CA3-CA1 Synaptic Plasticity 1

Question 1

Hippocampal damage causes

Answer 1

Anterograde amnesia up to 12 weeks after learning task (after which transferred to neocortex)

Question 2

HM

Answer 2

Hippocampus removed–> loss of declarative but not procedural memory

Question 3

Episodic memory in mammals involving the hippocampus

Answer 3

DIAGRAM

Question 4

First observation of LTP

Answer 4

Lomø discovered LTP in the perforant path input to dentate gyrus in the anaesthetized Norwegian rabbit hippocampus in vivo.
Terje Lomo
Initially, synaptic transmission monitored at <0.1Hz low frequency electrical stimulation of perforant path.
A brief high frequency 100Hz stimulus (HFS or “tetanus”) applied to same input for 3-4s.
Then, return to low frequency (<0.1Hz ) stimulation: observed in a steeper rise time (slope) of the extracelluar field EPSP.
Another result of the 100Hz stimulus was an increase in the number of cells firing action potentials (increased population spike - *).

Question 5

3 main hippocampal glutamatergic pathways

Answer 5

Perforant path fibre: perforant fibres- dentate granule cells
Mossy fibre: dentate granule cells-CA3 pyramidal cells
Schaffer collateral: CA3 pyramidal cells- CA1 pyramidal cells

Question 6

Best system for researching LTP

Answer 6

CA1 in vitro

Question 7

Hippocampal circuitry can be maintained in a slice

Answer 7

300 to 500μm thick

Question 8

Basic LTP experiment

Answer 8

Extra- and/or intracellular recording
from CA1 pyramidal neurone
Record synaptic responses to low frequency (LFS) Schaffer collateral stimulation <0.1Hz - control baseline.
Stimulate Schaffer collateral pathway at 100Hz for 1s - a high frequency stimulation (HFS).
Return to low frequency stimulation - post HFS responsiveness.

Question 9

After 30’ HFS

Answer 9

Changes in synaptic weight

Question 10

LTP at CA3 –> CA1 synapses

Answer 10

Monitoring at <0.1 Hz
Initial post-tetanic potentiation (PTP) or short-term potentiation (STP) - c.10 min.
Decays to a sustained level.
With a single 100Hz stimulus further decay to baseline at two hours. 1st hour - early LTP.
With four 100Hz stimulus (1 every five minutes) can lasts up to 24 hours - early LTP + late LTP

Question 11

Other induction protocols CA3–>CA1

Answer 11

“Theta burst stimulation (TBS)”

Fewer stimuli/”more physiological” compared to 100Hz for 1s HFS

Question 12

Induction of LTP is dependent on:

Answer 12

NMDA receptor activation: blocked with D-2-amino-5-phosphonovalerate (D-2-amino-5-phosphonopentanoate)
- a.k.a. APV or AP5. Postsynaptic rise intracellular Ca2+: block with intracellular injection of Ca2+ chelator EGTA.
Postsynaptic depolarization:
blocked by direct injection of negative current to hyperpolarized membrane potential and prevent depolarization during HFS.

Question 13

CA3–>CA1 LTP is mimicked by…

Answer 13

…brief pairing of low frequency 2Hz stimulation (20s) with depolarization to 0mV (no HFS)

Question 14

To induce LTP…

Answer 14

…a stimulus intensity threshold for induction must be reached

Question 15

Specificity of LTP

Answer 15

Input specific- LTP only induced at inputs active during HFS, and not adjacent inactive ones

Question 16

Cooperative LTP

Answer 16

The weaker stimulation of multiple pathways which

could not induce LTP on their own, can do so together

Question 17

Associative LTP

Answer 17

If weak stimulation of a single pathway is insufficient for LTP induction, subsequent pairing with strong stimulation of another independent pathway during HFS will induce LTP at both pathways

