Investigations on the physiological controls of water and saline intake in C57BL/6 mice

To examine the behavioral and neural control of body fluid homeostasis, water and saline intake of C57BL/6 mice was monitored under ad libitum conditions, after treatments that induce water or salt intake, and after ablation of the periventricular tissue of the anteroventral third ventricle (AV3V). Mice have nocturnal drinking that is most prevalent after the offset and before the onset of lights. When given ad libitum choice, C57BL/6 mice show no preference for saline over water at concentrations up to 0.9% NaCl and a progressive aversion to saline above that concentration. Systemic hypertonic saline, isoproterenol, and polyethylene glycol treatments are dipsogenic; however, systemic ANG II is not. Intracerebroventricular injections of both hypertonic saline and ANG II are dipsogenic, and diuretic treatment followed by a short period of sodium deprivation induces salt intake. After ablation of the AV3V, mice can be nursed to recovery from initial adipsia and, similar to rats, show chronic deficits to dipsogenic treatments. Taken together, the data indicate that mechanisms controlling thirst in response to cellular dehydration in C57BL/6 mice are similar to rats, but there are differences in the efficacy of extracellular dehydration-related mechanisms, especially for systemic ANG II, controlling thirst and salt appetite.     

The role of the perifornix area of the hypothalamus in forming alcoholic motivation in rats

In rats with alcohol motivation on the model of water deprivation with substitution of water for 20% ethanol solution, motivatiogenic "drinking centres" of the hypothalamus initiate in response to electro- and chemostimulation (acetylcholine) behavioural reactions of search and taking of alcohol and not of water. Electrolytic ablation of "thirst centres" of the perifornical hypothalamus area in rats with a formed attraction to alcohol is accompanied by a decrease of its taking during 3-5 days of observation. Microinjections of dipsogenic peptide angiotensin II, unlike acetylcholine administration, do not initiate taking of water or alcohol, but elicit only appearance of orienting-investigating, alimentary, sexual and other behavioural reactions. It is suggested that formation of alcohol motivation in these conditions is connected with a change of neurophysiological and neurochemical properties of the hypothalamus "thirst centres" initiating an active search and taking of alcohol and not of the water in rats with experimental alcoholism.     

Influences on water intake in the rat after lesions of the septal subareas

It has been suggested that the septum plays an inhibitory role in the behavioral function. Recent work has shown that the septum is heterogeneous from the neuroanatomical perspective. The present study examined the water intake of rats lesioned with kainic acid (0.5 microg/0.5 microl/site) on three septal subregions: anterior medial (MSa), posterior medial (MSp), and lateral (LS) sites. Drinking volume was enhanced mostly in rats with the MSp lesion, and so was locomotor activity. However, these two measures were not significantly correlated. This polydipsia induced by MSp lesion was also found in a chronic domain. Another experiment further determined the dipsogenic effects of polyethylene glycol (PEG; 20%) and hypertonic saline (1 M NaCl) in MSp lesioned rats. Water intake increased significantly after administration of the hypertonic saline treatment, but not after injection of PEG. However, this disparity approached a nonsignificant level 8 hr after thirst challenges were conducted. In addition to revealing septal hyperdipsia. these data suggest that the septal subareas can be functionally heterogeneous in drinking behavior. The dipsogenic response profiles for the cellular and extracellular thirst challenges could be differentially affected by kainate lesion in the MSp.     

Angiotensin, thirst, and sodium appetite: retrospect and prospect.

The fact that drinking in response to some hypovolemic stimuli was attenuated by nephrectomy but not by ureteric ligation led to the suggestion that the renal renin-angiotensin system may play a role in hypovolemic thirst. The isolation of a thirst factor from the kidney and the demonstration that this substance was renin supported the hypothesis. Subsequently, it was shown that the effects of renin on drinking were mediated through angiotensin II, which proved to be a potent dipsogenic substance when administered systemically or injected directly into the brain. Recently, it has been shown that angiotensin II, infused intravenously or through the carotid artery at rates that produce increases in plasma angiotensin II levels similar to those that occur in mild sodium depletion, causes the water-replete animal to drink. This discovery establishes that angiotensin is a physiological stimulus to drinking but it leaves open the question of the extent of the involvement of renal renin in normal thirst. Other unsolved problems are the role of cerebral isorenin in angiotensin thirst and its relationship with renal renin, and in view of its stimulating action on sodium intake when infused into the brain, whether angiotensin plays a significant role in sodium appetite.     

