Increased liking for salty foods in adolescents exposed during infancy to a chloride-deficient feeding formula.

In a model selected for its similarity to the hormonal consequences of sodium deficiency, food choices of 169 adolescents exposed during infancy to a chloride-deficient feeding formula were compared to those of their closest-aged siblings. Questionnaires completed by parents were used to assess food likes and dislikes. When a salty food was mentioned by parents as one craved by either child, exposed children were more likely than siblings to crave that food (p = 0.005). Frequencies of two of four salt-related dietary behaviors [adding salt to food before tasting (p = 0.03) and to atypical foods (p = 0.05)] were higher in exposed adolescents than in siblings, while frequencies of parallel sugar-related behaviors did not differ between the groups. Foods classified as being lower in saltiness were disliked by exposed children relative to siblings (p = 0.003), although ratings of foods higher in saltiness did not differ. Finally, when asked to rank eight foods in order of preference, ranks assigned by exposed children to salty foods tended (p = 0.07) to be higher than those of siblings. The data suggest a persistent effect of early experience on human salt preference. Additional studies are needed to determine whether salt intake is increased in this and other populations that suffer electrolyte depletion during early development.

Endocrine consequences of prenatal sodium depletion prepare rats for high need-free NaCl intake in adulthood.

Offspring of dams that were repeatedly sodium depleted during late pregnancy (at days E14, E17, and E20) expressed high need-free 3% NaCl intake in adulthood. Need-free 3% NaCl intake was greater in females, thereby respecting the sexual dimorphism of this behavior, and was increased further by successive sodium depletion in adulthood. Offspring from dams that had been sodium depleted while receiving the angiotensin-converting enzyme inhibitor, captopril, showed a need-free NaCl intake similar to that of control rats nonneonatally sodium depleted. Trunk blood taken from dams at day E18, i.e., 24 h after the second treatment, revealed that sodium depletion produced marked increases in the dam's plasma angiotensin (ANG) II and aldosterone that were not present when dams were treated with captopril during sodium depletion, even though both groups displayed a similar hyponatremia. We therefore propose that, during prenatal sodium depletion, the activation of the angiotensin-aldosterone system rather than the loss of sodium itself is responsible for the modification in need-free NaCl intake behavior. Finally, we suggest that, during pregnancy, ANG II may have an organizational effect on the neural substrate in the fetal brain that subserves subsequent NaCl intake behavior.

Effect of tachykinins on the need-free sodium intake of female rats: a continuous intracerebroventricular infusion study.

The present study investigated the effect of 24-h continuous ICV infusion of four different tachykinins on the enhanced need-free sodium intake induced by previous repeated sodium depletions in female rats. Female rats were employed because, in response to sodium depletions, they develop a higher need-free sodium intake than male rats. The following tachykinins were used: eledoisin, substance P (SP), [Sar9,Met(O2)11]SP and [Asp5,6,MePhe8]SP(5-11), also referred to as NH2-senktide, all at the same doses of 300 or 600 ng/h x 24 h. Food pellets, water, and 3% NaCl sodium solution were freely available. Eledoisin and NH2-senktide were more potent than SP in reducing the need-free sodium intake. On the other hand, [Sar9,Met(O2)11]SP had no effect. None of the tachykinins employed completely blocked the intake. Water intake was reduced, but this reduction was apparently a consequence of reduced intake of hypertonic sodium chloride solution, because at the same doses TKs did not inhibit water intake in a single-bottle test. Food intake remained unchanged at either dose used. These findings confirm previous studies in which pulse injection of the same drugs potently inhibited sodium intake. They also demonstrate that tachykinins endowed with high affinity for the NK3 receptor are the most potent in inhibiting sodium intake. Furthermore, these findings indicate that the tachykinins reduce the need-free sodium intake only during the infusion period, indicating that in these conditions they do not evoke either aversion for salt, or toxic consequences in the follow-up period.

Intracerebroventricular injection of renin in the neonatal rat reveals a precocious sodium appetite that is dissociated from renin-aroused thirst.

