The ontogeny of salt hunger in the rat

Accumbal μ-opioid receptors and salt taste-elicited hedonic responses in a rodent model of prenatal adversity, and their correlates using human functional genomics

Prenatal adversity is associated with behavioral obesogenic features such as preference for palatable foods. Salt appetite may play a role in the development of adiposity and its consequences in individuals exposed to prenatal adversity, and sodium consumption involves individual differences in accumbal µ-opioid receptors function. We investigated the hedonic responses to salt and the levels of µ-opioid receptors and tyrosine hydroxylase in the nucleus accumbens (Nacc) of pups from an animal model of prenatal dietary restriction. In children, we evaluated the interaction between fetal growth and the genetic background associated with the accumbal µ-opioid receptor gene (OPRM1) expression on sodium consumption during a snack test. Sprague-Dawley dams were randomly allocated from pregnancy day 10 to receive an ad libitum (Adlib) or a 50% restricted (FR) diet. The pups' hedonic responses to a salt solution (NaCl 2%) or water were evaluated on the first day of life. FR and Adlib pups differ in their hedonic responses to salt, and there were decreased levels of accumbal µ-opioid and p-µ-opioid receptors in FR pups. In humans, a test meal and genotyping from buccal epithelial cells were performed in 270 children (38 intrauterine growth restricted-IUGR) at 4 years old from a Canadian prospective cohort (MAVAN). The OPRM1 genetic score predicted the sodium intake in IUGR children, but not in controls. The identification of mechanisms involved in the brain response to prenatal adversity and its consequences in behavioral phenotypes and risk for chronic diseases later in life is important for preventive and therapeutic purposes.

Dissociable effects of dietary sodium in early life upon somatic growth, fluid homeostasis, and spatial memory in mice of both sexes

Postnatal growth failure is a common morbidity for preterm infants and is associated with adverse neurodevelopmental outcomes. Although sodium (Na) deficiency early in life impairs somatic growth, its impact on neurocognitive functions has not been extensively studied. We hypothesized that Na deficiency during early life is sufficient to cause growth failure and program neurobehavioral impairments in later life. C57BL/6J mice were placed on low- (0.4), normal- (1.5), or high- (3 g/kg) Na chow at weaning (PD22) and continued on the diet for 3 wk (to PD40). Body composition and fluid distribution were determined serially by time-domain NMR and bioimpedance spectroscopy, and anxiety, learning, and memory were assessed using the elevated plus maze and Morris water maze paradigms in later adulthood (PD63-PD69). During the diet intervention, body mass gains were suppressed in the low- compared with normal- and high-Na groups despite similar caloric uptake rates across groups. Fat mass was reduced in males but not in females fed low-Na diet. Fat-free mass and hydration were significantly reduced in both males and females fed the low-Na diet, although rapidly corrected after return to normal diet. Measures of anxiety-like behavior and learning in adulthood were not affected by diet in either sex, yet memory performance was modified by a complex interaction between sex and early life Na intake. These data support the concepts that Na deficiency impairs growth and that the amount of Na intake which supports optimal somatic growth during early life may be insufficient to fully support neurocognitive development.

Dietary Preference of Newly Weaned Pigs and Nutrient Interactions According to Copper Levels and Sources with Different Solubility Characteristics

Two feeding preference experiments and an in vitro assay were performed to assess the weaned pig preference for Cu doses and sources based on their sensorial perception and on the likely post-ingestive effects of Cu. At day 7 post-weaning, a total of 828 pigs were distributed into two different experiments. In Exp.1 (dose preference) a diet with a nutritional Cu level (15 mg/kg) of Cu sulfate (SF) was pair offered with higher Cu levels (150 mg/kg) of either SF or hydroxychloride (HCl). In Exp.2 (source preference), a diet supplemented with Cu-SF at 150 mg/kg was compared to a Cu-HCl (150 mg/kg) diet. At the short-term (day 7-9) and for the entire experimental week (day 7-14), pigs preferred diets with a high Cu level than with Cu at a nutritional dose (p < 0.05). Likewise, pigs preferred diets supplemented with a Cu-HCl source compared to diets with Cu-SF (p < 0.05). In vitro assay results showed a greater solubility and interaction of Cu-SF with phytic acid compared to Cu-HCl. In conclusion, pigs chose diets with higher levels of Cu probably to re-establish homeostasis after weaning. Pigs preferred diets with Cu-HCl compared to Cu-SF probably due to their solubilities and chemical differences.

