Insomniac Cavefish May Hold Clues to Human Sleep Disorders

Cavefish

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Blind Mexican cavefish sleep much less than closely related species that live near the surface, according to a study that involved shaking aquariums to keep fish awake.

By breeding the fish with their sighted counterparts, scientists determined that the difference in their sleep patterns is genetic. The discovery may help identify genes and pathways involved in insomnia and other sleep disorders in humans.

“Fish are really not so very different than humans,” said biologist Richard Borowksy of New York University, co-author of the study published April 7 in Current Biology.

Like all animals, fish need to sleep. But over years of working with fish, Borowsky noticed that Astyanax mexicanus seemed to stay much more active at night than other species. Because many species of cavefish have adapted genetically to life in the deep and dark, losing their eyesight and pigmentation, he wondered if their sleep habits were also genetic.

Borowsky’s team brought related but evolutionarily separated populations of A. mexicanus into the lab, including three groups of blind fish from different Mexican caves and a still-sighted group that lived near the surface.

To evaluate fish sleep patterns, the researchers first had to find a way to tell when fish are asleep. It turns out sleeping fish have a stereotypical posture: They stop moving, drop to the bottom of the tank and drop their tail. The scientists found that once a fish had been in this position for 60 seconds, it was definitely in a different state: Like rousing a sleepy teenager, it took three times as many taps on its tank to get a sleepy fish moving after that 60-second mark.

To verify that the fish were indeed sleeping, the researchers also explored what happened when they deprived the fish of sleep. They put the fish on top of a Vortex mixer set to its gentlest setting and had it vibrate for 10 seconds out of every minute, all night long. The next day, fish of all four species were far less active.

“It’s like they were out at night partying,” said co-author Eric Duboue. “The next day they had to sleep it off.”

They then monitored the different species of fish under simulated 12-hour day and night cycles and found that all three populations of cave fish slept way less than the surface fish. Cavefish slept an average of 110 to 125 minutes a day, while the surface fish averaged 800 minutes.

“It’s an extraordinary finding that this decrease in the amount of sleep occurred independently in the three different populations,” Borowsky said.

He suspects that the cavefish evolved to be awake more often because of their unique environments. At the surface, fish have to use energy to escape predators during the day, but have plenty of food available. It’s the opposite in a cave, where the fish are the top predators but food is scarce. A fish that’s awake more has a better chance of snagging the rare morsel as it wanders by.

To confirm that the differences in sleep habits they observed were genetic, the researchers bred the various cavefish populations with the surface fish. The offspring slept like cavefish, indicating a dominant gene for sleeplessness. A second generation, bred by mating that first hybrid generation, showed sleep behaviors in between the two populations. The researchers concluded that a few specific genes must be responsible for the sleep change.

The scientists say their research offers an important clue in the quest to unlock how genes control behavior in humans. It’s likely that the genes regulating sleep in fish are the same genes regulating sleep in humans, Borowsky said.

“We can now begin to ask why sleep is reduced,” said Joshua Gross, an evolutionary geneticist who studies cavefish at the University of Cincinnati and who was not involved in the study. “This study, and similar investigations, are critical for understanding how genes influence behaviors in the context of the natural world.”

In their next experiment, the researchers plan to pinpoint specific genes responsible for cavefish restlessness.

To sum it up, scientists found that genetics is a factor in fish insomnia. Now all they have to do is figure out if humans also have the same genetic tendencies. 

Article Source: http://www.wired.com/wiredscience/2011/04/insomniac-cavefish/