Jonny week 11-12 Flashcards
equation for probability of release?
mean no of vesicles involved in release/number of active sites
Pr = m/n
probability of release for NMJ?
0.1 to 0.2
low since vesicle fusion failure rate is high.
weight of numbers - more active zones than needed to ensure transmission.
more vesicles too
typical CNS synpases?
Vast majority are L-glutamatergic or GABAergic
For individual synapses, the EPSP/IPSP generated is very small ~ 0.01-1mV
Typical one or a few active sites per bouton/varicosity
Number of primed vesicles is small 2-10
Miniature EPSPs (mEPSPs) ~ 0.1mV (response to one vesicle)
Vesicle (or quantal) content smaller than NMJ (1,000-5,000 molecules)
For a “typical” excitatory synapse, the EPSP is a long way from reaching firing threshold even when the probability of release is high!
few like NMJ ie Climbing fibre input to Purkinje cells in cerebellum
factors affecting the probability of release?
Remember: the trigger for release is synaptotagmin binding Ca2+
shape of AP.
Open probability and rate of inactivation of the Ca2+ channels.
Number of release-ready (i.e. primed) vesicles at active zone.
Ca2+ responsiveness of these vesicles - Ca2+ sensitivity of release machinery
Number of primed vesicles
i/c Ca2+ levels
Resting levels of Ca2+
Ca2+ channel density - active zone area
Ca2+ channel open time, and the duration of depolarization - action potential shape
Release machnery-Ca2+ channel proximity
(threshold [Ca2+] 200μM)
i/c Ca2+ buffering (reuptake in smooth ER Ca2+ stores)
Ca2+ affinity of synaptotagmin and regulatory proteins
in the release machinery
measure of synaptic plasticity?
the change in amplitude of a postsynaptic response - synaptic strength or weight to the same level of presynaptic activation (same stimulus).
time frames of synaptic plasticity?
Short-term: milliseconds to minutes:
Rapid changes in synaptic event/process dynamics
Readily reversible soon after the inducing event (usually a change in presynaptic action potential frequency) ends
Long-term: hours, days, weeks:
Slow changes induced through activation of biochemical processes and changes in genomic expression → synaptic modification
More permanent and not rapidly reversible after inducing event ends
benefits of synaptic plasticity?
Development:
Synapse formation and stabilization
Pathway refinement - retention or loss
Cognition:
Allows network flexibility
Allows information storage - memory trace formation by strengthening/weakening of connections in neural networks
describe residual ca build up - synaptic plasticity.
As a result of the first AP, there is influx of Ca2+ via voltage-gated Ca2+ channels.
Ca2+ remains elevated for 10’s-100’s of ms, due to slow buffering and reuptake processes (Ca2+ store in smooth ER) in presynaptic terminals
The influx of Ca2+ during second AP combines with this residual elevation of Ca2+ level combines with to increase overall Ca2+ level
This increases probability of release, so over a number of synapses, more vesicles are likely to fuse…..
More likely to generate postsynaptic responses across a number of synapses → larger postsynaptic potential/current
describe presynaptic - ionotropicautoreceptor activation - synaptic plasticity
Following the first AP, the level of neuro- transmitter release leads to activation of presynaptic receptors
These can either be ionotropic or metabotropic
Some ionotropic presynaptic receptors are Ca2+ permeable cation channels - elevate background Ca2+ levels
The influx of Ca2+ during second AP combines with this elevation of Ca2+ level to increase overall Ca2+ level
Again, this increases release probability….
e.g. presynaptic Kainate, nicotinic ACh receptors NMDA-Rs
describe presynaptic vesicle depletion - synaptic plasticity
Each synapse has a small readily releasable pool (RRP) of vesicles (i.e. those that are primed)
The rate at which the RRP is replenished from a reserve pool can be relatively slow
The first AP leads to a reduction in RRP size
If not replenished quickly, then there is time period over which a reduced number of vesicle available for subsequent release and this decreases probability of release!
Hence, when the second AP arrives, the number of vesicles available for release is reduced → lower probability of release and hence smaller postsynaptic event
Typically occurs when pr is already high
describe presynaptic – metabotropicautoreceptor activation - synaptic plasticity
Following the first AP, the level of neurotransmitter release leads to activation of presynaptic autoreceptors - in this case they are metabotropic
Metabotropic - G protein-coupled receptors increase inactivation of Ca2+ channels (via βγ-dimer of G protein)
This reduces Ca2+ influx during the 2nd AP
Decreases release probability
Therefore, fewer vesicles fuse……
e.g. mGlu, GABAB receptors
what is postsynaptic ionotropicreceptor desensitization?
