Ripples in rats’ brains: The results show that brain cell activity can play a surprising role in the body’s metabolism.

Bursts of nerve cell activity, which record memories, may have an unexpected job outside the brain: to lower blood sugar levels.

New experiments show that immediately after a burst of pulses in the rat’s hippocampus, sugar levels in other parts of the body drop. These unexpected results, published Aug. 11 in the journal Nature, suggest that certain brain activities and metabolism are intertwined 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 was not involved in the study.

Neural shudders, called sharp wave pulsations, occur in people’s brains when they learn something new and retrieve memories (SN: 8/19/19). Pulsations are also noticeable during deep sleep. Such bursts of electrical activity are observed in sleeping mammals, birds and even lizards known as Australian dragons. The sharp waves of pulsations are thought to accompany the neural work of converting short-term knowledge into long-term memories.

Neuroscientist David Tingley wondered whether these signals might also alter something outside the brain. In collaboration with neurobiologist György Buzsáky of New York University’s Grossman School of Medicine and his colleagues, Tingley, who now works at Harvard University, attached continuous glucose monitors to the backs of rats. These devices, used by people with diabetes to track sugar levels in the fluid around their cells, are a good proxy for blood sugar levels. At the same time, the researchers measured the rats’ brain waves with electrodes implanted in the hippocampus, a brain structure that plays a key role in memory.

From time to time, the electrodes picked up clusters of pulsations. About 10 minutes after the pulsations appeared, sugar levels in the body dropped, the glucose monitors showed. “We saw these drops in the second rat, and in the third rat, and in the fourth rat,” Buzsaki says. “It was very consistent. The magnitude is small, but [the dips] are always present.”

To test whether this connection between pulsation and sugar lapses was simply a coincidence, the researchers forced nerve cells in the hippocampus to fire in response to light, creating an artificial pulsation. Of course, after this forced pulsation, the rats’ blood sugar levels dropped.

What’s more, when the researchers silenced the downward pulse signals with a drug that calms nerve cells in an area of the brain called the lateral septum, sugar levels did not drop. This suggests that these pulsations send signals that travel through the brain and ultimately tell the body to lower sugar levels.

“The whole thing was very surprising,” says Jan Born, a neurobiologist who studies metabolism at Tübingen University in Germany. One would expect a busy brain to require more energy in the form of sugar, not less, says Born, who wrote a commentary on the new work in the same issue of Nature. But here, “the brain is telling the body, ‘We don’t need that much energy, so lower your glucose levels. Why?” – Bourne says, “It’s hard to understand its function.”

Buzsaki wonders if these pulses might have originally evolved to help with metabolism. “At first they were beneficial to the body,” he suggests. Over time, the pulsations may have been attracted to other work, such as preserving memory.”

If this newly discovered link between brainwaves and metabolism exists in humans, Buzsaki says, it could provide an opportunity to influence sugar levels by regulating pulsation, which could prove useful for people with diabetes or other metabolic problems. The hippocampus is deep in the brain, but its activity can be altered by magnetic or electrical impulses in more accessible areas of the brain. However, altering pulses for metabolic purposes is a distant idea, Buzsaki warns.

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