Role of thromboxane receptors in the dipsogenic response to central angiotensin II

Central angiotensin II (ANG II) regulates thirst. Because thromboxane A2-prostaglandin H2 (TP) receptors are expressed in the brain and mediate some of the effects of ANG II in the vasculature, we investigated the hypothesis that TP receptors mediate the drinking response to intracerebroventricular (icv) injections of ANG II. Pretreatment with the specific TP-receptor antagonist ifetroban (Ifet) decreased water intake with 50 ng/kg icv ANG II (ANG II + Veh, 7.2 +/- 0.7 ml vs. ANG II + Ifet, 2.8 +/- 0.8 ml; n = 5 rats; P < 0.001) but had no effect on water intake induced by hypertonic saline (NaCl + Veh, 8.4 +/- 1.1 ml vs. NaCl + Ifet, 8.9 +/- 1.8 ml; n = 5 rats; P = not significant). Administration of 0.6 microg/kg icv of the TP-receptor agonist U-46,619 did not induce drinking when given alone but did increase the dipsogenic response to a near-threshold dose of 15 ng/kg icv ANG II (ANG II + Veh, 1.1 +/- 0.7 vs. ANG II + U-46,619, 4.5 +/- 0.9 ml; n = 5 rats; P < 0.01). We conclude that central TP receptors contribute to the dipsogenic response to ANG II.     

Brain angiotensinergic mediation of enhanced water consumption in lactating rats

The mechanism by which lactating rats increase fluid consumption to meet the demands of milk production is unknown. Because ANG II is the most potent dipsogenic stimulus known, this study examined whether angiotensinergic signaling plays a role in enhanced drinking in lactating rats. ANG II administered intracerebroventricularly caused a significantly greater dipsogenic response in lactating rats than in control rats, suggesting that dipsogenic responsivity to ANG II is enhanced in the brains of lactating rats. The angiotensin type 1 (AT1) ANG II receptor subtype antagonist SKF-108566, also given intracerebroventricularly, caused a significant reduction in water consumption in lactating rats, whereas it did not significantly affect water intake in control rats. In contrast, stimulation of drinking by the muscarinic agonist carbachol, also administered intracerebroventricularly, did not differ between lactating and control rats. Inhibition of drinking by the muscarinic antagonist atropine also did not differ significantly between lactating and control rats. These results suggest that the increased drinking in lactating rats involves an increased responsivity to ANG II in neurons that mediate dipsogenesis, as well as an enhancement in the amount of angiotensinergic input to these ANG II-responsive neurons.     

The role of renal nerves in the response to dipsogenic stimuli in the rat.

The effect of bilateral renal denervation on water intake and urine volume during specific thirst challenges was studied in rats. Renal denervation attenuated significantly the drinking response elicited by the administration of 30% polyethylene glycol (PG, extracellular challenge) but had no effect on the drinking response after an intracellular challenge (2.5 M NaCl) or after a 24-h water deprivation period. Furthermore, during a PG challenge total water intake was the same in two groups of rats, one with denervated kidneys and the other with beta-adrenergic neural activity in efferent renal nerves eliminated by blocking agents. Urine volumes were not affected by PG administration or water deprivation in denervated rats but were increased significantly after administration of 2.5 M NaCl. These results indicate that renal nerves play an important role in the physiological processes controlling extracellular thirst, and suggest that this role may be related to the neural control of release of renin.     

Peptide-protein complexes of angiotensins in the mechanisms of thirst

The comparative analysis of learned and natural drinking behavior under native angiotensin-I (A-I), angiotensin-II (A-II) and its protein-peptides complexes (PPC) administration in rats, was performed. Various effects of these substances on drinking behavior were observed. PPC of A-I became a dipsogenic agent as compared with the native one. Moreover, PPC of A-II facilitated selectively learned and not natural forms of drinking behavior. It's suggested that PPC of angiotensins play a specific role in endogenous mechanisms of thirst. An important function of this complexes due to their conformational properties and participation in realization of basic needs, is discussed.     

