Morphine enhancement of sucrose palatability: analysis by the taste reactivity test

The ability of morphine to modify sucrose palatability was assessed by the taste reactivity test. In Experiment 1, rats were injected with morphine (0.0, 0.5, 2.0, and 10.0 mg/kg, subcutaneously), 30 min before receiving a 10-min intraoral infusion of 2% or 20% sucrose solution. A dose of 2.0 mg/kg morphine enhanced ingestive reactions elicited by both concentrations of sucrose solution. In Experiment 2, the interval between morphine pretreatment and the taste reactivity test was manipulated. Rats given 2.0 mg/kg morphine 30 or 120 min before testing displayed enhanced ingestive reactions elicited by 20% sucrose solution during the first 5 min of a 10-min test. The results support the hypothesis that morphine enhances the hedonic assessment of sucrose solution.     

What is the role of dopamine in reward: hedonic impact, reward learning, or incentive salience?

What roles do mesolimbic and neostriatal dopamine systems play in reward? Do they mediate the hedonic impact of rewarding stimuli? Do they mediate hedonic reward learning and associative prediction? Our review of the literature, together with results of a new study of residual reward capacity after dopamine depletion, indicates the answer to both questions is 'no'. Rather, dopamine systems may mediate the incentive salience of rewards, modulating their motivational value in a manner separable from hedonia and reward learning. In a study of the consequences of dopamine loss, rats were depleted of dopamine in the nucleus accumbens and neostriatum by up to 99% using 6-hydroxydopamine. In a series of experiments, we applied the 'taste reactivity' measure of affective reactions (gapes, etc.) to assess the capacity of dopamine-depleted rats for: 1) normal affect (hedonic and aversive reactions), 2) modulation of hedonic affect by associative learning (taste aversion conditioning), and 3) hedonic enhancement of affect by non-dopaminergic pharmacological manipulation of palatability (benzodiazepine administration). We found normal hedonic reaction patterns to sucrose vs. quinine, normal learning of new hedonic stimulus values (a change in palatability based on predictive relations), and normal pharmacological hedonic enhancement of palatability. We discuss these results in the context of hypotheses and data concerning the role of dopamine in reward. We review neurochemical, electrophysiological, and other behavioral evidence. We conclude that dopamine systems are not needed either to mediate the hedonic pleasure of reinforcers or to mediate predictive associations involved in hedonic reward learning. We conclude instead that dopamine may be more important to incentive salience attributions to the neural representations of reward-related stimuli. Incentive salience, we suggest, is a distinct component of motivation and reward. In other words, dopamine systems are necessary for 'wanting' incentives, but not for 'liking' them or for learning new 'likes' and 'dislikes'.     

Taste avoidance, but not aversion, learning in rats lacking gustatory cortex.

Control rats rapidly learned to avoid drinking either a sucrose solution (Exp 1) or an NaCl solution (Exp 2) when the taste was paired with illness. These rats also produced aversive reactivity to each of these solutions in a taste reactivity test. Rats that lacked gustatory cortex (GC) learned to avoid drinking sucrose and NaCl, albeit at a slower rate than control rats. GC rats failed to display aversive reactivity to these tastes. The GC rats did show normal aversive reactivity to a strong quinine HCl solution during additional tests. It is suggested that the avoidance developed by GC rats did not entail a palatability shift of the conditional stimulus as it did in control rats. This altered learning strategy may account for the consistent learning deficits found in GC rats trained to avoid tastes. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Behavioral responses of the chronically instrumented sheep fetus to chemosensory stimuli presented in utero.

Fetal sheep were surgically prepared on Days 113–114 of gestation with an array of chronic instruments for recording electromyographic data (EMG) in oral-facial, axial, and limb muscles and heart rate (FHR). Fetuses also were fitted with an intraoral catheter for infusion of chemosensory fluids (isotonic saline, quinine, colostrum, sucrose) onto the surface of the tongue. Individual subjects received chemosensory infusions on Days 134–137. Fetuses showed consistent oral responses to quinine and milk, but did not respond to isotonic saline or sucrose. Different patterns of motor responses suggested that fetuses discriminated among different concentrations of quinine. The expression of tachycardia to quinine and bradycardia to milk also suggested differential responding to chemosensory fluids that differ in hedonic qualities Detailed characterization of fetal responses to these stimuli in utero confirm the functionality of the gustatory system in the sheep fetus near term. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Control of fluid palatability by exteroceptive Pavlovian signals.

