Water-restricted Rats Research Articles

Running-based food aversion learning in freely-fed and hydrated rats: Daily monitoring of running-induced nausea by measuring kaolin clay ingestion

The main objective of this research was to demonstrate food aversion learning in rats with unrestricted access to food and water, using wheel running as the unconditioned stimulus. Experiment 1 showed that the target-running paired training group consumed a statistically smaller amount of the target food (tteok rice cakes) compared to the target/running unpaired control group, but the decrease in consumption over days in the paired group was not fully supported by a statistical test. Experiment 2a improved the methodology by familiarizing rats with tteok before training, which resulted in both a statistically significant group effect and a statistically significant daily decrease in tteok consumption. Experiment 2b demonstrated that tteok aversion could be reacquired after an extinction phase. These experiments indicate that running-based tteok aversion in non-deprived rats is a valid example of Pavlovian conditioning and suggest that wheel running can cause similar effects in unrestricted rats as observed in food- or water-restricted rats. Additionally, daily measurements of kaolin clay ingestion suggested that wheel running induced nausea in the rats of these experiments.

Effects of a glucagon-like peptide-1 analog on appetitive and consummatory behavior for rewarding and aversive gustatory stimuli in rats

Glucagon-like peptide-1 (GLP-1) is an incretin hormone that is essential for the regulation of food intake and approved for the treatment of type 2 diabetes mellitus and obesity in humans. More recently, GLP-1 has been investigated for its ability to modulate motivation for food and drugs. Reward behavior can be divided into two components: ‘motivational’ (i.e., approach and consummatory behaviors) and ‘affective’ (i.e., perceived palatability). Studies show that GLP-1 analogs reduce the motivation to approach and consume palatable food, but the impact on affective responding is unknown. Thus, the present study tested the effect of the GLP-1 analog, Exendin-4 (Ex-4), on the appetitive response to intraorally delivered sucrose and quinine. Results showed that Ex-4 (2.4ug/kg ip) failed to alter passive drip, appetitive reactions (i.e., mouth movements, tongue protrusions, and lateral tongue protrusions) or aversive reactions (i.e., gapes) to sucrose. Paw-licking, however, was significantly reduced by Ex-4. Treatment with Ex-4 also failed to influence passive drip to quinine, but increased the latency to gape and reduced the total number of gapes emitted. In addition, Ex-4 reduced intake of quinine in water restricted rats, but did not reduce conditioned aversion (i.e., gapes) or avoidance (i.e., reduced intake) of a LiCl-paired saccharin cue. Thus, while Ex-4 had no effect on a learned aversion, it reduced approach and ingestion of sweet and bitter solutions, while leaving the appetitive affective response to the sweet almost intact, and the aversive affective response to the bitter reduced. Treatment with Ex-4, then, differentially modulates appetitive and consummatory components of reward, depending on the valence of the stimulus and whether its valence is learned or innate.

Habitual Preference for the Nondrug Reward in a Drug Choice Setting.

For adaptive and efficient decision making, it must be possible to select between habitual alternative courses of action. However, research in rodents suggests that, even in the context of simple decision-making, choice behavior remains goal-directed. In contrast, we recently found that during discrete trial choice between cocaine and water, water-restricted rats preferred water and this preference was habitual and inflexible (i.e., resistant to water devaluation by satiation). Here we sought to test the reproducibility and generality of this surprising finding by assessing habitual control of preference for saccharin over cocaine in non-restricted rats. Specifically, after the acquisition of preference for saccharin, saccharin was devalued and concurrent responding for both options was measured under extinction. As expected, rats responded more for saccharin than for cocaine during extinction, but this difference was unaffected by saccharin devaluation. Together with our previous research, this result indicates that preference for nondrug alternatives over cocaine is under habitual control, even under conditions that normally support goal-directed control of choice between nondrug options. The possible reasons for this difference are discussed.

A Free-Operant Reward-Tracking Paradigm to Study Neural Mechanisms and Neurochemical Modulation of Adaptive Behavior in Rats.

