Taste Responses in the Nucleus of the Solitary Tract in Saccharin-preferring and Saccharin-averse Rats

A minority of rats consistently reject the taste of sodium saccharinat concentrations that the majority find palatable. We choserats that selected either water (WP), or 0.03 M NaSaccharin(SP) in two-bottle preference tests and monitored single unitresponses to a range of taste qualities in the nucleus of thesolitary tract. WP rats gave significantly greater responsesto Na/Li salts and QHCI. Their responses to sugars were equalto those from SP rats. Total activity to NaSaccharin did notdiffer between the two groups, but its distribution across thethree identified neuron types did. The response was skewed fromone in which sugar (S) and sodium salt (N) participated nearlyequally (SP) to one dominated by the activity of N cells andnearly devoid of an S cell contribution (WP rats). Accordingly,the response profile for NaSaccharin was correlated nearly aswell with those of the sugars (+ 0.60) as with the Na/Li salts(+ 0.73) in SP rats, but was reshaped in WP rats to be nearlyidentical with those of the salts (+ 0.85) and unlike sugars(+ 0.30). In their heightened sensitivity to stimuli that humanscall salty and bitter, and in their rejection of the complextaste of NaSaccharin, WP rats showed many of the characteristicsof human tasters of PTC/PROP. Chem. Senses 21: 147–157,1996.     

Conditioned preference for sweet stimuli in OLETF rat: effects of food deprivation

The Otsuka Long-Evans Tokushima Fatty (OLETF) rat, an outbred strain of Long- Evans Tokushima Otsuka rat (LETO) that lacks CCK-1 receptor expression, is hyperphagic and develops obesity and type-2 diabetes. The present study sought to assess how OLETF rats alter intake, preference, and conditioned preference of palatable solutions after acute food deprivation. Our results show that after 24 h chow restriction, LETO rats increase both sucrose intake and two-bottle sucrose preference relative to their free-fed baseline, whereas OLETF rats do not increase sucrose intake (0.3 M or 1.0 M sucrose) or preference (1.0 M vs. 0.3 M sucrose) when they are food deprived. In contrast, OLETF rats exhibit a higher conditioned flavor preference when sucrose is used as unconditioned stimulus (US) relative to LETO rats, whether overnight food restricted (81% vs. 71% for OLETF and LETO rats, respectively) or free fed (82% vs. 54% for OLETF and LETO rats, respectively) during the test. When a noncaloric saccharin solution is used as US, OLETF rats show a higher preference for the saccharin-associated flavor relative to LETO rats when nondeprived (76% vs. 58% for OLETF and LETO rats, respectively); however, neither strain shows differential conditioned flavor preference for saccharin in the deprivation state during the test. These findings suggest that OLETF rats fail to integrate postabsorptive and orosensory effects of sucrose in a conditioning setting to influence intake. Thus, it appears that OLETF rats form preferences for sucrose based largely on orosensory and hedonic properties of the solution, rather than caloric value.     

Taste effects of some amino acids and glutamate compounds in the rat

Behavioral responses to five L-amino acids (Gly, Arg, Leu, Ala,Met) and five related L-glutamate compounds (MSG, MKG, MAG,Gln, GluHCl) were measured using 1-min taste reactivity andstandard 24-h, two-bottle preference tests. Taste reactivitytests measure the immediate pattern of ingestive and aversiveoral motor behavior elicited by direct oral infusion of tastestimuli. By permitting acute observations in non-deprived rats,taste reactivity tests are more sensitive to taste factors thanstandard long-term tests. Three stimulus concentrations of eachcompound were selected by behavioral and electrophysiologicalcriteria. Taste reactivity results often conflicted with standardintake results. In taste reactivity tests both Gly and MSG elicitingestive oral motor responses that increase with stimulus concentrationin the absence of aversive behavior. The opposite responseswere obtained using long-term intake tests; MSG and Gly preferenceratios actually decrease with increasing concentration. Thesedata suggest a reinterpretation of standard, longterm intaketests. Specifically, effects of taste versus post-oral stimulimay be distinguished by contrasting taste reactivity and two-bottlepreference tests. Differences in the pattern of oral motor behaviorselicited by the amino acid and glutamate compounds are alsodiscussed.     

