Role of peripheral angiotensin in salt appetite of the sodium-deplete rat

Control of fluid intake in dehydrated rats and evolution of sodium appetite

The objective of the present work is to examine from a new perspective the existence of causal factors not predicted by the classical theory that thirst and sodium appetite are two distinct motivations. For example, we ask why water deprivation induces sodium appetite, thirst is not "water appetite", and intracellular dehydration potentially causes sodium appetite. Contrary to the classical theory, we suggest that thirst first, and sodium appetite second, designate a temporal sequence underlying the same motivation. The single motivation becomes an "intervenient variable" a concept borrowed from the literature, fully explained in the text, between causes of dehydration (extracellular, intracellular, or both together), and respective behavioral responses subserved by hindbrain-dependent inhibition (e.g., lateral parabrachial nucleus) and forebrain facilitation (e.g., angiotensin II). A corollary is homology between rat sodium appetite and marine teleost thirst-like motivation that we name "protodipsia". The homology argument rests on similarities between behavior (salty water intake) and respective neuroanatomical as well as functional mechanisms. Tetrapod origin in a marine environment provides additional support for the homology. The single motivation hypothesis is also consistent with ingestive behaviors in nature given similarities (e.g., thirst producing brackish water intake) between the behavior of the laboratory rat and wild animals, rodents included. The hypotheses of single motivation and homology might explain why hyperosmotic rats, or eventually any other hyperosmotic tetrapod, shows paradoxical signs of sodium appetite. They might also explain how ingestive behaviors determined by dehydration and subserved by hindbrain inhibitory mechanisms contributed to tetrapod transition from sea to land.

Physiological and Transcriptomic Changes in the Hypothalamic-Neurohypophysial System after 24 h of Furosemide-Induced Sodium Depletion

BACKGROUND/AIMS

Furosemide is a loop diuretic widely used in clinical practice for the treatment of oedema and hypertension. The aim of this study was to determine physiological and molecular changes in the hypothalamic-neurohypophysial system as a consequence of furosemide-induced sodium depletion.

METHODS

Male rats were sodium depleted by acute furosemide injection (10 and 30 mg/kg) followed by access to low sodium diet and distilled water for 24 h. The renal and behavioural consequences were evaluated, while blood and brains were collected to evaluate the neuroendocrine and gene expression responses.

RESULTS

Furosemide treatment acutely increases urinary sodium and water excretion. After 24 h, water and food intake were reduced, while plasma angiotensin II and corticosterone were increased. After hypertonic saline presentation, sodium-depleted rats showed higher preference for salt. Interrogation using RNA sequencing revealed the expression of 94 genes significantly altered in the hypothalamic paraventricular nucleus (PVN) of sodium-depleted rats (31 upregulated and 63 downregulated). Out of 9 genes chosen, 5 were validated by quantitative PCR in the PVN (upregulated: Ephx2, Ndnf and Vwf; downregulated: Caprin2 and Opn3). The same genes were also assessed in the supraoptic nucleus (SON, upregulated: Tnnt1, Mis18a, Nr1d1 and Dbp; downregulated: Caprin2 and Opn3). As a result of these plastic transcriptome changes, vasopressin expression was decreased in PVN and SON, whilst vasopressin and oxytocin levels were reduced in plasma.

CONCLUSIONS

We thus have identified novel genes that might regulate vasopressin gene expression in the hypothalamus controlling the magnocellular neurons secretory response to body sodium depletion and consequently hypotonic stress.

Higher salt preference in heart failure patients

Heart failure (HF) is a complex syndrome that involves changes in behavioral, neural and endocrine regulatory systems. Dietary salt restriction along with pharmacotherapy is considered an essential component in the effective management of symptomatic HF patients. However, it is well recognized that HF patients typically have great difficulty in restricting sodium intake. We hypothesized that under HF altered activity in systems that normally function to regulate body fluid and cardiovascular homeostasis could produce an increased preference for the taste of salt. Therefore, this study was conducted to evaluate the perceived palatability (defined as salt preference) of food with different concentrations of added salt in compensated chronically medicated HF patients and comparable control subjects. Healthy volunteers (n=25) and medicated, clinically stable HF patients (n=38, NYHA functional class II or III) were interviewed and given an evaluation to assess their preferences for different amounts of saltiness. Three salt concentrations (0.58, 0.82, and 1.16 g/100 g) of bean soup were presented to the subjects. Salt preference for each concentration was quantified using an adjective scale (unpleasant, fair or delicious). Healthy volunteers preferred the soup with medium salt concentration (p=0.042), HF patients disliked the low concentration (p<0.001) and preferred the high concentration of salted bean soup (p<0.001). When compared to healthy volunteers, HF patients demonstrated a significantly greater preference for the soup with a high salt concentration (p=0.038). It is concluded that medicated, compensated patients under chronic treatment for HF have an increased preference for salt.

