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Melanocortin-4 receptor knockout (MC4RKO) mice are hyperphagic and develop obesity under free feeding conditions. We reported previously that MC4RKO mice did not maintain hyperphagia and as a result lost weight when required to press a lever to obtain food on a fixed ratio procurement schedule. To assess the generality of this result, we tested MC4RKO mice and their heterozygous and wild type littermates using progressive ratio (PR) schedules that are believed to be sensitive indicators of motivation. Mice lived in operant chambers and obtained all of their food (20mg pellets) via lever press responding. Food was available according to a PR schedule so that within a meal, food became progressively more costly, and we expected this would provide a stringent test of mechanisms controlling meal size. The schedule reset after either 3 or 20min of no responding, so defining meals, and the highest ratio completed before the reset was defined as the breakpoint. The average daily number of meals was lower and mean size of meals was higher at the 20 compared with the 3min reset condition. Mean daily food intake did not differ between the two reset criteria but did differ as a function of genotype, with MC4RKO mice eating about 25% more than heterozygous or wild type mice. Hyperphagia in the MC4RKO mice was characterized primarily by larger meals (higher breakpoints) and they emitted about twice as many responses as wild type mice. Thus, using a PR schedule, MC4RKO mice exhibit hyperphagia, and show a high level of motivation to support large meal sizes.

Previous studies have indicated that apelin, a novel peptide suggested to have some actions related to angiotensin peptides, has either a dipsogenic or an antidipsogenic effect and either increases or decreases blood pressure. The present study attempts to provide replication or understanding of these disparate effects. Neither central (lateral or third cerebral ventricle) nor peripheral (intravenous) administration of apelin induced water intake in sated rats, nor did it decrease water intake in deprived rats. It also had no effect on sodium appetite. Peripherally injected apelin had a hypotensive action in anesthetized rats, but had no consistent effect in awake, unrestrained rats. We conclude that apelin does not have reliable or robust effects on fluid intake or blood pressure in Sprague-Dawley rats under normal conditions, but discuss the possibility for a role of apelin in fluid homeostasis in selected physiological states.

Satiation of sodium appetite of rats was studied using a protocol of sodium loss induced by chronic diuretic treatment and daily salt consumption sessions. In the first experiment in which NaCl was freely available from a drinking spout, rats consumed substantially more hypertonic NaCl than their estimated physiological deficit. In the second and third experiments, a simulated foraging cost was imposed on obtaining NaCl. In these experiments, rats were required to perform a progressive ratio lever press task to procure aliquots of hypertonic NaCl, and under these conditions they showed a break point or satiation for NaCl when they had consumed an amount close to their estimated deficit. In the fourth experiment, we found that the chronic diuretic regimen produced hypovolemia along with large increases in plasma aldosterone concentration and renin activity, and that all of these parameters were reversed toward non-depleted control values at the time of behavioral satiation in the progressive ratio protocol. Examination of natriuresis revealed that excessive intake of NaCl had no lasting benefit in this protocol: NaCl consumed in excess of need on 1 day was completely lost at the subsequent diuretic treatment. In the final two experiments, we showed that oral preloads of NaCl had no significant effect on progressive ratio procurement of NaCl, but that oral preloads reduced subsequent oral drinking. The data are discussed in terms of definitions of satiation of appetite in regard to the tightly regulated domain of body fluid homeostasis, and by extension, their relevance to systems such as energy homeostasis where set points appear to be less tightly regulated.

For animals that cannot be housed socially for scientific reasons, such as rodents on food intake-energy expenditure studies, the provision of environmental enrichment may alleviate stress. However, the influence of environmental enrichment on food intake and energy expenditure is unresolved. The authors tested the effects of two environmental enrichment options on food intake and weight gain in singly housed mice. Their results may be helpful in developing rodent enrichment programs.

It is known that mice injected peripherally with ANG II do not show a drinking response but that cFos immunoreactivity (ir) is induced in brain regions similar to those in rats. We now show in Crl:CD1(ICR) mice that peripheral injection of the ANG II type 1 receptor antagonist losartan was sufficient to prevent this induction of Fos-ir in the subfornical organ (SFO). Injection of ANG II into the lateral cerebral ventricle produced a robust water intake in mice and induced Fos-ir in SFO, as well as in median preoptic (MnPO) and paraventricular (PVN) nuclei. Peripheral injection of losartan blocked this drinking response and prevented the induction of Fos-ir in each of these brain regions. Hypovolemia produced by polyethylene glycol (PEG) produced a robust water intake but no evidence of sodium appetite, and it induced Fos-ir in SFO, MnPO, and PVN. Peripheral injection of losartan did not affect this drinking response. Fos-ir induced by PEG in SFO and MnPO was reduced by treatment with losartan, while that induced in the PVN was further increased by losartan. Sodium depletion with furosemide and low-sodium diet produced a strong sodium appetite and induced Fos-ir in SFO and MnPO. Treatment with losartan completely blocked the sodium appetite, as well as the induction of Fos-ir in these brain regions. These data indicate that endogenous production of ANG II and action at forebrain receptors is critically involved in depletion-related sodium appetite in mice. The absence of an effect of losartan on PEG-induced drinking suggests the critical involvement of other factor(s) such as arterial or venous baroreceptor input, and we discuss how this factor could also explain why peripheral ANG II is not dipsogenic in mice.

We report the meal patterns of mice with the deletion of either the melanocortin type 3 or 4 receptors (MC3RKO or MC4RKO) compared with that of the wild type (WT) under conditions of varying foraging costs. Mice lived in two-lever operant chambers; the completion of a designated number of responses (termed procurement fixed ratio or PFR) on the "foraging" lever activated the other lever. On this second lever, the completion of a designated number of responses (termed consumatory fixed ratio or CFR) caused the delivery of a 20-mg food pellet. Animals could complete as many CFRs as they wished to constitute a meal, but whenever 10 min elapsed without pressing on this second lever, the meal was terminated and pressing on the "foraging" lever was again required to initiate a new meal. At lower PFRs, mice of all three genotypes took 5-7 well-defined meals per day of approximately 35 pellets/meal. At the highest PFR, mice of all three groups took about half this number of meals, with some increase in meal size, and total intake was slightly reduced. MC4RKO mice were obese compared with WT or MC3RKO but failed to eat more food in the operant chambers and, as a consequence, lost weight, regardless of PFR. Thus, changes in meal-taking strategies as a function of imposed foraging cost are not critically dependent on either MC3 or MC4 receptors, but these conditions did not allow us to study meal patterns in MC4RKO mice that are hyperphagic.

