Effects of melanin-concentrating hormone on licking microstructure and brief-access taste responses

The effects of intracerebroventricular application of melanin-concentrating hormone (MCH) on licking for sucrose, quinine hydrochloride (QHCl), and water solutions were evaluated in two experiments. In experiment 1, rats received 90-min access to sucrose and water solutions after MCH or vehicle microinjection to the third ventricle (3V). MCH increased intake largely through increases in the rate of licking early in the meal and in the mean duration of lick bursts, suggesting an effect on gustatory evaluation. Therefore, in experiment 2, brief access tests were used with a series of sucrose and QHCl concentrations to behaviorally isolate the effects of intracerebroventricular MCH on gustatory evaluation. MCH uniformly increased licking for all sucrose solutions, water, and weak concentrations of QHCl; however, it had no effect on licking for the strongest concentrations of QHCl, which were generally avoided under control conditions. Thus MCH did not produce nonspecific increases in oromotor activity, nor did it change the perceived intensity of the tastants. We conclude that MCH enhanced the gain of responses to normally accepted stimuli at a phase of processing after initial gustatory detection and after the decision to accept or reject the taste stimulus. A comparison of 3V NPY and MCH effects on licking microstructure indicated that these two peptides increased intake via dichotomous behavioral processes; although NPY suppressed measures associated with inhibitory feedback from the gut, MCH appeared instead to enhance measures associated with hedonic taste evaluation.     

Effects of hindbrain melanin-concentrating hormone and neuropeptide Y administration on licking for water, saccharin, and sucrose solutions

Melanin-concentrating hormone (MCH) and neuropeptide Y (NPY) are orexigenic peptides found in hypothalamic neurons that project throughout the forebrain and hindbrain. The effects of fourth ventricle (4V) infusions of NPY (5 microg) and MCH (5 microg) on licking for water, 4 mM saccharin, and sucrose (0.1 and 1.0 M) solutions were compared to identify the contributions of each peptide to hindbrain-stimulated feeding. NPY increased mean meal size only for the sucrose solutions, suggesting that caloric feedback or taste quality is pertinent to the orexigenic effect; MCH infusions under identical testing conditions failed to produce increases for any tastant. A second experiment also observed no intake or licking effects after MCH doses up to 15 microg, supporting the conclusion that MCH-induced orexigenic responses require forebrain stimulation. A third experiment compared the 4V NPY results with those obtained after NPY infusions (5 microg) into the third ventricle (3V). In contrast to the effects observed after the 3V NPY injections and previously reported forebrain intracerebroventricular (ICV) NPY infusion studies, 4V NPY failed to increase meal frequency for any taste solution or ingestion rate in the early phases of the sucrose meals. Overall, 4V NPY responses were limited to intrameal behavioral processes, whereas forebrain ICV NPY stimulation elicited both consummatory and appetitive responses. The dissociation between MCH and NPY effects observed for 4V injections is consistent with reports that forebrain ICV injections of MCH and NPY produced nearly dichotomous effects on the pattern of licking microstructure, and, collectively, the results indicate that the two peptides have separate sites of feeding action in the brain.     

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.     

Preference for sucralose predicts behavioral responses to sweet and bittersweet tastants

Rats can be classified as either sucralose avoiders (SA) or sucralose preferrers (SP) based on their behavioral responses in 2-bottle preference, 1-bottle intake, and brief-access licking tests. The present study demonstrates that this robust phenotypic variation in the preference for sucralose predicts acceptance of saccharin, an artificial sweetener with a purported concentration-dependent "bitter" side taste and a 0.25 M sucrose solution adulterated with increasing concentrations of quinine hydrochloride (QHCl). Specifically, SA displayed decreased preference for and intakes of saccharin (≥41.5 mM) and sucrose-QHCl (>0.5 mM QHCl) solutions, relative to SP. In a second experiment involving brief-access (30-s) tests, SP and SA did not differ in their unconditioned licking responses across a range of sodium chloride or QHCl solutions (0.03-1 mM). However, the acceptability threshold for sucrose was lower in SA, relative to SP (0.06 and 0.13 M, respectively). Our findings suggest that phenotypic differences in sucralose preference are indicative of a more general difference in the hedonic processing of stimuli containing "bittersweet" or "sweet" taste qualities.     

