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BMSc - Neuropharmacology and Behaviour > Synaptic Plasticity Mechanisms > Flashcards

Synaptic Plasticity Mechanisms Flashcards

1
Q

What is learning?

A

Process of acquiring information

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2
Q

What is memory?

A

Persistence of learning (storage and retrieval)

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3
Q

What are the different forms of memory?

A

Declarative:
Semantic; words, faces, objects
Episodic; snapshots of life events

Procedural:
Learning how something works

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4
Q

How do we know that the hippocampus is involved with memory?

A

Lots of studies; however recent one involved fMRI studies on London taxi driver

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5
Q

What is the definition of synaptic plasticity?

A

Persistent upregulation or downregulation of synaptic strength - LTP or LTD

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6
Q

When does plasticity occur?

A

Development
Learning and memory
Ageing
Response to trauma/ disease

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7
Q

What electrophysiology techniques can be used to record LTP?

A

fEPSP - downward deflection
intracellular recording - upward deflection
Whole cell patch clamp - downward deflection

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8
Q

What are 3 classical properties of LTP?

A

Specificity
Cooperativity
Associativity

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9
Q

Describe specificity in terms of LTP

A

LTP is specific to the tetanized (high frequency stimulation) pathways
Non-tetanized inputs, even convergent on the same dendritic region are not potentiated

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10
Q

Describe cooperativity in terms of LTP

A

LTP exhibits an intensity threshold whereby a weak stimulus is unable to induce LTP whereas a strong input can

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11
Q

Describe associativity in terms of LTP

A

A weak input will potentiate provided a strong convergent input is activated simultaneously
This feature is equated with classical conditioning, with the weak and strong inputs corresponding to the conditioned and non conditioned stimuli respectively

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12
Q

How can LTP be induced experimentally?

A

High frequency train of stimulation

Patterned stimulation

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13
Q

Describe the 4 LTP inducing stimulation protocols

A

Tetanic - 1000 Hz for 1s
Theta burst - short bursts at 5 Hz
Primed burst - very similar to theta burst
Realistic stimulation protocols

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14
Q

What are the 4 distinct phases of LTP induction?

A

Post-tetanic potentiation
Early LTP
Intermediate LTP
Late LTP

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15
Q

What are the 3 post synaptic glutamate receptors?

A

AMPA
NMDA
mGlu; metabotropic glutamate receptors linked to G proteins

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16
Q

Which glutamergic receptor is key to LTP induction?

A

NMDA - contribute little to basal synaptic transmission

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17
Q

What characteristics of NMDA receptors makes them key to LTP induction?

A

Under basal membrane potential; NMDA is blocked by Mg2+
Mg2+ remains until the membrane is depolarized to around +20 mV.

CALCIUM INFLUX AND POTTASIUM EFFLUX

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18
Q

Describe the action of high frequency stimulus in LTP

A

Activation of NMDA receptors
This allows for calcium influx
CaMK2, PKC induction
LTP induction

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19
Q

Is GABA involved with LTP

A

Yes; it acts as an inhibitory interneuron to prevent over excitation of the glutamergic synapse…the IPSP curtails the EPSP

GABA is released at the same time as glutamate to act on a combo of GABAA and GABAB to form this inhibitory post synaptic potential to limit the excitation at the synaptic connection

GABAB receptors (presynaptically) auto receptors whereby they act to limit the release of GABA during periods of sustained activation.

Train of excitation that is not limited by GABAergic inhibition due to activation of GABAB

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20
Q

Are there behavioural consequences of NMDAR blockade?

A

Inhibition of NMDAR blocks both LTP and spatial learning (Morris water maze)

Spatial learning is a specific hippocampal behavioural task

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21
Q

Is synaptic plasticity bidirectional?

A

Yes; LTP and LTD

High train frequency; sustained increase in magnitude of recordings = LTP

Low train frequency; sustained decrease of magnitude of recordings = LTD

This bidirectional nature is governed by modification of synaptic transmission mediated by AMPARs at glutamatergic synapses in area CA1 of the hippocampus. Calcium is the KEY regulator

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22
Q

Where is the crossover point between LTD and LTP?

A

Around 7-10 Hz

With calcium influx the main driver - dependent on NMDAR activation

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23
Q

What are the 3 distinct temporal phases of LTP?

A

1: dependent on post-translational modification of existing proteins i.e. phosphorylation
2: dependent on synthesis of new protein via existing mRNA - mRNA translation
3: dependent on synthesis of new protein and new mRNA - gene transcription in a somatic localization

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24
Q

Describe the 3 phases of temporal LTP in terms of time

A

Phase 1 (0-1hr) = covalent modification of existing synaptic proteins

Phase 2 (1-2hrs) = translation of pre-existing mRNA

Phase 3 (3hrs +) = gene induction, transcription, translation

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25
Q

Can we show experimentally that there are differences in these phases of LTP?