An operant task that may simulate natural foraging was used to study in Sprague-Dawley rats the need-free choice between water and various concentrations of NaCl solution, and the induction of sodium appetite during chronic deoxycorticosterone acetate (DOCA) treatment. The task imposed a fixed ratio foraging or procurement cost for access to each fluid. The relative preference for NaCl over water when both had the same procurement cost was similar to the preference/aversion function that has been reported for this strain of rat under free drinking conditions. As the procurement cost increased, the number of bouts taken per day of both water and NaCl decreased and the mean bout size increased. Administration of DOCA produced an increase in NaCl intake under all conditions, and this appetite was manifest as discrete episodes whose frequency and size were inversely related. Natural observations of sodium appetite in mammals, such as excursions to, often remote, mineral deposits, imply an episodic organization of this behavior. This report is the first laboratory simulation of environmental factors that affect the episodic or temporal organization of sodium appetite.

High elective intake of ethanol was achieved in rats by presenting ethanol in palatable vehicles. We simultaneously measured intake of food (chow) to assess the accuracy of caloric compensation for the energy in the alcoholic commodity. In the first study, we used beer; nonalcoholic beer was consumed in large amounts, and when 5% or 10% ethanol was added, intake amounted to approximately 10% of daily calories. In the second study, Polycose in either solution or a gel matrix was used as the palatable vehicle for ethanol. The intake of ethanol was even higher than in the beer study, particularly in the gel preparation. In all cases, both male and female rats showed accurate caloric compensation by a reduction in chow intake. In a final study, we showed that restricted time access to the Polycose-alcohol gel produced high elective intakes and substantial blood alcohol levels. Over 24 h, caloric compensation was again accurate. Thus, unlike some reports in humans, rats seem able to compensate accurately for alcohol calories and in particular when, as with most alcohol consumption by humans, these are presented in palatable vehicles.

This paper reviews the experiments, which demonstrate conclusively the involvement of the abdominal vagus nerve in normal expression of most aspects of thirst in rats, by Gerard P. Smith and his colleagues published between 1975 and 1984. The nature of that vagal contribution differs with the type of primary thirst signal. Thus, there is no clear or unitary answer concerning whether the contribution of the vagus nerve is purely sensory, or some general tonic action within the central nervous system. Subsequent studies using cFos mapping of intracellular dehydration in conjunction with vagotomy and/or hepatic manipulations are also reviewed and further illustrate the involvement of abdominal information, both in the initiation as well as the termination of drinking. Many of the questions that were raised by Smith during these pioneering studies remain unaddressed and unanswered.

Previously, we have shown that repeated administration of d-fenfluramine (D-F) to rats is associated with development of tolerance to the initial anorexia, but that in mice no such tolerance seems to occur. In the first study, we show that chronic administration of neither d-norfenfluramine (D-NF; the principal metabolite of D-F) nor the serotonin (5-HT) 2C receptor agonist m-chlorophenyl-piperazine (mCPP) is associated with the development of anorectic tolerance tested using a dessert protocol. However, compared with mice receiving these drugs for the first time, both of these chronic treatments were associated with a significant attenuation of Fos immunoreactivity (Fos-ir) in several brain regions, including bed nucleus of the stria terminalis, paraventricular hypothalamus, and central nucleus of amygdala. This attenuation is similar to that described previously in rats. Because loss of efficacy of these agents could be related to loss of 5-HT transporter (5-HTT) sites, their presumptive primary mode of action, in the final study we determined the effect of various, low-dose regimens of D-F and D-NF on 5-HT uptake in frontal cortex of mice and rats. We show in mice that D-F causes a greater loss of 5-HT uptake than D-NF, and that at the lowest dose regimen used uptake was unaffected in rats but was reduced in mice. The data are discussed in terms of the species difference in behavioral tolerance and differences in neurochemical profile of D-F and D-NF.

Using an outbred strain of mouse, we examined several characteristics of sodium appetite induced by depletion. We found that an appetite for 0.15 M NaCl solution was stimulated 24 h after injection of furosemide and access to a low-sodium diet, but not by low-sodium diet alone. When the duration of exposure to low-sodium diet was increased from 1 to 7 days, there was no additional effect on either the appetite or the blood plasma changes including elevated hematocrit ratio, protein and aldosterone concentrations, and plasma renin activity (PRA). Mice also showed an appetite for hypertonic (0.5 M) NaCl in solutions or in a gel matrix; the intakes of these two were comparable but the gel measurement was gravimetric so maybe more accurate. In the same study, we showed that single injections of either 10 or 40 mg/kg furosemide followed by a 24-h low-sodium diet produced similar appetites, but that 2.5 mg/kg had a submaximal effect. Lastly, we further validated the use of the gel matrix by showing in chronically depleted mice that intake was inversely related to NaCl concentration in the range 0.5-1.5 M, and that appetite was selective for sodium but not the anion with which it was paired.

The present studies examine some parameters involved in flavor avoidance learning, using LiCl to induce malaise, in a novel nondeprivation protocol that allows direct comparison between rats and mice. The procedure involves daily presentation of a gelatin dessert that contains carbohydrate (Polycose) and a distinctive food flavor. Regular chow is additionally available at all times. Both rats and mice showed robust intakes of these gels with little change of gram intake as concentration of Polycose was varied in the range 2-30%; at the highest concentration, the caloric yield was approximately 7% of normal daily intake in both species. Rats that were injected on three occasions with LiCl (0.75 meq/kg) 1 h after consumption of a flavored gel formed a complete and sustained conditioned flavor avoidance (CFA). In a two-flavor discrimination protocol, in which a second flavor was followed by injections of saline, rats showed complete avoidance of the LiCl-paired flavor and partial avoidance of the saline-paired flavor. Mice injected on three occasions with LiCl (6 meq/kg) 1 h after intake of a flavored gel formed a partial CFA; a more complete CFA was formed when there was no delay between removal of the flavor and the injection. Using this no-delay protocol, mice, like rats, showed avoidance of a saline-paired flavor in a two-flavor discrimination protocol, and the CFA was strong when the dose of LiCl was reduced to that used in rats (0.75 meq/kg). In comparable protocols, mice thus are able to form complete CFAs using low doses of LiCl that are comparable to CFAs observed in rats, but the interval between flavor and sickness over which associative learning can occur may be shorter in mice.

Characteristics of hydrochlorothiazide (HCZ)-induced sodium appetite were examined in rats fed with low sodium diet to which HCZ was added. Compared with controls, HCZ induced a robust appetite for 0.1 and 0.3 M NaHCO3. The intake of 0.3 M NaCl was only about one half that of NaHCO3, but was still greatly above control. In both cases, the intake of the sodium solution exceeded that of water, so the rats were taking a hypertonic mixture. The appetite was evident in both Sprague-Dawley and Long-Evans rats, but appeared to be more stable in the latter strain over at least 1 month. The diuretic and natriuretic effect of HCZ was sustained over this period. Plasma renin activity was elevated substantially in HCZ-treated rats, but aldosterone concentration was decreased. Administration of HCZ in food is a simple and reliable method for inducing a sustained, substantial, and need-based salt appetite in rats.

