Regulation of intakes of water and NaCl solutions in Fischer 344 rats: contrasts and comparisons between strains

Central AII evokes a normal sodium appetite in the Fischer rat, but its low spontaneous sodium intake may be related to reduced excitation and increased inhibition in septo-preoptic AII neurons

Fischer rats show a low or absent basal salt appetite and a reduced intake of salt solutions in response to peripherally administered angiotensin II (AII) when compared to other strains. We investigated spontaneous sodium intake, and sodium intake after intracerebroventricular (i.c.v.) AII and losartan, and septo-preoptic neuronal responses to AII and losartan, in age-matched male Fischer and Wistar rats. Spontaneous intake of 1.8% NaCl was lower in Fischers, but i.c.v. injection of 10 pmol AII produced similar 2 h intakes in a 2 h test period. Iontophoretic application of AII and losartan onto neurons in the septo-preoptic continuum revealed differences between the two strains of rat. In the Fischer rats only 11% of the spontaneously active neurons were sensitive to locally applied AII compared to approximately 30% in the Wistar. Local application of losartan produced neuronal inhibition in Fischer rats but neuronal excitation in Wistars. The central AII system appears to be regulated differently in these two strains, and may be related to the differences in their spontaneous sodium intake, but not to AII aroused sodium appetite.

Different vascular responsiveness to angiotensin II in two normotensive rat strains

Rats of the Fischer 344 (F344) and Wistar Kyoto (WKY) strains are known to present differences in stimuli responses involving the renin-angiotensin system and in cardiovascular responses to an acoustic startle stimulus. Here we compared the vascular reactivity to angiotensin II (ANG II) of these normotensive, inbred rat strains. Blood pressure (BP) and heart rate (HR) were recorded in conscious rats, before and after a neurohumoral blockade obtained by successive administration of chlorisondamine, enalapril, a V1-vasopressinergic receptor antagonist (Manning compound) and atropine methyl nitrate. BP was restored by a constant infusion of noradrenaline. Boluses of ANG II ranging from 0.001 to 1280 ng/kg were injected randomly. Average dose-response curves were established. After neurohumoral blockade, the minimum mean BP (MBP) produced by hydralazine (3 mg/kg, i.v.) and the maximum MBP produced by noradrenaline (60 microg/mL and 800 microL/min, i.v.) were used to reflect arterial wall structure. The maximal systolic blood pressure (SBP) and pulse pressure (PP) responses to ANG II were higher in F344 compared with WKY (+86 +/- 3 mmHg vs. +71 +/- 3 mmHg, P < 0.01 for SBP, +31 +/- 2 mmHg vs. +18 +/- 1 mmHg, P < 0.001 for PP). After the ANG II type 1 (AT1) receptor blocker valsartan, ANG II had no significant effect on BP. F344 and WKY exhibited the same maximum MBP in response to noradrenaline. However, MBP level following hydralazine was higher in F344 (F344: 48 +/- 2 mmHg vs. WKY: 37 +/- 3 mmHg, P < 0.01). The amplification in F344 of the vasoconstrictive response to ANG II mediated by AT1 receptors is compatible with a high number of AT1 receptors in this strain. In F344, the exaggerated systolic and PP responses to ANG II and the higher MBP level after hydralazine most likely reflect a structural modification of the arterial wall such as hypertrophic remodelling in F344.

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

Pregnancy and lactation are characterized by increases in NaCl intake, as determined by long-term consumption tests, which cannot examine the relative contribution of taste and postingestive factors to this phenomenon. Consequently, in this study, changes in NaCl preference during pregnancy and lactation were studied in nulliparous Long-Evans rats using a brief access test (lickometer). In Experiment 1, rats were maintained on a Na(+)-adequate diet (0.03% Na(+)), habituated to lickometer testing, and subsequently assessed during pregnancy and lactation with three 30-s exposures to each of seven taste solutions: 0.075 M sucrose (base), 0.089 M NaCl in base, 0.158 M NaCl in base, 0.281 M NaCl in base, 0.5 M NaCl in base, 0.158 M NaCl and 0.281 M NaCl. Results indicated higher lick rates to the 0.5 M NaCl in base, 0.158 M NaCl and 0.281 M NaCl solutions during late pregnancy and late lactation (Day 13 and beyond). In Experiment 2, a comparison of two diets differing in sodium content (0.03% vs. 0.3% Na(+)) determined that these changes in NaCl preference during pregnancy and lactation were unrelated to dietary sodium. Thus, the apparent increase in NaCl preference during pregnancy and lactation, independent of dietary sodium, suggests that this change in preference is not in response to physiological sodium need.

