Basic Taste Stimuli Research Articles

Taste Quality and Intensity of 100 Stimuli as Reported by Rats: The Taste–Location Association Task

The interpretation of neural activity related to sensory stimulation requires an understanding of the subject’s perception of the stimulation. Previous methods used to evaluate the perception of chemosensory stimuli by rodents have distinct limitations. We developed a novel behavioral paradigm, the taste–location association task, to complement these methods. First we tested if rats are able to learn associations between five basic taste stimuli and their spatial locations. This spatial task was based on four prototypical tastants and water. All four rats trained to perform the task reached levels of performance well above chance. Control trials demonstrated that the rats used only taste cues. Further, the learned stimulus set was resistant to interference, allowing for generalization experiments performed subsequently. We tested the rats’ gustatory generalizations of 100 tastants to the five trained stimuli, both regarding their taste qualities as well as intensity ratings. The taste profiles generated by these experiments contribute to the understanding of how perception of the specific taste stimuli relate to the perception of the five basic taste qualities in intact behaving rats. In this large taste space we found that intensity plays a major role. Furthermore, umami stimuli were not reported as being similar to other basic tastants. Our new paradigm enables neurophysiological studies of taste-based learning and memory in awake, freely moving animals.

Basic Taste Stimuli Elicit Unique Responses in Facial Skin Blood Flow

Facial expression changes characteristically with the emotions induced by basic tastes in humans. We tested the hypothesis that the five basic tastes also elicit unique responses in facial skin blood flow. Facial skin blood flow was measured using laser speckle flowgraphy in 16 healthy subjects before and during the application of basic taste stimuli in the oral cavity for 20 s. The skin blood flow in the eyelid increased in response to sweet and umami taste stimuli, while that in the nose decreased in response to a bitter stimulus. There was a significant correlation between the subjective hedonic scores accompanying these taste stimuli and the above changes in skin blood flow. These results demonstrate that sweet, umami, and bitter tastes induce unique changes in facial skin blood flow that reflect subjective hedonic scores.

NMDA receptor in intestinal enteroendocrine cell, STC-1

We earlier showed that intestinal STC-1 cells can respond to five basic taste stimuli. Here, we investigated which receptors or channels are involved in the sensing of umami in STC-1 cells. T1R family and metabotropic glutamate receptors were not involved. In contrast, we found that N-Methyl-D-aspartate (NMDA) receptors are expressed in STC-1 cells by RT-PCR analysis. Using a calcium-imaging technique, we also observed that a specific agonist NMDA induces the increase in the intracellular Ca concentration in STC-1 cells. We also found that pretreatment with NMDA enhances sweet-induced response in STC-1 cells, suggesting important roles of L-Glu-signaling in the chemosensory system of the gastrointestinal tracts.

Effect of gap junction blocker beta-glycyrrhetinic acid on taste disk cells in frog.

A gap junction blocker, 18beta-glycyrrhetinic acid (beta-GA), increased the membrane resistance of Ia, Ib and II/III cells of frog taste disk by 50, 160, and 300 M Omega, respectively, by blocking the gap junction channels and hemichannels. The amplitudes of gustatory depolarizing potentials in the disk cells for 4 basic taste stimuli were reduced to 40-60% after intravenous injection of beta-GA at 1.0 mg/kg. beta-GA of 1.0 mg/kg did not affect the resting potentials and the reversal potentials for tastant-induced depolarizing potentials in any taste disk cells. The percentage of cells responding to each of 4 basic taste stimuli and varying numbers of 4 taste qualities did not differ between control and beta-GA-treated taste disk cells. This implies that gustatory depolarizing response profiles for 4 basic taste stimuli were very similar in control and beta-GA-treated taste disk cells. It is concluded that beta-GA at 1.0 mg/kg reduced the amplitude of gustatory depolarizing potentials in taste disk cells by strongly blocking depolarizing currents flowing through the gap junction channels and hemichannels, but probably weakly affected the gustatory transduction mechanisms for 4 taste stimuli.