C57BL/6J lean and obese (lep -/-) mice were studied in a closed economy operant protocol that simulates foraging. A predetermined number of presses on a procurement lever (PFR) activated a consumatory lever on which presses would produce 20-mg food pellets. Mice could eat as much as they wished but, once no responding occurred for an elapsed 10-min period, the consumatory lever was inactivated and the procurement or foraging cycle began again. Under these conditions, as has been shown for rats and other species, mice initiated relatively discrete meals (about nine per day) at the lowest PFR, and the number of meals initiated per day decreased with increasing PFR. Meal size increased reciprocally, so that total intake was conserved across the range of PFR examined. Obese mice ate larger meals than lean mice at low PFR, and showed further increases but only at the highest PFRs. The small and inconsistent literature on meal patterns in mice is reviewed, and we discuss the utility of the present protocol to study the interactions between genetic and environmental economic factors, and their implications for the etiology of human obesity.

CD1 mice injected peripherally with either ANG I or ANG II failed to drink substantial amounts of water or NaCl, yet showed strong Fos immunoreactivity (ir) in subfornical organ (SFO). Mice injected with furosemide showed modest stimulation of NaCl intake either 3 or 24 h later, were hypovolemic, and showed elevated plasma renin activity (PRA). The pattern of Fos-ir in the brain after furosemide was similar to that seen after peripheral injection of ANG II. Mice became hypovolemic after subcutaneous injection of polyethylene glycol (PEG), showed large increases in PRA, aldosterone, and water intake, but did not show sodium appetite. PEG-treated mice had strong activation of SFO as well as other brain regions previously shown to be related to ANG-associated drinking in rats. ANG II appears to have a modified role in the behavioral response to fluid loss in mice compared with rats.

Rationale: The brain regions and receptor subtypes involved in water intake following central cholinergic stimulation have been incompletely characterized., Objectives: To examine whether drinking and brain Fos-immunoreactivity (ir) induced in rats by central administration of bethanecol is reversed by either the preferential M1 antagonist pirenzepine, the M3 antagonist 4-DAMP, or their combination., Methods: Male Sprague-Dawley rats were surgically implanted with cerebroventricular cannulae. The muscarinic agonist, bethanecol was used as the dipsogenic agent. Either nonselective (atropine) or selective muscarinic receptor antagonists were injected together with bethanecol to determine blockade of drinking. In parallel studies, Fos-ir was assessed in discrete brain regions., Results: Bethanecol-induced drinking was completely blocked by atropine or by a combination of pirenzepine and 4-DAMP; these latter antagonists alone produced sub-total inhibition of drinking. In contrast, water intake induced by angiotensin II was unaffected by combination of pirenzepine and 4-DAMP. Fos-ir was induced by bethanecol in many brain regions previously implicated in body fluid regulation, including subfornical organ and the magnocellular supraoptic and paraventricular hypothalamic nuclei. Induced Fos-ir was substantially but not completely prevented by co-injection of either pirenzepine or 4-DAMP, but their combination did not seem markedly more effective than either alone., Conclusions: Drinking induced by brain muscarinic receptor stimulation seems to proceed by a combination of M1 and M3 receptor subtypes. Drinking induced by angiotensin II occurs independently of this mechanism. Fos-ir induced in fluid-related brain regions by bethanecol either uses additional receptor type(s) or is less easily blocked than drinking behavior.

The present study examines whether repeated administration of dexfenfluramine (DFEN) to mice is associated with progressive attenuation of its anorectic action and induction of Fos-immunoreactivity (Fos-ir) in some brain regions, both known effects in rats. ICR:Cd-1 mice were adapted to receiving for 30 min daily a sweetened milk gelatin dessert in addition to ad libitum access to chow. Mice were then injected with either DFEN (5-10 mg/kg) or saline vehicle every other day. These doses of DFEN acutely suppressed intake by approximately 50% and this action was sustained for at least eight injections. The anorectic effect of DFEN was no different between drug-naïve and DFEN-pretreated mice under conditions that produce complete tolerance in rats. In addition, unlike in rats, the acute anorectic effect of m-chlorophenylpiperazine (mCPP) was unaffected by prior DFEN treatment. Mice were perfused after the intake test on the final experimental day and their brain processed for Fos-ir. DFEN induced Fos-ir in brain regions analogous to those reported in rats and, also, as in rats, DFEN pretreatments reduced substantially the Fos-ir induced by acute DFEN in regions including central nucleus of amygdala, bed nucleus of stria terminalis, and the paraventricular hypothalamus (magno- and parvocellular divisions; PVN). Thus, while DFEN pretreatments reduce the effects of DFEN in many brain regions in both rats and mice, mice are unlike rats insofar as there is no tolerance to the anorectic action, at least in the nondeprivation protocol used here.

Using dexfenfluramine as unconditional stimulus (US), the authors confirmed that sham-operated and area postrema (AP)-lesioned rats form comparable conditioned flavor avoidances. When lithium chloride (LiCI) was used as the US, AP-lesioned rats did not learn to avoid a drug-paired flavor conditional stimulus (CS+). Sham-operated, but not AP-lesioned, rats had low intakes of the placebo-paired flavor (CS-), which suggests that the lesions disrupted generalization of avoidance. Generalized avoidance in intact rats was similar when either sweetened milk or Polycose was used as the caloric vehicle for the CSs. When flavored gels of Polycose were used as CSs, C57BL/6J mice developed flavor avoidance with either LiCl or dexfenfluramine as US. Compared with rats, mice required higher doses of these agents, avoidance was not complete after many pairings, and there was no generalization to the CS-.

Feeding induced in rats by cerebroventricular (i.c.v.) injection of orphanin FQ was potently and dose-dependently reversed by peripheral injection of either the opioid antagonist naloxone or the cannabinoid CB(1) receptor antagonist SR 141716[N-piperidino-5-(4-chlorophenyl)-1-(2,4-dichlorophelyl)-4-methyl-3-pyrazole-carboxamine]. The combination of these two agents inhibited food intake in a manner suggestive of additivity or supra-additivity.

