Ontogeny of the ionic specificity of sodium appetite in the rat pup

Sodium-deficient adult rats prefer NaCl to other monochloride salts (e.g., Denton, 1991; Schulkin, 1991). However, it is not known when or how this specificity develops. Our experiments charted the development of the ionic specificity of sodium appetite aroused by sodium depletion or intracerebroventricular injection of renin. We compared intake of 3% NaCl to three other monochlorides, potassium (K), ammonium (NH₄), and lithium (Li), and calcium chloride (CaCl₂) at various ages between 72 hr postnatal and weaning. This revealed a biphasic developmental scheme: The adult pattern of discrimination between the salts emerges between 3 and 18 days of age. Subsequently, the preference for Na over the other salts increases into adulthood.©1994 John Wiley & Sons, Inc.     

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 rectifiying 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 © 1993 John Wiley & Sons, Inc.     

Angiotensin and salt appetite: physiological amounts of angiotensin given peripherally increase salt appetite in the rat

Two experiments were conducted in which adrenalectomized male Sprague-Dawley rats were maintained ad lib on distilled water, 3% saline, and sodium-free food. In Experiment 1, 45 rats were given 100, 200, 400, 800, and 1,000 micrograms/kg/day desoxycorticosterone acetate (DOCA) im for 5 days to determine the dose of DOCA that would produce the lowest voluntary saline intake, and 800 micrograms/kg/day was found to produce the nadir in saline intake. In Experiment 2, 40 rats were placed ad lib on distilled water, saline, and sodium-free food as described above, maintained on 800 micrograms/kg/day DOCA, and infused with 4, 25, 100 micrograms/kg/day angiotensin II (A II) or 0.9% saline. The three A II groups showed significant percentage changes in their saline intake above pre-A II levels; the saline control group showed no change in saline intake from pre-A II level. These results are interpreted to demonstrate the production of salt appetite in rats by peripheral administration of physiological doses of angiotensin II.     

The ontogeny of salt appetite aroused by depletion in the rat

The emergence of the rat's ability to respond behaviorally to a bodily sodium deficit was examined. Sucklings were depleted of bodily sodium by furosemide injections and their ability to replenish sodium by imbibing 3% NaCl solution was measured at different ages. The results suggest that the appetite for salt, as a response to sodium deficit, matures at weaning age.     

Ontogeny of polycose and sucrose appetite in neonatal rats

Rat pups 3 to 15 days of age were infused with sucrose or Polycose solutions (.03 or .3 M) through oral cannulas and their solution intake, mouthing behavior, and general locomotor activity were recorded. Overall, the pups displayed similar ingestive responses to Polycose and sucrose. They discriminated between .3 M sucrose and water as early as 6-days of age, and between .3 M Polycose and water as early as 9-days of age. However, at the .03 M concentration the pups responded to Polycose before they responded to sucrose. The results confirm previous reports that rat pups have an unlearned preference for sucrose and demonstrate that a similar preference exists for starch-derived polysaccharides. The Polycose appetite displayed by the neonatal rats is consistent with results obtained with adult animals which suggest that rats have taste receptors for starch-derived polysaccharides as well as for sugars.     

Multiple factors in the satiation of salt appetite

There is evidence in the literature that has been taken to show that, unlike hunger and thirst, salt appetite cannot be satiated in the absence of salt taste stimulation. The present study showed that repletion of body sodium in the absence of taste stimulation, that is, by gavage, can diminish subsequent saline intake. The satiating effects of gavage versus drinking of saline were studied at various intervals after repletion. For the first few hours, gastric loading was constantly less satiating than was drinking. But as the interval between gavage and testing was lengthened beyond 4-8 hr, the satiating effect began to increase until by 16 hr it was equal to that of drinking. The specificity of the satiating effect of saline gavage as a function of time between treatment and testing was also studied. There appeared to be a transient nonspecific blocking effect of solutes on solute intake which had a duration of less than 30 min. The satiating effect of saline gavage became specific after that time. The experiment suggests that there are multiple factors involved in the satiation of salt appetite--a taste factor, a short-latency post ingestional factor, and a long-latency postingestional factor.     

Subfornical organ participates in salt appetite

The effects of subfornical organ (SFO) lesions on salt and water intakes after sodium depletion were studied. Water and salt intakes were measured over 45 hr during a regimen that combined furosemide diuresis and access to low-sodium diet. Water was solely available for 23 hr after diuresis, and water and 0.3 M NaCl solution were available in choice for the next 22 hr. After diuresis, rats with SFO lesions drank significantly less water in 2 hr than controls but achieved equivalent water and sodium balances before salt access 20 hr later. After salt access, rats with SFO lesions drank significantly less saline and water in 2 hr than controls but had similar saline and water intakes over the next 20 hr. Thus, SFO lesions blunted acutely, but not chronically, saline and water intakes to sodium depletion, and the blunted intakes are not explainable by hydrational status.     

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.     

