Previous experiments in rodents showed that ablation of the septal brain region caused hyperdipsia. We investigated which part of the septal region needs ablation to produce hyperdipsia in sheep, and whether increased drinking was a primary hyperdipsia. Following ablation of the medial septal region (n = 5), but not parts of the lateral septal region (n = 4), daily water intake increased from 2.5-5 L/day up to 10 L/day for up to 3 months post-lesion. In hyperdipsic sheep, plasma osmolality increased on the first day post-lesion and body weight fell, suggesting that initial hyperdipsia was secondary to fluid loss. However hypemsmolality was not sustained long-term and plasma hypo-osmolality persisted from 0.5 to 3 months post-lesion. Acute dipsogenic responses to intravenous hypertonic saline, intravenous or intracerebroventricular angiotensin II, water deprivation for 2 days, or feeding over 5 h were not potentiated by medial septal lesions, showing that the rapid presystemic inhibitory influences that cause satiation of thirst upon the act of drinking were intact. However, hyperdipsic sheep continued to ingest water when hyponatremic (plasma [Na] was 127-132 mmol/1) and plasma osmolality was 262-268 mosmol/kg due to retention of ingested fluid resulting from intravenous infusion of vasopressin administered to maintain a basal blood level of antidiuretic hormone. The results show that septal lesion-induced hyperdipsia is not due to disruption of acute pre-systemic influences associated with drinking water that initiates rapid satiation of thirst. Rather, inhibitory influences of hyponatremia, hypo-osmolality or hypervolemia on drinking appear to be disrupted by medial septal lesions.

In response to dehydration, humans experience thirst. This subjective state is fundamental to survival as it motivates drinking, which subsequently corrects the fluid deficit. To elicit thirst, previous studies have manipulated blood chemistry to produce a physiological thirst stimulus. In the present study, we investigated whether a physiological stimulus is indeed required for thirst to be experienced. Functional MRI (fMRI) was used to scan fully hydrated participants while they imagined a state of intense thirst and while they imagined drinking to satiate thirst. Subjective ratings of thirst were significantly higher for imagining thirst compared with imagining drinking or baseline, revealing a successful dissociation of thirst from underlying physiology. The imagine thirst condition activated brain regions similar to those reported in previous studies of physiologically evoked thirst, including the anterior midcingulate cortex (aMCC), anterior insula, precentral gyrus, inferior frontal gyrus, middle frontal gyrus, and operculum, indicating a similar neural network underlies both imagined thirst and physiologically evoked thirst. Analogous brain regions were also activated during imagined drinking, suggesting the neural representation of thirst contains a drinking-related component. Finally, the aMCC showed an increase in functional connectivity with the insula during imagined thirst relative to imagined drinking, implying functional connectivity between these two regions is needed before thirst can be experienced. As a result of these findings, this study provides important insight into how the neural representation of subjective thirst is generated and how it subsequently motivates drinking behavior.

Much progress has been made during the past 30 years with respect to elucidating the neural and endocrine pathways by which bodily needs for water and energy are brought to conscious awareness through the generation of thirst and hunger. One way that circulating hormones influence thirst and hunger is by acting on neurones within sensory circumventricular organs (CVOs). This is possible because the subfornical organ and organum vasculosum of the lamina terminalis (OVLT), the sensory CVOs in the forebrain, and the area postrema in the hindbrain lack a normal blood‐brain barrier such that neurones within them are exposed to blood‐borne agents. The neural signals generated by hormonal action in these sensory CVOs are relayed to several sites in the cerebral cortex to stimulate or inhibit thirst or hunger. The subfornical organ and OVLT respond to circulating angiotensin II, relaxin and hypertonicity to drive thirst‐related neural pathways, whereas circulating amylin, leptin and possibly glucagon‐like peptide‐1 act at the area postrema to influence neural pathways inhibiting food intake. As a result of investigations using functional brain imaging techniques, the insula and anterior cingulate cortex, as well as several other cortical sites, have been implicated in the conscious perception of thirst and hunger in humans. Viral tracing techniques show that the anterior cingulate cortex and insula receive neural inputs from thirst‐related neurones in the subfornical organ and OVLT, with hunger‐related neurones in the area postrema having polysynaptic efferent connections to these cortical regions. For thirst, initially, the median preoptic nucleus and, subsequently, the thalamic paraventricular nucleus and lateral hypothalamus have been identified as likely sites of synaptic links in pathways from the subfornical organ and OVLT to the cortex. The challenge remains to identify the links in the neural pathways that relay signals originating in sensory CVOs to cortical sites subserving either thirst or hunger.

