by J. Travis
Since that discovery, adenosine has begun to intrigue more and more sleep investigators. Many studies in animals have shown that blocking adenosine's actions in the brain increases alertness, while injections of adenosine or similar compounds induce apparently normal sleep.
Now, by monitoring the brain chemistry of cats, a team of researchers has shown that natural adenosine concentrations in at least some parts of the brain build up during waking periods and decline during sleep. Moreover, the investigators report in the May 23 Science, adenosine concentrations in the brain shoot up dramatically in animals forced to stay awake.
The findings offer the most compelling evidence yet that adenosine is one of the long-sought factors whose fluctuating concentrations in the brain determine when an animal must sleep.
"That's fantastic. We predicted what they got," says Miodrag Radulovacki of the University of Illinois College of Medicine in Chicago, the earliest champion of the adenosine theory of sleep.
The new adenosine research may bolster a recent theory that the body's regular desire for sleep stems from the brain's periodic need to replenish low stores of energy.
Adenosine is a relatively simple, nitrogen-containing compound used widely by the body. It forms the core of adenosine triphosphate, or ATP, the energy-storage molecule that powers most of the biochemical reactions inside cells.
In the brain, adenosine secretion by cells such as neurons and glia often reflects how busy the cells are. "Areas that are active generate adenosine; areas that aren't active tend not to," says report coauthor Robert W. Greene of Harvard Medical School's Brockton (Mass.) Veterans Administration Medical Center.
This observation is central to a proposal put forth by H. Craig Heller of Stanford University and Joel H. Benington of St. Bonaventure University in Olean, N.Y. A few years ago, the pair pondered the mystery of why animals need to sleep. A host of theories, ranging from memory consolidation to aiding the immune system, have addressed this provocative issue.
Heller and Benington suggested that sleep occurs when some or all of an animal's brain becomes dangerously low on energy. Since the sleeping brain is much less active than the waking one, sleep allows the organ to replenish its energy, they proposed.
Adenosine could fit into this theory nicely, says Benington. Since adenosine secretion reflects brain cell activity, rising concentrations of this chemical may be how the organ gauges that it has been burning up its energy reserves and needs to shut down for a while.
To understand how adenosine induces sleep, Greene's team has studied the compound's effect on the brain's arousal centers. Cells in these centers have connections throughout the brain, and they help keep a body awake and alert. Test-tube studies of the cells conducted several years ago showed that adenosine "can actually shut them off," says Greene.
From that work, Greene's group hypothesized that increasing concentrations of adenosine near arousal centers might compel an organism to sleep.
In the new study, the scientists anesthetized cats and implanted electrodes in several regions of the brain, including arousal centers, to monitor neuronal activity. They also inserted small probes to sample the extracellular fluid.
The researchers found that adenosine concentrations near an arousal center were higher when the cats were awake than when they were asleep.
The scientists then deprived the cats of sleep by constantly playing with them. After 6 hours or so of playing, the felines were exhausted. "At the end of the sleep-deprivation period, they just sit there and look at you trying to get their attention," says Tarja Porkka-Heiskanen, a study coauthor.
At that point, adenosine concentrations in the cats' brains were, on average, double those observed when the cats had been awake for 2 hours. During 3 hours of "recovery" sleep, adenosine concentrations fell slowly.
Adenosine "is likely not the only sleep factor that exists in the brain. There may be others," notes Greene, adding that adenosine concentrations that normally induce sleep may be overcome sometimes by an excited or stressed animal.
Greene expects his group's work will stimulate renewed interest in adenosine, but he cautions against expecting a new sleeping pill soon. Even though the brain chemical induces a natural sleep that many current sleeping pills cannot duplicate, medicinal adenosine exerts several other effects. It can lower body temperature, alter blood pressure, and damage the heart.
The recognition of adenosine's importance in inducing sleep is gratifying, given that many scientists once discounted the connection, says Radulovacki.
The 64-year-old scientist notes that a friend recently asked him if he were happy that adenosine is now a hot topic. "I feel a lot better than when they ignored me. A scientist has to live long to reap the benefits," Radulovacki laughingly responded.
Porkka-Heiskanen, T., et al. 1997. Adenosine: A mediator of the sleep-inducing effects of prolonged wakefulness. Science 276(May 23):1265.
A special issue of the Sleep Research Society journal recently dealt with the adenosine and sleep connection. Online summaries of that issue's articles are available at: http://bisleep.medsch.ucla.edu/SRS/srs/radulovacki.htm, http://bisleep.medsch.ucla.edu/SRS/srs/benington.htm, and http://bisleep.medsch.ucla.edu/SRS/srs/greene.htm.
Robert W. McCarley
Harvard Medical School
940 Belmont Street
Brockton, MA 02401