Rationale: Recent studies in animals have implicated endogenous cannabinoids in the regulation of palatable food intake, but it is not yet clear to what extent pharmacological agents acting on this system may have sustained actions and applicability to different feeding protocols., Objectives: In the present study, we examine the effects of the cannabinoid CB1 receptor antagonist SR 141716 on food intake of rats, and its behavioral specificity. We examine whether tolerance develops to the anorectic actions of SR 141716, and whether it has either additive or synergistic actions with dexfenfluramine or naloxone., Methods: Undeprived rats were trained to eat a daily sweet milk dessert and on test days were administered single or combination drugs and intakes were recorded. In other studies, rats were deprived for 24 h of either food or water and intakes recorded after drug administration at the end of this time. In one study, rats were fed ad libitum chow with SR 141716 added., Results: SR 141716 (1-3 mg/kg) suppressed both palatable food intake in undeprived rats and food, but not water, intake after deprivation. Using an isobolographic analysis, SR 141716 had an additive anorectic effect with dexfenfluramine. In contrast, SR 141716 in combination with naloxone had a significantly supra-additive anorectic action. SR 141716 was also effective orally and no tolerance to its anorectic effect developed over 3 days., Conclusions: These data show that SR 141716 is an effective anorectic agent using both palatable foods and bland chow, and is selective because water intake was unaffected. SR 141716 is also effective orally and has an effect sustained for at least several days. There appears to be a synergistic interaction between opioid and cannabinoid systems in the regulation of feeding, whereas the combination of a serotonin releasing agent and the CB1 antagonist is additive.

Rationale: Studies of the effect of anorectic drugs such as fenfluramine in mice have indicated the desirability of using experimental protocols that do not involve deprivation., Objective: We have developed a non-deprivation or "dessert" protocol for use in mice that are maintained in standard housing conditions, and examine the effects of a serotonergic agent dexfenfluramine (DFEN), a dopaminergic agent phentermine (PHEN), and a selective norepinephrine uptake inhibitor thionisoxetine (TNIX) alone and in combination., Methods: Female C57BL/6J mice were adapted to 30 min daily presentation of a gelatinized form of sweetened milk using a holder that hooks over the side of the cage during tests; food spillage and contamination are minimal. Dose-inhibition curves were determined for DFEN, PHEN, and TNIX alone and for fixed ratio combinations of DFEN with either PHEN or TNIX., Results: Each drug produced a near linear dose-inhibition curve with the 50% inhibitory doses (DI50) of 5.6, 3.2 and 12.2 mg/kg, respectively. By isobolographic analysis, the effects of the drug combinations were strictly additive., Conclusion: The procedure described is highly suitable for testing anorectic drugs in mice and is adaptable to a variety of housing conditions and diets. The DFEN+ PHEN combination was additive, which contrasts with its reported supra-additive effect in rats.

Rationale: We have shown that the anorectic effect of dexfenfluramine (DFEN), an agent that acutely increases synaptic availability of serotonin (5-HT), shows complete tolerance after 2-3 prior applications when using acute feeding protocols and low dosages. It is unlikely this is due to either accumulative weight loss or presynaptic 5-HT depletion. In this study, we examined the possible contribution of 5-HT1B/2C receptors to behavioral tolerance by testing for cross tolerance between DFEN and the 5-HT1B/2C receptor agonists, m-chloro- and trifluoromethyl-substituted phenylpiperazines (mCPP and TFMPP). Additionally, we sought neuronal correlates of the behavioral changes by study of the induction of Fos-like immunoreactivity (ir) in discrete brain regions., Methods: Sprague-Dawley rats received two or three pre-injections, at 2-day intervals, of 2 mg/kg DFEN or vehicle. The rats were then food deprived for 24 h and, 30 min prior to a 1-h feeding test, received a s.c. injection of either DFEN, TFMPP (1 mg/kg), or mCPP (2 mg/kg). Additional groups received mCPP preinjections and test injection of either DFEN or mCPP. Rats in Fos-ir studies received similar injection regimens but were not food deprived and were perfused 1.5 h after the test injection., Results: DFEN-pretreated rats showed complete anorectic tolerance to DFEN, TFMPP, and mCPP. However, rats given this regimen of mCPP pretreatment were tolerant to neither mCPP nor DFEN. Fos-ir induced by DFEN in each brain region examined was either significantly reduced or abolished by prior DFEN injections. TFMPP induced less Fos-ir in these regions than DFEN and this was attenuated by prior DFEN., Conclusions: The behavioral data indicate that tolerance to DFEN anorexia is mediated partially or completely by functional subsensitivity at 5-HT1B and/or 5-HT2C receptors. The brain regions implicated include the paraventricular hypothalamus, medial striatum, lateral parabrachial nucleus, and nucleus of the solitary tract.

Free feeding rats given supplementary 1 h access per day to a palatable dessert test meal were tested for the anorectic effect of dehydroepiandrosterone alone or in combination with either the serotonin releasing agent dexfenfluramine or the norepinephrine uptake inhibitor thionisoxetine (LY 368975). Isobolographic analysis showed that the effect of dehydroepiandrosterone combined with either dexfenfluramine or thionisoxetine was within the range predicted for additivity of action.

We have shown previously that administration of angiotensin II (Ang II) produces an apparent decrease in thermoregulatory set point. Exposure to high salt diets either perinatally or later in life has been shown to increase pressor responsiveness to administration of Ang II, so in the present studies we examine whether high dietary NaCl would also increase the thermal responsiveness to Ang II. In the first study, we show that exposure to a basal NaCl diet (0.12%) during gestation through 4 weeks postnatally produced very large elevations in plasma renin activity (PRA) and aldosterone concentrations in the offspring. Exposure to high salt diet (3%) did not decrease the levels of these parameters below those fed mid salt diet (1%). In the second study, we show that rats raised through 4 weeks of age on basal diet, but then fed standard chow until adulthood, showed greater changes in tail skin (T(sk)) and colonic (T(c)) temperatures following administration of Ang II (200 microg/kg sc) than either mid- or high-salt-raised groups. In the third study, we confirmed this finding and extended it to show that rats raised on a very high salt diet (6%) also did not differ from the mid-salt group. In both studies, acute water intake measured in a separate test following administration of Ang II did not differ as a function of perinatal salt diet. In a fourth study, the period of exposure to the diets was extended from the perinatal period through adulthood and, surprisingly, there was no longer an enhanced thermal response to Ang II in basal diet rats compared with rats fed the very high salt diet. In the final study, rats raised on a regular diet but exposed only as adults to the test diets showed a nonsignificant trend toward a decreased thermal response in the basal group. Thus, dietary salt level may have opposite effects on Ang II effects on adult thermoregulation, depending on the age at the exposure.

