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 Taste is discriminating sensation of
soluble chemicals by the gustatory
system Olfaction is far more discriminating than
taste, and much of our subtle perceptual distincti
ons in flavors require integrating gustatory,
olfactory somatosensory information
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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.
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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
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ODORANT RECEPTOR STIMULATION OPENS cAMP-GATED
CATION CHANNEL
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RULES OF ODORANT RECEPTOR GENE EXPRESSION

• SOME RULES OF VERTEBRATE ODORANT RECEPTOR (OR)
  GENE EXPRESSION
• ARE SIMILAR TO ANTIBODY GENE EXPRESSION
• One olfactory neuron ----------------gt 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,

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OR GENES EXPRESSED IN EACH OF FOUR SECTORS OF
OLFACTORY EPITHELIUM
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CONVERGENCE OF AXONS EXPRESSING A SPECIFIC
ODORANT RECEPTOR TO ONE OR A FEW GLOMERULI IN
OLFACTORY BULB
OB
OE
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 93471998)
Peppermint odor activates a repertoire of
odorant receptors to stimulate a distinct set of
olfactory bulb glomeruli (from Guthrie et al,
PNAS 9033291993)
All M50 OR-expressing axons project to one
glomerulus, as detected by ISH (from Ressler et
al, Cell 7912451994)
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OLFACTORY INFORMATION IN GLOMERULI IS INTEGRATED
AND DISTRIBUTED TO DIFFERENT BRAIN CENTERS
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PHEROMONES ARE SPECIES-SPECIFIC ODORANTS SENSED
THROUGH A PARALLEL OLFACTORY SYSTEM
Specific pheromone receptors expressed in
dispersed sensory neurons within the veromonasal
organ
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OLFACTORY RECEPTORS ARE USED TO GUIDE AXONS TO
PROPER GLOMERULI OLFACTORY SENSORY AXONS LACKING
ODORANT RECEPTOR WANDER AND DIE
(from Wang et al, Cell 93471998)
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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 ---gt 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.
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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 S
WEET BITTER UMAMI
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DIFFERENT TASTE STIMULI USE DIFFERENT SIGNAL
TRANSDUCTION METHODS
SALTY-sensing taste cells express amiloride-sensit
ive sodium channels. Sodium in salts enters
through channel to depolarize cell. Potassium-typ
e 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.
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DIFFERENT TASTE STIMULI USE DIFFERENT SIGNAL
TRANSDUCTION METHODS
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
Ca2 through IP3
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 Ca2 through IP3 Other receptors couple
to gustducin, which activates cyclic nucleotide
phosphodiesterases A few bitter compounds act by
directly blocking leak potassium channels
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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.