16 - LTP and Memory Flashcards
“Mental exercise facilitates a greater development of … the nervous collaterals in the part of the brain in se. In this way, preexisting connections between groups of cells could be reinforced”
Modern parlance?
Strengthening of synapses may occur as a mechanism for learning
Cells that fire together, wire together. What molecular basis for this idea was shown?
NMDA receptor - coincidence detector at synapses
The hippocampus is critical for what process (memory)
Memory consolidation
converting STM into Long term memory
IMAGE SLIDE 4 (?)
How is long term memory classified?
Long term memory
Hippocampal neurons can act as ____ cells
Hippocampal neurons can act as place cells
-regions of space in which the cell shows preferential firing (ie fire when animal is in a specific location)
Four functions of the hippocampus
- memory formation
- spatial learning and spatial memory
- non-spatial learning such as visual recognition of objects
- forming associations between events occurring in close temporal succession to each other
2 hypotheses for hippocampal fxn
- you are here and not there
- Relational memory theory
- you are here in respect to other areas
What is long-term potentiation?
Brief bursts of high frequency stimuli caused a long-lasting increase in the size of subsequent EPSPs
ie synaptic strength increases instantaneously and stays increased for several hours - summation
What is long term depression?
Activity dependent reduction in the efficacy of neuronal synapses lasting hours or longer following a long-patterned stimulus
occurs when postsynaptic spikes precede presynaptic spikes by up to 20-50ms
Properties of LTP
- Cooperativity
- Need to activate enough afferent fibers (presynaptic axons) to elicit LTP - Associativity
- Pairing stimulation of weak and strong pathways elicits LTP - Specificity
- LTP is restricted to the stimulated pathway
how to record ltp
take baseline (20-30min)
pause stimulator
use pulse or more of 100hz to give high freq stim
ltp characterized by sharp/sudden increase in EPSPs
long-lasting ltp depends on protein synthesis translation - linked to long term memory in mice
Mechanisms of LTP: Post-synaptic alteration
Key mechanism for LTP expression:
Trafficking of AMPA receptors
ie ampa receptors added to post-synaptic membrane = amplifies sound
Mechanisms of LTP: post-synaptic alteration
What happens during tetanus? (high freq stimulus)
Unblocking of NMDA receptors → entry of Ca++ into post synaptic dendritic spine → activates kinases such as CaMKII, PKC, tyrosine kinases PKA
Additional properties that makes LTP excellent synaptic mechanism for memory storage:
- LTP is ____ (requires only a very brief burst of stimulation)
- LTP is ____ (can last for up to a year in intact lab animals)
- LTP-like plasticity occurs during ____
- Signaling requirements for ____ mirror those for LTP
- Like long-term memory, LTP requires ____
- LTP is rapidly induced (requires only a very brief burst of stimulation)
- LTP is long-lasting (can last for up to a year in intact lab animals)
- LTP-like plasticity occurs during associative learning
- Signaling requirements for memory storage mirror those for LTP
- Like long-term memory, LTP requires protein synthesis
2 Caveats to Ltp as synaptic mechanism for memory storage:
- LTP is required, but is not sufficient, for some forms of learning and memory.
- LTD is also critical for some forms of learning and memory, such as spatial memory formation in the water maze.
LTP and LTD induction involve different levels of ____ influx
LTP and LTD induction involve different levels of calcium influx
DESCRIBE the IMAGE
high frequency stim (HFS) leading to LTP → substantial increase in ____ → activates a number of ____ → culminates in ____
In contrast:
Lower frequency stimulation (LFS) leads to a more modest increase in ____ → preferentially activates ____which remove ____ from proteins and therefore favours ____ influx
high frequency stim leading to LTP → substantial increase in post-synaptic calcium → activates a number of protein kinases which phosphorylate other proteins → culminates in LTP
In contrast:
Lower frequency stimulation (LFS) leads to a more modest increase in calcium → preferentially activates protein phosphatases which remove *phosphate groups *from proteins and therefore favours LTD
What is the most intensely studied excitatory synapse in the mammalian brain?
Schaeffer Collateral Pathway - extends from area CA3 to pyramidal neurons of area CA1 in the hippocampus
The Schaeffer collateral pathway extends from ____ to ____ in the hippocampus
Schaeffer Collateral Pathway - extends from area CA3 to pyramidal neurons of area CA1 in the hippocampus
How does Hippocampal LTP show cooperativity, associativity and input specificity?
