week 2 - long term potentiation Flashcards
Describe Morice et al. (2008) Learning and memory study
mice with a downsyndrome model vs wild type
Mice with downsyndrome have less synaptic plasticity
Where is the hippocampus?
In the medial temporal lobe
What is another reason why the hippocampus is widely studied in learning and memory
Because it is very easily accessed electrophysiologically
You can even remove it from the brain and it’ll stay alive for much longer than other areas
What is anterograde amnesia
Inability to form new memories
What is retrograde amnesia
Loss of old memories
What is an intepretation of the fact that most hippocampal damage leads to amnesia of recent memories or new memories
Memories might start off in the hippocampus and move elsewhere
Describe frankland and bontempi (2005) ‘s study on hippocampal lesions
Hippocampal lesions impair memory in rodents in a time dependent way
Memory created in rodents
When hippocampus is lesioned soon after the memory is created the memory is lost
When hippocampus is lesioned after a month after the memory, the memory is generally not lost
This suggests that memories start in the hippocampus and move elsewhere
Describe Neves, Cooke and Bliss’s study into hippocampus circuitry
Most the input to the hippocampus comes into the dentate gyrus, flows to CA3 and flows to the hippocampus
The flow of information into the hippocampus in rodents is very simple
This is another reason why the hippocampus is a widely studied structure
How can you differentiate synaptic potentials and action potentials in the hippocampus in rodent hippocampal studies
Use a simple lamination (research more about this)
Because the hippocampus is simple, you can interpret what is synaptic vs. what is hippocampal, as well as being able to visualize the discrete fibre pathways
Describe a transverse hippocampal slice and why is this better in the hippocampus
You can look down at a slice of the rodent hippocampus and see the structure of the neural pathways
You can see what’s CA1, CA3 and you can stimulate different bits
This is better in the hippocampus cus in other brain areas the pathways cross over each other so you can’t isolate the different responses
What predictions would hebbian plasiticty theory generate - Q1
Activity dependence - Modification of synsapses requires co-activity between pre and post synaptic neurons
input specifity- only inputs co-active with the post synaptic cell will undergo modification
Cooperativtiy - Multiple inputs are required to drive plasticity inducing post-synaptic depolarization
Associativity - Weak inputs that are unable to depolarize the cell sufficiently enough, will still undergo change if paired with strong depolarizing inputs
Longetivty - Phenomenons should be long-lasting if they are going to serve memory
Describe intracellular vs. extracellular recording and benefits
Intracellular - inside the neuron. EPSP’s can be measured
Benefits = low noise recording from individual neurons
Extracellular - Away from the neuron e.g eeg
How does the steepness of the EPSP slope correlate with synaptic strength?
The steeper the slope of the EPSP field recording, the stronger the population synapses
Describe Bliss and Lomo’s 1973 LTP study
(Q1- Activity dependence)
Looked in the dentate gyrus recieving input
Recorded from a population of excitatory neurons in a aneastetised rabbit
One neuron stimulated and one recorded in each hemisphere
One hemisphere recieved high frequency electrical stimulation, the other recieved low frequency control pulses
Only the hemisphere that has been stimulated undergone LTP
LTP was measured as the field EPSP
This shows high frequency stimulation were required for increased synaptic strneght
This also shows that plasticity is activity dependent
How would you use intracellular recording to measure LTP (markram, 1997)
Q1 - Activity dependence
You can use spike timing dependent plastictiy
Stimulate with a single pulse the presynaptic input at the same time injecting the post synaptic neuron with current
If the presynaptic input is stimulate just before teh post synaptic cell we get a long term potentiation
The close the two stimulatoins are to being simultaneous, the stronger the LTP
Stimulating the post synaptic neuron first before the pre neuron leads to LTD - weakening of synapses
This shows timing of stimuli is important for LTP
This also shows activity is activity depndent
How would you demonstrate input specifity for synaptic plasticiy (beck et al, 2000)
(Q1 - Input plasticity)
tetanised vs. non tetanised input
Do more research on this (beck et al, 2000)
How would you demonstrate cooperativity and associativity in hebbian plasticity (Q1)
(McNaughton et al, 1978) Mcnaughton, 2003,
Barrioneuvo and brown, 1983
Provide both strong and weak electrical stimulation into the post synaptic neuron
If you stimulate just weak input, no LTP for that synapse
If both strong and weak input is stimulated, you do get LTP for the weak input synapse
How would you measure longetivity in hebbian plasticity (Q1)
Abraham et al 2002
Induced LTP in dente gyrus of rat by many stimulations
LTP lasted for 100 days in a rat
LTP even lasted for one rat 360 days, this is half of the rats life
What is the relationship between protein synthesis and LTP
Scharf et al, 2002 provided evidence, describe
Protein synthesis
Scharf et al found that if you block protein synthesis, LTP only lasts for an hour then returns back to baseline
If you don’t block protein synthsis, LTP is long lasting
What is the synaptic tagging hypothesis
Frey and morris 1998
New proteins get synthesised becasue genes are being expressed
However, because plasitcity is neuron specific, how do you ensure that the proteins only go to the relavent neurons?
