Neural Control and Modulation of Thirst, Sodium Appetite, and Hunger.

The function of central appetite neurons is instructing animals to ingest specific nutrient factors that the body needs. Emerging evidence suggests that individual appetite circuits for major nutrients-water, sodium, and food-operate on unique driving and quenching mechanisms. This review focuses on two aspects of appetite regulation. First, we describe the temporal relationship between appetite neuron activity and consumption behaviors. Second, we summarize ingestion-related satiation signals that differentially quench individual appetite circuits. We further discuss how distinct appetite and satiation systems for each factor may contribute to nutrient homeostasis from the functional and evolutional perspectives.

Hindbrain Double-Negative Feedback Mediates Palatability-Guided Food and Water Consumption.

Hunger and thirst have distinct goals but control similar ingestive behaviors, and little is known about neural processes that are shared between these behavioral states. We identify glutamatergic neurons in the peri-locus coeruleus (periLCVGLUT2 neurons) as a polysynaptic convergence node from separate energy-sensitive and hydration-sensitive cell populations. We develop methods for stable hindbrain calcium imaging in free-moving mice, which show that periLCVGLUT2 neurons are tuned to ingestive behaviors and respond similarly to food or water consumption. PeriLCVGLUT2 neurons are scalably inhibited by palatability and homeostatic need during consumption. Inhibition of periLCVGLUT2 neurons is rewarding and increases consumption by enhancing palatability and prolonging ingestion duration. These properties comprise a double-negative feedback relationship that sustains food or water consumption without affecting food- or water-seeking. PeriLCVGLUT2 neurons are a hub between hunger and thirst that specifically controls motivation for food and water ingestion, which is a factor that contributes to hedonic overeating and obesity.

Coupled Sensing of Hunger and Thirst Signals Balances Sugar and Water Consumption.

Hunger and thirst are ancient homeostatic drives for food and water consumption. Although molecular and neural mechanisms underlying these drives are currently being uncovered, less is known about how hunger and thirst interact. Here, we use molecular genetic, behavioral, and anatomical studies in Drosophila to identify four neurons that modulate food and water consumption. Activation of these neurons promotes sugar consumption and restricts water consumption, whereas inactivation promotes water consumption and restricts sugar consumption. By calcium imaging studies, we show that these neurons are directly regulated by a hormone signal of nutrient levels and by osmolality. Finally, we identify a hormone receptor and an osmolality-sensitive ion channel that underlie this regulation. Thus, a small population of neurons senses internal signals of nutrient and water availability to balance sugar and water consumption. Our results suggest an elegant mechanism by which interoceptive neurons oppositely regulate homeostatic drives to eat and drink.

Neural landscape diffusion resolves conflicts between needs across time.

Animals perform flexible goal-directed behaviours to satisfy their basic physiological needs1-12. However, little is known about how unitary behaviours are chosen under conflicting needs. Here we reveal principles by which the brain resolves such conflicts between needs across time. We developed an experimental paradigm in which a hungry and thirsty mouse is given free choices between equidistant food and water. We found that mice collect need-appropriate rewards by structuring their choices into persistent bouts with stochastic transitions. High-density electrophysiological recordings during this behaviour revealed distributed single neuron and neuronal population correlates of a persistent internal goal state guiding future choices of the mouse. We captured these phenomena with a mathematical model describing a global need state that noisily diffuses across a shifting energy landscape. Model simulations successfully predicted behavioural and neural data, including population neural dynamics before choice transitions and in response to optogenetic thirst stimulation. These results provide a general framework for resolving conflicts between needs across time, rooted in the emergent properties of need-dependent state persistence and noise-driven shifts between behavioural goals.

Dopaminergic error signals retune to social feedback during courtship.