The area postrema in hindbrain is a central player for regulation of drinking behavior in Japanese eels

It is recognized that fish will drink the surrounding water by reflex swallowing without a thirst sensation. We evaluated the role of the area postrema (AP), a sensory circumventricular organ (CVO) in the medulla oblongata, in the regulation of drinking behavior of seawater (SW) eels. The antidipsogenic effects of ghrelin and atrial natriuretic peptide and hypervolemia and hyperosmolemia (1 M sucrose or 10% NaCl) as well as the dipsogenic effects of angiotensin II and hypovolemia (hemorrhage) were profoundly diminished after AP lesion (APx) in eels compared with sham controls. However, the antidipsogenic effect of urotensin II was not influenced by APx, possibly due to the direct baroreflex inhibition on the swallowing center in eels. When ingested water was drained via an esophageal fistula, water intake increased 30-fold in sham controls but only fivefold in APx eels, suggesting a role for the AP in continuous regulation of drinking by SW eels. After transfer from freshwater to SW, APx eels responded normally with an immediate burst of drinking, but after 4 wk these animals showed a much greater increase in plasma osmolality than controls, suggesting that the AP is involved in acclimation to SW by fine tuning of the drinking rate. Taken together, the AP in the hindbrain of eels plays an integral role in SW acclimation, acting as a conduit of information from plasma for the regulation of drinking, probably without a thirst sensation. This differs from mammals in which sensory CVOs in the forebrain play pivotal roles in thirst regulation.     

Angiotensin and other peptides in the control of water and sodium intake

Several neuroactive peptides have been implicated in thirst and sodium appetite in different species; three peptides are considered here. The best established of these is the octapeptide angiotensin II, which when administered systemically or intracranially causes completely normal drinking behaviour in all vertebrates tested, including many mammals, four or five birds, one reptile and one bony fish. In the rat, in which the original experiments were carried out, injection of a few femtomoles of angiotensin II caused a brisk drinking response within a minute or so of injection at a time of day when the animal would usually be resting. The response is usually completed within 10 min and after the larger doses the amounts of water taken may approach what the animal would normally drink in the course of 24 h. Another response to intracranial angiotensin, seen so far only in the rat, is an increase in sodium appetite. This is slower in onset than thirst, lasts for many hours and the response tends to become greater with repeated injections of hormone. Naturally occurring increases in sodium appetite may be caused by angiotensin generated by the action of cerebral isorenin. A second neuroactive peptide that affects thirst is the undecapeptide eledoisin, which is found in the salivary glands of certain Mediterranean cephalopods. Eledoisin and, to a lesser extent, substance P, with which it is related, are potent intracranial dipsogens in the pigeon, producing behaviour that is indistinguishable from that produced by angiotensin. However, in contrast to the stimulatory action of angiotensin on drinking behaviour in all other vertebrate species tested, these substances specifically depress drinking in the rat. A third peptide that has been implicated in thirst is antidiuretic hormone (ADH). This hormone has a profound but indirect effect on water intake in diabetes insipidus. In the dog, however, ADH in physiological amounts may influence thirst mechanisms by direct action on the central nervous system. In this species, but not in the rat, ADH lowers the threshold of thirst in response to osmotic stimulation and also to infusion of angiotensin. Of these three peptides, and others not mentioned here, angiotensin II has the best claim to be regarded as a neuroactive peptide. It alone is always dipsogenic when injected into the brain and it also stimulates sodium appetite. Whether the effects of angiotensin, on thirst and sodium appetite should be regarded as manifestations of the activity of a classical endocrine system, of a paracrine system, of a neurotransmitter system, or of all of these, cannot be decided at present. But these actions of angiotensin, when considered with its other actions on the distribution and conservation of body fluid, show that the hormone is intimately concerned in extracellular fluid volume control.     

