Conditioned enhancement of flavor evaluation reinforced by intragastric glucose: I. Intake acceptance and preference analysis.

Rats acquire strong preferences for flavors that are paired with intragastric (IG) glucose infusions. The present study assessed the magnitude of this nutrient-conditioned change in flavor evaluation by comparing the strength of the rats' conditioned preference to their unconditioned preference for the sweet taste of sugar. Food-restricted rats were first trained (20 h/day) to associate one flavor (CS+) with IG 16% glucose infusions and another flavor (CS-) with IG water infusions. Then, in a series of 30 min/day two-bottle tests, the CS+ and CS- were separately offered in a choice vs. fructose of varying concentrations. The CS- was preferred to 2% fructose, but not 4% fructose. The CS+ was preferred to 2%, 4%, and 8% fructose and was isopreferred to 16% fructose. This CS+ vs. fructose preference profile was replicated and found to be independent of deprivation state. The intake-stimulating efficacy of the CS+ flavor and of fructose taste was also compared in one-bottle sham-feeding tests. Intakes of CS+ and of 16% fructose increased by similar amounts in the first sham-feeding test. However, with repeated testing, fructose intake continued to increase, whereas CS+ intake declined. This profile suggests extinction of the conditioned satiation response to fructose and extinction of the conditioned acceptance response to the CS+, respectively. These findings provide novel evidence for a robust learned shift in flavor evaluation reinforced by the postingestive actions of glucose.     

Conditioned flavor avoidance, preference, and indifference produced by intragastric infusions of galactose, glucose, and fructose in rats.

The postingestive reinforcing and satiating effects of intragastric (i.g.) infusions of 16% galactose, glucose, and fructose were compared in adult female rats. In Experiment 1, food-restricted rats were trained to drink (30 min/day) flavored solutions (the CS+Gal and CS+Glu) paired with intragastric (i.g.) infusions of galactose and glucose; other flavors (the CS-) were paired with IG water infusions. In subsequent choice tests, the rats strongly preferred (91%) the CS+Glu to the CS-, but avoided the CS+Gal (21%) in favor of the CS-. In Experiment 2, the rats were trained with a CS+Fru paired with i.g. fructose infusions and a CS- paired with i.g. water. In the choice test they consumed similar amounts of CS+Fru and CS- (CS+Fru preference = 51%). In other choice tests they preferred the CS+Glu (>80%) to CS+Fru and CS+Gal, and the CS+Fru (81%) to CS+Gal. Satiation tests were performed in Experiment 3 by adapting the rats to drink a 3% sugar + 0.2% saccharin solution paired with i.g. water infusions (30 min/day). On different test days 16% sugar instead of water was infused. IG galactose, glucose, and fructose produced comparable reductions in sugar+saccharin intake in the first test session. These findings demonstrate that, while the three sugars had similar satiating effects, they differed substantially in their postingestive flavor conditioning effects. The glucose and fructose results confirm prior data indicating that only glucose generates potent postingestive reinforcing stimuli. The galactose-induced flavor avoidance indicates that this sugar has a negative postingestive consequence. This may be due to the slow and incomplete hepatic metabolism of this sugar in adult rats. Conceivably, galactose intolerance may contribute to the lactose avoidance in adult animals.     

Conditioned flavor preference learning by intragastric administration of l-glutamate in rats

