The effects of restraint stress on intake of preferred and nonpreferred solutions in rodents

In these experiments we determined whether stress influenced intake of different flavored test solutions or only those that were preferred. In a series of studies, rats or hamsters were exposed to acute (1 h) or repeated (3 h/day for 3 days) restraint stress immediately followed by access to one of four tastants (saccharin, salt, citric acid, or quinine solutions) paired with water in a 24-h preference test. As rats prefer salt and hamsters do not, both species were used to test the effects of stress on preferred vs. nonpreferred solutions using the same stimulus. Acute restraint inhibited intake of saccharin in rats but had no effect on preference, indicating that suppression of intake was not due to changes in hedonic response. Restraint had no effect on saccharin intake of hamsters but significantly increased salt intake. However, as the preference ratio remained low for the solution (0.26), the stress-induced increase in salt intake was probably associated with a disturbance of sodium and fluid balance rather than a change in sensory perception. This was supported by stress having no effect on intake of nonpreferred solutions in rats or hamsters. Repeated restraint had no effect on salt or saccharin intake of rats when test solutions were presented after stress, but rats showed no preference for saccharin in a subsequent study in which the solution was associated with onset of stress. These results indicate that stress has specific effects on saccharin and salt intake that are not limited to preferred solutions.     

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.     

Effects of gustation and gastric loading on rapid calorie metering in rats

On gustation/intubation of nutritive sucrose or glucose solution, both ad lib and meal-time rats reduced their subsequent 1 hr calorie intake via stock diet. But gustation/intubation of calorically-inert water had no effect on 1 hr calorie intake of either group of rats. In contrast, on tasting calorically-inert but sweet saccharin, the 1 hr intake of meal-time rats was reduced, though not the intake of ad lib rats. However, on intubation of saccharin solution no reduction in 1 hr intake was shown by either meal-time or ad lib rats. Quinine also showed differential effects on calorie intake. On intubation, but not on gustation of quinine, both ad lib and meal-time rats evidenced increase in their 1 hr calorie intake. Daily (24 hr) calorie intake via stock diet of ad lib rats, in contrast to 1 hr intake was unaffected by gustation/intubation of any test solution, including nutritive sucrose and glucose. Meal-time rats daily (3 hr) intake, on the other hand, was similar to 1 hr calorie intake following test solution treatment. It was reduced on gustation/intubation of calorically-rich solution as well as on tasting the sweet, but calorically-inert saccharin. Taste/intubation of quinine did not cause any change in daily calorie intake of either ad lib or meal-time rats.     

Effects of age and drugs on food and fluid intake.

In young adults rats (5-month-old) d-amphetamine (2 mg/kg/d), administered on a long-term basis via drinking water, caused a moderate reduction in the intake of nutriments, which in part normalized within three weeks. Self-administration of a daily dose of 20 mg per kg diazepam over a period of 26 days led neither to hypodipsia nor to anorexia. Pentylenetetrazol (70 mg/kg/d) primarily produced a hypodipsia. The three drugs did not influence body weight. In 27-month-old rats d-amphetamine and pentylenetetrazol had the same qualitative effects. Intake of nutriments and the development of body weight were influenced more strongly than in young rats. Diazepam also had a marked effect in old animals. Nootropics (piracetam, pyrithinol, hydergin, centrophenoxin, aniracetam) had no effects on the parameters observed. When the agents were given in combination in both age groups the nootropic piracetam (230 mg/kg/d) weakened the effects induced by d-amphetamine, pentylenetetrazol or diazepam alone. The benzodiazepine, however, enhanced the loss of body weight and fluid intake in old rats caused by the stimulant or analeptic, whereas food intake remained unaffected. The results support the hypothesis that an organism's adaptivity to external and internal stimuli is reduced in later life. The behavior of young and old rats in the open field was not affected by any drug medication.     