Neonatal diuretic therapy may not alter Children's preference for salt taste

Human neonates are occasionally treated with diuretics, and we investigated whether this causes a long-term enhancement of salt preference. Salt preference was examined in children aged 4-11 years. Twenty one of the children had received furosemide therapy as preterm neonates, and 24 were preterm neonates from the same ward that had no furosemide therapy. No differences were found between the two groups in preferred concentration of NaCl in soup, in consumption of salty items, and in blood and urine sodium and creatinine. However, in a tested subsample, fractional excretion of sodium (FENa) was higher in the neonatally treated children, suggesting increased salt intake. Reported severity of morning sickness in the mother when pregnant with the child, the child's history of diarrhoea and vomiting and degree of dietary salt exposure were obtained by questionnaire. These variables also did not influence salt preference, or blood and urine sodium and creatinine, except for a correlation between dietary salt exposure and blood sodium concentration. We conclude that while the physiological evidence suggests increased salt intake in children treated neonatally with furosemide, more sensitive tests of salt preference at this age are required to reveal any influence early mineralofluid loss may have on salt preference in childhood.

Effects of lactation on sodium intake in Fischer-344 and Long-Evans rats

Rats of most strains are attracted to salt in low concentrations, and this attraction is increased by pregnancy and lactation. Nonreproducing Fischer-344 (F344) rats are unusual in that they avoid saline at all concentrations, raising the question of whether lactation alters their sodium appetite. Therefore, lactating and cycling Long-Evans and F344 rats were compared in their relative consumption of water and several concentrations of saline. We found that Long-Evans rats preferred saline, but F344 rats preferred plain water. In comparison with cycling rats, lactating Long-Evans rats markedly increased saline intake whereas F344 rats exhibited only a modest increase. Lactating rats of both strains increased total fluid intake, but this increase was primarily from water in F344 rats and from saline in Long-Evans rats. It was concluded that the aversion to saline characteristic of nonreproducing adult F344 rats continues during lactation. Their aversion to sodium in pup urine may account for the low levels of maternal anogenital licking observed in the F344 strain.

Sodium depletion and maternal separation in the suckling rat increase its salt intake when adult

To establish whether neonatal sodium depletion increases the adult's avidity for NaCl, 12-day-old suckling pups were injected with the natriuretic-diuretic furosemide (1 mg) while with their dams. The injections surged plasma aldosterone, and when the rats were adult (70 days), their spontaneous intake of 3% NaCl was increased. Additional experiments investigated whether maternal separation has a similar effect and could thus be a source of individual variation in salt intake of the adult. Fifteen-day-old pups were separated from their dams for 24 h in an incubator. When adult, their intake of 3% NaCl was increased. Availability of saline during maternal separation obviated the effect. The increase in adult intake of 3% NaCl was specific insofar as drinking of water was not increased similarly. The results show that the adult rat's avidity for sodium can be increased by postnatal natriuresis and possibly stress. The implications of the findings are discussed.

