No one should have to sleep with the fishes, but new research on zebrafish suggests that we sleep like them.
Sleeping zebrafish have brain activity similar to both deep slow-wave sleep and rapid eye movement, or REM, sleep thats found in mammals, researchers report July 10 in Nature. And the team may have tracked down the cells that kick off REM sleep.
The findings 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. Thats 150 million years earlier than scientists thought when they discovered that lizards sleep like mammals and birds (SN: 5/28/16, p. 9).
Whats more, sleep may have evolved underwater, says Louis C. Leung, a neuroscientist at Stanford University School of Medicine. “These signatures [of sleep] really have important functions — even though we may not know what they are — that have survived hundreds of millions of years of evolution.”
In mammals, birds and lizards, sleep has several stages characterized by specific electrical signals. During slow-wave sleep, the brain is mostly quiet except for synchronized waves of electrical activity. The heart rate decreases and muscles relax. During REM or paradoxical sleep, the brain lights up with activity almost like its awake. But the muscles are paralyzed (except for rapid twitching of the eyes) and the heart beats erratically.
For many years, scientists have known that fruit flies, nematodes, fish, octopuses and other creatures have rest periods reminiscent of sleep. But until now, no one could measure the electrical activity of those animals brains to see if that rest is the same as mammals snoozing.
Leung and colleagues developed a system to do just that in zebrafish by genetically engineering them to make a fluorescent molecule that lights up when it encounters calcium, which is released when nerve cells and muscles are active. By following the flashes of light using a light sheet microscope, the researchers tracked brain and muscle activity in the naturally transparent fish larvae.
The next task was to lull fish asleep under the microscope. In some experiments, the team added drugs that trigger either slow-wave or REM sleep in mammals to the fishs water. In others, researchers deprived fish of sleep for a night or tuckered the fish out with lots of activity during the day. Results from all the snooze-inducing methods were the same.
Sleeping fish have two distinct types of brain activity while sleeping, the team found. One, similar to slow-wave sleep, was characterized by short bursts of activity in some nerve cells in the brain. The researchers call that state slow-bursting sleep. REM-like sleep, which the researchers dubbed “propagating-wave sleep,” was characterized by frenzied brain activity that spreads like a wave through the brain. The researchers arent calling the sleep phases REM or slow-wave sleep because there are some minor differences between the way fish and mammals sleep.
SLEEP WAVE Using genetically engineered zebrafish, researchers watched fish sleep under the microscope. A molecule that lights up (red) when nerves and muscles become active was used to monitor brain and body functions. As the zebrafish prepares for REM-like sleep, a wave of activity travels toward the tail, making the fishs muscles go slack (about seven seconds in). Then another wave of activity (at about 11 seconds) sweeps up the fish. This wave is similar to ones that trigger rapid eye movement sleep in mammals.
A group of cells that line hollow spaces called ventricles deep in the brain seems to trigger that wave of REM-like brain activity. These ependymal cells dip fingerlike cilia into the cerebral spinal fluid that bathes the ventricles and the central nervous system. The cells appear to beat their cilia faster as amounts of a well-known, sleep-promoting hormone called melanin-concentrating hormone in the fluid increases, the researchers discovered.
Its unclear how the ependymal cells communicate with the rest of the brain to set off REM-like activity. Such cells are also present in mammals, but nRead More – Source