Salt appetite and lesions of the ventral part of the ventral median preoptic nucleus

Angiotensin receptors in the most ventral part of the ventral median preoptic nucleus (VVMnPO) or organum vasculosum laminae terminalis appear to be important for salt appetite to angiotensin in rats. If so, then small lesions of this region should reduce salt appetite that is dependent on angiotensin. In separate experiments, the lesion greatly reduced salt appetite after treatments with chronic oral captopril or sodium depletion. On the other hand, the VVMnPO lesion actually enhanced salt appetite to deoxycorticosterone acetate. The lesion did not affect water intake to water deprivation, combined food-water deprivation, isoproterenol, or hypertonic saline, and basal plasma osmolality and sodium values were normal. These experiments suggest that VVMnPO lesions selectively affect angiotensin-induced salt appetite without producing the gross hydrational deficits that occur with larger lesions of the ventral forebrain.     

Effects of SFO lesions on salt appetite during multiple sodium depletions.

A depletion of sodium may increase sodium intake by increasing the synthesis of angiotensin (ANG) II in the blood and by stimulating ANG II receptors in the subfornical organ (SFO) of the rat. Lesions of SFO reportedly reduce these intakes. The present experiments tested the effects of SFO lesions on salt appetite after three successive depletions. After a furosemide-induced natriuresis, Long-Evans rats had free access to water- and sodium-deficient diet for 22 h. Water and 0.3 M NaCl were given for 2 h, and then the rats received regular chow, water, and 0.3 M NaCl until the next injection 5 or 7 days later. SFO lesions reduced water intake 1-2 h after each furosemide injection but not during the overnight periods. The lesions did not affect salt appetite the next day, 24-26 h after furosemide, but they did prevent the expected increase in the chronic daily 0.3 M NaCl intake after repeated depletions. The second experiment was similar to the first except that three subcutaneous injections of 100 mg/kg captopril were given at 1, 18, and 20 h after furosemide for the second depletion only. After the first depletion, the results were similar to those of the same condition of the first experiment. After the second depletion, captopril greatly reduced water intake and salt appetite in all rats including those with SFO lesions. Thus, we found that the lesion reduced chronic intake, but we did not replicate results showing large effects of SFO lesions on acute salt appetite. This residual acute appetite after SFO lesion remains dependent on the synthesis of ANG II.     

Preoptic angiotensin and salt appetite

Two experiments were designed to test whether angiotensin (ANG) synthesis or receptor activation in the ventral preoptic region is critical for ANG-induced salt appetite in rats. In Experiment 1, infusions of ANG into the subfornical organ (SFO) produced water drinking without saline intake, but infusions near the organum vasculosum laminae terminalis (OVLT) produced both water and saline drinking. Thus, forebrain areas that support water drinking to ANG do not all support salt appetite. In Experiment 2, rats were given oral captopril (CAP) to enhance daily intake of water and saline solution by increasing ANG II synthesis in the brain. CAP microinjected into the SFO reduced CAP-enhanced water drinking without affecting saline intake, but CAP in the OVLT reduced enhanced saline intake without affecting water drinking. Thus, ANG acting in the OVLT, the most ventral part of the median preoptic nucleus, or other nearby structures is important for ANG-induced salt appetite.     

Forebrain sites of action for drinking and salt appetite to angiotensin or captopril

Three-hour infusions of angiotensin II (ANG II) agonists and antagonists were used to determine the relative sites of action of ANG in producing water drinking and salt appetite. In the first experiment, lateral ventricular (LV) but not fourth ventricular (4V) ANG II elicited water and saline intake, and LV but not 4V sarile, a competitive ANG II receptor blocker, reduced saline intake aroused by ip injections of 10 mg/kg furosemide and 6 mg/kg captopril. In the second experiment, water, but not saline, intake to furosemide-captopril treatment was reduced by sarile infusions into the subfornical organ (SFO). It is concluded that (a) brain ANG receptors for water and saline intakes are more accessible from the forebrain than the hindbrain ventricles and (b) receptors for water drinking, but not saline intake, after captopril reside in part in the SFO. Salt appetite appears to be dependent on ANG II receptors somewhere in the forebrain other than in the SFO.     