Hunger, thirst, loneliness and ambition determine the reward value of food, water, social interaction and performance outcome1. Dopamine neurons respond to rewards meeting these diverse needs2-8, but it remains unclear how behaviour and dopamine signals change as priorities change with new opportunities in the environment. One possibility is that dopamine signals for distinct drives are routed to distinct dopamine pathways9,10. Another possibility is that dopamine signals in a given pathway are dynamically tuned to rewards set by the current priority. Here we used electrophysiology and fibre photometry to test how dopamine signals associated with quenching thirst, singing a good song and courting a mate change as male zebra finches (Taeniopygia guttata) were provided with opportunities to retrieve water, evaluate song performance or court a female. When alone, water reward signals were observed in two mesostriatal pathways but singing-related performance error signals were routed to Area X, a striatal nucleus specialized for singing. When courting a female, water seeking was reduced and dopamine responses to both water and song performance outcomes diminished. Instead, dopamine signals in Area X were driven by female calls timed with the courtship song. Thus the dopamine system handled coexisting drives by routing vocal performance and social feedback signals to a striatal area for communication and by flexibly re-tuning to rewards set by the prioritized drive.

Sensory representation and detection mechanisms of gut osmolality change.

Ingested food and water stimulate sensory systems in the oropharyngeal and gastrointestinal areas before absorption1,2. These sensory signals modulate brain appetite circuits in a feed-forward manner3-5. Emerging evidence suggests that osmolality sensing in the gut rapidly inhibits thirst neurons upon water intake. Nevertheless, it remains unclear how peripheral sensory neurons detect visceral osmolality changes, and how they modulate thirst. Here we use optical and electrical recording combined with genetic approaches to visualize osmolality responses from sensory ganglion neurons. Gut hypotonic stimuli activate a dedicated vagal population distinct from mechanical-, hypertonic- or nutrient-sensitive neurons. We demonstrate that hypotonic responses are mediated by vagal afferents innervating the hepatic portal area (HPA), through which most water and nutrients are absorbed. Eliminating sensory inputs from this area selectively abolished hypotonic but not mechanical responses in vagal neurons. Recording from forebrain thirst neurons and behavioural analyses show that HPA-derived osmolality signals are required for feed-forward thirst satiation and drinking termination. Notably, HPA-innervating vagal afferents do not sense osmolality itself. Instead, these responses are mediated partly by vasoactive intestinal peptide secreted after water ingestion. Together, our results reveal visceral hypoosmolality as an important vagal sensory modality, and that intestinal osmolality change is translated into hormonal signals to regulate thirst circuit activity through the HPA pathway.

Thirst-driven hygrosensory suppression promotes water seeking in Drosophila.

Survival in animals relies on navigating environments aligned with physiological needs. In Drosophila melanogaster, antennal ionotropic receptors (IRs) sensing humidity changes govern hygrotaxis behavior. This study sheds light on the crucial role of IR8a neurons in the transition from high humidity avoidance to water-seeking behavior when the flies become thirsty. These neurons demonstrate a heightened calcium response toward high humidity stimuli in satiated flies and a reduced response in thirsty flies, modulated by fluctuating levels of the neuropeptide leucokinin, which monitors the internal water balance. Optogenetic activation of IR8a neurons in thirsty flies triggers an avoidance response similar to the moisture aversion in adequately hydrated flies. Furthermore, our study identifies IR40a neurons as associated with dry avoidance, while IR68a neurons are linked to moist attraction. The dynamic interplay among these neurons, each with opposing valences, establishes a preference for approximately 30% relative humidity in well-hydrated flies and facilitates water-seeking behavior in thirsty individuals. This research unveils the intricate interplay between sensory perception, neuronal plasticity, and internal states, providing valuable insights into the adaptive mechanisms governing hygrotaxis in Drosophila.

The cellular basis of distinct thirst modalities.