Gastric bypass reduces fat intake and preference

Roux-en-Y gastric bypass is the most effective therapy for morbid obesity. This study investigated how gastric bypass affects intake of and preference for high-fat food in an experimental (rat) study and within a trial setting (human). Proportion of dietary fat in gastric bypass patients was significantly lower 6 yr after surgery compared with patients after vertical-banded gastroplasty (P = 0.046). Gastric bypass reduced total fat and caloric intake (P < 0.001) and increased standard low-fat chow consumption compared with sham controls (P < 0.001) in rats. Compared with sham-operated rats, gastric bypass rats displayed much lower preferences for Intralipid concentrations > 0.5% in an ascending concentration series (0.005%, 0.01%, 0.05%, 0.1%, 0.5%, 1%, 5%) of two-bottle preference tests (P = 0.005). This effect was demonstrated 10 and 200 days after surgery. However, there was no difference in appetitive or consummatory behavior in the brief access test between the two groups (P = 0.71) using similar Intralipid concentrations (0.005% through 5%). Levels of glucagon-like peptide-1 (GLP-1) were increased after gastric bypass as expected. An oral gavage of 1 ml corn oil after saccharin ingestion in gastric bypass rats induced a conditioned taste aversion. These findings suggest that changes in fat preference may contribute to long-term maintained weight loss after gastric bypass. Postingestive effects of high-fat nutrients resulting in conditioned taste aversion may partially explain this observation; the role of GLP-1 in mediating postprandial responses after gastric bypass requires further investigation.     

No relationship between sequence variation in protein coding regions of the Tas1r3 gene and saccharin preference in rats

Nearly all mammalian species like sweet-tasting foods and drinks, but there are differences in the degree of 'sweet tooth' both between species and among individuals of the same species. Some individual differences can be explained by genetic variability. Polymorphisms in a sweet taste receptor (Tas1r3) account for a large fraction of the differences in consumption of sweet solutions among inbred mouse strains. We wondered whether mice and rats share the same Tas1r3 alleles, and whether this gene might explain the large difference in saccharin preference among rats. We conducted three experiments to test this. We examined DNA sequence differences in the Tas1r3 gene among rats that differed in their consumption of saccharin in two-bottle choice tests. The animals tested were from an outbred strain (Sprague-Dawley; experiment 1), selectively bred to be high- or low-saccharin consumers (HiS and LoS; experiment 2), or from inbred strains with established differences in saccharin preference (FH/Wjd and ACI; experiment 3). Although there was considerable variation in saccharin preference among the rats there was no variation in the protein-coding regions of the Tas1r3 gene. DNA variants in intronic regions were detected in 1 (of 12) outbred rat with lower-than-average saccharin preference and in the ACI inbred strain, which also has a lower saccharin preference than the FH/Wjd inbred partner strain. Possible effects of these intronic nucleotide variants on Tas1r3 gene expression or the presence of T1R3 protein in taste papillae were evaluated in the ACI and FH/Wjd strains. Based upon the results of these studies, we conclude that polymorphisms in the protein-coding regions of the sweet receptor gene Tas1r3 are uncommon and do not account for individual differences in saccharin preference for these strains of rats. DNA variants in intron 4 and 5 are more common but appear to be innocuous.     

Mouse taste preference tests: why only two bottles?

Two-bottle tests have been used extensively to measure the preference for taste and nutrient solutions but there has been little work with tests involving more than two bottles. Here, we compare the results obtained in two-bottle tests with those obtained in three- and six-bottle tests. In Experiment 1, we measured the preferences for 2 mM saccharin, 50 mM citric acid, 0.3 mM quinine hydrochloride and 75 mM NaCl displayed by 129X1/SvJ (129) and C57BL/6J (B6) mice. Mice drank more taste solution when they received two bottles providing taste solution and one providing water than when they received either a standard two-bottle test or two bottles providing water and one providing taste solution. The three-bottle tests also revealed the left spout side preferences of the 129 strain and were generally better at distinguishing between the 129 and B6 strains (i.e. were more sensitive) than were the two-bottle tests. In Experiment 2, we measured intakes and preferences in tests with six bottles, with one, two, three, four or five containing 75 mM NaCl and the rest containing water. NaCl preferences were monotonically related to the number of NaCl spouts available. A follow-up experiment found similar results whether the index of ingestion was volume intakes or licks. This argues that spillage cannot account for the effect of spout number on taste solution intake. Together, the results suggest that (i) the number of bottles of taste solution and water has a profound influence on taste solution intake and preference, and (ii) three-bottle tests may be more sensitive than two-bottle tests in many circumstances.     

