Overdrinking, swallowing inhibition, and regional brain responses prior to swallowing.
In humans, drinking replenishes fluid loss and satiates the sensation of thirst that accompanies dehydration. Typically, the volume of water drunk in response to thirst matches the deficit. Exactly how this accurate metering is achieved is unknown; recent evidence implicates swallowing inhibition as a potential factor. Using fMRI, this study investigated whether swallowing inhibition is present after more water has been drunk than is necessary to restore fluid balance within the body. This proposal was tested using ratings of swallowing effort and measuring regional brain responses as participants prepared to swallow small volumes of liquid while they were thirsty and after they had overdrunk. Effort ratings provided unequivocal support for swallowing inhibition, with a threefold increase in effort after overdrinking, whereas addition of 8% (wt/vol) sucrose to water had minimal effect on effort before or after overdrinking. Regional brain responses when participants prepared to swallow showed increases in the motor cortex, prefrontal cortices, posterior parietal cortex, striatum, and thalamus after overdrinking, relative to thirst. Ratings of swallowing effort were correlated with activity in the right prefrontal cortex and pontine regions in the brainstem; no brain regions showed correlated activity with pleasantness ratings. These findings are all consistent with the presence of swallowing inhibition after excess water has been drunk. We conclude that swallowing inhibition is an important mechanism in the overall regulation of fluid intake in humans.
Florey Institute of Neuroscience and Mental Health, University of Melbourne, Parkville, VIC 3010, Australia. Monash Biomedical Imaging, Monash University, VIC 3800, Australia; Monash Biomedicine Discovery Institute, Monash University, Melbourne, VIC 3800, Australia; Department of Medical Imaging and Radiation Sciences, Monash University, Melbourne, VIC 3800, Australia; michael.j.farrell@monash.edu ddenton@unimelb.edu.au. Monash Biomedical Imaging, Monash University, VIC 3800, Australia; Australian Research Council Centre of Excellence for Integrative Brain Function, Monash University, VIC 3800, Australia; Monash Institute of Clinical and Cognitive Neurosciences, Monash University, VIC 3800, Australia. Florey Institute of Neuroscience and Mental Health, University of Melbourne, Parkville, VIC 3010, Australia; Department of Physiology, University of Melbourne, Parkville, VIC 3010, Australia. Florey Institute of Neuroscience and Mental Health, University of Melbourne, Parkville, VIC 3010, Australia; Office of the Dean, Faculty of Medicine, Dentistry & Health Sciences, University of Melbourne, Parkville, VIC 3010, Australia; Baker International Diabetes Institute Heart and Diabetes Institute, Melbourne, VIC 3004, Australia michael.j.farrell@monash.edu ddenton@unimelb.edu.au.