The ability to respond flexibly to changing environmental circumstances is a hallmark of goal-directed behavior, and compromised flexibility is associated with a wide range of psychiatric conditions in humans, such as addiction and stress-related disorders. To identify neural circuits and transmitter systems implicated in the provision of cognitive flexibility, suitable animal paradigms are needed. Ideally, such models should be easy to implement, allow for rapid task acquisition, provide multiple behavioral readouts, and permit combination with physiological and pharmacological testing and manipulation. Here, we describe a paradigm meeting these requirements and employ it to investigate the neural substrates and neurochemical modulation of adaptive behavior. Water-restricted rats learned to emit operant responses for positive reinforcement (water reward) within minutes in a free-operant conditioning environment. Without further training, animals were able to track changes in the reward schedule. Given prior evidence that the medial prefrontal cortex (mPFC) and the dopaminergic system are required for flexible behavior, we aimed to assess both in more detail. Silencing of mPFC compromised flexible behavior when avoidance of punishment was required. Systemic injections of the D2-receptor agonist quinpirole and the D2-receptor antagonist eticlopride had complex, differential impacts on reward seeking and adaptive behavior.

Chronic water insufficiency induced kidney damage and energy dysregulation despite reduced food intake, which improved gut microbiota in female rats.

Water intake is recommended for weight loss, but the relationship between water intake and energy metabolism is not clear. We hypothesized that long-term water insufficiency would influence energy, glucose, and lipid metabolism while modulating gut microbiota. Female rats were provided with high-fat diets with different amounts of water and food intake for 6weeks as follows: water provided for 1h per day with food ad libitum (WRFA), water supply ad libitum plus pair feeding of with water restricted rats(WAFR), water restriction with ad libitum food for 3weeks and water and food intake ad libitum for 3weeks (WR-WA) and ad libitum supply of water and food (WAFA). Water intake in WRFA was about one-third of WAFR and WAFA, whereas food intake was lowered by 30% in WRFA and WAFR than WAFA. Body fat decreased in WRFA and WAFR, but WAFR decreased fat mass more than WRFA. Energy expenditure was lower in WRFA than WAFA and carbohydrate utilization was much higher in WRFA than the other groups. The peak serum glucose concentrations were lower in WAFA than the other groups and WRFA lowered serum insulin levels more than WAFA during OGTT. WRFA shrank the glomerulus with increased apoptotic cells and damaged renal tubules compared to the WAFA and WAFR. WR-WA also exhibited greater glomerular shrinkage and apoptosis that WAFA, but not as much WRFA, indicating that the kidneys were healing after water restriction damage. WRFA exacerbated dyslipidemia compared to the WAFA and WAFR groups. The gut microbiome was similarly modulated in WRFA and WAFR, compared to WAFA, but it was mainly affected by food intake, not water restriction in the host. WRFA and WAFR increased Bacteroidetes and decreased Firmicutes compared WAFA. In conclusion, chronic insufficient water intake induced renal damage, decreased energy expenditure, and exacerbated dyslipidemia in rats with reduced food intake. However, the reduction of food intake improved gut microbiome regardless of insufficient water intake and only minor effects on the microbiome were observed due to water restriction.

Effects of 5-HT1A, 5-HT2A and 5-HT2C receptor agonists and antagonists on responding for a conditioned reinforcer and its enhancement by methylphenidate.

These experiments examined the effects of selective 5-HT1A, 5-HT2A and 5-HT2C receptor ligands on responding for a conditioned reinforcer (CRf). Effects of these ligands were measured under basal conditions and following elevated dopamine (DA) activity produced by the DA reuptake inhibitor methylphenidate. Water-restricted rats learned to associate a conditioned stimulus (CS) with water in operant chambers. Subsequently, two response levers were made available; responding on one lever delivered the CS (now a CRf), while responding on the second lever had no consequences. The effects of agonist and antagonists of 5-HT1A (8-hydroxy-2(di-n-propylamino)tetralin hydrobromide (8-OH-DPAT) and N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-2-pyridinylcyclohexanecarboxamide (WAY100635)), 5-HT2A (DOI and M100907) and 5-HT2C (Ro60-0175 and SB242084) receptors on responding were examined alone, as well as in the presence of methylphenidate. Responding for a CRf was reduced by the agonists 8-OH-DPAT, DOI and Ro60-0175. 8-OH-DPAT also reduced responding for water and seemed to impair responding in a non-specific fashion. None of the receptor antagonists affected responding. Methylphenidate dose-dependently enhanced responding for a CRf, and this was attenuated by DOI and Ro60-0175. Conversely, the 5-HT2C receptor antagonist SB242084 potentiated the effect of methylphenidate. No evidence was found for a behaviourally selective effect of 5-HT1A receptor ligands on responding for a CRf. Activation of 5-HT2A receptors selectively inhibits responding for a CRf. 5-HT2C receptor ligands exerted bidirectional modulation of responding for a CRf, especially when DA activity was increased. This indicates that 5-HT2C receptor activity is an important modulator of DA-dependent reward-related behaviours.