Two experiments, with 15 male albino rats, investigated whether discrete auditory conditioned stimulus/stimuli (CS) that signal the availability or onset of taste unconditioned stimulus/stimuli (UCS) (sucrose, quinine) can control orofacial responses in the absence of those UCSs. In Exp I, one auditory stimulus (CS+) was paired with the delivery of a sucrose solution to the magazine floor, and another auditory stimulus (CS–) was never followed by sucrose. Following conditioning, oral infusions of water that were preceded by the CS+ were found to elicit more ingestive (sucrose-typical) orofacial responses than did water alone or water preceded by the CS–. In Exp II, the conditioned ingestive reactions to a signal for sucrose observed in Exp I again occurred, and conditioned aversive (quinine-typical) orofacial responses occurred in response to water infusions preceded by a former signal for quinine. Data suggest that perceived palatability may be influenced by Pavlovian associations involving exteroceptive CSs and illustrate the importance of supporting stimuli in modulating the effects of Pavlovian associations on behavior. (41 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Brainstem mediates diazepam enhancement of palatability and feeding: microinjections into fourth ventricle versus lateral ventricle

The hypothesis that benzodiazepine-induced hyperphagia is due to a specific enhancement of the palatability of foods has been supported by previous 'taste reactivity' studies of affective (hedonic and aversive) reactions to taste palatability. Diazepam and chlordiazepoxide enhance hedonic reactions of rats (rhythmic tongue protrusions, etc.) to sweet tastes in a receptor-specific fashion. A role for brainstem circuits has been indicated by a previous demonstration of the persistence of the taste reactivity enhancement by diazepam after midbrain decerebration. The present study examined whether benzodiazepine brainstem receptors are the chief substrates for palatability enhancement even in intact brains. We compared the effectiveness of benzodiazepine microinjections to elicit feeding and enhance hedonic reactions when delivered into either the lateral ventricle (forebrain) or the fourth ventricle (brainstem) of rats. The results show diazepam is reliably more effective at eliciting feeding and enhancing positive hedonic reactions to oral sucrose when microinjections are made in the fourth ventricle than in the lateral ventricle. We conclude that brainstem neural systems containing benzodiazepine-GABA receptors are likely to be the chief substrates for benzodiazepine-induced palatability enhancement.     

The ontogeny of feeding in rats: IV. Taste development as measured by intake and behavioral responses to oral infusions of sucrose and quinine.

Delivered infusions of sucrose or quinine through cannulas implanted in Charles River CD rat pups' mouths. Intake was measured, and behavioral responses were scored. Responsiveness to sweet and bitter tastes emerged over the 1st 2 wks of postnatal life. Pups showed discrimination between water and sucrose in their mouthing behavior and general activity from 3 days of age, but in their intake only from 6 days of age. Discrimination between water and quinine was not shown until 9 days of age in either behavior or intake. Even then, the stereotypic aversion reactions (paw treading, chin scraping) that characterize the adult response to quinine were not apparent until 12 days. By 15 days of age, preferences for sucrose and aversions to quinine were robust, resembled those of adult rats, and did not depend on previous experience with either solution. (31 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Amount of training and cue-evoked taste-reactivity responding in reinforcer devaluation.

