The Chemical Senses: Taste. Mary ET Boyle, Ph.D. Department of Cognitive Science, UCSD

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1 The Chemical Senses: Taste Mary ET Boyle, Ph.D. Department of Cognitive Science, UCSD

2 Remembrance of Things Past by Marcel Proust

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4 Most sensations are identified with a particular type of stimulus.

5 sugar on the tongue tastes sweet

6 How physical stimuli correlate with sensations is psychophysics

7 Major sensory modalities in humans are mediated by distinct Each class of receptor cell transforms one type of stimulus energy classes of receptor neurons located in specific sense organs. electrical signals that are encoded as trains of action potentials

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18 The primary function of the gustatory system is nutritional.

19 Taste: aesthetic and nutritive Is it the food safe? Gatekeeper sense Food interacts with receptor proteins on taste cells taste buds Transduce information about identity, concentration pleasantness and harmfulness of food. Signal to gastrointestinal system to receive and digest food salivation and swallowing or gagging and regurgitation

20 Sensory system goal: to generate internal representation of the outside world! Animals depend on the chemical senses to identify nourishment, poison, potential mate Not much has changed since primordial times! Chemical sensation Oldest and most common sensory system Humans to bacteria Chemical senses special sensory Gustation - taste Olfaction - smell Chemoreceptors task: to detect environmental signals mucus membranes; nerve endings on skin; digestive organs; arteries Odor-binding proteins on the antennae of dampwood termites mediate the transport of odoriferous chemicals to the olfactory nerves by encapsulating the hydrophobic scent chemicals in a water soluble coating.

21 Ominvore opportunistic eaters Innate preference for sweetness automatic acceptance An instinct to reject bitterness automatic rejection Inborn tastes? Nutritious vs. toxic Modify instinct enjoy coffee?? Deprived of essential salt? Internal deficiency crave food

22 Taste is a hot topic!

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26 Each Basic Taste has its own receptor! NaCl Acids - HCl K+,Mg++, quinine Sugars, etc Glutamate! elegant simplicity Saltiness Sourness Bitterness Sweetness Delicious Umami You are sour!

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28 salt sour bitter sweet Electrolytic balance we don t maintain a reservoir of salt we need to ingest salt daily. Protective factor acidic food change chemistry and destroy gut cells; Ripeness detector more sour = carbohydrates have not turned to sugars yet. Toxin detector many diverse molecules in the environment could be toxic; concentration of bitterness detector some bitter foods have medicinal properties. (Strychnine, arsenic, cyanide ) Energy-rich nutrient detector is useful to know which foods are a good and safe energy source and easily & rapidly absorbed. umami Amino-acid detector responds to L-glutamate found in protein foods (meats) and in some vegetables and fruits (tomatoes). Purpose? Identify dietary components

29 Endless chemicals = variety of tastes Taste receptors taste + smell = flavor Distinctive flavor Combination of Texture Temperature Pain Sensory modalities contribute to experience of food

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32 You don t just taste with your tongue! tongue, mouth, palate, pharynx, and epiglottis

33 Signals generated in the mouth are relayed through the brain stem and thalamus to the gustatory cortex.

34 Primary Gustatory Cortex Secondary Gustatory Cortex Thalamus VPM (Ventral Posterior Medial Nucleus) Frontal Operculum Insula Primary Gustatory Cortex Nucleus of the Solitary Tract Temporal Lobe

35 Primary Gustatory Cortex Thalamus VPM (Ventral Posterior Medial Nucleus) Frontal Operculum Secondary Gustatory Cortex Nucleus of the Solitary Tract

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37 Identify the following nerve a Vagus nerve b Facial nerve c Glossopharyngeal nerve d Cranial Nerve X

38 Identify the following nerve a Vagus nerve b Facial nerve c Glossopharyngeal nerve d Cranial Nerve IX

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40 Figure adapted from Bear et al 2007 & Purves, et al 2001

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42 VPM of thalamus Insular taste cortex and frontal operculum cortex ORGANIZATION OF THE TASTE SYSTEM Gustatory nucleus of the solitary tract Hypothalamus Amygdala Taste buds (anterior 2/3) Taste buds (posterior 1/3) Taste buds (epiglottis) Cranial nerve VII Cranial nerve IX Cranial nerve X Figure adapted from Purves, et al 2001

43 Taste pathways to thalamus and ctx are primarily ipsilateral Central taste pathways

44 Ageusia is the inability to detect and discriminate taste qualities. 3.2% of the German participants and 4.6% of the Norwegian participants were potential non-tasters of MSG.

