TASTE

Question 1

The many flavors we perceive are derived from the initial detection of 5 basic tastes:

Answer 1

– Detecting energy content (sweet, umami) – Sugars, e.g., fructose (in fruits) and sucrose (in white sugar)
– Some proteins, e.g., monellin (in serendipity berry)
– Artificial sweeteners, e.g., saccharin and aspartame
Amino acids taste savory (umami), e.g., glutamate or monosodium glutamate (MSG)
– Maintaining electrolyte balance (salty) Most salts taste salty, e.g., table salt, i.e., sodium chloride (NaCl)
– Monitoring pH level (sour) Most acids taste sour, e.g., hydrochloric acid (HCl)
– Avoiding toxins (bitter) – Simple ions, e.g., K+ and Mg2+
– Complex organic molecules, e.g., caffeine and quinine

Question 2

Tongue Anatomy

Answer 2

“Papillae” visible to naked eye.
Each papilla has from 1 to 100’s of microscopic “taste buds”.
Taste pore, on surface of tongue, exposed to mouth contents. This is where substances dissolved in saliva interact with taste cells
Each taste bud has 50-150 taste cells. A person typically has 2000-5000 taste buds.

Question 3

Taste stimuli depolarize taste cells

Answer 3

The membrane potential of taste cells
changes (depolarizes) when activated
by appropriate substance
Some taste cells respond primarily
to one basic taste; whereas other
cells respond to 2 or more tastes

Question 4

Salty taste mainly
due to sodium (Na+)

Answer 4

Na+ concentration needs to be quite high (≥10mM) to taste it. Salt-sensitive cells have Na+-selective channels.
This Na+ channel is always open (voltage insensitive). Increased Na+ in mouth leads to increased Na+ influx through Na+ channel. This depolarizes the cell,
causing calcium (Ca2+) influx through voltage-sensitive Ca2+ channels.
→Transmitter is released.

Question 5

Sour taste mainly due to
acidity (hydrogen ions, H+)

Answer 5

Acids dissolve in water producing H+ ions. Sour-sensitive cells have Na+ and K+ channels. H+ ions enter cells through Na+ channels. H+ binds to and blocks potassium (K+) channels. Blocking K+ channels reduces K+ efflux. H+ actions via Na+ and K+ channels depolarize cells. Ca2+ influx through voltage-sensitive Ca2+ channels.
→ Transmitter is released.

Question 6

Bitter, sweet and umami substances bind to GPCRs

Answer 6

– This initiates intracellular signaling events
– G-proteins stimulate enzyme called “phospholipase C”
– Phospholipase C produces inositol triphosphate (IP3)
Na+ influx into taste cells via Na+ channels
– IP3 opens type of Na+ channel unique to taste cells
– Na+ influx depolarizes taste cells
Increased Ca+ concentration inside taste cell
– IP3 triggers release of Ca+ from intracellular Ca+ storage sites
– This Ca+ activates channels permeable to adenosine triphosphate (ATP)
Transmitter release stimulates axons in cranial nerves
– ATP acts as the transmitter (but in unconventional way)
– ATP is released by diffusing through ATP-permeable channels

Question 7

GPCRs made of one or more protein subunits

Answer 7

– T1R and T2R genes encode for taste protein subunits
– Different genes encode for bitter, sweet and umami GPCRs

Bitter, sweet and umami proteins in different cells

Question 8

Bitter receptors

Answer 8

– 25 different types of bitter receptors
– These receptor types comprise the family of T2R proteins
– Bitter receptors likely made of two different T2R proteins

Question 9

Sweet receptors

Answer 9

– Only one type of sweet receptor
– Formed from two T1R proteins, i.e., T1R2 and T1R3

Question 10

Umami receptors

Answer 10

– Only one type of umami receptor
– Formed from two T1R proteins, i.e., T1R1 and T1R3

Question 11

Taste information is carried by 3 cranial nerves to the brainstem

Answer 11

Anterior 2/3 of tongue and palate send axons into cranial nerve VII
Posterior 1/3 of tongue send axons into cranial nerve IX
Throat regions (glottis, epiglottis, pharynx) send axons into cranial nerve X

Question 12

Some cells in cranial nerves are broadly-tuned; others are selective

Answer 12

CT = chorda tympani nerve (anterior tongue), branch of cranial nerve VII
GSP = greater superior petrosal nerve (palate), branch of cranial nerve VII
GP = glossopharangeal nerve (posterior tongue), cranial nerve IX
SLN = superior laryngeal nerve (throat), branch of cranial nerve X

Question 13

Left gustatory nucleus

Answer 13

Part of
“Nucleus of the
solitary tract”

Question 14

Broadly-tuned cells common in nucleus of solitary tract (NST)

Answer 14

“Broadly tuned” means
responds to a number of
different taste stimuli

i.e., gustatory nucleus in brainstem

Question 15

Cortical areas for taste processing

Answer 15

Secondary gustatory cortex in orbitofrontal cortex
Primary gustatory cortex in insula

orbitofrontal cortex is important for reward processing and valuations

Question 16

How can broadly-tuned cells discriminate between taste stimuli?

Answer 16

Problem: Individual cells commonly respond to more than one taste stimulus
– That is, individual cells may not be able to differentiate different taste stimuli
Solution: Ensemble (group) of cells may allow better discrimination of taste stimuli?
– As long as broadly-tuned cells have only partially overlapping responses to different stimuli
Imagine 4 cells. Each cell responds to more than one taste. Although 1 cell may poorly discriminate taste stimuli, taking into account the relative responses of all cells allows better taste discrimination.
E.g., robust response from the blue and green cells, but little/no response from pink and orange cells would unambiguously signal taste 1.

Question 17

A topographic map in GC? Recent evidence suggests selective GC cells?

GC = primary gustatory cortex

Answer 17

But recent work suggests that many GC cells may preferentially respond to one taste stimulus
– This selectivity contrasts with the broad tuning reported in older studies
– Does this reflect methodological differences? E.g., anesthetized vs behaving animals?
A topographic (gustotopic) map in GC has been reported
– Each taste encoded in separate region of GC
– (Right) “Hot spots” for sweet (green), salty (orange), bitter (red) and umami (yellow) in mouse GC

Question 18

Ensemble of cells in orbitofrontal cortex discriminates taste stimuli

Answer 18

Rat given water or sucrose in separate blocks of trials (this block structure allowed anticipation of taste stimuli)
Neural activity from ensemble of cells could discriminate between sucrose and water, even in anticipation period before licking/tasting

Question 19

Population of cells in gustatory cortex reflects level of satiation

Answer 19

Rat licks dispenser containing sucrose solution
Frequency of licking reflects level of satiation
Time between licking events called inter-trial interval (ITI)

Question 20

Specific satiety signaling in orbitofrontal cortex of monkeys

Answer 20

Response of orbitofrontal neurons decreased during consumption to satiety
– E.g., response to glucose decreased to near zero
This satiety effect was specific to the substance consumed to satiety
– E.g., although response to glucose decreased, there was little change in response to blackcurrant
This satiety effect was not due to peripheral adaptation
– No corresponding decrease in response from, e.g., nucleus of solitary tract in brainstem

Question 21

Multimodal integration in gustatory cortex in humans

Answer 21

Taste-odor integration in insula and
orbitofrontal cortex
Posterior insula only responded to water when subject thirsty
Taste-touch integration in insula (response to sucrose and viscous solution)
Pleasantness of taste-odor mixtures in orbitofrontal cortex
Anterior insula responded to water independent of thirst
Response to sucrose and fatty acid solution in orbito-frontal cortex