Effect of atrial natriuretic factor on central angiotensin II-induced responses in rats

The effect of intracerebroventricular (ICV) injection of atrial natriuretic factor (ANF) on drinking and pressor responses induced by centrally administered angiotensin II (AII) was examined in the rat. The ICV injection of ANF attenuated water intake induced by AII or 48-hr water deprivation. In contrast, ANF did not affect AII-induced pressor responses. The ICV injection of ANF did not cause recognizable change in blood pressure in spontaneously hypertensive rats or Wistar-Kyoto rats. These results suggested that ANF in the brain is involved in the central control of water intake. Brain ANF may be considered as a selective antagonist of the dipsogenic effect of AII but not its pressor effect.     

Antisense oligonucleotide inhibition of angiotensinogen in the brains of rats and sheep

Phosphorothioated antisense oligodeoxynucleotides (ODNs) that were complementary to various parts of the rat or sheep mRNA encoding angiotensinogen were synthesized by conventional techniques. Their effectiveness as blockers of angiotensinogen synthesis in the brain was tested by bioassay. This involved measuring the effect of centrally administered antisense ODNs on water drinking that occurred in response to intracerebroventricular injection of hog renin. Renin-induced drinking requires brain angiotensinogen for the generation of angiotensin I and then angiotensin II to stimulate thirst. Intracerebroventricular injection of an 18-mer antisense ODN (0.5 microg twice in 24 h) complementary to the 5'-end start codon for rat angiotensinogen mRNA caused a pronounced inhibition of renin-induced drinking. This effect appeared to be specific for this region of the codon because antisense ODNs directed against other regions of rat angiotensinogen mRNA were ineffective, and renin-induced drinking was not inhibited by intracerebroventricular injection of scrambled or mismatched sequences of the effective ODN or by intraperitoneal injection of it. Intracerebroventricular injection of antisense ODN (0.5 microg twice in 24 h) did not inhibit appetite or affect water drinking in response to some other dipsogenic stimuli, thus demonstrating the specificity of its action against renin-induced drinking. By contrast, intracerebroventricular administration of 625 microg of an antisense ODN directed against the corresponding 5'-end start codon region of sheep angiotensinogen mRNA did not inhibit intracerebroventricular renin-induced drinking in sheep. These data show that while intracerebroventricularly administered antisense may be used effectively in rodents, the method is not necessarily applicable in larger mammals.     

Effect of kassinin, neurokinin A and neurokinin B on drinking behaviour in the pigeon

Intracerebroventricular (i.c.v.) injection of kassinin produced a prompt and copious drinking response at doses of 10-1000 ng/pigeon, in the absence of other behavioural alterations or of changes in core temperature. Neurokinin A and B evoked drinking, but they were respectively 10 and 100 times less potent than kassinin. Intraperitoneal injection of kassinin elicited drinking, but at doses about 1000 X larger than the i.c.v. ones. The angiotensin antagonist [Sar1, Leu8]angiotensin II did not reduce drinking induced by i.c.v. kassinin, suggesting that its effect is not due to interaction with the central renin-angiotensin system. Moreover, the effect is apparently independent of the mechanisms controlling hypovolaemic and hyperosmotic thirst since exact additivity was found in the dipsogenic response when i.c.v. kassinin was administered in the presence of a hypovolaemic (subcutaneous (s.c.), polyethylene glycol) or hyperosmotic (s.c. hypertonic NaCl) dipsogenic stimulus. The present findings show that kassinin, neurokinin A and B share with the tachykinins already tested (eledoisin, physalaemin, substance P) a common dipsogenic action in pigeons. However, marked differences exist in their dipsogenic potency. This order of potency, eledoisin = kassinin = physalaemin greater than neurokinin A = substance P greater than neurokinin B, is not consistent with the tachykinin receptor subtypes so far proposed.     