Subfornical organ connectivity and drinking to captopril or carbachol in rats

These experiments were conducted to test whether drinking to ip captopril or to intraventricular carbachol requires an intact fiber system from the ventral subfornical organ (SFO). Wire-knife cuts were made through the wall of the third ventricle ventral to the SFO. Control rats had either sham lesions or histologically identified missed cuts. Rats with good cuts (a) drank less than either control group after ip injections of 4 mg/kg captopril, (b) drank normal amounts of 0.3 M NaCl solution when captopril was placed in the drinking water at 0.1 mg/ml, (c) drank less water but a normal amount of saline after 6 mg/kg captopril ip and 10 mg/kg furosemide diuretic ip, and (d) drank normal amounts of water after lateral ventricular injections of 1.2 or 4 nmol of carbachol. The results of the captopril experiments confirm predictions based on studies of SFO lesions and suggest that captopril causes water, but not saline, drinking via an angiotensin-related mechanism acting at the SFO. The carbachol experiment indicates either that the SFO is not a unique receptor site for ventricular carbachol or that the fibers mediating this response do not require the pathways through the ventral pole of the SFO.     

Sodium retention and salt appetite following deoxycorticosterone in hamsters

Previous research suggests that hamsters 1) fail to retain sodium after mineralocorticoid injections, 2) retain sodium after adrenalectomy equal to controls, and 3) do not develop salt appetite after mineralocorticoid or adrenalectomy. The present studies demonstrate sodium retention, body weight gain, hypernatremia, plasma volume expansion, and reduced fecal sodium excretion after daily injections of deoxycorticosterone acetate (DOCA). Salt appetite appeared after the 3rd and 4th days. Adrenalectomy caused reductions of sodium balance, plasma volume, and food intake, which were reversed by DOCA administration. Mineralocorticoids therefore represent one control of sodium metabolism in hamsters.     

Central administration of zinc reduces salt intake in rats

The aim of the present study was to investigate the effect of third ventricle injections of zinc on salt intake in rats in the three different experimental models where sodium appetite is increased: fluid deprivation, central angiotensinergic stimulation and sodium depletion. Adult Wistar male rats received third ventricle injections of Zn(Ac)2 in three different doses (0.03, 0.3 and 3.0 nmol/rat). Central angiotensinergic stimulation was achieved by third ventricle injections of angiotensin II in the dose of 25 ng/rat 30 min before central zinc administration. As expected, fluid deprivation, central angiotensinergic stimulation and sodium depletion significantly increased sodium appetite. Water intake was also enhanced after fluid deprivation and central angiotensinergic stimulation. After sodium depletion, no increase in water intake was observed. Third ventricle injections of zinc inhibited salt intake in all three experimental models studied. Water intake was also inhibited by central zinc administration after fluid deprivation and central angiotensinergic stimulation. Conversely, third ventricle injections of zinc were unable to modify food intake or body temperature. It is suggested that zinc, acting on central structures related to the control of body fluid homeostasis, inhibits the drive for salt intake that is normally observed during fluid deprivation, central angiotensinergic stimulation and sodium depletion.     

