SAPID Solutions and Food Intake in Repeated Dehydration and Rehydration Periods in Rats

In a previous report, it has been shown that water deprivation significantly affects the two-bottle taste preferences and one-bottle taste acceptance in rats when no food was available during tests. Since no food was available, the course of drinking was never interrupted by eating. Theoretically, if a rat faces a simultaneous choice between food and fluid, and if the course of drinking is interrupted by eating, these conditions might interfere with taste preferences, total fluid intake and eating in thirsty rats. The aims of the present experiments were: to ascertain whether food intake during both two-bottle preference and one-bottle acceptance tests in thirsty rats might be influenced by the palatability of the solutions; to verify whether the availability of food during tests influences taste preference and acceptance, and total fluid intake; to detect variations induced by dehydration on body weight and some plasma and urinary parameters that might interfere with food and fluid intake, taste preference and acceptance. Using naive rats, five groups of rats showing the same taste preferences for one of four prototypical tastes and water were selected. Then, both two-bottle preference (Expt 1) and one-bottle acceptance tests (Expt 2) were performed in rats deprived of water for either 12, 24, 36 or 48 h. The results showed that in both Expt 1 and Expt 2, inhibition of feeding and decrease of body weight during dehydration was very similar in all rats. The presence of food during the tests did not affect taste preference and acceptance. During Expt 1, after severe water deprivation (36 and 48 h), food intake was related to the palatability of the solution paired with water. When rats drank either NaCl or sucrose, they ate less food than rats drinking HCl, quinine, or water. In Expt 2, rats drinking NaCl solution as the only source of fluid ate significantly less food than all other groups. The intake of sucrose and/or NaCl solutions be may explained by two different post-ingestion effects (energetic and osmotic). Since rats drinking either sucrose or NaCl ate less food but drank more fluid, they had a significantly higher fluid/food intake ratio than that of rats who drank water, quinine, or HCl, who ate more food but drank less fluid. The increase of the fluid/food intake ratio in rats drinking sucrose or NaCl was directly correlated with the length of dehydration. Self-denial of food during dehydration may be responsible for overeating and overdrinking during the recovery period after tests. After dehydration lasting for 24 and 48 h, plasma [Na(+)], [protein], osmolality and haematocrit values increased but [K(+)] decreased. Urinary volume decreased but urinary [Na(+)] increased. These results are related to food and fluid intake, taste preference and acceptance after dehydration periods. Experimental Physiology (2001) 86.4, 489-498.     

The control of water and sodium chloride intake by postingestional and orosensory stimulation in water-deprived rats

We investigated the role of oropharyngeal and postingestional stimulation in the control of the intake of water and NaCl solutions by testing water-deprived rats under real- and sham-drinking conditions with water, 0.150, 0.225, and 0.300 M NaCl solutions. A series of real-drinking tests was given until intake stabilized followed by a series of sham-drinking tests with the same solution. When sham intake stabilized the concentration was increased and the series of real- followed by sham-drinking tests was repeated. Intake of water and 0.150 M NaCl in the first sham-drinking test was significantly greater than in the preceding real-drinking test and did not change with real- or sham-drinking experience. In contrast, intake of 0.225 and 0.300 M NaCl in the first sham-drinking test was not significantly greater than in the preceding real-drinking test but increased with sham-drinking experience. Real intake of 0.225 and 0.300 M NaCl following sham-drinking experience with a lower concentration declined significantly with real-drinking experience. These results show that postingestional stimulation plays a direct role in the control of the intake of water and isotonic saline with little or no orosensory contribution. In contrast, conditioned orosensory responsiveness played the central role in the control of the intake of the two hypertonic solutions with little or no direct contribution from postingestional stimulation. Postingestional stimulation, however, played an indirect role by serving as the unconditioned stimulus for the conditioned orosensory control.     