Synaptic tag hypothesis = Porteins get sent out all throughout the neuron, but you have a tag/binding partner at the synapses that have recently undergone change
WHat is the stages of LTP
Induction - the moment of change when the tetanus has been delivered, steep EPSP slope
Expression - Something in the synapse has changed
Maintenence - Maintenence of the synapse strength
What is the mechaism that detects induction (the special condition at which hebbian plasticity occurs) in glutamate synapses
The NMDA receptor is nessercary for the induction of LTP
Most synaptic potential comes in through AMPA receptors (in glutamatergic synapses)
NMDA is voltage gated meaning it only opens at a particular voltage. Normally magnesium is blocking the receptor but when theres enough voltage the magnesium leaves allowing a sharp influx of positively charged calcium ions
described NMDA receptors in resting and weakly stimulated vs strongly stimulated synaptes. (detecting expression)
How is this discovered experimentally
resting state = nmda closed
Weakly stimulated = nmda closed
Strongly stimulated = nmda opens
Experimentally, you can use drugs to block the different receptors and look at the effects
Describe collingridge et al (1983) NMDA receptor antagonist experiment
(ltp expression)
Used a drug AP5 into the cell bath whilst LTP was being induced
ap5 is a receptor antagonist
When ap5 was in bath, ltp didnt take place
When ap5 was washed away, ltp could take place
This shows that ap5 is nmda receptor dependent
Describe effects of NMDA antagonist on morris water maze performance
Morris et al, 1986
(ltp maintence)
When NMDA antagonists are used (injecting ap5 into hippocampus), performance on the morris water maze is strongly impaired
This shows that ltp is nmda dependent
It also shows that morris water maze performance is dependent on LTP
What impacts how strong the LTP is, when it happends
shepherd and hungair, 2007
AMPA receptors
You can use dominant negative system to block ampa receptor insertion
Do more research on this
Describe Rumpel et al 2005 study on AMPA and LTP
LTP is prevented by blocking ampa receptor insertion
Do more research on this
What is an inhibitory avoidace test
Uses operant conditioning
Mice have two compartment, one well lit and one dark
They want to go in the dark but they get a shock when they go in the compartment
If they stay in the light then they have a powerful memory of the electric shop
Describe mitsushima et al,s 2011 study
Learning is blocked by insetion of modified dominant negative ampa receptor tail
do more reserach on this
Describe PKMzeta as a mechanism for maintenence
sacktor, 2011
By phosphorylating ampa receptor you can change their conductance
PKMzeta is a consituent of a kinase that phosphorylates
if pkm is translated into being a functional kinase, it interacts with ampa insertion mechanisms to promote the insertion of ampas
Most synampses translationally repress PKM. When NMDA is activated and LTP occurs, the block of translation of PKM is lifted
This leads to a positive feedback loop where it supresses the blockade so the maintanence mechanism is continually being expressed at the synapse
Describe pastalkovaa et al 2006 study on pkmzeta and rat memory task
(for LTP maintence)
PKMzeta is required for hippocampus dependent memory
Do more research
describe Whitlock et al 2006 - learning induced synaptic potentiation occludes LTP
do more research on this
what are negative symptoms of schizohprenia?
Emotional, social withdrawal and anhedonia
describe dendritic spines in schizophrenia
Reduction in dendritic spines in pyramidal neurons (Glanz and Lewis, 2000)
However these are post mortem studies so we don’t know if its cause or consequence
How does PET image synapses in the live human brain?
A PET ligand called UCB binds to synaptic vescicle protein 2A which allows you to globally image synapses
(go over this, maybe ask chat gpt)
What has PET imaging shown us about synapses in schizophrenia
Give criticicisms
Individuals with schizophrenia show lower levels of pre-synaptic protein SV2
This does not give us single synapse resolution, so we cannot look at the level of the synapse.
Also because the individuals are alive, we cannot get much in the way of mechanistic insight
What evidence is there that links the genetics of schizophrenia with synaptic dysfunction?
Sullivan et al, 2012
Scz inividuals show gene variants which lead to altered gene expression, which leads to abberant cellular physiology, which leads to abnormal synaptic connectivity, which leads to abnormal circuit function.
