Presentation on the paper Dendritic spine changes associated with hippocampal long-term synaptic plasticity by Florian Engert & Tobias Bonhoeffer.

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Presentation transcript:

Presentation on the paper Dendritic spine changes associated with hippocampal long-term synaptic plasticity by Florian Engert & Tobias Bonhoeffer

What processes can be modelled by long-term enhancement of synaptic efficacy in the hippocampus? Neuronal plasticity Circuit reorganization Maybe 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 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

ExampleExample

Controls "Off-spot" regions: one case of new spines NMDA-receptor antagonist AP5 (50µM) -- > no LTP: no new spines Unsuccessful induction of LTP (e.g. only a short potentiation, <30 min): no new spines In 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 difficulties Uncertainty about the spatial location of the interesting synapses --> too large region to investigate Small and contradictive structural changes --> too large sample number > Big advantages of applied techniques!

Alternative (not contoversial) view Changes in spine shape influence synaptic strength (shortening and/or widening of the neck --> reduced resistance -->increased efficacy) Answer Maybe 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 required Functional --> 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, ; 1999)