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Imaging study sheds light on thirst and the brain (2-28-00)

This is your brain when youíre thirsty. This is your brain when youíre not.

Scientists at The University of Texas Health Science Center at San Antonio literally saw the difference after recording sophisticated >brain images that reveal more detail than ever about how our brains process information related to thirst and satiation.

Lead author Lawrence M. Parsons, Ph.D., and his American and Australian colleagues are reporting that the cerebellum, a portion of the brain near the brain stem, appears to optimize brain processing at critical moments such as when one becomes thirsty or has just satisfied a thirst. The findings of a study in 10 human subjects are reported in the Feb. 29 issue of the Proceedings of the National Academy of Science USA. Dr. Parsons is an assistant professor at the Health Science Centerís Research Imaging Center.

Scientists have long believed that the cerebellum coordinates muscle movement, and have ignored the possibility of its role in other functions. "Does the cerebellum directly tell us we are thirsty?" Dr. Parsons asked. "We donít think so. Itís not like a thermostat that detects a rapid change in temperature and causes the system to respond. Rather, the cerebellum helps the rest of the brain process information about the feelings and thoughts related to thirst."

He and his colleagues are testing the idea that the cerebellum is like a computer server that distributes information to branch computers. "When you are very thirsty or have just quenched the thirst, the brain initiates a great deal of processing to evaluate whether everything is okay and what is happening," he said. "The cerebellum, we think, helps the whole process to be efficient."

For example, the cerebellum and the hypothalamus, another key processing center, "talk" to each other in a sort of reciprocal connectivity. The hypothalamus is specifically involved in the physiological detection of thirst.

What happens if the cerebellum is damaged?

"Some people are born without cerebellums and no one can tell the difference." Dr. Parsons said. "This suggests that the cerebellum has a subtle influence on the rest of the brain. There is no evidence for a relationship between damage to the cerebellum and a personís experience of thirst. However, recent studies find that damage to the cerebellum does cause subtle impairments in the sensitivity or speed of mental processing, and we now believe thirst is one of those processes."

Scientists, working in the Research Imaging Center, studied healthy volunteers with a technique called Positron Emission Tomography (PET). During scanning, the researchers induced thirst intravenously by changing the concentration of sodium in the blood. The volunteers reported degrees of thirst depending on the salt concentration. Shortly after reaching strong thirst, the subjects were allowed to drink as much water as they wanted, and were scanned again. The scientists based their findings on the scans taken at different times.

The Research Imaging Center is a world-renowned center of PET, MRI (magnetic resonance imaging) and other brain mapping methods. The U.S. Department of Defense and the South Texas Veterans Health Care System provided the funding for the centerís PET system. The thirst study was supported by funding from the Robert J., Jr., and Helen C. Kleberg Foundation, the Harold G. and Leila Y. Mathers Charitable Foundation, the National Health and Medical Research Council of Australia, and the Howard Florey Biomedical Foundation of the United States.

Dr. Parsons collaborated with Derek Denton, Ph.D., of the Howard Florey Institute of Experimental Physiology and Medicine at The University of Melbourne, Australia. Co-authors from the Research Imaging Center include Jack L. Lancaster, Ph.D., professor, and Peter T. Fox, M.D., professor and director of the RIC. Other collaborators are from the Southwest Foundation for Biomedical Research in San Antonio.

Contact: Will Sansom