Rationale: We have shown previously that the anorectic effects of the catecholamine-releasing agent phentermine (PHEN) and the serotonin (5-HT)-releasing agent dexfenfluramine (DFEN) are greater than additive in rats. In the present study, we examined whether the norepinephrine-uptake inhibitors desmethylimipramine (DMI) and thionisoxetine (TNIX) have additive effects with either DFEN or with the 5-HT-uptake inhibitor fluoxetine (FLX). We also examined whether PHEN interacts with a postsynaptically acting 5-HT agonist., Methods: Undeprived rats were trained to eat a daily sweet-milk dessert and on test days were systemically administered single or combination drugs and the intakes recorded., Results: Both DMI and TNIX produced dose-related suppressions of food intake. However, by isobolographic analysis, they did not enhance the anorectic actions of either DFEN or FLX. In contrast, confirming and extending our previous work, PHEN had a greater potentiating effect on the anorectic actions of DFEN and FLX than TNIX. Further, the anorectic action of the 5-HT2c receptor agonist TFMPP was enhanced by PHEN., Conclusions: These and other data are consistent with the idea that 5-HT agents may work "upstream" of critical catecholaminergic synapses in the production of anorexia, and explain the diminished efficacy of norepinephrine-uptake inhibitors relative to PHEN. The implications for clinically useful anorectic agents are discussed briefly.

Dexfenfluramine (dF) and dexnorfenfluramine (dNF), its metabolite, are anorectic agents that release serotonin (5-HT) and may have a direct postsynaptic action. The effects on the anorectic effects of dF and dNF of either acute (p-chlorophenylalanine, PCPA) or chronic (5,7-dihydroxytryptamine, 5,7-DHT) brain 5-HT depletions were studied in rats and compared with the actions of a 5-HT uptake inhibitor (fluoxetine) and 5-HT(1B/2C) receptor agonists [1-(3-trifluoromethyl-phenyl)-piperazine and 1-(3-chlorophenyl) piperazine]. The anorexia caused by these agonists was enhanced in rats with 5,7-DHT lesions, possibly a result of receptor supersensitivity. In contrast, fluoxetine anorexia was somewhat reduced in one study and was unchanged in a second. Both dF and dNF anorexias were enhanced in rats with 5,7-DHT lesions. In contrast, the anorectic effects of either dF or dNF were unchanged in PCPA-treated rats relative to controls. Compared with controls, 5, 7-DHT-lesion rats showed greatly increased dF- and dNF-induced Fos-like immunoreactivity (ir) in the paraventricular (PVN) and supraoptic (SON) hypothalamic nuclei, and in the median preoptic area (MnPO), but were similar to controls in most other areas. PCPA pretreatment increased dF- and dNF-induced Fos-ir in the PVN, SON, and MnPO. In controls, equianorectic doses of dF and dNF induced Fos-ir in similar brain regions, but dNF produced relatively larger effects than dF in SON, PVN, and MnPO. The data are discussed in terms of multiple pathways in the anorectic actions of dF and dNF.

Most previous research has focused on the effects of single neurotransmitters and neuropeptides on ingestive behavior. An important next step in the advancement of the science of ingestive behavior is to gain an understanding of how these different systems interact with one another. The present article is designed as an introduction to interacting systems involved in the regulation of food intake. Specifically, we review recent research on several neurochemicals that have emerged as good candidates for further study of interactions because they appear to serve integrative roles.

Female rats were fed diets containing either a basal (0.12%), mid- (1%) or high (3%) level of NaCl during pregnancy and lactation. Plasma aldosterone was elevated approximately 5- and 15-fold in dams fed basal compared with either the mid- or high-NaCl diets at the end of both pregnancy and lactation (Postnatal Day 21), respectively. Dams fed basal diet and killed at the end of lactation had a higher density of angiotensin II receptors in the organum vasculosum laminae terminalis, paraventricular hypothalamus, and median preoptic nucleus than did rats fed either mid- or high-NaCl diets. Other dams, treated identically, were returned to rodent chow (approximately 0.2% NaCl) at the end of lactation for intake tests during the next week. Dams that had received basal diet did not differ from mid-NaCl and high-NaCl groups in sodium appetite induced by either acute sodium depletion or mineralocorticoid administration but showed the lowest spontaneous intake of NaCl solution.

Fenfluramine + phentermine was a widely used combination for weight loss. Fenfluramine and phentermine are believed to act via serotonin and catecholamines, respectively. To what extent these drugs interact has not been well-established. We compared the anorectic efficacy of a range of doses of the combination (using dexfenfluramine instead of fenfluramine) relative to a range of doses of the individual drugs in 90 min sweetened milk intake tests in deprived and nondeprived rats. Results were plotted on isobolograms to determine whether the anorectic effects of the combination were either additive or synergistic. Collectively, the isobolographic analysis revealed that: (1) under acute conditions, dexfenfluramine and phentermine interact for the most part in a synergistic manner, and (2) with the exception of phentermine alone, deprivation state at time of testing did not alter the efficacy of the anorectics. These findings suggest that combined drug treatment for obesity is a theoretical approach that merits further investigation.

When rats are treated with furosemide, there is a rapid natriuresis. However, increased sodium appetite does not occur until some time later. One hypothesis to explain this delay is that increased circulating levels of the hormones of sodium depletion prime or sensitize the brain circuits involved in sodium appetite, perhaps by induction of target gene(s). In the present study, we describe the time course of the temporal maturation of sodium appetite after furosemide treatment and the associated changes in plasma levels of ANG II and aldosterone and in plasma volume. Sodium appetite is modest 3 h after furosemide treatment, is increased after 12 h, and is still larger after 24 h. This pattern is evident with repeated testing. Plasma levels of aldosterone and plasma renin activity are substantially increased 3 h after furosemide treatment, and so the NaCl appetite cannot result simply from progressively increasing levels of these hormones. Furthermore, activation of the subfornical organ and the ventral lamina terminalis, assessed with c-Fos immunocytochemistry, did not differ across these three times. Metyrapone, an inhibitor of adrenal steroid synthesis, was used to examine sodium appetite in the absence of elevations in aldosterone after furosemide treatment. Although metyrapone effectively blocked the increase in aldosterone, it was without effect on the appetite 3 or 24 h after furosemide treatment. Furthermore, elevations of plasma aldosterone by the use of minipumps for several days before furosemide treatment did not prime or potentiate but instead tended to inhibit the induced sodium appetite, despite achieving levels of aldosterone and plasma renin activity typically associated with a robust sodium appetite. Infusions of DOCA gave a similar result. Lastly, minipump infusions of ANG II also did not potentiate sodium appetite. Thus neither addition nor subtraction of these hormones alone influenced sodium appetite under these conditions.