Salt need needs investigation

Expensive and extensive studies on the epidemiology of excessive Na intake and its pathology have been conducted over four decades. The resultant consensus that dietary Na is toxic, as well as the contention that it is less so, ignores the root cause of the attractiveness of salted food. The extant hypotheses are that most Na is infiltrated into our bodies via heavily salted industrialised food without our knowledge and that mere exposure early in life determines lifelong intake. However, these hypotheses are poorly evidenced and are meagre explanations for the comparable salt intake of people worldwide despite their markedly different diets. The love of salt begins at birth for some, vacillates in infancy, climaxes during adolescent growth, settles into separate patterns for men and women in adulthood and, with age, fades for some and persists for others. Salt adds flavour to food. It sustains and protects humans in exertion, may modulate their mood and contributes to their ailments. It may have as yet unknown benefits that may promote its delectability, and it generates controversy. An understanding of the predilection for salt should allow a more evidence-based and effective reduction of the health risks associated with Na surfeit and deficiency. The purpose of this brief review is to show the need for research into the determinants of salt intake by summarising the little we know.

Early high-sodium solid diet does not affect sodium intake, sodium preference, blood volume and blood pressure in adult Wistar-Kyoto rats

A high-Na diet may lead to the development of hypertension in both humans and rats; however, the causes of Na intake in amounts greater than physiologically needed as well as the mechanisms whereby high-Na food elevates blood pressure are not clear. Therefore, we decided to test the hypothesis that a high-Na diet introduced after suckling affects Na intake, food preference, resting blood pressure and blood volume in adult rats. Male Wistar-Kyoto (WKY) rats, 4 weeks old, were divided into three groups and placed on either a high-Na (3.28%), a medium-Na (0.82%) or a regular diet (0.22%) with the same energy content for 8 weeks. Subsequently, food preference, resting arterial blood pressure, blood volume, plasma osmolality and Na blood level were evaluated. When offered a choice of diets, all the groups preferred the regular chow, and there was no significant difference in total Na intake between the groups. When the rats experienced the change from their initial chow to a new one with different Na content, they continued to eat the same amount of food. Body weight, resting arterial blood pressure, blood volume, plasma osmolality and Na blood level were comparable between the groups. In conclusion, the results show that a high-Na diet introduced immediately after suckling does not affect Na preference and Na intake in adult WKY rats. Furthermore, the findings provide evidence that both blood volume and arterial blood pressure are highly protected in normotensive rats on a high-Na diet.

Early flavor learning and its impact on later feeding behavior

In this review, we first outline the role and mechanisms of the chemical senses (taste, smell, and chemical irritation) in the perception of the flavor of a food or beverage. We then describe research findings, much of them from our laboratories, on the ontogeny of flavor perception and the interacting roles of innate responses and learning in the establishment of flavor preference of infants and children. Broadly, taste (sweet, sour, salty, bitter, umami, or savory) preferences have a strong innate component. Sweet, umami, and salty substances are innately preferred, whereas bitter and many sour substances are innately rejected. Nevertheless, these innate tendencies can be modified by pre- and postnatal experiences. Volatile components of flavor, detected by the olfactory system, are strongly influenced by early exposure and learning beginning in utero and continuing during early milk (breast milk or formula) feedings. These experiences set the stage for later food choices and are important in establishing life-long food habits. As many of the diseases plaguing developed and developing societies involve excess consumption of some foods, an understanding of factors that determine choice and ingestion, particularly an understanding of the early factors, is important in designing strategies to enhance the health of the infant, child, and adult.

Prenatal imprinting of postnatal specific appetites and feeding behavior

Epigenetic influences on the fetus's genotype have been shown to occur during intrauterine life. Experimentally imposed extracellular dehydration in pregnant rats (a model for human hyponatremia caused by gravidic vomiting) brings about a dramatic enhancement of salt appetite not only in the dam, but also in offspring when they reach adulthood. This phenomenon has been verified in human newborn infants and adults whose mothers experienced nausea and/or vomiting during pregnancy. Alcohol consumption during pregnancy enhances its palatability for the offspring. Ingestion of olfactory test substances like anise or carrot by the mother during pregnancy gives rise to a preference for the same testants in the offspring. Under- or overnutrition in the pregnant mother appears to play a role in reprogramming the postnatal regulation of both feeding and fat reserves in offspring. Both maternal under- and overnutrition during pregnancy predispose the offspring to later development of obesity and type 2 diabetes mellitus. A careful examination of the systems concerned with the regulation of food intake, and the neurosubstances involved in such regulation, reveals some of the mechanisms by which maternal nutritional status can affect the offspring and their food-related behaviors.

