Synaptic Plasticity

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Synaptic Plasticity
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The term synaptic plasticity refers to the
variability of the strength of a signal
transmitted through a synapse.

• Facilitation
• The amplitude of the postsynaptic response
  increases when the postsynaptic cell is activated
  several times in quick succession
• Important Questions
• Presynaptic or Postsynaptic?
• Underlying Mechanisms?

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Time Course of Activity Induced Changes
Synaptic plasticity is classified according to
the duration over which the effect persists.
SHORT TERM CHANGESFacilitation appears
instantly, and is of short duration (100
ms)Depression recovers and Augmentation
dissipates within 10
secondsPost-tetanic potentiation (PTP) can last
for more than 10 minutesLONG TERM
CHANGESLong-term potentiation (LTP) and
Long-term Depression (LTD) last from minutes to
beyond 10 hours
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Short-term Changes in Signaling

• Most extensively studied at synapses in the
  peripheral nervous system (chick ciliary
  ganglion, skeletal muscle)
• Changes have also been demonstrated throughout
  the CNS
• Facilitation
• Augmentation
• Post-tetanic Potentiation (PTP)
• Depression
• Typically last for periods ranging from
  milliseconds (facilitation) to tens of minutes
  (PTP).

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Facilitation of Transmitter Release

• Most immediate effect of repetitive stimulation
  is synaptic facilitation
• Amplitudes of EPPs increase progressively
• The effect outlasts the stimulus train

Frog NMJ
Low Ca2
Cause increased mean number of quanta of
transmitter released by the presynaptic terminal,
probably by increasing the probability of release
and perhaps increasing the number of release
sites.
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Augmentation of Synaptic Transmission

• Slower phase of facilitation
• Increase in synaptic potential amplitude comes on
  more slowly than facilitation
• Decays over a much longer time period (time
  constant of 5-10s)

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Post-Tetanic Potentiation (PTP)

• Relatively long train of high frequency stimuli
  (Tetanus)
• Refers to increased transmitter (ACh) release
  from presynaptic terminal due to prior
  stimulation (similar to facilitation and
  augmentation)
• Differs from facilitation and augmentation in
  that its onset is considerably delayed
• (reaches maximum several seconds after
  stimulation ceases, lasts for tens of minutes
• Blocked by removal of calcium from bathing
  solution, but PTP occurs in the presence of TTX
  (w/ depolarizing pulses)

Curarized
Chick ciliary ganglion
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• The frog neuromuscular junction (NMJ) provides an
  excellent model for studying the role of
  receptors in synaptic transmission. The
  preparation has a large postsynaptic element,
  making it relatively easy to monitor changes in
  synaptic transmission in the form of end plate
  potentials (EPPs). Unlike action potentials, EPPs
  are not all-or-none responses instead, they
  reflect small changes in synaptic transmission.
  To observe EPPs, antagonists must be applied to
  the NMJ to compete with neurotransmitter binding
  to postsynaptic receptors. This competition
  prevents the depolarization of the postsynaptic
  membrane from reaching threshold and thus,
  eliminates action potentials.
• Curare is an example of a non-depolarizing muscle
  relaxant which blocks the nicotinic receptors,
  one of the two types of cholinergic
  (acetylcholine) receptors on the post synaptic
  membrane of the neuromuscular junction.

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Depression of Transmitter Release

• Synaptic depression can occur if the number of
  quanta released by a train is large
• Amplitudes of EPPs decrease progressively with
  repetitive stimulation
• This effect also outlasts the stimulus train (not
  shown)

Curarized
Frog NMJ
High Ca2
Thought to be caused by depletion of vesicles
from the presynaptic terminal during the
conditioning train, and reduced release efficacy.
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Short Term Synaptic Plasticity

• Synaptic enhancement (facilitation,
  augmentation, potentiation)
• ALL presynaptic mechanisms
• Increase in mean number of transmitter quanta
  without change in quantal size or postsynaptic
  effectiveness
• Increased probability of release and perhaps an
  increased number of release sites
• Crucial role of calcium
• Residual presynaptic intracellular calcium
• Synaptic depression
• MOSTLY presynaptic
• Depletion of pool of vesicles
• Decrease in number of transmitter quanta
• Decrease in probability of release and perhaps a
  reduced release efficacy

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Long-term Changes in Signaling

