Palatability profile in spontaneously hypertensive rats.

The spontaneously hypertensive rats (SHRs) have enhanced palatability for NaCl taste as measured by the increased number of hedonic versus aversive responses to intraoral infusion (1 mL/1 min) of 0.3 M NaCl, in a taste reactivity test in euhydrated condition or after 24 h of water deprivation + 2 h of partial rehydration (WD-PR). SHRs also ingested more sucrose than normotensive rats, without differences in quinine hydrochloride intake. Here, we investigated the palatability of SHRs (n = 8-10) and normotensive Holtzman rats (n = 8-10) to sucrose and quinine sulphate infused intraorally in the same conditions that NaCl palatability was increased in SHRs. SHRs had similar number of hedonic responses to 2% sucrose in euhydrated condition (95 ± 19) or after WD-PR (142 ± 25), responses increased when compared with normotensive rats in euhydrated condition (13 ± 3) or after WD-PR (21 ± 6). SHRs also showed increased number of aversive responses to 1.4 mM quinine sulphate compared with normotensive rats, whether in euhydrated condition (86 ± 6, vs. normotensive: 54 ± 7) or after WD-PR (89 ± 9, vs. normotensive: 40 ± 9). The results suggest that similar to NaCl taste, sweet taste responses are increased in SHRs and resistant to challenges in bodily fluid balance. They also showed a more intense aversive response in SHRs to bitter taste compared with normotensives. This suggests that the enhanced response of SHRs to taste rewards does not correspond to a decreased response to a typical aversive taste.

Participation of α2 -adrenoceptors in sodium appetite inhibition during sickness behaviour following administration of lipopolysaccharide.

Sickness behaviour, a syndrome characterized by a general reduction in animal activity, is part of the active-phase response to fight infection. Lipopolysaccharide (LPS), an effective endotoxin to model sickness behaviour, reduces thirst and sodium excretion, and increases neurohypophysial secretion. Here we review the effects of LPS on thirst and sodium appetite. Altered renal function and hydromineral fluid intake in response to LPS occur in the context of behavioural reorganization, which manifests itself as part of the syndrome. Recent data show that, in addition to its classical effect on thirst, non-septic doses of LPS injected intraperitoneally produce a preferential inhibition of intracellular thirst versus extracellular thirst. Moreover, LPS also reduced hypertonic NaCl intake in sodium-depleted rats that entered a sodium appetite test. Antagonism of α2 -adrenoceptors abolished the effect of LPS on sodium appetite. LPS and cytokine transduction potentially recruit brain noradrenaline and α2 -adrenoceptors to control sodium appetite and sickness behaviour.

Water deprivation-partial rehydration induces sensitization of sodium appetite and alteration of hypothalamic transcripts.

iSodium intake occurs either as a spontaneous or induced behavior, which is enhanced, i.e., sensitized, by repeated episodes of water deprivation followed by subsequent partial rehydration (WD-PR). In the present work, we examined whether repeated WD-PR alters hypothalamic transcripts related to the brain renin-angiotensin system (RAS) and apelin system in male normotensive Holtzman rats (HTZ). We also examined whether the sodium intake of a strain with genetically inherited high expression of the brain RAS, the spontaneously hypertensive rat (SHR), responds differently than HTZ to repeated WD-PR. We found that repeated WD-PR, besides enhancing spontaneous and induced 0.3 M NaCl intake, increased the hypothalamic expression of angiotensinogen, aminopeptidase N, and apelin receptor transcripts (43%, 60%, and 159%, respectively) in HTZ at the end of the third WD-PR. Repeated WD-PR did not change the daily spontaneous 0.3 M NaCl intake and barely changed the need-induced 0.3 M NaCl intake of SHR. The same treatment consistently enhanced spontaneous daily 0.3 M NaCl intake in the normotensive Wistar-Kyoto rats. The results show that repeated WD-PR produces alterations in hypothalamic transcripts and also sensitizes sodium appetite in HTZ. They suggest an association between the components of hypothalamic RAS and the apelin system, with neural and behavioral plasticity produced by repeated episodes of WD-PR in a normotensive strain. The results also indicate that the inherited hyperactive brain RAS is not a guarantee for sensitization of sodium intake in the male adult SHR exposed to repeated WD-PR.