Intestinal STC-1 cells respond to five basic taste stimuli

STC-1 cells have been established as an enteroendocrine cell line from the small intestine. By monitoring the level of intracellular Ca using a calcium-imaging technique, cellular responses of intestinal STC-1 cells to compounds of five basic tastants were investigated. Although this cell line was known to respond to bitter compounds, we found that compounds of the other four basic tastants also stimulate STC-1 cells. When solutions containing glutamate, sucrose, HCl, or NaCl were applied, the intracellular Ca concentration in STC-1 cells significantly increased. We thus demonstrated that the gastrointestinal system can sense all five basic taste stimuli, and that STC-1 cells emerge as a new cell model for studying the molecular mechanism of signaling pathways of the five basic tastants.

Relationships between insulin release and taste

Tasting sweet food elicits insulin release prior to increasing plasma glucose levels, known as cephalic phase insulin release (CPIR). The characteristic of CPIR is that plasma insulin secretion occurs before the rise of the plasma glucose level. In this experiment, we examined whether taste stimuli placed on the tongue could induce CPIR. We used female Wistar rats and five basic taste stimuli: sucrose (sweet), sodium chloride (salty), HCl (sour), quinine (bitter) or monosodium glutamate (umami). Rats reliably exhibited CPIR to sucrose. Sodium chloride, HCl, quinine, or monosodium glutamate did not elicit CPIR. The non-nutritive sweetener saccharine elicited CPIR. However, starch, which is nutritive but non-sweet, did not elicit CPIR although rats showed a strong preference for starch which is a source of glucose. In addition, we studied whether CPIR was related to taste receptor cell activity. We carried out the experiment in rats with bilaterally cut chorda tympani nerves, one of the gustatory nerves. After sectioning, CPIR was not observed for sweet stimulation. From these results, we conclude that sweetness information conducted by thistaste nerve provides essential information for eliciting CPIR.

Taste Receptor Cells Responding with Action Potentials to Taste Stimuli and their Molecular Expression of Taste Related Genes

Recent molecular biological studies have revealed many molecular aspects of taste transduction in receptor cells for each taste categories which are referred to as salty, sweet, sour, bitter and umami in human respectively. (Lindemann, 2001; Gilbertson and Boughter, 2003). For example, ion channels such as ENaC, ASIC and HCN mediate salty and sour taste and G-protein-coupled receptors such as T1Rs and T2Rs play a key role in sweet, bitter and umami taste. However, there is little evidence for involvements of these molecules in taste reception and transduction at the cellular level. It is necessary to examine both molecular and physiological properties in single taste receptor cell. In this study, by use of loose patch recording technique combined with single cell multiplex RT–PCR, we characterized single taste cells by their responses to four basic taste stimuli and expression of taste-related molecules.

Facial responses to four basic tastes in newborn rhesus macaques ( Macaca mulatta) and chimpanzees ( Pan troglodytes)

Newborn humans are known to show specific facial expressions in response to various kinds of taste stimuli and are presumed to be able to discriminate those kinds of tastes from just after birth. As the closest relatives to humans, the taste reactivity (threshold, preference and taste-elicited facial expression) of non-human primates has long been of great interest. To date, however, there have been few investigations in newborn non-human primates. In the present study, we investigated the facial expressions elicited in response to four basic taste stimuli, sweet, salty, sour and bitter, in the newborns of two non-human primate species, rhesus macaques and chimpanzees. The taste-elicited facial expressions were compared among the kinds of taste stimuli and between the two species. Rhesus macaques of less than 7 days old showed different patterns of facial expressions for water/sweet than for bitter, and chimpanzees less than 30 days old did so for sweet and bitter. The differences between these two species were evident in the presence and absence of certain facial expressions and the emerging patterns of certain components for each stimulus. In particular, chimpanzee response patterns to the bitter stimulus resembled to those of humans rather than rhesus macaques. Overall, rhesus macaques and chimpanzees responded differently to the same kinds of tastes, presumably reflecting differences in their evolutionary backgrounds.

