Disappearing Before Dawn
Gene expression studies are lending support to a new hypothesis for why everyone sleeps: to prune the strength or number of synapses.
By Kelly Rae Chi
t 10 a.m. on a frigid January, the lights automatically flicker on in a rat room at the University of Wisconsin–Madison's Research Park. Postdoc Erin Hanlon strolls in, still wearing her scarf from the trip to the lab, where she will spend the next hour or so with Telito, a rat. Telito's cage is tucked away in a television cabinet–like enclosure. He's freely moving but connected to a nearby computer by a bundle of wires emanating from the four tiny electrodes implanted into his cortex, held in place with screws and dental cement. She'll teach him to extend one paw through a plastic slot to grab a food reward—a task that will exercise a specific region of his brain. After 92 trials, she'll close the door behind her, let him nod off, and wait as the computer records the electrical brain waves of his slumber.
Hanlon is trying to replicate a similar 2004 experiment in humans performed by the same group, led by Chiara Cirelli and Giulio Tononi, which produced data that researchers are interpreting in two very different ways.1
In the experiment, the group asked human subjects to complete a motor task using a computer mouse while wearing a snug-fitting, high-density electroencephalogram (EEG) cap. After the participants performed the task, the researchers measured their sleep patterns. They noticed an interesting pattern in subjects' slow waves, electrical patterns of less than four waves per second that are thought to reflect the need for sleep. In general, people who are sleep-deprived tend to have more slow waves, and those waves are larger in amplitude than the slow waves of people who aren't sleep-deprived. In this experiment, slow waves were larger and occurred more often in the specific brain region used in the task, compared to other areas even within the same immediate brain region. And those subjects with the most active slow waves in that region seemed to perform better on the task the next day.
2,3 Downscaling at night would reduce the energy and space requirement of the brain, eliminate the weakest synapses, and help keep the strongest neuronal connections intact. This assumption is based on the principle in neuroscience that if one neuron doesn't fire to another very often, the connection between the two neurons weakens. By eliminating some of the unimportant connections, the body, in theory, eliminates background connections and effectively sharpens the important connections.
It's unclear how slow waves could affect synaptic strength at a molecular level, but Cirelli and Tononi suspect the slow-wave activity triggers a weakening of synapses, and the more slow waves, the more subsequent downscaling. Their belief stems from the timing of the slow waves, which swell early in the night and taper off. Plus, molecular and electrophysiological evidence indicates synapses are stronger at the beginning of the night and weakest after a long bout of sleep. To Cirelli and Tononi, the weakening of synapses overnight—which could also theoretically help people perform better on a task the next day—is the ultimate purpose of sleep.
"At this point, it's a hypothesis that demands our attention," says Robert Stickgold, an associate professor of psychiatry at Harvard Medical School in Boston, who says he still supports the theory that the purpose of sleep is to replay and consolidate memories. "Insofar as it's true—and there's no really strong evidence that it isn't—it's going to shape our whole understanding of sleep."
tarting in the late 1980s, Cirelli and Tononi, who also live together, began experimenting with cats and rats to try to decipher the molecular mechanisms of sleep homeostasis in the brain. Others in the field were looking at the effect of sleep on the activity of neurons in discrete brain areas, such as the hypothalamus and brainstem. But if sleep was really a core need of the brain, Cirelli and Tononi reasoned that it should be reflected in the molecules across entire regions of the brain, like the cortex.
In 1989, Constantine Pavlides and Jonathan Winson from the Rockefeller University in New York tested the idea that the same daytime patterns of neuronal firing in the hippocampus—a brain region important for learning and memory—occur during sleep. To do this, they used electrodes to record activity in the hippocampus of rats while they explored a rectangular box. The researchers saw increased neuronal activity during the task, and saw the same increases during both slow-wave sleep (non-REM) and REM sleep,4 suggesting the animal was replaying the memory of the task. It was around this time that the hypothesis emerged that sleep serves to replay and consolidate memories.
As more ideas came forward, many researchers began to think that sleep serves a variety of functions, such as conserving energy in the body, healing wounds, and synthesizing molecules that are depleted during the day. But Tononi found it hard to steer clear of the central issue, saying he was "romantically inclined" to believe that there might be a single core function of sleep. Over the next decade or so, he and Cirelli studied gene expression patterns, and their hypothesis began to emerge.
In the mid-1990s, they showed that genes coding for transcription factors such as c-fos, essentially markers of neuronal activation, are elevated in the later part of the waking day and lower during sleep in most brain regions. To them this finding indicated that broad changes in gene expression might occur across the brain during sleep. "That was the clue," says Cirelli. "If [transcription factors] change so dramatically between sleep and waking, that means that there are many other genes that can change between sleep and waking."5
At the time, Tononi was beginning to think about fundamental differences between sleep and wake states. Learning was an obvious choice, considering that animals learn while awake, not while asleep. Most forms of learning occur by strengthening of connections between neurons; this makes learning energetically costly to the brain. Stronger synapses consume more energy and space, and they require more cellular supplies, such as membranes to increase the surface area of contact (and chemical signaling).
According to Cirelli, neurons expend up to 80 percent of their energy on sending and receiving electrical signals, a process that adds and strengthens synaptic connections between neurons. This process could not continue indefinitely, they reasoned—at some point, the strength of those synapses would have to decrease. A global downscaling would shave off the weakest synapses, either in number or in size. This "pruning" would help sharpen the stronger connections, which, presumably, were more important in learning and retaining what you've learned.
In 2000, they screened for the activity of 10,000 gene transcripts in the rat brain to see which were associated with sleep. When they compared results from rats that were awake, sleep-deprived, or asleep for 8 hours, the scientists found 44 genes with increased expression during periods of wakefulness and/or sleep deprivation. Many of these genes were associated with synaptic plasticity, such as neurotransmitters and growth factors like brain-derived neurotrophic factor (BDNF).
Initially, Cirelli says, they were surprised at the findings. But then, as their hypothesis solidified, everything began to make sense. "Looking at this data in 2000, 2001, and the last one was in 2004, the pattern started coming out" that genes related to synaptic potentiation showed increased expression during waking hours, but not sleep, she says. Learning could not always be associated with stronger synapses, they reasoned—for one, sleep improves some aspects of learning, and synapses tend to weaken during sleep. Alternatively, the findings suggested, indirectly, that sleep was necessary to prune the number or strength of synapses down to baseline levels, and it is this process that boosts learning and memory. "It was not only true in the rat, but in the hamster and in the sparrows and mice that other people have described," she adds.
In contrast, Cirelli and Tononi, using adult rats, have seen reductions in CaMKII and other molecular and electrophysiological signs of increased synaptic strength in the cortex after sleep.3 Because Frank et al.'s data focus on local changes in the developing brain and the synaptic hypothesis is about the global adult brain, it's too soon to say with certainty whether the results contradict the hypothesis, Cirelli says.
"Now it may be that there [are] some differences in the type of plasticity that we're studying and what [Cirelli and Tononi are] attempting to study," Frank says. "If that's true, that means their theory can't be all encompassing," he adds. "The field as a whole will determine if these and other exceptions require a revision of [the hypothesis] or its demise."
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