- Cooperativity:
- The likelihood of inducing LTP increases when the number of stimulated fibres is increased
- Associativity:
- If two separate pathways (A and B) converging onto the same postsynaptic target cell are present, such that both can be stimulated to geve weak (A) and strong (B) inputs, then LTP can be induced in the weak pathway ONLY when its activity is associated or paired with activity from the strong pathway
- Specificity:
- LTP is evident only at activated synapses
What is typically measured during field potential recordings?
Initial slope of the EPSP
Voltage shown as downward deflection because tip of electrode is outside the cell
long-lasting ltp depends on ____- linked to long term memory in mice
long-lasting ltp depends on protein synthesis translation - linked to long term memory in mice
All AMPARs consist of ____ subunits, usually a dimer of ____ plus a dimer of ____
All AMPARs consist of *4 subunits, usually a dimer of GluR2 *plus a dimer of Glu⅓/4
Most AMPARs in the hippocampus contain ____ subunits
Most AMPARs in the hippocampus contain GluR2 subunits
GluR2 enables outward and inward currents to occur, depending on the membrane potential
Most AMPARs in the hippocampus contain GluR2 subunits
which enables:
Most AMPARs in the hippocampus contain *GluR2 *subunits
GluR2 enables outward and inward currents to occur, depending on the membrane potential
Activity induces insertion of ____ receptors which cause ____
Activity induces insertion of *GluR1/GluR2 heteromeric *receptors which cause LTP
Maintaining synaptic transmission at a given level involves replacing ____ receptors with ____
Maintaining synaptic transmission at a given level involves replacing *GluR1-containing receptors with GluR2 containing *receptors (eg GluR2/GluR3 heteromers)
AMPA receptors containing GluR1 help regulate ____ during LTP
AMPA receptors containing GluR1 help regulate insertion during LTP
Ampa receptors containing GluR2 control ____of receptors
Ampa receptors containing GluR2 control removal/endocytosis of receptors = LTD
NMDA receptors have ____ subunits; in hippocampus consist of two mandatory ____ subunits combined with ____ and/or ____
NMDA receptors have 4 subunits; in hippocampus consist of two mandatory NR1 subunits combined with *NR2A *and/or NA2B
NMDA receptors with NR2A subunits are predominantly at the ____
And NR2B receptors are mainly located ____
NMDA receptors with NR2A subunits are predominantly at the synapse
And NR2B receptors are mainly located distal from synapses
Research has shown that NR2A receptors are needed for ____ whereas NR2B receptors are needed for ____
Research has shown that NR2A receptors are needed for LTP induction whereas NR2B receptors are needed for LTD
Not all forms of LTP share the same mechanisms. Eg LTP is known to consist of at least 2 temporal phases:
An early phase (E-LTP)
A late phase (L-LTP)
L-LTP but not E-LTP requires ____ and activation of ____ for its stable expression
L-LTP but not E-LTP requires *protein synthesis (translation) *and activation of cAMP-dependent protein kinase (PKA) for its stable expression
When would synthesis of new proteins and activation of genes be required?
Only long-lasting changes in synaptic strength require the synthesis of new proteins and activation of genes
Making new long-term but not short-term memories requires:
Neural protein synthesis
L-LTP can last ____ and is induced by ____
E-LTP can last ____ and is induced by ____
L-LTP can last s*everal hours in slices *and is induced by strong electrical stimulation
E-LTP can last less than 2 hours and is induced by weaker stimulation
L-LTP has been shown to be strongly correlated with ____ (memory)
L-LTP has been shown to be strongly correlated with consolidation of long-term memory (memory)
LTP might be a neural representation of ____
long term memories
What is induction?
induction refers to the initiating events that trigger LTP
What processes are involved in induction?
- High frequency stimulation → releases glutamate from presynaptic terminals → binds to NMDA and AMPA receptors on surface of postsynaptic neurons → depolarization → relieves magnesium block of NMDA receptor channels → Ca++ passed into postsynaptic cell
- Entry of Ca++ into postsynaptic cell through voltage-gated ca++ channels and NMDA receptor channels required for induction of LTP
- Several protein kinases have been implicated in LTP induction
- Ca/Calmodulin-depended protein kinase II (CaMKII) and protein kinase C (PKC)
- Tyrosine kinases also seem to participate
- Pharmacological block of ____ receptors by application of ____ (such as ____)) prevents induction of LTP
- Valid for LTP at the ____ of area ____ (high densities of NMDA receptors)
- Pharmacological block of NMDA receptors by application of NMDA receptor antagonists (such as* D-2-amino-5-phosphonopentenoic acid* (AP5/APV)) prevents induction of LTP
- Valid for LTP at the Schaeffer collateral synapses of area CA1 (high densities of NMDA receptors)
What happens if calcium chelators are injected into postsynaptic cells?
induction of LTP is blocked → Ca++ entry into postsynaptic cell is required for induction of LTP
What is a critical trigger for LTP?
