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Sensory Encoding of Smell in the Olfactory System of MAMMALS

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Presentation on theme: "Sensory Encoding of Smell in the Olfactory System of MAMMALS"— Presentation transcript:

1 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

2 TODAY Compare / contrast! Gross Pathways Receptor Neurons Glomeruli
Output Neurons Higher centers

3 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)

4 Compare to Mammals Shared 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 Cortex DM Thalamus Behavioral outputs Lateral Horn Orbitofrontal ctx?

5 Quiz!

6 Olfactory Organs Mammals: 4 organs Insects: 2 organs
Main Olfactory Epithelium Vomeronasal Organ Grueneberg ganglion Septal organ of Masera Insects: 2 organs Antennae Maxillary Palps

7 Mammalian Olfactory Organs and Receptors

8 Main Olfactory Epithelium
MOE ORs for odor ID; functional genes Trace amine-associated receptors (TAARs) can detect volatile urine-based amines; 15 in mouse (social cues) Output to main olfactory bulb X X X

9 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

10 Gruenberg Ganglion X Trace amine-associated receptors (TAARs)
ONE V2R receptor Responsive to mechanical stimulation (sniffing / air puffs) Outputs to main olfactory bulb X

11 Septal Organ of Masera X ORs for general alerting
Responsive to mechanical stimulation (sniffing / air puffs) Outputs to main olfactory bulb X

12 Insect Olfactory Organs and Receptors

13 Antennae 60-340 Ors A few Grs (CO2)
Basiconic for odor recognition, repulsion behavior Ors A few Grs (CO2) Coleoconic (function unknown) Ionotropic receptors → derived from glutamate receptors! Trichoid for pheromones Keene & Waddel (2007)

14 Maxillary Palps Basiconic sensillia for taste enhancement
Keene & Waddel (2007)

15 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)

16 Review: Tuning Curves of ORNs
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 Hallem et al (2006)

17 ORN Activity vs Concentration
Kreher et al (2008)

18 ORN Activity vs Concentration
Kreher et al (2008)

19 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

20 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!

21 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 Odorant tuning curves Bruyne et al (2001)

22 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)

23 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)

24 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)

25 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

26 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

27 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 Mammalian Glomeruli Kandel, Jessel, Schwartz (2000)

28 Glomeruli: MAMMALIAN INSECT 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)

29 Review: Transformations
Two glomerular transformations: Increasing signal-to-noise Producing variable gain PN / Kenyon Cell Transformations: Decorrelation of ORN signals

30 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

31 Decorrelation

32 Sparse Coding in Kenyon Cells
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) Perez-Orive et al (2002)

33 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 ( coincident Pns) Loss of oscillations in bees → no “fine” discriminations Fig. 7 from Masse et al (2009)

34

35 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)

36 Higher-level pathways

37 Olfactory Learning Li et al (2008)


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