Age-related changes in thirst and associated neural activity in human subjects

Abstract

The intensity of thirst and the volume of fluid ingested in response to water deprivation decreases with advancing age, rendering the elderly vulnerable to dehydration. We have previously investigated changes in regional cerebral blood flow (rCBF) in young thirsty human subjects using both positron emission tomography (PET) and functional magnetic resonance imaging (fMRI). We have now extended these studies to the comparison of age-related changes in rCBF and drinking in response to thirst a stimulus in healthy men. Thirst was stimulated in 10 younger (23.7±2.8 years old) and 12 elderly (68.1±3.4 years) human male volunteers by intravenous infusion of hypertonic 0.5 M NaCl solution. rCBF was measured from regions of interest with PET. Thirst-related activations were generated by correlating rCBF with thirst ratings. Similar increases in plasma osmolality and ratings of thirst occurred in both groups after infusion of hypertonic NaCl. However, older subjects drank less water (153±29 ml) than the younger group (345±52 ml) in response to this stimulus. Thirst-related activation was evident in primary somatosensory and motor cortices, prefrontal cortex, anterior cingulate cortex (ACC), premotor cortex, and superior temporal gyrus in both groups. Post-drinking changes of rCBF in the ACC correlated with drinking volumes in both groups, but there was a steeper decrease in ACC rCBF relative to water drunk in the elderly. These results suggest that aging may be associated with changes in the satiation of thirst that may contribute to inadequate hydration in response to hypertonicity.