The Sense of Taste

1. The Sense of Taste

2. How tastes are detected by the tongue How taste information is processed by the brain 2

3. Function of Gustation • Designed to detect picomolar (10-12) concentrations of bitter substances • Detects molar concentrations of nutritious substances • Humans perceive sweet, sour, bitter, salty and umami • Olfaction and taste collaborate in determining flavor. • The somatosensory system establishes both localization and texture. 3

4. Organization of taste buds on the tongue 4

5. Taste buds contain 50-100 taste cells Taste cells are not neurons Short lifetimes 5

6. Similar molecules – different taste Saccharin O O O O N 2 N C H N H 3 N H S S S O O O O O O sweet bitter tasteless Aspartame

7. Very different molecules are bitter Cucurbitacin B Amarogentin Strychnine Bitter thresholds: 1.8 nM 0.6 mM 2 mM

8. Models of Taste Coding Model A: Across-fiber patterning Taste cells recognize multiple modalities; brain must compare different cells activity to know what the mouth tastes Model B: Labeled lines Taste cells recognize one modality; sweet cells fire and the brain perceives sweet 8

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10. Sweet taste Mutant mouse cannot detect saccharin Mutation is in a G protein coupled receptor Two other receptors are nearby T1R gene family = 3 genes 10

11. T1R taste receptors localize to the taste pore 11

12. Basic Techniques in Neurobiology Identifying ligands for receptors by calcium imaging • Put receptor gene + G15 into cells • When receptor is activated by ligand, • Activates G15 leading to Ca increase • 3. Calcium dye changes fluorescence G15 PLC—IP3 + DAG--- Ca release 12

13. T1R2 and T1R3 together recognize sugars + T1R2 +T1R3 + T1R2 + T1R3 13

14. T1Rs are selectively co-expressed in taste cells T1R1+3 &T1R3 T1R2+3 14

15. Mice lacking T1R2 or T1R3 do not detect sugars Expt: Give mice choice between water or water plus sugar normal 15 mutant

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19. The T2Rs are a family of novel taste GPCRs ~30 receptors found nearby genomic regions associated with bitter taste variations 19

20. Most T2Rs are co-expressed in the same cells 20

21. T2Rs function as receptors for bitter tastants + CYX + DEN + PROP PROP 21

22. There is no significant overlap between cells expressing sweet, bitter and amino acid receptors T1Rs T2Rs T1R3 T2Rs Cross sections of a tongue labeled with receptor-specific antibodies 22

23. Expression of T1Rs and T2Rs suggests that there are bitter cells and sweet cells Supports labeled line model of taste coding 23

24. Many taste signaling cascades have been proposed 24

25. T1Rs and T2Rs use a PLC-mediated cascade 25

26. Are T1Rs and T2Rs in different cells? Expt 1: knockout PLC--- mice can’t detect sweet or bitter Expt 2: put PLC back in under control of one T2R promoter what happens? 26

27. Taste cells innervated by 3 nerves facial (VII) glossopharyngeal (IX) vagus (X) Solitary tract nucleus of medulla hypothalamus, amygdala, gustatory cortex 27

28. Taste in fruit flies to study neural circuits • Taste with legs, wings, mouth • 60 taste receptor genes not related to TRs 28

29. Fly taste receptors are on legs, wings, mouth 29

30. Does the taste system map position? 30

31. Does the taste system map taste? 31

32. Fly taste system maps where the taste comes from (leg, mouth) what taste category the neuron recognizes (sweet, bitter) 32

33. Main points about taste • T1Rs recognize sugars • T2Rs recognize bitter substances • We recognize different modalities because • we have sugar cells, bitter cells, etc.. • Labeled line taste coding 33