week 4 - LTD Flashcards
Outline Kasai et al 2003 on glutamate sensivity and volume of dedritic spine head
Looked at post synaptic dendritic spine structure and expression of AMPA receptors
Compared a glutamate sensitivity map with images of spine structure/volume
The maximum glutamate sensitivity is significantly correlated with volume of dendritic spine head
This suggests that the number of AMPA receptors and Dendritic spine size is correlated.
This shows that functional and structural synaptic strengthening is related
Where are AMPA receptors located
In the central postsynaptic density. Thisis where the AMPA and NMDAR receptors are,
The mGlu receptors are more around the edge of the post synaptic terminal in the perisynaptic domain
What is the structure of AMPA receptors, and how can they be manipulated
They have four different subunits which can be alternatively spliced and RNA edited to form tetrameric ion channels
Step by step summary of synaptic transmission
An action potential travels down the axon and arrives at the presynaptic terminal.
The depolarisation opens voltage-gated calcium channels in the presynaptic membrane.
Ca²⁺ enters the terminal and rapidly accumulates in a small region near the channel (called the microdomain). Ca²⁺ concentration can reach ~10–25 μM and lasts about 300 µs.
Ca²⁺ binds to synaptotagmin, a Ca²⁺-sensitive protein on synaptic vesicles.
Binding of Ca²⁺ to synaptotagmin triggers vesicle fusion with the presynaptic membrane. A fusion pore opens, allowing the contents of the vesicle to be released.
Glutamate (the neurotransmitter) is released through the fusion pore and diffuses across the synaptic cleft.
Glutamate binds to AMPA receptors on the postsynaptic membrane.
AMPA receptor activation opens ion channels, allowing Na⁺ to enter the postsynaptic cell.
The Na⁺ influx depolarises the postsynaptic neuron, creating an excitatory postsynaptic current (EPSC).
what are mEPSC’s
mEPSCs are caused by spontaneous fusion of synaptic vesicles with the presynaptic membrane, even in the absence of an action potential. This spontaneous fusion releases neurotransmitter (like glutamate), which binds to postsynaptic receptors and generates a miniature EPSC.
How to suppress action potentials
With tetradoxin
Tetrodotoxin (TTX) is a potent neurotoxin that works by blocking voltage-gated sodium (Na⁺) channels in neurons and muscle cells.
What is a nanodomain, and how does it relate to mEPSC frequency and amplitude?
A nanodomain is a tiny cluster of AMPA receptors within the postsynaptic membrane, organised into dense hotspots beneath presynaptic release sites.
mEPSC amplitude increases when nanodomains contain more AMPA receptors, making each glutamate release more effective.
mEPSC frequency increases when there are more nanodomains or when they are better aligned with presynaptic release sites.
Though mEPSC frequency is often seen as a presynaptic measure, nanodomain organisation can influence it postsynaptically.
Describe two photon uncaging
Two-photon uncaging is a technique where focused laser light is used to break (“uncage”) a photolabile compound (e.g. caged glutamate), releasing an active neurotransmitter at a precise time and location.
It uses long-wavelength infrared light (e.g. 720–830 nm), which penetrates tissue deeply with minimal scattering.
Allows researchers to stimulate single synapses or spines with high spatial and temporal resolution.
Commonly used to study synaptic transmission, plasticity, and receptor localisation in brain slices or live neurons.
How to induce LTD and why
In essence, LFS induces LTD primarily by activating NMDARs, which triggers a calcium-dependent phosphatase signalling cascade that ultimately leads to the removal or modification of AMPA receptors at the synapse, reducing synaptic strength (As measured by EPSC field slope).
This contrasts with other stimulation protocols, such as high-frequency stimulation (HFS) or pairing protocols, which typically activate kinase pathways leading to LTP (synaptic strengthening), even though they also often involve NMDAR activation.