- very large studies with hundreds of thousands of individuals (GWAS, axome studies)
- GWAS look for SNP’s which are statistically more common in scz. This has allowed us to identify over 250 genetic variants that seem associated with the diagnosis. However they are quite weak, only increase risk of scz <1%
- Axome studies look for deletions and insertions at the chromosomal level and try to associate it with scz. Particular variants have been found e.g deletions in neuraxin 1 gene which codes for a synaptic adhesion protein. These variants are high penetrant but rare, meaning few have it but if you do then the risk is high
If you look at the function of a lot of the genes associated with scz, a lot of them encode for proteins that are found at synapses.
can animal models model scz?
PROS
- you can modify genes associated with scz to see the impact on behaviour
- Behavioural assays of scz symptoms are easy to conduct
- Its easy to target specific cell types and neural circuits involved in schizophrenia
CONS
- We cannot recapitulate the complex genetic backgrounds associated with scz. E.g it is likely that a combination of rare as well as common variants contribute
- Gene disruption does not always completely mimic human genetics, e.g a knockout does not always relate to the human mutation
- were working with mice not human neurons
- Not easy to ask mice if they are experiencing auditory hallucinations
Describe Stem cell models of synaptic dysfunction in scz
- these can investigate synaptic dysfunction in scz in HUMAN neurons
- adult somatic cells can be reprogrammed into Induced pluripotent stem cells (iPSC’s) which can be turned into neurons
- The cells retain the genetic makeup of the original human, so we can compare neuron cells from scz with healthy controls.
- You can do various studies with these cells, e.g compare physiological functioning, treat cells with drugs
APPROACHES TO IPSC’s
- Generate patient derived IPSC lines and use these to study the underlying neurobiology.
Describe patient derived IPSC approach in studying SCZq study
Brennand et al, 2011
They used a microarray to assess how many genes were expressed differently between patient and control lines. Nearly 600 genes were differently expressed, 25% of the dysregulated genes were associated with SCZ
They then compared the number of neurites that came out of the cell body in SCZ and Healthy neurons. SCZ iSPC’s showed fewer neurites than controls.
They also showed lower levels of synaptic protein PSD95, suggesting that the patient neurites were making fewer synaptic connections
They confirmed that the patient cells were making fewer synaptic connections by tracing synpatic connections. HOWVER, the was significant variation, regradless of HC or SCZ
HOWEVER
- There was a lot of individual variability, even within the control lines
- This meaning that whilst it showed that stem cells can recapitualite some aspects of the disorder, it did offer insight into underlying pathophysiology
Describe advantages and disadvatnages of the Patient Derived ISPC approach
ADV
- captures complex genetic background
- Donors have same known diagnosis
- allows modelling of genetic interactions
DISADV
- Hetergeneous genetic background due to different individuals
- Subtle/Gene specific phenotypes could be missed
- Large number of lines needed
Describe the isogenic ISPC approach to understanding differences in SCV.
E.g McTague et al, 2021
- You introduce mutations or deletions in ISPC’s to create isogenic neurons using gene editing, to geneticallyl correct mutations in the patient line
- This mean you have a corrected isogenic control, a patient line, and a healthy control line
- You can then conduct functional assays, Assesment of cellular dysfunction and degeneration and transcriptional analysis to identify disease phenotypes in patient vs. isogenic cells
Give an example of the isogenic IPSC approach
Wan et al (2014)
- identified a family where two sons had different disorders. One had schizophrenia and the other had major depression
- Both of them carried an identical genetic mutation (4 base pair deletion) in the DISC1 gene
- Generated IPSC’s from the sons and family members without the deletion and healthy non family member.
- proved that this deletion caused a frame shift in the c-terminal leading to an 80% loss of expression of the DISC1 protein
- THey carried out some experiments.
- They found that the original neurons from the sons had reduced levels of SV2a which is a synapse marker, suggesting they had a smaller number of synapses.
- Electrophysiological studies also found reduced EPSC amplitude, indicative of reduced synapse formation
- In the patient isogenic cells, they used gene editing to correct the 4 base pair deletion by re-introducing the base pairs
- In the healthy isogenic cells, they INTRODUCED the DISC1 mutation
- The isogenic patient cells DID NOT show the reduced levels of synapses that the original patient cells shpwed.
- The healthy isogenic cells with DISC1 mutation DID show reduced levels of synapses.
- This shows by introducing the mutation you can cause the phenotype, and by correcting the mutation you can rescue the phenotype
What are advantages and disadvantages of the isogenic approach
ADV.
- Allows for study of gene function in isogenic genetic background - few cell lines needed
- can engineer point mutations or deletions
- can link genes directly with cellular/molecular processes
DISADV.
- Single gene unlikely to be responsible for a complex genetic disorder
- Multiple genes with disease associated variants may interact
- Naturally occuring variant may only subtely impact gene function
What kind of neurons do we get from IPSC’s?
Brennand et al (2014)
- Compared gene expression between neural progenitor cells with differentiated neurons derived from IPSC’s
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