Functional correlates of activity in neurons projecting from the lamina terminalis to the ventrolateral periaqueductal gray

The lamina terminalis (LT) consists of the organum vasculosum of the LT (OVLT), the median preoptic nucleus (MnPO) and the subfornical organ (SFO). All subdivisions of the LT project to the ventrolateral periaqueductal gray (vlPAG). The LT and the vlPAG are implicated in several homeostatic and behavioral functions, including body fluid homeostasis, thermoregulation and the regulation of sleep and waking. By combining visualization of c-Fos protein and retrograde neuroanatomical tracer we have examined the functional correlates of LT-vlPAG projection neurons. Rats were injected with retrograde tracer into the vlPAG and, following a 1-week recovery period, they were subjected to either hypertonic saline administration (0.5 M NaCl, 1 mL/100 g i.p.), 24-h water deprivation, isoproterenol administration (increases circulating angiotensin II; 50 microg/kg s.c.), heat exposure (39 degrees C for 60 min) or permitted 180 min spontaneous sleep. Retrogradely labeled neurons from the vlPAG and double-labelled neurons were then identified and quantified throughout the LT. OVLT-vlPAG projection neurons were most responsive to hypertonic saline and water deprivation. SFO-vlPAG projection neurons were most active following isoproterenol administration, and MnPO-vlPAG projection neurons displayed significantly more Fos immunostaining following water deprivation, heat exposure and sleep. These results support the existence of functional subdivisions of LT-vlPAG-projecting neurons, and indicate three patterns of activity that correspond to thermal and sleep wake regulation, osmotic or hormonal stimuli.

Challenged sodium balance and expression of angiotensin type 1A receptor mRNA in the hypothalamus of Wistar and Dahl rat strains

The present study investigates the influence of a chronic high Na+ diet (8% Na+) on the expression of the angiotensin type 1A (AT1A) receptor gene in the lamina terminalis and paraventricular nucleus of the hypothalamus (PVH) in normotensive Wistar (W) rats, as well as in Dahl salt-resistant (DR) and Dahl salt-sensitive (DS) rats. Three weeks of 8% Na+ diet led to a higher blood pressure in DS rats compared to DR and W rats. Moreover, the high Na+ diet was correlated with a decreased expression of AT1A receptor mRNA in the median preoptic nucleus (MnPO) and in the PVH of DS rats, compared to DR and W rats. Contrastingly, the AT1A receptor mRNA expression was not altered by the high Na+ diet in the forebrain circumventricular organs of all the rat strains. Interestingly, a furosemide-induced Na+ depletion was correlated with an increased expression of AT1A receptor mRNA in the PVH, MnPO and SFO of both the DS and DR rats. It is concluded that chronic high Na+ diet did differently regulate the expression of AT1A receptor mRNA in two hypothalamic integrative centers for hydromineral and cardiovascular balance (the PVH and MnPO) in DS rats, compared to DR and W rats. However, the AT1A receptor mRNA expression was similarly regulated in DS and DR rats in response to an acute Na+ depletion, suggesting a distinct high Na+ -induced regulation of the AT1A receptor gene in the PVH and MnPO of DS rats.

Increased salt appetite in patients with congenital adrenal hyperplasia 21-hydroxylase deficiency

Salt appetite was investigated in 14 patients with congenital adrenal hyperplasia of the salt-wasting form (SW group), 12 patients with the simple virilized form who are not salt losing, and 18 healthy siblings. Salt appetite was evaluated by questionnaire, preference tests, and dietary analyses. The findings showed that SW who were not therapeutically normalized showed increased salt appetite but no change in sweet preference. Their salt appetite correlated with symptoms of salt wasting, namely, plasma renin activity, plasma K+, and urine Na+ and (inversely) with blood pressure. Sensitivity to the taste of NaCl was not altered. Factor analyses of a larger group confirmed the distinction between salt appetite and sweet preference, but intake of dietary Na+ and sweet carbohydrates and intake of salty and sweet snacks did not reflect distinct salt or sweet preferences. We confirm that putative perinatal dehydration, due to maternal nausea and vomiting during pregnancy, childhood vomiting, and diarrhea with occasional saline infusion, was related to increased salt appetite in adolescence. The findings suggest that salt appetite in humans is determined by interdependent, innate, physiological, and acquired attributes. Salt appetite in SW patients is an adaptive response mediated by the renin-angiotensin system, an innate predisposition to acquire salt preference (in anticipation of both sodium loss and its consequence), and imprinting by perinatal hyponatremic occurrences. Our findings contribute to understanding human salt intake, provide insight into the motivation for salt in patients with congenital adrenal hyperplasia 21-OH deficiency, and may point the way to improvements in therapeutic compliance in these patients.