Behavioral and physiological aspects of body fluid homeostasis in Fischer 344 rats

Previous studies have shown that Fischer 344 rats, unlike many other strains, have neither a spontaneous preference for dilute NaCl solutions nor an excessive consumption of it after sodium depletion. The present studies examine some characteristics of water intake in Fischer 344 rats. Their spontaneous water intake was only about 50% of that of age-matched Sprague-Dawley rats, and the water-to-food ratio was about 30% lower. When water was added to the food, Fischer 344 rats decreased their fluid intake by a corresponding amount, whereas Sprague-Dawley rats continued to drink substantial amounts. In the absence of food, Fischer 344 rats reduced their water intake by a greater fraction than rats of the Sprague-Dawley strain. Physiological changes during these studies were as expected from the behavioral data, except that plasma protein concentration was consistently 10% higher in Fischer 344 rats. In contrast to this economy in their spontaneous drinking, water intakes of Fischer 344 rats were comparable to Sprague-Dawley rats in response to water deprivation, and administration of either hypertonic NaCl or angiotensin II, and in a sham-drinking paradigm. However, following treatment with either isoproterenol or polyethylene glycol, Fischer 344 rats drank considerably less than Sprague-Dawley rats. Possible reasons for, and implications of, these strain differences in drinking are discussed.     

Regulatory drinking in rats with permanent access to a bitter fluid source.

Male rats were given a quinine adulterated fluid as their sole source of liquid for over 60 days. After a latency of a few days, fluid intake stabilised at about 20 ml/day, with the water to food ratio close to 1.0 ml/g. Bodyweights fell to 90 percent of control levels (rats drinking unadulterated water). A battery of dipsogenic challenges was administered. Compared to controls, the ingestive responses during elevated ambient temperature, hyperosmotic salty food, after fluid deprivation, and to extracellular fluid depletion, were all attentuated. Drinking to acute NaCl injections was totally abolished. The intake of the adulterated fluid was near zero during food deprivation, and when a vegetable and fruit diet was available. Body fluid changes were suggestive of a net dehydration in the quinine drinking rats. Parallels with self intravenous drinking rats and rats recovered from lateral hypothalamic lesions were considered, and possible fundamental differences between natural and nonatural thirst stimuli.     

Naloxone modifies sugar-water intake in rats drinking with open gastric fistulas

The effects of the opiate antagonist naloxone (NX) on fluid preference and intake were determined in rats drinking with chronically indwelling gastric fistulas. The subjects were tested both after 22.5 hr fluid deprivation, and no deprivation, with open fistulas (sham drinking), as well as with closed fistulas. Following an injection of either saline or NX (0.5–10.0 mg/kg, administered SC), or no injection, the subjects were given the choice to drink water or 10% sucrose, in a two-bottle test, for 1 hr/day. With open fistulas, and following fluid deprivation, the animals sham drank both sucrose and water, but had a strong preference for sucrose. When not fluid deprived, the same animals sham drank sucrose almost exclusively. NX significantly reduced sucrose intake by the sham drinking animals, in both the deprived and not deprived conditions, but did not modify fluid preference. These data support the idea that NX modifies affective reactivity to palatable solutions, and that NX's antidipsogenic actions are not due to feedback from post-absorptional events.     

Effects of small basolateral amygdala lesions on ingestion in the rat

Using a variety of tests, the effects of small bilateral electrolytic lesions restricted to either anterior (BLa) (n=8) or posterior basolateral (BLp) (n=8) nucleus of the amygdala on food and fluid ingestion were measured. BLa lesions increased latency to eat in a novel environment, relative to sham operated controls (n=9). Neither lesion affected intake of 0.1% HCl, either on first exposure or after pairing with IP LiCl (conditioned taste aversion). However, water intake of both lesion groups was higher than normal during limited access water deprivation. Ad lib food and water intake were normal in both groups, but BLa animals drank more water than shams in response to acute cellular dehydration (IP 2M NaCl). Ad lib 24 hour intake of a range of concentrations of sucrose (0.02 M, 0.2 M, 2 M) and NaCl (0.05 M, 0.2 M, 1 M) was normal in BLp animals, but the BLa group drank less of the normally most preferred concentration of NaCl (0.2 M) than shams. The BLa group did not increase food intake significantly after insulin administration. These lesion induced alterations are discussed in relation to previous studies on damage to basolateral amygdala, and possible anatomical substrates for the effects are briefly discussed.     