Importance of lipolysis in oral cavity for orosensory detection of fat

Lingual lipase is usually secreted from von Ebner's glands, although there is great variation between species. Lingual lipase is thought to be an auxiliary enzyme for fat digestion and absorption in mammals; however, the reason for lipolysis in the oral cavity is not known. We focused on the gustatory sense and investigated the significance of lingual lipase in the perception of a fat taste by using orlistat, a potent lipase inhibitor. Five-minute two-bottle preference tests demonstrated that the addition of orlistat diminished the preference for triacylglycerides but not for free fatty acids. Radioactive triolein applied on rats' circumvallate papilla revealed that lingual lipase was released continuously to generate significant amounts of fatty acids and other lipolytic products within 1-5 s, which was enough time to taste fat. These findings suggest that lingual lipase is released to perceive the taste of triacylglycerides and to find nutritive lipids in food.     

Increased oral and decreased intestinal sensitivity to sucrose in obese, prediabetic CCK-A receptor-deficient OLETF rats

CCK-A receptor-deficient Otsuka Long-Evans Tokushima fatty (OLETF) rats are hyperphagic and develop obesity and Type 2 diabetes. In this strain, taste preference functions have not been investigated. Therefore, a series of short-access, two-bottle tests were performed in age-matched prediabetic OLETF and nonmutant Long-Evans Tokushima Otsuka (LETO) rats to investigate preference for sucrose (0.03, 0.1, 0.3, or 1.0 M) presented with a choice of water. To discern orosensory from postgastric factors that may contribute to this preference, in a separate experiment, rats were allowed to sham feed sucrose in the absence or presence of duodenal sucrose infusion (0.3, 0.6, or 1.0 M). In the two-bottle real-feeding tests, OLETF rats exhibited a greater preference for 0.3 M sucrose (91.2 +/- 1.7 and 78.5 +/- 3.4% for OLETF and LETO, respectively; P < 0.01) and 1.0 M sucrose (65.3 +/- 1.2 and 57.5 +/- 2.7% for OLETF and LETO, respectively; P < 0.05) than LETO rats. OLETF rats also sham fed less of the lowest (0.03 M; 33.8 +/- 4.8 and 58.3 +/- 7.3 ml for OLETF and LETO, respectively; P < 0.05) and more of the highest (1.0 M; 109.9 +/- 6.5 and 81.0 +/- 3.9 ml for OLETF and LETO, respectively; P < 0.01) concentration of sucrose relative to LETO rats. Finally, intraduodenal sucrose infusions (0.6 and 1.0 M) produced a smaller reduction of 0.3 M sham sucrose intake [14.1 +/- 8.1 vs. 52.5 +/- 3.3 ml and 49.4 +/- 8.0 vs. 82.4 +/- 3.2 ml for 0.6 M (P < 0.01) and 1.0 M (P < 0.05) infusions in OLETF and LETO, respectively]. These findings demonstrate that OLETF rats display an increased preference for sucrose, an effect that is at least partially influenced by the orosensory stimulating effect of sucrose. This enhanced responsiveness to oral stimulation, coupled with the deficit in responding to the postingestive feedback of intestinal sucrose, may contribute additively to the development of hyperphagia and weight gain in OLETF rats.     

Response of Pigs to Bitter-tasting Compounds

Two-bottle preference tests were done to determine whether pigsdetect bitter-tasting compounds. Four standard bitter compoundsand nine bitter-tasting pharmaceutical compounds were tested.Pigs detect and avoid taste compounds that humans perceive asbitter-tasting. A dose-response to varying concentrations ofbitter tastants can be measured. Chem. Senses 22: 129–132,1997.     