Short-term cross-sensitizion of need-free sugar intake by combining sodium depletion and hypertonic NaCl intake.

History of sodium depletion cross-sensitizes the effects of drugs of abuse. The objective of the present study was to find out if history of sodium depletion also cross-sensitizes a natural reward such as sugar intake in the rat. Sodium depletion was induced by furosemide combined with removal of ambient sodium for 24 h; it was repeated seven days later. The depletion was immediately followed by 0.3 M NaCl intake in a sodium appetite test (active sodium repletion). Seven days after the last depletion, hydrated and fed (need-free) sucrose-naïve animals were offered 10% sucrose in a first 2-h sucrose test. The sucrose test was repeated once a day in a series of five consecutive days. History of sodium depletion enhanced sucrose intake in the first and second tests; it had no effect from the third to fifth sucrose test. The effect on the initial sucrose intake tests disappeared if the rats did not ingest 0.3 M NaCl in the sodium appetite test. Prior experience with sucrose intake in need-free conditions had no effect on sodium appetite. History of intracellular dehydration transiently influenced sucrose intake in the first sucrose test. We found no evidence for thirst sensitization. We conclude that history of dehydration, particularly that resulting from sodium depletion, combined to active sodium repletion, produced short-term cross-sensitization of sucrose intake in sucrose-naïve rats. The results suggest that the cross-sensitization of sucrose intake related with acquisition of sugar as a novel nutrient rather than production of lasting effects on sugar rewarding properties.

Role of the lateral parabrachial nucleus in the control of sodium appetite.

In states of sodium deficiency many animals seek and consume salty solutions to restore body fluid homeostasis. These behaviors reflect the presence of sodium appetite that is a manifestation of a pattern of central nervous system (CNS) activity with facilitatory and inhibitory components that are affected by several neurohumoral factors. The primary focus of this review is on one structure in this central system, the lateral parabrachial nucleus (LPBN). However, before turning to a more detailed discussion of the LPBN, a brief overview of body fluid balance-related body-to-brain signaling and the identification of the primary CNS structures and humoral factors involved in the control of sodium appetite is necessary. Angiotensin II, mineralocorticoids, and extracellular osmotic changes act on forebrain areas to facilitate sodium appetite and thirst. In the hindbrain, the LPBN functions as a key integrative node with an ascending output that exerts inhibitory influences on forebrain regions. A nonspecific or general deactivation of LPBN-associated inhibition by GABA or opioid agonists produces NaCl intake in euhydrated rats without any other treatment. Selective LPBN manipulation of other neurotransmitter systems [e.g., serotonin, cholecystokinin (CCK), corticotrophin-releasing factor (CRF), glutamate, ATP, or norepinephrine] greatly enhances NaCl intake when accompanied by additional treatments that induce either thirst or sodium appetite. The LPBN interacts with key forebrain areas that include the subfornical organ and central amygdala to determine sodium intake. To summarize, a model of LPBN inhibitory actions on forebrain facilitatory components for the control of sodium appetite is presented in this review.

Chronic administration of catalase inhibitor attenuates hypertension in renovascular hypertensive rats.