In two experiments, rats received minimal (16) pairings of one auditory conditioned stimulus (CS) cue with a sucrose reinforcer, and extensive (112) pairings of another auditory CS with that reinforcer. After sucrose was devalued by pairing it with lithium chloride in some rats (Devalue groups) but not others (Maintain groups), taste reactivity (TR) and other responses to unflavored water were assessed in the presence of the auditory CSs alone. The minimally trained CS controlled substantially more evaluative TR responses than the extensively trained CS. Those TR responses were hedonic (positive) in the Maintain groups, but aversive (negative) in the Devalue groups. By contrast, food cup entry and other responses thought not to reflect evaluative taste processing were controlled more by the extensively trained cue. These responses were reduced by sucrose devaluation comparably, regardless of the amount of training. The results suggest rapid changes in the content of learning as conditioning proceeds. Early in training, CSs may be capable of activating preevaluative processing of an absent food reinforcer that includes information about its palatability, but that capability is lost as training proceeds. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Hunger alters the expression of acquired hedonic but not sensory qualities of food-paired odors in humans.

To test whether expression of hedonic and sensory odor qualities acquired by association with sweet and bitter tastes depend on hunger state, hungry volunteers experienced odors paired with sucrose, quinine, or water and then were tested under different hunger states manipulated with energy preloads. Acquired liking for sucrose-paired odors was evident following a low-energy or control preload but not a high-energy preload; however, odor sweetness increased in all preload conditions. Acquired dislike and increased bitterness of quinine-paired odors were independent of preloading. These data demonstrate hunger-dependent expression of acquired liking for flavors through flavor-flavor associations in humans and demonstrate independence between acquired hedonic and sensory qualities of odors. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Effect of the growth rate of Pseudomonas aeruginosa biofilms on the susceptibility to antimicrobial agents: beta-lactams and fluoroquinolones

The combined effects of meal cost and food flavor on meal size were studied with a method that avoided the covariation of nutrient composition and caloric density with palatability. As rats (Rattus norvegicus) drank flavored fluids (unpalatable 0.05% sucrose octaacetate [SOA], neutral 0.05% saccharin, and palatable 2% Polycose + 0.2% saccharin [P + S]), liquid diet was infused intragastrically. Relative to saccharin, rats with free access ate 10% more calories in larger meals while consuming P + S and initially ate fewer calories in smaller but more frequent meals while drinking SOA. Other rats lever-pressed to begin meals, which halved meal number and doubled meal size relative to the free-access group. Although foraging rats also ate larger P + S meals and smaller SOA meals, the changes did not affect total intake. Without the usual differential postingestive effects of foods that differ in palatability, making food more costly blunts rats' response to its flavor.     

Le r?le du chlorhydrate de quinine dans l'inhibition conditionnée du réflexe tarsal chez Drosophila melanogaster.

The proboscis extension reflex (PER) can be elicited by applying a sucrose stimulation to the tarsus of a walking fly. This reflex decreases in frequency with repetition, presumably habituation, a nonassociative learning. If each sucrose stimulation is followed by a bitter stimulation, quinine chloride, the PER declines more rapidly, probably the result of conditioning, an associative learning. The present work shows that quinine chloride does not always inhibit PER suppressions but depends on the moment of delivery, being most effective when presented immediately after a sucrose stimulation. A bitter stimulus presented before, or simultaneously with a sucrose stimulation is less effective than habituation to sucrose alone. This experiment provides evidence for an interpretation in terms of cognitive association. The model of learning is not a Pavlovian conditioning as advanced by Médioni and Vaysse (1975), but corresponds to the punishment paradigm of Dyal and Corning (1973). (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Palatability and foraging cost interact to control caloric intake.

The combined effects of meal cost and food flavor on meal size were studied with a method that avoided the covariation of nutrient composition and caloric density with palatability. As rats (Rattus norvegicus) drank flavored fluids (unpalatable 0.05% sucrose octaacetate [SOA], neutral 0.05% saccharin, and palatable 2% Polycose?+?0.2% saccharin [P?+?S]), liquid diet was infused intragastrically. Relative to saccharin, rats with free access ate 10% more calories in larger meals while consuming P?+?S and initially ate fewer calories in smaller but more frequent meals while drinking SOA. Other rats lever-pressed to begin meals, which halved meal number and doubled meal size relative to the free-access group. Although foraging rats also ate larger P?+?S meals and smaller SOA meals, the changes did not affect total intake. Without the usual differential postingestive effects of foods that differ in palatability, making food more costly blunts rats' response to its flavor. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Effects of the benzodiazepine receptor inverse agonist Ro 15-4513 on the ingestion of sucrose and sodium saccharin solutions: A microstructural analysis of licking behavior.