Salt Appetite Is Reduced by a Single Experience of Drinking Hypertonic Saline in the Adult Rat

Salt appetite, the primordial instinct to favorably ingest salty substances, represents a vital evolutionary important drive to successfully maintain body fluid and electrolyte homeostasis. This innate instinct was shown here in Sprague-Dawley rats by increased ingestion of isotonic saline (IS) over water in fluid intake tests. However, this appetitive stimulus was fundamentally transformed into a powerfully aversive one by increasing the salt content of drinking fluid from IS to hypertonic saline (2% w/v NaCl, HS) in intake tests. Rats ingested HS similar to IS when given no choice in one-bottle tests and previous studies have indicated that this may modify salt appetite. We thus investigated if a single 24 h experience of ingesting IS or HS, dehydration (DH) or 4% high salt food (HSD) altered salt preference. Here we show that 24 h of ingesting IS and HS solutions, but not DH or HSD, robustly transformed salt appetite in rats when tested 7 days and 35 days later. Using two-bottle tests rats previously exposed to IS preferred neither IS or water, whereas rats exposed to HS showed aversion to IS. Responses to sweet solutions (1% sucrose) were not different in two-bottle tests with water, suggesting that salt was the primary aversive taste pathway recruited in this model. Inducing thirst by subcutaneous administration of angiotensin II did not overcome this salt aversion. We hypothesised that this behavior results from altered gene expression in brain structures important in thirst and salt appetite. Thus we also report here lasting changes in mRNAs for markers of neuronal activity, peptide hormones and neuronal plasticity in supraoptic and paraventricular nuclei of the hypothalamus following rehydration after both DH and HS. These results indicate that a single experience of drinking HS is a memorable one, with long-term changes in gene expression accompanying this aversion to salty solutions.     

A trial of assessment of the interrelated action of metabolic and sensory factors in the choice of food by rats in various states of protein and calcium nutrition

The strategy of food choice by adult rats was studied after giving previously to them a balanced diet, a non-protein diet and/or a diet with calcium excess. Tests were carried out for food preference offering to the rats semi-purified diets differing in the contents of protein and calcium. Besides that, the taste of the food was changed adding to it saccharose, quinine, mint and caramel. Depending on the nutritional state of the rats significant differences were found in their preference for foods which was related to the general direction of homeostasis recovery. In protein malnutrition diets were chosen containing protein, while calcium content and taste were of secondary importance.     

Sour taste preferences of children relate to preference for novel and intense stimuli

Previous research has suggested that some children have a preference for sour tastes. The origin of this preference remains unclear. We investigated whether preference for sour tastes is related to a difference in rated sour intensity due to physiological properties of saliva, or to an overall preference for intense and new stimuli. Eighty-nine children 7-12 years old carried out a rank-order procedure for preference and category scale for perceived intensity for four gelatins (i.e. 0.0 M, 0.02 M, 0.08 M, 0.25 M added citric acid) and four yellow cards that differed in brightness. In addition, we measured their willingness to try a novel candy and their flow and buffering capacity of their saliva. Fifty-eight percent of the children tested preferred one of the two most sour gelatins. These children had a higher preference for the brightest color (P < 0.05) and were more likely to try the candy with the unknown flavor (P < 0.001) than children who did not prefer the most sour gelatins. Preference for sour taste was not related with differences in rated sour intensity, however those who preferred sour taste had a higher salivary flow (P < 0.05). These findings show that a substantial proportion of young children have a preference for extreme sour taste. This appears to be related to the willingness to try unknown foods and preference for intense visual stimuli. Further research is needed to investigate how these findings can be implemented in the promotion of sour-tasting food such as fruit.     

