L6 - Making Maps Flashcards
Hierarchy of the visual system
Object
Lens
Retina
Tectum
Nasal axons go
Posterior
Temporal axons go
Anterior
Main conclusion from the stripe assay
Posterior tectime makes a non permissive factor that repels temporal retinal axons
How was it determined that it was the temporal axons avoiding a repellant factor in posterior stripes
Activity abolished by heating posterior but NOT anterior membranes
Posterior membranes cause temporal growth cones to collaspe in vitro
What is the repellant factor produced in the posterior tectum
Two ephrins
Describe how the ephrins are expressed in the posterior tectum
Expressed in a gradient from lo anteriorly to hi posteriorly
Aside from ephrins there is a second gradient seen in the retina … what is this … describe the set up of this gradient
Eph receptor for ephrins A2 and A5
Expressed in a counter gradient from high temporally to low nasally
What is the effect of knocking out both ephrin A2 and A5
Temporal neurones project they axons to the posterior tectum and the topogrpahic map is distorted
Ephrin A2/A5 double knockout mice
Fail to make topographic maps
Describe how this tectum map is made in rodents
Intitial growth of all fibres throughout the tectum
Axon branching and synaptogenesis - THIS IS SENSITIVE TO THE EPHRIN GDT
Competition for synaptic partners involving electrical activity
Electrical activity can____________________ to _______________
Modulate responses
Guidance cues
Who conclued that electrical activity can modulate responses to guidance cues
Ming et al 2001
Describe what Ming et al 2001 concluded
Demonstrated that electrical stimulation can enhance of reverse the response to guidance cues
In whaat case does electrical stimulation enhance the response to guidance cues
Netrin
In whaat case does electrical stimulation reverse the response to guidance cues
MAG
Electrical sitmualtion leads to an increase in _________ in a _________ dependent manner
Increase cAMP
Ca dependent
So more stimulation leads to
More stable Ca elevation
Refinemnt of connections is …..
Activity dependent
In lower vert. where the intial map in the tectum is topographic - coarse grained and axons make contacts over large and overlapping areas of the tectum … What is seen at later stages?
Some contacts are lost and the map becomes more precise
The refinement of the topogrpahic map depends on what (2) things
Activity AND
Competiton
Cells that wire together
Fire together
The ‘cells that wire together fire together’ describes what mechanism
Mechanism of synapose elimination which involves the localised release of neurotrohpic factors - THIS IS THOUGHT TO BE ENHANCED WHEN TWO CELLS FIRE AT THE SAME TIME
Give an example of where the ‘wire together fire together’ does not explain development in that region
In the embryonic tectim (Superior colliculus) mapping takes place before the animal encounters any light
The embryonic retina is
Spontaneously active
In mammals what two compoenents work together to form the topographic map
Ephrins and electrical activity
As axons reach the tectum what happens to the retina
Becomes spontaneously active
What could one hypothesise regarding the retina becomming spontaneously active
Could be that Hebbian principles are already operating (as in the occular domainance columns)
Experimental evidence to support the fact that Hebbian principles already operating
Na channel blocker - TTX - blocks AP gen - blocks map refinement
Geneticially block nAChR - no map refinement
When are early exuberant projections pruned back by in mice
Post natal day 8
Describe what is seen in mie which lack the B2 subunit of nAChR
Uncorrelated RGC activity (retinal waves are disrupted_ and their topogrpahic maps do not refine
TTX applied …
Axon refinenment DOES NOT occur
Describe the links to electrical activity and neurotrophins
Enhanced retinal activity - upreg BDNF
BDNF promotes RGC axon branching ONLY WHERE Eph-Ephrin interactions are low
Downregulation EphrinA5
Increase in branching
____________ and ________ interact physically in a__________________
Ephrins and Trks
Neurotrophin dependent manner
Where else are topogrpahic maps used?
In the visual systems - RGCs - LGN
Also uses ephrins
Describe how you represent a non-spatial sense like smell?
Olfactor information detected in the olfactory epithelium
1000+ olf receptors but each neurone only expresses the one
Receptor expression is dispersed in the nasal epithelium how does this compare to the olfactory bulb
Axons become organised in the olfactory bulb
Describe how mappying occurs from epithelium to the bulb
Neurones expressing the same receptor are able to converge on the same glomerulus
What is IRES
Internal ribosome entry site
What can IRES be used for
To express more than one gene
LacZ encodes what
Beta galactosidase enxyme
Receptor expression governs __________
Guidance
What experiment could you use to demonstrate that where axons go is determined by which receptor is expressed
RECEPTOR SWAP EXPERIMENTS
When the P2 coding region is swapped for M71 axons go to normal M71 glomerulus
What is the strucutre of olfactory receptors
7 TMD GPCR like molecules
Early guidance of olfacotry neruones is ….
