Neuroscience Journal Club

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Neuroscience Journal Club

• Experience-dependent plasticity of dendritic
  spines in the developing rat barrel cortex in
  vivo
• Balazs Lendvai, Edward A. Stern, Brian Chen
  Karel Svoboda

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Processing of Sensory Informations
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BRAIN what for?

• ELABORATION OF STIMULA
• COLLECTION OF (SENSORY) INPUTS
• PROCESSING OF INFO (Cortex) ? RECORDING
• SPAWNING A (MOTOR) OUTPUT

Analogous to neurons processing
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BRAIN what for?

• MEMORIZATION
• ENCODING analysis of sensorial informations
• STORAGE keep a copy or permanent recording of
  coded informations
• RETRIEVAL following use of the information
  stored, in order to behave in a certain way or to
  solve a problem.

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What is memory?
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Where is memory stored?

• Where in the brain are memories stored?
• How do we know this?
• How does the brain store electrical patterns of
  activity with cells?

• Lashley K. lesioned various portion of the
  cortex to localize the one responsible for
  mnemonic retention
• H.M. following a surgery, he suffered from
  anterograde amnesia
• Penfield W. electrical stimulation of cortex
  surface

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PENFIELD

• Hebb rule for Synaptic Plasticity (1946)
  synaptic facilitation can derive from each
  experience
• The trace (persistence or repetition of a
  reverberatory activity) tends to induce lasting
  cellular changes that adds to its stability and
  that can be retrieved several years later through
  an electrical current, without loosing any detail

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The substrate of memory is dendritic spines
maybe
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How is long-term memory stored?

• Neurons form circuits where electrical signals
  (spikes) propogate between synapses
• Once a circuit is stimulated, under certain
  circumstances it is easier to stimulate again

• Reverberating circuits
• Long term potentiation
• example of an electrical stimulation causing
  permanent change
• Inhibitors of protein synthesis or calcium
  signals prevent LTP

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A glutamatergic postsynaptic membrane containing
AMPA and NMDA subtypes of glutamate receptor.
(A). Glutamate molecules released from the
pre-synaptic terminal diffuse across the synaptic
cleft and bind to both sub-types of receptor,
opening AMPA receptor channels producing an
excitatory post-synaptic potential (EPSP). (B).
High concentrations of glutamate produce strong
depolarisation of the post-synaptic membrane,
resulting in the expulsion of magnesium ions from
the NMDA receptor channel, and allowing influx of
Na and Ca2 ions (C).
Meccanismi cellulari di apprendimento e memoria
LTP e LTD
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Cellular mechanisms of learning and memory LTP
and LTD
? Repeated stimulus leaves a TRACE ? Nervous
track repeatedly crossed can be subjected to LTP
? Activation of cascade biochemical mechanisms
that determine STRUCTURAL MODIFICATIONS of the
circuit itself
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• In the nucleus PKA and MAPK phosphorylate and
  activate the cAMP response element-binding (CREB)
  protein and remove the repressive action of
  CREB-2, an inhibitor of CREB-1. CREB-1 in turn
  activates several immediate-response genes that
  lead to the expression of LTP/memory effector
  proteins with the growth of new synaptic
  connections.
• A typical sensory neuron in the intact Aplysia
  has about 1200 synaptic varicosities. Following
  long-term sensitization, the number more than
  doubles to about 2600 with time the number
  returns to about 1500.

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SUMMARY

• ? Repeated stimulus leaves a trace
• ? Nervous track that is repeatedly crossed can be
  subjected to LTP
• ? Activation of cascade biochemical mechanisms
  that determine structural modifications of the
  circuit itself
• ? Activation/deactivation of synapses
• ? Protrusion/retraction of spines/filopodia

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Cellular mechanisms of learning and memory LTP
and LTD
From Science, 2006
? Activation/deactivation of synapses ?
Protrusion/retraction of spines/filopodia
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LTP and SPINES

• More than 90 of excitatory axodendritic synapses
  in the mammalian cortex occur on small dendritic
  appendages called spines
• Filopodia and spines sprout in response to strong
  synaptic stimuli that produce LTP, suggesting
  that such motility may be an important aspect of
  activity-dependent synaptic plasticity

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• Do changes in neuronal structure underlie
  cortical plasticity?1,2

1. Bailey, C. H. Kandel, E. R. Annu. Rev.
Physiol. 55, 397426 (1993). 2. Buonomano, D. V.
Merzenich, M. M. Annu. Rev. Neurosci. 21,
149186 (1998).
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In vivo 2P imaging

• Images from 0 to 100 ?m depth
• 100 ? 100 ?m area
• 1 ?m z stack
• 512 ? 512 resolution
• Integration time 5 ?sec x pixel
• 50mW _at_ 935 nm
• 120 fs pulsewidth
• Laser wavelength 935nm