Deoxycorticosterone stimulates the activity of nicotinamide adenine dinucleotide phosphate-diaphorase/nitric oxide synthase immunoreactivity in hypothalamic nuclei of rats

Mineralocorticoids (MC) play an important role in development of salt appetite. Part of this effect involves the hypothalamic paraventricular (PVN) and supraoptic (SON) nuclei, in which MC treatment increases arginine vasopressin (AVP) synthesis and release. Since the AVP system is also modulated by nitric oxide (NO), we studied if deoxycorticosterone acetate (DOCA) treatment changed the number of nicotinamide adenine dinucleotide phosphate-diaphorase (NADPH-d) active neurons and neuronal NO synthase (nNOS)-immunoreactive (IR) cells in the PVN and SON. After four injections of DOCA (10 mg/rat per day), rats developed a salt appetite and increased NADPH-d active and nNOS-IR neurons in both nuclei. A single DOCA injection did not change salt consumption or nNOS-IR cells, but increased the number of NADPH-d positive neurons in the PVN only. Therefore, while acute MC treatment stimulated the activity of pre-existing enzyme, chronic steroid treatment recruited additional neurons showing nNOS immunoreactivity/NADPH-d activity. These data suggest a role for NO produced in the PVN and SON in DOCA stimulatory effects on AVP mRNA and salt appetite.     

Atrial natriuretic peptide in the subfornical organ reduces drinking induced by angiotensin or in response to water deprivation

Three experiments tested whether the subfornical organ (SFO) could be a site of action for the antidipsogenic effects of atrial natriuretic peptide (ANP) in rats. Pretreatment with 100, 230, or 500 pmol ANP in the SFO reduced drinking induced by 10 pmol angiotensin II in the SFO. Drinking in response to water deprivation was reduced by ANP in rats having cannulas in or near the SFO, but not in rats having cannulas distant from the SFO or in the ventricles. Finally, ANP had no effect on eating or drinking after food deprivation, suggesting that the rats in the other experiments were not acutely incapacitated. The SFO may mediate the central effects of ANP on drinking induced by angiotensin or in response to water deprivation and could play a similar role in the central effects of ANP on salt appetite, diuresis, vasopressin secretion, and blood pressure.     

Alpha-adrenergic control of ANG II-induced drinking in the rat median preoptic nucleus

The present study was carried out to investigate the contribution of angiotensinergic and catecholaminergic systems in the median preoptic nucleus (MnPO) to the drinking response elicited by activation of the subfornical organ (SFO) following injections of angiotensin II (ANG II) in the awake unrestrained rat. Microinjection of ANG II into the SFO caused the drinking response. The water intake caused by ANG II injected into the SFO was significantly enhanced by previous injections of phenylephrine, an alpha-adrenoceptor agonist, but not by isoprenaline, a beta-adrenoceptor agonist. Previous injections of either saralasin, a specific ANG II antagonist, or phentolamine, an alpha-adrenoceptor antagonist, into the MnPO significantly attenuated the water intake caused by the ANG II injection into the SFO. Similar injections of timolol, a beta-adrenoceptor antagonist, or the vehicle into the MnPO had no significant effect on the drinking response. These results show the involvement of both angiotensinergic and alpha-adrenergic systems in the MnPO in the drinking response induced by ANG II acting at the SFO, and imply that the alpha-adrenergic system may serve to enhance the neural inputs including the angiotensinergic inputs from the SFO, thereby causing increased dipsogenic response.     

Forebrain lesions differentially affect drinking elicited by dipsogenic challenges and injections of muscimol into the median raphe nucleus

Injections of muscimol into the median raphe nucleus (MR) elicit intense drinking in normally hydrated rats. To determine whether this response is dependent on forebrain systems mediating other aspects of water intake, the authors examined the effects of lesions of the subfornical organ (SFO), median preoptic nucleus (MnPO), lateral preoptic area (LPO), or lateral hypothalamus (LH) on the drinking. Lesions of the SFO or LH attenuated muscimol-elicited drinking, whereas lesions of the MnPO or LPO increased water intake after the treatment. All of the lesion groups showed a deficit in drinking to injections of polyethylene glycol and at least one of the doses of hypertonic saline. Only the SFO- and LH-lesioned groups showed a suppression of drinking to systemic injections of angiotensin II, suggesting that the drinking elicited by intra-MR injections of muscimol may involve changes in the central circuits mediating angiotensin-induced drinking.     