The role of dose and restriction state on morphine-, cocaine-, and LiCl-induced suppression of saccharin intake: A comprehensive analysis

Rats avoid intake of a taste cue when paired with a drug of abuse or with the illness-inducing agent, lithium chloride (LiCl). Although progress has been made, it is difficult to compare the suppressive effects of abused agents and LiCl on intake of a gustatory conditioned stimulus (CS) because of the cross-laboratory use of different CSs, different unconditioned stimuli (USs), and different doses of the drugs, different conditioning regimens, and different restriction states. Here we have attempted to unify these variables by comparing the suppressive effects of a range of doses of morphine, cocaine, and LiCl on intake of a saccharin CS using a common regimen in non-restricted, food restricted, or water restricted male Sprague-Dawley rats. The results showed that, while the putatively aversive agent, LiCl, was effective in suppressing intake of the taste cue across nearly all doses, regardless of restriction state, the suppressive effects of both morphine and cocaine were greatly reduced when evaluated in either food or water restricted rats. Greater sensitivity to drug was revealed, at very low doses, when testing occurred in the absence of need (i.e., when the rats were non-restricted). Together, these results provide the first uniform and comprehensive analysis of the suppressive effects of morphine, cocaine, and LiCl as a function of dose and restriction state. In the present case, the suppressive effects of morphine and cocaine are found to differ from those of LiCl and, in some respects, from one another as well.

Responding for a conditioned reinforcer, and its enhancement by nicotine, is blocked by dopamine receptor antagonists and a 5-HT2C receptor agonist but not by a 5-HT2A receptor antagonist

An aspect of nicotine reinforcement that may contribute to tobacco addiction is the effect of nicotine to enhance the motivational properties of reward-associated cues, or conditioned stimuli (CSs). Several studies have now shown that nicotine enhances responding for a stimulus that has been paired with a natural reinforcer. This effect of nicotine to enhance responding for a conditioned reinforcer is likely due to nicotine-induced enhancements in mesolimbic dopaminergic activity, but this has not been directly assessed. In this study, we assessed roles for dopamine (DA) D1 or D2 receptors, and two serotonin (5-HT) receptor subtypes known to modulate DA activity, the 5-HT2C or 5-HT2A subtypes, on nicotine-enhanced responding for a conditioned reinforcer. Water-restricted rats were exposed to Pavlovian conditioning sessions, where a CS was paired with water delivery. Then, in a second phase, animals were required to perform a novel, lever-pressing response for presentations of the CS as a conditioned reinforcer. Nicotine (0.4mg/kg) enhanced responding for the conditioned reinforcer. To examine potential roles for dopamine (DA) and serotonin (5-HT) receptors in this effect, separate groups of animals were used to assess the impact of administering the D1 receptor antagonist SCH 23390, D2 receptor antagonist eticlopride, 5-HT2C receptor agonist Ro 60-0175, or 5-HT2A receptor antagonist M100907 on nicotine-enhanced responding for conditioned reinforcement. SCH 23390, eticlopride, and Ro 60-0175 all reduced responding for conditioned reinforcement, and the ability of nicotine to enhance this effect. M100907 did not alter this behavior. Together, these studies indicate that DA D1 and D2 receptors, but not 5-HT2A receptors, contribute to the effect of nicotine to enhance responding for a conditioned reinforcer. This effect can also be modulated by 5-HT2C receptor activation.

