1 bi / cns 150 lecture 20 wednesday, november 13, 2013 olfaction: vertebrates / worms / insects...
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Bi / CNS 150 Lecture 20 Wednesday, November 13, 2013
Olfaction: vertebrates / Worms / insects
Henry Lester
Reading: Kandel Chapter 32, pp 625-636 (not taste)
(What is olfaction?)
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Proust, Remembrance of Things Past
“as soon as I had recognized the taste of the piece of madeleine soaked in her decoction of lime-blossom which my aunt used to give me (although I did not yet know and must long postpone the discovery of why this memory made me so happy) immediately the old grey house upon the street, where her room was, rose up like a stage set to attach itself to the little pavilion opening on to the garden which had been built out behind it for my parents (the isolated segment which until that moment had been all that I could see); and with the house the town, from morning to night and in all weathers, the Square where I used to be sent before lunch, the streets along which I used to run errands, the country roads we took when the weather was fine . . . “
Olfactory memory
The nose can detect and (in principle) classify thousands of different compounds.
The ‘mapping’ of these compounds probably occurs by matching to memory templates stored in the brain; thus, a smell is categorized based on one’s previous experiences of it and on the other sensory stimuli that correlate with its appearance.
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Recognition of chemicals by the olfactory system
The nose can distinguish very similar compounds as different smells.
An example: the two stereoisomers of carvone smell like spearmint and caraway.
This implies that there are stereoisomer-specific carvone receptors.
An earlier argument for proteins!
Carvone
Stereo center
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Anatomy of the mammalian olfactory system
In many mammals (rodent shown here), the
olfactory organs within the nose are split
into the main olfactory epithelium (MOE)
and the vomeronasal organ (VNO).
MOE neurons project to the main olfactory
bulb (MOB).
VNO neurons project to the accessory
olfactory bulb (AOB).
MOB output neurons project to regions of
cortex, while AOB output neurons project
only to the (ventral) amygdala.
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Cells of the mammalian main olfactory epithelium
Olfactory neurons have
apical dendrites with long
ciliary extensions, where
the transduction
components are located.
Cilia are embedded in the
mucus layer.
Olfactory neurons turn over
and are replaced every 60
days.
Axon
Olfactorysensory neuron
Dendrite
Cilia
Basalcells
Supportingcells
Mucus
To olfactory bulb
Figure 32-2
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Olfactory receptor proteins in vertebrates and most other phyla
Odorants bind to 7-helix (G-protein coupled) receptors.
In mice, >1000 genes (2-3% of genes!) encode these receptors.
Receptor sequences also are quite variable, especially in putative odorant-binding helices.
Thus, the repertoire is extremely diverse.
In mammals, each neuron probably expresses only a single receptor.
7GTP GDP + Pi
Effector: membrane-bound
enzymeoutside
Odorant binds to receptor
activatesG protein
The start of the G protein pathway in vertebrate olfaction
How fast?100 ms to 10 s
How far?Probably less 1 m
inside
Part of Fig. 32-3
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cytosol
The usual GPCR pathwaykinase
phosphorylatedprotein
cAMPCa2+
intracellularmessenger
receptor
tsqiG protein
enzymechannel effector
membrane
from Lecture 12
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Intracellular messengers bind to proteins
kinases
phosphorylatedprotein
A few ion channels(olfactory system,
retina)
N
NN
N
NH2
O
OHO
HH
O
P-O
O
cyclic AMP (cAMP)
Ca2+ and
but in a previous lecture, we said . . .intracellularmessenger
Ca2+ cAMPcGMP
Previous lecture
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The GPCR pathway in anolfactory cell
channel
receptor
tolfqiG protein
enzymechannel effector
intracellularmessenger
Ca2+ cAMPcGMP
Very similar to Gs
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Olfactory neurons have cAMP-activated Na+/Ca2+ Channels
Excised “inside-out” patch allows access to the inside surface of the membrane
no cAMP no channel openings
+cAMP
+cAMP
closed
open
receptor
qiG protein
channel
ts
enzymechannel effector
intracellularmessenger
Ca2+ cAMPcGMP
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More about olfactory channels and their role in olfactory transduction
Olfactory cAMP-gated channels are permeable to Na+ and Ca2+
Thus, odorant binding causes depolarization of the olfactory neuron through
Na+ entry.
