NaCl preference increases during pregnancy and lactation: assessment using brief access tests

Sex Differences in Salt Appetite: Perspectives from Animal Models and Human Studies

Salt ingestion by animals and humans has been noted from prehistory. The search for salt is largely driven by a physiological need for sodium. There is a large body of literature on sodium intake in laboratory rats, but the vast majority of this work has used male rats. The limited work conducted in both male and female rats, however, reveals sex differences in sodium intake. Importantly, while humans ingest salt every day, with every meal and with many foods, we do not know how many of these findings from rodent studies can be generalized to men and women. This review provides a synthesis of the literature that examines sex differences in sodium intake and highlights open questions. Sodium serves many important physiological functions and is inextricably linked to the maintenance of body fluid homeostasis. Indeed, from a motivated behavior perspective, the drive to consume sodium has largely been studied in conjunction with the study of thirst. This review will describe the neuroendocrine controls of fluid balance, mechanisms underlying sex differences, sex differences in sodium intake, changes in sodium intake during pregnancy, and the possible neuronal mechanisms underlying these differences in behavior. Having reviewed the mechanisms that can only be studied in animal experiments, we address sex differences in human dietary sodium intake in reproduction, and with age.

Sex differences in fat taste responsiveness are modulated by estradiol

Sex as a biological variable has been the focus of increasing interest. Relatively few studies have focused, however, on differences in peripheral taste function between males and females. Nonetheless, there are reports of sex-dependent differences in chemosensitivity in the gustatory system. The involvement of endogenous changes in ovarian hormones has been suggested to account for taste discrepancies. Additionally, whether sex differences exist in taste receptor expression, activation, and subsequent signaling pathways that may contribute to different taste responsiveness is not well understood. In this study, we show the presence of both the nuclear and plasma membrane forms of estrogen receptor (ER) mRNA and protein in mouse taste cells. Furthermore, we provide evidence that estrogen increases taste cell activation during the application of fatty acids, the chemical cue for fat taste, in taste receptor cells. We found that genes important for the transduction pathway of fatty acids vary between males and females and that these differences also exist across the various taste papillae. In vivo support for the effect of estrogens in taste cells was provided by comparing the fatty acid responsiveness in male, intact female, and ovariectomized (OVX) female mice with and without hormone replacement. In general, females detected fatty acids at lower concentrations, and the presence of circulating estrogens increased this apparent fat taste sensitivity. Taken together, these data indicate that increased circulating estrogens in the taste system may play a significant role in physiology and chemosensory cellular activation and, in turn, may alter taste-driven behavior.NEW & NOTEWORTHY Using molecular, cellular, and behavioral analyses, this study shows that sex differences occur in fat taste in a mouse model. Female mice are more responsive to fatty acids, leading to an overall decrease in intake and fatty acid preference. These differences are linked to sex hormones, as estradiol enhances taste cell responsiveness to fatty acids during periods of low circulating estrogen following ovariectomy and in males. Estradiol is ineffective in altering fatty acid signaling during a high-estrogen period and in ovariectomized mice on hormone replacement. Thus, taste receptor cells are a direct target for actions of estrogen, and there are multiple receptors with differing patterns of expression in taste cells.

Decrease in sweet taste response and T1R3 sweet taste receptor expression in pregnant mice highlights a potential mechanism for increased caloric consumption in pregnancy

While much is known on how the maternal diet affects offspring fitness, less is known on the role of taste in guiding and promoting food intake during this crucial period. Women have intense food cravings and exhibit altered taste preferences during pregnancy, however the mechanistic details underlying these changes are presently unclear. We performed longitudinal brief-access taste testing in female mice before, during, and after pregnancy, along with quantitative PCR on taste buds and morphological analysis of taste tissues from pregnant and non-pregnant mice. Sucrose licking response decreased progressively during pregnancy compared to that prior to mating, with partial recovery in the post-partum period. No change in taste morphology was evident between pregnant and non-pregnant mice, however a notable decrease in T1R3 sweet taste receptor mRNA expression was recorded in pregnant dams. We conclude that altered taste preferences during pregnancy likely result from changes in the expression profile of taste buds in the mother, which may promote a less healthy diet while expecting.