Previous research on the ontogeny of sodium appetite in the rat has shown that sodium deficit first engenders sodium intake at 12 days of age, whereas direct stimulation of the brain renin--angiotensin system by intracranial injection of renin increases intake of NaCl solution as early as 3 days postnatally. Similar activation of brain angiotensin also increases thirst, so that the specificity of the precocious sodium intake remains undetermined. In this article we report experiments that dissociate neonatal renin-evoked sodium appetite and thirst, and establish the specificity of the appetite. Our findings confirm that sodium appetite can first be discerned at 3 days of age, and show that it rapidly develops until 12 days of age. During this developmental window, renin-evoked sodium appetite is dissociated from thirst because (a) NaCl is preferred to water, (b) the appetite develops faster than thirst, and (c) 3-day-old renin-stimulated pups will avidly lick dry NaCl. These results show that activation of brain angiotensin in the 3-day-old rat pup evokes a precocious and specific sodium appetite.

Lesions of the central nucleus of the amygdala. I: Effects on taste reactivity, taste aversion learning and sodium appetite.

Bilateral damage to the central nucleus of the amygdala (CeAX) in the rat blunts need-induced NaCl intake and abolishes daily need-free NaCl intake when measured with a two-bottle test. Such a deficit could be the result of impaired taste function. To assess the taste function of the CeAX rat various taste stimuli were introduced directly into the oral cavity and taste-elicited oral motor responses were measured. Oral motor responses elicited by 0.62 M and 0.13 M sodium chloride, 0.3 M sucrose and 0.01 M citric acid, were similar in control and CeAX rats. Additionally CeAX and control rats acquired a taste aversion for fructose or maltose when either was paired with LiCl. Finally, in CeAX rats, like in control rats, the pattern of oral motor responses to 0.5 M NaCl was dependent on internal state; sodium depletion dramatically altered taste-elicited oral motor behavior. These results suggest that, in the rat, the deficits in NaCl intake behavior that follow CeAX do not appear to be a result of dramatic changes in gustatory function.

Lesions of the central nucleus of the amygdala. II: Effects on intraoral NaCl intake.

Bilateral lesions of the central nucleus of the amygdala (CeAX) disrupt both need-free and need-induced NaCl intake. Quite surprisingly, in response to sodium depletion, the same rats who fail to augment their NaCl consumption dramatically increase the numbers of oral motor behaviors associated with ingestion that they display when NaCl is presented intraorally. The present study attempts to resolve these apparently contradictory results by measuring the intake of a NaCl solution delivered directly into the mouth. Controls enhanced their intraoral intake of 0.5 M NaCl in response to sodium depletion while CeAX rats did not. CeAX rats, however, showed discriminative intake responses to tastes. Like controls, CeAX rats promptly rejected 0.3 mM quinine infusions and ingested 1.0 M sucrose for prolonged periods. In addition, food deprivation enhanced the intraoral intake of a dilute sucrose solution in both CeAX and control rats. Thus, in the CeAX rat some tastes and some internal states modulate intake as they do in intact rats. These results are consistent with the hypothesis that central nucleus of the amygdala damage interferes with the consummatory phase of NaCl intake behaviors.

Salt appetite consequent on sodium depletion in the suckling rat pup.

The ontogeny of the behavioral ability to compensate for sodium deficit was studied in the rat. The experiments showed that: 1) Before weaning age, sodium-depleted pups will increase their avidity for 3% NaCl solution; 2) the ability to select and drink a salt solution in response to a sodium deficit continues to evolve between 17-24 days of age, and that pups at these ages will modify their intake of salt and water as do adult rats when rectifying plasma osmolality; 3) The increased appetite for sodium is evident even when depleted preweanlings are dehydrated and provided with solid NaCl tablets to lick, showing that sodium appetite and hydrational status are already dissociated at this age; and finally, 4) sodium depletion first induces an increase in intake of orally infused 3% NaCl solution in 12-day-old pups. The picture of the development of salt appetite in the suckling rat that these findings present is of a precocious competence to meet a challenge to sodium homeostasis. In this respect salt appetite emerges in parallel to the other ingestive behaviors.

Medial region of the amygdala: involvement in adrenal-steroid-induced salt appetite.

We report the following observations: first that cell body damage induced by ibotenic acid within the medial region of the amygdala resulted in the abolition of corticosterone-potentiated, aldosterone-induced salt appetite. Sodium depletion-induced salt appetite was not impaired by the lesion, nor was angiotensin-dependent salt appetite via captopril treatment. These results provide evidence that the medial region of the amygdala has a major role in the natriorexigenic effects of the adrenal steroids.

Central tachykinin injection potently suppresses the need-free salt intake of the female rat.