AIMS: Reactive oxygen species like hydrogen peroxide (H2O2) are produced endogenously and may participate in intra- and extracellular signaling, including modulation of angiotensin II responses. In the present study, we investigated the effects of chronic subcutaneous (sc) administration of the catalase inhibitor 3-amino-1,2,4-triazole (ATZ) on arterial pressure, autonomic modulation of arterial pressure, hypothalamic expression of AT1 receptors and neuroinflammatory markers and fluid balance in 2-kidney, 1clip (2K1C) renovascular hypertensive rats. MATERIALS AND METHODS: Male Holtzman rats with a clip occluding partially the left renal artery and chronic sc injections of ATZ were used. KEY FINDINGS: Subcutaneous injections of ATZ (600 mg/kg of body weight/day) for 9 days in 2K1C rats reduced arterial pressure (137 ± 8, vs. saline: 182 ± 8 mmHg). ATZ also reduced the sympathetic modulation and enhanced the parasympathetic modulation of pulse interval, reducing the sympatho-vagal balance. Additionally, ATZ reduced mRNA expression for interleukins 6 and IL-1ß, tumor necrosis factor-α, AT1 receptor (0.77 ± 0.06, vs. saline: 1.47 ± 0.26 fold change), NOX 2 (0.85 ± 0.13, vs. saline: 1.75 ± 0.15 fold change) and the marker of microglial activation, CD 11 (0.47 ± 0.07, vs. saline, 1.34 ± 0.15 fold change) in the hypothalamus of 2K1C rats. Daily water and food intake and renal excretion were only slightly modified by ATZ. SIGNIFICANCE: The results suggest that the increase of endogenous H2O2 availability with chronic treatment with ATZ had an anti-hypertensive effect in 2K1C hypertensive rats. This effect depends on decreased activity of sympathetic pressor mechanisms and mRNA expression of AT1 receptors and neuroinflammatory markers possibly due to reduced angiotensin II action.

GABAergic and glutamatergic transmission reveals novel cardiovascular and urinary bladder control features in the shell nucleus accumbens.

The shell Nucleus Accumbens (NAcc) projects to the lateral preoptic area, which is involved in the central micturition control and receives inputs from medullary areas involved in cardiovascular control. We investigated the role of GABAergic and glutamatergic transmission in the shell NAcc on intravesical pressure (IP) and cardiovascular control. Male Wistar rats with guide cannulas implanted bilaterally in the shell NAcc 7 days prior to the experiments were anesthetized with 2% isoflurane in 100% O2 and subjected to cannulation of the femoral artery and vein for mean arterial pressure (MAP) and heart rate recordings (HR) and infusion of drugs, respectively. The urinary bladder (UB) was cannulated for IP measurement. A Doppler flow probe was placed around the renal arterial for renal blood flow (RBF) measurement. After the baseline MAP, HR, IP and RBF recordings for 15 min, GABA or bicuculline methiodate (BMI) or L-glutamate or kynurenic acid (KYN) or saline (vehicle) were bilaterally injected into the shell NAcc and the variables were measured for 30 min. Data are as mean ± SEM and submitted to Student́s t test. GABA injections into the shell NAcc evoked a significant fall in MAP and HR and increased IP and RC compared to saline. L-glutamate in the shell NAcc increased MAP, HR and IP and reduced RC. Injections of BMI and KYN elicited no changes in the variables recorded. Therefore, the GABAergic and glutamatergic transmissions in neurons in the shell NAcc are involved in the neural pathways responsible for the central cardiovascular control and UB regulation.

Sympathetic and angiotensinergic activity in spontaneously hypertensive rats treated with 3-amino-1,2,4-triazole.

Previous studies from our laboratory have shown that the pressor response to intracerebroventricular (icv) administered ANG II in normotensive rats or spontaneously hypertensive rats (SHRs) is attenuated by increased central H2O2 concentration, produced either by direct H2O2 icv injection or by increased endogenous H2O2 centrally in response to local catalase inhibition with 3-amino-1,2,4-triazole (ATZ). In the present study, we evaluated the effects of ATZ administered peripherally on arterial pressure and sympathetic and angiotensinergic activity in SHRs. Male SHRs weighing 280-330 g were used. Mean arterial pressure (MAP) and heart rate (HR) were recorded in conscious freely moving SHRs. Acute intravenous injection of ATZ (300 mg/kg of body weight) did not modify MAP and HR during the next 4 h, however, the treatment with ATZ (300 mg/kg of body weight twice per day) for 3 days reduced MAP (144 ± 6, vs. saline, 183 ± 13 mmHg), without changing HR. Intravenous hexamethonium (ganglionic blocker) produced a smaller decrease in MAP 4 h after ATZ (-25 ± 3, vs saline -38 ± 4 mmHg). Losartan (angiotensinergic AT1 receptor blocker) produced a significant depressor response 4 h after ATZ (-22 ± 4, vs. saline: -2 ± 4 mmHg) and in 3-day ATZ treated SHRs (-25 ± 5, vs. saline: -9 ± 4 mmHg). The results suggest that the treatment with ATZ reduces sympathetic activity in SHRs and simultaneously increases angiotensinergic activity.