Acute sodium deficit triggers plasticity of the brain angiotensin type 1 receptors

The brain renin-angiotensin system (bRAS) is involved in the control of hydromineral balance. However, little information is available on the functional regulation of the bRAS as a consequence of sodium deficit in the extracellular fluid compartments. We used a pharmacological model of acute Na+ depletion (furosemide injections) to investigate changes of a major component of the bRAS, the hypothalamic angiotensin type 1A (AT(1A)) receptors. Furosemide induced a rapid and long-lasting expression of the AT(1A) mRNA in the subfornical organ, the median preoptic nucleus (MnPO), and the parvocellular division of the paraventricular nucleus (pPVN). Na+ depletion increased the number of cells expressing AT(1A) mRNA in the pPVN, but not in the MnPO. The enhancement of AT(1A) mRNA expression was associated with an increase in AT(1) binding sites in all the regions studied. It is of interest that in the paraventricular nucleus, the majority of the neurons expressing AT(1A) mRNA also showed an increase in metabolic activity (Fos-related antigen immunoreactivity [FRA-ir]). By contrast, in the MnPO, we observe two distinct cell populations. Our data demonstrated that an acute Na+ deficit induced a functional regulation of the hypothalamic AT(1A) receptors, indicating that these receptors are subject to plasticity in response to hydromineral perturbations.

Circulating angiotensin II mediates sodium appetite in adrenalectomized rats

We investigated the role of circulating ANG II in sodium appetite after adrenalectomy. Adrenalectomized rats deprived of their main access to sodium (0.3 M NaCl) for 9 h drank 14.1 +/- 1.5 ml of the concentrated saline solution in 2 h of access. Intravenous infusion of captopril (2.5 mg/h) during the last 5 h of sodium restriction reduced sodium intake by 77 +/- 12% (n = 5) without affecting the degree of sodium depletion and hypovolemia incurred during deprivation. Functional evidence indicates that this dose of captopril blocked production of ANG II in the peripheral circulation, but not in the brain; that is, injection of ANG I into the lateral brain ventricle stimulated intake of both water and 0.3 M NaCl. Intravenous infusion of ANG II (starting 10-15 min before 0.3 M NaCl became available) in adrenalectomized, captopril-treated rats restored both sodium intake and blood pressure to values seen in rats not treated with captopril. Longer (20 h) infusions of captopril in 22-h sodium-restricted rats also blocked sodium appetite, but reduced or prevented sodium depletion. Intravenous infusion of ANG II after these long captopril infusions stimulated sodium intake, but intake was less than in controls not treated with captopril. These results indicate that most or all of the sodium appetite of adrenalectomized rats is mediated by circulating ANG II.

NaCl appetite in two strains of rat reported to be resistant to mineralocorticoid-induced hypertension

Both Long-Evans (LE) and Wistar-Furth (WF) strains of rat are known to be resistant to development of hypertension by mineralocorticoid (MC) treatment. MC-induced hypertension is, in part, mediated by the brain. We have examined another aspect of central MC action, the induction of NaCl appetite in these strains, by using the more common Sprague-Dawley (SD) and Wistar (WS) strains for comparison. In the first experiment, LE and SD rats were administered three treatments known to induce an appetite for NaCl solution in rats. Administration of deoxycorticosterone acetate (DOCA) increased the intake of 0.45 M NaCl in both strains, but the amounts consumed were about 2-fold greater in LE rats than in SD rats. Administration of captopril also increased NaCl intake, but there were no differences between LE and SD rats. NaCl depletion with furosemide induced NaCl appetite in both strains, but the amounts consumed were about 2-fold greater in LE rats than in SD rats. In the second experiment, adult male WF and WS rats were administered DOCA, enalapril, or furosemide and NaCl appetite was determined. Both strains showed comparable NaCl appetite during each of these treatments. However, during a 5-week regimen of DOCA with only NaCl-KCl solution to drink, uninephrectomized WF rats consumed less than WS rats. Thus, despite reported resistance to MC-induced hypertension, neither LE nor WF strains of rats showed correspondingly marked deficits in induced NaCl appetite.