The preference for foods or fluids in rats is partly dependent on its postingestive consequences. Many studies have investigated postingestive effect of high caloric substances, such as carbohydrate or fat. In this study, we examined postingestive effect of l-glutamate at the preferable concentration using conditioned flavor preference paradigm. Adult male rats with chronic intragastric (IG) cannula were trained to drink a flavored solution (conditioned stimulus; CS+) paired with IG infusion of nutrient solution and another flavored solution (CS−) with IG distilled water infusion on alternate days. The nutrient solution was 60 mM monosodium l-glutamate, sodium chloride or glucose. Before and after conditioning, rats received 30 min two-bottle choice tests for CS+ and CS− solution. All groups exhibited no significant preference for CS+ in pre-test period. By the last half of conditioning period, intake of CS+ solution was significantly higher than that of CS− in MSG group, but not in NaCl and glucose groups. After conditioned, the MSG group showed significantly higher intake and preference for CS+ solution (69.9%), while the NaCl and glucose group did not show any significant intake and preference for CS+ solution (50.9%, 43.5%, respectively). These results indicate that the amino acid l-glutamate at a preferable concentration has a positive postingestive effect as demonstrated by its ability to condition a flavor preference. The mechanism(s) for this positive effect could be through a direct effect on gut Glu receptors rather than the provision of calories or glucose from metabolized Glu; Further studies are needed to test these hypotheses.     

Conditioned acceptance and preference but not altered taste reactivity responses to bitter and sour flavors paired with intragastric glucose infusion

Nutrient-conditioned flavors preferences are thought to involve an increase in flavor palatability (hedonic evaluation). Consistent with this view is the recent finding that a sweet flavor paired with intragastric glucose infusions elicited more hedonic taste reactivity (TR) responses than did an alternative sweet flavor paired with intragastric water. The generality of this finding was examined by conditioning preferences for inherently avoided nonsweet flavors. Rats were trained in 20 h/day and then 30 min/day sessions with a CS+ flavor (sour citric acid or bitter sucrose octaacetate) paired with intragastric 16% glucose infusion, and the opposite flavor (CS-) paired with intragastric water. Glucose conditioning increased the CS+ acceptance in one-bottle tests and produced a 95% CS+ preference in two-bottle sessions. Yet, TR responses to brief intraoral infusions of the two CS flavors did not differ, even after extensive testing. Subsequent choice tests revealed that a 1% fructose solution was preferred to the CS-, whereas the CS+ was preferred to 1% and 2% fructose and equally preferred to 4%, 8%, and 16% fructose. These results indicate that strong nutrient-conditioned flavor preferences are not always associated with increased flavor palatability as measured by TR tests. Therefore, nonhedonic processes, perhaps increased incentive salience, appear to mediate the enhanced preference and acceptance conditioned by postingestive nutrient actions.     

Taste reactivity responses elicited by reinforcing drugs: a dose-response analysis.

A series of 3 experiments with Sprague-Dawley rats measured taste reactivity (TR) responses elicited by sucrose that was paired on 5 occasions with various doses of d-amphetamine (0, 1, 2, 5, or 10 mg/kg), nicotine (0, 0.4, 0.8, 1.2, or 2.0 mg/kg), or morphine (0, 2, 8, 20, or 80 mg/kg). The TR responses elicited by flavors paired with each of these drugs were compared with those elicited by flavors paired with lithium. The only doses of d-amphetamine and nicotine that effectively conditioned aversive TR responses were high doses that have also been demonstrated to be incapable of producing a place preference. Extremely high doses of morphine were incapable of producing aversive TR responses. It is suggested that aversive TR responses are only produced by doses of agents that are not reinforcing in other paradigms.     

Conditioned changes in pain reactivity: II. In search of the elusive phenomenon of conditioned hyperalgesia

Previous research suggests that a stimulus that has been paired with an aversive event can elicit either an increase (hyperalgesia) or decrease (hypoalgesia) in pain reactivity in rats. Attempts have recently been made to isolate the variables that determine the direction of the conditioned response. Very little evidence has been found for conditioned hyperalgesia when a spinally mediated measure of pain reactivity (the tail-flick test) was used. In the present study, the impact of a conditioned stimulus was assessed using a procedure modeled after one in which conditioned hyperalgesia was obtained with the tail-flick test (Davis & Hendersen, 1985, Experiment 4). With this training paradigm, a discrete conditioned stimulus was found to elicit hypoalgesia but not hyperalgesia.     