A critique on the theory of homeostasis.

The objective of this critique is to demonstrate that the theory of "internal environment" (TIE) does not support the theory of "homeostasis" (TOH). We review and conclude that remains valid the concept of "internal environment", which corresponds anatomically to the extracellular fluid (ECF) that bathes tissue cells. The Claude Bernard's classification of "life", a corollary of the TIE under a strict "reactive" paradigm, we then interpret as a classification of how animals behave in response to environmental changes. According to such interpretation, the two theories agree that, when facing changes in the external environment, animals with "free" behavior regulate essential metabolism factors present in the ECF. These are "internalized environmental factors" or IEF (temperature, O2, water, and basic organic and inorganic "nutrients"), a marine legacy of the evolution of the body fluid compartments. However, we show that have empirical and logical shortcomings key inferences derived from the TIE. Such inferences representing traditional premises of TOH we summarize here in two axioms: "if free behavior then regulated IEF" and "all behavioral mechanisms regulate the IEF". In addition, whereas "stability" means "free behavior versus dormancy" in TIE, it means "tissue cells that resist destruction" in TOH. This leads to inevitable contradictions, here discussed at length, that reduce the scope of TOH. We might be in need of a theory that considers not only where TIE and TOH are superficially valid, but also where they crucially diverge, in order to explain "stability" as applied to physiology and behavior.

Involvement of V1-type vasopressin receptors on NaCl intake by hyperosmotic rats treated with muscimol in the lateral parabrachial nucleus.

GABAA receptor activation with agonist muscimol in the lateral parabrachial nucleus (LPBN) induces 0.3 M NaCl intake. In the present study, we investigated water and 0.3 M NaCl intake in male adult rats treated with losartan (angiotensin AT1 receptor antagonist) or MeT-AVP (V1-type vasopressin receptor antagonist) combined with muscimol or methysergide (5-HT2 antagonist) into the LPBN in rats treated with intragastric 2 M NaCl. After 2 M NaCl load and bilateral injections of muscimol (0.5 nmol/0.2 µL) into the LPBN, rats ingested water and 0.3 M NaCl. The pre-treatment of the LPBN with MeT-AVP (1 nmol/0.2 µL) but not losartan (50 µg/0.2 µL) in muscimol treated rats reduced 0.3 M NaCl intake. The pre-treatment of the LPBN with MeT-AVP did not modify the increased 0.3 M NaCl intake in rats treated with methysergide (4 µg/0.2 µL), suggesting that the effect of MeT-AVP was not due to non-specific inhibition of ingestive behavior. The results suggest that endogenous vasopressin in the LPBN facilitates the effects of GABAergic activation driving cell-dehydrated male rats to ingest 0.3 M NaCl.

Water deprivation-induced sodium appetite and differential expression of encephalic c-Fos immunoreactivity in the spontaneously hypertensive rat.

The spontaneously hypertensive rat (SHR) has an intense consumption of NaCl solution. Water deprivation (WD) followed by water intake to satiety induces partial rehydration (PR)-the WD-PR protocol-and sodium appetite. In the present work, WD produced similar water intake and no alterations in arterial pressure among spontaneously hypertensive rat (SHR), Wistar-Kyoto, and Holtzman strains. It also increased the number of cells with positive c-Fos immunoreactivity (Fos-IR) in the lamina terminalis and in the hypothalamic supraoptic (SON) and paraventricular (parvocellular, PVNp) nucleus in these strains. The WD and WD-PR produced similar alterations in all strains in serum osmolality and protein, plasma renin activity, and sodium balance. The SHR ingested about 10 times more 0.3 M NaCl than normotensives strains in the sodium appetite test that follows WD-PR. After WD-PR, the Fos-IR persisted, elevated in the lamina terminalis of all strains but notably in the subfornical organ of the SHR. The WD-PR reversed Fos-IR in the SON of all strains and in the PVNp of SHR. It induced Fos-IR in the area postrema and in the nucleus of the solitary tract (NTS), dorsal raphe, parabrachial (PBN), pre-locus coeruleus (pre-LC), suprachiasmatic, and central amygdalar nucleus of all strains. This effect was bigger in the caudal-NTS, pre-LC, and medial-PBN of SHRs. The results indicate that WD-PR increases cell activity in the forebrain and hindbrain areas that control sodium appetite in the rat. They also suggest that increased cell activity in facilitatory brain areas precedes the intense 0.3 M NaCl intake of the SHR in the sodium appetite test.