45 This study suggests that L- glutamate may interact with a series of receptors at the surface of taste cells. The coding of the L-glutamate taste signal could result from the cooperation of these various receptors and, therefore, depending on genetic peculiarities, human subjects might perceive or not the proper umami taste. O. Lugaz, A.-M. Pillias and A. Faurion (2002) Chem. Senses

46 Cranial Nerve VII - Facial Somatic motor Special sensory Movement of muscles of facial expression Sensation of taste in anterior 2/3 of the tongue Recall, cranial nerves III-XII are like spinal nerves, in that they contain axons of the PNS. A single nerve can have fibers performing different functions. PNS (both somatic and visceral sensory) axons are afferent because the carry the information to a the CNS. In Latin: afferent= carry to ; efferent= carry from

47 Cranial Nerve IX - Glossopharyngeal Somatic motor Visceral motor Special sensory Visceral sensory Special sensory receptors are located in sense organs Movement of muscles in the throat Parasympathetic control of salivary glands Sensation of taste in posterior 1/3 of the tongue Detection of blood pressure changes in the aorta

48 Recall: First order neuron is a sensory neuron that delivers sensations to the CNS.

49 Chandrashekar, J. (2006) et al. Nature 444,

50 Is the tongue map an urban myth? Tip of the tongue Sweetness Back of the tongue Bitterness Sides of tongues There is no tongue map! All basic Saltiness modalities & sourness are present in all areas of the tongue! Chandrashekar, J. (2006) et al. Nature 444,

51 Photo: Shahbake, M. (2008)

52 Taste (TRC taste receptor cells) is in the papillae (of the beholder!) Papillae-taste sensitive structures Circumvallate papillae Foliate papillae Fungiform papillae Each papilla has one to several hundred taste buds. Taste buds have the TRC! Taste Receptor Cells are the taste detectors. Chandrashekar, J. (2006) et al. Nature 444,

53 What are papillae anyway? circumvallate papillae Each papilla has one to several hundred taste buds. A taste bud is a cluster of taste receptor cells. Each taste bud has 50 to 150 taste receptor cells. A person typically has taste buds although exceptional cases have as few as 500 or as many as 20,000.

54 Taste bud - Every bud can sense a variety of tastes. Taste buds are made of a variety of taste receptor cells, and each cell can sense different chemicals. Tiny finger-like structures on each cell are called microvilli and they have hundreds of receptors on them.

55 TRC Taste Receptor Cell are in the bud Taste Bud Reminds you of Cluster of TRCs Chandrashekar, J. (2006) et al. Nature 444,

56 Taste receptor cells Basal cells TRCs Taste pore Apical end near surface of tongue Chemically sensitive part of TRC Microvilli project into pore TRC synapse with gustatory afferent axons Gustatory afferent axons Synapse Microvilli TRC chemical and electrical synapse onto basal cells (note to self TRCs are not neurons, but they act like them!)

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58 When a TRC is activated by an appropriate tastant, its membrane potential changes, usually depolarizing. The voltage shift is called the receptor potential. Receptor potentials Action potentials

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60 Slowly adapting mechanoreceptor responds as long as the pressure is applied to the skin. A rapidly adapting mechanoreceptor responds at the beginning and end of the stimulus, Firing rates of sensory neurons convey information about stimulus intensity and time course Signals the rate at which the probe is applied and removed The total number of action potentials discharges during the stimulus is proportional to the amount of pressure applied to the skin. Figures adapted from: Mountcastle et al (1966)

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67 How is Taste Coded? It is not yet known if a particular taste is due to the response of a specific nerve fiber (specificity coding) or if it results from the pattern of firing across several different nerve fibers (i.e., distributed coding). There is some evidence in favor of both types of neural coding.

68 Labeled Line Encoding predicts that individual taste receptor cells will respond to only a single taste quality. bitter sweet sweet MSG MSG Information about each taste quality is transmitted by separate afferent pathways to the gustatory cortex. bitter bitter MSG MSG bitter sweet bitter sweet MSG

69 Across-fiber pattern-coding model Individual taste cells respond to different taste qualities. Information is transmitted by afferent fibers that have broadly overlapping response spectra. The code for a particular quality is determined by a pattern of activity across all of the afferent nerve fibers.

