Background noise can do more than distract. In certain situations--like the firing of neurons--noise can enhance a signal. Now researchers have shown for the first time that the coordinated activity of some brain cells may depend on a critical level of such noise. A video premiered today at the annual meeting of the American Physical Society in Kansas City, Missouri, shows cultured rat brain cells apparently acting in concert, producing spiral waves of chemical activity. The findings could, perhaps, have implications for human epilepsy.
Biologist Ann Cornell-Bell of Viatech Imaging in Ivoryton, Connecticut, put on display star-shaped cells, called astrocytes, from the rat hippocampus, a brain region associated with long-term memory. Astrocytes play a housekeeping role in the brain, mopping up excess neurotransmitters, and Cornell-Bell followed them as they performed this task. The video relies on a cascade of chemical reactions: When a neurotransmitter called kainate binds to the surface of an astrocyte, a molecular floodgate opens and sodium ions rush in. To compensate, the "excited" cell pumps in calcium ions. Cornell-Bell monitored the astrocytes with a dye that glows in the presence of calcium.
Kainate added to the cultured astrocytes lit them up; the glow quickly spread to neighboring cells and the wave spiraled like a pinwheel. The amount of kainate controls how many astrocytes spontaneously light up and thus produces the background noise. Too much or too little yielded short-lived waves that propagated to only a few cells. But moderate amounts triggered large, stable, and long-lasting waves. "Noise in neural systems can play a useful role" by enhancing transmission, says team member Frank Moss, a physicist at the University of Missouri in St. Louis.
The findings may one day have implications for the treatment of epileptics. During seizures, neurons fire too rapidly, flooding the brain with neurotransmitters. When Cornell-Bell's team added kainate to astrocytes grown from human tissue removed during surgery for epilepsy, the cells glowed in chaotic patterns. The researchers speculate that increased firing during seizures creates too much noise for the coordinated action of astrocytes--and perhaps decreases their efficiency in sopping up neurotransmitters. Experts are intrigued by the spiral waves but dubious about their relevance to epilepsy. At least, says Steven Schiff, a neuroscientist at Children's National Medical Center in Washington, D.C., "they're fun to look at."