Anti-hypertensive effect of hydrogen peroxide acting centrally.

Intracerebroventricular (icv) injection of hydrogen peroxide (H2O2) or the increase of endogenous H2O2 centrally produced by catalase inhibition with 3-amino-1,2,4-triazole (ATZ) injected icv reduces the pressor responses to central angiotensin II (ANG II) in normotensive rats. In the present study, we investigated the changes in the arterial pressure and in the pressor responses to ANG II icv in spontaneously hypertensive rats (SHRs) and 2-kidney, 1-clip (2K1C) hypertensive rats treated with H2O2 injected icv or ATZ injected icv or intravenously (iv). Adult male SHRs or Holtzman rats (n = 5-10/group) with stainless steel cannulas implanted in the lateral ventricle were used. In freely moving rats, H2O2 (5 µmol/1 µl) or ATZ (5 nmol/1 µl) icv reduced the pressor responses to ANG II (50 ng/1 µl) icv in SHRs (11 ± 3 and 17 ± 4 mmHg, respectively, vs. 35 ± 6 mmHg) and 2K1C hypertensive rats (3 ± 1 and 16 ± 3 mmHg, respectively, vs. 26 ± 2 mmHg). ATZ (3.6 mmol/kg of body weight) iv alone or combined with H2O2 icv also reduced icv ANG II-induced pressor response in SHRs and 2K1C hypertensive rats. Baseline arterial pressure was also reduced (-10 to -15 mmHg) in 2K1C hypertensive rats treated with H2O2 icv and ATZ iv alone or combined and in SHRs treated with H2O2 icv alone or combined with ATZ iv. The results suggest that exogenous or endogenous H2O2 acting centrally produces anti-hypertensive effects impairing central pressor mechanisms activated by ANG II in SHRs or 2K1C hypertensive rats.

Activation of alpha(2)-adrenoceptors in the lateral hypothalamus reduces pilocarpine-induced salivation in rats.

Anti-hypertensive drugs that act on central alpha(2)-adrenoceptors and imidazoline receptors usually cause dry mouth in patients. A central area important for the control of salivary secretion and also for the effects of alpha(2)-adrenoceptor activation is the lateral hypothalamus (LH). Therefore, in the present study we investigated the effects of the injections of moxonidine (an alpha(2)-adrenoceptor and imidazoline agonist) alone or combined with RX 821002 (alpha(2)-adrenoceptor antagonist) into the LH on the salivation induced by intraperitoneal (i.p.) pilocarpine (cholinergic muscarinic agonist). Male Holtzman rats with stainless steel cannula implanted into the LH were used. Saliva was collected using pre-weighted small cotton balls inserted into the animal's mouth under ketamine anesthesia. Salivation induced by i.p. pilorcarpine (4micromol/kg of body weight) was reduced by the injection of moxonidine (10 and 20nmol/0.5microl) into the LH (222+/-46 and 183+/-19mg/7min, vs. vehicle: 480+/-30mg/7min). The inhibitory effect of moxonidine on pilocarpine-induced salivation was abolished by prior injections of RX 821002 (160 and 320nmol/0.5microl) into the LH (357+/-25 and 446+/-38mg/7min). Injections of the alpha(1)-adrenoceptor antagonist prazosin (320nmol/0.5microl) into the LH did not change the effects of moxonidine. The results show that activation of alpha(2)-adrenoceptors in the LH inhibits pilocarpine-induced salivation, suggesting that LH is one of the possible central sites involved in the anti-salivatory effects produced by the treatment with alpha(2)-adrenoceptor agonists.

Catalase blockade reduces the pressor response to central cholinergic activation.