Independent ingestion and microstructure of feeding patterns in infant rats lacking CCK-1 receptors

Otsuka Long-Evans Tokushima fatty (OLETF) rats are a strain of Long-Evans Tokushima Otsuka (LETO) rats that do not express CCK-1 receptors, developing in adulthood, hyperphagia, obesity, and non-insulin-dependent diabetes mellitus (NIDDM). We examined weight gain and meal patterns during a 30-min independent ingestion test on postnatal days 2-4 and again on days 9-11 in OLETF and LETO rat pups. OLETF pups were significantly heavier compared with their LETO controls at both ages, and they consumed significantly more of the sweet milk diet. The difference in intake can be attributed to a significant increase in meal size and duration. Number of clusters and bursts of licking within a meal were greater in OLETF rat pups, with no difference between strains in burst and cluster size. Interlick interval (ILI) was not significantly different between OLETF and LETO pups. This measure decreased on days 9-11 compared with days 2-4 in both strains. Latency to start feeding was significantly shorter on days 2-4 in OLETF vs. LETO pups, but this difference disappeared at the second test at the older age. Two- to four-day-old OLETF pups consumed a larger volume of milk during the first minute of feeding, and their initial lick rate and decay of lick rate were significantly larger compared with their LETO controls. Lack of CCK-1 receptors, or other OLETF-related abnormalities, therefore, resulted in a satiation deficit, leading to increased meal size, hyperphagia, and increased weight gain as early as 2-4 postnatal days.     

Initial licking responses of mice to sweeteners: effects of tas1r3 polymorphisms

Recent studies have established that the T1R3 receptor plays a central role in the taste-mediated ingestive response to sweeteners by mice. First, transgenic mice lacking the gene for T1R3, Tas1r3, show dramatically reduced lick responsiveness to most sweeteners. Second, strains with the taster allele of Tas1r3 (T strains) are more sensitive to low sweetener concentrations than strains with the nontaster allele (NT strains) and consume greater quantities of low- to midrange concentrations of sweeteners during 24-h tests. We asked how Tas1r3 polymorphisms influence the initial licking responses of four T strains (FVB/NJ, SWR/J, SM/J, and C57BL/6J) and four NT strains (BALB/cJ, 129P3/J, DBA/2J, and C3H/HeJ) to two sweeteners (sucrose and SC-45647, an artificial sweetener). We used the initial licking response as a measure of the taste-mediated ingestive response because its brief duration minimizes the potential contribution of nontaste factors (e.g., negative and positive postingestive feedback). Further, we used two complimentary short-term intake tests (the brief-access taste test and a novel 1-min preference test) to reduce the possibility that our findings were an epiphenomenon of a specific testing procedure. In both tests, the T strains were more responsive than the NT strains to low concentrations of each sweetener. At higher concentrations, however, there was considerable overlap between the T and NT strains. In fact, the initial licking response of several NT strains was more vigorous than (or equivalent to) that of several T strains. There was also considerable variation among strains with the same Tas1r3 allele. We conclude that Tas1r3 polymorphisms contribute to strain differences in initial lick responsiveness to low but not high concentrations of sweeteners.     