Question 18

Hebb’s rule and LTP at hippocampal synapses

Answer 18

Synaptic weight goes UP if there is repeated activity in a presynaptic input coincident with postsynaptic firing.
A single 100Hz high-frequency stimulus train satisfies this requirement: Presynaptic input is undergoing sustained activation at 100Hz; Depending on whether there is strong single pathway or multiple weak pathway activation, there should be sufficient postsynaptic depolarization to cause action potential firing.
As this is the major induction requirement for both CA3–>CA1 LTP and EC–>DG LTP - both are said to be Hebbian

Question 19

Hebbian synapses and associativity

Answer 19

Hebb’s predictions allow for a number of weak presynaptic inputs to contribute to the generation of the postsynaptic depolarization of membrane potential that leads to change as long as Hebb’s rule is met.
You would expect to see this type of co-operativity and associativity at Hebbian synapses.

Question 20

LTP at other synapses

Answer 20

EC-DG (perforant path cf. Lomo & Morris)- Layer II/III and V entorhinal cortex EC-CA1 (perforant path)- Layer II/III neocortex
DG-CA3 (mossy fibre pathway)
CA3-CA3 (associational/commisural fibre pathway)
CA1-subiculum

Question 21

Early stages of CA3-CA1 LTP depends on:

Answer 21

Activation of PKC
block by peptide PKC19-31 PKC antibodies.
Activation of CaMKII
block by peptide CaMKII273-302
Blocking peptides - pseudosubstrates for calmodulin: truncated amino acid sequences are calmodulin-binding regions - out-compete the endogenous kinase - prevent activation

Question 22

LTP is associated with the increased phosphorylation of…

Answer 22

…the AMPA receptor GluR1 subunit

at Ser831. Both PKC and CaMKII can target this site.

Question 23

Ser831 phosphorylation increases…

Answer 23

…conductance of AMPA receptor ion
channels, ­ current flow through channels and hence EPSC amplitude (thus EPSP larger as proportional to current flowing through the channel)

Question 24

AMPA receptor trafficking- insertion

Answer 24

c.15 min following LTP induction. there is insertion of AMPA receptors (AMPARs) into the postsynaptic density (PSD) region from extrasynaptic reverse pools.
This leads to a further ­ in macroscopic conductance, beyond ­ in microscope conductance associated with individual AMPAR ion channels (last slide).
Dependent on the phosphorylation status of GluR1 subunit S845P: - insertion into plasma membrane - primed S831P: - translocation into PSD - potentiated

Question 25

A presynaptic locus for LTP?

Answer 25

Debated but potential involvement of intercellular signaling molecules:
free diffusible messengers such as NO (nitric oxide), arachidonic acid and PAF (platelet activating factor)
membrane bound trans-synaptic extracellular proteins - integrins, ephrin/EPH receptor
(>100 potential mediators so far)

Question 26

LTP anatomical changes- spine splitting

Answer 26

Hippocampal slice (in organotypic culture): apply TBS to CA3→CA1 pathway
Reconstruction of a three dimensional model from serial sections under e.m.
Synapses likely to be involved in LTP identified by presence of elevated postsynaptic Ca2+ levels compared to control (chemically-induced Ca2+ precipitation protocol).
↑ No. of multiple spines connecting same axon and dendrite

Question 27

Mediation of spine splitting

Answer 27

Likely to be local mechanism involving the cytoskeleton (unlikely to be genomic)
(DIAGRAM)

Question 28

Late stages of CA3-CA1 LTP (pharmacology)

Answer 28

Blocked by: H89-selective PKA inhibitor PKA thus PKA involved(via↑cAMP)
Blocked by: Anisomycin-translational inhibitor thus new protein involved Actinomycin D; - transcriptional inhibitor thus new mRNA involved

Question 29

Late stages of CA3-CA1 LTP

Answer 29

1) Increase postsynaptic [Ca2+]
2) Ca2+-activation of PKA
3) Phosphorylation of CREB
4) CREB binding to CRE
5) CREB binding to CBP linked RNA polymerase II – starts transcription
6) New mRNA of IEGs: transcription factors - C/EBP effector proteins - BDNF
7) IEG transcription factors
8) New mRNA of LRGs (a.k.a. LEGs)
9) New protein synthesis
10) Long-term synaptic changes, Synaptogenesis