An analysis of 5'-inosine and 5'-guanosine monophosphate taste in rats

Inosine monophosphate (IMP) and guanosine monophosphate (GMP) elicit an umami taste in humans and synergistically increase the intensity of the umami taste of monosodium glutamate (MSG). Conditioned taste aversion (CTA) studies in rodents indicate that these nucleotides and MSG elicit quite similar tastes, but recent physiological evidence suggests that these nucleotides and MSG may not activate the same population of taste receptors and therefore may not elicit identical taste qualities. This study reports the findings of several behavioral experiments with rats that compared the taste properties of IMP and GMP with each other and with those of MSG. Well-trained rats were able to detect both nucleotides at nanomolar concentrations, but they did not respond to either nucleotide in two-bottle preference tests or brief-access CTA tests at concentrations less than 0.5 mM. Discrimination experiments found that the tastes of these nucleotides could not be discriminated from each other, but both could be discriminated from MSG, even when the taste of Na(+) was controlled. Overall, these experiments indicate the taste properties of the two 5'-ribonucleotides are quite similar to each other, and even though they may elicit an umami sensation, these sensations are not identical to the taste of MSG.     

Overeating by young obesity-prone and lean rats caused by tastes associated with low energy foods

Objective: Childhood obesity is a prominent health problem that may involve early learning about tastes and the energy content of foods. We tested the hypothesis that food tastes predictive of low energy content cause overeating in young animals. Research Methods and Procedures: Juvenile and adolescent (4‐ and 8‐week‐old) male JCR:LA‐cp lean (+/cp or +/+) and obesity‐prone (cp/cp) rats were given sweet (saccharin) and salty (sodium chloride) gelatin cubes made with starch (high caloric) or no starch (low caloric) for 16 days of taste conditioning. After 10 hours of food deprivation, rats received pre‐meals with flavors that had been paired or unpaired with high caloric content during taste conditioning, followed immediately by measurement of chow intake at regular meals. Results: Our findings show that both lean (+/cp) and obesity‐prone (cp/cp) juvenile rats ate more regular chow after a pre‐meal with a flavor associated with low caloric value than after a similar pre‐meal with a flavor predictive of high caloric content. This effect occurred with juvenile rats but not with adolescents. Discussion: Data from our study indicate that the subversion of the relationship between taste and caloric content disrupts the normal physiological and behavioral energy balance of juvenile rats, resulting in overeating that is independent of genetic disposition for obesity.     

CD36 gene deletion reduces fat preference and intake but not post-oral fat conditioning in mice

Several findings suggest the existence of a "fatty" taste, and the CD36 fatty acid translocase is a candidate taste receptor. The present study compared fat preference and acceptance in CD36 knockout (KO) and wild-type (WT) mice using nutritive (triglyceride and fatty acid) and nonnutritive (Sefa Soyate oil) emulsions. In two-bottle tests (24 h/day) naive KO mice, unlike WT mice, displayed little or no preference for dilute soybean oil, linoleic acid, or Sefa Soyate emulsions. At high concentrations (2.5-20%), KO mice developed significant soybean oil preferences, although they consumed less oil than WT mice. The postoral actions of fat likely conditioned these preferences. KO mice, like WT mice, learned to prefer a flavored solution paired with intragastric soybean oil infusions. These findings support CD36 mediation of a gustatory component to fat preference but demonstrate that it is not essential for fat-conditioned flavor preferences. The finding that oil-naive KO mice failed to prefer a nonnutritive oil, assumed to provide texture rather than taste cues, requires explanation. Finally, CD36 deletion decreased fat consumption and enhanced the ability of the mice to compensate for the calories provided by their optional fat intake.     

On the relationship between sweet taste and seasonal body weight changes in a primate (Microcebus murinus)

