Psychophysical Assessment of the Role of the T1R2 and T1R3 Protein Subunits in Taste Responsiveness to Polycose and Putative T1R2+3 Taste Receptor Ligands

• Other Authors: Treesukosol, Yada (authoraut)
• Format: Others
• Language: English; English
• Published: Florida State University
• Subjects: Psychology
• Online Access: http://purl.flvc.org/fsu/fd/FSU_migr_etd-1526

Taste is important in regulating ingestive behavior and can be thought to serve roles in three main functional domains: stimulus identification, hedonics and physiological reflexes. The first step in the process of taste sensation involves the chemical solution coming into contact with taste receptor cells, ultimately stimulating activity in afferent fibers sending signals to the brain. Recent discoveries in the rodent model have identified candidate taste receptors thought to be involved in detecting sweet taste stimuli. Findings from studies using mice with certain T1R subunits ablated in a genetic knock-out (KO) preparation, point to a critical role for the subunits T1R2 and T1R3 in heterodimers that respond to sweeteners. Nevertheless, without rigorous psychophysical testing, there is insufficient data in the literature to conclusively claim that T1R2+3 is the only receptor that mediates the taste sensations generated by sweeteners. Polycose is a polysaccharide mixture that contains glucose polymers of varying lengths. Rodents show a preference for both sucrose and Polycose but these compounds are thought to elicit non-identical taste qualities. If indeed T1R2+3 mediates sucrose taste but sucrose and Polycose elicit separate percepts, it would follow that T1R2 KO and T1R3 KO mice would show reduced responses to sucrose but not to Polycose. In the following set of studies, T1R2 KO and T1R3 KO mice and their wild-type (WT) littermate controls, were behaviorally tested to address the following questions: (i) Are the T1R2 and T1R3 subunits necessary to maintain normal unconditioned licking responses to Polycose? (ii) What is the optimal glucose chain-length to stimulate the postulated polysaccharide receptor? (iii) Are T1R2 and T1R3 necessary for the detection of sucrose and Polycose when extraneous cues are minimized? In the first study (Chapter 2), mice were tested in a series of brief-access tests to compare their behavioral responses to Polycose with sucrose and Na-saccharin. Each compound was presented for 3 sessions in a Latin-Square order. Both KO groups initiated significantly fewer trials than WT mice to Na-saccharin and sucrose but there were no genotype differences for the number of trials initiated to Polycose. While WT mice monotonically increased their licking as a function of concentration for all 3 stimuli, both KO groups displayed severely blunted concentration-dependent changes in licking to sucrose and Na-saccharin. The KO groups tested after Polycose exposure demonstrated some degree of concentration-dependent licking of sucrose, likely attributable to learning related to prior postingestive experience. In striking contrast, the KO mice displayed concentration-dependent licking of Polycose, similar to their WT controls. This outcome demonstrates that the T1R2 and T1R3 proteins are individually unnecessary for normal affective taste responses to Polycose in a brief-access test. In the second study (Chapter 3), mice were tested in three brief-access taste tests to glucose, maltose (2 glucose units), maltotriose (3 glucose units) or Polycose (a mixture of varying lengths of glucose polymers) to address what the optimal stimulus is for the postulated polysaccharide taste receptor. Both KO groups displayed severely abolished licking responses to glucose across all testing sessions. The KO mice showed blunted responses to maltose and maltotriose but in the third session, some KO mice showed concentration-dependent licking, likely attributable to the association of postingestive consequences. In contrast, KO mice displayed concentration-dependent licking to Polycose, evident in the first session, similar to that of WT mice. Thus the optimal ligand for the postulated polysaccharide receptor likely possesses more than 3 glucose moieties. To address the issue of ligand detectability while minimizing factors such as hedonics and postingestive cues that have been shown to influence responsiveness (eg. Chapter 2 and 3), mice were tested in a two-response operant discrimination procedure (Chapter 4). These mice were trained to lick a response ball upon sampling a taste solution and to lick another response ball upon sampling water. Correct responses were reinforced with water and incorrect responses were punished with a time-out. Testing was conducted with a modified descending method of limits procedure across sessions. Interestingly, KO mice were not able to discriminate 1.0 M sucrose from water, even after 17 sessions. These same animals showed normal NaCl detection thresholds and competence in the discrimination task. The results provide strong evidence that the T1R2 and T1R3 subunits are individually necessary for the detection of sucrose. In contrast, both KO groups showed concentration-dependent functions to Polycose. Whether the presence of at least one of these T1R subunits is necessary for the normal responsiveness and detectability of Polycose, remains to be determined. Alternatively, there may be novel taste receptor(s) that subserve polysaccharide taste as has been suggested in the literature. === A Dissertation Submitted to the Department of Psychology in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy. === Fall Semester, 2010. === October 29, 2010. === Psychophysics, Taste, Polycose, Sweet, T1R2, T1R3, Receptors === Includes bibliographical references. === Alan C. Spector, Professor Directing Dissertation; Jasminka Ilich-Ernst, University Representative; Robert Contreras, Committee Member; Thomas Houpt, Committee Member; Colleen Kelley, Committee Member.