Effects of thirst-inducing stimuli on consumption of ethanol solutions by golden hamsters

Two experiments were performed to examine the acute effects of thirst-inducing stimuli upon the intake of tap water and ethanol solutions by golden hamsters, a species which avidly consumes ethanol solutions. In Experiment 1, three groups of adult male hamsters (n = 6/group) were maintained on Purina chow and tap water; hamsters in two of the groups also had access to one of two ethanol solutions (15% or 30%, v/v). Animals were deprived at various times of either one or both fluids for 24 hr, and then either one or both fluids were presented during a 2-hr drinking test. Total water intake increased substantially following both selective water deprivation and total fluid deprivation whenever tap water was available during the drinking test, but no significant changes occurred when only the ethanol solution was available. Both total fluid deprivation and selective ethanol deprivation produced similar increases in ethanol consumption, but selective water deprivation did not, suggesting that the temporary removal of tap water has little direct effect upon ethanol intake in hamsters, at least at the ethanol concentration levels studied here. In Experiment 2, thirst was induced by a subcutaneous injection of 10% saline (1 ml/100 g). This procedure produced large increases in total water intake whenever tap water was available during the drinking test, but ethanol intake did not change under any circumstances. These results suggest that factors that acutely enhance water intake have little or no effect upon the ethanol consumption of golden hamsters.     

Intraventricular neuropeptide Y injections stimulate food intake in lean, but not obese Zucker rats.

We examined the effect of acute third intraventricular (IVT) injections of either saline or NPY (0.95, 3.0, 9.5, or 30.0 micrograms in 1 microliter) on the 1-, 4-, and 22-hour postinjection food and water intake of female obese (fa/fa), heterozygous lean (Fa/fa), and homozygous lean (Fa/Fa) Zucker rats. None of the doses of NPY had an effect on either food or water intake of fa/fa rats. A significant increase of food intake was seen in Fa/Fa rats at 1 and 4 hours after the 3.0 micrograms injection of NPY and at 1, 4, and 22 hours after the 9.5 micrograms injection of NPY. Both 3.0 and 9.5 micrograms of NPY also stimulated 1- and 4-hour postinjection food intake of Fa/fa rats, although this effect was significant only at 4 hours after the 3.0 micrograms dose. NPY had a less reliable effect on water intake; 3.0 micrograms of NPY stimulated 1-hour postinjection water intake of Fa/fa rats and 4-hour postinjection water intake of Fa/Fa rats. These results indicate that lean, but not obese Zucker rats, respond by eating more to centrally administered NPY. This deficit is similar to the effects seen with IVT insulin injections and may be a result of a common receptor-mediated mechanism.     

Intraventricular injection of tachykinin NK3 receptor agonists suppresses the ingestion of NaCl-associated tastes

The present experiments evaluated whether a salty taste was required for injections of a neurokinin-3 (NK3) receptor agonist (senktide) to suppress intake or whether senktide would reduce the intake of tastes that are predictive of NaCl. During training, different groups of rats were given access to 1% almond + water, 1% almond + 0.3 M NaCl, or 1% almond + 0.1 M sucrose. On the test day, rats were administered intraventricular injections of either saline or 200 ng senktide and then given access to 1% almond + water. Senktide had no effect on the intake of the water-associated or sucrose-associated almond. In contrast, senktide significantly reduced the intake of NaCl-associated almond. Senktide had no effect on almond intake by water-deprived rats. These results show that activation of NK3 receptors reduces the intake of NaCl and of a neutral taste that is predictive of sodium but not of calories.     

The effect of sodium load on the development of hypertension, plasma renin and kininogen in rats with renal artery constriction

Effects of sodium load on the development of hypertension, plasma renin activity (PRA) and kininogen were studied in rats with renal artery constriction and untouched contralateral kidney. After the operation or sham-operation, 0.9% NaCl or water were given as drinking fluid. A marked hypertension (systolic pressure greater than 150 mmHg) developed in all operated rats on saline, but only in 2/3 of operated rats on water. In none of the sham-operated controls did systolic pressure exceed 150 mmHg during 7 postoperative weeks. Within the operated group on water, hypertensive rats had significantly higher PRA values than normotensive animals (P less than 0.05). Salt load slightly suppressed the PRA in sham-operated rats but not in animals with constriction renal artery, compared to sham-operated controls on water. The operated rats on salt excess had higher plasma kininogen levels than the operated normotensive rats on water (P less than 0.05), but there were no other significant differences in kininogen values between different study groups, regardless of whether blood pressure was increased or not. The results indicate that in this form of hypertension, the high blood pressure can be maintained without any increase in PRA if animals are subjected to a sodium load which sensitizes vascular beds to angiotensin. The increase in plasma kininogen, suggesting suppression of kallikrein-kinin system, is unlikely associated with the increase of blood pressure.     