μ-Opioid modulation in the rostral solitary nucleus and reticular formation alters taste reactivity: evidence for a suppressive effect on consummatory behavior

The neural control of feeding involves many neuromodulators, including the endogenous opioids that bind μ-opioid receptors (MORs). Injections of the MOR agonist, Damgo, into limbic and hypothalamic forebrain sites increase intake, particularly of palatable foods. Indeed, forebrain Damgo injections increase sucrose-elicited licking but reduce aversive responding (gaping) to quinine, suggesting that MOR activation may enhance taste palatability. A μ-opioid influence on taste reactivity has not been assessed in the brain stem. However, MORs are present in the first-order taste relay, the rostral nucleus of the solitary tract (rNST), and in the immediately subjacent reticular formation (RF), a region known to be essential for consummatory responses. Thus, to evaluate the consequences of rNST/dorsal RF Damgo in this region, we implanted rats with intraoral cannulas, electromyographic electrodes, and brain cannulas aimed at the ventral border of the rNST. Licking and gaping elicited with sucrose, water, and quinine were assessed before and after intramedullary Damgo and saline infusions. Damgo slowed the rate, increased the amplitude, and decreased the size of fluid-induced lick and gape bouts. In addition, the neutral stimulus water, which typically elicits licks, began to evoke gapes. Thus, the current results demonstrate that μ-opioid activation in the rNST/dorsal RF exerts complex effects on oromotor responding that contrast with forebrain effects and are more indicative of a suppressive, rather than a facilitatory effect on ingestion.     

Effect of i.c.v. infusion of the alpha-MSH agonist MTII on meal patterns in male rats following nicotine withdrawal

The present study explored the role of endogenous alpha-MSH in the alteration of meal patterns induced by nicotine (NIC) withdrawal. Male Sprague Dawley rats bearing third ventricle cannulas were placed in computerized food intake monitors. On days 1-21, the rats were given 4 mg/kg/day of NIC or saline (SAL) in four equal i.p. doses during the dark period. NIC suppressed (P < 0.05) food intake only during the first week. The normalization of food intake occurred when the reduced meal size of the NIC injected rats was countered by an increase in meal number. Despite the normalization of 24-h food intake, body weight in NIC rats was decreased (P < 0.05) for 21 days. On day 22, the rats were divided into 4 groups (n's = 7-8 each) and injected into the third ventricle with various doses of the alpha-MSH agonist MTII or artificial cerebrospinal fluid (aCSF): SAL + aCSF, SAL + MTII, NIC + aCSF, NIC + MTII. Infusion of MTII (30 ng/rat) suppressed (P < 0.01) dark phase food intake in both groups, but the NIC + MTII group ate (P < 0.05) more than the SAL + MTII group. Meal number during the dark phase was suppressed by MTII, but the NIC + MTII group took significantly more meals that the SAL + MTII group. Infusion of MTII suppressed meal size in SAL and NIC treated rats, but this effect was attenuated in NIC treated rats. All meal parameters normalized by the day after i.c.v. infusion. These data indicate that NIC treatment differentially affects the neural controls of meal number and meal size and attenuates the suppression by MTII of meal number and meal size.     

Rapid stimulus-bound suppression of intake in response to an intraduodenal nonnutritive sweetener after training with nutritive sugars predicting malaise

In a previous report (Schier et al., Am J Physiol Regul Integr Comp Physiol 301: R1557-R1568, 2011), we demonstrated with a new behavioral procedure that rats exhibit stimulus-bound suppression of intake in response to an intraduodenal (ID) bitter tastant predicting subsequent malaise. With the use of the same modified taste aversion procedure, the present experiments evaluated whether the sweet taste properties of ID stimuli are likewise detected and encoded. Thirsty rats licked at sipper spouts for hypotonic NaCl for 30 min and received brief (first 6 min) yoked ID infusions of either the same NaCl or an isomolar lithium chloride (LiCl) solution in each session. An intestinal taste cue was mixed directly into the LiCl infusate for aversion training. Results showed that rats failed to detect intestinal sweet taste alone (20 mM Sucralose) but clearly suppressed licking in response to a nutritive sweet taste stimulus (234 mM sucrose) in the intestine that had been repeatedly paired with LiCl. Rats trained with ID sucrose in LiCl subsequently generalized responding to ID Sucralose alone at test. Replicating this, rats trained with ID Sucralose in compound with 80 mM Polycose rapidly suppressed licking to the 20 mM Sucralose alone in a later test. Furthermore, ID sweet taste signaling did not support the rapid negative feedback of sucrose or Polycose on intake when their digestion and transport were blocked. Together, these results suggest that other signaling pathways and/or transporters engaged by caloric carbohydrate stimuli potentiate detection of sweet taste signals in the intestine.     