In humans, activity in the anterior midcingulate cortex (aMCC) is associated with both subjective thirst and swallowing. This region is therefore likely to play a prominent role in the regulation of drinking in response to dehydration. Using functional MRI, we investigated this possibility during a period of "drinking behavior" represented by a conjunction of preswallow and swallowing events. These events were examined in the context of a thirsty condition and an "oversated" condition, the latter induced by compliant ingestion of excess fluid. Brain regions associated with swallowing showed increased activity for drinking behavior in the thirsty condition relative to the oversated condition. These regions included the cingulate cortex, premotor areas, primary sensorimotor cortices, the parietal operculum, and the supplementary motor area. Psychophysical interaction analyses revealed increased functional connectivity between the same regions and the aMCC during drinking behavior in the thirsty condition. Functional connectivity during drinking behavior was also greater for the thirsty condition relative to the oversated condition between the aMCC and two subcortical regions, the cerebellum and the rostroventral medulla, the latter containing nuclei responsible for the swallowing reflex. Finally, during drinking behavior in the oversated condition, ratings of swallowing effort showed a negative association with functional connectivity between the aMCC and two cortical regions, the sensorimotor cortex and the supramarginal gyrus. The results of this study provide evidence that the aMCC helps facilitate swallowing during a state of thirst and is therefore likely to contribute to the regulation of drinking after dehydration.

secretion. We mapped the sites in the brain that are activated by i.v. infusion of a dipsogenic dose of relaxin (25 tig/h) by immu- w; nohistochemically detecting Fos expression. Relaxin administra- to tion resulted in increased Fos expression in the subfornical organ O (SFO), organum vasculosum of the lamina terminalis (OVLT), me- bc dian preoptic nucleus, and magnocellular neurons in the supraoptic ot and paraventricular nuclei. Ablation of the SFO abolished relaxin- de induced water drinking, but did not prevent increased Fos expres- or sion in the OVLT, supraoptic or paraventricular nuclei. Although el ablation of the OVLT did not inhibit relaxin-induced drinking, it did br cause a large reduction in Fos expression in the supraoptic nucleus ac and posterior magnocellular subdivision of the paraventricular SI nucleus. In vitro single-unit recording of electrical activity of neu- se rons in isolated slices of the SFO showed that relaxin (10-7 M) th added to the perfusion medium caused marked and prolonged to increase in neuronal activity. Most of these neurons also re- la sponded to 10-7 M angiotensin II. The data indicate that blood- nc borne relaxin can directly stimulate neurons in the SFO to initiate stl water drinking. It is likely that circulating relaxin also stimulates re neurons in the OVLT that influence vasopressin secretion. These two circumventricular organs that lack a blood-brain barrier may have regulatory influences on fluid balance during pregnancy in Ar rats. st] ar

Due to the importance of dietary sodium and its paucity within many inland environments, terrestrial animals have evolved an instinctive sodium appetite that is commensurate with sodium deficiency. Despite a well-established role for central opioid signaling in sodium appetite, the endogenous influence of specific opioid receptor subtypes within distinct brain regions remains to be elucidated. Using selective pharmacological antagonists of opioid receptor subtypes, we reveal that endogenous mu-opioid receptor (MOR) signaling strongly drives sodium appetite in sodium-depleted mice, whereas a role for kappa (KOR) and delta (DOR) opioid receptor signaling was not detected, at least in sodium-depleted mice. Fos immunohistochemistry revealed discrete regions of the mouse brain displaying an increased number of activated neurons during sodium gratification: the rostral portion of the nucleus of the solitary tract (rNTS), the lateral parabrachial nucleus (LPB), and the central amygdala (CeA). The CeA was subsequently targeted with bilateral infusions of the MOR antagonist naloxonazine, which significantly reduced sodium appetite in mice. The CeA is therefore identified as a key node in the circuit that contributes to sodium appetite. Moreover, endogenous opioids, acting via MOR, within the CeA promote this form of appetitive behavior.

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.