Breastfeeding and women's interest in specific food tastes

To determine whether breastfeeding alters women's interest in eating foods of different taste categories, we surveyed women at their 6-week post-partum check-up, asking them to rate their interest in eating various foods. Regardless of whether women responded in English or Spanish, they indicated greatest interest in eating sweet-tasting foods and least interest in eating sour-tasting foods, independent of whether they were breastfeeding. In general, the interest in eating foods of all taste qualities foods was increased in women who were breastfeeding; however, interest in eating salty and sour foods was not altered by breastfeeding in Spanish respondants. It is noteworthy that interest in eating foods of specific taste categories correlated with ratings of hunger in women who were not breastfeeding, but not in women who were breastfeeding. Thus, although breastfeeding women had a greater interest in eating foods of all taste categories, their interest does not appear to be driven solely by hunger. Finally, independent of breastfeeding, the interest in eating specific foods within taste categories differed between English and Spanish respondants, with Spanish respondants reporting greater interest in eating both nuts and bananas compared to English respondants. Together, these findings represent an initial approach to assess the impact of breastfeeding on interest in eating different types of food, and of how reproductive status and cultural differences may interact to affect food preferences and thereby to alter food choices.

Development and evaluation of an oral fast disintegrating anti-allergic film using hot-melt extrusion technology

The main objective of this novel study was to develop chlorpheniramine maleate orally disintegrating films (ODF) using hot-melt extrusion technology and evaluate the characteristics of the formulation using in vitro and in vivo methods. Modified starch with glycerol was used as a polymer matrix for melt extrusion. Sweetening and saliva-simulating agents were incorporated to improve palatability and lower the disintegration time of film formulations. A standard screw configuration was applied, and the last zone of the barrel was opened to discharge water vapors, which helped to manufacture non-sticky, clear, and uniform films. The film formulations demonstrated rapid disintegration times (6-11s) and more than 95% dissolution in 5min. In addition, the films had characteristic mechanical properties that were helpful in handling and storage. An animal model was employed to determine the taste masking of melt-extruded films. The lead film formulation was subjected to a human panel for evaluation of extent of taste masking and disintegration.

High salt intake as a multifaceted cardiovascular disease: new support from cellular and molecular evidence

Scientists worldwide have disseminated the idea that increased dietary salt increases blood pressure. Currently, salt intake in the general population is ten times higher than that consumed in the past and at least two times higher than the current recommendation. Indeed, a salt-rich diet increases cardiovascular morbidity and mortality. For a long time, however, the deleterious effects associated with high salt consumption were only related to the effect of salt on blood pressure. Currently, several other effects have been reported. In some cases, the deleterious effects of high salt consumption are independently associated with other common risk factors. In this article, we gather data on the effects of increased salt intake on the cardiovascular system, from infancy to adulthood, to describe the route by which increased salt intake leads to cardiovascular diseases. We have reviewed the cellular and molecular mechanisms through which a high intake of salt acts on the cardiovascular system to lead to the progressive failure of a healthy heart.

Postmating Circuitry Modulates Salt Taste Processing to Increase Reproductive Output in Drosophila

To optimize survival and reproduction, animals must match their nutrient intake to their current needs. Reproduction profoundly changes nutritional requirements, with many species showing an appetite for sodium during reproductive periods. How this internal state modifies neuronal information processing to ensure homeostasis is not understood. Here, we show that dietary sodium levels positively affect reproductive output in Drosophila melanogaster; to satisfy this requirement, females develop a strong, specific appetite for sodium following mating. We show that mating modulates gustatory processing to increase the probability of initiating feeding on salt. This postmating effect is not due to salt depletion by egg production, since abolishing egg production leaves the sodium appetite intact. Rather, the salt appetite is induced need-independently by male-derived Sex Peptide acting on the Sex Peptide Receptor in female reproductive tract neurons. We further demonstrate that postmating appetites for both salt and yeast are driven by the resultant silencing of downstream SAG neurons. Surprisingly, unlike the postmating yeast appetite, the salt appetite does not require octopamine, suggesting a divergence in the postmating circuitry. These findings demonstrate that the postmating circuit supports reproduction by increasing the palatability of specific nutrients. Such a feedforward regulation of sensory processing may represent a common mechanism through which reproductive state-sensitive circuits modify complex behaviors across species.