Describe what this means
Activitiy independent
Even in the absence of the ligand each receptor has a characterisitc basal signalling activity
Neurones expressing the same olfacotr receptor will have
Similar basal cAMP level (adenylate cyclase dependent)
Similar basal cAMP level (adenylate cyclase dependent) in ORS determines
Via CREB
This level of transcription of a family of guiace cues and receptors
What types of guidsance cues are turned on
Robo/Slit
Neurophillin
The guidance cues transcribed are known as
Type 1 molecules
Type 1 molecule transcription results in
Characteristic type 1 protein level - receptors and cues
Are associated with expression of a particular olfactory receptor
Determines the mapping in the olfactor bulb
Disruption of guidance cue expression
Disrupts regional mapping in olfacotry bulb
Describe how continuous maps are converted to discrete maps
Axons entering the olfacotry bulb are pre sorted due to type 1 cue/receptor interactions
Activity independent
Cue expression switches with time so that early entering axons then guid the later entering
Sorting into glomeruli - activity dep -
Activity drives higher cAMP levels which turns on expression of type 2 cues
These interactions sort axons expression same ORs into groups to form glomeruli
How do we know gomeruli sorting is activity dependent
Activity blocking reagents block the sorting
What are some examples of type 2 cues
Homophillic adhesion molecules (Kirrels and contactins)
OR mutual replellents (Ephs and ephrins)
SUMARY TO THE BULB
EARLY
Activity independent, pre target sorting
Olfactory receptor specific expression of TYPE 1 molecules
SUMMARY TO THE BULB
KATE
Activity dependent glomerulus sorting
Olfactory receptor specific expression of type 2 molecules
What is different when compared to the visual system about the pisiform cortex neurones
No spatial organisation
What is interesting about spatial organisation of piriform cortex cells and mitral cells
They exhibit no spatial organisation
PC
Piriform cortex
PC neurons respond to multiple ….
Structurally dissimilar odorants
The pirifrom cortex was suggested to be the
Site of olfactory learning
Who tested the idea that the piriform cortex was the site of the olfactory leaening
What tool did they use
Choi et al 2011
Using optogenetics
Describe the experimental startergy of Choi et al (2011) …
Introduce ChR2 into a subset of neruones
Can allow the PC neruones to fire without input from the mitral cells
Stimulation of ChR2+ neurones with light paired with an aversive/appetitive stimulus in unconditioned animals
After conditioning test whether light along
What type of learning did Chois et al (2011) stratergy depend on
Classical associatev learning
3 ways that ChR2 could be introduced into Piriform cortex neurones - pros/cons of each
1) Use a synapsin promoter - hits 50% of the cells
2) Infect floxed ChR2 into mouse into which Cre is driven by an Emx1 promoter restricted to excitatory neurones - hits 50%
3) Insert floxed ChR2 (which is flipped) at the same time as virus expressing Cre driven by synapsin - inverted floxing - much lower expresion rate (10%)
Inverted floxing
Flip the origenation so that Cre will invert and insert a gene - NOT DELETE IT
ChR2 has been shown to be able to
Conditon adversive behaviour
How does ChR2 condition adversive behaviour
Photostimulation of ChR2 expression neurones in PC - CONDITIONED STIMULUS was paired with a footshock - UNCONDITONED STIMULUS
- on only one side of the chamber to conditon the animals
Animals exhibit flight behavour to CS alsone by only when ChR2 was present in the PC neurones
Conditioning with odorants and PS together showed that either PS or odorants could elicit flight
What is the experimental eviidence that ChR2 photostiulation is able to drive appetitive behaviours?
Mice trained to take water in response to odorant could be retrained to respond to the PS
Male mice could be trained to associate presence of a female with either an odour as the CS of with PS as the CS
Evidene to suggest piriform cortex neruones are plastic in associative learning capacity
Same set of ChR2 expressing PC neurones can be re trained in either direction
Distinct set of ChR2 expressing PC neurones can be trainined and retrained ti elicit different behaviours
This shows the piriform cortex is a VERY PLASTIC SUBSTRATE
Does the evidence presented here prove the PC is the site of odorant learning?
No — it just shows the PC can be used for associative learning
Are all responses to odorants learned?
A small subset of odours elicit innate responses
E,g. TMT from fox = fear
Spatially invartiant projections from OB to cortical amygdala that may be involved
