Scientists seeking the origins of sleep may have uncovered important clues in the Australian bearded dragon. By tracing sleep-related neural signals to a specific region of the lizard’s brain—and linking that region to a mysterious part of the mammalian brain—a new study suggests complex sleep evolved even earlier in vertebrate evolution than researchers thought. The work could ultimately shed light on the mechanisms behind sleep—and pave the way for studies that may help humans get a better night’s rest.
“Answers to the questions raised and reframed by this research seem extremely likely to be significant in many ways, including clinically,” says Stephen Smith, a neuroscientist at the Allen Institute who was not involved with the new study.
Mammals and birds have two kinds of sleep. During rapid eye movement (REM) sleep, eyes flutter, electrical activity moves through the brain, and, in humans, dreaming occurs. In between REM episodes is “slow wave” sleep, when brain activity ebbs and electrical activity synchronizes. This less intense brain state may help form and store memories, a few studies have suggested.
In 2016, Gilles Laurent, a neuroscientist at the Max Planck Institute for Brain Research, discovered that reptiles, too, have both kinds of sleep. Every 40 seconds, central bearded dragons (Pogona vitticeps) switch between the two sleep states, he and his colleagues reported.
No one had figured out which part of the brain drove these slow wave patterns in mammals, birds, or reptiles. So, Laurent’s team used electrodes to track down electrical activity associated with slow wave patterns in slices of dissected bearded dragon brains. (Such electrical activity often persists after death.) They soon homed in on a small part of the dorsal ventricular ridge, a region located toward the front of the lizard’s brain with an unknown function—until now.
Then came the unexpected revelation. Laurent’s postdoc Maria Tosches, now an assistant professor at Columbia University, and two graduate students had been assessing gene activity in cells from different parts of the lizard’s brain and comparing that activity with that of a mouse brain. The set of genes active in the reptilian brain region that generated the “slow wave” pattern closely resembled that in the mouse claustrum, an irregular sheet of nerve cells deep in the brain with connections throughout the forebrain. The resemblance indicated that reptiles, too, had a claustrum.
“The claustrum has been a mystery for quite a while,” says Yang Dan, a neuroscientist at the University of California, Berkeley, who studies sleep circuitry in mouse brains. Other researchers have suggested it as the source of consciousness. But few have considered it important for sleep. And none had thought it existed in reptiles.
Excited about possibly connecting the neural region to sleep, Laurent’s team traced this putative claustrum’s connections to the rest of the lizard’s brain. Like the mammalian claustrum, this one connects to many parts of the brain, including areas involved with sleep, Laurent and his colleagues report today in Nature. When they damaged the claustrum, the dragon still slept, but no slow wave pattern was generated. “This paper really figured out where the slow waves originate,” Dan says.
Laurent now thinks he can sketch out a slow wave sleep scenario that makes sense. In reptiles and possibly other vertebrates, a claustrum doesn’t start or stop sleep, but responds to signals from a sleep command center deeper in the brain. Then, it generates the slow wave pattern and transmits it to other parts of the brain.
Laurent’s team also found a claustrum in a distant reptilian relative, the Trachemys scripta turtle, leading the researchers to conclude that the brain region predates the evolution of reptiles. Indeed, the new work suggests the claustrum and its role in sleep date back 320 million years to the ancestor of birds, other reptiles, and mammals, Laurent says.
That, says Terrence Sejnowski, a computational neuroscientist at the Salk Institute for Biological Studies, means the claustrum’s role in mammalian sleep should be investigated, especially in humans. “If the claustrum is important for sleep in reptiles, it might also be important for sleep [and sleep disorders] in humans.” However far down that path researchers get, the new work drives home the value of studying sleep in different species, Smith says, while showing how reptiles are “a major window” into vertebrate brain evolution.