Detection of maltodextrin and its discrimination from sucrose are independent of the T1R2 + T1R3 heterodimer

TRPM4 and PLCβ3 contribute to normal behavioral responses to an array of sweeteners and carbohydrates but PLCβ3 is not needed for taste-driven licking for glucose

The peripheral taste system is more complex than previously thought. The novel taste-signaling proteins TRPM4 and PLCβ3 appear to function in normal taste responding as part of Type II taste cell signaling or as part of a broadly responsive (BR) taste cell that can respond to some or all classes of tastants. This work begins to disentangle the roles of intracellular components found in Type II taste cells (TRPM5, TRPM4, and IP3R3) or the BR taste cells (PLCβ3 and TRPM4) in driving behavioral responses to various saccharides and other sweeteners in brief-access taste tests. We found that TRPM4, TRPM5, TRPM4/5, and IP3R3 knockout (KO) mice show blunted or abolished responding to all stimuli compared with wild-type. IP3R3 KO mice did, however, lick more for glucose than fructose following extensive experience with the 2 sugars. PLCβ3 KO mice were largely unresponsive to all stimuli except they showed normal concentration-dependent responding to glucose. The results show that key intracellular signaling proteins associated with Type II and BR taste cells are mutually required for taste-driven responses to a wide range of sweet and carbohydrate stimuli, except glucose. This confirms and extends a previous finding demonstrating that Type II and BR cells are both necessary for taste-driven licking to sucrose. Glucose appears to engage unique intracellular taste-signaling mechanisms, which remain to be fully elucidated.

Dietary experience with glucose and fructose fosters heightened avidity for glucose-containing sugars independent of TRPM5 taste transduction in mice

OBJECTIVE

Experience with metabolically distinct sugars, glucose and fructose, enhances attraction to the orosensory properties of glucose over fructose. To gain insight into which sensory signals are affected, we investigated how this nutritive learning reshapes behavioral responding to various sugars in brief access taste tests in C57BL6/J (B6) mice and assessed whether sugar-exposed mice lacking the TRPM5 channel involved in G-protein coupled taste transduction could acquire these types of preferences for glucose-containing sugars.

METHODS

B6, TRPM5 knockout (KO), and TRPM5 heterozygous (Het) mice were given extensive access to water (sugar naïve) or 0.316, 0.56, and 1.1 M glucose and fructose (sugar-exposed) and then tested, whilst food deprived, for their relative avidities for glucose, fructose, sucrose, maltose, and/or a non-metabolizable glucose analog in a series of taste tests.

RESULTS

Sugar-exposed B6 mice licked relatively more for glucose than fructose, driven by an increased avidity for glucose, not an avoidance of fructose, and licked more for maltose, compared to their sugar-naïve counterparts. Sugar-exposed B6 mice did not lick with such avidity for a non-metabolizable glucose analog. TRPM5 KO mice took longer to acquire the sugar discrimination than the Het controls, but both groups ultimately licked significantly more for glucose than fructose. Het mice displayed clear preferential licking for sucrose over fructose, while licking comparably high for glucose, sucrose, and maltose. KO mice licked significantly more for maltose than sucrose.

CONCLUSIONS

Collectively, the findings suggest that ingestive experience with glucose and fructose primarily reprograms behavioral responding to a TRPM5-independent orosensory signal generated by glucose-containing sugars.

Novel approaches to the study of viscosity discrimination in rodents

Texture has enormous effects on food preferences. The materials used to study texture discrimination also have tastes that experimental animal can detect; therefore, such studies must be designed to exclude taste differences. In this study, to minimize the effects of material tastes, we utilized high- and low-viscosity forms of carboxymethyl cellulose (CMC-H and CMC-L, respectively) at the same concentrations (0.1-3%) for viscosity discrimination tests in rats. In two-bottle preference tests of water and CMC, rats avoided CMC-H solutions above 1% (63 mPa·s) but did not avoid less viscous CMC-L solutions with equivalent taste magnitudes, suggesting that rats spontaneously avoided high viscosity. To evaluate low-viscosity discrimination, we performed conditioned aversion tests to 0.1% CMC, which initially showed a comparable preference ratio to water in the two-bottle preference tests. Conditioning with 0.1% CMC-L (1.5 mPa·s) did not induce aversion to 0.1% CMC-L or CMC-H. However, rats acquired a conditioned aversion to 0.1% CMC-H (3.6 mPa·s) even after latent inhibition to CMC taste by pre-exposure to 0.1% CMC-L. These results suggest that rats can discriminate considerably low viscosity independent of CMC taste. This novel approach for viscosity discrimination can be used to investigate the mechanisms of texture perception in mammals.

