The role of sucrose-sensitive neurons in ingestion of sweet stimuli by hamsters

CALHM1 Deletion in Mice Affects Glossopharyngeal Taste Responses, Food Intake, Body Weight, and Life Span

Stimulation of Type II taste receptor cells (TRCs) with T1R taste receptors causes sweet or umami taste, whereas T2Rs elicit bitter taste. Type II TRCs contain the calcium channel, calcium homeostasis modulator protein 1 (CALHM1), which releases adenosine triphosphate (ATP) transmitter to taste fibers. We have previously demonstrated with chorda tympani nerve recordings and two-bottle preference (TBP) tests that mice with genetically deleted Calhm1 (knockout [KO]) have severely impaired perception of sweet, bitter, and umami compounds, whereas their sour and salty tasting ability is unaltered. Here, we present data from KO mice of effects on glossopharyngeal (NG) nerve responses, TBP, food intake, body weight, and life span. KO mice have no NG response to sweet and a suppressed response to bitter compared with control (wild-type [WT]) mice. KO mice showed some NG response to umami, suggesting that umami taste involves both CALHM1- and non-CALHM1-modulated signals. NG responses to sour and salty were not significantly different between KO and WT mice. Behavioral data conformed in general with the NG data. Adult KO mice consumed less food, weighed significantly less, and lived almost a year longer than WT mice. Taken together, these data demonstrate that sweet taste majorly influences food intake, body weight, and life span.

Cycloheximide: no ordinary bitter stimulus

Cycloheximide (CyX), a toxic antibiotic with a unique chemical structure generated by the actinomycete, Streptomyces griseus, has emerged as a primary focus of studies on mammalian bitter taste. Rats and mice avoid it at concentrations well below the thresholds for most bitter stimuli and T2R G-protein-coupled receptors specific for CyX with appropriate sensitivity are identified for those species. Like mouse and rat, golden hamsters, Mesocricetus auratus, also detected and rejected micromolar levels of CyX, although 1mM CyX failed to activate the hamster chorda tympani nerve. Hamsters showed an initial tolerance for 500microM CyX, but after that, avoidance of CyX dramatically increased, plasticity not reported for rat or mouse. As the hamster lineage branches well before division of the mouse-rat lineage in evolutionary time, differences between hamster and mouse-rat reactions to CyX are not surprising. Furthermore, unlike hamster LiCl-induced learned aversions, the induced CyX aversion neither specifically nor robustly generalized to other non-ionic bitter stimuli; and unlike adverse reactions to other chemosensory stimuli, aversions to CyX were not mollified by adding a sweetener. Thus, CyX is unlike other bitter stimuli. The gene for the high-affinity CyX receptor is a member of a cluster of five orthologous T2R genes that are likely rodent-specific; this "CyX clade" is found in the mouse, rat and probably hamster, but not in the human or rabbit genome. The rodent CyX-T2R interaction may be one of multiple lineage-specific stimulus-receptor interactions reflecting a response to a particular environmental toxin. The combination of T2R multiplicity, species divergence and gene duplication results in diverse ligands for multiple species-specific T2R receptors, which confounds definition of 'bitter' stimuli across species.

Peripheral gustatory processing of sweet stimuli by golden hamsters

Behaviors and taste-nerve responses to bitter stimuli are linked to compounds that bind T2 receptors expressed in one subset of taste-bud receptor cells (TRCs); and behavioral and neural responses to sweet stimuli are linked to chemical compounds that bind a T1 receptor expressed in a different TRC subset. Neural and behavioral responses to bitter-sweet mixtures, however, complicate the ostensible bitter and sweet labeled lines. In the golden hamster, Mesocricetus auratus, quinine hydrochloride, the bitter prototype, suppresses chorda tympani (CT) nerve responses to the sweet prototype: sucrose. This bitter-sweet inhibition was tested with concentration series of sucrose and dulcin, a hydrophobic synthetic sweetener that hamsters behaviorally cross-generalize with sucrose. Dulcin, sucrose and other sweeteners activate one subset of CT fibers: S neurons; whereas, quinine activates a separate subset of CT fibers: E neurons. Whole-nerve and S-neuron CT responses to a sweetener concentration series, mixed with 0, 1, 3 and 10 mM quinine, were measured for 0-2.5 s transient and/or 2.6-10 s steady-state response periods. Ten-sec total single-fiber records, aligned at response onset, were averaged for 100 ms bins to identify response oscillations. Quinine inhibition of dulcin and sucrose responses was identical. Each log molar increment in quinine resulted in equivalent declines in response to either sweetener. Furthermore, sucrose response decrements paralleled response increments in quinine-sensitive CT neurons to the same quinine increases. A 1.43 Hz bursting rhythm to the sweeteners was unchanged by quinine inhibition or decreases in sweetener concentration. Taste-bud processing, possibly between-cell inhibition and within-cell negative feedback, must modify signals initiated by T1 receptors before they are transmitted to the brain.