PLCbeta2-independent behavioral avoidance of prototypical bitter-tasting ligands

Using a brief-access taste assay, we show in the present report that although phospholipase C beta2 knockout (PLCbeta2 KO) mice are unresponsive to low- and midrange concentrations of quinine and denatonium, they do significantly avoid licking higher concentrations of these aversive compounds. PLCbeta2 KO mice displayed no concentration-dependent licking of the prototypical sweetener sucrose but were similar to wild-type mice in their responses to citric acid and NaCl, notwithstanding some interesting exceptions. Although these findings confirm an essential role for PLCbeta2 in taste responsiveness to sucrose and to low- to midrange concentrations of quinine and denatonium in mice as previously reported, they importantly suggest that higher concentrations of the latter two compounds, which are bitter to humans, can engage a PLCbeta2-independent taste transduction pathway.     

Greater superficial petrosal nerve transection in rats does not change unconditioned licking responses to putatively sweet taste stimuli

The greater superficial petrosal nerve (GSP), innervating taste buds in the palate, is known to be exceptionally responsive to sucrose, especially compared with the responsiveness of the chorda tympani nerve (CT). However, whereas transection of the CT (CTX) alone has little or no effect on unconditioned licking responses to many "sweet" stimuli, the impact of GSP transection (GSPX) alone is equivocal. To further examine the role of the GSP on licking responses to putatively sweet-tasting substances, brief-access taste tests were conducted in nondeprived rats before and after sham surgery (SHAM) or CTX or GSPX. A range of concentrations of sucrose, L-alanine, glycine, and L-serine, with and without 1.0 mM inosine monophosphate (IMP) added, were used. All groups showed significant concentration-dependent increases in licking to all stimuli presurgically and postsurgically. CTX decreased licking responses relative to SHAM rats in the first sucrose test. There was also a group x concentration interaction for L-alanine, but post hoc tests did not reveal its basis. Other than this, there were no significant differences among the surgical groups. Interestingly, rats with GSPX tended to initiate fewer trials than SHAM rats. Overall, after GSPX, the remaining gustatory nerves are apparently sufficient to maintain concentration-dependent licking responses to all stimuli tested here. The disparity between our results and others in the literature where GSPX reduced licking responses to sucrose is possibly related to differences in surgical technique or test trial duration.     

Glossopharyngeal nerve transection does not alter taste reactivity to sucrose conditioned to be aversive

Glossopharyngeal nerve (GL) transection in rats is known to markedly reduce gaping, a stereotypical aversive oromotor behavior, in response to intraorally delivered quinine. In this experiment we tested whether GL transection would reduce gaping in response to an otherwise palatable stimulus (sucrose) but conditioned to be aversive. Sprague-Dawley rats were implanted with intraoral cannulae. Five received bilateral transection of the GL and five served as sham-operated controls. Water-deprived rats were presented with 0.3 M sucrose for 15 min immediately followed by an injection of 0.15 M LiCl on three occasions. Rats were then habituated to the taste reactivity chamber and intraoral fluid infusion for 3 days, and tested on day 4 with a 1 ml infusion (1 min) of 0.3 M sucrose. All rats drank negligible amounts of sucrose by the third conditioning session and there were no differences in sucrose intake between the groups. There were no significant differences in gapes, or any other measured oromotor response, to sucrose between GL-transected and sham-operated rats. These results show that the GL is not a necessary afferent limb for gaping in response to conditionally aversive taste compounds.     