Ghrelin reduces hypertonic saline intake in a variety of natriorexigenic conditions

Ghrelin is a gut peptide that has been studied extensively for its role in food intake and energy balance. More recent studies show that ghrelin reduces water intake in rats and some non-mammalian species. Despite the importance of the regulation of NaCl intake in body fluid homeostasis, the effects of ghrelin on saline intake have not been investigated. Accordingly, we tested the effect of ghrelin on water and 1.8% NaCl intake in two-bottle test conditions with the following five stimuli that increase hypertonic saline intake: central angiotensin II administration; 24 h fluid deprivation; water deprivation followed by partial rehydration; dietary sodium deficiency; and polyethylene glycol administration combined with dietary sodium deficiency. We found that ghrelin attenuated saline intake stimulated by angiotensin II, by water deprivation followed by partial rehydration and by dietary sodium deficiency. We did not detect an effect of ghrelin on saline intake after 24 h fluid deprivation without partial rehydration or after the combination of polyethylene glycol and dietary sodium deficiency. The finding that ghrelin reduced hypertonic saline intake in some, but not all, natriorexigenic conditions mirrors the previously published findings that in one-bottle tests of drinking, ghrelin reduces water intake in only some conditions. The results provide evidence for a new role for ghrelin in the regulation of body fluid homeostasis.     

Pregastric food-contingent stimulation elicits drinking in the absence of systemic dehydration in the rat

Adult male Sprague-Dawley rats surgically fitted with a stainless steel gastric cannula were prepared following 24-h food deprivation for sham feeding liquid diet with open gastric fistula. Sham feeding (pregastric food-contingent stimulation) of 5, 10, 20 or 40 ml of liquid diet elicited water intake that was not in proportion to volume of liquid diet sham fed. Rats sham feeding between 1 and 68 ml of sweetened milk showed no evidence of cellular or extracellular dehydration as measured by plasma osmolality and hematocrit, respectively. Subcutaneous injection of 100 mg/kg captopril, a dose sufficient to block conversion of angiotensin I to angiotensin II in brain and periphery, inhibited drinking elicited by sham feeding without effect on sham feeding. These results demonstrate that pregastric food-contingent stimulation elicits water intake that is not in proportion to amount of liquid diet sham fed and that occurs in the apparent absence of systemic dehydration. That such drinking could depend upon endogenous angiotensin II provokes consideration of a role for angiotensin II in the mediation of a pregastric (perhaps histaminergic) mechanism that initiates drinking in advance of postprandial dehydration.     

Role of superior laryngeal nerve and Fos staining following dehydration and rehydration in the rat

Immunohistochemistry for Fos was used to determine the role of the superior laryngeal nerve in conscious rats following water deprivation and rehydration. Adult male rats were subjected to either unilateral superior laryngeal nerve section (SLNX) or sham surgery. Two weeks later rats from each surgical group were water deprived for 48 h or water deprived for 46 h and given access to water for 2 h prior to perfusion. Controls were allowed ad libitum access to water. Brains were processed for Fos using a commercially available antibody. Changes in plasma osmolality and hematocrit were not significantly different between SLNX and sham following any of the treatments. Water intake in rats was not significantly affected by SLNX. In the supraoptic nucleus (SON) of sham rats, water deprivation significantly increased Fos staining while water intake following dehydration prevented this increase. Water deprivation significantly increased Fos staining in the SON of SLNX rats. Following water intake after 46 h water deprivation in SLNX rats, Fos staining in the ipsilateral SON was significantly greater than the contralateral SON and significantly lower than 48 h water deprivation. In the nucleus of the solitary tract (NTS) of sham rats, both water deprivation and water intake produced significant increases in Fos staining bilaterally compared to euhydrated controls. In SLNX rats, water deprivation significantly increased Fos in both ipsilateral and contralateral NTS that was not different from sham rats. SLNX significantly decreased Fos staining in the ipsilateral NTS of rats given access to water after dehydration compared to the corresponding sham treated rats. Fos staining was not affected in the contralateral NTS of SLNX rats given access to water after dehydration. This suggests that the superior laryngeal nerve contributes to changes in Fos staining in the NTS and SON following water intake in dehydrated rats.     

Water adulteration with citric acid: effects on drinking and responsivity to regulatory challenges

Rats with permanent access to a water supply adulterated with citric acid (CA) displayed persistent reductions in fluid intake and fluid/food ratios; and at appropriate concentrations of CA, they also exhibited lowered body weights and drinking deficits after fluid deprivation and after hypertonic NaCl injections. Unlike rats in previous investigations that were forced to consume quinine adulterated water, CA drinkers exhibited less disruption in their ingestive behavior following regulatory challenges, diminished short-term but greater long-term abilities in responding to hypovolemia, and an ability to increase fluid intake after fluid deprivation plus hypertonic NaCl. These data reveal that substances which degrade the taste quality of water do not exert a unitary influence on fluid intake, and they further underscore the complexity of ecological factors involved in controlling drinking behavior.     