Imaging into brain cortex of a P90 GFP-M
transgenic mouse Mice are developed in J. Sanes
Lab
Resolution of individual dendritic spines
The movie shows the population of GFP labeled
neurons.
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Methods

• Infection of neocortical neurons in vivo with
  SINEGFP with injection into brain parenchyma
• Sensory deprivation trimming
• Intracellular recording in vivo

In vivo high-resolution imaging of barrel cortex
neurons infected with SIN-EGFP (Sindbis virus
containing the gene for Enhanced GFP).
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Methods

• Time lapse two photon microscopy
• In vivo imaging of the structural dynamics of
  dendritic spines and filopodia in the intact
  brain
• Piramidal neurons in layer 2/3 of developing
  (P8-18) rat barrel cortex

Objectives

• Modulating sensory inputs by trimming whiskers
  changes the response properties of neurons.
• Examine the effects of the rat's sensory
  experience on the structure and dynamics of spiny
  protrusions as a substrate of experienced-dependen
  t plasticity

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Barrel Cortex
Spatial arrangement of the whiskers on the rats
face matrix of large hairs represented in these
brain areas by a topographically similar matrix
of cell rings. (A, B) Barrels aggregates of
cell rings in layer IV of the cerebral cortex .
Barrel cortex area in the somatosensory cortex
(C) where neurons are grouped in barrel- like
arrangements, with a hollow center of lesser cell
density surrounded by a circle of higher cell
density. IMP one-to-one relationship between
each vibrissa and its corresponding barrel.
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Barrel Cortex 2P Time-lapse Imaging

• Characterization of dendritic protrusions high
  resolution 2PLSM images.
• Quantification of motility lenght of individual
  protrusions vs time
• Description of structural dynamics for
  individual protrusion average change of lenght
  per sampling interval (mm per 10 min)

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Results

• High mobility in vivo dendritic protrusions are
  dynamic (changed lenght, shape,
  appeared/disappeared) over timescales of 10 min
  and over lengths of mm
• Largest motility in the youngest animals (P8-12)
• ? less filopodia

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Effects on the structure and dynamics of spiny
protrusions

• Whiskers trimming 1-3 days before imaging
• Comparison of LOCATIONS control, deprived,
  specificity
• Comparison of AGES
• during (P11-13), before (P8-10), after (P14-16)
  synaptogenesis (whiskers use in exploratory
  behaviour)

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DYNAMICS

• Protrusive motility is modulated by previous
  experience
• AGE only during a brief critical period, P1113,
  deprivation caused a large decrease in motility
• LOCATION effects of sensory deprivation are
  specific to the deprived region of the cortex.

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STRUCTURE AND DENSITY

• Sensory deprivation does NOT change the average
  structure (distributions of lengths, distr. among
  different morphological classes) or density of
  dendritic protrusions (in all ages)

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Effects on the development of receptive fields

• Recordings of membrane potential dynamics of
  regular spiking neurons in P1416 rats
• Measurement of PSPs amplitudes in response to
  deflections of single whiskers (SW or PW)
• RESULTS
• Principal whisker response was smaller than in
  control animals but the surround was stronger and
  broader
• Sensory deprivation has a profound effect on the
  TUNING of sensory maps of layer 2/3 pyramidal
  neurons.

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Effects on network synaptic activity

• Measurement and computation of the distribution
  of MP to see if experience-dependent changes in
  spontaneous synaptic activity drive changes in
  protrusive motility
• RESULTS
• NO long-lasting effects on network synaptic
  activity
• Experience-dependent changes in motility are
  coupled more directly to the history of sensory
  activity

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Results summary

• HIGH BASAL motility in spines and filopodia they
  appeared, disappeared or changed shape over tens
  of minutes
• Experience-dependent modulation of dendritic
  motility (synaptic lifetimes) is limited to a
  sharp critical period (P1113)
• Does sensory experience drive this motility?
• Yes Sensory deprivation markedly (40) reduced
  protrusive motility in deprived regions of the
  barrel cortex
• No Whisker trimming did NOT change the density,
  length or shape of spines and filopodia

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Results summary

• ELECTROPHYSIOLOGICAL MEASUREMENTS effects on
  synaptic ACTIVITY
• Sensory deprivation spanning the critical period
  is associated with defective development of layer
  2/3 sensory maps
• Sensory deprivation perturbs the
  experience-dependent rearrangements of synaptic
  connections required to form precise sensory maps
  

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Long Term Depression
LTP can be saturated You only have a finite
number of synapses Your brain is in danger of
getting full Therefore you need LTD
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THANKS FOR YOUR ATTENTION!!!
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