The work stands off on its own: there is almost no one nearby, the nearest neighbor at a distance of 0.485. Thematically it is closest to research on large language models, genes, explainable AI and self-learning, but in essence it is pure neurophysiology of aging. Around it, quite a lot has already been done on individual nodes: how networks find critical points on their own, how biological recognition switches smoothly, how robots learn in chaos. This paper fills a gap: for the first time, systematically, at the level of specific neurons and behavior, it shows how the entire thirst system ages as a whole.
Mice of different ages (from 2 to 30 months) of both sexes were studied at several levels. Plasma and urine osmolality, urine volume, vasopressin and copeptin levels were measured. Eating and drinking behavior was recorded using automatic feeders and lickometers with sub-second resolution. The activity of neurons in the subfornical organ (SFO) and paraventricular hypothalamus (PVH) was recorded by fiber photometry with the calcium indicator GCaMP6s. Dopamine in the nucleus accumbens was measured with the GRAB-DA3m sensor. To test causality, optogenetic stimulation of SFO neurons and operant lever tasks were used.
In old mice (26–30 months, which corresponds to 73–81 years in humans), blood plasma is more concentrated: 330 versus 312 mOsm/kg in young males. Their urine is more dilute (1716 versus 2201 mOsm/kg), and urine volume is higher. Urinary vasopressin level is reduced (409 versus 665 pg/ml), and kidney sensitivity to its analog dDAVP falls. After a meal, PVH neurons that secrete vasopressin activate more slowly (18.3 versus 13.7 minutes to half-maximum), and the link between food intake and the start of drinking breaks down: the delay in old males is 21.3 minutes versus 4.0 in young ones. At the same time, after 24-hour water deprivation, old mice drink more (795 versus 489 licks in males), press the lever harder for water (89 versus 55 presses) and show a longer dopamine release in response to water (74.8 versus 38.1 seconds).
Unlike neighboring works that examine individual mechanisms of aging, this one for the first time assembles the full picture: how several nodes of a single homeostatic system age at once. The main difference from its neighbors is not a new method but the scale: simultaneous coverage of physiology, behavior and neural dynamics across the entire lifespan. This makes it possible to see that the thirst system ages unevenly: some pathways (presystemic signals from the mouth and gut) degrade, while others (systemic blood signals) remain preserved or even strengthen.
The topic could develop toward the search for molecular causes of age-related changes in the sensory afferents of the oral cavity and gastrointestinal tract. Perhaps the same mechanisms that worsen vision and hearing with age also damage the receptors that tell the brain about food and drink. Another direction is to study the time course of dehydration after water deprivation: how quickly osmolality rises and blood volume falls in old animals.
The results are important for gerontology and clinical medicine: they explain why older people often drink at the wrong time and suffer from dehydration. They may also help in developing hydration strategies for elderly patients and in understanding age-related changes in motivation.
Measure the dynamics of osmolality and blood volume after water deprivation in young and old mice. Check how aging affects the sensory neurons that innervate the mouth and gut.
The work is directly connected to an unsolved problem in gerontology: why some homeostatic systems age faster than others. It shows that degradation and compensatory strengthening can coexist within a single system, which complicates the search for a single "cause of aging".
Old mice often drop and scatter their food, so to eat the same amount they need to fuss with the pellets longer. This does not prevent them from eating normally, but it delays the start of drinking after a meal.
Not published on arXiv. Here are its page, the PDF and the formalization: you can check it yourself, not only read our account.