sensory encoding of smell in the olfactory system of mammals
DESCRIPTION
Sensory Encoding of Smell in the Olfactory System of MAMMALS (reviewing “Olfactory Perception: Receptors, Cells, and Circuits” by Su et al, 2009) Ben Cipollini COGS 160 May 13, 2010. TODAY. Compare / contrast! Gross Pathways Receptor Neurons Glomeruli Output Neurons Higher centers. - PowerPoint PPT PresentationTRANSCRIPT
Sensory Encoding of Smellin the Olfactory System of MAMMALS
(reviewing “Olfactory Perception: Receptors, Cells, and Circuits” by Su et al, 2009)
Ben CipolliniCOGS 160
May 13, 2010
TODAY Compare / contrast!
Gross Pathways Receptor Neurons Glomeruli Output Neurons Higher centers
Gross Pathways
ORNs in antennae
Projection neurons from antenna lobe to lateral horn and mushroom body
Glomeruli in antenna lobe mediate most receptor-specific processing
Kenyon cells in mushroom body have sparse representation of odors for associative learning
Lateral horn has place-specific processing of sensory-motor associations
Keene & Waddel (2007)
Compare to MammalsShared Feature Insect Mammal
ORNs Antenna + maxillary palps
Olfactory epithelium
Glomeruli Antenna lobe Olfactory bulb
Output cells Projection Neurons
Mitral cells & Tufted cells
Classification & learning
Mushroom body Piriform CortexDM Thalamus
Behavioral outputs Lateral Horn Orbitofrontal ctx?
Quiz!
Olfactory Organs
Mammals: 4 organs Main Olfactory
Epithelium Vomeronasal Organ Grueneberg ganglion Septal organ of
Masera
Insects: 2 organs Antennae Maxillary Palps
Mammalian OlfactoryOrgans and Receptors
Main Olfactory Epithelium
MOE ORs for odor ID; 250-1200
functional genes
Trace amine-associated receptors (TAARs) can detect volatile urine-based amines; 15 in mouse (social cues)
Output to main olfactory bulbX
XX
Vomeronasal Organ
V1Rs (urine) for conspecific recognition, male sexual behavior, maternal aggression, regulation of female estrous cycles, stress level indicator
V2Rs (sweat and urine) for pregnancy blocking, individual / gender identity, aggression (from males), stress (from females)
Formyl Peptide Receptors (immune system) for health status
Outputs to accessory olfactory bulb
Gruenberg Ganglion
Trace amine-associated receptors (TAARs)
ONE V2R receptor
Responsive to mechanical stimulation (sniffing / air puffs)
Outputs to main olfactory bulb
X
Septal Organ of Masera
ORs for general alerting
Responsive to mechanical stimulation (sniffing / air puffs)
Outputs to main olfactory bulb
X
Insect Olfactory Organs and Receptors
Antennae
Keene & Waddel (2007)
Basiconic for odor recognition, repulsion behavior
60-340 Ors
A few Grs (CO2)
Coleoconic (function unknown)
Ionotropic receptors → derived from glutamate receptors!
Trichoid for pheromones
Maxillary Palps
Keene & Waddel (2007)
Basiconic sensillia for taste enhancement
The Evolutionary Story
Insects Finding homologies in species of the same
order can be challenging Probably fast evolution Mechanism (duplication & variation vs.
modification) unknown
NOTE: loss of a single OR doesn't necessarily eliminate associated behavior Ensemble encoding Different ORs coding for an odor at different
concentrations (helps with variable gain)
Review: Tuning Curves of ORNs
Hallem et al (2006)
Odorants are identified by the pattern of receptors activated
Including inhibition of tonic firing
Individual receptors are activated by subsets of odorants
Receptors lie along a smooth continuum of tuning breadths
Broadly tuned receptors are most sensitive to structurally similar odorants
Higher concentrations of odorants elicit activity from greater numbers of receptors
Odor intensity as well as odor identity is represented by the number of activated receptors
ORN Activity vs Concentration
Kreher et al (2008)
ORN Activity vs Concentration
Kreher et al (2008)
Evolution II: Pseudogenization
Humans 20-30% of ORs 90% of VRN1s 100% (so far) of
VRN2s (only 20 genes exist)
Aquatic vertebrates Only have OR class I
Terrestrial vertebrates Have OR class I & II
Dolphins Have class I 100% of OR class II
pseudogenized
For no particular reason...
3 cool properties of ORNs that were discussed in this paper: Temporal tuning curves Antagonistic ORNs! Insect ORNs are actually really weird!