The effects of the benzodiazepine receptor partial inverse agonist Ro 15-4513 on consumption of either a 1% sucrose solution or a 0.1% sodium saccharin solution in nondeprived male rats was examined. A video-recording approach was adopted in which licks were counted in a frame-by-frame analysis. Ro 15-4513 (1–10 mg/kg) caused a significant decrease in the intake of both sucrose and saccharin solutions that was associated with a reduction in the initial rate of licking. There was a decrease in the total duration of drinking, total licks, and number of bouts for both sucrose and saccharin. For sucrose, mean bout duration was significantly reduced, although this was not so for saccharin. Intrabout lick rate, the latency to engage in drinking, and the postdrinking time were not affected for either sucrose or saccharin. These data are consistent with previous evidence that strongly suggests that benzodiazepines influence palatability. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Hedonic and sensory characteristics of odors conditioned by pairing with tastants in humans.

Animals readily acquire positive odor-taste hedonic associations, but evidence for this in humans remains weak and was explored further. Retronasal pairing of odors with sucrose or salty stimuli (Experiment 1) increased the rated sweetness of sucrose-paired odors without altering liking, although changes in odor pleasantness correlated with sucrose liking. Experience of odors with sucrose or quinine by sweet likers (Experiment 2) found increased pleasantness and sweetness for sucrose-paired odors, whereas quinine-paired odors became less liked and more bitter. Odor-sucrose pairings in sweet likers and dislikers (Experiment 3) found increased sweetness in both groups but increased odor liking only in likers. These data suggest that evaluative and sensory learning are dissociable and that evaluative changes are sensitive to individual differences in sweet liking. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Persistence of Preference for a Flavor Presented in Simultaneous Compound With Sucrose.

Rats exposed to a simultaneous compound of a flavor and sucrose subsequently exhibited a preference for the flavor over water. This preference persisted across repeated testing even though the flavor was presented in the absence of sucrose. The preference did, however, extinguish if the rats were hungry when trained or tested, or if they had been reexposed to sucrose between training and test. Though failing to extinguish the preference, presentation of the flavor outside the compound protected it from the effects of sucrose devaluation, indicating that these presentations extinguished the within-compound association between the flavor and sucrose. The authors conclude that the hedonic reaction elicited by sucrose imbues the flavor with the same hedonic properties, and these properties maintain the preference independently of the flavor-sucrose association. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Alternating ingestive and aversive consummatory responses suggest a two-dimensional analysis of palatability in rats.

Examined the hedonic response to a taste which is usually regarded as the product of a central integration of gustatory afferent information that ends in a single decision about the nature and intensity of the response to be given. The response is often characterized as a point lying along a single dimension of palatability, stretching from strongly positive to strongly negative. Two experiments dealing with consummatory responses in male Sprague-Dawley rats suggested that the final response is not made on the basis of a single central analysis of taste information, but rather is the result of a competition between 2 separate systems that are activated by tastes. A single oral infusion of a taste solution may elicit rapid alternation between ingestive and aversive consummatory responses. Such alternation is better interpreted as due to a simultaneous activation of 2 palatability dimensions than as a reflection of neutral palatability. When increases in the magnitude of aversive responses are produced by taste mixtures, there is not necessarily a reciprocal decrease in ingestive responses. This asymmetry supports the hypothesis of independent palatability dimensions. (26 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Comparison of ovarian and hypothalamic obesity syndromes in the female rat: Effects of diet palability on food intake and body weight.