Repeated sodium depletions produce a persistent, enhanced need-free salt intake in the rat, particularly in the female. The neurochemical mechanisms underlying the phenomenon are still unknown. The present studies evaluated the effect on the enhanced need-free salt intake of the female rat (1) of pharmacological interference with the natriorexigenic hormones angiotensin II and aldosterone and (2) of the central injection of the tachykinin peptides, which are endowed with antinatriorexic activity. The need-free salt intake of the female rat is not modified by treatment with the angiotensin-converting enzyme inhibitor captopril or by the aldosterone receptor antagonist RU-28318. On the other hand, the behavior is highly sensitive to the inhibitory effect of central tachykinins, suggesting the possibility that need-free salt intake might be linked to modification (down-regulation) of the inhibitory tachykininergic system.

Sex and sodium intake in the rat.

Female rats drink more 3% NaCl solution than do males, both when they need sodium (need-induced sodium intake or sodium appetite) and when they do not (need-free sodium intake). The sexual dimorphism of sodium intake is a secondary sexual characteristic because after castration at 1 day of age male rats drank 3% NaCl in adulthood in a manner similar to that of females in both the need-free and need-induced state, and females given long-term, neonatal testosterone drank low, malelike volumes of 3% NaCl on a daily need-free basis, but their response to sodium depletion was unchanged. This sexual dimorphism of sodium intake seems to be governed by testosterone that has been converted in the brain to estrogen because treatment of Day 1 castrated females with a nonaromatizable androgen, dihydrotestosterone, did not change either their need-free or their need-induced 3% NaCl intake. Castration in adulthood of male and female rats did not change their sodium consumption. However, when castrated adults received testosterone, need-free intakes of NaCl were suppressed in both sexes, but the suppression of 3% NaCl intake occurred only while the steroid was present. Exogenous testosterone also lowered the need-induced sodium intake of adult castrated females. Thus, in castrated adults, need-free intake was actively suppressed by exogenous testosterone in both sexes, whereas need-induced intake of NaCl was suppressed only in females. These data indicate that sodium intake in the rat is a sexually dimorphic behavior that is organized neonatally and can be actively suppressed in adulthood by testosterone.

Control of salt intake by steroids and cerebral peptides.

Steroids (aldosterone and testosterone) and peptides of cerebral origin (angiotensin II and the tachykinins) control the salt intake of the rat. They arouse or suppress the behaviour and produce life-long enhancements of NaCl intake. Need-induced salt intake (salt appetite or salt hunger), which is the consequence of sodium deficiency, is aroused by a synergy within the brain of cerebral angiotensin II and aldosterone. And prior episodes of sodium depletion produce enhancements of subsequent salt appetites, but only if the prior depletions were accompanied by angiotensin II and aldosterone action. Need-free salt intake, which occurs daily when the rat is in positive sodium balance, is inherently high in the rat and is organized in the perinatal period by aromatized testosterone which suppresses the intake of the male. It is also enhanced by prior activations of angiotensin II and aldosterone. Both need-induced and need-free salt intake are suppressed by intracranial tachykinins. Non-mammalian tachykinins (eledoisin, physalaemin, kassinin) are both antidipsogenic and antinatriorexigenic, but amino-senktide, an analogue of the mammalian tachykinin substance P with selective affinity for the NK 3 receptor, appears to be a selective antinatriorexigenic agent, and could provide a rational therapy for chronic overconsumption of salt.

The anteroventral wall of the third ventricle and the angiotensinergic component of need-induced sodium intake in the rat.

Because the anteroventral wall of the third ventricle (AV3V) has been implicated in the control of sodium intake it was studied in rats with damage of the AV3V region in the sodium-replete and sodium-deplete states, and when they were treated with either pulse intracerebroventricular (pICV) injection of renin to activate brain angiotensin or daily subcutaneous injections of deoxycorticosterone (DOCA). The response of rats with AV3V damage to sodium depletion was retarded and the excess sodium intake that is induced by pICV renin was absent, but their daily need-free sodium intake and the sodium intake that is induced by DOCA were essentially normal. The results suggest that the AV3V is responsible for the angiotensinergic, but not the aldosteronergic, component of the ANG II/ALDO synergy that controls need-induced sodium appetite in the rat. The integrity of the AV3V is not necessary for daily need-free sodium intake or for its enhancement by sodium depletions, suggesting that ANG II is probably not important for these phenomena. But ANG II action within the AV3V might be important for the rapid burst of sodium intake that immediately follows a period of depletion.

Deficits in NaCl ingestion after damage to the central nucleus of the amygdala in the rat.