Ca2+ also enters and activates a Cl - channel, increasing depolarization (ECl
is near zero in these cells).
This process stimulates the olfactory neuron to fire action potentials.
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Expression zones of 4 individual olfactory receptors(rat nose, coronal section)
The olfactory turbinates display four ‘expression zones’.
Each receptor is expressed in a small, randomly
distributed subset of neurons within one of the 4 zones
.
As there are ~1000 receptors, about 1/250 of neurons
within a zone express each receptor.
Neurons within each expression zone send axons to a
different quadrant of the olfactory bulb.
Another gene class, expressed in all olfactory neuronsFigure 32-5
K20
K20
L45
A16
Olfactoryreceptor
Olfactoryepithelium
Olfactorybulb
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Projections to the olfactory
bulb
Olfactory neurons send axons to the glomeruli (synaptic balls shielded by glia) of the olfactory bulb.Olfactory neurons excite mitral cells, which are the bulb output cells.
Olfactory sensory neuron
Mitral cell
Periglomerular cell
Tuftedcell
Inhibitory
Figures 32-1, 32-6
To lateral olfactory tractlike a bishop’s miter (hat)
perforated (Latin)
glomus, ball of yarn (Latin)
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Projections to specific glomeruli
Neurons expressing a specific
olfactory receptor project their
axons to a single glomerulus in
each half-bulb.
Axons converge from many
directions onto the target.
This projection specificity is at
least partly determined by the
receptor itself, but the
mechanisms are unknown.
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Mice: glomeruli connected to neurons expressing I7, a receptor for octanol, respond
to octanol.
This was also the case if the I7 glomerulus was moved to the wrong place in the bulb
by transplacing the I7 gene into the genomic locus for another receptor.
Glomerular odorant responses: Ca2+ imaging in a fish
Individual glomeruli are selectively activated by specific odorants.
In fish, “odorants” are soluble amino acids.
Imaging studies now show that specific glomeruli in mammals are also activated in response to odorants.
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Maps of mitral cell projections to higher olfactory areas
Piriform cortex neurons receive projections from mitral cells corresponding to
many glomeruli that receive input from ORNs expressing different receptors.
Mitral cells also project to olfactory tubercle and other areas.
Integration of odorant responses and odorant identification may take place in
cortex, although some integration is also likely to occur in the bulb.
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The vomeronasal organ
The VNO is thought to respond to pheromones.
It is a cup-shaped organ near the front of the rodent nose; its neurons are divided into basal and apical (near the lumen) layers.
The microvilli of the VNO neurons face the lumen.
Neurons in the apical layer express the G protein α subunit Gαi2, while those in the basal layer express Gαo.
The transduction channel and the receptors are located on the microvilli at the edge of the lumen.
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VNO receptor molecules
The 2 distinct families of VNO G protein-coupled receptors are all unrelated to MOE receptors.
Each VNO neuron probably expresses only one receptor, as in the MOE.
Figure 32-9
V2Rs (~100 genes in the rodent) are
expressed in a random pattern by basal layer
neurons (Go-expressing neurons). V2Rs have
large N-terminal extracellular domains.
V1Rs (~180 genes) are expressed by
different subsets of neurons within the
apical layer (Gi-expressing neurons).
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The GPCR pathway in a VNO cell
channel
receptor
tsqiG protein
enzymechannel effector
intracellularmessenger
Ca2+ cAMPcGMPIP3
DAG
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VNO signal transduction
Like Alberts 15-36© Garland
phosphatidyl inositol4,5 bisphosphate = PI(4,5)P2
TRPC2 channel
(like the GPCR lecture)
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Response characteristics of VNO neurons
VNO neurons respond to urine.
Some neurons selectively respond to urine from mice of the same sex,
others to urine of the opposite sex.