Results of 2 experiments with 96 CFE female rats show that both ovariectomy and hypothalamic knife cuts produced hyperphagia and obesity in Ss. The ovarian obesity, however, unlike hypothalamic obesity, was virtually independent of diet palatability. Ovariectomized Ss became obese on quinine-adulterated diets which completely blocked hypothalamic obesity, and they displayed little further weight gain when given a high-fat diet which greatly potentiated hypothalamic obesity. Ovarian and hypothalamic obesity were also found to be additive irrespective of diet condition when both surgical treatments were combined in the same S; that is, ovariectomy increased the food intake and body weight of knife-cut Ss given the quinine or high-fat diet. In contrast to their dissimilar feeding effects, ovariectomy, hypothalamic cuts, and the combined surgeries did not differentially alter the aversion to a 0.01% quinine solution. Results indicate that ovarian obesity and hypothalamic obesity represent 2 different feeding disorders and are mediated by separate neural mechanisms. The functional nature of these disorders is discussed in light of recent body weight set point interpretations. (40 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

The role of quinine chlorhydrate in the conditioned inhibition of the tarsal reflex in Drosophila melanogaster

The proboscis extension reflex (PER) can be elicited by applying a sucrose stimulation to the tarsus of a walking fly. This reflex decreases in frequency with repetition, presumably habituation, a nonassociative learning. If each sucrose stimulation is followed by a bitter stimulation, quinine chloride, the PER declines more rapidly, probably the result of conditioning, an associative learning. The present work shows that quinine chloride does not always inhibit PER suppressions but depends on the moment of delivery, being most effective when presented immediately after a sucrose stimulation. A bitter stimulus presented before, or simultaneously with a sucrose stimulation is less effective than habituation to sucrose alone. This experiment provides evidence for an interpretation in terms of cognitive association. The model of learning is not a Pavlovian conditioning as advanced by Medioni and Vaysse (1975), but corresponds to the punishment paradigm of Dyal and Corning (1973).     

Temporal and Qualitative Dynamics of Conditioned Taste Aversion Processing: Combined Generalization Testing and Licking Microstructure Analysis.

The pattern of licking microstructure during various phases of a conditioned taste aversion (CTA) was evaluated. In Experiment 1, rats ingested lithium chloride (LiCl) for 3 trials and were then offered sodium chloride (NaCl) or sucrose on 3 trials. A CTA to LiCl developed and generalized to NaCl but not to sucrose. CTA intake suppression was characterized by reductions in burst size, average ingestion rate, and intraburst lick rate, and increases in brief pauses and burst counts. Compared with previous studies, LiCl licking shifted from a pattern initially matching that for normally accepted NaCl to one matching licking for normally avoided quinine hydrochloride by the end of the 1st acquisition trial. In Experiment 2, a novel paradigm was developed to show that rats expressed CTA generalization within 9 min of their first LiCl access. These results suggest that licking microstructure analysis can be used to assay changes in hedonic evaluation caused by treatments that produce aversive states. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Increased liking for a solution is not necessary for the attenuation of neophobia in rats.

Recent evidence suggests that liking and wanting of food rewards can be experimentally dissociated (e.g., Berridge, 1996); this dissociation extends to attenuated neophobia in the present study. Rats tend to eat less of a novel food than a familiar food, a phenomenon called neophobia. The present experiments evaluated whether attenuation of neophobia by prior exposure reflects enhanced liking of the flavor using the Taste Reactivity (TR) test. In Experiment 1, rats given five 10-s TR trials with water or various concentrations of saccharin solution (0.1%, 0.2%, 0.5%) did not show a change in the number of hedonic reactions displayed across trials. However, in a subsequent consumption test from a bottle containing 0.25% saccharin solution, rats with no prior saccharin exposure (group water) consumed less than rats with prior saccharin exposure; that is they displayed neophobia. In Experiment 2, whether rats received five 10-s TR trials with water or 0.5% saccharin solution, they did not display a difference in hedonic reactions to 0.25% saccharin solution in two 5-min TR test trials. These results suggest that the attenuation of neophobia is evidenced as an increase in the tendency to approach a bottle containing the flavored solution (wanting), but not as an enhanced liking of that solution. (PsycINFO Database Record (c) 2010 APA, all rights reserved)