Effect of sex chromosome complement on sodium appetite and Fos-immunoreactivity induced by sodium depletion

Previous studies indicate a sex chromosome complement (SCC) effect on the angiotensin II-sexually dimorphic hypertensive and bradycardic baroreflex responses. We sought to evaluate whether SCC may differentially modulate sexually dimorphic-induced sodium appetite and specific brain activity due to physiological stimulation of the rennin angiotensin system. For this purpose, we used the "four core genotype" mouse model, in which the effect of gonadal sex and SCC is dissociated, allowing comparisons of sexually dimorphic traits between XX and XY females as well as in XX and XY males. Gonadectomized mice were sodium depleted by furosemide (50 mg/kg) and low-sodium diet treatment; control groups were administered with vehicle and maintained on normal sodium diet. Twenty-one hours later, the mice were divided into two groups: one group was submitted to the water-2% NaCl choice intake test, while the other group was perfused and their brains subjected to the Fos-immunoreactivity (FOS-ir) procedure. Sodium depletion, regardless of SCC (XX or XY), induced a significantly lower sodium and water intake in females than in males, confirming the existence in mice of sexual dimorphism in sodium appetite and the organizational involvement of gonadal steroids. Moreover, our results demonstrate a SCC effect on induced brain FOS-ir, showing increased brain activity in XX-SCC mice at the paraventricular nucleus, nucleus of the solitary tract, and lateral parabrachial nucleus, as well as an XX-SCC augmented effect on sodium depletion-induced brain activity at two circumventricular organs, the subfornical organ and area postrema, nuclei closely involved in fluid and blood pressure homeostasis.

Salt preference of nursing mothers is associated with earlier cessation of exclusive breastfeeding

Successful breastfeeding is predicated on its initial success. Salt appetite during lactation may be relevant to breastfeeding success because sodium is essential for development of foetus and neonate. Here we examined whether maternal salt preference might facilitate breastfeeding. Nursing mothers (n=327) were categorized as high, medium or low salt preferring, and the relationship to persistence of exclusive breastfeeding during the first 25 days postnatal was evaluated. Contrary to expectation, we find that mothers with low salt preference persisted in breastfeeding beyond day 7 postnatal in comparison to mothers with high salt preference, and mothers with high salt preference had the shortest exclusive breastfeeding duration up to postnatal day 25. Awareness of this among health workers and nursing mothers could contribute to successful breastfeeding.

Curt Richter and the female rat

Richter fully appreciated the fundamental biological importance of sex differences, in particular the challenges of female reproductive function to the two classes of behavior that most engaged him, endogenous rhythms and "self-regulatory" behaviors. Indeed, his contributions in these areas justify honoring him as one of the founders of behavioral neuroendocrinology. During the 1920s Richter, together with Wang, Kinder and other students, performed elegant phenomenological and mechanistic studies of sex differences in rats' spontaneous locomotor activity, nest building, and food intake. All of these behaviors display rhythms entrained to the ovarian cycle in female rats, and Richter's analyses of them formed the bases of many current areas of behavioral neuroscience. Slightly later, Richter made fundamentally important discoveries related to nutrient self-selection in pregnant and lactating rats, including changes in micronutrient and macronutrient selection. These data played a major role in the development of his concept of behavioral regulation of homeostasis. Unfortunately, some of these discoveries are rarely recalled. This is both historically regrettable and potentially an impediment to contemporary research progress.

Sex differences in electrophysiological and behavioral responses to NaCl taste

We tested the hypothesis that sex differences in preference for NaCl are attributable to estrogen-mediated alterations in gustatory processing. Electrophysiological responses of the chorda tympani nerve to NaCl were blunted by estrogen treatment in ovariectomized female rats, suggesting that females are less sensitive to concentrated NaCl solutions during high estrogen conditions. In contrast, after a taste aversion was conditioned to 150-mM NaCl, estrogen- and oil-treated ovariectomized rats generalized the aversion to a lower concentration of NaCl than did males, suggesting that females are more sensitive to the taste of dilute NaCl solutions regardless of estrogen. Thus, sex differences in NaCl preferences may be attributable to differences in NaCl taste processing that involve both acute and developmental effects of estrogen.