Intracerebroventricular (icv) injection of hydrogen peroxide (H2O2), a reactive oxygen species, or the blockade of catalase (enzyme that degrades H2O2 into H2O and O2) with icv injection of 3-amino-1,2,4-triazole (ATZ) reduces the pressor effects of angiotensin II also injected icv. In the present study, we investigated the effects of ATZ injected icv or intravenously (iv) on the pressor responses induced by icv injections of the cholinergic agonist carbachol, which similar to angiotensin II induces pressor responses that depend on sympathoexcitation and vasopressin release. In addition, the effects of H2O2 icv on the pressor responses to icv carbachol were also tested to compare with the effects of ATZ. Normotensive non-anesthetized male Holtzman rats (280-300 g, n = 8-9/group) with stainless steel cannulas implanted in the lateral ventricle were used. Previous injection of ATZ (5 nmol/1 µl) or H2O2 (5 µmol/1 µl) icv similarly reduced the pressor responses induced by carbachol (4 nmol/1 µl) injected icv (13 ± 4 and 12 ± 4 mmHg, respectively, vs. vehicle + carbachol: 30 ± 5 mmHg). ATZ (3.6 mmol/kg of body weight) injected iv also reduced icv carbachol-induced pressor responses (21 ± 2 mmHg). ATZ icv or iv and H2O2 icv injected alone produced no effect on baseline arterial pressure. The treatments also produced no significant change of heart rate. The results show that ATZ icv or iv reduced the pressor responses to icv carbachol, suggesting that endogenous H2O2 acting centrally inhibits the pressor mechanisms (sympathoactivation and/or vasopressin release) activated by central cholinergic stimulation.

Central muscarinic and LPBN mechanisms on sodium intake.

Central cholinergic activation stimulates water intake, but also NaCl intake when the inhibitory mechanisms are blocked with injections of moxonidine (α2 adrenergic/imidazoline agonist) into the lateral parabrachial nucleus (LPBN). In the present study, we investigated the involvement of central M1 and M2 muscarinic receptors on NaCl intake induced by pilocarpine (non-selective muscarinic agonist) intraperitoneally combined with moxonidine into the LPBN or by muscimol (GABAA agonist) into the LPBN. Male Holtzman rats with stainless steel cannulas implanted bilaterally in the LPBN and in the lateral ventricle were used. Pirenzepine (M1 muscarinic antagonist, 1 nmol/1 µl) or methoctramine (M2 muscarinic antagonist, 50 nmol/1 µL) injected intracerebroventricularly (i.c.v.) reduced 0.3 M NaCl and water intake in rats treated with pilocarpine (0.1 mg/100 g of body weight) injected intraperitoneally combined with moxonidine (0.5 nmol/0.2 µL) into the LPBN. In rats treated with muscimol (0.5 nmol/0.2 µL) into the LPBN, methoctramine i.c.v. also reduced 0.3 M NaCl and water intake, however, pirenzepine produced no effect. The results suggest that M1 and M2 muscarinic receptors activate central pathways involved in the control of water and sodium intake that are under the influence of the LPBN inhibitory mechanisms.

Serotonergic receptor blockade in the lateral parabrachial nucleus: different effects on hypertonic and isotonic NaCl intake.

Hypertonic NaCl intake is produced by serotonin receptor antagonism in the lateral parabrachial nucleus (LPBN) of dehydrated rats or in rats pretreated with a mineralocorticoid, for example deoxycorticosterone (DOCA), that receive an intracerebroventricular injection (icv) of angiotensin II (ang II). The objective of the present work was to find out whether these two mechanisms are also involved with isotonic NaCl intake. Serotonin receptor blockade by methysergide in the LPBN (4 microg/0.2 microl bilaterally) had no effect on 0.15 M NaCl (methysergide: 19.3+/-5.2 ml/60 min; vehicle: 19.3+/-4.2 ml/60 min; n=7) or water (methysergide: 3.4+/-1.4 ml/60 min; vehicle 2.2+/-0.6 ml/60 min) intake induced by systemic diuretic furosemide combined with low dose of captopril (Furo/Cap). Methysergide treatment 4 days later in the same animals produced the expected enhancement in the 0.3 M NaCl intake induced by Furo/Cap (methysergide: 16.6+/-3.5 ml/60 min; vehicle: 6.6+/-1.5 ml/60 min). Similar result was obtained when another group was tested first with 0.3 M NaCl and later with 0.15 M NaCl. Isotonic NaCl intake induced by icv ang II was however enhanced by prior DOCA treatment. A de novo hypertonic NaCl intake was produced in another group by the same combined treatment. The results suggest that a facilitatory mechanism like the mineralocorticoid/ang II synergy may enhance NaCl solution intake at different levels of tonicity, while the action of an inhibitory mechanism, like the LPBN serotonergic system, is restricted to the ingestion at hypertonic levels.

