Marion E. Frank, Ph.D. Professor Center for Chemosensory Sciences

1. University of Connecticut Graduate SchoolMEDS 371: Systems Neuroscience2011Chemosensory Systems: Taste Marion E. Frank, Ph.D. Professor Center for Chemosensory Sciences Oral Health & Diagnostic Sciences School of Dental Medicine

2. GUSTATORY SYSTEM Purpose of Taste—Detect Nutrients and Poisons 1 of 3 Chemical Senses, Taste---other Smell and Chemesthesis—Chemical activation of Somesthesis Cool of Menthol, Hot of Capsaicin 1 Chemical may activate all 3 senses, e.g. Ethanol

3. Chemicals that Taste: Water Soluble Quality to Humans, [Also Good-Bad, Post-Ingestional]

4. TASTE QUALITIES SOUR acetic acid citric acid HCl SWEET sucrose fructose saccharin SALTY NaCl KCl Na2SO4 NH4Cl UMAMI: MSG BITTER quinineHCl MgSO4 caffeine Taste Tetrahedron

5. 5 NEW TASTES 1. 2 NEW CARBOHYDRATE: MALTODEXTRIN & STARCH 2. FATS: FREE FATTY ACIDS, eg. CAPROIC, LINOLEIC 3. CALCIUM SPECIFIC 4. WATER • Taste Criteria • Logical purpose • Defined stimulus class • Transduction mechanism • Gustatory system involvement • Unique perception • Behavioral/physiological effect.

6. Taste stimuli 1. Taste receptors 2. Taste cells activated 3. Signals relayed to taste nerve Taste bud 4. Taste nerve transmits Signals to brain. Taste Coding Logic according to Charles Zuker From Yarmolinsky DA, Zuker CS, Ryba NJP. 2009. Common sense about taste: from mammals to insects. Cell 139: 234-244.

7. Pattern Coding---2 Ways Non-specific Receptor Cells Non-specific Nerve Fibers

8. Chorda tympani S Neuron: Responses to Sucrose • Sweet Rhythm---Temporal Coding? From Frank et al., 2005

9. Candidate Taste Receptors GPCRs Ion Channels From Yamolinsky et al., 2009

10.  gustducin 2 Kinds of Initial Taste Transduction GPCR and Ion-channel

11. [Zhao, Zhang, Hoon, Chandrashekar, Erlenbach, Ryba & Zucker; Cell, 2003] Aspartame activates human but not mouse T1R2+3 1. Mouse with hT1R2+3 Receptor in Sweet Cells aspartame TR determines Chemical Selectivity.

12. RASSL = Receptor Activated Soley by Synthetic Ligand 2. Mouse with RASSL in Sweet Cells opioid TRC Activation determines Appetitive Behavior.

13. Taste: Species Variation (Diversification) T1R in Carnivores Carnivore evolutionary tree showing branching of Feliform and Caniform about 60 MYA. Dish = Diet Carnivore Insectivore Omnivore Herbivore Piscivore From Li, X. Pseudogenization: Cats no T1R2 (sweet taste) (Li et al, 2006) and Pandas (Carnivora) no T1R1 (glutamate taste) (Zhao et al, 2010).

14. Species Differences: Bitter to Humans: Ionic or Non-ionic Sensitivity differences Human Qui: 10M Dntn: 10nM SOA: 7M Caff: 3mM Ionic Hamster Qui: 0.3mM Dntn: 1mM SOA: 1mM Caff: 3mM Non-Ionic Taste the same to humans

15. Species Differences: 2 “Bitters” in Hamsters Conditioned Taste Aversion CS: Conditioned stimuli CS: Qui = 1 mM QuinineHCl, Den = 3 mM Denatonium benzoate. MgS = 180 mM MgSO4, Caf = 100 mM Caffeine, SOA = 1.5 mM Sucrose octa-acetate. From Frank et al., 2004.

16. Taste Bud Type I, II, III Receptor cells Taste Receptor Cells 10-day Lifespan I: dark, glia-like II: T1 & T2 Rs III: synapse with nerve.

17. Cell Types in Taste Bud Pannexin 1 hemichannels- ATP From Finger, 2005

18. FromChaudhari N, Roper SD. 2010. The cell biology of taste. J Cell Biol 190:285-296. I. Type I cells degrade/absorb neurotransmitters and may clear extracellular K+ that accumulates after action potentials (shown as bursts) in receptor and presynaptic cells. II. Receptor cell. Taste compounds induce release of ATP through pannexin1 (Panx1) hemichannels. The extracellular ATP excites ATP receptors (P2X, P2Y) on sensory nerve fibers and on taste cells. Presynaptic cells, in turn, release serotonin (5-HT), which inhibits receptor cells. III. Presynaptic cell. Sour stimuli directly activate presynaptic cells. Only presynaptic cells form ultrastructurally identifiable synapses with nerves.