Fluid intake is an essential innate behaviour that is mainly caused by two distinct types of thirst1-3. Increased blood osmolality induces osmotic thirst that drives animals to consume pure water. Conversely, the loss of body fluid induces hypovolaemic thirst, in which animals seek both water and minerals (salts) to recover blood volume. Circumventricular organs in the lamina terminalis are critical sites for sensing both types of thirst-inducing stimulus4-6. However, how different thirst modalities are encoded in the brain remains unknown. Here we employed stimulus-to-cell-type mapping using single-cell RNA sequencing to identify the cellular substrates that underlie distinct types of thirst. These studies revealed diverse types of excitatory and inhibitory neuron in each circumventricular organ structure. We show that unique combinations of these neuron types are activated under osmotic and hypovolaemic stresses. These results elucidate the cellular logic that underlies distinct thirst modalities. Furthermore, optogenetic gain of function in thirst-modality-specific cell types recapitulated water-specific and non-specific fluid appetite caused by the two distinct dipsogenic stimuli. Together, these results show that thirst is a multimodal physiological state, and that different thirst states are mediated by specific neuron types in the mammalian brain.

A gut-to-brain signal of fluid osmolarity controls thirst satiation.

Satiation is the process by which eating and drinking reduce appetite. For thirst, oropharyngeal cues have a critical role in driving satiation by reporting to the brain the volume of fluid that has been ingested1-12. By contrast, the mechanisms that relay the osmolarity of ingested fluids remain poorly understood. Here we show that the water and salt content of the gastrointestinal tract are precisely measured and then rapidly communicated to the brain to control drinking behaviour in mice. We demonstrate that this osmosensory signal is necessary and sufficient for satiation during normal drinking, involves the vagus nerve and is transmitted to key forebrain neurons that control thirst and vasopressin secretion. Using microendoscopic imaging, we show that individual neurons compute homeostatic need by integrating this gastrointestinal osmosensory information with oropharyngeal and blood-borne signals. These findings reveal how the fluid homeostasis system monitors the osmolarity of ingested fluids to dynamically control drinking behaviour.

Dynamic processing of hunger and thirst by common mesolimbic neural ensembles.

The nucleus accumbens (NAc) is a canonical reward center that regulates feeding and drinking but it is not known whether these behaviors are mediated by same or different neurons. We employed two-photon calcium imaging in awake, behaving mice and found that during the appetitive phase, both hunger and thirst are sensed by a nearly identical population of individual D1 and D2 neurons in the NAc that respond monophasically to food cues in fasted animals and water cues in dehydrated animals. During the consummatory phase, we identified three distinct neuronal clusters that are temporally correlated with action initiation, consumption, and cessation shared by feeding and drinking. These dynamic clusters also show a nearly complete overlap of individual D1 neurons and extensive overlap among D2 neurons. Modulating D1 and D2 neural activities revealed analogous effects on feeding versus drinking behaviors. In aggregate, these data show that a highly overlapping set of D1 and D2 neurons in NAc detect food and water reward and elicit concordant responses to hunger and thirst. These studies establish a general role of this mesolimbic pathway in mediating instinctive behaviors by controlling motivation-associated variables rather than conferring behavioral specificity.

An integrative sensor of body states: how the mushroom body modulates behavior depending on physiological context.

The brain constantly compares past and present experiences to predict the future, thereby enabling instantaneous and future behavioral adjustments. Integration of external information with the animal's current internal needs and behavioral state represents a key challenge of the nervous system. Recent advancements in dissecting the function of the Drosophila mushroom body (MB) at the single-cell level have uncovered its three-layered logic and parallel systems conveying positive and negative values during associative learning. This review explores a lesser-known role of the MB in detecting and integrating body states such as hunger, thirst, and sleep, ultimately modulating motivation and sensory-driven decisions based on the physiological state of the fly. State-dependent signals predominantly affect the activity of modulatory MB input neurons (dopaminergic, serotoninergic, and octopaminergic), but also induce plastic changes directly at the level of the MB intrinsic and output neurons. Thus, the MB emerges as a tightly regulated relay station in the insect brain, orchestrating neuroadaptations due to current internal and behavioral states leading to short- but also long-lasting changes in behavior. While these adaptations are crucial to ensure fitness and survival, recent findings also underscore how circuit motifs in the MB may reflect fundamental design principles that contribute to maladaptive behaviors such as addiction or depression-like symptoms.