A

Yes; specific inhibitors of translation and transcription
Anisomycin = translation (inhibition of early LTP)
Actinomycin D = transcription (inhibition of late LTP)

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26
Q

Describe phase 1 of LTP

A

Inhibition of a variety of kinases (blockade of phosphorylation) inhibits the early phase of LTP;

PKA, PKC, PKG, ERK, CaMKII, CaMKIV, PYK2, Fyn

Implies multiple parallel pathways, all of them essential for full expression of plasticity response

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27
Q

What drives the activation of kinases in early LTP?

A

NMADAR mediated Ca2+ rise engages kinases (LTP)/ phosphatase (LTD) that are key for LTP and LTD

28
Q

How can phase 1 be modelled?

A 
Inhibit the function of specific proteins; 
Receptor antagonists
Enzyme inhibitors
K/o mice
Use of inactive/ dominant negative forms
29
Q

Describe the mechanisms for the maintenance of early phase synaptic plasticity

A

Phosphorylation of receptors

Receptor trafficking

30
Q

What is the composition of many AMPARs?

A

GluA1 and GluA2

31
Q

What is the impact of phosphorylation of AMPARs on LTP/ LTD?

A

Phosphorylation of GluA1 at s831 and s845 both increase the conductance and affect receptor trafficking (receptor expression at plasma membrane or golgi apparatus)

32
Q

Describe the differences in phosphorylation of AMPARs between LTD or LTP

A

LTD; dephosphorylation at ser845

LTP; phosphorylation at ser831

33
Q

What is the evidence that GluA receptors are trafficked during LTP?

A

Indirect; studies that block post synaptic interactions with regulatory/ trafficking proteins e.g. PICK/GRIP. Evidence from silent synapses

Direct; immunohistochemically detection of surface GluA receptors (culture) and from imaging fluorescent GluA chimeras (SEP-GluA1)

34
Q

What controls AMPA receptor trafficking?

A

TARPS (transmembrane AMPA receptor regulatory proteins) - example include stargazin

TARPS control AMPAR receptor trafficking and lateral surface diffusion of AMPARs
TARPs also modulate the electrophysiological (biophysical) properties of AMPARs

Control GluA1

35
Q

What phosphorylates S831 and S845 respectively?

A

S831; CaMKII

S845; PKA

36
Q

What are the actions of GRIP and PICK?

A

GRIP; AMPAR stabilization at the synapse

PICK; AMPAR receptor trafficking in endocytosis/ LTD and subunit composition regulation

37
Q

What is the role of NSF?

A

Binds to GluA2

Disrupts PICK1 binding and synaptic insertion

38
Q

What do AMPARs lacking GluA2 subunits show?

A

Pronounced inward rectification due to voltage dependent block of the channel by polyamines at positive membrane potentials

39
Q

How can the post synaptic receptor numbers be increased?

A

Increase in receptor delivery
Lateral diffusion
Reduce removal of receptor

40
Q

Is the k/o out of CaMKII sufficient to completely inhibit LTP?

A

No; LTP still occurs, albeit at a less intense amplitude

The addition of a NMDAR antagonist to this k/o mice completely prevent LTP however

41
Q

Is PKM-zeta required for LTP?

A

Form of PKC
Pharmacological inhibition of PKM zeta with ZIP blocks LTP
PKM-zeta activity is necessary and sufficient for LTP maintenance

42
Q

What AMPA receptor subunit allows for the permeability of calcium?

A

GluA2 lacking or GluA2 containing an R subunit

43
Q

What is the role of polyamines in the inward rectification seen in AMPARs lacking GluA2 subunits?

A

Bind to the channel pore at positive potentials and block the flow via AMPA receptor channel

44
Q

Describe the difference in current voltage relationship between GluA2 lacking or GluA2 containing AMPAR receptors

A

GluA2 lacking = rectifying I/V curve due to blockade via polyamines
GluA2 containing = linear I/V curve

45
Q

Is it just AMPARs that are moved towards and away from the synapses during synaptic plasticity?

A

No; evidence that NMDARs are mobile
Selective blocker of NMDA receptors (MK-801), only blocks active, open NMDARs.
After application of MK-801, there was complete blockage of synaptic transmission

46
Q

How can the movement of AMPARs during plasticity be monitored?

A

Fluorescent imaging - monitoring in real time

Genetically engineered receptors such as GFP
Or
Tack single molecules with cyanide dye or quantum dots. Recognize components of receptor to the tag with a marker

47
Q

Are there differences in lateral diffusion of glutamate receptors during development?

