Abstract
The motivation to seek and consume water is an essential component of human fluid–electrolyte homeostasis, optimal function, and health. This review describes the evolution of concepts regarding thirst and drinking behavior, made possible by magnetic resonance imaging, animal models, and novel laboratory techniques. The earliest thirst paradigms focused on single factors such as dry mouth and loss of water from tissues. By the end of the 19th century, physiologists proposed a thirst center in the brain that was verified in animals 60 years later. During the early- and mid-1900s, the influences of gastric distention, neuroendocrine responses, circulatory properties (i.e., blood pressure, volume, concentration), and the distinct effects of intracellular dehydration and extracellular hypovolemia were recognized. The majority of these studies relied on animal models and laboratory methods such as microinjection or lesioning/oblation of specific brain loci. Following a quarter century (1994–2019) of human brain imaging, current research focuses on networks of networks, with thirst and satiety conceived as hemispheric waves of neuronal activations that traverse the brain in milliseconds. Novel technologies such as chemogenetics, optogenetics, and neuropixel microelectrode arrays reveal the dynamic complexity of human thirst, as well as the roles of motivation and learning in drinking behavior.
Highlights
Water comprises over 80% of human brain, cardiac, skeletal muscle, kidney and gastrointestinal tissues [1]; it is the medium in which metabolism, excretion, absorption, secretion and diffusion occur
Because water is essential for the survival of humans, selective pressures have forged mechanisms that regulate total body water (TBW) volume during periods of abundance, perturbation and insufficiency
Because little is known about human neural networks that produce the thirst sensation and motivate humans to drink, it is reasonable to ask, “What are the functions of activated brain loci in Tables 2–5?” In response to this question, Table 6 presents a variety of evidence-based functions and sensations that have been associated with swallowing, thirst, taste, smell, and somatosensory information
Summary
Water comprises over 80% of human brain, cardiac, skeletal muscle, kidney and gastrointestinal tissues [1]; it is the medium in which metabolism, excretion, absorption, secretion and diffusion occur. In contrast to homeostatic thirst, non-homeostatic thirst (Figure 3) is influenced by the taste and temperature of a fluid (i.e., alliesthesia [102,103]), mouth dryness, gastric distention, meal contents, meal timing [105], idiosyncratic learned, preferences, occupational schedules, social interactions, and cultural norms [75,98] Both physiologists and cognitive neuroscientists have recognized that dehydrated humans drink to satiation rapidly across 3–10 min [74,106], causing decreased thirst and motivation to drink, well before elevated plasma osmolality and sodium return to normal concentrations Expression of a reporter gene, ex vivo radiolabeling of co-incubated target cells, and transfer of cells for in vivo PET imaging [118]
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