SGK1-dependent salt appetite in pregnant mice

Aim: Pregnancy is typically paralleled by substantial increase in maternal extracellular fluid volume, requiring net accumulation of water and NaCl. The positive water and salt balance is accomplished at least in part by increased uptake of salt secondary to enhanced salt appetite. Little is known about the underlying cellular mechanisms. Stimulation of salt appetite by mineralocorticoids, however, is known to be dependent on the serum‐ and glucocorticoid‐inducible kinase SGK1. Methods: To test for a role of SGK1 in the stimulation of salt appetite during pregnancy, fluid intake was recorded in pregnant SGK1 knockout mice (sgk1^?/?) and their wild type littermates (sgk1^+/+). The mice were offered two bottles, one with plain water and the other with isotonic saline. Results: In early pregnancy, i.e. up to 10 days prior to parturition, the sgk1^+/+ mice displayed a significant preference for saline, whereas the sgk1^?/? mice preferred water. Accordingly, the water intake was significantly smaller and saline intake was significantly larger in sgk1^+/+ mice than in sgk1^?/? mice and the preference for water was significantly stronger in sgk1^?/? mice than in sgk1^+/+ mice. Plasma aldosterone levels were higher in sgk1^?/? mice than in sgk1^+/+ mice, a difference contrasting the enhanced salt appetite of sgk1^+/+ mice. Conclusions: SGK1 participates in the stimulation of salt appetite during pregnancy.     

Sensitization of salt appetite is associated with increased "wanting" but not "liking" of a salt reward in the sodium-deplete rat

To examine the role of incentive sensitization in the potentiation of salt appetite by prior depletions, the authors assessed the motivation to obtain salt ("wanting") and the palatability of salt ("liking") independently in salt-sensitized rats. Breakpoint on a progressive ratio reinforcement schedule was used to measure salt wanting and taste reactivity was used to measure salt liking in rats with and without a history of Na+ depletion. Salt-sensitized rats displayed higher breakpoints relative to controls. However, a history of Na+ depletion was not associated with a greater positive shift in taste reactivity measures. The data suggest that these components of reward are separable in this model and support the general proposition that sensitization may alter wanting but not liking.     

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 kappa opioid receptors modulate salt appetite in rats

The role of the central opioid system in the control of water and salt intake is complex, with both stimulatory and inhibitory effects having been observed. The aim of the present study was to investigate the participation of the central κ-opioid receptors in the control of salt appetite. Male Wistar rats were submitted to two different experimental protocols: sodium deficit produced by the diuretic, furosemide, and brain angiotensinergic stimulation in rats under normal sodium balance. Lateral ventricle (LV) injections of Nor-binaltorphimine (Nor-BNI) at different doses (5, 10 and 20 nmol) inhibited hypertonic saline solution (1.5%) intake in sodium-depleted rats. The salt appetite induced by an LV injection of angiotensin II (Ang II) (10 ng) was also blocked by Nor-BNI injections into the LV, while no significant change was observed in water intake. Furthermore, the decrease in salt intake seems not to have been due to a general inhibition of locomotor activity or to any change in palatability, since central administration of Nor-BNI failed to modify the intake of a 0.1% saccharin solution when the animals were submitted to a "dessert test" or to induce any significant locomotor deficit in the open-field test. Also the central administration of Nor-BNI was unable to modify blood pressure in sodium-depleted animals. The present results suggest that activation of endogenous κ-opioid receptors modulates salt appetite induced by sodium depletion and by central angiotensinergic stimulation in rats.     

Change in salt appetite due to rehydration level in rats

Thermally dehydrated rats were given a choice of tap water and saline (0.9% or 1.8% NaCl), and the change in the salinity of their choice during rehydration was measured up to 15 hr. The rats consumed more water than saline for about 2 hr after the start of fluid replacement (about 55 mEq/l), while they consumed more saline than tap water (about 120 mEq/l) thereafter. Urine output and urinary Na output were only about 20% of their intake during the initial 4 hr of rehydration, while after 4 hr the output became almost equal with the intake. The change in salt intake occurred when about 90% of Na loss and 60% of fluid loss was regained. The results indicate that rats choose dilute salt solution to lower plasma osmolality during the initial period of the rehydration and then regain body fluid isotonically. Urine volume and urinary Na output increased only after volume repletion. Thus, osmoregulation with salt appetite has priority over fluid volume regulation in restitution from thermal dehydration.     