Physiological Needs: Sensations and Predictions in the Insular Cortex.

Physiological needs create powerful motivations (e.g., thirst and hunger). Studies in humans and animal models have implicated the insular cortex in the neural regulation of physiological needs and need-driven behavior. We review prominent mechanistic models of how the insular cortex might achieve this regulation and present a conceptual and analytical framework for testing these models in healthy and pathological conditions.

Thirst and drugs: A study in the World Health Organization's pharmacovigilance database.

Thirst is a complex physiological compensatory mechanism but could also be associated with drugs. This association was poorly investigated previously. Using the WHO global pharmacovigilance database, Vigibase®, disproportionality analyses potential associations between exposure to drugs and thirst reports were performed. All reports of thirst in adults between 01/01/2000 and 31/12/2023 were included. Results are expressed as reporting odds ratio (ROR). Analysis of the 3186 reports of thirst (978 'serious') allowed, first, to confirm the association between thirst and exposure to vasopressin antagonists (tolvaptan), lithium, gliflozins (dapagliflozin, empagliflozin), pregabalin and antimuscarinic drugs (glycopyronium, oxybutynin, tiotropium). Second, new safety signals were described with monoamine reuptake inhibitors (antidepressants: duloxetine, venlafaxine; anti-obesity agent: sibutramine), antipsychotic (olanzapine), glucocorticoid (prednisolone), diuretic (furosemide) drugs as well with ribavirin or sodium oxybate. This study is the first to offer a list of drugs associated with thirst in humans.

The weight, urine colour and thirst Venn diagram is an accurate tool compared with urinary and blood markers for hydration assessment at morning and afternoon timepoints in euhydrated and free-living individuals.

The weight, urine colour and thirst (WUT) Venn diagram is a practical hydration assessment tool; however, it has only been investigated during first-morning. This study investigated accuracy of the WUT Venn diagram at morning and afternoon timepoints compared with blood and urine markers. Twelve men (21 ± 2 years; 81·0 ± 15·9 kg) and twelve women (22 ± 3 years; 68·8 ± 15·2 kg) completed the study. Body mass, urine colour, urine specific gravity (USG), urine osmolality (UOSM), thirst and plasma osmolality (POSM) were collected at first-morning and afternoon for 3 consecutive days in free-living (FL) and euhydrated states. Number of markers indicating dehydration levels were categorised into either 3, 2, 1 or 0 WUT markers. Receiver operating characteristics analysis calculated the sensitivity and specificity of 1, 2 or 3 hydration markers in detecting dehydration or euhydration. Specificity values across morning and afternoon exhibited high diagnostic accuracy for USG (0·890-1·000), UOSM (0·869-1·000) and POSM (0·787-0·990) when 2 and 3 WUT markers were met. Sensitivity values across both timepoints exhibited high diagnostic accuracy for USG (0·826-0·941) and UOSM (0·826-0·941), but not POSM in the afternoon (0·324) when 0 and 1 WUT markers were met. The WUT Venn diagram is accurate in detecting dehydration for WUT2 and WUT3 based off USG, UOSM and POSM during first-morning and afternoon. Applied medical, sport and occupational practitioners can use this tool in field settings for hydration assessment not only at various timepoints throughout the day but also in FL individuals.

Hierarchical neural architecture underlying thirst regulation.

Neural circuits for appetites are regulated by both homeostatic perturbations and ingestive behaviour. However, the circuit organization that integrates these internal and external stimuli is unclear. Here we show in mice that excitatory neural populations in the lamina terminalis form a hierarchical circuit architecture to regulate thirst. Among them, nitric oxide synthase-expressing neurons in the median preoptic nucleus (MnPO) are essential for the integration of signals from the thirst-driving neurons of the subfornical organ (SFO). Conversely, a distinct inhibitory circuit, involving MnPO GABAergic neurons that express glucagon-like peptide 1 receptor (GLP1R), is activated immediately upon drinking and monosynaptically inhibits SFO thirst neurons. These responses are induced by the ingestion of fluids but not solids, and are time-locked to the onset and offset of drinking. Furthermore, loss-of-function manipulations of GLP1R-expressing MnPO neurons lead to a polydipsic, overdrinking phenotype. These neurons therefore facilitate rapid satiety of thirst by monitoring real-time fluid ingestion. Our study reveals dynamic thirst circuits that integrate the homeostatic-instinctive requirement for fluids and the consequent drinking behaviour to maintain internal water balance.

