Lecture 2 - Taste Pt. 1

Question 1

What two senses create our sensation of flavor?

Answer 1

taste and smell; what is going on in the tongue and the odor comes from the retro nasal pathway

Question 2

What are the 5 taste qualities and which ones are attractive and which are aversive?

Answer 2

Attractive
-sweet - sucrose (energy source)
-umami - monosodiumglutamate (components of protein); does not activate cortical pathways so we do not really detect it but it might be a bit salty because of the sodium which is a separate set of taste cell
-salty - NaCl (electrolyte)

Aversive
-sour - protons, acids (citrus and unripe fruits)
-bitter - many different chemical most of which are toxins

Question 3

What are the taste receptor cells; how many epithelial cells do they have and how many tastes are they dedicated to?

Answer 3

-taste buds
-50-100
-dedicated to one of the five taste

Question 4

How are taste buds or taste receptor cells different from olfactory sensory neurons and what are they more similar to?

Answer 4

-different since they do not have an axon and are similar to photoreceptors cells in the eyes since they have a change in membrane potential and release neurotransmitters which synapse onto afferent nerve fibers

Question 5

How often does the taste system regenerate?

Answer 5

continuously

Question 6

Where are circumvallate taste buds found?

Answer 6

-on the back of the tongue

Question 7

Where are the fungiform taste buds found?

Answer 7

-on the front side of tongue

Question 8

Where are foliate taste buds found?

Answer 8

-on the sides of your tongue

Question 9

How many taste receptor cells/epithelial cells per taste bud?

Answer 9

50-100

Question 10

Do taste receptor cells have axons and what are they innervated by?

Answer 10

-no axons
-innervated by fibers originating in taste ganglia that travel in cranial nerves

Question 11

What is the nature of the taste receptors and what chemical do they detect (need to clone them)?

Answer 11

-sweet receptor, umami receptor, bitter receptor, etc.

Question 12

On what animals was the sweet receptor cloned?

Answer 12

mouse mutant that was less sensitive to sweet; they were inbred to make them less susceptible to sweet and they could identify the sea locus which is how they identified the gene

Question 13

What type of receptors are sweet and umami?

Answer 13

GPCRs - means they signal through G proteins like odorant receptors

Question 14

What two subunits are the sweet and umami receptors composed of?

Answer 14

two subunits - one shared and one unique - the ligand binds to one component of the receptor causing a conformational change

Question 15

How many receptors does sweet have?

Answer 15

T1R2 and T1R3

Question 16

How many receptors does umami have?

Answer 16

T1R1 and T1R3

Question 17

How was it proven that T1R2 and T1R3 respond to sweet?

Answer 17

-the receptors were transfected or introduced into tissue culture cells and the red indicates responding cells to different sucrose concentrations in calcium imaging
-need T1R2 and T1R3 to generate a respond to moderate concentrations of sucrose and to saccharin

Question 18

Since the sweet and umami receptors are GPCRs are they calcium permeable?

Answer 18

-not they are not calcium permeable; the calcium elevation is due to coupling of the taste receptor to the signaling pathways that activate intracellular IP3 receptors which release calcium from the ER or intracellular stores
-GPCR —-> PIP3 split by phospholipase C —-> becomes IP3 and DAG (DAG with calcium activates PKC)

Question 19

How was it tested to determine whether T1R2 and T1R3 were necessary or not to generate a response to sweet?

Answer 19

-a knockout experiment was performed
-they performed a nerve recording in different knockout animals; the nerve is an integrations of APs and we sum it to get an integrated nerve response
-T1R1 knockout caused no effect cause that is the umami receptor
-T1R2 and T1R3 knockout caused desensitization to sugar and both being knocked out caused complete eradication of the sweet receptor

Question 20

How many bitter receptors are there and what are they called?

Answer 20

30 receptors for bitter and that are all GPCRs and they are called T2Rs

Question 21

What is each bitter receptor sensitive to?

Answer 21

one or more bitter tasting chemicals (quinine, caffeine, etc)

Question 22

What do all bitter cells express and what does that cause?

Answer 22

they express all 30 bitter receptors which can generate a signal leading to transmitter release which is why we cannot distinguish among different chemicals that taste bitter

Question 23

Why is the bitter taste not differentiated from an evolutionary perspective?

Answer 23

-it indicates we should not eat something

Question 24

Where do sweet and umami receptors bind their ligand compared to the bitter receptor?

Answer 24

-sweet and umami ind their ligand in the extracellular part of the protein while the bitter receptor binds its ligand in the TM domain

Question 25

How does bitter, sweet, and umami signal transduction work?

Answer 25

1. ligand binds to GPCR
2. activates the G protein
3. G protein activates PLC (PLCB2)
4. PLC breaks down PIP3 into IP3 and DAG
5. IP3 opens the calcium channel on the ER
6. intracellular calcium goes up
7. the TRPM5 ion channels is opened by the higher intracellular calcium
8. sodium rushes in depolarizing the cell

Question 26

What are the two elements unique to taste cells?

Answer 26

-the TRPM5 sodium channel
-PLCB2

Question 27

What is used to perform mouse taste tests?

Answer 27

the brief access lickometer
-the animal is presented with each stimulus for 5-10 seconds and the licks are counted
-if presented with a bitter stimulus licks decrease if presented with a sweet stimulus lick will increase
-two bottle taste preference

two bottle taste preference
-animal is given two bottles over a period of 30mins to 24hours and total fluid consumed is measured

Question 28

What happened two TRPM5 KO mice and the PLCB2 knockout mice?

Answer 28

they could not taste bitter, sweet, or umami

Question 29

What are the two models for taste coding?

Answer 29

1. cells are dedicated to one of the 5 tastes and signaling to brain is hard wired
2. cells express a mix of receptors and the brain sorts it all out

Question 30

How was it tested whether the same taste cells mediate response to bitter and sweet?

Answer 30

PLCB2 rescue experiment
1. eliminate all signaling by removing PLCB2 so the animals cannot taste bitter, sweet, or umami
2. add PBLC2 back under the promoter of a bitter receptor in a transgenic mouse
3. Does this restore the sensitivity to sweet? (if no then the cells that make the bitter receptor are different than those that make the sweet receptor; if yes then some cells make the sweet and bitter receptor)

Conclusion: that cells that make bitter receptors and those that make sweet and umami receptors are distinct

Question 31

What two elements needed to be introduced into a transgenic mouse for the PLCB2 rescue experiment to determine whether or not the same taste cells mediate response to bitter and sweet?

Answer 31

1. the taste receptor promoter or T1R or T2R
   -directs the expression of the trans gene to cells that are making the taste receptor (T2R is the bitter receptor)
2. transgene - this could be GFP, TRPM5, PLCB2, another GPCR (you can put in the GFP to make sure the animal is making the PLCB2)

Question 32

Can we make an animals avoid a random substance by putting the receptor for that substance into the bitter receptor expressing cells and can we make an animal become attracted to a substance by putting the receptor for that substance into sweet-receptor expressing cells?

Does the cell type determine the behavioral output?

Answer 32

Put the unnatural receptor RASSL for tasteless compound spiradoline under T2R bitter receptor promoter and found that the transgenic animals acid the agonist (spiradoline)

(note that in this study the Dox or doxycycline is used to induce the RASSL expression)

Question 33

What is a common taste polymorphism that is a dominant trait?

Answer 33

taters of PTC

Question 34

What allelic variation underlies variation in sensitivity to PTC?

Answer 34

in the Tas2R38 gene

-PAV is the taster and AVI is the nontatser