Ontogeny of enkephalin-like immunoreactivity in the rat hippocampus

The postnatal development of leucine5-enkephalin-like immunoreactivity within the hippocampal formation of the rat has been analyzed using immunocytochemical techniques. From the day of birth to postnatal day three, no intrinsic hippocampal elements exhibit immunoreactivity although labeled axons are found within the fimbria, within the alveus, and in the vicinity of the angular bundle. On postnatal day 4, a few immunoreactive hippocampal neurons can be seen in stratum radiatum of the region CA3 and by postnatal day 8, within the hilus, strata pyramidale and oriens of regio superior, and the subiculum. There is a dramatic increase in the incidence of immunoreactive perikarya between postnatal days 8 and 10 in all fields as well as the appearance of labeled neurons in CA1 stratum pyramidale and stratum granulosum of the dentate gyrus. Two days after the first appearance of immunoreactive perikarya, intensely immunoreactive neurons, labeled much more extensively than is ever seen in the adult, are encountered in each subfield of the hippocampus. The spatio-temporal order in both the emergence of perikaryal immunoreactivity and the transient appearance of intensely immunoreactive neurons follows that of neurogenesis, with immunoreactivity developing 12-14 days after the peak period of last cell division for a given hippocampal region. The incidence of immunoreactive perikarya in the dentate gyrus was quantified in rat pups ranging from postnatal days 8 to 19. The appearance of labeled neurons followed the spatio-temporal gradients that have been described for neurogenesis in this region as well. Immunoreactive perikarya emerged in the suprapyramidal stratum granulosum prior to their emergence in the infrapyramidal zone and in the temporal pole of the dentate earlier than in the mid-dorsoventral dentate. The lateral perforant path and mossy fiber axons, seen to exhibit enkephalin-like immunoreactivity in the adult hippocampal formation, differ in their relative maturity at the age immunoreactivity first appears. Immunoreactivity appears as early as postnatal day 4 in the lateral perforant path, an age at which these axons are just growing into their target field while it is not found within the mossy fibers until after postnatal day 10, an age at which mossy fiber bouton elaboration is well advanced and physiologically competent mossy fiber synapses with the regio inferior pyramidal cells have been established. The latter observation indicates that enkephalin is not necessary for synaptic transmission at the mossy fiber synapse.     

Ontogeny of thirst in the infant rat.

The development of water drinking in response to thirst stimuli was found to be precocious and sequential in the neonatal rat. It was studied in suckling rats with a technique that took advantage of their vigorous consummatory reflexes and minimized appetitive responding. Responsiveness to the following thirst challenges appeared in the suckling rat with a clear timetable: cellular dehydration at 2 days of age, hypovolemia induced by colloid dialysis at 4 days, and beta-adrenergic activation at 6 days. Responsiveness to deprivation from mother's milk was vigorous at the earliest day tested (2 days) but could not be distinguished from hunger. Rats weaned, isolated, and tested as miniadults responded appetitively to thirst challenges as early as 16 days of age. But they required a warm environment, optimum access to the water source, and sufficient time to respond. As revealed by consummatory responding, the controls of water drinking mature weeks before they are required. They precede the maturation of capacities for sustained appetitive responding. These appear in the last week of the suckling period, yielding animals that are prepared for adult, thirst-motivated behavior at the onset of weaning.     

Ontogeny of Peyer's patches of the rat

The development of lymphoid and nonlymphoid cells in Peyer's patches (PP) of the rat was investigated using light microscopical methods (routine histological techniques, enzyme histochemistry and immunohistochemistry). In newborn rats PP were mainly populated by T lymphocytes and Ia-positive nonlymphoid cells, which most likely are interdigitating cells. At about 12 days after birth the B and T cells were localized in defined regions, the follicular (FA) and interfollicular area (IFA), respectively. Compartmentalization within the FA started about 14 days after birth. The first signs of the development of secondary follicles were seen from about 18 days onward. PP obtained their mature structure at about 4 weeks after birth. It is suggested that after PP had developed fully, cells having cytoplasmic IgA migrate via the high endothelial venules (HEV) to the lamina propria of the intestine; cIgM and IgG cells seem to develop locally within the FA.     

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.     

Ontogeny of the VATER kidney in a rat model

By exposing rat fetuses to adriamycin prenatally, a rat model of VATER association has been created. Absence of the fetal bladder is prominent and the kidneys show features of chronic obstruction with hydronephrosis/hydroureter, loss of parenchyma, fewer glomeruli, and less differentiation. The aim of this study was to elucidate this rat model, to determine exactly when the changes in the kidneys develop, hopefully thereby to expand our understanding of congenital obstructive uropathy. Timed-pregnant Sprague-Dawley rats were injected intraperitoneally with adriamycin on days 6-9 of gestation. The control group received saline. Fetuses were recovered on gestational days (GDs) 20, 19, 18, 17, 16, 15, 14, 12, and 10 (total, 120 control, 121 treated). Macroscopic features were determined. Serial sections were then taken and stained with hematoxylin and eosin. Comparisons were made under light microscopy. The metanephric kidney first became apparent at GD12. The development of the control and treated kidneys appeared similar till GD18. Beyond this day, the treated kidneys exhibited increasing degrees of distension of Bowman's capsule, ducts, and subsequently pelvis and ureter. There were fewer levels of glomeruli, which were also less differentiated. Less differentiation was also noted in the medulla, and with time this became thin in comparison to the control kidneys. By GD20, the renal pelvis was grossly dilated with a blunted papilla, and the renal parenchyma was thin. Prenatal exposure of rat fetuses to adriamycin results in kidneys that are chronically obstructed, as the majority of the fetuses show absence of the bladder. Absence of renal dysmorphology until GD18, when urine is first produced, suggests strongly that the effect of adriamycin on the kidney is indirect, via agenesis of the bladder and secondary to backpressure from early urine production. This is a unique, simple, and reliable model of fetal obstructive uropathy and will be very useful to facilitate further investigation into its pathophysiology and to explore new treatment options.