A Novel Psychophysical Procedure for Bitter Taste Assessment in Rats

A persistent problem with attempts to examine bitter taste mechanismshas been the lack of adequate behavioral methodology providingdata which parallels that obtained from physiological investigations.We developed a brief contact procedure to assess the abilityof rats to detect the presence of a weak bitter compound dissolvedin a strong sucrose solution. Male Fischer 344 rats were trainedto drink immediately to multiple 10-s presentations of acetaminophen(2, 8, 32, 128 mM), chlorpheniramine maleate (1, 3, 9, 27 mM)L-tryptophan (13.5, 27, 54, 108 mM), pseudoephedrine hydrochloride(1, 4, 16, 64 mM) and quinine hydrochloride (0.008, 0.04, 0.2,1.0 mM) dissolved in 0.8 M sucrose. The number of licks to sucroseand water were also measured. A microcomputer controlled stimuluspresentations and measured the animal's licks of each solutionduring each 10-s presentation. The responses to the bitter +sucrose mixture were significantly decreased at most concentrationswith increasing levels of the bitter component. This was truefor all five bitter-tasting compounds, but over different concentrationranges relatively unique to each compound. The present studyis the first to characterize the sensory effects of acetaminophen,pseudoephedrine, and chlorpheniramine maleate, all purportedto taste bitter to humans. These results demonstrate rats' acuteability to discriminate by taste not only the presence but theconcentration of a dilute bitter compound dissolved in a strongsucrose solution. Chem. Senses 20: 305–312, 1995.     

The effects of a phagostimulant and a deterrent on the microstructure of feeding byManduca sexta caterpillars

Caterpillars of the hawkmothManduca sexta Johan. (Lepidoptera: Sphingidae) were offered discs of agar containing one of a range of concentrations of sucrose or of quinine, and the microstructure of the resulting feeding behaviour was quantified. As sucrose concentration increased caterpillars ate more by increasing the rate of biting and the lengths of meals. Bite frequency increased, after a latency of seconds to minutes, over the entire range tested (0.005 M to 0.5 M) whereas meal length reached a maximum at 0.1 M. Larvae spent less time feeding on agar containing 1 mM quinine than on quinine-free agar. This was accomplished by a reduction in the size of meals; numbers of chewing bouts in meals were lower. No other feeding behaviours were affected. In addition there was no orderly relationship between quinine concentration and changes in feeding behaviours. These observations suggested that postingestive effects were important in decreasing feeding. This paper is dedicated to the memory of Vincent G. Dethier     

Contribution of alpha-gustducin to taste-guided licking responses of mice

We examined the necessity of alpha-gustducin, a G protein alpha-subunit expressed in taste cells, to taste-mediated licking responses of mice to sapid stimuli. To this end, we measured licking responses of alpha-gustducin knock-out (Gus-/-) mice and heterozygotic littermate controls (Gus+/-) to a variety of 'bitter', 'umami', 'sweet', 'salty' and 'sour' taste stimuli. All previous studies of how Gus-/- mice ingest taste stimuli have used long-term (i.e. 48 h) preference tests, which may be confounded by post-ingestive and/or experiential effects of the taste stimuli. We minimized these confounds by using a brief-access taste test, which quantifies immediate lick responses to extremely small volumes of sapid solutions. We found that deleting alpha-gustducin (i) dramatically reduced the aversiveness of a diverse range of 'bitter' taste stimuli; (ii) moderately decreased appetitive licking to low and intermediate concentrations of an 'umami' taste stimulus (monosodium glutamate in the presence of 100 microM amiloride), but virtually eliminated the normal aversion to high concentrations of the same taste stimulus; (iii) slightly decreased appetitive licking to 'sweet' taste stimuli; and (iv) modestly reduced the aversiveness of high, but not low or intermediate, concentrations of NaCl. There was no significant effect of deleting alpha-gustducin on licking responses to NH4Cl or HCl.     