The relationship between obesity and taste, especially sweettaste, has been and is of interest. From this point of viewof a small primate, the lesser mouse lemur (Microcebus murinus),is of particular interest. It goes through a yearly cycle ofphysiological changes, one of which is an extreme variationin body weight of up to 100%. This occurs concomitantly withsignificant changes of the animal's liking for sucrose; measuredby two-bottle preference tests, the threshold for sucrose changesfrom 28–45 in lean to 77–105 mM in obese animals.It is possible that a change in peripheral taste sensitivitymight be the cause for these preference changes. To test thispossibility we studied the ability of M.murinus to taste sucrosewith electrophysiological and conditioned taste aversion techniques.The electrophysiological recordings were obtained from the chordatympani proper nerve in two heavy and three lean animals. Wedid not record any difference between the two groups in theirneural response to a series of sucrose concentrations. Conditionedtaste aversion experiments with 200 mM sucrose as conditioningstimulus and 50 and 200 mM sucrose as test stimuli gave similarresults. No difference was found between three heavy and fourlean animals; both groups rejected the sucrose concentrations.The results support the notion that the seasonal variationsin preference threshold to sucrose were unrelated to the abilityof M.murinus to taste sucrose.     

Bottle-choice tests in Sprague-Dawley rats using liquid diets that differ in oil and sucrose contents

Bottle choice tests using liquid diets were done with Sprague-Dawley (SD) rats. SD rats ingested more oil-and-sucrose-enriched milk (hi-fat) and less oil-enriched milk (hi-fat-no-carb) than sucrose-enriched (hi-carb) milk by two-bottle choice tests after they were habituated to liquid diets for 4 days. Chronic food restriction didn't increase hi-fat ingestion but hi-fat-no-carb. Rats ingested less without habituation, and overnight food deprivation increased intake. This increment was maintained after rats were free-fed. The difference in fat content of the maintenance diet had little effect on fat preference. These results showed SD rats prefer a sweet and fatty liquid diet than a sweet and lean liquid diet. Habituation and food restriction were more important than the composition of the maintenance diet to demonstrate a clear preference for the fatty liquid diet.     

Role of hydroxycinnamic acids in food flavor: a brief overview

The influence of hydroxycinnamic acids (HCAs) on food flavor is reviewed. In coffee, whole-grain foods and related model systems, the HCAs have been reported to contribute to the flavor profile by multiple mechanisms, such as to impart taste attributes, to generate aroma and taste-active compounds by phenolic degradation, as well as to alter the mechanisms of the Maillard reaction and related flavor development. Consequently the role of HCAs on the flavor properties of food products is complex, multifaceted and can be related to the chemistry and fate of HCAs during thermal processing.     

Food preference of young rhesus monkeys fed marginally zinc deficient diets

Zinc deprivation has been shown to produce hypogeusia in rhesus monkeys but the possible consequences of altered taste sensitivity for food preference and selection is not known. We studied 13 rhesus monkeys (2.5 to 3 years of age) for food preference in a structured choice situation. These animals were fed two levels of dietary zinc: marginally zinc deprived (fed 4 μg zinc/g diet from conception and throughout development;N=6), or control (fed a 100 μg zinc/g diet throughout the study;N=7). Preference for familiar vs unfamiliar food items, order of food preferences, persistence in retrieval of preferred and nonpreferred foods, and preference under four deprivation periods were examined. Animals were offered a choice of two food items, both containing minimal zinc levels, and food choice and latency of choice were measured. Results indicate that animals fed the marginal zinc diets had reduced preference for unfamiliar foods relative to controls and different patterns of food preference. These results may be relevant to maintenance of appropriate food selection in marginally zinc deprived populations.     

Radiation-induced taste aversion: effects of radiation exposure level and the exposure-taste interval

Radiation-induced taste aversion has been suggested to possibly play a role in the dietary difficulties observed in some radiotherapy patients. In rats, these aversions can still be formed even when the radiation exposure precedes the taste experience by several hours. This study was conducted to examine whether increasing the radiation exposure level could extend the range of the exposure-taste interval that would still support the formation of a taste aversion. Separate groups of rats received either a 100 or 300 R gamma-ray exposure followed 1, 3, 6, or 24 h later by a 10-min saccharin (0.1% w/v) presentation. A control group received a sham exposure followed 1 h later by a 10-min saccharin presentation. Twenty-four hours following the saccharin presentation all rats received a series of twelve 23-h two-bottle preference tests between saccharin and water. The results indicated that the duration of the exposure-taste interval plays an increasingly more important role in determining the initial extent of the aversion as the dose decreases. The course of recovery from taste aversion seems more affected by dose than by the temporal parameters of the conditioning trial.