• In the CNS, repetitive activity produces changes
  in synaptic efficiency that last much longer than
  seen at peripheral synapses - ranging from
  minutes to hours.
• Hippocampal LTP best studied of any form of
  plasticity. Much of the research predicated on
  assumption that hippocampal LTP is the mechanism
  for learning.
• Cortex both LTP and LTD of pyramidal cell
  excitatory synapses
• Amygdala LTP closely linked to fear
  conditioning
• Cerebellum mostly LTD of Purkinje cell EPSPs.
  Some LTP at Purkinje cell excitatory synapses
  and LTP of inhibitory synapses
• May represent neural substrates for learning and
  memory
• Long-Term Potentiation
• Long-Term Depression

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LTP and LTDin vitro vs. in vivo

• Acute Brain Slice Prep, Slice culture,
  Co-Cultured cells
• Limitations mimics an intact system
• removal of normal inputs and milieu
• addition of blockers such as picrotoxin or
  tetrodotoxin
• lack normal outputs
• Advantages clear and interpretable response
• Single EPSP/IPSP is unequivocal its there or
  it isnt
• No contamination from other inputs
• Intact anesthetized or freely moving animal
• Dont know effective stimulation
• Can study effects of stimulation on behavior

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Long-term Potentiation
Why the hippocampus?

• First described by Bliss and Lomo (1973) at
  glutamatergic synapses in the hippocampal
  formation.
• High frequency stimulation of inputs to dentate
  gyrus cells produces an increase in the amplitude
  of EPSPs lasting for hours or days.
• Homosynaptic LTP
• The LTP effect also observed in neocortex.

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Long-term Potentiation in CA1
Requires only a brief tetanus, is input specific,
and can last many weeks
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Associative LTP and Learning?
Associative LTP is the strengthening of the
connection between two neurons that have been
simultaneously active
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Associative LTP
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Mechanism(s) for LTP in CA1
Increased effectiveness of existing postsynaptic
AMPA receptors, perhaps by phosphorylation. PKC
phosphorylation of the AMPA receptor changes the
protein in some way that increases the ionic
conductance of the channel. Insertion of
completely new AMPA receptors into the
membrane Changes to the structure of the
synapse- new buds form on postsynaptic dendrites,
axons sprout and form multiple synapses.
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Significance of Changes in Synaptic Efficacy

• LTP (and LTD) are of particular interest because
  learning and memory are thought to involve
  long-term changes in synaptic efficacy.
• A number of correlations have been shown between
  spatial learning in intact animals and LTP in
  hippocampal slices (ie., both blocked by NMDA or
  mGlu Receptor antagonists)
• LTP in amygdala strongly associated with aversive
  (fear) conditioning
• rats trained to associate foot shock with a
  sound exhibit an exaggerated auditory startle
  reflex
• cells in the amygdala display LTP-like
  increase in their synaptic responses to
  stimulation of auditory inputs.
• both are blocked by NMDA receptor antagonists.

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Types of Long-term Depression
Linden Connor, 1995
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Cerebellar Anatomy

• EXCITATORY
• Parallel Fibers (gr c.)
• Climbing Fibers
• INHIBITORY
• Purkinje Cells
• Stellate Cells
• Basket Cells

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Long-term Depression in the Cerebellum
After pairing, there is an LTD of the response to
parallel fiber stimulation
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Mechanism of LTD in the Cerebellum
AMPA receptors are internalized Postsynaptic
effect
CF activates Purkinje Cell, Na entry depolarizes
the dendrite, and voltage-gated Ca2 channels are
activated. PF activation (glutamate) also
increases Na entry, through AMPA receptors. The
glutamate also directly activates mGluRs in the
membrane. This generates DAG which activates
PKC. PKC phosphorylates proteins--somehow
leading to a decreased number of AMPA receptors
in the postsynaptic membrane.
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Hebbian Rules for Synaptic ModificationDonald
Hebb(1940s)

• When the presynaptic axon is active, and at the
  same time the postsynaptic neuron is strongly
  activated by other inputs, then the synapse
  formed by the presynaptic axon is strengthened
• Neurons that fire together wire together
• When the presynaptic axon is active, and at the
  same time the postsynaptic neuron is weakly
  activated by other inputs, then the synapse
  formed by the presynaptic axon is weakened
• Neurons that fire out of sync lose their link