C57BL/6J and DBA/2J mice vary in lick rate and ingestive microstructure

Fluid licking in mice is an example of a rhythmic behavior thought to be under the control of a central pattern generator. Inbred strains of mice have been shown to differ in mean or modal interlick interval (ILI) duration, suggesting a genetic-based variation. We investigated water licking in the commonly used inbred strains C57BL/6J (B6) and DBA/2J (D2), using a commercially available contact lickometer. Results from 20-min test sessions indicated that D2 mice lick at a faster rate than B6 mice (10.6 licks/s vs. 8.5 licks/s), based on analysis of the distribution of short-duration ILIs (50-160 ms). This strain difference was independent of sex, extent of water deprivation or total number of licks. D2 mice also displayed a faster lick rate when the strains were tested with a series of brief (5 s) trials. However, when ingestion over the entire 20-min session was analyzed, it was evident that D2 mice had an overall slower rate of ingestion than B6 mice. This was because of the tendency for D2 mice to have more very long pauses (>30 s) between sequences of licking bursts. Overall, it appeared that D2 mice licked more efficiently, ingesting more rapidly during excursions to the spout that were fewer and farther between.     

Ongoing ingestive behavior is rapidly suppressed by a preabsorptive, intestinal "bitter taste" cue

The discovery that cells in the gastrointestinal (GI) tract express the same molecular receptors and intracellular signaling components known to be involved in taste has generated great interest in potential functions of such post-oral "taste" receptors in the control of food intake. To determine whether taste cues in the GI tract are detected and can directly influence behavior, the present study used a microbehavioral analysis of intake, in which rats drank from lickometers that were programmed to simultaneously deliver a brief yoked infusion of a taste stimulus to the intestines. Specifically, in daily 30-min sessions, thirsty rats with indwelling intraduodenal catheters were trained to drink hypotonic (0.12 M) sodium chloride (NaCl) and simultaneously self-infuse a 0.12 M NaCl solution. Once trained, in a subsequent series of intestinal taste probe trials, rats reduced licking during a 6-min infusion period, when a bitter stimulus denatonium benzoate (DB; 10 mM) was added to the NaCl vehicle for infusion, apparently conditioning a mild taste aversion. Presentation of the DB in isomolar lithium chloride (LiCl) for intestinal infusions accelerated the development of the response across trials and strengthened the temporal resolution of the early licking suppression in response to the arrival of the DB in the intestine. In an experiment to evaluate whether CCK is involved as a paracrine signal in transducing the intestinal taste of DB, the CCK-1R antagonist devazepide partially blocked the response to intestinal DB. In contrast to their ability to detect and avoid the bitter taste in the intestine, rats did not modify their licking to saccharin intraduodenal probe infusions. The intestinal taste aversion paradigm developed here provides a sensitive and effective protocol for evaluating which tastants-and concentrations of tastants-in the lumen of the gut can control ingestion.     

On the sense of taste in two Malagasy Primates (Microcebus murinus and Eulemur mongoz)

The relationship between phylogeny and taste is of growing interest.In this study we present recordings from the chorda tympaniproper (CT) nerve of two lemuriforme primates, the lesser mouselemur (Microcebus murinus) and the mongoose lemur (Eulemur mongoz),to an array of taste stimuli which included the sweeteners acesulfame-K,alitame, aspartame, D-glucose, dulcin, monellin, neohesperidindihydrochalcone (NHDHC), saccharin, sodium superaspartame, stevioside,sucralose (TGS), sucrose, suosan, thaumatin and xylitol, aswell as the non–sweet stimuli NaC1, citric acid, tanninand quinine hydrochloride. In M.murinus the effects of the tastemodifiers gymnemic acid and miraculin on the CT response wererecorded. Conditioned taste aversion (CTA) experiments in M.murinusand two-bottle preference (TBP) tests in E.mongoz were alsoconducted. We found that all of the above tastants except thaumatinelicited a CT response in both species. The CTA technique showedthat M.murinus generalized from sucrose to monellin but notto thaumatin. The intake of aspartame, ranging in concentrationfrom 0.1 to 30 mM was measured in E.mongoz with TBP tests. Atno concentration did we see a preference, but there was a significantrejection of 10 and 30 mM aspartame (P