The relationship between specific gustatory nerve activity and central patterns of taste-evoked neuronal activation is poorly understood. To address this issue within the first central synaptic relay in the gustatory system, we examined the distribution of neurons in the nucleus of the solitary tract (NST) activated by the intraoral infusion of quinine using Fos immunohistochemistry in rats with bilateral transection of the chorda tympani (CTX), bilateral transection of the glossopharyngeal nerve (GLX), or combined neurotomy (DBLX). Compared with nonstimulated and water-stimulated controls, quinine evoked significantly more Fos-like-immunoreactive (FLI) neurons across the rostrocaudal extent of the gustatory NST (gNST), especially within its dorsomedial portion (subfield 5). Although the somatosensory aspects of fluid stimulation contributed to the observed increase in FLI neurons, the elevated number and spatial distribution of FLI neurons in response to quinine were remarkably distinguishable from those in response to water. GLX and DBLX produced a dramatic attenuation of quinine-evoked FLI neurons and a shift in their spatial distribution such that their number and pattern were indiscernable from those observed in water-stimulated controls. Although CTX had no effect on the number of quinine-evoked FLI neurons within subfield 5 at intermediate levels of the gNST, it produced intermediate effects elsewhere; yet, the spatial distribution of the quinine-evoked FLI neurons was not altered by CTX. These findings suggest that the GL provides input to all FLI neurons responsive to quinine, however, some degree of convergence with CT input apparently occurs in this subpopulation of neurons. Although the role of these FLI neurons in taste-guided behavioral responses to quinine remains speculative, their possible function in oromotor reflex control is considered.

Pregnant rats were infused with phentermine plus dexfenfluramine from days 3 through 17 of gestation. Control rats were either pair-fed or were fed ad libitum. There were no effects of prenatal drug treatment on number of offspring, their birth weights, or on their motor coordination assessed at 11 days of age. Mothers and pups were sacrificed 21 days postpartum. Drug-treated mothers, but not their pups, showed a reduced density of serotonergic axons in the hippocampus compared with controls. 25% of the pups from the prenatal drug group showed mitral valve thickening.

The present study examined whether the area postrema and adjacent nucleus of the solitary tract (AP/NTS) is necessary for the expression of anorexia to two classes of anorectic agent. The first agent is the serotonergic agonist, dexfenfluramine (DFEN) and the second is the pancreatic peptide, amylin. Rats were prepared with either aspiration lesions of the AP/NTS or a sham operation. Rats with such lesions (APX) displayed normal anorexia following administration of DFEN, but the anorectic effect of amylin was completely eliminated. The magnitude of a conditioned flavor aversion to DEN was similar in APX and sham operated controls but, unlike controls, APX rats did not reduce total intake in the two-bottle preference test. Finally, the induction of Fos-like immunoreactivity (Fos-ir) following either DFEN or amylin was examined in both APX and sham operated groups. Both agents induced Fos-ir in the AP and/or NTS of sham operated rats, and this region was entirely absent in the APX rats. DFEN-induced Fos-ir was reduced greatly in the PVN of APX rats, but appeared normal in several other regions surveyed, including the central nucleus of the amygdala and the dorsal striatum. In contrast, amylin-induced Fos-ir was reduced in many rostral brain regions of APX rats. These data indicate that neither the anorexia nor the flavor aversion that are produced by DFEN are dependent upon the AP, and in particular that Fos-ir induced by DFEN in the LPBE is not due to afferents from the AP/NTS. In contrast, the anorectic effect of amylin seems to be due principally to its direct action at the AP/NTS., (Copyright 1999 Elsevier Science B.V.)

Water deprivation induces expression of the immediate early gene c-fos in specific brain regions, most likely as a result of the activation of cells that are responsive to changes in osmolality and/or blood volume. We hypothesized that the magnitude of c-fos expression would be a function of both the duration of water deprivation and the time of day at which the deprivation started. This study was designed to examine the pattern of Fos-like immunoreactivity (FLI) following water deprivation in rats under normal light/dark conditions (nLD) and reverse light/dark conditions (rLD). Rats were deprived of water but not food either for 0, 5, 16, 24 or 48 h. As expected, hematocrit ratio (HCT), osmolality (OSM), plasma renin activity (PRA) and weight loss increased as a function of duration of water deprivation. In non-deprived rats (0 h), very little FLI was observed in most brain regions. The number of cells showing FLI increased with duration of water deprivation in the supraoptic nucleus (SON), paraventricular nucleus (PVN), organum vasculosum laminae terminalis (OVLT), median preoptic nucleus (MnPO) and subfornical organ (SFO) in both nLD and rLD conditions. However, the pattern of FLI differed between nLD and rLD conditions. Compared to corresponding nLD groups after 5 or 24-h water deprivation, rLD groups had significantly more FLI in SON and PVN, and higher PRA and HCT. Also, weight loss and FLI in the MnPO were greater after 5 h, and FLI in the SFO was greater after 24 h under rLD compared to nLD conditions. Our findings indicate that the magnitude of c-fos expression, and change in weight and plasma parameters were a function of both the duration of water deprivation and the time of day at which the deprivation started. This may result from ingestion of food early in the deprivation periods during the rLD tests, thus producing greater change in osmolality and blood volume., (Copyright 1999 Elsevier Science B.V.)

Lateral cerebroventricular (LCVT) administration of the alpha-MSH agonist analog Nle4DPhe7alpha-MSH (NDP-MSH) inhibited food intake in food-deprived rats, but did not inhibit water intake in water-deprived rats. When NDP-MSH was administered into the fourth ventricle (4CVT), comparable suppressions of food intake were observed. LCVT and 4CVT administration of NDP-MSH also reduced spontaneous 24 h food intake. LCVT injection of NDP-MSH greatly attenuated food intake stimulated in sated rats by acute CVT administration of neuropeptide Y (NPY). These and other data suggest that alpha-MSH is an important endogenous regulator of food intake, possibly acting downstream of NPY. In an attempt to assess further the sites of action of NDP-MSH, a qualitative mapping study of Fos-like immunoreactive (Fos-ir) neurons was performed following LCVT administration of NDP-MSH. LCVT injection of NDP-MSH induced Fos-ir in several forebrain regions including cortex, striatum, bed nucleus of the stria terminalis, paraventricular nucleus of the hypothalamus and arcuate nucleus. The combination of NPY and NDP-MSH did not produce obvious antagonism or cancellation of effects in any region examined. Thus, the site(s) of action of NDP-MSH on food intake remain to be clarified.

Recently, a combination of the anorectics fenfluramine (FEN) and phentermine (PHEN) has been used to treat obesity. While each of these agents has been investigated in animals, little is known concerning the effects of the combination on ingestive behavior and body weight. In the present experiments, we report: (1) the effects of acute administration of dexfenfluramine (DFEN) and PHEN individually and in combination on sweetened milk intake and body weight in non-deprived rats and (2) the effects of chronic administration (7 day minipump) of DFEN, PHEN, and their combination on daily food intake and body weight both during and after the treatment period. Additionally, the effects of the 5-HT2C agonist 1-[3-(trifluoromethyl)-phenyl]piperazine (TFMPP) alone and in combination with PHEN on food intake and body weight were assessed. Both acute and chronic administration of DFEN and PHEN revealed that in combination they are more effective than when given individually. However, the DFEN/ PHEN combination does not appear to exert effects that are selective for food intake because water intake was markedly suppressed in water-deprived rats. PHEN alone or in combination with either DFEN or TFMPP also produced increased activity or alertness during the day when controls normally were asleep. While anorectic combinations such as DFEN/PHEN may be effective at promoting weight loss and reducing food intake, future studies on their specificity, safety and efficacy are warranted.