A

Yes; very motile early on in development which reduces as time progresses and there is further neuronal maturation

48
Q

Describe the receptor trafficking model

A

Receptors can move distant from synaptic cleft to insert into the membrane

They can also diffuse across whereby they can be internalized

Once internalised, the receptors can be degraded or recycled

The receptors can be synthesised and transported within the post synaptic membrane where they can then be inserted

49
Q

Describe the molecular basis of late phase LTP

A

mRNA translation/ local protein synthesis - involves mRNA present in the dendrites (synapse specificity of plasticity via tetanized synapses maintained)

50
Q

Which two mRNAs present in the dendrites have been demonstrated to be involved with phase 2 of synaptic plasticity?

A

MAP2; microtubule associated protein 2 modulates microtubule dynamics and so promotes outgrowth of processes

CaMKII; subunit involved in NMDAR dependent signalling

51
Q

How is stimulation of local protein synthesis achieved from these mRNAs (MAP2, CaMKII)?

A

Inhibition of mRNA degradation; leading to elevated mRNA levels and increase synthesis of the corresponding protein

Phosphorylation of ribosomal proteins near the synapse leading to more efficient translation

Phosphorylation of ribosomal proteins triggered by ERK

52
Q

What role does the alteration of glutamate receptor number on the post synaptic membrane have on synaptic plasticity?

A

Key in the strengthening of excitatory post synaptic potentials

53
Q

How can the glutamate receptor number be increased on the post synaptic membrane?

A

Phase 1; increase protein delivery (membrane insertion/ trafficking, lateral mobility)

Phases 2/3: increase protein synthesis (local protein synthesis, somatic synthesis (general protein trafficking)

54
Q

How is phase 3 of synaptic plasticity occured?

A

Synthesis of new protein and new mRNA

Gene transcription is activated by transcription factors

Transcription factors bind to specific target sites in genome to regulate transcription of a target gene (increase = activator, decrease = repressor)

55
Q

Describe different types of transcription factors

A

Constitutive TFs = present in an inactive form in cytoplasm, to be activated following the stimulus

Inducible TFs; activated as target genes of constitutive TFs and they themselves then stimulate transcription of their own target “late response” genes

56
Q

What constitutive TFs are involved in synaptic plasticity?

A

CREB
Elk1
NFkappaB

57
Q

What inducible TFs are involved in synaptic plasticity?

A

zif268 (early growth response1 - erg1)

junB (component of AP1)

58
Q

Describe the role of CREB activation in synaptic plasticity?

A

CREB can binds to DNA sequences called cAMP response elements (CRE) to increase or decrease transcription of a downstream gene

59
Q

Describe the role of PKA in LTP

A

Genetic overexpression of PKA inhibition in mouse hippocampus

2x100Hz protocol to induce early LTP = no impact

4x100Hz protocol to induce late LTP = block of LTP particularly at late stages in comparison with WT mice

This had impact on spatial memory tasks (hippocampal based learning task)

60
Q

How are IEGs (inducible TFs) involved in LTP?

A

Calcium activates kinases - this is a short term effect by phosphorylating receptors

This activated kinase will enter the nucleus to cause phosphorylation of the transcription factor

This results in binding to IEG DNA to initiate transcription

IEG product e.g. FOS acts as a transcription factor initiates synthesis of other proteins

61
Q

Is zig/268 (inducible TF) a causal step in LTP?

A

High frequency stim will induce zif/268 about 15 mins after stimulus with a peak induction at 30-60 mins

MK801 (NMDA antagonist) will block LTP and the zif/268 induction

If you stimulate an inhibitory pathway to block LTP you also inadvertently block zif/268 induction

Sub-threshold stimulus paradigm incapable to induce LTP, there will be no induction of zif/268

At around 60 min time point in zif/286 animals there is not difference in LTP

However, around 2-3 days later, there are specific deficits in knock down animals in comparison with control with subsequent behavioural deficit in morris water maze

62
Q

What CREB target late response genes act to maintain LTP in the long run

A

BDNF - promotes neuronal survival, neurite outgrowth and further plasticity

63
Q

What NFkB target late response gene acts to maintain LTP in the long run?

A

Neuronal cell adhesion molecule (NCAM) which promotes neurite outgrowth and maintenance of synaptic architecture

64
Q

What Zif/268 target late response genes act to maintain LTP in the long run

A

Proteasome genes

Agrin/ gephyrin which act to cluster receptors and stabilize synapses

65
Q

What morphological changes occur to formulate long term memories?

A

Changes in synaptic morphology occur rapidly after triggering of synaptic plasticity

Morphological plasticity probably contributes to phases 1-3 being sustained by different mechanisms at each phase

66
Q

What changes are proposed to occur throughout LTP?

A

Enlargement of post synaptic density

Synaptic splitting - one presynaptic area infiltrates 2 synapses which then go on to form 2 independent synapses

67
Q

What neuromodulators have been linked to synaptic plasticity?

A 
Leptin - leptin receptors 
DA - D1 
2AG - CB1 (cannabinoid receptors)
ACh - M1 (GPCR)
5-HT2c
Histamine - H1

BMSc - Neuropharmacology and Behaviour (27 decks)

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