Neuron types, receptors, behavior, and taste quality

Neurophysiological studies on chorda tympani (CT) single fibers and behavioral studies on generalization of learned aversions in hamsters (Mesocricetus auratus) are reviewed. The work on hamsters is compared to work on other species, including the laboratory rat and several primate species, including humans. This body of data demonstrates associations between response profiles of physiologically defined specialist CT neurons and behavioral stimulus generalizations on one hand, and characteristics of putative taste receptors, on the other. Response profiles of generalist CT neurons are similarly associated with receptor characteristics, but are not associated with specific behavioral discriminations. The associations of peripheral nerve data with both receptor and behavior strongly suggest specific codes for "sucrose-like" and "NaCl-like" taste qualities. Definitive conclusions regarding "patterns" or "labeled lines" requires an understanding of mechanisms of central neural processing of the several specialist and generalist taste-afferent inputs.

Chorda tympani responses in two inbred strains of mice with different taste preferences

Behavioral studies suggest that there are significant differences in the taste systems of the inbred mouse (Mus musculus) strains: C57BL/6J (B6) and DBA/2J (D2). In an attempt to understand the biological basis of the behavioral differences, we recorded whole-nerve chorda tympani responses to taste solutions and compared the results to intake of similar solutions in nondeprived mice. Stimuli included a test series composed of 0.1 M sodium chloride, 0.3 M sucrose, 10 mM sodium saccharin, 3 mM hydrochloric acid, and 3 mM quinine hydrochloride, as well as concentration series for the same substances. Neural activity of the chorda tympani that was evoked by sucrose, saccharin, or NaCl was greater in B6 than D2 mice; and neural threshold for sucrose was lower in B6 mice, but neural thresholds for HCl and quinine were lower in D2 mice. B6 mice drank more sucrose and saccharin but less quinine than D2 mice; thus, sucrose and saccharin preference were positively correlated, but NaCl and quinine aversiveness were negatively correlated with the chorda tympani results. Nonetheless, genes involved in the structuring of taste receptors and/or the chordae tympani, which transduce taste stimuli having diverse perceptual qualities, differ for the two mouse strains.

Gustatory responses of the hamster Mesocricetus auratus to various compounds considered sweet by humans

The taste of 30 compounds was studied in the golden hamster with three different methods: single-fiber recordings, two-bottle preference (TBP), and conditioned taste aversion (CTA) tests. On the whole, the results showed that the sense of taste in the hamster differs in many respects from that in humans because, of 26 tested compounds known as sweet to humans, 11 had no taste or tasted differently. The results also supported the notion that activity in S-fibers elicits liking and activity in Q- or H-fibers rejection. Specifically hierarchial cluster analysis of 36 single fibers from the chorda tympani proper nerve separated N-, H-, and S-clusters consisting of 11 sucrose-, 14 NaCl-, and 11 citric-best fibers. Ace-K, cyanosuosan, N-4-cyanophenyl-N'-cyanoguanidineacetate (CCGA), -tryptophan, N-3, 5-dichlorophenyl-N'-(S)-alpha-methylbenzylguanidineacetate (DMGA), saccharin, SC-45647, and suosan stimulated only the S-fibers, were significantly preferred in TBP tests, and generalized to sucrose in the CTA tests. Ethylene glycol stimulated the N-fibers in addition to the S-fibers. This explains its generalization to sucrose in CTA. Its toxicity may contribute to its rejection in TBP tests. Sodium cyclamate stimulated a few N- but no S-fibers, which may explain the nondiscriminatory TBP and CTA results. Glycine elicited its largest response in the S-fibers, although it also stimulated other fibers. The resulting mixed taste sensation may explain why it was not preferred in TBP, although it generalized to sucrose in the CTA. Alitame, aspartame, N-4-cyanophenylcarbamoyl--aspartyl-(R)-alpha-methylbenzylamine (CAM), N-4-cyanophenylcarbamoyl-(R, S)-3-amino-3-(3, 4-methylenedioxyphenyl) propionic acid (CAMPA), N-(S)-2-methylhexanoyl--glutamyl-5-amino-2-pyridinecarbonitrile (MAGAP), N-1-naphthoyl--glutamyl-5-amino-2-pyridinecarbonitrile (NAGAP), NHDHC, superaspartame, and thaumatin were among the compounds considered sweet by humans that gave no response, were not discriminated in the TBP test, and gave no generalization in the CTA tests.

