Brief exposure to high temperatures has lasting effect on nervous system

February 1999

Researchers at the University of Chicago, in collaboration with scientists at Queens University in Ontario, Canada, have shown for the first time that brief exposure to high temperatures has long-lasting physiological effects on the nervous system. These changes, which the researchers measured in locusts, may be what enables the animals to fly in very hot environments.

The study, published in the February issue of the Journal of Neurophysiology, is also the first to use a new technique allowing scientists to measure changes in neuronal activity in insects' minute brains.

"Scientists have suspected that heat shock has adaptive properties for all kinds of animals, we just haven't had any direct evidence until now," says Nino Ramirez, PhD, assistant professor of organismal biology and anatomy at the University of Chicago. Ramirez and R.M. Robertson, PhD, professor of biology at Queens University, decided to look into how heat exposure affects the locust's ability to fly.

"Locusts that are transferred from cooler places to a warm desert have trouble flying and some even die," says Ramirez. "But locusts exposed to brief periods of high temperatures and later released into the desert can fly normally. We thought that previous heat exposure might have changed the properties of the neurons somehow," says Ramirez.

To test the idea that heat shock induces lasting modifications of neuronal properties, Ramirez and Robertson exposed 50 locusts to 45 degree Celsius temperatures (about 112 degrees Fahrenheit) for 3 hours, while control locusts were kept at room temperature (about 70 degrees Fahrenheit) for the same amount of time. Heat-shocked locusts were allowed 6 to 24 hours to recover.

"We couldn't measure activity in single neurons because they are too tiny," Ramirez says. "We had no choice but to measure from slices of the brain which are large enough to preserve relatively intact neuronal activity, but thin enough for us to insert measuring devices. To do this, we had to create a whole new procedure for taking slices of the insect's brain, something that no one had done before," says Ramirez.

Cells in the slices were kept alive in a Petri dish and Ramirez monitored the neurons using tools thinner than a human hair. What the researchers found surprised them. "We expected to see some changes in the flow of calcium from the neurons, but instead, we found that potassium outflow from the neurons of heat-shocked locusts was greatly reduced," says Ramirez.

The significance of this difference is not yet known, but Ramirez suspects that it might reduce the sensitivity of the locust's flight motor neurons, allowing the insect to fly in ultra-high temperatures. "There is definitely adaptive significance to the change in potassium levels, although more investigation is required to determine exactly how the changes influence the locust's ability to fly in extreme heat," Ramirez explains.

Next, Ramirez wants to see if heat shock has the same neuroprotective role in invertebrates. "If we see the same effect in mice we can start trying to figure out exactly how heat shock plays this protective role," says Ramirez. "Then we might be able to mimic those protective effects in cases where the nerves are undergoing trauma, like in stroke patients where the nerves are starved and attacked by free radicals."

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