Clinical Characteristics of Adipsic Diabetes Insipidus.

OBJECTIVE: Adipsic diabetes insipidus (ADI) is a life-threatening disease. It is characterized by arginine vasopressin deficiency and thirst absence. Data about clinical characteristics of ADI were scarce. This study investigated the clinical features of hospitalized ADI patients. METHODS: A retrospective study was conducted of hospitalized ADI patients admitted to the Endocrinology Department of Huashan Hospital between January 2014 and December 2021, and compared with central diabetes insipidus (CDI) patients with normal thirst. RESULTS: During the study period, there were a total of 507 hospitalized CDI patients, among which 50 cases were ADI, accounting for 9.9%. Forty percent of ADI patients were admitted due to hypernatremia, but there were no admissions due to hypernatremia in the control group. The lesions of ADI patients were more likely to be located in the suprasellar area (100% vs 66%, P < .05). Higher prevalence of hypothalamic dysfunction (76% vs 8%, P < .001), central hypothyroidism (100% vs 90%, P = .031), hyperglycemia (66% vs 32%, P < .001), dyslipidemia (92% vs 71%, P = .006), and hyperuricemia (64% vs 37%, P = .003) was found in the ADI group than in the control group. The proportions of hypernatremia were higher in the ADI group both at admission and at discharge (90% vs 8%, 68% vs 8%, respectively, both with P < .001), contributing to higher prevalence of complications, such as renal insufficiency, venous thrombosis, and infection. CONCLUSION: ADI patients were found with higher prevalence of hypernatremia, hypopituitarism, hypothalamic dysfunction, metabolic disorders, and complications, posing a great challenge for comprehensive management.

Is increased subjective thirst associated with greater interdialytic weight gains, extracellular fluid and dietary sodium intake?

BACKGROUND: Some previous studies have reported an effect of increasing subjective thirst and interdialytic weight gains (IDWG), and that this may be influenced by nonadherence to dietary sodium restrictions, whereas others reported no such association. As such we wished to review the effect of self-reported thirst on IDWGs and dietary sodium intake. METHODS: Dialysis patients were asked to complete visual analogues thirst, distress thermometer (DT) scores and complete a sodium food frequency questionnaire (SFFQ). IDWG and pre and post dialysis volumes were measured with multifrequency bioelectrical impedance. RESULTS: One hundred and eleven patients completed the questionnaires and had bioimpedance measurements: 63% male, mean age 63.8 ± 16.1 years, 33% diabetic with a median thirst score 3 (0-5) and SFFQ 52.0 ± 18, and IDWG 2.1 ± 1.3%. Thirst was associated with DT (r = 0.28, p = 0.004) and negatively with age (r = -0.31, p < 0.001), but not SFFQ, IDWG, extracellular water, or dialysate sodium, or dialysate to plasma gradient. Patients with higher thirst scores were younger (58.0 ± 15.2 vs. 69.4 ± 15.0 years, p < 0.001) with higher DT scores (5 [2-7] vs. 2 [0-5], p < 0.001). On multivariate logistic analysis, only age was associated with self-reported thirst (odds ratio 0.95, 95% confidence limits 0.92-0.98, p < 0.001). CONCLUSION: We found that subjective thirst was greater for younger patients and those who reported higher levels of distress, but no association with IDWGs, dietary sodium intake, or dialysate sodium. However, most of our patients followed the dietary advice, as evidenced by the low SFFQ scores and % IDWGs. Whether thirst increases distress or distress increases subjective thirst remains to be determined.