Physiological correlates of attenuated salt appetite in Fischer 344 rats

Dietary NaCl deprivation stimulates a robust salt appetite in Wistar rats but has little influence on this behavior in rats of the Fischer 344 (F344) strain. To examine physiological substrates of attenuated salt appetite in F344 rats, several pertinent measures of renal function and fluid homeostasis were made in Wistar and F344 rats eating normal and NaCl-deplete diets. Physiological adjustments to NaCl deprivation were similar between the two strains; however, F344 rats showed smaller increases in plasma renin activity (PRA) than their Wistar counterparts. In addition, F344s decreased urinary sodium excretion more rapidly than Wistar rats in response to deprivation. The present studies also revealed several strain differences in baseline fluid and electrolyte regulation. Relative to the Wistar strain, F344 rats were characterized by high baseline PRA, increased arginine vasopressin (AVP) excretion, decreased urine volume, and diminished thirst. We propose that AVP and oxytocin activation may reduce salt preference and suppress the development of salt appetite in F344 rats.

Angiotensin II and fluid ingestion in old rats

Fluid ingestion was studied in Fischer 344/Brown Norway F1 rats aged 3, 12, 20, and 24 months of age. There was an age-related decrease in fluid ingestion when fluid intake was measured over 24 h. After water deprivation, 24- and 20-month-old rats drank less than 3- and 12-month-old rats. Twelve, 20-, and 24-month-old rats had less fluid intake associated with food deprivation than did 3-month-old rats. Three month old rats drank more fluid after angiotensin II than did 12-, 20-, and 24-month-old rats when expressed as fluid intake per kg body weight. These studies confirm that the rat is a reasonable model to study age-related hypodipsia.

Ontogeny of preference and aversion to salt in Fischer 344 rats and Syrian hamsters

Fischer 344 (F344) rats and Syrian hamsters, unlike rats of the Sprague-Dawley or Wistar strains, exhibit an aversion to dilute NaCl solutions as adults. In the present studies, using the anterior intraoral infusion method, it is shown that both F344 rats and hamsters accept more dilute NaCl than either water or concentrated NaCl (i.e., show a "single bottle" preference/aversion function identical to Sprague-Dawley rats) at both 6 and 18 days of age. At 18 days of age, they also show a preference for dilute NaCl over water in a simultaneous choice paradigm. In contrast, only 1-3 days later when these animals are weaned, they strongly reject NaCl solutions from sipper tubes in favor of water. It is unlikely that an abrupt discontinuity in neurosensory development can fully explain this dramatic reversal in salt acceptance.

Calcium deprivation increases NaCl intake of Fischer-344 rats

Fischer-344 rats fed Ca2(+)-deficient diet for 63 days increased intake of 0.3 M NaCl solution from control levels of approximately 8 ml/day to greater than 60 ml/day. During the same period, rats fed Na(+)-deficient diet drank approximately 11 ml/day. These results indicate that Fischer-344 rats, which generally spurn NaCl, drink large amounts of it when Ca2+ deprived.

Comparison of the dipsogenic responsiveness of Long-Evans and Sprague-Dawley rats

The dipsogenic responsiveness of the Sprague-Dawley (albino) and Long-Evans (hooded) strains of rats was compared. The responsiveness of the angiotensin II pathway for induction of drinking was assessed by means of acute administration of the beta-adrenoceptor agonist, isoproterenol (25 micrograms/kg, SC), angiotensin I (150 micrograms/kg, SC), and angiotensin II (150 micrograms/kg, SC). The results show that rats of the Long-Evans (hooded) strain drink less water than those of the Sprague-Dawley strain in response to acute administration of either isoproterenol or angiotensin I. However, there was no difference between groups in water intake resulting from administration of angiotensin II. This suggests that the Long-Evans strain has less capacity to convert angiotensin I to angiotensin II than the Sprague-Dawley strain, but that responsiveness of the Long-Evans strain to angiotensin II is similar to that of the Sprague-Dawley strain. The responsiveness of the osmoreceptor pathway for induction of drinking was also assessed both by measurement of the drinking response to acute administration of hypertonic saline (1% body weight of 1 M NaCl solution, IP) and a 24-hour dehydration. The results show that rats of the Long-Evans strain drink less water in response either to administration of hypertonic saline or to dehydration than those of the Sprague-Dawley strain. This suggests that responsiveness to stimulation of the osmoreceptor pathway of the Long-Evans strain induces less drinking than in the Sprague-Dawley.(ABSTRACT TRUNCATED AT 250 WORDS)