Pharmacological activation of kappa opioid receptors: aversive effects in adolescent and adult male rats

The dynorphin (DYN)/kappa opioid receptor (KOR) system is involved in the dysphoric properties of drugs of abuse. Given that adolescents show reduced sensitivity to aversive effects of many drugs, alterations in the DYN/KOR system may contribute to the prevalence of drug use during adolescence. The present study was designed to assess dysphoric properties of a selective kappa agonist, U62,066, in adolescent and adult rats using both conditioned taste aversion (CTA) and conditioned place aversion (CPA) paradigms. For CTA, water-restricted rats were administered U62,066 following 30 min access to a saccharin solution, with subsequent saccharin consumption used to index aversion. For CPA, animals were allowed access to both compartments of a two-compartment chamber for a 15-min pre- and post-conditioning test. For conditioning, subjects were administered U62,066 prior to confinement to one side of the chamber and saline prior to confinement to the other side for a total of four pairings. Overall, adolescents displayed reduced sensitivity to the kappa agonist relative to adults. Adults demonstrated taste aversions to the 0.2 and 0.3 mg/kg doses of U62,066, whereas adolescents did not display aversions to any tested doses. Adults demonstrated a place aversion to the 0.1 and 0.2 mg/kg dose of U62,066 when paired with the preferred side of the conditioning chamber. Adolescents did not display aversions to any of the doses tested. Reduced sensitivity to DYN/KOR system activation during adolescence may be a contributing factor to the age-typical insensitivity to aversive properties of drugs commonly abused by adolescents.

Gene expression in the thin limbs of Henle of the waterrestricted rat

Sodium and water homeostasis and the formation of concentrated urine depend upon the generation of a steep osmotic gradient within the renal medulla. Although the mechanism of this process remains controversial, the descending thin limbs (DTLs) and ascending thin limbs (ATLs) of the loops of Henle play important roles. Our goal was to identify genes that may be involved in the urine concentrating mechanism and to quantify their transcript levels in the DTLs and ATLs. Using real‐time PCR, we analyzed transcripts in isolated DTLs and ATLs of water‐restricted rats. Claudin 4 (CLDN4), sodium‐dependent glucose transporter 1 (NaGLT1), sodium glucose cotransporter 1 (SGLT1), and sodium myo‐inositol cotransporter 1 (SMIT1) were present in both types of tubules and their levels increased in the ATLs after 72h of water restriction. Chloride channel (Clcnka) mRNA in the ATLs and urea transporter mRNA (UT‐A2) in the DTLs increased. We also identified a novel variant of SGLT1 and four variants of the arginine vasopressin receptor 2 (AVPR2). Our results indicate that in addition to the previously characterized aquaporins and urea transporters, several other genes and their products may be involved in the movement of ions and solutes in the thin limbs of Henle. Further studies will determine how these genes contribute to urine concentration. NIDDK DK083338

A Cholinergic Mechanism for Reward Timing within Primary Visual Cortex

Neurons in rodent primary visual cortex (V1) relate operantly conditioned stimulus-reward intervals with modulated patterns of spiking output, but little is known about the locus or mechanism of this plasticity. Here we show that cholinergic basal forebrain projections to V1 are necessary for the neural acquisition, but not the expression, of reward timing in the visual cortex of awake, behaving animals. We then mimic reward timing invitro by pairing white matter stimulation with muscarinic receptor activation at a fixed interval and show that this protocol results in the prolongation of electrically evoked spike train durations out to the conditioned interval. Together, these data suggest that V1 possesses the circuitry and plasticity to support reward time prediction learning and the cholinergic system serves as an important reinforcement signal which, invivo, conveys to the cortex the outcome of behavior.

Multiple Processes Underlie Benzodiazepine-Mediated Increases in the Consumption of Accepted and Avoided Stimuli

