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April 27, 2011

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Small groups of brain cells may “sleep” if you don’t, study finds

April 27, 2011
Courtesy of the National Institute of Mental Health
and World Science staff

A new study in rats may shed light on how sleep-de­prived lifestyles im­pair func­tion­ing with­out peo­ple real­iz­ing it, sci­en­tists say. The more rats are sleep-de­prived, the more some of their neu­rons, or in­forma­t­ion-pro­cessing brain cells, take cat­naps – with con­se­quent de­clines in task per­for­mance.

Even though the an­i­mals are awake and ac­tive, brain­wave meas­ures re­veal that scat­tered groups of neu­rons in the think­ing part of their brain, or cor­tex, are briefly fall­ing asleep, re­search­ers ex­plain.

Rats play with ob­jects in  their cages. (Cred­it: Giu­lio Tononi, U. of Wisconsin-Madison)
“Such tired neu­rons in an awake brain may be re­spon­si­ble for the at­ten­tion lapses, poor judg­ment, mis­take-prone­ness and ir­ri­ta­bil­ity that we ex­pe­ri­ence when we haven’t had enough sleep, yet don’t feel par­tic­u­larly sleepy,” said Giu­lio To­no­ni of the Uni­vers­ity of Wis­con­sin-Ma­di­son. “Strik­ingly, in the sleep-de­prived brain, sub­sets of neu­rons go of­fline in one cor­tex ar­ea but not in an­oth­er – or even in one part of an ar­ea and not in an­oth­er.”

To­no­ni and col­leagues re­port their find­ings on­line in the April 28 is­sue of the jour­nal Na­ture.

Pre­vi­ous stud­ies had hinted at such lo­cal snooz­ing with pro­longed wake­ful­ness. Yet lit­tle was known about how un­der­ly­ing neu­ronal ac­ti­vity might be chang­ing.

Members of Tonon­i’s group tracked elec­tri­cal ac­ti­vity at sites in the cor­tex as they kept rats awake for sev­er­al hours. They put new ob­jects in­to their cages – col­or­ful balls, boxes, tubes and odor­ous nest­ing ma­te­ri­al from oth­er rats. The sleep­i­er the rats got, more sub­sets of cor­tex neu­rons switched off, seem­ingly ran­dom­ly, in var­i­ous places. 

These “tired” neu­rons’ elec­tri­cal pro­files re­sem­bled those of neu­rons dur­ing deep sleep, though the rats were awake as con­firmed by brain read­ings and be­hav­ior, the re­search­ers said. The nap­ping neu­rons’ ac­ti­vity, they added, was anal­o­gous to lo­cal lapses seen in some forms of ep­i­lep­sy, and it cor­re­lat­ed with poorer per­for­mance on tasks such as reach­ing for food.

Groups of neu­rons go­ing of­fline with long­er wake­ful­ness is, in many ways, the mir­ror im­age of pro­gres­sive changes that oc­cur dur­ing re­cov­ery sleep fol­low­ing a pe­ri­od of sleep de­priva­t­ion, To­no­ni and col­leagues said. To­no­ni sug­gests that both serve to main­tain equi­lib­ri­um – part of the com­pen­sa­to­ry mech­a­nisms that reg­u­late sleep need. Just as sleep de­priva­t­ion pro­duces a brain-wide state of in­sta­bil­ity, it may al­so trig­ger lo­cal in­sta­bil­ity in the cor­tex, he added, pos­sibly by de­plet­ing lev­els of brain chem­i­cal mes­sen­gers. So tired neu­rons might nod off as part of an energy-saving or re­stor­a­tive pro­cess for over­loaded neu­ronal con­nec­tions.

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A new study in rats may shed light on how sleep-deprived lifestyles impair functioning without people realizing it, scientists say. The more rats are sleep-deprived, the more some of their neurons, or information-processing brain cells, take catnaps – with consequent declines in task performance. Even though the animals are awake and active, brainwave measures reveal that scattered groups of neurons in the thinking part of their brain, or cortex, are briefly falling asleep, researchers explain. “Such tired neurons in an awake brain may be responsible for the attention lapses, poor judgment, mistake-proneness and irritability that we experience when we haven’t had enough sleep, yet don’t feel particularly sleepy,” said Giulio Tononi of the University of Wisconsin-Madison. “Strikingly, in the sleep-deprived brain, subsets of neurons go offline in one cortex area but not in another – or even in one part of an area and not in another.” Tononi and colleagues report their findings online in the April 28 issue of the journal Nature. Previous studies had hinted at such local snoozing with prolonged wakefulness. Yet little was known about how underlying neuronal activity might be changing. Tononi’s group tracked electrical activity at sites in the cortex as they kept rats awake for several hours. They put new objects into their cages – colorful balls, boxes, tubes and odorous nesting material from other rats. The sleepier the rats got, more subsets of cortex neurons switched off, seemingly randomly, in various places. These “tired” neurons’ electrical profiles resembled those of neurons during deep sleep, though the rats were awake as confirmed by brain readings and behavior, the researchers said. The napping neurons’ activity, they added, was analogous to local lapses seen in some forms of epilepsy, and correlated with poorer performance on tasks such as reaching for food. Groups of neurons going offline with longer wakefulness is, in many ways, the mirror image of progressive changes that occur during recovery sleep following a period of sleep deprivation, Tononi and colleagues said. Tononi suggests that both serve to maintain equilibrium – part of the compensatory mechanisms that regulate sleep need. Just as sleep deprivation produces a brain-wide state of instability, it may also trigger local instability in the cortex, he added, possibly by depleting levels of brain chemical messengers. So tired neurons might nod off as part of an energy-saving or restorative process for overloaded neuronal connections.