These studies examined the NaCl intake behaviors of rats with bilateral electrolytic lesions of the central nucleus of the amygdala (CeAX). Daily need-free intake of 3% NaCl was abolished by CeAX even in rats in which it had been enhanced preoperatively by a history of repeated sodium depletions but was slightly restored by three successive postoperative sodium depletions. CeAX rats drank water but not 3% NaCl to high doses of DOCA and to the activation of cerebral angiotensin II, and expressed small but reliable salt intake (need-induced salt intake or salt appetite) after postoperative sodium depletions. Other ingestive behaviors (water drinking, intake of food and 5% sucrose) were normal. When given decreasing concentrations of NaCl solution the CeAX rats rejected them until the concentration reached 0.2%. These findings suggest that lesions to the central nucleus of the amygdala produce a global impairment in salt intake behaviors that is possibly due to an alteration in the central processing of the salt taste signal.

Transection of the stria terminalis without damage to the medial amygdala does not alter behavioural sodium regulation in rats.

Damage to the medial region of the amygdala has been shown to impair mineralocorticoid-induced sodium appetite, while leaving intact sodium appetite induced through sodium depletion. This effect may result from the interruption of the flow of information through the stria terminalis (ST), a neural pathway linking the medial amygdala with the ventral forebrain. We determined the effect of transecting the ST of the rat, at a point remote from the medial amygdala, on sodium appetite induced with the administration of mineralocorticoids and with the natriuretic furosemide. Similar to control and amygdala lesioned rats, rats with ST knife-cuts displayed a normal sodium appetite following treatment with furosemide. However, unlike medial amygdala lesions, transection of the ST alone did not block mineralocorticoid-induced sodium appetite. Therefore, the inability of mineralocorticoids to induce a salt appetite in medial amygdala lesioned rats does not result from damage to the stria terminalis.

Blood-borne and cerebral angiotensin and the genesis of salt intake.

These experiments reevaluate earlier work in which salt intake was evoked by blood-borne angiotensin II. That work is inconsistent with recent demonstrations that cerebral, not blood-borne, angiotensin II is the synergist with aldosterone in arousing salt intake in the rat. We show, first, that the pharmacological doses of angiotensin II that were used in the earlier work are natriuretic (and dipsogenic). They cause urinary sodium losses that precede and exceed sodium intake. Second, we show that the excess sodium intake that is associated with pharmacological doses of intravenous angiotensin is not caused by endogenous aldosterone. Last, we show that this excess sodium intake is abolished by intracerebroventricular captopril thereby suggesting that it is caused by activation of cerebral angiotensin II and harmonizing its mechanism with current concepts.

Neuroendocrine factors in salt appetite.

We dedicate this paper to Curt P. Richter, father of the study of salt appetite, who died recently at the age of 94. Richter first demonstrated that the adrenalectomized rat's voracious appetite for salt kept it alive (1936) and showed the same in humans (1940). Our first paper in 1955 demonstrated that salt appetite was an innate response to salt depletion. Since then, we have pursued the notion that the neuroendocrine consequences of sodium depletion create a brain state that raises salt appetite. In Epstein's laboratory, it was shown that angiotensin and aldosterone, the hormones of salt retention in the periphery, act synergistically in the brain to produce salt appetite in the rat. Block either hormone and the appetite is reduced by half; block both and the appetite is eliminated despite severe bodily need. With repeated depletions or treatments of the brain with angiotensin and aldosterone, salt ingestion increases, reaching an asymptote by the third depletion. Need-free intake of NaCl also increases, especially in female rats which ingest more NaCl than male rats. In Stellar's laboratory, running speed to salt solutions in a runway is used as a measure of salt appetite. When the appetite is raised with large doses of DOCA, a mimic of aldosterone, rats run rapidly for a taste of strong salt solutions as high as 24% (almost 4 molar). Using ingestion as a measure, the role of the atrial natriuretic peptide (ANP), an antagonist of angiotensin's physiological effect, was investigated as a modulator of salt appetite. When angiotensin is involved is producing salt appetite, following sodium depletion by a diuretic combined with a low-salt diet, ANP reduced salt intake by 40%. When salt appetite was raised by DOCA, however, ANP either had no effect or reduced salt ingestion by only 10%. The subfornical organ, the lateral preoptic area, and the central and medial nuclei of the amygdala are being investigated as major components of the limbic circuit underlying salt appetite produced by the actions of angiotensin, aldosterone and ANP in the brain.