Tuning Curves of ORNs (New):Temporal Dynamics
Different ORNs can have different temporal dynamics (even for the same odor)
A single ORN can have different temporal dynamics to different odors
Bruyne et al (2001)
Odorant tuning curves
Combinatorics: Antagonistic Inhibition
The perceived magnitude of an odorant mixture was neither additive nor a simple average of its components
Fell between these limits, due to:
Masking (i.e. modification of perceived odor) or counteraction (i.e. reduction of odor intensity).
Mixing some odorants led to the emergence of novel perceptual qualities that were not present in each individual odorant
Suggests that odorant mixture interactions occurred at some levels in the olfactory system
Observed at presynaptic ORN axons in olfactory bulb
Oka et al (2004)
Insects ORNs are CRAAAZY!
Insect odor receptors have 7 transmembrane domains and have long been assumed to be GPCRs.
BUT we see major major differences!
No G protein mutant has been found to suffer a severe loss of olfactory function.
The topology of the insect Ors is inverted relative to GPCRs.
Each OR also appears to form a heteromultimer with Or83b
A canonical OR (with Or83b), can form a “ligand-gated cation channel”
Due to an odorant-induced, rapidly developing, transient inward current, independent of G protein signaling
A second, slower and larger component to the odorant-induced inward current
Slower both in onset and decay kinetics
Is sensitive to inhibition by a GDP analog Siegel et al (1999)
ORN Transduction: “canonical” Odorant binds to the odor
receptor
Odor receptor changes shape and binds/activates an “olfactory-type” G protein
G protein activates the lyase - adenylate cyclase (LAC)
LAC converts ATP into cAMP
cAMP opens cyclic nucleotide-gated ion channels
Calcium and sodium ions to enter into the cell, depolarizing the ORN
• Calcium-dependent chlorine channels contribute to depolarization as well
G protein turned off by GDP Firestein & Menini (1999)
Review: Projection Neurons
Live in antenna lobe (~200 per)
Receive input from ALL ORNs of a single class (~50; ~25 from each side)
Despite convergent input, show broader odorant tuning than ORNs
Project out to “higher centers”: mushroom body & lateral horn
Tufted & Mitral Cells
Live in olfactory bulb
Receive input from ALL ORNs of a single class from a single side
Like insect projection neurons, show broader odorant tuning than ORNs
Like insect projection neurons, project out to “higher centers”
NOTE: only mitral cells project to posterior piriform cortex
Review: Glomeruli
In Antenna Lobe, one per odorant “class” (50)
Consist of: Axons of ORNs
Dendrites of projection neurons
Neurites (axons and dendrites) of local neurons
ORN inputs all from same “class”, come bilaterally
Kandel, Jessel, Schwartz (2000)
Mammalian Glomeruli
Glomeruli:
50:1 convergence
Pns input from 1
Interglomerular inhibition (local neurons)
Intraglomerular inhibition (local neurons)
5000:1 convergence
M/T input from 1
Interglomerular inhibition (granule cells)
Intraglomerular inhibition (juxtaglomerular cells)
INSECT MAMMALIAN
Review: Transformations
Two glomerular transformations:
Increasing signal-to-noise
Producing variable gain
PN / Kenyon Cell Transformations:
Decorrelation of ORN signals
Variable Gain Revisited Broader tuning widths and nonlinear amplification among projection neurons
are mainly due to strong ORN-projection neuron synapses
How?
Low Activity Amplification: Weak presynaptic ORN activity is sufficient to trigger robust neurotransmitter release and cause substantial PN responses.
High Activity Fall-off: Strong ORN activity leads to depletion of synaptic neurotransmitter.
How about mammals?
The strong synapses : due to presence of numerous synaptic vesicle release sites and a high release probability
High probabilities of vesicle release have also been found in the mammalian olfactory bulb
Sparse Coding in Kenyon Cells
Perez-Orive et al (2002)
IN THE LOCUST PNs (columns)
respond to most odorants; KCs (columns) respond to very few
“Population sparseness” - % of cells that do NOT respond to an odor (rows)
How Do Locust KCs Become Sparse?
High convergence (400:1, 50% of PNs!)
Weak unitary synaptic connections
Synaptic integration in (oscillatory) time windows
Voltage-gated channels amplify coincident spikes
High spiking threshold (50-100 coincident Pns)
Loss of oscillations in bees → no “fine” discriminations
Fig. 7 from Masse et al (2009)
Higher-level pathways
Posterior Piriform cortex does classification (like Mushroom body!)
Cells within PPC project to MOST areas that are connected to
This includes feedback projections to olfactory bulb
Johnson et al (2000)
Higher-level pathways
Olfactory Learning
Li et al (2008)