish sleep is similar to that of mammals, says neuroscientist Paul Shaw of the University of Washington School of Medicine in St. Louis. In all the bodies known to doze, "sleep is manifest everywhere" in the body, he says.
Future experiments could show why lack of sleep or lack of Z contributes to people's health problems, such as obesity, heart disease and diabetes.