Suppression of food intake by intragastric glucose in rats with impaired glucose tolerance

Three experiments were performed to examine the relationship between impaired glucose tolerance and food intake. In the first experiment, normal rats that were given long-term insulin treatment, which was then withdrawn, ate less than controls when refed after food deprivation. Despite reduced intakes, rats previously treated with insulin became more hyperglycemic than controls during refeeding. In the second experiment, intragastric glucose injections reduced food intakes to a similar degree in control rats and in rats experiencing insulin withdrawal even though glucose loading produced a much greater increase in plasma glucose level in previously insulin-treated rats. In the third experiment, intragastric loads of a glucose polymer, Polycose, reduced food intake to the same degree in normal rats and in streptozotocin-diabetic rats both two and sixteen days after insulin withdrawal when diabetic rats were, respectively, hypo- and hyperphagic. The results show that impaired glucose tolerance does not appreciably alter the suppressive effects of glucose loading on food intake. Other effects of glucose administration, besides those on insulin-dependent glucose utilization, appear to reduce food intake after glucose loading.     

All-or-none suppression of glucose sham feeding by an intragastric mixed meal in rats.

Rats were permitted to sham feed a 1 M glucose solution after varying delays (0-40 min) following the intragastric (IG) infusion of a large, nutritionally adequate meal. (a) The meal affected sham feeding in an all-or-none way. After such a meal, sham feeding was either suppressed almost entirely, or it was not suppressed at all as compared with a no-meal control condition. (b) When it occurred, the suppression was short-lived: As little as a 20-min delay after the meal could suffice to change its suppressant effect from "all" to "none." This implies in turn that the much longer suppression found under other conditions is not a product of systemic inhibition on readiness to ingest. (c) The duration of the suppression appeared to decrease with successive exposures to the experimental conditions. After several such exposures, some rats showed no postprandial suppression at all, even immediately after the IG meal.     

Effects of estrogen upon feeding inhibition produced by intragastric glucose loads

Two studies were performed to determine if estrogen could potentiate the inhibitory effects of glucose on feeding behavior. In the first experiment, ovariectomized rats were injected with either 6 μg of estradiol benzoate (EB) or with sesame oil, and two days later were given 5 ml intragastric loads of either 40% glucose or 13.5% urea by gavage. Gavage was performed after a 12-hr fast, during the light period. Food intake (FI) was measured at hourly intervals, starting 2 hr after gavage and reaccess to food as well as daily throughout the experiment. In a second experiment, rats received a similar treatment, except that gavage was performed in an unfasted condition, during the dark period. It was found that (1) a 12-hr fast abolishes anorexic effects of EB, but leaves the inhibitory effects of glucose intact, (2) EB does not potentiate short-term effects of glucose, and (3) EB enhances delayed (22 hr after gavage) effects of glucose upon feeding. These findings are consistent with the hypothesis that EB modified the long-term control of feeding behavior, probably at the level of the ventromedial hypothalamus.     

Energy density and macronutrient composition determine flavor preference conditioned by intragastric infusions of mixed diets

In prior studies rats preferred a flavor (CS+HF) paired with intragastric (IG) infusions of a high-fat diet to a flavor (CS+HC) paired with a high-carbohydrate diet, yet just the opposite preference was observed with pure-nutrient infusions. The present study tested the hypothesis that variations in nutrient density as well as composition influence flavor learning. Animals were trained (22 h/day) with IG infusion of milk-based high-fat and high-carbohydrate liquid diets paired with intakes of flavored, noncaloric CS+ solutions. A third flavor, the CS-, was paired with water infusion. Standard chow was available ad libitum. The rats preferred both CS+ flavors to the CS-, whether the infused diets were dense (HF and HC, 2.1 kcal/ml) or dilute (hf and hc, 0.5 kcal/ml), indicating that all diet infusions were reinforcing. They consumed the CS+hc and CS+hf equally in training, and preferred the CS+hc, showing that at low-energy density carbohydrate was more reinforcing than fat. In contrast, CS+HF intake exceeded that of CS+HC in training, and the rats preferred the CS+HF to the CS+HC. In further tests the rats preferred the CS+HF to the CS+hc, the CS+HF to the CS+hf, and the CS+HC to the CS+hc; i.e., when the diets differed in energy density the flavors associated with the more concentrated infusions were preferred. In the absence of influence by flavor cues from the nutrients themselves, rats' preferences for flavors associated with diets high in fat or carbohydrate are dependent on energy density. The differential satiating effects of fat and carbohydrate may contribute to these density-dependent preferences.     