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.     

5-HT2 and 5-HT3 receptors in the lateral parabrachial nucleus mediate opposite effects on sodium intake

The present study investigated the role of several 5-HT receptor subtypes in the lateral parabrachial nucleus (LPBN) in the control of sodium appetite (i.e. NaCl consumption). Male Holtzman rats had cannulas implanted bilaterally into the LPBN for the injection of 5-HT receptor agonists and antagonists in conjunction with either acute fluid depletion or 24-h sodium depletion. Following these treatments, access to 0.3 M NaCl was provided and the intakes of saline and water were measured for the next 2 h. Bilateral injections of the 5-HT2A receptor antagonist, ketanserin or the 5-HT2C receptor antagonist, mianserin into the LPBN increased 0.3 M NaCl intake without affecting water intake induced by acute fluid-depletion. Bilateral injections of the 5-HT2B receptor agonist, BW723C86 hydrochloride, had no effect on 0.3 M NaCl or water intake under these conditions. Treatment of the LPBN with the 5-HT2B/2C receptor agonist, 2-(2-methyl-4-clorophenoxy) propanoic acid (mCPP) caused dose-related reductions in 0.3 M NaCl intake after 24 h sodium depletion. The effects of mCPP were prevented by pretreating the LPBN with the 5-HT2B/2C receptor antagonist, SDZSER082. Activation of 5-HT3 receptors by the receptor agonist, 1-phenylbiguanide (PBG) caused dose-related increases in 0.3 M NaCl intake. Pretreatment of the LPBN with the 5-HT3 receptor antagonist, 1-methyl-N-[8-methyl-8-azabicyclo (3.2.1)-oct-3-yl]-1H-indazole-3-carboxamide (LY-278,584) abolished the effects of PBG, but LY-278,584 had no effects on sodium or water intake when injected by itself. PBG injected into the LPBN did not alter intake of palatable 0.06 M sucrose in fluid replete rats. The results suggest that activation of the 5-HT2A and 5-HT2C receptor subtypes inhibits sodium ingestion. In contrast, activation of the 5-HT3 receptor subtype increases sodium ingestion. Therefore, multiple serotonergic receptor subtypes in the LPBN are implicated in the control of sodium intake, sometimes by mediating opposite effects of 5-HT. The results provide new information concerning the control of sodium intake by LPBN mechanisms.     

Role of reactive oxygen species in contraction-mediated glucose transport in mouse skeletal muscle

Studies from our laboratory and others show that oestrogen reduces angiotensin II (Ang II)-induced water intake by ovariectomized rats. Elimination of endogenous oestrogen by ovariectomy causes weight gain that can be reversed or prevented by oestrogen replacement. Changes in body weight modify cardiovascular responses to Ang II but whether such changes have similar effects on central and behavioural responses to Ang II is unknown. The goal of this study was to evaluate the contributions of oestrogen and weight loss to isoproterenol (isoprenaline; Iso)-induced Fos immunoreactivity (IR) and to angiotensin type 1 (AT1) receptor mRNA in forebrain regions implicated in the control of fluid balance. Isoproterenol significantly increased Fos IR in the hypothalamic paraventricular and supraoptic nuclei, the subfornical organ (SFO), and the organum vasculosum of the lamina terminalis, but had no effect on AT1 mRNA expression. However, both Iso-induced Fos IR and the AT1 mRNA were attenuated in the SFO of the oestrogen and weight loss groups compared with that of the control group. Consequently, we examined the effect of weight loss on Iso-induced water intake and plasma renin activity (PRA) and found that weight loss decreased water intake after Iso, but had no effect on PRA. Thus, we propose that weight loss decreases Ang II-elicited water intake in the female rat by down-regulating the expression of the AT1 receptor.     