New FindingsWhat is the central question of this study? Sodium appetite is controlled by conserved neuronal transmitter-receptor systems. Here, we tested the contribution made by relaxin family peptide 3 receptor (RXFP3), the cognate G-protein-coupled receptor for the neuropeptide relaxin-3.What is the main finding and its importance? Intracerebroventricular infusion of an RXFP3 antagonist reduced in a dose-dependent manner the volume of 0.3m NaCl consumed by sodium-depleted C57Bl/6J (wild-type) mice. This effect was absent in sodium-depleted Rxfp3 knockout mice, and RXFP3 antagonist infusion did not alter water consumption in wild-type mice subjected to multiple thirst tests, indicating both the pharmacological and the physiological specificity of observed effects. Our findings identify endogenous relaxin-3-RXFP3 signalling as a modulator of sodium appetite.Overconsumption of highly salted foods is common in Western diets and contributes significantly to metabolic disorders such as hypertension, renal dysfunction and diabetes. Sodium appetite, or the desire of terrestrial animals to seek and consume sodium-containing salts, is a behaviour mediated by a set of evolutionarily conserved neuronal systems. In these studies, we tested whether this instinctive behavioural drive is influenced by the G-protein-coupled relaxin family peptide3 receptor (RXFP3), the cognate receptor for the neuropeptide relaxin-3, because relaxin-3-RXFP3 signalling can modulate arousal, motivation and ingestive behaviours. Intracerebroventricular (i.c.v.) infusion of the selective RXFP3 antagonist, R3(B1-22)R, reduced in a dose-dependent manner the volume of 0.3m NaCl solution consumed when offered to sodium-depleted C57Bl/6J wild-type mice, relative to vehicle-treated control animals. Notably, i.c.v. R3(B1-22)R infusion did not alter 0.3m NaCl consumption relative to vehicle in sodium-depleted Rxfp3 knockout mice, confirming the pharmacological specificity of this effect. Furthermore, i.c.v. R3(B1-22)R did not alter the volume of water consumed by wild-type mice in three tests where water drinking was the normal physiological response, suggesting that the ability of R3(B1-22)R to reduce activated salt appetite is specific and not due to a generalized reduction in drinking behaviour. These findings identify, for the first time, that endogenous relaxin-3-RXFP3 signalling is a powerful mediator of salt appetite in mice and further elucidate the functional role of the relaxin-3-RXFP3 system in the integrative control of motivated behaviours.

The instinct of thirst was a cardinal element in the successful colonization by vertebrates of the dry land of the planet, which began in the Ordovician period about 400 million y ago. It is a commonplace experience in humans that drinking water in response to thirst following fluid loss is a pleasant experience. However, continuing to drink water once thirst has been satiated becomes unpleasant and, eventually, quite aversive. Functional MRI experiments reported here show pleasantness of drinking is associated with activation in the anterior cingulate cortex (Brodmann area 32) and the orbitofrontal cortex. The unpleasantness and aversion of overdrinking is associated with activation in the midcingulate cortex, insula, amygdala, and periaqueductal gray. Drinking activations in the putamen and cerebellum also correlated with the unpleasantness of water, and the motor cortex showed increased activation during overdrinking compared with drinking during thirst. These activations in motor regions may possibly reflect volitional effort to conduct compliant drinking in the face of regulatory mechanisms inhibiting intake. The results suggestive of a specific inhibitory system in the control of drinking are unique.

This issue of The Journal of Physiology, recording aspects of the fiftieth anniversary of the Howard Florey Institute, tells of how a successful basic scientific investigation attracted the enthusiastic interest of several Australian and overseas philanthropic leaders. It led to the building of arguably the world's best laboratories for long term survival experiments on large animals, and incorporated state of the art radiochemical and molecular biology laboratories.

Studies in sheep have shown that renal excretion of sodium may be under osmoregulatory control. When sheep become dehydrated, or are infused intravenously with hypertonic saline, they increase renal Na excretion in addition to secreting vasopressin and developing a thirst. These natriuretic, antidiuretic, and dipsogenic responses to dehydration and hypertonicity can be greatly reduced by lowering the cerebrospinal fluid NaCl concentration or by prior ablation of tissue in the anterior wall of the third ventricle. Lowering of cerebrospinal fluid NaCl concentration also prevents postprandial natriuresis which normally occurs in association with a postprandial increase in plasma Na concentration and tonicity. We propose that there is a cerebral osmoregulatory control of Na excretion which may interact with volume influences from the cardiovascular system to regulate renal Na output. The effector mechanism from brain to kidney mediating such cerebral control of Na excretion is probably hormonal.