Developmental Changes in Sugar Responses of the Chorda Tympani Nerve in Preweanling Rats

To clarify developmental changes in the gustatory system of the rat, integrated taste responses from the chorda tympani (CT) nerve were recorded and analyzed at different postnatal ages. The response magnitude was calculated relative to the response to the standard, 0.1 M NH4Cl. Even at 1 week of age, the CT responded well to all tested 0.1 M chloride salts (NH4Cl, NaCl, LiCl, KCl, RbCl and CsCl). The responses to 0.1 M NaCl and LiCl increased with increasing age of the rat while response magnitudes to KCl, RbCl and CsCl did not change up to 8 weeks. At 1 week, the integrated response pattern was quite similar to that in adult rats for NaCl, HCl and quinine hydrochloride (QHCl). The concentration-response functions for NaCl, HCl, QHCl and sucrose at 2 weeks were essentially the same as those at 8 weeks. These results suggest that taste buds in the 2-week-old rat are functionally mature for the detection of the four basic taste stimuli. The relative magnitude of the responses to the various sugars was smaller at 1 week compared to the adult rat and reached a maximum at weeks 3-4, then decreased gradually with age. Among the six sugars, sucrose was the most effective followed by lactose. From weeks 1-4, the magnitude of the integrated taste response to fructose was smaller than that to lactose except at 3 weeks of age. Maltose, galactose and glucose were less potent stimuli than the other sugars tested. The response magnitude to lactose at 4 weeks had decreased compared to that for the other sugars. Taste responses to the sugars in preweanling and adult rats were not cross-adapted by the individual sugars. These results suggest that after 1 week of age during postnatal development in the rat, taste information from the CT rapidly increases in its importance for feeding behavior.

Taste laterality in the split brain.

Two patients with corpus callosum resection, one complete and the other sparing the genu and the rostrum, were tested for discrimination of three basic taste stimuli (sour, bitter, salty) applied to the right or left sides of the tongue. Responses were made by pointing with either hand to written words or images of visual objects corresponding to the stimuli, a language-based discrimination. In both patients, response accuracy was significantly above chance for both hemitongues but there was a significant advantage for the left side. Reaction time was shorter for left stimuli than for right stimuli but the difference was not significant. Eight normal controls matched for age with the patients performed equally well with right and left hemitongue stimuli and so did a third callosotomy patient with sparing of the posterior callosum, including the splenium. Tactile and visual tests showed that the left hemisphere was responsible for language-based responses in the first two patients. The results confirm and extend previous findings in another callosotomy patient, indicating that: (i) taste information from either side of the tongue can reach the left hemisphere in the absence of the corpus callosum; (ii) the ipsilateral input from the tongue to the left hemisphere is more potent functionally than the contralateral input and (iii) in the normal brain, the corpus callosum, specifically its posterior part including the splenium, appears to equalize the effects of the ipsilateral and contralateral gustatory inputs on the left hemisphere. Taken together with evidence about lateralized taste deficits following unilateral cortical lesions, the results also suggest that the gustatory pathways from tongue to cortex are bilaterally-distributed with an ipsilateral predominance that may be subject to individual variations.

C-Fos-like immunoreactivity in the brainstem following gastric loads of various chemical solutions in rats

The distribution of c-Fos-like immunoreactivity (c-FLI) in the lower brainstem especially in the area postrema (AP), nucleus of the tractus solitarius (NTS) and parabrachial nucleus (PBN) was examined following gastric loads of various chemical solutions in rats. An aliquot of 7.5 ml of each stimulus was intragastrically infused, and c-FLI was detected. The most remarkable c-FLI was induced by LiCl, lactose and ethanol which are known to be effective unconditioned stimuli in conditioned taste aversions. Polycose and disaccharides such as sucrose and maltose induced more c-FLI than monosaccharides such as glucose, fructose and galactose. Relatively low levels of c-FLI were observed for other sweeteners such as saccharin, glycine and alanine, and other basic taste stimuli such as NaCl, HCl, quinine and umami substances. Each stimulus induced a similar proportion of c-FLI among the subnuclei of the NTS, but not in the PBN, where chemicals effective in inducing conditioned taste aversions elicited stronger c-FLI in the external lateral subnucleus, and those in inducing conditioned taste preferences such as Polycose and glucose elicited stronger c-FLI in the dorsal lateral subnucleus. Vagotomy reduced c-FLI to about 50% for LiCl stimulation and to about 30% for sucrose stimulation, suggesting that LiCl has a larger proportion of extravagal inputs than sucrose.

