DRINKING CAUSED BY CONSTRICTION OF THE INFERIOR VENA CAVA IN THE RAT

The awareness of thirst: proposed neural correlates

The neural and endocrine bases of the generation of thirst are reviewed. Based on this review, a hierarchical system of neural structures that regulate water conservation and acquisition is proposed. The system includes primary sensory-receptive areas; secondary sensory structures (circumventricular organs), which detect levels of hormones, including angiotensin II and vasopressin, which are involved in generating thirst; preoptic and hypothalamic structures; and an area within the ventrolateral quadrant of the periaqueductal gray matter. Hodological and other data are used to determine the hierarchical organization of the system. Based on studies of the effects of lesions to various structures within the hierarchy of the system, it is proposed that the awareness of thirst in rodents is either entirely or predominantly due to neuronal activities in a subsection of the ventrolateral periaqueductal gray matter. It is also hypothesized that the awareness of thirst in primates is due to neuronal activities in both the ventrolateral periaqueductal gray and in a region within the medial prefrontal and anterior cingulate cortex.

Angiotensin, thirst, and sodium appetite

Angiotensin (ANG) II is a powerful and phylogenetically widespread stimulus to thirst and sodium appetite. When it is injected directly into sensitive areas of the brain, it causes an immediate increase in water intake followed by a slower increase in NaCl intake. Drinking is vigorous, highly motivated, and rapidly completed. The amounts of water taken within 15 min or so of injection can exceed what the animal would spontaneously drink in the course of its normal activities over 24 h. The increase in NaCl intake is slower in onset, more persistent, and affected by experience. Increases in circulating ANG II have similar effects on drinking, although these may be partly obscured by accompanying rises in blood pressure. The circumventricular organs, median preoptic nucleus, and tissue surrounding the anteroventral third ventricle in the lamina terminalis (AV3V region) provide the neuroanatomic focus for thirst, sodium appetite, and cardiovascular control, making extensive connections with the hypothalamus, limbic system, and brain stem. The AV3V region is well provided with angiotensinergic nerve endings and angiotensin AT1 receptors, the receptor type responsible for acute responses to ANG II, and it responds vigorously to the dipsogenic action of ANG II. The nucleus tractus solitarius and other structures in the brain stem form part of a negative-feedback system for blood volume control, responding to baroreceptor and volume receptor information from the circulation and sending ascending noradrenergic and other projections to the AV3V region. The subfornical organ, organum vasculosum of the lamina terminalis and area postrema contain ANG II-sensitive receptors that allow circulating ANG II to interact with central nervous structures involved in hypovolemic thirst and sodium appetite and blood pressure control. Angiotensin peptides generated inside the blood-brain barrier may act as conventional neurotransmitters or, in view of the many instances of anatomic separation between sites of production and receptors, they may act as paracrine agents at a distance from their point of release. An attractive speculation is that some are responsible for long-term changes in neuronal organization, especially of sodium appetite. Anatomic mismatches between sites of production and receptors are less evident in limbic and brain stem structures responsible for body fluid homeostasis and blood pressure control. Limbic structures are rich in other neuroactive peptides, some of which have powerful effects on drinking, and they and many of the classical nonpeptide neurotransmitters may interact with ANG II to augment or inhibit drinking behavior. Because ANG II immunoreactivity and binding are so widely distributed in the central nervous system, brain ANG II is unlikely to have a role as circumscribed as that of circulating ANG II. Angiotensin peptides generated from brain precursors may also be involved in functions that have little immediate effect on body fluid homeostasis and blood pressure control, such as cell differentiation, regeneration and remodeling, or learning and memory. Analysis of the mechanisms of increased drinking caused by drugs and experimental procedures that activate the renal renin-angiotensin system, and clinical conditions in which renal renin secretion is increased, have provided evidence that endogenously released renal renin can generate enough circulating ANG II to stimulate drinking. But it is also certain that other mechanisms of thirst and sodium appetite still operate when the effects of circulating ANG II are blocked or absent, although it is not known whether this is also true for angiotensin peptides formed in the brain. Whether ANG II should be regarded primarily as a hormone released in hypovolemia helping to defend the blood volume, a neurotransmitter or paracrine agent with a privileged role in the neural pathways for thirst and sodium appetite of all kinds, a neural organizer especially in sodium appetit