What study should i read to know about reduction of neuronal activity - the function of certain types of learning
lee et al (2006)
Inducing LTP vs. LTD
Typical Induction Protocols:
◦
LTP is typically induced by a brief patterned stimulus, such as high-frequency stimulation (HFS) (e.g., 1 second stimulation at 100 Hz) [14, figure 7c].
◦
LTD is typically induced by prolonged periods of low-frequency stimulation (LFS) (e.g., 15 minutes of stimulation at 1 Hz, or 900 stimuli at 1–3 Hz). Other protocols like pairing baseline synaptic stimulation with depolarization, spike-timing dependent plasticity (STDP), or application of receptor agonists can also induce LTD
How does Tau and pTau relate to LTP and LTD
- Pathways for LTP and LTD
LTP pathway (Synapse strengthening):
Triggered by high-frequency stimulation or strong activity.
Involves activation of PI3K → Akt1, which promotes cell survival and growth.
This pathway reduces phosphorylation of Tau, helping maintain healthy synaptic structure and plasticity.
LTD pathway (Synapse weakening):
Triggered by low-frequency stimulation.
Activates GSK-3 (Glycogen Synthase Kinase-3) and caspases, which are associated with synapse pruning or weakening.
GSK-3 phosphorylates Tau, producing pTau (phosphorylated Tau) — a hallmark of neurodegenerative pathology when it becomes excessive.
What evidence supports reduced activity as a marker of object recognition memory?
Human fMRI studies (Lee et al., 2006) show lower perirhinal activation during repeated object viewing.
Rodent tasks (Brown & Aggleton, 2001) show rats explore novel objects more, and neuronal activity is higher for new stimuli.
Together, these findings show that recognition learning involves reduced neural responses to familiar stimuli.
How does LTD support reversal learning in the hippocampus?
Reversal learning requires updating a memory by suppressing an old association and forming a new one.
LTD (Long-Term Depression) weakens synaptic strength at the site of the old memory (e.g., the previous escape hole location).
This synaptic weakening enables the animal to successfully learn the new location.
Without LTD, the old memory persists, leading to impaired reversal learning.
This shows that reduced synaptic activity is an essential mechanism for flexible learning.
How does LTD support reversal learning, and what task demonstrates this?
In the spatial reversal learning task, animals learn the location of an escape hole on a circular platform (like a Barnes maze).
After learning the location, the escape hole is moved to a new position.
Successful learning of the new location requires forgetting the old one.
This is supported by LTD (Long-Term Depression), which weakens synapses encoding the old location.
With LTD, animals adapt and find the new hole (successful reversal).
Without LTD, animals continue searching the old spot (impaired reversal).
This shows that synaptic weakening is critical for updating memories and enabling flexible learning.
What happens to AMPA receptors during long-term depression (LTD)?
During LTD, AMPA receptors (AMPARs) are removed from the postsynaptic membrane through endocytosis.
Receptors first diffuse out of the postsynaptic density and are then endocytosed at the lateral edges of the spine.
Once internalised, AMPARs are either:
Recycled back to the membrane later
Or sent for degradation, reducing synaptic strength.
This process contributes to the persistent weakening of synaptic transmission characteristic of LTD.
What happens to AMPA receptors during long-term potentiation (LTP)?
During LTP, AMPA receptors (AMPARs) are inserted into the postsynaptic membrane to strengthen synaptic transmission.
Receptors are delivered from intracellular pools to the postsynaptic density (PSD).
This insertion increases the number of functional AMPARs at the synapse, enhancing sensitivity to glutamate.
The result is a long-lasting increase in synaptic strength, supporting learning and memory.
How does LTD lead to AMPA receptor endocytosis at the synapse?
LTD is triggered by Ca²⁺ influx through NMDA receptors.
Step 1: Ca²⁺ causes NSF (N-ethylmaleimide-sensitive factor) to dissociate from the GluA2 subunit of AMPARs.
Step 2: AP2 (Adaptor Protein 2) binds GluA2 and recruits clathrin, initiating clathrin-mediated endocytosis.
