Effect of aging on regional cerebral blood flow responses associated with osmotic thirst and its satiation by water drinking: A PET study

*Howard Florey Institute,
^†Centre for Neuroscience,
**Department of Physiology, and
^††Office of the Dean, Faculty of Medicine, Dentistry, and Health Sciences, University of Melbourne, Parkville, Victoria 3010, Australia;
^‡National Ageing Research Institute, Parkville, Victoria 3052, Australia;
^§Research Imaging Center, University of Texas Health Science Center, San Antonio, TX 78229-3904;
^¶Southwest Foundation for Biomedical Research, P.O. Box 760549, San Antonio, TX 78245-0549;
^‖School of Medicine, Flinders University, Adelaide, South Australia 5001, Australia; and
^‡‡Baker Heart Research Institute, Alfred Hospital, Prahran, Victoria 3181, Australia

Contributed by D. A. Denton, November 7, 2007 (received for review August 10, 2007)

Abstract

Levels of thirst and ad libitum drinking decrease with advancing age, making older people vulnerable to dehydration. This study investigated age-related changes in brain responses to thirst and drinking in healthy men. Thirst was induced with hypertonic infusions (3.1 ml/kg 0.51M NaCl) in young (Y) and older (O) subjects. Regional cerebral blood flow (rCBF) was measured with positron emission tomography (PET). Thirst activations were identified by correlating rCBF with thirst ratings. Average rCBF was measured from regions of interest (ROI) corresponding to activation clusters in each group. The effects of drinking were examined by correlating volume of water drunk with changes in ROI rCBF from maximum thirst to postdrinking. There were increases in blood osmolality (Y, 2.8 ± 1.8%; O, 2.2 ± 1.4%) and thirst ratings (Y, 3.1 ± 2.1; O, 3.7 ± 2.8) from baseline to the end of the hypertonic infusion. Older subjects drank less water (1.9 ± 1.6 ml/kg) than younger subjects (3.9 ± 1.9 ml/kg). Thirst-related activation was evident in S1/M1, prefrontal cortex, anterior midcingulate cortex (aMCC), premotor cortex, and superior temporal gyrus in both groups. Postdrinking changes of rCBF in the aMCC correlated with drinking volumes in both groups. There was a greater reduction in aMCC rCBF relative to water drunk in the older group. Aging is associated with changes in satiation that militate against adequate hydration in response to hyperosmolarity, although it is unclear whether these alterations are due to changes in primary afferent inflow or higher cortical functioning.

Footnotes

^§§To whom correspondence may be addressed. E-mail: gary.egan{at}florey.edu.au or ddenton{at}unimelb.edu.au
Author contributions: M.J.F., P.A.P., M.M., P.T.F., D.A.D., and G.F.E. designed research; M.J.F., R.S., P.A.P., P.T.F., and J.B.-W. performed research; M.J.F., F.Z., R.S., and G.F.E. analyzed data; and M.J.F., M.M., D.A.D., and G.F.E. wrote the paper.
The authors declare no conflict of interest.