Characteristics of thirst and sodium appetite in mice (Mus musculus)

The intakes of water and sodium chloride (NaCl) solution were examined in mice following treatment with agents that either stimulate or mimic various components of the renin-angiotensin-aldosterone system. Injections of either angiotensin II (Ang II) or isoproterenol produced very little water intake compared with the robust responses to either intracellular dehydration or to extracellular dehydration induced by treatment with polyethylene glycol (PEG). In studies on appetite for NaCl solution, mice exhibited no spontaneous preference for 0.15 M NaCl solution over water and did not change this preference during treatment with deoxycorticosterone acetate (DOCA), a sodium-deficient diet, or after adrenalectomy. Plasma concentrations of aldosterone were increased in intact mice fed a sodium-deficient diet but were not eliminated by adrenalectomy. However, acute treatment with furosemide in combination with a sodium-deficient diet stimulated an appetite for NaCl solution. Chronic oral administration of an angiotensin I (Ang I) converting enzyme inhibitor failed to induce a NaCl appetite. These findings show that mice are refractory to the induction of either water or NaCl intake by stimuli of the renin-angiotensin-aldosterone system, stimulation that is highly effective in rats; this suggests that there may be major differences among rodents in the hormonal determinants of behaviors related to hydromineral homeostasis.     

Taste preference and acceptance in thirsty and rehydrated [correction of dehydrated] rats

Experiments were designed to determine whether water deprivation affects taste preferences and/or taste acceptance. In experiment 1, both five- and two-bottle preference tests were performed in normally hydrated rats to permit the selection of five groups of rats showing the same taste preference for one of four prototypical tastes. Subsequently, in the same groups of rats, taste preferences were determined by a two-bottle test (experiment 2), and taste acceptance by a one-bottle test (experiment 3), following 12, 24, 36, and 48 h of water deprivation. After both 12 and 24 h of dehydration, during the first 10 min of the tests of experiment 2, all rats ingested greater volumes of either NaCl or sucrose solution than water, but more water than either HCl or quinine solution, and the differences were very significant (P<.0001). After 36 or 48 h of dehydration, the differences became very small and, in some cases, the P-values were at the lowest or borderline level of the significance, suggesting that dehydrated rats poorly discriminate the nature of the fluid drunk. During the 11-60 min interval, all rats preferred either sucrose or NaCl to water, but water to either HCl or quinine. Experiment 3 was performed to ascertain whether the need for fluid might overcome the palatability of solutions. All rats, dehydrated for 36 or 48 h, after 10 min of exposure, drank equal amounts of fluid, independent of its palatability. During the 11-20 and 21-60 min interval, the fluid intake of rats changed in accordance with the palatability of the solution available. In conclusion, severe thirst in rats may override the palatability of the solutions, and the thirst drive may be so strong that they do not reject fluids because body fluid balance would be severely compromised.     

Altered spontaneous and osmotically induced drinking for fowls with permanent access to dilute quinine

Fowls were given a dilute quinine solution as their sole source of fluid, and effects on normal ingestion and on drinking responses to dipsogenic stimuli were examined. Compared to controls with water, daily fluid intakes were depressed by 25% with quinine. There was no significant effect of quinine on food intake, but growth was suppressed slightly. Drinking in response to hypertonic NaCl injections was attenuated with quinine, and whereas an initial peak in water intake was seen directly after hypertonic NaCl injection, this was absent with quinine. Moreover, increases in quinine intake after hypertonic NaCl injections were insufficient to restore normal osmolality. Plasma analyses indicated that birds drinking quinine were permanently dehydrated and, unlike birds with water, they appeared to reduce the hyperosmolality induced by hypertonic NaCl injections in the absence of drinking. In contrast, drinking responses to angiotensin were generally similar with quinine and water, although birds drinking quinine tended to stop sooner with the highest dose of angiotensin. These results support previous suggestions that osmotic thirst is of primary importance in control of normal drinking, but also demonstrate that birds tolerate a degree of dehydration if a suitable fluid source is unavailable.     