Both Long-Evans (LE) and Wistar-Furth (WF) strains of rat are known to be resistant to development of hypertension by mineralocorticoid (MC) treatment. MC-induced hypertension is, in part, mediated by the brain. We have examined another aspect of central MC action, the induction of NaCl appetite in these strains, by using the more common Sprague-Dawley (SD) and Wistar (WS) strains for comparison. In the first experiment, LE and SD rats were administered three treatments known to induce an appetite for NaCl solution in rats. Administration of deoxycorticosterone acetate (DOCA) increased the intake of 0.45 M NaCl in both strains, but the amounts consumed were about 2-fold greater in LE rats than in SD rats. Administration of captopril also increased NaCl intake, but there were no differences between LE and SD rats. NaCl depletion with furosemide induced NaCl appetite in both strains, but the amounts consumed were about 2-fold greater in LE rats than in SD rats. In the second experiment, adult male WF and WS rats were administered DOCA, enalapril, or furosemide and NaCl appetite was determined. Both strains showed comparable NaCl appetite during each of these treatments. However, during a 5-week regimen of DOCA with only NaCl-KCl solution to drink, uninephrectomized WF rats consumed less than WS rats. Thus, despite reported resistance to MC-induced hypertension, neither LE nor WF strains of rats showed correspondingly marked deficits in induced NaCl appetite.

Nitric oxide (NO) has been implicated centrally as an inhibitory neuromodulator, acting in short feedback loops. Neurons capable of NO synthesis have been localized in various thirst-related hypothalamic nuclei. Intracerebroventricular (icv) injection of L-arginine (L-arg), the precursor for NO, has previously been shown to attenuate both dehydration- and angiotensin II (Ang II)-induced drinking behavior. The present study further examines the effects of L-arg on drinking. We confirmed that icv administration of L-arg (50 microg) reduced water intakes induced by both 24 h water deprivation and icv Ang II (250 ng). We additionally showed that L-arg inhibited the water intake induced by peripheral injection of Ang II and the intake of 0.3 M NaCl following 24 h sodium depletion. We demonstrated the behavioral specificity of L-arg treatment by showing that it did not inhibit the intake of sucrose in food deprived rats and did not act as an unconditional stimulus for the formation of a conditioned taste aversion. These results lend further support to the idea that NO plays a role in modulating fluid balance and drinking behavior.

A comprehensive review of the literature through mid-1997 is presented on the application of immediate early gene mapping to problems related to brain mechanisms of fluid homeostasis and cardiovascular regulation in mammals. First, the basic mechanisms of fluid intake and the principles and pitfalls of immediate early gene mapping are briefly introduced. Then, data from several principal paradigms are reviewed. These include fluid deprivation and intracellular dehydration, both of which are associated with thirst and water intake. The contributions of peripheral sodium receptors, and of both hindbrain and forebrain integrative mechanisms are evaluated. Extracellular dehydration, and associated aspects of both thirst and sodium appetite are then reviewed. The contributions of both structures along the lamina terminalis and the hypothalamic magnocellular neurosecretory groups figure prominently in most of these paradigms. Effects of hypotension and hypertension are discussed, including data from the endogenous generation and the exogenous application of angiotensin II. Lastly, we summarize the contribution of the early gene mapping technique and consider briefly the prospects for new advances using this method.

The capacity of aging rats to defend body fluid homeostasis in response to a variety of dipsogenic and natriorexigenic stimuli was assessed. Male and female rats of both the Fischer 344 (FR) and Sprague-Dawley (SD) strains were used and tested at target ages of approximately 5, 10, 15, and 20 mo in both longitudinal and cross-sectional studies. There were no consistent age-related declines in water intake in response to water deprivation or acute administration of hypertonic NaCl; angiotensin (ANG) I, II, III; or isoproterenol. Likewise, there were no major impairments in either urinary excretion of the hypertonic NaCl load or excretion of water or hypotonic NaCl loads, although the latter were excreted more slowly in the older cohorts. The preference/aversion functions for NaCl solutions differed between SD and FR rats, but did not change with age except in male FR rats that lost their aversion to dilute NaCl at 20 mo of age. Intake of hypotonic NaCl solution after acute sodium depletion (furosemide treatment) showed a partial decline with age, and the older rats sustained larger estimated sodium deficits after a 6-h repletion period. A more complete age-related decline was observed in the intake of hypertonic NaCl stimulated by chronic dietary administration of a kininase II inhibitor (ramipril). Male rats of 15-20 mo of age showed no ramipril-induced sodium appetite. Brain ANG II receptor density, determined by autoradiography, declined by almost 50% in the paraventricular nucleus at 20 mo of age and declined slightly in the organum vasculosum laminae terminalis but did not decline in either the supraoptic nucleus or subfornical organ. Thus the major deficits in fluid intake in aging rats are related to salt appetite; the mechanism was not identified definitively.

The patterns of Fos-like immunoreactivity (Fos-ir) in rat brain were compared following treatment of rats with two anorectic 'gut' peptides. Central administration of GLP-1 produced dose-related increases in Fos-ir in the area postrema (AP) and caudal nucleus of the solitary tract (cNTS) as well as strong activation in the lateral parabrachial nucleus (LPBE), hypothalamic paraventricular nucleus (PVN), bed nucleus of the stria terminalis (BNST) and central nucleus of the amygdala (CeA). At centrally-active doses, peripheral administration of GLP-1 did not induce Fos-ir in brain. In contrast, peripheral administration of amylin produced strong Fos-ir in the AP and cNTS, as well as the BNST and CeA, but not in the PVN. The common activation of the LPB-BNST-CeA by these and other previously-studied anorectics suggest this is an important circuit involved in satiety.