Suppression and enhancement of in vitro lymphocyte reactivity by factors in rat submandibular gland extracts.

The addition of crude extracts from rat submandibular (SM) glands to murine spleen and lymph node cell cultures stimulated with concanavalin A (Con A) induced either suppression (at high concentrations) or further stimulation (at lower concentrations) or further stimulation (at lower concentrations) of proliferative activity. Gel filtration of the extracts revealed that suppressive activity was due to factors of molecular weight in the 50,000-96,000 range, while stimulation was due to factors in the 13,000-35,000 molecular weight range. The suppressor activity of the higher molecular weight fractions was not due to a reduction of cell viability or of the uptake of tritiated thymidine. This was demonstrated by the fact that the addition of IL-2 to the cultures completely reversed the suppressive effect. Further fractionation of the suppressive and of the stimulatory gel filtration fractions with the chromatofocusing technique led to the identification of a single fraction with suppressor activity and of multiple discrete fractions with stimulatory activity.     

Taste reactivity analysis of 6-hydroxydopamine-induced aphagia: implications for arousal and anhedonia hypotheses of dopamine function

The deficits in feeding and drinking that result from 6-hydroxydopamine (6-OHDA) lesions of the mesostriatal dopamine system are often explained using either sensorimotor arousal or anhedonia hypotheses. Sensorimotor arousal hypotheses posit that dopamine systems facilitate the capacity of sensory stimuli to activate any motor output. The anhedonia hypothesis suggests that dopamine systems amplify the hedonic impact of positive reinforcers. Natural palatability-dependent ingestive and aversive actions, which are emitted by rats to tastes, provide a sensitive test that can discriminate between these hypotheses: A reduction of sensorimotor arousal should diminish the ability of tastes to elicit any actions, whereas anhedonia should shift the balance between positive and aversive actions. To directly compare these two hypotheses, taste reactivity was examined in rats made aphagic by intranigral 6-OHDA injections. The results did not support either of these predictions: Taste reactivity was essentially unchanged. The persistence of normal taste reactivity argues against both an anhedonia and a global sensorimotor arousal interpretation and provides further evidence that the capacity for hedonics can be neurologically dissociated from motivated appetitive behavior. An incentive attribution hypothesis that can account for the results is discussed, along with its implications for a wide range of phenomena associated with dopamine depletion.     

A Mathematical Model of ATP Secretion by Type II Taste Cells

Neuroscience and Behavioral Physiology - The functional unit of the taste system in mammals is the taste bud, which is a heterogeneous population of 50–80 different cells, including types I,...     

Glass reinforced hydroxyapatite for hard tissue surgery--part II: in vitro evaluation of bone cell growth and function.

Hydroxyapatite (HA)-based materials are considered to be potentially useful as bone implant materials, particularly those reinforced with glass to improve mechanical strength. However, the precise effects of glass-reinforced HA on the growth and functions of bone cells are still unclear. The present study has therefore examined the response of human osteoblast-like cells to HA and HA reinforced with two different proportions of glass, namely 2.5% and 5%. All materials enabled the cells to attach and proliferate during 7 days in culture and, although the growth was less than on control plastic surfaces, there was no deleterious effect of the 5% glass composite compared with HA alone. Flow cytometry analysis showed that there was no effect on cell size and granularity, but there were marked and highly selective changes in the expression of certain connective tissue proteins. Thus, while bone sialoprotein and osteonectin were down-regulated on HA alone, the expression of these antigens was relatively enhanced on the composite materials, and collagen type I was also up-regulated on the glass-reinforced HA. Thus, modulation of the glass composition of HA materials could be used to produce not only improved mechanical strength, but also enhanced biocompatibility.     