Biobehavior of the human love of salt

We are beginning to understand why humans ingest so much salt. Here we address three issues: The first is whether our salt appetite is similar to that in animals, which we understand well. Our analysis suggests that this is doubtful, because of important differences between human and animal love of salt. The second issue then becomes how our predilection for salt is determined, for which we have a partial description, resting on development, conditioning, habit, and dietary culture. The last issue is the source of individual variation in salt avidity. We have partial answers to that too in the effects of perinatal sodium loss, sodium loss teaching us to seek salt, and gender. Other possibilities are suggested. From animal sodium appetite we humans may retain the lifelong enhancement of salt intake due to perinatal sodium loss, and a predisposition to learn the benefits of salt when in dire need. Nevertheless, human salt intake does not fit the biological model of a regulated sodium appetite. Indeed this archetypal 'wisdom of the body' fails us in all that has to do with behavioral regulation of this most basic need.

Reinforcer-specificity of appetitive and consummatory behavior of rats after Pavlovian conditioning with food reinforcers

We examined the reinforcer-specificity of Pavlovian conditioning in the control of appetitive and consummatory behaviors in Pavlovian-to-instrumental transfer, cue-potentiated eating, and devaluation procedures. Rats received pairings of one conditioned stimulus with sucrose and another conditioned stimulus with maltodextrin. In Experiment 1, rats were also trained to earn sucrose for one instrumental response and maltodextrin for another. In a transfer test, the Pavlovian cues enhanced the rate of instrumental responding more when the food reinforcer predicted by the instrumental response and the Pavlovian cue were consistent than when they were inconsistent, but both cues enhanced both responses. In Experiment 2, sated rats' consumption of each food was potentiated in the presence of a cue for that food, but not in the presence of a cue for the other food. In Experiment 3, one food was devalued by pairing it with lithium chloride, prior to testing food consumption and food-cup directed behaviors. The food cues selectively controlled food-cup related behaviors, regardless of the presence of the devalued or nondevalued foods in the food cup. Together, these results are consistent with the view that conditioned cues modulate appetitive and consummatory behaviors with increasing levels of specificity. The closer an action comes to ingestion, the more it is controlled by sensory properties conveyed by learned cues. These data are discussed in the context of allostatic regulation of food foraging and intake.

Lowest neonatal serum sodium predicts sodium intake in low birth weight children

Forty-one children aged 10.5 +/- 0.2 years (range, 8.0-15.0 yr), born with low birth weight of 1,218.2 +/- 36.6 g (range, 765-1,580 g) were selected from hospital archives on the basis of whether they had received neonatal diuretic treatment or as healthy matched controls. The children were tested for salt appetite and sweet preference, including rating of preferred concentration of salt in tomato soup (and sugar in tea), ratings of oral spray (NaCl and sucrose solutions), intake of salt or sweet snack items, and a food-seasoning, liking, and dietary questionnaire. Results showed that sodium appetite was not related to neonatal diuretic treatment, birth weight, or gestational age. However, there was a robust inverse correlation (r = -0.445, P < 0.005) between reported dietary sodium intake and the neonatal lowest serum sodium level (NLS) recorded for each child as an index of sodium loss. The relationship of NLS and dietary sodium intake was found in both boys and girls and in both Arab and Jewish children, despite marked ethnic differences in dietary sources of sodium. Hence, low NLS predicts increased intake of dietary sodium in low birth weight children some 8-15 yr later. Taken together with other recent evidence, it is now clear that perinatal sodium loss, from a variety of causes, is a consistent and significant contributor to long-term sodium intake.

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.

Ontogeny of urine preference and its relationship to NH4Cl preference and sodium hunger in suckling rat pups

We chart the postnatal ontogeny of urine preference in the suckling rat. Twelve-day-old sucklings, when offered urine, NH4Cl, or NaCl, ingest more urine and NH4Cl than NaCl. When rendered sodium hungry by ivc renin or by sodium depletion, these sucklings prefer urine and NH4Cl to NaCl, dilute urine, or an NaCl and KCl mineral mix equimolar to urine; however, by 18 days of age, urine and NH4Cl are no longer preferred to NaCl. Hence, urine preference in the suckling may be specific and preparatory for the variety of purposes urine preference serves in the adult rat, and it might guide the pup to urinary sodium in the nest. Since preference for urine and NH4Cl covary during postnatal development, the high preference for NH4Cl in midterm sucklings might be because its ammonium flavor is similar to urine.

Palatability: response to nutritional need or need-free stimulation of appetite?