Sodium intake by hyperosmotic rats treated with a GABA(A) receptor agonist into the lateral parabrachial nucleus.

Inhibitory mechanisms in the lateral parabrachial nucleus (LPBN) and central GABAergic mechanisms are involved in the regulation of water and NaCl intake. Besides increasing fluid depletion-induced sodium intake, the activation of GABA(A) receptors with muscimol into the LPBN also induces ingestion of 0.3 M NaCl in normonatremic, euhydrated rats. It has been suggested that inhibitory mechanisms activated by osmotic signals are blocked by GABA(A) receptor activation in the LPBN, thereby increasing hypertonic NaCl intake. Therefore, in the present study we investigated the effects of muscimol injected into the LPBN on water and 0.3 M NaCl intake in hyperosmotic cell-dehydrated rats (rats treated with an intragastric load of 2 M NaCl). Male Wistar rats with stainless steel cannulas implanted bilaterally into the LPBN were used. In euhydrated rats, muscimol (0.5 nmol/0.2 microl), bilaterally injected into the LPBN, induced ingestion of 0.3 M NaCl (24.6+/-7.9 vs. vehicle: 0.5+/-0.3 ml/180 min) and water (6.3+/-2.1 vs. vehicle: 0.5+/-0.3 ml/180 min). One hour after intragastric 2 M NaCl load (2 ml), bilateral injections of muscimol into the LPBN also induced 0.3 M NaCl intake (22.1+/-5.2 vs. vehicle: 0.9+/-0.8 ml/210 min) and water intake (16.5+/-3.6 vs. vehicle: 7.8+/-1.8 ml/210 min). The GABA(A) antagonist bicuculline (0.4 nmol/0.2 microl) into the LPBN reduced the effect of muscimol on 0.3 M NaCl intake (7.1+/-2.1 ml/210 min). Therefore, the activation of GABA(A) receptors in the LPBN induces ingestion of 0.3 M NaCl by hyperosmotic cell-dehydrated rats, suggesting that plasma levels of renin or osmolarity do not affect sodium intake after the blockade of LPBN inhibitory mechanisms with muscimol.

Opioid and α2 adrenergic mechanisms are activated by GABA agonists in the lateral parabrachial nucleus to induce sodium intake.

The activation of GABA, opioid or α2 adrenergic mechanisms in the lateral parabrachial nucleus (LPBN) facilitates hypertonic NaCl intake in rats. In the present study, we combined opioid or α2 adrenergic antagonists with GABA agonists into the LPBN in order to investigate if NaCl intake caused by GABAergic activation in normohydrated rats depends on opioid or α2-adrenergic mechanisms in this area. Male Holtzman rats with stainless steel cannulas implanted bilaterally in the LPBN were used. Bilateral injections of muscimol or baclofen (GABAA and GABAB agonists, respectively, 0.5 nmol/0.2 µl) into the LPBN induced strong ingestion of 0.3 M NaCl (45.8 ±â€¯7.3 and 21.8 ±â€¯4.8 ml/240 min, respectively) and water intake (22.7 ±â€¯3.4 and 6.6 ±â€¯2.5 ml/240 min, respectively). Naloxone (opioid antagonist, 150 nmol/0.2 µl) into the LPBN abolished 0.3 M NaCl and water intake to muscimol (2.0 ±â€¯0.6 and 0.9 ±â€¯0.2 ml/240 min, respectively) or baclofen (2.3 ±â€¯1.1 and 0.8 ±â€¯0.4 ml/240 min, respectively). RX 821002 (α2 adrenoceptor antagonist, 10 nmol/0.2 µl) into the LPBN reduced 0.3 M NaCl intake induced by the injections of muscimol or baclofen (26.6 ±â€¯8.0 and 10.1 ±â€¯4.9 ml/240 min, respectively). RX 821002 reduced water intake induced by muscimol (7.7 ±â€¯2.9 ml/240 min), not by baclofen. The results suggest that sodium intake caused by gabaergic activation in the LPBN in normohydrated rats is totally dependent on the activation of opioid mechanisms and partially dependent on the activation of α2 adrenergic mechanisms in the LPBN.