Step 3: In some cases, PICK1 binds GluA2, causing GRIP/ABP to dissociate — this loosens the receptor’s anchoring and promotes endocytosis.
Result: Fewer AMPARs at the postsynaptic membrane, leading to reduced synaptic strength — the hallmark of LTD.
How does Amyloid-β (Aβ) cause aberrant synaptic plasticity in the hippocampus?
A:
Amyloid-β oligomers disrupt normal synaptic plasticity in the hippocampus, a key region for learning and memory.
Aβ impairs LTP (long-term potentiation), preventing synaptic strengthening.
It enhances LTD (long-term depression), promoting excessive synaptic weakening and loss.
Aβ also disrupts AMPA/NMDA receptor trafficking and calcium signalling, further impairing plasticity.
These changes contribute to memory deficits and synaptic dysfunction in Alzheimer’s disease.
How do Tau isoforms and phosphorylation relate to synapse weakening?
Tau is a microtubule-associated protein encoded by the MAPT gene on chromosome 17.
Alternative splicing of MAPT produces six Tau isoforms, differing by the number of N-terminal inserts (N1, N2) and microtubule-binding repeats (R1–R4).
These isoforms are involved in microtubule stabilisation and synaptic function.
Tau has many post-translational modification sites, especially for phosphorylation (S/T/Y).
Excess phosphorylation (pTau) disrupts Tau’s function, leading to synapse weakening, impaired plasticity, and contributing to neurodegeneration (e.g., in Alzheimer’s disease).
How does Tau phosphorylation contribute to synapse weakening?
Under basal conditions, Tau is present in dendrites and spines at low phosphorylation levels (pTau).
A brief Ca²⁺ rise activates GSK-3β, which phosphorylates Tau at serine 396.
This phosphorylation is crucial for triggering AMPA receptor endocytosis via the GluA2–PICK1 interaction, leading to synaptic weakening.
Additional mechanisms may involve changes in microtubule stability and/or regulation of NMDA receptor function via Fyn kinase trafficking.
How do neurotoxic pathogens disrupt synaptic plasticity?
They aberrantly activate the Caspase-3–GSK-3β signalling cascade, which promotes synapse weakening.
At the same time, they inhibit synapse-strengthening pathways like those supporting LTP.
GSK-3β phosphorylates Tau, producing pTau, which accumulates at synapses.
pTau drives AMPA receptor endocytosis and microtubule destabilisation, leading to excessive synapse weakening.
This shifts the neuron from a healthy basal state to aberrant plasticity, ultimately causing synapse loss and contributing to neurodegeneration.
What techniques are used in system-based proteomics to identify targets in Alzheimer’s disease?
Mass Spectrometry-Based Proteomics
→ Identifies and quantifies proteins in human post-mortem brain tissue (Control, AsymAD, AD).
Quantitative Proteomics
→ Measures protein abundance changes across disease stages to detect up- or down-regulated proteins.
Bioinformatics Categorisation
→ Groups proteins by cell type and function (e.g. synaptic, inflammatory, mitochondrial) to reveal disrupted pathways.
Target & Candidate Identification
→ Highlights key biological systems altered in AD, such as:
↑ In AD: Inflammatory, Myelination, RNA Binding/Splicing
↓ In AD: Synaptic, Mitochondrial, Cytoskeletal
Summary of this lecture
Changes to the synaptic expression of AMPA receptors (AMPAR) are
considered a major component of synaptic plasticity and synaptic
connectivity.
* Long-term depression (LTD) is a physiological form of synaptic
plasticity which leads to a reduction of synaptic transmission.
* AMPAR endocytosis is a key molecular mechanism of LTD.
* The glycogen synthase kinase-3 beta (GSK-3β)-mediated
phosphorylation of Tau (pTau) may be a general pathway to regulate
physiological LTD.
* Alzheimer’s disease associated pathogens can cause pathological
plasticity (eg., aberrant-excess LTD expression), this leads inhibition
of long-term potentiation (LTP) and a simultaneous increase in LTD in
the hippocampus resulting in synapse weakening