Potentiation of angiotensin II-induced drinking by glucocorticoids is a specific glucocorticoid type II receptor (GR)-mediated event

Earlier studies showed that pretreatment (3 and 6 h) of rats with the glucocorticoid hormone, dexamethasone, potentiated the drinking response to either central or peripheral administration of angiotensin II (AII). In the present study the specificity and mechanisms of this potentiation were examined. Intraperitoneal (i.p.) injection of rats with the pure glucocorticoid agonist, RU 28362 (0.4-1.6 mg/kg; 3-24 h), resulted in a time- and dose-dependent potentiation of the drinking responses to either peripherally (100 micrograms/kg, s.c.) or centrally (10 ng) injected AII, similar to the effects of dexamethasone. Drinking induced by central injection of carbachol (200 ng) was unaltered by pretreatment with RU 28362, suggesting that potentiation by this compound was specific for AII. The potentiation of AII-induced drinking by either dexamethasone or RU 28362 was completely abolished by pretreatment with the glucocorticoid Type II receptor (GR) antagonist, RU 38486 (2 mg/kg, i.p.), but not by the mineralocorticoid Type I receptor (MR) blocker, mespirenone (2 mg/kg, i.p.). Taken together, these results indicate that the glucocorticoid-induced potentiation of AII-induced drinking is mediated via GR. Associated with the fact that glucocorticoids potentiate AII-induced drinking is the observation that these steroids also potentiate AII-induced urine output. This enhancement of urine output may explain in part the potentiation in drinking behavior. Possible mechanisms are discussed.

The importance of thirst in maintenance of fluid balance

Plasma osmolality is maintained within very narrow limits by the control of water intake via thirst and water output via secretion of vasopressin. Osmoreceptors are situated in the brain, but on the blood side of the blood-brain barrier in a circumventricular organ. These regions are stimulated by an increase in plasma osmolality and form the most important input to cause thirst and drinking. Cardiopulmonary and arterial baroreceptors sensitive to blood volume and blood pressure also can be important, so hypovolaemic events such as haemorrhage can stimulate thirst. Both raised plasma osmolality and reduced blood volume contribute to thirst and vasopressin secretion following water deprivation. The importance of the nucleus medianus in the neural circuitary involved in integrating thirst should be emphasized. Mechanisms which stop drinking are different from those which initiate it, and oropharyngeal metering of the volume of fluid consumed provides the important input. There are a number of situations in humans where thirst thresholds and sensitivities are altered. The elderly have higher thirst thresholds and this can cause symptoms of dehydration. Increased drinking is seen in congestive heart failure, renal hypertension and certain cerebral lesions. Thirst thresholds are set at lower levels in pregnancy and in the luteal phase of the menstrual cycle and may contribute to fluid retention in these situations.

Arousal of a specific and persistent sodium appetite in the rat with continuous intracerebroventricular infusion of angiotensin II

1. Prolonged exposure of the brain of the normal Na-replete rat to angiotensin II produced a marked and persistent Na appetite. In a first series of experiments, short-term, repeated systemic injections of isoprenaline or renin (both of which raise circulating angiotensin levels), and repeated intracranial injections of angiotensin II evoked increased ingestion of 2 . 7% NaCl. In the second series of experiments, continuous infusions of angiotensin II directly into the brain evoked extremely large intakes of 3% NaCl. 2. In addition to large intakes of hypertonic NaCl some rats drank daily volumes of water that exceeded their body weight. 3. Not only did the animals drink large volumes of 3% NaCl some rats drank daily volumes of water that exceeded their body weight. 3. Not only did the animals drink large volumes of 3% NaCl during continuous angiotensin II infusion, but after termination of the infusion they continued to ingest NaCl at a rate comparable to that of the adrenalectomized rat. In most of the animals the persistent NaCl intake diminished over several days, but other animals continued to drink NaCl for as long as their intake was measured (up to 7 months). 4. The response to continuous infusion of angiotensin II was dose-dependent. Both water and 3% NaCl intake increased over a dose range of 6 ng h-1 to 6000 ng h-1. The persistence of the sodium appetitite was also dose-dependent across the same range of doses. 5. Angiotensin-induced salt appetite is specific for Na. Animals did not drink 0 . 5 M-NH4Cl and only occasionally drank minimal amounts of 0 . 5 M-KCl during continuous infusion. 6. The large water turnover was not responsible for the Na appetite. Rats given access to 3% NaCl only during infusion of angiotensin copiously. Animals that were not infused but were given saccharine-flavoured water in order to increase their water intakes did not drink 3% NaCl offered at the same time even though fluid intake was high. Rats that did not receive intracranial infusions but were infused intragastrically with volumes of water equal to or exceeding the amounts that were drunk during angiotensin infusion did not drink the 3% NaCl but did drink some water. 7. Records of the drinking by rats infused with angiotensin show that firstly the onset of drinking after the start of angiotensin infusion varied from animal to animal, secondly, NaCl drinking was not temporally linked to water intake, although this was observed occasionally, and thirdly, most of the drinking occurred during the night although angiotensin was infused continuously throughtout the nychthemeron. 8. Therefore, increases in angiotensin levels, probably with other factors such as increased levels of aldosterone or ACTH, result in Na appetite. The hormonal changes may alter the animals' preception of salt making it more acceptable. By means that are not yet understood the increased accceptability of salt persists after the termination of angiotensin infusion.