The traditional view of palatability was that it reflected some underlying nutritional deficit and was part of a homeostatically driven motivational system. However, this idea does not fit with the common observation that palatability can lead to short-term overconsumption. Here, we attempt to re-evaluate the basis of palatability, first by reviewing the role of salt-need both in the expression of liking for salty tastes, and paradoxically, in dissociating need from palatability, and second by examining the role of palatability in short-term control of appetite. Despite the clarity of this system in animals, however, most salt (NaCl) intake in man occurs in a need-free state. Similar conclusions can be drawn in relation to the palatability of food in general. Importantly, the neural systems underlying the hedonic system relating to palatability and homeostatic controls of eating are separate, involving distinct brain structures and neurochemicals. If palatability was a component of homeostatic control, reducing need-state should reduce palatability. However, this is not so, and if anything palatability exerts a stronger stimulatory effect on eating when sated, and over-consumption induced by palatability may contribute to obesity. Differential responsivity to palatability may be a component of the obese phenotype, perhaps through sensitisation of the neural structures related to hedonic aspects of eating. Together, these disparate data clearly indicate that palatability is not a simple reflection of need state, but acts to promote intake through a distinct hedonic system, which has inputs from a variety of other systems, including those regulating need. This conclusion leads to the possibility of novel therapies for obesity based on modulation of hedonic rather than homeostatic controls. Potential developments are discussed.

Episodes of water deprivation enhance daily hypertonic NaCl intake in rats

Water and 1.8% NaCl intake was recorded daily in adult male rats (N = 6) submitted to four water deprivations plus four sodium appetite tests, each at the end of each 7-day interval, or in controls (non-deprived, N = 6). Water deprivation was achieved by removing water and 1.8% NaCl for 24 h. Water was then offered for 2 h. At the end of this period, 1.8% NaCl was also offered in addition to water (sodium appetite test). Average daily 1.8% NaCl intake was enhanced from 5.2 +/- 1.0 to 15.7 +/- 2.5 ml from the first to the fifth week in the experimental group and was unchanged in the control group. Daily water intake was not altered in either group. Thus, repeated episodes of water deprivation enhance daily NaCl intake.

The moth and the aspen tree: sodium in early postnatal development

Over the past 25 years, our perception of the neonatal kidney has changed markedly from its being a "limited" organ compared with that of the adult to being extraordinarily well adapted in its role in maintaining homeostasis and making possible the rapid somatic growth necessary during this critical period of life. The present review focuses on the physiologic adaptations by the neonatal kidney in the maintenance of a positive sodium balance, which is necessary for normal growth not only in mammals but also in moths. There is a fine interplay between the developing brain, heart, thyroid, adrenals, and sympathetic nervous system, all converging on the kidney to conserve sodium, which is limited in the diet. The renin-angiotensin system plays a central role in this response and is balanced by developmental changes in the renal response to atrial natriuretic peptide, all of which contribute to sodium conservation. Over the next 25 years, advances in molecular genetics will doubtless elucidate many more facets of the mechanisms underlying neonatal sodium homeostasis. This will be particularly important as the survival of ever smaller preterm infants improves steadily.

Does previous protein feeding affect the response of sheep towards foods that differ in their rumen availability, but not content, of nitrogen?

The objective of the experiment was threefold: (1) to test whether an 'unlearned appetite' for dietary protein exists in sheep, (2) whether such an appetite and subsequent diet selection depend on the degree of previous protein deprivation, and (3) whether the N-source in the foods offered as choice influences diet selection. Differences in protein deprivation were achieved by feeding sheep with food either with high (HP) or low (LP) protein. Sheep were fed on Food LP, either for the same period of time as sheep on Food HP or until they reached the same live-weight (LW) as sheep on Food HP. Following the feeding regimes that induced differences in animal state, sheep were given a choice between a novel low-protein food (T) or Food T supplemented with isonitrogenous amounts with one of three nitrogen sources: urea (U), casein (C), or formaldehyde-treated casein (TFC). Diet selection measured in the short-term or over the first few days did not provide any evidence in support of an unlearned appetite for protein by sheep of different states. In fact, diet selection by all animals was characterized by an avoidance of the foods supplemented with the three nitrogen sources. This avoidance was strongest for the food supplemented with formaldehyde-treated casein. Selection of considerable amounts of supplemented foods was gradual and consistent only after animals gained experience of them, i.e. were allowed to consume a single supplemented food for a period of 7 days. Following this period, animals that had previously consumed Food LP for the same period of time as animals on Food HP selected a higher proportion of the supplemented food than the other sheep. The results support the view that there is no unlearned appetite for protein in sheep, and that control over diet selection is learned.

Self-selection of copper-containing diets by copper-deficient and overloaded rats

To determine the effect of copper status on the preference for copper-containing food, male Sprague-Dawley rats were weaned to a copper-deficient, copper-adequate or high-copper diet. Four weeks later, alterations in copper status were confirmed by measurement of liver copper concentrations. Rats (n=10) were then given the choice between a copper-adequate or a copper-restricted diet of similar composition. Preference scores indicated that control rats preferred copper-deficient food. Preferences of rats on the high-copper diet were not different from control rats, suggesting that copper overload did not alter the pattern of selection. However, 7 of the 10 copper-deficient rats ate 80% or more of their intake as copper-adequate food resulting in preference scores that were significantly different from controls (p<0.03). This alteration in preference was corrected within 1 day of access to copper-adequate food.