Effects of exercise and anion on intake of sodium solutions in Syrian hamsters.

Previous studies have shown that hamsters have low spontaneous intakes of NaCl solutions and are refractory to induction of salt appetite. In order to examine the generality of these results, the intakes of NaCl and NaHCO3 solutions are reported for hamsters housed either in normal laboratory conditions (sedentary) or with free access to exercise wheels. Spontaneous salt intakes, as well as those induced by acute sodium depletion and treatment with either the angiotensin converting enzyme inhibitor, enalapril, or deoxycorticosterone acetate (DOCA), were measured. The intakes of NaCl and NaHCO3 were similar. Salt intakes tended to be lower in exercising than in sedentary groups. Neither acute sodium depletion nor administration of enalapril increased salt intake systematically, but treatment with DOCA increased intakes of both salt and water. These results are contrasted with the efficacy of all three treatments to induce salt appetite in rats.     

Angiotensin-converting enzyme in subfornical organ mediates captopril-induced drinking

These experiments tested whether angiotensin-converting enzyme (ACE) located within the subfornical organ (SFO) participates in the generation of water intake during peripheral ACE blockade with captopril (CAP). Lesions of the SFO virtually abolished drinking in response to intraperitoneal CAP injection. Intracranially injected CAP suppressed drinking induced by intraperitoneal CAP more completely with direct SFO injection compared with intraventricular or control tissue injections. This central captopril treatment did not alter the drinking response to subcutaneous hypertonic saline. Intraventricular injections of the angiotensin II (ANG II) receptor blocker sarile reduced drinking during oral captopril treatment in rats rehydrating from water deprivation. The results indicate that (a) the SFO mediates drinking caused by peripheral ACE inhibition; (b) the ACE located within the SFO may locally convert ANG I to ANG II, which then stimulates thirst; and (c) central ANG II receptors mediate thirst caused by peripheral ACE inhibition.     

Water and NaCl intake of chicks as mediated by angiotensin II, renin, or salt deficiency.

Water, feed and NaCl intakes were measured in response to angiotensin II (ANGII) injected SC and ICV, and renin injected ICV, as well as to dietary salt deficiency in female broiler chicks (Gallus domesticus). In the first experiment, SC (100 micrograms/bird) and ICV (10 micrograms/bird) ANGII injection resulted in increased initial water intake. An additive effect on drinking was noted in response to consecutive daily SC injections. In addition, feed efficiency and growth were depressed following repeated ANGII injections (p less than 0.001). In a 2nd experiment, ICV ANGII stimulated increased cumulative water intake through 18 hours postinjection (p less than 0.05). Intake of 3.0% NaCl solution and feed was unaffected through 48 hours. Renin (1 microgram ICV) failed to affect cumulative water intake up to 48 hours postinjection. In the third experiment, dietary salt deficiency reduced feed intake after just 48 hours on salt-deficient diets (p less than 0.01), and growth and feed efficiency were significantly impaired (p less than 0.001) through 20 days of age. Intakes of NaCl solutions (0.8, 0.7 or 0.6%), however, were unaffected in salt-deficient vs. control birds. While the chicks in these experiments demonstrated a consistent drinking response to ANGII when injected peripherally or centrally, a salt appetite could not be elicited in these birds by components of the renin-angiotensin system or by dietary salt depletion.     

The apparent dependence of salt appetite in the pigeon on endogenous angiotensin II