The relationship between elevated water intake and oedema associated with congestive cardiac failure in the dog

1. In the dog constriction of the thoracic region of the inferior vena cava increases water intake and extracellular and plasma fluid volumes. 2. Restriction of water intake to the pre-operative level for 2 weeks reduces the measured extracellular fluid volume to the pre-operative level. 3. Administration of the competitive angiotensin inhibitor, saralasin acetate, to two dogs in congestive cardiac failure following thoracic caval constriction markedly reduced their water intake. 4. These results suggest that increased fluid intake is probably important in the aetiology of the oedema associated with congestive cardiac failure, probably through the renin-angiotensin system.

Renin-angiotensin role in thirst: paradoxical enhancement of drinking by angiotensin converting enzyme inhibitor

A competitive angiotensin converting enzyme antagonist SQ 20, 881 (SQ), was used to examine the role of the renin-angiotensin system in putative renin-dependent thirst in the albino rat. Significant enhancement of "renin-dependent" as well as renin-independent drinking was observed in the presence of peripheral SQ. Intraventricular SQ obviated this enhancement of drinking but did not affect the water intake caused by the original stimulus itself, whereas it sharply reduced drinking evoked by peripheral renin in nephrectomized rats. Prior renin depletion likewise had no influence on so-called renin-dependent thirst.

The role of a renal thirst factor in drinking induced by extracellular stimuli

1. Rats in normal fluid balance drank water 1-2 hr after complete ligation of the inferior vena cava either above or below the renal veins. At the same time there was a fall in urine flow and excretion of electrolyte, especially after caval ligation above the renal veins, so that the animals ended the initial 6 hr period in positive fluid balance.2. Caval ligation was relatively ineffective as a stimulus to drinking after bilateral nephrectomy, but was effective in rats made anuric by ureteric ligation.3. Rats subjected to caval ligation and offered a choice between water and 1.8% saline (w/v) drank water, despite the increasing hypotonicity of the body fluids thereby resulting.4. During the secondary polydipsia, which generally occurred on about the third day after caval ligation as renal function was recovering, there was an increased preference for 1.8% saline.5. Constriction of the aorta above the renal arteries, or constriction of both renal arteries, also caused drinking, oliguria and the development of positive fluid balance.6. Constriction of the aorta below the renal arteries, or after nephrectomy, was ineffective as a stimulus to drinking.7. Saline extracts of renal cortex caused rats in normal water balance to drink. Activity was destroyed by boiling the extract for 10 min. Renal medullary and hepatic extracts were without effect on drinking.8. It proved impossible to separate dipsogenic and pressor activities of renal extracts during the different stages of fractionation which lead to the production of renin; disappearance of one activity was invariably accompanied by disappearance of the other.9. Dipsogenic and pressor actions were greater in nephrectomized rats than in normal rats.10. Both extractable dipsogenic factor and extractable pressor activity were reduced by treating the rat with DOCA and saline for several weeks beforehand.11. The renal dipsogen therefore has similar properties to renin. It may prove to be identical with renin, particularly in view of the fact that angiotensin also stimulates drinking.12. Adrenalectomy did not affect drinking induced by renin or by caval ligation.13. It is concluded that the renin angiotensin system may play a role in the genesis of the thirst which follows certain extracellular stimuli.