Early oral hydration on demand in postanesthesia care unit effectively relieves postoperative thirst in patients after gynecological laparoscopy: a prospective randomized controlled trial.

BACKGROUND: Postoperative thirst is one of the most intense, common and easily ignored subjective discomforts in patients after gynecological surgery. This study aimed to investigate whether early oral hydration on demand in the postanesthesia care unit (PACU) after gynecological laparoscopy under general anesthesia can appease postoperative thirst and increase patient comfort. METHODS: Participants were randomized into the intervention and control groups. Patients in the intervention group were allowed to achieve early oral hydration on demand in the PACU if they were evaluated as fully conscious, with stable vital signs, grade 5 muscle strength, and well-recovered cough and swallowing reflex. However, the total amount of water intake throughout the entire study should not exceed 0.5mL/kg. During the study, the frequency of water intake, the total amount of water intake and adverse events were accurately recorded. The control group was managed according to the routine procedures and began to drink water 2 h after anesthesia. The intensity of thirst and subjective comfort in patients were assessed using the visual analog scale (VAS) when they entered and left the PACU. RESULTS: No statistically significant differences were identified in age, height, weight, body mass index, pre-operative fasting time, duration of surgery, intraoperative fluid intake, intraoperative blood loss, intraoperative urine volume, and thirst intensity and subjective comfort scores between the groups before intervention (P > 0.05). After intervention, the VAS score for thirst intensity in the intervention group significantly decreased (P < 0.05), and the VAS score for subjective comfort in the intervention group significantly increased (P < 0.05). No adverse events were detected in both groups during the entire study. CONCLUSION: Early oral hydration on demand in the PACU can safely and effectively relieve postoperative thirst in patients, and improve patient comfort after gynecological laparoscopy. TRIAL REGISTRATION: This single-center, prospective, randomized controlled trial was registered at the Chinese Clinical Trial Center on April 27, 2023. The registration number of this study is ChiCTR2300070985.

The efficacy and safety of modified ultraearly oral hydration for alleviating thirst in patients after thoracoscopic surgery: a prospective randomized controlled trial.

OBJECTIVE: Postoperative fasting following thoracoscopic surgery can cause intense thirst and oral discomfort. However, there is currently no research on ultraearly oral hydration (UEOH) in middle-aged or elderly patients after thoracoscopic surgery. The aim of this study was to investigate the effectiveness and safety of UEOH for improving oral discomfort after thoracoscopic surgery. METHODS: This single-center prospective double-blind randomized controlled trial was conducted from April 2022 to November 2023. A total of 64 middle-aged and elderly patients who underwent the first thoracoscopic surgery on the day were enrolled at our institution. Postoperatively, in the Postanesthesia Care Unit (PACU), patients were randomly assigned at a 1:1 ratio to either the UEOH group or the standard care (SC) group. The primary outcome was the patient's thirst score at 6 h after surgery. Secondary outcomes included the incidence of postoperative oral discomfort; pain scores; the occurrence of adverse reactions such as nausea, vomiting, regurgitation and aspiration; anxiety scores on the first postoperative day; the time to first flatus; and recovery satisfaction scores. RESULTS: The demographic and surgical characteristics were similar between the two groups. Patients in the UEOH group had lower thirst scores 6 h after surgery than did those in the SC group(16.1 ± 6.70 vs. 78.4 ± 8.42, P < 0.01). The incidence of postoperative oral discomfort (P < 0.01), anxiety scores on the first postoperative day (P<0.05), and time to first flatus (P<0.05) were better in the UEOH group. Additionally, the incidences of adverse reactions, such as postoperative nausea, vomiting, regurgitation and aspiration, were similar between the two groups (P>0.05). CONCLUSION: For middle-aged and elderly patients undergoing thoracoscopic surgery, the use of a modified UEOH protocol postoperatively can improve thirst and promote gastrointestinal recovery without increasing complications. TRIAL REGISTRATION: This single-center, prospective, RCT has completed the registration of the Chinese Clinical Trial Center at 07/12/2023 with the registration number ChiCTR2300078425.