Elevation of postsynaptic calcium
Injection of calcium into postsynaptic cells produces potentiation and blocking Calcium blocks LTP
What Protein Kinases have been implicated in LTP induction?
- Calcium/calmodulin-dependent protein kinase II (CaMKII)
- Protein kinase C (PKC)
- Tyrosine kinases
- PKA
How do we know that CaMKII and PKC are involved in LTP induction?
Because blockers of these kinases prevents induction and maintenance of LTP
How do we know tyrosin kinases participate in LTP induction?
B/c pharmacological block of these kinases by a specific inhibitor of tyrosin kinases (genistein) prevents LTP induction
How does PKA enable induction of L-LTP?
By inhibiting protein phosphatases q and 2A (PA1 /2A)
Phosphatases dephosphorylate proteins - known that phosphatase activation suppresses LTP and favors LTD
By inhibiting PP1 /2A PKA allows L-LTP to be induced through suppression of
An inhibitory constraint exerted by these phosphatases
PKA and MAP kinases (aka ERK) modulate the ____ to control ____ which is critical for ____
PKA and MAP kinases (aka ERK) modulate the translational machinery to control initiation of translation (protein synthesis) which is critical for initiating long-lasting potentiation
ERK = extracellular signal-regulated kinase
Overall, phosphorylation of proteins in postsynaptic neurons by ____, ____, ____ and ____ is essential for induction of various forms of LTP
Overall, phosphorylation of proteins in postsynaptic neurons by CamKII, PKC, PKA, and tyrosine kinases is essential for induction of various forms of LTP
What are 3 possible methods of expression?
Once LTP is triggered, the synaptic enhancement needs to be established or expressed:
- Increased number of effective release sites in presynaptic terminals (uncovering of “silent” synapses after LTP induction)
- Increased probability fo transmitter secretion because of AP broadening or enhanced excitation-secretion coupling in nerve terminals (presynaptic mechanism)
- An increase in # of postsynaptic receptors and/or increased receptor sensitivity to transmitter
Blocking ____ in hippocampal slices prevents expression of L-LTP
Blocking protein synthesis (translation) in hippocampal slices prevents expression of L-LTP
What are two drugs that are used to block translation (and thus can prevent expression of L-LTP)
Anisomycin and cycloheximide
Blocking transcription with an inhibitor (eg ____) prevents _____ without affecting induction of early-LTP. What does this suggest about transcription?
Gene expression (transcription is required for L-LTP
Blocking transcription with an inhibitor (eg actinomycin-D) prevents expression of L-LTP without affecting induction of early-LTP. What does this suggest about transcription?
Regulating translation is important for controlling the ____
Regulating translation is important for controlling the persistence of LTP
How is rapid, local control of long-lasting plasticity at individual synapses accomplished?
Plasticity-related proteins can be produced rapidly and locally (in dendrites) after electrical stimulation.
Dendritic expression of some proteins is increased within 5 min after LTP induction
This allows for rapid local control of long-lasting plasticity
How to cells respond to external stimuli?
by regulating the translational efficiencies of specific mRNAs to synthesize proteins
What is translation initiation
Translation initiation (TI) is rate-limiting and most translational control mechanisms act on TI
These predominantly involve the phosphorylation of “eukaryotic initiation factors” (eIFs) that assemble TI complexes that promote ribosomal binding to mRNAs - required step for TI
How is TI measured?
The levels of expression of specific phopho-eIFs are used as measures of TI
(eIFs = eukaryotic initiation factors)
What is one key rate-limiting step during TI
The ability of eIF4E to form eIF4F initiation complex
TI is facilitated by eIF4F
What does eIF4F consist of and what binding is critical towards the formation of 4F complex and TI?
eIF4F consists of eIF4A, 4E, and 4G
4E binding to 4G is a critical step towards 4F complex formation and TI
In the basal state, 4E is sequestered by an ____
In the basal state, 4E is sequestered by an inhibitory binding protein (4EBP)
What increases 4EBP phosphorylation and what does it allow?