Endotoxin-induced enhancement of glucose influx into murine peritoneal macrophages via GLUT1.

Hypoglycemia is among the most injurious metabolic disorders caused by endotoxemia. In experimental endotoxemia with lipopolysaccharide (LPS) in animals, a marked glucose consumption is observed in macrophage-rich organs. However, the direct effect of LPS on the uptake of glucose by macrophages has not been fully understood, and the present study was undertaken to shed light on this point. The consumption and uptake of glucose, as measured with 2-deoxy-D-[3H]glucose, by murine peritoneal exudate macrophages in culture were accelerated two- to threefold by stimulation with 3 ng of LPS per ml. The rate of glucose uptake reached a plateau after 20 min of stimulation and remained at the maximum as long as LPS was present. Northern (RNA) blot analysis with cDNA probes for five known isoforms of glucose transporter (GLUT) revealed that the expression of GLUT by macrophages was restricted to the GLUT1 isoform during LPS stimulation and the amount of GLUT1 mRNA was increased by the stimulation. These results suggest that macrophage responses to LPS are supported by a rapid and sustained glucose influx via GLUT1 and that this is a participating factor in the development of systemic hypoglycemia when endotoxemia is prolonged.     

Eosinophil infiltration and enhancement of airway reactivity by leukocyte chemotactic factor, formyl-methionyl-leucyl-phenylalanine (fMLP), in guinea pigs.

N-formyl-L-methionyl-L-leucyl-L-phenylalanine (fMLP) is a bacterial-derived chemotactic factor for eosinophils and neutrophils. This study is aimed to examine whether or not eosinophil infiltration induced by intra-airway administration of fMLP causes the damage of the bronchial epithelium and results in airway hyperresponsiveness in normal non-sensitized guinea pigs. In normal guinea pigs fMLP administered by aerosol inhalation or intratracheal injection caused significant infiltration of eosinophils in the tracheal mucosa and enhanced bronchial reactivity to inhaled histamine 6 and 24 hours after exposure. Electron microscopic examination showed damage of the alignment of the epithelial cells in the bronchial mucosa in fMLP-treated guinea pigs. PAF antagonists CV3988 and WEB2086 and a 5-lipoxygenase inhibitor (AA-861) did not prevent fMLP induced eosinophil infiltration, which suggests that fMLP caused eosinophil infiltration mainly by its chemotactic activity, not by the release of platelet activating factor (PAF) or leukotrienes in this experimental condition. These results showed that in normal guinea pigs a bacteria-derived chemoattractant of fMLP could reproduce a sequence of eosinophil infiltration and airway hyperresponsiveness, similar to the inflammatory pathophysiology after antigen challenge in sensitized animals. We concluded that eosinophil infiltration induced by either immunological or non-immunological mechanisms can cause airway damage and airway hyperresponsiveness.     

Morphine- and naltrexone-induced modification of palatability: analysis by the taste reactivity test.

We used the taste reactivity (TR) test, a direct measure of the hedonic properties of a tastant, to assess in Sprague-Dawley rats the ability of morphine (an opiate agonist) and naltrexone (an opiate antagonist) to modify the palatability of a bitter quinine solution and a sweet sucrose solution. Morphine reduced the aversive hedonic properties of both novel and familiar quinine solution (0.05% and 0.1%) but did not modify the palatability of 20% sucrose solution. Naltrexone reduced the positive hedonic properties of sucrose solution (2% and 20%) but did not modify the palatability of 0.05% quinine solution. The pattern of results suggests that the modification of feeding produced by opiate agonists and antagonists may be mediated by an hedonic shift in the palatability of the tastant.