19. Distribution of Taste Buds in Mouse Oral Cavity From Yamolinsky et al., 2009

20. Diagram of Rat Tongue cv fn fo fn : fungiform, fo : foliate, cv : circumvallate

21. [Glossopharyngeal] [X n. to Pharynx] 3 Cranial Nerves areTaste Nerves [Geniculate ganglion] Place taste buds are suggests function, CT easier to dissect than GL

22. Chorda Tympani Generalist and Specialist Neurons CT = chorda tympani Generalists H neurons Specialists N neurons S neurons

23. N E N E + Amiloride + CT nerve Activity Activity Na+ K+ NH4+ Na+ K+ NH4+ Na+ K+ NH4+ Na+ K+ NH4+ NaCl CTA Behavior Behavior Na+ K+ NH4+ Na+ K+ NH4+ Amiloride block of ENaC changes taste of NaCl in rodents. Rodent Na+ taste is more specific than human salty taste. The GL does not use ENaC to detect Na+; CT and GSP do.

24. Rat Glossopharyngeal Nerve Recording From Frank et al., 2008

25. Rodent Taste Pathways Humans no PbN relay From Yamolinsky et al., 2009

26. Convergence of Taste Information on Brainstem Neurons CN VII CT = chorda tympani nerve GSP = greater superficial petrosal nerve NTS = n. solitary tract P = palate T = tongue PT= : same stimulus PT : different stimuli Role of Inhibition? GABAergic and glycinergic inhibitory neurotransmitters in NTS

27. Human Central Taste Pathways N. VII = facial nerve N. IX = glossopharyngeal nerve N. X = vagus nerve

28. Dealing with Natural Settings A simple way to simulate natural tasting in humans. Vary Timing and Concentration of Multiple Distinct Stimuli A taste example. • 4 Stimuli Presented in Pairs: Adapt-Test. • N = 100 mM NaCl • S = 300 mM sucrose • NS = Mix of N and S • 0 = Water • 10 subjects identified 2 replicates of 16 test stimuli after 5-sec adaptation. • Stimuli were “Taste Puffs” to the tip of the tongue.

29. Within the binary mixture, the salt taste was less identifiable than the sugar taste of sucrose [dotted horizontal lines]. Sugar was just about perfectly identified as a single, mixture or EXTRA component. The salt taste was better identified as an EXTRA component after adaptation to sucrose. As AMBIENT mixture components, sugar and salt were even less salient than self-adapted single components. Characteristic tastes of sugar and salt) were readily identified when preceded by water (dotted horizontal lines) or after cross-adaptation but after self-adaptation or mixture-adaptation tastes were less salient. Mixture Suppression and Selective Adaptation A way to study how the brain uses peripheral labeled lines?

30. Convergence of Sensory Inputs in Orbitofrontal Cortex From Rolls, 2004

31. Gustatory System Summary • We perceive sweet, salty, sour and bitter taste qualities. • There are 3 types (I, II, III) of taste-bud receptor cells (TRC). • TRC transduce chemicals in aqueous solution, using either GPCR: T1R for sweet, T2R for bitter; or ion channels: TRP for sour, ENaC salty in rodents. • TRC turn over and there is a neuron-taste bud neuro-trophism. • Primary afferent neurons are specialists or generalists for taste quality. Specialists are labeled lines sending specific quality information from the receptors to the brain. • CNS processing of taste information begins in the nucleus of the solitary tract where neurons receive convergent input, show inhibition, and project to VPMpc in the thalamus. • Thalamic taste neurons project to primary taste cortex in anterior insula and frontal operculum. • Taste-quality discrimination and learning require thalamo-cortical pathways, supplemented by pathways to amygdala and hypothalamus to add motivational and hedonic features to tastes.

32. SUC ALC ALC ALC WAT WAT WAT WAT AN INTERESTING PAPER Free-Access Drinking Gulick D, Green AI. 2010. Role of caloric homeostasis and reward in alcohol intake in Syrian golden hamsters Physiol Behav. 101: 518-526. Caloric Value EXPERIMENT 1 Preference for alcohol or an ascending sucrose concentration 14-day Baseline (2-bottles); 3rd bottle  Concentration every 5 days, Measure 4 days, skipping 1st of 5.

33. Stimulus Compounds Ethanol and sucrose are metabolized for energy. 15% ethanol and 0.614 M sucrose are equi-caloric. Saccharin is non-caloric; 4 mM saccharin and 614 mM sucrose are preferred by hamsters. Ethanol caloric value is 7 calories, Sucrose 4 calories, Food 3.4 calories per gram.

34. ETHANOL INTAKE Ethanol Intake falls as Sucrose concentration rises in the 3rd bottle. Saccharin has no effect. 2-way ANOVA. Sucrose Concentration, Group and Concentration by Group Interaction, all 3, p<0.001. Decreased alcohol consumption with sucrose but not saccharin shows hamster alcohol consumption is tied to caloric content. Suppression of alcohol intake by a sucrose solution of lower caloric content supports a role for reward value in alcohol consumption.

35. Results of our pilot study using conditioned taste aversions (CTA) suggests that ethanol is not bitter but “SWEET” to golden hamsters. Generalizations from 10% ethanol to 10 stimuli (TS) were tested with intake (mL) ratios for each ethanol-conditioned animal to mean control intake for water-conditioned animals. • Test Stimuli • Water • 5%,10%, 20% Ethanol • 10% Isopropyl Alcohol • 100 mM Sucrose • 10 mM Vanillin • 10 ppm Capsaicin • 10 mM Caffeine • 1 mM Quinine·HCl • The reward value of • sugar and ethanol may evoke sweet signs for calories in hamsters. Ratios for alcohols, sucrose and capsaicin fell beneath the ratio for control water.

36. The End