Unlike ORNs, their responses are narrowly tuned; no neurons were ever
observed to respond to more than one compound.
A behavioral assay: mice produce ultrasonic calls (‘whistling’) in response
to contact with urine from the opposite sex; production of these calls
requires both the VNO and the MOE.
In TRPC2 knockout mice, VNO neurons do not respond to urine; and mice
do not vocalize in response to urine
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AOB projections to the brain
Mitral cells in the AOB have apical dendrites that arborize in multiple glomeruli.
The AOB projects to the amygdala (directly), and the hypothalamus (via the
amygdala).
The projections from the rostral and caudal AOB halves are superimposed in the
amygdala.
This implies that integration of pheromone signals may take place primarily in the
AOB.
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Generalities about main olfactory system and vomeronasal system function
The main olfactory system mediates cortical responses to volatile odorants,
and these cortical responses are used to drive conscious behavior (food-
seeking, predator avoidance, etc).
The VN system is thought to mediate unconscious responses to water-soluble
pheromone compounds found in urine and secretions of other individuals.
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Loss of VNO signaling eliminates aggressive responses to intruders
TRPC2 knockout males mate normally with females.
Remarkably, though, they also mount males, which control mice never do.
The TRPC2 knockout phenotype suggests that the ‘default’ pathway in the absence of VNO input is to mate with everything.
VNO input causes male mice to fight rather than attempt to mate
Normal male mice attack intruders introduced into their territory, especially intruders swabbed with male pheromone.
TRPC2 knockout mice lack this response.
TRP2 = TRPC2
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Chemical nature of pheromones
The various pheromones include
prostaglandins in fish,
androstenone in pigs, and
protein ligands such as hamster aphrodisin.
In most cases, however, individual pure compounds don’t elicit strong responses.
Natural pheromones are mixtures of many substances,
perhaps combinations of (protein carriers) plus (bound small organic compounds).
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A genetic model system: nematode olfaction
C. elegans can chemotax toward and away from volatile attractants and repellents.
It uses only two pairs of neurons, AWA and AWC, to respond to volatile attractants.
It has many olfactory receptors, however, so each chemosensory neuron must express many of these.
The ODR-10 receptor is expressed in AWA and localized to its dendrite.
ODR-10 is a receptor for the odorant diacetyl (2,3 butanedione).
Worms lacking ODR-10 are not attracted to diacetyl.
Figure 32-11
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ODR-10 phenotype
ODR-10 is specific for diacetyl and does not respond to 2,3-pentanedione,
which differs by only one methylene group.
ODR-10 mutants still chemotax to 2,3-pentanedione
The AWC cell has receptors for 2,3-pentanedione
Deletion of AWC destroys chemotaxis to 2,3-pentanedione
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ODR-10 recognizes components of a metabolic pathway
What is the selective advantage of a worm’s response to diacetyl?
Diacetyl results from respiration by certain bacteria.
These bacteria often use citrate as a carbon source.
ODR-10 also recognizes the metabolic intermediates citrate and pyruvate.
Diacetyl is a volatile signature compound for certain bacterial species.
Many other bacteria do not make diacetyl but do make acetoin or lactate as
respiratory endproducts.
Diacetyl attraction thus allows the worm to recognize specific food sources at
a distance in the soil.
Citrate and pyruvate (nonvolatile) interactions with ODR-10 may provide
taste-like recognition of these bacteria after the worm arrives at their colony.
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A Recent Surprise: Insect Olfactory Receptors are Probably Ligand-gated Channels
Encoded by one of ~ 60 genes
An auxiliary subunit, common to most insect olfactory receptors.
Also called Or83b, Or1, Or2, and Or7.
Terminology is converging on “Orco”
For structure of the Drosophila
olfactory system, see Fig. 32-10
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“Metabotropic signalling in vertebrates provides a rich panoply of positive and negative regulation, whereas ionotropic signalling in insects enhances processing speed.” Kaupp, Nature Revs. Neuro, 2010
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End of Lecture 20
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