Ripples of nerve cell activity that lock in memories may have an unexpected job outside of the brain: dropping blood sugar levels in the body.

Soon after a burst of ripples in a rat’s hippocampus, a brain structure that plays a key role in memory, sugar levels elsewhere in the body dipped, new experiments show. The curveball results, reported August 11 in Nature, suggest that certain types of brain activity and blood sugar control – a key part of metabolism – are entwined in surprising and mysterious ways.

“This paper represents a significant advance in our understanding of how the hippocampus modulates metabolism,” says Elizabeth Gould, a neuroscientist at Princeton University who wasn’t involved in the study.

Neural shudders are called sharp-wave ripples zig and zag in the brains of people as they learn new things and draw memories back up (SN: 9/14/19, p.14). Ripples also feature prominently during deep sleep and are thought to accompany the neural work of transforming short-term knowledge into long-term memories.

Neuroscientist David Tingley, now at Harvard University, wondered whether these signals might also change something outside of the brain. Tingley and colleagues fitted continuous glucose monitors onto the backs of eight rats. The researchers simultaneously measured the rats’ brain waves with electrodes implanted in the hippocampus.

About 10 minutes after a bout of ripples, blood sugar levels in the body fell, the monitors showed. “We saw these dips in the second rat, and the third rat, and the fourth rat,” says coauthor Gyorgy Buzsaki, a neuroscientist at New York University Grossman School of Medicine. “It was super consistent. The magnitude is small, but [the dips] are always there.”

Rats’ blood sugar levels also dropped after the team induced bouts of bran ripples using light. When the researchers jammed the ripples’ signals with a drug that quiets nerve cells, blood sugar levels did not drop. That suggests these ripples send signals that ping-pong through the brain and ultimately tell the body to reduce its sugar.

“All of this was very surprising,” says neuroscientist Jan Born of the University of Tubingen in Germany. You might expect busy bran at work to call for more energy, in the form of sugar, not less, says Born, who co-wrote a commentary on the study in Nature.

Buzsaki speculates that these ripples might have evolved to aid in metabolism. As time passed, the ripples may have been pulled in on memory storage.

If this newfound link between brain waves and metabolism exists in people, it might hint at a way to influence sugar levels by tweaking ripples, Buzsaki says, an idea that could be useful for people with diabetes and other metabolic problems.

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