LECTURE 18:  OLFACTION AND TASTE
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LECTURE 18: OLFACTION AND TASTE. REQUIRED READING: Kandel text, Chapter 32. Smell and Taste are the chemical senses Smell (olfaction) is the discriminating sensation of volatile chemical odorants by the olfactory system

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LECTURE 18: OLFACTION AND TASTE

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Lecture 18 olfaction and taste

LECTURE 18: OLFACTION AND TASTE

REQUIRED READING: Kandel text, Chapter 32

Smell and Taste are the chemical senses

Smell (olfaction) is the discriminating sensation of volatile chemical odorants by the

olfactory system

Taste is discriminating sensation of soluble chemicals by the gustatory system

Olfaction is far more discriminating than taste, and much of our subtle perceptual

distinctions in flavors require integrating gustatory, olfactory & somatosensory information


Lecture 18 olfaction and taste

ODORS ARE DETECTED BY NASAL OLFACTORY SENSORY NEURONS

Apical dendrite of sensory neuron projects through support cells to nasal cavity, and

is capped by dendritic cilia projecting into specialized mucus in the cavity

Olfactory sensory neurons are fairly short-lived (1-2 months), and regenerate from basal stem cells

Each sensory neuron responds to a single odorant or a specific repertoire of chemically related odorants

An odor is ENCODED by the specific combination of neurons which respond to it

Sensory neurons respond to odorant by inward current flow, which depolarizes neuron.

There is a relationship between odorant concentration and size/duration of inward current;

sufficient depolarization triggers action potential.


Lecture 18 olfaction and taste

OLFACTORY SENSORY NEURONS EXPRESS ODORANT RECEPTORS

Clue to discovery of odorant receptors: Odors trigger cAMP synthesis in

olfactory sensory neurons

Linda Buck and Richard Axel reasoned odorant receptors were G-protein-coupled receptors

They searched for novel GPCRs expressed in subsets of olfactory sensory neurons

using the techniques of reverse transcription + polymerase chain reaction (RT-PCR)

combined with in situ hybridization (ISH)

Method led to discovery of

~1000 odorant receptor genes in mammals,

each encoding a 7-TM GPCR

Odorant receptors can be classified into

subfamilies, each having somewhat

greater amino acid sequence similarity


Lecture 18 olfaction and taste

ODORANT RECEPTOR STIMULATION OPENS cAMP-GATED CATION CHANNEL


Lecture 18 olfaction and taste

RULES OF ODORANT RECEPTOR GENE EXPRESSION

SOME RULES OF VERTEBRATE ODORANT RECEPTOR (OR) GENE EXPRESSION

ARE SIMILAR TO ANTIBODY GENE EXPRESSION

One olfactory neuron ----------------> One OR gene expressed

Allelic exclusion: Only one of two alleles of an OR gene expressed in a neuron;

Allelic choice is random

MORE RULES

Each OR gene expressed in neurons interspersed within one of four domains of

nasal olfactory epithelium

All axons from sensory neurons expressing the same receptor converge on

one or a few glomeruli within the olfactory bulb

THEREFORE, CELLS FOR DETECTING AN ODOR ARE DISPERSED IN EPITHELIUM,

AND ALL DETECTION OF THE ODOR IS GATHERED AND SUMMATED

INTO A SPECIFIC CLUSTER OF OLFACTORY BULB NEURONS


Lecture 18 olfaction and taste

OR GENES EXPRESSED IN EACH OF FOUR SECTORS OF OLFACTORY EPITHELIUM


Lecture 18 olfaction and taste

CONVERGENCE OF AXONS EXPRESSING A SPECIFIC ODORANT RECEPTOR

TO ONE OR A FEW GLOMERULI IN OLFACTORY BULB

OE

OB

The P2 OR gene was genetically

tagged with to coexpress a

Tau-bGAL protein, which binds to

axon microtubules and is detected

With X-GAL.

