Taste (2 Lectures)

Question 1

What is taste?

Answer 1

Activation of oral taste cells (vertebrates) or gustatory receptor neurons (insects) which contain GRs

Not the same as flavour (=combined experience of olfaction and gustation)

Question 2

Types of taste

Answer 2

Bitter, sweet, sour, salt, umami, kokumi (enhancement via calcium channels)

Question 3

GRs in anemone

Answer 3

Involved in development - may be original function of GRs

Question 4

Peripheral spatial coding of GRs

Answer 4

NO peripheral organisation (i.e. on tongue - no sweet area etc). There are, however, structural differences and different thresholds in different regions.

Question 5

Mammalian taste

Answer 5

Tongue and soft palate contain Rs. Send signals along nerve (chorda tympani), synapse at petrosal ganglion BEFORE entering brain (key different to olfaction).

Question 6

Papillae types

Answer 6

Circumvallate, foliate and fungiform - contain tastebuds. Each bud contains 3 cells: type 1 (glia-like), type 2 (receptor), type 3 (presynaptic). These are NOT neurons (release neurotransmitters but don’t have axons). We smell with neurons but don’t taste with neurons.
Varied no. of cells in buds; density positively correlates with sensitivity

Question 7

Taste bud structure

Answer 7

Type 1: 50% of cells in the taste bud, respond to salt.
Type 2: sugar/bitter/umami
Type 3: sour - only cell type that forms synapses with afferent nerve fibres

Question 8

Sour

Answer 8

Acid = protons (H+).
Blocks K+ channels –> depol. –> Ca2+ entry –> transmitter release

Difficult to identify sour cells because many ion channels not involved in sour taste and pH sensitive.

Question 9

Salt

Answer 9

Na+ enters via ENaC (epithelial Na Channels) –> depol. –> Ca2+ entry –> transmitter release via connection with type 2 cells.

Often sensitive to amiloride (won’t function) but not in humans. Humans only have amiloride-insensitive salt sense, generally respond to higher salt levels. Unknown R

Question 10

Type 2 cells

Answer 10

Rs similar to GPCRs
Bitter: T2R
Sweet: T1R2 + T1R3
Umami: T1R1 + T1R3
(same subunit more to do with structure, other to do with binding).

T2Rs and T1Rs have common intracellular pathway - TRPM5 (ion channel, allows sodium in) and CALHM1 (voltage-gated ATP release channel - ATP released after sodium enters)

Question 11

T2Rs (sharks, mice, humans, dolphins, reptiles, panda)

Answer 11

Bitter taste. 50-80 types of T2R gene with different ligand profiles. Absent in sharks - can’t taste bitter (bitter taste evolved after split of bony fish).

In mice, single type 2 cells express more than one type of T2R.

Humans: genetic variation for detection of PTC and PROP (sprouts). T2R38 associated with PTC tasting, multiple alleles involved in detection. Women more likely to be tasters.

Dolphins can’t taste bitter but reptiles can.

Pandas have more bitter-detecting TAS2R genes than carnivores (bamboo = bitter). In giant panda, T1R1 (umami) became pseudogene around same time it became a herbivore.

Question 12

Cats can’t taste sweet

Answer 12

Their T1R2 is a pseudogene. Also in tiger and cheetah.

Question 13

Next stage after taste buds

Answer 13

Taste buds innervated by afferent fibres –> brainstem and thalamus
Most ganglion neurons tuned to single taste qualities

Question 14

How long do taste cells live

Answer 14

5-20 days, continually replenished. How is representation in the brain maintained?
Bitter and sweet cells express specific guidance molecules (SEMA3A and 7A). KOs show more multituned brain neurons.

Question 15

Fish taste

Answer 15

Teleosts (major gropu of bony fish) have tastebuds in 5 pops: oral, palatal, laryngeal, branchial, cutaneous, barbels
Density and distribution depends on ecology and age

Question 16

Bird taste

Answer 16

Initially assumed to have limited gustation due to inertial feeding (limited tongue/food contact).
<5% taste buds on tongue, 70% on palate, 25% on lower mandible

No functional T1R2 (sweet) in any known bird

But… hummingbirds = nectar feeders. Like sugar and aspartame. T1R1/T1R3 detects umami in other birds, T1R1 mutated in hummingbird to detect sugar.

Question 17

Slugs and snails

Answer 17

Don’t like salt (can taste it). Recordings from taste neurons show much higher amp of response at night than day. Swapping light and dark –> shift in response (response is under circadian control).

Snails good at learning associations with salt.

Question 18

Drosophila taste

Answer 18

Many GR types expressed in single taste neurons.
Different life stages have different taste genome.
Receptors in terminal organ, ventral organ and pharyngeal sensilla. Project to sub-oesophogeal ganglion.

Have sweet and bitter sensing cells on their wings.