Hyperphagia is a reported side effect of anxiolytic benzodiazepines such as chlordiazepoxide (CDP). Prior research has focused primarily on the ingestive responses to sweet or solid foods. We examined CDP effects on licking for normally accepted and avoided taste solutions across a range of concentrations. The effect of CDP (10 mg/kg) versus saline on the licking patterns of water-restricted rats for water and 3 concentrations of sucrose, saccharin, NaCl, monosodium glutamate (MSG), citric acid, and quinine (Q-HCl) solutions was evaluated during 1 h tests. CDP increased meal size for all tastants except citric acid. Analysis of licking microstructure revealed 3 dissociable effects of CDP. CDP affected oromotor coordination as indicated by a uniform increase in the modal interlick interval for all stimuli. CDP increased meal size as indicated by shorter pauses during consumption of water, MSG, and weaker saccharin concentrations, and by fewer long interlick intervals (250-2000 ms) for normally avoided tastants. CDP also increased meal size by increasing burst size, burst duration, and the initial rate of licking for most solutions, suggesting increased hedonic taste evaluation. CDP did not affect variables associated with postingestive feedback such as meal duration or number of bursts, and the results also suggest that CDP did not enhance the perceived taste intensity. We hypothesize that the reduction of pause duration is consistent with an increased motivation to sample the stimulus that synergizes with changes in taste-mediated responsiveness to some but not all stimuli to yield increases in the consumption of both normally accepted and avoided taste stimuli.

Water Restriction and Fluid Temperature Alter Preference for Water and Sucrose Solutions

The role of diet temperature in ingestive behavior is poorly understood. We examined the importance of stimulus temperature and water-restriction state on the preference for and intake of water and sucrose. Using custom-designed equipment that allows us to monitor and maintain solution temperatures during testing (±0.1 °C), we conducted a series of 2-bottle preference tests (10 °C water vs. sucrose 10-40 °C) and brief access tests (10-40 °C water and sucrose). Water-restricted rats preferred cold water over any sucrose concentration (0.0-1.0 M) if the sucrose was 30 or 40 °C, whereas the same rats preferred sucrose at all concentrations and temperatures when unrestricted suggesting that the water-restriction state interacts with temperature preference. In a series of brief-access tests using a Davis Rig (MS-180), rats reduced licking to cold sucrose compared with 20 °C sucrose, suggesting that unlike water, cold temperature reduced the palatability of sucrose.

Flavor preferences conditioned by post-oral infusion of monosodium glutamate in rats

Monosodium glutamate (MSG), the prototypical umami source, can enhance preference for associated flavors in humans and rodents. Although MSG flavor preference has been attributed to its taste, vagally-mediated post-oral detection has also been demonstrated. Recent studies showed that water-restricted rats acquired a preference for a flavor paired with intragastric (IG) infusion of 60mM MSG in rats. The present study extends this work by comparing MSG-based flavor conditioning in water- and food-restricted rats and testing the persistence of flavor preferences. Rats with IG catheters drank flavored solutions paired with volume-matched infusions of 60mM MSG or water in daily 30-min sessions. Two training/test cycles were conducted, each with eight one-bottle training sessions followed by two two-bottle preference tests without infusions. Food- and water-restricted groups displayed similar preferences for the MSG-paired flavor. When non-reinforced testing was continued after the second cycle, the food-restricted group sustained its preference across three 2-day tests, but water-restricted rats lost their preference. Other food-restricted rats learned to prefer a flavor paired with intraduodenal infusion, indicating that gastric stimulation by MSG is not required. A third experiment showed that adding 2mM of the nucleotide inosine monophosphate to the IG infusion of MSG did not significantly enhance flavor conditioning. Because MSG-based flavor preferences can be obtained with infusions that bypass the stomach, the site for detecting MSG reinforcement may be intestinal.

Flavor preferences conditioned by post-oral infusion of monosodium glutamate in rats

Monosodium glutamate (MSG), the prototypical umami source, can enhance preference for associated flavors in humans and rodents. Although MSG flavor preference has been attributed to its taste, vagally-mediated post-oral detection has also been demonstrated. Recent studies showed that water-restricted rats acquired a preference for a flavor paired with intragastric (IG) infusion of 60 mM MSG in rats. The present study extends this work by comparing MSG-based flavor conditioning in water- and food-restricted rats and testing the persistence of flavor preferences. Rats with IG catheters drank flavored solutions paired with volume-matched infusions of 60 mM MSG or water in daily 30-min sessions. Two training/test cycles were conducted, each with eight one-bottle training sessions followed by two two-bottle preference tests without infusions. Food- and water-restricted groups displayed similar preferences for the MSG-paired flavor. When non-reinforced testing was continued after the second cycle, the food-restricted group sustained its preference across three 2-day tests, but water-restricted rats lost their preference. Other food-restricted rats learned to prefer a flavor paired with intraduodenal infusion, indicating that gastric stimulation by MSG is not required. A third experiment showed that adding 2 mM of the nucleotide inosine monophosphate to the IG infusion of MSG did not significantly enhance flavor conditioning. Because MSG-based flavor preferences can be obtained with infusions that bypass the stomach, the site for detecting MSG reinforcement may be intestinal.