Neural Isolation of the Olfactory Bulbs Severely Impairs Taste-Guided Behavior to Normally Preferred, But Not Avoided, Stimuli

Here we systematically tested the hypothesis that motivated behavioral responsiveness to preferred and avoided taste compounds is relatively independent of the olfactory system in mice whose olfactory bulbs (main and accessory) were surgically disconnected from the rest of the brain [bulbotomy (BULBx)]. BULBx was confirmed histologically as well as functionally with the buried food test. In brief access taste tests, animals received 10-s trials of various concentrations of a taste compound delivered quasirandomly. BULBx C57BL/6 (B6) mice displayed severely blunted concentration-dependent licking for the disaccharide sucrose, the maltodextrin Maltrin, and the fat emulsion Intralipid relative to their sham-operated controls (SHAM B6). Licking for the noncaloric sweetener saccharin was also blunted by bulbotomy, but less so. As expected, mice lacking a functional “sweet” receptor [T1R2+T1R3 knockout (KO)] displayed concentration-dependent responsiveness to Maltrin and severely attenuated licking to sucrose. Like in B6 mice, responsiveness to both stimuli was exceptionally curtailed by bulbotomy. In contrast to these deficits in taste-guided behavior for unconditionally preferred stimuli, BULBx in B6 and KO mice did not alter concentration-dependent decreases for the representative avoided stimuli quinine and citric acid. Nor did it temper the intake of and preference for high concentrations of affectively positive stimuli when presented in long-term (23-h) two-bottle tests, demonstrating that the surgery does not lead to a generalized motivational deficit. Collectively, these behavioral results demonstrate that specific aspects of taste-guided ingestive motivation are profoundly disturbed by eliminating the anatomic connections between the main/accessory olfactory bulbs and the rest of the brain.

Electrophysiological responses to sugars and amino acids in the nucleus of the solitary tract of type 1 taste receptor double-knockout mice

Strong evidence supports a major role for heterodimers of the type 1 taste receptor (T1R) family in the taste transduction of sugars (T1R2+T1R3) and amino acids (T1R1+T1R3), but there are also neural and behavioral data supporting T1R-independent mechanisms. Most neural evidence for alternate mechanisms comes from whole nerve recordings in mice with deletion of a single T1R family member, limiting conclusions about the functional significance and T1R independence of the remaining responses. To clarify these issues, we recorded single-unit taste responses from the nucleus of the solitary tract in T1R double-knockout (double-KO) mice lacking functional T1R1+T1R3 [KO1+3] or T1R2+T1R3 [KO2+3] receptors and their wild-type background strains [WT; C57BL/6J (B6), 129X1/SvJ (S129)]. In both double-KO strains, responses to sugars and a moderate concentration of an monosodium glutamate + amiloride + inosine 5'-monophosphate cocktail (0.1 M, i.e., umami) were profoundly depressed, whereas a panel of 0.6 M amino acids were mostly unaffected. Strikingly, in contrast to WT mice, no double-KO neurons responded selectively to sugars and umami, precluding segregation of this group of stimuli from those representing other taste qualities in a multidimensional scaling analysis. Nevertheless, residual sugar responses, mainly elicited by monosaccharides, persisted as small "sideband" responses in double-KOs. Thus other receptors may convey limited information about sugars to the central nervous system, but T1Rs appear critical for coding the distinct perceptual features of sugar and umami stimuli. The persistence of amino acid responses supports previous proposals of alternate receptors, but because these stimuli affected multiple neuron types, further investigations are necessary.NEW & NOTEWORTHY The type 1 taste receptor (T1R) family is crucial for transducing sugars and amino acids, but there is evidence for T1R-independent mechanisms. In this study, single-unit recordings from the nucleus of the solitary tract in T1R double-knockout mice lacking T1R1+T1R3 or T1R2+T1R3 receptors revealed greatly reduced umami synergism and sugar responses. Nevertheless, residual sugar responses persisted, mainly elicited by monosaccharides and evident as "sidebands" in neurons activated more vigorously by other qualities.