Acute hypertension inhibits thirst stimulated by ANG II, hyperosmolality, or hypovolemia in rats

The present study sought to determine whether increases in arterial blood pressure inhibited drinking behavior evoked by ANG II, hyperosmolality, or hypovolemia in rats. Cumulative water intakes in 60- or 90-min tests and latency to the first lick were recorded as indexes of thirst. During intravenous infusions of 100 ng. kg(-1). min(-1) ANG II, attenuation of the induced increases in arterial pressure with the arteriolar vasodilator diazoxide resulted in greater water intakes and shorter latencies to drink. Drinking behavior stimulated by intravenous infusion of hypertonic saline was significantly inhibited by increases in arterial pressure caused by intravenous infusion of phenylephrine or endothelin-1, and this inhibition of drinking was proportional to the induced increase in pressure. Upon termination of the phenylephrine infusion, mean arterial pressure returned to basal values, and drinking was restored. Phenylephrine-induced increases in arterial pressure also inhibited drinking behavior in response to hypovolemia that could not be explained by differences in plasma renin activity, plasma protein concentration, or plasma osmolality. Thus increases in arterial pressure inhibit water drinking behavior in response to each of these three thirst stimuli in rats.     

Inhibition of thirst when dehydrated rats drink water or saline

The present experiments sought to identify the physiological signals that inhibit thirst when dehydrated rats drink water or NaCl solution. Rats were deprived of drinking fluid but not food overnight. When allowed to drink again, the dehydrated animals consumed water or saline (0.05 M, 0.10 M, 0.15 M, or 0.20 M NaCl solution) almost continuously for 5-8 min before stopping. The volumes consumed were similar regardless of which fluid they ingested, but blood analyses indicated that increased plasma osmolality and decreased plasma volume, or both, still remained when drinking terminated. These results suggest that the composition of the ingested fluid is less significant than its volume in providing an early signal that inhibits thirst and fluid consumption by dehydrated rats. Analyses of the gastrointestinal tracts revealed that the cumulative volume in the stomach and small intestine correlated highly with the amount consumed regardless of which fluid was ingested. These and other results suggest that the volume of fluid ingested by dehydrated rats is sensed by stretch receptors detecting distension of the stomach and small intestine, which provide an early inhibitory stimulus of thirst.     

Mode of consumption plays a role in alleviating hunger and thirst

While studying the effect of structure on satiety, effects of mode of consumption, additional water to drink, and thirst have been neglected. The objective was to assess effects of structure, mode of consumption of food, and additional drinking of water on fullness and thirst. In study 1, 20 subjects (BMI 22.5 ± 0.5 kg/m(2); age 21.4 ± 3 years) underwent consumption conditions; SEW: solids to eat + 750 ml water to drink; LEW: liquefied soup to eat including 500 ml water + 250 ml water separately to drink; LDW: the same as LEW but served as drinks; SE, LE, and LD: the same as previous but without water to drink. In study 2, a subset of subjects underwent consumption conditions: solid carbohydrate, solid protein, solid fat: the same as SEW, but for each macronutrient separately; liquefied carbohydrate, liquefied protein, liquefied fat: the same as LEW, but for each macronutrient separately. Appetite, insulin concentration, glucose concentration, and ghrelin concentration were measured. Eating, independent of structure, suppressed desire to eat more than drinking (P < 0.01). Drinking water separately vs. water consumption in the food suppressed thirst more (P < 0.001). Regarding protein, satiety was higher in the solid vs. liquefied condition, while blood parameters were not significantly different. Only after drinking a meal most subjects (80%) wanted to consume more of the same meal, in order to alleviate hunger (63%) or quench thirst (37%). We conclude that mode of consumption plays a role in alleviating hunger and thirst. Subjects required further consumption after drinking the meal, motivated by hunger or thirst, showing that drinking a meal causes confusion that may imply a risk of overconsumption.     

Atriopeptin prevents angiotensin II-stimulated glucose utilization in the subfornical organ

Previous studies have shown that angiotensin II (ANG II) increases glucose utilization in the subfornical organ and stimulates drinking behavior. We investigated with the deoxyglucose method whether atriopeptin III, an atrial natriuretic peptide (ANP), would prevent this enhanced glucose metabolism and interfere with the drinking response in the presence of ANG II. Two rat models with high circulating levels of ANG II were studied: the homozygous Brattleboro and ANG II-infused Sprague-Dawley rats. ANP decreased the normally enhanced glucose utilization in the subfornical organ in the Brattleboro rat and inhibited ANG II-stimulated glucose metabolism in the subfornical organ of Sprague-Dawley rats. This effect was accompanied by decreased ANG II-stimulated water intake. These findings indicate that ANP may act at the level of subfornical organ to antagonize the dipsogenic action of ANG II.     