All axons converge to a single glomerulus

(from Wang et al, Cell 93:47;1998)

Peppermint odor activates

a repertoire of odorant

receptors to stimulate a

distinct set of olfactory bulb glomeruli

(from Guthrie et al, PNAS 90:3329;1993)

All M50 OR-expressing

axons project to

one glomerulus,

as detected by ISH

(from Ressler et al,

Cell 79:1245;1994)


Lecture 18 olfaction and taste

OLFACTORY INFORMATION IN GLOMERULI IS INTEGRATED AND

DISTRIBUTED TO DIFFERENT BRAIN CENTERS


Lecture 18 olfaction and taste

PHEROMONES ARE SPECIES-SPECIFIC ODORANTS SENSED THROUGH

A PARALLEL OLFACTORY SYSTEM

Specific pheromone receptors

expressed in dispersed

sensory neurons within

the veromonasal organ


Lecture 18 olfaction and taste

OLFACTORY RECEPTORS ARE USED TO GUIDE AXONS TO PROPER GLOMERULI:

OLFACTORY SENSORY AXONS LACKING ODORANT RECEPTOR WANDER AND DIE

(from Wang et al, Cell 93:47;1998)


Lecture 18 olfaction and taste

OLFACTORY RECEPTORS ARE USED TO GUIDE AXONS TO PROPER GLOMERULI:

CHANGING ODORANT RECEPTOR EXPRESSED IN A NEURON CHANGES ITS PROJECTION

When different odorant receptors are

engineered to be expressed in cells that

would normally express the P2 receptor,

the axons of these neurons project to

a new glomerular “address”.

In P3 ---> P2 neurons, axons project to

where P3 neuron axons go.

But for other misexpressions, the

glomerular address is different from

both that of P2 or of the replacement OR.

Therefore, while the specific OR is a

determinant of axonal pathfinding,

it is NOT the only determinant.


Lecture 18 olfaction and taste

ORGANIZATION OF THE TASTE BUD

Each taste bud contains ~100 taste cells

Mature taste cells are very short-lived,

and are continuously regenerated

from basal cells

Apical microvilli of taste cells are

exposed to saliva through the taste pore

Tasty substance is sensed at microvilli

by several mechanisms, but always

induces depolarization and

action potential generation

Taste cell action potential releases

neurotransmitter which activates

gustatory afferent fiber

Taste cells can detect

one of five known tastes:

SOUR

SALTY

SWEET

BITTER

UMAMI


Lecture 18 olfaction and taste

DIFFERENT TASTE STIMULI USE DIFFERENT SIGNAL TRANSDUCTION METHODS

SALTY-sensing taste cells express

amiloride-sensitive sodium channels.

Sodium in salts enters through channel

to depolarize cell.

Potassium-type salts also stimulate

these cells because of leak potassium

channels and change in EK

SOUR-sensing taste cells express

proton-sensitive potassium

leak channels.

Acid H+ ions (protons) block

these potassium channels,

reducing gK and

depolarizing cell.


Lecture 18 olfaction and taste

DIFFERENT TASTE STIMULI USE DIFFERENT SIGNAL TRANSDUCTION METHODS

BITTER-sensing taste cells use 7-TM receptors coupled to various G proteins. Bitter sensation is method for recognizing TOXIC compounds.

There is a family of related bitter receptors.

Some receptors couple to Gq which activates

PLC to increase Ca+2 through IP3

Other receptors couple to gustducin, which

activates cyclic nucleotide phosphodiesterases

A few bitter compounds act by directly blocking

leak potassium channels

SWEET-sensing taste cells use 7-TM

receptor coupled to Gs.

Sugars act through Gs to produce

cAMP, and PKA phosphorylates

and closes potassium leak channels,

causing depolarization

Alternatively, some substances

(artificial sweeteners) bind receptors

coupled to Gq which activates

PLC to increase Ca+2 through IP3


Lecture 18 olfaction and taste

TRANSMISSION OF TASTE INFORMATION TO THE BRAIN

AND PERCEPTION OF TASTE

Integration of taste stimuli first occurs in afferant gustatory fibers, since each fiber receives input

from multiple taste cells of different types. Each gustatory afferent fiber has a response profile to 5 tastes

Taste stimuli project to the gustatory cortex, where it is consciously perceived.

Taste stimuli are integrated with somatosensory inputs to generate perception of where tasty substance is.

Taste perception is also shaped by parallel olfactory input;

the somatosensory stimulus “fools” us to perceive the olfaction as part of the taste.


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