70 Response profiles of individual chorda tympani axons to four different stimuli (indicated by the four different colors). The numbers indicate individual axons. The responses reflect the net activity for 5 seconds after application of each tastant. Axons 1-10 respond best to sweet tastant. Axons respond best to NaCl along with axons Axons respond best to HCl Remap data Purves, et al (2001) Neuroscience

71 Across fiber encoding Each row summarizes the response of all 40 axons to a single taste stimulus. Each distinct tastant has a unique pattern of activation. It appears that the population-response is what determines the sensation associated with each tastant. Purves, et al (2001) Neuroscience

72 each taste quality is specified by the activity of nonoverlapping cells and fibers. Labeled line encoding model TRC tuned to respond to a single taste modality TRC innervated by single afferent axon Axon signal to brain taste perception Across Fiber Encoding - I Multimodal TRCs: TRCs are tuned to multiple taste qualities 1 TRC responding to multiple tastants 1 afferent axon Across Fiber Encoding - II TRCs are tuned to a single taste quality A single afferent axon will carry information for more than one taste modality Figures adapted from Chandrashekar, J. (2006) et al. Nature 444,

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76 Mechanisms of Taste Transduction Tastant mechanisms of action: Directly pass through ion channels (salt and sour) Bind to and block ion channels (sour) Bind to G-protein-coupled receptors in the membrane that activate second messenger system open ion channels (bitter, sweet, umami) Salt-sensitive taste cells Membrane potential depolarization VGCC xmtr release Special Na + selective channel Blocked by the drug amiloride

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79 Comparison of Salt and Sour tastants receptor potentials: 1. Na+ directly passes into amiloridesensitive Na channel. 2. Na+ inward current diffuses down it s concentration gradient and creates a receptor potential that triggers VGCC and VGNa channels to open. 3. Inward Ca++ current 4. 5-HT transmitter release. 5. Signal to gustatory afferent axon. 1. H+ is permeable to amiloridesensitive Na channel. 2. Causes an inward current causing the membrane to depolarize. 3. H+ blocks a special K+ channel. This action distinguishes it from the effect of Na. 4. It also blocks an outward current further depolarizing the membrane triggering the VNa and VGCC causing serotonin transmitter release Salt Na + Sour H +

80 five basic taste qualities: Salty Sour Sweet Umami Bitter G-Protein Coupled Receptor family (GPCRs) Foster, S. R. et al (2104)

81 Taste Receptors G-Protein Coupled Receptor family (GPCRs) Type1 T1R Type2 T2R Foster, S. R. et al (2104)

82 G-Protein Coupled Receptor family: taste receptors type 1 and type 2 The T1R family has 3 members within the Glutamate/family C GPCR group sweet umami The T2R family consists of 25 highly divergent GPCRs that mediate bitter taste. bitter Foster, S. R. et al (2104)

83 Transduction mechanism for bitter, sweet, umami 1. G-protein coupled (dimers) receptors T1R and T2R 2. Bitter receptors signal poison danger! 3. Over 30 different taste receptors but the brain cannot readily distinguish between the bitter tastants. 4. Same 2 nd messenger system for bitter, sweet, umami! G-proteins activates phospholipase-c With increases synthesis of inositol triphosphate (IP 3 ) IP 3 triggers release Ca ++ from calcium stores, Taste specific Na + ion channel depolarization and xmtr release. The main transmitter is ATP diffusing through ATP-permeable channel

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87 Bitterness: Families of taste receptor genes - TIR and T2R Bitter: quinine-sensitive K+ channel

88 Sweet receptors T1R2+T1R3 Expressed in different taste cells MSG Sugar Sweetener Sweet and umami are mediated by GPCRs T1R1, T1R2 & T1R3 distantly related to metabotropic glutamate receptor.

89 Transduction mechanisms in a generic taste cell In general, the greater the tastant concentration, the greater the depolarization of the taste cell. The apical surface contains both channels and G-proteincoupled receptors that are activated by chemical stimuli. The basolateral surface contains voltage-gated Na +, K +, and Ca 2+ channels, as well as all the machinery for synaptic transmission mediated by serotonin.

90 Mechanisms of Taste Transduction Sweetness Sweet tastants natural and artificial are detected by the same taste receptor protein. Sweet receptors T1R2+T1R3 Expressed in different taste cells

91 Mechanisms of Taste Transduction Umami Detects amino acids Only one type Formed by: T1R1+T1R3

92 Mechanisms of Taste Transduction Bitter Poison detectors Difficult to distinguish different bitter tastants About 25 types of bitter receptors: Family of T2Rs Dimers consisting of two different T2Rs 2 nd messenger pathway Bi Bitter receptors: dimers of T2Rs

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94 Concluding Remarks Transduction mechanisms Gustation Similar to the signaling systems used in every cell of the body Common sensory principles - broadly tuned cells Population coding Timing of action potentials May represent sensory information in ways not yet understood

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