Enhanced angiotensin II induced sodium appetite in renovascular hypertensive rats.

Renovascular hypertensive 2-kidney, 1-clip (2K1C) rats have an increased activity of the renin-angiotensin system and an initial transitory increase in daily water and NaCl intake. However, the dipsogenic and natriorexigenic responses to angiotensin II (ANG II) have not been tested yet in 2K1C rats. Therefore, in the present study, we evaluated water and 0.3 M NaCl intake induced by water deprivation (WD)-partial rehydration (PR) or intracerebroventricular (icv) ANG II in 2K1C rats. In addition, the cardiovascular changes to these treatments were also evaluated. Male Holtzman rats received a silver clip around the left renal artery to induce 2K1C renovascular hypertension. In the 5th week, a group of animals received a guide cannula in the lateral ventricle for icv injections. Daily water intake increased from the 3rd week after surgery and remained elevated until the 6th week (last recording week), whereas daily 0.3 M NaCl intake transiently increased from the 2nd to the 5th week after surgery. On the 6th week, in spite of comparable daily 0.3 M NaCl intake between 2K1C and sham rats, WD-PR and icv ANG II induced an increased 0.3 M NaCl intake in 2K1C rats. Water intake induced by WD-PR, not by icv ANG II, also increased in 2K1C rats. The increase in arterial pressure to WD-PR or icv ANG II was similar in sham and 2K1C rats. Therefore, these results suggest that 2K1C rats are more responsive to the natriorexigenic effects of ANG II, whereas other responses to ANG II are not modified.

Enhancement of meal-associated hypertonic NaCl intake by moxonidine into the lateral parabrachial nucleus.

alpha2-Adrenoceptor activation with moxonidine (alpha2-adrenergic/imidazoline receptor agonist) into the lateral parabrachial nucleus (LPBN) enhances angiotensin II/hypovolaemia-induced sodium intake and drives cell dehydrated rats to ingest hypertonic sodium solution besides water. Angiotensin II and osmotic signals are suggested to stimulate meal-induced water intake. Therefore, in the present study we investigated the effects of bilateral injections of moxonidine into the LPBN on food deprivation-induced food intake and on meal-associated water and 0.3M NaCl intake. Male Holtzman rats with cannulas implanted bilaterally into the LPBN were submitted to 14 or 24h of food deprivation with water and 0.3M NaCl available (n=6-14). Bilateral injections of moxonidine (0.5nmol/0.2microl) into the LPBN increased meal-associated 0.3M NaCl intake (11.4+/-3.0ml/120min versus vehicle: 2.2+/-0.9ml/120min), without changing food intake (11.1+/-1.2g/120min versus vehicle: 11.2+/-0.9g/120min) or water intake (10.2+/-1.5ml/120min versus vehicle: 10.4+/-1.2ml/120min) by 24h food deprived rats. When no food was available during the test, moxonidine (0.5nmol) into the LPBN of 24h food-deprived rats produced no change in 0.3M NaCl intake (1.0+/-0.6ml/120min versus vehicle: 1.8+/-1.1ml/120min), nor in water intake (0.2+/-0.1ml/120min versus vehicle: 0.6+/-0.3ml/120min). The results suggest that signals generated during a meal, like dehydration, for example, not hunger, induce hypertonic NaCl intake when moxonidine is acting in the LPBN. Thus, activation of LPBN inhibitory mechanisms seems necessary to restrain sodium intake during a meal.