Neurohormonal control of salt intake in the rat.

Steroids (aldosterone and testosterone) and peptides of cerebral origin (angiotensin II and the tachykinins) control the salt intake of the rat. They arouse or suppress the behavior and they produce lifelong enhancements of NaCl intake. Need-induced salt intake (salt appetite or salt hunger), which is the consequence of sodium deficiency, is aroused by a synergy within the brain of cerebral angiotensin II and aldosterone. And prior episodes of sodium depletion produce enhancements of subsequent salt appetites, but only if the prior depletions were accompanied by angiotensin II and aldosterone action. Need-free salt intake, which occurs daily when the rat is in positive sodium balance is also enhanced by prior activations of angiotensin II and aldosterone. Both need-induced and need-free salt intake are suppressed by intracranial tachykinins. Nonmammalian tachykinins (eledoisin, physalaemin, kassinin) are both antidipsogenic and antinatriorexigenic, but amino-senktide, an analog of the mammalian tachykinin substance P with selective affinity for the NK 3 receptor, appears to be a selective antinatriorexigenic agent, and could provide a rational therapy for chronic over-consumption of salt.

Transient and lasting effects of reproductive episodes on NaCl intake of the female rat.

Following recovery from experimental sodium depletion, both the need-free and the future need-induced NaCl intakes of the rat are increased. The present experiments asked whether a naturally occurring episode of sodium need, pregnancy and lactation, would also enhance NaCl intake. The daily 0.3 M NaCl intake of Long-Evans rats increased from 18.2 ml (prior to pregnancy) to 28.7 ml during pregnancy, and increased further to 36.2 ml during lactation. The daily intakes remained increased after the weaning of the second litter, at 28.5 ml/24h for females which had access to 0.3 M NaCl during the reproductive episodes (DAM + Na) and at 27.6 ml/24h for those which did not (DAM). When they were subsequently sodium depleted, the need-induced NaCl intake of the DAM, but not the DAM + Na group, was significantly increased compared to virgin female rats. The sodium depletion-induced water intakes of both groups were significantly increased. Neither the need-free nor the need-induced intakes of their offspring (as adults) were increased. In summary, pregnancy and/or lactation, in the absence of a sodium deficit, produced increased intakes of NaCl which persisted beyond the reproductive episodes.

Salt appetite in rat pups: ontogeny of angiotensin II-aldosterone synergy.

Preweanling rats were tested to determine whether angiotensin II (ANG II) and aldosterone (Aldo) act synergistically to enhance salt appetite at 12 and 17 days. Twelve-day-old pups received one of four hormone treatments in four doses: 1) ANG II only [1, 2, 10, or 100 ng pulse intracerebroventricular (icv)], 2) Aldo only (1, 2, 10, or 40 micrograms/day sc), 3) Aldo + ANG II (four individual doses combined), or 4) vehicle. Seventeen-day-old rats received the same treatments in two doses (2 or 100 ng ANG II; 2 or 40 micrograms Aldo). Pups were presatiated with milk through anterior oral catheters and then given either 4% NaCl or water for 30 min. Intake was assessed by body weight change. At both ages, ANG II enhanced salt (and water) intake, and Aldo enhanced salt (but not water) intake. Minimum effective doses were comparable to those reported for adults. ANG II-Aldo synergy was absent at 12 days and present at 17 days, when salt intake was 590% greater than the summed intakes evoked by ANG II and Aldo alone. The neural mechanisms for ANG II-Aldo synergy thus mature later than those mediating the hormone's individual actions in arousing salt appetite.

Peripheral angiotensin II is not the cause of sodium appetite in the rat.

Activation of the renin-angiotensin system has been shown to arouse both water and sodium intake in the rat. The following experiments examine the contributions of the peripheral and central renin-angiotensin systems to the expression of sodium appetite in the adrenal-intact rat. Firstly, we find that, unlike intracerebroventricular angiotensin II, intravenous administration of the hormone at dipsogenic doses does not arouse sodium intake in the sodium replete rat when given alone or in combination with systemic mineralocorticoid pretreatment. Secondly, in the sodium depleted rat, we find that interference with central, but not peripheral, angiotensin II action suppresses sodium appetite. Together, these data confirm recent evidence which demonstrates that it is angiotensin II of cerebral origin, not angiotensin II of renal origin, that is necessary for the expression of sodium appetite.