Differential effects of intraventricular injections of tachykinin NK1 and NK3 receptor agonists on normal and sham drinking of NaCl by sodium-deficient rats.

The effects of lateral ventricular injections of succinyl-[Asp6, N-Me-Phe8]-substance P (SENK; 25, 100, 200 ng), a tachykinin NK3 receptor agonist, and [Sar9, Met(O2)11]-substance P (Sar Met; 100, 200 ng), an NK1 receptor agonist, on normal (gastric fistula closed) and sham drinking (gastric fistula open) of hypertonic NaCl by sodium-deficient rats were compared. Intraventricular injections of Sar Met had no effect on NaCl intake in either condition. Injections of 100 ng and 200 ng SENK caused an equal suppression of NaCl intake in the 2 fistula conditions. The latency to drink was not affected, but the initial lick rate was significantly lower and decayed more rapidly after 100 ng SENK than after saline or 25 ng SENK. The results show that (a) the tachykinin subtypes are not equally involved in the control of need-induced salt intake; (b) negative feedback from the stomach and distal gastrointestinal tract is not required for intraventricular injections of SENK to suppress sodium appetite; (c) the activation of NK3 receptors decreases the oral excitatory influence of hypertonic NaCl in sodium-deficient rats.     

Oroesophageal factors in the patterning of drinking.

Eleven albino rats were fitted with chronic gastric fistulas to determine the effect of stomach draining on the pattern of drinking. Each animal was given ten two-hr drinking tests with the gastric fistula open on every other test. On the first open fistula test intake was five times greater than that of the following closed fistula test. However, the first fistula opening did not alter the temporal position of the first pause in intake. Across subsequent open, but not closed fistula tests, consumption increased further so that on the last test open fistula intake was ten times greater than in closed fistula intake. On the basis of these results it is suggested that the early portion of the drinking sequence is under the control of conditioned oroesophageal stimuli while the later phases of the sequence are controlled by stimuli arising from the alimentary canal and/or blood.     

Post-ingestive signals and satiation of water and sodium intake of male rats

This study investigated the role of post-ingestive signals in the satiation of thirst or salt appetite. Post-ingestive signals, defined as those arising from the passage of fluid into the duodenum and proximal jejunum, were manipulated by implanting rats with gastric fistulas. After recovery, rats were water deprived and the following day gastric fistulas were opened (sham-drinking) or closed (control). Deprivation-induced thirst significantly increased water intake with sham-drinking rats consuming four-fold more than controls after 120 min access. Subsequently, rats were given sodium deficient chow for 48 h and the next day were administered furosemide and urine was collected. Twenty-four hours later, gastric fistulas were manipulated and rats were given water and 0.5 M NaCl and intakes were measured. After 120 min of access, rats were sacrificed and plasma sodium (pNa) and plasma-renin-activity (PRA) were measured. Furosemide resulted in a loss of 2.2 mEq of sodium in urine and sham-drinking rats consumed significantly more water and 0.5 M NaCl when compared to controls. At 120 min sham-drinking rats consumed 7.5 mEq of sodium nearly twice that of controls but had significantly lower pNa and significantly increased PRA. Interestingly, the ratio of water to 0.5 M NaCl intake was similar in both groups, with each making a mixture of ≈ 0.25 M NaCl. The results suggest that post-ingestive signals are necessary for the satiation of thirst and salt appetite.     

Control of feeding during saline consumption and fluid deprivation in hamsters

Cellular dehydration induced by water deprivation or hypertonic saline injection reduces feeding in a variety of species. Normal feeding in rats is maintained during isotonic saline consumption by increasing the intake of saline compared to the usual intake of water. Hamsters do not show the spontaneous preference for isotonic saline noted in rats, even after adrenalectomy. In the present investigation, feeding by hamsters was depressed during both isotonic and hypertonic saline consumption compared to the usual feeding with water. Saline intakes did not exceed water intakes under similar conditions. When fluid intakes were elevated by prior fluid deprivation, feeding rates increased at all concentrations of saline after a delay proportional to the osmolality of the solution. Positive 24-hr sodium balances were always associated with saline consumption. Water and hypertonic saline injections reduced feeding, and the fluid loads were excreted very slowly. When hamsters were fluid deprived prior to injections, saline totally suppressed feeding, while water increased feeding compared to sham injected controls. It is concluded that cellular dehydration produces a reduction of feeding in hamsters drinking isotonic or hypertonic saline. Reduced feeding with isotonic saline consumption results from the failure of hamsters to increase their ad lib intake of that solution. The prolonged retention of both sodium and fluid after saline consumption or injection suggests that further saline intake may be inhibited by an expansion of the extracellular space.     