Drinking spout orifice size affects licking behavior in inbred mice

Using a lickometer, we assessed the effect of drinking spout orifice size on the licking behavior of inbred mice [C57BL/6J, SWR/J, 129P3/J and DBA/2J]. Animals licked from drinking spout sipper tubes that had what were defined as either a large (2.7 mm) or a small (1.5 mm) orifice. Mice took approximately twice as many licks from a stationary single small orifice drinking spout than when licking from a spout with a large orifice during separate 30-min sessions. However, their total intake volume was approximately the same. We calculated that mice received a mean of 0.55 muL per lick from the drinking tubes with a small orifice and a mean of 1.15 muL per lick from the drinking tubes with a large orifice. Thus, the animals appear to have regulated their fluid intake by proportionally adjusting their licking as a function of the lick volume. On average, this regulation occurred through modulation of the size of licking bursts and not their frequency. However, strain differences in compensation strategy were observed. When licking was restricted to a series of 5-s trials in a 30-min brief access test session, the smaller orifice size increased the range of responsiveness that was expressed. Mice increased their average licks per trial by 20% and took 60% more trials when licking from a spout with a small orifice. Interestingly, when the orifice size was quasi-randomly varied within a brief access session, licking was greater from large orifice drinking spouts, suggesting that water delivered from the two orifice sizes differs in its reinforcement efficacy. These findings demonstrate that drinking spout orifice size can significantly influence experimental outcomes in licking tests involving mice and care should be taken in controlling this variable in testing the effects of taste or other factors on ingestive behavior.

Dietary ethinyl estradiol exposure during development causes increased voluntary sodium intake and mild maternal and offspring toxicity in rats

Exogenous estrogen exposure during development often results in behavioral masculinization and/or defeminization of genetic females. Genetic males may be defeminized, hypermasculinized or even demasculinized after similar treatment. Here, pregnant Sprague-Dawley rats consumed phytoestrogen-free diets containing 0, 1, 5 or 200 ppb EE(2) beginning on gestational day (GD) 7. Offspring were weaned to the same maternal diet and maintained gonadally intact. There were mild effects on body weight and food consumption in dams of the 200 ppb group and their offspring weighed less at birth than those of the control group; however, gross assessments of nursing behavior were normal in all dietary groups. Postweaning, offspring of the 200 ppb group weighed less and consumed less food than controls. There were no EE(2)-related effects on open-field activity (tested at postnatal days (PND) 22-24, 43-45 and 64-66), play behavior (tested at PND 35), running wheel activity (PND 63-77) or intake of a 0.3% saccharin-flavored solution (PND 69-71). Intake of a 3.0% sodium chloride-flavored solution on PND 73-75 was increased in both male and female offspring of the 200 ppb group relative to same-sex controls, an effect that is reportedly estrogen mediated. Sodium chloride-flavored solution intake is a sexually dimorphic behavior for which female rats consume more than males. Here, while EE(2) exposure had few effects on the conventional tests of sexually dimorphic behaviors, exposure to 200 ppb in the diet appeared to feminize genetic males and hyperfeminize genetic females with regard to sodium intake.

Circadian rhythms of neuroendocrine dopaminergic neuronal activity in ovariectomized rats

Prolactin (PRL) secretion is inhibited by dopamine (DA) released from hypothalamic neuroendocrine neurons designated tuberoinfundibular (TIDA), tuberohypophyseal (THDA) and periventricular hypophyseal (PHDA) dopaminergic (DAergic) neurons. Since PRL is secreted in many physiological states with a circadian rhythm, the purpose of these experiments was to determine if patterns of neuroendocrine DAergic neuronal activity in rats are also circadian. The activity of neuroendocrine DAergic neurons, defined as DA turnover rate in nerve terminals and quantitated as the ratio of DOPAC (a primary DA metabolite) to DA content, was measured by high-performance liquid chromatography with electrochemical detection (HPLC-EC) in these populations of DA neurons of OVX rats. TIDA neurons exhibit a rhythm of activity in a light:dark cycle which free-runs in constant dark (DD) and is entrained by light, indicating that TIDA neuronal activity is circadian. THDA and PHDA neurons also display daily rhythms entrained to a photoperiod and PHDA neuronal activity free-runs in DD with a period of approximately 24 h. However, a significant rhythm of THDA neuronal activity was not detected under DD. In the OVX rat, the activities of TIDA and PHDA neurons, but not THDA neurons, describe all the characteristics of a circadian rhythm as they are both entrained by light, but only TIDA and PHDA neurons maintain a significant rhythm of activity under DD.