Effects of lesions of the ventral ventral median preoptic nucleus or subfornical organ on drinking and salt appetite after deoxycorticosterone acetate or yohimbine.

Lesions of the ventral ventral median preoptic nucleus (VVMnPO) enhanced daily salt appetite induced by subcutaneous (sc) injections of deoxycorticosterone acetate (DOCA) but did not affect acute salt appetite or water intake after sc injections of 5 mg/kg of the alpha-2-adrenoreceptor blocker yohimbine. Lesions of the subfornical organ (SFO) or its rostral fiber pathways had no effect on fluid intakes during DOCA treatments but significantly reduced water intake after yohimbine. These findings extend those of a previous report (Fitts, Tjepkes, & Bright, 1990) of enhanced DOCA-induced saline intake in VVMnPO-lesioned rats and demonstrate that the effect is specific to lesions of the VVMnPO. The mechanism of the thirst and salt intake elicited by yohimbine is still unclear, but the SFO and its fiber pathways appear to be important for the expression of the water drinking component. Neither lesion reliably affected yohimbine-induced salt appetite.     

Lipopolysaccharide reduces sodium intake and sodium excretion in dehydrated rats

The objective of this study was to find out if lipopolysaccharide (LPS) administered intraperitoneally affects sodium and water intake and renal excretion in dehydrated rats. LPS (0.3-5 mg/kg b.w.) inhibited 0.3 M NaCl intake induced by subcutaneous injection of the diuretic furosemide (FURO, 10 mg/kg b.w.) combined with the angiotensin converting enzyme inhibitor, captopril (CAP, 5 mg/kg b.w.). Only the highest doses of LPS (2.5 and 5 mg/kg) inhibited water intake induced by FURO/CAP. LPS (0.6 mg/kg) reduced urinary volume and sodium excretion, but had no effect on mean arterial pressure or heart rate of rats treated with FURO/CAP. LPS (0.3-5.0 mg/kg) abolished intracellular thirst and reduced by 50% the urine sodium concentration of rats that received 2 ml of 2 M NaCl by gavage. LPS (0.3-5.0 mg/kg) also reduced thirst in rats treated with FURO alone (10 mg/rat sc). The results suggest that LPS has a preferential, but not exclusive, inhibitory effect on sodium intake and on intracellular thirst. The inhibition of hydro-mineral intake and the antinatriuresis caused by LPS in dehydrated rats may contribute to the multiple effects of the endotoxin on fluid and electrolyte balance and be part of the strategy to cope with infections.     

Inhibition of drinking in water-deprived rats by combined central angiotensin II and cholinergic receptor blockade.

The effect of blockade of central angiotensin II (AII) receptors and cholinergic receptors on thirst induced by water deprivation was studied in Sprague-Dawley rats and rats with hereditary hypothalamic diabetes insipidus (DI). Neither central AII nor cholinergic blockade alone affected drinking. Antagonism of both receptors simultaneously, however, significantly inhibited water intake of both Sprague-Dawley and DI rats. This inhibitory effect was not observed in water-deprived, nephrectomized rats. The combined antagonism on water intake was specific, since milk intake in hungry rats was not affected by simultaneous AII and cholinergic blockade. Isorenin concentrations in brain tissue were at control levels in water-deprived, nephrectomized, and non-nephrectomized Sprague-Dawley rats but were increased in water-deprived DI rats. The results suggest that angiotensin and cholinergic receptors in the brain have a physiological role in thirst. Thirst is maintained when either receptor is intact, but reduced when both receptors are inhibited by antagonists. They are independently capable of maintaining thirst.     

Increased alcohol selection in rats after alcohol drinking paired with recovery from thiamine deficiency

Repeated pairings of novel alcohol solutions (5% or 7.5% w/v) with IP injections of 267 microgram/kg thiamine in thiamine-deficient male Sprague-Dawley rats resulted in the ingestion of pharmacologically active doses of alcohol in association with recovery. Mean alcohol intake in a postrecovery fluid choice situation exceeded metabolic capacity for 7--10 days; such intake was not observed in nondeficient pair-fed rats or in formely deficient rats whose recovery was not paired with alcohol drinking. Alcohol was self-selected in the presence of water under conditions where the rats had no experience with water while deficient, but the presence of 0.1% saccharin as a postrecovery alternative to alcohol was sufficient to abolish the elevated intake. Absolute alcohol intake and blood alcohol levels on recovery days and during self-selection were comparable with 5% and 7.5% alcohol solutions.     