Changes in salt intake after abdominal vagotomy: Evidence for hepatic sodium receptors

Several reports have suggested that the mammalian liver contains neural receptors, innervated by the vagus nerve, that monitor the sodium concentration and osmolarity of the portal circulation. These reports have been concerned primarily with either the neurophysiological identification of these receptors or their role in the short term control of urine output. Inasmuch as relatively little is known about the role of these receptors to consummatory behavior, we investigated the effects of hepatic vagotomy in rats on sodium intake as well as on sodium output. Hepatic vagotomized (HV) rats drank less NaCl solution (0.03, 0.1, 0.3M) in 24 hr during a two-bottle test with water than sham operated rats. Comparable differences in the intakes of either water, KCl or glucose solutions were not found. The two groups of rats did not differ in their intakes of water or 0.3M NaCl after an injection of either an osmotic load (IP, 2 M NaCl, 1% BW), deoxycorticosterone acetate (SC, 5 mg) along with furosemide (SC, 10 mg), or after 10 days of sodium deprivation. Urinary sodium output was reduced in HV rats during sodium deprivation but not when the rats had adequate levels of sodium in their diet. Because circadian patterns of water and food intake as well as body weight growth of hepatic vagotomized rats were similar to those of control rats, general malaise due to surgery and generalized deficits in motivation were ruled out as explanations for the depressed daily drinking of NaCl solutions. These findings support the existence of hepatic sodium receptors and their possible involvement to the control of sodium regulation.     

Drinking by portacaval shunted rats to regulatory challenges

Male rats received an experimental portacaval shunt (PCS) which diverted all of the hepatic-portal blood from the liver into the vena cava. Several months postsurgery, the regulation of fluid balance was evaluated in PCS and sham operated rats. PCS and Sham groups exhibited increased drinking in response to water deprivation and to an osmotic load, however, in both situations the PCS rats drank more than their surgical controls. Despite this difference in intake (adjusted for differences in body weight), the initial bout parameters of the PCS group following osmotic challenge were not appreciably different from those of the Sham group. The reduction of plasma volume with polyethylene glycol treatment elicited drinking in both PCS and Sham groups, with the former group consuming more fluid than the latter after polyethylene glycol but not after vehicle treatment. The results indicate that even after portacaval anastomosis rats were capable of increasing drinking in response to regulatory challenges and that the greater spontaneous fluid intake of PCS rats relative to controls which has often been observed persists during such compensatory responses.     

Drinking to intracellular dehydration following vagotomy in rats

Abdominally vagotomized rats maintained on a solid diet drank less and had longer latencies to drink than sham vagotomized rats following IP injection of an osmotic load (0.75 M NaCl, 1% BW). However, these two groups did not differ in latency or water intake following injection of isotonic saline. Since both vagotomized and control rats drank more water and had shorter latencies following injection of hypertonic saline than after isotonic saline, vagotomy apparently attenuated but did not abolish osmotic drinking. Maintenance on a liquid diet and a brief fast prior to testing (to ensure an empty stomach) did not alter these results, indicating that the impairment of gastric emptying of solid food that accompanies total abdominal vagotomy cannot account for the attenuation of osmotically induced drinking. Furthermore, this deficit was seen even when intracellular dehydration was produced at different times during the circadian cycle and when water presentation was delayed 0.5 hr postinjection. In addition, vagotomized rats drank less than control rats following 16-hr water deprivation and exhibited a lower water-food ratio on ad lib regimen. However, vagotomized and sham vagotomized rats exhibited the same relative day-night difference in water consumption as well as short latency response to thermal pain, which with other results indicates that vagotomy did not result in a general impairment of behavior. These findings suggest that osmotic perturbations are detected by the viscera and the information conveyed to the brain via afferent vagus nerves.