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Brain-Imaging Study Tries to Discover Why We Lose Our Sense of Thirst as We Age
| 05.10.2004

SAN ANTONIO -- The sense of thirst declines as people age, a phenomenon that can be especially dangerous for the elderly during the summer.

Scientists in Texas and Australia are collaborating on a study to find out what happens in the brain that causes the sense of thirst to wane.

“Many elderly people should feel thirsty, but they don’t, and because they’re not thirsty, they don’t drink enough liquids,” said Dr. Robert Shade, associate scientific director and chairman of the Department of Physiology and Medicine at the Southwest Foundation for Biomedical Research, based in San Antonio. “As a result, they can become volume depleted or dehydrated.”

Shade said that may be one significant reason why the elderly are more adversely affected by heat stress, which can send many to the hospital for emergency medical care and even lead to death, as was seen in the summer 2003 heat wave in France that killed thousands.

Currently, scientists and physicians do not understand why people’s sense of thirst declines with age. One strong possibility is a “cognitive disconnect.” As cognitive function declines with aging, it may be that something goes awry in the process of how a physical stimulus sends a signal to the brain and the brain interprets that signal to elicit the appropriate response. But where does that disconnect occur? And what exactly is causing it? That still needs to be determined.

“Clearly, either many elderly people are not aware of being thirsty – at least not to the degree they should be – or if they are aware of it, it isn’t registering correctly and they’re misinterpreting the symptoms,” said Dr. Peter Fox, director of the Research Imaging Center, the University of Texas Health Science Center at San Antonio. “Perhaps they’re uncomfortable, but they can’t put their finger on why. So it’s not clear what exactly is going on, but it is clear that when you lose the urgency of your thirst drive, you die. So this is a big risk.”

Shade and Fox are working with Dr. Derek A. Denton, emeritus research professor and emeritus director, the Howard Florey Institute for Experimental Physiology and Medicine, The University of Melbourne, Australia. A grant to the Florey Institute from the G. Harold and Leila Y. Mathers Charitable Foundation, New York, is funding the thirst study.

For 15 years, Drs. Shade and Denton have collaborated on animal studies exploring the physiology of salt-and-water metabolism and salt-and-water appetite. In 1997, they teamed up with Dr. Peter Fox, with resources and expertise in brain imaging and an understanding of the brain mechanisms of physiology and cognition. With their combination of expertise and resources, these researchers were in a unique position to explore the physiological interaction of cerebral function and the autonomic nervous system, particularly as it relates to thirst.

They have conducted several studies of a “normal” population, healthy young adults, who agreed to undergo PET scans of their brains while experiencing different levels of thirst. These studies served to document which areas of the brain are activated as the need for fluids increases and decreases; the strength of brain signals and their reception in areas of the brain related to perception, attention and cognition; and which areas of the brain are affected by either thirst, physical stress, or both.

Now that they have the data from younger adults, the investigators are recruiting an equal number of healthy senior citizens, age 65 and up, to participate in the same study. Researchers can then compare their brain scans to those of their younger counterparts and hopefully identify parts of the thirst-signaling mechanism that are not working properly in the elderly.

“We’re going to look at where in the brain signals are getting lost as the brain first receives a signal from a thirst stimulus, then transfers that signal to the area of the brain where consciousness lies,” said Dr. Shade. “We also will look at differences in the degree of brain activation in the elderly as compared to young people. Perhaps we’ll see that the right areas of the brain are activated in the elderly, but the degree of that activation is less. That would be very informative. It would give us a clue to where the differences lie and what the problem is.”

From there, researchers expect that follow-up studies in animals will be needed to learn more about the physiological factors causing the breakdowns in the thirst mechanism, as well as how to design treatments to correct that breakdown.

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