Ethanol consumption and taste preferences in C57BL/6ByJ and 129/J mice

Mice of the C57BL/6ByJ (B6) and 129/J (129) strains were offered different concentrations of taste solutions in 48-hr, two-bottle choice tests. In comparison with the 129 strain, the B6 strain had higher preferences for ethanol, sucrose, and citric acid. They had lower preferences for NaCl and similar preferences for capsaicin and quinine hydrochloride. These data are consistent with the hypothesis that the higher ethanol intake by B6 mice depends, in part, on higher hedonic attractiveness of its sweet taste component.

Analysis of polysaccharide taste in hamsters: behavioral and neural studies

A series of studies was carried out in hamsters (Mesocricetus auratus) to determine whether polysaccharides have behavioral and neurophysiological characteristics that distinguish them from simple sugars. Behavioral studies utilized solutions of glucose, maltose, sucrose, Polycose, and glycogen in two-bottle preference tests and in tests of generalization of conditioned taste aversions. Multiunit and single-unit responses of the chorda tympani nerve were studied with the same stimuli. Neural responses to Polycose and glycogen were found to be generated primarily by ionic contaminants. Dialysis or deionization dramatically reduced electrophysiological responses, a result consistent with occurrence of Polycose and glycogen sensitivity in electrolyte-sensitive nerve fibers. Effects of treatment with the Na + -channel blocker amiloride and cross-adaptation were also consistent with neural responses generated by ionic contaminants. Hamsters showed strong preferences for the sugars and Polycose, a mixture of glucose polymers with alpha-1,4 linkages, and even stronger preferences for a glycogen preparation. Conditioned flavor aversions were established to glycogen, sucrose, and maltose, but no aversion was learned to 3.2% Polycose. The learned aversion to maltose partly generalized to glycogen and sucrose, but sucrose and glycogen did not cross-generalize. Deionization did not affect the preferences for Polycose and glycogen but removal of contaminants of mol.wt. < or = 7000 Da greatly reduced preference for glycogen. In conclusion, glycogen itself, after removal of low molecular weight contaminants, is a poor taste stimulus in hamsters, both behaviorally and neurophysiologically. However, Polycose is highly preferred by hamsters but gives little chorda tympani response after removal of ionic contaminants. In alert animals, the action of salivary amylase on polysaccharides may produce simpler, detectable taste stimuli.

Structure and function of gustatory neurons in the nucleus of the solitary tract. I. A classification of neurons based on morphological features

Prior investigations in other laboratories have provided convincing evidence that the neurons of the rostral nucleus of the solitary tract (rNST) can be grouped according to their physiological response properties or morphologic features. The present study is based on the premise that the response properties of gustatory neurons are related to, and perhaps governed by, their morphology and connectivity. In this first phase of our ongoing investigation of structure-function relationships in the rNST of the rat, we have used intracellular injection of neurobiotin to label individual physiologically characterized gustatory neurons. A total of 63 taste-sensitive neurons were successfully labeled and subjected to three-dimensional quantitative and qualitative analysis. A cluster analysis using six morphologic features (total cell volume, soma area, mean segment length, swelling density, spine density, and number of primary dendrites) was used to identify six cell groups. Subsequent analyses of variance and posthoc comparisons verified that each of these six groups differed from all others with respect to at least one variable, so each group was "typified" by at least one of the six morphologic features. Neurons in group A were found to be the smallest neurons in the sample. The cells in group B had small somata and exhibited the highest swelling density of any group. Group C neurons were distinguished by dendrites with long, spine-free branches. These dendrites were significantly longer than those of any other group except Group F. The neurons in group D had more primary dendrites than any other group. Group E neurons possessed dendrities with the lowest swelling density but the most spines of any group. The cells in group F were the largest neurons in our sample and possessed the largest somata of any group. Thus overall cell size and density of dendritic spines and swellings were found to be particularly important variables in this classification scheme. Our preliminary results suggest that the number and density of dendritic spines (as well as other morphologic features) may be related to a given neuron's most effective stimulus, indicating that it will indeed be possible to use the criteria established in the present investigation to derive structure-function relationships for gustatory neurons in the rNST.

The sense of taste: neurobiology, aging, and medication effects

The sense of taste is an oral chemical sense in mammals that is involved in the choice of foods. Initial transduction of taste stimuli occurs in taste buds, which are distributed in four discrete fields in the oral cavity. Medications can affect the taste buds and ion channels in taste-bud cell membranes involved in stimulus transduction. The sense of taste gradually declines with aging, with bitter taste most affected. Neural circuits that mediate taste in primates include cranial nerves VII, IX, and X, the solitary nucleus in the brain stem, the ventroposteromedial nucleus of the thalamus, and the insular-opercular cortex. The central taste pathways process taste information about sweet, salty, sour, and bitter stimuli serially and in parallel. Medications associated with "metallic" dysgeusia and taste losses affect the taste system via unknown mechanisms.