Central angiotensin induction of fetal brain c-fos expression and swallowing activity

The present study examined physiological and cellular responses to central application of ANG II in ovine fetuses and determined the fetal central ANG-mediated dipsogenic sites in utero. Chronically prepared near-term ovine fetuses (130 +/- 2 days) received injection of ANG II (1.5 microg/kg icv). Fetuses were monitored for 3.5 h for swallowing activity, after which animals were killed and fetal brains were perfused for subsequent Fos staining. Intracerebroventricular ANG II significantly increased fetal swallowing in near-term ovine fetuses (1.1 +/- 0.2 to 4.5 +/- 1.0 swallows/min). The initiation of stimulated fetal swallowing activity was similar to the latency of thirst responses (drinking behavior) elicited by central ANG II in adult animals. ANG II evoked increased Fos staining in putative dipsogenic centers, including the subfornical organ, organum vasculosum of the lamina terminalis, and median preoptic nucleus. Intracerebroventricular injection of ANG II also caused c-fos expression in the fetal hindbrain. These results indicate that an ANG II-mediated central dipsogenic mechanism is intact before birth, acting at sites consistent with the dipsogenic neural network. Central ANG II mechanisms likely contribute to fetal body fluid and amniotic fluid regulation.     

Arterial baroreceptors mediate the inhibitory effect of acute increases in arterial blood pressure on thirst

The present study sought to determine whether arterial baroreceptor afferents mediate the inhibitory effect of an acute increase in arterial blood pressure (AP) on thirst stimulated by systemically administered ANG II or by hyperosmolality. Approximately 2 wk after sinoaortic denervation, one of four doses of ANG II (10, 40, 100, or 250 ng. kg(-1) x min(-1)) was infused intravenously in control and complete sinoaortic-denervated (SAD) rats. Complete SAD rats ingested more water than control rats when infused with 40, 100, or 250 ng x kg(-1) x min(-1) ANG II. Furthermore, complete SAD rats displayed significantly shorter latencies to drink compared with control rats. In a separate group of rats, drinking behavior was stimulated by increases in plasma osmolality, and mean AP was raised by an infusion of phenylephrine (PE). The infusion of PE significantly reduced water intake and lengthened the latencies to drink in control rats but not in complete SAD rats. In all experiments, drinking behavior of rats that were subjected to sinoaortic denervation surgery but had residual baroreceptor reflex function (partial SAD rats) was similar to that of control rats. Thus it appears that arterial baroreceptor afferents mediate the inhibitory effect of an acute increase in AP on thirst stimulated by ANG II or hyperosmolality.     

Presystemic influences on thirst, salt appetite, and vasopressin secretion in the hypovolemic rat

The present studies investigated the influence of presystemic signals on the control of thirst, salt appetite, and vasopressin (VP) secretion in rats during nonhypotensive hypovolemia. Rats were injected with 30% polyethylene glycol (PEG) solution, deprived of food and water overnight, and then allowed to drink water, 0.15 M NaCl, or 0.30 M NaCl. The PEG treatment, which produced 30-40% plasma volume deficits, elicited rapid intakes in an initial bout of drinking, but rats consumed much more 0.15 M NaCl than water or 0.30 M NaCl. In considering why drinking stopped sooner when water or concentrated saline was ingested, it seemed relevant that little or no change in systemic plasma Na(+) concentration was observed during the initial bouts and that the partial repair of hypovolemia was comparable, regardless of which fluid was consumed. In rats that drank 0.15 M NaCl, gastric emptying was fastest and the combined volume of ingested fluid in the stomach and small intestine was largest. These and other observations are consistent with the hypothesis that fluid ingestion by hypovolemic rats is inhibited by distension of the stomach and proximal small intestine and that movement of dilute or concentrated fluid into the small intestine provides another presystemic signal that inhibits thirst or salt appetite, respectively. On the other hand, an early effect of water or saline consumption on VP secretion in PEG-treated rats was not observed, in contrast to recent findings in dehydrated rats. Thus the controls of fluid ingestion and VP secretion are similar but not identical during hypovolemia.