Blockade of endogenous angiotensin II (ANG II) biosynthesis by intramuscular administration of the angiotensin converting enzyme inhibitor captopril (1 or 10 mg/kg b.w.t.) completely suppressed salt appetite induced by sodium depletion in the pigeon. The effect was selective since captopril did not reduce deoxycorticosterone (DOCA)-induced salt appetite nor water drinking to ANG II and eledoisin. Blockade of brain ANG II receptors by pulse intracerebroventricular (pICV) injection of the ANG II receptor antagonist [Sarcosine1, isoleucine8] ANG II produced a marked, although partial, inhibition of salt appetite. The inhibition was quantitatively similar to the effectiveness of the ANG II receptor blockade, as measured by the suppression of drinking to pICV ANG II. Blockade of brain aldosterone (ALDO) receptors by pICV injections of the mineralocorticoid receptor antagonist RU-28318 did not significantly suppress depletion-induced appetite at doses that markedly reduced DOCA-induced salt appetite. These findings suggest that the pigeon might be completely dependent on ANG II for the expression of depletion-induced salt appetite. This is in contrast with what has been found in the rat, in which blockade of both ANG II and ALDO are necessary to suppress the appetite.     

Deficits in behavioral responding to regulatory challenges after lesions of ventrobasal thalamus in rats

Electrolytic lesions of the posteromedial portion of the ventrobasal thalamic complex of rats impaired the feeding response to glucoprivation produced by systemic injection of 2-deoxy-D-glucose and the sodium appetite induced by injections of desoxycorticosterone, and produced abnormally low water intake in the absence of food. Drinking elicited by intracellular dehydration was not consistently affected. Lesions of the ventral anterior nucleus of the thalamus only impaired the feeding response to glucoprivation. The behavioral changes observed after posterior ventrobasal thalamic damage may be related to a disruption of gustatory, visceral, and somatosensory thalamic connections with the corpus striatum and neocortex.     

Sodium appetite decreased by central angiotensin blockade

Disturbances in body water and electrolytes that trigger sodium appetite, such as sodium depletion or hypovolemia, are potent activators of the renin-angiotensin system. In the absence of an actual deficit in body fluids, angiotensin injections are adequate to stimulate increased sodium ingestion. To assess whether angiotensin is a significant mediator of sodium appetite induced by acute alterations in body fluids, sodium intake was examined in rats during central or peripheral angiotensin blockade. Central blockade of angiotensin receptors by intracerebroventricular (ICVT) injection of the analogue antagonist saralasin decreased (but did not eliminate) sodium intake after polethylene glycol-induced hypovolemia or sodium depletion resulting from dialysis against glucose. Conversely, peripheral blockade of angiotensin converting enzyme with orally active captopril potentiated rather than decreased sodium appetite and stimulated water intake after sodium depletion. This increased water and salt intake after peripheral inhibition of converting enzyme was reversed, however, by concurrent central blockade of angiotensin receptors. These data support the hypothesis that angiotensin participates in sodium appetite associated with acute alteration in body fluids. Furthermore, the brain is the site at which angiotensin exerts its influence on sodium appetite. While the involvement of angiotensin of brain origin is not ruled out, the change in sodium appetite after peripheral blockade of converting enzyme suggests that circulating angiotensin derived from renal renin may interact with central angiotensin receptors regulating sodium appetite.     

Attenuated drinking response induced by angiotensinergic activation of subfornical organ projections to the paraventricular nucleus in estrogen-treated rats

The present study was carried out to examine whether estrogen modulates the drinking response caused by activation of neural pathways from the subfornical organ (SFO) to the hypothalamic paraventricular nucleus (PVN) in the female rat. Microinjection of angiotensin II (ANG II) into the SFO elicited drinking in ovariectomized female rats that were treated with either propylene glycol (PG) vehicle or estradiol benzoate (EB). The amount of water intake induced by the ANG II injection was significantly greater in the PG-treated than in the EB-treated animals. In both groups, previous injections of either saralasin, an ANG II antagonist, or phentolamine, an alpha-adrenoceptor antagonist, bilaterally into the PVN resulted in the significant attenuation of the drinking response to ANG II, whereas similar injections of saline vehicle into the PVN were without effect. These results suggest that the circulating estrogen may act to reduce the drinking response that is mediated through angiotensinergic and alpha-adrenergic mechanisms in the PVN in response to angiotensinergic activation of SFO efferent projections.