Sham drinking in rats: osmotic and volumetric manipulations

Male rats, fitted with indwelling gastric fistulas, were tested with either a closed fistula (normal drinking) or an open fistula (sham drinking) following either intracellular dehydration (with NaCl), extracellular dehydration (with PEG), or the combination of the two stimuli. In normal drinking tests, the combined stimulus induced an intake of water that was the sum of the individual effects. In sham-drinking tests, both NaCl- and PEG-treated rats drank up to twice that of their normal drinking counterparts, but those given NaCl + PEG showed no such increase. Various body fluid measurements confirmed the persistence of the respective dehydration states at the end of sham-drinking sessions. The relatively poor or absent sham drinks after these stimuli are contrasted with vigorous sham drinking following fluid deprivation. In a second experiment, sham drinking following NaCl injections did not improve with repeated testing, but intake after intravenous infusion of NaCl did show some increase but was still modest and slow compared with that after fluid deprivation.     

Increase in plasma renin activity,water intake and blood pressure following application and reapplication of a renal artery clip

Application of a renal artery clip in rats with an undisturbed contralateral kidney caused a sustained increase in blood pressure and a transient rise of plasma renin activity and water intake. The response of blood pressure, plasma renin activity and water intake was augmented after reapplication of the clip to normotensive declipped rats (renal hypertensive rats, from which the clip had been removed 24 h before the reapplication). The time-course of the changes of blood pressure, plasma renin activity and water intake were similar after the initial application as after reapplication of the clip. Administration of an inhibitor (SQ 14.225) of the converting enzyme abolished the increase in blood pressure and water intake after reapplication of the clip. These data indicate a critical role of renin in the rise of blood pressure and water intake after initial application of a renal artery clip as well as after reapplication of the clip to declipped rats.     

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.     

The role of adrenal or testicular hormones in voluntary ethanol and NaCl intake of crowded and individually housed rats

The effect of adrenalectomy or castration on the ingestive behaviour of 10% ethanol, 0.5 M NaCl, water, and food was investigated in 2 models of increased/high ethanol consumption (1) adult male rats, previously individually housed with low ethanol intake, moved crowded housing and (2) individually housed adult male rats with high ethanol intake in the absence of any known aetiology. In study 1, rats that had been previously individually housed were paired with an animal in a small cage with ad libitum access to 10% ethanol intake, 0.5 M NaCl, water, and food at all times. Rats significantly increased 10% ethanol intake after they were pair-housed. The pairs were either adrenalectomized, castrated or sham operated. Neither adrenalectomy nor castration resulted in a significant change in 10% ethanol intake. 0.5 M NaCl intake was elevated and food intake and body weight were decreased in adrenalectomized rats. In study 2, rats that consumed large amounts of ethanol in the absence of any known aetiology remained in individual housing. Ethanol intake was decreased subsequent to either adrenalectomy or castration; adrenalectomy resulted in an increase in 0.5 M NaCl intake. These results suggest that the influence of adrenal or testicular hormones on ethanol intake is situation dependent. The increase in ethanol intake induced by placing animals in crowded housing appears to be independent of these hormones.     

Use of oral antibiotics in studies of ingestive behavior in rats.

The occurrence of chronic respiratory disease (CRD) in animal colonies is a problem encountered in many laboratories. Sixteen female albino rats were used to investigate the effects on food and water intake of the oral antibiotics most often used to treat CRD. Four concentrations (0.01, 0.05, 0.25, and 0.50%) of Tetracycline hydrochloride (TH) and Sodium sulfamethazine (SS) were presented at different times mixed with food or water. The animals were permitted to choose between each drugged and normal food or water combination for 4 days. Both drugs seriously affected fluid intake when placed in solution. When mixed with food, only SS produced a preference-aversion function and increased total daily food and water intake regardless of concentration. The presence of TH in the food had no effect on food choice or on total daily food intake at any concentration.