HFS-induced L-LTP increases 4EBP phosphorylation, which frees 4E to bind to 4G to form the 4F complex
In mouse hippocampus, genetic deletion of an isoform of 4EBP, 4EBP2 increases ____ and converts ____ into persistent, translation-dependent L-LTP following just one 100Hz train
In mouse hippocampus, genetic deletion of an isoform of 4EBP, 4EBP2 increases 4F complex formation and converts* short-lasting, translation-independent E-LTP* into persistent, translation-dependent L-LTP following just one 100Hz train
What would cause L-LTP induced by multiple trains of 100Hz stimulation to be reduced, along with reduced long-term fear memory?
Genetic deletion of an isoform of 4EBP (4EBP2) which would increase 4F complex formation
In the mouse hippocampus, genetic deletion of an isoform of 4EBP, 4EBP2, increases 4F complex formation and converts short-lasting, translation-independent E-LTP into persistent, translation-dependent L-LTP following just one 100Hz train - reduces L-LTP induced by multiple 100Hz trains and reduced long term fear memory.
What does this indicate?
Indicates that increased translation initiation in 4EBP2 knockout mice is detrimental to this form of LTP and to long-term memory
Thus tight regulation of translation initiation is needed for normal expression of LTM
Which two key kinases regulate 4EBP in hippocampal neurons?
ERK and mTOR (mammalian target of rapamycin)
How does mTOR regulate 4EBP?
mTOR directly phosphorylates 4EBP to release 4E from repression and promote formation of 4F initiation complex
What phosphorylates ERK and what does it allow?
MEK (MAPK/ERK kinase) phosphorylates ERK, which then phosphorylates (activates) Mnk1 (MAPK-interacting kinase-1)
What is the function of Mnk1?
How is it activated
MEK phosphorylates ERK- phosphorylates (activates → Mnk1
Mnk1 phosphorylates 4E → promotes 4E binding to 4G → enhances formation of the 4F initiation complex
____ gates ERK-dependent phosphorylation of 4E and translation initi
*mTOR *gates ERK-dependent phosphorylation of 4E and translation initi
L-LTP is associated with regulation of a critical translation factor ____ by ERK and mTOR
L-LTP is associated with regulation of a critical translation factor* eIF4E* by ERK and mTOR
L-LTP is associated with regulation of a critical translation factor eIF4E by ____ and ____
L-LTP is associated with regulation of a critical translation factor eIF4E by ERK and mTOR
How are signalling pathways responsible for induction and expression of hippocampal LTP identified? (2 approaches)
- Genetic approach
- create genetically-modified mice with specific protein kinases or phosphatases knocked our or inhibited in their hippocampi
- measure behaviour and synaptic physiology - Pharmacological approach
- several drugs that inhibit specific biochemical signalling pathways (many are membrane permeable - easy to apply to hippocampal slices)
- Block activation of specific protein kinases or protein phosphatases that are believed to be important for LTP induction or expression
What is the time difference between Amygdala fear conditioning and the hippocampus spatial learning?
- Hippocampus and spatial learning
- done with Morris maze
- Takes weeks of training
- Amygdala fear conditioning
- remember task after just one session lasting about 3 minutes
What area of the brain is required for fear conditioning?
the amygdala - lesions of the amygdala impair fear conditioning
What part of the brain is required for making emotional memories?
Amygdala
What experiment provided a convincing demonstration that LTP can occur in vivo during and after learning?
Recording EPSPs in the amygdala → these EPSPs became potentiated when auditory tone was paired with foot shock
Potentiation persisted after training
-unpaired random delivery of shock and tone resulted in less potentiation
Thus LTP in the amygdala can occur in awake rats as a consequence of behavioural learning and memory
Strong stimulation (such as in long-term potentiation) preferentially activates:
PKA → carries out a number of processes including recruitment of transcription in nucleus / production of mRNAs → translated into proteins related to post-synaptic growth
The ERK pathway is important for ____
Translation (protein synthesis)
PKA suppresses ____
PKA suppresses PP1
-tips scale in favour of Ltp
PKA suppresses ____ through inhibitor-1
PKA suppresses PP1
PKA suppresses Protein Phosphatase-1 (PP1) through ____
PKA suppresses Protein Phosphatase-1 (PP1) through inhibitor-1
Which promoter drives transgene expression in forebrain, hippocampus?
CaMK2-alpha promoter
- produce transgenic mice that overexpress genes
Antagonists to which receptor blocks water maze learning and hippocampal LTP?
NMDA Receptor Antagonist (DL-AP5)
Contextual memory requires ____ and ____
Contextual memory requires hippocampus and amy
Cued memory requires
the amygdala
Which site of the amygdala is critical for integrating the CS (tone) and US (footshock)
Lateral nucleus of the amygdala
