Hints of another signaling system in brain.
Astrocytes, the star-shaped cells that surround and support nerve cells in the brain and spinal cord, can propagate chemical waves over long distances in response to a chemical messenger found in the brain, researchers report. This intriguing discovery suggests that astrocytes, like nerve cells, may carry information between various parts of the central nervous system (CNS).
Astrocytes guide fetal neuron development, "digest" neurotransmitters and regulate the blood flow in the central nervous system. They belong to a class of cells called glia, which together outnumber nerve cells 10 to one in the brain and constitute about half the brain's volume. But until recently, says Bill Chiu, a neuroscientist at the University of Wisconsin-Madison, "people thought of glial cells as silent."
In the past few years this premise has begun to crumble, as researchers accumulated evidence that astrocyte surfaces bear receptors for glutamate, a common neurotransmitter in the brain. Experiments showed that these cells respond to glutamate by releasing stored calcium ions, but the function of the calcium releases remained unclear.
Ann H. Cornell-Bell, Steven M. Finkbeiner and their colleagues at the Yale University School of Medicine in New Haven, Conn., used a fluorescent calcium indicator to observe in unprecedented detail the dynamics of calcium release in cultured, glutamate-stimulated astrocytes. The astrocytes responded by unleashing waves of ions that spread like ripples inside the cells. Wave periods ranged from 9 seconds to 30 minutes depending upon the experimental conditions.
Other waves spread beyond the cells, triggering chemical ripples in nearby astrocytes. The longest wave initiated reactions in 59 other astrocytes in 51 seconds before leaving the microscope's field of view, the group reports in the Jan. 26 SCIENCE. "With receptors on these cells and a signaling system working between them, a potential communication system exists," Finkbeiner told SCIENCE NEWS.
Researchers have seen periodic release of calcium in many other types of cells, but the observation in astrocytes is "potentially very exciting," says Chiu, because astrocytes appear interconnected in the brain and often nestle near glutamate-secreting nerve junctions. "Now we can propose that glutamate activity in one part of the brain may be passed elsewhere [via astrocytes]," he says. "That adds a potential new dimension to signaling in the brain."
The Yale researchers do not yet know what kind of information the ionic pulses may encode. They hope to gain some clues by extending their experiments to slices of living brain tissue.
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|Date:||Jan 27, 1990|
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