Thermal sensation has both discriminative and hedonic components. The neural network associated with thermal discrimination has been described, but the representation of the hedonic component of thermal sensation in the brain has yet to be demonstrated. This study measured regional cerebral blood flow (rCBF) with Positron Emission Tomography in human participants during whole-body cooling and warming. Ratings of the hedonic dimension of thermal sensation were correlated with rCBF and compared to brain activation maps of skin and core temperature changes. The hedonic dimension of thermal sensation was represented in a widely distributed network that included somatosensory, limbic, paralimbic, and associative cortices. Changes in rCBF associated with ratings of the hedonic dimension of thermal sensation were contrasted with changes in rCBF associated with skin and core temperature to identify brain regions that were uniquely activated by the hedonic dimension of thermal sensation. The contrast between the correlates of the hedonic dimension of thermal sensation and the correlates of skin and core temperatures identified a region in the posterior part of the mid cingulate cortex (pMCC). The independent relationship between rCBF changes in the pMCC with ratings of the hedonic dimension of thermal sensation may indicate an important role for the pMCC in the integration of temperature-related signals from receptors in the skin and core.

Sodium appetite is an instinct that involves avid specific intention. It is elicited by sodium deficiency, stress-evoked adrenocorticotropic hormone (ACTH), and reproduction. Genome-wide microarrays in sodium-deficient mice or after ACTH infusion showed up-regulation of hypothalamic genes, including dopamine- and cAMP-regulated neuronal phosphoprotein 32 kDa (DARPP-32), dopamine receptors-1 and -2, α-2C- adrenoceptor, and striatally enriched protein tyrosine phosphatase (STEP). Both DARPP-32 and neural plasticity regulator activity-regulated cytoskeleton associated protein (ARC) were up-regulated in lateral hypothalamic orexinergic neurons by sodium deficiency. Administration of dopamine D1 (SCH23390) and D2 receptor (raclopride) antagonists reduced gratification of sodium appetite triggered by sodium deficiency. SCH23390 was specific, having no effect on osmotic-induced water drinking, whereas raclopride also reduced water intake. D1 receptor KO mice had normal sodium appetite, indicating compensatory regulation. Appetite was insensitive to SCH23390, confirming the absence of off-target effects. Bilateral microinjection of SCH23390 (100 nM in 200 nL) into rats’ lateral hypothalamus greatly reduced sodium appetite. Gene set enrichment analysis in hypothalami of mice with sodium appetite showed significant enrichment of gene sets previously linked to addiction (opiates and cocaine). This finding of concerted gene regulation was attenuated on gratification with perplexingly rapid kinetics of only 10 min, anteceding significant absorption of salt from the gut. Salt appetite and hedonic liking of salt taste have evolved over >100 million y (e.g., being present in Metatheria). Drugs causing pleasure and addiction are comparatively recent and likely reflect usurping of evolutionary ancient systems with high survival value by the gratification of contemporary hedonic indulgences. Our findings outline a molecular logic for instinctive behavior encoded by the brain with possible important translational–medical implications.

The pattern of regional brain activation in humans during thirst associated with dehydration, increased blood osmolality, and decreased blood volume is not known. Furthermore, there is little information available about associations between activation in osmoreceptive brain regions such as the organum vasculosum of the lamina terminalis and the brain regions implicated in thirst and its satiation in humans. With the objective of investigating the neuroanatomical correlates of dehydration and activation in the ventral lamina terminalis, this study involved exercise-induced sweating in 15 people and measures of regional cerebral blood flow (rCBF) using a functional magnetic resonance imaging technique called pulsed arterial spin labeling. Regional brain activations during dehydration, thirst, and postdrinking were consistent with the network previously identified during systemic hypertonic infusions, thus providing further evidence that the network is involved in monitoring body fluid and the experience of thirst. rCBF measurements in the ventral lamina terminalis were correlated with whole brain rCBF measures to identify regions that correlated with the osmoreceptive region. Regions implicated in the experience of thirst were identified including cingulate cortex, prefrontal cortex, striatum, parahippocampus, and cerebellum. Furthermore, the correlation of rCBF between the ventral lamina terminalis and the cingulate cortex and insula was different for the states of thirst and recent drinking, suggesting that functional connectivity of the ventral lamina terminalis is a dynamic process influenced by hydration status and ingestive behavior.