We have previously shown that peripheral administration of bradykinin (BK) induces water intake in rats acutely pretreated with captopril, a kininase II inhibitor. We now show that BK-induced drinking is also observed in rats treated chronically with dietary captopril, and that this is reversed by Hoe 140, a BK receptor antagonist. Both acute and chronic captopril in combination with BK caused a large increase in plasma renin activity. Fos-like immunoreactivity (Fos-in used as a marker of cellular activation) was induced by BK + captopril in regions of the brain previously associated with action of angiotensin (Ang) II, including the circumventricular organs and the magnocellular hypothalamic nuclei. However, while water intake induced by peripheral administration of Ang I was potentiated by acute administration of captopril, it was suppressed by chronic captopril treatment. Fos-IR induced in brain by Ang I was not markedly affected by either acute or chronic treatment with captopril. The simultaneous occurrence of potentiated drinking to BK and inhibited drinking to Ang I following chronic treatment with captopril suggest that different mechanisms of action are involved. In order to further examine this possibility, rats were given lesions of the anterodorsal third ventricle region. Lesions that completely abolished the water intake following administration of Ang II only partly attenuated water intake induced by BK + captopril. Further, Fos-IR induced by BK + captopril was only partly (31%) reduced in the supraoptic and paraventricular nuclei of lesioned rats compared with sham operated controls. We suggest that at least two mechanisms, one Ang-related, underlie drinking after BK+captopril.

The purpose of this study was to determine whether beta-adrenergic receptors are involved in the elevation of blood pressure, tachycardia, and cardiac hypertrophy in rats chronically exposed to cold (5 degrees C). Four groups of rats were used. Two groups of rats were exposed to 5 degrees C and the other 2 groups were kept at 25 degrees C. In each temperature condition, one group received regular food while the other received food to which a nonspecific beta-adrenoceptor antagonist, d,l-propranolol, was added. The blood pressure of the untreated, cold-exposed group increased significantly within 3 weeks of exposure to cold and remained elevated until the end of the experiment. Chronic treatment with d,l-propranolol decreased the rate of cold-induced elevation of blood pressure at doses of 1.0 and 1.2 g/kg of food, and produced a complete reversal of hypertension at a higher dose ( 1.5 g/kg of food). Propranolol also prevented tachycardia in cold-exposed rats. The fact that propranolol decreased the elevation of blood pressure and prevented tachycardia suggests that both beta1- and beta2-adrenoceptors are blocked in cold-exposed rats. Propranolol, however, did not affect cold-induced cardiac hypertrophy.

We assessed the ability of acute peripheral administration of the AT-1 receptor antagonist, losartan, to reverse both the water intake and Fos-immunoreactivity (Fos-ir) induced in rats by either peripheral or cerebroventricular (i.c.v.) administration of angiotensin (Ang) II. We compared this with endogenous generation of Ang II during either hypovolemia or hypotension. Relatively low doses of losartan blocked the dipsogenic effect of peripherally administered exogenous Ang II, but a higher dose (20 mg/kg) was needed to block the dipsogenic effect of i.c.v.-administered Ang II. Fos-ir induced by i.c.v. Ang II was attenuated in SFO and SON by 10-20 mg losartan/kg given peripherally, but Fos-ir in the MnPO and PVN was unaffected. These findings suggest that losartan has limited permeability into the brain. We used peripheral losartan to assess the contribution of Ang II to water intake and Fos-ir responses to peripheral injection of either polyethylene glycol (PEG; a colloid that produces non-hypotensive hypovolemia) or isoproterenol (hypotensive agent). Water intakes were unaffected by the higher dose of losartan given s.c. Intraperitoneal injection of EXP 3174, the active metabolite of losartan that may more readily penetrate the blood-brain barrier, inhibited isoproterenol-, but not PEG-induced water intakes. Fos-ir was induced by PEG and isoproterenol in several regions of the brain also activated by Ang II. Fos-ir was greatly attenuated in the SFO by losartan following administration of PEG, but not isoproterenol, and was either unaffected or increased in SON and PVN after either agent. These data suggest that the increased circulating Ang II following PEG or isoproterenol acts at the SFO and is more readily reversible by losartan in normotensive (PEG) than in hypotensive (isoproterenol) states. Non-Ang neural input to the SON and PVN, presumably from baroreceptors, appears to be sufficient to produce strong Fos-ir in these regions, as well as to engage drinking.

In most forms of experimentally induced hypertension in rats, females develop a less severe form of the disease than males. The objective of the present study was to compare the two genders with respect to the development of cold-induced hypertension. The results of the study indicate that both males and females develop comparable elevations of blood pressure and at approximately the same rate. Thus, the blood pressures of both groups increased significantly within 2 weeks of exposure to cold and reached similar maximal levels by the seventh week. The dipsogenic responsiveness of both groups of cold-exposed rats to acute administration of the peptide hormone, angiotensin II (AngII), was increased to approximately the same extent above that of warm-adapted counterparts, suggesting an increase in the responsiveness to AngII in the brain. To assess this possibility, the induction of the oncogene, cFos, was studied in brain following IV infusion of AngII (333 ng/kg/min). Fos-like immunoreactivity (FLI) was greater (p < 0.01) in subfornical organ, supraoptic and paraventricular hypothalamic nuclei of both cold-exposed groups compared to warm-adapted controls. Thus, both male and female rats have similar elevations of blood pressure as well as increased dipsogenic and FLI responsiveness to administration of AngII during chronic exposure to cold.

Water intake was examined in young (3-5 months) and old (20-24 months) rats following peripheral administration of either angiotensin (Ang) II or captopril+bradykinin (BK). Relative to body weight, intake after Ang II showed no age-related difference, while that after captopril+BK was markedly reduced in the old rats. In other rats that were not allowed to drink after captopril+BK, the induction of Fos-like immunoreactivity (IR) was much lower in the subfornical organ, supraoptic and median preoptic nuclei of old rats compared with their young counterparts. Intake of 0.15 M NaCl during chronic dietary administration of enalapril was robust in young rats but was much reduced in old rats, as was their plasma renin activity. Thus, reduced thirst and sodium appetite are found in conjunction with kininase II (angiotensin-converting enzyme) inhibitors in aging rats, possibly because they fail to generate the high levels of circulating renin seen in young rats under these conditions.

Several studies are reviewed in which behavioral aspects of angiotensin (Ang) II on fluid intake have been compared with induction of the immediate early gene product, Fos, as a marker of neuronal activation in rat bain. Either peripheral or central administration of Ang II induced Fos along the lamina terminalis (SFO, MnPO, AV3V) and in the magnocellular neurosecretory groups (SO, PVH). A similar pattern is seen with central injection of renin. Both pharmacological and antisense oligonucleotide probe studies indicate that an AT1 receptor is involved, probably with the initial transduction in the SFO. Treatments that induce sodium appetite all induce Fos along the lamina terminalis, but usually not in the SO or PVN. Kininase II inhibitors, such as captopril, acutely potentiate drinking to Ang I, but after chronic exposure they may inhibit water intake. In contrast, the dipsogenic effect of bradykinin which is manifest in the presence of acute captopril remains unaffected by chronic administration. This suggests that the sodium appetite that appears with chronic captopril treatment may depend in part on peptides other than Ang.