Neuronal sleep patterns appeared at least 450 million years ago, according to researchers

Researchers at the Stanford University School of Medicine have discovered that neuronal signatures in sleepy zebrafish are analogous to those of humans, suggesting that brain activity has evolved there at least 450 million years ago, before creatures come out of the ocean.

Scientists have known for over 100 years that fish enter the state of sleep, but until now they did not know if their sleep resembled that of land animals.

The researchers found that when they sleep, zebrafish can display two states similar to those seen in mammals, reptiles and birds: slow-wave sleep and REM sleep, or fast-moving sleep. eyes. This discovery marks the first time that these brain structures have been recorded in fish.

"It takes years off the evolution of neuronal sleep signatures," said postdoctoral researcher Louis Leung, Ph.D.

An article describing the research will be published on July 10 in Nature. Philippe Mourrain, Ph.D., associate professor of psychiatry and behavioral sciences, is the principal author. Leung is the main author.

To study zebrafish, the usual inhabitants of the aquarium, also known as danios, researchers have constructed a fluorescent plate microscope capable of performing complete imaging of the fish body with a single-cell resolution . They recorded cerebral activity while the fish were sleeping in an agar solution that immobilized them. They also observed the heart rate, eye movements and muscle tone of the sleeping fish with the help of a fluorescence polysomnography that they developed.

They named the observed states of sleep: "slow sleep", analogous to slow wave sleep, and "propagative wave sleep", analogous to REM sleep. Although fish do not move their eyes during REM sleep, the signatures of the brain and muscles are similar. (The fish do not close their eyes when they sleep because they have no eyelids.)

Sleep like fish

The researchers found another similarity between fish and human sleep. By genetically disrupting the functioning of the hormone-concentrating melanin, a peptide that regulates the sleep-wake cycle, and by observing neuronal expressions during fish sleep, the researchers determined that the signaling of this hormone regulated sleep fish spreading as in sleep. mammals.

The other aspects of their sleep resemble those of terrestrial vertebrates, said Mourrain: The fish remain motionless, their muscles relax, their cardiorespiratory rhythms slow down and they do not react to the approach.

"They lose muscle tone, their heartbeats decrease, they do not respond to stimuli.The only real difference is the lack of rapid eye movements during REM sleep," said Mourrain, although he added: "The rapid movement of the eyes is not a good criterion of this state, and we prefer to call paradoxical sleep, because the brain looks awake when we are asleep."

Although scientists can not say with certainty that all animals are sleeping, this seems to be a universal need in vertebrates and invertebrates. Animals will die if they are deprived of sleep long enough and people who do not get enough sleep suffer from mental problems such as memory failures and impaired judgment, as well as an increased risk of mental disorders. such as obesity and hypertension.

The exact benefits of sleep, however, remain a mystery. "It's an essential function," said Mourrain, "but we do not know exactly what that does."

He added that sleep disorders are linked to most neurological disorders such as autism spectrum disorders, Fragile X syndrome and Alzheimer's and Parkinson's diseases. "Sleep disturbances are an aggravating factor of these disorders," said Mourrain. It is essential to develop this animal model to study sleep functions at the cellular level, including neuronal connectivity and DNA repair, to understand the physiopathological consequences of sleep disturbances, he added. .

This discovery means that sleep research can be conducted on zebrafish, which is easy to study, in part because it is transparent. They breed quickly, are inexpensive to maintain, and are one inch long. Drug testing requires only the addition of chemicals to their water.

"Since the neural signatures of fish are essentially the same as ours, we can use the information about them to generate new drug trials," said Leung. He added that mice, which often perform the function of human research, are nocturnal and are a less relevant model for our sleep.

"Since zebrafish are diurnal like humans, it may be more accurate biologically to compare the sleep of fish with that of humans," Leung said.