Artificial neural networks estimate the contribution of taste neurons to coding

Taste qualities are believed to be coded in the activity of populations of taste neurons. However, it is not clear whether all neurons are equally responsible for coding. To clarify the point the relative contribution of each taste neuron to coding was assessed by constructing simple three-layer neural networks with input neurons that represent cortical taste neurons of the rat. The networks were trained by the back-propagation learning algorithm to classify the neural response patterns to the basic taste stimuli (sucrose, HCl, quinine–hydrochloride, and NaCl). The networks had four output neurons representing the basic taste qualities, the values of which provide a measure for similarity of test stimuli to the basic taste stimuli. We estimated relative contributions of input neurons to the taste discrimination of the network by examining their significance S j , which is defined as the sum of the absolute values of the connection weights from the jth input neuron to the hidden layer. When the input neurons with a smaller S j (e.g., 15 out of 39 input units) were “pruned” from the trained network, the ability of the network to discriminate the basic taste qualities was not greatly affected. On the other hand, the taste discrimination of the network progressively deteriorated much more rapidly with pruning of input neurons with a larger S j . These results suggest that cortical taste neurons differentially contribute to the coding of taste qualities. Input neurons with a larger S j tended to be with a larger variation of neural discharge rates to the basic taste stimuli. The variation of neural discharges may be important in the coding of taste qualities.

Gustatory neural coding in the cortex of the alert cynomolgus macaque: the quality of bitterness.

We sought to define the gustatory neural representation in primates for stimuli that humans describe as predominantly bitter. Thus we analyzed the responses of single neurons from the insular cortex of two alert, male cynomolgus macaques in response to the oral application of four basic taste stimuli (glucose, NaCl, HCl, and quinine HCl) and fruit juice, and to a series of 15 other chemicals to which humans ascribe a bitter component. Gustatory neurons occupied a volume of 109 mm3 across an area of 4.0 mm in the anterposterior plane, 4.4 mm in the mediolateral, and 6.2 mm in the dorsoventral. Taste cells represented 161 (8.6%) of the 1881 neurons tested for chemical sensitivity. Fifty of these could be monitored throughout the delivery of the entire stimulus series, and their responses constitute the data of this study. The mean spontaneous discharge rate of the cortical gustatory cells was 3.2 +/- 3.3 spikes/s (range = 0.2-17.7 spikes/s). The mean breadth-of-tuning coefficient was a moderate 0.77 +/- 0.15 (range = 0.25-0.99). Forty-eight neurons responded to taste stimuli with excitation, and two responded with inhibition. Forty-one of the 50 neurons were able to be classified into one of four functional types based on their responses to the four basic stimuli used here. These were sugar (n = 22), salt (n = 7), acid (n = 7), and quinine (n = 5). A two-dimensional space was generated from correlations among the response profiles elicited by the stimuli array. The 16 bitter chemicals formed a coherent group that was most closely related to HCl, moderately to NaCl, and bore no relationship with glucose. Within the bitter stimuli, six formed a subgroup that was most separated from all nonbitter chemicals: quinine HCl, phenlythiocarbamide, propylthiouracil, caffeine, theophylline, and phenylalanine. Humans describe these stimuli as rather purely bitter. Of the remaining 10 bitter compounds, 4 were on the fringe of the bitter group leading to NaCl: MgCl2, CaCl2, NH4Cl, and arginine. Humans characterize these as bitter-salty. Three were on the fringe leading to HCl: urea, cysteine and vitamin B1. Humans call these bitter-sour. The remaining three (nicotine, histidine, and vitamin B2) occupied the center of the bitter group. Taste quality, inferred from the position of each stimulus in the space, correlated well with human descriptions of the same stimuli, reinforcing the value of the macaque as a neural model for human gustation.