The Functional and Neurobiological Properties of Bad Taste

The gustatory system serves as a critical line of defense against ingesting harmful substances. Technological advances have fostered the characterization of peripheral receptors and have created opportunities for more selective manipulations of the nervous system, yet the neurobiological mechanisms underlying taste-based avoidance and aversion remain poorly understood. One conceptual obstacle stems from a lack of recognition that taste signals subserve several behavioral and physiological functions which likely engage partially segregated neural circuits. Moreover, although the gustatory system evolved to respond expediently to broad classes of biologically relevant chemicals, innate repertoires are often not in register with the actual consequences of a food. The mammalian brain exhibits tremendous flexibility; responses to taste can be modified in a specific manner according to bodily needs and the learned consequences of ingestion. Therefore, experimental strategies that distinguish between the functional properties of various taste-guided behaviors and link them to specific neural circuits need to be applied. Given the close relationship between the gustatory and visceroceptive systems, a full reckoning of the neural architecture of bad taste requires an understanding of how these respective sensory signals are integrated in the brain.

Characterization of mouse chorda tympani responses evoked by stimulation of anterior or posterior fungiform taste papillae

Different gustatory papilla types vary in their locations on the tongue. Distinctions have often made between types, but variation within fungiform papillae has seldom been explored. Here, regional differences in fungiform papillae were investigated by flowing solutions selectively over either an anterior fungiform (AF, tongue tip) or a posterior fungiform (PF, middle third) region as taste-evoked activity was measured in the chorda tympani nerve of C57BL/6J (B6) mice. Significantly larger responses were evoked by NaCl applied to the AF than PF region, and the ENaC blocker amiloride reduced the NaCl response size only for the former. Umami synergy, based on co-presenting MSG and IMP, was larger for the AF than PF region. The regions did not differ in response size to sour chemicals, but responses to l-lysine, l-arginine, sucrose, and tetrasodium pyrophosphate were larger for the AF than PF region. Thus, fungiform papillae on the tongue tip differed from those found further back in their transduction mechanisms for salty and umami compounds. Gustatory sensitivity also showed regional variation, albeit with a complex relationship to palatability and taste quality. Overall, the data support a regional organization for the mouse tongue, with different functional zones for the anterior, middle, and posterior thirds.

Taste sensitivity to a mixture of monosodium glutamate and inosine 5'-monophosphate by mice lacking both subunits of the T1R1+T1R3 amino acid receptor

The taste of l-glutamate and its synergism with 5'-ribonucleotides is thought to be primarily mediated through the T1R1+T1R3 heterodimer in some mammals, including rodents and humans. While knockout (KO) mice lacking either receptor subunit show impaired sensitivity to a range of monosodium glutamate (MSG) concentrations mixed with 2.5 mM inosine 5'-monophosphate (IMP) in amiloride, wild-type (WT) controls can detect this IMP concentration, hindering direct comparison between genotypes. Moreover, some residual sensitivity persists in the KO group, suggesting that the remaining subunit could maintain a limited degree of function. Here, C57BL/6J, 129X1/SvJ, and T1R1+T1R3 double KO mice ( n = 16 each to start the experiment) were trained in a two-response operant task in gustometers and then tested for their ability to discriminate 100 µM amiloride from MSG (starting with 0.6 M) and IMP (starting with 2.5 mM) in amiloride (MSG+I+A). Testing continued with successive dilutions of both MSG and IMP (in amiloride). The two WT strains were similarly sensitive to MSG+I+A ( P > 0.8). KO mice, however, were significantly impaired relative to either WT strain ( P < 0.01), although they were able to detect the highest concentrations. Thus, normal detectability of MSG+I+A requires an intact T1R1+T1R3 receptor, without regard for allelic variation in the T1R3 gene between the WT strains. Nevertheless, residual sensitivity by the T1R1+T1R3 KO mice demonstrates that a T1R-independent mechanism can contribute to the detectability of high concentrations of this prototypical umami compound stimulus.