Dehydration-induced increase in aquaporin-2 protein abundance is blocked by nonsteroidal anti-inflammatory drugs

It is now well established that the antidiuretic response to vasopressin is modulated by changes in aquaporin-2 (AQP2) expression in response to hydration status. While vasopressin itself is one signal driving expression, other signals also play a part. In this study, we planned to investigate whether prostaglandins, known to modulate AQP2 trafficking, may play a role in this process. Male Wistar rats were kept in metabolic cages, with either free access to water and food, or were given 15 g of food gelled with water, such that they were fluid restricted or fluid loaded. The effects of oral administration of two structurally different NSAIDs, indomethacin and ibuprofen, and a COX-2-selective NSAID, meloxicam, on urine output and AQP2 expression were investigated in kidneys removed under terminal anesthesia. All the NSAIDs decreased AQP2 expression significantly in water-restricted rats but did not significantly alter PGE excretion. In water-loaded rats, the effects were less marked, and meloxicam had no significant effect. Consistent with this, ibuprofen prevented the increase in AQP2 expression seen in response to dehydration. These results demonstrate that NSAIDs decrease AQP2 protein abundance, particularly during adaptation during dehydration. This may be of particular significance in older and critically ill patients, who are prone to dehydration.

Subdiaphragmatic Vagotomy Prevents Drinking-Induced Reduction in Plasma Corticosterone in Water-Restricted Rats

Dehydrated rats exhibit a rapid inhibition of the hypothalamic-pituitary-adrenal axis after rehydration. Drinking activates vagal afferents that project to neurons in the nucleus tractus solitarius (NTS). We hypothesized that when dehydrated rats drink, vagal afferents stimulate NTS neurons initiating inhibition of hypothalamic-pituitary-adrenal activity. Experiments assessed NTS activity by measuring Fos expression. Rats were water restricted for 1 or 6 d, limiting access to water to 30 min/d in the morning. Drinking after single or repeated restriction increased Fos, demonstrating increased NTS activity. We next examined the contribution of the vagus by comparing hormonal responses after total subdiaphragmatic vagotomy or sham surgery. Water restriction for 6 d increased plasma arginine vasopressin (AVP), ACTH, and adrenal and plasma corticosterone in both groups. In sham rats, drinking reduced plasma AVP, ACTH, adrenal and plasma corticosterone by 7.5 min. In total subdiaphragmatic vagotomy rats, whereas drinking reduced plasma AVP, ACTH, and adrenal corticosterone, drinking did not reduce plasma corticosterone. To identify the source of vagal activity, hormonal responses to restriction-induced drinking were measured after common hepatic branch vagotomy (HBV). Although pituitary hormonal responses were not affected by HBV, the adrenal and plasma corticosterone responses to water restriction were reduced; in addition, drinking in HBV rats decreased adrenal corticosterone without changing plasma corticosterone. These data indicate that an intact vagus is necessary to reduce plasma corticosterone when water-restricted rats drink and that the common hepatic vagal branch contributes to the response. These findings implicate the vagus in augmenting rapid removal of circulating corticosterone during relief from stress.

Central G-alpha subunit protein-mediated control of cardiovascular function, urine output, and vasopressin secretion in conscious Sprague-Dawley rats.