GABAergic inhibition and modifications of taste responses in the cortical taste area in rats

Using multibarrel electrodes, recordings were made in the cortical taste area (CTA), specifically in the granular and dysgranular parts of the insular cortex (areas GI and DI), of urethane-anesthetized rats. The effects of an iontophoretic application of γ-aminobutylic acid (GABA) and bicuculline methiodide (BMI), a specific antagonist to the GABA A receptor, were tested. GABA decreased background discharges in ca. 69% of 509 neurons in both areas, and in ca. 58% of 64 taste neurons. BMI antagonized the inhibitory action of GABA in CTA neurons and facilitated background discharges in ca. 51% of the 390 neurons tested, including ca. 69% of the 52 taste neurons, which indicates that CTA neurons have GABA A receptors to receive inhibitory inputs from interneurons. In both areas, the effects of BMI (6–20 nA) on taste responses of the 85 CTA neurons (49 and 36 in areas GI and DI, respectively) to the four basic taste stimuli were examined: 65 neurons were recognized in the absence of BMI, whereas 20 only in the presence of the drug. BMI increased taste responses in 25 of the former group and changed the type of their response profiles in 25 including 12 neurons whose responses were increased. It also changed the best stimulus in 34 neurons. The drug affected the receptive fields in almost all cases examined ( n=23) and increased the size in 78.2% when the value for all four basic taste stimuli were totaled. New receptive fields were uncovered by BMI in varying regions of the oral cavity depending on the taste stimulus. But the drug decreased taste responses in several neurons ( n=8). These findings indicate that the GABAergic inhibitory system apparently contributes to modifying or selecting taste information in both areas of the CTA.

Effects of dietary biotin on enhanced sucrose intake and enhanced gustatory nerve responses to sucrose seen in diabetic OLETF rat.

We used the sucrose preference test and taste nerve recording to investigate the effect of dietary biotin on the abnormal sucrose taste sensitivity and preferences seen during the course of diabetes mellitus. For this, we used Otsuka Long-Evans Tokushima fatty (OLETF) rats. The chorda tympani nerve (CT nerve) response to sucrose (> 1 M) was of greater relative magnitude in OLETF rats than in non-diabetic control (Long-Evans Tokushima Lean, LETO) rats, but the responses to other basic taste stimuli (such as HCl, quinine-HCl and L-glutamic acid) did not differ between the two groups. In behavioral experiments using a two-bottle preference test, solution intake for sucrose (> 50 mM) was higher in OLETF rats than in LETO rats. The neural responses to sucrose (1.5-2 M) in OLETF rats were lower when given a biotin-high diet (BH-OLETF) than when given a biotin-basal diet (BB-OLETF), but this was not true of the other basic tastes. However, there were no significant differences between BH-OLETF and BB-OLETF rats in terms of sucrose solution intake. These findings suggest that the enhanced sugar sensitivity observed in OLETF rats is probably the result of a genetic difference between OLETF and LETO rats, though the discrepancy can be modified by the dietary biotin level.

Increase in Inositol 1,4,5-Trisphosphate Levels of the Fungiform Papilla in Response to Saccharin and Bitter Substances in Mice

Inositol 1,4,5-trisphosphate (IP₃) levels of the mouse fungiform papilla and nonsensory epithelial tissue in response to various taste stimuli were measured by use of a competitive protein binding assay. Stimulation of the tongue with 20 mM saccharin induced a significant increase in IP₃ levels of the fungiform papillae without affecting those of the nonsensory epithelial tissue, whereas stimulations with 20 mM quinine and denatonium raised IP₃ levels in not only the fungiform papilla but slightly also nonsensory epithelial tissue. No such increases in IP₃ levels were observed to lower concentrations of saccharin, denatonium and quinine, and other basic taste stimuli, such as 0.5 M sucrose, 0.1 M NaCl and 10 mM HCl. Pretreatment of the tongue with pronase, a proteolytic enzyme which inhibits sweet responses, completely eliminated the increase in IP₃ levels induced by saccharin, but not those induced by quinine or denatonium. These results suggest that an IP₃ pathway in taste cells of the mouse fungiform taste bud is involved in the transduction not only for denatonium and quinine, but also for 20 mM saccharin. The increase of IP₃ induced by saccharin, but not by denatonium and quinine, may occur through pronase-sensitive membrane components.