Effects of L-arginine on angiotensin II-related water and salt intakes

Nitric oxide (NO) has been implicated centrally as an inhibitory neuromodulator, acting in short feedback loops. Neurons capable of NO synthesis have been localized in various thirst-related hypothalamic nuclei. Intracerebroventricular (icv) injection of L-arginine (L-arg), the precursor for NO, has previously been shown to attenuate both dehydration- and angiotensin II (Ang II)-induced drinking behavior. The present study further examines the effects of L-arg on drinking. We confirmed that icv administration of L-arg (50 microg) reduced water intakes induced by both 24 h water deprivation and icv Ang II (250 ng). We additionally showed that L-arg inhibited the water intake induced by peripheral injection of Ang II and the intake of 0.3 M NaCl following 24 h sodium depletion. We demonstrated the behavioral specificity of L-arg treatment by showing that it did not inhibit the intake of sucrose in food deprived rats and did not act as an unconditional stimulus for the formation of a conditioned taste aversion. These results lend further support to the idea that NO plays a role in modulating fluid balance and drinking behavior.

Schedule-induced polydipsic consumption of hypertonic NaCl solutions: effects of chlordiazepoxide

Rats were exposed to daily, 3-h sessions of schedule-induced polydipsia (SIP) in which either water, hypertonic NaCl solution (1.5% or 2.2%), or concurrent water and 1.5% NaCl were available. Each condition was in effect for several, consecutive weeks. Presession subcutaneous injections of chlordiazepoxide (CDZP) produced dose-related increases in the polydipsic ingestion of both NaCl solutions but had smaller and less certain effects on water consumption. Under the concurrent-fluid presentation condition, CDZP primarily increased NaCl solution consumption. Conditions generating SIP may function to attenuate what might be viewed as the punishing effects of ingesting highly hypertonic NaCl solutions, thereby permitting the chronic self-administration of large, daily amounts. Like many benzodiazepines, CDZP can attenuate the effects of punishment and thereby also increase NaCl solution ingestion. The two sources of punishment attenuation may be additive, with both differentially effecting greater increases in hypertonic NaCl ingestion, compared to water, when both fluids are presented concurrently.

The neuroendocrinology of thirst and salt appetite: visceral sensory signals and mechanisms of central integration

This review examines recent advances in the study of the behavioral responses to deficits of body water and body sodium that in humans are accompanied by the sensations of thirst and salt appetite. Thirst and salt appetite are satisfied by ingesting water and salty substances. These behavioral responses to losses of body fluids, together with reflex endocrine and neural responses, are critical for reestablishing homeostasis. Like their endocrine and neural counterparts, these behaviors are under the control of both excitatory and inhibitory influences arising from changes in osmolality, endocrine factors such as angiotensin and aldosterone, and neural signals from low and high pressure baroreceptors. The excitatory and inhibitory influences reaching the brain require the integrative capacity of a neural network which includes the structures of the lamina terminalis, the amygdala, the perifornical area, and the paraventricular nucleus in the forebrain, and the lateral parabrachial nucleus (LPBN), the nucleus tractus solitarius (NTS), and the area postrema in the hindbrain. These regions are discussed in terms of their roles in receiving afferent sensory input and in processing information related to hydromineral balance. Osmoreceptors controlling thirst are located in systemic viscera and in central structures that lack the blood-brain barrier. Angiotensin and aldosterone act on and through structures of the lamina terminalis and the amygdala to stimulate thirst and sodium appetite under conditions of hypovolemia. The NTS and LPBN receive neural signals from baroreceptors and are responsible for inhibiting the ingestion of fluids under conditions of increased volume and pressure and for stimulating thirst under conditions of hypovolemia and hypotension. The interplay of multiple facilitory influences within the brain may take the form of interactions between descending angiotensinergic systems originating in the forebrain and ascending adrenergic systems emanating from the hindbrain. Oxytocin and serotonin are additional candidate neurochemicals with postulated inhibitory central actions and with essential roles in the overall integration of sensory input within the neural network devoted to maintaining hydromineral balance.