The role(s) of central Galpha-proteins in the regulation of cardiovascular and renal function is unknown. We examined how inhibition/downregulation of central Galphai/Galphao, Galphaz or Galphaq proteins altered the characteristic cardiovascular (depressor), renal excretory (diuretic), and plasma AVP (inhibitory) responses to intracerebroventricular injection of nociceptin/orphanin FQ (N/OFQ) in rats. Before investigation, rats were pretreated intracerebroventricularly with saline vehicle (5 microl, 48 h, n=6), pertussis toxin (PTX; 48-h, 1 microg, n=6), or Galphaz, Galphaq, or scrambled oligodeoxynucleotide (ODN) (25 microg, 24 h, n=6 per group). On the study day, intracerebroventricular N/OFQ (5.5 nmol) or vehicle (5 microl) was injected into pretreated conscious rats. Mean arterial pressure (MAP) and heart rate (HR) were recorded, and urine was collected for 90 min. In vehicle or scrambled ODN groups, intracerebroventricular N/OFQ decreased MAP and HR and produced water diuresis (sensitive to UFP-101, N/OFQ receptor antagonist). The hypotension and bradycardia, but not diuresis, to N/OFQ were abolished in PTX-pretreated rats. In contrast, intracerebroventricular ODN pretreatment markedly blunted (Galphaz) or augmented (Galphaq) the diuresis to intracerebroventricular N/OFQ. In separate studies, the action of central N/OFQ to decrease plasma AVP levels in naïve water-restricted rats was differentially altered by intracerebroventricular Galphaz ODN (blunted) and Galphaq ODN (augmented) pretreatment. These studies demonstrate central Galphai/Galphao activity mediates intracerebroventricular N/OFQ's cardiovascular depressor function. Alternatively, central Galphaz (inhibitory) and Galphaq (stimulatory) activity differentially modulates AVP release to control the pattern of diuresis to intracerebroventricular N/OFQ. These findings highlight the novel selective central Galpha-subunit protein-mediated control of cardiovascular vs. renal excretory function.

Application of an operant generalization paradigm to characterize the perceptual quality of common taste compounds in a rat model.

To circumvent limitations associated with the conditioned taste aversion generalization paradigm, we previously developed an operant taste generalization procedure to make inferences on the taste quality of chemical compounds in rodent models. In this procedure, 4 groups ( n =6) of water-restricted rats were trained in a two-response operant discrimination task to make one response after sampling an assigned standard taste compound (various concentrations of either NaCl, sucrose, quinine hydrochloride, or citric acid) or a different response if the taste compound was any of the remaining 3 comparison stimuli. Correct responses were reinforced with access to water and errors were punished with a time-out. Here, novel test compounds were presented to these trained animals without a programmed consequence and the degree of generalized responding to the standard versus comparison stimuli was quantified. Results indicated that concentrations of sodium gluconate (0.376 M and 0.668 M) were treated predominantly as NaCl-like; maltose (0.421 M) and fructose (0.421 M) were treated predominantly as sucrose-like; denatonium (0.131 and 0.360 mM) was treated predominantly as quinine-like; and tartaric acid (10.4 and 28.2 mM) was treated predominantly as citric acid-like. Monosodium glutamate produced a profile suggesting a sucrose-like and sodium-like quality, whereas KCl produced a complex profile that was predominantly represented by quinine-like, citric acid-like, and NaCl-like qualities. Water and lower concentrations of maltose and fructose had a significant quinine-like component suggesting the presence of a weak “bitter” taste. These findings demonstrate the utility of this procedure in assessing how rodents perceptually categorize taste compounds. Supported by NIH Grants F31-DC007301 [CLG] and R01-DC01628 [ACS].

Obestatin Inhibits Vasopressin Release and Thirst

We have previously reported that obestatin, a product of post-translational processing of the ghrelin prohormone, significantly inhibits thirst in a variety of animal models (AJP 292:, 2007). To further investigate potential CNS actions of obestatin on fluid and electrolyte homeostasis, we examined the effects of i.c.v. administration of the peptide on salt appetite and vasopressin (AVP) secretion. Saline drinking in response to polyethylene glycol-induced hypovolemia was not altered by obestatin in a dose that did significantly reduced water drinking in this and previously reported models. On the other hand, i.c.v. administration of obestatin in overnight water restricted rats significantly reduced AVP levels in peripheral plasma in a time and dose-dependent fashion. Our previous patch clamp recordings have shown that subfornical organ neurons respond to the direct application of obestatin, and thus the actions of the peptide may occur as a consequence of actions at circumventricular organs and not the hypothalamus. These results further our hypothesis that obestatin acts within the CNS to modulate the behavioral and endocrine mechanisms controlling fluid balance.