Broad tuning of rat taste cells for four basic taste stimuli.

The breadth of responsiveness of rat taste cells to the four basic taste stimuli was studied using the entropy measure (H) proposed by Smith and Travers. H values range from 0.0 for narrow tuning to 1.0 for broad tuning. Based on the responses of depolarizing receptor potentials of 26 rat taste cells to the four basic taste stimuli, taste cells were classified into nine NaCl-best, four Q-HCl (quinine-HCl)-best, 10 HCl-best and three sucrose-best cells. NaCl-best cells were narrowly tuned to the four basic taste stimuli, but the other three stimuli-best cells were broadly tuned to the stimuli. In all, 85% of the taste cells responded to more than one of four basic taste stimuli. The mean H values for NaCl-best, Q-HCl-best, HCl-best and sucrose-best cells were 0.285, 0.832, 0.781 and 0.796 respectively. The mean H value for all 26 taste cells was 0.621. This was larger than H in rat gustatory fibers. Transformation of large H values in taste cells into small H values in taste fibers may be due to a non-random interaction between taste cells and taste fibers during the synaptic formation. Broad tuning properties of rat taste cells suggest that the across-taste cell response pattern may play an important role in taste quality coding mechanisms.

Responses of primate cortical neurons to unitary and binary taste stimuli.

1. The responses of 126 neurons in primary gustatory cortices of two rhesus monkeys were recorded during sapid stimulation of the tongue with 18 taste stimuli. Ten of these stimuli were dissolved in distilled water (DW): 1.0 M sucrose (Suc), 0.1 M and 0.03 M sodium chloride (NaCl), 0.003 M hydrochloric acid (HCl), 0.001 M quinine hydrochloride (QHCl), 0.03 M monosodium glutamate (MSG), 0.03 M polycose, 0.3 M glycine, 0.1 M proline, and 0.1 M malic acid. Seven other stimuli were dissolved in 0.03 M MSG; the last stimulus was a mixture of 1.0 M Suc and 0.03 M NaCl. 2. The average spontaneous rate (2.2 +/- 0.2 spikes/s, mean +/- SE) and response to DW (2.5 +/- 0.2) of these 126 neurons was low but within the range previously reported for neurons in primate taste cortex. Suc was the most effective stimulus for 24.1% of the neurons tested followed by NaCl (15.7%), QHCl (14.8%), HCl (11.1%), MSG (10.2%), and other miscellaneous unitary gustatory stimuli (8.3%). Binary taste mixtures were the most effective stimuli for 15.7% of the sample. The net responses (corrected for DW, in spikes/s) for Suc-best (3.3), NaCl-best (4.3), HCl-best (3.4), QHCl-best (2.3), and MSG-best (4.1) were sluggish, but comparable with that reported previously. 3. The response breadth of the 82 neurons that responded best to either Suc, NaCl, HCl, or QHCl measured with the entropy coefficient indicated a moderate response breadth for these neurons (mean = 0.79; range = 0.30-0.98). According to the response criteria adopted in this experiment (water response +/- 1.96 SD), however, 81 of these 82 neurons (98.1%) responded to only one or two of the four basic taste stimuli. The disparity between the entropy- and criterion-based measures of response derive from the nature of the two statistics. Adjustments that would make the entropy statistic less inclusive and the definition of a response according to statistical criteria less exclusive would increase their concordance. 4. Three multivariate statistics (cluster, principal axis factor, and multidimensional analysis) were used to analyze the data. Cluster analysis enabled us to divide the 82 taste neurons into groups on the basis of response similarity. Each of the four largest groups was dominated by neurons that responded best to one of the four basic taste stimuli: Suc, NaCl, QHCl, and HCl (ranked in descending order); the fifth largest cluster contained neurons that responded best to MSG. Principal axis factor analysis demonstrated that 80.8% of the total variance could be accounted for by three factors. Neurons responding best to Suc, NaCl, and QHCl each were closely associated with one of those three factors, but the loadings of the HCl-best neurons were evenly distributed across all three factors. The communality coefficient of these three factors was > 80% for the Suc-, NaCl-, HCl-, and QHCl-best neurons; the MSG-best neurons, by comparison, had very few high loadings on any of these three factors and a correspondingly low communality coefficient of 40.4%, a difference that was statistically significant from the other four groups. Thus the three factors related to Suc-, NaCl-, HCl-, and QHCl-best neurons are not relevant to MSG-best neurons. We used multidimensional analysis to arrange the neurons that responded best to Suc, NaCl, HCl, QHCl, and MSG into five loosely arranged and partially overlapping clusters. A multidimensional space based on stimulus similarity showed that MSG was as different from the four basic taste stimuli as they were from one another. 5. Mixture suppression, a common observation in human psychophysical experiments, was examined at the neurophysiological level by including binary tastants in the stimulus battery. The average response of 19 Suc-best neurons to 1.0 M Suc (4.1 spikes/s) decreased to near 0 when the solvent was changed from DW to either 0.03 M MSG or 0.03 M NaCl. Similar decrements were observed in NaCl- and MSG-best neurons tested with Suc/NaCl mixtures.

