Presentation on the paper

Dendritic spine changes associated with

hippocampallong-term

synaptic plasticity

by

Florian Engert&

Tobias Bonhoeffer

What processes can be modelled by long-term enhancement of synaptic efficacy in the

hippocampus?

Neuronal plasticityCircuit reorganizationMaybe learning and memory

Do the changes in synaptic magnitude have any morphological reason on the subcellular level?

(I.e. is there any connection between function and structure?)

Let's have a closer look at some particular regions of the postsynaptic dendrite!

Techniques used

Local superfusion technique to concentrate on only one active synapse

Two-photon imaging to "photograph" the changing dendrite with reduced phototoxic damage

Results

Long-term functional enhancement of synapses in area CA1 causes spine growth on the postsynaptic dendrite,which does not occur in other regions or after solely short-term magnitude changes

Experimental setup

• Pyramidal neuron in CA1 (Vrest<-65mV)impaled with an intracellular recording electrode& filled with the fluorescent dye calcein

Electrical stimulation of the Schaffer collaterals-->EPSPs

• Blocking solution of 10µM Cd^2+ &

0.8µM Ca^2+ stops transmitter release

• Only a small area of about 30µm diameter

is spared --> superfusion spot

• Relevant synapses are found• Baseline synaptic transmission is recorded

• LTP(depolarization + new stimuli)

6 high-resolution 3-D image stacks of the postsynaptic neuron per hour

Example

New <-->disappearing

spinesActive <-->"off-spot"

regions

Controls

• "Off-spot" regions: one case of new spinesNMDA-receptor antagonist AP5 (50µM) --> no LTP: no new spines Unsuccessful induction of LTP(e.g. only a short potentiation, <30 min): no new spinesIn all three conditions: randomly disappearing spines

• ´Blind´observer

• Correlation between the emergence of new spines and

an increase in synaptic efficacy

• Slight anticorrelation between the disappearence of spines and an increase in synaptic

efficacy

• (t-test)

Studies only of long-lasting changes

What about short-term LTP & LTD?

• Technical difficultiesUncertainty about the spatial location of the interesting synapses--> too large region to investigateSmall and contradictive structural changes --> too large sample number---------> Big advantages of applied techniques!

Alternative (not contoversial) viewChanges in spine shape influence synaptic strength(shortening and/or widening of the neck --> reduced

resistance -->increased efficacy)

AnswerMaybe they occur in addition to the numerical changes

& below the given spatial resolution

???________Do the emerging spines contain any active synapses?

AAA_______Schould be expected so in congruity with the others

???________How are the first effects of LTP to explain if the initial

morphological changes detected arise no earlier than 30min after LTP

induction? AAA________At first: transient changes,

then: structural and permanent ones

???_____Does the disapperance of spines depend on synaptic activity

or general synaptic decay(a kind of regulation of synaptic density over the

lifetime)?

AAA______Data support the latter alternative,but probably some functional changes depend on

synaptic activity

Summary

• LTP in hippocampus --> new dendritic spines???____ Also new synapses (formation within an hour after the stimulus)_____???More experiments requiredFunctional --> physiological & structural changes

Note added in proof

• Supporting evidence from another experimental set up:Strong synaptic acticity --> new dendritic processes(M. Maletic-Savatic,R.Malinow& K.Svoboda,Science 283,1923-1927;1999)