With the 1st AP, the level of transmitter release leads to initial activation of postsynaptic receptors
This is then followed by postsynaptic receptor desensitization - they enter an inactive/closed state that cannot be reactivated (rapid EPSC decay)
Recovery from this process occurs slowly
When the 2nd AP triggers transmitter release, the recovery from desensitization has been only partial so fewer receptors are available for activation
Reduces postsynaptic responsiveness
e.g. AMPA, GABAA receptors
Again, typically occurs when pr is already high.
declarative memory?
explicit
conscious
knowing what
facts, events
non declarative memory?
implicit
do not have a conscious component
knowing how
skills, priming, classical conditioning, habituation
object recognition where?
hippocampus.
Removing hippocampus in epileptic patient: memory loss
Morris Water Maze?
hidden platform on water, mice remember where it is
origins of LTP?
norwegian rabbit hippocampus
low freq stimulation, then a high frequency stimulus for 3-4s (tetanus).
Return to low freq stimulation after several hours.
a steeper rise time of the extracellular field EPSP.
increased no of cells firing APs.
LTP at CA3 –> CA1 synapses?
Electrical stimulation of Schaffer collateral pathway.
Record synaptic responses to low frequency Schaffer collateral stimulation
early and late term potentiating?
repeat HFS multiple times, ie 4 times 5 minutes apart.
remains for up to 24 hours.
component phases of LTP?
Induction phase:
- the initiation of LTP by HFS 100Hz stimulus.
Transient / early phase:
- the reversible Early-LTP following induction.
Consolidated / late phase:
- the permanent changes that then maintain Late-LTP.
describe the LTP CA3-CA1 induction phase.
dependent upon NMDA receptor activation, blocked with AP5 (antagonist).
prevents induction phase from occurring and thus LTP.
there is a postsynaptic rise of intracellular Ca, Ca chelator EGTA blocks LTP.
can induce LTP by postsynaptic depolarization at the same time as the synapse activating.
blocked by direct injection of negative current to hyperpolarized membrane potential and prevent depolarization during HFS.
how do NMDA receptors contribute at hyperpolarised membranes?
barely, blocked by Mg
receptors responsible for baseline transmission?
Baseline neurotransmission - LFS
LTP induction and AMPA/NMDA receptors?
HFS produces a large depolarization via AMPA receptor activation due to temporal summation.
Unblocked NMDA receptors cause further depolarization and allows Ca2+ to enter the postsynaptic spine.
This removes the Mg2+ NMDA receptor block as membrane negativity less attractive to Mg2+.
L-glutamate, activates both AMPA and NMDA receptors during LTP induction.
Also activates voltage-gated Ca2+ channels.
AP5 depresses summated EPSPs evoked at 100Hz
AP5 activation after induction leads to what?
no blocking of LTP.
independent of NMDA receptor activation?
PKC and CaMKII do what?
AMPA receptors are targetted by both to be phosphorylated.
Phosphorylation increases conductance of AMPA receptor ion channels, increased current flow through channels and hence macroscopic EPSC amplitude.
EPSP is larger as it is proportional to current flowing through channel.
can be blocked to stop LTP by PKC19-31 or CaMKII273-302
PKA A inhibitors do what to late LTP?
H89, inhibits LTP.
Anisomycin - translational inhibitor, new protein synthesized
Actinomycin D - transcriptional inhibitor, new mRNA synthesized
cAMP does what?
induces PKA
problems with LTP?
If synaptic weights can only be increased long term:
then eventually all synapses would become fully potentiated
synaptic weights increased to a ceiling point where no further increase in synaptic efficacy could occur.
This would not be much use as the basis for learning or memory
However, could maintain system flexibility through the disassociation (re-setting) to allow subsequent re-association of synaptic connectivity
Requires a means of depressing synaptic transmission or reversing LTP, long-term depression (LTD).
LTD at CA3-CA1?
Electrical stimulation of Schaffer collateral pathway.
1hz not 100hz for 15 mins, return to baseline.
responsiveness of 80% of control levels
LTD depends on what?
AP5 still blocks so depends on NMDA receptors.
Also depends on:
Postsynaptic membrane potential depolarization!
Increase in postsynaptic intracellular Ca2+
how to stop LTD?
sensitive to inhibitors of protein prosphatase (dephosphorylating proteins)
expression blocked by:
Okadaic acid - ser/thr protein phosphatase 1 and 2A (PP1 & PP2A) inhibitor.
FK506 - calcineurin (ser/thr protein phosphatase 2B or PP2B) inhibitor.
LTD/LTS similarities?
Both LTP and LTD blocked by AP5 (both NMDA-dependent), and require depolarization and Ca2+ entry.
LTD/LTS why do each occur?
a difference in activation threshold for protein phosphatases vs kinase
> 5uM intracellular calcium leads to PKA