Enhanced gustatory neural responses to sugars in the diabetic db/db mouse

Sweet taste sensitivity in a genetic model of diabetes, the db/db mouse, in which a single major gene defect leads to the expression of diabetes and obesity, was studied by examining chorda tympani nerve responses to various taste stimuli, including sugars. The chorda tympani responses to four sugars, sucrose, fructose, glucose, and maltose, in adult db/db mice showed greater relative magnitudes and lower thresholds than those in adult lean mice, but responses to other basic taste stimuli, such as NaCl, HCl, and quinine HCl, were not different in the two groups. Behavioral experiments using a two-bottle preference test demonstrated that taste preference scores for the four sugars at suprathreshold concentrations, except 1.0 M, were higher in db/db than in control mice. Infant mice of 7-9 days of age possessing the genotype db/db also exhibited greater neural responses and lower thresholds for sugars than infant control mice, whereas streptozotocin-induced adult diabetic mice possessing the genotype +/+ did not exhibit larger sugar responses. These findings suggest that the enhanced sugar sensitivities observed in db/db mice are probably determined by a single major gene, db. The db gene may act on a common factor(s) involved in the stimulus-secretion coupling in the pancreatic B cell and the taste cell of db/db mice.

Pruning of rat cortical taste neurons by an artificial neural network model.

1. Taste qualities are believed to be coded in the activity of ensembles of taste neurons. However, it is not clear whether all neurons are equally responsible for coding. To clarify the point, the relative contribution of each taste neuron to coding needs to be assessed. 2. We constructed simple three-layer neural networks with input units representing cortical taste neurons of the rat. The networks were trained by the back-propagation learning algorithm to classify the neural response patterns to the basic taste stimuli (sucrose, HCl, quinine hydrochloride, and NaCl). The networks had four output units representing the basic taste qualities, the values of which provide a measure for similarity of test stimuli (salts, tartaric acid, and umami substances) to the basic taste stimuli. 3. Trained networks discriminated the response patterns to the test stimuli in a plausible manner in light of previous physiological and psychological experiments. Profiles of output values of the networks paralleled those of across-neuron correlations with respect to the highest or second-highest values in the profiles. 4. We evaluated relative contributions of input units to the taste discrimination of the network by examining their significance Sj, which is defined as the sum of the absolute values of the connection weights from the jth input unit to the hidden layer. When the input units with weaker connection weights (e.g., 15 of 39 input units) were "pruned" from the trained network, the ability of the network to discriminate the basic taste qualities as well as other test stimuli was not greatly affected. On the other hand, the taste discrimination of the network progressively deteriorated much more rapidly with pruning of input units with stronger connection weights. 5. These results suggest that cortical taste neurons differentially contribute to the coding of taste qualities. The pruning technique may enable the evaluation of a given taste neuron in terms of its relative contribution to the coding, with Sj providing a quantitative measure for such evaluation.